Paleostress directions near two low-angle normal faults: Testing mechanical models of weak faults and off-fault damage
Paleostress directions near two low-angle normal faults: Testing mechanical models of weak faults and off-fault damage
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两个低角度正断层附近的古应力方向：测试弱断层和断层损伤的力学模型
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发表时间：
2015
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通讯作者：
J. Selverstone
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作者：
G. Axen;A. Luther;J. Selverstone
许多大滑移断层，例如圣安德烈亚斯断层和低角度正断层（LANF），相对于其周围环境或实验室摩擦测量结果似乎较弱，并且在区域应力场中的滑移方向较差。一些模型试图解释滑动和/或此类断层形成的机制。其他模型将断层周围的损坏解释为由于断层或地震破裂传播或非平面断层上的滑动造成的。这些模型中的大多数都明确预测了近断层应力场。
挖出的低角度正断层下盘是测试此类模型的有利天然实验室，因为它们暴露了经过脆塑性转变的岩石以及全部或部分震源地壳。我们提出了由裂缝和滑移线方向数据反演得出的简化古应力张量，这些数据主要取自两个 LANF（南加州的 Whipple 和 West Salton 滑脱断层）上下盘的断层核心和裂缝损伤带。除了地壳最上面几公里之外，沿着这些断层的摩擦弱物质可能并不重要，并且孔隙流体压力可能从未达到岩石静压值。
大多数结果表明，断层与近断层最大压应力（σ1）方向成较大角度，总体符合安德森拉伸应力场。我们的研究结果支持上地壳滑移的“强三明治”力学模型，其中法向摩擦 LANF 嵌入更强的环境中，并以与 σ1 成大角度的滑移，以及穿过脆性地壳厚度的应力旋转模型，在脆塑性转变附近适度骤降 σ1，前提是某些机制允许断层在下盘挖出时以平缓的倾角穿过脆性地壳传播。与断层成中等角度的古 σ1 矢量稀疏，可能反映中地壳中形成的早期损伤，而 σ1 和脱离之间的角度适中或在沿走 LANF 或地震破裂传播期间。颗粒流导致的库仑塑性预测了 σ1 ～45° 处的断层，但没有得到很好的支持，因为许多与 LANF 成中等角度的古 σ1 矢量来自碎裂断层核下方的裂缝。我们的结果与“弱三明治”模型不一致，该模型预测由于局部孔隙流体压力或弹性变化，σ1 会重新定向到损伤区和/或断层核心内的断层的小角度（〜30°）。非平面断层上的滑移引起的破裂与我们的古应力结果基本一致。然而，所研究的 LANF 的粗糙度尚不清楚，但它们可能具有非常低的粗糙度。该波状断层模型中用于约束预期损伤区域的应力状态与根据现场测量在强夹心模型中推断的应力状态几乎相同。受损区的裂缝可能没有记录上倾断层或地震破裂传播，这对于地震传播尤其如此，但沿走向传播可能在某些地点控制了破裂。一些古应力场可能与滑移平行轴周围的分离折叠有关。
Many large-slip faults, such as the San Andreas fault and low-angle normal faults (LANFs), appear to be weak relative to their surroundings or to laboratory friction measurements, and to be poorly oriented for slip in the regional stress field. Several models seek to explain the mechanics of slip and/or formation of such faults. Other models explain damage around faults as due to fault or earthquake rupture propagation or slip on nonplanar faults. Most of these models explicitly predict the near-fault stress field.
Exhumed footwalls of low-angle normal faults are advantageous natural laboratories for testing such models because they expose rocks that passed through the brittle-plastic transition and all or part of the seismogenic crust. We present reduced paleostress tensors derived from inversion of fracture and slip-line orientation data taken mainly from the fault cores and fractured damage zones in the upper footwalls of two LANFs, the Whipple and West Salton detachment faults of southern California. Frictionally weak materials probably were not significant along these faults except in the uppermost few kilometers of the crust, and pore-fluid pressure probably never approached lithostatic values.
Most results show that the faults were at a high angle to the near-fault maximum compressive stress (σ1) direction, in general accord with Andersonian extensional stress fields. Our results support a “strong-sandwich” mechanical model for slip in the upper crust, in which normal-friction LANFs are embedded in stronger surroundings and slip at high angles to σ1 and models of stress rotation across the thickness of the brittle crust, with moderately plunging σ1 near the brittle-plastic transition, provided that some mechanism allows the faults to propagate through the brittle crust at gentle dips as the footwalls are exhumed. Paleo-σ1 vectors oriented at moderate angles to the faults are sparse and may reflect early damage formed in the midcrust, while the angle between σ1 and the detachment was moderate or during alongstrike LANF or earthquake rupture propagation. Coulomb plasticity due to granular flow, which predicts faults at ∼45° to σ1, is not well supported because many paleo-σ1 vectors with moderate angles to the LANFs are from fractures below the cataclastic fault cores. Our results are inconsistent with “weak-sandwich” models that predict reorientation of σ1 to low angles (∼30°) to the fault within the damage zone and/or fault core due to local pore-fluid pressure or elasticity changes. Fracturing due to slip on non-planar faults is generally consistent with our paleostress results. However, the roughness of the LANFs studied is not known, but they may have very low roughness. The stress state used in this wavy-fault model to constrain the expected damage region is nearly identical to that inferred in the strong-sandwich model from field measurements. Fractures in the damage zone probably do not record up-dip fault or earthquake rupture propagation, which is expected especially for earthquake propagation, but along-strike propagation may have controlled fracturing at some sites. Some paleostress fields are probably related to folding of the detachments about slip-parallel axes.