Regional mapping of the crustal structure in southern California from receiver functions (original) (raw)
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Deep structure of southern California
Physics of the Earth and Planetary Interiors, 2007
We used 214,210 P-wave arrival times from 7536 local earthquakes and 16,470 travel-time residuals from 332 teleseismic events recorded by the southern California Seismic Network to determine a detailed three-dimensional (3D) P-wave velocity structure of the crust and mantle down to 600 km depth beneath southern California. In this study, we have taken into account the Moho topography under this region determined by a previous receiver-function study. We found that the undulations of the Moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle. When the Moho topography is taken into account, ray paths and travel times can be computed more accurately. The tomographic images show a very heterogeneous structure in the crust and upper mantle under southern California. The velocity structure in the shallow depth correlates well with the surface geological features. Three major anomalies in the upper mantle are revealed clearly beneath the southern Sierra Nevada, Salton Trough and Transverse Ranges. Compared with the previous result, both the shape and size of the three anomalies show some differences. Our result revealed two prominent low-velocity (low-V) anomalies above the high-velocity (high-V) anomalies in the upper mantle under southern Sierra Nevada and the Transverse Ranges. We consider that the upper-mantle anomaly under the Transverse Ranges was formed through convergence and sinking of the entire subcrustal lithosphere. Vertical motions and removal of the dense crust root caused the high-V anomaly under the southern Sierra Nevada. The prominent low-V anomalies above the two high-V anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs. The Salton Trough low is the response to the lithospheric extension when the Pacific plate was rifted away from the North American plate.
Crustal structure of northeastern California
Journal of Geophysical Research, 1986
In 1981, the U.S. Geological Survey conducted a seismic refraction survey of northeastern California designed to characterize the structure in four geologic provinces: the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces. The survey consisted of north-south lines in the Klamath Mountains and Modoc Plateau provinces, northwest-southeast lines centered on Mount Shasta and Medicine Lake volcano, and an east-west line linking all the profiles. All lines except the east-west line ranged in length from 125 to 140 km, contained three shot points, and were recorded by 100 instruments. The east-west line was 260 km long, contained six shot points, and was recorded by 200 instruments. The Klamath and Modoc lines yielded the simplest models. The Klamath model is finely layered from the surface to at least 14-km depth, consisting of a series of high-velocity layers (6.1-6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km downward to an unknown depth. The Modoc model, in contrast, is thickly layered and has lower velocity at all depths down to 25 km. The uppermost layer, 4.5 km thick, consists of low-velocity material (2-4.5 km/s). Velocity beneath this layer is much higher (6.2 km/s) and increases slowly with depth. A small velocity step (to 6.4 km/s) is seen at 11 km, and a larger step (to 7.0 km/s) is seen at 25 km depth. Moho is probably 38-45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains. Models for the Shasta and Medicine Lake lines show special features including low velocity (less than 3.5 km/s) in the edifice of Mount Shasta but high velocity (5.6 km/s) at shallow depth (1-2 km) under the summit of Medicine Lake volcano. The model for the east-west line consists of a western part similar to the Klamath model, an eastern part similar to the Modoc model, and laterally changing velocity structure in between, underlying the Cascade Range. This model was converted to a density model, and observed Bouguer gravity data were matched. A general decrease in Bouguer gravity values eastward may be explained by a general decrease in the density of crustal layers and does not require a change in crustal thickness. Beneath the 4.5-km-thick surficial layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for a refraction line in the Sierra Nevada. It is unlike velocity models for rift areas, to which the Modoc Plateau has been likened by some authors. We theorize that beneath its surficial volcanic and sedimentary rocks, the Modoc Plateau is underlain by a basement of granitic and metamorphic rocks that are the roots of ancient magmatic arc(s). The fine layering in the Klamath model is consistent with the imbricate structure of the Klamath Mountains. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet corresponds to a velocity step from 6.5 to 6.7 km/s at 7-km depth in our model. The 6.7 km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, mostly amphibole schists, which crop out west of the Trinity ultramafic sheet. Deeper velocity layers can likewise be correlated to terranes that crop out farther west. In our geologic cross section of northeastern California, derived from our velocity-density model for the east-west line, the Klamath Mountains are underlain by folded and thrust-faulted slices of oceanic crust. The Modoc Plateau and westernmost Basin and Range province are underlain by a section of volcanic and sedimentary rocks overlying granitic and metamorphic rocks, all tilted westward between an inferred fault under Medicine Lake volcano and the Surprise Valley fault. In the Cascade Range, geologic units appear to be discontinuous, and structures include horsts, grabens, and a 10-km step downward to the east in the 7 km/s layer. The latter step may represent a fault, fold, intrusion, or a combination of any of the three. Apparently, the Cascade Range, a modern magmatic arc, is developed across the suture region between the stack of oceanic rock layers underlying the Klamath Mountains and the inferred roots of magmatic arc(s) underlying the Modoc Plateau.
The nature of deep crustal structures in the Mojave Desert, California
Geophysical Journal International, 1987
The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion. The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.
