Shallow structure of the Yadong-Gulu rift, southern Tibet, from refraction analysis of Project INDEPTH common midpoint data

Refracted arrivals on International Deep Profiling of Tibet and the Himalaya (INDEPTH) common midpoint (CMP) data provide insight into the shallow structure of the Yadong-Gulu rift in southern Tibet. This is the first such information available for the Neogene-Quaternary rifts on the Tibetan Plateau. Geologic cross sections across three separate valleys within the rift have been constructed from these data and available surface geological information. In each case, the valleys are floored by asymmetric, low-velocity sedimentary fills ranging from similar to 100 m to similar to 1.5 km in thickness. Beneath all three, the basement appears to dip gently to the east (similar to 5 degrees). Seismic reflection data from one of these valleys (Nyinzhong) show a similar to 30 degrees east dipping reflector within the basement beneath the valley that projects upward to the normal-sense Nyainqentanglha detachment exposed in the range immediately to the west. Field investigations by INDEPTH personnel have additionally shown the existence of previously unrecognized normal-sense shear zones bordering the other two valleys. The newly discovered shear zones are 200 to > 500 m thick and in both instances are characterized by moderately to steeply west dipping mylonites with top-to-west shear sense. Metamorphic minerals within the shear zones indicate at least amphibolite grade metamorphism with increasing metamorphic grade down section. The seismic data, coupled with the new geologic information and geomorphologic data from Landsat imagery, suggest that the structure of the northern Yadong-Gulu rift is comparable to other well-documented rift systems in which steep range-front normal faults terminate at, or sole into, moderately dipping normal-sense detachments at depth that have undergone substantial footwall uplift and rotation during extension. This geometry implies that the extension of the Tibetan Plateau crust has been accommodated at depth by lateral flow within the middle crust. It also suggests that E-W extension of the plateau may have been substantially greater than has generally been presumed.