Deep reflection surveying in central Tibet: lower-crustal layering and crustal flow

Details Deep reflection surveying in central Tibet: lower-crustal layering and crustal flow Deep reflection surveying in central Tibet: lower-crustal layering and crustal flow

Jan 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004geoji.156..115r&link_type=abstract

Geophysical Journal International, Volume 156, Issue 1, pp. 115-128.

Physics

12

Scientific paper

In the summer of 1998, a series of multichannel, deep seismic reflection tests were conducted in central Tibet around 89°E as part of the third phase of Project INDEPTH, a multidisciplinary study of the structure of the core of the Tibetan Plateau. Short seismic reflection sections from four locations spanning the Jurassic Banggong-Nujiang Suture show several features: thin (<2 km), sedimentary cover sequences; an unreflective upper crust down to approximately 25 km depth; strongly reflective lower crust down to 65 km depth (22 s traveltime) and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection Moho. These profiles are unlike those in parts of the Yadong-Gulu Rift to the south where few deep reflections exist and reflection profiles are dominated by high-amplitude reflection bright-spots at 6 s which have been interpreted as fluid bodies, either water or magma. Analyses of seismic amplitude decay on individual shot records confirm source-generated energy at traveltimes beyond 22 s, thus indicating that the change in reflection character at the Moho is not the result of a loss of signal penetration. The depths to the top and bottom of the lower-crustal reflectivity match both a mid-crustal velocity increase and the Moho determined for this area from refraction profiling. Lower-crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower-crustal flow in Tibet. However, Cretaceous-age magmatic intrusions may explain at least part of the observed reflectivity.

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