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

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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 constrain 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

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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-margin 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 Mountains. comprise the largely crystalline core of the Alleghanian orogen. Here, we detail a brief overview of the geologic setting of each province to provide a ■■ GEOLOGIC AND METHODOLOGIC BACKGROUND foundation for the interpretation of thermochronology results. The Appalachian Plateau hosts subhorizontal autochthonous Paleozoic Geologic Background strata in the western region of the Appalachian foreland basin (Fig. 2). The eastern portion of the plateau exhibits long-wavelength (~15 km), low-amplitude Along our orogen-perpendicular transect (Fig. 2, A-A′), the westernmost (~200 m) folds formed during the convergence of Laurentia and Gondwana at Appalachian Plateau and Valley and Ridge physiographic provinces collectively the late stages of the Alleghanian orogeny (Cardwell et al., 1968; Hatcher, 2010). form the Appalachian foreland, which consists of siliciclastic sequences that The western region of the plateau exposes subhorizontal Pennsylvanian and were deposited during the Late Ordovician Taconic orogeny, the Middle to Late early Permian sedimentary rocks derived from the Alleghanian highlands that Devonian Acadian orogeny, and the late Carboniferous–Permian Alleghanian experienced shortening along the Burning Springs thrust and layer-parallel orogeny (Drake et al., 1989; Hatcher et al., 1989; Osberg et al., 1989; Ettensohn, shortening (Root, 1996; Whitaker and Bartholomew, 1999). The province is 2008; Hatcher, 2010). Syntectonic basin strata are onlapped by carbonate, shale, bordered to the east by the Allegheny structural front, which is the westward and sandstone sequences that were deposited during periods of tectonic boundary of Alleghanian thrust faulting in the adjacent Valley and Ridge Prov- quiescence, and they are underlain by a Cambrian–Late Ordovician synrift ince (Gwinn, 1964; Mitra, 1987). and passive-margin succession and series of foreland basin sequences (Read, The Valley and Ridge Province forms an archetypal thin-skinned fold-and- 1989). Toward the east, the Blue Ridge and Piedmont physiographic provinces thrust belt within the eastern half of the Appalachian foreland basin (Fig. 2;

80°30’W 80°00’W 79°30’W 79°00’W 78°30’W Quaternary Alluvium A West Virginia Virginia Triassic Newark Supergroup CA3 433-358 Monongahela Grp. CA4 CA1 376-234 Conemaugh Grp. 455-373 Penn. CA5 Alleghany Grp. 336-298 North Mt. fault Harrisonburg CA1-CA3 Pottsville Grp. 38°30’N CA2 425-373 Mauch Chunk Fm. CA6 296-209 Miss. Greenbriar Limestone Pocono Grp. Pulaski fault Blue Ridge fault CA4 Hampshire Fm. Chemung Fm. Staunton Braillier Fm. Appalachian Plateau Devonian Province Mt. Run fault CA10 Millboro Shale 280-248 CA8 CA7 Charlottesville 186-121 Oriskany Sandstone 298-217 Helderberg Grp.

38°00’N Tonoloway Fm. Valley and Ridge Wills Creek Fm. Province Allegheny A’ Silurian Williamsport Fm. structural front CA5 McKenzie Fm. Blue Ridge CA9 Clinton Fm. Province Juniata Fm. Piedmont CA6 Tuscarora Sandstone Province Shores Melange Zone - Chopawamsic fault CA10 Martinsburg Fm. CA9 323-244 Edinburg Fm. 40 kilometers Ordovician Bowens Creek fault New Market Limestone

37°30’N St. Clair fault Beekmantown Fm ZHe Dates (This Study) Existing ZFT Existing AFT Dates (Ma) Dates (Ma) Conococheague Fm. App. Plateau Sample Samples name (Naeser et al., 2016) (Roden, 1991) Elbrook Fm. Valley & Ridge 145 - 299 33 - 66 Cambrian Waynesboro Fm. Blacksburg Samples CA1 Saltville thrust 455-373 299 - 541 66 - 145 Blue Ridge CA7 Chilhowee Grp. 541 - 952 145 - 299 Roanoke Sample Catoctin Fm. Piedmont ZHe date single-grain Cities Thrust faults Swift Run Fm. Sample date range Precambrian CA8 Basement

Figure 2. (A) Simplified geologic map of the central Appalachian Mountains colored by unit age and shaded by relief. The range of single-grain zircon (U-Th)/He (ZHe) dates within samples is shown in colored circles, and previously reported apatite fission-track (AFT; Roden, 1991) and zircon fission-track (ZFT; Naeser et al., 2016) dates are also indicated. Major regional faults (Cardwell et al., 1968; Kulander and Dean, 1986; Rader and Wilkes, 2001) and the boundaries of the Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateau physiographic provinces are also shown. (B) Stratigraphy of the central Appalachian Mountains (Cardwell et al., 1968; Rader and Wilkes, 2001). Penn.—Pennsylvanian; Miss.—Mis- sissippian; Grp.—Group; Fm.—Formation.

Gwinn, 1964; Perry, 1978; Kulander and Dean, 1986; Wilson and Shumaker, 1992; easternmost Great Valley subprovince juxtaposes Cambrian–Ordovician car- Evans, 2010; Hatcher, 2010; Lammie et al., 2020). Deformation is expressed as bonate sequences atop the folded Ordovician–Mississippian roof sequence a roof rock sequence of long-wavelength (~15 km), high-amplitude (>2 km) (Kulander and Dean, 1988; Evans, 1989, 2010). folds in Ordovician through Carboniferous strata (Rader and Wilkes, 2001). The Blue Ridge Province is a topographically prominent massif that forms The roof sequence is underlain by a duplex of Cambrian through Ordovician the western half of the crystalline core of the Appalachian orogen (Fig. 2). carbonate rocks that form antiformal stacks spatially correlated with folds The Blue Ridge exposes a west-verging anticlinorium that is thrust atop Cam- in the overlying roof strata (Evans, 1989). While few emergent faults exist in brian–Ordovician carbonates of the Valley and Ridge Province by the Blue the western Valley and Ridge Province, the North Mountain thrust fault in the Ridge thrust (Tollo et al., 2004). The axis of the Blue Ridge antiform consists

of Grenville-aged (ca. 1.2–1.0 Ga) igneous and metamorphic basement rocks The total accumulated radiation damage of a grain depends on the effec- intruded by Neoproterozoic plutons and overlain by Neoproterozoic meta tive uranium concentration (eU, the weighted alpha productivity, where eU = sedimentary rocks of the Lynchburg Group, Neoproterozoic metavolcanics [U] + 0.235 × [Th]), and the time since a grain has cooled beneath the zircon rocks of the Catoctin and Swift Run Formations, and Cambrian metasedi- radiation damage annealing zone (~250–350 °C; Ginster et al., 2019). Thus, eU mentary rocks and siliciclastic strata of the Chilhowee Group (Rader and is a useful damage proxy for samples with a shared thermal history. At low Evans, 1993). The Blue Ridge is bounded to the east by an ~750-km-long, radiation damage, caused by low eU concentrations and/or minimal damage northeast-striking system of subparallel Alleghanian right-slip mylonitic shear accumulation time, 4He diffusivity decreases with progressive damage accu-

zones, including the Bowens Creek fault (Gates, 1987) and Mountain Run fault mulation, which increases the effective TC (Guenthner et al., 2013). As damage (Fig. 2; Pavlides et al., 1983; Pavlides, 1987, 1989, 1994; Evans and Milici, 1994). continues to accumulate, 4He diffusivity reaches a minimum and crosses the

