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• 1. [Article] Alpine glacier and pluvial lake records of late pleistocene climate variability in the western United States

  Citation × Citation Citation Abstract Copy Title: Alpine glacier and pluvial lake records of late pleistocene climate variability in the western United States Author: Licciardi, Joseph M. Title: Alpine glacier and pluvial lake records of late pleistocene climate variability in the western United States Author: Licciardi, Joseph M. Close

• 2. [Article] Late Pleistocene and postglacial sedimentation and stratigraphy of deep-sea environments off Oregon

  Citation × Citation Citation Abstract Copy Title: Late Pleistocene and postglacial sedimentation and stratigraphy of deep-sea environments off Oregon Author: Duncan, John Russell Jr Title: Late Pleistocene and postglacial sedimentation and stratigraphy of deep-sea environments off Oregon Author: Duncan, John Russell Jr Close

• 3. [Article] The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada

  The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene ...

  Citation × Citation Citation Abstract Copy Title: The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada Author: Wendell, William Glen The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene to Pleistocene volcanic rocks and volcanic derived sedimentary rocks. The volcanic rocks include rhyolite, rhyolite welded tuffs, and basalt. Sedimentary rocks are volcanic conglomerates, sandstones, siltstones, and mudstones with subordinate amounts of diatomite, lignite, opaline chert, and volcanic tuff. Unconsolidated deposits include Quaternary alluvium, landslide debris, and playa sediments. The entire sequence is 2200± feet thick. Rock units within the thesis area can be correlated with units of the same age throughout northeastern California, northwestern Nevada, central and southern Oregon, and western Idaho. The principal structural features within the area are the Virgin Valley syncline, Big Mountain-McGee Mountain anticline, and the tilted fault-block mountain that gives rise to the Big Mountain- McGee Mountain complex. The deformation that produced the broad, gentle folds and fault-block mountain were part of the Basin and Range orogeny. The episode of greatest faulting occurred during Pleistocene time, producing the present features of the Basin and Range structure within the thesis area and the surrounding region. Title: The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada Author: Wendell, William Glen The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene to Pleistocene volcanic rocks and volcanic derived sedimentary rocks. The volcanic rocks include rhyolite, rhyolite welded tuffs, and basalt. Sedimentary rocks are volcanic conglomerates, sandstones, siltstones, and mudstones with subordinate amounts of diatomite, lignite, opaline chert, and volcanic tuff. Unconsolidated deposits include Quaternary alluvium, landslide debris, and playa sediments. The entire sequence is 2200± feet thick. Rock units within the thesis area can be correlated with units of the same age throughout northeastern California, northwestern Nevada, central and southern Oregon, and western Idaho. The principal structural features within the area are the Virgin Valley syncline, Big Mountain-McGee Mountain anticline, and the tilted fault-block mountain that gives rise to the Big Mountain- McGee Mountain complex. The deformation that produced the broad, gentle folds and fault-block mountain were part of the Basin and Range orogeny. The episode of greatest faulting occurred during Pleistocene time, producing the present features of the Basin and Range structure within the thesis area and the surrounding region. Close

• 4. [Article] Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon

  This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination ...

