Titanite Petrochronology Records Secular Temperature and Fluid Evolution During Ductile Deformation: An Example From Late Cretaceous Shear Zones in the Eastern Transverse Ranges

Quantifying the timing and conditions of ductile deformation is essential for quantitative models of lithospheric deformation. Yet, directly constraining these variables and documenting how they change during a single deformation event remain difficult. We present titanite microstructural, zoning, trace‐element, and U‐Pb data from


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The titanite U-Pb system is one of the best candidates to integrate constraints on the timing and conditions of deformation in ductile shear zones. Titanite is a common accessory mineral in many crustal rocks (Frost et al., 2000) and has a high apparent U-Pb closure temperature as defined by datasets from nature (∼750°C; Hartnady et al., 2019;. Titanite dates are therefore more likely to record (re)crystallization than cooling at all but granulite-facies metamorphic conditions. Titanite has a propensity to recrystallize in the presence of metamorphic fluids, which resets U-Pb dates and provides a direct constraint on the timing of fluid infiltration (e.g., . Zr contents in titanite can also be used to document the temperature of (re)crystallization (Hayden et al., 2008). Titanite has been empirically shown to respond to deformation by synkinematic neocrystallization (Papapavlou et al., 2017) and/or undergoing crystal-plastic deformation (Bonamici et al., 2015;Gordon et al., 2021;Kavanagh-Lepage et al., 2023;Moser et al., 2022), providing a means to link the timing to the temperature and fluid conditions of deformation. However, how titanite responds to and records the timing and conditions of ductile deformation-including whether dislocation creep or fluid-driven recrystallization is the primary mechanism by which dates are reset during deformation-is known only in a limited subset of ductiley deformed rocks (Bonamici et al., 2015;Gordon et al., 2021;Kavanagh-Lepage et al., 2023;Moser et al., 2022;Papapavlou et al., 2017). This limits the application of titanite U-Pb petrochronology to structural problems.
This contribution presents an integrated titanite U-Pb, microstructure, zoning, and trace-element data set from Late Cretaceous, sub-m-scale ductile shear zones hosted in the Jurassic Cottonwood pluton, located within Joshua Tree National Park and the Eastern Transverse Ranges, CA. These shear zones are ideal for resolving how titanite responds to and records the timing and conditions of events in ductile shear zones as their deformation age is (a) independently constrained by the crystallization age of deformed aplite dikes and (b) significantly younger than the intrusion age of the host pluton. These factors make it clear which titanite textures, trace-element compositions, and dates are related to shear-zone activity. We evaluate the usefulness of titanite petrochronology to document: (a) the timing, temperature, and fluid compositions of deformation; (b) the evolution of these conditions with ongoing deformation; and (c) the diversity of weakening mechanisms that may be concomitant during a single, short-lived deformation episode. The data set also addresses the relative roles of fluids and lattice reorientation in resetting U-Pb dates in plastically deformed titanite.

Geologic Setting and Field Observations
The Cottonwood Pass area of Joshua Tree National Park spans the eastern Hexie Mountains and the western Eagle Mountains of the Eastern Transverse Ranges (Figure 1). Here, exposed bedrock comprises Cordilleran arc rocks typical of the Eastern Transverse Ranges and Mojave crustal province basement ( Figure 1; Barth et al., 2008;Economos et al., 2021;Needy et al., 2009;Powell, 2001;Wooden & Miller, 1990). In the field area (Figure 1b), Proterozoic gneisses (Powell, 2001) were intruded by the granitic to granodioritic Cottonwood pluton at ∼155 Ma (Barth et al., 2008) followed by the granitic Smoke Tree Well pluton and granodioritic Porcupine Wash pluton at ∼75 Ma (Needy et al., 2009). Previous Al-in-hornblende thermobarometry indicates that the Porcupine Wash Pluton was emplaced at ∼11 km depth (Needy et al., 2009). There are no emplacement depth estimates for the Cottonwood or Smoke Tree Well plutons. Mesozoic, Cordilleran arc magmatism ended in the Eastern Transverse Ranges and central Mojave region at ∼74 Ma (Economos et al., 2021).
Our observations focus on a series of discrete, cm-to dm-scale mylonitic shear zones hosted in the Jurassic Cottonwood pluton (Figure 2a). These shear zones are anastomosing at the outcrop-scale, bifurcating around relatively undeformed host blocks of the Cottonwood pluton ( Figure 2b). In general, the shear zones are (a) gently to moderately dipping, broadly NE-SW striking with oblique, normal-sense kinematics, or (b) subvertical, ESE-WNW striking with sinistral kinematics (where shear sense could be determined; Figures 2c-2f; Figure S1 in Supporting Information S1). However, the strike of the gently to moderately dipping shears is variable at the outcrop scale and across the field area ( Figure S1 in Supporting Information S1). Lineations plunge gently in both the moderately dipping and subvertical shears ( Figure S1 in Supporting Information S1). The shear zones are variably subparallel to and offset a suite of aplite dikes (Figures 2c-2h). Some offset dikes are continuous for several meters within individual shear zones (Figures 2e and 2f). Field evidence, including scar folding of the host pluton at the dike margins, indicates that the dikes are ductiley deformed within the shear zones (Figures 2g  and 2h). Centimeter-thick quartz veins are common in both the gently to moderately dipping shear zones and are broadly oriented subparallel to the mylonitic foliation; nowhere were they observed to cut the shear zones at a high angle (Figures 2g and 2h). The microstructures of the quartz veins in thin section illustrate that they are plastically deformed (Figure 2k).

Sample Descriptions and Petrography
We present titanite data from two Cottonwood pluton mylonites sampled from two discrete shear zones ( Figure S1 in Supporting Information S1). Sample 1 (Figure 3 and Figure S1 in Supporting Information S1; 33.807231 N, 115.803005 W) is from the gently dipping shear zone shown in Figure 2b; sample 2 ( Figure 3 and Figure S1 in Supporting Information S1; 33.807193 N, 115.803447 W) is from the subvertical shear zone shown in Figures 2e and 2f that offsets an aplite dike. Both samples contain plagioclase and alkali feldspar, quartz, biotite, titanite, ilmenite, magnetite, and minor zircon and apatite ( Figure 3 and Figure S2 in Supporting Information S1). Feldspar porphyroclasts in sample 1 have core-mantle microstructures, with some larger grains displaying irregular or serrated grain boundaries (Figures 3a and 3e). These microstructures are compatible with bulge recrystallization accompanied by a component of subgrain rotation recrystallization. In sample 2, feldspar is fine-grained and porphyroclasts are absent (Figures 3c and 3f). Quartz deformation microstructures in both samples are consistent with recrystallization via subgrain rotation and/or grain boundary migration ( Figure 3e). Subdomains of larger quartz grains that parallel the foliation are present in both samples (Figures 3a and 3f); the microstructures of these larger quartz grains are consistent with late bulge recrystallization. Titanite grains in the two samples range from euhedral to subhedral, several-hundred μm micron-long grains that typically have their long axes aligned with the foliation (Figures 3a and 3b; Figure  S2 in Supporting Information S1) to several-hundred μm micron-long titanite aggregates that are ubiquitously aligned with the foliation, wherein individual grains are ≤50 μm long (Figures 3c and 3d; Figure S2 in Supporting Information S1). The euhedral to subhedral and aggregate grains are proximal to one another and distributed throughout the thin section, with as little as ∼1,500 μm separating the two textures ( Figure S3 in Supporting Information S1). Both textural types are located within biotite seams that parallel the foliation (Figures 3a-3d; Figure S2 in Supporting Information S1). The euhedral to subhedral titanite grains are only present in sample 1, whereas titanite aggregates are present in both samples ( Figure S2 in Supporting Information S1).

