Many low-angle normal faults (dip ≤30°) accommodate tens of kilometers of crustal extension, but their mechanics remain contentious. Most models for low-angle normal fault slip assume vertical maximum principal stress σ1, leading many authors to conclude that low-angle normal faults are poorly oriented in the stress field (≥60° from σ1) and weak (low friction). In contrast, models for low-angle normal fault formation in isotropic rocks typically assume Coulomb failure and require inclined σ1 (no misorientation). Here, a data-based, mechanical-tectonic model is presented for formation of the Whipple detachment fault, southeastern California. The model honors local and regional geologic and tectonic history and laboratory friction measurements. The Whipple detachment fault formed progressively in the brittle-plastic transition by linking of “minidetachments,” which are small-scale analogs (meters to kilometers in length) in the upper footwall.Minidetachments followed mylonitic anisotropy along planes of maximum shear stress (45° from the maximum principal stress), not Coulomb fractures. They evolved from mylonitic flow to cataclasis and frictional slip at 300–400 °C and ∼9.5 km depth, while fluid pressure fell from lithostatic to hydrostatic levels. Minidetachment friction was presumably high (0.6–0.85), based upon formation of quartzofeldspathic cataclasite and pseudotachylyte. Similar mechanics are inferred for both the minidetachments and the Whipple detachment fault, driven by high differential stress (∼150–160 MPa). A Mohr construction is presented with the fault dip as the main free parameter. Using “Byerlee friction” (0.6–0.85) on the minidetachments and the Whipple detachment fault, and internal friction (1.0–1.7) on newly formed Reidel shears, the initial fault dips are calculated at 16°–26°, with σ1 plunging ∼61°–71° northeast. Linked minidetachments probably were not well aligned, and slip on the evolving Whipple detachment fault probably contributed to fault smoothing, by off-fault fracturing and cataclasis, and to formation of the fault core and fractured damage zone.Stress rotation may have occurred only within the mylonitic shear zone, but asymmetric tectonic forces applied to the brittle crust probably caused gradual rotation of σ1 above it as a result of: (1) the upward force applied to the base of marginal North America by buoyant asthenosphere upwelling into an opening slab-free window and/or (2) basal, top-to-the-NE shear traction due to midcrustal mylonitic flow during tectonic exhumation of the Orocopia Schist. The mechanical-tectonic model probably applies directly to low-angle normal faults of the lower Colorado River extensional corridor, and aspects of the model (e.g., significance of anisotropy, stress rotation) likely apply to formation of other strong low-angle normal faults.

中文翻译：

---

强烈的低角度法向断层是如何形成的：加利福尼亚州东南部的Whipple支队

许多低角度正断层（倾角≤30°）可容纳数十公里的地壳延伸，但其力学仍存在争议。大多数低角度法向断层滑动模型都假定垂直最大主应力为σ1，许多作者得出这样的结论：低角度法向断层在应力场中取向较弱（与σ1≥60°），并且弱（低摩擦）。相比之下，各向同性岩石中低角度法向断层形成的模型通常假设库仑破裂，并且需要倾斜的σ1（无取向）。在这里，提出了一个基于数据的机械构造模型，用于形成加利福尼亚东南部的Whipple脱离断层。该模型适用于当地和地区的地质和构造历史以及实验室的摩擦测量。Whipple脱离断层是在脆塑性过渡过程中通过“上层下盘”的小型“类比”（长为米至千米）的小型模拟的联系而逐渐形成的。小型类比沿着最大剪切应力（45°）平面上的各向异性出现。从最大主应力），而不是库仑骨折。它们在300-400°C和约9.5 km的深度处从糊状流演变成分解和摩擦滑移，而流体压力则从岩石静压水平下降到静水压力水平。基于形成石英辉石的催化裂隙和假速溶质，微分离摩擦力可能很高（0.6-0.85）。对于微分离和Whipple分离断层，都可以推断出类似的机制，这是由高差应力（〜150-160 MPa）驱动的。提出了一个以故障倾角为主要自由参数的摩尔结构。利用微动和威普尔断层的“贝利摩擦”（0.6-0.85），以及新形成的里德尔剪切机的内摩擦（1.0-1.7），计算出初始断层倾角为16°-26°，σ1下降为〜东北61°–71°。相连的微型裂隙可能没有很好地对齐，并且正在演化的Whipple裂隙中的滑移可能通过断层破裂和催化作用导致断层平滑，并形成了断层核心和破裂的破坏区，应力旋转可能仅发生在肌层剪切带，但是施加于脆性地壳的不对称构造力可能导致σ1在其上方逐渐旋转，其原因是：（1）浮力软流圈上升到无板块的开口窗和/或（2）构造掘尸过程中由于中壳壳层的泥浆流所引起的向上的力Orocopia Schist。力学构造模型可能直接适用于科罗拉多河下游延伸走廊的低角度正断层，而模型的各个方面（例如，各向异性的重要性，应力旋转）很可能适用于其他强低角度正断层的形成。