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And Post-Alleghanian Exhumation of the Central Appalachian Mountains from Zircon (U-Th)/He GEOSPHERE, V

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And Post-Alleghanian Exhumation of the Central Appalachian Mountains from Zircon (U-Th)/He GEOSPHERE, V Research Paper GEOSPHERE Spatially variable syn- and post-Alleghanian exhumation of the central Appalachian Mountains from zircon (U-Th)/He GEOSPHERE, v. 17, no. 4 thermochronology https://doi.org/10.1130/GES02368.1 Luke C. Basler1, Jaclyn S. Baughman1, Michelle L. Fame1, and Peter J. Haproff2 8 figures; 2 tables; 1 set of supplemental files 1Department of Earth and Oceanographic Science, Bowdoin College, 255 Maine Street, Brunswick, Maine 04011, USA 2Department of Earth and Ocean Sciences, University of North Carolina Wilmington, 601 South College Road, Wilmington, North Carolina 28403, USA CORRESPONDENCE: [email protected] ABSTRACT 1997; Lock and Willett, 2008); however, reconstructing the timing, rates, and CITATION: Basler, L.C., Baughman, J.S., Fame, M.L., and Haproff, P.J., 2021, Spatially variable syn- and spatial variability of exhumation during and immediately after these geologic post-Alleghanian exhumation of the central Appala- To assess spatial and temporal patterns of Phanerozoic orogenic burial and processes is often difficult. During orogen development, exhumation typically chian Mountains from zircon (U-Th)/He thermochro- subsequent exhumation in the central Appalachian Mountains, we present depends on the magnitude of crustal loading, which drives rapid erosion of nology: Geosphere, v. 17, no. 4, p. 1151–1169, https:// doi .org /10.1130 /GES02368.1. mid-temperature zircon (U-Th)/He (ZHe; closure temperature [TC] = 140–200 °C) thickened crust and rugged topography within active mountain belts (Reiners dates for 10 samples along a 225 km, strike-perpendicular transect spanning the and Brandon, 2006). During postcollisional rifting, exhumation is regulated by Science Editor: Andrea Hampel Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic lithospheric thinning and edge-driven asthenospheric convection, which can Associate Editor: Christopher J. Spencer provinces in West Virginia and western Virginia. Ranges of single-grain ZHe cause rift-flank uplift, erosion, and subsequent exhumation. The magnitude of dates exhibit an eastward younging trend from 455–358 Ma in the Pennsylvanian exhumation generally decreases away from the rift margin (Steckler, 1985; Buck, Received 18 November 2020 Appalachian Plateau to 336–209 Ma in the Valley and Ridge, 298–217 Ma in the 1986), though extensional faulting can still result in rapid exhumation hundreds Revision received 26 February 2021 Accepted 30 April 2021 Blue Ridge, and 186–121 Ma in the Piedmont. Within the Pennsylvanian Appa- of kilometers inboard of the margin, as seen along the modern Red Sea Rift lachian Plateau, detrital ZHe dates are older than corresponding depositional (Szymanski et al., 2016). Exhumation during both orogen development and Published online 21 June 2021 ages, thus limiting postdepositional burial temperatures to less than 160 °C. rifting is regulated by key structural and lithospheric dynamics, and thus recon- These ZHe dates capture predepositional mid-Paleozoic cooling signatures, structing the exhumation record provides essential geologic information about indicating provenance from either recycled Taconic or Acadian basin strata or the spatial and temporal evolution of an orogen (McQuarrie and Ehlers, 2018). mid-Paleozoic Appalachian terranes. Across the Valley and Ridge and western Within the central Appalachian Mountains, the timing and rates of exhu- Blue Ridge provinces, reset Permian detrital ZHe dates feature flat date-effective mation driven by the late Carboniferous–Permian Alleghanian orogeny and uranium correlations that suggest rapid Alleghanian cooling initiating prior to subsequent rifting of the Atlantic Ocean either are not known or remain rel- 270 Ma. ZHe dates within the Valley and Ridge are more than 100 m.y. older than atively unconstrained (e.g., Roden, 1991; Evans, 2010). Across the foreland, previously reported regional apatite fission-track dates, reflecting a protracted maximum thermal indicators convey first-order information about the quan- period of stable post-Alleghanian thermal conditions within the foreland. By tity and spatial patterns of postdepositional exhumation. For instance, the contrast, post-Triassic single-grain ZHe dates in the interior Piedmont document surficial exposure of more thermally mature sedimentary rocks toward the rapid postrift cooling, likely resulting from both the relaxation of an elevated east (Ruppert et al., 2010; Repetski et al., 2014) indicates that the magnitude geothermal gradient and exhumation from rift-flank uplift. The spatial discon- of postdepositional exhumation increased toward the hinterland, assuming