Search
Search Results
-
Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. ...
Citation Citation
- Title:
- Reconstructing the Physical Record of a Four-Million-Year Volcanic System : Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon
- Author:
- DiGiulio, Jennifer
Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. 171 mapped stratigraphic units over an area of 150 km² reveal four periods of volcanic activity resulting in diverse composition lavas ranging from ~48 to 72 wt% SiO₂. Eruptive periods are divided into (1) ~4.0 - 0.78 Ma; (2) 0.78 - 0.3 Ma; (3) 0.3 - 0.02 Ma; and (4) eruptions of the past 13,000 years. Repeated glaciations over the past 800,000 years have sculpted the landscape of the MJA and include the Pleistocene glaciations of Jack Creek (early Wisconsin) and Cabot Creek (late Wisconsin), and the Holocene glaciation of Jefferson Park. Anomalously glassy lava flows, columnar jointing, and streamlined shapes are lithologic evidence of intraglacial eruptions in numerous units, including the andesite of Whitewater Creek (Qawc), andesite and dacite of Park Butte (Qapb; Qdpb), and the basaltic andesite of Whiskey Creek (Qbawh). Mineral compositions and textures are highly variable among the four units. Patchy to oscillatory zoning in plagioclase and dissolution cores and partially resorbed rims in amphiboles indicate pervasive disequilibrium conditions. Feldspar (~An₃₅₋₉₈) and amphibole (~1.1-1.5 ͥ ͮ Al) compositions are relatively continuous across a broad range, and pyroxene compositions are typically ~En₄₂₋₄₉ and En₆₅₋₇₄. Phenocryst assemblages of units Qawc, Qapb, Qdpb, and Qbawh were probed to assess pressure and temperature conditions of pre-eruptive magmas in the MJA. Estimates from amphiboles, feldspars, and pyroxenes indicate temperatures ranging from ~650 to 1100 °C. Pressure estimates indicate crystallization depths of ~3 to 25 km, with the majority of crystallization occurring between ~15 and 25 km. Pyroxene temperatures (Putirka model) are always the highest, (~950-1150 °C) and plagioclase-amphibole pair (HBAS model) temperatures are the lowest (~650-875 °C), with amphibole temperatures (Ridolfi model) falling in between those ranges. Calculated partition coefficients of Sr and Ba from this study range from 1.5 to 6.75 and 0.12 to 1.00, respectively, in close agreement with calculated partition coefficients of Bindeman et al. (1998) and Dohmen and Blundy (2014). Reconstructed Sr concentrations range from 227 to 799 µg/g, which is inconsistent with the melting of a Sr-rich andesite end-member produced at 30-40 km depth as proposed by Conrey et al. (2001). The diverse spread of data reported here suggests complex petrologic mixing processes predominantly occurring in the midto upper crust beneath the MJA and contributes to the understanding of pre-eruptive magmatic conditions in the Cascade volcanic arc
-
322. [Article] Seismotectonics of the Himalayas and the Tibetan Plateau : moment tensor analysis of regional seismograms
This thesis presents a detailed seismotectonic investigation of the Himalayan region and the Tibetan plateau as part of project HiCLIMB to explore the state of stress and the kinematics of the world’s ...Citation Citation
- Title:
- Seismotectonics of the Himalayas and the Tibetan Plateau : moment tensor analysis of regional seismograms
- Author:
- Baur, Jan R.
