HR: 17:30h
AN: S14A-07    [Abstracts]
TI: Imaging Delamination Beneath Tien Shan Using Multiple-ScS Reverberations
AU: * Revenaugh, J
EM: justinr@umn.edu
AF: Geology and Geophysics University of Minnesota, 310 Pillsbury Dr. S.E., Minneapolis, MN 55105, United States
AU: Turner, S
EM: turners@stu.beloit.edu
AF: Department of Geology Beloit College, 700 College St, Beloit, WI 53511, United States
AB: Mantle reflectivity mapping through the use of multiple-ScS reverberations has proven to be very effective, especially for the transition zone discontinuities. However, most studies of this kind have examined paths crossing ocean basins and few have sampled sub-continental mantle (e.g. Sipkin and Revenaugh, 1994). In large part, this is explained by the fact that the relatively thin and homogeneous oceanic lithosphere coupled with lower upper mantle Q result in much less scattered wave interference than the thick, heterogeneous lithosphere and high Q of the continents. But given sufficient deep seismicity, the method does provide useful results. Here we apply it to the Tien Shan region using recordings captured by the PASSCAL Tien Shan Experiment. Convergent features such as crustal shortening and magmatic underplating have resulted in extremely complex lithosphere (Vinnik et al., 2004). As with all previous applications of multiple ScS reverberation mapping, we identify the 410-km and 660-km discontinuities. For paths connecting events in Hindu Kush and the Tien Shan network, we observe several other reflectors, including a shear impedance decrease at approximately 110 km depth, which coincides well with receiver function imaging of the base of the seismic lid; and a large shear impedance increase at 300 km depth. The latter, which we dub S, is 50% larger than the 660 km discontinuity and is by far and away the largest reflector we have ever seen in the upper mantle in 25 years of looking. The feature is robust: it appears in all subsets of the data, is not affected by changes in processing parameters and can be observed in individual seismograms (unlike most other mantle reflectors). Having determined that it is real, it remains to understand its origin. A reasonable explanation is a piece of delaminated lithosphere that has broken off due to extreme lithospheric thicknening. Delamination has previously been suggested by Chen et al. (1997) from tomography and the thin seismic lid. If correct, the delaminated material must either geometrically focus the shear reverberations or have a highly unusual velocity contrast with surrounding material likely requiring some decompression melting in material rising to fill the void.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
SC: Seismology [S]
MN: 2007 Fall Meeting