tinuity between stable synrift thermal conditions in the Valley and Ridge and a spatially consistent geothermal gradient. However, deciphering the timing, rapid cooling in the Piedmont suggests that rift-flank uplift and cooling were rates, and therefore precise mechanisms responsible for this pattern of differ- concentrated outboard of the foreland within the Piedmont province. ential exhumation requires reconstructing the thermal history of surface rock. Mid- to low-temperature thermochronology serves as a powerful tool to con- strain time-temperature paths, yet within the central Appalachian Mountains, ■ INTRODUCTION a thermal history gap spanning the late Carboniferous–Permian Alleghanian orogeny to Triassic–Jurassic rifting exists within prior thermochronologic data Continent-continent convergence and postorogenic rifting induce differential sets. In postorogenic settings, low-temperature apatite fission-track (AFT; clo- exhumation across an orogen (e.g., Tucker and Slingerland, 1994; Arne et al., sure temperature [TC] = 80–120 °C; Donelick et al., 2005) and apatite (U-Th)/He This paper is published under the terms of the (AHe; TC = 40–90 °C; Farley and Stockli, 2002) thermochronology typically can CC-BY-NC license. Luke C. Basler https://orcid.org/0000-0003-0368-413X be applied to determine the synorogenic and synrift rates and spatial patterns © 2021 The Authors GEOSPHERE | Volume 17 | Number 4 Basler et al. | Central Appalachian exhumation Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/4/1151/5362213/1151.pdf 1151 by guest on 26 September 2021 Research Paper of exhumation (e.g., Ehlers and Farley, 2003; Enkelmann and Garver, 2016). Thermochronometers Maximum Temp. Indicators 0 However, the central Appalachian Mountains have been consistently exhum- (U-Th)/He Fission CAI %Ro ing along a passive margin for greater than 180 m.y., and low-temperature Track 1 2 3 4 (<120 °C) thermochronology does not capture higher-temperature thermal 50 AHe histories of orogen development (e.g., Spotila et al., 2004; McKeon et al., 2014; PRZ 1 AFT Shorten and Fitzgerald, 2019). Furthermore, mid-temperature detrital zircon fis- 100 2 PAZ sion-track (ZFT; TC = ~220–260 °C; Fig. 1; Yamada et al., 1995) dates are not reset by postdepositional thermal conditions (Naeser et al., 2016) and thus do not 150 3 provide information about postburial exhumation rates. Here, we employed zir- ZHe con (U-Th)/He (ZHe; TC = 140–200 °C; Reiners, 2005) thermochronology to target PRZ the thermal history gap between the AFT and ZFT thermochronometers, which 200 Temperature (°C) corresponds to exhumation driven by the late Paleozoic Alleghanian orogeny ZFT in the western Appalachians and Triassic–Jurassic rifting in the eastern Appa- 4 250 PAZ lachians. Addressing the thermal imprint of these events informs the broader understanding of transitions from active orogen growth to postrift passive-mar- gin sedimentation, refines existing thermal constraints, and demonstrates the 300 utility of mid-temperature detrital ZHe dating in deciphering the burial and Figure 1. Temperature sensitivity of the apatite (U-Th)/He (AHe) exhumation histories of ancient fold-and-thrust belts. and zircon (U-Th)/He (ZHe) partial helium retention zone (PRZ; We present ZHe data for 10 samples along an orogen-perpendicular tran- Farley and Stockli, 2002; Reiners, 2005; Shuster and Farley, 2009; Guenthner et al., 2013), and apatite fission-track (AFT) and zircon sect across the central Appalachian foreland and orogenic core (Fig. 2, A-A′), fission-track (ZFT) partial annealing zones (PAZ; Yamada et al., spanning the Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont 1995; Donelick et al., 2005; Ketcham et al., 2007). The PRZ and physiographic provinces within Virginia and West Virginia. We began by draw- PAZ correspond to the closure temperature “window” of these ing first-order inferences from basic depositional constraints and single-grain thermochronometers. Conodont alteration indices (CAI) classify a range of maximum burial temperatures through changes in ZHe date-effective uranium concentration (eU) correlations. Next, we exploited conodont fossil color (Epstein et al., 1977). Vitrinite reflectance existing structural, thermochronologic, and maximum thermal data to per- (VR) records maximum burial temperatures through diagenetic form inverse thermal modeling of the ZHe data set. Finally, we constructed an changes in hydrocarbon-bearing vitrinite. VR values (%Ro) are overview of the transect-parallel trends in the timing and rates of exhumation. from the basin%Ro model of Nielsen et al. (2017), based on a reheating and cooling rate of 2.8 °C/m.y. Our results constrain spatially variable and geologically reasonable syn- and post-Alleghanian orogeny exhumation rates and augment the existing trove of structural and geochronologic data in the Appalachian
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