Titanite Petrochronology Approach and Methods
Titanite U-Pb dates have been demonstrated to record a variety of tectonothermal processes depending on the P−T and deformation history of a rock (Bonamici et al., 2015;Frost et al., 2000;Garber et al., 2017;Hartnady et al., 2019;Hayden et al., 2008;Moser et al., 2022;Spencer et al., 2013;Walters et al., 2022). For example, U-Pb dates may reflect one or more phases of igneous and/or metamorphic titanite neocrystallization (Spencer et al., 2013;Stearns et al., 2016). Thermally mediated volume diffusion of radiogenic Pb in titanite is negligible at temperatures <750°C Kohn, 2017;Stearns et al., 2016) and there are few clear examples of Pb volume diffusion in natural titanite data sets (Bonamici et al., 2015;Garber et al., 2017). Titanite further recrystallizes via dissolution-reprecipitation recrystallization, wherein a fluid reacts with a crystal, replaces the original grain with the same or similar phase, alters the trace-element composition, and variably resets the U-Pb dates Moser et al., 2022;Walters et al., 2022). The development of crystal-plastic deformation microstructures can also affect U-Pb dates in titanite and other minerals, with dislocations and/or subgrain boundaries variably trapping or serving as fast diffusion pathways for Pb and other trace-elements, or forming efficient conduits for fluids to recrystallize grain interiors (Bonamici et al., 2015;Fougerouse et al., 2021;Gordon et al., 2021;Kavanagh-Lepage et al., 2023;Moser et al., 2022;Odlum et al., 2022;Piazolo et al., 2016;Reddy et al., 2006).
Each of these processes produces diagnostic intragrain spatial patterns among U-Pb dates, trace-element concentrations, and microstructures that can be used to identify the process responsible for the U-Pb date and trace-element distributions (Bonamici et al., 2015;Cherniak, 1993;Garber et al., 2017;Holder, Hacker, Seward, Kylander-Clark, & Holder, 2019;Kohn, 2017;Moser et al., 2022;Spencer et al., 2013;Stearns et al., 2016;Walters et al., 2022;Walters & Kohn, 2017). We combine backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) data, laser-ablation split-stream ICP-MS (LASS) analyses of Figure 1. (a) Regional geologic map of the Eastern Transverse Ranges. After Barth et al. (2008). (b) Geologic map of the Cottonwood Pass area of Joshua Tree National Park. Cottonwood pluton (CW); Porcupine Wash pluton (PW); Smoke Tree Well pluton (STW). Yellow star denotes study location. After Powell (2001) and Barth et al. (2008); includes new mapping by the authors. U-Pb dates and trace-element concentrations together with electron probe microanalysis (EPMA) measurements of F contents to relate titanite U-Pb dates to zoning, microstructures, and trace-element compositions to differentiate among the above processes. All data were acquired in standard thickness, polished thin sections. A detailed discussion of the effect of volume diffusion on titanite U-Pb dates and trace-element contents is included in Supporting Information S1 (Text S1, Figure S4).

Scanning Electron Microscopy (BSE and EBSD)
Scanning electron microscopy (SEM) data were acquired on FEI Quanta field emission source SEM at the University of California, Santa Barbara. The BSE images were acquired using a 10 kV accelerating voltage and 1-5 nA beam current. Electron backscatter diffraction (EBSD) data were collected and indexed using an HKL Technology Nordlys II EBSD camera and Oxford/HKL Channel 5 Flamenco software with a 20 kV accelerating voltage, 1-5 nA beam current, and at a ∼15 mm working distance. The polished sample surface was tilted 70° with respect to the electron beam. EBSD data were acquired using an electron-beam rastering approach over a 2 × 2 μm grid. The data were processed using Oxford/HKL Channel 5 Tango application; single pixels whose orientations deviated significantly with respect to neighboring pixels were removed, and unindexed pixels were assigned average orientations if at least six of the neighboring pixels were indexed as titanite. Relative misorientation maps were created using Oxford/HKL Channel 5 Tango and titanite pole figures were made using MTEX in MATLAB (Bachmann et al., 2010;Nolze & Hielscher, 2016).
The U-Th-Pb and trace-element data were reduced using the Iolite 2.5 plugin for IgorPro 6.37 with the "U-Pb Geochronology3" (for U-Th-Pb isotopes) and "Trace Elements" (for trace-element concentrations, assuming 19.2 wt % Ca in the unknown titanite analyses) data reduction schemes (Paton, 2011). Integrations excluded the first two and last two seconds of each analysis prior to the downhole fractionation correction; integrations for unknowns were also trimmed to select only homogeneous date and trace-element portions to remove the effects of inclusions. Reported uncertainties for 206 Pb/ 238 U and 207 Pb/ 206 Pb include additional uncertainty propagated in quadrature to make secondary titanite reference material BLR a single population (i.e., MSWD = 1). These additional uncertainties were 2% for 207 Pb/ 206 Pb and 3.1% for 206 Pb/ 238 Pb. All reported titanite dates are 207 Pb/ 206 Pb-corrected 206 Pb-238 U dates using a common 207 Pb/ 206 Pb of 0.86 ± 0.05, as defined by the 207 Pb/ 206 Pb intercept of unanchored regression through the youngest population of titanite U-Pb analyses; this value is within the uncertainty of the Stacey and Kramers (1975) 207 Pb/ 206 Pb intercept for the measured dates (0.84).