  Citation × Citation Citation Abstract Copy Title: Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon Author: Punke, Michele Leigh This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination of the stratigraphic record can reveal depositional signatures that provide insights into the geomorphic and tectonic forces active within coastal river basins. Three case studies from the southern Oregon coast illustrate the complex relationship between tectonics and geomorphic processes along an active margin coast such as Oregon's Cascadia subduction zone. This work illustrates that the differential preservation of late Pleistocene-age terrestrial deposits in Oregon's coastal landscape, and the early cultural sites they may contain, is not random but can be closely related to larger tectonogeomorphic processes operating at local and regional scales. Detailed subsurface investigation of one case study site, the lower Sixes River valley, reveals a complex history of depositional environment evolution in relation to geomorphic and tectonic forces. Litho- and biostratigraphic data sets are used to develop a depositional environment reconstruction for the lower Sixes River site through time. This reconstruction of the depositional environment from the late Pleistocene to the present indicates a transgressive evolution that differs from models of transgressive coastal facies and from other studied Northwest coast estuarine life histories. Factors such as eustatic sea level rise, regional and local tectonic alteration of the landscape, sediment supply, or valley morphology may have played roles in the creation and preservation of this atypical depositional sequence. Litho- and biostratigraphic evidence of six Cascadia subduction zone earthquakes younger than 6200 cal yr BP is recorded in a long sediment core from the Sixes River valley. All six of these events correlate with events previously reported for the area by Kelsey et al. (2002). At least five additional plate boundary earthquakes lowered tidal marshes and freshwater wetlands prior to 6200 cal yr BP. The presence within the lower Sixes River valley of an intertidal environment capable of recording Cascadia subduction zone earthquakes dating to the late Pleistocene and early Holocene has not been found at any other location on the Northwest coast. Title: Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon Author: Punke, Michele Leigh This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination of the stratigraphic record can reveal depositional signatures that provide insights into the geomorphic and tectonic forces active within coastal river basins. Three case studies from the southern Oregon coast illustrate the complex relationship between tectonics and geomorphic processes along an active margin coast such as Oregon's Cascadia subduction zone. This work illustrates that the differential preservation of late Pleistocene-age terrestrial deposits in Oregon's coastal landscape, and the early cultural sites they may contain, is not random but can be closely related to larger tectonogeomorphic processes operating at local and regional scales. Detailed subsurface investigation of one case study site, the lower Sixes River valley, reveals a complex history of depositional environment evolution in relation to geomorphic and tectonic forces. Litho- and biostratigraphic data sets are used to develop a depositional environment reconstruction for the lower Sixes River site through time. This reconstruction of the depositional environment from the late Pleistocene to the present indicates a transgressive evolution that differs from models of transgressive coastal facies and from other studied Northwest coast estuarine life histories. Factors such as eustatic sea level rise, regional and local tectonic alteration of the landscape, sediment supply, or valley morphology may have played roles in the creation and preservation of this atypical depositional sequence. Litho- and biostratigraphic evidence of six Cascadia subduction zone earthquakes younger than 6200 cal yr BP is recorded in a long sediment core from the Sixes River valley. All six of these events correlate with events previously reported for the area by Kelsey et al. (2002). At least five additional plate boundary earthquakes lowered tidal marshes and freshwater wetlands prior to 6200 cal yr BP. The presence within the lower Sixes River valley of an intertidal environment capable of recording Cascadia subduction zone earthquakes dating to the late Pleistocene and early Holocene has not been found at any other location on the Northwest coast. Close

• 5. [Article] Constraints of the origin of the middle Pleistocene transition from the glacial sedimentary record of the north-central United States

  This dissertation focuses on the role of ice sheets in the transition during the middle Pleistocene (-1.2 Ma) from 41-kyr glacial cycles to 100-kyr glacial cycles. This research evaluates the hypothesis ...

