Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith

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.

Methods
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 206Pb/238U date for analyses <1100 Ma, and the 207Pb/206Pb date for those >1100 Ma. For the former, discordance is calculated as the percent difference between the 207Pb/235U and 206Pb/238U dates. Corrections for minor amounts of common Pb in zircon were made on 206Pb/238U dates following the methods of Tera and Wasserburg (1972) using measured 207Pb/206Pb and 238U/206Pb 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 207Pb/206Pb dates due to large uncertainties in measured 204Pb.

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 207Pb/206Pb-corrected 206Pb/238U 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 TiO2 to be one.  For samples that lack rutile we assume a value of 0.6 based on the presence of ilmenite.