Spreading process of the Mariana Trough

马里亚纳海槽是一个典型的洋内弧后盆地，具有多种特征。我们汇编了大量马里亚纳海槽的地表地球物理数据（水深测量、磁力和重力），这些数据是在过去几年日本的几次航行中收集的。地形特征和地幔布格异常（MBA）显示了明显的区域差异。22 N以北显示出极低的MBA，表明裂陷的初期。21 N和14 N之间的中心区域显示出特征性的缓慢扩张特征，存在中谷新火山带和沿着轴的“牛眼”MBA。14 N以南地区表现为EPR型轴向起伏形态，MBA值低于中部。构造演化可以通过构造展布的方向、非转换偏移的轨迹以及矢量地磁异常场确定的地壳年龄来推断。地壳年龄表明，北方、中部和南部的海底扩张分别开始于4 Ma、6 Ma和3 Ma。北方和中部地区的传播速度一般较慢（约20毫米/年），而南部地区则较快（约35毫米/年）。在17.5N以北，伸展轴方向随年龄的变化逐渐发生顺时针15 °的变化，而在17.5N以南，伸展轴方向则发生在3.5-3.0Ma之间，从N35 W到N5 W。我们研制了一种新型的海底电磁仪（OBEM），它体积小，易于使用Filloux型电极。在马里亚纳地区进行的一年OBEM观测的初步结果表明，马里亚纳岛弧下存在深度约70 km的高导电带，马里亚纳海槽下存在厚度约10 km的低导电带。

The Mariana Trough is a typical intra-oceanic back-arc basin that shows a variety of features. We have compiled extensive surface geophysical data (bathymetry, magnetic and gravity) of the Mariana Trough, which were collected during several Japanese cruises over the last few years. Topographic features and the mantle Bouguer anomaly (MBA) reveal distinct differences between regions. The north of 22N shows extremely low MBA indicating an incipient rifting. The central region between 21N and 14N shows characteristic slow-spreading features; existence of median valley neovolcanic zone and "Bull's eyes" MBA along the axes. The south of 14N shows an EPR type axial relief morphology and lower MBA than that in the central. Tectonic evolution can be inferred from the direction of spreading fabric, the trace of non-transform offsets, and by crustal ages identified by vector geomagnetic anomaly field. The crustal ages indicate that the seafloor spreading started at 4Ma, 6Ma and 3Ma at the northern end, the central, and the southern end, respectively. The spreading rates are generally slow (~20 mm/yr) in the northern and central areas, while they are fast (~35 mm/yr) in the south. Spreading axis directions have gradually changed by 15 degrees clockwise with age in the north of 17.5N, while they occurred at 3.5-3.0Ma from N35W to N5W just south of 17.5N. On the other hand, they are relatively constant in the southern end.We have developed a new type of ocean bottom electro-magnetometer (OBEM), which is small and easy to handle with Filloux-type electrodes. Preliminary results of an OBEM observation for one year in the Mariana region suggest that very conductive zone (about 70km in depth) below the Marianas island arc and low conductivity zone with thickness of a few ten km beneath the Mariana Trough.