  Citation × Citation Citation Abstract Copy Title: Constraints of the origin of the middle Pleistocene transition from the glacial sedimentary record of the north-central United States Author: Roy, Martin This dissertation focuses on the role of ice sheets in the transition during the middle Pleistocene (-1.2 Ma) from 41-kyr glacial cycles to 100-kyr glacial cycles. This research evaluates the hypothesis that the middle Pleistocene transition (MPT) was related to the glacial erosion of a regolith mantle and the subsequent exposure of fresh crystalline bedrock. This issue is addressed through the study of glacial sedimentary sequences in Iowa, Nebraska, Kansas, and Missouri. Testing the hypothesis first required the establishment of a stratigraphic framework for pre-Illinoian glacial sequences. A chronology developed around till compositional data and chronological constraints provided by paleomagnetic measurements on glacigenic sediments, and three volcanic ashes, indicate that the midcontinent tills can be grouped under three categories representing at least seven ice advances: two older groups of reverse-polarity tills containing a low and intermediate proportion of clasts and minerals derived from crystalline bedrock, respectively, and one younger group of normal-polarity tills enriched in crystalline materials. The bulk geochemistry of the silicate fraction of the midcontinent tills was then used to evaluate the character of the rock source eroded by ice sheets. The results show a general trend in which geochemical indices, from oldest to youngest tills, fall parallel to a mixing line defined by the composition of a weathered and fresh crystalline rock sources, respectively. The content in meteoric 10Be of tills also supports the existence of a regolith and its glacial erosion by -1.3 Ma, thus concordant with the onset of the MPT. Marine records of strontium, hafnium, and osmium isotopes provide additional support for the hypothesis. The nature of the till compositional changes was further addressed through a provenance study based on 504 40Ar/39Ar ages measured on individual K-feldspar grains retrieved from tills that span the last 2 Myr. Most samples yielded ages identical to those of numerical ages of the Churchill Province, thereby constraining deposition by ice from the western (Keewatin) sector of the Laurentide ice sheet. These results rule out a change in provenance, and thus show support for a change in the composition of the Churchill bedrock source during the late Cenozoic. Title: Constraints of the origin of the middle Pleistocene transition from the glacial sedimentary record of the north-central United States Author: Roy, Martin This dissertation focuses on the role of ice sheets in the transition during the middle Pleistocene (-1.2 Ma) from 41-kyr glacial cycles to 100-kyr glacial cycles. This research evaluates the hypothesis that the middle Pleistocene transition (MPT) was related to the glacial erosion of a regolith mantle and the subsequent exposure of fresh crystalline bedrock. This issue is addressed through the study of glacial sedimentary sequences in Iowa, Nebraska, Kansas, and Missouri. Testing the hypothesis first required the establishment of a stratigraphic framework for pre-Illinoian glacial sequences. A chronology developed around till compositional data and chronological constraints provided by paleomagnetic measurements on glacigenic sediments, and three volcanic ashes, indicate that the midcontinent tills can be grouped under three categories representing at least seven ice advances: two older groups of reverse-polarity tills containing a low and intermediate proportion of clasts and minerals derived from crystalline bedrock, respectively, and one younger group of normal-polarity tills enriched in crystalline materials. The bulk geochemistry of the silicate fraction of the midcontinent tills was then used to evaluate the character of the rock source eroded by ice sheets. The results show a general trend in which geochemical indices, from oldest to youngest tills, fall parallel to a mixing line defined by the composition of a weathered and fresh crystalline rock sources, respectively. The content in meteoric 10Be of tills also supports the existence of a regolith and its glacial erosion by -1.3 Ma, thus concordant with the onset of the MPT. Marine records of strontium, hafnium, and osmium isotopes provide additional support for the hypothesis. The nature of the till compositional changes was further addressed through a provenance study based on 504 40Ar/39Ar ages measured on individual K-feldspar grains retrieved from tills that span the last 2 Myr. Most samples yielded ages identical to those of numerical ages of the Churchill Province, thereby constraining deposition by ice from the western (Keewatin) sector of the Laurentide ice sheet. These results rule out a change in provenance, and thus show support for a change in the composition of the Churchill bedrock source during the late Cenozoic. Close

• 6. [Article] Seismic sequence stratigraphy and tectonic evolution of southern hydrate ridge

  A 3D seismic volume was acquired summer 2000 over the southern end of Hydrate Ridge (FIR), an anomalously shallow ridge 100 km offshore Newport, Oregon. The survey followed a succession of scientific expeditions ...

