Wang Zhikai, Singh Satish C, Prigent Cécile, Gregory Emma P M, Marjanović Milena
Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France.
Nat Geosci. 2022 Sep 1;15:741-746. doi: 10.1038/s41561-022-01003-3. Epub 2022 Aug 25.
Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending down to ~60 km depth below sea level. We performed 3D thermal modelling that suggests the anomaly is likely to be due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere-asthenosphere boundary, causing significant thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high temperature melt may occur beneath the ridge axis.
转换断层调节岩石圈板块之间的横向运动，引发大地震。在远离活跃转换边界的地方，以前的大洋转换断层也形成了覆盖洋底的断裂带。然而，这些断层的深部结构仍然是个谜。在此，我们展示了来自赤道大西洋罗曼什转换断层的超长偏移地震数据，该数据表明存在一个低速异常，其延伸至海平面以下约60千米深处。我们进行了三维热模拟，结果表明该异常可能是由于向下至约16千米深处广泛的蛇纹石化作用，其下方是一个水化的剪切糜棱岩带，延伸至32千米深处。这些水被认为源自渗入断层深处的海水衍生流体。在32千米以下被解释为一个低温、水致熔融带，该带抬高了岩石圈 - 软流圈边界，导致转换断层处的岩石圈显著变薄。转换断层处存在变薄的岩石圈可以解释在转换断层处观察到的火山活动、增厚的地壳和转换内部扩张中心。这也表明在洋中脊轴下方可能发生水致熔体与高温熔体的迁移和混合。
