GeoScienceWorld Lithosphere Volume 2020, Article ID 8881315, 25 pages https://doi.org/10.2113/2020/8881315

Research Article Zircon (U-Th)/He Thermochronologic Constraints on the Long-Term Thermal Evolution of Southern and Western Texas

1 1 1 2 Nathan Z. Reade, Julian M. Biddle, Jason W. Ricketts , and Jeffrey M. Amato

1Department of Geological Sciences, The University of Texas at El Paso, 500 West University Ave., El Paso, Texas 79968, USA 2Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA

Correspondence should be addressed to Jason W. Ricketts; [email protected]

Received 20 November 2019; Revised 20 February 2020; Accepted 2 July 2020; Published 1 September 2020

Academic Editor: Sarah Roeske

Copyright © 2020 Nathan Z. Reade et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0).

Zircon (U-Th)/He (ZHe) dates are presented from eight samples (n = 55) collected from three ranges including the Carrizo and Franklin Mountains in western Texas and the in southern New Mexico. ZHe dates from Proterozoic crystalline rocks range from 6 to 731 Ma in the Carrizo Mountains, 19 to 401 Ma in the Franklin Mountains, and 63 to 446 Ma in the Cookes Range, and there is a negative correlation with eU values. These locations have experienced a complex tectonic history involving multiple periods of uplift and reburial, and we use a combination of forward and inverse modeling approaches to constrain plausible thermal histories. Our final inverse models span hundreds of millions of years and multiple tectonic events and lead to the following conclusions: (1) Proterozoic exhumation occurred from 800 to 500 Ma, coinciding with the break-up of Rodinia; (2) elevated temperatures at approximately 100 Ma occurred during final development of the Bisbee basin and are a likely result of elevated heat flow in the upper crust during continental rifting; (3) a pulse of cooling associated with Laramide shortening is observed from 70 to 45 Ma in the Cooks Range and 80 to 50 Ma in the Franklin Mountains, whereas the Carrizo Mountains were largely unaffected by this event; and (4) final cooling to near-surface temperatures began 30–25 Ma at all three locations and was likely a result of extension. These data help to bridge the gap between higher and lower temperature isotopic systems to constrain complex thermal histories in tectonically mature regions.

1. Introduction respectively [3, 4]. Multidiffusion domain (MDD) analysis of K-feldspar using the 40Ar/39Ar system [5] could fill in Thermochronology is a powerful tool to constrain the ages the temperature gap between biotite 40Ar/39Ar and apatite and durations of past geologic events because exhumation (U-Th)/He, although it has been shown to be problematic leads to cooling, the timing of which is recorded by different in some instances (e.g., [6–8]), whereas MDD analysis of isotopic systems. Cooling ages are interpreted in context of muscovite shows promise [9, 10]. Zircon fission-track, sensi- closure temperatures of different minerals (e.g., [1]) in which tive to temperatures of ~270–210°C [11], is a widely used diffusion of radiogenic daughter isotopes slows significantly thermochronometer to partially fill this temperature range, below a known temperature range. However, there can be a although there can still be a gap in the thermochronologic gap in the dates obtained from high-temperature thermo- history between 40Ar/39Ar methods and apatite (U-Th)/He chronologic systems such as titanite U-Pb, hornblende because of variations in closure temperature related to 40Ar/39Ar, and mica 40Ar/39Ar, which record the timing of cooling rate. cooling from ~600 to 300°C [2], versus low-temperature sys- Limitations in deciphering thermochronologic histories tems such as apatite fission-track and (U-Th)/He, which related to this gap may be overcome by the work of record the timing of cooling below 120–60°C and 90–30°C, Guenthner et al. [12], who describe a helium diffusion

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model that incorporates an important relationship between New Mexico and the Franklin Mountains and Carrizo zircon (U-Th)/He (ZHe) dates and radiation damage accu- Mountains of west Texas (Figure 1). ZHe dates for each sam- mulation and annealing in zircon crystals. This relation- ple have significant intrasample ZHe date variability which ship is typically expressed by intrasample ZHe dates that we use to produce a series of forward and inverse thermal his- sometimes span hundreds of millions of years. For grains tory models. We also present ZHe data from several detrital that have experienced identical thermal histories, differ- samples and discuss the ways in which these data compli- ences in ZHe dates are governed by differences in the ment ZHe dates from crystalline samples. Our new thermal effective uranium (eU = U +0:235Th) that are reflected as history models capture multiple pulses of cooling and reheat- either positive or negative ZHe date-eU relationships. ing that can be directly tied to known geologic events and These relationships result from differences in the ability provide an important link between disparate higher- of each grain to retain helium that are dependent on eU temperature 40Ar/39Ar cooling ages and lower-temperature values. Positive and negative ZHe date-eU trends are the apatite fission-track and (U-Th)/He datasets. Finally, we cumulative result of the overall thermal history and the evaluate the use of both inverse and forward models to com- effects of radiation damage on helium diffusion, and these pare the long-term thermal history of rocks in this region and allow for continuous thermal histories to be reconstructed. determine an improved application of the ZHe thermochro- Helium diffusion models in zircon suggest a temperature nology method. sensitivity window ranging from approximately 210 to 50°C [12, 13], which partially overlaps with zircon 2. Key Tectonic Events of the Region fission-track and K-feldspar 40Ar/39Ar data at the higher range and with apatite fission-track and (U-Th)/He tech- From 1.8–1.6 Ga, Paleoproterozoic growth of southwest niques at the lower end. The large ZHe temperature sensitiv- Laurentia occurred by progressive accretion of arc terranes, ity window makes it possible to constrain more complete and including the Yavapai and Mazatzal Provinces [29–31], continuous thermal histories, offering an opportunity to where final assembly of the supercontinent Rodinia occurred bridge the gap between higher and lower temperature during Grenville tectonism in the late Mesoproterozoic methods. Previous studies have relied on this intrasample (Figure 1) [32]. The Grenville orogeny in southwestern spread in ZHe dates to investigate a wide range of geologic Laurentia records arc accretion and continent-continent col- processes, such as the Proterozoic thermal history of rocks lision between 1350 and 980 Ma [33]. Some of the Grenville (e.g., [14]), timing of Laramide exhumation [15], and devel- orogenic foreland is exposed in western Texas in the Carrizo opment of the South American passive margin [16]. How- Mountains [33, 34]. The ca. 1380–1327 Ma Carrizo Moun- ever, additional studies are needed to further assess the tain Group (Figure 1(b)), composed of immature clastic efficacy of this method. rocks, within-plate rhyolitic volcanic rocks, and minor car- Proterozoic crystalline rocks of southwestern North bonates, likely records rifting of continental crust within a America have experienced a complex tectonic history that back-arc basin during overall convergence [35–37]. The spans over a billion years (e.g., [17]). In southwestern New Franklin Mountains (Figure 1(b)) record continued sedimen- Mexico and western Texas, this history includes, but is not tation within this back-arc basin from ca. 1260–1240 Ma [36] limited to, Mesoproterozoic and Neoproterozoic assembly until continent-continent collision ca. 1150–1120 Ma [36]. and break-up of the supercontinent Rodinia, Pennsylvanian- North of the Grenville deformation front (Figure 1), magma- Ancestral Rocky Mountain deformation, widespread tism in the Franklin Mountains is represented by ~1.1 Ga plu- Paleozoic and Mesozoic sedimentation, latest - tonic and volcanic rocks, including the Red Bluff granite Laramide compression, and culminating with which was targeted in this study for ZHe dating [38, 39]. Pet- Neogene extension related to development of the Rio Grande rological and geochemical studies classify the Red Bluff gran- rift (e.g., [18–22]). ite as a within-plate, A-type granite and support a model 40Ar/39Ar cooling ages from Proterozoic crystalline rocks where these rocks were emplaced within a more regional in New Mexico typically range from approximately 1600 to strain field dominated by NW-SE shortening and orthogonal 1000 Ma and reflect thermal pulses during intracontinental NE-SW extension related to Grenville convergence [39, 40]. tectonism and plutonism (e.g., [23, 24]). However, apatite Final shortening along the southern margin of Laurentia fission-track and (U-Th)/He studies from the same region occurred between ~1035 and 980 Ma [33, 41]. in New Mexico yield dates that are typically younger than Models for the breakup of Rodinia suggest diachronous about 100 Ma, reflective of cooling during exhumation asso- disassembly with early rifting occurring between 780 and ciated with the Laramide orogeny and Rio Grande rift exten- 680 Ma, followed by the main rifting phase between 620 sion [25–28]. These two disparate datasets each provide brief and 550 Ma [18, 21, 42, 43]. Supercontinent breakup may snapshots into a more complex thermal history, but a contin- have also been accompanied by a major pulse of denudation uous thermal record remains lacking, particularly in the recorded in ZHe datasets [14]. Breakup was followed by range from about 1000 Ma to younger than 100 Ma. Thus, deposition of Paleozoic passive margin sediment on Precam- this region of the southwest U.S. is an exceptional natural brian basement (e.g., [44]) to create the Great , laboratory for investigating long-term thermal histories a globally significant feature in the rock record. In southern using ZHe thermochronologic methods. New Mexico and western Texas, Proterozoic crystalline rocks Here, we present the first ZHe dates from three ranges in beneath the Great Unconformity typically consist of 1.45- southern New Mexico and west Texas: the Cookes Range in 1.35 and some 1.1 Ga granites intruded into 1.68-1.65

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Quaternary fault

Cenozoic volcanic rocks Cenozoic intrusive rocks Paleozoic - Mesozoic rocks Precambrian rocks

0 25 50 100 Km

(a)

Permian Hueco Cookes Range Franklin Mtns. Carrizo Mtns. limestone C-O Bliss Fm. Cretaceous rocks 05 km C-O Steeruwitz Bliss Fm. thrust C-O 16CR01 17FR04 Bliss Fm. Permian 1.1 Ga 18VH01 18VH02 Abo Fm. RBG CMG 2 3 CMG Eocene 17FR03 1 105°W CR Rubio Peak Fm. 1.1 Ga 15FR02 N RBG 1.1 Ga Mesozoic

RBG CM 17FR02 Paleozic 05 km CR, FM, 16CR02 1.1 Ga Hazel Fm. Proterozoic Bliss Fm. 1.26 Ga Allamore and granite 05 km N Additional unit map

CM Tumbledown Fms. 1.38-1.29 Ga Carrizo patterns for Fig. 1B Mountain Group (b)

Figure 1: (a) Simplified geologic map of southern New Mexico, western Texas, and northern Chihuahua, Mexico. Sample locations for zircon (U-Th)/He thermochronology are shown as red circles. Inset shows Proterozoic provinces in red from Whitmeyer and Karlstrom [31] and regional tectonic framework of the Ancestral Rocky Mountains, Mexican Border rift, Laramide deformation, and Rio Grande rift. Dark blue stippled pattern is Ancestral Rocky Mountain basins, and purple shading is Ancestral Rocky Mountain uplifts, modified from Leary et al. [90]. Black line with ticks is the northern boundary of the Mexican Border rift [50]. (b) More detailed geologic relationships and sample locations for the Cookes Range (CR), Franklin Mountains (FM), and Carrizo Mountains (CM). The Great Unconformity is shown as green, red, and purple lines, where different colors are based on the age of the overlying unit as labeled. Numbers refer to geochronologic and thermochronologic constraints used in thermal history modeling: (1) U-Pb zircon crystallization age of 1111 ± 43 Ma from the Thunderbird Group rhyolite [38]. (2) U-Pb zircon crystallization age of 1120 ± 35 Ma from the Red Bluff granite [39]. (3) 40Ar/39Ar hornblende and muscovite cooling ages of ca. 1035 Ma from the Carrizo Mountain Group [41]. Cookes Range geology is modified from the Geologic Map of New Mexico [91]. Franklin Mountains geology is modified from Harbour [66] and Lucia [92]. Carrizo Mountains geology is modified from King et al. [93] and Davis and Mosher [33]. Fm.: formation; RBG: Red Bluff granite; CMG: Carrizo Mountain Group; C-O: Cambrian-.

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supracrustal rocks of the Mazatzal Province [17]. These rocks southern New Mexico suggests that many samples farther are generally overlain by the Cambrian-Ordovician Bliss to the west have been completely reset, and all ZHe dates Sandstone, which define a Great Unconformity that encom- are Oligocene-Miocene [25, 56, 57]. These studies found passes 600 m.y. to more than 1,000 m.y. of missing time. that along this EW transect, the Cookes Range was the Late Paleozoic deformation in western North America westernmost location to preserve older ZHe dates, resulted in the development of the Ancestral Rocky although it does contain Oligocene intrusive and extrusive Mountains (e.g., [19, 45]). In the Cookes Range (Figure 1), rocks (Figure 1(b)) that may have partially affected the Ancestral Rocky Mountain deformation is evident by a near ZHe data (as described in more detail below). Therefore, absence of sedimentation from approximately 320 to mountain ranges east of and including the Cookes Range 290 Ma [19]. In contrast, the Franklin Mountains record near were selected because they are removed from the effects continuous sedimentation during this timeframe [46], sug- of Oligocene magmatism and likely retain a record of gesting minimal exhumation. In west Texas, deformation the older, long-term thermal history. locally stripped early Paleozoic sedimentary rocks and Whole-rock samples were processed using standard exposed Proterozoic basement which was subsequently cov- mineral separation techniques to isolate the zircon fraction. ered by early Permian rocks [47]. Zircon separates were inspected with a petrographic micro- Starting in the Late , a continental rift developed scope equipped with a digital camera used for measuring in the region of northern Sonora, Mexico, and southern grain dimensions, including length, width, and depth. Ideal Arizona (e.g., [48]). The resulting rift basin was originally zircon crystals for ZHe thermochronology are euhedral with termed the Bisbee basin [49] and now referred to as the no inclusions and a minimum diameter of 70 μm. Five to ten Mexican Border Rift (Figure 1) [50]. Marine strata have of the best zircons from each sample were hand-picked and Tethyan indicating a connection to the Gulf of Mexico loaded into Nb tubes for analysis. All samples were analyzed (e.g., [51]). Volcanogenic material such as mafic volcanic at the Thermochronology Research and Instrumentation rocks and pillow basalts were deposited in this rift [52–54]. Laboratory at CU Boulder (CU TRaIL). Several of the samples from this study were collected from Since all samples yield a range of ZHe dates and eU the northern rift shoulder in southwestern New Mexico. values, a forward modeling approach was first used to con- Rifting in this region ended by the middle Cretaceous based strain the long-term thermal history of each mountain range on subsidence studies indicating a transition from rift- since crystallization of the sample. Forward modeling allows related thermal subsidence to the formation of a foreland for the calculation of a thermochronometric age from a basin beginning in the Albian [50, 55]. determined time-temperature path, using helium diffusion Late Mesozoic to Eocene deformation during the Lara- or annealing kinetics [58]. Multiple hypothetical time- mide orogeny involved northeast-directed crustal shortening temperature paths are investigated that are based on known that resulted in a series of uplifts and basins [22]. These geologic constraints, including formation of the Great basins and uplifts trend northwest in southern New Mexico Unconformity, periods of Paleozoic and Mesozoic sedimen- and extend into northern Mexico and western Texas; our tation, and possible exhumation during Ancestral Rocky Carrizo Mountains study site lies at the easternmost extent Mountain, Laramide, and Rio Grande rift deformation events of Laramide deformation (Figure 1). These structures were (Table S1, Fig. S1). For each of these paths, we vary extensively dissected by younger continental extension maximum or minimum temperatures achieved during related to development of the Rio Grande rift [22]. Initial different segments of the sample’s thermal history. ZHe rifting in southern New Mexico began during the Oligocene date-eU curves are calculated from each t-T path using a to produce a series of fault-block uplifts and basins that cre- Matlab script (Guenthner, pers. comm., 2018) that ate the modern topographic grain of the region [20]. The incorporates the zircon radiation damage accumulation and Cookes Range is bounded on three sides by normal faults annealing model (ZRDAAM) from Guenthner et al. [12]. related to development of the Rio Grande rift, and the Inputs include a specific time-temperature path, zircon eU Franklin Mountains and Carrizo Mountains are both associ- values, and zircon grain size. These date-eU paths are then ated with active normal faults that document continued compared to ZHe data. Model date-eU paths that are deformation in this region. drastically different than the observed data are not viable thermal histories. By varying different segments of the 3. Methods thermal history, the model date-eU paths from each sample are incrementally adjusted to provide a range of permissible Uplifts within the southern Rio Grande rift, such as the thermal histories. This approach assumes that the only Carrizo Mountains, Franklin Mountains, and Cookes Range, control on ZHe dates is helium diffusion related to crystal expose Proterozoic crystalline basement that has been damage and annealing that evolves through time. However, exhumed to the surface in the footwalls of Cenozoic normal the spread in ZHe dates could also be influenced by other faults. Samples were collected from crystalline basement factors such as crystal size, U and Th zoning in the zircon rocks and clastic coarse-grained sandstone or granule con- crystal, and implantation of helium from neighboring glomerate sedimentary rocks because they likely contain grains (e.g., [59–61]). Although possible He implantation is abundant zircon crystals. In addition, sample locations were generally not known, and zoning information is not targeted to avoid possible effects of widespread Oligocene typically obtained in ZHe studies, the effects of crystal size plutonism. Sample collection along an E-W transect across on resulting date-eU curves can be investigated. To do this,

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we use a method similar to other workers (e.g., [14, 62, 63]) These are combined with 18 ZHe dates previously reported in which three separate date-eU curves are modeled to create [25, 56], including nine dates from the Franklin Mountains an envelope within which the ZHe dates should lie. To create and nine dates from the Cookes Range (Figure 2). In total, the three curves, we use the mean grain size ± 2 standard this project incorporates 55 individual grain ZHe dates deviations. from eight samples to investigate patterns in long-term We also use inverse modeling to further refine possible thermal histories spanning hundreds of millions of years thermal histories. Whereas forward modeling was used to (Table 1). test tens of possible paths, an inverse modeling approach Ten ZHe dates from two samples were obtained from the can test tens of thousands of possible paths and explore addi- Mesoproterozoic Hackett Peak Formation of the Carrizo tional possible t-T paths that are permissible by the ZHe data. Mountain Group in the northern Carrizo Mountains Inverse modeling allows for the calculation of t-T paths that (Figures 1(b) and 2, Table 1). These data show a wide match measured thermochronometric ages to within a spec- range in both ZHe date (6–731 Ma) and eU concentration ified amount of statistical error [58]. We use HeFTy v. 1.9.3 (52–1729 ppm). There is also a well-defined negative ZHe [4], which uses a Monte Carlo method to plot possible date-eU correlation that shows a steep trend until eU time-temperatures paths based on entered data. Input data values of approximately 400 ppm. ZHe dates show a slight into HeFTy includes U, Th, Sm concentrations, grain radius, positive relationship with grain radius. The oldest ZHe measured age, and age uncertainty. HeFTy requires a set dates correspond to the largest grains, suggesting a possi- of temperature-time parameters for modeling, such as start ble grain size control on ZHe dates. times, end times, and temperatures. All inverse models Twenty-nine ZHe dates from four samples were obtained explore a total of 50,000 paths, where resulting “accept- from the Mesoproterozoic Red Bluff granite and the able” paths have goodness-of-fit parameters >0.05, and Cambrian-Ordovician Bliss Sandstone in the Franklin “good” paths have goodness-of-fit criteria >0.5. Complete Mountains (Figures 1(b) and 2). ZHe dates from the Red model constraints and inputs are available in Table S1 Bluff granite have a large range from 19 to 401 Ma. Their and Figure S1 and follow methods outlined in Flowers eU concentrations show a large spread as well, ranging from et al. [64]. 63 to 828 ppm. Together, these data have a slightly negative, Finally, we explore the significance of detrital ZHe dates although poorly defined, ZHe date-eU correlation, where from samples collected from the Cookes Range and Franklin older ZHe dates typically correspond to lower eU values. Mountains. In contrast to igneous rock samples, partially However, individual samples from the Franklin Mountains reset detrital zircon grains only share a common post- do not always show a negative trend, and some (17FR03) depositional thermal history, but each individual grain may show more of a flat trend. The majority of crystalline zircon still retain an older, unique thermal imprint. As a result, they grains from the Franklin Mountains cluster within a narrow should not necessarily lie along a single date-eU curve, but size range of 35-55 μm such that a ZHe date-grain size trend will instead have potentially highly variable ZHe dates [15]. is not observed. ZHe dates from the Bliss Sandstone show a Analysis of detrital grains entails constructing inheritance ZHe date range of 62–649 Ma, with eU concentrations envelopes for each location, following methods outlined in ranging from 90 to 374 ppm. Many of these ZHe dates Reiners et al. [65] and Guenthner et al. [15]. The inheritance are older than ZHe dates from crystalline basement sam- envelope is created by combining a zero-inheritance date-eU ples, and together, they define a negative trend date-eU curve with multiple maximum inheritance curves that trend. This sample displays a wider range in crystal sizes assume different zircon crystallization ages. The zero- than the crystalline grains, where most of the grains are inheritance date-eU curve is constructed from the thermal older and larger. However, ZHe dates do not appear to history that begins at the depositional age of the detrital sam- increase with larger crystals. ple and assumes no inherited radiation damage at that time, a A total of 16 ZHe dates are presented from the Cookes situation that can be achieved by either complete resetting Range (Figures 1(b) and 2). Ten ZHe dates are from a of detrital grains at the time of deposition or zero U-Pb Proterozoic granite exposed in Rattlesnake Ridge along ages at the time of deposition. Maximum inheritance the southern edge of Cookes Range. These ZHe dates range curves incorporate the effects of inherited radiation dam- from 63 to 446 Ma with an eU range of 232–945 ppm. These age by assuming zircon crystallization at surface tempera- data show a steep negative ZHe date-eU relationship at tures prior to the depositional age. These grains then lower eU values, which transitions to a flatter trend at share an identical thermal history as the zero-inheritance higher eU values. curve after deposition. When combined, these different An additional six ZHe dates were obtained from the curves yield an inheritance envelope that should contain Permian from the Cookes Range. ZHe dates the observed ZHe dates if the t-T path is a plausible result range from 44 to 130 Ma, with corresponding eU values from of the sample’s thermal history. 56 to 351 ppm. Although this sample does not yield as large of an eU range, these ZHe dates have a slight negative trend 4. ZHe Date-eU and Date-Radius Patterns where older ZHe dates correspond to lower eU values. In contrast to Franklin Mountains samples, detrital grains from A total of 37 new ZHe dates are presented, including 10 dates the Cookes Range are mostly younger than crystalline zircon from the Carrizo Mountains, 20 dates from the Franklin grains. Individual ZHe dates do not seem to show a well- Mountains, and seven dates from the Cookes Range. defined trend with grain size.

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500 500 Cookes Range (n = 16) 400 400

300 300

200 200 ZHe data (Ma) ZHe data (Ma)

100 100

0 0 0 200 400 600 800 1000 0 10 20 30405060 70

eU (ppm) Radius (�m)

16CR01 (sedimentary) Tis study 16CR02 (igneous) Biddle et al. (2018) 800 800 Franklin Mountains 700 (n = 29) 700 600 600

500 500

400 400

300 300 ZHe data (Ma) ZHe data (Ma) 200 200

100 100

0 0 0 200 400 600 800 1000 0 2010 30 40 50607080 90

eU (ppm) Radius (�m) 15FR03 (igneous) 17FR04 (igneous) 17FR02 (sedimentary) Tis study 17FR03 (igneous) Biddle et al. (2018) 800 800 Carrizo Mountains 700 (n = 10) 700 600 600

500 500

400 400

300 300 ZHe data (Ma) ZHe data (Ma) 200 200

100 100

0 0 0 400 800 1200 1600 2000 0310 20 04050760 0

eU (ppm) Radius (�m)

18VH01 (metamorphic) 18VH02 (metamorphic)

Figure 2: ZHe date-eU and ZHe date-radius plots for samples collected from the Cookes Range, Franklin Mountains, and Carrizo Mountains. All errors are plotted at 2σ and include uncertainties on U, Th, and He measurements, as well as uncertainty for the alpha-ejection correction. Squares are data from Biddle et al. [25].

5. Forward Modeling Approach paths are used to test which periods of burial or uplift have the largest effect on the resulting ZHe date-eU pattern and Hypothetical time-temperature paths were constructed for which events have relatively minor effects. To do this, we crystalline samples at each location for two purposes used a Matlab routine that incorporates the zircon radiation (Figures 3–5). First, these paths are used to eliminate thermal damage accumulation and annealing model (ZRDAAM) histories that are incompatible with the observed ZHe dates from Guenthner et al. [12]. Forward model outputs are calcu- and begin to refine possible thermal histories. Second, these lated ZHe date-eU curves that are predicted from the input

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Sample Mass (μg) Spherical radius (μm) 4He (nmol/g) U (ppm) Th (ppm) Sm (ppm) eU (ppm) Raw date (Ma) Ft Corrected date (Ma) 2σ Carrizo Mountains 18VH01: Carrizo Mountain Group, metavolcanic. 31.0589°N-104.9611°W, Elev. 1419 m NRz1 2.58 40.5 243.7 71.9 97.8 0.6 94.8 458.82 0.72 628.14 19.75 NRz2 6.76 56.9 383.6 109.7 68.0 0.6 125.7 539.59 0.80 667.97 27.87 NRz4 8.35 59.7 292.2 99.5 73.8 0.9 116.8 446.26 0.81 547.34 43.21 NRz5 10.46 61.9 202.5 59.3 66.7 1.4 75 481.22 0.81 586.15 18.95 NRz7 4.09 44.3 358.2 303.4 221.8 4.9 355.6 183.89 0.75 245.37 4.12 NRz8 10.02 62.5 183.0 47.4 24.6 0.6 53.2 604.60 0.82 730.86 40.82 NRz9 3.53 47.7 278.7 146.6 159.0 1.1 184 274.54 0.76 359.13 9.80 NRz10 3.77 47.7 352.9 148.5 206.5 1.2 197 323.41 0.76 423.54 17.00 18VH02: Carrizo Mountain Group, metavolcanic. 31.0393°N-104.8971°W, Elev. 1321 m NRz4 4.23 44.6 224.8 323.3 442.1 2.6 427.2 96.72 0.75 129.49 4.06 NRz5 8.73 61.8 43.5 1389.0 1447.5 2.3 1729.1 4.66 0.81 5.73 0.14 Franklin Mountains 15FR03: Red Bluff granite. 31.8340°N-106.4810°W, Elev. 1421 m NRz1 2.9 44.5 136.4 275.6 235.2 1.7 330.9 75.90 0.74 101.76 3.03 NRz2 1.96 38.6 156.1 222.1 113.6 1.8 248.7 115.26 0.71 161.41 3.56 NRz3 3.09 46.0 152.0 254.1 155.6 1.6 290.7 96.17 0.75 127.16 4.42 NRz4 2.14 39.4 58.8 101.9 145.1 1.4 136.0 79.64 0.71 112.02 4.36 NRz5 2.33 41.8 234.6 217.1 117.8 1.4 244.8 175.06 0.73 237.96 10.05 NRz6 1.32 35.0 141.5 178.8 97.0 0.9 201.6 128.75 0.68 187.35 4.43 NRz7 5.29 48.6 253.4 212.9 134.3 1.6 244.5 189.13 0.77 245.35 5.59 NRz8 4.17 45.6 276.0 138.0 109.1 1.4 163.6 304.68 0.75 401.87 14.97 NRz9 3.3 42.8 101.6 175.5 93.2 1.1 197.4 94.68 0.74 127.70 3.11 NRz10 1.67 37.0 77.7 174.1 109.2 1.3 199.8 71.72 0.70 102.43 5.92 17FR02: Bliss Sandstone. 31.8006°N-106.4747°W, Elev. 1381 m ∗z1 4.5 48.7 247.6 122.6 87.3 7.8 143.1 312.3 0.8 404.7 38.4 ∗z2 5.3 51.2 97.8 302.7 304.0 14.9 374.1 48.3 0.8 62.2 5.7 ∗z3 3.2 44.3 249.8 77.1 57.8 0.0 90.7 489.8 0.7 649.0 89.1 NRz1 17.9 79.3 357.7 118.1 152.8 8.3 154.0 416.07 0.85 486.09 9.00 NRz2 8.8 64.3 425.1 175.7 181.3 8.6 218.4 350.49 0.82 425.31 15.09 NRz3 11.4 67.8 151.1 87.1 116.4 3.3 114.5 239.94 0.83 289.17 7.96 NRz4 7.0 60.2 521.0 286.2 237.1 3.9 341.9 276.00 0.81 339.64 7.25 7 on 03 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8881315/5293569/8881315.pdf 8 Table 1: Continued.

Sample Mass (μg) Spherical radius (μm) 4He (nmol/g) U (ppm) Th (ppm) Sm (ppm) eU (ppm) Raw date (Ma) Ft Corrected date (Ma) 2σ 17FR03: Red Bluff granite. 31.8397°N-106.4859°W, Elev. 1434 m ∗z1 5.3 51.9 173.6 601.0 166.2 1.4 640.0 50.1 0.8 63.9 5.7 ∗z2 3.0 40.8 186.6 292.3 228.4 1.2 346.0 99.1 0.7 137.0 8.9 ∗z3 3.0 43.0 11.6 132.9 82.7 1.0 152.4 14.2 0.7 19.2 2.5 NRz1 3.34 44.4 640.4 707.0 398.6 23.2 800.7 146.44 0.75 195.53 8.04 NRz2 1.3 34.9 133.0 657.1 728.5 1.3 828.3 29.67 0.68 43.70 0.71 NRz3 2.97 46.8 180.7 141.3 194.2 10.1 187.0 176.45 0.75 233.03 10.24 NRz4 2.97 46.8 581.3 632.1 575.7 1.3 767.4 138.81 0.76 182.84 6.16 NRz5 3.78 50.5 7.2 52.7 44.5 0.5 63.1 21.17 0.77 27.37 1.16 17FR04: Red Bluff granite. 31.8931°N-106.4758°W, Elev. 1477 m ∗z1 2.0 34.8 92.8 138.8 79.9 7.0 157.5 108.2 0.7 158.5 31.4 ∗z2 4.7 50.0 123.7 94.4 75.0 3.6 112.0 201.2 0.8 259.6 24.3 ∗z3 5.6 50.9 46.6 81.1 57.2 0.0 94.6 90.6 0.8 116.6 11.6 NRz1 3.97 40.8 55.4 170.0 125.2 2.7 199.4 51.26 0.72 70.99 2.24 Cookes Range 16CR01: Permian Abo Formation (sandstone). 32.4466°N-107.6906°W, Elev. 1495 m ∗z1 6.6 50.5 48.4 189.3 107.8 0.4 214.7 41.6 0.8 53.7 6.1 ∗z2 3.3 45.7 66.2 159.5 94.4 1.8 181.6 67.2 0.8 89.2 8.3 ∗z3 2.2 39.2 37.6 61.3 57.8 0.5 74.8 92.5 0.7 129.5 22.6 ∗z4 4.9 47.3 24.2 48.2 33.3 0.0 56.0 79.7 0.8 104.8 12.9 ∗z5 3.0 41.3 94.1 176.7 87.3 5.6 197.3 87.8 0.7 120.3 18.9 ∗z6 4.0 50.6 64.2 319.6 132.1 0.0 350.7 33.9 0.8 43.6 3.7 16CR02: Precambrian granite. 32.3919°N-107.7148°W, Elev. 1435 m ∗z1 1.8 39.1 297.3 286.1 87.0 2.0 306.5 177.1 0.7 245.7 33.0 ∗z2 2.0 39.1 222.5 280.8 81.1 14.9 299.8 136.0 0.7 189.0 31.1 ∗z3 2.3 41.0 164.5 255.5 85.4 4.5 275.5 109.7 0.7 150.1 21.6 NRz1 2.47 42.5 204.0 289.1 61.3 4.2 303.5 123.33 0.74 166.32 7.72 NRz2 3.16 42.2 321.9 219.8 51.0 2.9 231.7 251.93 0.74 338.56 9.36 NRz3 2.57 43.3 318.4 819.1 448.9 34.9 924.6 63.50 0.74 85.66 2.61 NRz4 5 48.8 254.9 217.0 96.8 2.8 239.8 193.88 0.77 250.60 6.82 NRz5 5.25 51.0 264.5 225.5 88.2 7.5 246.2 195.87 0.78 249.93 6.11 NRz6 8.31 62.1 218.0 772.2 55.5 14.1 785.3 51.29 0.82 62.61 2.23

NRz7 5.43 54.5 555.6 261.0 81.2 16.6 280.1 356.21 0.79 445.80 11.02 Lithosphere ∗ZHe data from Biddle et al. [25]. Lithosphere 9

0 1000 900 Rapid exhumation at 1050-1000 Ma 100 predicts oldest ZHe dates that match 800 the observed dates and sharp 200 700 decrease to low dates by eU values Paleozoic 600 of 400 ppm. 300 burial 500 Proterozic Ancestral Rocky 400

400 Mountain cooling (Ma) date ZHe exhumation 300 Mesozoic burial 500 200 Laramide/Rio Grande rif cooling 100 600 0 14001200 1000 800 600 400 200 0 0 400 800 1200 1600 2000 Time (Ma) eU (ppm) (a) 0 1000 900 100 800 200 1050-1000 700 Ma cooling 600 300 500 400 400 ZHe date (Ma) date ZHe 300 500 200 100 600 0 14001200 1000 800 600 400 200 0 0 400 800 1200 1600 2000 Time (Ma) eU (ppm)

110 150 120 160 130 170 140 180 (b) 0 1000 900 100 800 200 1050-1000 700 Ma cooling 600 300 500 400 400 ZHe date (Ma) date ZHe 300 500 200 100 600 0 14001200 1000 800 600 400 200 0 0 400 800 1200 1600 2000 Time (Ma) eU (ppm)

130 170 140 180 150 190 160 200 (c) 0 1000 900 100 800 200 1050-1000 700 Ma cooling 600 300 500 400 400 ZHe date (Ma) date ZHe 300 500 200 100 600 0 14001200 1000 800 600 400 200 0 0 400 800 1200 1600 2000 Time (Ma) eU (ppm)

60 100 70 110 80 120 90 130 (d)

Figure 3: Preliminary forward models for the Carrizo Mountains. The models are constructed using an iterative approach by individually varying different segments of the path. (a) Three hypothetical paths that vary the timing of Proterozoic cooling and calculated ZHe date- eU curves. The blue path yields the oldest ZHe dates and is used in the following iterations. (b) Eight hypothetical t-T paths that vary the maximum Paleozoic burial temperature. Shallower burial temperatures yield a better match to the observed ZHe dates, and a temperature of 120°C is used in the next iteration. (c) Eight hypothetical t-T paths that vary the maximum Mesozoic burial temperature. A burial temperature of 170°C matches the negative slope in the ZHe data and is used in the final iteration. (d) Eight hypothetical paths that vary the post-Laramide pre-Rio Grande rift temperature. These paths have little effect on the resulting date-eU curves.

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0 1200 Proterozic 50 1000 exhumation Paleozoic 100 burial 800 150 Mesozoic 600 200 burial

ZHe date (Ma) 400 250 Laramide/Rio 200 300 Grande rif cooling 350 0 1000 800 600 400 200 0 0 200 400 600 800 1000 Time (Ma) eU (ppm)

80 160 100 180 120 200 140 220 (a) 0 800 50 700 100 600 500 150 400 200 300 ZHe date (Ma) 250 200 300 100 350 0 1000 800 600 400 200 0 0 200 400 600 800 1000 Time (Ma)

40 120 60 140 80 160 100 180 (b) 0 800 50 700 100 600 500 150 400 200 300 ZHe date (Ma) 250 200 300 100 350 0 1000 800 600 400 200 0 0 200 400 600 800 1000 Time (Ma)

40 120 60 140 80 160 100 180 (c) 0 800 50 700 Precambrian burial histories have 600 100 little efect on the ZHe age-eU pattern 500 150 400 200 Proterozoic burial 300 ZHe date (Ma) 250 and exhumation 200 300 100 350 0 1000 800 600 400 200 0 0 200 400 600 800 1000 Time (Ma) (d)

Figure 4: Preliminary forward models for the Franklin Mountains. The models are constructed using an iterative approach by individually varying different segments of the path. (a) Eight hypothetical paths that vary the maximum Mesozoic temperature. A higher temperature of 180°C closely aligns with the observed data and is used in the next iteration. (b) Eight hypothetical paths that vary the maximum end-Paleozoic temperatures. The temperatures are held constant from 250 to 150 Ma because lower Cretaceous rocks are presumed to unconformably overlie Permian rocks in the Franklin Mountains [66]. An intermediate temperature of 120°C crosses the average grain dates and is used in the following iteration. (c) Eight hypothetical paths vary the end-Laramide pre-Rio Grande rift temperature. A lower temperature of 60°C is used in the final iteration. (d) Eight hypothetical paths that test possible Proterozoic burial and exhumation prior to deposition of Paleozoic sediments. These paths seem to have little effect on the resulting date-eU curves.

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Figure 5: Preliminary forward models for the Cookes Range. The models are constructed using an iterative approach by individually varying different segments of the path. (a) Eight hypothetical paths vary the maximum Paleozoic temperature. No paths match the observed data, but higher burial temperatures of 160°C result in a closer fit to the data. This temperature is used in the following iteration. (b) Eight hypothetical paths that vary maximum Mesozoic temperatures. A temperature of 160°C yields a date-eU curve that closely aligns with the negative slope of the observed data. (c) Eight paths that vary end-Laramide pre-Rio Grande rift temperatures. The resulting date-eU curves suggest that lower temperatures yield a better fit to the data, and a temperature of 80°C is used in the final iteration. (d) Five paths that vary the timing of Proterozoic cooling. Early cooling yields date-eU curves that are a better fit to the observed data.

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data. Hypothetical time-temperature paths were constructed (Figure 3(c)). These t-T paths yield date-eU curves with a for each mountain range by varying the amount of burial or wide range in maximum ZHe dates and widths. When exhumation from different deformational events in that area. compared to the observed data, there is a trade-off between Specific geologic constraints for each location are provided in matching the oldest ZHe dates and matching the negative Table S1 and Figure S1. Each ZHe date-eU curve calculated date-eU slope of the data. For example, a temperature of from an input time-temperature is then compared to the 160°C predicts oldest ZHe dates that match the observed ZHe data collected from each location. Below, we first data, while a temperature of 170-180°C predicts a negative present preliminary forward models for each of the three slope that closely follows the data. Of the calculated date- study locations. Then, we refine these results and produce eU curves, a maximum Mesozoic burial depth of 170°C best-fit models that represent the closest match to the gives a predicted curve that somewhat matches the steep observed data acquired through this approach. negative date-eU slope in the observed data, as well as the kink where the slope changes to a much shallower trend. fi 5.1. Preliminary Forward Models The nal set of representative paths was constructed to vary the transition from Laramide shortening-related uplift 5.1.1. Carrizo Mountains. Four sets of representative t-T to Rio Grande rift extensional uplift. To do this, we vary paths were constructed for the Carrizo Mountains that each the temperature at which the sample resided after Laramide varies a different segment of the history. The first set of rep- uplift and prior to Rio Grande rift uplift (Figure 3(d)). Eight resentative paths varies the timing of Proterozoic exhuma- thermal histories are examined that vary the temperature tion prior to deposition of Paleozoic strata (Figure 3(a)). between 60 and 130°C from 40 to 30 Ma. All of the paths Three possible thermal histories are examined to test how are nearly identical, suggesting that the overall ZHe date-eU sensitive ZHe data are to Proterozoic exhumation. These pattern is not as sensitive to this younger deformational event three paths all cool through the range of 500 to 350°Cat in the Carrizo Mountains. 1035 Ma, consistent with 40Ar/39Ar hornblende and musco- vite data [41]. From there, they diverge to include early exhu- 5.1.2. Franklin Mountains. Neogene faulting and tilting of the mation to 15°C at 1000 Ma (blue path), intermediate Franklin Mountains expose Proterozoic granite along the exhumation from 300 to 15°C from 800 to 700 Ma (teal path), eastern base of the range (Figure 1(b)). A total of three Prote- and late exhumation from 300 to 15°C from 600 to 500 Ma rozoic samples from the Franklin Mountains were collected (yellow path) (Figure 3(a)). The post-500 Ma segments of from various locations along the range. Although they were these paths are identical. These three paths each yields a all collected from similar elevations, calculated depths for date-eU curve that is notably different than the others, sug- each sample beneath the Great Unconformity varied. For gesting that the Proterozoic cooling history imparts a signif- example, although samples 15FR03 and 17FR03 resided at icant influence on the resulting date-eU relationship. Of the paleodepths of 361 m and 229 m, respectively, sample three proposed cooling scenarios, rapid exhumation to the 17FR04 was located at a significantly greater depth of surface from 1050 to 1000 Ma predicts maximum ZHe dates approximately 1587 m. For modeling purposes, samples that are similar to the observed dates, although it does not 15FR03 and 17FR03 were combined into a single sample match the negative slop of the data. However, this result sup- because they likely experienced near-identical thermal histo- ports rapid cooling followed by prolonged residence at the ries, and together, these grains show a larger range in eU Earth’s surface. values that allows for tighter constraints on the thermal his- The second set of representative paths builds off the first tory. ZHe dates from sample 17FR04 were excluded, and no iteration by using 1050–1000 Ma exhumation from attempt was made to model this sample, in part, because only Figure 3(a). Here, we vary the depth of maximum Paleozoic four ZHe dates are available for this sample with limited burial after formation of the Great Unconformity (Table S1, spread in eU values (Figure 2, Table 1). Figure 3(b), and Figure S1). Eight thermal histories are Four sets of representative paths were constructed for the examined that vary the maximum burial temperature from Franklin Mountains (Figure 4). The four sets of paths are 110 to 180°C at 325 Ma, signifying maximum burial before presented in the order of decreasing date-eU curve variabil- Ancestral Rocky Mountain uplift. The resulting ZHe date- ity. For example, Mesozoic burial yields date-eU curves that eU patterns vary slightly in width, but there is a larger vary widely in terms of maximum ZHe date and the location variation in the maximum ZHe age. However, maximum of the steep negative trend, whereas different Proterozoic burial depths of less than 140°C all yield almost identical paths have little effect on date-eU curves. The first set of rep- date-eU paths, suggesting that this dataset is insensitive to resentative paths was constructed to vary the amount of lower temperatures. Modeled date-eU paths of 110–140°C Mesozoic burial after regional uplift ceased in the Cretaceous predict maximum ZHe dates that are most similar to the (Figure 4(a)). Eight paths are examined that vary the maxi- observed data. mum burial temperature between 80 and 220°C at 80 Ma, sig- The third set of representative paths incorporates 1050– nifying maximum burial before Laramide uplift. These paths 1000 Ma Proterozoic cooling and uses a maximum Paleozoic produce date-eU curves with marked differences. The most burial temperature of 120°C. This iteration varies depths of notable differences between these curves are the maximum Mesozoic burial after Ancestral Rocky Mountain uplift ZHe date and the position of the negative slope, suggesting (Table S1; Figure S1). Eight thermal histories are examined that this segment of the path has a significant effect on the that vary maximum depth from 130 to 200°Cat80Ma final ZHe date-eU curve. Of the eight proposed t-T paths,

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burial to 180°C yields a date-eU curve that closely aligns with observed ZHe date-eU relationships do not show this pedi- the observed data. ment, the higher burial temperatures provide a better match The second set of representative paths was constructed to because the maximum ZHe dates are similar to the observed vary the amount of Paleozoic burial after the Great Uncon- dates, whereas lower burial temperatures predict ZHe dates formity. This path includes a period of constant temperature older than 1000 Ma. from 250 to 150 Ma because in the Franklin Mountains, The second set of paths was constructed to vary the lower Cretaceous rocks unconformably overlie Permian amount of Mesozoic burial after Ancestral Rocky Mountain rocks, suggesting a period of no deposition [66]. Eight ther- uplift. These paths use a Paleozoic burial temperature con- mal histories are examined that vary the maximum burial straint of 160°C (Figure 5(a)). Eight thermal histories are temperature between 40 and 180°C from 250 to 150 Ma examined that vary the temperature from 60 to 200°C (Figure 4(b)). A minimum temperature of 40°C was chosen (Figure 5(b)). The resulting date-eU curves show dramatic to represent minimal Paleozoic burial, and a maximum tem- variability. At lower burial temperatures, a prominent perature of 180°C was chosen because it is near the higher pediment is observed, although this pediment gradually end of the ZHe temperature sensitivity window. The result- diminishes at higher temperatures. A Mesozoic burial tem- ing date-eU curves are similar at eU values greater than perature of 160°C produces a date-eU curve that lies along 400 ppm. At lower values, the curves diverge to form peaks the path of observed ZHe dates and captures the broad at different ZHe dates. Although multiple resultant date-eU date-eU pattern at intermediate and high eU values curves roughly match the observed ZHe dates, an intermedi- (200–1000 ppm). ate burial temperature of 120°C produces a curve that bisects In the third iteration, Paleozoic and Mesozoic burial tem- the data. peratures are held at 160°C, and the transition from Laramide Next, the transition from Laramide shortening-related shortening to Rio Grande rift extensional uplift is varied from uplift to Rio Grande rift extensional uplift is varied 20 to 160°C (Figure 5(c)). Lower temperatures of 20–120°C (Figure 4(c)). Eight possible thermal histories are examined produce nearly identical date-eU curves. The highest temper- that vary post-Laramide temperatures between 40 and ature of 160°C predicts oldest ZHe dates that are too young to 180°C at 40 Ma. Higher burial temperatures yield date-eU match the observed data. Because lower temperatures appear curves that are nearly flat. Burial temperatures less than to be insensitive to the resulting date-eU pattern, a tempera- about 120°C produce curves with little variation, yet these ture of 80°C closely follows the observed dates and is used in curves predict oldest ZHe dates that more closely align with the final iteration. the observed data. A temperature of 60°C is selected because Finally, the three previous iterations are used to constrain it produces a curve with the highest ZHe peak. the Phanerozoic segments of the t-T history, and the Protero- In the final iteration, the Proterozoic thermal history is zoic uplift history is varied (Figure 5(d)). Five possible histo- examined (Figure 4(d)). All of the possible paths are forced ries are evaluated that vary the timing of exhumation to the to near-surface temperatures shortly after crystallization surface. Three paths of either early cooling or constant cool- because the Red Bluff granite intrudes the overlying ing are nearly identical at eU values greater than 250 ppm. At Thunderbird Rhyolite in some locations, suggesting it was lower values, they diverge. t-T paths involving rapid cooling emplaced at shallow levels in the crust. From here, eight dif- younger than 1000 Ma produce distinctively broad date-eU ferent paths are examined that postulate that the Red Bluff patterns. The three early cooling scenarios yield date-eU granite may have been buried and then exhumed back to curves that are similar to the observed ZHe date-eU trends. the surface prior to deposition of Paleozoic strata. Maximum The only significant difference between these three date-eU burial temperatures vary from 15 to 200°C. In addition, two curves is the predicted maximum ZHe date. However, different times of exhumation are investigated, including no ZHe grains are available with eU values lower than exhumation from 800 to 750 Ma and exhumation from 650 200 ppm where these paths diverge. to 550 Ma. These different t-T paths give predicted date-eU curves with little variability, suggesting that the observed 5.1.4. Summary of Preliminary Forward Models. The above ZHe date-eU values are not strongly affected by the Protero- analysis serves as a useful guide for interpreting ZHe datasets. zoic thermal history of these samples. This is consistent with For all three locations, the final forward models are non- the observed ZHe dates, which are mostly younger than unique such that our approach to refine the thermal history 250 Ma. only manages to produce a single t-T path out of many. However, this approach does highlight which segments of 5.1.3. Cookes Range. Four sets of representative paths were the t-T history the resulting date-eU curve is most sensitive constructed for Cookes Range. The first set of paths was con- to. For example, at all three locations varying the magni- structed to vary the amount of Paleozoic burial after the Great tude of Mesozoic burial has a drastic effect on the resulting Unconformity. Eight thermal histories are examined that vary date-eU curves. In the Carrizo Mountains and Cookes the maximum burial depth to temperatures that range from Range, the timing of Proterozoic exhumation also produces 60 to 200°C at 325 Ma, signifying maximum burial before date-eU curves that show significant differences. In contrast, Ancestral Rocky Mountain uplift (Figure 5(a)). The higher at all three locations, the post-Laramide/pre-Rio Grande rift temperature t-T paths yield date-eU curves with prominent temperature does not appear to significantly affect the flat pediments at intermediate eU values. This pediment date-eU curves. This analysis highlights some of the uses disappears with lower burial temperatures. Although the of a forward modeling technique for understanding which

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Figure 6: Best-fit forward models for (a) the Carrizo Mountains, (b) the Franklin Mountains, and (c) Cookes Range. These models use the iterative results from the preliminary forward models in Figures 3–5. For each model, a date-eU envelope is created by using the average grain size ðspherical radiusÞ ± 2 standard deviations. For Cookes Range, three plausible t-T paths are presented because the observed ZHe data do not constrain the low eU segment of the date-eU curve.

events may be preserved in the thermal record and which and implantation are unknown, for some samples, grain events the data are not sensitive to. Below, we use these results size does seem to be related to ZHe date (Figure 2). Here, and incorporate the possible effects of grain size to produce we produce refined forward model t-T paths for each loca- our best-fit forward models. tion that incorporate possible ZHe date effects related to differences in grain size. For each t-T path, we create three 5.2. Best-Fit Forward Models. The preliminary forward separate date-eU curves using the average grain size ± 2 modeling efforts above fail to yield a single t-T path that standard deviations, based on similar published methods fi adequately aligns with the observed date-eU patterns (e.g., [14, 62, 63]). These curves de ne a date-eU envelope ff (Figures 3–5). These results suggest that the effects of radi- that shows the combined e ects of radiation damage and ation damage and annealing on helium diffusion through grain size. the crystal lattice [12] may not be the only control on the observed ZHe dates. As described above, other factors that 5.2.1. Carrizo Mountains. For the Carrizo Mountains, we use could possibly influence ZHe dates are crystal size, U and the average grain size of 53 ± 17 μm (±2 standard deviations) Th zoning in the zircon crystal, and implantation of helium to create a date-eU envelope of the best-fit preliminary curve from neighboring grains. Although the effects of zoning (Figure 6(a)). This t-T path includes cooling to surface

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temperatures from 1050 to 1000 Ma, early Paleozoic burial to that after metamorphism, the sample remained at elevated 120°C, exhumation to surface temperatures by 285 Ma, temperatures until rapid uplift at approximately 1035 Ma Paleozoic through Mesozoic reheating to a maximum tem- based on 40Ar/39Ar hornblende and muscovite ages [41]. perature of 170°C at 80 Ma, and a post-Laramide temperature ZHe data also document the timing of this pulse of exhuma- of 60°C before final cooling to surface temperatures. This tion and suggest that the sample was cooled to near-surface best-fit t-T path yields a date-eU envelope that encompasses temperatures at this time (Figure 7(a)). However, some t-T all but the oldest ZHe date. paths also suggest that the sample could have remained at shallow levels in the crust and was further exhumed to the fi 5.2.2. Franklin Mountains. The best- t forward model for the surface during a second event at approximately 600– Franklin Mountains samples uses an average zircon grain 500 Ma. Increasing temperatures during Paleozoic sedimen- μ size of 43 ± 10 m to produce a date-eU envelope tation are not well constrained by the model, as maximum (Figure 6(b)). This t-T path includes no Proterozoic burial, burial temperatures from the nine t-T paths range from 50 Paleozoic burial depths to a temperature of 120°C, renewed ° ° to 180 C prior to Ancestral Rocky Mountain uplift which Mesozoic burial to a maximum temperature of 180 C, post- exposed Proterozoic basement to the surface by around ° fi Laramide residence at a temperature of 60 C, and nal uplift 285 Ma. The Mesozoic segment of the inverse model is tightly fi to the surface during Rio Grande rift extension. The best- tt- constrained and suggests that the sample reached maximum T path yields a date-eU envelope that encompasses nine of temperatures of 150–160°C. Inverse modeling shows mini- the 18 total ZHe dates. This model fails to account for the mal cooling associated with the Laramide orogeny, and most oldest as well as the youngest ZHe dates at low eU values. paths remain at temperatures >130°C until approximately The large spread in ZHe dates from 19 to 402 Ma at eU values 30 Ma, when the sample was rapidly cooled to near-surface <200 ppm suggests that other factors, such as crystal zoning, temperatures. may also significantly affect ZHe dates in the Franklin Mountains. 6.2. Franklin Mountains. A total of 13 good paths were obtained from inverse modeling of samples collected from 5.2.3. Cookes Range. The best-fit forward model for the the Red Bluff granite (Figure 7(b)). Field relationships sug- Cookes Range uses zircon grain sizes of 46 ± 9 μm gest that this granite was emplaced at relatively shallow (Figure 6(c)). Unlike the other two locations, here, we show crustal levels, as it intrudes into and cuts an ignimbrite within the results of three separate best-fit models. Two models the Thunderbird Group. These volcanic rocks are interpreted involve early cooling, whereas the third model is constant to be the erupted equivalent of the Red Bluff granite, which is cooling from the time of crystallization. Each of the resulting supported by geochemical evidence [67] and overlapping date-eU envelopes are nearly identical at intermediate and geochronologic ages [38]. Inverse models suggest that the high eU values, but diverge at low eU values where no ZHe Red Bluff granite was then reheated to temperatures ranging dates are available. The resulting best-fit t-T paths encompass from 50 to 250°C. While this large uncertainty likely suggests nearly all of the observed ZHe dates. However, the yellow and the ZHe data from the Franklin Mountains may be largely green paths are slightly skewed towards lower eU grains and insensitive to the Proterozoic history, the observation that do not encompass the oldest ZHe date or the three grains at all good paths require some amount of heating is consistent eU values of ~300 ppm. The blue path appears to yield a with a model of possible Proterozoic reburial following crys- slightly better fit with the overall ZHe date-eU trend. tallization. All 13 paths show relatively monotonic reheating 6. Inverse Modeling Approach throughout the Paleozoic and Mesozoic, and converge within a temperature range of 155–190°C by the time of incipient Laramide deformation. These paths then cool rapidly from The forward modelling method described above is successful ° at finding possible t-T paths that yield date-eU curves that approximately 80 to 50 Ma to temperatures less than 100 C. are consistent with the observed data. However, with that A period of slower cooling is observed beginning at 50 Ma, – fi approach, it is only feasible to investigate a limited number and by 25 15 Ma, all paths show a nal rapid pulse of cooling of possible t-T paths. Here, an inverse modeling approach to surface temperatures. is used to continue to refine the thermal histories and explore additional possible t-T paths that may have been overlooked 6.3. Cookes Range. The inverse model for the Cookes Range during the forward modeling analysis. In this section, we yielded a total of nine good paths that are consistent with present a single inverse model for each of the three study the available geologic constrains and synthetic grains locations. The inverse models use synthetic grains that repre- (Figure 7(c)). Following crystallization of this sample, all sent binned averages of the observed ZHe data. This is neces- t-T paths show rather monotonic Proterozoic cooling sary because of the large number of individual ZHe dates for towards surface temperatures. However, all t-T paths pre- each location and the limited number of inputs available in serve a pulse of more rapid cooling between 800 and HeFTy. Complete inverse modeling methods and data inputs 500 Ma that terminated at near-surface temperatures. As are provided in Table S1 and Figure S1. with previous inverse models from the Carrizo and Franklin Mountains, the Paleozoic thermal history of this sample is 6.1. Carrizo Mountains. The final inverse model for the relatively unconstrained. However, the nine paths suggest Carrizo Mountains produced nine paths with a good fitto the sample reached maximum burial temperatures that range the input synthetic grains (Figure 7(a)). These paths suggest from 85 to 180°C prior to Ancestral Rocky Mountain uplift.

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0 1000 900 100 800 200 700 300 600 500 400 400 Carrizo Mountains

ZHe date (Ma) 300 500 Paths tried: 50,000 Acceptable paths: 50 200 600 Good paths: 9 100 700 0 1800 120014001600 1000 800 600 400 200 0 0 400 800 1200 1600 2000

Time (Ma) eU (ppm)

Synthetic grains used in thermal history modeling ZHe dates from samples 18VH01 and 18VH02 (a) 0 500 50 Franklin Mountains 400 100 Paths tried: 50,000 150 Acceptable paths: 298 300 Good paths: 13 200 250 200 ZHe date (Ma) 300 100 350 400 0 1800 14001600 1200 1000 800 600 400 200 0 0 200 400 600 800 1000

Time (Ma) eU (ppm)

Synthetic grain used in thermal history modeling ZHe dates from samples 15FR03 and 17FR03 (b) 0 700

50 600 100 500 150 400 200 300 250 Cookes

Range ZHe date (Ma) 200 300 Paths tried: 50,000 350 Acceptable paths: 81 100 Good paths: 9 400 0 1800 14001600 1200 1000 800 600 400 200 0 0 200 400 600 800 1000

Time (Ma) eU (ppm)

Synthetic grains used in thermal history modeling ZHe dates from sample 16CR02 (c)

Figure 7: Inverse modeling results for (a) the Carrizo Mountains, (b) the Franklin Mountains, and (c) Cookes Range. For each inverse model, geologic constraints and assumptions are shown as blue boxes. Panels on the right show the synthetic grains used in red and the predicted date-eU curve for each “good” path that was produced during inverse modeling. Synthetic grains were created by averaging the observed ZHe data. Complete modeling assumptions, inputs, and details on creating synthetic grains are provided in Table S1 and Figure S1. The red dashed lines in each plot are the best-fit forward models from each location.

The uplift history during the Ancestral Rocky Mountain during renewed heating during Mesozoic burial, the paths deformational event is also relatively unconstrained, with converge to a maximum temperature of 140–180°C. This uplift temperatures ranging from 40 to 165°C. However, sample shows a strong component of cooling that is coeval

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1465 Ma 1785 Ma 1250 Ma 0 800 50 700 Cookes Range 100 600 150 500 200 400 250 300 ZHe date (Ma) date ZHe 1465 Ma 300 200 1785 Ma Deposition of 1250 Ma Zero inheritance 350 Abo Formation 100 400 0 1800 120014001600 1000 800 600 400 200 0 0 200 400 600 800 1000

Time (Ma) eU (ppm) (a) 1850 Ma 1465 Ma 1710 Ma 0 800 50 700 1250 Ma 1625 Ma 100 600 Franklin 150 Mountains 500 200 400 250 300

Deposition of (Ma) date ZHe 1710 300 Bliss Sandstone 200 1250 Ma Ma 1465 Ma Zero inheritance 350 100 1625 Ma 1850 Ma 400 0 1800 120014001600 1000 800 600 400 200 0 0 200 400 600 800 1000

Time (Ma) eU (ppm) (b)

Figure 8: Detrital inheritance curves for sandstone samples collected in the Cookes Range and Franklin Mountains. (a) Cookes Range detrital ZHe dates and inheritance curves. The vertical green bar is the time of deposition of the Abo Formation. Gray lines are the “good” paths, and the red curve is the best-fit path from inverse modeling. The red path is used to create the zero-inheritance curve, and maximum inheritance curves are calculated using observed peaks in detrital zircon U-Pb ages from the Abo Formation [69]. (b) Franklin Mountains detrital ZHe dates and inheritance curves. The vertical green bar is the time of deposition of the Bliss Sandstone. Gray lines are the “good” paths, and the red curve is the best-fit path from inverse modeling. The red path is used to create the zero-inheritance curve, and maximum inheritance curves are calculated using observed peaks in detrital zircon U-Pb ages from the Bliss Sandstone [72].

with Laramide deformation. All paths display rapid cooling An additional seven ZHe dates were obtained from the to lower temperatures of 50–70°C beginning 60–70 Ma and Cambrian-Ordovician Bliss Sandstone in the Franklin ending by approximately 45 Ma. The sample remained Mountains, including three presented by Biddle et al. within this temperature range until approximately 25 Ma, [25]. Ideally, more detrital zircon grains would have been when it was cooled to near-surface temperatures. selected, but these samples did not yield sufficient zircon grains that were euhedral and of sufficient size. 7. Detrital Sample Inheritance Curves For both samples, we rely on the inverse modeling con- straints described above to aid with the construction of inher- Two Paleozoic detrital samples were collected for ZHe ther- itance envelopes. To do this, we use the best-fit path from the mochronology from the Cookes Range and the Franklin inverse models as the assumed post-depositional thermal his- Mountains. ZHe data from these locations were obtained in tory of the detrital samples (Figure 8). The best-fit path order to compare to ZHe data from crystalline basement highlighted in red in Figure 8 represents the thermal history samples, test whether zircon grains had been reset after depo- used to create the zero inheritance curves. The maximum sition, and test for possible influences of magmatism. A total inheritance curves are created using published detrital zircon of six ZHe dates were obtained from the Permian Abo U-Pb ages for the Permian Abo Formation and Cambrian- Formation from the Cookes Range [25]. Here, the Abo Ordovician Bliss Sandstone in southern New Mexico. Below, Formation consists of fine- to medium-grained sandstone. we describe each sample in more detail.

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7.1. Cookes Range. The Abo Formation in the Cookes Range 8. Discussion lies approximately 600 m above the Proterozoic granite based on thicknesses of older Paleozoic units [68]. Assuming a The analysis above incorporates multiple methods to con- geothermal gradient of 25°C/km, this is equal to a temper- strain a ZHe date-eU dataset. Southern New Mexico and ature difference of 16°C. This difference is likely minimal western Texas have experienced a prolonged and complex in the resulting date-eU curves such that the thermal his- tectonic history and, while the ZHe dataset may not be sensi- tory of the Proterozoic granite is likely similar to the post- tive to all tectonic and sedimentation events, the datasets do depositional thermal history of the overlying detrital sam- constrain parts of the overall history with varying confidence. ple. The zero-inheritance date-eU curve that is constructed Below, we first comment on similarities and differences using the best-fit path from the inverse model is combined between the forward and inverse modeling methods. We with three other maximum inheritance curves to create an then use the inverse modeling and detrital inheritance curve inheritance envelope (Figure 8(a)). The maximum inheri- results to explore the more regional tectonic significance. tance curves use zircon crystallization ages of 1250 Ma, These results are used to comment on multiple tectonic 1465 Ma, and 1785 Ma, based on peaks in U-Pb detrital events in the study area spanning more than a billion years zircon ages from the Abo Formation in southern New of Earth history. Mexico [69]. When compared with the inheritance enve- lopes, detrital ZHe dates all fall along the zero-inheritance curve or below the inheritance envelope (Figure 8(a)). If the 8.1. Refining the Zircon (U-Th)/He Method: Comparison of post-depositional thermal history is correct, then these Forward and Inverse Models. Our modeling approach uses observations would suggest that the three grains that lie along two methods for investigating thermal histories using ZHe the zero-inheritance curve could be derived from a ca. 280– date-eU relationships. The forward modeling approach was 300 Ma source. However, probability density plots do not designed to incrementally narrow the possible thermal his- show abundant Paleozoic detrital zircon U-Pb ages [69]. This tory by varying separate segments of the t-T path. This would also not account for the three ZHe dates that fall below approach was successful at producing best-fit paths for each the inheritance envelope. Alternatively, all ZHe detrital region that yield date-eU curves that are consistent with the grains could be derived from Proterozoic sources if the observed ZHe data (Figure 6). However, because the entire post-depositional history is modified to include a thermal thermal history of each region is considered (over a billion pulse to partially reset ZHe dates. A granodioritic stock and years of time), the best-fit paths are inevitably simplistic, several basaltic and dacitic dikes and sills with reported allowing for the possibility that additional t-T paths might K-Ar and 40Ar/39Ar ages that range from 38 to 45 Ma exist that are also consistent with the data. Our next attempt [68, 70, 71] intrude Paleozoic and Mesozoic rocks in the to constrain each thermal history was to use an inverse model Cookes Range [68], possibly resulting in partial resetting approach to test for this possibility by investigating a total of of zircon grains. Partial resetting of detrital ZHe dates 50,000 paths per sample, rather than tens of paths with the from the Abo Formation is also supported by the date- forward model approach. eU plots in Figure 2, where Abo Formation ZHe dates In the Carrizo Mountains, the best-fit forward model and are all younger than the Proterozoic ZHe dates at low the inverse modeling paths are similar (Figure 7(a)), although eU values. the inverse modeling results allow for more complexity in the sample’s thermal history. For example, the inverse modeling paths suggest that the sample may have remained at shallow 7.2. Franklin Mountains. The Bliss Sandstone sample was depths in the crust from 1000 to 600 Ma, followed by a sec- collected approximately 300 m stratigraphically above the ond pulse of uplift. The inverse modeling paths also allow Proterozoic samples used for inverse modeling, suggesting for a wider range of Paleozoic burial temperatures prior to a temperature difference of only 7–8°C, assuming a geother- Ancestral Rocky Mountain uplift. The largest difference mal gradient of 25°C/km. The best-fit inverse path should between the two modeling results is during 80–40 Ma therefore be a good approximation of the postdepositional Laramide deformation. The forward model predicts signifi- thermal history of the Bliss Sandstone (Figure 8(b)). cant Laramide uplift, followed by a smaller pulse of Rio Maximum inheritance curves were constructed using zircon Grande rift exhumation. The inverse model results, however, crystallization ages of 1250 Ma, 1465 Ma, 1625 Ma, 1710 Ma, suggest that Laramide exhumation was minimal, and much and 1850 Ma based on U-Pb detrital zircon age probability of the final cooling was accomplished within the last 30 Ma plots from the Bliss Sandstone in southern New Mexico (Figure 7(a)). [72]. The resulting inheritance envelope encompasses all In the Franklin Mountains, the best-fit forward model but one ZHe date from the Bliss Sandstone (Figure 8(b)). and the inverse model paths are also similar for most of the These observations are consistent with the zircon grains in thermal history (Figure 7(b)). The biggest inconsistency the Bliss Sandstone in the Franklin Mountains being between the two is from 1000 to 500 Ma, where the best-fit derived from Proterozoic sources, similar to other regions forward model remains at surface temperatures, whereas of southern New Mexico. These results also suggest that the inverse model paths all show some amount of reheating the post-depositional t-T path for the Bliss Sandstone (red prior to Paleozoic deposition. However, both approaches curve in Figure 8(b)) is a plausible solution for its thermal predict similar Mesozoic burial temperatures followed by history. more recent cooling to surface temperatures.

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Similar to the other two locations, comparison of forward coinciding with the breakup of Rodinia and the breakup of and inverse modeling results from the Cookes Range also Pangaea. They suggest a genetic relationship between conti- shows the widest discrepancy during the Proterozoic nental uplift/denudation and the supercontinent cycle that (Figure 7(c)). Whereas our forward modeling analysis is also observed in rocks from southern New Mexico and yielded multiple Proterozoic cooling paths that are consistent western Texas. with the observed data, the inverse modeling results are most consistent with steady cooling from 1600 to 500 Ma, rather 8.3. Ancestral Rocky Mountain Deformation. In southern than earlier pulses of rapid cooling. The inverse models also New Mexico and western Texas, Ancestral Rocky Mountain suggest that the Proterozoic history of this region is more intracontinental deformation resulted in a series of uplifts complex than the simplistic forward models, possibly involv- and sedimentary basins (e.g., [19]). The sedimentary record ing multiple pulses of cooling separated by hundreds of mil- preserved in Ancestral Rocky Mountain basins suggests dia- lions of years. The Phanerozoic segment of the forward chronous subsidence histories that young to the west, a pat- model is consistent with the inverse model results, although tern that is interpreted to reflect diachronous closure of the the inverse model paths allow for more variability. Ouachita suture [74]. Within the study area, the Pedregosa Overall, the comparison between forward and inverse and Orogrande basins experienced similar times of peak sub- modeling results is similar at all three locations. However, sidence rates from approximately 300 to 285 Ma [74]. Inverse in instances such as this where the goal is to constrain the models for the Carrizo Mountains and Cookes Range include entire thermal history from ZHe data, the forward modeling Paleozoic constraint boxes that allow for a wide range of approach is overly simplistic and is unable to explore the full burial temperatures (50–300°C) from 450 to 290 Ma range of possible thermal histories. Thermal histories from (Figure 7). Good paths are not sensitive to peak burial tem- these regions are complex and a thorough investigation is peratures in either model other than they could not have not plausible using a forward model approach where tens been heated to temperatures >180°C. These paths are also of paths are investigated. An inverse modeling approach that not sensitive to the timing of reheating, with paths reaching tests thousands of paths is better equipped to fully explore peak temperatures from 450 to 300 Ma. possible thermal histories in order to more confidently inter- These observations suggest that, at least in this study area, pret the results. As a result, we will refer to the inverse model ZHe datasets are not sensitive to either the timing or magni- results during the remainder of the discussion below. tude of Ancestral Rocky Mountain deformation (Figure 9). Instead, ZHe date-eU patterns are largely controlled by the 8.2. Proterozoic Exhumation. In the Carrizo Mountains, timing and magnitude of both older and younger events that inverse modeling results combined with existing 40Ar/39Ar affected the thermal history of these rocks. These observa- data [41] suggest rapid cooling to temperatures <150°Cat tions are useful for possible future studies. In order to inves- 1030–1000 Ma (Figure 7(a)). This period of cooling is similar tigate burial and exhumation related to the Ancestral Rocky to 40Ar/39Ar cooling ages and likely reflects movement along Mountain event using ZHe thermochronology, we suggest the Streeruwitz thrust during continent-continent collision either focusing on a location with a less complicated tectonic (Figure 1(b)). All but three of the paths remain at tempera- history so that the date-eU pattern is largely controlled by tures higher than approximately 75°C until 600 Ma. These Ancestral Rocky Mountain deformation or combining ZHe results are consistent with sedimentological analyses of the data with additional datasets that constrain either the older synorogenic Hazel Formation in the footwall of the Streeru- or younger tectonic history. witz thrust. Metavolcanic and metasedimentary clasts derived from the Carrizo Mountain Group are notably absent 8.4. Mesozoic Rifting. Southwestern New Mexico, southeast- in the Hazel conglomerate, suggesting that these rocks were ern Arizona, and northern Sonora, Mexico experienced rift- most likely not exhumed to the Earth’s surface at this time ing starting in the Late Jurassic which created the Bisbee [73]. Instead, most inverse paths preserve a second pulse of basin [48, 49]. The rift basin strata in southern New Mexico cooling to near-surface temperatures from 600 to 500 Ma reached a maximum thickness of about 2800 m by Albian (Figure 7), which coincides with the final breakup of Rodinia time [48]. Near the base of the section (TSA1 of Lawton (Figure 9). Although slightly less constrained, final exhuma- et al., in press), these strata are interlayered with astheno- tion in the Franklin Mountains and Cookes Range occurred spherically derived mafic volcanic rocks [52]. The strata on within a similar timeframe. In the Franklin Mountains, the the flanks of the basin are much thinner or nonexistent, as Red Bluff granite was emplaced at shallow depths and subse- the rift shoulder was a topographic high during basin devel- quently buried, consistent with geochemical studies of the opment. The samples from this study were all along the Red Bluff granite that suggest it formed within an extensional inferred edge of the rift (Lawton et al., in press), but they still stress field [39]. Good paths for the Franklin Mountains and could have experienced the elevated geothermal gradients Cookes Range both preserve a pulse of cooling to near- that are common in continental rifts (e.g., [75, 76]). In addi- surface temperatures between approximately 800 and tion, the injection of mafic magmas into the upper crust in 500 Ma (Figures 7(b) and 7(c)). These results are similar to the areas near our study area could have been a factor in con- previous ZHe thermochronologic investigations. DeLucia ducting heat from the mantle. Gradients in rifts can exceed et al. [14] presented ZHe data and inverse modeling results 60°C/km [77] but can decrease rapidly after extension ends. from the Ozark Plateau of Missouri that document pulses These elevated geothermal gradients are a likely cause of the in exhumation from 850 to 680 Ma and 225 to 150 Ma, maximum temperatures of all three sample locations. In the

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Grenville Orogeny Ancestral Rocky Laramide (Final assembly Mountain Orogeny of Rodinia) Breakup Mexican RGR of Rodinia YO MO Border Rif 0 Carrizo 100 Mountains 200

300

400

500

600 0 100 Franklin Mountains 200

300

400 0 Cookes 100 Range 200

300

400 (n=55) 40Ar/39Ar data (Shaw et al., 2005) AFT and AHe data (Donahue, 2016) ZHe data (this study)

140016001800 1200 1000 800 600 400 200 0

Time (Ma)

Figure 9: Tectonic summary of the thermal history of each study site. Black lines are “good” paths from the inverse modeling. Vertical bars of different colors represent the duration of main tectonic events to have affected southern New Mexico and western Texas. Kernel density estimation (KDE) plots on the bottom show compiled 40Ar/39Ar data [24], apatite fission-track and (U-Th)/He data [94] and ZHe data from this study. All KDE diagrams were plotted using DensityPlotter [95]. YO: Yavapai orogeny; MO: Mazatzal orogeny; RGR: Rio Grande rift.

Bisbee basin, rifting was followed by shortening resulting in structures do not extend to the Carrizo Mountains, where the creation of a basin in Albian time dominated by dynamic Laramide deformation consists of small-amplitude folds subsidence in response to arc-continent collision in Mexico and minor thrust faults [81]. (Lawton et al., in press). Thus, geothermal gradients likely Laramide magmatism in southwestern New Mexico reverted to normal values by 100 Ma in the study area. occurred in several pulses between about 75–70 Ma and 60–55 Ma, and 45–40 Ma [82, 83]. Evidence includes tuffs 8.5. Laramide Orogeny and Magmatism. Southern New [82], andesite boulders in basins [84], plutonic rocks such Mexico and western Texas are at the eastern limit of Lara- as the Sylvanite complex [79], and various intrusions associ- mide deformation. In southern New Mexico, NE-directed ated with ore deposits [85]. Our inverse modeling results are shortening produced a series of NW-trending basins and designed to include only exhumation and cooling during uplifts that range in age from Late Cretaceous to Eocene Laramide uplift, although the nonuniqueness of these models [22]. Most, if not all, Laramide faults are reactivated normal does not preclude some amount of Laramide reheating. In faults that developed during the formation of the Mexican addition to the exposed volcanic and plutonic rocks, deeper Border rift [22, 78], and many thrust faults have fault throws Laramide plutons could also have affected ZHe dates. For that exceed several kilometers (e.g., [79]). Faults and folds example, Murray et al. [86] provide numerical models that related to Laramide shortening follow the Texas-Mexico bor- suggest midcrustal plutons can reset low-temperature ther- der towards Big Bend National Park [80]. However, these mochronologic ages in upper crustal rocks. This is a result

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that is not explored here, but could also be a contributing fac- refine these results and can test tens or hundreds of thou- tor in the observed ZHe data. sands of possible t-T paths. Cooling that is coeval with the Laramide orogeny is pre- Our thermal history inverse modeling results lead to the served in inverse models from the Cookes Range and the following conclusions: (1) the Red Bluff granite in the Frank- Franklin Mountains. In the Cookes Range, paths show a lin Mountains was likely buried and exhumed prior to depo- pulse of cooling beginning 60–70 Ma and ending by 45 Ma sition of Paleozoic sediments, and the unconformity at this (Figure 9). The Franklins preserve a similar time of cooling location is therefore a compound erosional surface; (2) Prote- from approximately 80 to 50 Ma. Although scattered Lara- rozoic exhumation at all three locations occurred between mide basins in southern New Mexico were present during 800 and 500 Ma, coinciding with the break-up of Rodinia; the Late Cretaceous, the majority of basins and uplifts devel- (3) all three study sites record elevated temperatures at oped largely during the Paleocene–Eocene [22], overlapping approximately 100 Ma that likely reflects elevated heat flow with the inverse models from the Cookes Range and Franklin during continental rifting to form the Mesozoic Bisbee basin. Mountains. In contrast, the Carrizo Mountains show almost These results suggest that elevated heat flow within the rift no cooling during this timeframe, and Cenozoic cooling to extended into the rift shoulder, rather than being confined near-surface temperatures was delayed until after 30 Ma to the rift axis; (4) Laramide cooling occurred from 70 to (Figure 9). These observations are consistent with the loca- 45 Ma in the Cookes Range and 80 to 50 Ma in the Franklin tion of the Carrizo Mountains at the edge of the belt of Mountains. In contrast, the Carrizo Mountains shows no Laramide shortening where total deformation was minimal. cooling during Laramide shortening, which likely reflects its location at the edge of observed Laramide deformation; and 8.6. Rio Grande Rift. The Neogene Rio Grande rift of south- (5) all three study sites preserve a pulse of cooling that begins ern New Mexico disrupted all previous tectonic elements at 30–25 Ma, simultaneous with observed cooling in the cen- and imparted the present topographic grain in the landscape. tral and northern segments of the rift. At the latitude of El Paso, Texas, the rift makes a sudden bend In contrast, these data provide almost no information on and structures south of here trend NW-SE instead of N-S as the timing or magnitude of Ancestral Rocky Mountain defor- they do in the central and northern segments of the rift. In mation. These data provide important information on the Colorado and New Mexico, combined apatite fission-track broad thermal history of southern New Mexico and western and apatite (U-Th)/He data suggest that extension was Texas and highlight the effectiveness of using the ZHe largely synchronous across this region, and these data record method in tectonically complex regions. These data also help a main pulse of cooling from 25 to 10 Ma that is interpreted to bridge the gap between lower temperature systems such as to reflect extensional exhumation (e.g., [28, 87–89]). apatite (U-Th)/He and fission-track and higher temperature Inverse models from all three study locations preserve a methods of thermochronology such as 40Ar/39Ar. pulse of cooling that is coeval with the development of the Rio Grande rift. Rapid cooling began at approximately Data Availability 25 Ma in the Cookes Range, 25–15 Ma in the Franklin Moun- tains, and 30 Ma in the Carrizo Mountains (Figure 9). The data supporting the results of this study include zircon Although this timeframe is only a fraction of the entire ther- (U-Th)/He dates, grain sizes, and U, Th, Sm, and He concen- mal history investigated (over a billion years), it seems to trations. This information, along with sample locations, is have a significant effect on the resulting ZHe date-eU curve included in Table 1. at each location. These results are consistent with previously published low-temperature thermochronologic data and Conflicts of Interest modeling from the southern rift that suggest that the south- fl ern segment of the rift was active at the same time as the cen- The authors declare that they have no con icts of interest. tral and northern segments and that cooling of rocks was accomplished through fault exhumation rather than through Acknowledgments magmatic injection [28, 87]. JWR was supported by NSF-EAR 1624538 and JMA was sup- ported by NSF-EAR 1624575. ZHe data were obtained from 9. Conclusions the (U-Th)/He thermochronology lab at CU Boulder. We thank Rebecca Flowers and James Metcalf for their help with A total of 55 individual grain ZHe dates are presented from data acquisition and discussions and Michelle Gavel for assis- three ranges in southern New Mexico and western Texas. tance with mineral preparation. Two anonymous reviewers ZHe dates span hundreds of millions of years and record provided very useful feedback that helped improve the qual- long-term thermal histories of Proterozoic crystalline rocks. ity of this manuscript. These rocks have experienced a prolonged tectonic history involving multiple periods of cooling and reheating. A com- Supplementary Materials bination of forward and inverse modeling techniques was used to investigate plausible thermal histories of these sam- Table S1: thermal history model inputs and assumptions for ples. We find that although a forward modeling approach is forward and inverse models. Figure S1: description of geo- advantageous for quickly comparing several dissimilar ther- logic constraints used in forward and inverse models. mal histories, an inverse modeling approach can further (Supplementary Materials)

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