  Citation × Citation Citation Abstract Copy Title: Seismic sequence stratigraphy and tectonic evolution of southern hydrate ridge Author: Chevallier, Johanna A 3D seismic volume was acquired summer 2000 over the southern end of Hydrate Ridge (FIR), an anomalously shallow ridge 100 km offshore Newport, Oregon. The survey followed a succession of scientific expeditions aimed at studying the gas hydrates present in the shallow subsurface that gave the name to the ridge. This thesis consists of a seismic sequence analysis of the high-resolution (125 Hz) 3D survey. Identification of seismic units and interpretation of depositional sequences observed on the seismic sections is presented. The sequence analysis is compared with the results from nine sites cored during ODP Leg 204 during summer 2002. The first objective is to document in detail the stratigraphy of the ridge so that we can compare it with the gas hydrate distribution. The second is to reconstruct the structural evolution through time of this complex anticline as inferred from the depositional history. The result is a time series of structural evolutionary cross-sections as well as a series of paleo-bathymetric maps revealing the development of and interplay between the structures now buried in the subsurface of southern HR. The structural evolutionary diagrams show the existence of three anticlines, interpreted as thrust-related folds. They formed at the deformation front and controlled the distribution and deformation of the sediments during the Pleistocene. The current southern HR started its uplift less than 0.5 Ma. A seismic relict in the form of a double BSR is a witness to the evolution of the gas hydrate system of HR. It confirms the recent uplift of the ridge and consequent shallowing of the base of the gas hydrate stability zone (GHSZ). Further detailed studies of the stratigraphy reveal stratigraphic controls on the fluid flow, which in turn control the distribution of gas hydrates. Analysis of the amplitude map of the bottom-simulating reflector (BSR), which is a proxy for the free gas distribution, shows a relationship between anticlinal features within the older strata (older than 1.6 Ma) and strong amplitude anomalies of the BSR, which confirm previous observations suggesting a very low permeability for the young slope-basin sediments and an accumulation of gas within the older sediments underneath. Title: Seismic sequence stratigraphy and tectonic evolution of southern hydrate ridge Author: Chevallier, Johanna A 3D seismic volume was acquired summer 2000 over the southern end of Hydrate Ridge (FIR), an anomalously shallow ridge 100 km offshore Newport, Oregon. The survey followed a succession of scientific expeditions aimed at studying the gas hydrates present in the shallow subsurface that gave the name to the ridge. This thesis consists of a seismic sequence analysis of the high-resolution (125 Hz) 3D survey. Identification of seismic units and interpretation of depositional sequences observed on the seismic sections is presented. The sequence analysis is compared with the results from nine sites cored during ODP Leg 204 during summer 2002. The first objective is to document in detail the stratigraphy of the ridge so that we can compare it with the gas hydrate distribution. The second is to reconstruct the structural evolution through time of this complex anticline as inferred from the depositional history. The result is a time series of structural evolutionary cross-sections as well as a series of paleo-bathymetric maps revealing the development of and interplay between the structures now buried in the subsurface of southern HR. The structural evolutionary diagrams show the existence of three anticlines, interpreted as thrust-related folds. They formed at the deformation front and controlled the distribution and deformation of the sediments during the Pleistocene. The current southern HR started its uplift less than 0.5 Ma. A seismic relict in the form of a double BSR is a witness to the evolution of the gas hydrate system of HR. It confirms the recent uplift of the ridge and consequent shallowing of the base of the gas hydrate stability zone (GHSZ). Further detailed studies of the stratigraphy reveal stratigraphic controls on the fluid flow, which in turn control the distribution of gas hydrates. Analysis of the amplitude map of the bottom-simulating reflector (BSR), which is a proxy for the free gas distribution, shows a relationship between anticlinal features within the older strata (older than 1.6 Ma) and strong amplitude anomalies of the BSR, which confirm previous observations suggesting a very low permeability for the young slope-basin sediments and an accumulation of gas within the older sediments underneath. Close

• 7. [Article] Timescales of large silicic magma systems : implications from accessory minerals in Pleistocene lavas of the Altiplano-Puna Volcanic Complex, central Andes

  Constraining the development, evolution, and timescales of large silicic magma systems is important to understanding the development of granite batholiths, the relationships between volcanoes and their ...

