Abstract

A geophysical survey including bathymetric swath mapping and magnetic vector measurements was conducted at four locations in the Mariana Trough during R/V Yokosuka cruise Y96-13 Leg 2 in order to better understand its tectonic history. In the southern Mariana Trough, swath bathymetry revealed that a N-S or NNE-SSW trending graben extends northward at least to 14¡05'N, and the graben is morphologically similar to the rift valley at 18¡N except for the smaller height of the rift wall. However, magnetic anomaly patterns do not show any evidence for E-W spreading in this region, but suggest NE-SW trending magnetic structure. In the central Mariana Trough at 18¡N, a rift valley and other morphologies typical of seafloor spreading are well developed. Magnetic vector anomalies suggest that seafloor spreading started at about 4 Ma in this region, and the spreading half-rate has decreased with time from about 2.5 cm/yr during the Gauss Chron to 0.9 cm/yr during the Brunhes Chron. In the Central Grabens at about 20¡N, a row of N-S or NNW-SSE trending small ridges with positive Z-component magnetic anomalies was discovered. It is probably the neovolcanic zone of a recently developed spreading axis. In the northern Mariana Trough at 21¡30'N, magnetic anomaly patterns closely resemble to those in the 18¡N region were observed, suggesting the occurrence of seafloor spreading between 21¡N and 22¡N north of the Central Grabens.