The Blue Ridge is also truncated in the east by a southeast-dipping, brittle damage percolation threshold, after which a precipitous drop in TC is observed normal fault that forms the western boundary of the Triassic Scottsville Basin (Nasdala et al., 2004; Reiners, 2005; Guenthner et al., 2013). These radiation (Roberts, 1928; Bailey et al., 2014). damage effects manifest as spans of ZHe dates correlated with eU, as individ- The Piedmont Province is the low-relief eastern half of the Appalachian ual grains possess disparate effective closure temperatures that each access crystalline core, and it consists of an amalgamation of mostly peri-Gondwanan different portions of the sample’s thermal history, and that collectively capture terranes that were thrust over the Laurentian margin during the Acadian and rates of cooling along the time-temperature (t-T) path. Rapid exhumation Taconic orogenies. These terranes contain Mesoproterozoic to Neoproterozoic through the zircon helium partial retention zone (PRZ) will cool grains through and Paleozoic metamorphic and igneous rocks, are sporadically intruded by their individual “closure temperatures” at a similar time, thereby producing a Taconic and Alleghanian plutons, and are overlain by unmetamorphosed Tri- flat date-eU correlation. In contrast, once sufficient damage has accumulated assic–Jurassic rift-fill sedimentary rocks (Rader and Evans, 1993; Hughes et al., for zircon to cross the damage percolation threshold, slow exhumation, iso- 2014). Piedmont terranes are bounded by numerous Taconic to Alleghanian thermal holding within the PRZ, or reheating and partial 4He loss can yield a right-slip shear zones, many of which utilized preexisting zones of weakness steep, negative date-eU correlation. such as ophiolitic mélange complexes. These right-slip systems include the Thermal modeling software, HeFTy v.1.9.3 (Ketcham, 2005), tracks time- Brookneal fault zone, the Shores mélange zone (Brown, 1988), the Chopawam- evolving zircon 4He diffusion kinetics using the zircon radiation damage sic fault along the eastern margin of the Western Piedmont terrane (Gates et accumulation and annealing model (ZRDAAM; Guenthner et al., 2013), and al., 1986), and the Spotsylvania fault zone along the eastern margin of the it can constrain the various thermal histories that satisfy ZHe date, eU, and Chopawamsic terrane (Bailey et al., 2004). grain-size data. Within HeFTy, thermal history paths can be further constrained through the integrated vitrinite reflectance model of Nielsen et al. (2017), and by constructing t-T constraint boxes that force thermal paths through known Detrital Zircon (U-Th)/He Thermochronology and Maximum Thermal geologic constraints, such as depositional ages or U-Pb crystallization dates. Indicators However, interpretation and thermal modeling of ZHe dates from sedimentary rocks are complicated by the fact that detrital zircons do not necessarily share Mid-temperature ZHe thermochronology is a powerful tool used to recon- the same predepositional thermal history, as grains may have different prove- struct the timing and rates of orogenic exhumation, constrain provenance, nance. Consequently, grains from a single sample may have variable inherited and determine maximum burial conditions (e.g., Reiners, 2005; Cecil et al., damage accumulation and/or 4He retention prior to deposition, such that for 2006; Powell et al., 2016). Careful interpretation of ZHe data requires an accu- some thermal histories, it may be inappropriate to model grains together. For rate characterization of 4He diffusion kinetics, which depend strongly on the samples that may lack a shared predepositional thermal history, we consid- time-evolving accumulated radiation damage, and to a lesser extent on grain ered a range of damage and 4He inheritance histories to explore the role and size and shape (Reiners et al., 2002; Nasdala et al., 2004; Guenthner et al., 2013). complexity that variable t-T histories may have on detrital ZHe data patterns, Thermal histories may generate positive correlations between date and grain particularly for ZHe dates that were not fully reset by postdepositional ther- size and/or date and aspect ratio owing to the dependence of 4He diffusion mal conditions (Guenthner et al., 2014; Powell et al., 2016). We also carefully on grain dimensions and anisotropic c-axis (fast) versus a-axis (slow) zircon interpreted our model outputs in the context of known flaws in the ZRDAAM, 4He diffusion (Reiners and Farley, 2001; Anderson et al., 2020). Grain size and which may under- or overestimate 4He diffusivity depending on specific sample crystal morphology can cause tens of degrees Celsius difference in the effective and thermal history characteristics.

TC, defined as the temperature of the sample at the calculated date (Dodson, Within basins, maximum thermal indicators, including vitrinite reflectance 1973). Radiation damage is often the dominant control on 4He diffusivity and (VR), conodont alteration indices (CAI), and fluid inclusion microthermometry,

can lead to >100 °C differences in effective TC, especially in protracted thermal form an additional method to determine maximum burial conditions, recon- settings (e.g., Johnson et al., 2017; Baughman and Flowers, 2020). struct eroded overburdens, and model thermal histories (Epstein et al., 1977;

Evans, 1995; Evans and Battles, 1999; Reed et al., 2005; Rowan, 2006; Shorten conditions in presently exposed Valley and Ridge rocks likely exceeded those and Fitzgerald, 2019). VR uses the temperature-sensitive diagenetic alteration experienced by Appalachian Plateau rocks. of vitrinite to classify a maximum reheating temperature (Fig. 1; Sweeney Within the Valley and Ridge and Appalachian Plateau Provinces, existing ZFT, and Burnham, 1990; Nielsen et al., 2017), CAI records a range in maximum AFT, and AHe thermochronologic dates bracket a significant temporal gap in the temperatures from diagenetic changes in conodont fossil color (Epstein et al., t-T history. Higher-temperature ZFT dates within the central and northern Appa- 1977), and fluid inclusion microthermometry targets the composition of veins lachian foreland are not reset or partially reset by postdepositional reheating to determine conditions during formation. All are sensitive to temperatures (Fig. 2, small circles; Lakatos and Miller, 1983; Johnsson, 1985, 1986; Roden et al., from ~100 °C to 300 °C, which are typical of foreland basins experiencing 1993; Montario and Garver, 2009; Naeser et al., 2016), corroborating maximum >4 km of burial. thermal indicators, but providing little information about the postdepositional An integrated approach combining thermochronology with maximum exhumation history. By contrast, lower-temperature AFT and AHe thermo- thermal indicators serves as an effective method with which to reconstruct chronometers are fully reset, but they generally capture only post-Triassic the thermal history of a sedimentary basin, as each technique targets distinct exhumation (Roden, 1991; Roden et al., 1993; Blackmer et al., 1994; Boettcher characteristics of a t-T path (e.g., Kamp et al., 1996; Crowhurst et al., 2002; Reed and Milliken, 1994; Reed et al., 2005; Shorten and Fitzgerald, 2021). These data et al., 2005; Evans et al., 2014; Shorten and Fitzgerald, 2019). For instance, burial sets typically exhibit an eastward younging trend (Fig. 2, diamonds), and they temperatures derived from maximum thermal indicators can further refine have been interpreted to reflect differential cooling during a mid-Cretaceous rates of thermochronologically calculated cooling by forcing t-T paths through pulse of exhumation. Within the southern Pennsylvania Appalachian basin, a specific temperature. Similarly, thermochronology can constrain the timing Blackmer et al. (1994) modeled AFT and VR data to infer that post-Alleghanian of peak thermal conditions using maximum thermal data, and regional-scale thermal conditions were relatively stable, but this stability may have been pre- isograd maps created from maximum thermal data can provide estimates ceded by a rapid late Permian cooling pulse. However, the thermal signature of of overburdens and/or geothermal gradients that aid in the interpretation of the late Carboniferous–Permian Alleghanian orogeny is not directly captured thermochronologic data. within these thermochronologic data. Our mid-temperature ZHe data set falls between the temperature sensitivities of the AFT and ZFT thermochronometers and bridges the corresponding thermal history gap. Prior Thermal Constraints Within the central Appalachian Blue Ridge and Piedmont Provinces, existing thermochronology is limited, but it suggests a thermal discontinuity cor- Sedimentary samples within the Appalachian foreland and western half of responding to orogen-perpendicular changes in lithology and structure. In the Blue Ridge Province experienced a thermal history encompassing deposi- particular, ZFT dates from Naeser et al. (2016) displayed a sharp eastward tion at surficial temperatures, reheating by burial, and subsequent exhumation younging trend perpendicular to the strike of the Blue Ridge Province, from and cooling, which can be reconstructed using both maximum thermal indi- Precambrian (617–582 Ma) ZFT dates not reset by burial within Cambrian Chil- cators and thermochronology. In contrast, thermal history paths for primarily howee Group samples to significantly younger mid- to late Paleozoic dates from crystalline samples within the Piedmont and Blue Ridge Provinces exhibit cool- the eastern crystalline core of the Blue Ridge (Fig. 2, small circles). Within the ing from metamorphic temperatures and, owing to their crystalline lithology, interior of the Piedmont Province further north in Maryland, higher-temperature 40 39 are reliant on thermochronology for thermal history reconstructions. Here, we muscovite (TC = 500 ± 50 °C) and hornblende (TC = 400 ± 50 °C) Ar/ Ar dates provide an overview of prior thermal constraints and identify gaps targeted document cooling following the Taconic and Acadian orogenies (Kunk et al., by our ZHe data set. 2005). In the Piedmont and Newark rift basin of Pennsylvania and Maryland,

Within the central Appalachian foreland, the magnitude of postdeposi- synrift (ca. 220–180 Ma) titanite fission-track (TC = 265–310 °C; Coyle and Wagner, tional overburden exerts a primary control on maximum postdepositional 1998) and ZFT dates indicate a thermal pulse during rifting, potentially driven thermal conditions. In the Appalachian Plateau, Pennsylvanian VR isograd by a steepening of the thermal gradient up to 60 °C/km (Roden and Miller, maps exhibit an eastward increase in thermal maturity, thereby suggesting 1991; Kohn et al., 1993; Steckler et al., 1993) along with localized reactivation greater burial depths toward the hinterland (Ruppert et al., 2010). VR and fluid of Paleozoic transpressional faults and Triassic extensional faults. inclusion microthermometry data suggest that maximum burial temperatures reached ~150 °C immediately west of the Allegheny deformation front (Reed et al., 2005; Evans, 2010; Ruppert et al., 2010). Within the Valley and Ridge ■■ METHODS Province, maximum thermal conditions display significant spatial and stratigraphic variability related to the development of thrust duplexes, deposition We collected 10 samples along a 225-km-long, orogen-perpendicular in “piggyback basins,” and the passage of warm orogenic fluids (Zhang and transect across the Appalachian foreland basin and orogenic interior of West Davis, 1993; Evans, 2010; Repetski et al., 2014). However, maximum thermal Virginia and western Virginia (Fig. 2; Table 1), spanning the Appalachian

