Structures around the Tinjar‐West Baram Line in northern Kalimantan and seafloor spreading in the proto‐South China Sea

Abstract

AbstractThe Tinjar‐West Baram Line is a NW‐trending trans‐lithospheric fault in northern Kalimantan; its northwestern extension into the South China Sea (SCS) is the West Baram Line. In this paper, we propose that the geodynamic processes of the proto‐South China Sea (PSCS) played a key role in the formation and evolution of the Tinjar‐West Baram Line, based on previous studies of the strata, crustal thicknesses, gravity anomalies and other characteristics of the blocks adjacent to the Tinjar‐West Baram Line and the palaeomagnetic‐based plate reconstructions of the PSCS. The Tinjar‐West Baram Line has a close link to the subduction of the PSCS. Structural restoration reveals that (1) before 35 Ma, the Tinjar‐West Baram Line was a NE‐trending transform fault, which is consistent with the NE‐trending strike‐slip faults widely distributed in the East Asian Continental Margin, in the PSCS. (2) From the perspective of tectonic evolution, the extinction of the PSCS and the spreading of the SCS drove the Luconia Block in the northern SCS to accrete to the western side of the Tinjar‐West Baram Line. This process resulted in a contrast of crustal rocks adjacent to the Tinjar‐West Baram Line; to the east of this line, the Nansha Trough is oceanic crust, whereas to the west of this line the Luconia Block has an affinity with continental crust. The velocity and thickness of the crust show great differences on either side of the Tinjar‐West Baram Line. (3) The kinematic analysis of the Tinjar‐West Baram Line reveals that the collisional orogeny between the Luconia and Kalimantan blocks happened at 45–37 Ma on the west of the Tinjar‐West Baram Line; on its eastern side, the Nansha Trough was subducting south towards Kalimantan Island. During the interval 35–10 Ma, the kinematics of the Tinjar‐West Baram Line shows a feature of dextral strike‐slip faulting. (4) The earlier published palaeomagnetic data show that the Kalimantan Block experienced a counterclockwise rotation of about 50° during the period from 25 to 10 Ma. In addition, the exert counterclockwise rotation of about 20° of the Tinjar‐West Baram Line happened due to the resistance of the Luconia Block. Therefore, the Tinjar‐West Baram Line changed from an early NE‐trending to late NW‐trending structure which results in its present day tectonic framework. Thus, we suggest that the Tinjar‐West Baram Line was originally a NE‐trending transform fault of the PSCS extending to the continental crust. Subsequently, the Tinjar‐West Baram Line became the western border of the PSCS, accompanying the collisional orogeny between the Luconia and Kalimantan blocks. Copyright © 2016 John Wiley & Sons, Ltd.