TECTONIC ACTIVITRY PHASES IN CENOZOIC PERIOD ALONG THE CAO BẰNG - TIÊN YÊN FAULT ZONE, TIÊN YÊN - LẠNG SƠN SECTION

Truong Thanh Phi

Hanoi Univesity of Natural Resourses and Envinronment

Corresponding author: thanhgislab@gmail.com

Abstract: The Cao Bang - Tien Yen fault zone, Tien Yen - Lang Son section is about 100 km long, running in the NW-SE direction, the northeastern part of the Red River fault zone. The field survey was conducted at 21 locations, including the description of lithological characteristics, fracture orientation measurement, stratigraphic displacement, and the evidence of striation motion on the fault surface. The analytical results of 59 striations on the fault surface along the fault zone and section identified that the lateral strike-slip stress states with the four compression phases E-W, NESW, NW-SE and N-S. From the field observation, in combination with previous studies, the main tectonic phases are arranged in the order of directions: 1) NW-SE; 2) E-W; 3) NE-SW and 4) N-S. In particular, the first compression phase of the NW-SE direction severely destroyed the old rocks of the Jurassic age, encountered at many survey locations along the fault zone and section; the second compression phase of the E-W direction occurred during the Cenozoic time, causing the left displacement of the Red River fault zone in the Oligocene-Miocene time and the left motion along the Cao Bang - Tien Yen fault zone formed the Neogene sedimentary basins: Cao Bang, That Khe, Lang Son and Na Duong; the third compression phase of NE-SW direction occurred during the Mid-late Miocene, causing a tectonic inversion of the NW-SE faults in the northern part of the Red River basin, which is located in the southeast area of the fault zone; the final compression phase of the N-S direction, occurred during the Pliocene-Quaternary time, caused the right motion along the Cao Bang - Tien Yen and the Red River fault zones.

Keywords: Cao Bằng-Tiên Yên and Red River fault zones,Cao Bang basin, Neogene Na Duong basin,

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