TABLE 1. SELECTED GEOLOGIC INFORMATION FOR CENTRAL APPALACHIAN SAMPLES Sample Single-grain zircon Elevation Latitude Longitude Unit/formation† Depositional period Lithology Physiographic (U-Th)/He date range* (m) (°N) (°W) province (Ma) CA1 455–373 317 38.6111 80.5656 Kanawha Formation, Pottsville Group Middle Pennsylvanian Sandstone Appalachian Plateau CA2 425–373 477 38.4805 80.4541 New River Formation, Pottsville Group Late Pennsylvanian Sandstone Appalachian Plateau CA3 433–358 1079 38.6171 80.1076 Kanawha Formation, Pottsville Group Middle Pennsylvanian Sandstone Appalachian Plateau CA4 376–234 1291 38.4758 79.6998 Hampshire Formation Devonian Sandstone Appalachian Plateau CA5 336–298 1089 38.4247 79.5986 Cacapon Sandstone Silurian Sandstone Valley and Ridge CA6 296–209 794 38.3582 79.5496 Tuscarora Sandstone Silurian Sandstone Valley and Ridge CA7 298–217 531 38.0405 78.8738 Chilhowee Group Cambrian Phyllite Blue Ridge CA8 186–121 111 37.9839 78.3109 Potomac terrane N.A.§ Quartzofeldspathic schist Piedmont CA9 323–244 677 37.5422 80.3538 McKenzie Formation, Clinton Group Silurian Sandstone Valley and Ridge CA10 280–248 925 38.0495 79.7949 Martinsburg Formation Ordovician Sandstone Valley and Ridge *3-4 zircon (U-Th)/He dates. †Cardwell et al. (1968) and Rader and Evans (1993). §Not applicable.

Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic provinces. and ellipsoid grain morphologies, respectively. Single-grain ZHe date error The elevation of sampling sites varied from 317 m in the Piedmont to 1291 m represents 2σ propagated analytical uncertainty. For rounded sedimentary in the Appalachian Plateau. Table 1 presents a summary of geological informa- grains yielding ZHe dates older than depositional ages, special correction tion for all samples. Samples CA1, CA2, and CA3 consist of coarse sandstones factors have been developed to account for abrasion during transport and collected from the Pennsylvanian Pottsville Group in the Appalachian Plateau deposition (Rahl et al., 2003). However, as Appalachian foreland basin strata interior, while sample CA4 is a sandstone from the Devonian Hampshire Forma have been subject to multiple episodes of basin recycling (e.g., Eriksson et al., tion in the easternmost Appalachian Plateau. Within the Valley and Ridge 2004; Park et al., 2010), it is unreasonable to pinpoint when abrasion occurred, Province, samples CA5, CA6, and CA9 are Silurian sandstones, while sample and thus we only applied standard ellipsoidal alpha-correction factors to non- Spatially variable syn- and post-Alleghanian exhumation of the CA10 is an Ordovician sandstone from the Martinsburg Formation. Sample reset rounded grains. central Appalachian Mountains from zircon (U-Th)/He CA7 from the Blue Ridge Province is phyllite of the Cambrian Chilhowee Group, thermochronology and sample CA8 from the Piedmont Province is quartzofeldspathic schist of Luke C. Basler1, Jaclyn S. Baughman1, Michelle L. Fame1, and Peter J. Haproff2 1Bowdoin College Department of Earth and Oceanographic Science, 255 Maine Street, the Laurentian-affinity Potomac terrane (Hughes et al., 2014). ■■ RESULTS Brunswick, ME 04011

2University of North Carolina, Wilmington, Department of Earth and Ocean Sciences, 601 South ZHe analyses were conducted at the University of Colorado at Boulder Ther- College Road, Wilmington, NC 28403 mochronology Research and Instrumentation Laboratory (CU TRaIL). Analyzed Single-grain ZHe data are shown in Figure 3 and reported in Table 2. Single-

SUPPLEMENTAL MATERIAL single zircon grains were isolated using standard mineral separation tech- grain ZHe dates exhibit a clear younging trend toward the east, from 455–373 Ma niques, and 3–4 suitable grains per sample were handpicked, photographed, in the westernmost Appalachian Plateau sample (CA1) to 186–121 Ma in the Figure S1: Photomicrographs of zircon grains. Figure S2: ZHe date versus aspect ratio plot. and measured under a Leica M165 binocular microscope with an attached easternmost Piedmont sample (CA8; Fig. 4). West of the Allegheny structural Figure S3: ZHe date versus elevation plot. 1 Figure S4: ZHe date versus radius plot. calibrated digital camera (Fig. S1 displays photomicrographs of analyzed front, detrital ZHe dates predate or span depositional ages, whereas east of the Figure S5: Forward model simulations of ZRDAAM zircon damage annealing. Figure S6: Forward model “inheritance envelopes” for Appalachian Plateau samples. zircon grains). Individual grains were put into a Nb packet and placed into an front, detrital ZHe dates postdate depositional ages. ZHe dates do not display Figure S7: Forward model “inheritance envelopes” for Valley and Ridge samples. −8 4 Figure S8: Inverse model results for models run without reheating constraint boxes. ASI Alphachron He extraction line under vacuum (3 × 10 torr). To extract He a clear single-grain date-aspect ratio or date-elevation relationship (Figs. S2 gas, packets were heated twice with a diode laser to 800–1100 °C. Released and S3 [see footnote 1]). 4He was purified using SAES getter methods, spiked with3 He, and measured Single-grain ZHe dates of Middle and Lower Pennsylvanian sandstones using a Balzers PrismaPlus GM6220 quadrupole mass spectrometer. After from the Appalachian Plateau Province (CA1, CA2, CA3) range from 455 to 1 Supplemental Material. Photomicrographs of zircon degassing, grains were treated with 235U and 230Th tracers and dissolved using 358 Ma, predate depositional ages (Fig. 4, blue circles), and generally feature grains; graphs of zircon (U-Th)/He date vs. aspect ratio, sample elevation, and grain radius; second-order an acid vapor HF and HCl dissolution method. U and Th were measured on negative date-eU correlations (Fig. 3, orange circles). Sample CA1 displays a forward and inverse model simulations, including an Agilent 7900 quadrupole inductively coupled plasma–mass spectrometer negative, gently sloped date-eU correlation spanning from 435 Ma at 148 ppm “inheritance envelopes”; inverse thermal model in- (ICP-MS). Orthorhombic and ellipsoidal geometries were used for alpha-ejec- eU to 373 Ma at 2012 ppm eU (Fig. 3A, orange circles), whereas sample CA3 put table. Please visit https://doi.org/10.1130/GEOS 4 .S.14522328 to access the supplemental material, and tion corrections to account for He ejected from grain edges (Ketcham et al., yields a slightly steeper negative date-eU correlation from 432 Ma at 602 ppm contact [email protected] with any questions. 2011), and for zircon mass, volume, and concentration calculations for euhedral eU to 398 Ma at 991 ppm eU (Fig. 3A, bold orange circles). Sample CA2 exhibits

500 400 400 Acadian Orogeny Taconic Orogeny Acadian Orogeny

400 Acadian Orogeny 350 300 Alleghanian Orogeny Alleghanian Orogeny 300 Alleghanian Orogeny 300 200 200 250 CA1 CA5 ZHe date (Ma) ZHe date (Ma) ZHe date (Ma) Valley Appalachian CA2 CA6 100 and Blue 100 Plateau 200 CA7 CA3 Ridge CA9 Ridge

CA4 CA10 Piedmont CA8 0 150 0 0 500 1000 1500 2000 2500 0 250 500 750 1000 1250 0 500 1000 1500 2000 2500 eU (ppm) eU (ppm) eU (ppm)

Figure 3. Single-grain zircon (U-Th)/He (ZHe) date vs. effective uranium (eU) plots for the (A) Appalachian Plateau Province, (B) Valley and Ridge Province, and (C) Blue Ridge and Piedmont Provinces. Vertical error bars represent analytical date error, and horizontal bars represent uniform 15% eU error to account for differences in observed from idealized morphologies (Guenthner et al., 2016; Baughman et al., 2017). The gray shaded regions indicate the duration of major geologic events.