Electron Probe Microanalysis (EPMA)
Quantitative analyses of major and select trace elements in titanite were performed using a Cameca SX100 electron microprobe equipped with five wavelength dispersive spectrometers and Thermo NSS EDS system. The data were acquired with a 15 kV accelerating voltage, 40 nA beam current, and 5 μm beam diameter. Si K α , Ca K α , and Ti K α X-rays were acquired using EDS. Fe K α X-rays were analyzed on an LLIF; Mn K α , Ce L α , Y L α , and Nb L α on an LPET; Al K α on an LTAP; and F K α and O K α on PC0 crystal. Counting times for both peak and background intensities were 20 s for Y and Nb; 36 s for F and O, 50 s for Mn; 80 s for Ce, Al, and Fe; and 162 s for Si, Ca and Ti. X-ray intensity reference materials included natural and synthetic minerals and glasses. All X-ray intensities were acquired and quantified using Probe for EPMA from Probe Software Inc. The stoichiometry of each analysis was determined using the workflow outlined in Oberti et al. (1991) with all Fe assumed as ferric.

Zr-In-Titanite Thermometry
Zr-in-titanite temperatures were calculated using the Zr concentrations analyzed by LASS and the Hayden et al. (2008) Zr-in-titanite thermometer. We assumed a pressure of 0.3 ± 0.1 GPa for the calculation from previous estimates of the emplacement depth of the Cretaceous Porcupine Wash pluton (Needy et al., 2009), which cuts the Cottonwood pluton and was synchronous with aplite dike intrusion and ductile shearing. We further adopted SiO 2 = 1 and TiO 2 = 0.75 ± 0.25 based on the presence of quartz and titanite, but the absence of rutile (Kapp et al., 2009). We report two uncertainties for each of the temperature estimates: one that includes the uncertainties on the Zr concentrations only (assuming identical pressure and TiO 2 among all measured titanite spots), and a second that includes the uncertainties on the pressure and TiO 2 estimates.

U-Pb Dates and Textures
Titanite LASS analyses (n = 245) were acquired from 14 grains from the two samples (8 sub-to euhedral grains and 4 aggregates from sample 1, 2 aggregates from sample 2; Figure S2 in Supporting Information S1, Table S2). The compiled U-Pb data in Tera-Wasserburg concordia space define populations at 151 ± 2 Ma (anchored to initial 207 Pb/ 206 Pb = 0.86, MSWD = 1.2, 108 analyses; Figure 4) and 75 ± 1 Ma (unanchored, MSWD = 1.2, 87 analyses; Figure 4). A subset of analyses (n = 50; Figure 4) fall between these populations. Three key relationships among grain morphology, zoning, and microstructure characterize these distinct date populations: 1. All dates from the titanite aggregates belong to the ∼75 Ma population ( Figure 5 and Figure S5 in Supporting Information S1); no age inheritance is observed. BSE images reveal that individual grains within the aggregates are concentrically zoned, have minor lobate-cuspate rims, or are nearly featureless in BSE (Figures 5a and 5b; Figure S5 in Supporting Information S1). Many individual titanite grains in the aggregates are cored by <10 μm diameter ilmenite inclusions (Figures 5a and 5b; Figure S5 in Supporting Information S1) and all aggregates are spatially associated with ilmenite and/or magnetite (Figures 5a and 5b; Figures S2 and S5 in Supporting Information S1). Triple-junction, ∼120° grain boundaries are common in the titanite aggregates, though these are more evident (and grain boundaries are straighter) in aggregates from sample 2 than in sample 1 (Figures 5a and 5b; Figure S5 in Supporting Information S1). EBSD data of the aggregates demonstrate that the misorientations between individual grains within the aggregates are >10° (Figures 5c and 5d; Figure S5 in Supporting Information S1); low-angle boundaries (LABs; 1-10°) are common within individual grains, with aggregates from sample 1 preserving more LABs than those from sample 2 (Figures 5c and 5d; Figure S5 in Supporting Information S1). Pole figures reveal the dispersion of the crystallographic axes and poles to crystallographic planes in predominantly great circles; this is particularly evident in the <010> pole figures, in which poles that lie close to the primitive are dispersed along it (Figures 5e and 5f; Figure S5 in Supporting Information S1). Unanchored Tera-Wasserburg regression of all U-Pb analyses from the aggregate titanite grains from both samples (n = 57) defines a single population at 76 ± 1 Ma (MSWD = 1.2; Figure 5g).  (e, f) EBSD relative misorientation maps of the two grains shown in (a) and (b) demonstrating that the lattices of these grains are not bent at scales detectable by the 25 μm laser-ablation spot. Circles in right grain are laser-ablation spots; the EBSD data for this grain were collected subsequent to LASS analyses. (g, h) Tera-Wasserburg concordia diagrams of all U-Pb analyses (n = 118) from unbent, sub-to euhedral titanite grains. Ellipses are colored by date population (same as LASS spots) in (g) and by texture (core or rim) in (h).
Information S1); a subset of dates from the rims are intermediate between the 151 Ma and 75 Ma populations (5 of 25 analyses; Figures 6c, 6d, 6g, and 6h; Figure S6 in Supporting Information S1). The cores of these grains are characterized by sector and/or oscillatory zoning (Figures 6a and 6b; Figure S6 in Supporting Information S1) and largely yield 151 Ma dates (Figures 6c, 6d, 6g, and 6h; Figure S6 in Supporting Information S1). EBSD misorientation maps demonstrate that five of the seven sub-to euhedral grains analyzed are effectively unbent (Figures 6e and 6f; Figure S6 in Supporting Information S1). The outermost ∼20 μm of some of these grain tips are rotated by up to ∼6°, but we were unable to sample these domains for LASS analyses due to their small size. 3. EBSD misorientation maps reveal that two of the analyzed sub-to euhedral grains display up to 10° of intragrain misorientation (Figures 7a and 7b). Like the unbent sub-to euhedral grains (e.g., Figure 6), BSE images demonstrate that these grains contain BSE-dark, lobate-cuspate rims with 75 Ma dates (Figures 7c and 7d). Unlike the unbent grains, however, BSE-dark domains cut across the interiors of bent grains, resulting in convoluted and patchy zoning (Figures 7c and 7d). Date and misorientation are demonstrably related in these complexly zoned portions (Figures 7e and 7f). In one of the bent grains, 15 of 17 analyses from the misoriented grain tip fall between the 151 Ma population and the 75 Ma population (Figures 7a and 7e). In the other bent grain, dates young with increasing misorientation and 5 of the 8 analyses that belong to the 75 Ma population lie entirely within lobate-cuspate, BSE-dark domains that cut across the interior and misoriented portion of the grain (Figures 7d and 7f). In considering the complete data set, the majority of dates that fall between 151 Ma and 75 Ma population (33 of 50 analyses) come from the two bent grains (cf. Figures 6g and 7g).

