Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide‐angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong‐Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P‐wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along‐strike to the east based on prior fan profiling; (3) a thick high‐velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid‐crustal low‐velocity layer in the P‐wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide‐angle profile with other seismic results allows a cross‐section of Moho depths to be constructed across Tibet. This cross‐section shows that crustal thickness tends to decrease from south to north, with values of 70–80 km south of the middle of the Lhasa terrane, 60–70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low‐density, hot upper mantle with respect to southern Tibet.

We have undertaken a series of controlled field experiments to develop seismoelectric experimental methods for near-surface applications and to improve our understanding of seismoelectric phenomena. In a set of off-line geometry surveys (source separated from the receiver line), we place seismic sources and electrode array receivers on opposite sides of a man-made target (two sand-filled trenches) to record separately two previously documented seismoelectric modes: (1) the electromagnetic interface response signal created at the target and (2) the coseismic electric fields located within a compressional seismic wave. With the seismic source point in the center of a linear electrode array, we identify the previously undocumented seismoelectric direct field, and the Lorentz field of the metal hammer plate moving in the earth’s magnetic field. We place the seismic source in the center of a circular array of electrodes (radial and circumferential orientations) to analyze the source-related direct and Lorentz fields and to establish that these fields can be understood in terms of simple analytical models. Using an off-line geometry, we create a multifold, 2D image of our trenches as dipping layers, and we also produce a complementary synthetic image through numerical modeling. These images demonstrate that off-line geometry (e.g., crosswell) surveys offer a particularly promising application of the seismoelectric method because they effectively separate the interface response signal from the (generally much stronger) coseismic and source-related fields.

[1] During the summer of 1998, active-source seismic data were collected along a transect running 400 km NNW-SSE across the central Tibetan Plateau as the third phase of project INDEPTH (International Deep Profiling of Tibet and the Himalaya). The transect extends northward from the central Lhasa block, across the Jurassic Bangong-Nujiang Suture (BNS) at 89.5°E, to the central Qiangtang block. A seismic velocity model for the transect to $25 km depth produced by inversion of P wave first arrivals on $3000 traces shows (1) a $50-km-wide region of low velocity (at least 5% less than surrounding velocities) extending to the base of the model at the BNS; (2) sedimentary cover for the southern Qiangtang block that is $3.5 km thick; (3) a distinct interface between sedimentary cover and Qiangtang basement or underplated Jurassic melange in the central Qiangtang block; and (4) evidence that the Bangoin granite extends to a depth of at least 15 km. The BNS has little geophysical signature, and appears unrelated to the $5 km northward shallowing of the Moho which is associated with the BNS in central Tibet. Geophysical data along the main INDEPTH III transect show little evidence for widespread crustal fluids, in contrast to the seismic ''bright spots'' found in southern Tibet and to magnetotelluric evidence of fluid accumulations in eastern Tibet. A comparison between the global average and Tibetan velocity-depth functions offers constraints for models of plateau uplift and crustal thickening. Taken together with the weak geophysical signature of the BNS, these velocitydepth functions suggest that convergence has been accommodated largely through pure-shear thickening accompanied by removal of lower crustal material by lateral escape, likely via ductile flow. Although we cannot resolve the details, we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of Tibet as a series of accreted terranes.