GENESIS OF OCEANIC PLATEAUS AND SEAMOUNTS IN THE PACIFIC OCEAN——A CASE STUDY OF SHATSKY RISE

Although there are many large oceanic plateaus and seamounts in the northwest Pacific,the Shatsky Rise is a unique one,due to not only its tectonic location,but also the significance for understanding the genesis of oceanic plateaus in the region.It is also important to the study of the mechanism of origin of other rises in the northwest Pacific.There are three hypotheses proposed for the formation of the Shatsky Rise:(1) plume-head hypothesis,(2) ocean ridge tectonic hypothesis,and (3) meteorite impact hypothesis.All these hypotheses need a large-scale thermal anomaly from the deep,and a vehicle for transportation of the lower plume material to the base of plate.To the hypotheses of (1) and (2),a RRR triple junction separating the Izanagi,Farallon,and Pacific plates is a key.The formation of the northwest Pacific rises is related to the changes in the speed of plate movement and the jump and reorganization of the ridge/triple junction.This paper illustrates the possible tectonic events and mechanism during the period while the Shatsky Rise formed according to pre-existing geophysical and geochemical data,such as magnetic lineation,drilling cores,and the rose digrams of dip direction of joints and contacts of igneous rocks from FMS logging during the IODP324 cruise.It is suggested that the Tamu and Shirshov massifs are consistent with the seafloor spreading theory while the Ori massif is formed by aplume-head,but the possibility of ridge-plume interaction is not excluded.