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• 1. [Article] Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

  The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province ...

  Citation × Citation Citation Abstract Copy Title: Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197 Author: Keller, Randall A., Duncan, Robert A. The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar-39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2s) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N-R-N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n-22r-22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot. Title: Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197 Author: Keller, Randall A., Duncan, Robert A. The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar-39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2s) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N-R-N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n-22r-22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot. Close

• 2. [Article] Post-K/PB younger ⁴⁰Ar–³⁹Ar ages of the Mandla lavas: Implications for the duration of the Deccan volcanism

  This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/lithos/

  Citation × Citation Citation Abstract Copy Title: Post-K/PB younger ⁴⁰Ar–³⁹Ar ages of the Mandla lavas: Implications for the duration of the Deccan volcanism Author: Shrivastava, J. P., Duncan, Robert A., Kashyap, Mamta This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/lithos/ Title: Post-K/PB younger ⁴⁰Ar–³⁹Ar ages of the Mandla lavas: Implications for the duration of the Deccan volcanism Author: Shrivastava, J. P., Duncan, Robert A., Kashyap, Mamta This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/lithos/ Close

• 3. [Article] Age progressive volcanism in the New England seamounts and the opening of the Central Atlantic Ocean

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Age progressive volcanism in the New England seamounts and the opening of the Central Atlantic Ocean Author: Duncan, Robert A. copyrighted by American Geophysical Union Title: Age progressive volcanism in the New England seamounts and the opening of the Central Atlantic Ocean Author: Duncan, Robert A. copyrighted by American Geophysical Union Close

• 4. [Article] New 40Ar/39Ar age progression for the Louisville hot spot trail and implications for inter–hot spot motion

  In this study we present 42 new ⁴⁰Ar/³⁹Ar incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart ...

  Citation × Citation Citation Abstract Copy Title: New 40Ar/39Ar age progression for the Louisville hot spot trail and implications for inter–hot spot motion Author: Gee, Jeffrey S., Lonsdale, Peter F., Colwell, Lauren E., Koppers, Anthony A. P., Mahoney, John J., Duncan, Robert A., Gowen, Molly D. In this study we present 42 new ⁴⁰Ar/³⁹Ar incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian‐Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (~80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age‐progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian‐Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter–hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0°S, 134.5°W versus 52.4°S, 137.2°W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and hot spot motions, therefore provide a better fit to the new Louisville age data. The fit is particularly good for seamounts younger than 50 Ma, a period for which there is little predicted motion for the Louisville hot spot and little inter–hot spot motion with Hawaii. However, discrepancies in the Louisville age‐distance record prior to 50 Ma indicate there is an extra source of inter–hot spot motion between Louisville and the other Pacific hot spots that was not corrected for in the global S04G model. Finally, based on six new ⁴⁰Ar/³⁹Ar age dates, the 169°W bend in the Louisville seamount trail seems to have formed at least 3 Myr before the formation of the Hawaiian‐Emperor bend. The timing of the most acute parts of both bends thus appears to be asynchronous, which would require other processes (e.g., plume motions) than a global plate motion change between 50 and 47 Ma to explain these two observations. Title: New 40Ar/39Ar age progression for the Louisville hot spot trail and implications for inter–hot spot motion Author: Gee, Jeffrey S., Lonsdale, Peter F., Colwell, Lauren E., Koppers, Anthony A. P., Mahoney, John J., Duncan, Robert A., Gowen, Molly D. In this study we present 42 new ⁴⁰Ar/³⁹Ar incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian‐Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (~80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age‐progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian‐Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter–hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0°S, 134.5°W versus 52.4°S, 137.2°W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and hot spot motions, therefore provide a better fit to the new Louisville age data. The fit is particularly good for seamounts younger than 50 Ma, a period for which there is little predicted motion for the Louisville hot spot and little inter–hot spot motion with Hawaii. However, discrepancies in the Louisville age‐distance record prior to 50 Ma indicate there is an extra source of inter–hot spot motion between Louisville and the other Pacific hot spots that was not corrected for in the global S04G model. Finally, based on six new ⁴⁰Ar/³⁹Ar age dates, the 169°W bend in the Louisville seamount trail seems to have formed at least 3 Myr before the formation of the Hawaiian‐Emperor bend. The timing of the most acute parts of both bends thus appears to be asynchronous, which would require other processes (e.g., plume motions) than a global plate motion change between 50 and 47 Ma to explain these two observations. Close

• 5. [Article] Erosion by rivers and transport pathways in the ocean: A provenance tool using 40Ar--39Ar incremental heating on fine-grained sediment

  We use Ar-40-Ar-39 incremental heating to fingerprint bulk fluvial sediment entering the northeast Pacific Ocean, with the long-term intent of tracking sediment source and transport changes from the terrestrial ...

