Collaborative Research: Shear Zone and Detachment Formation during Syn-convergent Extension in the Andes: Cordillera Blanca, Peru

造山的主要过程是地球的缩短和变厚。在构造板块交汇的相对狭窄的造山带上。在过去二十年中，人们越来越认识到在山带内地壳伸展与缩短同时发生的重要性。在包括喜马拉雅山脉和安第斯山脉在内的活动山带中已经发现了这种同辐合伸展的例子，许多研究人员也在古代山带形成的岩石中发现了类似过程的证据。该项目的目标是记录在秘鲁安第斯山脉的科迪勒拉布兰卡西南边缘的活动断层系统中正在进行的同步收敛扩展的机制。在这个现代构造上收集的数据将有助于研究人员解释来自古代造山带和现代山带的同辐合伸展的证据。科迪勒拉布兰卡是研究这些过程的理想构造，因为伸展是最近的（600万年），区域构造制度在断层演化过程中保持相当稳定，并且在整个200公里长的拆离中下盘的岩石相似。这些因素将有助于比较来自分离不同部分的数据，并使我们能够推断过去的变形过程和速率。同辐合伸展按相对于山带的伸展方向大致可分为两大类：造山带平行伸展是指地壳平行于山带走向（垂直于缩短方向）的伸展；横向伸展是指沿山带走向的伸展（与缩短方向平行）。造山带平行伸展一般发生在与山脉垂直的断层上，而横向伸展则发生在与山带平行的构造上。横向延伸提出了一个难题：地壳如何在同一时间、同一方向上缩短和延伸？答案似乎与地壳在不同层次上的缩短和伸展的分割有关。通过研究科迪勒拉·布兰卡在挖掘过程中随时间的演变，我们将试图阐明在整个安第斯山脉持续整体缩短的同时允许延伸的条件。本项目将重点研究剪切带尺度过程，包括：(1)在挖掘至地壳浅层的过程中，厚度、应变速率、差流应力和温度是如何演变的；(2)主剥离的变形与地震活跃构造的滑动之间是如何关联的？通过仔细整合结构几何制图、运动学研究、变形机制分析、温度估计和古地壳测量学，我们希望重建一条剖面，显示在变形和挖掘过程中，剪切带的强度和力学行为在不同深度是如何变化的。我们的研究结果将有助于研究人员更好地理解现代和古代山带在地壳伸展过程中伸展断层的演化过程。这项研究是肯塔基大学诺克斯维尔分校和蒙大拿州立大学研究人员共同努力的结果。除了项目的科学目标外，该项目还支持STEM学科的研究生和本科生的培训，并根据研究成果为公共宣传工作做出贡献。

The dominant process in mountain building is shortening and thickening of the Earth?s crust across relatively narrow orogenic belts where tectonic plates converge. During the past two decades there has been growing appreciation of the importance of crustal extension occurring within mountain belts at the same time as shortening. Examples of such syn-convergent extension have been identified in active mountain belts including the Himalaya and Andes and many researchers have also found evidence for similar processes in rocks that formed in ancient mountain belts. The goal of this project is to document the mechanisms of ongoing syn-convergent extension within an active fault system that bounds the southwestern margin of the Cordillera Blanca in the Peruvian Andes. Data collected on this modern structure will help researchers interpret evidence for syn-convergent extension from ancient orogenic events as well as modern mountain belts. The Cordillera Blanca is the ideal structure to study these processes because extension is very recent ( 6 million years), the regional tectonic regime has remained fairly constant during the evolution of the fault and the rocks in the footwall are similar along the entire 200 km length of the detachment. These factors will facilitate comparison of data from different parts of the detachment and allow us extrapolate processes and rates of deformation into the past. Syn-convergent extension can be broadly divided into two general types according to the direction of extension relative to the mountain belt: orogen parallel extension involves stretching of the crust parallel to the trend of the mountain belt (perpendicular to the direction of shortening); transverse extension involves stretching across the trend of the mountain belt (parallel to the direction of shortening). Orogen parallel extension generally occurs on faults oriented perpendicular to the mountains while transverse extension occurs on structures parallel to the mountain belt. Transverse extension poses something of a conundrum: How can the crust shorten and extend at the same time and in the same direction? The answer seems to involve a partitioning of shortening and extension at different levels in the crust. By studying the evolution of the Cordillera Blanca through time as it is exhumed we will attempt to elucidate the conditions that allow extension while overall shortening continues across the Andes. This project will focus on shear-zone-scale processes including: (1) How have the thickness, strain rate, differential flow stress and temperature evolved during exhumation to shallower crustal levels, and (2) How is deformation on the master detachment related to slip on seismically active structures? By carefully integrating field mapping of structural geometry, kinematic studies, analysis of deformation mechanisms, temperature estimates and paleopiezometry, we hope to reconstruct a profile showing how the strength and mechanical behavior of the shear zone varied at different depths during deformation and exhumation of the Cordillera Blanca. Our research will provide results that will help researchers to better understand how extensional faults evolve during crustal extension in modern and ancient mountain belts. The research represents a collaborative effort between researchers at the University of Kentucky Knoxville and Montana State University. In addition to the scientific goals of the project, the project is supporting the training of graduate and undergraduate students in an STEM discipline and contributing to public outreach efforts based on the research outcomes.