Trace-Element Chemistry and Zr-In-Titanite Temperatures
The trace-element compositions and Zr-in-titanite temperatures systematically vary among these date-texture groupings. Aggregates from sample 1 are depleted in nearly all REEs (except for Eu), Y, Fe, Zr, Hf, Nb, and Ta when compared to the sector and oscillatory-zoned cores in the same sample ( Figure 8a). For example, the median Zr and Ce concentrations in the cores are 342 and 2,445 ppm, respectively, whereas the median concentrations of these elements in the aggregates (in sample 1) are 155 ppm (Zr) and 926 ppm (Ce; Figure 8a). The individual trace-element concentrations of the aggregates from the two samples are broadly comparable, though Fe is demonstrably higher, and V and Cr are lower in aggregates from sample 2 compared to sample 1 (Figure 8b). BSE-dark rims in sample 1 are depleted in the REEs, Y, Sr, Mg, Mn, Fe, Hf, Zr, Nb, and Ta and enriched in Cr, Sc, V, and Al compared to the oscillatory and sector-zoned cores from the same grains ( Figure 8c). For instance, the median Zr and Ce concentrations of the rims are 52 ppm (Zr) and 90 ppm (Ce), which are ∼300 and ∼2,000 ppm lower than the median values for the cores in the same grains ( Figure 8c).
Chondrite-normalized REE profiles of the titanite cores and aggregates from both samples have similar flat shapes with La depletions (Figure 9a), though the aggregates have lower total REE. The cores exhibit a pronounced negative Eu anomaly; Eu anomalies are comparatively minor in the rims as well as the aggregates from both samples, with the aggregates and rims from sample 1 yielding a slight negative Eu/Eu* and aggregates from sample 2 yielding a slight positive Eu/Eu* (Figure 9a). M-HREE abundances in the rims from sample 1 are similar to those from the aggregates in both samples ( Figure 9a). LREE are significantly depleted in the rims in sample 1 compared to the cores and aggregates from both samples (Figure 9a).
Fluorine abundances cluster into two distinct populations: (a) F is generally higher in the titanite rims from sample 1 compared to oscillatory/sector zoned cores and aggregates from both samples and (b) F contents are similar among aggregates from both samples and the cores from sample 1 (Figure 9b; Table S3). A subset (3 of 31) of rim F analyses yielded values comparable to cores and aggregates, whereas 4 of 24 F analyses from the aggregates in sample 1 are comparable to values from rims ( Figure 9b; Table S3). Rims also have systematically higher F/(Al + Fe 3+ ) than the titanite aggregates and cores (Figure 9b; Table S3). F is positively correlated with Al + Fe 3+ (Figure 9b), consistent with the coupled substitution of Al + Fe for Ti and F + OH for O in titanite (Carswell et al., 1996;Franz & Spear, 1985).
Calculated Zr-in-titanite temperatures from the sector-and oscillatory-zoned cores range from ∼680 to 760°C (Figure 9c). The temperatures from the aggregates in both samples are predominantly ∼640-710°C (Figure 9c). The BSE-dark, lobate-cuspate rims record temperatures of ∼600-620°C (Figure 9c). Although these temperature differences cannot be resolved if the full external uncertainties in pressure and TiO 2 are considered, the Zr concentrations of these date-texture populations do differ outside their 2σ analytical uncertainty (Figure 9c).
Nb/Ta is similar among the cores and aggregates from both samples (Figure 9d). In general, Nb/Ta values are higher in the rims than in the cores and the aggregates, though some rim analyses yielded Nb/Ta values that  overlap those from cores and aggregates. Rim and aggregate Zr/Hf ratios are similar to but slightly higher than those from cores ( Figure 9e).

Aplite Dike Zircon U-Pb Data
We acquired zircon U-Pb data from the aplite dike offset by the shear zone in Figures 2e and 2f to provide an independent constraint on the deformation age of the Cottonwood shear zones (Figure 10 and Figure S7 in Supporting Information S1; Table S4). The dated aplite dike sample is undeformed and was sampled ∼10 m from the main trace of the shear zone ( Figure S1 in Supporting Information S1) in an effort to date processes related to crystallization, rather than deformation, of the dike. The zircon U-Pb data were acquired at the Arizona LaserChron Center; the zircon U-Pb methods and data treatment are presented in Supporting Information S1 (Text S1). Concordant zircon U-Pb analyses from the aplite dike fall into three groups. Three concordant analyses from oscillatory zoned zircon yield Proterozoic dates between ∼1.8 and ∼1.6 Ga (Figures 10a and 10b).  Ten concordant analyses from oscillatory and sector-zoned zircon cores yield a coherent, Jurassic population at 148 ± 3 Ma (MSWD = 2.1; Figures 10c and 10d). The youngest, coherent zircon U-Pb date population is defined by five concordant analyses with a weighted mean 206 Pb-238 U age of 75 ± 2 Ma (MSWD = 0.9; Figures 10e  and 10f). The analyses that define this 75Ma population include sector-zoned cores, oscillatory zoned cores, and oscillatory zoned rims (Figure 10f).

Igneous Titanite Crystallization and Shear Zone Deformation Age
The bimodality in titanite U-Pb dates from the two samples ( Figure 4) suggests that these data record the timing of punctuated processes at ∼151 and ∼75 Ma. Dates that make up the ∼151 Ma population are exclusively located within the sector-and oscillatory-zoned cores of the sub-to euhedral grains (Figures 6, 7 and Figure S6 in Supporting Information S1). This age also overlaps prior constraints on the intrusion age of the Cottonwood pluton (155 ± 2 Ma; Barth et al., 2008), suggesting that the 151 Ma population represents the igneous titanite crystallization age in the Cottonwood pluton ( Figure 11a). The younger, ∼75 Ma titanite population is indistinguishable from the youngest coherent population of zircon U-Pb dates from the aplite dike ( Figure 10). The analyses that comprise the ∼75 Ma zircon population in the aplite dike are from sector-zoned cores, oscillatory zoned cores, and oscillatory zoned rims (Figure 10e). Combined with the fact that the aplite dike sample is undeformed and was sampled from outside the shear zone, this suggests that the ∼75 Ma zircon population represents the crystallization age of the dike. As the aplite dikes are offset by the shear zones, their crystallization age provides an independent constraint that deformation must have occurred after or at 75 Ma. Given this context, the ∼75 Ma titanite population likely reflects the timing of shear zone activity. Each of the three titanite textural groupings described in Section 5.1.1 can be linked to separate but related processes active in the ductile shear zones at ∼75 Ma.

