Collaborative Research: Discrete vs. Continuous Continental Deformation and the Role of the Lower Crust in the Tien Shan

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0309927RoeckerThis award provides support for an active seismic reflection program, and follow up passive seismic and MT surveying in the western Tien Shan orogen, Kyrgystan and NW China. The Tien Shan is a spectacular example of intracontinental thick-skinned compressional deformation associated with the India-Eurasia collision, taking up about half of this collision's deformation, and it is active. Existing results show a concentration of deformation near the northern and southern margins of the Tien Shan, inhomogeneous resistivity structure including high resistivity zones extending to Moho depths, and GPS deformation measurements which correlate with geological measurements estimated around the major deforming faults and cumulative over the orogen. The new work is set up to allow a choice between two distinct means of accommodating the compressional deformation in the lower crust; one by discrete faulting to uppermost mantle depths, and the other by continuous ductile thickening in the lower crust. A cornerstone of the effort is a dense CMP reflection profile to search for crustal scale fault zones traceable to the upper mantle as a verification of the first model. Additional passive seismic work is advocated to test for higher-T mantle upwellings penetrating toward the Moho presumably as a means of stimulating lower crustal ductility and to search for upper mantle interfaces. MT work is aimed at reprocessing and inverting older Soviet-era MT data stated to be of fair-good quality to provide coincident coverage with the proposed seismic line, and to collect additional sites into the Tarim Basin to improve sampling and resolve possible structures associated with the Tarim-Tien Shan transition. ***