Abstract
The Okinawa Trough (OT) is an incipient back-arc basin, but its crustal nature is still controversial. Gravity inversion along with sediment and lithospheric mantle density modeling are used to map the regional Moho depth and crustal thickness variations of the OT and its adjacent areas. The gravity inversion result shows that the crustal thicknesses are 17–22 km at the northern OT, 11–19 km at the central OT, and 7–19 km at the southern OT. Because of the crust with a thickness larger than 17 km, the slow southward arc movement, and scarce contemporaneous volcanisms, the northern OT should be in the stage of early back-arc extension. All of the moderate crustal thickness, high heat flow, and intense volcanism at the central OT indicate that this region is probably in the transitional stage from the back-arc rifting to the oceanic spreading. A crust that is only 7 km thick, lithosphere strength as low as the mid-ocean ridge, and MORB-similar basalts at the southern OT demonstrate that the southern OT is at the early stage of seafloor spreading.
Highlights
Crust is the outermost solid shell of the Earth and the crust types vary significantly among different tectonic units, such as continental margins, oceanic basins, and island arcs
Where gm is the mantle residual gravity anomaly that reflects Moho undulations; gb and gs are the gravity anomaly induced by seawater and sediment, respectively, when taking continental crust density (2.7 g/cm3) as the background; gt is the anomaly that originates from mantle density perturbations because of the thermal expansion when taking the normal mantle density (3.3 g/ cm3) as the background; go is the gravity anomaly caused by the other sources, and this anomaly is relatively small and can be ignored (Bai et al, 2014; Kusznir et al, 2018)
Since the NW-SE-trending Kerama and Tokara faults, which separate the northern-central-southern Okinawa Trough (OT), are thought as the result of the subduction of the high and buoyant topography in the Philippines Sea Plate (Sibuet et al, 1998; Gungor et al, 2012) and no evidence can demonstrate that pre-rifting OT has varying lithospheric strength, we suggest that the diffuse rifting in the OT is due to the topographic high subduction
Summary
Crust is the outermost solid shell of the Earth and the crust types vary significantly among different tectonic units, such as continental margins, oceanic basins, and island arcs. The Okinawa Trough (OT) is a back-arc basin developed under the area of East China Sea. a number of studies have been performed to examine its crustal structure (Iwasaki et al, 1990; Nakamura et al, 2003; Gungor et al, 2012; Klingelhoefer et al, 2012; Shang et al, 2017; Qi et al, 2020), it is still controversial if the nature of its crust is continental, transitional, or oceanic (Liu et al, 2016) (Figure 1). Seismic data suggest that the southern OT is characterized by well-developed symmetric deep faults, while more diffuse rifting occurred in the north (Gungor et al, 2012). It indicates that the forces along the OT are uneven (Doo et al, 2018). The third stage is the initial divergence indicated by the newborn oceanic crustal spreading (Liu et al, 2016)
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