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

造山的主要过程是在构造板块汇聚的相对狭窄的造山带上缩短和加厚地球--S地壳。在过去二十年中，越来越多的人认识到地壳伸展在山脉带内与缩短同时发生的重要性。在包括喜马拉雅和安第斯山脉在内的活动山带中已经发现了这种同步收敛伸展的例子，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秘鲁安第斯山脉Cordillera Blanca西南边缘的活动断裂系统内正在进行的同步会聚伸展的机制。收集到的关于这一现代构造的数据将帮助研究人员解释古代造山事件和现代山带的同步会聚伸展证据。Cordillera Blanca是研究这些过程的理想构造，因为伸展是最近的(600万年)，在断裂的演化过程中，区域构造体制保持了相当的稳定，沿整个滑脱200公里长度的下盘中的岩石是相似的。这些因素将有助于比较来自分离的不同部分的数据，并使我们能够外推过去的变形过程和速率。根据相对于山带的伸展方向，同向伸展大致可分为两种一般类型：造山带平行伸展是指地壳平行于山带的走向(垂直于缩短方向)；横向伸展是指横跨山带的走向(平行于缩短方向)的伸展。造山带的平行伸展一般发生在垂直于山脉的断层上，而横向伸展一般发生在平行于山带的构造上。横向伸展提出了一个难题：地壳如何在同一时间同一方向缩短和伸展？答案似乎涉及到地壳中不同级别的缩短和伸展的划分。通过研究Cordillera Blanca在出土过程中随时间的演变，我们将试图阐明在安第斯山脉继续整体缩短的同时允许扩展的条件。该项目将重点研究剪切带尺度的过程，包括：(1)折返过程中厚度、应变率、差流应力和温度是如何演变到较浅的地壳水平的；(2)主滑脱上的变形与地震活动构造上的滑动有什么关系？我们希望通过将构造几何学、运动学研究、变形机制分析、温度估计和古地层测量的野外填图结合起来，重建剪切带在变形和折返过程中在不同深度的强度和力学行为变化的剖面。我们的研究结果将帮助研究人员更好地了解现代和古代山带在地壳伸展过程中伸展断层的演化。这项研究代表了肯塔基诺克斯维尔大学和蒙大拿州立大学的研究人员之间的合作努力。除了该项目的科学目标外，该项目还支持对STEM学科的研究生和本科生进行培训，并根据研究成果为公众宣传工作做出贡献。

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.