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

合作研究：安第斯山脉同步汇聚伸展过程中的剪切带和脱离形成：秘鲁科迪勒拉布兰卡

• 批准号: 1220237
• 负责人: Micah Jessup
• 金额: $ 19.02万
• 依托单位: University of Tennessee Knoxville
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-15 至 2017-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1220237&HistoricalAwards=false
• 关键词: Collaborative; Research; Shear; Zone; Detachment

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.

造山运动的主要过程是地球的缩短和增厚？在构造板块汇聚的相对狭窄的造山带中，地壳的厚度会增加。在过去20年中，人们越来越认识到在山脉地带内地壳拉张与缩短同时发生的重要性。在喜马拉雅山和安第斯山脉等活动山区地带已经发现了这种同会聚伸展的例子，许多研究人员还在古代山区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白山系西南缘的活动断层系统内的同向会聚伸展的机制。在这个现代结构上收集的数据将有助于研究人员解释古代造山事件以及现代山脉的同会聚延伸证据。科迪勒拉布兰卡是研究这些过程的理想结构，因为伸展是最近的（600万年），区域构造体制在断层演化过程中保持相当稳定，下盘的岩石在整个200 km的拆离长度上沿着是相似的。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。 同会聚伸展根据相对于山带的伸展方向可大致分为两大类：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;横向伸展是指横跨山带走向（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走向的断裂上，而横向伸展则发生在平行于山脉带走向的构造上。横向伸展带来了一个难题：地壳如何在同一时间向同一方向缩短和伸展？答案似乎涉及地壳不同层次上缩短和伸展的划分。通过研究随着时间的推移，当它被挖掘出来时，我们将试图阐明允许延伸的条件，而整体缩短继续跨越安第斯山脉。本项目将侧重于剪切带规模的过程，包括：（1）在地壳较浅层次的折返过程中，厚度、应变率、流动应力差和温度是如何演变的，以及（2）主拆离体的变形与地震活动构造上的滑动有何关系？通过仔细整合现场测绘的结构几何学，运动学研究，变形机制，温度估计和paleozometry的分析，我们希望重建一个剖面，显示剪切带的强度和力学行为如何在不同的深度变化在变形和折返的科迪勒拉布兰卡。我们的研究结果将有助于研究人员更好地了解现代和古代山区地壳伸展过程中伸展断层的演变。 这项研究代表了肯塔基州诺克斯维尔大学和蒙大拿州州立大学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。