KINEMATIC EVOLUTION ACROSS NORTHERN TIBET AND IMPLICATIONS FOR THE HIMALAYAN-TIBETAN OROGEN

The northern margin of the Tibetan Plateau was constructed by several Cenozoic thrust systems, including the North Qaidam and Qilian Shan thrust belts, which have been active since the Eocene. Active shortening in northern Tibet is evenly distributed across the thrust systems, which contrasts sharply to many sections of the Himalaya where most of the slip budget is accommodated along the thrust front. We attribute the overall architecture of the northern plateau to the dry climate and weak basal detachment along a pre-existing south-dipping subduction-mélange complex. Geodesy provides a comprehensive understanding of how plate convergence is partitioned across the Himalayan-Tibetan orogen (HTO); of the ~40 mm/yr present-day convergence across the orogen, ~20% is accommodated across the northern thrust systems whereas ~50% occurs across the Himalaya. Plate-circuit reconstructions and plate-convergence-rate calculations tell us that HTO convergence rates have decreased from ~120 mm/yr at the time of initial India-Asia collision to the present-day value of ~40 mm/yr. In the simplest scenario, this requires that shortening rates across intra-plateau thrust belts have decreased accordingly. Here we present a model to convey how these rates have varied through time for specific thrust systems. For example, we model shortening rates across the Qilian Shan thrust belt that have decreased from ~15 mm/yr in the Eocene to 6 mm/yr today, which yields an integrated shortening value of ~400 km. These estimates match our existing, yet limited, geologic restorations, and thus this method can provide useful bounds to guide and test field investigations. We further discuss how integrated shortening rates across the entire HTO should exist within external bounds of India-Asia convergence. That is, estimated Himalayan shortening rates, which are prevalent in the literature through the application of different thermochronometers, should be compatible with rate estimates in central and northern Tibet under some estimated bulk convergence rate. Where discrepancies exist, alternative tectonic models can be applied, such as aspects of the Greater Indian Basin hypothesis, “double subduction” prior to collision, or continental subduction, which require different orogen-scale convergence-rate vs time paths.