Synkinematic Titanite Growth at 75 Ma
Dates from the titanite aggregates define a single population that is the same as the maximum deformation age defined by the aplite dikes. This single date population may reflect either complete recrystallization and U-Pb date resetting of Jurassic igneous titanite during plastic deformation at ∼75 Ma or titanite neocrystallization at 75 Ma. Gordon et al. (2021) describe titanite with similar textures in Caledonian shear zones in Norway's Western Gneiss Region. The absence of a single date population in some of these titanite were interpreted to reflect partial to complete resetting of titanite U-Pb dates as a result of plastic deformation (Gordon et al., 2021). We therefore expect that any titanite grain that experienced deformation-induced date resetting would likely retain some inherited component of the original crystallization age. However, there is no trace of an inherited, ∼151 Ma igneous crystallization age in the titanite aggregate grains in this study ( Figure 5g). Instead, the single population recorded by the Cretaceous titanite aggregates is more consistent with a single growth event at 75 Ma. In addition, the aggregates are spatially associated with magnetite and/or ilmenite, with some individual grains within the aggregates preserving small ilmenite cores (Figures 3c, 3d, 5a, and 5b; Figures S2, S5 in Supporting Information S1). These textures suggest that the titanite aggregates grew from the breakdown of ilmenite and/or magnetite. Further, all aggregates are aligned with the foliation in both samples (Figures 3c, 3d, and 5; Figures S2 and S5 in Supporting Information S1). Together, these data and observations suggest that the aggregate grains formed as a result of synkinematic titanite neocrystallization at 75 Ma, rather than recrystallization of Late Jurassic igneous titanite (Figure 11b).
The microstructure of the aggregates implies that these titanite grains deformed plastically as they crystallized ( Figure 5 and Figure S5 in Supporting Information S1). Many individual aggregate grains contain low-angle grain boundaries (Figures 5c and 5d; Figure S5 in Supporting Information S1), and crystallographic axes as well as poles to planes are dispersed along great circles (Figures 5e and 5f; Figure S5 in Supporting Information S1). These data are consistent with the deformation of these titanite aggregates by dislocation creep, most likely subgrain rotation recrystallization (Lloyd et al., 1997). The polygonal nature of the aggregate grains indicates that temperatures remained high enough after deformation ceased to cause grain boundary area reduction (Bons & Urai, 1992; Gordon et al., 2021).

Titanite Dissolution-Reprecipitation During Deformation
The sub-to euhedral titanite grains preserve BSE-dark, lobate-cuspate rims that predominantly yield ∼75 Ma dates ( Figure 6 and Figure S6 in Supporting Information S1). Such textures are consistent with recrystallization via dissolution-reprecipitation, a process by which fluids drive recrystallization, changing the trace-element composition of a grain but preserving the grain shape (Geisler et al., 2007;Putnis, 2009). Strikingly similar textures have been observed in numerous other titanite grains interpreted to have recrystallized via dissolutionreprecipitation (Garber et al., 2017;Moser et al., 2022;Walters et al., 2022). The dates from the lobate-cuspate rims are identical (within uncertainty) to the timing of deformation recorded by the synkinematic titanite aggregates ( Figure 5). These observations imply that the sub-to euhedral, protolith igneous titanite recrystallized via interface-coupled dissolution-reprecipitation during ductile deformation at ∼75 Ma (Figure 11c). Some rims are nearly continuous around entire titanite grains, and we do not observe any textures indicative of dissolution-reprecipitation creep ("core and beard" structures; Wintsch & Yi, 2002). Therefore, fluid-driven recrystallization was most likely interface-coupled and did not directly cause titanite deformation.

Fluid-Driven Recrystallization in Bent Grains
Dates and microstructure are correlated in the two bent titanite grains (Figure 7). For example, in one bent grain, dates from the bent grain tip scatter between the igneous titanite crystallization age and the deformation age of the shear zone (Figures 7a, 7c and 7e). In the other bent titanite grain, dates young with increasing lattice bending and only 1 out of 17 analytical spots preserves the igneous crystallization age of the grain (Figures 7b,  7d and 7f). These observations suggest that lattice bending played some role in resetting U-Pb dates in the bent titanite grains. Lattice bending may either directly facilitate Pb loss via dislocation-assisted diffusion (Gordon et al., 2021;Piazolo et al., 2016) or these microstructures may indirectly affect dates, serving as efficient pathways for fluids to infiltrate and recrystallize bent domains (e.g., Moser et al., 2022). The spatial association of lattice bending and fluid-related zoning makes it difficult to evaluate the direct effects of lattice bending on the U-Pb dates in these titanite . Patchy and convoluted BSE-dark domains (consistent with their recrystallization by fluids) are present in the bent grain tip of the larger bent grain (Figure 7c). In addition, BSE-dark compositional domain cuts across the entirety of the smaller of the two bent grains and is continuous with a BSE-dark lobate-cuspate rim, whose geometry is consistent with having formed via dissolutionreprecipitation recrystallization (Figure 7d). Further, it is only in bent grains that BSE-dark, fluid-related zoning is observed in grain interiors rather than only at grain margins (cf. Figures 6 and 7). Though it is not possible to determine whether lattice bending directly affected dates in these titanite, the consistent spatial relationship between lattice bending and fluid-related zoning makes it clear that the bent titanite grain tips facilitated enhanced fluid-mediated recrystallization.
Dates from the bent grains are distributed between the igneous titanite crystallization age and the deformation age, rather than producing a bimodal distribution of dates (Figures 7e-7g). This date span can be explained by either mechanical mixing of fully fluid-reacted 75 Ma domains and unreacted, igneous 151 Ma domains or a range of partial to complete resetting of U-Pb dates by some combination of lattice bending and fluid-driven recrystallization. To evaluate the effects of mechanical mixing versus partial resetting, we compared dates to their corresponding trace-element concentrations for the two bent grains. Mechanical mixing should produce mixing lines between the ∼151 Ma population (i.e., cores) and the ∼75 Ma population (i.e., rims) for every measured trace element. The relationship between dates and trace-element concentrations in the bent grains could plausibly be explained by mixing for some elements, but not for every element measured ( Figure S8 in Supporting Information S1). For example, Ce concentrations decrease monotonically with younging date in the bent grains ( Figure  S8 in Supporting Information S1; consistent with mechanical mixing), but V concentrations in areas of convoluted zoning retain the composition of unreacted cores ( Figure S8 in Supporting Information S1). These data are consistent with the variable resetting of dates and trace-element signatures (or "decoupled" dates and trace elements), that is commonly observed in both fluid-recrystallized Moser et al., 2022;Walters et al., 2022) and plastically deformed titanite (Gordon et al., 2021;Kavanagh-Lepage et al., 2023;Moser et al., 2022). This indicates that the range of dates in the bent titanite at least partly result from partial resetting by dissolution-reprecipitation and/or deformation, though the small size of some fluid-reacted domains indicates that some mechanical mixing likely occurred. The interpretation of partially reset dates in bent domains indicates that, if there were any direct effects of lattice bending on U-Pb dates and trace-element concentrations, the presence and motion of dislocations that facilitated this lattice bending was not wholly sufficient to completely reset the titanite U-Pb and trace-element systematics.

