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Title: Lg Attenuation Anisotropy Across the Western US

 [1]; ORCiD logo [1];  [1]; ORCiD logo [1]
  1. Los Alamos National Laboratory
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Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
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USDOE National Nuclear Security Administration (NNSA)
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Conference: Amercian Geophysical Union ; 2017-12-11 - 2017-12-15 ; New Orleans, Louisiana, United States
Country of Publication:
United States

Citation Formats

Phillips, William Scott, Rowe, Charlotte Anne, Stead, Richard J., and Begnaud, Michael Lee. Lg Attenuation Anisotropy Across the Western US. United States: N. p., 2017. Web.
Phillips, William Scott, Rowe, Charlotte Anne, Stead, Richard J., & Begnaud, Michael Lee. Lg Attenuation Anisotropy Across the Western US. United States.
Phillips, William Scott, Rowe, Charlotte Anne, Stead, Richard J., and Begnaud, Michael Lee. 2017. "Lg Attenuation Anisotropy Across the Western US". United States. doi:.
title = {Lg Attenuation Anisotropy Across the Western US},
author = {Phillips, William Scott and Rowe, Charlotte Anne and Stead, Richard J. and Begnaud, Michael Lee},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2017,
month =

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  • The western margin of the North American plate includes subduction, transform faulting and a migrating triple junction. The authors have measured polarization azimuths ([phi]) and delay-times ([delta]t) of split shear waves. Stations located close to the northern end of the San Andreas fault, near the San Francisco Bay area yielded well-constrained but azimuthally varying splitting parameters. These can be explained by a model consisting of two anisotropic layers: an upper layer with fast direction parallel to the strike of the San Andreas fault and a lower layer with E-W fast direction. Both layers have average delay-times of 1[+-]0.3 s. Themore » authors have found that an east-west fast feature is also present beneath stations in the Sierra-Nevada, the Mojave Desert and the Los Angeles area. These latter measurements do not require more than one layer. The E-W fast layer diminishes near the southern edge of the Gorda plate. The authors interpret their measurements as caused by single or double layers of homogeneously, transversely anisotropic material with horizontal symmetry axes due to strain-induced preferred orientation of olivine in the upper mantle. They suggest that the fault-parallel fast layer is the result of finite strain associated with the transform plate motion between the North American and Pacific plates. The deeper layer with E-W fast direction can not be associated with known surface tectonic features. One possible mechanisms for this E-W fast feature is that it may be related to the passage of the trailing edge of the Farallon plate as the slab migrated northward beneath central California. The shear associated with the different motion between the slab and the asthenosphere may cause mineral alignment leading to shear-wave splitting.« less
  • A retrodeformable cross section that integrates surface and subsurface data across the Ventura basin and western Transverse Ranges illustrates the structural style and evolution of Oligocene to Holocene age structures. Three deformational events are recognized: Oligocene to early Miocene compression, late Miocene through Pliocene normal faulting, and Pleistocene to Holocene compression. Oligocene to early Miocene compression caused uplift and formation of a large antiform north of the Santa Ynez fault in the central part of the range. This antiform is interpreted to be a ramp-related fold associated with movement on a southwest-verging blind thrust fault. Late Miocene through Pliocene agemore » normal faulting along the Oak Ridge fault formed the southern boundary of the Ventura basin. The basin formed by simple block rotation along the normal fault and was filled by up to 6 km of Pliocene and Pleistocene clastic sediment. During Pleistocene to Holocene compression, the Ventura basin and western Transverse Ranges were deformed by both north- and south-verging thrust faults and related folds. The Ventura Avenue anticline is interpreted to be the result of imbricate thrust faulting in the Rincon and Monterey formations. Along the Oak Ridge trend, thrust-related folds rotated and reactivated the late Miocene and Pliocene age normal faults. A restoration of the regional transect documents 34% or 35 km of shortening by thrusting and folding during the Pleistocene to Holocene phase of compression.« less
  • The Cenomanian-Turonian boundary is noted by a global positive excursion in the carbon isotopic signature. Sediments deposited during this interval in Europe are rich in organic carbon this interval in Europe are rich in organic carbon. These data suggest unusually high productivity which should be expressed within the composition of the plankton. The authors have examined samples from three sections in the Western Interior Seaway from Manitoba, Colorado, Arizona, that represent nearshore and offshore calcareous litho facies. They analyzed the foraminifera and calcareous nannofossils to determine how biostratigraphic and paleoecologic events affected the makeup of faunal and floral assemblages. Themore » extinction of the calcareous nannofossil P. albianus occurs at the first appearance of the planktonic foraminifer W. archeocretacea, which marks the base of the boundary interval. Nearshore foraminiferal assemblages that include planktonic taxa, characteristically contain a benthic fauna composed of both arenaceous and calcareous species. During these blooms though, planktonics disappear as does the calcareous component of the benthic fauna, most notably the nodosariids. Co-occurring shifts in the relative abundance of the planktonic foraminifera and the calcareous nannofossils in the more offshore facies are also evident. Finally when the authors compare the faunal data with the record of carbon and oxygen isotopes, they find that the timing of the biostratigraphic events already described is coincident with the positive excursion of the carbon isotope.« less
  • Cretaceous strata in Arizona, New Mexico, and southern Colorado were deposited in the southern part of the asymmetric Western Interior basin and consist of east- and northeastward-prograding clastic wedges of shoreface sandstones and coastal-plain deposits separated by thick tongues of west- and southwestward-extending marine shale and, on the eastern shelf, by limestone. The interlayering of the two primary facies (sandstone and shale) is the result of either regional subsidence, eustasy, sediment supply, or a combination of these. Constructing and comparing regional cross sections will aid in accomplishing one of the goals of the Western Interior Cretaceous (WIK) project of themore » International Union of Geological Science's Global Sedimentary Geology Program, which is to identify the global effects of eustasy. Preserved Cretaceous strata in this part of the Western Interior basin range from a few hundred feet thick on the east to about 7000 ft on the west. A cross section across this area reveals facies relations, major sequence boundaries, unconformities, and available biostratigraphic and chronostratigraphic data for the sections preserved in the present Laramide basins. Sequence boundaries occur in the middle late Albian, at the base of the late Turonian, and within the latest Turonian. Younger sequence boundaries, if present, are less obvious, and their associated unconformities have not been identified.« less