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（测深、磁场和重力），这些数据是在过去几年中日本几次巡航中收集的。地形特征和地幔布格异常（MBA）显示出明显的区域差异。北纬22度以北的MBA极低，表明隆升初期。在21N ~ 14N之间的中部地区表现出典型的缓慢扩张特征；沿轴线存在中谷新火山岩带和“牛眼”MBA。14N以南为EPR型轴向起伏形态，MBA较中部低。构造演化可以通过扩展构造方向、非变换偏移量迹和矢量地磁异常识别的地壳年龄来推断。地壳年龄表明，北端、中部和南端的海底扩张分别开始于4Ma、6Ma和3Ma。北部和中部的蔓延速度一般较慢（~20 mm/yr），而南部的蔓延速度较快（~35 mm/yr）。扩散轴方向在17.5N以北随年龄逐渐顺时针变化15度，而在17.5N以南从N35W到N5W发生在3.5 ~ 3.0 ma。另一方面，它们在南端相对恒定。我们研制了一种新型的海底电磁力仪（OBEM），该仪器体积小，易于操作，采用filloux型电极。在马里亚纳地区进行的为期一年的obm观测初步结果表明，马里亚纳岛弧下方存在极导电性区（深度约70公里），马里亚纳海槽下方存在厚度为几十公里的低导电性区。

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.