Oceanic transform faults formed at mid‐ocean ridges are intrinsic features of modern plate tectonics. Nevertheless, numerical mantle convection models typically struggle to reproduce the strike‐slip movement observed along transform faults on Earth. Instead, mantle convection models tend to produce mostly convergent and divergent plate boundaries. Based on regional visco‐(elasto)‐plastic thermomechanical models it has been demonstrated that a strong strain‐induced weakening of rocks has to be assumed in order to initiate and stabilize the characteristic orthogonal ridge‐transform spreading patterns. However, the physical origin of such intense rheological weakening remains unclear. Considering that in nature oceanic transform faults show a strongly reduced grain size, a potentially strong influence of grain size reduction processes on the rheological strength of these structures can be assumed. Employing 3‐D thermomechanical visco‐plastic models, we study the effect of grain size reduction on oceanic transform fault initiation. Our results show that ductile weakening induced by grain size reduction indeed results in sufficient localization to initiate a transform fault. Without any additional weakening mechanisms, transform faults in our models remain stable up to 2 Myr. We identify parameters that affect stability and longevity of the transform fault during the initiation phase, such as the grain damage formulation and grain growth prefactor. In our models, transform faults initiate in the brittle crust and propagate downward, thus indicating a top‐down initiated localization process. The observed grain size, rheology, and strain rate inside the shear zone of our models agree well with observations in nature; however, the longevity of natural examples cannot be reached.

中文翻译：

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减小晶粒尺寸会引发变形故障吗？-来自3D数值研究的见解

在洋中脊形成的海洋转换断层是现代板块构造的内在特征。但是，数值地幔对流模型通常很难再现沿地球上的转换断层观察到的走滑运动。相反，地幔对流模型往往会产生大部分收敛和发散的板块边界。基于区域粘弹性（弹塑性）塑性热力学模型，已经证明必须假定岩石发生了强烈的应变诱发弱化，才能启动和稳定特征正交脊转换分布模式。但是，这种强烈的流变性减弱的物理原因仍不清楚。考虑到自然界中的海洋转换断层显示出明显减小的晶粒尺寸，可以假设晶粒减小过程对这些结构的流变强度的潜在强大影响。利用3D热力学粘塑性模型，我们研究了晶粒尺寸减小对大洋相变断层起爆的影响。我们的结果表明，由晶粒尺寸减小引起的韧性减弱确实导致了足够的局部化，从而引发了变形断层。如果没有任何其他减弱机制，我们模型中的变换故障将保持稳定，直到2 Myr。我们确定了在初始阶段会影响转换故障的稳定性和寿命的参数，例如晶粒损伤的制定和晶粒长大的因素。在我们的模型中，转换断层在脆性地壳中开始并向下传播，从而表明了自顶向下的起始定位过程。观察到的晶粒尺寸 我们模型的剪切区内的流变学和应变率与自然界的观察结果非常吻合；但是，自然例子的寿命无法达到。