Fault plane solutions of earthquakes and active tectonics of the Tibetan Plateau and its margins

Abstract

SUMMARY Fault plane solutions of earthquakes within and on the margins of the Tibetan Plateau show diverse styles of faulting and deformation, with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike-slip faulting resulting in roughly east-west crustal extension within the plateau. The direction of overthrusting of the Himalaya onto the Indian Shield is radially outward, varying from southwest in the western Himalaya to south-southeast in the east. Assuming that the Indian Shield behaves rigidly, this requires a west-northwest divergence of western Tibet from southeastern Tibet at a rate of 18 ± 9 mm yr−1, comparable with the rate of convergence at the Himalaya. Fault plane solutions of earthquakes in the southern portion of the Tibet Plateau consistently show large components of normal faulting on roughly north-striking planes and corroborate such extension. Within the high plateau, where elevations exceed 5000 m, normal and strike-slip faulting occur so that an overall east-southeast-west-northwest extension of the region (at about 10 mm yr−1) is partitioned into roughly equal parts of crustal thinning and north-northeast-south-southwest crustal shortening (about 5 mm yr−1). In general, strike-slip faulting characterizes solutions for earthquakes within eastern Tibet, where mean elevations drop below 4500–5000 m, but the orientations of the strike-slip faults vary across the region. In central Tibet, left-lateral slip occurs on planes trending roughly northeast, but for earthquakes farther east, the orientations of that plane become progressively east-west and then southeast. This variation in orientation implies a rotation of material along curved left-lateral shear zones. Thus, the eastward extrusion of Tibet appears to be facilitated not only by rapid left-lateral shear, but also by large clockwise rotations of the material in eastern Tibet. The rate of eastward extrusion of material in eastern Tibet, relative to the Tarim Basin to its north, is roughly 30–40 mm yr−1. Fault plane solutions of earthquakes in the northern and eastern margins of Tibet show large components of thrust faulting, with the P-axes, oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau. The orientation of this crustal shortening is northeast-southwest on the northeastern margin, east-west on the eastern margin, and northwest-southeast in the Longmenshan on the southeastern margin. Thus, at least some of the extrusion of eastern Tibet out of India's northward path into Asia is absorbed by crustal shortening on the margins of the plateau. The variation from normal faulting in the high Tibetan Plateau, where elevations exceed 5000 m, to dominantly strike-slip faulting farther east where elevations are lower, and then to thrust faulting on the margins of the plateau, where elevations drop below 3000 m, surely results, at least in part, from a decrease in the value of the vertical stress: the magnitude of the east-west compressive stress need not vary across the plateau.