  Citation × Citation Citation Abstract Copy Title: Erosion by rivers and transport pathways in the ocean: A provenance tool using 40Ar--39Ar incremental heating on fine-grained sediment Author: Duncan, Robert A., Pisias, Nicklas G., VanLaningham, Sam We use Ar-40-Ar-39 incremental heating to fingerprint bulk fluvial sediment entering the northeast Pacific Ocean, with the long-term intent of tracking sediment source and transport changes from the terrestrial system to the marine environment through time. We show reproducible age spectra from individual rivers accounting for the majority of sediment delivered to the Pacific margin. Two tests are performed to confirm the validity of the bulk sediment Ar-40-Ar-39 incremental heating measurements and to address why polymineralic sediment might yield concordant age steps. The first model tests, in light of bulk mineralogy and diffusion of Ar from silicates, whether measured K/Ca spectra (measured from Ar-39 and Ar-37, respectively) are consistent with typical values for K- and Ca-bearing minerals. Calculations show that the bulk mineralogy is reflected in the outgassing K/Ca spectra and identify plagioclase as the dominant mineral contributing to the plateau-defining portion of the age spectra. A second model predicts bulk sediment ages from integrated bedrock cooling age-area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin. Calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for. Overall, this technique shows considerable promise, not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems. Title: Erosion by rivers and transport pathways in the ocean: A provenance tool using 40Ar--39Ar incremental heating on fine-grained sediment Author: Duncan, Robert A., Pisias, Nicklas G., VanLaningham, Sam We use Ar-40-Ar-39 incremental heating to fingerprint bulk fluvial sediment entering the northeast Pacific Ocean, with the long-term intent of tracking sediment source and transport changes from the terrestrial system to the marine environment through time. We show reproducible age spectra from individual rivers accounting for the majority of sediment delivered to the Pacific margin. Two tests are performed to confirm the validity of the bulk sediment Ar-40-Ar-39 incremental heating measurements and to address why polymineralic sediment might yield concordant age steps. The first model tests, in light of bulk mineralogy and diffusion of Ar from silicates, whether measured K/Ca spectra (measured from Ar-39 and Ar-37, respectively) are consistent with typical values for K- and Ca-bearing minerals. Calculations show that the bulk mineralogy is reflected in the outgassing K/Ca spectra and identify plagioclase as the dominant mineral contributing to the plateau-defining portion of the age spectra. A second model predicts bulk sediment ages from integrated bedrock cooling age-area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin. Calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for. Overall, this technique shows considerable promise, not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems. Close

• 6. [Article] Geochronology of Galapagos seamounts

  Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of ...

  Citation × Citation Citation Abstract Copy Title: Geochronology of Galapagos seamounts Author: Christie, David M., Duncan, Robert A., Sinton, Christopher W. Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of these features. The areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 Ma. Age-distance relationships are consistent with a decrease in the velocity of the Nazca plate relative to the Galápagos hotspot during the past 9 m.y., most likely at ~5 Ma when there was a change in Pacific-hotspot motion. Alternatively, the plate velocity remained constant and volcanism at a single seamount endured up to 7 m.y. lavas from seamounts along the Wolf-Darwin lineament to the northwest of the Galápagos Platform are young (<1 Ma), with the youngest measured ages closest to the platform. Title: Geochronology of Galapagos seamounts Author: Christie, David M., Duncan, Robert A., Sinton, Christopher W. Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of these features. The areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 Ma. Age-distance relationships are consistent with a decrease in the velocity of the Nazca plate relative to the Galápagos hotspot during the past 9 m.y., most likely at ~5 Ma when there was a change in Pacific-hotspot motion. Alternatively, the plate velocity remained constant and volcanism at a single seamount endured up to 7 m.y. lavas from seamounts along the Wolf-Darwin lineament to the northwest of the Galápagos Platform are young (<1 Ma), with the youngest measured ages closest to the platform. Close

• 7. [Article] Insights into the late Cenozoic configuration of the laurentide Ice Sheet from ⁴⁰Ar/³⁹Ar dating of glacially transported minerals in midcontinent tills

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Insights into the late Cenozoic configuration of the laurentide Ice Sheet from ⁴⁰Ar/³⁹Ar dating of glacially transported minerals in midcontinent tills Author: Hemming, Sidney R., Duncan, Robert A., Clark, Peter U., Roy, Martin copyrighted by American Geophysical Union Title: Insights into the late Cenozoic configuration of the laurentide Ice Sheet from ⁴⁰Ar/³⁹Ar dating of glacially transported minerals in midcontinent tills Author: Hemming, Sidney R., Duncan, Robert A., Clark, Peter U., Roy, Martin copyrighted by American Geophysical Union Close

• 8. [Article] Implications of a nonlinear ⁴⁰Ar/³⁹Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Implications of a nonlinear ⁴⁰Ar/³⁹Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots Author: Koppers, Anthony A. P., Steinberger, Bernhard, Duncan, Robert A. Copyrighted by American Geophysical Union. Title: Implications of a nonlinear ⁴⁰Ar/³⁹Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots Author: Koppers, Anthony A. P., Steinberger, Bernhard, Duncan, Robert A. Copyrighted by American Geophysical Union. Close

• 9. [Article] Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027.

  Citation × Citation Citation Abstract Copy Title: Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites Author: Ford, Mark T., Duncan, Robert A., Grunder, Anita L. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027. Title: Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites Author: Ford, Mark T., Duncan, Robert A., Grunder, Anita L. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027. Close

• 10. [Article] Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas

  The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya ...

  Citation × Citation Citation Abstract Copy Title: Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas Author: Denyer, Percy, Duncan, Robert A., Sinton, Christopher W. The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jaco, and the Quepos Peninsula using Ar-40-Ar-39 radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88-90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83-84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid-ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau [Donnelly, 1994]. The Jacó lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ~64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks, Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jacó lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume. Title: Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas Author: Denyer, Percy, Duncan, Robert A., Sinton, Christopher W. The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jaco, and the Quepos Peninsula using Ar-40-Ar-39 radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88-90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83-84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid-ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau [Donnelly, 1994]. The Jacó lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ~64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks, Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jacó lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume. Close

• 11. [Article] Effective elastic thickness of the lithosphere along the Easter Seamount Chain

  Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2-min altimetry-derived global gravity grid are used to determine the effective elastic ...

