Origin of transform faults in back‐arc basins: examples from Western Pacific marginal seas

Abstract

AbstractTransform faults in back‐arc basins are the key to revealing the evolution of marginal seas. Four marginal basins in the Western Pacific, i.e. the South China Sea (SCS), Okinawa Trough (OT), West Philippine Basin (WPB) and Shikoku‐Parece Vela Basin (SPVB), were studied to redefine the strikes and spatial distribution of transform faults or fracture zones. Based on high‐resolution tectonomorphology, gravity and magnetic anomalies, pattern of magnetic lineations, seismic profiles, geometry of basins and palaeomagnetic data, together with analyses of regional geological setting, plate reconstruction and geodynamic analysis, this paper suggests that all the transform faults in the four marginal basins are in general NNE‐trending. Moreover, by comparing with the contemporary structural framework of the East Asian Continental Margin, we propose new models concerning marginal seas spreading and have revised the previous Cenozoic plate reconstruction models related to the East Asian Continental Margin and the Western Pacific marginal seas. There are three possible origins of these NNE‐trending transform faults. 1. Inheriting the orientation of the strike‐slip faults at the rifting continental margin (e.g. the SCS and OT). The real strike of transform faults should not be NW but NNE. The large‐scale NNE‐trending dextral strike‐slip faults distributed in the continental shelf of the SCS control a series of pull‐apart basins of the SCS. Due to a higher degree of pull‐apart, oceanic crust began to open. Then they evolved into the NNE‐trending transform faults in the SCS and could also be regarded as a natural extension of the NNE‐trending strike‐slip faults in the South China Block (SCB). The geodynamic mechanism of the OT is similar to that of the SCS. Consequently, transform faults of the OT should also be NNE‐trending, which is not perpendicular to the spreading axis but instead displays oblique spreading. 2. Izu‐Bonin‐Mariana (IBM) Trench retreat to the NNE and NE. Subduction rollback to the NE and NNE produced the NE‐ and NNE‐striking horizontal tensile stress, resulting in the rifting of the Kyushu‐Palau Ridge (KPR), controlling the spreading of the SPVB and forming the NE‐ and NNE‐trending transform faults. This also involves oblique spreading. 3. The later overall rotation of the Philippine Sea Plate (PSP). Since 25 Ma, the WPB has rotated clockwise about 40°. Therefore the NW‐ and NNW‐trending transform faults that formed at the later spreading stage have rotated to be the near‐N–S‐ or NNE‐striking faults. These transform faults are almost perpendicular to the spreading axis. Copyright © 2016 John Wiley & Sons, Ltd.