This thesis presents a detailed seismotectonic investigation of the Himalayan region and the Tibetan plateau as part of project HiCLIMB to explore the state of stress and the kinematics of the world’s largest continental collision zone. Using full regional waveforms for moment tensor inversion, source parameters for 107 earthquakes were determined with moment-magnitudes (Mw) ranging from 3.5 to 6.3. The significant decrease in magnitude threshold with respect to previous studies was accomplished through the usage of broadband data from the HiCLIMB, HIMNT, and Bhutan temporary networks. Combining the results from this study with previously published earthquake source parameters, the investigation focuses on three topics: (1) Deformation along the front of the Himalayan arc associated with the Main Himalayan Thrust (2) Extension in the southern Tibetan plateau, and (3) Location and stress orientation of intermediate-depth earthquakes. Thrust event epicenters along the Himalayan front closely coincide with the 3500 m topography contour. These earthquakes can be associated with elastic strain accumulation near the lower tip of the locked part of the MHT due to tectonic loading from its creeping down-dip extension. Centroid depths and nodal plane dips of these thrust events are inconsistent with slip merely on the main detachment and indicate significant deformation in the vicinity of the MHT. Especially in far western Nepal, nodal plane dips are systematically steeper and slip during these events might play a role in the formation of asperities and barriers on the detachment surface. The P-axes azimuths of the thrust events along the Himalayan arc deviate considerably from a mere circular geometry. Spatial filtering of the regional topography reveals that slip of events in the footwall as well as the hanging wall aligns perpendicular to the mountain range on a 50 km wavelength scale. The topography-perpendicular alignment of the slip direction on planes with significant inclination suggests that these thrust events contribute considerably to the mountain building process and to the formation of the local shape of the arc. Deformation on the southern Tibetan plateau is dominated by shallow normal faulting in the upper 15 km of the crust. The extensional direction, while generally trending east-west, shows an apparent transition from arc parallel in the Tethyan Himalaya to northward convex in the southern Lhasa terrane. North of about N31º, deformation changes to strike-slip prevalence. The locations of changes in faulting patterns coincide with changes in geometry of the underthrusting Indian crust revealed by receiver function images. This correlation indicates a significant influence of basal traction on shallow crustal faulting processes. This study provides additional evidence that most intermediate-depth seismicity occurs beneath the Moho, signifying a strong upper mantle. Faulting in the upper mantle is dominated by strike-slip faulting with northerly trending P-axes. The maximum horizontal compressive stress axes of mantle earthquakes align with the direction of the India-Eurasia convergence and imply a relation of this deformation to the subduction process.
-
323. [Article] Seismic performance of diagrid steel structures using single and double friction mass dampers
The steel diagrid structural system is a recent load bearing and lateral resisting structural system for tall building structures that is relatively unexplored in the western United States. One possible ...Citation Citation
- Title:
- Seismic performance of diagrid steel structures using single and double friction mass dampers
- Author:
- Ramadhan, Garlan
The steel diagrid structural system is a recent load bearing and lateral resisting structural system for tall building structures that is relatively unexplored in the western United States. One possible reason for the little use of diagrid systems in earthquake prone regions is the lack of guidelines and application examples illustrating the design and analysis of these structures. In this work, a prototype building with 72 stories is used as an example for which the design and analysis of the diagrid system is performed. To mitigate the possible large displacement and base shear demands that these structures may undergo under seismic events, two new design solutions consisting of one or two friction tuned mass damper (TMD) units are explored. In the first solution, a TMD is placed on the top four stories of the building and is tuned to reduce the contribution of the fundamental mode of vibration of the structure. The second solution uses a double TMD system, in which a second TMD unit - tuned to the second period of the structure – is added at mid-height of the building. Using a nonlinear finite element model of the tuned mass damper, the effectiveness of the friction mass damper design is studied. The mass damper system consists of a concrete tank containing sand or water. The tank is placed in between the building reinforce concrete structural core and the exterior steel diagrid system. This mass damper is connected to the structure using friction pendulum isolators which are chosen due to their ability to undergo large deformations. The models are then subjected to earthquake ground motions from historical shallow crustal and subduction-zone events. Parametric studies are carried out to optimize the mass damper design in improving the seismic performance of the building. Optimization of the seismic performance is assessed in terms of minimization of inter-story drift ratios, base and story forces, as well as floor absolute accelerations. The results show that the single TMD system can reduce significantly the peak base reaction and inter-story drift envelopes. Addition of the second TMD provides further improvements in terms of reducing the peak base reactions, while also producing notable reductions in peak absolute floor accelerations, which are not observed when only one TMD unit is used.
-
-
325. [Article] Geology and geochemistry of the Little Walker Volcanic Center, Mono County, California
Detailed mapping and geochemical analysis of Oligocene to early Pliocene volcanic rocks in the Little Walker volcanic center, Mono County, California have revealed a complex eruptive history. After eruption ...Citation Citation
- Title:
- Geology and geochemistry of the Little Walker Volcanic Center, Mono County, California
- Author:
- Priest, George R.