  Citation × Citation Citation Abstract Copy Title: Timescales of large silicic magma systems : implications from accessory minerals in Pleistocene lavas of the Altiplano-Puna Volcanic Complex, central Andes Author: Tierney, Casey R. Constraining the development, evolution, and timescales of large silicic magma systems is important to understanding the development of granite batholiths, the relationships between volcanoes and their plutonic underpinnings, and the development of the continental crust. The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic volcanism over a ~11 Ma time-span that climaxed about 4 Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6 Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-Th/U-Pb data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km³, and are distributed over a roughly elliptical area of almost 2000km² centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. ⁴⁰Ar/³⁹Ar ages from biotite reveals eruption ages from 108±6 to 120±5 ka while more accurate sanidine for some of the domes reveal eruption ages from 87±4 to 97±2 ka. SIMS U-series crystallization ages from the rims of 215 zircon crystals from the domes show a similar age spectra from dome to dome, with common peaks in zircon ages at ~100ka and ~220ka. Furthermore, the ages reveal a fairly continuous spread of ages from near eruption to >300ka indicating that the residence time of this magma body was likely over a similar time interval. Ubiquitous andesitic inclusions evidence a vital role for recharge in sustaining and maybe eventual eruption of these magmas. Lastly, the interiors of crystals with rim U-Th secular equilibrium ages were re-analyzed and have yielded U-Pb ages of up to 3.5 Ma. The presence of these older interiors suggests that the source region of these magmas retained a record of an earlier history dating back to the last supervolcanic eruption in the region from the nearby Pastos Grandes caldera. This suggests that the thermal history of the system precluded complete resorption of antecrysts. The volcanological, petrological, temporal and spatial coherence of this series of eruptions combined with the similar ⁴⁰Ar/³⁹Ar and zircon age spectra argue for a long-lived and unitary magma chamber revealing perhaps the waning of this major continental magma system. Title: Timescales of large silicic magma systems : implications from accessory minerals in Pleistocene lavas of the Altiplano-Puna Volcanic Complex, central Andes Author: Tierney, Casey R. Constraining the development, evolution, and timescales of large silicic magma systems is important to understanding the development of granite batholiths, the relationships between volcanoes and their plutonic underpinnings, and the development of the continental crust. The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic volcanism over a ~11 Ma time-span that climaxed about 4 Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6 Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-Th/U-Pb data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km³, and are distributed over a roughly elliptical area of almost 2000km² centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. ⁴⁰Ar/³⁹Ar ages from biotite reveals eruption ages from 108±6 to 120±5 ka while more accurate sanidine for some of the domes reveal eruption ages from 87±4 to 97±2 ka. SIMS U-series crystallization ages from the rims of 215 zircon crystals from the domes show a similar age spectra from dome to dome, with common peaks in zircon ages at ~100ka and ~220ka. Furthermore, the ages reveal a fairly continuous spread of ages from near eruption to >300ka indicating that the residence time of this magma body was likely over a similar time interval. Ubiquitous andesitic inclusions evidence a vital role for recharge in sustaining and maybe eventual eruption of these magmas. Lastly, the interiors of crystals with rim U-Th secular equilibrium ages were re-analyzed and have yielded U-Pb ages of up to 3.5 Ma. The presence of these older interiors suggests that the source region of these magmas retained a record of an earlier history dating back to the last supervolcanic eruption in the region from the nearby Pastos Grandes caldera. This suggests that the thermal history of the system precluded complete resorption of antecrysts. The volcanological, petrological, temporal and spatial coherence of this series of eruptions combined with the similar ⁴⁰Ar/³⁹Ar and zircon age spectra argue for a long-lived and unitary magma chamber revealing perhaps the waning of this major continental magma system. Close

• 8. [Article] Late Pleistocene Slip Rate along the Panamint Valley Fault System, Eastern California

  The Eastern California Shear Zone (ECSZ) is a broad zone of dextral shear inboard of the North American - Pacific plate boundary. Despite decades of study, the significance of a mismatch between geodetic ...

