Tracking the Late Mesozoic Strike-Slip System of the Western U.S. Cordillera

大规模走滑断裂是包括北美科迪勒拉在内的造山带发育的重要过程。这些断层可以容纳大量的地体位移，并以复杂的方式打乱它们的相对位置，导致地体的拼贴关系难以辨认。造山带的区域走滑断裂也可以对古代板块相互作用提供重要的见解。在北美科迪勒拉地区，前新生代走滑断裂作用被认为在地块的搬运和组装过程中发挥了重要的构造作用，但所提出的走滑期的具体情况却陷入了不确定性和争议之中。关键问题包括走滑边界的位置，以及沿走滑边界位移的数量、意义和时间。这些不确定性必须得到解决，才能对前新生代科迪勒拉进行可靠的古地理和古构造重建。该项目的重点是内华达西部剪切带系统（WNS），这是最近在俄勒冈州东南部和内华达州西北部发现的一个主要构造边界。目前沿WNS获得的数据表明，它在中生代晚期容纳了几个100公里的右移的外部地体，并且它可能连接了另外两个重要的中生代晚期构造边界，爱达荷州西部的鲑鱼河缝合线和加利福尼亚州内华达山脉中南部的莫哈韦-雪湖断层。这个项目正在通过各种新的研究更严格、更广泛地检验这些结论。如果有关边界的初步假设是正确的，将对北美科迪勒拉的古地理和古构造重建和解释产生深远的影响。本项目具体研究内容如下：WNS的详细制图、构造和地质年代学研究正在向南扩展到内华达州西部的Gerlach地区，这将扩大对边界的了解，使其走向距离超过200公里。(2)从Gerlach地区到俄勒冈州东南部，确定了WNS沿线微地体的分布、起源和意义。这些地层和构造上截然不同的岩石构造组合可能是沿WNS走滑体系的吻合链向北拖拽的断裂片。(3)正在对所提出的跨走滑系统的地体联系进行验证，以确定推断的位移意义和位移量是否有效，以及推断的WNS、莫哈韦-雪湖断裂和鲑鱼河缝合带之间的连续性是否能够得到更严格的支持。(4)各种地质年代学研究正在确定沿西北缘走滑位移和变形的时间。

Large-scale strike-slip faulting is increasingly recognized as an important process in the development of orogenic belts, including the North American Cordillera. These faults can accommodate substantial displacement of terranes and shuffle their relative positions in complex ways, resulting in a collage of terranes whose relationship is difficult to decipher. Regional strike-slip faulting in orogenic belts can also yield crucial insight into ancient plate interactions. In the North American Cordillera, pre-Cenozoic strike-slip faulting is believed to have played an important structural role in terrane transport and assembly, but the specifics of proposed strike-slip episodes are mired in uncertainty and controversy. Key questions involve the location of strike-slip boundaries, and the amount, sense, and timing of displacement along them. These uncertainties must be resolved before any reliable paleogeographic and paleotectonic reconstructions of the pre-Cenozoic Cordillera can be generated.This project is focusing on the western Nevada shear zone system (WNS), a recently identified major tectonic boundary in southeast Oregon and northwest Nevada. Data obtained so far along the WNS suggest that it accommodated several 100 km dextral displacement of outboard terranes in the late Mesozoic, and that it likely connects up with two other important late Mesozoic tectonic boundaries, the Salmon River suture of western Idaho and the Mojave-Snow Lake fault of the central and southern Sierra Nevada in California. This project is testing these conclusions more rigorously and extensively by a variety of new studies. If initial hypotheses concerning the boundary are correct, it will have far-reaching implications for paleogeographic and paleotectonic reconstructions and interpretations of the North American Cordillera.Specific research being done in this project is as follows. (1) Detailed mapping, structural, and geochronologic studies of the WNS are being extended south into the Gerlach area of western Nevada, which will expand knowledge of the boundary to a strike distance of over 200 km. (2) Distribution, origin, and significance of microterranes along the WNS are being determined from the Gerlach area to southeast Oregon. These stratigraphically and structurally distinct lithotectonic assemblages may be fault slices dragged northward along anastomosing strands of the WNS strike-slip system. (3) Proposed terranes linkages across the strike-slip system are being tested to determine whether the inferred sense and amount of displacement is valid, and whether the inferred continuity between the WNS, Mojave-Snow Lake fault, and Salmon River suture can be more rigorously supported. (4) Timing of strike-slip displacement and deformation along the WNS are being determined by a variety of geochronologic studies.