Detailed mapping and geochemical analysis of Oligocene to early Pliocene volcanic rocks in the Little Walker volcanic center, Mono County, California have revealed a complex eruptive history. After eruption of widespread rhyolitic ash flows of the Valley Springs Formation in the Oligocene, Miocene to early Pliocene volcanism of the western Great Basin and northern Sierra Nevada was dominated by eruption of calc-alkalic, andesitic lavas bearing abundant hydrous mafic phenocrysts, and, thus, high H₂O contents. These kinds of calc-alkaline magmas are associated with most of the major epithermal Au-Ag districts of the western Great Basin. A highly potassic latitic pulse of volcanism occurred at the Little Walker volcanic center about 9.5 m.y. ago during the ongoing calc-alkalic activity. The latitic series is unusually enriched in K and other incompatible elements, as well as Fe compared to the surrounding calc-alkaline rocks. The latites have mineralogic evidence of much lower H₂O content than the calc-alkaline lavas; yet early latitic magmas were rich enough in volatiles to produce very large, welded ash-flow sheets (e.g., the Eureka Valley Tuff). Rapid evacuation of the magma reservoir beneath the Little Walker center during the ash-flow activity resulted in formation of the Little Walker caldera. Intracaldera volcanism culminated with extrusion of viscous, phenocryst-rich plug domes and coulees of transitionally calc-alkaline, low-K latite lava of the Lavas of Mahogany Ridge. The low-K latite series is severely depleted in all incompatible elements relative to early latitic rocks and has mineralogic, geologic, and trace element evidence of higher H₂O content relative to early latites. Significant phenocrystic hornblende, association with hydrothermal alteration, and high Eu⁺³ /Eu⁺² all suggest significant H₂O concentration in the low-K latite magmas. These rocks probably come from a source region intermediate between that of the calc-alkaline and latite series. Trace and major element data favor generation of latitic magmas from a primitive mantle diapir. The diapir rose into a subduction zone that was actively generating widespread calc-alkalic lavas throughout the region from hydrous mantle and, possibly, lower crustal sources. The latite magmas were drier and hotter than the calc-alkaline magmas, but were also enriched in volatiles, particularly CO₂, and incompatible elements from their undepleted mantle source. Rising latitic magmas may have gained additional incompatible elements by wall rock reaction and zone refining of upper mantle and lower crustal rocks. Extensive qualitative trace element evidence of crystal fractionation shows that incompatible elements may have been further concentrated by variable amounts of crystal settling. High-pressure (plagioclase-poor, pyroxene-rich) fractionation of the early, dry latitic series produced low-Ca-Mg latites with high Fe/Mg and A1₂0₃ but low Si0₂. Low-pressure (plagioclase rich) differentiation of the early latitic magmas produced quartz latite ash flows with high Si0₂ and moderate Fe/Mg, while low-pressure differentiation of hydrous low-K latite magmas yielded silicic low-K latite and quartz latite lavas with low Fe/Mg. More extensive separation of olivine relative to pyroxenes at low pressures and increased stability of subsilicic hydrous crystals and Fe-Ti oxides in the hydrous magmas account for changes in differentiation trends caused by Ptotal and PH₂O variations. Lack of giant welded ash-flow sheets in the hydrous calc-alkaline series and common eruption of such ash flows from volcanic centers with rather anhydrous magmas led to the conclusion that H₂0/CO₂ as well as total volatile content are critical controls on the likelihood of large scale, hot ash-flow eruptions. Giant, hot ash-flow sheets and associated calderas are favored in magmas with low H₂0/CO₂ and high total volatile content. Basaltic and latitic volcanism in areas of thick sialic crust, where crystal fractionation is extensive are, therefore, the best sources of giant ash-flow sheets. H₂0/CO₂ and total volatile content were also critical controls of the probability of hydrothermal ore deposition. Magmas with high H₂0/CO₂ and moderate total volatile contents are most favored for ore deposition, because such magmas tend to form mesozonal or epizonal plutons rather than volcanic rocks. Plutonic crystallization of hydrous magma will yield a fluid phase capable of transferring incompatible metals and geothermal heat to ground water. If permeable structures and rocks are present, as in a caldera, widespread mineralization will be favored, but there may be no genetic relation between ore-forming magmas and magmas which produce calderas.
