The transition from continental to lithospheric breakup recorded in proto-oceanic crust: Insights from the NW South China Sea

Abstract

The formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive ocean-continent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT.

Late Ediacaran to Early Cambrian Breakup Sequences and Establishment of the Eastern Laurentian Passive Margin, Newfoundland, Canada

“…Humber margin strata in the model of van Staal et al (2013) were deposited following the Cambrian Series 2 onset of thermal subsidence, ~20-30 Myr after hyperextension and the late Ediacaran rift-related magmatism in the Canadian Appalachians. An unexplored element of van Staal et al (2013)'s hypothesis is that some magmapoor margins are established by a two-step process with crustal breakup before mantle breakup (e.g., Huismans & Beaumont, 2011 and stratigraphic studies have accordingly recognized crustal and mantle breakup sequences in basins that record the transition between the timing of rupture and onset of thermal subsidence (Alves & Cunha, 2018;Chao et al, 2023;Soares et al, 2012Soares et al, , 2014. In combination with the new depositional age, sediment provenance, and stratigraphic interpretations reported herein, we use modern magma-poor rift analogues to propose that the Labrador and Curling groups comprise parts of crustal and mantle breakup sequences deposited along eastern Laurentia.…”