SYN- TO POST-OROGENIC EVOLUTION OF COLLISIONAL MOUNTAIN SYSTEMS: INVESTIGATING THE POTENTIAL FOR CRUSTAL FLOW USING THERMOCHRONOLOGY AND NUMERICAL MODELS

This dissertation is composed of three distinct manuscripts which collectively investigate processes that contribute to the late evolution of collisional mountain systems —specifically, the ancient Appalachian-Caledonian system. In the first paper, 40Ar/39Ar thermochronological data are used to constrain the timing of exhumation of the Scandian orogenic wedge of northern Scotland. Muscovite and amphibole samples yield dates of ca. 420-411 Ma, consistent with cooling after peak orogenesis. During this cooling phase, dates from both systems in individual thrust sheets show an increase in cooling rate in the later stage of exhumation; in the orogenic core, the cooling rate increases by 150-300%. This increase is less pronounced in the immediate flanking sheets, and not present in the most foreland-directed sheet. These rates imply very high unroofing rates during final exhumation. Neither the increase in, nor magnitude of, these rates is consistent with the presumed decrease in erosion rates once collision has ceased. Thus, a connection to a proposed lower crustal flow mechanism in the East Greenland Caledonides is postulated; this forms the basis for the next chapter, which aims to rule out erosion as the only driver for denudation of the Scottish orogen. The second chapter presents results of numerical models which are used to simulate erosion, isostasy, and crustal thermal evolution in the Scandian wedge during tectonic uplift (2-D model) and post-collisional decay (1-D and 2-D models). First, interpreted late syn-orogenic uplift rates are used to (1) define the geothermal gradient of the crust just prior to the decay phase, and (2) determine the initial topography and erosion rate of the orogen in the 1-D model. Erosion and isostasy of the orogen over 10 Myr leaves high topography in the orogen core, and total erosion is not sufficient to fully exhume exposed rocks in the time required by the data. The thermal model results in geothermal gradients of 20-40 degrees C km-1, which confirms estimates based on a constant 25 degrees C km-1 gradient in the first chapter. The 2-D model, which uses constraints from the 1-D model, confirms the results and indicates that denudation of the Scandian wedge likely required geodynamic assistance to accomplish the post-orogenic topography and exhumation rates implied by data. Finally, the third chapter presents thermochronologic data for the Blue Ridge (BR) and Inner Piedmont (IP) of the Southern Appalachians. The IP has been proposed as a relict Neoacadian crustal channel, but the timing and extent of high-grade metamorphism remain debated and must be constrained if the hypothesis is to be tested. Amphibole 40Ar/39Ar dates for the BR and IP show (1) Neoacadian dates in the IP which get younger to the southwest, and (2) no direct evidence for post-Taconic high-grade metamorphism in the BR. The dates in the BR are consistent with the role of the BR as the “buttress” of the proposed crustal flow system. The date trend in the IP suggests that high-temperature conditions existed in the channel during the timing of proposed flow, as well as earlier cooling in the northeast prior to southwestern-directed deflection along the Brevard Fault Zone. All dates are older than the main Alleghanian event, and therefore do not support later overprinting. Taken together, these dates provide the constraint needed for further examination of the system and comparison to the hypothesized lower crustal flow mechanism in the Scottish Caledonides