Mantle flow and lithosphere-asthenosphere coupling beneath the southwestern edge of the North American craton: constraints from shear-wave splitting measurements

High-quality broadband seismic data recorded by the USArray and other stations in the southwestern United States provide a unique opportunity to test different models of anisotropy-forming mechanisms in the vicinity of a cratonic edge. Systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations. The western and southern edges of the North American craton show edge-parallel fast directions with larger-than-normal splitting times, and the continental interior is characterized by smaller splitting times spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of North America. Except for a small area in the vicinity of the Llano Uplift in central Texas, no systematic azimuthal variations of the splitting parameters are observed, suggesting that a single layer of anisotropy with horizontal axis of symmetry can adequately explain the observations. Estimation of the depth of the source of the observed anisotropy using spatial coherency of the splitting parameters indicates that the observed anisotropy mostly originate from the upper asthenosphere, through simple shear between the partially coupled lithosphere and asthenosphere --Abstract, page iv

Seismic Anisotropy and Mantle Flow beneath the Northern Great Plains of North America

A diverse set of tectonic features and the recent availability of high-quality broadband seismic data from the USArray and other stations on the northern Great Plains of North America provide a distinct opportunity to test different anisotropy-forming mechanisms. A total of 4138 pairs of well-defined splitting parameters observed at 445 stations show systematic spatial variations of anisotropic characteristics. Azimuthally invariant fast orientations subparallel to the absolute plate motion (APM) direction are observed at most of the stations on the Superior Craton and the southern Yavapai province, indicating that a single layer of anisotropy with a horizontal axis of symmetry is sufficient to explain the anisotropic structure. For areas with simple anisotropy, the application of a procedure for estimating the depth of anisotropy using spatial coherency of splitting parameters results in a depth of 200–250 km, suggesting that the observed anisotropy mostly resides in the upper asthenosphere. In the vicinity of the northern boundary of the Yavapai province and the Wyoming Craton, the splitting parameters can be adequately explained by a two-horizontal layer model. The lower layer has an APM-parallel fast orientation, and the upper layer has a fast orientation that is mostly consistent with the regional strike of the boundary. Based on the splitting measurements and previous results from seismic tomography and geodynamic modeling, we propose a model involving deflecting of asthenosphere flow by the bottom of the lithosphere and channeling of flow by a zone of thinned lithosphere approximately along the northern boundary of the Yavapai province

Seismic stratigraphic interpretation and reservoir characterization of the Mamuniyat Formation in the B-NC115 field, Murzuk Basin, Libya

Paleozoic succession in Libya offers much potential for scientific investigation and oil industry exploration due to the prolific reserve of hydrocarbons contained within the rocks of that region. The Paleozoic stratigraphy in Libya is divided into two mega sequences, a lower sequence extending from Cambrian to Silurian and upper sequence extending from Devonian to Permian. Mamuniyat Formation, a late Ordovician outcrop succession which is the main reservoir in Murzuk Basin, is composed of massive cross-bedded sandstone, most of which is generally medium to coarse grain. In B-NC115 field, the Mamuniyat Formation shows varied thickness which is a result of paleotopographic highs of glacial related origin. This formation is divided into three sequences based on the lithofacies and well log data pattern. The upper sequence, which was deposited when sea level fell, is the producing unit that has more than 100 ft (30.40 m) of net pay in some areas with good porosity and permeability. This sequence has three facies, and has thickness of 200 ft (60.96 m) of sand, interbeded with shales. They range from one foot to several feet and are recognized by log pattern and also field observation. Depth, structure, and isochron maps addressed the paleohigh structure, which is a combination of structure and stratigraphic feature. The reservoir shows good porosity and permeability range from 9% to 12%, and 500-1000 mD respectively. Seismic interpretation showed the progressive onlapping of the Silurian shale units over the Mamuniyat paleohighs, which known as the paleoglacial related trap. Although there is no significant fault signature in the field, a possible normal-faulted anticline has been recognized. Crossplot analysis and petrophysical results indicate the heterogeneity of the field. Oil-water contact has been determined by the resistivity logs at -3052 ft SS (922.02 m) --Abstract, page iii

Pervasive Double-Layer Anisotropy beneath the Central Tien Shan and Its Geodynamic Implications