-
326. [Article] Genesis of the El Salvador porphyry copper deposit, Chile and distribution of epithermal alteration at Lassen Peak, California
The El Salvador porphyry copper deposit in the Indio Muerto district of northern Chile has been geologically investigated for more than 60 years and provides one of the best bases for understanding similar ...Citation Citation
- Title:
- Genesis of the El Salvador porphyry copper deposit, Chile and distribution of epithermal alteration at Lassen Peak, California
- Author:
- Lee, Robert G. (Robert George)
The El Salvador porphyry copper deposit in the Indio Muerto district of northern Chile has been geologically investigated for more than 60 years and provides one of the best bases for understanding similar environments of ore formation elsewhere in the world. Fourteen new zircon U/Pb isotopic ages obtained via in situ SHRIMP-RG analysis are here coupled with previous geological studies to allow refinement of the timing of Eocene porphyry magma emplacement responsible for copper and molybdenum mineralization that occurs in several ore bodies within the district. The earliest intrusions are rhyolites that crop out throughout the district, but are more abundant in the north. In contrast, the later granodiorite porphyries were emplaced only in the central and southern parts of the district. Two age periods of mineralization have been documented using zircon U/Pb geochronology. The low grade and small copper deposit at Old Camp in the northern district is associated with a quartz porphyry intrusion that yielded an age of 43.6 ± 0.5 Ma, whereas the main copper molybdenum deposit at Turquoise Gulch is associated with emplacement of the granodioritic L porphyry plug that yielded an age of 42.0 ± 0.5 Ma. The final intrusion is a series of latite porphyry dikes, which post-date ores and yielded a U/Pb zircon age of 41.6 ± 0.5 Ma. Inherited Eocene zircons with ages from ~45 Ma to ~47 Ma are found within younger porphyry intrusions and likely formed via magmatic recycling of older intrusions. Therefore, the zircon U/Pb ages suggest magmatism spanned approximately 5 million years from 47 to 42 Ma, with hydrothermal copper-molybdenum ores dominantly forming during the final stages of porphyry emplacement. Geochemical analyses by XRF, ICP-MS, electron microprobe and laser-ablation ICP-MS define a wide range of major, minor and trace element contents for the Eocene porphyry intrusions within the district. The early rhyolite and quartz porphyry intrusions have rare earth contents with strong negative europium anomalies and relatively low Sr/Y and Sm/Yb ratios consistent formation via fractional crystallization of plagioclase-rich mineral assemblages from more mafic parental melts. The granodiorite porphyries have no europium anomalies and a wide range of Sr/Y and Sm/Yb ratios that support an origin via fractional crystallization of garnet, hornblende ± titanite, and minor plagioclase from an andesitic parental melt. The granodiorite intrusions at M Gulch – Copper Hill are ~1 m.y. older than and have less evolved trace element ratios than the younger granodiorite intrusions associated with the main mineralization event. The evolving Eocene intrusions are the result of lower to middle crust melts ascending to mix with silica-rich differentiated melts derived from fractional crystallization of older andesitic magmas. Progressive decrease of Eu/Eu* ratios in the zircons with decreasing age gives direct evidence in support of the hypothesis that the main ore mineralization is directly related to the evolution of the upper crustal magma reservoir to progressively more oxidized conditions. A second goal of this study was to document the mineralogy and zonation of altered wall rock at Lassen National Volcanic Park in northern California, in order to understand the pressure, temperature, fluid composition, and epithermal processes along the southern flank of Lassen Peak. Extensive epithermal wall rock alteration occurs along the southern flank of the Cascadia volcano and includes both active and fossil geothermal systems. Geologic mapping coupled with mineral identification using a portable infrared spectrometer and X-ray diffraction outline several hydrothermal systems within the park. Currently active, steam-heated acid sulfate alteration is characterized by kaolinite, alunite, opal, and cristobalite with accessory iron sulfates. The active hydrothermal zones are proximal to thermal pools and fumaroles at Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell. At least three fossil systems occur within andesite lavas and flow breccias of the eroded Pleistocene Brokeoff Volcano. Intermediate argillic alteration occurs at higher elevations on the flanks of the eroded volcano and is characterized by mixed layer illite-smectite, quartz, pyrite, and albite. Propylitic alteration occurs within the eroded lower elevations of Little Hot Springs Valley and is characterized by chlorite, calcite, quartz, pyrite, illite, albite, and rare epidote. Also present to a lesser extent is an advanced argillic alteration defined by pyrophyllite, dickite, alunite, kaolinite, and quartz formed at Pilot Pinnacle.
