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Latest Triassic - earliest Jurassic geodynamic evolution of the Southeastern Asia and mass extinction event

Golonka J., Krobicki M.

Volumina Jurassica

2006

Vol. 4, no. 1

281-282

The most significant geodynamic process in the Southeastern Asia was convergent Late Triassic event which was the continuation of Indosinian Orogeny and induced consolidation of Chinese blocks. Also, Indochina and Indonesia were sutured to South China, and Qiangtang block approached the Eurasian margin. Siliciclastic shallow marine shale sedimentation covered the western part of South China Plate, while the central and eastern parts were uplifted. Siliciclastic sedimentation prevailed. The collisional Triassic events continued between North China and Mongolia. Volcanics and collisional granites were common in both parts of the newly formed plate as well as on the Siberian margin of the Mongol-Okhotsk Ocean. Continental clastics and volcanics prevailed in the area between Africa and North America near Triassic/Jurassic boundary. The earliest Jurassic was the time of complete assembly of eastern Pangea and of onset of the break-up of the supercontinent and separation of Gondwana and North America. The colour of these rocks changed from red to grey along the Triassic/Jurassic boundary due to increased wetness and global transition from oxic to anoxic conditions. In Indochina block the Upper Triassic rocks are deformed and include in the thrust system. The Indosinian Orogeny continued after its collisional phase. In Indochina late phase of Indosinian Orogeny lasted around 50 million years until earliest Jurassic. This phase included back-arc deposits, synorogenic rocks and postorogenic molasse. Several phases of the orogeny could be distinguished based on metamorphic, plutonic and structural events. Carbonate sedimentation predominated along the Neotethyan margins, between 35°N and 35°S latitudes. The northwestern Neotethys region consisted of numerous horst blocks capped by carbonate platforms with adjacent grabens filled with deeper-water black mudstone and organic-rich shale facies. Reef communities were rare at that time. The time around the Triassic/Jurassic boundary marked the very important extinction event. From the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction of biotas: 1. The closure of Paleotethys and assembly of the Asian part of Pangea. This event caused the origin of new subduction zone along the Neotethys margin first in its western, later in the eastern part. Neotethys went from spreading to subducting phase. The new volcanic arcs were born along the subduction line. 2. The break-up of Pangea in the future Central Atlantic area and transition from rifting to drifting phase. 3. The very extensive volcanism and origin of Large Igneous Provinces type of basalts. This volcanism was related to the break-up of Pangea and caused rising amount of atmospheric CO2. 4. Sea level fluctuation. Drop of sea-level was related to the assembly of Pangea and was followed immediately by the rise caused by the Pangea break-up. It affected especially carbonate platforms and reefs. 5. Anoxia. It was related to the formation of restricted basins and nutrient oversupply caused perhaps by volcanic activity and sea-level fluctuations. All the above mentioned events are bound together by close relationships. Plate tectonic activity caused palaeogeographic and palaeoclimatic change, which triggered mass extinction.