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1. Temporal and spatial variations in magmatism and transpression in a Cretaceous arc, Median Batholith, Fiordland, New Zealand

   https://doi.org/10.1130/L1073.1  

   Buriticá, Luisa F. ; Schwartz, Joshua J. ; Klepeis, Keith A. ; Miranda, Elena A. ; Tulloch, Andy J. ; Coble, Matthew A. ; Kylander-Clark, Andrew R.C. ( September 2019 , Lithosphere)

   Abstract We investigated the interplay between deformation and pluton emplacement with the goal of providing insights into the role of transpression and arc magmatism in forming and modifying continental arc crust. We present 39 new laser-ablation–split-stream–inductively coupled plasma–mass spectrometry (LASS-ICP-MS) and secondary ion mass spectrometry (SIMS) 206Pb/238U zircon and titanite dates, together with titanite geochemistry and temperatures from the lower and middle crust of the Mesozoic Median Batholith, New Zealand, to (1) constrain the timing of Cretaceous arc magmatism in the Separation Point Suite, (2) document the timing of titanite growth in low- and high-strain deformational fabrics, and (3) link spatial and temporal patterns of lithospheric-scale transpressional shear zone development to the Cretaceous arc flare-up event. Our zircon results reveal that Separation Point Suite plutonism lasted from ca. 129 Ma to ca. 110 Ma in the middle crust of eastern and central Fiordland. Deformation during this time was focused into a 20-km-wide, arc-parallel zone of deformation that includes previously unreported segments of a complex shear zone that we term the Grebe shear zone. Early deformation in the Grebe shear zone involved development of low-strain fabrics with shallowly plunging mineral stretching lineations from ca. 129 to 125 Ma. Titanites in these rocks are euhedral, are generally aligned with weak subsolidus fabrics, and give rock-average temperatures ranging from 675 °C to 700 °C. We interpret them as relict magmatic titanites that grew prior to low-strain fabric development. In contrast, deformation from ca. 125 to 116 Ma involved movement along subvertical, mylonitic shear zones with moderately to steeply plunging mineral stretching lineations. Titanites in these shear zones are anhedral grains/aggregates that are aligned within mylonitic fabrics and have rock-average temperatures ranging from ∼610 °C to 700 °C. These titanites are most consistent with (re)crystallization in response to deformation and/or metamorphic reactions during amphibolite-facies metamorphism. At the orogen scale, spatial and temporal patterns indicate that the Separation Point Suite flare-up commenced during low-strain deformation in the middle crust (ca. 129–125 Ma) and peaked during high-strain, transpressional deformation (ca. 125–116 Ma), during which time the magmatic arc axis widened to 70 km or more. We suggest that transpressional deformation during the arc flare-up event was an important process in linking melt storage regions and controlling the distribution and geometry of plutons at mid-crustal levels. 

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2. The initiation and growth of transpressional shear zones through continental arc lithosphere, southwest New Zealand

   Klepeis, K.A. ; Schwartz, J.J. ; Miranda, E.A. ; Lindquist, P. ; Jongens, R. ; Turnbull, R. ; and Stowell, H.H. ( September 2022 , Tectonics)

   Structural analyses combined with new U-Pb zircon and titanite geochronology show how two Early Cretaceous transpressional shear zones initiated and grew through a nearly complete section of continental arc crust during oblique convergence. Both shear zones reactivated Carboniferous faults that penetrated the upper mantle below Zealandia's Median Batholith but show opposite growth patterns and dissimilar relationships with respect to arc magmatism. The Grebe-Indecision Creek shear zone was magma-starved and first reactivated at ∼136 Ma as an oblique-reverse fault, along which an outboard batholith partially subducted beneath Gondwana. This system nucleated at or above ∼20 km depth and propagated downward at 2–3 mm yr−1, accumulating at least 35–45 km of horizontal (arc-normal) shortening by ∼124 Ma. In contrast, the magma-rich George Sound shear zone first reactivated in the lower crust (∼55 km depth) at ∼124 Ma and grew upward at ∼3 mm yr−1, reaching the upper crust by ∼110 Ma. In this latter system, magmatism influenced shear zone architecture and drove its growth while subduction and oblique convergence ended. As magma entered the roots of the system and began to solidify, deformation was driven out of the lower crust and into the middle crust where the system widened by a factor of three when fold-thrust belts formed on either side of a steep, central transpressional shear zone. This study illustrates how the reactivation of structural weaknesses localizes deformation at all depths in the lithosphere and shows how magma-deformation feedbacks influence shear zone connectivity and built a batholith from the bottom up. 

