Thermochronologic constraints on the late Cenozoic exhumation history of the Gurla Mandhata metamorphic core complex, Southwestern Tibet

Abstract

How the Tibetan plateau is geodynamically linked to the Himalayas is a topic receiving considerable attention. The Karakoram fault plays key roles in describing the structural relationship between southern Tibet and the Himalayas. In particular, considerable debate exists at the southeastern end of the Karakoram fault, where its role is interpreted in two different ways. One interpretation states that slip along the dextral Karakoram fault extends eastward along the Indus-Yalu suture zone, bypassing the Himalayas. The other interprets that fault slip is fed southward into the Himalayan thrust belt along the Gurla Mandhata detachment (GMD). To evaluate these competing models, the late Miocene history of the GMD was reconstructed from thermokinematic modeling of zircon (U-Th)/He data. Three east-west transects reveal rapid cooling of the GMD footwall from 8.0 ± 1.3 Ma to 2.6 ± 0.7 Ma. Model simulations show a southward decrease in slip magnitude and rate along the GMD. In the north, initiation of the GMD range between 14 and 11 Ma with a mean fault slip rate of 5.0 ± 0.9 mm/yr. The central transect shows an initiation age from 14 to 11 Ma with a mean fault slip rate of 3.3 ± 0.6 mm/yr. In the south, initiation began between 15 and 8 Ma with a mean fault slip rate of 3.2 ± 1.6 mm/yr. The initiation ages and slip rates match the Karakoram fault across several timescales, supporting the idea that the two are kinematically linked. Specifically, the data are consistent with the GMD acting as an extensional stepover, with slip transferred southward into the Himalayas of western Nepal.