Patterns 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.