a scattered date-eU correlation, although eU is bounded between 210 and ■■ DISCUSSION 385 ppm, and the date-radius plot exhibits a positive date-radius trend (Fig. 3A, cross-hatched orange circles; Fig. S4 [footnote 1]). In contrast to Pennsylvanian Controls on 4He Diffusivity and Thermal Conditions strata, a Devonian sandstone (CA4) collected from the easternmost Appala- chian Plateau, adjacent to the Allegheny structural front, features single-grain Radiation damage exerts a primary control on 4He diffusivity within our ZHe dates spanning the depositional age of the unit. Sample CA4 exhibits a ZHe samples. Negative date-eU correlations for samples CA2, CA3, CA5, CA8, steep, negative date-eU correlation from 357 Ma at 267 ppm eU to 233 Ma at and CA10 are consistent with sufficient damage accumulation as to cross the 511 ppm eU (Fig. 3A, dotted orange circles). zircon damage percolation threshold, such that increased damage causes 4 Within the Valley and Ridge Province, single-grain ZHe dates (CA5, CA6, increased He diffusivity and a lower effective TC (Reiners, 2005). Samples CA9, CA10) are Carboniferous through Triassic, postdate depositional ages, CA1, CA4, and CA7 display parabolic date-eU correlations, suggesting that feature primarily flat, negatively trending date-eU correlations, and display a accumulated damage within individual grains spans the damage percolation slight eastward younging trend in single-grain date ranges, from 336–298 Ma threshold (Guenthner et al., 2013). Regional and within-sample date-eU cor- (CA5) to 296–209 Ma (CA6; Fig. 4). Samples CA6 and CA10 exhibit flat date-eU relations will be interpreted in detail within subsequent sections. While grain

correlations between 250 and 300 Ma and 197 and 767 ppm eU (Fig. 3B). size and aspect ratio cause up to 10 °C difference in TC (Reiners et al., 2002; Sample CA6 yielded an anomalously young zircon grain date of 209 Ma at Anderson et al., 2020), these effects are likely masked by the strong control of 197 ppm eU. Photomicrographs indicate the presence of a large inclusion, radiation damage on diffusivity. Nonetheless, two samples (CA2 and CA6) lack- and this grain is therefore excluded from subsequent interpretation (Fig. S1 ing significant spread in eU exhibit positive date-radius correlations (Fig. S4), [footnote 1]). Sample CA5 exhibits slightly older Carboniferous single-grain and minor data dispersion within other samples may be explained in part by dates and a slightly positive date-radius correlation (Fig. 3B, cross-hatched effects of grain radius and morphology on 4He diffusion. blue circles; Fig. S4), while sample CA9 exhibits a negative date-eU correlation For Appalachian foreland samples, we infer that ZHe dates primarily reflect from 323 Ma at 206 ppm eU to 244 Ma at 552 ppm eU (Fig. 3B, blue circles). cooling during orogen development and subsequent exhumation. However, Blue Ridge Province sample CA7 yielded four single-grain dates that range tectonically elevated geothermal gradients and warm orogenic fluids may form from 298 to 217 Ma and exhibit a positive, then gradually negative, ZHe date-eU a supplementary control on both ZHe dates and maximum thermal indicators. correlation (Fig. 3C, red circles). To the east, Piedmont Province sample CA8 Nonetheless, no samples were collected from the Devonian interval identified yielded four single-grain dates that range from 186 to 121 Ma, feature a flat, as a region of warm orogenic fluid flow by Evans and Battles (1999), and Penn- gradually negative date-eU correlation, and display a weak positive date-radius sylvanian VR, Devonian VR, Devonian CAI, and Ordovician CAI isograd maps correlation (Fig. 3C, purple circles; Fig. S4). do not feature anomalous west-bulging salients across our transect (Ruppert

TABLE 2. ZIRCON (U-Th)/He DATA FROM THE CENTRAL APPALACHIAN MOUNTAINS Sample Geometry* Length Width Radius† Mass U ± Th ± Sm ± eU§ He ± Th/U Uncorrected ±# Ft** Corrected ± (μm) (μm) (μm) (μg) (ppm) (ppm) (ppm) (ppm) (nmol/g) date date (Ma) (Ma) CA1 R 101.8 46.1 27.1 0.5 538.0 16.5 281.8 7.2 8.0 1.5 605.2 885.3 6.1 0.52 265.1 7.3 0.57 455.4 23.5 R 74.1 47.2 26.5 0.4 1863.9 21.1 632.8 7.4 56.7 46.7 2012.7 2368.1 11.6 0.34 214.1 2.4 0.57 373.4 8.1 R 87.4 47.7 27.6 0.5 140.0 3.9 163.6 2.5 8.4 1.2 179.4 260.0 1.0 1.16 262.8 5.6 0.57 452.4 18.5

CA2 R 206.6 85.9 50.1 3.6 183.4 3.4 114.3 1.7 0.8 0.2 210.2 367.4 1.2 0.62 315.4 5.0 0.76 410.2 12.8 R 202.0 85.2 50.8 3.6 205.6 10.4 104.6 1.2 1.7 0.4 230.2 419.4 1.7 0.51 328.4 14.5 0.77 424.7 37.0 E 233.4 72.5 47.3 4.5 347.6 6.3 135.7 1.1 2.9 0.2 379.5 651.7 1.7 0.39 309.9 5.0 0.76 403.5 13.3 E 166.5 63.3 39.8 2.3 242.8 4.9 94.4 1.4 1.6 0.3 265.0 397.0 1.6 0.39 271.3 5.0 0.72 373.2 13.8

CA3 E 159.3 70.7 42.3 2.6 903.1 18.3 374.7 13.5 5.5 0.6 991.1 1621.5 5.7 0.42 295.6 5.5 0.74 398.4 14.8 R 169.9 67.8 39.9 1.9 711.2 25.3 275.5 5.2 14.3 1.5 776.0 1091.6 3.7 0.39 255.1 8.1 0.71 358.1 22.2 R 122.9 77.7 42.7 1.8 525.8 11.8 328.2 3.6 2.8 0.3 602.9 1054.5 3.1 0.62 315.6 6.1 0.72 432.5 16.4 R 162.6 85.3 49.2 2.9 827.8 11.1 268.5 2.6 4.1 0.4 890.9 1375.9 4.3 0.32 279.4 3.5 0.76 364.5 9.1

CA4 R 134.2 83.3 47.0 2.3 434.7 10.0 257.9 7.8 4.2 0.4 495.3 677.1 2.0 0.59 248.1 5.0 0.75 329.7 13.2 E 131.1 71.3 40.4 2.0 314.6 4.1 227.8 2.9 1.8 0.6 368.2 556.3 1.9 0.72 273.7 3.2 0.72 376.1 8.7 E 123.7 62.5 36.5 1.5 418.9 9.3 391.1 3.5 8.2 0.6 510.8 454.0 1.7 0.93 162.5 2.9 0.69 233.5 8.4 E 170.2 60.6 38.6 2.2 222.3 6.7 188.4 3.4 2.9 0.4 266.5 374.5 1.5 0.85 255.0 6.4 0.71 357.2 18.0

CA5 R 197.2 118.2 67.0 6.7 170.5 1.9 99.1 3.8 5.4 0.6 193.8 297.5 1.0 0.58 277.9 3.1 0.82 336.4 7.4 R 273.1 103.3 62.5 7.1 179.1 5.5 178.1 3.3 14.6 0.7 221.0 312.6 1.0 0.99 256.6 6.3 0.81 316.3 15.4 R 172.3 111.5 62.4 5.2 165.9 4.5 116.2 5.4 9.1 0.6 193.2 257.3 1.4 0.70 241.8 5.9 0.81 297.8 14.4

CA6 E 139.1 65.3 38.7 1.9 382.7 5.0 179.5 2.4 3.2 0.4 424.9 493.1 1.6 0.47 211.3 3.0 0.71 294.6 8.3 E 155.3 66.8 40.9 2.3 180.9 5.8 68.0 1.3 35.9 65.1 196.9 164.4 0.5 0.38 152.6 4.4 0.73 208.8 12.2 R 155.4 96.5 54.1 3.5 413.5 15.3 273.1 7.8 4.5 0.4 477.7 610.5 2.4 0.66 232.3 7.3 0.78 296.4 18.6 R 180.5 91.4 53.4 3.7 178.3 1.8 133.5 7.0 2.8 0.4 209.6 254.6 0.9 0.75 221.0 2.7 0.78 283.2 6.8

CA7 E 198.5 58.3 38.3 2.5 374.9 5.1 119.5 4.9 1.3 0.3 403.0 341.6 1.2 0.32 155.1 2.1 0.71 217.3 5.8 E 162.2 63.9 39.7 2.3 801.0 14.8 532.7 9.1 4.7 0.7 927.2 1036.7 4.8 0.66 203.8 3.3 0.72 282.4 9.3 R 103.8 60.6 34.5 0.9 1626.0 31.2 903.5 15.0 28.8 1.1 1838.3 1580.9 7.4 0.56 157.3 2.7 0.66 237.0 7.9 R 95.9 64.5 35.8 1.0 561.8 16.6 373.1 17.2 10.1 1.3 649.5 717.3 3.2 0.66 201.3 5.2 0.67 297.9 15.1

CA8 E 339.6 96.7 60.7 11.2 80.5 1.2 29.3 0.6 0.5 0.1 87.4 72.1 0.3 0.36 151.0 2.1 0.81 185.5 5.1 E 243.2 142.9 78.7 14.0 168.3 1.7 88.7 1.0 0.7 0.1 189.1 153.1 0.3 0.53 148.2 1.3 0.85 173.4 3.1 E 219.7 117.3 63.9 8.5 196.2 2.7 104.6 3.1 1.2 0.1 220.7 171.9 0.7 0.53 142.7 1.8 0.82 173.4 4.4 R 160.1 83.2 48.4 2.7 1302.8 20.1 482.6 18.4 7.6 0.5 1416.2 704.3 2.5 0.37 91.5 1.3 0.76 120.7 3.5

CA9 R 99.8 54.4 28.5 0.7 404.3 11.4 626.7 9.5 38.3 2.6 551.6 431.6 1.5 1.55 143.3 5.9 0.58 244.1 9.1 R 86.9 55.2 28.9 0.6 176.7 11.8 124.3 4.4 1.4 0.8 205.9 219.4 0.7 0.70 194.3 21.7 0.60 322.8 33.4 R 84.8 65.1 33.3 0.8 216.4 4.0 243.5 4.0 10.4 2.8 273.6 253.1 0.6 1.13 169.1 4.9 0.64 261.1 7.3

CA10 E 80.1 44.0 24.7 0.5 727.2 25.6 183.8 8.2 2.9 3.1 770.4 601.7 2.4 0.25 143.1 9.3 0.57 248.1 15.8 E 81.6 47.8 25.7 0.5 433.3 18.3 113.5 4.3 2.5 3.2 460.0 416.2 2.2 0.26 165.4 12.9 0.58 280.0 21.0 E 105.0 41.8 26.1 0.6 505.71 11.3 327.7 4.4 4.1 1.9 582.7 490.0 2.4 0.6 153.9 6.0 0.59 260.6 10.1 *E—euhedral, R—rounded. †Equivalent spherical radius. §Effective uranium concentration weighting Th and U for alpha productivity, where eU = [U] + 0.235 × [Th]. #2σ analytical uncertainty based on measurement of U, Th, Sm, He, and grain dimensions. **Alpha correction factor of Ketcham et al. (2011), assuming euhedral orthorhombic or ellipsoidal geometry.