-
327. [Article] Geology of the Elsie-lower Nehalem River area, south-central Clatsop and northern Tillamook counties, northwestern Oregon
The middle Eocene Tillamook Volcanics form the oldest rock unit in the Elsie-lower Nehalem River area. K-Ar age determinations and age constraints imposed by foraminiferal and calcareous nannofossil assemblages ...Citation Citation
- Title:
- Geology of the Elsie-lower Nehalem River area, south-central Clatsop and northern Tillamook counties, northwestern Oregon
- Author:
- Mumford, Daniel Franklin
The middle Eocene Tillamook Volcanics form the oldest rock unit in the Elsie-lower Nehalem River area. K-Ar age determinations and age constraints imposed by foraminiferal and calcareous nannofossil assemblages of overlying sedimentary strata indicate an absolute age of about 42 Ma for the uppermost Tillamook Volcanics. Major oxide values indicate that the upper Tillamook Volcanics are highly fractionated high Fe-Ti tholeiitic basalts and basaltic andesites. These volcanics were erupted in a developing forearc under an extensional plate tectonic setting and formed a moderately large oceanic island. These subaerial flows are predominantly aphyric to plagioclase-augite porphyritic and have a pilotaxitic flow texture. Epochs of both normal and reverse magnetic polarity are recorded. Thermal subsidence related to the end of the volcanism resulted in deposition of the transgressive late Eocene Hamlet formation over the "Tillamook island". The informal Hamlet formation consists of three members. From oldest to youngest they are: the Roy Creek member, the Sunset Highway member, and the Sweet Home Creek member. Three lithofacies are present in the Roy Creek member. The stratigraphically lowest of these consists of basaltic boulder-pebble conglomerate and locally fossiliferous pebbly basaltic sandstones which were deposited in a high energy nearshore environment around rocky basaltic headlands and sea stacks of the Tillamook Volcanics. Molluscan fossils in this lithofacies are correlative to the middle to late Eocene "CowlitzCoaledo" fauna. Successively overlying lithofacies are a very coarseto coarse-grained shallow marine basaltic sandstone lithofacies and a medium- to fine-grained basaltic sandstone lithofacies. This fining upward sequenced documents progressive deepening of the depositional basin. Framework clasts in all three Roy Creek member lithofacies were predominantly derived from the Tillamook Volcanics. Pore-filling diagenetic chlorite, smectite (nontronite), calcite, and zeolite (clinoptilolite and heulandite) cements severely reduce the porosity of Roy Creek member sandstones. The Sunset Highway member of the Hamlet formation conformably overlies the Roy Creek member in eastern Clatsop and western Columbia counties and pinches out to the west at about the longitude of the Nehalem River in T. 4 N., R. 8 W.. The Sunset Highway member is predominantly composed of interbedded micaceous arkosic sandstone, lithic arkose, and muddy micaceous arkosic siltstone with a few beds of basaltic sandstone and basaltic debris flow breccias. The dominant micaceous arkosic composition of the Sunset Highway member reflects a distant extrabasinal granitic-metamorphic provenance and contrasts with that of the locally derived underlying basaltic Roy Creek member. Low angle trough cross-bedding, hummocky bedding, and microcross-laminations in fine to medium-grained arkosic sandstones are interpreted to have been produced by large storm-generated waves and on a high energy inner shelf. Thin interbeds of bioturbated mudstone and mollusc-bearing bioturbated sandstones formed during periods of fairweather conditions and during lower sedimentation rates. Rare matrix supported, basaltic debris-flow breccias and basaltic sandstones were derived from nearby basaltic headlands and by rivers draining the Tillamook Volcanics. Minor secondary intraparticle porosity occurs with some primary intergranular porosity in relatively matrix-free Sunset Highway member arkosic sandstones. However, much of the porosity and permeability of these potential sandstones has been reduced by diagenetic smectite coatings on framework grains and potassium feldspar overgrowths of feldspars. The mudstone-dominated Sweet Home Creek member was conformably deposited on the Sunset Highway member in eastern Clatsop and western Columbia counties. In western Clatsop County the Sweet Home Creek member directly and conformably overlies the Roy Creek member due to pinch out of the Sunset Highway member. Upper Narizian to lowermost Refugian benthic foraminiferal assemblages from this unit indicate outer shelf to upper slope sedimentation and continued subsidence of the depositional (Astoria) basin. Micromicaceous and carbonaceous silty mudatone dominates this unit but thin-bedded micaceous arkosic turbidite sandstones are present in the lower part, and