  Citation × Citation Citation Abstract Copy Title: Effective elastic thickness of the lithosphere along the Easter Seamount Chain Author: Kruse, Sarah E., Duncan, Robert A., Liu, Zhengrong J., Naar, David F. Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2-min altimetry-derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the Easter Seamount Chain (ESC). Forward modeling, admittance, and coherence methods yield consistent results. With the exception of the eastern and western ends of the ESC the effective elastic thickness along the chain is similar to 1-4 km. The thin elastic thickness for the majority of the ESC seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the Nazca plate seafloor age. The elastic thickness southeast of the Nazca fracture zone is similar to 6 km, apparently because of the seafloor age discontinuity across the fracture zone. The elastic thickness near the San Felix Island, at the eastern end of the ESC, is even greater (similar to 11 km), which is compatible with the estimated seafloor age at the time of loading. A slight increase in the effective elastic thickness of the far western part of the ESC suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center. These findings combined with published geochemistry support a hotspot origin for the ESC, complicated by large-scale plate boundary reorganizations and channeling of plume material to the East Pacific Rise. Title: Effective elastic thickness of the lithosphere along the Easter Seamount Chain Author: Kruse, Sarah E., Duncan, Robert A., Liu, Zhengrong J., Naar, David F. Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2-min altimetry-derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the Easter Seamount Chain (ESC). Forward modeling, admittance, and coherence methods yield consistent results. With the exception of the eastern and western ends of the ESC the effective elastic thickness along the chain is similar to 1-4 km. The thin elastic thickness for the majority of the ESC seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the Nazca plate seafloor age. The elastic thickness southeast of the Nazca fracture zone is similar to 6 km, apparently because of the seafloor age discontinuity across the fracture zone. The elastic thickness near the San Felix Island, at the eastern end of the ESC, is even greater (similar to 11 km), which is compatible with the estimated seafloor age at the time of loading. A slight increase in the effective elastic thickness of the far western part of the ESC suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center. These findings combined with published geochemistry support a hotspot origin for the ESC, complicated by large-scale plate boundary reorganizations and channeling of plume material to the East Pacific Rise. Close

• 12. [Article] Temporal variations in plate convergence and eruption rates in the Western Cascades, Oregon

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Temporal variations in plate convergence and eruption rates in the Western Cascades, Oregon Author: Verplanck, Emily P., Duncan, Robert A. copyrighted by American Geophysical Union Title: Temporal variations in plate convergence and eruption rates in the Western Cascades, Oregon Author: Verplanck, Emily P., Duncan, Robert A. copyrighted by American Geophysical Union Close

• 13. [Article] Age of the Batoka basalts, norther Zimbabwe, and the duration of Karoo Large Igneous Province magmatism

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Age of the Batoka basalts, norther Zimbabwe, and the duration of Karoo Large Igneous Province magmatism Author: Briden, J. C., Jones, D. L., MacNiocaill, C., Duncan, Robert A., Randall, D. E. copyrighted by American Geophysical Union Title: Age of the Batoka basalts, norther Zimbabwe, and the duration of Karoo Large Igneous Province magmatism Author: Briden, J. C., Jones, D. L., MacNiocaill, C., Duncan, Robert A., Randall, D. E. copyrighted by American Geophysical Union Close

• 14. [Article] DuncanTimingCompositionContinental.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinental.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinental.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 15. [Article] DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 16. [Article] DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 17. [Article] Evolution of the Walvis Ridge-Rio Grande rise hot spot systen : implications for African and South American plate motions over plumes

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Evolution of the Walvis Ridge-Rio Grande rise hot spot systen : implications for African and South American plate motions over plumes Author: O'Connor, John M., Duncan, Robert A. copyrighted by American Geophysical Union Title: Evolution of the Walvis Ridge-Rio Grande rise hot spot systen : implications for African and South American plate motions over plumes Author: O'Connor, John M., Duncan, Robert A. copyrighted by American Geophysical Union Close

• 18. [Article] Geochronology of age-progressive volcanism of the Oregon High Lava Plains : implications for the plume interpretation of Yellowstone

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Geochronology of age-progressive volcanism of the Oregon High Lava Plains : implications for the plume interpretation of Yellowstone Author: Deino, Alan L., Grunder, Anita L., Duncan, Robert A., Jordan, Brennan T. copyrighted by American Geophysical Union Title: Geochronology of age-progressive volcanism of the Oregon High Lava Plains : implications for the plume interpretation of Yellowstone Author: Deino, Alan L., Grunder, Anita L., Duncan, Robert A., Jordan, Brennan T. copyrighted by American Geophysical Union Close

• 19. [Article] ⁴⁰Ar-³⁹Ar chronology for the volcanic history of the Southeast Greenland rifted margin

  Results of ⁴⁰Ar–³⁹Ar Ar dating constrain the age of the submerged volcanic succession, part of the seaward-dip ping reflector sequence of the Southeast Greenland volcanic rifted margin, recovered during ...

