South China Sea: From collapse to breakup via core complex, wide rift and narrow rift
Abstract
Before continental rifting, the South China Sea continental margins formed part of an Andean-type orogenic belt developed in association with the subduction of Paleopacific plate from Early Jurassic to Late Cretaceous. In the Late Jurassic, slab retreat resulted in orogenic collapse and widespread extensions in the South China continent, and the seaward migration and localisation of extension ultimately led to continental lithosphere breakup in the early Oligocene. Therefore, the South China Sea has evolved from orogenic collapse through wide continental rifting to continental breakup. However, how this geological evolution proceeded remains enigmatic. Here we use numerical experiments (see Figure) to show that the collapse of a thick (~50 km) and warmer orogenic crust delivers metamorphic core complex (MCC) in a dominantly wide rift mode of continent extension. This evolution is the outcome of the coeval formation of crustal-scale detachment faults, crustal boudinage and mobilisation of the deep crust. Importantly, during this stage, the weak lower crust decouples the upper crust from the upper mantle, which favours MCC and wide rift. This protracted (~30 to 40 Myr) period of wide continental rifting is followed by a shorter (~5 to 10 Myr) narrow rift mode, leading to continental breakup. This second stage is made possible by the cooling of the lithosphere, leading to the embrittlement of the lower crust and its coupling with the mantle. Our experiment reveals that a single phase of extension (i.e., constant extensional velocity) can deliver polyphase extension in which the three main modes of continental extension (Buck, 1991) are expressed. Here, the polyphase extension is not the result of changing driving stresses but that of the evolving rheology during the cooling of an extending orogenic crust. As the highly extended southern and northern margins of the South China Sea display the key structural features of the Basin and Range Province (wide rift and MCC), we propose that they developed following orogenic collapse and wide rift mode of continental extension. We also suggest that the opening of the South China Sea, 32 Myr ago, represents the next step in the collapse to rifting evolution of the Basin and Range Province.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.T22B..02R