Tectonics And Evolution Of The Okinawa Trough

As a backarc basin rifted on the eastern margin of the Eurasian plate, theOkinawa Trough is conspicuous because of its unique position, intense tectonic andmagmatic activities, complicated geodynamics and the resource and environmentvalues which has been highlightened by the development of economics and society.As a part of the973program, the study of this thesis was aimed to reveal the deep andshallow structures and tectonic evolution of the Okinawa Trough, which is importantnot only for the marine rights and interests, but also the resource strategy of China.Large amount of gravity, magnetic, seismic reflection and regional geology datawere collected and sorted for the study of structural characters and tectonic evolution.For the gravity and magnetic data, the anomaly characters were described andinterpreted firstly. Then the deep and shallow structures were studied by using themethod of upward-continuation and directional derivative calculation, respectively.The Moho depth was calculated and the distribution of the relatively large fault beltsand magmatic bodis were deduced. Seismic stratigraphy and structural studies werecarried out by using the multi-channel seismic reflection profiles. According to theinterpretation results of the profiles, the ages, thicknesses and distributions ofsedimentary strata were ascertained and some structural stytles were detected.Characteristics and properties of several main tectonic units were redetermined, andnew tectonic map was compiled. Through restoration, the evolution process of thenorthern-middle part of the trough was proved to be different from the southern part,and the possible influence factors were discussed.The Okinawa Trough is characterized by both high free-air and Bouguer gravityanomalies but relatively large water depth, implying the upwelling of mantle materialwith high density. Both the distribution patterns of the total-field and pole-reductedmagnetic anomales in the East China Sea show as longitudinal belts in E-W andblocks in N-S, which are different from those of the Philippine Sea area, but theanomaly distribution patterns of the Ryukyu Arc and Okinawa Trough are influencedby the latter.The upward-continuation results of the Bouguer anomaly reveal the deepstructures clearly. Calculation results of the Moho depth shows that there is a Mohorise under the Okinawa Trough, and the Moho is deeper in the northern part than thesouthern, which is mirror-symmetric to the submarine topography. The Moho depth is14-18km in the south,18-24km in the middle and24km in the north. The NW-SEoriented fault belts in the study area are arc-shaped, which implies that one fault beltsmay consists of several en echelon faults. These fault belts are sub-parallel to eachother in the shelf area but change their directions in the trough and arc areas fromnorth to south, and generally normal to the trends of the trough. Therefore we suggestthe arc-shape was generated by the migration of the tectonic stress field during theevolution of the East China Sea. Several discontinuous magnetic belts were alsorecognized in the eastern margin, western margin and axial part of the trough and under the shelf margin.Seismic profiles show that the acoustic basement of the Okinawa Trough isoverlained by the Miocene, Pliocene and Quaternary strata, but change with position.The Miocene and Pliocene are thick in the middle and northern part, however, thesouthern Okinawa Trough is mainly filled with Quaternary sediments, thin and lentoidPliocene and Miocene are only preserved in some small areas. Large mount of faults,magmatic bodies, submarine canyons and buried canyons are found on the profiles inand around the trough, reflecting the intense tectonic and magmatic activities and sealevel changes during the evolution of the Okinawa Trough. Through correlationsbetween different profiles, the Longwang Belt locating in the west margin of thetrough or under the continental slop was suggested to be a discontinuous basementrise, separating the continent front depression to the west and the Okinawa basin tothe east, and was formed by block faulting and magmatism. The Longwang Belt andthe main part of the Diaoyudao Belt was terminated by the NW-SE trendingYushan-Kumi fault. South of the fault, there is a basement rise called “MianhuaBelt”under the western part of the trough and the continental margin, and the belt wassuggested to composed of the submerged southern Diaoyudao Belt and Pliocene-Quaternary magmatic rocks.Gravity inversion, refraction/wide-angle reflection, magmatic and other data allshows that the crust of the Okinawa Trough has thinned and intruded by basic andultrabasic rocks from mantle. Therefore most part of the crust should belong toocean-beared continental crust, but some places, especially in the southern OkinawaTrough, newborn oceanic crust has formed, and further geophysical survey mayreveal them.Bounded by the Yushan-Kumi fault belt, the evolution history of themiddle-northern Okinawa Trough to the north is different from the southern OkinawaTrough to the south. The initial rifting of the former started from the continent frontdepression in middle-Miocene, but the main rifting process migrated eastward to theOkinawa Trough in late-Miocene and lasting to present. The initial rifting of thesouthern Okinawa Trough started from late-Miocene, but in Pliocene the riftingsuspend and the regional subsidence happed. At the boundary of Pliocene andPleistocene, after short doming, the southern Okinawa Trough entered fast riftingstage and now, in drifting stage. The differences of the subduction, the Taiwanmountain building process and the indentation of the Gagua Plateau with Ryukyu Arcwere suggested to be the main controlling factors of the tectonic evolution.