  Citation × Citation Citation Abstract Copy Title: ⁴⁰Ar-³⁹Ar chronology for the volcanic history of the Southeast Greenland rifted margin Author: Tegner, Christian, Duncan, Robert A. Results of ⁴⁰Ar–³⁹Ar Ar dating constrain the age of the submerged volcanic succession, part of the seaward-dip ping reflector sequence of the Southeast Greenland volcanic rifted margin, recovered during Leg 163. At the 3°Ne 6 drilling transect, the fully normally magnetized volcanic units at Holes 989B (Unit 1) and 990A (Units 1 and 2) are dated at 57.1 ± 1.3 Ma and 55.6 ± 0.6 Ma, respectively. This correlates with a common magnetochron, C25n. The underlying, reversely magnetized lavas at Hole 990A (Units 3–13) yield an average age of 55.8 ± 0.7 Ma and may correlate with C25r. The argon data, however, are also consistent with eruption of the lavas at Site 990 during the very earliest portion of C24. If so, the normally polarized unevittsohav be correlated to a cryptochron ( e.g., C24r -11 at ~55.57 Ma). The lavas at Holes 989B and 990A have typical oceanic compositions, implying that final plate separation between Greenland and northwest Europe took place at ~56 Ma. The age for Hole 989B lava is younger than expected from the seismic interpretations, posing questions about the structural evolution of the margin. An age of 49.6 ± 0.2 Ma for the basaltic lava at Site 9 88 (~6°N) points to the importance of postbreakup tholeiitic magmatism at the rifted margin. Together with results from Leg 152, a virtually complete time frame for ~12 m.y. of pre-, syn-, and postbreakup volcanism during rifted margin evolution in Southeast Greenland can now be assembled. This time frame includes continental type volcanism at ~6 1–60 Ma, synbreakup volcanism beginning at ~57 Ma, and postbreakup volcanism at ~49.6 Ma. These discrete time windows coincide with distinct periods of tholeiitic magmatism from the onshore East Greenland Tertiary Igneous Province and is consistent with discrete mantle-melting events triggered by plume arrival (~ 61–60 Ma) under central Greenland, continental breakup (~57–54 Ma), and passage of the plume axis beneath the East Greenland rifted margin after breakup (~50–49 Ma ), respectively. Title: ⁴⁰Ar-³⁹Ar chronology for the volcanic history of the Southeast Greenland rifted margin Author: Tegner, Christian, Duncan, Robert A. Results of ⁴⁰Ar–³⁹Ar Ar dating constrain the age of the submerged volcanic succession, part of the seaward-dip ping reflector sequence of the Southeast Greenland volcanic rifted margin, recovered during Leg 163. At the 3°Ne 6 drilling transect, the fully normally magnetized volcanic units at Holes 989B (Unit 1) and 990A (Units 1 and 2) are dated at 57.1 ± 1.3 Ma and 55.6 ± 0.6 Ma, respectively. This correlates with a common magnetochron, C25n. The underlying, reversely magnetized lavas at Hole 990A (Units 3–13) yield an average age of 55.8 ± 0.7 Ma and may correlate with C25r. The argon data, however, are also consistent with eruption of the lavas at Site 990 during the very earliest portion of C24. If so, the normally polarized unevittsohav be correlated to a cryptochron ( e.g., C24r -11 at ~55.57 Ma). The lavas at Holes 989B and 990A have typical oceanic compositions, implying that final plate separation between Greenland and northwest Europe took place at ~56 Ma. The age for Hole 989B lava is younger than expected from the seismic interpretations, posing questions about the structural evolution of the margin. An age of 49.6 ± 0.2 Ma for the basaltic lava at Site 9 88 (~6°N) points to the importance of postbreakup tholeiitic magmatism at the rifted margin. Together with results from Leg 152, a virtually complete time frame for ~12 m.y. of pre-, syn-, and postbreakup volcanism during rifted margin evolution in Southeast Greenland can now be assembled. This time frame includes continental type volcanism at ~6 1–60 Ma, synbreakup volcanism beginning at ~57 Ma, and postbreakup volcanism at ~49.6 Ma. These discrete time windows coincide with distinct periods of tholeiitic magmatism from the onshore East Greenland Tertiary Igneous Province and is consistent with discrete mantle-melting events triggered by plume arrival (~ 61–60 Ma) under central Greenland, continental breakup (~57–54 Ma), and passage of the plume axis beneath the East Greenland rifted margin after breakup (~50–49 Ma ), respectively. Close

• 20. [Article] High-resolution ⁴⁰Ar/³⁹Ar dating of the oldest oceanic basement basalts in the western Pacific basin

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: High-resolution ⁴⁰Ar/³⁹Ar dating of the oldest oceanic basement basalts in the western Pacific basin Author: Staudigel, Hubert, Koppers, Anthony A. P., Duncan, Robert A. Copyrighted by American Geophysical Union. Title: High-resolution ⁴⁰Ar/³⁹Ar dating of the oldest oceanic basement basalts in the western Pacific basin Author: Staudigel, Hubert, Koppers, Anthony A. P., Duncan, Robert A. Copyrighted by American Geophysical Union. Close

• 21. [Article] Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/.

  Citation × Citation Citation Abstract Copy Title: Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti Author: Krawl, Kyle, Kent, Adam J. R., Duncan, Robert A., Loewen, Matthew W. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Title: Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti Author: Krawl, Kyle, Kent, Adam J. R., Duncan, Robert A., Loewen, Matthew W. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Close

• 22. [Article] Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier ...

  Citation × Citation Citation Abstract Copy Title: Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/ Title: Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/ Close

• 23. [Article] Five million years of compositionally diverse, episodic volcanism : construction of Davidson Seamount atop an abandoned spreading center

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Five million years of compositionally diverse, episodic volcanism : construction of Davidson Seamount atop an abandoned spreading center Author: Huard, J. J., Lonsdale, P., Davis, A. S., Paduan, J. B., Duncan, Robert A., Clague, D. A., DeVogelaere, Castillo, P. R. copyrighted by American Geophysical Union Title: Five million years of compositionally diverse, episodic volcanism : construction of Davidson Seamount atop an abandoned spreading center Author: Huard, J. J., Lonsdale, P., Davis, A. S., Paduan, J. B., Duncan, Robert A., Clague, D. A., DeVogelaere, Castillo, P. R. copyrighted by American Geophysical Union Close

• 24. [Article] Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland

  This is the publisher’s final pdf. The published article is copyrighted by Dansk Geologisk Forening and can be found at: http://www.2dgf.dk/.

  Citation × Citation Citation Abstract Copy Title: Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland Author: Pedersen, Asger K., Larsen, Lotte M., Watt, W. Stuart, Sorensen, Erik Vest, Duncan, Robert A. This is the publisher’s final pdf. The published article is copyrighted by Dansk Geologisk Forening and can be found at: http://www.2dgf.dk/. Title: Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland Author: Pedersen, Asger K., Larsen, Lotte M., Watt, W. Stuart, Sorensen, Erik Vest, Duncan, Robert A. This is the publisher’s final pdf. The published article is copyrighted by Dansk Geologisk Forening and can be found at: http://www.2dgf.dk/. Close

• 25. [Article] The Cobb-Eickelberg seamount chain : hotspot volcanism with mid-oean ridge basalt affinity

  Cobb hotspot, currently located beneath Axial seamount on the Juan de Fuca ridge, has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products ...

