DOI: 10.5061/dryad.jh9w0vtn6
title: Data from: Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith

description: Structural analyses combined with U‐Pb zircon petrochronology show the
 influence of arc magmatism on the evolution of two transpressional shear
 zones in the deep root of the Late Cretaceous Southern California
 batholith.  The mid-crustal Black Belt and lower-crustal
 Cucamonga shear zones (eastern San Gabriel Mountains) formed at ~84 Ma
 shortly after a large mass of tonalite and granodiorite intruded the lower
 crust.  Both shear zones were active until at least ~74 Ma and
 probably until 72-70 Ma.  In the mid-crustal shear zone,
 rheological contrasts between mingling magmas localized deformation at
 dike margins.  The deformation began as hypersolidus flow in
 partially crystallized dikes and then transitioned to deformation below
 the solidus when alternations between viscous creep and brittle faulting
 produced interlayered pseudotachylyte, cataclasite, and mylonite. As the
 dikes solidified, strain hardening drove shear zone growth and created
 thin (10-30 m) high-strain zones and faults that are widely spaced across
 ~1 km. In contrast, the lower-crustal Cucamonga shear zone was
 magma-starved, lacks the variety of shear zone fabrics exhibited by its
 mid-crustal counterpart, and formed by the reactivation of a pre-existing
 fabric that records pure reverse displacements at 124-93 Ma.  The
 two shear zones created a partitioned style of intra-arc transpression
 where sinistral-reverse (mostly arc-parallel with some arc-oblique)
 displacements were accommodated on moderately dipping faults and shear
 zones and arc-normal shortening was accommodated by coeval
 folds.  This study shows how a magmatic surge influenced the
 architecture and style of Late Cretaceous transpression in the Southern
 California batholith, including the evolution of high-strain zones that
 record alternating episodes of brittle, ductile, and hypersolidus
 deformation.  The results illustrate how magmatism localizes
 strain on deep-crustal faults during orogenesis and oblique convergence.
description: We conducted U-Pb analyses on 11 zircon-bearing samples to
 determine the timing of magmatism, metamorphism, and deformation in the
 Black Belt and Cucamonga shear zones. Some zircons exhibited interior core
 and rim domains that reflect a history of deformation and metamorphism at
 amphibolite- to granulite-facies conditions.  The
 methods used in this study closely follow those outlined in Kylander-Clark
 et al. (2013) and are described in detail by Schwartz et al. (2024b). 
 U-Pb ratios were collected at the University of California, Santa Barbara
 using a Nu Plasma multi-collector inductively coupled plasma mass
 spectrometer (MC-ICPMS) with a Photon Machines 193 ArF excimer laser with
 HelEx cell. Spot size and frequency were 35 µm and 4 Hz, respectively. 
 The primary standard (91500) was accessed every 10 analyses to correct for
 in-run fractionation of Pb/U and Pb isotopes. A secondary standard
 (Temora-2) was analyzed every ~10 analyses to assess data reproducibility.
 Uncertainties are reported as 2SE internal calculated from Iolite and
 IsoplotR (Paton et al., 2010; Vermeesch, 2018).  We assigned a 2%
 uncertainty to all dates for interlab comparisons to account for the
 reproducibility of standards. Zircon ages are reported
 using the <sup>206</sup>Pb/<sup>238</sup>U date
 for analyses &lt;1100 Ma, and the
 <sup>207</sup>Pb/<sup>206</sup>Pb date for those
 &gt;1100 Ma. For the former, discordance is calculated as the percent
 difference between the
 <sup>207</sup>Pb/<sup>235</sup>U and
 <sup>206</sup>Pb/2<sup>38</sup>U dates.
 Corrections for minor amounts of common Pb in zircon were made on
 <sup>206</sup>Pb/<sup>238</sup>U dates following
 the methods of Tera and Wasserburg (1972) using measured
 <sup>207</sup>Pb/<sup>206</sup>Pb and
 <sup>238</sup>U/<sup>206</sup>Pb ratios and an
 age-appropriate Pb isotopic composition of Stacey and Kramers (1975).
  Zircons with large common Pb corrections (e.g., analyses interpreted as
 having ~20% or greater contribution from common Pb) were discarded. No
 corrections were made on
 <sup>207</sup>Pb/<sup>206</sup>Pb dates due to
 large uncertainties in measured <sup>204</sup>Pb.
