Discovery of Mega‐Sheath Folds Flooring the Liwan Subbasin (South China Sea): Implications for the Rheology of Hyperextended Crust

Abstract

Sheath folds recognized on kilometer scale are rare and only described in salt tectonics in seismic data. Here, we present mega‐sheath folds spectacularly imaged in a 3‐D seismic survey in the seismic basement flooring the hyperextended Liwan subbasin in the northern margin of the South China Sea. The sheath folds show eye structures delimited by anastomosing discontinuities. The axial planes of the sheath folds are subparallel to a rift‐related, extensional detachment surface separating hyperextended seismic basement from the syn‐rift sedimentary sequence. The sheath folds as well as the anastomosing discontinuities formed during hyperextension by ductile shearing of the forearc inherited and prestructured metasediments. The ductile shearing eventually resulted in the metasediments derived from midcrust unroofed at the footwall of an extension detachment fault. The discovery of mega‐sheath folds and ductile deformation within the basement conflicts with the general assumption that hyperextended domains are in the brittle field. The new seismic observations show the importance of forearc inherited, prestructured anisotropic basement controlling the crustal rheology during hyperextension.

The syn‐rift tectono‐stratigraphic record of rifted margins (Part II): A new model to break through the proximal/distal interpretation frontier

“…Rifting may not always initiate on a thermally and gravitationally equilibrated lithosphere (Manatschal, Chenin, Lescoutre, et al., 2021; Zhang et al., 2020) and/or the pre‐rift sediments may display variable thicknesses inherited from previous tectonic events. In Western Europe, for instance, the Alpine Tethys rifting overprinted an area that was formerly affected by the Meliata‐Vardar rifting, which is expressed by a thickening of the pre‐Alpine Tethys rifting sedimentary sequence to the east (Manatschal, Chenin, Ghienne, et al, 2021 and references therein).…”

New Insights Into the Rift‐To‐Drift Process of the Northern South China Sea Margin Constrained by a Three‐Dimensional Wide‐Angle Seismic Velocity Model

“…For example, HVLC has been observed confined in the northeastern SCS margin (Nissen, Hayes, Buhl, et al, 1995;Wang et al, 2006;Yan et al, 2001), although its formation mechanism and age were debated due to the different periods of magmatism on the continental slope (Cheng et al, 2021;Fan et al, 2017;Yan et al, 2006). The hyper-extended continental crust controlled by detachment faults have also been documented in the Pearl River Mouth Basin (Deng et al, 2020;Lei et al, 2019;Yang et al, 2018;Zhang et al, 2020). However, the other important characteristics such as SDRs typical for magma-rich margin and exhumed serpentinized mantle related to magma-poor margin have yet been reported, which make it controversial about the nature of the northern SCS margin and subsequently confused about its formation mechanism.…”

New Insights into the Rift-to-Drift Process of the Northern South China Sea Margin Constrained by a Three-dimensional OBS Seismic Velocity Model

“…For example, HVLC has been observed confined in the northeastern SCS margin (Wang et al, 2006;Yan et al, 2001;Nissen et al, 1995a), although its formation mechanism and age are debated due to the different periods of magmatism on the continental slope (Cheng et al, 2021;Fan et al, 2017;Yan et al, 2006). The hyper-extended continental crust controlled by detachment faults have also been documented in the Pearl River Mouth Basin (Deng et al, 2020;Zhang et al, 2020;Lei et al, 2019;Yang et al, 2018).…”