  Citation × Citation Citation Abstract Copy Title: The Cobb-Eickelberg seamount chain : hotspot volcanism with mid-oean ridge basalt affinity Author: Desonie, Dana L., Duncan, Robert A. Cobb hotspot, currently located beneath Axial seamount on the Juan de Fuca ridge, has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products of the hotspot, from eight volcanoes in the Cobb-Eickelberg seamount (CES) chain, show a westward age progression away from the hotspot and a westward increase in the age difference between the seamounts and the crust on which they formed. These results are consistent with movement of the Pacific plate over a fixed Cobb hotspot and eventual encroachment by the westwardly migrating Juan de Fuca ridge. CES lavas are slightly enriched in alkalies and incompatible elements relative to those of the Juan de Fuca ridge but they have Sr. Nd, and Pb isotopic compositions virtually identical to those found along the ridge. Therefore, Cobb hotspot is a stationary, upper mantle melting anomaly whose volcanic products show strong mid-ocean ridge basalt (MORB) affinity. These observations can be explained by low degrees of partial melting of entrained heterogeneous upper mantle MORB source material within a thermally driven lower mantle diapir or by an intrinsic MORB-like composition of the deeper mantle source region from which northeast Pacific plumes rise. Title: The Cobb-Eickelberg seamount chain : hotspot volcanism with mid-oean ridge basalt affinity Author: Desonie, Dana L., Duncan, Robert A. Cobb hotspot, currently located beneath Axial seamount on the Juan de Fuca ridge, has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products of the hotspot, from eight volcanoes in the Cobb-Eickelberg seamount (CES) chain, show a westward age progression away from the hotspot and a westward increase in the age difference between the seamounts and the crust on which they formed. These results are consistent with movement of the Pacific plate over a fixed Cobb hotspot and eventual encroachment by the westwardly migrating Juan de Fuca ridge. CES lavas are slightly enriched in alkalies and incompatible elements relative to those of the Juan de Fuca ridge but they have Sr. Nd, and Pb isotopic compositions virtually identical to those found along the ridge. Therefore, Cobb hotspot is a stationary, upper mantle melting anomaly whose volcanic products show strong mid-ocean ridge basalt (MORB) affinity. These observations can be explained by low degrees of partial melting of entrained heterogeneous upper mantle MORB source material within a thermally driven lower mantle diapir or by an intrinsic MORB-like composition of the deeper mantle source region from which northeast Pacific plumes rise. Close

• 26. [Article] A captured island chain in the coast range of Oregon and Washington

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: A captured island chain in the coast range of Oregon and Washington Author: Duncan, Robert A. copyrighted by American Geophysical Union Title: A captured island chain in the coast range of Oregon and Washington Author: Duncan, Robert A. copyrighted by American Geophysical Union Close

• 27. [Article] Radiometric dating of Young MORB using the ⁴⁰Ar-³⁹Ar incremental heating method

  copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Radiometric dating of Young MORB using the ⁴⁰Ar-³⁹Ar incremental heating method Author: Hogan, L. G., Duncan, Robert A. copyrighted by American Geophysical Union. Title: Radiometric dating of Young MORB using the ⁴⁰Ar-³⁹Ar incremental heating method Author: Hogan, L. G., Duncan, Robert A. copyrighted by American Geophysical Union. Close

• 28. [Article] Temporal and geochemical variability of volcanic products of the Marquesas hotspot

  copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Temporal and geochemical variability of volcanic products of the Marquesas hotspot Author: Desonie, Dana L., Duncan, Robert A., Natland, J. H. copyrighted by American Geophysical Union. Title: Temporal and geochemical variability of volcanic products of the Marquesas hotspot Author: Desonie, Dana L., Duncan, Robert A., Natland, J. H. copyrighted by American Geophysical Union. Close

• 29. [Article] The timing and duration of the Karoo igneous event, southern Gondwana

  A volcanic event of immense scale occurred within a relatively short period in early Jurassic time over large regions of the contiguous Gondwana supercontinent. In southern Africa, associated remnants ...

  Citation × Citation Citation Abstract Copy Title: The timing and duration of the Karoo igneous event, southern Gondwana Author: Duncan, A.R., Duncan, Robert A., Marsh, J.S., Hooper, P.R., Rehacek, J. A volcanic event of immense scale occurred within a relatively short period in early Jurassic time over large regions of the contiguous Gondwana supercontinent. In southern Africa, associated remnants of thick volcanic successions of lava flows and extensive dike and sill complexes of similar composition have been grouped together as the Karoo Igneous Province. Correlative volcanic and plutonic rocks occur in Antarctica and Australia as the Ferrar Province. Thirty-two new Ar-40/Ar-39 incremental heating experiments on feldspars and whole rocks from Namibia, South Africa and East Antarctica produce highly resolved ages with a vast majority at 183 +/- 1 Ma and a total range of 184 to 179 Ma. These are indistinguishable from recent, high-resolution Ar-40/Ar-39 and U-Pb age determinations reported from the Antarctic portion of the province. Initial Karoo volcanism (Lesotho-type compositions) occurred across the entire South African craton. The ubiquitous distribution of a plexus of generally nonoriented feeder dikes and sills intruding Precambrian crystalline rocks and Phanerozoic sediments indicates that these magmas penetrated the craton over a broad region. Lithosphere thinning of the continent followed the main pulse of igneous activity, with volcanism focused in the Lebombo-Nuanetsi region, near the eventual split between Africa and Antarctica. Seafloor spreading and dispersion of east and west Gondwana followed some 10-20 m.y. afterward. The volume of the combined Karoo-Ferrar province (similar to 2.5 x 10(6) km(3)) makes it one of the largest continental flood basalt events. The timing of this event correlates with a moderate mass extinction (Toarcian-Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian-Triassic or Cretaceous-Tertiary boundaries associated with the Siberian and Deccan flood basalts events, respectively. The difference may be due to the high southerly latitude and somewhat lower eruption rates of the Karoo event. Title: The timing and duration of the Karoo igneous event, southern Gondwana Author: Duncan, A.R., Duncan, Robert A., Marsh, J.S., Hooper, P.R., Rehacek, J. A volcanic event of immense scale occurred within a relatively short period in early Jurassic time over large regions of the contiguous Gondwana supercontinent. In southern Africa, associated remnants of thick volcanic successions of lava flows and extensive dike and sill complexes of similar composition have been grouped together as the Karoo Igneous Province. Correlative volcanic and plutonic rocks occur in Antarctica and Australia as the Ferrar Province. Thirty-two new Ar-40/Ar-39 incremental heating experiments on feldspars and whole rocks from Namibia, South Africa and East Antarctica produce highly resolved ages with a vast majority at 183 +/- 1 Ma and a total range of 184 to 179 Ma. These are indistinguishable from recent, high-resolution Ar-40/Ar-39 and U-Pb age determinations reported from the Antarctic portion of the province. Initial Karoo volcanism (Lesotho-type compositions) occurred across the entire South African craton. The ubiquitous distribution of a plexus of generally nonoriented feeder dikes and sills intruding Precambrian crystalline rocks and Phanerozoic sediments indicates that these magmas penetrated the craton over a broad region. Lithosphere thinning of the continent followed the main pulse of igneous activity, with volcanism focused in the Lebombo-Nuanetsi region, near the eventual split between Africa and Antarctica. Seafloor spreading and dispersion of east and west Gondwana followed some 10-20 m.y. afterward. The volume of the combined Karoo-Ferrar province (similar to 2.5 x 10(6) km(3)) makes it one of the largest continental flood basalt events. The timing of this event correlates with a moderate mass extinction (Toarcian-Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian-Triassic or Cretaceous-Tertiary boundaries associated with the Siberian and Deccan flood basalts events, respectively. The difference may be due to the high southerly latitude and somewhat lower eruption rates of the Karoo event. Close

