LATE PERMIAN TO LATE TRIASSIC PALEOGEOGRAPHIC EVOLUTION OF THE SOUTH CHINA AND INDOCHINA BLOCKS, SOUTH EAST ASIA

Camille Rossignol (1) , Sylvie Bourquin (1) , Erwan Hallot (1) , Marc Poujol (1) , Francoise Roger (2) , Marie-Pierre Dabard (1) , El Al.

1 GR - Géosciences Rennes
2 Géosciences Montpellier

The timing, spatial pattern and mechanisms that controlled the biotic recovery following the endPermian mass extinction, during the Early and Middle Triassic, are still under debate. In South East Asia (SEA), an Early Triassic biotic recovery has been documented in both terrestrial and marine ecosystems. SEA thus appears as a key area to document how life recovered after the end-Permian mass extinction. However, some aspects of the paleogeography and geodynamic evolution of SEA during the late Permian to the Late Triassic, those are likely to have influenced the biotic recovery in this area, remain debated. The debates concern, in particular, the formation of the Indosinian mountain belt that resulted from the continental collision between the South China and Indochina blocks, representing the main plates that composed SEA at that time. Sedimentary basins, through the sedimentary successions and the nature of the deposits, reflect the geology of the area from which the sediments were derived and provide valuable record of hinterland tectonism. To document the building of the Indosinian mountain belt and associated paleoenvironments, we investigated two sedimentary basins: the Sam Nua and the Song Da basins (present-day northern Vietnam), located, respectively, to the south and to the north of the inner zones of the Indosinian orogen (i.e., the Nam Co and the Song Ma units). Using sedimentological and dating analyses (foraminifers biostratigraphy and UPb dating on detrital zircon), we provide a new stratigraphic framework for these basins and propose a geodynamic evolution of the present-day northern Vietnam. During the late Permian to the Early Triassic, the Sam Nua Basin was opened to marine influences, and was mainly supplied by volcaniclastic sediments originating from an active volcanic activity. Geochemical investigations, combined with sedimentological and structural analyses, support an arcrelated setting for this magmatism, indicating the presence of a continental arc on the northern margin of the Indochina plate. During the Middle to the Late Triassic, the Sam Nua Basin underwent erosion that lead to the formation of a major unconformity resulting from the dismantling of the Indosinian Truong Son belt, built after the continental collision between the South China and the Indochina blocks. The sedimentation resumed at the end of the Late Triassic, with the deposition of very coarse material, emplaced under continental setting in a syn-to postorogenic foreland basin. To the north, the Song Da Basin is characterized by strongly diachronous formations, deposited during the Early and the Middle Triassic over a basal unconformity. These formations correspond to marine platform limestone and continental deposits, and represent the infilling of a foreland basin. Together, the Song Da and Sam Nua basins thus document the geodynamic evolution and the paleogeography of SEA during the late Permian to the Late Triassic. The South China and the Indochina blocks were separated by an oceanic domain, that closed progressively until the Middle Triassic and resulted in the formation of the Indosinian mountain belt. These highly dynamic tectonic changes were associated with paleoenvironment variations that have likely impacted the biosphere turnover during the Triassic.

Domains: Stratigraphy

Camille Rossignol, Sylvie Bourquin, Erwan Hallot, Marc Poujol, Francoise Roger, et al.. LATE PERMIAN TO LATE TRIASSIC PALEOGEOGRAPHIC EVOLUTION OF THE SOUTH CHINA AND INDOCHINA BLOCKS, SOUTH EAST ASIA. International Meeting of Sedimentology 2017, Oct 2017, Toulouse, France. pp.781. ⟨insu-01718147⟩