Structural and Thermal Analysis of an Upper Crustal Low-angle Normal Fault in the Sevier Orogen, Southern Nevada

中新世摩门峰拆离断层是沿着内华达州南部盆岭省塞维尔造山带前缘的一条低角度正断层。它经常被引用为世界上最好的大位移（大于20公里）正断层的例子之一，它的形成和滑动倾角小于25度，因此它的起源与脆性断裂和滑动如何发生在与最大主应力成大角度的平面上的问题有关。近年来，拆离作用被解释为一个灾难性的重力滑动块体系统或一个具有有限构造滑动的大规模溶解带。这些假说挑战了大位移拉张滑脱是地壳重要构造单元的观点。该项目的目标是利用地质测绘、地层学和最先进的低温测温方法，为这些相互竞争的假设提供关键检验。摩门峰拆离带最初的大滑移假说是基于白垩纪构造标志物的重建，并预测现在暴露在拆离带下盘的前塞维尔逆冲斜坡的构造起伏和坡度与上盘保存的大型单斜构造具有相同的构造起伏和坡度。研究小组将在对该系统知之甚少的上盘部分进行地质测绘和地层厚度测量，以便与下盘进行定量比较。将使用两种独立的方法，用低温测温法测试滑脱、滑坡和溶解模型的预测：（1）磷灰石-锆石对的（U-Th）/He热年代学，以限制拆离下盘的时间-温度历史，以及（2）新的凝块同位素碳酸盐温度计，以区分滑坡模型的预测（冷下盘脉系、断层面热断层泥）和拆离模式（热下盘脉系、热断层泥）。无论这些和其他各种结构假说的测试结果如何，断层岩和脉系统的低温碳酸盐测温是一个新领域，因此，拟议的工作具有潜在的变革性影响。我们理解地震和断层力学的主要障碍之一是非正式地称为应力悖论。应力悖论简单地说，就是地球上沿着断层面的力太小，不允许断裂或持续的摩擦滑动，但显然断层移动并产生地震。目前还没有达成共识，为什么会这样，因此这个问题是地质学和地球物理学的一个主要研究焦点领域。应力悖论对于一类被称为低角度正断层的断层最为尖锐，这种断层适应地壳的水平延伸。该项目旨在测试位于内华达州南部的摩门峰拆离断层的起源，该断层是世界上暴露最好的低角度正断层之一。几个研究小组提出了他们声称的证据，证明摩门教峰脱离根本不是一个低角度的正断层，从而质疑任何这样的断层甚至存在的概念。研究小组将使用标准地质技术和最先进的地球化学技术，调查沿拆离带沿着变形的时间、深度和温度，预计这将证伪一个或多个提出的假设。

The Miocene Mormon Peak detachment is a low-angle normal fault along the Sevier orogenic front in the Basin and Range province in southern Nevada. It is often cited as one of the best examples in the world of a large-displacement (greater than 20 kilometers) normal fault that both formed and slipped at dips of less than 25 degrees, and therefore its origin is relevant to the problem of how brittle fracture and slip can occur on planes oriented at a high angle to the maximum principal stress. In recent years, the detachment has alternatively been interpreted as a system of catastrophically emplaced gravity slide blocks or a zone of large-scale dissolution with limited tectonic slip. These hypotheses challenge the notion that large-displacement extensional detachments are important tectonic elements in the earth's crust. The objective of this project is to use geological mapping, stratigraphy and state-of-the-art low-temperature thermometric methods to provide critical tests for these competing hypotheses. The initial large-slip hypothesis for the Mormon Peak detachment is based on reconstruction of Cretaceous structural markers, and predicts that the structural relief and slope of the frontal Sevier thrust ramp, now exposed in the footwall of the detachment, has the same structural relief and slope as a large monoclinal structure preserved in the hanging wall. The research team will conduct geological mapping and stratigraphic thickness measurements in the poorly understood hanging wall part of the system for quantitative comparison with the footwall. Predictions of the detachment, landslide and dissolution models will be tested with low-temperature thermometry using two independent approaches: (1) (U-Th)/He thermochronometry on apatite-zircon pairs to constrain the time-temperature history of the detachment footwall, and (2) the new clumped isotope carbonate thermometer to distinguish between predictions of the landslide model (cold footwall vein systems, hot gouge on the fault surface) and detachment model (warm footwall veins and warm gouge). Regardless of the outcome of these and other tests for various structural hypotheses, low-temperature carbonate thermometry of fault rocks and vein systems is a new field, and the proposed work accordingly has the potential for transformative impact.One of the primary obstacles to our understanding of the mechanics of earthquakes and faulting is informally referred to as the stress paradox. The stress paradox is simply that the forces in the earth along fault planes are too low to permit either fracture or continued frictional sliding, yet obviously the faults move and generate earthquakes. There is at present no consensus as to why this is the case, and hence the problem is a major research focus area in geology and geophysics. The stress paradox is most acute for a class of faults known as low-angle normal faults, which accommodate horizontal extension of the earth's crust. This project is designed to test competing hypotheses for the origin of one of the best-exposed low-angle normal faults in the world known as the Mormon Peak detachment, located in southern Nevada. Several research teams have presented evidence they claim demonstrate that the Mormon Peak detachment is not a low-angle normal fault at all, and thereby question the notion that any such faults even exist. The research team will use standard geological techniques and state-of-the-art geochemical techniques to investigate the timing, depth and temperature of deformation along the detachment, which is expected to falsify one or more of the proposed hypotheses.