• 30. [Article] Spatial and temporal distribution of a C-like asthenoshpheric component in the Rano Rahi Seamount Field, East Pacific Rise, 15°-19°S

  copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Spatial and temporal distribution of a C-like asthenoshpheric component in the Rano Rahi Seamount Field, East Pacific Rise, 15°-19°S Author: Sinton, J. M., Mahoney, J. J., Duncan, Robert A., Hall, L. S. copyrighted by American Geophysical Union. Title: Spatial and temporal distribution of a C-like asthenoshpheric component in the Rano Rahi Seamount Field, East Pacific Rise, 15°-19°S Author: Sinton, J. M., Mahoney, J. J., Duncan, Robert A., Hall, L. S. copyrighted by American Geophysical Union. Close

• 31. [Article] Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume Author: McBirney, Alexander R., White, William M., Duncan, Robert A. copyrighted by American Geophysical Union Title: Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume Author: McBirney, Alexander R., White, William M., Duncan, Robert A. copyrighted by American Geophysical Union Close

• 32. [Article] Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models Author: Roperch, Pierrick, Chauvin, Annick, Duncan, Robert A. copyrighted by American Geophysical Union Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models Author: Roperch, Pierrick, Chauvin, Annick, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 33. [Article] ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81

  ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project ...

  Citation × Citation Citation Abstract Copy Title: ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81 Author: Duncan, Robert A., Sinton, Christopher W. ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project Leg 81, southwest Rockall Plateau (56°N). Both of these legs drilled into thick sections of submerged lava flows, known as seaward-dipping reflector sequences, that are part of the Tertiary North Atlantic Volcanic Province (NAVP). Results show that subaerial volcanism began at the southeast Greenland Margin as early as 6162 Ma with the eruption of continentally contaminated lavas. After an apparent hiatus, volcanism continued with the subaerial eruption of noncontaminated, mid-ocean-ridge basalt (MORB) -like lavas. A sill or flow was emplaced at 52 Ma into the sediments overlying the basement lavas at the most seaward site (918). The MORB-like lavas of the Rockall Plateau gave poor results, although the data suggest that they erupted at 5758 Ma and may be contemporaneous with the compositionally similar oceanic lavas of Leg 152. Compiled ages from the NAVP indicate that most of the lavas were erupted in two distinct magmatic episodes. The first occurred prior to 60 Ma with volcanism at the southeast Greenland Margin, in West Greenland, and the northwest British Isles. This early stage of widespread volcanism most likely records the arrival of the Iceland mantle plume. The second phase of magmatism began about 57 Ma with voluminous basaltic eruptions centralized along the eventual line of continental separation. Magmatism at this stage was associated with continental breakup and asthenospheric upwelling with probable thermal input from the mantle plume. Title: ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81 Author: Duncan, Robert A., Sinton, Christopher W. ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project Leg 81, southwest Rockall Plateau (56°N). Both of these legs drilled into thick sections of submerged lava flows, known as seaward-dipping reflector sequences, that are part of the Tertiary North Atlantic Volcanic Province (NAVP). Results show that subaerial volcanism began at the southeast Greenland Margin as early as 6162 Ma with the eruption of continentally contaminated lavas. After an apparent hiatus, volcanism continued with the subaerial eruption of noncontaminated, mid-ocean-ridge basalt (MORB) -like lavas. A sill or flow was emplaced at 52 Ma into the sediments overlying the basement lavas at the most seaward site (918). The MORB-like lavas of the Rockall Plateau gave poor results, although the data suggest that they erupted at 5758 Ma and may be contemporaneous with the compositionally similar oceanic lavas of Leg 152. Compiled ages from the NAVP indicate that most of the lavas were erupted in two distinct magmatic episodes. The first occurred prior to 60 Ma with volcanism at the southeast Greenland Margin, in West Greenland, and the northwest British Isles. This early stage of widespread volcanism most likely records the arrival of the Iceland mantle plume. The second phase of magmatism began about 57 Ma with voluminous basaltic eruptions centralized along the eventual line of continental separation. Magmatism at this stage was associated with continental breakup and asthenospheric upwelling with probable thermal input from the mantle plume. Close

• 34. [Article] Tahiti: Geochemical evolution of a French Polynesian volcano

  The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system ...

