USING REMOTE SENSING TO CONNECT OUTCROP-SCALE UHP PETROLOGY TO AN OROGEN-SCALE TECTONIC PROBLEM

The northern margin of Qaidam Basin in northern Tibet is characterized by discontinuous exposures of early Paleozoic ultrahigh-pressure (UHP) metamorphic rocks ductilely interfolded with a Cambrian ophiolite sequence. The UHP exposures are separated by 10s-100s of km in a spatial distribution controlled by Cenozoic faults that thrust the Paleozoic rocks over Neogene and Quaternary sediments. Collectively, the discontinuous UHP exposures, referred to as the North Qaidam UHP terrane, have been commonly inferred to represent a large body of consistent age formed by a single tectonic event. Emplacement of the ophiolite sequence has been similarly interpreted. The relationship between the various exposures of UHP and ophiolite units places important constraints on tectonic models of northern Tibet. However, the suggested genetic link between the separate exposures remains speculative partly as a consequence of a lack of detailed geologic mapping and petrologic analysis of these remote locations. Remote sensing techniques, coupled with existing field mapping for one UHP/ophiolite locality, provide a way to evaluate the prevailing single-body hypothesis. Specifically, ASTER imagery was used to produce spectral maps of four localities along the northern Qaidam Basin margin – Dulan, Xitie Shan, Luliang Shan, and Qing Shan. The spectrally-identified lithologic units were confirmed by petrographic analysis of samples previously collected in the field at the Luliang Shan locality, and the Luliang Shan spectral map pattern was verified against the results of detailed geologic field mapping. Lithologic identifications and geologic maps for the other three sparsely-studied localities were produced based on correlation with the Luliang Shan spectral mapping. Where available, petrologic information for the other three sites was used for independent verification. Preliminary observations suggest that spectral mapping can be used to discriminate ophiolite subunits and to distinguish felsic UHP rock from younger granitic intrusions. The data suggest that the spectral characteristics of the ophiolite units in the separate exposures are similar and may indicate a single ophiolite sequence. The correlation of the UHP units is less clear, which may be a result of heterogeneities in the UHP continental crust.