rare, thin basaltic turbidites are present in the upper half. X-ray diffraction analysis shows that the dominant clay minerals in the Sweet Home Creek member niudstone are smectite (montmorillonite), kaolinite, and illite. The Cole Mountain basalt (informal) intrudes and locally overlies the Sweet Home Creek member. This caic-alkaline basaltic andesite is thought to have formed in a compressional plate tectonic regime and been emplaced on the outer shelf and upper slope as shallow irregular sills and dikes and minor submarine pillow basalt-hyaloclastite complexes. Siliceous nodules associated with pillowed units locally contains a few per cent pyrite and are associated with small areas of high-grade supergene copper-silver mineralization. The normally polarized Cole Mountain basalt is chemically, petrographically, and lithologically distinct from the Tillamook Volcanics and Grande Ronde Basalt of the Columbia River Basalt Group. The uppermost Narizian and Refugian (late Eocene) Jewell member of the Keasey Formation disconformably overlies the Cole Mountain basalt and Sweet Home Creek member. A thin basal glauconitic sandstone-siltstone reflects a period of reduced sedimentation under slightly reducing conditions and marks the disconformity. The unit primarily consists of laminated to thin bedded tuffaceous mudstone with a few thin tuff beds, small micaceous arkosic sandstone channels and clastic dikes. Clay minerals in the Jewell member are dominated by smectite (montmorillonite), with minor kaolinite and illite (degraded mica) in the lower part of the unit. Benthic foraminiferal assemblages in the unit indicate bathyal or slope depths and have been assigned to the lower Refugian to upper Narizian stages. In the middle Miocene, irregular dikes and sills of the Grande Ronde Basalt of the Columbia River Basalt Group intruded the late Eocene sedimentary strata in the thesis area. Two magneto-chemical types of Grande Ronde Basalt, N2/low MgO-low Ti02 and N2/ high MgO, were identified in the thesis area. These were geochemically and magnetically correlated to subaerial flows of magneto-chemical types IA and 5A of Mangan and others (1986) on the Columbia Plateau. The intrusions or invasive flows are interpreted to have been derived from voluminous plateau eruptions by invasion into soft, unconsolidated Neogene sediments at the marine/coast interface and then into the more brittle but ductile Paleogene strata of the area as first proposed by Beeson and others (1979). Uplift of the Coast Range was initiated in the late Miocene as a result of rapid offshore underthrusting in the subduction zone (Snavely and others, 1983). This has resulted in subaerial erosion and exposure of the faulted and gently folded forearc ridge and deposition of Quaternary alluvial gravels and sands along major rivers and creek in the thesis area. The dominant structural features of the Elsie-lower Nehalem River area are generally down-to-the-north, east-west-trending high angle faults with oblique offset and a conjugate set of oblique slip northwest-trending right-lateral and northeast-trending left-lateral faults. Folds are broad and relatively minor. The major east-west-trending fault pattern may have been initially produced by extensional stresses related to subsidence of the "Tillamook island". The conjugate strike-slip fault pattern may have been created by partial coupling of the forearc basin with oblique subduction of the Farallon plate. Other than timber, locally used rock aggregate from small quarries is the only resource that has been realized in the thesis area. Most quarries are developed in dikes and sills of Grande Ronde Basalt and the aggregate is used to macadamize logging roads. Diagenetic events have resulted in significant loss of porosity and permeability of potential reservoir sandstones in the area. The most favorable targets are relatively matrix-free micaceous arkosic sandstones in the Sunset Highway member, but these have been breached by erosion in the eastern part of the thesis area and pinch out in the western part of the area where potential mudstone cap rocks (e.g., Hamlet and Keasey formations) are present. Mudstones in the area contain woody-structured kerogen and average about 1% total organic carbon. These potential source rocks are generally thermally immature but have locally been baked by basaltic intrusions. This results in elevated vitrinite reflectance values (in the oil window and beyond) and, therefore, the mudstones may be potential source rocks for methane generation. Mineralized fault zones have substantial width and length but do not appear to carry anomalous concentrations of any metals other than arsenic. High-grade supergene copper-silver mineralization associated with Cole Mountain basalt intrusions has been documented but appears to be very localized and is not thought to be a viable exploration target.