  Citation × Citation Citation Abstract Copy Title: Tahiti: Geochemical evolution of a French Polynesian volcano Author: Fisk, Martin R., White, William M., Duncan, Robert A., Nielsen, Roger L. The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (~1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and the late-stage, valley-filled phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg#60-64), while the later valley-filled lavas are basanitic (Mg#64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also changes systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this ~0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0 .9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield-building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25-50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognized a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radically zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted viscously coupled sheath. A correlation between Pa/Ce and isotope ratio suggest that the Society plume contains deeply recycled continental material. Title: Tahiti: Geochemical evolution of a French Polynesian volcano Author: Fisk, Martin R., White, William M., Duncan, Robert A., Nielsen, Roger L. The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (~1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and the late-stage, valley-filled phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg#60-64), while the later valley-filled lavas are basanitic (Mg#64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also changes systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this ~0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0 .9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield-building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25-50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognized a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radically zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted viscously coupled sheath. A correlation between Pa/Ce and isotope ratio suggest that the Society plume contains deeply recycled continental material. Close

• 35. [Article] Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record

  This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/.

  Citation × Citation Citation Abstract Copy Title: Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record Author: Wessel, Paul, Duncan, Robert A., Mahoney, John J., Ray, Jyotisankar, Naar, David F., Ray, Jyotiranjan S. This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. Title: Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record Author: Wessel, Paul, Duncan, Robert A., Mahoney, John J., Ray, Jyotisankar, Naar, David F., Ray, Jyotiranjan S. This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. Close

• 36. [Article] Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion

  copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion Author: Chauvin, Annick, Duncan, Robert A., Bonhommet, Norbert, Levi, Saul copyrighted by American Geophysical Union. Title: Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion Author: Chauvin, Annick, Duncan, Robert A., Bonhommet, Norbert, Levi, Saul copyrighted by American Geophysical Union. Close

• 37. [Article] Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range Author: Cox, Allan, Magill, James, Duncan, Robert A. copyrighted by American Geophysical Union Title: Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range Author: Cox, Allan, Magill, James, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 38. [Article] Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine Author: Roperch, Pierrick, Duncan, Robert A. copyrighted by American Geophysical Union Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine Author: Roperch, Pierrick, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 39. [Article] Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System Author: Watts, A. B., Weissel, J. K., Duncan, Robert A., Larson, R. L. copyrighted by American Geophysical Union Title: Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System Author: Watts, A. B., Weissel, J. K., Duncan, Robert A., Larson, R. L. copyrighted by American Geophysical Union Close

• 40. [Article] Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau

  In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar ...

  Citation × Citation Citation Abstract Copy Title: Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau Author: Spadea, Piera, Tarney, John, Marriner, Giselle F., Nivia, Alvaro, Duncan, Robert A., Kerr, Andrew C, Kempton, Pamela D. In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar to 90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high-MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The Ar-40/Ar-39 ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)(n) > 1 and (ε(Nd))(i) < +4.1) to relatively depleted ((La/Nd)(n) < 1 and (ε(Nd))(i) > +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas. Title: Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau Author: Spadea, Piera, Tarney, John, Marriner, Giselle F., Nivia, Alvaro, Duncan, Robert A., Kerr, Andrew C, Kempton, Pamela D. In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar to 90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high-MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The Ar-40/Ar-39 ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)(n) > 1 and (ε(Nd))(i) < +4.1) to relatively depleted ((La/Nd)(n) < 1 and (ε(Nd))(i) > +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas. Close

• 41. [Article] Seamount morphology in the Bowie and Cobb hot spot trails, Gulf of Alaska

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Seamount morphology in the Bowie and Cobb hot spot trails, Gulf of Alaska Author: Dziak, Robert P., Keller, Randall A., Chaytor, Jason D., Duncan, Robert A. copyrighted by American Geophysical Union Title: Seamount morphology in the Bowie and Cobb hot spot trails, Gulf of Alaska Author: Dziak, Robert P., Keller, Randall A., Chaytor, Jason D., Duncan, Robert A. copyrighted by American Geophysical Union Close

• 42. [Article] Hotspots, mantle plumes, flood basalts, and true polar wander

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Hotspots, mantle plumes, flood basalts, and true polar wander Author: Duncan, Robert A., Richards, M. A. copyrighted by American Geophysical Union Title: Hotspots, mantle plumes, flood basalts, and true polar wander Author: Duncan, Robert A., Richards, M. A. copyrighted by American Geophysical Union Close

• 43. [Article] Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor

  Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge ...

  Citation × Citation Citation Abstract Copy Title: Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor Author: Mahoney, J. J., Pyle, D. G., Duncan, Robert A., Christie, David M. Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87SR/86SR (0.7025-0.7029) and 206Pb/204Pb (18.80-19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030-0.7035), low 206Pb/204Pb (17.99-18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma. Title: Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor Author: Mahoney, J. J., Pyle, D. G., Duncan, Robert A., Christie, David M. Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87SR/86SR (0.7025-0.7029) and 206Pb/204Pb (18.80-19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030-0.7035), low 206Pb/204Pb (17.99-18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma. Close

• 44. [Article] Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: implications for the lithospheric cracking hypothesis for the origin of intraplate, non-hotspot volcanic chains

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: implications for the lithospheric cracking hypothesis for the origin of intraplate, non-hotspot volcanic chains Author: Scheirer, Daniel S., Forsyth, Donald W., Harmon, Nicholas, Duncan, Robert A. copyrighted by American Geophysical Union Title: Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: implications for the lithospheric cracking hypothesis for the origin of intraplate, non-hotspot volcanic chains Author: Scheirer, Daniel S., Forsyth, Donald W., Harmon, Nicholas, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 45. [Article] ⁴⁰Ar/³⁹Ar geochronology of silicic and basic volcanic rocks on the margins of the North Atlantic

  At the submerged margins of the North Atlantic, andesitic to dacitic and basaltic volcanic rocks occur together. The silicic rocks were derived by processes requiring the presence of continental crust (crustal ...