600 accumulation and annealing model, however, requires shared predepositional Precambrian thermal histories. To evaluate predepositional thermal histories, we created 500 ZHe date > depositional age Cambrian forward-model inheritance “envelopes” (e.g., Guenthner et al., 2014; Powell Ordovician et al., 2016) predicting ZHe dates for variable durations of radiation damage ZHe date < depositional age Silurian accumulation across a range of geologically reasonable burial depths (Figs. S6 400 Devonian and S7 [footnote 1]). Most grains for Appalachian Plateau samples CA1–CA4 CA5 CA1 CA2 fall between the 320 and 500 Ma inheritance lines at a geologically reasonable CA3 Carboniferous 300 burial temperature of 140 °C, suggesting that grains began accumulating radi- CA10 ation damage during this interval. Similarly, ZHe dates from sample CA10 fall CA9 near the 950 Ma inheritance line at burial temperature of 190 °C. These models CA4 200 CA6 CA7 demonstrate that within-sample variability in predepositional thermal histories is limited and unlikely to significantly alter thermal history interpretations. Zircon (U-Th)/He date (Ma) 100 CA8 Second, models required a surficial temperature of 20 °C during the deposi- Blue tional range of the formation. Third, models incorporated t-T constraint boxes Appalachian Plateau Valley and Ridge Ridge Piedmont requiring postdepositional reheating. Within Pennsylvanian Appalachian Pla- 0 teau samples (CA1–CA3), where prior VR data were available, %R (Ruppert et 0 50 100 150 200 250 o al., 2010) was entered directly into HeFTy using the Basin%Ro model of Nielsen WNW Distance from A to A’ (km) ESE et al. (2017). Reheating temperature constraints were left purposefully broad

Figure 4. Plot of single-grain zircon (U-Th)/He (ZHe) dates vs. distance along line A-A′ (25–220 °C) to allow for exploration by the model. All other sedimentary samples (Fig. 2). Sample locations are projected along strike to the transect, and dates are were from stratigraphic intervals without existing VR data, and temperature color-coded by depositional age, corresponding to colored geologic period boxes. constraints of reheating were constructed using VR and CAI data from strata Vertical bars delineate Appalachian physiographic provinces across the transect. bounding the sampled rock unit, described in detail in Table S1 (footnote 1). Prior work constraining the timing of maximum thermal conditions is limited, and we applied a broad early to middle Permian (299–260 Ma) temporal con- et al., 2010; Repetski et al., 2014), which have been interpreted as regions of straint within the Valley and Ridge Province. The 299 Ma bound is specified orogenic fluid flow in the Anthracite Basin of southwest Pennsylvanian and by VR, CAI, and micro-inclusion thermometry data (Epstein et al., 1977; Evans, elsewhere in the foreland (Zhang and Davis, 1993; Evans and Battles, 1999). 2010; Repetski et al., 2014) that indicate the magnitude of burial exceeded the reconstructed stratigraphic thickness through the Carboniferous. The 260 Ma bound is conservatively drawn from late Permian synfolding paleomagnetic Thermal Modeling data suggesting that folding, which likely postdated maximum reheating temperatures, occurred in the range of ca. 280–260 Ma (Lewchuk et al., 2002; Cox Inverse thermal modeling of ZHe data was performed using the ZRDAAM et al., 2005; Elmore et al., 2006). Within the Appalachian Plateau Province, we (Guenthner et al., 2013) implemented in HeFTy v1.9.3 (Ketcham, 2005). Figure 5 required Permian maximum thermal conditions consistent with a burial model displays inverse thermal model results for all samples. A comprehensive descrip- by Reed et al. (2005) and the presence of likely Permian rocks within western tion of our modeling approach and sources of external data is detailed in Table S1 Maryland (Brezinski and Conkwright, 2013). As a comparison to t-T paths found (footnote 1). We applied four types of thermal constraints in which t-T paths were using the above reheating constraints, Figure S8 displays thermal history out- forced to take into account zircon U-Pb crystallization dates, depositional ages, puts for models with significantly expanded reheating constraints. reheating constraints, and prior low-temperature thermochronology. Finally, we required cooling through boxes representing the upper bound First, models began at a temperature of 400 °C at 1250–950 Ma, a temporal of the AFT or AHe partial annealing and partial retention zones (120 °C and range coeval with the Grenville orogeny and corresponding to a significant 80 °C), respectively, when existing dates were collocated with our ZHe samples peak in zircon U-Pb date probability distributions within sampled formations (Roden, 1991; Reed et al., 2005). A lack of published technical data precludes (Reed et al., 2005; Park et al., 2010). While previously reported U-Pb dates for integrating these samples directly into HeFTy. All models ended at surface Silurian and Cambrian samples fall exclusively (>99%) within this Grenville thermal conditions of 20 °C. time span, Middle and Lower Pennsylvanian, Upper Devonian, and Ordovician Four known concerns with the ZRDAAM are relevant to the interpretation samples (CA1–CA4 and CA10) harbor a significant population (~10%–30%) of of our thermal model simulations. First, ZRDAAM does not account for aniso- Paleozoic or pre-Grenville zircon grains, suggesting grain provenance from tropic 4He diffusion in zircon, although this factor exerts a secondary control on 4 multiple source terranes. Thermal modeling based on a radiation damage He diffusivity corresponding to ~10 °C differences in TC (Anderson et al., 2020),

Reheating box Existing AHe constraint Time (Ma) Time (Ma) 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 0 500 0 350 CA1: Appalachian Plateau CA6: Valley and Ridge 1I 1I 400 300 100 100 Pennsylvanian deposition 300 250 Model begins at zircon 200 200 200 200 U-Pb date range Grain omitted ZHe date (Ma) Temp. (°C) ZHe Date (Ma) Temp. (°C) 100 from model due 150 300 Sample currently 300 to inclusion at surface 0 0 0 500 1000 1500 2000 2500 0 300 600 900 400 eU (ppm) 400 eU (ppm) 0 500 0 350 CA2: Appalachian Plateau CA9: Valley 1I 400 and Ridge 1I 300 100 100 300 250 200 200 200 200 100 ZHe date (Ma) Temp. (°C) Temp. (°C) 150 300 0 300 ZHe Date (Ma) 0 300 600 900 1200 0 eU (ppm) 0 300 600 900 400 400 eU (ppm) 0 500 0 350 CA3: Appalachian Plateau CA10: Valley 400 and Ridge 300 100 100 300 250 200 200 200 200

Temp. (°C) 100 Temp. (°C) ZHe date (Ma) ZHe Date (Ma) 150 300 300 0 0 0 300 600 900 1200 0 300 600 900 400 eU (ppm) 400 eU (ppm) 0 500 0 500 CA4: Appalachian Plateau 400 1I 400 100 100 300 300

200 200 200 200 100 Temp. (°C) ZHe date (Ma) Temp. (°C) 100 ZHe Date (Ma) 300 300 0 0 0 300 600 900 1200 CA7: Blue Ridge 0 500 1000 1500 2000 2500 eU (ppm) 400 400 eU (ppm) 0 350 1200 1100 1000 900 800 700 600 500 400 0 500 CA5: Valley and Ridge 1I 300 Time (Ma) 400 100 100 250 300 200 200 200 200 Temp. (°C) Temp. (°C)

ZHe Date (Ma) 100 150 ZHe Date (Ma) 300 300 0 CA8: 0 0 300 600 900 Piedmont 0 500 1000 1500 2000 2500 400 eU (ppm) 400 eU (ppm) 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 300 200 100 0 Time (Ma) Time (Ma)

Figure 5. Inverse thermal history model outputs for 10 zircon (U-Th)/He (ZHe) samples using the zircon radiation damage accumulation and annealing model (Guenthner et al., 2013) implemented in HeFTy (Ketcham, 2005). Green lines indicate acceptable-fit time-temperature paths, purples lines represent good-fit paths, and the black line denotes the best-fit path. Thermal history paths are forced through blue boxes representing U-Pb crystallization dates, depositional ages, postdepositional reheating constraints, existing thermochronologic dates, and modern surficial temperatures. Date-effective uranium (eU) plots compare single-grain ZHe dates with the date-eU correlation predicted by the corresponding best-fit thermal history path. A full description of inverse model input parameters and sources of data is detailed in Table S1 (text footnote 1). AHe—apatite (U-Th)/He.