 Cathodoluminescence images were obtained using a FEI Quanta
 scanning electron microscope before and after ablation to evaluate
 analyzed areas and compare them with growth textures. Where possible, we
 targeted all growth domains and report
 <sup>207</sup>Pb/<sup>206</sup>Pb-corrected
 <sup>206</sup>Pb/<sup>238</sup>U ages of
 texturally homogeneous populations. In a few cases where a laser spots
 overlapped multiple domains we report the data in tables but did not
 considered them in weighted mean calculations. Concordia plots and
 error-weighted average ages are shown in the accompanying journal
 article. Trace elements were measured simultaneously
 with U-Pb isotopes by LA-SF-ICPMS using Zr as the internal standard and
 nominal values of 43.14 % Zr. Trace element data were reduced using Iolite
 (Paton et al., 2010, 2011) and concentrations calculated relative to
 NIST-612 as a primary standard. BHVO-2G was analyzed as a secondary
 standard to assess reproducibility of the data. For zircon, model
 Ti-in-zircon temperatures were calculated using the Ferry and Watson
 (Ferry and Watson, 2007) calibration. Samples of granulite-facies rocks
 from the Cucamonga terrane contain rutile, allowing us to estimate the
 activity of TiO<sub>2</sub> to be one.  For samples that lack
 rutile we assume a value of 0.6 based on the presence of
 ilmenite.
description: # Data from: Influence of magmatism on the architecture of transpressional
 faults and shear zones in the deep crust of the Late Cretaceous Southern
 California batholith
 [https://doi.org/10.5061/dryad.jh9w0vtn6](https://doi.org/10.5061/dryad.jh9w0vtn6) ## Description of the data and file structure All isotopic, geochemical, and trace element data were collected as part of a study to determine the timing of magmatism, metamorphism, and deformation in the Black Belt and Cucamonga shear zones in the southeastern San Gabriel Mountains in Southern California. Some zircons exhibited interior core and rim domains that reflect a history of deformation and metamorphism at amphibolite- to granulite-facies conditions. Where possible, we used zircon trace-element concentrations and ratios, particularly U (ppm), U/Th, Gd/Yb, and Dy/Yb, to distinguish metamorphic age populations. For more information on methods and an interpretation of the data, please see the publication in the journal *GSA Bulletin* by Klepeis et al. (2025). ### Files and variables #### File: Supplementary_Table_1_U-Pb_zircon_FINAL.xlsx **Description:** The table in the data sheet tab titled 'Table 1. U-Pb data unknowns' presents all U-Pb isotope data for the 11 samples analyzed plus the standards used. All standards are listed at the bottom of the table. The zircon standards include: Plesovice(Ples), which is used as a zircon U-Pb age standard; 91500, which is a primary zircon reference standard used for age normalization and reproducibility; Australian zircon suite (AusZ), which is a zircon age standard that is used as a secondary check on accuracy; GJ-1 zircon (GJ), which is a widely used zircon standard, the Malinau/Malim zircon suite (Mali) which is an additional age validation standard. Trace element standards include NIST 612 (N612), which is a synthetic silicate glass standard (see more information below), and BHVO, which is a natural basalt reference standard (see more information below). In the 'Table 1. U-Pb data unknowns' tab, n/a means not applicable. This refers to the lack of best ages and absolute error calculations for the standards N612 and BVHO. This information isn't applicable because these are non-zircon trace element standards, so no age is calculated. In contrast, the zircon standards (91500, AusZ, GJ, Mali, Plesovice) all have well-known ages, so best age and absolute error calculations are included. In the Table 1 data tab, n.d. means not detected. Only 1 average value and a standard deviation for the trace element analyses are provided for each sample in columns AP and AQ, respectively (the rest of the cells are blank). The tab titled 'NIST 612' shows data that used the widely used synthetic glass standard from the U.S. National Institute of Standards and Technology. The standard is commonly analyzed to calibrate trace-element concentrations during LA-ICP-MS work. The tab titled 'BHVO' reports data from a well-characterized natural basalt reference material (from Hawaii). This is used as a secondary standard to monitor analytical accuracy and precision. In the BHVO data tab, n.d. means not detected. Isotopic ratios and Apparent ages are not corrected for common lead. For the 206Pb/238U ages, common lead was corrected by inferring the initial Pb-composition from the Stacey and Kramers (1975) two stage isotope evolution model (Vermeesch, 2018). Analyses with greater than 10% uncertainty in 207Pb/206Pb age (1-sigma) or 5% uncertainty in 206Pb/238U age (1-sigma), 20% discordance, and/or 5% reverse discordance are excluded. Accepted ages were calculated by using 206Pb/238U ages for grains younger than 1100 Ma and 207Pb/206Pb ages for grains older than 1100 Ma. ##### Variables * U, U/Th, 238U/206U, 207Pb/206Pb, 207Pb/206Pb vs 238U/206Pb isotopic ratios, apparent ages and absolute errors. * Trace-element data for P, Ti, V, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta Abbreviations include: ppm is parts per million, abs is absolute, Ma is millions of years, abs err is absolute error, error corr is error correction. ## Code/software Uncertainties were calculated using Iolite and IsoplotR (Paton et al., 2010; Vermeesch, 2018)