  Citation × Citation Citation Abstract Copy Title: ⁴⁰Ar/³⁹Ar geochronology of silicic and basic volcanic rocks on the margins of the North Atlantic Author: Hitchen, K., Duncan, Robert A., Sinton, C. W. At the submerged margins of the North Atlantic, andesitic to dacitic and basaltic volcanic rocks occur together. The silicic rocks were derived by processes requiring the presence of continental crust (crustal anatexis and/or contamination of mafic magmas) while the majority of the basaltic lavas had little or no contact with continental crust. We report ⁴⁰Ar–³⁹Ar incremental heating ages for several dacitic and basaltic rocks recovered from three offshore localities of the North Atlantic Igneous Province. Dacitic lavas and tuffs at the southeast Greenland margin and trachytic lavas in the Scottish Hebrides erupted contemporaneously with basaltic lavas at 62–61 Ma. In contrast, the silicic lavas from the northern Rockall Trough (offshore western Scotland) and the Vøring Plateau (offshore Norway) erupted at ~ 55 Ma followed shortly by basaltic volcanism. At this time, silicic magmatism at the southeast Greenland margin had ceased and only oceanic basalts were erupted. Similarly, ~ 55 Ma lavas on the southwest Rockall Plateau are wholly basaltic. The compositions of all of the dated silicic volcanic rocks are consistent with derivation from partial melting of either continental crust or sediments. The heat necessary for partial melting appears to have been provided by basaltic magmas. Therefore, the existence of the silicic rocks indicates the presence of continental crust as well as a stable tectonic environment that allowed the stagnation and pooling of basaltic melts within the crust. With this in mind, it is apparent that at 62–60 Ma, both western and eastern sides of the present North Atlantic margins were characterized by extensional environments within continental crust that were restrictive to the passage of mafic magmas. By 55 Ma, at the time of continental breakup, the proximal margins at southeast Greenland and the Rockall Plateau were devoid of continental crust. But the presence of 55 Ma silicic magmatism on the eastern North Atlantic margin can be attributed to a broader zone of magmatism and sediment-filled Mesozoic rift basins. Title: ⁴⁰Ar/³⁹Ar geochronology of silicic and basic volcanic rocks on the margins of the North Atlantic Author: Hitchen, K., Duncan, Robert A., Sinton, C. W. At the submerged margins of the North Atlantic, andesitic to dacitic and basaltic volcanic rocks occur together. The silicic rocks were derived by processes requiring the presence of continental crust (crustal anatexis and/or contamination of mafic magmas) while the majority of the basaltic lavas had little or no contact with continental crust. We report ⁴⁰Ar–³⁹Ar incremental heating ages for several dacitic and basaltic rocks recovered from three offshore localities of the North Atlantic Igneous Province. Dacitic lavas and tuffs at the southeast Greenland margin and trachytic lavas in the Scottish Hebrides erupted contemporaneously with basaltic lavas at 62–61 Ma. In contrast, the silicic lavas from the northern Rockall Trough (offshore western Scotland) and the Vøring Plateau (offshore Norway) erupted at ~ 55 Ma followed shortly by basaltic volcanism. At this time, silicic magmatism at the southeast Greenland margin had ceased and only oceanic basalts were erupted. Similarly, ~ 55 Ma lavas on the southwest Rockall Plateau are wholly basaltic. The compositions of all of the dated silicic volcanic rocks are consistent with derivation from partial melting of either continental crust or sediments. The heat necessary for partial melting appears to have been provided by basaltic magmas. Therefore, the existence of the silicic rocks indicates the presence of continental crust as well as a stable tectonic environment that allowed the stagnation and pooling of basaltic melts within the crust. With this in mind, it is apparent that at 62–60 Ma, both western and eastern sides of the present North Atlantic margins were characterized by extensional environments within continental crust that were restrictive to the passage of mafic magmas. By 55 Ma, at the time of continental breakup, the proximal margins at southeast Greenland and the Rockall Plateau were devoid of continental crust. But the presence of 55 Ma silicic magmatism on the eastern North Atlantic margin can be attributed to a broader zone of magmatism and sediment-filled Mesozoic rift basins. Close

• 46. [Article] Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/.

  Citation × Citation Citation Abstract Copy Title: Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench Author: Wright, Dawn J., Meffre, Sebastian, Bloomer, Sherman H., Duncan, Robert A., Hauff, Folkmar, Falloon, Trevor J., Hoernle, Kaj, Crawford, Tony J. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Title: Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench Author: Wright, Dawn J., Meffre, Sebastian, Bloomer, Sherman H., Duncan, Robert A., Hauff, Folkmar, Falloon, Trevor J., Hoernle, Kaj, Crawford, Tony J. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Close

• 47. [Article] Evidence for diffuse exension of the Pacific plate from Pukapuka ridges and cross-grain gravity lineations

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Evidence for diffuse exension of the Pacific plate from Pukapuka ridges and cross-grain gravity lineations Author: Mammerickx, Jacqueline, Levitt, Daniel A., Johnson, Catherine L., Sandwell, David T., Lynch, Mary Ann, Duncan, Robert A., Winterer, Edward L. copyrighted by American Geophysical Union Title: Evidence for diffuse exension of the Pacific plate from Pukapuka ridges and cross-grain gravity lineations Author: Mammerickx, Jacqueline, Levitt, Daniel A., Johnson, Catherine L., Sandwell, David T., Lynch, Mary Ann, Duncan, Robert A., Winterer, Edward L. copyrighted by American Geophysical Union Close

• 48. [Article] Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise

  This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by American Geophysical Union and can be found at: http://www.agu.org/journals/gc...

  Citation × Citation Citation Abstract Copy Title: Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise Author: Grevemeyer, Ingo, Haase, Karsten M., Brandl, Philipp A., Duncan, Robert A., Stroncik, Nicole, Regelous, Marcel This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Title: Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise Author: Grevemeyer, Ingo, Haase, Karsten M., Brandl, Philipp A., Duncan, Robert A., Stroncik, Nicole, Regelous, Marcel This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Close