Features Of Gravity And Magnetic Field And Their Tectonic Implications Of The Okinawa Trough

Okinawa trough is a back-arc basin on the edge of the East China Sea shelf generated by plate subduction.It has the same structural and gravity and magnetic features of "East-West zoning,North-South blocking" with the East China Sea.The superposition of different control due to plate subduction has created its unique features in the aspects of crustal structure,tectonic features,fault development,magmatic activity,etc.,and the study of its tectonic features is of great significance to the study of the evolution mechanism of the back-arc basin.In this paper,based on the data pattern that gravity and magnetic data is the main data supplemented by other geological-geophysical data,we use various methods such as the method of reduced to the pole,boundary identification methods,variable density interface constraint inversion method,and the effective elastic thickness estimation method based on the coherence method to study and discuss the basic features of the gravity and magnetic field,the distribution of faults and igneous rocks,the crustal structure and property,and spatial differences in the structural features and evolution process of the Okinawa trough.We use various boundary identification methods and seismic profiles to infer 7primary faults,83 secondary faults,and 47 igneous rocks,we have come to an understanding on the basis of clarifying the internal structural framework of the Okinawa Trough that the strike-slip direction changes of the primary faults indicate different evolution history of the north and south sections of the trough.The distribution of secondary faults and igneous rocks together indicate that the south section of the trough is more rifted than the north section,and may have entered the seafloor expansion period of the typical back-arc basin.The results of Moho depth,crustal thickness and distribution of extension factor in Okinawa Trough are obtained by using the variable density interface constraint inversion method.The Moho depth changes from 16 km to 26 km,and the crustthickness is 12-22 km.Both of them decrease from north to south.The extension factor changes between 1.4-2.5,and increases from north to south,indicating that the Okinawa Trough has suffered a strong crustal thinning.Through the joint inversion of three typical profiles,it can be concluded that the upper and middle crust in the north section of the trough is obviously thinned,and the lower crust in the south section is also obviously thinned,which proves that the rifting in the south section is stronger than that in the north section.The distribution difference of sedimentary strata indicates that the time of large-scale rifting in the south and north section of the trough is not synchronous.In this paper,the effective elastic thickness of the lithosphere calculated by the coherence method varies between 9-30 km,which decreases from north to south,and the strength of the lithosphere also decreases gradually.Compared with the heat flow distribution and earthquake events data,the results show that the thermal tectonic activity has been gradually stronger from north to south,with relatively frequent major earthquakes,and the total tectonic activity follow this pattern,too.The Okinawa trough also has the features that the deep tectonic activity of the south is stronger than that in the north,however the shallow tectonic activity of the north is stronger than that in the south.Based on the above results and understandings,this paper summarizes the spatial differences of tectonic features of Okinawa Trough,and concludes that the main reasons for the spatial differences in the evolution process are as the following: 1.The block rotation in the south section of the trough,2.The stagnation of the southern part of the trough in the Miocene,3.The asynchronous large-scale rifting in the north and south sections of the trough.Finally,a set of simple geological models is established to show the process.