The Crustal Structure of Central Tibet Based on Local Earthquake Records and a Reinterpretation of Seismic Data Along the INDEPTH III Profile

Recent thermochronologic data from the Annapurna region of central Nepal show very young ages in the footwall of the Main Central Thrust (MCT), with late Miocene-Pliocene Th/Pb monazite ages especially common 5-15 kilometers south of the MCT (Catlos et al., 2001). These data can be explained with several different models, including: 1) Reactivation of the MCT and its splays in Miocene-Pliocene time with motion on some faults continuing to the present (e.g. Catlos et al., 2001); 2) Growth of a duplex beneath the MCT that passively lifted both the footwall and the hangingwall of the MCT in Miocene-Pliocene time (e.g. DeCelles et al., 2001); 3) A combination of MCT reactivation and duplex growth. When combined with structural mapping, our pilot dataset of in situ monazite Th/Pb ages from hangingwall and footwall rocks in the Madi Khola allows us to begin to validate aspects of some models and eliminate others. Our structural and thermochronologic interpretations are enhanced by Nd isotopic analyses that accurately constrain the location of the MCT (defined as the crustal boundary between Lesser Himalayan and Greater Himalayan rocks). We use Nd isotopes to map the MCT across the 60 kilometers between the Modi and Marsyandi Rivers based on previous studies that showed that Lesser Himalayan rocks contain more radiogenic Nd than Greater Himalayan rocks. In addition to identifying potential structural modifications of the MCT, our map will be useful for future studies in the Annapurna region because the critical task of locating the MCT is not always straightforward in the field.