Evolution of Temperature and Fluid Composition During Deformation
Calculated Zr-in-titanite temperatures from the titanite aggregates from both samples and the lobate-cuspate rims are predominantly ∼600-700°C, with most temperatures from the rims ∼30°C (or ∼30 ppm Zr) lower than those from the titanite aggregates (Figure 9c). Assuming equilibrium partitioning of Zr in titanite, this difference may reflect an increase in pressure, a difference in quartz, zircon, or rutile activity, or a true temperature difference between the two titanite populations. If the temperature remained constant, a 30-ppm difference in Zr concentrations would require an increase in pressure of ∼0.2 GPa between aggregate and rim (re)crystallization, or about ∼7.5 km of burial in timescales undetectable by the precision afforded by LASS for these dates (i.e., ∼1-3 Myr). This is geologically unreasonable and can be ruled out as the primary cause. The presence of quartz and zircon indicate that the SiO 2 and ZrSiO 4 of the system are both 1; the varying Zr-in-titanite temperatures are thus not readily explained by underestimating SiO 2 or overestimating ZrSiO 4 . The aggregate grains are consistently associated with ilmenite and magnetite, whereas the rims on the sub-to euhedral grains are not. This suggests that the difference in Zr content between the two textures could reflect localized TiO 2 variation. However, TiO 2 would have to have been less than ∼0.5 in the aggregates to account for the totality of the Zr concentration offset. This is unlikely as the presence of titanite alone suggests that TiO 2 is greater than ∼0.5 for both textures (Ashley & Law, 2015;Chambers & Kohn, 2012;Kapp et al., 2009). Local scale TiO 2 heterogeneity is also less plausible given that the two titanite textures are distributed throughout the sample and located proximal to one another ( Figure S3 in Supporting Information S1). Alternatively, the lower Zr contents in the rims could reflect ∼30°C of cooling between synkinematic titanite aggregate crystallization and recrystallization of the titanite rims. This would imply that the titanite aggregates record early deformation at temperatures >630°C, with rim recrystallization occurring as the system cooled (Figures 11b and 11c). Since the dates from aggregates and rims are indistinguishable, cooling must have occurred over timescales undetectable by the precision afforded by LASS (i.e., ∼1-3 Myr). We therefore find it most plausible that the two ∼75 Ma titanite textures with varying Zr content faithfully record the temperature evolution of the shear zone as deformation progressed.
The two titanite textures with the same age (aggregates and rims) but different trace-element compositions and apparent temperatures (Figures 8 and 9) require that two distinct titanite (re)crystallization reactions occurred at timescales shorter than LASS uncertainties (i.e., ∼1-3 Myr). These reactions could reflect either simultaneous titanite (re)crystallization that occurred as a result of localized equilibrium or a rock-wide sequence of reactions involving phases that were progressively out of equilibrium as the system evolved. If the two grain types reflect localized rather than rock-wide equilibrium, then there should be a clear textural control on the spatial distribution of the aggregates and euhedral grains, in which all reactants responsible for the titanite-forming reaction were spatially clustered. However, the aggregates and euhedral grains occur adjacent to a variety of different phases and both grain types are located within biotite foliations (Figure 3 and Figure S2 in Supporting Information S1). The only textural distinction between the sub-euhedral grains and the aggregates is that the aggregates are consistently associated with ilmenite and/or magnetite. If the growth of titanite aggregates from ilmenite and/or magnetite was the overarching control on the chemical differences between the titanite textures, then the titanite aggregates should be REE depleted when compared to the rim compositions (as the REE concentrations are negligible in both ilmenite and magnetite, e.g., Shepherd et al., 2022). However, the LREEs are enriched in the aggregates compared to the rims, and M-HREE concentrations are comparable between aggregates and rims ( Figure 9a). Though there may have been some localized effects on the trace-element compositions, these observations suggest that spatially localized equilibrium could not have been the only control on the difference in trace-element composition between the two titanite textures.
The zoning textures and trace-element compositions of the aggregates and rims are instead consistent with a sequence of reactions as the system evolved and different phases became progressively out of equilibrium. Some individual grains within the aggregates have BSE dark, lobate-cuspate rims (Figure 5a) with F contents and Zr-in-titanite temperatures comparable to those from the rims in the sub-to euhedral grains (Figures 9b and 9c; Figure S10 in Supporting Information S1). These observations demonstrate that aggregates did experience some interface-coupled dissolution-reprecipitation recrystallization after their initial crystallization at ∼75 Ma, although fluid-recrystallization was clearly more extensive in the sub-to euhedral igneous titanite (cf. Figures 5  and 6). These textural relationships require that the aggregate grains grew first, followed by the formation of the lobate-cuspate rims on all pre-existing titanite (aggregates and sub-to euhedral grains), indicating that the two textures represent time-progressive reactions.
Several lines of evidence indicate that both of the titanite-producing reactions occurred under fluid-present conditions. As noted above, the lobate-cuspate geometry of the rims suggests that the fluids directly drove the recrystallization of the sub-to euhedral grains (Figure 6; e.g., Putnis, 2009). The titanite aggregates clearly contain volatiles (e.g., F; Figure 9b) that must have been sourced from a fluid phase, as their textural setting indicates they grew from the breakdown of anhydrous phases (ilmenite and/or magnetite; Figure 5). Further, a grain-boundary fluid is required to transport many of the chemical species involved in the reaction (e.g., Ca + REE) to the nucleation site of the titanite aggregates. We therefore suggest that the trace-element concentrations in both the aggregates and the rims, at least in part, reflect the composition of the fluids that accompanied these sequential reactions during deformation.
For example, the increase in F compositions and F/[Al + Fe 3+ ] in the rims compared to the aggregates most likely represents the evolution of volatiles in the system, and thus changing fluid compositions, as deformation progressed (Figure 9b). Elevated F contents alone could be explained by rims enriched in Al (as F incorporation into titanite is complexed with Al and is thus "Al limited"); however, the increased F/[Al + Fe 3+ ] indicates a difference in absolute F contents of the system between aggregate and rim (re)crystallization. Further, several aspects of the mineralogy of the mylonites, as well as the geochemistry of the fluid-reacted titanites, are consistent with a predominantly hydrous fluid accompanying deformation. The mylonites lack carbonate minerals that would indicate high CO 2 fluid pressures. In addition, the samples contain other hydrous phases (biotite, apatite) that most likely (re)crystallized during deformation. Further, the presence of titanite over rutile and ilmenite is consistent with high 2 (Frost et al., 2000;Kohn, 2017). Finally, the loss of REEs and high field-strength elements in fluid-recrystallized rims is consistent with their recrystallization in the presence of a hydrous, halogen-bearing fluid (Migdisov et al., 2009;Rapp et al., 2010). We thus suggest that titanite chemistry records the evolution of the fluid that accompanied deformation from a hotter (640-710°C), hydrous, comparatively F-depleted fluid (during aggregate crystallization; Figure 11b) to a cooler (600-620°C), more halogen-rich brine (during rim recrystallization; Figure 11c).
The fluid composition evolution could reflect the closed-system partitioning of elements into phases that co-crystallized with the titanite or an open-system change in fluid composition. The rim-forming reaction resulted in the enrichment of F, increased F/[Al + Fe 3+ ], depletion of LREEs, elevated Nb/Ta, and unchanged Zr/Hf in the rims compared to the aggregate titanite grains (Figures 8 and 9). Co-crystallization of biotite should result in LREE depletion (Mahood et al., 1983) and similar Zr/Hf ratios between the two textures (Nash & Crecraft, 1985). However, co-crystallizing biotite would not drive titanite enrichment in F (as biotite preferentially incorporates F over OH; Zhang et al., 2022) or elevated titanite Nb/Ta ratios (as D Nb /D Ta is higher for biotite than for titanite; Acosta-Vigil et al., 2010;Nash & Crecraft, 1985;Prowatke & Klemme, 2005;Stepanov & Hermann, 2013). Similarly, co-crystallizing apatite could deplete LREEs (Prowatke & Klemme, 2006), but F contents, Nb/Ta, and Zr/Hf should all decrease in titanite that (re)crystallized with apatite (Mathez & Webster, 2005;Prowatke & Klemme, 2006). Alternatively, if the fluids were sourced from the Smoke Tree Well pluton (which intruded at ∼75 Ma), then the change in fluid composition could reflect the evolving open-system fluid composition as the pluton crystallized. Fluorine is preferentially incorporated into the residual melt in igneous systems rather than the exsolved fluid phase, such that the F-enriched fluids recorded by the titanite rims could represent a transition from pluton-derived fluids to aplite-derived fluids. However, late-stage exsolved magmatic fluids typically only evolve to high F contents if systems are significantly F-enriched (Dolejs & Zajacz, 2018). Regardless of how or why the fluid composition changed, the elevated F contents and increased F/[Al + Fe 3+ ] contents in the titanite rims are consistent with the fluid evolving toward a more F-rich brine with ongoing deformation and early cooling (Figures 11b and 11c).
A few analyses from lobate-cuspate BSE dark rims retain trace-element compositions, including F, Zr, and Nb/Ta, that are similar to cores and the aggregate grains (Figures 9b-9d). These data indicate that either some rim development commenced at the same temperature and fluid conditions as synkinematic titanite aggregate neocrystallization, or that trace-element contents were incompletely reset from their core compositions. If these values record "early" rim development (i.e., prior to cooling and evolution of fluid to more F-enriched brine, synchronous with aggregate crystallization), then the same analyses with "core/aggregate-like" Zr contents should also retain "core/aggregate-like" Nb/Ta. This, however, is not the case, as several spots with low Nb/ Ta (i.e., <40) also yield low Zr contents (i.e., <630°C or <60 ppm; Figure S9 in Supporting Information S1). Further, the three EPMA analyses from rims with F contents similar to aggregates and cores are located near LASS analyses that yielded incompletely reset U-Pb dates ( Figure S10 in Supporting Information S1), suggesting that the trace-element contents were similarly incompletely reset. These data indicate that the few rim spots with "core/aggregate-like" trace-element compositions reflect incomplete resetting rather than early rim development.

Implications for Dating Deformation With Titanite
Previous titanite datasets demonstrate that titanite U-Pb petrochronology can be a useful tool to constrain the timing of ductile deformation (Bonamici et al., 2015;Gordon et al., 2021;Kavanagh-Lepage et al., 2023;Moser et al., 2022;Papapavlou et al., 2017). Here, dates from plastically deformed, synkinematic titanite aggregates provide an unequivocal deformation age for the studied shear zones. In addition, the trace-element compositions of the titanite aggregates and rims document the temperature of early deformation and early cooling of the system as deformation continued, as well as the evolving volatile contents of the fluid that accompanied deformation ( Figure 11). Titanite petrochronology is therefore a powerful approach to not only date ductile shear zones but also document the temperature and fluid compositions of deformation and how these conditions evolve within a single deformation event.
Our integrated data set also demonstrates that titanite petrochronology is capable of capturing the diversity of weakening mechanisms and resulting complex rheological evolution that may occur over the course of a single phase of deformation. The timing of shear zone activity, documented by the age of the titanite aggregates, indicates that ductile deformation accompanied the intrusion of the Porcupine Wash and Smoke Tree Well plutons (Figure 1b), which would have provided a heat source and a mechanism for thermal weakening. In addition, deformed, neocrystallized, synkinematic titanite aggregates acted as a weak phase during deformation, deforming via dislocation creep ( Figure 5). Further, hydrous fluids are capable of modifying rock strength through the crystallization of new phases and/or hydrolytic weakening (Griggs, 2010;Kronenberg, 1994;Paterson, 1989;J. Tullis & Yund, 1985;White & Knipe, 1978). We note that even small water-rich fluid volumes having been empirically shown to strongly affect viscosity in silicate rocks (Griggs, 1967). Tracking how the content and composition of fluids evolve during ductile deformation, such as recorded by the titanite petrochronology data in this study, is thus essential for documenting how rock strength evolves as the conditions of deformation change. Future studies should further investigate how these and other factors that directly cause crustal weakening are recorded by titanite dates, microstructures, and trace-element compositions.
Prior work on deformed titanite suggests that the development of dislocation-related microstructures may occasionally, but not always, result in complete resetting of U-Pb dates (Gordon et al., 2021;Moser et al., 2022). In this contribution, two bent titanite yield a range of dates between the crystallization age of the Cottonwood pluton and the deformation age of the shear zones, with one bent grain tip producing dates that predominantly fall between these populations (Figure 7). These data illustrate that lattice bending alone is not always sufficient to wholly reset U-Pb dates. The precise reason(s) for the variable effects of the presence and motion of dislocations on the mobility of Pb in titanite remain unclear and likely relate to several variables beyond the scope of this study. However, we note that the duration of deformation may play a significant role. In other, much larger (i.e., km-scale) mylonitic shear zones, titanite U-Pb dates have been shown to be completely reset as a result of lattice reorientation and subgrain development during deformation episodes lasting up to ∼10 Myr . In contrast, the broad synchroneity of dike emplacement, synkinematic titanite growth, and fluid-driven recrystallization, as well as the lack of date variation in the titanite rims and aggregates, suggest that the Cottonwood shear zones record a punctuated (e.g., ∼1-3 Myr) phase of high-temperature deformation and fluid flow on cmto dm-scale shear zones. Further work, however, is required to test this hypothesis.
In addition, evidence of fluid-mediated recrystallization that was simultaneous with deformation in previous studies has made it challenging to discriminate between the relative roles of fluids and the direct effects of dislocations on U-Pb dates (Gordon et al., 2021;Moser et al., 2022). In the data set presented here, the bent grains are the only ones with fluid-related zoning that cuts across grain interiors, rather than only being developed at grain margins (Figures 7c and 7d; cf. Figure 6). These observations illustrate a clear relationship between lattice bending and the extent of fluid-related recrystallization. Though there may be direct effects of lattice bending on U-Pb dates, it is clear that in some cases fluid-related recrystallization is ultimately required to reset U-Pb dates in titanite with bent crystal lattices.

