The Late Cenozoic tectonic evolution of Gurla Mandhata, Southwest Tibet

Abstract

How strain within the Tibetan plateau is geodynamically linked to that within the Himalayan thrust belt is a topic receiving considerable attention. The right-lateral Karakoram fault plays key roles in models describing the structural relationship between southern Tibet and the Himalaya. Considerable debate exists at the southeastern end of the Karakoram fault, where the role of the Karakoram fault is interpreted in two very different ways. One interpretation states that slip along the Karakoram fault extends eastward along the Indus-Yalu suture zone, thereby bypassing the Himalayan thrust belt to its north. The other, interprets that a significant component of the slip is fed southward into the Himalayan thrust belt along the Gurla Mandhata detachment. To evaluate this debate, the late Miocene fault slip rate history of the Gurla Mandhata detachment system is reconstructed from thermokinematic modeling with Pecube of zircon (U-Th)/He and biotite and muscovite 40Ar/39Ar thermochronometric ages. This slip rate history is then compared to that of the Karakoram fault. Zircon (U-Th)/He thermochronometric data from 3 east-west footwall transects reveal cooling of the Gurla Mandhata footwall through the zircon partial retention zone, from 8.01±1.31 Ma to 2.56±0.7 Ma. Results from ~21,100 Pecube models show a southward progression of decreasing fault slip magnitude and rate along the Gurla Mandhata detachment system. The northern transect modeling results show an initiation age from 14-11 Ma with a mean fault slip rate of 5.0±0.9 mm/yr. The central transect modeling results show an initiation age from 14-11 Ma with a mean fault slip rate of 3.3±0.6 mm/yr. The southern transect modeling results show an initiation age from 15-8 Ma with a mean fault slip rate of 3.2±1.6 Ma. These fault initiation ages and fault slip rate results match estimates obtained for the Karakoram fault across several timescales, supporting the idea that the two are kinematically linked. Specifically, the data are consistent with the Gurla Mandhata detachment acting as a right-step extensional stepover along which the Karakoram fault slip is transferred into the Himalayan thrust belt of western Nepal.