The deep crustal structure of the Mojave Desert, California, from Cocorp seismic reflection data

Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N‐S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two‐way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross‐section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment‐basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present‐day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E‐W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

Section: Regional Geology and Teci'onicsmentioning

confidence: 99%

Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data

Li¹,

Henyey²,

Silver³

1992

“…It is juxtaposed against its upper plate by a latestage high-angle fault. The Rand and Portal Ridge schists can be linked by structure contours (Figure 1) constructed from surface relations and a series of seismic studies [Cheadle et al, 1986;Li et al, 1992;Malin et al, 1995;Yan et al, 2005]. The contour pattern indicates that the Rand fault and underlying schist lie beneath the entire western Mojave in a broad synform structure.…”

Section: Geologic Settingmentioning

confidence: 99%

“…In contrast, the well-known Cima volcanic field lies to the NNE above a region expected to be underlain by substantial Ludington et al [2005], Dokka [1983], and Malin et al [1995]. Rand Fault structure contours are based on the work of Cheadle et al [1986], Li et al [1992], Malin et al [1995], and Yan et al [2005]. Young (<5 Ma) mafic volcanic rocks throughout the Mojave Desert, including the basalts from Dish Hill and Cima, have isotopic compositions indicative of an asthenospheric source [Livaccari and Perry, 1993].…”

Section: Geologic Settingmentioning

confidence: 99%

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

Luffi¹,

Saleeby²,

Lee³

et al. 2009

[1] Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (e Nd = À13 to À6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (e Nd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25-28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction.

“…The Transverse Ranges were formed as a result of the compressional stresses associated with the Big Bend in the San Andreas fault. The complex structure and tectonics of the region have been studied using plate motions [Anderson, 1971], leveling data [Thatcher, 1976], geology [Yeats, 1981;Yeats, 1983;Namson and Davis, 1988a,b], seismicity and kinematics [Corbett and Johnson, 1982;Bird and Rosenstock, 1984;Savage et al, 1986;Nicholson et al, 1986] and seismic reflection and seismicity studies Kanamori, 1977, 1978;Humphreys et al, 1984;Webb and Kanamori, 1985;Cheadle et al, 1986;Huang et al, 1996;Ryberg and Fuis, 1998]. …”

Section: Introductionmentioning

confidence: 99%

Fine structure of the rupture zone of the April 26 and 27, 1997, Northridge aftershocks

Venkataraman¹,

Mori²,

Kanamori³

et al. 2000