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In situ stress control on microcrack generation and macroscopic extensional fracture in exhuming bedrock

机译:采掘基岩微裂纹产生与宏观延伸断裂的原位应力控制

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[1] Crustal stresses beneath evolving alpine landscapes result from a combination of tectonic strain, bedrock exhumation, and topography. The stress field is regulated by elastic material properties and the brittle strength of critically stressed elements, particularly those within preexisting faults and intact rock. Combining Byerlee's law for crustal stresses with a recently developed trilinear fracture envelope, we propose an extension of the critically stressed crust concept to constrain in situ stresses through microcrack generation and extensional fracture propagation. A compilation of 814 global in situ stress measurements suggests microcrack development will limit long-term rock strength, and maximum differential stresses in the upper ~1 km of the crust can be controlled by cohesive bedrock behavior. Using an elastoplastic, 2-D finite-difference model, we approximate in situ stress development within landscapes undergoing high rates of exhumation in both normal and reverse tectonic regimes. Critical near-surface stresses in these environments are defined by the microcrack initiation threshold, estimated to be roughly one third of the unconfined compressive strength of intact rock, while stresses deeper in the crust adhere to Byerlee's law. Our models indicate that exhumation-induced stresses limited by long-term rock strength are the primary contributor to the near-surface stress field, while topographic relief reduces stress near valley axes. Simulating glacial erosion then allows us to illustrate a path-dependent relationship between critical stress development, fracture formation, and geomorphic processes. We find that glacial unloading can generate new microcracks in near-surface bedrock, resulting in unstablemacroscopic extensional (or "exfoliation") fracture propagation during incision of U-shaped alpine valleys.
机译:[1]不断变化的高山景观下的地壳应力是由于构造应变,基岩掘出和地形共同作用的结果。应力场由弹性材料特性和临界受压元件的脆性强度来调节,特别是在已存在的断层和完整岩石中的那些元件。结合Byerlee地壳应力定律和最近开发的三线性裂缝包络线,我们提出了对临界应力地壳概念的扩展,以通过微裂纹的产生和扩展的裂缝传播来限制原地应力。对814个全球原位应力测量值的汇编表明,微裂纹的发展将限制长期的岩石强度,并且地壳上部〜1 km处的最大差分应力可以通过粘性基岩行为来控制。使用弹塑性二维有限差分模型,我们可以估算在正向和反向构造状态下经历高掘出速率的景观中的原位应力发展。在这些环境中,关键的近地表应力由微裂纹萌生阈值确定,估计约为完整岩石无侧限抗压强度的三分之一,而地壳深处的应力则遵循拜耳里定律。我们的模型表明,受长期岩石强度限制的发掘引起的应力是近地表应力场的主要因素,而地形起伏减小了谷轴附近的应力。然后,通过模拟冰川侵蚀,我们可以说明临界应力发展,裂缝形成和地貌过程之间的路径相关关系。我们发现冰川卸载可能会在U型高山山谷切开过程中在近地表基岩中产生新的微裂纹,从而导致不稳定的宏观扩展(或“剥落”)裂缝扩展。

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