Abstract
This integrated tectonic study reveals the basin-forming and deforming processes on an active margin. Southwest Japan (SWJ) is an island arc under the influence of oblique subduction of the Philippine Sea plate, which has provoked dextral slips on the arc-bisecting Median Tectonic Line (MTL). Plio-/Pleistocene sediments in Beppu Bay, a tectonic depression at the westernmost portion of the MTL, are categorized into lower (5 ~ 0.7 Ma), upper (0.7 ~ 0.3 Ma), and an auxiliary uppermost (0.3 Ma ~ present) unit in ascending order. Detailed seismic interpretation demonstrates that major structures in the deep interior of the basin are an older half-graben under a strong N-S extensional regime and a younger pull-apart sag that developed in a right-stepping part of the MTL as a result of late Quaternary-enhanced strike-slip rates on the fault. Sediments within the pull-apart have been deformed by later inversion events as a contraction phase arose. Conspicuous deformation of the hanging wall of the low-angle detachment of the basin was successfully reproduced by numerical modeling. Based on a discrete element method, this suggests that structural differences in the deformed sedimentary layers are caused by differences in the dip angles of the faults. Remarkable temporal changes in tectonic regimes around Beppu Bay and other areas of SWJ are probably related to transient modes of convergence, including the migration of the Euler pole, of the Philippine Sea plate since ca. 6 Ma.
Highlights
Subduction of oceanic plates results in deformation of continental margins and formation of various tectonic basins
Spatiotemporal changes in fault activity, migration of the depocenters in Beppu Bay, and emergence of the newly found inversion phase are to be linked to the tectonic regimes on the convergent margin
Its complicated evolutionary process records the transitions of tectonic stress on the Philippine Sea plate convergent margin
Summary
Subduction of oceanic plates results in deformation of continental margins and formation of various tectonic basins. Spatial/temporal variations in sedimentary basin morphology on an active margin give clues to changes in the convergence modes of oceanic plates and to regional tectonic history. Beppu Bay (Figure 1), located in central Kyushu in southwest Japan (hereafter referred to as SWJ), is a typical example of a tectonically controlled sedimentary basin under the influence of the Philippine Sea plate convergence. A tension graben, the Hohi Volcanic Zone (Figure 1; hereafter referred to as the HVZ), which includes the Beppu Bay, emerged in central Kyushu at ca. The evolutionary process of the HVZ has been controlled by the activity of a large bisecting fault in SWJ, namely, the Median Tectonic Line (hereafter referred to as the MTL). Changes in the obliquity of the Philippine Sea plate motion have provoked thrusting and dextral slips on the MTL, which initiated as a regional crustal break in the Cretaceous east Eurasian margin and whose active segment has shrunk during the late Quaternary as suggested by geomorphological features
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
