Stability of the Crust-Mantle Structures and Tectonics of the Island Arc and Trench System

Abstract

In order to discuss the stability of crustal structure deduced from seismic refraction measurements, the relationship between the depth to the Moho and the pressure acting on it is investigated by analysis of approximately 360 refraction data obtained in both continental and oceanic regions. It is found that there is a linear relationship between the depth to the Moho and pressure in vast areas extending from continents to ocean basins. The isostatic pressure versus depth relationship of the Moho is determined from about 200 refraction data obtained in stable areas. The division of data into two groups, one in stable areas and the other in active tectonic areas, is based mainly on the occurence of shallow and intermediate depth earthquakes. If the pressure deviation from the isostatic relationship exceed 95% confidence levels, these values are denoted pressure anomalies of the Moho, and the crust-mantle structures of such pressure anomalies are judged to be out of isostatic equilibrium. Because the pressure anomalies may correspond to the vertical component of forces acting on the rocks of unstable crust-mantle structures, inferences about the force mechanisms of tectonic processes acting on the island arc and trench systems are discussed through investigations of the pressure section on the Moho. The pressure sections on the Moho are presented for the Kuril-Kamchatka, Japan, Izu-Ogasawara, Honshu-Mariana, Tonga and Puerto Rico island arcs and associated trench systems. From those investigations, it is revealed that tectonism in the island arc and trench systems is controlled by forces acting in opposite directions; i.e., there is an upward force of about 500kg/cm 2 acting on the upper mantle rock under the island arc and a downward force of about 1000kg/cm 2 acting on the upper mantle rock in trench.