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

中新世摩门峰滑脱是内华达州南部盆岭省Sevier造山带前缘的一条低角度正断层。它经常被认为是世界上最好的大位移(大于20公里)正断层的例子之一，该断层在倾角小于25度时形成并滑动，因此它的起源与与最大主应力成大角度的平面如何发生脆性断裂和滑动有关。近年来，该滑脱被解释为灾害性侵位的重力滑动块体系统或构造滑动有限的大规模溶蚀带。这些假说挑战了大位移伸展拆离是地壳中重要构造元素的观点。该项目的目标是使用地质制图、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测试。摩门峰滑脱的最初大滑动假说是基于白垩纪构造标志的重建，并预测现在暴露在滑脱下盘中的前缘Sevier逆冲斜坡的结构起伏和斜率与保存在上盘中的大型单斜构造具有相同的结构起伏和斜率。研究小组将在该系统鲜为人知的上盘部分进行地质测绘和地层厚度测量，以便与底盘进行定量比较。滑脱、滑坡和溶解模式的预测将通过两种独立的方法用低温测温进行检验：(1)磷灰石-锆石对的(U-Th)/He温度计时，以限制滑脱下盘的时间-温度历史，以及(2)新的块状同位素碳酸盐温度计，以区分滑坡模式(冷下盘脉系统，断层表面热断层泥)和滑脱模式(热下围脉和热泥)的预测。不管对各种构造假说的这些和其他测试的结果如何，断层岩和脉系的低温碳酸盐测温是一个新的领域，因此拟议的工作具有潜在的变革影响。我们理解地震和断层机制的主要障碍之一被非正式地称为应力悖论。应力悖论简单地说就是，沿断层面的地力太低，既不允许破裂，也不允许持续的摩擦滑动，但很明显，断层移动并产生地震。关于为什么会这样，目前还没有达成共识，因此这个问题是地质学和地球物理学的一个主要研究重点领域。对于一类被称为低角度正断层的断层来说，应力悖论最为尖锐，这种断层适应了地壳的水平伸展。该项目旨在检验关于位于内华达州南部的摩门峰支队的世界上暴露最好的低角度正断层之一的起源的相互竞争的假说。几个研究小组已经提出了证据，他们声称证明了摩门峰滑脱根本不是一个低角度的正断层，从而质疑任何这样的断层存在的想法。研究小组将使用标准地质技术和最先进的地球化学技术来调查沿滑脱带变形的时间、深度和温度，预计这将证伪一个或多个提出的假说。

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.