Evidence for small-scale mantle convection in the upper mantle.pdfVIP

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. 0, XXXX, doi:10.1029/2002JB002039, 2003 1 Evidence for small-scale mantle convection in 2 the upper mantle beneath the Baikal rift zone 3 Stephen S. Gao and Kelly H. Liu 4 Department of Geology, Kansas State University, Manhattan, Kansas, USA 5 Paul M. Davis and Phillip D. Slack 6 Department of Earth and Space Sciences, University of California, Los Angeles, California, USA 7 Yuliy A. Zorin, Valentina V. Mordvinova, and Vladimir M. Kozhevnikov 8 Institute of the Earth’s Crust, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia 9 Received 18 June 2002; revised 2 October 2002; accepted 28 January 2003; published XX Month 2003. 10 [1] Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long 11 profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped 12 lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift 13 axis (‘‘central high’’). An area with early arrivals relative to the stable Siberian platform of 14 up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis (‘‘flank 15 lows’’). While the location of the central high is approximately fixed in the vicinity of the 16 rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving 17 rays. We use three techniques to invert the travel time residuals for velocity anomalies 18 beneath the profile. Two of the techniques assume an isotropic velocity structure, and one 19 of them considers a transversely isotropic velocity model with a vertical axis of symmetry. 20 We use independent geophysical observations such as gravity, active source seismic 21 exploration, and crustal thickness measurements to compare the applicability of the 22 models. Other types of g