From orogeny to rifting: the role of inherited structures during the formation of the South China Sea

Abstract

Cenozoic rifting in the South China Sea developed after a Mesozoic Andean-type orogeny (i.e., Yanshanian orogen) which led to structural, compositional, and thermal inheritance.These inherited lithospheric weaknesses can control the inception and evolution of rifting, as well as the final architecture of the rifted continental margin. In order to better understand these processes, recent studies have utilized seismic profiles, drill cores, and geochronological analysis to identify Mesozoic strata, magmatic rocks related to a former arc, and pre-Cenozoic fault systems in the region. These findings reveal that the pre-rift lithosphere was heterogeneous and that inherited structures affected the subsequent Cenozoic rift evolution.Here we use multi-stage models to investigate the impact of tectonic inheritance on the spatiotemporal evolution and final rift margin architecture in the South China Sea. We employ a numerical forward model that includes a two-way coupling strategy (Neuharth et al., 2022) linking the geodynamic code ASPECT and the landscape evolution model FastScape. We reproduce the first-order kinematic evolution of the South China Sea by imposing accordion type models of continental collision, followed by extension. We present a reference model that incorporates orogenic topography, thrust fault distribution, and the architecture of the rifted margin, while also accounting for realistic crustal thicknesses, heat flow, and lithosphere-asthenosphere boundary (LAB) properties. This model was derived by conducting a systematic evaluation of a suite of models that varied in terms of lithosphere rheology, convergence velocity, heat production, erosion rate, and random initial noise distribution.Our reference model reproduces a range of observations including continental collision, post-orogenic collapse, continental rifting and lithospheric breakup. During orogeny, the lithosphere undergoes thrust faulting, and crustal thickening, leading to the formation of inherited weakness in the crust. From orogenic collapse to continental rifting, pre-existing thrust faults serve as nucleation sites for normal faults, and their interaction with later rift-related normal faults can locally modify the regional stress field. During rifting, low-angle detachment faults which connect the reactivated thrust faults contribute to the overall deformation of the lithosphere. In this model, crustal thickening led to increasing temperature, which resulted in a more ductile lower crust with a rheological transition from brittle to ductile deformation. This thermal weakening of the lower crust allows for increased deformation and strain accommodation during lithospheric stretching. The presence of pre-existing thrust faults and a more ductile lower crust ultimately led to the formation of wide rifted margin of the South China Sea. We suggest that this finding is applicable to other post-orogenic, wide rifts worldwide, such as the Basin and Range Province, the Aegean Sea and the West Anatolian extensional system.[1] Neuharth, D., Brune, S., Wrona, T., Glerum, A., Braun, J., & Yuan, X. (2022). Evolution of rift systems and their fault networks in response to surface processes. Tectonics, 41(3), e2021TC007166.