ON THE FORMATION AND EVOLUTION OF THE MESOZOIC-CENOZOIC ACTIVE CONTINENTAL MARGIN AND ISLAND ARC TECTONICS OF THE WESTERN PACIFIC OCEAN
Guo Lingzhi;Shi Yangshen;Ma Ruishi Department of Geology Nanjing University
The Mesozoic-Cenozoic active continental margins and island arcs are the most spectacular morphotectonic expressions and mobile parts of the plate-tectonic regimes, and are ofgreat geotectonic significance. As early as in 1885, E. Suess recognized the fundamental dif-ferences between two quite different types of continental margins: the active (Pacific) forthe concordant and the passive (Atlantic) for the discordant. Since the advent and advan-cement of the concept of the plate-tectonics, the above classification of continental marginsis not adequate to elucidate the actual diversity of continental margins. In 1972, K. J. Hsuadded a third, i. e. the Mediteranean-type continental margin, "whose complex history has been tried at predominantly lateral motions between the Europian and African plates duringthe last 200 million years". Moreover, the Pacific type continental margins might also befurther divided into three subtypes: (1) the composite ttench-island arc-back basin subtype;(2) the Cordilleran ubtype ipcluding transform-fault and convergent margin (trench-arc) system which demarks the boundary between continent and ocean; and (3) the Andeansubtype including the trench which is at the edge of the mountain arc. The region under study comprises the Zhejiang-Fujian-Guangdong mainland coastal re-gion and Taiwan Province, as well as the East China Sea, Taiwan Strait, South China Seaof southeastern China, Korean Peninsula, Sea of Japan, Japan Islands, Ryukyu Islands,Philippine Islands and their adjacent areas. This study mainly deals with the tectonic for-mation and evolution of the active continental margins and island-arcs of the western Pa-cific Ocean.Their evolution might be summarized in four stages: 1. On the basis of rock types, stratification, colour index, rhythmic features, synge-netic structures and faunal assemblages of Carboniferous marine strata in Nanxi village ofFuding county, Fujian province, the authors found that these Carboniferous marine sedi-ments are of typical miogeosynclinal flysc ormation. The late Paleozoic eugeosynclinalvolcano-sedimentary formations have also been found in Taiwan Province (Dai Nan'aogroup), Sakhalin Islands, Japan Islands, Philippine Islands, northern Kalimantan, Indo-ChinaPeninsula, etc. In the light of the above-mentioned facts we are led to the conclusion thatthe basement underlying the late Mesozoic volcanics belongs to the Hercynian-Indosiniangeosynclinal folded belts which constitute a single tectonic entity framed with the ancientAsian continental margins. 2. During the Yenshanian orogeny an andean-subtype active continental margin oc-curred at the edge of the East Asia continental block around the Tethys and Western Pa-cific Ocean, which constitutes a late Mesozoic trench-mountain arc system similar to that ofthe western coast (South American trench-Andes mountain arc system) of south America. 3. During the Late Cretaceous and early Tertiary (Early Himalayan orogeny), dueto the activation, motion and subduction of the Pacific plate, the microspreading was iti-ated along the active continental margins where the offshore island arcs and the back-arcbasins were started to form, and the tectonic differentiation of continental margins (And-ean subtype and island-arc subtype) occurred at the same time. 4. During the late Tertiary and Quaternary (Late Himalayan orogeny), in particular.Owing to the oceanward migration of the subduction zone, the mordern morpho-tectonicfeatures of the composite trench-island arc-back arc basin systems have taken place at lastand the active plate-tectonic processes are still going on at present.