-
328. [Article] Subsurface densities & lithospheric flexure of the Himalayan foreland in Pakistan, interpreted from gravity data
Gravity data along a N-S profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretation of the gross structure of the foreland fold and thrust belt of the Himalaya. ...Citation Citation
- Title:
- Subsurface densities & lithospheric flexure of the Himalayan foreland in Pakistan, interpreted from gravity data
- Author:
- Duroy, Yannick
Gravity data along a N-S profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretation of the gross structure of the foreland fold and thrust belt of the Himalaya. In northern Pakistan large deviations from Airy Isostatic equilibrium are observed. An excess of mass characterizes the northern Kohistan arc and a deficit of mass underlies a broad area extending from southern Kohistan to the Salt Range, while to the south a slight excess of mass seems to prevail in the region of the Sargodha ridge. This anomalous distribution of mass can be understood if the Indian elastic plate, which is assumed to overlie an inviscid fluid, is flexed down under the weight of both the overthrust mountains and the sediments eroded off the mountain and deposited in the foredeep basin. In many respects the intracontinental subduction of India beneath the Himalaya is similar to island arc formation, including the seismically active Sargodha ridge, an outer topographic rise analogous to the flexural bulge encountered seaward of oceanic trenches. Analysis of Bouguer gravity anomalies along a profile extending from the Sargodha ridge to the main mantle thrust (MMT) show that most of the negative-southward gravity gradient can be attributed to crustal thickening, while short wavelength anomalies are produced by lateral variation of density within the northward thickening sedimentary wedge. In the Sargodha ridge area, an additional contribution of about 25 mgals appears to be due to excess of mass at lower crustal or upper mantle levels. The Moho discontinuity is interpreted to bulge up beneath the Sargodha high, then gradually increase in dip from 1° to 3° beneath the Salt Range and Potwar Plateau (approximately equal to the change in dip of the basement surface). The Moho slope changes from upwardly convex to upwardly concave beneath southern Kohistan. Finally, north of the Main Mantle Thrust (MMT) it bends down again, but at a steeper angle of about 15°. Shorter wavelength anomalies superimposed on the regional gradient are modeled in terms of upper crustal density changes, including those due to: 1) offsets of the basement surface, 2) variable thickness of the Eocambrian evaporite sequence that forms the basal decollement, 3) thrusting and folding of relatively high density, older parts of the stratigraphic section to higher structural levels, particularly in the Salt range and northern Potwar plateau, and 4) thickening of the low density Neogene molasse sequence into the axis of the Soan Syncline, a structural depression between the Salt range and northern Potwar plateau. Subsurface densities of the overthrust wedge, as well as the definition of the shape of the top surface of the Indian plate interpreted from gravity, place bounds on the flexural rigidity of such a plate and the forces that deform it. In northern Pakistan the flexural rigidity of the elastic Indian plate (D = 4.0 x 10²³ Nm) is a factor of 10 smaller than the current values interpreted for the central and eastern Himalaya. Because of the small elastic thickness interpreted for the Indian plate in Pakistan (He [approximately] 30 km), the Bouguer gravity gradient is steeper than in the Himalaya of India. Moreover the maximum flexural stresses are concentrated within the crust which may account for the seismic activity of the Sargodha ridge and southern Kohistan. At the end of the Indian elastic plate (arbitrarily chosen at the MMT), a large positive vertical shear stress, S₀ = 9.2 x 10¹² N/m, is applied to account for the topographic load north of the MMT. In addition, to fit the gravity constraints it was necessary to apply a strong negative bending moment, M₀ = -0.85 x 10¹⁸ N, at the end of the plate. The negative bending moment is due to the combined effect of the northward migration of the Indian plate and the southward differential compressional force generated by the crustal rocks stacked beneath the northern Kohistan arc. Consequently, in southern Kohistan the surface of the Indian plate is concave. The upper portion of the elastic plate is therefore under compressional regime, while the lower portion is subject to extentional stress. High flexural stresses are probably the primary source of the Hazara seismic zone where incipient reverse faulting seems to take place. In contrast, the pronounced convexity developed along the flexural bulge can account for 1) tensional stress in the upper part of the Indian plate, large enough to produce basement normal faults interpreted beneath the Salt range and Sargodha ridge; and 2) compressional stress in the lower portion of the crust that cause the excess of mass and seismicity beneath the Sargodha Ridge.