The Tien Shan is an intraplate mountain belt situated between two stable Precambrian continental blocks, the Tarim Basin and the Kazakh shield. Contrasting mechanisms have been proposed to explain shear-wave splitting measurements obtained in the central part of Tien Shan, including N-S shortening originated from the India-Eurasia collision, mantle plume, small-scale mantle convection, and relative motion between the lithosphere and the underlain asthenosphere. Virtually all of the previous studies presented and interpreted their results in the form of station-averaged splitting parameters. Such a practice is valid only for the simplest form of anisotropy, i.e., a single layer with a horizontal axis of symmetry. In addition, all the studies only used the SKS phase in the 84-130 degree epicentral distance range. Consequently, only events from a narrow back-azimuthal range were used, making it impossible for identifying the existence of complex anisotropy which is diagnosed by systematic azimuthal variations of the resulting splitting parameters. Here we present a comprehensive analysis of newly-measured individual (rather than station-averaged) splitting parameters at about 40 broadband stations with data available at the IRIS Data Center. A total of about 900 pairs of well-defined SKS, SKKS, and PKS splitting parameters were obtained by using a robust shear-wave splitting measurement procedure that we developed (Liu, 2009, G-cubed). The use of all the three XKS phase and the full epicentral distance range (up to 180 degree) dramatically improved the azimuthal coverage, and provided clear evidence for the existence of pervasive double-layer anisotropy, as demonstrated by systematic azimuthal variations of the splitting parameters with a 90-degree periodicity. Ongoing effort to grid-search the four parameters suggests that beneath most of the stations with sufficient azimuthal coverages, the upper layer is in the lithosphere with an approximately E-W fast direction, which is parallel to the strike of the mountain belt, and the lower layer is in the NE-SW direction which might be related to mantle flow associated with the relative motion between the lithosphere and the asthenosphere

Pervasive Complex Seismic Azimuthal Anisotropy beneath the Western United States Orogenic Zone

Whether the western US Orogenic Zone is underlain by a single layer or double layer anisotropy is still a debated issue. A careful examination of NA-SWS-1.1, a coherent shear-wave splitting database for North America [Liu, 2009] finds that the splitting parameters at the majority of the stations in the western US with adequate azimuthal coverage demonstrate a clear azimuthal variation with a 90 degree periodicity. Specifically, events from the southwest show a splitting time that is about 60% of that for events from the west and northwest. In terms of fast directions, events from the southwest have a fast direction that is greater than 90 degree, while those from the west and northwest result in mostly E-W fast directions. This particular pattern of azimuthal dependence breaks down locally in tectonically anomalous areas such as the Yellowstone and the Great Basin where a high velocity lithospheric drip has recently been proposed [West et al., 2009]. One of the simplest explanations of this apparent two-layer phenomenon is the existence of a stratified mantle flow system consisting of two layers of plastic flow, as recently suggested in the Afar depression in Ethiopia. Alternatively, one layer of anisotropy could reside in the lithosphere and the other in the asthenosphere. However, this is an unlikely model because the western US has a thin lithosphere with spatially varying dominant tectonic characteristics, yet the azimuthal variation of the splitting parameters seems similar across a large area. We fitted the apparent splitting parameters under a two-layer model using the procedure of Silver and Savage [1994], and found a two-layer model in which the lower layer has a nearly E-W fast direction which is parallel to the absolute motion direction of the NA plate, and the upper layer has a fast direction that is approximately parallel to the San Andreas fault (SAF). These observations support a pervasive two-layer anisotropy beneath the western US orogenic zone, and suggest the existence of SAF-parallel lithospheric or asthenospheric fabric extending at least 800 km from the Pacific-North American plate boundary

Mantle Flow and Lithosphere--Asthenosphere Coupling beneath the Southwestern Edge of the North American Craton: Constraints from Shear-Wave Splitting Measurements

High-quality broadband seismic data recorded by the USArray and other stations in the southwestern United States provide a unique opportunity to test different models of anisotropy-forming mechanisms in the vicinity of a cratonic margin. Systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations. The western and southern edges of the North American craton show edge-parallel fast directions with larger-than-normal splitting times, and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of North America. At the majority of the stations, no significant systematic azimuthal variations of the splitting parameters are observed, suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations. The spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere. Based on previous results of seismic tomography and geodynamic modeling, we propose a model involving deflecting of asthenospheric flow by the cratonic root as the cause of the observed edge-parallel fast directions and large splitting times along the western and southern edges of the North American craton