Genesis And Structural Evolution Of The Shatsky Rise In The Northwest Pacific Ocean

In the past years, as the development of DSDP、ODP、IODP and based onpetrology, isotope geochemistry and structural geology, many geologists have studiedoceanic plateaus systematically. This paper focuses on the formation and tectonicevolution of the Shatsky Rise based on the coring data of IODP324cruise in2009.Previous results proposed three ideas on the formation of the Shatsky Rise. The firstone is that its formation has a close relationship with ridge spreading, because theShatsky Rise locates at the intersection of Japanese magnetic lineation and Hawaiimagnetic lineration. The second one is mantle plume head model, according to thevolume and duration of formation of the massifs. And other geologists suggest thatdecompression melting of mantle material caused by meteorite impact products themassifs. Three models have their advantages and disadvantages.87Sr/86Sr data of igneous veins at three sites which were cored in IODP324cruisereveal that the ages of veins formation are about100Ma,30-40Ma less than the agesof the wall rocks. It may be the result of slower cooling than normal oceanic crust.Mantle plume rears and then causes the upwelling of asthenosphere material, thereleased exert heat makes the long duration of calcite vein crystalization. In the othercase, the spreading of ridges may be influenced by mantle plume and then induces theformer`s regular jumping.Under the microscope for studying vein textures of two sites, we conclude thatthere are mainly three types of vein growth: symmetric growth from median line tovein walls, unilateral growth from one wall to the other, and the irregular infillinggrowth. For the second type, median line moves as time goes by. Veins of Site1350have several obvious growth stages, implying underwent several stages ofcrystalization. While the infilling veins of Site1346are mostly calcites, the growthdirection is difficult to be identified. As the depth increases, the width and openingdirection of veins change. In general, the gash types of veins growth are tensional joints and shear joints. Combined with FMS logging results such as radial distributionof joints in the Ori Massif, exert heat may result from the mantle plume tail rearsunder the Ori Massif.Based on the isotope geochemistry and micrograph of veins, combined with theFMS results and former studies, this paper puts forward to a new model of genesis ofthe Shatsky Rise: ridge-plume interaction and mantle plume tail model. In the longestreversal polarity time, the P-F-I (Pacific-Farallon-Izanagi)tripe junction was catchedby mantle plume head to induce a jumping of eastward800km and clockwise rotationof30°, inducing lava eruption to form the southern Tamu Massif; the Ori Massif wasformed by mantle plume tail catching triple junction while it moved to nearby. Thenorthern Tamu Massif, the Shirshov massif, the Papanin Ridge and the OnjinSeamounts may result from.the jumpings of triple junction.The formation age of the Shatsky Rise is late Jurassic and early Cretaceous, animportant time when global tectonic pattern switches greatly. This research willprovide significant information for the other oceanic plateaus. During J3-K1, thecontinental margins of East China was under compressional environment, producing aseries of compressional basins. During K2, the motion direction of the Pacific Platechanged from NW (Late Jurassic) to NNW. Assumed that the Hawaii Hotspot wasfixed, the motion direction of the Pacific Plate before47Ma was NNW-directed, laterfollowing NWW-directed until now.