Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30’E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems.

大洋滑脱断层在调节缓慢-超慢速扩张的洋中脊板块分歧方面发挥着核心作用。位于东经 64°30' 的超慢速扩张的西南印度洋中脊 (SWIR) 的近非岩浆区域中的连续翻转滑脱断层可容纳约 100% 的板块分歧，且海底大部分为超镁铁质光滑海底。在这里，我们提供了由海底地震仪记录的微震活动数据，显示在几乎无岩浆的光滑海底下，轴向脆性岩石圈的厚度约为 15 公里，并不比附近的火山海底或更多岩浆 SWIR 分离系统厚。 。我们的数据表明，具有正常震源机制的微地震与活动滑脱断层和相对上盘断层的地震成像损伤区位于同一地点。上盘地震活动水平明显高于慢速-超慢山脊的更多岩浆分离所记录的地震活动水平，这可能是近非岩浆翻转分离系统的独特特征。