Department of Geosciences

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Browsing Department of Geosciences by Subject "0404 Geophysics"

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    Along-fault migration of the Mount McKinley restraining bend of the Denali fault defined by late Quaternary fault patterns and seismicity, Denali National Park & Preserve, Alaska
    Burkett, Corey A.; Bemis, Sean P.; Benowitz, Jeff A. (Elsevier, 2016-12-14)
    The tallest mountain in North America, Denali (formerly Mount McKinley, 6,190 m), is situated inside an abrupt bend in the right-lateral strike-slip Denali fault. This anomalous topography is clearly associated with the complex geometry of the Denali fault, but how this restraining bend has evolved in conjunction with the regional topography is unknown. To constrain how this bend in the Denali fault is deforming, we document the Quaternary fault-related deformation north of the Denali fault through combined geologic mapping, active fault characterization, and analysis of background seismicity. Our mapping illustrates an east–west change in faulting style where normal faults occur east of the fault bend and thrust faults predominate to the west. The complex and elevated regional seismicity corroborates the style of faulting adjacent to the fault bend and provides additional insight into the change in local stress field in the crust adjacent to the bend. The style of active faulting and seismicity patterns define a deforming zone that accommodates the southwestward migration of this restraining bend. Fault slip rates for the active faults north of the Denali fault, derived from offset glacial outwash surfaces, indicate that the Mount McKinley restraining bend is migrating along the Denali fault at a late Pleistocene/Holocene rate of ~ 2–6 mm/yr. Ongoing thermochronologic and structural studies of the Mount McKinley restraining bend will extend these constraints on the migration and evolution of the restraining bend deeper in time and to the south of the Denali fault.
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    Geodynamic investigation of a Cretaceous superplume in the Pacific ocean
    Xue, Jing; King, Scott D. (Elsevier, 2016-08-01)
    The similarity in both age and geochemistry of the Ontong-Java, Hikurangi, and Manihiki plateaus suggests that they formed as a single superplateau from a unique mantle source. We investigate the necessity of a thermal superplume to form the Great Ontong-Java plateau at about 120 Ma using 3D spherical models of convection with imposed plate reconstruction models. The numerical simulations show that the giant plateau which formed as a result of melting due to the interaction of a plume head and the lithosphere would have been divided into smaller plateaus by spreading ridges, and end up at the present locations of Ontong-Java, Manihiki, and Hikurangi plateaus as well as a fragment in the western Caribbean. By comparing temperature and melt fraction between models with and without an initial thermal superplume, we propose that a Cretaceous superplume in Pacific at 120 Ma is required to form large igneous plateaus.
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    Hotspot swells revisited
    King, Scott D.; Adam, Claudia (Elsevier, 2014-10-01)
    The first attempts to quantify the width and height of hotspot swells were made more than 30. years ago. Since that time, topography, ocean-floor age, and sediment thickness datasets have improved considerably. Swell heights and widths have been used to estimate the heat flow from the core-mantle boundary, constrain numerical models of plumes, and as an indicator of the origin of hotspots. In this paper, we repeat the analysis of swell geometry and buoyancy flux for 54. hotspots, including the 37 considered by Sleep (1990) and the 49 considered by Courtillot et al. (2003), using the latest and most accurate data. We are able to calculate swell geometry for a number of hotspots that Sleep was only able to estimate by comparison with other swells. We find that in spite of the increased resolution in global bathymetry models there is significant uncertainty in our calculation of buoyancy fluxes due to differences in our measurement of the swells' width and height, the integration method (volume integration or cross-sectional area), and the variations of the plate velocities between HS2-Nuvel1a (Gripp and Gordon, 1990) and HS3-Nuvel1a (Gripp and Gordon, 2002). We also note that the buoyancy flux for Pacific hotspots is in general larger than for Eurasian, North American, African and Antarctic hotspots. Considering that buoyancy flux is linearly related to plate velocity, we speculate that either the calculation of buoyancy flux using plate velocity over-estimates the actual vertical flow of material from the deep mantle or that convection in the Pacific hemisphere is more vigorous than the Atlantic hemisphere.
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    Slip partitioning along a continuously curved fault: Quaternary geologic controls on Denali fault system slip partitioning, growth of the Alaska Range, and the tectonics of south-central Alaska
    Bemis, Sean P.; Weldon, Ray J.; Carver, Gary A. (Geological Society of America, 2015-06-01)
    Active transpressional fault systems are typically associated with the development of broad zones of deformation and topographic development; however, the complex geometries typically associated with these systems often make it difficult to isolate the important boundary conditions that control transpressional orogenic growth. The Denali fault system is widely recognized as transpressional due to the presence of the Denali fault, a major, active, right-lateral fault, and subparallel zones of thrust faults and fault-related folding along both the north and south flanks of the Alaska Range. Measured Quaternary and Holocene slip rates exist for the Denali fault system and portions of the adjacent thrust system, but the partitioning of fault slip between contractional and translational components of this transpressional system has not been previously studied in detail. Exploiting the relatively simple geometry of the Denali fault, we analyze the style and distribution of active faulting within the Alaska Range to define patterns of strain accommodation and determine how contractional and translational strain is partitioned across the Denali fault system. As the trace of the Denali fault curves by-70° across central Alaska, the mean strike of the thrust system to the north remains subparallel to the Denali fault, while to the south, the few faults with known or suspected Quaternary offset are oblique to the Denali fault. This relationship suggests that as the Denali fault system accommodates local fault-parallel strike slip, it partitions the residual part of the regional NW-directed plate motion into NW-SE shortening south of the Denali fault and shortening perpendicular to the Denali fault to the north. The degree of slip partitioning is consistent with a balanced slip budget for the two primary faults that contribute displacement to the Denali fault system (the eastern Denali fault and Totschunda fault). The current obliquity of displacement south of the Denali fault is the result of the late Cenozoic development of the Totschunda fault, which provides a more direct connection for the transfer of strain from the Fairweather transform fault to the Denali fault system. The transmitted strain is partitioned into right-lateral slip on the Denali fault and into Denali fault-normal shortening that is accommodated by thrust faulting in the Alaska Range and distributed left-lateral slip faulting within interior Alaska to the north.