Journal of Geophysical Research, 1992
A 30 km-long N-S seismic reflection line was shot by California Consortium for Crust Studies (CALCRUST) across the southern Mojave Desert and onto the northern flank of the San Bernardino Mountains in sot, them California. On the northern end of the seismic section, the reflectivity increases markediy in the midcrust at a depth corresponding to a two-way travel time of 4 to 5 s (12-15 km), suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust. The high reflectivity disappears at about 8 s (24 kin) and may be correlated with a change in seismic velocity in the lower crust from 6.3 km/s to 6.8 km/s. A band of reflectivity between 9.5 and 10 s (27-30 km) is believed to represent the Moho. The midcrustal relectivity transition and Moho both deflect downward toward the San Bernardino Mountains uplift over the entire length of the profile. The deflection of the midcrustal transition (12 ø) appears greater than that of the Moho (6ø), resulting in a thinning of the lower crust to the south beneath the uplift. In addition, the midcrustal transition coincides with the base of the seismogenic zone (brittle-ductile transition?) which is also dipping southward beneath the San Bernardino Mountains, while the Moho deflection is consistent with elastic flexure resulting from edge loading by the San Bernardino Mountains which have been thrust over the Mojave block. It is suggested that the thinning of the lower crust beneath the San Bernardino Mountains is a result of north directed ductile flow in response to loading by the over thickened upper crest. Since a portion of the load is transmitted through the lower crust to the Moho, the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift (_•2 m.y.).
The deep crustal structure of the Mojave Desert, California, from Cocorp seismic reflection data
Tectonics, 1986
COCORP seismic reflection .. profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low-angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The uppermost horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona-Rand-Orocopia Schist. This reflecting horizon 1Now at
Two lithospheric profiles across southern California derived from gravity and seismic data
Journal of Geodynamics, 2007
We present two detailed 2-D density transects for the crust and uppermost mantle across southern California using a linear gravity inversion technique. This technique parameterizes the crust and upper mantle as a set of blocks that are based on published geologic and seismic models. Each block can have a range of densities that are constrained where possible by borehole measurements, seismic velocities, and petrologic data. To further constrain the models, it is assumed that the lithosphere is close to isostatic equilibrium at both ends of the profiles, in the deep ocean and east of the Mojave Desert. We calculate the lithostatic pressure variations field for the whole cross section to rule out the geophysically insignificant solutions. In the linear equation, ρ = a + bV (V, seismic P-wave velocity; ρ, density), which approximates the mantle density-velocity (ρ-V) relationship, different coefficients for b were evaluated. Lower coefficients (b < 0.2) correspond to an almost purely thermally perturbed mantle, while higher coefficients (b > 0.3) imply that other effects, such as composition and/or metamorphic changes, play an important role in the mantle. Density models were constructed with the coefficient b ranging from 0 to 0.6. The results indicate that a high b value in the mantle ρ-V relationship is associated with less dense crust in the Mojave block and more dense crust in the Catalina schist block. In the less dense Mojave block, the average density of the whole crust is ∼2.75 g/cm 3 , while that of the lower crust is ∼2.72 g/cm 3 . These densities imply a high silica content in the crust, and a minor fraction of basic rock in the lower crust, or perhaps the absence of a basaltic layer altogether. By comparison, the average density of a typical continental stable platform is ∼2.85 g/cm 3 . Models with higher b coefficients (0.5-0.6) are characterized by a large isostatic imbalance. On the other hand, lower b values (0-0.2) require a consolidated whole crust density in the Mojave Desert of ∼2.78 g/cm 3 , and a lower crust density of ∼2.89 g/cm 3 with mostly basaltic composition. This contradicts the observed, lower V p /V s -ratio in the Mojave Desert associated with mostly felsic and low-density crust. Models with lower b coefficients (0.1-0.2) are characterized by an absence of local Airy compensation beneath the San Gabriel Mountains at the LARSE-1 profile. These, and other non-gravity arguments, suggest optimal solutions to the mantle ρ-V relation of b ∼ 0.2-0.4. This, in turn, means that both thermal and petrological effects occur inside the downwelling of the uppermost mantle high velocity body located beneath the Transverse Ranges. During the development of this mantle downwelling, the basaltic layer of the Mojave block was likely eroded and pulled down into the high velocity body. Those basaltic fragments may have been transformed into eclogites, and this metamorphic change implies a higher b-coefficient density-velocity relationship than would be expected for a purely thermal process. Published by Elsevier Ltd.
Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias
Journal of Geophysical Research, 2001
We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, México, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ϳ31ЊN latitude. Depth to the Moho is ϳ33 (Ϯ3) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ϳ40 (Ϯ4) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ϳ15-18 (Ϯ2) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ϳ25Њ. This range of Moho depths within the Peninsula Ranges, as well as the sharp ϳeast-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.
A tomographic image of mantle structure beneath Southern California
Geophysical Research Letters, 1984
We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X-ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image.