and we did not observe a strong correlation between date and aspect ratio Prot. Camb.Ord. Sil. Dev. Carb. Perm. Tri. Jur. Cret. Cen. within our data set (Fig. S2 [footnote 1]). Second, ZRDAAM likely overestimates 0 Valley Ridge Eastern App. Plateau the diffusivity of high-damage, high-eU grains, resulting in inverse t-T paths Deposition that may overestimate cooling or underestimate the temperature required Central App. ? 50 Plateau to reset ZHe dates (Johnson et al., 2017). We were able to fitt -T paths to our AHe highest eU grains and carefully interpreted model outputs. Third, ZRDAAM Blue Ridge 100 underestimates the amount of radiation damage required to cross the damage AFT percolation threshold (Gautheron et al., 2020), thereby underestimating 4He diffusivity and returning thermal history paths that are hotter and/or heated 150 1%R for a longer duration than geologically required. ZHe Temp. o Piedmont Sensitivity Fourth, ZRDAAM anneals radiation damage at temperatures lower than 2%R 200 o that reported by studies of experimental damage annealing (Ginster et al., 3%Ro

2019). To evaluate the impact of excessive damage annealing, we performed (°C) Temperature 4%Ro forward modeling comparing the radiation damage accumulated by a reheated 250 zircon to that of a grain continually accumulating radiation damage at the surface (Fig. S5). The difference in alpha dose between these two thermal 300 histories represents the amount of annealing predicted by ZRDAAM for the 600 500 400 300 200 100 0 reheated grain. Within the Appalachian Plateau Province, geologically reason- Time (Ma) able reheating at temperatures below 170 °C caused <5% difference in alpha Figure 6. Composite time-temperature paths of five selected regions dose, suggesting that excessive damage annealing has a negligible effect on including the Pennsylvanian Appalachian Plateau (yellow), eastern De- the inverse thermal models (Fig. S5, blue paths). For the Valley and Ridge vonian Appalachian Plateau (green), central Valley and Ridge (blue), Blue and Blue Ridge sample paths, reheating to 250 °C caused a 25% difference in Ridge (red), and Piedmont (purple). Shaded boxes represent cooling through prior thermochronologic dates (Roden, 1991). Horizontal arrows alpha dose when compared to a surficially held sample (Fig. S5, green paths). denote approximate vitrinite reflectance (%Ro) at selected temperatures. An unknown portion of this 25% likely represents “true” annealing; however, The recent (<100 Ma) thermal history of each region is beyond the reso- any excessive annealing would reinforce our primary interpretation of fast lution of this figure and is omitted. AHe—apatite (U-Th)/He; ZHe—zircon Alleghanian exhumation. If grains in fact possess additional radiation dam- (U-Th)/He; AFT—apatite fission track. Time periods: Prot—Proterozoic; Camb—Cambrian; Ord—Ordovician; Sil—Silurian; Dev—Devonian; age, more rapid cooling through the ZHe PRZ would be required to yield the Carb—Carboniferous; Perm—Permian; Tri—Triassic; Jur—Jurassic; Cret— observed flat date-eU correlations. Cretaceous; Cen—Cenozoic.

Regional Interpretations front, predate Pennsylvanian depositional ages (Fig. 4, blue circles), indicating that maximum postdepositional temperatures did not fully reset ZHe dates. Within each physiographic province, we first interpret our data set using This constraint suggests that postdepositional reheating did not significantly basic depositional constraints and zircon (U-Th)/He data systematics, including exceed 160 °C, the lower range of the ZHe PRZ, a result in good agreement an analysis of date-eU correlations. Next, we obtain inverse model thermal with prior estimates of maximum thermal conditions using VR and micro-in- model results, presented in Figures 5 and 6, to corroborate first-order inter- clusion thermometry (Reed et al., 2005; Evans, 2010). This constraint imposes pretations and further constrain the timing and rates of burial and exhumation. a ceiling on the magnitude of post-Pennsylvanian burial in the Appalachian Then, we summarize the along-transect description of trends in burial, Allegha- Plateau originating from Alleghanian clastic input (Fig. 8C), and it likewise nian exhumation rates, and rift-flank exhumation. The following discussion limits the magnitude of unroofing, as significant postburial exhumation would is guided by a spatial representation of exhumation patterns in Figure 7 and produce surficial exposures of rocks yielding ZHe dates fully reset by burial sequential cross sections in Figure 8. temperatures. A weak younging trend in single-grain ZHe dates toward the hinterland (samples CA1 to CA3) may indicate an eastward-increasing magnitude of partial resetting consistent with eastward-increasing post-Pennsylvanian Appalachian Plateau burial depths and associated maximum thermal conditions. Zircons within the Pennsylvanian Appalachian Plateau likely experienced All single-grain ZHe dates (11 grains) of the three sandstone samples from rapid predepositional Taconic or Acadian cooling. This is shown by flat to the Appalachian Plateau (CA1, CA2, CA3), west of the Allegheny structural negative date-eU correlations for samples CA1, CA2, and CA3, indicating that

80°30’W 79°30’W 78°30’W

152 78 0.94 155 51 0.67

168 31 0.42 156 39 0.53

199 103 1.33 38°30’N

216 135 1.51 Figure 7. Elevation map visualizing selected inverse-model thermal information for sedimentary samples CA1–CA7, CA9, and CA10. For each 212 137 1.69 252 164 1.74 zircon (U-Th)/He date, the leftmost circle (red) is Sample CA8 colored to denote maximum postdepositional (crystalline) temperatures within the average good-fit inverse model time-temperature (t-T) path. The

38°00’N Valley and Ridge Appalachian Plateau center (green) and right-most (blue) circles Province Blue Ridge Piedmont Province Province Province indicate the magnitude (°C) and average rate (°C/m.y.) of cooling between maximum post- Elevation (m) depositional thermal conditions and 200 Ma in 1482 40 Kilometers average good-fit inverse modelt -T paths. 29 201 90 1.01 KEY Within the average good-fit inverse model t-T path: Max. burial Magnitude of Average cooling rate Max. Magnitude Average temp (°C) cooling by between max. temp 37°30’N burial of cooling cooling 200 Ma (°C) and 200 Ma (°C/Ma) temp rate 130 30 0.3 180 75 0.8 214 96 1.31 230 120 1.3 280 165 1.8

samples cooled rapidly through disparate temperature sensitivities of individual and a subsequent span in single-grain temperature sensitivities correlated to zircon grains spanning the ZHe PRZ. Dates are broadly similar to early Paleozoic eU. Compared to ZHe dates not reset within Pennsylvanian rock units further ZFT dates reported by Naeser et al. (2016), suggesting predepositional cooling west, partial resetting of this Devonian sandstone requires greater maximum

from temperatures exceeding ~240 °C, which is the approximate effective TC of thermal conditions. This was likely caused by a lower stratigraphic position, the ZFT thermochronometer (Yamada et al., 1995). Our single-grain ZHe dates roughly 1 km below Pennsylvanian Pottsville Formation samples (CA1–CA3), are generally coeval with the timing of the Taconic and Acadian orogenies, as well as a greater depth of post-Pennsylvanian burial toward the east, con- suggesting that the Alleghanian orogeny drove erosion of Acadian or Taconic sistent with a west-tapering sedimentary wedge in the Appalachian Plateau clastic wedges, which likely contained syntectonically cooled zircons, causing during adjacent Valley and Ridge deformation (Fig. 8C; Ruppert et al., 2010). the westward transport and redeposition of these grains within Pennsylvanian The present exposure of this Devonian sample previously subject to deeper strata. Alternatively, these predepositional cooling signatures could reflect the burial depths also suggests a greater magnitude of postdepositional exhu- Alleghanian exhumation and erosion of Acadian and Taconic metamorphic core mation toward the east. This partially reset ZHe sample cannot independently terranes that had previously cooled past the ZHe effective closure temperature. document the timing of this exhumation, however, because the date reflects Single-grain ZHe dates for sample CA4 (four grains; 376–233 Ma), an Upper the time-integrated sum of partially diffused predepositional relict helium and Devonian sandstone from the easternmost Appalachian Plateau Province, span postdepositional accumulated helium. the depositional age, indicating partial resetting by postdepositional reheat- Good-fit HeFTy inverse thermal model paths for Pennsylvanian Appalachian ing. Partial resetting is consistent with a steep date-eU correlation (Fig. 3A), a Plateau ZHe dates integrated with prior regional VR data (Fig. 5; CA1, CA2, result of the differential accumulation of radiation damage prior to reheating CA3) exhibit fast predepositional cooling and require a reheating temperature

? CA1 CA2 CA4 CA4 CA5 CA6 BRT 0 2 4 Late Carboniferous 6 8 Depth (km) 10

CA7 ? NMT BRT 0 2 Early - Mid Permian 4 6 8 Depth (km) 10

NMT BRT 0 2 ASF Mid - Late Permian 4 6 8 Depth (km) 10

NMT EXPLANATION ASF BRT CA8 0 Ordovician through Permian 2 sedimentary rocks Present Day 4 6 Cambrian through Ordovician 8 carbonates Depth (km) 10 Early Cambrian sedimentary rocks through Precambrian basement