  Citation × Citation Citation Abstract Copy Title: Late Pleistocene Slip Rate along the Panamint Valley Fault System, Eastern California Author: Choi, Na Hyung The Eastern California Shear Zone (ECSZ) is a broad zone of dextral shear inboard of the North American - Pacific plate boundary. Despite decades of study, the significance of a mismatch between geodetic velocities and geologic fault slip rates across the ECSZ remains incompletely understood. Geodetically determined interseismic strain across the ECSZ has been determined to be 9-11 mm/yr, but across much of the ECSZ, the geologic rates sum to only about 50% that rate. At the latitude of 36°-37°N, geologic slip rates have been constrained along the length of Owens Valley and Death Valley fault systems; however, the neighboring Panamint Valley fault system (PVFS) has limited geologic slip data. Existing estimates of Late Pleistocene - Holocene slip rates along the PVFS are relatively slow (~2-3 mm/yr) and come from a single site along the southernmost fault segment. In contrast, geodetic measurements through GPS and InSAR suggest that the northernmost segment of the PVFS, the Hunter Mountain fault, may experience slip rates as high as 6 mm/yr. Offset Holocene geomorphic markers along the Hunter Mountain fault have been proposed to record 3-4 mm/yr of slip, albeit with a qualitative age constraint; however, slip rates along the central PVFS are completely lacking. In this thesis, I present results from an investigation of displaced alluvial fan surfaces along the central PVFS, at the mouth of Jail Canyon. This site is characterized by a relict channel system associated with a large alluvial fan complex: The channel is displaced dextrally across a ~500 m wide zone of distributed deformation. Restoration of the channel with the aid of LiDAR digital topography suggests 118 m of net slip, oriented 303°, across the PVFS. The degree of soil development and the degraded morphology of channel walls place bounds on the abandonment ages of displaced surfaces. Soil ages are based on a set of soils in twelve alluvial deposits of known age in Panamint Valley and the Mojave Desert (Hoffman, 2009). Profile development indices (PDI) of these soils suggest a strong correlation of soil development with age; interpolation of this empirical relationship to soils at Jail Canyon suggests that soils in the displaced surfaces are likely 11-22 ka (and up to 29 ka within 95% confidence). To model the age of the offset channel walls at Jail Canyon, a range of diffusivity constants, which controls the rate of change in elevation, was locally calibrated for Panamint Valley. Calibration with fault scarps and wave-cut scarps of known age suggests a best-estimate diffusivity range of 1.2 - 2.5 m²/ka. Applying this range to the degradation of channel risers at Jail Canyon results in a best-estimate age of 9 - 19 ka, consistent with soil ages. Using an age range of 20-30 ka, bounded by the conservative end of age estimate from each method, the minimum slip rate along this segment of the PVFS is 4-6 mm/yr. This rate is faster than any existing geologic slip rate along the PVFS and confirms previous suggestions that (1) the PVFS carries a large fraction of dextral shear in the ECSZ north of the Garlock Fault, (2) slip along the PVFS has been accelerating in the past 3-4 Ma, and (3) all three major fault systems north of the Garlock decrease in slip southward despite the steady