Collaborative Research: Did Miocene Core Complexes in Western Arizona Initiate as Late Cretaceous Extensional Shear Zones?

在北美科迪勒拉山脉，变质核杂岩通常被解释为记录了第三纪大规模伸展的单一阶段。然而，越来越多的证据表明，许多科迪勒拉核心杂岩经历了白垩纪的变形，从根本上影响了他们的第三纪构造发展。特别是中第三纪核杂岩中的糜棱岩可能重新激活了晚白垩世伸展剪切带。该项目的主要目标是确定晚白垩世变形和中新世核心复杂的发展在亚利桑那州西部的Harcuvar和Buckskin-Rawhide山脉之间的关系。这项研究测试的假设，在这些核心复杂的下板剪切带是复合结构，在晚白垩世伸展开始。研究将评估下板块结构的年龄和构造意义，并评估晚白垩世构造如何影响中新世核心复合体延伸的位置，几何形状和风格。研究目标将通过多学科方法实现，包括地质测绘、显微构造分析、U-Pb地质年代学、40 Ar/39 Ar热年代学和石英中钛地质测温法。Harcuvar和Buckskin-Rawhide核心复合体是解决这一问题的杰出场所，因为下板块的几个部分没有被中新世应变覆盖，并且因为糜棱岩和非糜棱岩下板块岩石之间存在明确的横切关系。此外，这些都是一些被广泛接受的核心复合体形成模型的经典地点。对核心复合体的研究继续在我们理解基本构造过程中发挥核心作用，包括大规模伸展的起源和机制以及主要断层带的长期行为。这项研究的预期结果将提供洞察到许多这些伸展过程。例如，核心复合体的位置和核心复合体结构的低角度几何形状可能受到较早的构造活动的强烈影响。此外，如果核杂岩伸展开始于晚白垩世，那么中第三纪拆离断层的滑动速率和位移可能比原先认为的要低。在核杂岩中发现晚白垩世伸展作用对理解拉腊米造山运动期间科迪勒拉山脉的构造演化也具有重要意义。

In the North American Cordillera, metamorphic core complexes are generally interpreted to record a single phase of large-magnitude Tertiary extension. However, growing evidence suggests many Cordilleran core complexes underwent Cretaceous deformation that fundamentally influenced their Tertiary tectonic development. In particular, mylonites in mid-Tertiary core complexes may have reactivated Late Cretaceous extensional shear zones. The primary goal of this project is to determine the relationship between Late Cretaceous deformation and Miocene core complex development in the Harcuvar and Buckskin-Rawhide Mountains in western Arizona. This study tests the hypothesis that lower plate shear zones in these core complexes are composite structures that initiated during Late Cretaceous extension. Research will evaluate the age and tectonic significance of lower plate fabrics and assess how Late Cretaceous structures influenced the location, geometry, and style of Miocene core complex extension. Research goals will be accomplished through a multidisciplinary approach involving geologic mapping, microstructural analysis, U-Pb geochronology, 40Ar/39Ar thermochronology, and Ti-in-quartz geothermometry. The Harcuvar and Buckskin-Rawhide core complexes are outstanding sites to address this topic because several portions of the lower plate are not overprinted by Miocene strain and because clear cross-cutting relationships are present between mylonitic and non-mylonitic lower plate rocks. In addition, these are classic localities where some of the widely accepted models of core complex formation were developed. Research on core complexes continues to play a central role in our understanding of fundamental tectonic processes, including the origins and mechanics of large-magnitude extension and the long-term behavior of major fault zones. The anticipated results of this research will provide insight into many of these extensional processes. For example, the locations of core complexes and the low-angle geometry of core complex structures may have been strongly influenced by older tectonic activity. In addition, if core complex extension initiated during the Late Cretaceous, the slip rate and displacement on mid-Tertiary detachment faults may be lower than originally believed. The discovery of Late Cretaceous extension in core complexes would also have important implications for understanding the tectonic evolution of the Cordillera during the Laramide orogeny.