A magnetotelluric survey in northern Pakistan contains effects from both 2‐D and 3‐D structure. Identification of the effects of the sediments along the Indus River valley using 3‐D modeling permitted the selection of modes that could be accurately inverted with 2‐D inversions. The resulting 2‐D model reveals generally resistive (> 500 ohm‐m) upper crust (0–8 km), a more conductive (30–50 ohm‐m) middle to lower crust (8–40 km), and a resistive (> 300 ohm‐m) upper mantle. Shallow crustal (< 10 km) conductors correlate with a hydrothermally altered fault zone and/or carbonaceous metamorphic rocks near the Raikot fault. A prominent midcrustal conductor located beneath Nanga Parbat is required to fit the data, but its depth, dimensions, and conductivity are poorly constrained by existing data.
A 450 km long north‐south magnetotelluric profile spanning the Tien Shan from Kazakhstan to western China reveals lateral variations in the resistivity of the mantle lithosphere to depths of 140 km. Minimum changes of one order of magnitude in this depth range result from variations in temperature or composition, or both. Higher resistivities beneath a central portion of the range where the Moho is half as deep as elsewhere in the Tien Shan indicate a strong lithospheric block. We propose that this block protects this region of thin crust from appreciable deformation and instead transmits stresses due to India‐Asia plate convergence from the southern to the northern parts of the range.
A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.
Is erosion important to the structural and petrological evolution of mountain belts?The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics.At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse.Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.
Abstract Vertical electrical soundings were used to map the presence or absence of an aquitard separating a shallow, contaminated aquifer from a deeper, uncontaminated one in San Bernardino Valley, California. Correlation of vertical electrical soundings with lithologie logs from adjacent wells allowed us to also map local variations in the elevation of the water table Comparison of known waste sites, the distribution of the aquitard, and elevation of the water table yielded probable directions of contaminant transport. We conclude that there is a significant hazard to municipal water wells due to possible transport of contaminants between the shallow and deeper aquifers.
Magnetotelluric (MT) data in Long Valley, California, are principally sensitive to the complex structure in the rocks filling the caldera. While the overall shapes of the sounding curves are set by the deep crustal structure and mantle, the lateral variations of that response can be entirely explained using structure of the upper 1600 m. A conductive body at depths from 1350 to 1600 m is present in the northern, southern, and western moats and beneath the resurgent dome. This conductor effectively masks structures at intermediate depths. We believe that this conductor represents fractured Bishop Tuff within the caldera and possibly graphitic metasediments beneath it. Electrical logs from wells penetrating these formations are used to corroborate our interpretation. A large, conductive (5 ohm m) body of magma is precluded by the MT data. The response to a spherical chamber of 4‐km radius at a depth of 10 km is several times larger than the mismatch between our modeled and observed sounding curves. Small (2‐km radius), isolated pockets of magma may be present, however. We have developed a systematic approach to interpreting MT data in a complex volcanic environment. We clustered sites on the basis of similar responses to perceive patterns in spite of severe distortions by local heterogeneity. Our model was built from the surface downward, incorporating other published geophysical surveys and a time domain electromagnetic survey released by Unocal. Three‐dimensional modeling was employed from the start, and our interpretation was based primarily upon the phases. This systematic approach of stripping off the known, shallow structure in order to study the intermediate and deeper structure was quite successful. An important finding of our study is that the phase is sensitive to shallow (1350 m), three‐dimensional structure even at frequencies of 0.01 Hz. Three‐dimensional modeling simply cannot be avoided in complex geological environments.
Three high‐resolution seismic reflection lines were acquired in the northern part of the San Jacinto graben. The graben, a pull‐apart basin formed by a dilational right step of the San Jacinto fault zone, has been previously interpreted as a simple rhombochasm. The reflection survey located at least one significant and previously unidentified intrabasin fault, referred to here as the Farm Road strand. This fault lies approximately halfway between the Claremont and Casa Loma strands of the San Jacinto fault zone. At the north end of the basin, the southwestern boundary of the graben is interpreted to be the newly identified Farm Road strand and not the Casa Loma strand as was previously thought. The identification of this intrabasin fault allows us to infer that the San Jacinto basin comprises coalescing subbasins and is not a simple pull‐apart basin with an unusually large length:width ratio. The distances between the en echelon Casa Loma, Farm Road, and Claremont strands are between 1 and 2 km. This close spacing would likely permit an earthquake rupture to jump between strands and thus propagate through the San Jacinto basin.
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