geodetic velocities. Title: Late Pleistocene Slip Rate along the Panamint Valley Fault System, Eastern California Author: Choi, Na Hyung The Eastern California Shear Zone (ECSZ) is a broad zone of dextral shear inboard of the North American - Pacific plate boundary. Despite decades of study, the significance of a mismatch between geodetic velocities and geologic fault slip rates across the ECSZ remains incompletely understood. Geodetically determined interseismic strain across the ECSZ has been determined to be 9-11 mm/yr, but across much of the ECSZ, the geologic rates sum to only about 50% that rate. At the latitude of 36°-37°N, geologic slip rates have been constrained along the length of Owens Valley and Death Valley fault systems; however, the neighboring Panamint Valley fault system (PVFS) has limited geologic slip data. Existing estimates of Late Pleistocene - Holocene slip rates along the PVFS are relatively slow (~2-3 mm/yr) and come from a single site along the southernmost fault segment. In contrast, geodetic measurements through GPS and InSAR suggest that the northernmost segment of the PVFS, the Hunter Mountain fault, may experience slip rates as high as 6 mm/yr. Offset Holocene geomorphic markers along the Hunter Mountain fault have been proposed to record 3-4 mm/yr of slip, albeit with a qualitative age constraint; however, slip rates along the central PVFS are completely lacking. In this thesis, I present results from an investigation of displaced alluvial fan surfaces along the central PVFS, at the mouth of Jail Canyon. This site is characterized by a relict channel system associated with a large alluvial fan complex: The channel is displaced dextrally across a ~500 m wide zone of distributed deformation. Restoration of the channel with the aid of LiDAR digital topography suggests 118 m of net slip, oriented 303°, across the PVFS. The degree of soil development and the degraded morphology of channel walls place bounds on the abandonment ages of displaced surfaces. Soil ages are based on a set of soils in twelve alluvial deposits of known age in Panamint Valley and the Mojave Desert (Hoffman, 2009). Profile development indices (PDI) of these soils suggest a strong correlation of soil development with age; interpolation of this empirical relationship to soils at Jail Canyon suggests that soils in the displaced surfaces are likely 11-22 ka (and up to 29 ka within 95% confidence). To model the age of the offset channel walls at Jail Canyon, a range of diffusivity constants, which controls the rate of change in elevation, was locally calibrated for Panamint Valley. Calibration with fault scarps and wave-cut scarps of known age suggests a best-estimate diffusivity range of 1.2 - 2.5 m²/ka. Applying this range to the degradation of channel risers at Jail Canyon results in a best-estimate age of 9 - 19 ka, consistent with soil ages. Using an age range of 20-30 ka, bounded by the conservative end of age estimate from each method, the minimum slip rate along this segment of the PVFS is 4-6 mm/yr. This rate is faster than any existing geologic slip rate along the PVFS and confirms previous suggestions that (1) the PVFS carries a large fraction of dextral shear in the ECSZ north of the Garlock Fault, (2) slip along the PVFS has been accelerating in the past 3-4 Ma, and (3) all three major fault systems north of the Garlock decrease in slip southward despite the steady geodetic velocities. Close