Tectonic Significance of the Cottonwood Shear Zones
The limited range of Cretaceous titanite U-Pb dates suggests that the Cottonwood shear zones represent a short-lived pulse of Cretaceous crustal weakening; here we consider the possible tectonic drivers of these features. One explanation is that these structures represent a punctuated, mid-crustal response to regional tectonism. However, field observations and structural data demonstrate that the geometries of these shear zones vary at the m and tens of m scale, and their complex geometries and kinematics are not readily explained by a prevailing regional tectonic stress field ( Figure S1 in Supporting Information S1). Further, though activity on the Cottonwood shear zones was synchronous with the end of Cretaceous magmatism (Economos et al., 2021) and the onset of extension and exhumation in the broader Mojave (Wells & Hoisch, 2008), other regional, Late Cretaceous shear zones in the southwest Cordillera have km-scale mylonite zones (Cawood et al., 2022;Friesenhahn, 2018;Wells & Hoisch, 2008), and/or host evidence for voluminous fluid flux (such as economic mineral deposits; Cawood et al., 2022;Wells & Hoisch, 2008). As such, the scale and expression of the Cottonwood shear zones appear to be regionally unique. These observations are inconsistent with the formation of the Cottonwood shear zones as regional tectonic structures, which are expected to have consistent kinematics and geometries, as well as a similar age and deformation style as other Late Cretaceous, regional shear zones in the southwest Cordillera.
Alternatively, the Cottonwood shear zones may have formed in response to local plutonism, such as the intrusion of the ∼75 Ma Porcupine Wash and Smoke Tree Well plutons (Figure 1b; Needy et al., 2009). In this model, the shear zones accommodated the emplacement of the plutons themselves and/or represent ephemeral weakening of the host rocks from the thermal/fluid pulse associated with plutonism. Such a causal relationship between plutonism and shearing predicts several relationships. First, deformation along the shear zones would be broadly coeval with plutonism. Second, the duration of deformation is expected to mirror the duration of pluton emplacement, which for the Cottonwood area plutons should be short-lived based on the limited age range of the local Late Cretaceous plutons (Figure 1b; ∼75 Ma plutons only, Needy et al., 2009). A causal relationship between plutonism and deformation also predicts a spatial relationship between the plutons and shear zones; shear zone density would likely increase with proximity to the pluton due to the localized stress field and/or the thermal/fluid aureole surrounding the pluton. Evidence for weakening as a result of fluid influx may include the localization of veins and dikes along shear zones and/or evidence for fluid-driven recrystallization within the shear zones.
Our data nominally agree with these predictions and support the notion that these ductile shear zones developed in response to local Cretaceous plutonism. The titanite and zircon U-Pb data reveal that shearing and fluid flow in the Cottonwood shear zones-and intrusion of the aplite dikes, Porcupine Wash, and Smoke Tree Well plutons-were geologically synchronous and short-lived (i.e., ∼1-3 Myr based on the uncertainties associated with laser-ablation analyses), suggesting that these processes were concomitant. Further, mapping and field observations reveal that the shear zones are broadly clustered within tens of m of the Cottonwood-Porcupine Wash contact, though further work is required to confirm their spatial extent. Finally, quartz veins that subparallel the mylonitic fabric (Figures 2g, 2h, 3a, and 3d) testify to high fluid pressures synchronous with shearing, and the ∼75 Ma fluid-recrystallized rims in the titanite provide evidence for localized fluid flow that was contemporaneous with deformation. Our data and observations are therefore compatible with the cm-to dm-scale Cottonwood shear zones reflecting a transient phase of deformation, heating, and fluid flow that accompanied the emplacement of the Porcupine Wash and/or Smoke Tree Well plutons, reflecting localized rather than regional crustal weakening. This interpretation suggests that the weakening effects of magmatism can be punctuated and highly localized, even in long-lasting, high-volume arc segments such as the southwest Cordillera. However, additional data, including high precision geochronology from the plutons, dikes, and shear zones, as well as more extensive mapping, are required to fully investigate these possible links between plutonism and deformation.

Conclusions
Titanite petrochronology documents the timing and evolving temperature and fluid composition of deformation in sub-m-scale, Cretaceous ductile shear zones hosted in the Jurassic Cottonwood pluton, Eastern Transverse Ranges. Neocrystallized titanite aggregates are spatially associated with ilmenite and magnetite and yield a single population of U-Pb dates at ∼75 Ma. Microstructural evidence of dislocation creep indicates that these grains deformed plastically during synkinematic neocrystallization. Sub-to euhedral titanite preserve oscillatory-and sector-zoned cores with ∼151 Ma dates that record the timing of igneous titanite crystallization in the host Cottonwood pluton. Lobate-cuspate rims on these grains yield predominantly ∼75 Ma dates that record the age of fluid-mediated recrystallization in the presence of F-enriched, hydrous fluids. Titanite with bent crystal lattices yield a range of ∼151 to 75 Ma dates that are correlated with lattice bending and fluid-related, patchy and convoluted zoning. These grains demonstrate that in some instances, lattice bending alone may be insufficient to fully reset U-Pb dates and that fluids may be the ultimate mechanism by which dates are reset in some titanite deformed by dislocation creep. Deformation and fluid flow at ∼75 Ma within these shear zones was synchronous with the intrusion of aplite dikes, as well as the Porcupine Wash and Smoke Tree Well plutons. Together with the irregular geometries and kinematics of the shear zones, these data are consistent with a punctuated phase of high-temperature deformation that accompanied local pluton intrusion in the mid crust at ∼75 Ma.
Titanite aggregates and fluid-recrystallized rims record different Zr-in-titanite temperatures and F contents despite their indistinguishable ages. The aggregates document early deformation at ∼640-710°C under hydrous, F-depleted fluid conditions, whereas rims formed later at ∼600-620°C in the presence of a comparatively F-enriched fluid. These data suggest that the fluid that accompanied deformation evolved to a more halogen-enriched brine as the system began cooling during ongoing deformation. Titanite petrochronology is therefore a powerful tool to date deformation, constrain changes in temperature and fluid compositions during ductile shearing, and document the complex variables related to crustal weakening and the rheological evolution of ductile shear zones.

Data Availability Statement
Titanite and zircon U-Pb data as well as sample IGSNs are available in the Geochron data repository at http:// www.geochron.org/dataset/html/geochron_dataset_2023_04_24_sIbH7. The titanite trace-element data have been archived in the EarthChem repository (Moser, 2023).