Crustal Tilting and Differential Exhumation of Gangdese Batholith Revealed by Bedrock Pressures
Abstract
The exhumation history of the Gangdese Batholith, southern Tibet, bears information on how magmatism and tectonics interact with the surface processes in a long-lived magmatic orogen. The present-day Gangdese Batholith (Lhasa-Nyingchi section) exposes deeper rocks to the east, suggesting the batholith represents a tilted crustal section. In this study, we applied Al-in-hornblende barometry to Gangdese rocks to obtain pluton emplacement pressures. Our results, together with existing bedrock pressure data, reveal the regional paleo-depth pattern in the Gangdese Batholith. The western Lhasa section exposes plutons emplaced at 2-3 kbar whereas the eastern part, near Nyingchi, exposes crust recording 6-12 kbar pressure, consistent with an eastward increase in host rock metamorphic grade and peak pressures. We then couple pressure data with new and published ages to constrain the exhumation history of the Gangdese Batholith. Results show that since 100 Ma, the upper crust experiences minimal exhumation except for a short pulse in the Miocene. In contrast, the middle-lower crust records a complex exhumation and burial history reflecting major tectonic events including the development of a continental arc and continent-continent collision. Since 20 Ma, the eastern section near Nyingchi undergoes fast and focused exhumation (> 40 km), which is likely related to the exhumation of the Eastern Himalayan Syntaxis. Conversely, the western section near Lhasa shows limited exhumation (< 10 km). Such dramatic differential exhumation in the E-W direction tilted the crust to the present-day exposure levels. By combining the exhumation history with results from kinematic modeling, we infer a thickening strain rate of ~10 -15 s -1 and average surface erosion rate of ~0.5 km/Myr during the Late Cretaceous Gangdese continental arc phase, similar to the Sierra Nevada and Peninsular Range arcs in the North American Cordillera. Our results also support a southern Lhasa terrane with 50-60 km thick crust and 4-5 km elevation during Late Cretaceous. Lastly, this study demonstrates that during the evolution of a magmatic orogen, the upper and middle-lower crust behaves differently, and the exhumation history reflects integrated tectonic, magmatic, and surface processes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V31D0157Y
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 1037 Magma genesis and partial melting;
- GEOCHEMISTRY;
- 1115 Radioisotope geochronology;
- GEOCHRONOLOGY;
- 8104 Continental margins: convergent;
- TECTONOPHYSICS