A naturally constrained stress profile through the middle crust in an extensional terrane
A naturally constrained stress profile through the middle crust in an extensional terrane
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DOI：
10.1016/j.epsl.2010.11.044
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发表时间：
2011-03-01
影响因子：
5.3
通讯作者：
Platt, John P.
中科院分区：
文献类型：
--
作者：
Behr, Whitney M.;Platt, John P.
我们提出了一种方法，利用古地壳测量法、钛-石英热压测量法（TitaniQ）和二维热模拟，在已发掘的中地壳岩石区域中构建自然约束应力剖面。以惠普尔山变质核杂岩下盘为例。WMCC中的岩石最初在20 km深度处受分布韧性剪切变形，然后随着下盘的冷却和向脆性-韧性转变（BDT）方向发展，逐渐被局部韧性剪切带覆盖，最终被离散脆性断裂覆盖。在挖掘过程中，不断增加的应变局部化和冷却使得早期的微观结构得以保存，因此WMCC中的岩石代表了温度-应力空间（以及推断的深度-应力空间）中的几个点。我们确定了足够多的这些应力-深度点，构建了一个完整的流过中地壳至类似20km深度的流动应力剖面，并由此得出了该地区中新世伸展期间脆性上地壳环境应力的区域估计，以及该地区脆性-韧性过渡的峰值强度。最大差应力达到136mpa，就在脆-韧转变的下方，深度约为9km。在假定垂直最大主应力和近静水孔隙流体压力的条件下，上地壳的应力水平符合Byerlee定律，表明25度倾向Whipple断层的摩擦系数近似于0.4。在20km深度处，差应力减小到10 ~ 20mpa。对于活跃变形区域的典型应变速率(10(-1)2到10(-15)/s)，我们的应力剖面被Hirth等人（2001）的湿石英岩流动规律所包围，而Rutter和Brodie（2004）的流动规律高估了这一特定区域的强度。(C) 2010 Elsevier By。版权所有。
We present a method in which paleopiezometry, Ti-in-quartz thermobarometry (TitaniQ), and 2-D thermal modeling are used to construct a naturally constrained stress profile through the middle crust in an area of exhumed mid-crustal rocks. As an example, we examine the footwall of the Whipple Mountains metamorphic core complex (WMCC). Rocks in the WMCC were initially deformed at similar to 20 km depth by distributed ductile shear, and were then progressively overprinted by localized ductile shear zones and eventually by discrete brittle fracture as the footwall was cooled and exhumed toward the brittle-ductile transition (BDT). Increasing strain localization and cooling during exhumation allowed earlier microstructures to be preserved, and rocks in the WMCC therefore represent several points in temperature-stress space (and by inference depth-stress space). We identify enough of these stress-depth points to construct a complete profile of the flow stress through the middle crust to a depth of similar to 20 km, from which we derive regional estimates of the ambient stresses in the brittle upper crust, and the peak strength at the brittle-ductile transition in this region during Miocene extension.Maximum differential stress reached similar to 136 MPa just below the brittle-ductile transition at a depth of similar to 9 km. Stress levels are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures, and suggest a coefficient of friction on the 25 degrees-dipping Whipple fault of similar to 0.4. Differential stress decreases to 10-20 MPa at 20 km depths ark. similar to 500 degrees C. For strain rates typical of actively deforming regions (10(-1)2 to 10(-15)/s), our stress profile is bracketed by the Hirth et al. (2001) flow law for wet quartzite, whereas the flow law of Rutter and Brodie (2004) overestimates the strength of this particular region. (C) 2010 Elsevier By. All rights reserved.