20 km Zircon (U-Th)/He sample location and PRZ

Figure 8. Sequential cross sections along line A-A′ (Wilson and Shumaker, 1985; Evans, 1989, 2010; Rowan, 2006; Lammie et al., 2020). Red circles denote locations of zircon (U-Th)/He samples, while the shaded red box represents the zircon (U-Th)/He partial helium retention zone (PRZ), assuming a consistent geothermal gradient of 25 °C/km and surface temperature of 20 °C. ASF—Allegheny structural front; BRT—Blue Ridge thrust; NMT—North Mountain thrust.

not exceeding ~160 °C, corroborating the above interpretations derived from Valley and Ridge date-eU correlations and depositional constraints. Good-fit model t-T paths display a short cooling pulse immediately following maximum postdeposi- All single-grain ZHe dates (13 grains) for Silurian and Ordovician strata in tional temperatures, a feature that likely satisfies maximum thermal conditions the Valley and Ridge Province postdate depositional ages (Fig. 4; CA5, CA6, required by VR data without a prolonged duration of heating that would reset CA9, CA10), suggesting postdepositional reheating in excess of 190–200 °C, ZHe dates. This pulse of cooling could represent a small quantity of exhuma- which is the upper range of the ZHe PRZ. Compared to non-reset ZHe dates tion toward the end of the Alleghanian orogeny and is in agreement with a in Appalachian Plateau Pennsylvanian strata, complete resetting of Valley and thermal reconstruction by Evans (1995). However, the cooling signal within Ridge samples resulted from structurally lower positions below the Devonian our models is fairly weak and could alternatively have been caused by known Acadian-related clastic wedge, as well as greater quantities of postdeposi- flaws of the ZRDAAM. tional overburden during overthrusting and/or clastic input related to the North HeFTy inverse t-T paths for Devonian sample CA4 show isothermal holding Mountain thrust sheet (Fig. 8B). Moreover, compared to non-reset ZHe dates of at temperatures within the ZHe PRZ (Fig. 5), consistent with a prolonged dura- Pennsylvanian strata in the Appalachian Plateau Province, the modern expo- tion of partial resetting at thermal conditions hotter than those experienced sure of Ordovician and Silurian strata previously subjected to greater burial by Pennsylvanian samples further west. suggests a greater magnitude of postdepositional exhumation toward the east.

Single-grain ZHe dates coeval with Alleghanian orogenic growth, in con- the initiation of rifting (Fig. 6). Stable thermal conditions during this interval cert with flat date-eU correlations (Fig. 3B), suggest rapid syn-Alleghanian argue against significant rift-associated cooling or reheating within the Valley cooling of the Valley and Ridge Province. Flat date-eU correlations developed and Ridge Province, and indicate that the decay of Alleghanian topography as grains quickly cooled through the ZHe PRZ, yielding a similar date across within the Valley and Ridge may have largely ceased by the end of the Permian. a span in single-grain temperature sensitivities. No clear trend exists among ZHe dates along strike, across strike, and at deeper stratigraphic levels (Fig. 2). Collectively, these cooling signatures indicate rapid Alleghanian exhumation Blue Ridge of the Valley and Ridge coincident with the west-trending development of surficial folds and underlying duplexes during the middle Permian (Fig. 8). Sample CA7 from the Cambrian Chilhowee Group of the western Blue Ridge These data suggest maximum thermal conditions were achieved in the early Province yielded reset single-grain ZHe dates that are generally contempo- to middle Permian, and that sedimentation and/or thrust loading of the Val- raneous with the late stages of the Alleghanian orogeny. Similar to foreland ley and Ridge ceased by the middle to late Permian and was supplanted by basin samples from the Valley and Ridge Province, the fully reset grains from rapid erosion of presumably elevated terrain (Slingerland and Furlong, 1989). the Blue Ridge capture a synorogenic exhumation signature as foreland basin This result is consistent with syndeformational paleomagnetic fold data sug- and underlying passive-margin sedimentary rocks were uplifted and exhumed. gesting middle to late Permian folding within the central Appalachian Valley A parabolic to negatively trending date-eU correlation, maintained up to eU and Ridge Province (Stamatakos et al., 1996; Cox et al., 2005; Elmore et al., values of 1838 ppm, with presumably high damage (Figs. 3C and 5), suggests 2006). Single-grain dates from Silurian sandstone sample CA6 are notably significant, rapid cooling through the ZHe PRZ during the late Permian. Struc- older (336–298 Ma) than prior estimates for the initiation of cooling within tural reconstructions of the central Appalachians indicate that the activation the western Valley and Ridge Province, suggesting that it may retain partial of the Blue Ridge thrust likely preceded the activation of the North Mountain quantities of predepositional helium. thrust and other thrusts to the west (Fig. 8A), suggesting that cooling may HeFTy inverse thermal model paths for Valley and Ridge samples are con- have initiated in the Blue Ridge prior to that in the Valley and Ridge (Evans, sistent with the above interpretations and further refine the timing, duration, 1989). However, we did not observe any significant difference between Valley and magnitude of the Alleghanian cooling signal (Figs. 5 and 6, blue paths). and Ridge and Blue Ridge ZHe dates, indicating at minimum that both were Good-fit paths for samples CA6, CA9, and CA10 require cooling to have initiated rapidly exhuming in the late Permian. However, it is possible that, within the by 270 Ma, a constraint that overlaps with the syndeformation paleomag- Blue Ridge Province, an earlier cooling signal exists at temperatures exceeding netic fold test data within the western Valley and Ridge spanning from 280 to the upper bound of the ZHe PRZ (~200 °C). 260 Ma. Together, these data suggest that the Wills Mountain anticline and Good-fit HeFTy inverse model paths for Blue Ridge sample CA7 feature a underlying duplex in the region likely formed by 280–270 Ma and were actively rapid cooling pulse during the late Permian (Fig. 5). Good-fit t-T paths do not exhuming by 270 Ma, coincident with regional west-directed shortening and precisely constrain the timing of cooling initiation, but they do require cooling formation of the Long Ridge anticline to the east (Fig. 8C; Evans, 2010). The below 125 °C by 200 Ma. However, the ZRDAAM tends to overestimate the elevated topography of the Wills Mountain anticline and adjacent structures diffusivity of high-damage zircons, which may force t-T paths to overestimate shed sediment westward on the Appalachian Plateau, suggesting maximum the magnitude of cooling within this sample. burial of the plateau occurred after 270 Ma. Moreover, folds in the easternmost Appalachian Plateau, including the Elkins Valley anticline to the west, also likely formed after 270 Ma as a result of overburden-induced increases in pore-fluid Piedmont pressure (Evans, 2010). Sample CA9, located 100 km southwest of the study transect within the St. Clair thrust sheet, displays a steeper date-eU correlation A quartzofeldspathic schist from the interior of the Piedmont Province consistent with slower rates of cooling. This may reflect along-strike spatial yielded single-grain ZHe dates ranging from 186 to 121 Ma, i.e., significantly variability in Alleghanian exhumation rates, or, alternatively, it could be due younger than single-grain dates of 298–217 Ma from the Blue Ridge Province to zircon grains that were only partially reset by burial temperatures. sampled ~60 km to the west. A flat, slightly negative date-eU correlation (Fig. 3C, Within Valley and Ridge samples CA6, CA9, and CA10, all good-fit model purple circles) suggests rapid Jurassic cooling after the initiation of rifting, paths require cooling to at least 140 °C by 250 Ma, and best-fit model paths tend which is further shown within the inverse thermal model t-T paths (Fig. 5). This to display even greater amounts of cooling by the end of the Permian (Fig. 5). cooling may have been caused by two linked mechanisms. First, rift-induced

Together with Cretaceous AFT (TC = ~80–120 °C) dates in the Valley and Ridge lithospheric thinning and asthenospheric edge convection may have driven Province and easternmost Appalachian Plateau Province (Roden, 1991), our ther- rift-flank uplift and subsequent exhumation inboard of the rift margin during mal models suggest an ~100 m.y. period of relatively stable thermal conditions and immediately after rifting (ca. 200–150 Ma), as illustrated by the presence of from the end of the Permian to the beginning of the Cretaceous, coeval with “great escarpments” along rift shoulders (Steckler, 1985; Buck, 1986; Spotila et al.,

