HR: 0830h
AN: S11C-0308 [PDF]
TI: Receiver functions in central Tibet, implications for crustal structure and anisotropy
AU: * Ozacar, A
EM: ozacar@geo.arizona.edu
AF: Department of Geosciences,
University of Arizona, Gould-Simpson Building, 1040E. Fourth St., Tucson, AZ 85721-0077 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences,
University of Arizona, Gould-Simpson Building, 1040E. Fourth St., Tucson, AZ 85721-0077 United States
AU: Gilbert, H J
EM: hgilbert@geo.arizona.edu
AF: Department of Geosciences,
University of Arizona, Gould-Simpson Building, 1040E. Fourth St., Tucson, AZ 85721-0077 United States
AU: Guynn, J
EM: jguynn@geo.arizona.edu
AF: Department of Geosciences,
University of Arizona, Gould-Simpson Building, 1040E. Fourth St., Tucson, AZ 85721-0077 United States
AB:
The dense INDEPTH III seismic array, which extended from the Lhasa terrane across the Banggong-Nujjang suture (BNS) into the
Qiangtang terrane in central Tibet, yielded a large, high quality teleseismic data set. Results from stacking and migration
of receiver functions with various frequencies reveal that (1) the crustal thickness ranges between 65-70 km. (2) A small
($\sim5$ km) Moho offset is observed when using a constant crustal Vp/Vs ratio of 1.73 for the entire profile. On the other
hand, using a higher Vp/Vs ratio of 1.80 for the Qiangtang terrane, as suggested from the timing of multiples, produces a
nearly flat Moho. (3) For higher frequencies, the amplitudes of the P to S conversions at the Moho (PmS) diminish along a
$\sim100$ km wide zone across the BNS, which implies a gradational velocity contrast. (4) A shallow ($\sim10$ km)
low-velocity zone (LVZ) appears $\sim40$ km south of BNS and is coincident with high conductivity observed from
magnetotelluric studies. (5) In the mid-crust a strong continuous LVZ is present at $\sim30$ km depth in the southern section
and becomes shallower ($\sim15$ km) and more pronounced to the north. The northward increasing trend of the Vp/Vs ratio, the
low Q values, and the large depth change of the mid-crustal layer observed from wide-angle seismic data are spatially
consistent with this LVZ. (6) In general, the lower crust has more intra-crustal arrivals with large amplitudes under the
Qiangtang terrane than in the Lhasa terrane. (7) The presence of dipping layers and/or seismic anisotropy is evident in the
crust near the BNS based on azimuthally varying amplitudes and large tangential energy. The sharp onset of strong SKS
splitting suggests crustal anisotropy with fast polarization directions varying from E-W to NE-SW. These splitting
observations align well with the direction of shear in the present-day crustal strain field in Tibet. In order to constrain
the origin and geometry of crustal anisotropy, observed variations in receiver functions with backazimuth will be modeled for
hexagonal anisotropy parameters by using a global minimization technique. This approach will provide additional information
on crustal fabrics, and in conjunction with geologic data, will enable us to distinguish crustal flow from fossilized fabrics
of older tectonic events.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting