# An Assessment of Monazite Fission-Track Thermochronology as a Proxy for Low-Magnitude Cooling, Catalina-Rincon Metamorphic Core Complex, AZ, USA

Gilby Jepson, Barbara Carrapa, Sean Jones, Barry P Kohn, Andrew J W Gleadow, Sarah W M George, Caden J Howlett, Kerry Gallagher, Alex N Frickenstein, George Gehrels, et al.

► To cite this version:

Gilby Jepson, Barbara Carrapa, Sean Jones, Barry P Kohn, Andrew J W Gleadow, et al. An Assessment of Monazite Fission-Track Thermochronology as a Proxy for Low-Magnitude Cooling, Catalina-Rincon Metamorphic Core Complex, AZ, USA. Geochemistry, Geophysics, Geosystems, 2025, 26 (4), pp.e2024GC011881. 10.1029/2024GC011881. insu-05045708

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Distributed under a Creative Commons Attribution 4.0 International License
# Geochemistry, Geophysics, Geosystems®

## RESEARCH ARTICLE

10.1029/2024GC011881

**Key Points:**

* Age-elevation relationships suggest monazite fission-track (MFT) can record unique events correlated with low-magnitude cooling

* The annealing model for MFT does not address all the sources of annealing and overestimates its temperature sensitivity

* MFT ages in the Catalina-Rincon Mountains, AZ, correlate with a period of Pliocene-Pleistocene precipitation in the southwest USA

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

G. Jepson,
gjepson@ou.edu

**Citation:**

Jepson, G., Carrapa, B., Jones, S., Kohn, B. P., Gleadow, A. J. W., George, S. W. M., et al. (2025). An assessment of monazite fission-track thermochronology as a proxy for low-magnitude cooling, *Catalina-Rincon Metamorphic Core Complex, AZ, USA. Geochemistry, Geophysics, Geosystems*, 26, e2024GC011881. https://doi.org/10.1029/2024GC011881

Received 9 SEP 2024
Accepted 15 MAR 2025

**Author Contributions:**

**Conceptualization:** Gilby Jepson,
Barbara Carrapa, Sean Jones, Barry
P. Kohn, Andrew J. W. Gleadow

**Data curation:** Gilby Jepson

**Formal analysis:** Gilby Jepson,
Barbara Carrapa, Barry P. Kohn, Andrew
J. W. Gleadow, Alex N. Frickenstein

**Funding acquisition:** Gilby Jepson,
Barbara Carrapa, Barry P. Kohn, Andrew
J. W. Gleadow

© 2025 The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## An Assessment of Monazite Fission-Track Thermochronology as a Proxy for Low-Magnitude Cooling, Catalina-Rincon Metamorphic Core Complex, AZ, USA

Gilby Jepson¹,² ID, Barbara Carrapa², Sean Jones³, Barry P. Kohn³ ID, Andrew J. W. Gleadow³ ID, Sarah W. M. George¹,², Caden J. Howlett², Kerry Gallagher⁴ ID, Alex N. Frickenstein⁵, George Gehrels², and Antoine Triantafyllou⁶

¹School of Geosciences, University of Oklahoma, Norman, OK, USA, ²Department of Geosciences, University of Arizona, Tucson, AZ, USA, ³School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Melbourne, VIC, Australia, ⁴Géosciences Rennes, UMR6118, CNRS, Université de Rennes, Rennes, France, ⁵Stephenson School of Biomedical Engineering, University of Oklahoma, Norman, OK, USA, ⁶Lyon Geology Laboratory - Earth, Planets and Environment (LGL-TPE), Université Claude Benardé Lyon 1, ENS de Lyon, UMR-CNRS, UMR, Villeurbanne, France

**Abstract** Conventional low-temperature thermochronology can resolve rock cooling over geological timescales (>1 Myr) typically associated with ~6–2 km of erosion, often induced by tectonic processes. Lower magnitude erosional events, however, produced by surface processes remain difficult to resolve. Here, we apply monazite fission-track (MFT) thermochronometry to the tectonically well-constrained Catalina-Rincon metamorphic core complex (MCC) to assess its ability to resolve low-temperature cooling in the upper-crust over ~10⁶ years timescales. Established low-temperature thermochronology (apatite fission-track [AFT] and apatite and zircon [U-Th-Sm]/He) record timing of MCC and subsequent Basin and Range tectonic exhumation (26–20 Ma and 15–12 Ma, respectively). Monazite fission-track data were obtained from samples collected in two vertical elevation profiles. The eastern transect displays a Late Miocene—Pliocene age-elevation trend and implies a two-stage cooling history related to late-stage footwall uplift associated with Basin and Range normal faulting (~7.5–5.1 Ma) and subsequent Pliocene—Pleistocene erosion (~4.0–3.0 Ma). The northwestern transect data suggest a single period of rapid Pliocene—Pleistocene cooling (~2.8–1.0 Ma). Thermal history modeling, however, fails to find a solution that satisfies the MFT annealing model with the AFT annealing and (U-Th-Sm)/He diffusion models. This suggests that the present MFT thermal annealing model does not account for all sources of annealing. We suggest that Pliocene—Pleistocene MFT ages may record a period of climate-enhanced erosion during a known phase of increased precipitation associated with the development of the North American Monsoon.

**Plain Language Summary** It is very difficult to date the modern topography. This is because the modern topography represents an aggregate of surface processes (e.g., storms, rivers) and tectonic processes (e.g., faults) acting over geologic time (millions of years). We are very good at dating the modern surface through geomorphology and faults through low-temperature thermochronology. However, these tectonic and surface processes remain disconnected as the upper 1–2 km of the Earth's surface is very difficult to resolve. In this manuscript, we present an assessment of a relatively new method which has the possibility of dating that key window, monazite fission-track (MFT). Using the uranium to fission-track decay system in monazite, we collected samples from the Catalina-Rincon mountains in southern Arizona, USA. The MFT dating records ages that are significantly younger than our previously used methods for dating faults. These results are confounded by elevated concentrations of U and Th, mineral growth zones, and our present understanding of how damage is preserved in monazite. Despite these issues, this work suggests that the MFT ages correspond to a period of increased precipitation in the Pliocene—Pleistocene which is associated with the development of large river-valleys in the Catalina-Rincon Mountains.

## 1. Introduction

Modern topography reflects the interplay between tectonic and surface processes (e.g., England & Molnar, 1990). Rock uplift is facilitated by contractionsal or extensional faults which produce relief (e.g., Beaumont et al., 1996; Fitzgerald et al., 1995; Stockli et al., 2000). The relief can be concurrently or subsequently modified by surface processes including precipitation, river incision, and glaciation (e.g., Herman et al., 2013; Thiede & Ehlers, 2013).
**Investigation:** Gilby Jepson,
Barbara Carrapa, Sean Jones, Barry
P. Kohn, Andrew J. W. Gleadow, Sarah
W. M. George, Caden J. Howlett,
Kerry Gallagher, George Gehrels,
Antoine Triantafyllou

**Methodology:** Gilby Jepson,
Barbara Carrapa, Sean Jones, Barry
P. Kohn, Andrew J. W. Gleadow, Sarah
W. M. George, Caden J. Howlett, Alex
N. Frickenstein, George Gehrels,
Antoine Triantafyllou

**Project administration:** Gilby Jepson,
Barbara Carrapa, Barry P. Kohn, Andrew
J. W. Gleadow

**Resources:** Gilby Jepson

**Software:** Kerry Gallagher,
George Gehrels

**Supervision:** Gilby Jepson,
Barbara Carrapa

**Validation:** Gilby Jepson

**Visualization:** Gilby Jepson, Barry
P. Kohn, Andrew J. W. Gleadow, Sarah
W. M. George

**Writing – original draft:** Gilby Jepson

**Writing – review & editing:**
Gilby Jepson, Barbara Carrapa,
Sean Jones, Barry P. Kohn, Andrew
J. W. Gleadow, Sarah W. M. George,
Caden J. Howlett, Kerry Gallagher, Alex
N. Frickenstein, George Gehrels,
Antoine Triantafyllou

As a result, present-day landscapes represent an aggregate of polyphase erosional histories as a response to tectonic and paleoclimate changes over geologic time. A given landscape will then reflect the impact and relative importance and the associated variance of these different factors. This makes isolating specific mechanisms and their associated timing and duration challenging (e.g., Avouac & Burov, 1996; Blythe et al., 2007; Burbank et al., 2003; Willett & Brandon, 2002; Whipple, 2009).

A series of sedimentological, geomorphic, and geo-thermochronologic methodologies are employed to resolve the evolution of the modern relief. For example, geomorphic indices such as knickpoints and channel steepness provide quantifiable constraints on landscape evolution (e.g., Duvall et al., 2004; Crosby & Whipple, 2006; Hoke et al., 2015). Integrated with cosmogenic radionuclides such as ¹⁰Be and ²⁶Al, these geomorphic indices can resolve relief forming processes on timescales of 10²–10⁵ years to depths of up to 10s, perhaps 100s, of meters (e.g., Kudriavtseva et al., 2023; Schaefer et al., 2022; Tanski et al., 2024). On the 10⁶–10⁹-year timescale, low-temperature thermochronology has long been used to constrain erosion in response to both tectonic and climate driven surface processes (e.g., Reiners & Brandon, 2006; Schildgen & van der Beek, 2019; Thiede et al., 2004). However, established low-temperature thermochronometers such as apatite (U-Th-Sm)/He (AHe) and apatite fission-track (AFT) are typically sensitive to cooling through the ~120–40°C window, which roughly corresponds to crustal depths of ~6–2 km (e.g., Braun et al., 2006). These systems generally remain insensitive to cooling in the upper 2–1 km. This shallow depth range is important in terms of influencing and thus constraining the development and form of modern relief (Figure 1). Thus, there remains a significant temporal (and spatial) disconnect between the methods used to document modern relief and the methods used to quantify tectonic and surface processes through geologic time. In order to better understand surface processes and the history of modern relief, we require a geo-thermochronometer which is sensitive to cooling over temperatures <50°C on timescales between 10⁵ and 10⁷ years. Apatite ⁴He/³He thermochronology and quartz electron spin resonance have been used to document cooling through this key time-temperature window (King et al., 2020; Shuster and Farley, 2004, 2005; Shuster et al., 2005; Tsukamoto et al., 2017). However, the specific laboratory requirements and inherited complications have hampered their widespread usage (e.g., King et al., 2016; Shuster & Farley, 2005).

Monazite fission-track (MFT) is a low-temperature thermochronometer which has a broad partial annealing zone (PAZ) (140–70°C over 10⁶ years; Jones et al., 2021; Weise et al., 2009). In practice, however, it is suggested that MFT records geological timescale annealing primarily over temperatures of <50°C (Jones et al., 2023). As a result, MFT has the potential for constraining rock cooling through the upper ~2–1 km of the Earth's crust. Here, we apply MFT to the Catalina-Rincon metamorphic core complex (MCC), which has a well-documented tectonic history. Integrated with published low-temperature thermochronology from established methods, we apply MFT in conjunction with monazite U-Th-Pb and trace element analysis, apatite U-Pb analysis, zircon and apatite (U-Th-[Sm])He analysis, AFT analysis, and thermal history modeling to assess the potential of MFT as a tool to inform us about upper-crustal cooling.

## 2. Geologic Background

### 2.1. Tectonic Setting

Subduction of Farallon oceanic lithosphere beneath the North American plate in the Late Jurassic generated a magmatic arc and retroarc shortening, causing thickening of North American crust and the development of a regional foreland basin system to the east of the fold-thrust belt (e.g., DeCelles, 2004; Dickinson et al., 1996; Ducea & Barton, 2007; Gehrels et al., 2009; Hamilton, 1969; Oldow et al., 1989; Weil & Yonkee, 2012). Beginning in Late Cretaceous time, flat-slab subduction led to a period of arc-magmatic quiescence in the Sierra Nevada batholith and the generation of basement-involved, thick-skinned structures which partitioned the regional foreland basin (e.g., Bird, 1984; Caylor et al., 2021; Coney & Reynolds, 1977; Constenius et al., 2003; DeCelles, 2004; Saleeby, 2003; Schwartz et al., 2023). This period of subhorizontal subduction led to the generation of regionally widespread peraluminous magmatism (e.g., Bahadori et al., 2022; Chapman et al., 2020; Favorito & Seedorff, 2018) responsible for the emplacement of the Paleocene-Eocene Wilderness Suite, which is exposed throughout the Catalina MCC (Fornash et al., 2013).

Beginning during Eocene time, foundering and westward roll-back of the Farallon slab initiated and is correlated with a regional flare-up and west-southwest-ward migration of silicic back-arc magmatism (e.g., Best et al., 2016; Coney & Reynolds, 1977; Constenius et al., 2003; Copeland et al., 2017; Feeley, 2003). Subsequently, the thickened North American crust and associated high topography (~3,000 m) became gravitationally unstable and
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic block diagram modified after Ault et al. (2019) illustrating low-temperature thermochronometers and examples of relief forming processes. FT—fission-track, He—(U-Th-[Sm])/He, ESR—electron spin resonance.</figcaption></figure>

underwent gravitational collapse forming extensional basins and exhuming metamorphic core complexes from Alberta, Canada to southern Arizona along long-wavelength, low-angle normal (detachment) faults (e.g., Cassel et al., 2018; Davis et al., 2023; Flansburg & Stockli, 2023; Howlett et al., 2021; Wells & Hoisch, 2008; Zuza & Cao, 2022; Zuza et al., 2025).

The Santa Catalina and Rincon mountains in southern Arizona are the product of Oligocene-Miocene MCC tectonics overprinted by subsequent Miocene-Pliocene Basin and Range extension (e.g., Arca & Johnson, 2010; Davis et al., 2023; Dickinson, 1991; Spencer et al., 2022). Exhumation of the Catalina MCC is controlled by the Catalina detachment fault (Figure 2; e.g., Dickinson, 1991). Slip along the Catalina detachment fault initiated in the early Oligocene (~30 Ma; Spencer et al., 2022) and cuts late Proterozoic and Paleozoic sedimentary, metasedimentary, and igneous rocks, exposing Proterozoic to Cenozoic igneous and metamorphic rocks to the upper crust (e.g., Arca & Johnson, 2010; Ducea et al., 2020; Fornash et al., 2013). The main phase of mylonitization associated with exhumation along the Catalina detachment occurred at ~27–22 Ma, synchronous with the emplacement of a series of intrusive magmatic bodies and leucocratic melts (Davis et al., 2023; Ducea et al., 2020; Fornash et al., 2013; Terrien, 2012). Mylonitization ceased by ~22 Ma, however, detachment fault related exhumation continued until ~20 Ma as recorded by multiple low-temperature thermochronometric samples collected from the exposed mylonitic edifice (Davis et al., 2023; Fayon et al., 2000; Jepson et al., 2021, 2022; Keith et al., 1980; Scoggin et al., 2021).

During Miocene time, crustal thinning transitioned from low-angle detachment faulting to a period of high-angle normal faulting associated with Basin and Range extension (e.g., Atwater & Stock, 1998; Dickinson, 2002; McQuarrie & Wernicke, 2005). Across southern Arizona, Basin and Range extension initiated at ~18 Ma and
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Topographic map of the Catalina-Rincon metamorphic core complex (MCC) with major structures after Arca and Johnson (2010). Monazite fission-track, apatite fission-track, zircon fission-track, apatite (U-Th-Sm)/He (AHe), and zircon (U-Th)/He (ZHe) are from this study and Jepson et al. (2022).</figcaption></figure>

extended into the Pliocene (e.g., Dickinson, 1991; McQuarrie & Wernicke, 2005). The mylonitic edifice of the Catalina MCC was cut by a series of high-angle normal faults including the Pirate and Martinez Ranch faults (Figure 2; Davis et al., 2004). Exhumation along footwalls and associated hanging-wall uplift largely occurred between ~15 and 12 Ma and is recorded throughout the exposed Catalina MCC rocks and in synextensional basins (e.g., Dickinson, 1991; Fayon et al., 2000; Jepson et al., 2022; Scoggin et al., 2021). However, evidence of minor Pliocene brittle faulting and exhumation has also been reported (Scoggin et al., 2021), and low-magnitude seismicity continues to the present.

**3. Methods**

**3.1. Sampling**

To assess if and how MFT might be used as a proxy for low-magnitude, upper-crustal cooling, we collected 10 samples along two elevation profiles from the Catalina-Rincon MCC (Table 1). The first transect was collected from the Proterozoic Oracle granite and the Paleocene Wilderness suite mylonite along the northwestern exposure of the Catalina detachment fault footwall (Figure 2). The northwestern transect comprises samples SP-01 (2,258 m), WP-01 (2,004 m), Mz554 (1,608 m), VAT12 (1,270 m), and C2 (934 m) over an elevation range of ~1,300 m. The eastern transect was collected from the Wilderness suite granites of the Catalina-Rincon MCC along the footwall and hanging-wall of the Basin and Range Martinez Ranch Fault (Figure 2; Davis et al., 2004). The eastern transect comprises samples R4 (1,850 m), R3 (1,767 m), R2 (1,582 m), R1 (1,433 m), and R5 (1,327 m) with a total elevation range of ~500 m. Monazite, apatite, and zircon crystals were extracted following standard mineral separation procedures (e.g., Kohn et al., 2019).
<caption>**Table 1**</caption>

AFT—Apatite Fission-Track; AUPb—Apatite U-Pb; Age—Crystallization Age of the Rock; AHe—Apatite (U-Th-Sm)/He; Elev.—Elevation in Meters Above Sea Level; Lat.—North Latitude; Long.—East Longitude Using Coordination System WSM 84; MFT—Monazite Fission-Track; Th-Pb—Monazite Th-Pb; ZHe—Zircon (U-Th)/He

<table><thead><tr><th>Sample</th><th>Method</th><th>Lithology</th><th>Age</th><th>Lat.</th><th>Long.</th><th>Elev. (m)</th></tr></thead><tbody><tr><td colspan="7">Northwestern transect</td></tr><tr><td>SP-01</td><td>MFT/AHe/ZHe/Th-Pb</td><td>Oracle Granite</td><td>Proterozoic</td><td>32.399</td><td>-110.689</td><td>2,258</td></tr><tr><td>WP-01</td><td>MFT/AHe/ZHe/Th-Pb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.367</td><td>-110.718</td><td>2,004</td></tr><tr><td>Mz 554</td><td>MFT/AHe/ZHe/Th-Pb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.354</td><td>-110.723</td><td>1,608</td></tr><tr><td>KJJ-09-07</td><td>AHe/ZHe</td><td>Leatherwood Granodiorite</td><td>Cretaceous</td><td>32.452</td><td>-110.752</td><td>2,337</td></tr><tr><td>VAT12</td><td>MFT/AHe/ZHe/Th-Pb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.353</td><td>-110.858</td><td>1,270</td></tr><tr><td>C2</td><td>MFT/AHe/AFT/Th-Pb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.313</td><td>-110.742</td><td>934</td></tr><tr><td colspan="7">Eastern transect</td></tr><tr><td>R1</td><td>MFT/AHe/AFT/Th-Pb/AUPb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.162</td><td>-110.502</td><td>1,433</td></tr><tr><td>R2</td><td>MFT/AHe/AFT/Th-Pb/AUPb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.16</td><td>-110.508</td><td>1,582</td></tr><tr><td>R3</td><td>MFT/AHe/AFT/Th-Pb/AUPb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.159</td><td>-110.512</td><td>1,767</td></tr><tr><td>R4</td><td>MFT/AHe/AFT/Th-Pb/AUPb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.156</td><td>-110.518</td><td>1,850</td></tr><tr><td>R5</td><td>MFT/AHe/AFT/Th-Pb/AUPb</td><td>Wilderness Granite Suite</td><td>Eocene</td><td>32.16</td><td>-110.496</td><td>1,327</td></tr></tbody></table>

## 3.2. Monazite Fission-Track, U-Th-Pb, and Trace Element Analysis

Fission-track thermochronology is based on the physical damage caused to minerals via the spontaneous decay of $^{238}$U into two equivalently sized isotopes (Hurford & Green, 1983). Thus, all minerals which contain $^{238}$U can theoretically be used for fission-track dating. Due to the necessity of determinable $^{238}$U concentrations and the requirement of established etching and annealing protocols, fission-track methods have traditionally been confined to minerals such as apatite, zircon, and titanite (Fleischer & Price, 1964; Gleadow et al., 1976; Naeser & Dodge, 1969). Fission-tracks have been observed in monazite since the 1970s (Shukoljukov & Komarov, 1970). However, as a consequence of the high neutron capture cross section of elevated Gd concentrations in monazite (the largest of any element), the thermal-neutron induced decay of $^{235}$U, implicit in the external detector method for fission-track dating, is markedly reduced (Weise et al., 2009) and severe nuclear heating during irradiation make this method impossible. This has hampered the development of the MFT method. However, the advent of laser ablation fission-track (LAFT) dating has made fission-track dating of monazite more practical (Fayon, 2011; Hasebe et al., 2004; Weise et al., 2009). Furthermore, studies to improve the etching protocol and to calibrate the PAZ have led to the MFT system being further investigated for use in geological applications (Jones et al., 2019, 2021, 2022, 2023; Weise et al., 2009).

Monazite grains were scattered and mounted in epoxy, polished to reveal an internal section, and etched in 6 M HCl at ~90°C for 60 min following a protocol outlined by Jones et al. (2019, Figure S1 in Supporting Information S1). Plastic 1-inch ring forms were used to prevent monazite loss due to epoxy expansion during the etching process. Jones et al. (2022) refined the etching protocol to suggest that the cores of monazite latent fission-tracks are etched close to their maximum length after 5 min. Further, Jones et al. (2022) have argued that no significant track annealing occurs until >>90 min of etching at 90°C, and hence the 60 min etch time used here is acceptable. Monazite fission-tracks were counted in crystals that were approximately oriented on their (100) face and measured using an Olympus BX51 microscope with an associated digitizing tablet at the University of Arizona fission-track laboratory (Dumitru, 1993) and on a Zeiss Axio Imager 2 microscope and Fijjiontrack ImageJ plugin (Lang et al., 2023) at the Structure, Tectonics, OU Thermochronology (STOUT) laboratory at the University of Oklahoma (OU).

Monazite confined track length measurement protocols are less-well established than those for zircon and apatite (e.g., Donelick et al., 2005; Yamada et al., 1995). Here, we followed the literature available for apatite, zircon, and titanite, which documents the influence of measurement choices and fission-track features on confined length
distributions (e.g., Jonckheere et al., 2007; Ketcham & Tamer, 2021; Tamer & Ketcham, 2023; Yamada et al., 1995). Confined track length measurements were performed on linear tracks which did not intersect with any grain defects (e.g., fractures) and were <15° dip from the surface (Donelick et al., 2005; Laslett et al., 1982). Terminations were measured from the mid-way point along the track tip margin of tracks with tips that were relatively wide indicating sufficient etching (e.g., Tamer & Ketcham, 2023). We note that as the etching protocol for MFT is nascent, “sufficient etching” remains a qualitative judgment. Confined track lengths were not corrected for c-axis projection (Ketcham et al., 2007). In samples with low numbers of spontaneous fission-tracks, we opted for the “all measurable” approach outlined by Tamer and Ketcham (2023).

Monazite <sup>238</sup>U concentration, U-Th-Pb dates, and trace element concentrations for this study were acquired in two analytical runs. The first set of monazite isotopic and trace element data were collected using a Photon Machine Analyte G2 Excimer 193 nm laser ablation (LA) system connected to a Thermo Element2 single-collector High Resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) at the Arizona LaserChron Center. The laser energy density of ~5.6 j/cm<sup>2</sup>, a repetition rate of 7 hz, and a spot diameter of 7 µm were used. An ablation was run for 135 shots. Along with <sup>235</sup>U, <sup>232</sup>Th, and <sup>204–208</sup>Pb, trace elements <sup>88</sup>Sr and <sup>89</sup>Y, and rare earth elements (REE: <sup>139</sup>La, <sup>140</sup>Ce, <sup>146</sup>Nd, <sup>147</sup>Sm, <sup>153</sup>Eu, <sup>157</sup>Gd, <sup>163</sup>Dy, <sup>172</sup>Yb, and <sup>175</sup>Lu) were measured. Monazite standard 44069 was used as a primary standard for the reduction of Th-Pb ratios and trace element concentrations and monazite standard 554 was used as a secondary standard to check accuracy (Harrison et al., 1999; Liu et al., 2012). Mass reduction and isotope ratio determination were performed using an in-house Excel spreadsheet (E2agecalcMON). Detailed methods are available at www.laserchron.org (U-Th-Pb analytical methods—monazite) and Text S1 in Supporting Information S1 and Table S1.

The second set of monazite isotopic and trace element data were collected using an Elemental Scientific Laser New Wave 213 nm solid state LA system connected to a PerkinElmer NexION2000 ICP-MS. The LA-ICP-MS is run by the Mass Spectrometry, Proteomics & Metabolomics Core Facility, which is part of the Department of Chemistry and Biochemistry, University of Oklahoma (OU). A laser energy density of ~5.6 j/cm², a repetition rate of 10 hz, and a spot diameter of 7 µm were used. An ablation was run for 120 shots. At OU, we focused on the collection of radiogenic isotopes <sup>204</sup>Pb, <sup>206</sup>Pb, <sup>207</sup>Pb, <sup>208</sup>Pb, <sup>232</sup>Th, and <sup>238</sup>U, with limited determination of trace elements (<sup>89</sup>Y) and REEs (<sup>139</sup>La). Monazite standard 44069 was used as a primary standard for Th-Pb ratios and trace element concentrations and monazite standard 554 was used as a secondary standard to check accuracy (Harrison et al., 1999; Liu et al., 2012). Mass concentration (X_Trace_Elements_IS) and isotope ratio (U-Pb_Geochronology) determination were performed using Iolite v4 (Barla et al., 2023; Paton et al., 2011). Detailed methods are in Text S2 in Supporting Information S1.

Monazite fission-track ages were calculated using a modified LA fission-track equation after Gleadow et al. (2015) and Vermeesch (2017) using an initial track-length of 10.6 µm (Jones et al., 2019; Weise et al., 2009) and a monazite density of 5.22 g/cm³ (Jones et al., 2023; Nasdala et al., 2020). The fission-track parent <sup>238</sup>U concentration was determined using LA-ICP-MS as described above. The <sup>232</sup>Th isotope also undergoes spontaneous fission, but this is usually not considered an issue in apatite, titanite, and zircon due to the relatively low concentration and long spontaneous fission decay constant of Th. Monazite contains significant (~8.0 wt%) concentrations of Th (Rapp & Watson, 1986); however, given the relatively young rock crystallization age (Paleogene-Eocene) and the extremely long <sup>232</sup>Th spontaneous fission decay constant of 1.2 × 10<sup>-21</sup> yr<sup>-1</sup>, particularly relative to that of <sup>238</sup>U (~7.0–8.7 × 10<sup>-17</sup> yr<sup>-1</sup>; Guedes et al., 2003), we consider the influence of possible <sup>232</sup>Th tracks to be negligible (Holden & Hoffman, 2000). Due to the absence of a suitably characterized MFT standard, single-grain ages were calculated using the absolute method (Gleadow et al., 2015; Vermeesch, 2017). Monazite fission-track central ages were determined using a Radialplotter (Galbraith, 2005; Vermeesch, 2009).

### 3.3. Apatite Fission-Track, U-Pb, and Trace Element Analysis

Within apatite, the damage caused by spontaneous fission is annealed on geologic timescales at temperatures typically >120°C and remains relatively stable at temperatures <60°C (e.g., Green et al., 1986; Stephenson et al., 2006). Apatite grains were mounted in epoxy, polished to reveal an internal section, and etched in 5.5M HNO₃ at 20 ± 1°C for 20 ± 1 s following the protocol of Donelick et al. (2005). Fission-tracks were counted and
measured at the STOUT Laboratory on a Zeiss Axio Imager 2 using the Fijjiontrack ImageJ plugin for fission-track analysis (Lang et al., 2023).

Apatite <sup>238</sup>U concentration, U-Pb ages, and trace element concentrations for this study were acquired using an Elemental Scientific Laser New Wave 213 nm solid state LA system connected to a PerkinElmer NexION2000 ICP-MS at the Mass Spectrometry, Proteomics & Metabolomics core, University of Oklahoma. The laser energy density was ~3.9 j/cm<sup>2</sup>, a repetition rate of 10 Hz, a spot diameter of 30 µm, and ablation was run for 165 shots. At OU, we focused on the collection of radiogenic isotopes <sup>204</sup>Pb, <sup>206</sup>Pb, <sup>207</sup>Pb, <sup>208</sup>Pb, <sup>232</sup>Th, and <sup>238</sup>U, with limited determination of trace elements (<sup>88</sup>Sr and <sup>89</sup>Y) and REEs (<sup>139</sup>La and <sup>140</sup>Ce). Apatite standard MAD2 was used as a primary standard for U-Pb ratios (Thomson et al., 2012), NIST610 was used as the primary standard for trace element concentrations (Barla et al., 2023), and McClure and Durango apatite (McDowell et al., 2005; Schoene & Bowring, 2006) were run as secondary standards to check accuracy and perform a session-zeta correction (Vermeesch, 2017).

Apatite fission-track ages were calculated using a modified fission-track equation after Hasebe et al. (2004), Gleadow et al. (2015), and Vermeesch (2017). Durango apatite was used as a session-zeta to normalize single-grain fission-track ages and correct for instrumental drift (Vermeesch, 2017). Central ages (Galbraith, 2005) were determined using a Radialplotter (Vermeesch, 2009).

### 3.4. Zircon and Apatite (U-Th-[Sm])/He Analysis

The (U-Th-[Sm])/He thermochronometry system is based on the accumulation and diffusion of radiogenic <sup>4</sup>He. The closure temperature for He in apatite is typically between ~80–40°C and in zircon below ~180°C (e.g., Braun et al., 2006). Apatite and zircon separation was performed at the University of Arizona following standard separation techniques and analyses carried out at the University of Melbourne. Both apatites and zircons were picked using an Olympus SZX12 binocular microscope, then submerged in ethanol and checked under polarized light to remove grains with inclusions and damage. Appropriate grains were subsequently imaged and measured before being loaded into acid-treated platinum (Pt) capsules. The (U-Th-[Sm])/He analytical method followed the protocol outlined by Gleadow et al. (2015), except that for ZHe analysis <sup>233</sup>U and <sup>229</sup>Th spikes were used.

Apatite grains were placed in vacuum and degassed at ~900°C for 5 min via a Coherent Quattro FAP 820 nm diode laser with a fiber-optic coupling to the sample chamber. The He content was measured via isotope dilution using a pure <sup>3</sup>He spike calibrated against an independent <sup>4</sup>He standard using a Balzers quadrupole (Prisma QMS 200) mass spectrometer.

Zircon grains were placed in vacuum and degassed at ~1,300°C for 20 min via a Coherent Quattro FAP 820 nm diode laser with a fiber-optic coupling to the sample chamber. A hot blank was run after each extraction to verify complete outgassing. All helium analyses were performed on single grains.

Degassed apatite grains were removed from the laser chamber, dissolved in HNO<sub>3</sub> (still in their Pt capsules) and analyzed for parent isotopes via an Agilent 7700X ICP Mass Spectrometer. Zircon grains, however, were extracted from their Pt capsules, transferred to Parr bombs, spiked with <sup>233</sup>U and <sup>239</sup>Th, and digested in small volumes (0.3–0.5 ml) at 240°C for 40 hr in HF. These were run alongside standard solutions containing the same spike concentrations as samples and treated with the same procedure along with a series of unspiked reagent blanks. A seconding bombing in HCl for 24 hr at 200°C followed to dissolve fluoride salts. Zircon solutions were dried, dissolved in HNO<sub>3</sub> and diluted in H<sub>2</sub>O down to 5% acidity for analysis of <sup>238</sup>U, <sup>235</sup>U, and <sup>232</sup>Th via ICP-MS.

Analyses for parent isotopes were calibrated using the international rock standard BCR-2, and a Mud Tank Carbonatite apatite solution (for apatite), and international rock standards BCR-2 and BHVO-2 (for zircons). Raw apatite He ages were corrected for α-ejection using the approach of Farley et al. (1996), while for zircon He data the correction prior to age calculation followed the approach of Hourigan et al. (2005). U- and Th-isotope ratios were measured to a precision of 2% (but was typically better than 1%) and on average the precision for both individual apatite and zircon (U-Th-[Sm])/He ages is estimated at ~6% or less. Durango apatite (McDowell et al., 2005) and Fish Canyon Tuff zircon (Gleadow et al., 2015) were also run as “unknowns” with each batch of apatite or zircon samples and served as internal checks on analytical accuracy.
<table><caption>**Table 2**
Summary Monazite U-Th-Pb Ratios, Ages, and Geochemistry of Samples From the Catalina-Rincon Metamorphic Core Complex, AZ, USA</caption><thead><tr><th>Sample</th><th>n</th><th>U (ppm)</th><th>Th (ppm)</th><th>eU<sup>a</sup></th><th><sup>208</sup>Pb/<sup>232</sup>Th ± 2se<sup>b</sup></th><th><sup>207</sup>Pb/<sup>235</sup>Th ± 2se<sup>b</sup></th><th>Th-Pb age ± 1se (Ma)<sup>c</sup></th><th>MSWD<sup>d</sup></th><th>La (ppm)</th><th>Y (ppm)</th><th>Gd/Yb</th><th>Dy/Yb</th></tr></thead><tbody><tr><td>WP-01</td><td>21</td><td>403</td><td>41,073</td><td>9,774</td><td>0.00207 ± 4.2</td><td>0.0793 ± 13.4</td><td>40.8 ± 0.5</td><td>59</td><td>21,532</td><td>160,071</td><td>29.5</td><td>11.1</td></tr><tr><td>Mz554</td><td>18</td><td>127</td><td>42,985</td><td>9,765</td><td>0.00221 ± 4.1</td><td>0.1000 ± 16.9</td><td>43.2 ± 0.6</td><td>43</td><td>187,570</td><td>19,323</td><td>29.8</td><td>9.7</td></tr><tr><td>SP-01</td><td>13</td><td>471</td><td>35,450</td><td>8,059</td><td>0.00221 ± 3.9</td><td>0.0797 ± 11.8</td><td>43.9 ± 0.5</td><td>81</td><td>145,701</td><td>16,061</td><td>26.4</td><td>10.1</td></tr><tr><td>VAT12</td><td>19</td><td>2,038</td><td>42,500</td><td>11,561</td><td>0.00125 ± 4.7</td><td>0.0315 ± 7.8</td><td>45.7 ± 0.6</td><td>120</td><td>160,678</td><td>25,077</td><td>35.5</td><td>12.4</td></tr><tr><td>C2</td><td>9</td><td>2,925</td><td>56,100</td><td>34,987</td><td>0.00236 ± 5.2</td><td>0.0490 ± 9.0</td><td>48.6 ± 2.2</td><td>0.5</td><td>145,100</td><td>20,100</td><td>—</td><td>—</td></tr><tr><td>R1</td><td>14</td><td>555</td><td>144,000</td><td>34,987</td><td>0.00132 ± 5.1</td><td>0.1400 ± 5.0</td><td>26.4 ± 1.1</td><td>2.4</td><td>220,500</td><td>16,300</td><td>—</td><td>—</td></tr><tr><td>R2</td><td>16</td><td>552</td><td>82,100</td><td>20,112</td><td>0.00137 ± 7.6</td><td>0.0280 ± 18.0</td><td>26.6 ± 0.9</td><td>3.6</td><td>161,500</td><td>8,860</td><td>—</td><td>—</td></tr><tr><td>R3</td><td>14</td><td>1,065</td><td>119,000</td><td>29,467</td><td>0.00136 ± 6.6</td><td>0.0175 ± 16.4</td><td>26.4 ± 1.1</td><td>2.4</td><td>157,400</td><td>12,300</td><td>—</td><td>—</td></tr><tr><td>R4</td><td>19</td><td>820</td><td>113,000</td><td>27,854</td><td>0.00139 ± 6.8</td><td>0.0214 ± 9.3</td><td>27.3 ± 0.9</td><td>0.8</td><td>163,700</td><td>8,590</td><td>—</td><td>—</td></tr><tr><td>R5</td><td>16</td><td>732</td><td>38,800</td><td>9,774.4</td><td>0.00148 ± 5.9</td><td>0.0002 ± 13.0</td><td>28.7 ± 1.2</td><td>1.2</td><td>141,900</td><td>16,800</td><td>—</td><td>—</td></tr></tbody></table>

Note. Rare earth element ratios are chondrite normalized after Sun and McDonough (1989). <sup>a</sup>Effective uranium concentration. <sup>b</sup>Standard error (%). <sup>c</sup>Standard error (absolute). <sup>d</sup>Mean weighted square deviation.

# 4. Results

## 4.1. Monazite U-Th-Pb, Major and Trace Element Analysis

Four monazite samples were analyzed for U-Th-Pb ratios and trace and REE concentrations at the University of Arizona. Due to known issues around Cenozoic monazite disequilibrium in the <sup>238</sup>U → <sup>206</sup>Pb decay scheme as a result of excess <sup>230</sup>Th, we utilize the <sup>232</sup>Th → <sup>208</sup>Pb decay scheme for monazite Th-Pb ages (Larson et al., 2011; Schärer, 1984). Sample SP-01 yielded a Th-Pb age of 43.9 ± 0.5 Ma (n = 13). Sample WP-01 yielded a Th-Pb age of 40.8 ± 0.5 Ma (n = 21). Sample Mz554 yielded a Th-Pb age of 43.2 ± 0.6 (n = 18). Sample VAT12 yielded two populations of Th-Pb ages: an older population with a Th-Pb age of 45.7 ± 0.6 Ma (n = 8) and a younger population with a Th-Pb age of 24.0 ± 0.3 Ma (n = 11). Detailed single-grain geochemistry and U-Th-Pb ratios are in Table 2, Figure S2 and S3 in Supporting Information S1, and Tables S2 and S3.

Six monazite samples were analyzed for U-Th-Pb ratios and trace and REE concentrations at the University of Oklahoma. Sample C2 yielded a Th-Pb age of 48.6 ± 2.2 Ma (n = 9). Sample R1 yielded a Th-Pb age of 25.3 ± 1.2 Ma (n = 14). Sample R2 yielded a Th-Pb age of 26.6 ± 0.9 Ma (n = 16). Sample R3 yielded a Th-Pb age of 26.4 ± 1.1 Ma (n = 14). Sample R4 yielded a Th-Pb age of 27.3 ± 0.9 Ma (n = 19). Sample R5 yielded a Th-Pb age of 28.7 ± 1.2 Ma (n = 16). Detailed single-grain geochemistry and U-Th-Pb ratios are in Table 2, Figure S3 in Supporting Information S1, and Tables S2 and S3.

## 4.2. Apatite U-Pb and Trace Element Analysis

Five apatite samples were analyzed for U-Pb ratios and trace and REE concentrations at the University of Oklahoma. Sample R1 yielded a MSWD U-Pb age of 27.4 ± 1.2 Ma (n = 20). Sample R2 yielded a MSWD U-Pb age of 30.0 ± 1.5 Ma (n = 19). Sample R3 yielded an MSWD U-Pb age of 27.8 ± 1.7 Ma (n = 12). Sample R4 yielded a MSWD U-Pb age of 27.3 ± 2.0 Ma (n = 20). Sample R5 yielded an MSWD U-Pb age of 27.2 ± 0.6 Ma (n = 20). Detailed single-grain geochemistry and U-Th-Pb ratios are in Table 3, Figure S4 in Supporting Information S1, and Table S4.

## 4.3. Apatite and Zircon (U-Th-[Sm])/He Analyses

Apatite and zircon (U-Th-[Sm])/He dates were measured for nine samples from the northwestern and eastern transects across the Catalina-Rincon MCC. Many of these samples (n = 6) contain grains which are older than their host-rock crystallization age (57 Ma, Fornash et al., 2013) and inconsistent with previously published AHe dates from the same locality (Jepson et al., 2022). The AHe data presented in Jepson et al. (2022) followed a variation on the (U-Th-Sm)/He methodology where multiple apatite grains for a single aliquot were mechanically abraded to reduce dispersion due to possible excess He implantation (after Spiegel et al., 2009). In grains where there has been significant He implantation, this procedure would lead to younger ages than would be obtained
<table><caption>**Table 3**
Summary of Apatite U-Th-Pb Ratios, Ages, and Geochemistry of Samples From the Catalina-Rincon Metamorphic Core Complex</caption><thead><tr><th>Sample</th><th>n</th><th>U (ppm)</th><th>Th (ppm)</th><th>eU<sup>a</sup></th><th><sup>207</sup>Pb/<sup>235</sup>U ± 2se<sup>b</sup></th><th><sup>206</sup>Pb/<sup>238</sup>U ± 2se<sup>b</sup></th><th>U-Pb age ± 2se (Ma)<sup>c</sup></th><th>MSWD<sup>d</sup></th><th>Sr/Y</th><th>La/CeN</th></tr></thead><tbody><tr><td>R1</td><td>20</td><td>60.6</td><td>3.6</td><td>61.5</td><td>3.38 ± 0.61</td><td>0.035 ± 0.0059</td><td>27.4 ± 1.1</td><td>340</td><td>0.049</td><td>0.798</td></tr><tr><td>R2</td><td>16</td><td>37.3</td><td>10.9</td><td>39.8</td><td>5.69 ± 1.30</td><td>0.050 ± 0.0095</td><td>30.1 ± 1.5</td><td>300</td><td>0.057</td><td>0.727</td></tr><tr><td>R3</td><td>14</td><td>21.8</td><td>12.6</td><td>25.8</td><td>9.17 ± 1.97</td><td>0.096 ± 0.019</td><td>27.8 ± 1.7</td><td>220</td><td>0.02</td><td>0.829</td></tr><tr><td>R4</td><td>19</td><td>25.2</td><td>26.7</td><td>31.9</td><td>7.44 ± 1.70</td><td>0.068 ± 0.014</td><td>27.3 ± 2.0</td><td>250</td><td>0.039</td><td>0.735</td></tr><tr><td>R5</td><td>16</td><td>8.4</td><td>1.5</td><td>8.7</td><td>10.45 ± 3.69</td><td>0.098 ± 0.029</td><td>27.2 ± 0.7</td><td>410</td><td>0.24</td><td>0.764</td></tr></tbody></table>

Note. Rare earth element ratios are chondrite normalized after Sun and McDonough (1989). <sup>a</sup>Effective uranium concentration. <sup>b</sup>Standard error (absolute). <sup>c</sup>Standard error (absolute). <sup>d</sup>Mean weighted square deviation.

from unabraded grains. There are a number of variables that can alter (U-Th-[Sm])/He ages including radiation damage, spherical equivalent grain radius, grain fragmentation, U-Th zonation, U- and Th-bearing inclusions, He implantation, chemical composition and the effects of crystal imperfections (e.g., Ault & Flowers, 2012; Brown et al., 2013; Gautheron et al., 2012; Spiegel et al., 2009; Vermeesch et al., 2007; Wildman et al., 2016; Zeitler et al., 2017). These variables tend to cause older ages; thus, to account for this scatter, we followed the first quartile age (FQA) approach outlined by He et al. (2021) for both apatite and zircon helium data (Figure 3). Although this approach is advised for samples with many single-grain ages, Kapp et al. (2023) have shown that the FQA approach can assist in resolving over dispersed data sets with fewer aliquots. Results are presented from highest to lowest of the vertical elevation profile. The northwestern transect yielded ZHe FQAs between ~24–15 Ma and AHe FQAs between ~28–10 Ma (Figure 3; Table 4).

Apatite (U-Th-Sm)/He dates were collected from five samples in the eastern transect. Due to the highly dispersed nature of samples R1 and R2 and in light of the issues discussed above, we have chosen to report the youngest single-grain. Sample R1 yielded AHe ages between 123 and 30 Ma (n = 5) with the youngest grain age of 30.5 ± 1.9 Ma. Sample R2 yielded AHe ages between 308 and 38 Ma (n = 5) with the youngest grain age of 37.8 ± 2.3 Ma. The remaining samples (R3, R4, and R5) yielded AHe FQAs between ~28–20 Ma (Figure 3; Table 4).

## 4.4. Apatite Fission-Track Analysis

Five samples from the eastern transect were selected for AFT analysis (Table 5, Figure 4). Results are presented from highest to lowest of the vertical elevation profile. Sample R4 yielded a central age of 27.0 ± 2.6 Ma (n = 20; P[χ²] = 0.28) and a mean track length (MTL) of 13.6 ± 1.4 µm (n = 35). Sample R3 yielded a central age of 24.1 ± 3.4 Ma (n = 12; P[χ²] = 0.93) and a MTL of 13.9 ± 1.7 µm (n = 16). Sample R2 yielded a central age of 23.6 ± 1.9 Ma (n = 20; P[χ²] = 0.32) and a MTL of 13.7 ± 1.1 µm (n = 40). Sample R1 yielded a central age of 22.5 ± 1.6 Ma (n = 19; P[χ²] = 0.24) and a MTL of 13.5 ± 1.4 µm (n = 57). Sample R5 yielded a central age of 34.8 ± 4.2 Ma (n = 20; P[χ²] = 0.81) and insufficient confined track lengths to obtain an MTL. For detailed AFT data see Figure S5 in Supporting Information S1 and Table S4 (Kohn et al., 2024).

## 4.5. Monazite Fission-Track Analysis

Five samples from the northwestern transect were selected for MFT analysis (Table 5 and Figures 4a–4j). Results are presented from top to bottom of the vertical elevation profile. Sample SP-01 yielded a central age of 2.6 ± 0.3 Ma (n = 12; P[χ²] = 0.00) and a MTL of 10.7 ± 1.6 µm (n = 75). Sample WP-01 yielded a central age of 1.9 ± 0.2 Ma (n = 17; P[χ²] = 0.00) and a MTL of 10.5 ± 1.1 µm (n = 51). Sample Mz554 yielded a central age of 2.8 ± 0.3 Ma (n = 15; P[χ²] = 0.99) and a MTL of 10.7 ± 1.4 µm (n = 46). Sample VAT12 yielded a central age of 0.9 ± 0.1 Ma (n = 22; P[χ²] = 0.00) and a MTL of 10.6 ± 1.3 µm (n = 51). Sample C2 yielded a central age of 1.1 ± 0.2 Ma (n = 9; P[χ²] = 0.00) and a MTL of 8.9 ± 0.9 µm (n = 6).

Five samples from the eastern transect were selected for MFT analysis (Table 4 and Figures 4k–4s). Sample R4 yielded a central age of 7.7 ± 0.7 Ma (n = 17; P[χ²] = 0.00) and a MTL of 9.9 ± 1.3 µm (n = 37). Sample R3 yielded a central age of 4.5 ± 0.4 Ma (n = 14; P[χ²] = 0.09) and a MTL of 9.7 ± 1.4 µm (n = 26). Sample R2 yielded a central age of 5.1 ± 0.5 Ma (n = 16; P[χ²] = 0.02) and a MTL of 9.4 ± 1.4 µm (n = 37). Sample R1 yielded a central age of 4.4 ± 0.4 Ma (n = 15; P[χ²] = 0.00) and a MTL of 9.0 ± 0.9 µm (n = 35).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Boxplots of single grain apatite (U-Th-Sm)/He ages (A = Eastern transect and B = Northwestern transect) and zircon (U-Th)/He (C) indicating first quartile and youngest single-grain ages.</figcaption></figure>

yielded a central age of 4.8 ± 1.3 Ma (n = 14; P[χ²] = 0.00) and insufficient confined lengths to obtain an MTL.
Sample R5 yielded a central age of 4.5 ± 0.4 Ma (n = 14; P[χ²] = 0.09) and a MTL of 10.1 ± 1.4 µm (n = 20).
Single grain MFT data are available in Table S2.

**4.6. Thermal History Modeling**

Thermal history modeling was performed in two stages, the first using AFT ages, confined track lengths, AHe, and ZHe data from this study and Jepson et al. (2022), and the second combining these established low-temperature thermochronometric data with our new MFT (single-grain ages and confined length measurements) data. Here, we used QTQt software (version 5.8.6) to determine the thermal history of the samples. This software implements a Bayesian trans-dimensional Markov Chain Monte Carlo approach (Gallagher, 2012) to sample time-temperature paths that adequately predict the measured data for a sample. In this study, we have data available from two vertical profiles with limited horizontal separation between the samples for a given profile. Consequently, the samples for a given profile could be modeled together as this tends to reinforce the real signal and reduces the influence of random noise (Gallagher et al., 2005). We use the AFT annealing model after Ketcham et al. (2007), the AHe diffusion model after Flowers et al. (2009), and the MFT fanning annealing model after Jones et al. (2021). An initial unconstrained exploratory was first undertaken, followed by runs modified through the addition of a geologic constraint. Our constraint (t = 27.5 ± 2.5 Ma at 200 ± 50°C) was based on the onset of detachment faulting in the Catalina-Rincon MCC established by Spencer et al. (2022) and Peters et al. (2003), using temperatures consistent with those identified by Jepson et al. (2022). A large number of iterations (200,000) were run in order to generate a distribution of thermal history models. The general prior was set to 25.7 ± 25.7 Ma at 70 ± 70°C.
<table><caption>**Table 4**
Single-Grain Zircon and Apatite (U-Th-[Sm]/He) Ages of Samples From the Catalina-Rincon Metamorphic Core Complex, AZ, USA</caption><thead><tr><th>Sample no.</th><th>Analysis no.</th><th>He#</th><th><sup>4</sup>He gas (ncc)</th><th>Mass (mg)</th><th>Mean FT<sup>a</sup></th><th>U (ppm)</th><th>Th (ppm)</th><th>Sm (ppm)</th><th>Th/U</th><th>[eU] (ppm)<sup>b</sup></th><th>Uncorrected age (Ma)</th><th>Corrected age (Ma)</th><th>Error ±1s (Ma)</th><th>Grain length (mm)</th><th>Grain half-width (mm)</th><th>Grain morphology<sup>c</sup></th></tr></thead><tbody><tr><td colspan="17">Southwestern Trasect - Zircon (U-Th)/He</td></tr><tr><td>KJJ-09-7</td><td>22424</td><td>75666</td><td>2.199</td><td>0.0080</td><td>0.78</td><td>133.5</td><td>87.5</td><td>-</td><td>0.66</td><td>154.4</td><td>14.6</td><td>18.7</td><td>0.9</td><td>268.8</td><td>46.4</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>22426</td><td>75713</td><td>3.526</td><td>0.0069</td><td>0.79</td><td>216.4</td><td>91.5</td><td>-</td><td>0.42</td><td>238.1</td><td>17.6</td><td>22.3</td><td>1.1</td><td>203.7</td><td>54.7</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>22427</td><td>75716</td><td>7.845</td><td>0.0093</td><td>0.81</td><td>491.3</td><td>111.1</td><td>-</td><td>0.22</td><td>518.9</td><td>13.4</td><td>16.5</td><td>0.8</td><td>251.7</td><td>53.7</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>22428</td><td>75719</td><td>2.820</td><td>0.0071</td><td>0.77</td><td>175.2</td><td>119.8</td><td>-</td><td>0.68</td><td>204.2</td><td>16.0</td><td>20.8</td><td>1.1</td><td>260.7</td><td>44.1</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>23429</td><td>79134</td><td>6.263</td><td>0.0123</td><td>0.81</td><td>176.8</td><td>102.0</td><td>-</td><td>0.57</td><td>201.0</td><td>20.8</td><td>25.7</td><td>1.3</td><td>314.2</td><td>52.8</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>23430</td><td>79137</td><td>2.839</td><td>0.0070</td><td>0.78</td><td>181.8</td><td>90.8</td><td>-</td><td>0.50</td><td>204.2</td><td>16.3</td><td>20.9</td><td>1.1</td><td>241.1</td><td>46.7</td><td>2T</td></tr><tr><td>SP-01</td><td>23418</td><td>79122</td><td>18.625</td><td>0.0107</td><td>0.82</td><td>748.1</td><td>121.4</td><td>-</td><td>0.01</td><td>779.2</td><td>18.4</td><td>22.4</td><td>1.2</td><td>284.5</td><td>52.7</td><td>2T</td></tr><tr><td>SP-01</td><td>23419</td><td>79125</td><td>24.252</td><td>0.0074</td><td>0.77</td><td>1.714.1</td><td>130.9</td><td>-</td><td>0.01</td><td>1.742.1</td><td>15.5</td><td>20.1</td><td>1.1</td><td>307.9</td><td>39.9</td><td>2T</td></tr><tr><td>SP-01</td><td>23420</td><td>79128</td><td>33.400</td><td>0.0129</td><td>0.82</td><td>1.215.3</td><td>112.5</td><td>-</td><td>0.01</td><td>1.240.2</td><td>17.2</td><td>20.9</td><td>1.1</td><td>322.9</td><td>53.1</td><td>2T</td></tr><tr><td>SP-01</td><td>23421</td><td>79131</td><td>39.736</td><td>0.0097</td><td>0.79</td><td>1.532.8</td><td>132.9</td><td>-</td><td>0.01</td><td>1.561.7</td><td>21.5</td><td>27.3</td><td>1.5</td><td>325.2</td><td>44.7</td><td>2T</td></tr><tr><td>WP-01</td><td>23412</td><td>79104</td><td>1.769</td><td>0.0065</td><td>0.75</td><td>138.1</td><td>86.1</td><td>-</td><td>0.62</td><td>159.7</td><td>14.0</td><td>18.7</td><td>6.0</td><td>310.8</td><td>37.0</td><td>2T</td></tr><tr><td>WP-01</td><td>23413</td><td>79107</td><td>3.720</td><td>0.0077</td><td>0.76</td><td>285.0</td><td>42.3</td><td>-</td><td>0.15</td><td>295.1</td><td>13.5</td><td>17.8</td><td>3.0</td><td>346.1</td><td>37.6</td><td>2T</td></tr><tr><td>WP-01</td><td>23414</td><td>79110</td><td>2.267</td><td>0.0039</td><td>0.74</td><td>316.0</td><td>64.3</td><td>-</td><td>0.20</td><td>331.5</td><td>14.4</td><td>19.5</td><td>2.8</td><td>195.3</td><td>38.9</td><td>2T</td></tr><tr><td>WP-01</td><td>23415</td><td>79113</td><td>2.668</td><td>0.0099</td><td>0.78</td><td>94.7</td><td>58.3</td><td>-</td><td>0.61</td><td>108.3</td><td>20.5</td><td>26.2</td><td>1.5</td><td>352.2</td><td>42.8</td><td>2T</td></tr><tr><td>WP-01</td><td>23416</td><td>79116</td><td>13.916</td><td>0.0110</td><td>0.80</td><td>680.1</td><td>112.3</td><td>-</td><td>0.16</td><td>708.6</td><td>14.7</td><td>18.3</td><td>1.2</td><td>344.5</td><td>46.2</td><td>2T</td></tr><tr><td>Mz554</td><td>22419</td><td>75651</td><td>9.597</td><td>0.0216</td><td>0.83</td><td>246.3</td><td>20.9</td><td>-</td><td>0.08</td><td>250.7</td><td>14.6</td><td>17.5</td><td>0.9</td><td>494.4</td><td>52.7</td><td>2T</td></tr><tr><td>Mz554</td><td>22420</td><td>75654</td><td>10.448</td><td>0.0124</td><td>0.81</td><td>393.8</td><td>41.8</td><td>-</td><td>0.11</td><td>403.8</td><td>17.1</td><td>20.7</td><td>1.0</td><td>304.5</td><td>54.4</td><td>2T</td></tr><tr><td>Mz554</td><td>22421</td><td>75657</td><td>28.404</td><td>0.0473</td><td>0.87</td><td>334.5</td><td>46.9</td><td>-</td><td>0.14</td><td>346.0</td><td>14.3</td><td>16.4</td><td>0.8</td><td>485.7</td><td>77.0</td><td>1T</td></tr><tr><td>Mz554</td><td>22422</td><td>75660</td><td>8.419</td><td>0.0178</td><td>0.81</td><td>170.6</td><td>30.9</td><td>-</td><td>0.18</td><td>178.0</td><td>21.9</td><td>27.0</td><td>1.4</td><td>519.8</td><td>45.9</td><td>2T</td></tr><tr><td>Mz554</td><td>22423</td><td>75663</td><td>7.134</td><td>0.0085</td><td>0.78</td><td>402.3</td><td>41.9</td><td>-</td><td>0.10</td><td>410.6</td><td>16.8</td><td>21.5</td><td>1.1</td><td>321.9</td><td>41.7</td><td>2T</td></tr><tr><td>VAT 12</td><td>22414</td><td>75636</td><td>11.675</td><td>0.0064</td><td>0.78</td><td>994.2</td><td>16.8</td><td>-</td><td>0.02</td><td>996.5</td><td>15.0</td><td>19.3</td><td>1.0</td><td>257.6</td><td>41.8</td><td>2T</td></tr><tr><td>VAT 12</td><td>22415</td><td>75639</td><td>7.520</td><td>0.0041</td><td>0.73</td><td>1.098.5</td><td>29.3</td><td>-</td><td>0.03</td><td>1.111.6</td><td>13.6</td><td>18.6</td><td>0.9</td><td>240.8</td><td>34.0</td><td>2T</td></tr><tr><td>VAT 12</td><td>22416</td><td>75642</td><td>42.893</td><td>0.0075</td><td>0.80</td><td>2.757.5</td><td>44.0</td><td>-</td><td>0.02</td><td>2.759.5</td><td>17.0</td><td>21.3</td><td>1.1</td><td>236.3</td><td>49.5</td><td>2T</td></tr><tr><td>VAT 12</td><td>22417</td><td>75645</td><td>32.698</td><td>0.0076</td><td>0.78</td><td>2.016.3</td><td>28.7</td><td>-</td><td>0.01</td><td>2.033.1</td><td>17.4</td><td>22.3</td><td>1.1</td><td>294.3</td><td>42.0</td><td>2T</td></tr><tr><td>VAT 12</td><td>22418</td><td>75648</td><td>36.721</td><td>0.0099</td><td>0.81</td><td>1.816.6</td><td>25.6</td><td>-</td><td>0.01</td><td>1.825.9</td><td>16.7</td><td>20.6</td><td>1.0</td><td>287.6</td><td>49.8</td><td>2T</td></tr><tr><td colspan="17">Fish Canyon Tuff standard<sup>a</sup></td></tr><tr><td>FCT</td><td>22467</td><td>75710</td><td>5.391</td><td>0.0046</td><td>0.76</td><td>415.8</td><td>205.0</td><td>-</td><td>0.49</td><td>461.0</td><td>20.9</td><td>27.5</td><td>1.4</td><td>186.1</td><td>45.2</td><td>2T</td></tr><tr><td>FCT</td><td>22465</td><td>75559</td><td>3.429</td><td>0.0035</td><td>0.73</td><td>380.7</td><td>181.3</td><td>-</td><td>0.47</td><td>427.7</td><td>18.8</td><td>25.8</td><td>1.3</td><td>187.1</td><td>37.9</td><td>2T</td></tr><tr><td>FCT</td><td>22466</td><td>75597</td><td>5.876</td><td>0.0034</td><td>0.73</td><td>549.1</td><td>332.0</td><td>-</td><td>0.60</td><td>628.9</td><td>22.6</td><td>30.9</td><td>1.6</td><td>173.1</td><td>39.9</td><td>2T</td></tr><tr><td>FCT</td><td>23094</td><td>77738</td><td>5.413</td><td>0.0056</td><td>0.77</td><td>327.9</td><td>169.2</td><td>-</td><td>0.51</td><td>366.5</td><td>21.7</td><td>28.1</td><td>1.5</td><td>202.2</td><td>47.5</td><td>2T</td></tr><tr><td>FCT</td><td>23096</td><td>77744</td><td>7.571</td><td>0.0060</td><td>0.79</td><td>385.4</td><td>224.4</td><td>-</td><td>0.58</td><td>441.3</td><td>23.5</td><td>29.7</td><td>1.5</td><td>188.8</td><td>54.4</td><td>2T</td></tr></tbody></table>
<table><caption>**Table 4**
*Continued*</caption><thead><tr><th>Sample no.</th><th>Analysis no.</th><th>He#</th><th><sup>4</sup>He gas (ncc)</th><th>Mass (mg)</th><th>Mean FT<sup>a</sup></th><th>U (ppm)</th><th>Th (ppm)</th><th>Sm (ppm)</th><th>Th/U</th><th>[eU] (ppm)<sup>b</sup></th><th>Uncorrected age (Ma)</th><th>Corrected age (Ma)</th><th>Error ±1s (Ma)</th><th>Grain length (mm)</th><th>Grain half-width (mm)</th><th>Grain morphology<sup>c</sup></th></tr></thead><tbody><tr><td colspan="16"><strong>Southwestern Trasect - Apatite (U-Th-Sm)/He</strong></td></tr><tr><td>KJJ-09-7</td><td>22440</td><td>76584</td><td>0.081</td><td>0.0039</td><td>0.69</td><td>5.9</td><td>24.3</td><td>33.8</td><td>4.07</td><td>8.2</td><td>14.2</td><td>20.6</td><td>1.5</td><td>200.1</td><td>48.6</td><td>2T</td></tr><tr><td>KJJ-09-7</td><td>22441</td><td>76587</td><td>0.079</td><td>0.0056</td><td>0.74</td><td>4.4</td><td>15.7</td><td>36.9</td><td>3.59</td><td>8.0</td><td>14.0</td><td>18.9</td><td>1.4</td><td>214.3</td><td>50.8</td><td>0T</td></tr><tr><td>KJJ-09-7</td><td>22442</td><td>76590</td><td>0.076</td><td>0.0060</td><td>0.75</td><td>3.4</td><td>13.0</td><td>30.4</td><td>3.80</td><td>6.5</td><td>15.6</td><td>20.8</td><td>1.5</td><td>199.5</td><td>54.6</td><td>0T</td></tr><tr><td>KJJ-09-7</td><td>22443</td><td>76593</td><td>0.109</td><td>0.0059</td><td>0.75</td><td>4.6</td><td>20.1</td><td>47.6</td><td>4.37</td><td>9.4</td><td>15.6</td><td>20.8</td><td>1.5</td><td>185.4</td><td>56.3</td><td>0T</td></tr><tr><td>SP-01</td><td>23422</td><td>79079</td><td>0.122</td><td>0.0059</td><td>0.78</td><td>9.2</td><td>2.9</td><td>209.3</td><td>0.31</td><td>9.9</td><td>14.6</td><td>18.7</td><td>1.2</td><td>159.9</td><td>60.8</td><td>0T</td></tr><tr><td>SP-01</td><td>23423</td><td>79082</td><td>0.121</td><td>0.0150</td><td>0.82</td><td>2.3</td><td>3.1</td><td>178.5</td><td>1.32</td><td>3.0</td><td>14.8</td><td>18.1</td><td>1.1</td><td>273.2</td><td>73.8</td><td>0T</td></tr><tr><td>SP-01</td><td>23425</td><td>79092</td><td>0.328</td><td>0.0331</td><td>0.87</td><td>3.4</td><td>2.0</td><td>243.2</td><td>0.58</td><td>3.9</td><td>13.8</td><td>15.9</td><td>1.0</td><td>292.6</td><td>106.2</td><td>0T</td></tr><tr><td>SP-01</td><td>23426</td><td>79095</td><td>0.072</td><td>0.0065</td><td>0.77</td><td>4.1</td><td>4.9</td><td>171.7</td><td>1.19</td><td>5.3</td><td>13.3</td><td>17.6</td><td>1.1</td><td>204.6</td><td>56.3</td><td>0T</td></tr><tr><td>WP-01</td><td>23407</td><td>79064</td><td>0.136</td><td>0.0153</td><td>0.83</td><td>3.6</td><td>2.2</td><td>138.8</td><td>0.61</td><td>4.1</td><td>13.9</td><td>16.8</td><td>1.1</td><td>210.9</td><td>85.0</td><td>0T</td></tr><tr><td>WP-01</td><td>23408</td><td>79067</td><td>0.113</td><td>0.0093</td><td>0.80</td><td>4.7</td><td>3.2</td><td>197.2</td><td>0.69</td><td>5.5</td><td>14.1</td><td>17.6</td><td>1.1</td><td>143.2</td><td>80.4</td><td>0T</td></tr><tr><td>WP-01</td><td>23409</td><td>79070</td><td>0.123</td><td>0.0086</td><td>0.80</td><td>5.6</td><td>2.0</td><td>203.2</td><td>0.36</td><td>6.1</td><td>15.2</td><td>19.0</td><td>1.1</td><td>189.9</td><td>67.2</td><td>0T</td></tr><tr><td>WP-01</td><td>23410</td><td>79073</td><td>0.052</td><td>0.0089</td><td>0.79</td><td>1.6</td><td>4.8</td><td>203.7</td><td>2.94</td><td>2.8</td><td>10.9</td><td>13.7</td><td>0.9</td><td>157.8</td><td>75.0</td><td>0T</td></tr><tr><td>WP-01</td><td>23411</td><td>79076</td><td>0.281</td><td>0.0056</td><td>0.77</td><td>19.9</td><td>4.2</td><td>198.0</td><td>0.21</td><td>20.9</td><td>18.3</td><td>23.8</td><td>1.5</td><td>188.6</td><td>54.4</td><td>0T</td></tr><tr><td>Mz554</td><td>22434</td><td>76566</td><td>0.715</td><td>0.0148</td><td>0.83</td><td>3.3</td><td>1.3</td><td>91.6</td><td>0.40</td><td>3.6</td><td>90.6</td><td>109.1</td><td>6.3</td><td>259.0</td><td>75.4</td><td>0T</td></tr><tr><td>Mz554</td><td>22435</td><td>76569</td><td>0.183</td><td>0.0140</td><td>0.83</td><td>3.3</td><td>3.0</td><td>174.8</td><td>0.88</td><td>4.1</td><td>19.6</td><td>23.6</td><td>1.8</td><td>176.8</td><td>88.9</td><td>0T</td></tr><tr><td>Mz554</td><td>22436</td><td>76572</td><td>0.232</td><td>0.0163</td><td>0.84</td><td>2.6</td><td>1.0</td><td>213.5</td><td>0.37</td><td>2.8</td><td>25.8</td><td>30.8</td><td>2.4</td><td>203.7</td><td>89.2</td><td>0T</td></tr><tr><td>Mz554</td><td>22437</td><td>76575</td><td>0.083</td><td>0.0115</td><td>0.80</td><td>0.9</td><td>1.3</td><td>201.2</td><td>1.33</td><td>1.2</td><td>20.6</td><td>25.7</td><td>1.8</td><td>259.7</td><td>66.3</td><td>0T</td></tr><tr><td>Mz554</td><td>22438</td><td>76578</td><td>0.150</td><td>0.0081</td><td>0.81</td><td>3.8</td><td>3.1</td><td>129.8</td><td>0.81</td><td>4.6</td><td>27.1</td><td>33.4</td><td>2.5</td><td>227.3</td><td>74.2</td><td>1T</td></tr><tr><td>Mz554</td><td>23427</td><td>79098</td><td>0.107</td><td>0.0055</td><td>0.74</td><td>2.2</td><td>3.0</td><td>265.9</td><td>1.37</td><td>2.9</td><td>31.1</td><td>42.1</td><td>3.1</td><td>235.0</td><td>48.5</td><td>0T</td></tr><tr><td>Mz554</td><td>23428</td><td>79101</td><td>0.166</td><td>0.0119</td><td>0.82</td><td>6.6</td><td>3.3</td><td>121.3</td><td>0.49</td><td>7.4</td><td>13.7</td><td>16.7</td><td>1.1</td><td>198.0</td><td>77.3</td><td>0T</td></tr><tr><td>VAT-12</td><td>22429</td><td>76534</td><td>0.812</td><td>0.0275</td><td>0.86</td><td>14.5</td><td>2.1</td><td>77.3</td><td>0.14</td><td>15.0</td><td>15.5</td><td>18.0</td><td>1.6</td><td>333.3</td><td>90.6</td><td>0T</td></tr><tr><td>VAT-12</td><td>22430</td><td>76537</td><td>0.164</td><td>0.0119</td><td>0.83</td><td>9.6</td><td>2.5</td><td>132.6</td><td>0.26</td><td>10.2</td><td>10.0</td><td>12.1</td><td>0.8</td><td>164.5</td><td>84.8</td><td>0T</td></tr><tr><td>VAT-12</td><td>22431</td><td>76557</td><td>0.184</td><td>0.0236</td><td>0.86</td><td>5.4</td><td>1.5</td><td>38.6</td><td>0.27</td><td>5.7</td><td>10.6</td><td>12.4</td><td>1.3</td><td>196.8</td><td>109.3</td><td>0T</td></tr><tr><td>VAT-12</td><td>22432</td><td>76560</td><td>0.197</td><td>0.0190</td><td>0.85</td><td>7.1</td><td>2.5</td><td>76.8</td><td>0.35</td><td>7.7</td><td>10.3</td><td>12.1</td><td>1.0</td><td>251.4</td><td>86.7</td><td>0T</td></tr><tr><td>VAT-12</td><td>22433</td><td>76563</td><td>0.172</td><td>0.0132</td><td>0.83</td><td>5.7</td><td>1.5</td><td>128.9</td><td>0.27</td><td>6.1</td><td>15.0</td><td>18.1</td><td>1.6</td><td>202.1</td><td>80.5</td><td>0T</td></tr><tr><td colspan="16"><strong>Durango apatite standard<sup>d</sup></strong></td></tr><tr><td>Durango</td><td>22659</td><td>76540</td><td>5.576</td><td>0.0141</td><td>1.00</td><td>18.6</td><td>357.1</td><td>274.2</td><td>19.04</td><td>103.6</td><td>30.9</td><td>30.9</td><td>2.1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Durango</td><td>22660</td><td>76662</td><td>4.102</td><td>0.0117</td><td>1.00</td><td>16.7</td><td>313.9</td><td>287.7</td><td>18.70</td><td>91.2</td><td>30.9</td><td>30.9</td><td>1.8</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Durango</td><td>23449</td><td>79085</td><td>1.437</td><td>0.0039</td><td>1.00</td><td>15.3</td><td>338.7</td><td>195.7</td><td>22.01</td><td>95.4</td><td>31.3</td><td>31.3</td><td>1.5</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Durango</td><td>23394</td><td>78973</td><td>4.133</td><td>0.0115</td><td>1.00</td><td>15.0</td><td>331.9</td><td>186.6</td><td>21.99</td><td>96.9</td><td>31.2</td><td>31.2</td><td>1.5</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Durango</td><td>23395</td><td>78976</td><td>3.840</td><td>0.0099</td><td>1.00</td><td>16.6</td><td>354.8</td><td>242.8</td><td>21.17</td><td>100.8</td><td>31.2</td><td>31.2</td><td>1.3</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>
<table><caption>**Table 4**
*Continued*</caption><thead><tr><th>Sample no.</th><th>Analysis no.</th><th>He#</th><th><sup>4</sup>He gas (ncc)</th><th>Mass (mg)</th><th>Mean FT<sup>a</sup></th><th>U (ppm)</th><th>Th (ppm)</th><th>Sm (ppm)</th><th>Th/U</th><th>[eU] (ppm)<sup>b</sup></th><th>Uncorrected age (Ma)</th><th>Corrected age (Ma)</th><th>Error ±1s (Ma)</th><th>Grain length (mm)</th><th>Grain half-width (mm)</th><th>Grain morphology<sup>c</sup></th></tr></thead><tbody><tr><td colspan="17"><em>Eastern Transect - Apatite (U-Th-Sm)/He</em></td></tr><tr><td>R-1</td><td>28202</td><td>96927</td><td>0.433</td><td>0.00669</td><td>0.79</td><td>14.9</td><td>32.4</td><td>254.0</td><td>2.18</td><td>22.6</td><td>23.3</td><td><strong>29.5</strong></td><td><strong>1.8</strong></td><td>200.6</td><td>57.6</td><td>0T</td></tr><tr><td>R-1</td><td>28203</td><td>96930</td><td>1.052</td><td>0.00342</td><td>0.73</td><td>17.0</td><td>51.4</td><td>275.0</td><td>3.02</td><td>29.2</td><td>85.6</td><td><strong>117.5</strong></td><td><strong>7.3</strong></td><td>171.7</td><td>44.5</td><td>0T</td></tr><tr><td>R-1</td><td>28204</td><td>96933</td><td>0.717</td><td>0.00432</td><td>0.75</td><td>13.4</td><td>59.1</td><td>342.1</td><td>4.40</td><td>27.5</td><td>49.0</td><td><strong>65.0</strong></td><td><strong>4.0</strong></td><td>170.4</td><td>50.2</td><td>0T</td></tr><tr><td>R-1</td><td>28205</td><td>96936</td><td>1.164</td><td>0.00475</td><td>0.78</td><td>16.9</td><td>36.0</td><td>247.8</td><td>2.14</td><td>25.5</td><td>78.1</td><td><strong>100.5</strong></td><td><strong>6.2</strong></td><td>146.6</td><td>56.8</td><td>0T</td></tr><tr><td>R-1</td><td>28206</td><td>96939</td><td>1.053</td><td>0.00534</td><td>0.79</td><td>13.6</td><td>43.5</td><td>224.0</td><td>3.20</td><td>24.0</td><td>66.8</td><td><strong>84.9</strong></td><td><strong>5.3</strong></td><td>143.0</td><td>61.0</td><td>0T</td></tr><tr><td>R-2</td><td>28207</td><td>96942</td><td>0.334</td><td>0.00352</td><td>0.71</td><td>9.5</td><td>43.8</td><td>260.3</td><td>4.59</td><td>19.9</td><td>38.6</td><td><strong>54.2</strong></td><td><strong>3.4</strong></td><td>206.1</td><td>41.2</td><td>0T</td></tr><tr><td>R-2</td><td>28208</td><td>96945</td><td>0.363</td><td>0.00191</td><td>0.71</td><td>19.6</td><td>62.4</td><td>381.4</td><td>3.18</td><td>34.5</td><td>44.9</td><td><strong>63.7</strong></td><td><strong>3.9</strong></td><td>130.7</td><td>48.2</td><td>1T</td></tr><tr><td>R-2</td><td>28209</td><td>96948</td><td>0.884</td><td>0.00357</td><td>0.75</td><td>14.9</td><td>40.6</td><td>257.4</td><td>2.74</td><td>24.6</td><td>81.9</td><td><strong>109.0</strong></td><td><strong>6.8</strong></td><td>137.0</td><td>50.9</td><td>0T</td></tr><tr><td>R-2</td><td>28210</td><td>96951</td><td>3.009</td><td>0.00399</td><td>0.76</td><td>11.2</td><td>62.1</td><td>230.9</td><td>5.55</td><td>26.0</td><td>234.3</td><td><strong>308.3</strong></td><td><strong>19.1</strong></td><td>133.5</td><td>54.5</td><td>0T</td></tr><tr><td>R-2</td><td>28211</td><td>96954</td><td>0.352</td><td>0.00315</td><td>0.70</td><td>29.7</td><td>19.4</td><td>217.4</td><td>0.65</td><td>34.3</td><td>26.5</td><td><strong>37.8</strong></td><td><strong>2.3</strong></td><td>235.8</td><td>36.5</td><td>0T</td></tr><tr><td>R-3</td><td>28213</td><td>96957</td><td>0.219</td><td>0.01706</td><td>0.86</td><td>2.7</td><td>4.2</td><td>113.5</td><td>1.57</td><td>3.7</td><td>27.8</td><td><strong>32.3</strong></td><td><strong>2.0</strong></td><td>206.2</td><td>90.7</td><td>0T</td></tr><tr><td>R-3</td><td>28214</td><td>96963</td><td>4.321</td><td>0.01925</td><td>0.85</td><td>16.9</td><td>22.4</td><td>234.0</td><td>1.32</td><td>22.2</td><td>81.6</td><td><strong>96.2</strong></td><td><strong>6.0</strong></td><td>324.7</td><td>76.8</td><td>0T</td></tr><tr><td>R-3</td><td>28215</td><td>96966</td><td>0.269</td><td>0.02664</td><td>0.88</td><td>3.3</td><td>3.8</td><td>116.4</td><td>1.13</td><td>4.2</td><td>19.1</td><td><strong>21.7</strong></td><td><strong>1.3</strong></td><td>247.6</td><td>103.4</td><td>0T</td></tr><tr><td>R-3</td><td>28216</td><td>96969</td><td>0.137</td><td>0.01143</td><td>0.82</td><td>1.3</td><td>4.4</td><td>63.7</td><td>3.35</td><td>2.3</td><td>40.4</td><td><strong>49.0</strong></td><td><strong>3.0</strong></td><td>231.1</td><td>70.2</td><td>0T</td></tr><tr><td>R-4</td><td>28217</td><td>96972</td><td>0.357</td><td>0.00504</td><td>0.77</td><td>15.4</td><td>35.3</td><td>305.9</td><td>2.30</td><td>23.8</td><td>24.2</td><td><strong>31.2</strong></td><td><strong>1.9</strong></td><td>167.7</td><td>54.7</td><td>0T</td></tr><tr><td>R-4</td><td>28218</td><td>96975</td><td>0.469</td><td>0.01163</td><td>0.82</td><td>11.8</td><td>18.8</td><td>223.8</td><td>1.59</td><td>16.3</td><td>20.1</td><td><strong>24.4</strong></td><td><strong>1.5</strong></td><td>255.8</td><td>67.2</td><td>0T</td></tr><tr><td>R-4</td><td>28219</td><td>96978</td><td>0.737</td><td>0.01678</td><td>0.85</td><td>11.8</td><td>26.1</td><td>280.4</td><td>2.21</td><td>18.0</td><td>19.7</td><td><strong>23.3</strong></td><td><strong>1.4</strong></td><td>260.6</td><td>80.0</td><td>0T</td></tr><tr><td>R-4</td><td>28220</td><td>96981</td><td>0.452</td><td>0.00485</td><td>0.77</td><td>20.2</td><td>16.6</td><td>223.0</td><td>0.82</td><td>24.2</td><td>31.3</td><td><strong>40.9</strong></td><td><strong>2.5</strong></td><td>203.4</td><td>48.7</td><td>0T</td></tr><tr><td>R-4</td><td>28221</td><td>96984</td><td>0.693</td><td>0.01085</td><td>0.84</td><td>14.6</td><td>17.8</td><td>231.0</td><td>1.22</td><td>18.8</td><td>27.5</td><td><strong>32.9</strong></td><td><strong>2.0</strong></td><td>183.7</td><td>76.7</td><td>0T</td></tr><tr><td>R-5</td><td>28222</td><td>96987</td><td>0.091</td><td>0.00656</td><td>0.80</td><td>5.3</td><td>2.5</td><td>85.2</td><td>0.47</td><td>5.9</td><td>18.9</td><td><strong>23.7</strong></td><td><strong>1.5</strong></td><td>203.3</td><td>56.7</td><td>0T</td></tr><tr><td>R-5</td><td>28223</td><td>96990</td><td>0.128</td><td>0.00805</td><td>0.79</td><td>6.3</td><td>8.1</td><td>92.7</td><td>1.28</td><td>8.2</td><td>15.7</td><td><strong>19.9</strong></td><td><strong>1.2</strong></td><td>268.8</td><td>54.6</td><td>0T</td></tr><tr><td>R-5</td><td>28224</td><td>96993</td><td>0.498</td><td>0.01863</td><td>0.85</td><td>9.5</td><td>6.5</td><td>113.8</td><td>0.69</td><td>11.0</td><td>19.7</td><td><strong>23.1</strong></td><td><strong>1.4</strong></td><td>314.6</td><td>76.7</td><td>0T</td></tr><tr><td>R-5</td><td>28225</td><td>96996</td><td>0.651</td><td>0.00321</td><td>0.71</td><td>9.1</td><td>24.9</td><td>164.7</td><td>2.74</td><td>15.0</td><td>109.0</td><td><strong>152.5</strong></td><td><strong>9.5</strong></td><td>217.0</td><td>45.5</td><td>1T</td></tr><tr><td>R-5</td><td>28226</td><td>96999</td><td>0.811</td><td>0.00448</td><td>0.76</td><td>8.9</td><td>18.6</td><td>273.7</td><td>2.10</td><td>13.3</td><td>108.9</td><td><strong>143.7</strong></td><td><strong>8.9</strong></td><td>181.2</td><td>49.6</td><td>0T</td></tr><tr><td colspan="17"><em>eDurango apatite standard</em></td></tr><tr><td>Durango</td><td>28227</td><td>97002</td><td>5.015</td><td>-</td><td>1.00</td><td>-</td><td>-</td><td>-</td><td>23.08</td><td>-</td><td>33.0</td><td><strong>33.0</strong></td><td><strong>2.0</strong></td><td>-</td><td>-</td><td>-</td></tr><tr><td>Durango</td><td>28228</td><td>97005</td><td>2.817</td><td>-</td><td>1.00</td><td>-</td><td>-</td><td>-</td><td>20.66</td><td>-</td><td>32.3</td><td><strong>32.3</strong></td><td><strong>2.0</strong></td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote><sup>a</sup><em>T</em><sub>1</sub> is the α-ejection correction after Farley et al. (1996). <sup>b</sup>Effective uranium concentration (U ppm + 0.238 Th ppm). <sup>c</sup>Grain morphology - 0T = no terminations, 1T = one termination, 2T = 2 terminations. <sup>d</sup>(U-Th-Sm)/He age for Durango apatite reference standard is 31.02 ± 1.01 Ma (1σ) (McDowell et al., 2005). <sup>e</sup>(U-Th)/He age for the Fish Canyon Tuff zircon reference standard is 28.4 ± 0.4 Ma (Gleadow et al., 2015).</footnote>
<caption>**Table 5**</caption>

Summary of Apatite and Monazite Fission-Track Densities, Ages, and Geochemistry of Samples From the Catalina-Rincon Metamorphic Core Complex, AZ, USA

<table><thead><tr><th>Sample</th><th>g<sup>a</sup></th><th>Ns<sup>b</sup></th><th>ρs × 10<sup>5c</sup></th><th>U ± 1sd (ppm)<sup>d</sup></th><th>Dpc/Dpar (μm)<sup>e</sup></th><th>P(χ<sup>2</sup>)<sup>f</sup></th><th>Age ± 1sd (Ma)<sup>g</sup></th><th>MTL ± 1sd (μm)<sup>h</sup></th><th>n<sup>i</sup></th></tr></thead><tbody><tr><td colspan="10">Southwestern Transect—Monazite Fission-Track</td></tr><tr><td>SP-01</td><td>12</td><td>260</td><td>6.4</td><td>525 ± 6</td><td>0.85</td><td>0.00</td><td>2.55 ± 0.31</td><td>10.7 ± 1.6</td><td>75</td></tr><tr><td>WP-01</td><td>17</td><td>224</td><td>4.2</td><td>462 ± 4</td><td>0.99</td><td>0.00</td><td>1.92 ± 0.20</td><td>10.5 ± 1.1</td><td>51</td></tr><tr><td>Mz554</td><td>15</td><td>105</td><td>13.8</td><td>135 ± 1.4</td><td>0.87</td><td>0.99</td><td>2.77 ± 0.27</td><td>10.5 ± 1.1</td><td>46</td></tr><tr><td>VAT12</td><td>22</td><td>588</td><td>11.5</td><td>2,261 ± 26</td><td>0.84</td><td>0.00</td><td>0.91 ± 0.11</td><td>10.6 ± 1.3</td><td>53</td></tr><tr><td>C2</td><td>9</td><td>249</td><td>17.1</td><td>2,100 ± 560</td><td>0.70</td><td>0.00</td><td>1.07 ± 0.24</td><td>8.9 ± 0.9</td><td>6</td></tr><tr><td colspan="10">Eastern Transect—Monazite Fission-Track</td></tr><tr><td>R4</td><td>19</td><td>798</td><td>30.4</td><td>820 ± 110</td><td>0.80</td><td>0.00</td><td>7.38 ± 0.65</td><td>9.9 ± 1.3</td><td>37</td></tr><tr><td>R3</td><td>14</td><td>336</td><td>28.2</td><td>1,065 ± 195</td><td>0.70</td><td>0.09</td><td>4.49 ± 0.42</td><td>9.7 ± 1.4</td><td>26</td></tr><tr><td>R2</td><td>16</td><td>316</td><td>14.1</td><td>552 ± 79</td><td>0.80</td><td>0.02</td><td>5.09 ± 0.48</td><td>9.4 ± 1.4</td><td>37</td></tr><tr><td>R1</td><td>14</td><td>96</td><td>24.6</td><td>555 ± 405</td><td>0.70</td><td>0.00</td><td>4.80 ± 1.30</td><td>—</td><td>—</td></tr><tr><td>R5</td><td>19</td><td>403</td><td>12.6</td><td>732 ± 94</td><td>0.70</td><td>0.00</td><td>2.98 ± 0.39</td><td>10.1 ± 1.4</td><td>20</td></tr><tr><td colspan="10">Eastern Transect—Apatite Fission-Track</td></tr><tr><td>R4</td><td>20</td><td>181</td><td>3.6</td><td>25 ± 2</td><td>1.80</td><td>0.28</td><td>27.0 ± 2.6</td><td>13.6 ± 1.4</td><td>35</td></tr><tr><td>R3</td><td>12</td><td>59</td><td>2.8</td><td>22 ± 2</td><td>1.90</td><td>0.93</td><td>24.1 ± 3.4</td><td>13.9 ± 1.7</td><td>16</td></tr><tr><td>R2</td><td>19</td><td>209</td><td>3.7</td><td>37 ± 3</td><td>1.80</td><td>0.32</td><td>23.6 ± 1.9</td><td>13.7 ± 1.1</td><td>40</td></tr><tr><td>R1</td><td>20</td><td>351</td><td>6.4</td><td>61 ± 4</td><td>1.70</td><td>0.24</td><td>22.5 ± 1.6</td><td>13.5 ± 1.4</td><td>57</td></tr><tr><td>R5</td><td>20</td><td>82</td><td>1.7</td><td>8 ± 1</td><td>0.70</td><td>0.81</td><td>34.8 ± 4.2</td><td>—</td><td>—</td></tr></tbody></table>

# 5. Qualitative Interpretation

## 5.1. Monazite Th-Pb and Apatite U-Pb Data

The northwestern transect yields two age populations for the monazite Th-Pb ages of ~49–40 Ma (Eocene) and ~24 Ma (Oligocene-Miocene, Figure S3 in Supporting Information S1). The eastern transect yields consistent apatite U-Pb and monazite Th-Pb ages of ~30–25 Ma (Oligocene, Figures S3 and S4 in Supporting Information S1). The Eocene population is consistent with the emplacement of the Wilderness Suite peraluminous plutonism, which has been extensively documented throughout the Catalina-Rincon MCC (Davis et al., 2023; Ducea et al., 2020; Fornash et al., 2013). However, Eocene monazite Th-Pb ages obtained from the Proterozoic Oracle Granite (sample SP-01) suggest that the emplacement of the Wilderness Suite caused U-Th-Pb age resetting by either dissolution-[re]crystallization or Pb diffusion of monazites within the host Proterozoic granites. Regionally, the Eocene plutonism is correlated with a period of widespread crustal anatexis which emplaced peraluminous granitoids in MCCs throughout the North American Cordillera (e.g., Chapman et al., 2021, 2024; Miller & Bradfish, 1980).

The Oligocene monazite Th-Pb and apatite U-Pb ages yielded in this study are consistent with the timing for the onset of detachment faulting within the Catalina-Rincon MCC (~30–26 Ma; Figures S3 and S4 in Supporting Information S1; Peters et al., 2003; Spencer et al., 2022). Oligocene plutons (~27–25 Ma) have been documented within the Catalina-Rincon MCC and are interpreted to be the product of syn-detachment melting and emplacement (Davis et al., 2023; Ducea et al., 2020). The Oligocene monazite Th-Pb and apatite U-Pb ages from the eastern transect (samples R1-R5) are interpreted to be caused by either dissolution-[re]crystallization or Pb diffusion during detachment faulting (Davis et al., 2023; Jepson et al., 2021). An alternative is that the eastern transect granite (mapped as Eocene Wilderness suite; Arca & Johnson, 2010) is part of the Oligocene Catalina Intrusive suite, which has been identified elsewhere in the Catalina-Rincon MCC (Davis et al., 2023; Ducea

<footnote><sup>a</sup>Number of grains analyzed per sample. <sup>b</sup>Sum of tracks counted. <sup>c</sup>Density of spontaneous tracks counted. <sup>d</sup>Median concentration of 238uranium. <sup>e</sup>Median length of the etch pits (Dpar = apatite, Dpc = monazite) in μm. <sup>f</sup>Probability that single grain ages belong to the same population. <sup>g</sup>Central age (Ma) for AFT. <sup>h</sup>Mean track length of confined track lengths measured. <sup>i</sup>Number of confined tracks measured.</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Monazite fission-track radial plots and confined track length histograms. Std. Dev. is the standard deviation, D<sub>pc</sub> is the etch pit diameter.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Age-elevation plots for the southwestern (a) and eastern (b) transects through the Catalina-Rincon metamorphic core complex. The low-temperature thermochronology data are from Jepson et al. (2022) and this study, AFT is apatite fission-track, AHe is apatite (U-Th-Sm)/He, ZHe is zircon (U-Th)/He, and MFT is monazite fission-track. Dashed lines outline the dispersed age-elevation trends. Solid blue lines are fitting using a linear regression model.</figcaption></figure>

et al., 2020). Due to the susceptibility of U-Th-Pb ages in phosphate phases to be modified, zircon U-Pb dating would be required to adequately resolve the crystallization age.

Ducea et al. (2020) reported Eocene zircon U-Pb ages from a leucocratic dyke which crosscuts the mylonitic fabric and suggested that the mylonite in the Catalina-Rincon MCC may have represented an inherited feature prior to Oligocene detachment faulting. Monazites from the same sample (VAT12) yielded both Eocene and Oligocene Th-Pb ages (Figure S3 in Supporting Information S1). Given the presence of Oligocene monazite Th-Pb ages, we suggest that the Eocene zircon U-Pb ages identified by Ducea et al. (2020) represent an inherited age and may not reflect the timing of the cross-cutting relationship.

## 5.2. Apatite Fission-Track and Apatite and Zircon (U-Th-[Sm])/He Data

Apatite fission-track and apatite and zircon (U-Th-[Sm])/He ages obtained in this study are consistent with previously published low-temperature thermochronometric data collected throughout the Catalina-Rincon MCC (Davis et al., 2023; Fayon et al., 2000; Jepson et al., 2021, 2022; Keith et al., 1980; Scoggin et al., 2021). The northwestern transect yields late Oligocene and early Miocene ages (~25–12 Ma), which are in agreement with exhumation associated with late-stage detachment faulting and Basin and Range high angle normal faulting (Davis et al., 2004; Dickinson, 1991). Combining the low-temperature thermochronometric data from the northwestern Catalina-Rincon MCC in this study with those of Jepson et al. (2022), we find that our new data is consistent with the dispersed age-elevation profile recorded along the exposed detachment footwall (Figure 5a).

The eastern transect yielded AFT data, which display a late Oligocene (~27–23 Ma) age-elevation trend (Figure 5b). The lowest elevation sample (R5) yielded a late Eocene age (~35 Ma), which is older than its associated apatite U-Pb and monazite Th-Pb ages (~27–29 Ma) and out of sequence with the age-elevation trend. However, this sample was collected in the hanging-wall of the Martinez Ranch Fault (Davis et al., 2004). Thus, its age likely reflects a mixed cooling-age which has been identified at relatively higher structural levels in the Catalina-Rincon MCC (Jepson et al., 2022). Even following the FQA protocol (He et al., 2021), the AHe data in the eastern transect yielded high dispersion and many single-grain AHe ages which are older than their corresponding apatite U-Pb and monazite Th-Pb ages (Figure 3). This suggests that those AHe ages which are older than their associated apatite U-Pb and monazite Th-Pb ages are likely spurious and will not be discussed further.

## 5.3. Monazite Fission-Track Data

All 10 MFT ages obtained in this study yield central ages which are significantly younger (>10 Myr) than their corresponding AHe, ZHe, and AFT ages, supporting laboratory estimates of a ≤46–25°C nominal temperature sensitivity over ~10⁶ yr (Jones et al., 2019, 2023; Weise et al., 2009). However, out of the 10 samples analyzed for
MFT, only two (samples Mz554 and R3) pass the P(χ²) test (Figure 4). Failure to pass the P(χ²) at a given confidence level suggests overdispersion of single-grain ages relative to uncertainty on a given central age (Galbraith, 2005). This may be due to poorly constrained (or even unknown) physical effects. For example, established fission-track systems such as AFT have found that etch-pit length (Dpar) reflects a geochemical control on annealing (e.g., Sobel & Seward, 2010) and more specifically increased Cl concentrations have been shown to be associated with higher annealing temperatures (e.g., Green et al., 1985; Issler et al., 2021). These geochemical controls cause differences in annealing rates and can lead to overdispersion of ages for a given sample. Concerning MFT, Jones et al. (2023) suggested that some possible sources of uncertainty might include relatively higher precision in the ²³⁸U concentrations that the MFT system may be undergoing differential annealing at surface temperatures (140–70°C monazite partial annealing zone [MPAZ]), perhaps with a chemical control reflected in major and trace element concentrations, or radiation enhanced annealing of tracks in monazite.

### 5.3.1. Possible Effects of Geochemistry on Monazite Fission-Track Ages

To assess the role of monazite geochemistry on annealing, we compared pooled single-grain MFT ages to etch-pit length (Dpc), effective uranium (eU) (²³⁸U + 0.238 * ²³²Th), Y, and La (Figure 6). The etch-pit length is known to reflect bulk compositional factors in other minerals (e.g., apatite). We observe a scattered, yet relatively flat trend of etch-pit length with MFT age (Figure 6a), which suggests that such a bulk estimate of monazite geochemistry may not reflect annealing as closely as it does for apatite (e.g., Carlson et al., 1999; Issler et al., 2021). An inverse relationship between etch time and radiation damage has been proposed by Nakajima et al. (2024). The single grain Dpc consistency (0.82 ± 0.17 µm) observed in this study compared to published results (Jones et al., 2022) suggests that etch times were likely sufficient. We note that etch-pit length and width in monazite are similar (Jones et al., 2022); thus, we report the observed longest axis as Dpc.

Monazite geochemistry is controlled by two main substitutions:

$$2\text{REE}^{3+} \leftrightarrow \text{Th}^{4+} + \text{Ca}^{2+} \quad (1)$$

$$\text{REE}^{3+} + \text{P}^{5+} \leftrightarrow \text{Th}^{4+} + \text{Si}^{4+} \quad (2)$$

where U⁴⁺ may substitute for Th⁴⁺ (Engi, 2017; Spear & Pyle, 2002). Jones et al. (2023) observed a correlation between MFT single-grain age and Si⁴⁺ indicating a tentative geochemical control on annealing. Here, we compared MFT age to La (ppm) as well as Y (ppm), as Y³⁺ may substitute for REE³⁺ (Engi, 2017; Spear & Pyle, 2002). Yttrium concentrations in monazites are frequently zoned, reflecting co-existing garnet growth and breakdown (e.g., Kylander-Clark et al., 2013; Pyle et al., 2001). We observe an inverse correlation between Y concentration and MFT single-grain ages (Figure 6c). In addition, there is a diffuse relationship between lower La concentrations and younger ages; however, we suggest that this might reflect the substitution between REE³⁺ and Y³⁺ (Figure 6d). The monazite Th-Pb standard (Mz554) passes the P(χ²) test (0.99; Figure 4e; Liu et al., 2012). Although perhaps this is not surprising as geochronological standards tend to be more chemically homogeneous than unknown samples, this supports the proposition that monazite geochemistry influences MFT age dispersion. Thus, we suggest that Y concentration may play a role in the intra-sample single-grain age dispersion observed in MFT data, but this requires further investigation beyond the scope of this study.

### 5.3.2. Monazite Confined Fission-Track Length Distributions

In fission-track thermochronology, the distribution of confined fission-track lengths primarily reflect temperatures experienced by a sample in its transit through and below the PAZ (e.g., Green et al., 1986), while apparent ages contain information on timing or duration of the recorded thermal history. Here, we compare length distribution interpretations established for the apatite PAZ to possible cooling pathways through the MPAZ. Due to the broad MPAZ temperature range (140–70°C), confined track length distributions in monazite may be more representative of apatite confined track length distributions from within the apatite PAZ (120–60°C; Gleadow et al., 1983, 1986). However, we acknowledge that correlations between confined track length behavior in the apatite and monazite PAZs remain a proxy until further empirical and laboratory calibration has been carried out.

Four samples from the northwestern transect yield similar monazite confined fission-track mean lengths of ~10.6 µm (Figure 4), which is close to the lengths (~10.60 ± 0.19 µm) of implanted tracks determined by Jones
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Monazite fission-track single-grain ages compared to; (a) etch-pit length (Dpc), (b) effective uranium, (c) Y concentration (log scale), and (d) La concentration. Green lines are fitted splines using generalized additive models and linear regression model (c) and gray shading is one standard error.</figcaption></figure>

et al. (2021), suggesting limited annealing through the MPAZ. Samples SP-01, WP-01, and Mz554 from the higher-elevation portion of the northwestern transect show track length histograms with negatively skewed distributions. In apatite, negatively skewed distributions are indicative of relatively slow monotonic cooling through the PAZ (Green et al., 1989). The two low elevation samples from the northwestern transect (VAT12 and C2) yield a broader, more unimodal distribution, which in apatite is indicative of fast cooling through the PAZ (Green et al., 1989; Figure 4), albeit sample C2 has too low a confined track count (n = 6) to be meaningful. Coupled with the age-elevation trend observed in the northwestern transect, the length distributions suggest a single period of cooling in the Pliocene—Pleistocene (<5 Ma; Figure 4). Based on the correlation with apatite confined track length distributions and the age-elevation profile, we suggest that the northwestern MFT transect underwent a single period of cooling from the upper to the lower MPAZ (≤46–25°C) at ~3–1 Ma.

The age-elevation and MTL relationship observed in the eastern transect suggests that two periods of cooling are preserved, an earlier phase of cooling (latest Miocene) at higher elevations, which transitions to a phase of more recent cooling at lower elevations (Pliocene). In the eastern transect, the upper three samples (R4, R3, and R2) yield shorter MTL (~9.7 µm) compared to the northwestern transect, suggesting longer residence at higher MPAZ temperatures (Figure 4). The lowest sample yields a longer MTL (~10.1 µm), suggesting a period of faster cooling through the MPAZ. Further, there is a relationship between elevation and the skewness of the MTL
<figure><img src="image_9.png" /><figcaption>**Figure 7.** A combined monazite fission-track age-confined mean track length plot for the Catalina-Rincon metamorphic core complex. The blue line is fitted using a linear model with a quadratic function. Sample C2 is excluded from the model fit due to insufficient confined track lengths (n = 6).</figcaption></figure>

histograms, with the highest elevation sample (R4) showing a positively skewed distribution, separated by a bimodal or unimodal distribution (sample R3), from negatively skewed distributions observed in samples R2 and R5 (Figure 4). In apatite, this switch in length distributions is interpreted to represent a transition from prolonged residence in, or slow cooling through, the PAZ to more recent fast cooling. Using apatite confined lengths as a proxy, we interpret the eastern MFT transect to preserve a transition from slow Miocene-Pliocene cooling (~8–4 Ma) to enhanced Pliocene cooling (~3 Ma) through the MPAZ.

The transition from slow Miocene to faster Pliocene-Pleistocene cooling is supported when combining the MFT age and MTL data from northwestern and eastern transects (Figure 7). This approach is frequently used with the AFT method to identify a concave up pattern (or “boomerang” trend) which represents a transition from slow to fast regional cooling (Gallagher & Brown, 1997). For the MFT data, we observe shorter lengths (<10 µm) and late Miocene—Pliocene central ages (~8–4 Ma) transitioning to longer lengths (>10 µm) and Pliocene—Pleistocene central ages (~3–1 Ma; Figure 7). Note that sample C2 is excluded due to a low number of confined tracks (n = 6). In the AFT system, the shorter length part of the boomerang represents a period of slow, protracted cooling, and the longer length part of the boomerang represents the onset of more rapid cooling (Gallagher & Brown, 1997; Gleadow et al., 2002). Following this framework, we suggest that the Catalina MFT age and length data represent the onset of faster cooling through the MPAZ during the Pliocene.

### 5.3.3. The Impact of Partial Annealing at Surface Temperatures

Given the broad temperature range inferred for the monazite PAZ (140–70°C) at 10⁶ yr time scales, it is clear that the MFT system remains open and is still undergoing annealing at surface temperatures (Figure S1 in Supporting Information S1; Jones et al., 2021, 2023). In an assessment of heat production in the Catalina-Rincon MCC, Ketcham (1996) determined a geothermal gradient of 20.4–24°C/km for the top 15 km. The southwestern USA is presently in a hot, arid environment with elevated mean annual surface temperatures in the basins of 20–25°C and peak summertime daily temperatures of ~38°C (Fick & Hijmans, 2017). However, the Catalina-Rincon MCC is exposed to comparatively lower mean annual surface temperatures of 10–15°C (Fick & Hijmans, 2017). As is standard practice, rock samples collected in this study were taken from ~5–10 cm below the surface, which provides minor insulation from surface heating (e.g., Reiners et al., 2007; Reiter, 2005). However, surface temperatures of <10–15°C are still sufficiently high to cause ambient temperature annealing (Figure S1 in Supporting Information S1; Jones et al., 2021).
In the southwestern USA, local lapse rates (change of surface temperature with elevation) are typically ~7.5°C/km (Meyer, 2007). Our samples show a maximum elevation difference of ~1,300 m ($\Delta T$ = ~10°C), given that the MFT system does undergo significant annealing at surface temperatures; this difference might be sufficient to explain the relationship between central age and elevation. The northwestern transect yielded a Pliocene—Pleistocene age-elevation trend (~2.8–1.0 Ma) and an associated gradient of ~0.89 ± 0.31 mm/yr (Figure 5a). The eastern region yielded a late Miocene—Pliocene age-elevation trend (~7.5–3.0 Ma) and an associated gradient of ~0.12 ± 0.04 mm/yr (Figure 5b). The age-elevation relationship is consistent with a qualitative estimate of the cooling rate provided by the boomerang age-MTL relationship, which shows relatively slow Miocene—Pliocene cooling (~7.5–3.0 Ma) and relatively rapid Pliocene cooling (~3–1 Ma, Figure 7). The occurrence of younger MFT ages (<3 Ma) at the highest elevations (SP-01) and unique age and MTL distributions in each transect leads us to suggest that, although surface temperature likely induces some degree of annealing, the MFT ages in this study primarily reflect cooling through the upper part of the monazite PAZ.

# 6. Quantitative Interpretation

## 6.1. Thermal History Modeling

As described earlier, we performed thermal history modeling to assess the behavior of the MFT annealing model outlined in Jones et al. (2021). Results of the thermal history modeling are compared to our qualitative assessment of the MFT data above and a previously established tectonic evolution of the Catalina-Rincon MCC (Davis et al., 2023; Dickinson, 1991; Jepson et al., 2022). The northwestern thermal history model was based on four samples (SP-01, WP-01, Mz554, C2) in an elevation profile (maximum vertical separation of ~1,324 m) in the southwest Catalina-Rincon MCC (Figure 2). Both apatite and monazite single-grain ages, confined track lengths, (U-Th-[Sm])/He data, and modeling parameters are presented in Tables 4 and 5, and Tables S2, S4, and S5. Sample UoM0422-05 from Jepson et al. (2022) was collected from the same locality as sample C2 and provided the AFT data. The thermal history model which was run without MFT yields a time-temperature (tT) pathway that is consistent with the interpretation of Jepson et al. (2022) and displays rapid cooling from the initial constraint (27.5 ± 2.5 Ma and 200 ± 50°C) to ~80–40°C at 23 Ma, followed by a period of protracted cooling from ~80–40°C to ~20 ± 20°C at 0 Ma (Figure 8a). The thermal history model, which was run with the MFT data, yields a tT pathway that shows a period of rapid cooling from ~180–120°C to ~60°C at ~3.5 Ma followed by more protracted cooling from ~60°C to ~20°C at 3–1 Ma. The timing (~3–1 Ma) of rapid cooling is consistent with our qualitative estimates (Figures 5 and 7), however the magnitude of cooling at ~3.5 Ma ($\Delta T$ = 120–60°C) is not consistent with tectonic history of the Catalina-Rincon MCC (e.g., Davis et al., 2023; Dickinson, 1991) and fails to find a solution which honors all the thermochronology data together (Figure 8b inset). The poor fit of the better established AFT and AHe methods (Flowers et al., 2009; Ketcham et al., 2007) suggests that the present MFT annealing model may not adequately capture the amount of annealing observed in the northwestern transect.

The eastern thermal history model was based on five samples (R4, R3, R2, R1, and R5) along an elevation profile (maximum vertical separation of ~523 m) in the eastern Catalina-Rincon MCC (Figure 2). The thermal history model, which was run without MFT, yields a time-temperature (tT) pathway that is consistent with that of Jepson et al. (2022) and displays rapid cooling from the initial constraint (27.5 ± 2.5 Ma and 200 ± 50°C) to ~110–90°C at ~25 Ma, followed by an extended period of protracted cooling from ~110–90°C to ~20 ± 20°C at 0 Ma (Figure 8c). The thermal history model with MFT yields a two-stage tT pathway. Initially, there is slow cooling from ~80–70°C to ~50–40°C at ~8–7 Ma, followed by a period of rapid cooling from ~50–40°C to ~20 ± 20°C at ~5–3 Ma (Figure 8d). This thermal history model for the eastern transect is consistent with the established geologic framework, which documents slow cooling through the upper-crust during the Miocene (e.g., Davis et al., 2023; Jepson et al., 2022). The timing (~3 Ma) and magnitude ($\Delta T$ = ~30°C) of the second phase of cooling are in good agreement with the MFT age-elevation data and qualitative interpretations based on confined track length distributions (Figures 5 and 7). As with the northwestern profile, the fit for the AFT and AHe data, however, remains poor (Figure 8d inset), suggesting that the MFT annealing model may not be adequate in replicating all sources of annealing.

Radiation damage from the elevated concentrations of radiogenic elements has been suggested as a potential influence on annealing kinetics. Radiation-enhanced annealing has been suggested as playing a role in producing younger ages in zircon and AFT thermochronometers generally (e.g., Hendriks & Redfield, 2005; Kasuya & Naeser, 1988). However, the typically low concentration of $^{238}$U and $^{232}$Th in apatite limits the impact of any such
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Thermal history models for the northwestern and eastern transects through the Catalina-Rincon metamorphic core complex. (a) Thermal history model and associated predicted versus observed for the northwestern transect without monazite fission-track (MFT) data and (b) thermal history model and associated predicted versus observed including MFT. (c) Thermal history model and associated predicted versus observed for the eastern transect without MFT and (d) thermal history model and associated predicted versus observed including MFT. Ellipses in the predicted versus observed age plots for (b) and (d) highlight the failure of these models to find a solution consistent with the thermal annealing models for all of the low-temperature thermochronology systems, and are consistent only with the MFT data. AFT = apatite fission-track, MTL = mean track length, AHe = apatite (U-Th-Sm)/He, LL is log likelihood. Modeling was performed using QTQt 5.8.6 with the initial starting constraint of 200 ± 50°C at 27.5 ± 2.5 Ma based on published constraints in Peters et al. (2003), Jepson et al. (2022), and Spencer et al. (2022).</figcaption></figure>

radiation enhanced annealing (e.g., McDannell et al., 2019). Laboratory annealing experiments on fission-track stability in zircon have found only limited increases in track reduction with higher radiogenic element concentrations (e.g., Sales et al., 2023; Tagami & Matsu'ura, 2019). In monazite, the very high $^{238}$U and $^{232}$Th concentrations are suggested to facilitate annealing as a function of alpha recoil (e.g., Meldrum et al., 1998; Nasdala et al., 2020). Jones et al. (2023) identified a strong correlation between increased eU and younger intra-sample age populations, suggesting that radiation-enhanced annealing may be operating and therefore has a potential influence on MFT age dispersion. Increased radiation damage is also correlated with etching rate, as young, low radiation damage monazites exhibit higher chemical resistance (Nakajima et al., 2024). Despite observing no correlation between increased eU and younger ages (Figure 6b), additional annealing due to high radiogenic element concentrations may be a factor in reconciling the overestimated cooling found using the present MFT annealing model.

The combined AFT, AHe, and MFT thermal history models show a large overestimation of the magnitude of cooling for the northwestern transect, and a minor overestimation in the magnitude of cooling for the eastern transect (Figures 8b and 8d). Yet across both transects, the timing and pathway resolved by thermal history modeling is consistent with our qualitative estimates established through age-elevation and age-MTL relationships (Figures 5 and 7). Even if there is a geochemical control on annealing, it remains that annealing is a temperature-controlled process. The overestimation of cooling suggests that the MFT annealing model
established by Jones et al. (2021) is overestimating the MPAZ or that there are temperature independent controls on annealing. Previous estimates on MFT annealing have proposed a linear fanning model for extrapolation from laboratory to geological timescales (Jones et al., 2021; Weise et al., 2009). A fanning curvilinear model, however, has been proposed for apatite annealing data and predicts a lower resistance to annealing on geological timescales (Carlson et al., 1999; Ketcham et al., 1999). Thus, we suggest that radiation damage, intragrain geochemistry, and further annealing experiments (e.g., borehole samples) are required to explain the discrepancy between the qualitative observations and the thermal history modeling.

# 7. Discussion

## 7.1. Pre-Metamorphic Core Complex Exhumation (~40 Ma)

The role of middle to lower-crustal processes has been increasingly documented as influencing the development of MCCs within the North American Cordillera (e.g., Konstantinou et al., 2013; Levy et al., 2023; Zuza & Cao, 2022). In the Catalina-Rincon MCC, Ducea et al. (2020) and Jepson et al. (2022) have both documented pre-MCC signals, which suggest a multi-stage development. In the eastern transect, we observe a series of AHe single-grain ages and one AFT central age which are older than 35 Ma (Figure 3 and Table 4). However, the AHe and AFT ages are older than their corresponding monazite Th-Pb and apatite U-Pb ages, suggesting that this apparent Eocene age reflects spurious cooling ages or excess He. Thus, we do not observe any convincing evidence of pre-MCC detachment exhumation.

## 7.2. Metamorphic Core Complex Exhumation (30–19 Ma): Detachment Faulting

The present-day geomorphology of the Catalina-Rincon MCC is dominated by the SW dipping low-angle detachment fault, which juxtaposes mylonitic lower-plate rocks against brittle upper-plate rocks (e.g., Davis, 1987; Davis & Coney, 1979). The onset of detachment faulting is constrained to 30–26 Ma based on syn-extensional basin deposits and fault-tilted ash flow tuffs (Peters et al., 2003; Spencer et al., 2022) and had ceased by ~22 Ma as constrained by an undeformed mafic dyke cutting the mylonitic fabric (Davis et al., 2023). The majority (~80%) of established low-temperature thermochronometric data (AFT, AHe, and ZHe) collected in this study yielded ages between ~30 and ~20 Ma (Figure 9) and thus coincided with detachment faulting. Additionally, we observe low-temperature thermochronometric data that are within error of associated monazite Th-Pb and apatite U-Pb ages. Davis et al. (2023) and Jepson et al. (2021) also observed a similar overlap between high- and low-temperature thermochronometric data. Overall, this implies rapid cooling from middle-to shallow-crustal temperatures (~600–100°C; Figures 8a and 8c) at a rate difficult to achieve with surface processes. These results confirm that tectonic exhumation through syn- and post-detachment faulting is the major mechanism for mid-to upper-crustal cooling in the Catalina-Rincon MCC (e.g., Davis et al., 2023; Ducea et al., 2020; Fayon et al., 2000). Consistent with similar studies from the nearby Coyote and Pinaleño MCC systems (Chapman et al., 2024; Gottardi et al., 2020).

## 7.3. Basin and Range Exhumation (17–11 Ma): High-Angle Normal Faulting

The oldest MFT age (~7.4 Ma, R4) obtained in this study is structurally highest on the footwall of the Martinez Ranch Fault and preserves relatively short and positively skewed confined track lengths, indicative of slow cooling (Figure 4). This is reflected in the eastern transects MFT age-elevation relationship and the combined MFT age-elevation plot (Figures 5 and 7). Several high-angle normal faults (e.g., the Martinez Ranch Fault and the Pirate Fault; Figure 2) cut the Catalina-Rincon MCC edifice and are associated with regional Basin and Range extension (~15–12 Ma, e.g., Davis et al., 2004; Dickinson, 1991; Singleton et al., 2019). Evidence from thermochronometric and fault-offset data suggests that the Basin and Range normal faulting led to ~5–2.5 km of denudation/exhumation (e.g., Davis et al., 2004; Fayon et al., 2000; Jepson et al., 2022). Fayon et al. (2000) suggested that differential erosion of footwall uplifts caused by Basin and Range normal faulting may explain the diffuse trend of established low-temperature cooling ages of ~20–10 Ma identified in this study. Scoggin et al. (2021) obtained Miocene (~15–8 Ma) Mn- and Fe-oxide (U-Th)/He ages from the Catalina—Rincon MCC and correlated them with fluid circulation and oxide recrystallization associated with Basin and Range faulting. We suggest that our late Miocene MFT age reflects slow exhumation in response to post-Basin and Range footwall uplift (Figures 8 and 9).
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Schematic figure of the Catalina-Rincon metamorphic core complex modified after Rosenkranz et al. (2018) indicating timing of zircon (U-Th)/He (ZHe), apatite (U-Th-Sm)/He (AHe), apatite fission-track, and monazite fission-track data and their interpreted exhumation mechanisms.</figcaption></figure>

## 7.4. Low-Magnitude Pliocene—Pleistocene Erosion (~5–1 Ma): A Climate Signal?

The majority (90%) of MFT data across both the northwestern and eastern transects yield Pliocene—Pleistocene ages (5.1–1.0 Ma, Figure 4). The eastern transect suggests a period of Pliocene exhumation (~5–3 Ma), which is ~20 Myr younger than exhumation determined using established low-temperature thermochronometers in both the eastern and northwestern transects (Figures 8 and 9). The northwestern MFT profile yields an elevated apparent exhumation rate of 0.89 ± 0.31 mm/yr at ~2.8–1.0 Ma (Figure 5). The observed period of young (<3 Ma), elevated exhumation (>0.5 mm/yr) occurs when the region is considered to be tectonically quiescent as extension had migrated to the western margin of North America by ~6 Ma (e.g., McQuarrie & Wennicke, 2005). Thus, it is unlikely that these MFT ages are a direct response to deformation or tectonic exhumation.

We suggest that the ~3–1 Ma cooling may have been facilitated by climate enhanced erosion associated with changes in the North American paleoclimate (Figure 10). During the Miocene, the δ¹⁸O record from the southwestern USA shows a gradual shift toward higher values associated with increased summer rainfall and enhanced evaporation rates, similar to the present day (Figure 10; Latorre et al., 1997; Licht et al., 2017). Beginning in the Pliocene, continental ostracod data indicate an increase in seasonal precipitation relative to modern (Forester, 1991; Ibarra et al., 2018). This change is supported by the δ¹⁸O record which shows a decreasing trend from 6 Ma to 4.5 Ma, interpreted to represent a wetter climate (Mack et al., 1994; Smith et al., 1993). Enhanced precipitation and a stronger monsoon were maintained until ~2.5–1.4 Ma before returning to a period of increased aridity at ~1.4–0.7 Ma (Figure 10; Bhattacharya et al., 2022; Ibarra et al., 2018; Mack et al., 1994; Slate et al., 1996). The return to the modern arid climate would have reduced modern erosion rates preserving the Pliocene—Pleistocene signal (e.g., Sobel et al., 2003). Climatically enhanced erosion has been recorded in the Pliocene-Pleistocene both in the southwestern USA and globally (e.g., Herman & Champagnac, 2016; Mack & Leeder, 1999). Hence, the correlation between the onset of increased precipitation in the southwestern USA and the observed MFT ages suggests that cooling recorded by MFT in the Catalina-Rincon MCC could be related to Pliocene climate-enhanced erosion (Figure 10). Our results highlight the potential of MFT to record climate enhanced erosion and possibly date young relief-forming processes.

## 8. Conclusions

Using the first recorded MFT age-elevation and age-MTL relationships, we show two periods of low-magnitude cooling in the Catalina-Rincon MCC. An early period of late Miocene—early Pliocene (~7.5–3 Ma) slow exhumation is related to late-stage footwall uplift associated with Basin and Range normal faulting. This was followed by a limited period of Pliocene—Pleistocene (~3.0–1.0 Ma) rapid exhumation (~0.89 mm/yr), which
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Kernel density estimate of thermochronometric ages in the Catalina-Rincon metamorphic core complex from 20 Ma to present compared against major tectonic events and the δ¹⁸O record of the global ocean (Westerhold et al., 2020; Zachos et al., 2001) and the southwestern USA (Latorre et al., 1997; Licht et al., 2017; Mack et al., 1994; Smith et al., 1993, Table S6).</figcaption></figure>

we attribute to monsoon intensification in the southwestern USA. Although the MPAZ encompasses surface temperatures, the MFT age measurements are nonetheless structured with clear variations with both elevation and track length, indicating that they do preserve a record of their thermal history. Qualitative interpretations from these relationships provide insight into the pathway through the near-surface shallow crust. Our findings suggest that the current, purely thermal annealing model for MFT data overestimates temperature change, yet reasonably reconstructs the timing deduced from qualitative observations. This may indicate an additional unquantified contribution to fission-track annealing in monazite. We also suggest that high intra-sample age dispersion may be a function of a geochemical control with higher Y and lower La concentrations correlating with younger single-grain MFT ages, similar to Si content being related to older single-grain MFT ages reported by Jones et al. (2023). Notwithstanding the early stage of our understanding of the MFT system, its potential as a new low-temperature thermochronometer opens an exciting window into exploring interactions among relief generation, climate, and surface processes.
## Acknowledgments

Gilby Jepson was supported by the donors of the American Chemical Society Petroleum Research Fund under Grant 66957-DNI8. Barbara Carrapa acknowledges NSF-EAR Tectonics 1919179. George Gehrels acknowledges NSF-EAR 2050246 for the support of the Arizona Laserchron Center. Antoine Triantafyllou acknowledges the CNRS-INSU and the Tellus-SYSTER program for the AIRCOO project. We thank scientific editor Whitney Behr and an anonymous reviewer for their insightful comments which improved this work. The University of Melbourne thermochronology laboratory receives funding under Project A3.51 of the AuScope program (www.auscope.org.au) of the Australian National Collaborative Research Infrastructure Strategy (NCRIS). Dr. Steven Foster and the University of Oklahoma Mass Spectrometry, Proteomics, & Metabolomics (MSPM) Core Facility are thanked for providing instrumentation and assisting with data collection.

## Data Availability Statement

Data and figures supporting the conclusions presented here are available on the Zenodo data repository via Jepson (2025).

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