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

在北美科迪勒拉，变质岩心杂岩体通常被解释为记录了第三纪大震级的单期伸展。然而，越来越多的证据表明，许多科迪勒拉岩心杂岩体经历了白垩纪的变形，这从根本上影响了它们的第三纪构造发育。特别是中第三纪岩心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亚利桑那州西部Harcuvar和Buckskin-Rawhide山脉晚白垩世变形与中新世岩心杂岩发育之间的关系。本研究验证了这些岩心杂岩体下板块剪切带是晚白垩世伸展期形成的复合构造的假设。研究将评估下板块构造的年龄和构造意义，并评估晚白垩世构造如何影响中新世岩心杂岩体伸展的位置、几何形状和样式。研究目标将通过多学科方法完成，包括地质填图、微观结构分析、U-Pb地质年代学、40Ar/39Ar热年代学和Ti-in-quartz地球温度学。哈库瓦尔和鹿皮-生皮岩心杂岩是解决这一问题的杰出地点，因为下板块的一些部分没有中新世的套印，而且在糜棱质和非糜棱质下板块岩石之间存在明确的交叉关系。此外，这些地区是一些被广泛接受的岩心复杂地层模型发展的经典地区。对岩心杂岩体的研究继续在我们对基本构造过程的理解中发挥着核心作用，包括大震级伸展的起源和力学以及主要断裂带的长期行为。这项研究的预期结果将为许多这些伸展过程提供见解。例如，岩心杂岩的位置和岩心杂岩的低角度几何结构可能受到较老的构造活动的强烈影响。此外，如果岩心杂岩体的伸展始于晚白垩世，则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。在岩心杂岩体中发现晚白垩世伸展，对认识拉腊胺造山运动时期科迪勒拉的构造演化具有重要意义。

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.