Precollisional tectonics and terrain amalgamation offshore southern Taiwan: Characterizations from reflection seismic and potential field data

Abstract

Sponsored by the Chinese National Fundamental Research and Development Program in 2001, Guangzhou Marine Geological Survey launched out a long geophysical survey from the northeastern part of the South China Sea (SCS), through the Luzon Arc, to the Huatung Basin and the Gagua Ridge. Based on high-resolution seismic data from this survey, combined with gravimetric and magnetic modeling, a systematic effort is made to the study of the regional geodynamics offshore southern Taiwan. By focusing particularly on precollisional tectonic interactions between adjacent geological units and their tectonic affiliations, this study can help reveal early arc-continent collisional processes that formed the Taiwan orogen. The construction of the Manila accretionary prism and its eastward progressive deformation indicate that the subduction of SCS have experienced multiple phases of increased activity. Active precollisional crustal shortening within the Northern Luzon Trough resulted in tilting of sedimentary layers at angles between 6° and 13°. But the shortening induced by tilting accounts for only a tiny part of regional total crustal compression. The eastern flank of the Luzon Arc appears to be more active than the rest, evidenced by active faulting and folding in the intra-arc basins on the eastern flank. Magnetic modeling/inversion shows that the Luzon Arc may have experienced multiple phases of magmatic activities, causing lateral magnetic inhomogeneity. Bouguer gravity anomalies and gravity modeling indicate that the Huatung Basin has anomalously higher crustal and upper mantle densities than those of SCS and the Luzon Arc. In addition, there is a large bathymetric difference between the Huatung Basin and the northeastern part of SCS basin. These observations argue against early hypothesis that the Huatung Basin and the northeastern part of SCS basin may once have belonged to one single oceanic crust, in part or in whole. The Gagua Ridge, as a sliver of uplifted oceanic crust, may be related to a transient northwestward subduction of the western Philippine plate. All evidences point to the argument that the region offshore southern Taiwan is experiencing multiple terrain amalgamation, which is a classical model for continental growth.

New geophysical constraints on a failed subduction initiation: The structure and potential evolution of the Gagua Ridge and Huatung Basin

“…The flexure of the WPB oceanic crust forms a deep sedimentary trough that is continuous along the length of GR in free‐air gravity data (Figures c, c, and b) and is also visible in the velocity models as a depression of low‐velocity material (2.5–4.0 km/s) to the east of GR. Previous studies [ Deschamps et al ., ; Li et al ., ] observe an asymmetric gravity profile associated with the GR and corresponding sedimentary trench that are very similar to type examples of gravity profiles acquired across active subduction systems. We observe a similar asymmetry in the gravity profiles coincident to transect T1B, T2, and T4A (Figures b, c, and ).…”

“…[] and Li et al . []. Initial deformation at the fracture zone may have resulted from same the clockwise rotation of the WPB identified by Deschamps et al .…”

“…Furthermore, gravity modeling from an E‐W transect across the basin from Li et al . [] requires ocean crust of average thickness (∼5–6 km) to satisfy the observed gravity anomaly.…”

“…[] analyzed interstation phase velocities between OBS stations off the eastern coast of Taiwan and found that in the short‐period range, the phase velocities were comparable to those characterizing the 15–30 Ma Parece‐Vela Basin and therefore similar in age to the SCS. However, there are large differences in bathymetry, and higher crustal and mantle densities in the HB compared to the SCS [ Li et al ., ]. It has also been identified that the HB is 500–1000 m shallower than the Eocene age WPB, despite claims that the HB is much older.…”

South China Sea Tectonics

“…It developed from Cenozoic continental margin rifting, and its central portion is floored with oceanic crust. Despite its relatively small size and short evolutionary history, the SCS has undergone nearly a complete Wilson cycle from continental breakup to seafloor spreading to subduction and is well suited for studying various plate boundary activities, such as continental margin rifting (e.g., Hayes and Nissen, 2005), seafloor subduction (the Manila Trench; e.g., Li et al, 2007b), strike-slip faulting (the Red River fault; e.g., Leloup et al, 2001;Clift and Sun, 2006), and active orogenic processes (Taiwan; e.g., Huang et al, 2001) (Figure F1).…”