• 9. [Article] Snow Peak, OR: late Miocene to early Pliocene volcanism in the central Cascadia forearc

  Snow Peak is a voluminous shield volcano located ~50 km west of the axis of the modern Cascade arc. A previously published K-Ar age estimate indicates that the Snow Peak lavas are ~3 Ma. Subsequently, ...

  Citation × Citation Citation Abstract Copy Title: Snow Peak, OR: late Miocene to early Pliocene volcanism in the central Cascadia forearc Author: Hatfield, Ashley K. Snow Peak is a voluminous shield volcano located ~50 km west of the axis of the modern Cascade arc. A previously published K-Ar age estimate indicates that the Snow Peak lavas are ~3 Ma. Subsequently, the preliminary research framework for studying the Snow Peak lavas focused on constraining the petrogenesis of the primitive lava compositions, thereby addressing the chemical and thermal structure of the Cascadia forearc mantle. However, ⁴⁰Ar/³⁹Ar age dating conducted for this study indicates that the Snow Peak lavas are ~5.5 – 6 Ma. Consequently, the research framework undertaken for this thesis focused on constraining the petrogenetic pathway of the Snow Peak lavas, thereby addressing the volcano-tectonic development of the early High Cascades. ⁴⁰Ar/³⁹Ar age results for the spatial range of the Snow Peak volcanic center indicate that the volcano formed between ~5.5 and 6 Ma. Whole-rock major and trace element geochemical results indicate that Snow Peak is a monogenetic, basaltic volcanic center that unconformably overlies the Western Cascades. Petrographic analysis coupled with end-member mineral chemistry results indicate that phenocryst phases consist of olivine, pyroxene, and plagioclase. Decompression crystal fractionation of this phase assemblage explains the observed liquid line of descent. Phase equilibria modeling in COMAGMAT of a selected primitive, and a selected daughter sample, indicate that differentiation of the Snow Peak lavas began ~8-10 kb, and ~1150°C – 1200°C, and the Snow Peak daughter formed ~1-2 kb and ~1050°C – 1150°C. A plagioclase hygrometer indicates that the Snow Peak lavas contain ~2 wt % H₂O. These results are consistent with melting of moderately enriched mantle and addition of a relatively small amount of LILE-enriched slab-derived material. The timing of volcanism at Snow Peak correlates to the development of an intra-arc rift, marked by the eruption of voluminous mafic lava flows, such as the Deschutes Formation, and early Mt. Jefferson basalts. Mafic underplating may have played a greater role in Cascadia volcanism during such periods of intra-arc rifting, and such an extensional tectonic setting could facilitate the eruption of basaltic lavas. Additionally, Snow Peak aligns with the NW-trending Brother's Fault zone, although the Snow Peak center lies on the western side of the arc axis. Snow Peak magma may have formed in response to decompression associated with uplift along the Brother's Fault zone, with a fluid flux from the downgoing slab. Title: Snow Peak, OR: late Miocene to early Pliocene volcanism in the central Cascadia forearc Author: Hatfield, Ashley K. Snow Peak is a voluminous shield volcano located ~50 km west of the axis of the modern Cascade arc. A previously published K-Ar age estimate indicates that the Snow Peak lavas are ~3 Ma. Subsequently, the preliminary research framework for studying the Snow Peak lavas focused on constraining the petrogenesis of the primitive lava compositions, thereby addressing the chemical and thermal structure of the Cascadia forearc mantle. However, ⁴⁰Ar/³⁹Ar age dating conducted for this study indicates that the Snow Peak lavas are ~5.5 – 6 Ma. Consequently, the research framework undertaken for this thesis focused on constraining the petrogenetic pathway of the Snow Peak lavas, thereby addressing the volcano-tectonic development of the early High Cascades. ⁴⁰Ar/³⁹Ar age results for the spatial range of the Snow Peak volcanic center indicate that the volcano formed between ~5.5 and 6 Ma. Whole-rock major and trace element geochemical results indicate that Snow Peak is a monogenetic, basaltic volcanic center that unconformably overlies the Western Cascades. Petrographic analysis coupled with end-member mineral chemistry results indicate that phenocryst phases consist of olivine, pyroxene, and plagioclase. Decompression crystal fractionation of this phase assemblage explains the observed liquid line of descent. Phase equilibria modeling in COMAGMAT of a selected primitive, and a selected daughter sample, indicate that differentiation of the Snow Peak lavas began ~8-10 kb, and ~1150°C – 1200°C, and the Snow Peak daughter formed ~1-2 kb and ~1050°C – 1150°C. A plagioclase hygrometer indicates that the Snow Peak lavas contain ~2 wt % H₂O. These results are consistent with melting of moderately enriched mantle and addition of a relatively small amount of LILE-enriched slab-derived material. The timing of volcanism at Snow Peak correlates to the development of an intra-arc rift, marked by the eruption of voluminous mafic lava flows, such as the Deschutes Formation, and early Mt. Jefferson basalts. Mafic underplating may have played a greater role in Cascadia volcanism during such periods of intra-arc rifting, and such an extensional tectonic setting could facilitate the eruption of basaltic lavas. Additionally, Snow Peak aligns with the NW-trending Brother's Fault zone, although the Snow Peak center lies on the western side of the arc axis. Snow Peak magma may have formed in response to decompression associated with uplift along the Brother's Fault zone, with a fluid flux from the downgoing slab. Close