The San Andreas fault (California, USA) is near vertical at shallow (<10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of the Garlock Fault and sustains the geometric complexity of the fault.

中文翻译：

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受粘性下地壳流影响的地壳压断断层几何形状

圣安德烈亚斯断层（美国加利福尼亚州）在浅层（<10 公里）深度接近垂直。沿圣安德烈亚斯断层进行的地球物理调查表明，在 10-20 公里的深度，它在西部横断山脉附近向西南倾斜约 50-70°，在圣戈尔戈尼奥地区向东北倾斜。我们使用三维热机械模型研究断层沿走向几何变化的可能起源。对于被推压断层隔开的两个区块，我们的模型显示粘性的下地壳物质从高粘性地块移动到低粘性地块。由于脆韧转变深度处的简单剪切流，粘性下地壳中的断层-法向流使断层旋转。这与初始断层倾角方向无关。用于模拟南加州下地壳的流变学变化已通过独立观察得到验证。由于较低的地壳粘度变化导致的块体挤压促进了 Garlock 断层的形成并维持了断层的几何复杂性。