-
329. [Article] Sedimentation, economic enrichment and evaluation of heavy mineral concentrations on the southern Oregon continental margin
Heavy minerals can contain potentially economic amounts of metals as both matrix and trace constituents. Such minerals appear as unconsolidated black sands on the continental shelf off southwest Oregon ...Citation Citation
- Title:
- Sedimentation, economic enrichment and evaluation of heavy mineral concentrations on the southern Oregon continental margin
- Author:
- Bowman, Kenneth Charles Jr
Heavy minerals can contain potentially economic amounts of metals as both matrix and trace constituents. Such minerals appear as unconsolidated black sands on the continental shelf off southwest Oregon and along the Oregon coast. Two diverse energies are considered in this investigation. Environmental energy of the depositional regimen, Part I; energy involved in crystallization of transition metals from a magma, Part III. In Part II, an analytical scheme for the evaluation of opaque oxides is proposed, and an examination of the results as applied to two samples is presented. Part I The unconsolidated black sands on the Oregon continental margin have been profoundly affected by tectonic uplift aid by cyclic erosive transgression and regression. Progressive enrichment in heavy minerals from the Klamath Mountains has apparently occurred during each glacio-eustatic regression of the Pleistocene seas, each regression a period of intensified erosion and sediment transport. Subsequent erosive transgressions selectively sort and redistribute these heavy minerals into paralic beach and nearshore deposits. Uplift of the coast and shelf implies that the heavy minerals were reworked during the Holocene transgression into concentrations of greater extent and higher ore tenor than relict deposits of earlier transgressions in upraised Pleistocene terraces. Extrapolation of ore reserve values from the terraces by "Mirror Image" concepts might seriously underestimate the potential of offshore deposits. Offshore heavy mineral concentrations should be coincident with observed submarine terraces. Part II An analytic scheme was developed to investigate opaque oxides in two samples; one from the Pleistocene terraces; the other from near the present shelf edge. Analyses involving X-ray diffraction techniques, atomic absorption and neutron activation established the mineralogy and elemental distribution in magnetically separated diagnostic splits. Chrome spinel, ilmenite and magnetite comprise the opaque oxide fraction in both samples. Correlation studies of these analyses suggest: (1) Chromium is a matrix metal of chrome spinel and is diadochic into magnetite. (2) Iron appears in all opaque oxides and in increasing amounts with increasing magnetic susceptibility. (3) Titanium is a matrix metal in ilmenite, and diadochic into chrome spinel and magnetite. (4) Nickel and ruthenium are diadochic into and correlated to the spinel structure; i.e. to chrome spinel and magnetite. (5) Osmium appears to be correlated to chromium. (6) Zinc is limited to spinel in these samples. Part III Goldschmidt's and Ringwood's criteria for diadochy often fail to explain the distribution of the transition metals because crystal field effects are not considered. Favored d[superscript n] configurations, e.g. octahedrally coordinated, low spin d⁶ cations in the spinel minerals, result in shortened interatomic distance and significantly strengthened cation-ligand bonds, possibly affecting the distribution of such metal cations. The octahedral site preference energy parameter (OSPE) has been used to explain distributional behavior of the first (3d) transition series metals. OSPE calculations for four low spin d⁶ cations - Co(III), Ru(II), Rh(III), and Pt(IV) - give significantly high values for this parameter. High OSPE valued transition metal cations possibly form stable proto-mineral oxide complexes in the magma which persist through crystallization. These associations predetermine the enrichment of transition metals in oxide minerals and act as nuclei during cooling and solidification. Subduction of oxidized and hydrolyzed near-surface rocks down a Benioff zone provides progressively higher Eh in the magma, a variety of cation oxidation states, and water for sepentinization of ultramafic rocks. The distribution of the platinum metals in a strongly reducing magma environment should be different than in the oxidizing magma proposed for the Klamath ultramafics. The OSPE parameter offers an explanation for the observed distribution of platinum group metals in spinel minerals from this investigation, in chromites from Uralian dunitic massifs and the Stillwater complex; and of iridium from the Great Lake Doleritic Sheet, Tasmania. Chrome spinel from Oregon had twice the concentration of ruthenium, and one-third the amount of osmium as similar Uralian chromite deposits. The first significant concentration of ruthenium in magnetite is herein reported recommending continued research into the platinum metal distribution in southwest Oregon.
-
Water and geology: how does geology control where you find and how you use water? / Roddey, James -- Through the eyes of the state geologist / Beaulieu, John D. -- What is groundwater? -- Geology and groundwater ...
Citation Citation
- Title:
- Cascadia : a quarterly publication of the Oregon Department of Geology & Mineral Industries, volume 2, number 1 (Winter/Spring 2002)
- Author:
- Oregon. Dept. of Geology and Mineral Industries
- Year:
- 2002, 2005
Water and geology: how does geology control where you find and how you use water? / Roddey, James -- Through the eyes of the state geologist / Beaulieu, John D. -- What is groundwater? -- Geology and groundwater -- Who owns and manages Oregon's water? -- Recent geologic efforts related to groundwater -- A groundwater case study: Catherine Creek and the Upper Grande Ronde Valley -- McKenzie - Willamette River confluence project