On The Evolutional Model Of The Tectonic Geomorphology Of The Okinawa Trough

The Okinawa Trough (OT) is the solitary active backarc basin in the world that is formed by extensional rifting in the continental marginal zone and is in the early stage of evolution. Contrast analyses of geological structures, Cenozoic stratigraphy and geophysical fields between the OT and its surrounding areas (i.e., the East China Sea shelf, the southern Japan Arc, the Ryukyu trench-arc system, the Philippine Sea basin and the Taiwan-Luzon collision zone) are conducted. The tectonic geomorphological types are classified based on the theory of plate tectonics. And a tectonic geomorphological map of the OT and its surrounding areas is compiled. Based on these, the crustal properties, geological structures and stratigraphic patterns of the OT are examined in details; the tectonic geomorphological characteristics and distribution patterns of the OT are delineated, the morphogenic mechanism, geomorphologic ages and evolutionary history of the OT are determined, and the evolutional model of the tectonic geomorphology of the OT is established. The following remarks are concluded. (1) The formation, evolution and distribution patterns of the tectonic geomorphologic features of the OT are controlled by the tectonic movements and magmatic activities caused by the subduction and collision between the Philippine Sea plate (PH) and the Eurasia plate (EA). (2) The tectonic geomorphologic patterns of the OT is controlled by two sets of structural lines parallel and perpendicular to the Ryukyu trench subduction zone respectively, forming the geomorphologic zonation parallel to, and the geomorphologic blocking perpendicular to the subduction zone, called E-W zonation and N-S blocking respectively. The E-W geomorphologic zonation parallel to the subduction zone is formed under the extensional stress field in the backarc zone of the Ryukyu trench-arc-backarc system, consisting of a series of geomorphologic zones extending in the direction parallel to the trending of the OT, with a series of en echelon rifting depressions as the central zone. The N-S geomorphological blocking perpendicular to the structural line of the subduction zone is due to the different thicknesses and properties of the crusts in the northern, central and southern OTs, which are related to the different velocities of the relative movement between the PH and the EA, the different intersection angles between the PH movement direction and the trending of the subduction zone, and owing to the upward butting of the ridges and plateaus on the subducting PH to the overriding plate. In term of tectonic geomorphology, the northern, central and southern OTs are divided by the two great clinoforms located at 27°45′-28°02′N and 26°25′-26°35′N, respectively. (3) The formation and evolution of the OT have experienced three major tectonic movements, called the first, second and third phases of the OT movements respectively. Each of the phases is related to the changes of the direction and/or mode of the relative movements between the PH and the EA. The first phase of the OT movement occurred from the late Miocene to the early Pliocene, which caused the former island arc at the margin of the east Asia mainland folded and upwarped. A series of rifting grabens formed along the axial zone of the upwarped fold, which are the embryonic forms of the OT. As a result, the Ryukyu land mass was separated apart from the east Asia mainland by the original OT. The second phase of the OT movement occurred from the end of the Pliocene to early Pleistocene, which is characterized by large-scale normal faulting and magmatisms. This phase of the OT movement caused the original grabens formed during the first phase to link up as a united trough basin and extend southwestwards to northern Taiwan Island. A series of en echelon rifting grabens were formed along the axis of the trough. The major tectonic geomorphologic pattern of the OT was established. The third phase of the OT movement happened from late Pleistocene to Holocene, which is characterized mainly by less-scale normal faul...