Several thermally activated mechanisms have been proposed to explain the dynamic weakening of faults at fast, seismic slip rates v > 0.1 m s⁻¹. Nevertheless, direct constraints from microstructural observations on the operation of such weakening mechanisms are still lacking for most rock forming minerals. This includes olivine, a very refractory mineral relevant for seismic activity occurring in peridotite massifs. Here, we report mechanical data and microstructures of olivine aggregates deformed in a rotary shear apparatus at slip rates from 10⁻² m s⁻¹ to 1 m s⁻¹. A number of 34 shear deformation experiments were performed under axial stresses of 20 MPa and at room temperature. Samples are composed of either synthetic iron-free nanocrystalline forsterite powder (initial grain size = 0.07 μm) or natural iron-bearing olivine powder from San Carlos or San Bernardino (Fo₉₁, Arizona, USA; initial grain size = 70 ± 2 μm). Shear deformation lasts from a few seconds up to 5 min and ends after 0.03 to 3 m of slip. Independently of the initial grain size or composition, friction coefficients decrease from peak values of 0.8 to values below 0.4, after only 0.1 m of slip. The recovered samples were characterized by scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. For all samples, before the onset of dynamic weakening, the deformation localizes in a principal shear zone, well-marked in the natural olivine aggregates by a rapid decrease of grain size down to <1.7 μm. Variations in texture, fractured grains and the formation of a fine-grained matrix support cataclastic flow at an early stage of deformation, responsible for the foremost grain size reduction. The samples display well-defined zones with partially sintered microstructure, and shape preferred orientation. The initial texture of the cold-pressed starting material is rapidly reduced to a near random distribution after only 0.03 m of slip. All crystal preferred orientations (CPO) remain weak, with a J-index ≲2. Based on the microstructures, the dominant deformation mechanism may be enhanced effective diffusion (by surface and grain boundary diffusion) causing mechanical weakening and a viscous behavior, which is then temperature-dependent under sub-solidus conditions. We find no microstructural evidence for frictional melting, flash heating or thermal decomposition. Our results advocate that dynamic weakening of faults is independent of initial grain size, and can be achieved after displacements as low as 0.1 m without production of frictionally induced-melt.

已经提出了几种热激活机制来解释在快速地震滑动率v  > 0.1 m s ^-1时断层的动态减弱。然而，对于大多数成岩矿物来说，仍然缺乏来自微观结构观察对这种减弱机制的作用的直接约束。其中包括橄榄石，这是一种非常难熔的矿物，与橄榄岩块体中发生的地震活动有关。在这里，我们报道了在旋转剪切装置中滑移速率从10 ^-2  m s ^-1到1 m s ^-1的橄榄石聚集体的力学数据和微观结构。在20 MPa的轴向应力和室温下进行了34次剪切变形实验。样品由合成的无铁纳米晶镁橄榄石粉（初始粒度= 0.07μm）或来自圣卡洛斯或圣贝纳迪诺的天然含铁橄榄石粉（Fo _{91）组成。}，美国亚利桑那州；初始粒度= 70±2μm）。剪切变形持续几秒钟到5分钟，并在0.03到3 m的滑动后结束。与初始晶粒尺寸或成分无关，仅在滑动0.1 m后，摩擦系数便从0.8的峰值减小到0.4以下的值。通过扫描电子显微镜，电子反向散射衍射和透射电子显微镜对回收的样品进行表征。对于所有样品，在动态减弱开始之前，变形都集中在一个主要的剪切带中，这种现象在天然橄榄石聚集体中表现出明显的特征，其晶粒尺寸迅速减小至<1.7μm。质地的变化，破裂的晶粒和细晶粒基体的形成在变形的早期阶段支持碎裂流，这是导致晶粒尺寸减小的主要原因。样品显示出清晰界定的区域，具有部分烧结的微观结构，并且形状优先。仅0.03 m的滑移后，冷压原材料的初始质地便迅速降低至接近随机分布。所有晶体的首选取向（CPO）仍然很弱，具有J指数≲2 .基于微观结构，主要的变形机制可能是有效扩散（通过表面和晶界扩散）增强的，从而导致机械弱化和粘性行为，然后在次固相条件下取决于温度。我们没有发现摩擦熔化，急速加热或热分解的微观结构证据。我们的研究结果表明，断层的动态弱化与初始晶粒大小无关，并且可以在位移低至0.1 m后实现，而不会产生摩擦诱导的熔体。