Collaborative Research/RUI: The Role of Orogen Parallel Fault Systems in the Late Exhumation History of a Convergent Orogen

摘要先前公布的40 Ar/ 39 Ar矿物年龄和新的磷灰石裂变径迹年龄模式显示了阿巴拉契亚北部两大造山带平行断裂系统明显的时间-温度不连续性。这些不连续性，横跨缅因州诺伦贝加断层系统的一部分和佛蒙特州-新罕布什尔州边界的阿蒙努苏断层，强烈表明这些断层系统的部分在白垩纪晚期或更早的时间进行了显著的垂直运动（可能高达2-3公里）。这两个断裂系统都位于阿卡迪亚造山带的最高坡度部分的侧面，在这两个例子中，冷却时代的模式都表明，造山带核心的岩石相对于造山带两侧的下落岩石向上移动。本文概述了一项横跨这两个断裂系统的低温热年代学研究（磷灰石裂变径迹和(U-Th)/He），目的是：确定沿这些造山带平行断裂系统的低温热年代学不连续的区域范围。跨走向采样穿越将允许PI测试一个有效的假设，该假设提出这些先前建立的造山带平行软弱带在造山带上容纳了显著的造山后均衡隆起。确定低温不连续是否局限于造山带边缘，或者造山带核心内的其他构造是否也可能容纳造山带后的位移。确定这些结构活跃了多长时间。初步的磷灰石裂变径迹测年穿越了Norumbega和amonoose断层系统，但尚未确定整个构造的同期冷却时间。迄今为止的结果表明，这两个构造的一部分一直活跃到晚白垩纪，但没有确定更低的年龄限制。PI建议将较低的闭合温度(U-Th)/He技术（~ 60-70℃闭合温度）纳入他们的研究中，这将提供后期冷却时间的信息，并揭示当前并置在这些断层上的岩石是否在晚白垩纪/早第三纪同时冷却。估算这些造山带平行断裂系统的造山后位移幅度，并确定它们在造山带发掘史上的重要性。本研究结果不仅将为阿巴拉契亚北部地区的发掘历史提供必要的信息，而且将对了解造山带后挖掘的机制和过程产生更广泛的影响。几乎所有的造山带都与造山带平行软弱带有关，这些构造在山带的晚期发掘史中起什么作用尚不清楚。对两个造山带两侧断裂带的低温热年代学研究将为这些构造在造山带后挖掘中的作用提供急需的信息。这项拟议的研究将涉及两个具有低温热年代学、区域构造学和阿巴拉契亚北部断层系统研究经验的PI之间的合作。重要的是，两所院校的本科生将完全融入该项目，并将参与采样、样本处理、数据分析和解释。

AbstractPatterns of previously published 40 Ar/ 39 Ar mineral ages and new apatite fission track ages determined by the PIs show distinct time-temperature discontinuities across two major orogen-parallel fault systems in the northern Appalachians. These discontinuities, across a portion of the Norumbega fault system in Maine and the Ammonoosuc fault along the Vermont-New Hampshire border, strongly suggest that portions of these fault systems accommodated significant (perhaps up to 2-3 km) vertical movement in Late Cretaceous or younger time. Both of these fault systems flank the highest grade portions of the Acadian orogen and in both instances the patterns of cooling ages suggest that rocks in the core of the orogen moved up relative to down-dropped rocks on the flanks of the orogen. This proposal outlines a collaborative low-temperature thermochronological study (apatite fission-track and (U-Th)/He) across these two fault systems with the goals of: oDetermining the regional extent of the low-temperature thermochronological discontinuities along these orogen-parallel fault systems. Across strike sampling traverses will allow the PI's to test a working hypothesis that proposes these previously established orogen-parallel zones of weakness accommodated significant differential post-orogenic isostatic uplift across the orogen.oDetermining if the low-temperature discontinuities are confined to the margins of the orogen or whether other structures within the core of the orogen may also be accommodating post-orogenic displacements.oDetermining how long these structures have been active. Preliminary apatite fission track dating traverses across both the Norumbega and Ammonoosuc fault systems have yet to establish a time of contemporaneous cooling across the structures. Results to date suggest portions of both these structures have been active into the Late Cretaceous, but no lower age limit has been established. The PI's propose to incorporate the lower closure temperature (U-Th)/He technique (~ 60-70 o C closure temperature) into their study which will provide information on the timing of cooling at later times and reveal whether the rocks currently juxtaposed across these faults cooled contemporaneously during the Late Cretaceous/Early Tertiary.oEstimating the magnitude of post-orogenic displacement along these orogen-parallel fault systems and determining how important they were in the exhumation history of the orogen.The results of this study will not only provide much needed information on the exhumation history of the northern Appalachians, but will have a broader impact on the understanding of mechanisms and processes associated with post-orogenic mountain belt exhumation. Orogen-parallel zones of weakness are associated with nearly all orogenic belts and what role these structures play in the late exhumation history of mountain belts is unclear. The proposed low-temperature thermochronological study of two orogen flanking fault zones will provide much needed information on the role of these structures in post-orogenic exhumation. This proposed research will involve collaboration between two PI's with experience in low-temperature thermochronology, regional tectonics, and studies of northern Appalachian fault systems. Importantly, undergraduate students from both institutions will be fully integrated into the project and will be involved in sampling, sample processing, data analysis and interpretation.