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 to ~60 km below sea level. We performed three-dimensional thermal modelling that suggests the anomaly is probably due to extensive serpentinization down to ~16 km, overlying a hydrated, shear mylonite zone down to 32 km. 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 substantial 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.

转换断层适应岩石圈板块之间的横向运动，产生大地震。远离活跃转换边界，前大洋转换断层也形成覆盖海底的断裂带。然而，这些断层的深层结构仍然是神秘的。在这里，我们展示了赤道大西洋 Romanche 转换断层的超长偏移地震数据，表明存在延伸至海平面以下约 60 公里的低速异常。我们进行了 3D 热模型，表明异常可能是由于广泛的蛇纹石化（低至约 16 公里），覆盖低至 32 公里的水合剪切糜棱岩带。水被认为来自渗透到断层深处的海水衍生流体。32公里以下被解释为低温，水致熔融带抬高了岩石圈-软流圈边界，导致转换断层处岩石圈显着变薄。在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火山活动、增厚的地壳和转换内扩展中心。这也表明，水致熔体与高温熔体的迁移和混合可能发生在脊轴下方。