2004). Second, the relaxation of an elevated geothermal gradient, estimated at in the east (Fig. 6). Within the Blue Ridge and Valley and Ridge Provinces, ZHe 55–60 °C/km during rifting further north in the Newark Basin (Kohn et al., 1993), data constrain rapid Permian cooling initiating before 270 Ma, and cooling to may also have contributed to the cooling signal. Rapid rift-induced cooling is at least 140 °C by 250 Ma from maximum temperatures exceeding 190 °C. We supported by fully reset Jurassic ZFT and titanite FT dates within Mesozoic rift interpret this rapid cooling pulse to represent rapid syntectonic exhumation basins along the length of the North American rift margin (Kohn et al., 1993; of the actively deforming fold-and-thrust belt. Steckler et al., 1993). These data also correlate to a pulse of postrift offshore By contrast, Pennsylvanian Appalachian Plateau samples located west of sedimentation in the Baltimore Canyon Trough (Pazzaglia and Brandon, 1996) the Allegheny structural front exhibit ZHe dates that limit maximum thermal as well as an accelerated postrift cooling signal observed in low-temperature conditions during burial to less than 160 °C, which likely occurred after 270 Ma AFT data sets (Roden, 1991; Roden et al., 1993; Shorten and Fitzgerald, 2021). (Fig. 8C). When compared to early to mid-Mesozoic AFT dates (246–171 Ma; Roden, 1991) within the region, this constraint both limits the total quantity of Alleghanian exhumation and suggests moderate-to-slow postorogenic rates Orogen-Perpendicular Burial and Exhumation Trends of cooling. Our inverse thermal models did identify a potential Alleghanian cooling pulse within Pennsylvanian samples (CA1, CA2, and CA3), which may Burial Trends within the Appalachian Foreland represent limited synorogenic exhumation within the Appalachian Plateau. However, this feature is poorly constrained and may alternatively be an artifact The eastward increase in maximum burial temperatures, from <160 °C in of the modeling approach. the Pennsylvanian Appalachian Plateau to 150–180 °C in the eastern Devo- Using average good-fitt -T paths, spatial patterns of maximum postdepo- nian Appalachian Plateau and to >190 °C in the Valley and Ridge and Blue sitional thermal conditions, average Alleghanian cooling rates, and the total Ridge Provinces (Fig. 7), is broadly consistent with prior maximum thermal magnitude of Alleghanian cooling are visualized in Figure 7. Consistent with estimates (e.g., Epstein et al., 1977; Reed et al., 2005; Evans, 2010; Ruppert above interpretations, this map indicates that the magnitude and average et al., 2010; East et al., 2012). Our model-output maximum postdepositional rates of Alleghanian cooling increase toward the east, which thus indicates temperatures within the Pennsylvanian Appalachian Plateau, however, are surficial exposures of older, more thermally mature rocks in the eastern Valley 15–25 °C hotter than previous estimates. This discrepancy is likely a function and Ridge Province.

of the basin%Ro model of Nielsen et al. (2017), which at low VR values yields

warmer maximum thermal estimates than those derived from the Easy%Ro model of Sweeney and Burnham (1990). Postorogenic Rift-Shoulder Exhumation Spatial and temporal trends in maximum thermal conditions reflect differential burial based on stratigraphic level, proximity to the orogenic front, Piedmont Province sample CA8 records a rapid postrift cooling event at ca. and structural history, presented visually in Figure 8. Within the Valley and 150 Ma, which we attribute to rift-flank exhumation and the postrift relaxation Ridge Province, maximum thermal conditions may have been regulated by the of the geothermal gradient. Acceptable-fit thermal model paths constrain early to mid-Permian emplacement of the North Mountain thrust sheet, which cooling rates of 2.3–3.4 °C/m.y. between 200 and 120 Ma, which are in good either thickened the section that includes footwall rocks of the easternmost agreement with previously reported zircon and titanite fission-track studies Valley and Ridge Province or served as the source of sediments shed to the of the Piedmont Province (Kohn et al., 1993; Kunk et al., 2005) and correlate west (Fig. 8B; Evans, 2010). Footwall samples closer to the ramp of the North to a documented pulse of offshore sedimentation (Poag and Sevon, 1989; Mountain thrust or at lower stratigraphic levels in the footwall consequently Pazzaglia and Brandon, 1996). achieved hotter maximum thermal conditions. In the Appalachian Plateau By contrast, our ZHe dates within the Appalachian Plateau, Valley and Ridge, Province, maximum thermal conditions display a similar westward-decreasing and Blue Ridge Provinces, interpreted in the context of prior thermochronol- trend consistent with a west-tapering sedimentary wedge. However, maximum ogy, indicate stable synrift thermal conditions. Thermal models for samples thermal conditions in the Appalachian Plateau were likely achieved in the late CA7 and CA9 within the Blue Ridge and Valley and Ridge Provinces (Fig. 5), Permian, after the Valley and Ridge Province, and coeval with the erosion respectively, both require cooling beneath 140 °C by 250 Ma, a temperature and westward deposition of material from the Alleghany highlands (Fig. 8C). just above the ~120 °C effective closure temperature of the AFT thermochronometer. Within the Valley and Ridge Province, AFT dates are generally 150 Ma or younger (Roden, 1991), suggesting that averaged Triassic cooling rates Alleghanian Exhumation in the Foreland in the foreland were less than 0.25 °C/m.y. Remarkably stable postorogenic, synrift thermal conditions indicate that rift-induced cooling was concentrated ZHe data interpreted in the context of prior thermochronology provide evi- outboard of the foreland within Piedmont terranes. This eastward decrease dence that the magnitude and rates of Alleghanian exhumation were greatest in syn- and postrift exhumation is further supported by Mesozoic, fully reset

AFT dates within the Appalachian Plateau and Valley and Ridge Provinces that (1) Within the Appalachian Plateau Province, detrital ZHe dates were not young toward the hinterland (Roden, 1991). Reed et al. (2005) also detailed a reset by the latest episode of burial and instead record rapid predepo- similarly slow postorogenic early Mesozoic cooling rate of 0.5 °C/m.y further sitional cooling coeval with the Taconic and Acadian orogenies, thereby basinward within the Appalachian Plateau Province. suggesting provenance from either pre-Carboniferous foreland basin Blue Ridge sample CA7, yielding single-grain dates of 298–217 Ma, is dis- strata or exhumed pre-Alleghanian terranes. tanced only ~60 km from sample CA8, which yielded single-grain dates of (2) Within the Valley and Ridge and Blue Ridge Provinces, fully reset ZHe 186–121 Ma. This difference in cooling dates within the Blue Ridge Province dates are contemporaneous with the Alleghanian orogeny and display was previously noted in a ZFT study by Naeser et al. (2016), who exploited it flat date-eU correlations. This result is consistent with rapid Alleghanian as a tracer of provenance for coastal plain sediments. Our more detailed ther- exhumation initiating prior to 270 Ma, the magnitude and rates of which mal models suggest that Jurassic extension accommodated by lower-crustal increase toward the east. thinning and upper-crustal normal faulting resulted in localized exhumation in (3) Within the Valley and Ridge Province, existing AFT dates are generally the western portion of the Piedmont Province. Although speculative, several >100 m.y. younger than our ZHe dates, reflecting a prolonged period of rel- upper-crustal structures located along the eastern margin of the Blue Ridge atively stable post-Alleghanian thermal conditions from ca. 250 to 145 Ma. and within the Piedmont Province may have accommodated Triassic–Juras- (4) Within the Piedmont Province, single-grain ZHe dates range from 186 to sic rifting and exhumation of sample CA8. Such structures could include 121 Ma and exhibit a flat date-eU correlation, suggesting rapid cooling the Mountain Run fault zone along the western margin of the Blue Ridge during Mesozoic rifting, which was likely driven by rift-flank uplift and Province, or the Lakeside-Spotsylvania fault zone and the Brookneal fault lessening of the geothermal gradient. zone–Shores mélange zone within the Piedmont Province (Gates et al., 1986; (5) There is an ~100 m.y. difference in ZHe dates between samples col- Gates, 1987; Evans and Milici, 1994; Bailey et al., 2004). Detailed field-based lected from the adjacent Blue Ridge and Piedmont Provinces (~60 km structural studies along these candidate structures and higher-resolution ZHe horizontal distance). This result is consistent with rift-induced uplift of data across the Blue Ridge and Piedmont Provinces are needed to evaluate the Piedmont accommodated along existing structural discontinuities these hypotheses. between the two provinces. The spatial patterns of exhumation within our ZHe data set conform to a (6) Detrital ZHe dates that are not reset or are partially or fully reset by tectonic model of orogen development and subsequent rifting (Fig. 8). Within burial document pre-, syn-, and postorogenic exhumation signals and the Appalachian fold-and-thrust belt, rates and total quantities of exhumation demonstrate the ability of ZHe dating to reconstruct higher-temperature increase toward the hinterland (Fig. 6), where the magnitude and duration of thermal histories of fold-and-thrust development within orogens that deformation and crustal thickening were greatest. Rapid syntectonic rates of experience a significant duration and magnitude of postorogenic decay. exhumation are collocated with orogen-scale Alleghanian surface folds and underlying thrust duplexes, suggesting that significant paleotopography and relief were maintained in the deformed foreland (Fig. 8C). As topography ACKNOWLEDGMENTS decayed following the cessation of convergent tectonism, a protracted period We thank Mark Evans, Ryan McKeon, and an anonymous reviewer for valuable comments that improved the manuscript. We also thank Patrick Finnerty for assistance with collecting samples, of relatively stable thermal conditions existed within the foreland. The onset Shona Ortiz for help with sample preparation, and James Metcalf at the University of Colorado of Atlantic rifting drove cooling along the outer rift margin, likely as a result Boulder Thermochronology Research and Instrumentation Laboratory (CU TRaIL) for laboratory of rift-flank uplift and exhumation, and the eventual relaxation of an elevated training and assistance. This work was funded by a Bowdoin Faculty Development Committee geothermal gradient. However, relatively stable thermal conditions persisted (FDC) award to J.S. Baughman, and a Bowdoin Hughes Family Research Fellowship and Grua/ O’Connell Research Award to L.C. Basler. Samples were collected from the ancestral homelands within the foreland during Atlantic rifting to the east, suggesting that the ther- of the Calicuas and Manahoac peoples. mal imprint of rifting did not penetrate significantly into the foreland.

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