摘要
The Tibetan lithosphere plays an important role in global geodynamics, Geophysical evidence from gravity, geoidal, seismic, geothermal, and magnetotelluric (MT) observations provides important insights on the structure of the lithosphere, with implications for rheology and tectonics.The largest amplitudes of 1°× 1° negative Bouguer anomdlies in Tibet appear to renect a relatively soft, multilayered, low density lithosphere beneath the plateau. A density-contrast interface (possible base of the lithosphere), obtained by gravity inversion, shows a maximum depth of about l35 km near the ITS (Yaluzangbu suture), decreasing toward both southern and northern borders, with steeper slope in southern Tibet than in north central Tibet. The Himalayan collision zone corresponds to a north-dipping slope of Indian lithosphere plunging below the Higher Himalayas at the maximum dip angle of 36° Lateral variations between southern and north central Tibet are evidenced from both geoidal, using spherical harmonic differences of degrees 18O and 20,and seismic analyses using attenuation, anisotropy, and velocities including the Vnp distribution.The wave-like pzttern of positive and negative lateral changes of crustal density suggests a convergent (compressive) zone in the Himalayas and southern Tibet and a divergent (extensional)zone in north central Tibet, respectively. The lowest Pn velocity of 7.78~7. 83 km s-1 and low S wave velocities in the upper mantle are observed in north central Tibet, whereas high P-and. Swave velocities in the upper mantle, including high Vpn, are found in southern Tibet. Geothermal modeling reveals an heterogeneous thermal structure: southern Tibet has a relatively cold mantle,while northern Tibet has a relatively hbt mantle. The differences between southern and northern Tibet with respect to many geophysical observations should be taken into account for updating and improving thermorheological models of the lithosphere. The strctural features of the Tibetan lithosphere suggest a possible geedynamical model involving the northward penetration of the In dian plate.