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3. The Initiation and Growth of Transpressional Shear Zones Through Continental Arc Lithosphere, Southwest New Zealand

   https://doi.org/10.1029/2021TC007097  

   Klepeis, Keith A. ; Schwartz, Joshua J. ; Miranda, Elena ; Lindquist, Peter ; Jongens, Richard ; Turnbull, Rose ; Stowell, Harold ( September 2022 , Tectonics)

   Structural analyses combined with new U‐Pb zircon and titanite geochronology show how two Early Cretaceous transpressional shear zones initiated and grew through a nearly complete section of continental arc crust during oblique convergence. Both shear zones reactivated Carboniferous faults that penetrated the upper mantle below Zealandia's Median Batholith but show opposite growth patterns and dissimilar relationships with respect to arc magmatism. The Grebe‐Indecision Creek shear zone was magma‐starved and first reactivated at ∼136 Ma as an oblique‐reverse fault, along which an outboard batholith partially subducted beneath Gondwana. This system nucleated at or above ∼20 km depth and propagated downward at 2–3 mm yr⁻¹, accumulating at least 35–45 km of horizontal (arc‐normal) shortening by ∼124 Ma. In contrast, the magma‐rich George Sound shear zone first reactivated in the lower crust (∼55 km depth) at ∼124 Ma and grew upward at ∼3 mm yr⁻¹, reaching the upper crust by ∼110 Ma. In this latter system, magmatism influenced shear zone architecture and drove its growth while subduction and oblique convergence ended. As magma entered the roots of the system and began to solidify, deformation was driven out of the lower crust and into the middle crust where the system widened by a factor of three when fold‐thrust belts formed on either side of a steep, central transpressional shear zone. This study illustrates how the reactivation of structural weaknesses localizes deformation at all depths in the lithosphere and shows how magma‐deformation feedbacks influence shear zone connectivity and built a batholith from the bottom up.

    

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4. STYLES AND HISTORY OF CONVERGENT MARGIN DEFORMATION IN THE SOUTHERN CALIFORNIA BATHOLITH DURING THE LATE CRETACEOUS BEGINNING OF THE LARAMIDE OROGENY

   https://doi.org/10.1130/abs/2023AM-394531  

   Baskin, Jillyan ; Klepeis, Keith ; Schwartz, Joshua ; Miranda, Elena ; Robles, Francine ; Mora-Klepeis, Gabriela ( October 2023 , Geological Society of America Abstracts with Programs)

   In the eastern San Gabriel Mountains, located north of Los Angeles, California, the late Cenozoic Cucamonga thrust has uplifted and exposed the lower crustal root of the Mesozoic Southern California Batholith. We use structural data and U-Pb zircon analyses from these exposures to document changes in the style of intra-arc deformation in the batholith as the Laramide Orogeny began during the Late Cretaceous (at or after ~90 Ma). At the base of the uplifted section, a 4 km-thick package of metasedimentary rock records the intrusion of amphibolite, charnokite and other dikes of probable Jurassic to Early Cretaceous age. The oldest gneissic fabrics (S1, S2) in these rocks record Early Cretaceous partial melting, granulite-facies metamorphism, and top-to-the-S and -SE (present day reference frame) reverse motion on surfaces that dip moderately to the N and NW. These structures define a D1/D2 thrust system that formed on the trench side of the arc and was active during the Early Cretaceous. From 89-77 Ma this thrust system was reactivated by oblique-slip shear zones (D3) that record sinistral-reverse displacements on N- and NW-dipping surfaces. The timing of deformation in these latter shear zones is indicated by the age of 90-85 Ma syn-kinematic intrusions of the Tonalite of San Sevaine Lookout. After emplacement of the tonalite, the lower crustal section was deformed by a series of S-vergent, overturned folds. The emplacement of granodioritic dikes into the axial planes of some of these folds suggests that they formed during the latest stages of D3 transpression and tonalite emplacement. Superimposed on all these structures are a series of ductile-to-brittle thrust faults and folds that appear to be related to formation of the late Cenozoic Cucamonga thrust fault at the southern edge of the San Gabriel mountains. These data show that the Southern California Batholith in the San Gabriel Mountains records a tectonic transition from Early Cretaceous reverse faulting and crustal imbrication on the trench side of the arc to Late Cretaceous transpression and oblique sinistral-reverse deformation during a magmatic flare-up from 89-77 Ma. Another major episode of shortening and crustal imbrication occurred during the late Cenozoic when the Cucamonga thrust uplifted the San Gabriel block. 

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5. Tectonic and magmatic construction of lower crust in the southern California batholith

   Schwartz, J.J. ( May 2024 , Geological Society of America Abstracts with Programs)

   We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, a ~ 500-km-long, paleo-arc segment of the Mesozoic California arc that lies between the southern Sierra Nevada batholith and northern Peninsular Ranges Batholith. We focus on the Cucamonga and San Antonio terranes located in the eastern San Gabriel Mountains where the deep root of the Mesozoic arc is exhumed by the Quaternary Cucamonga thrust fault. This lower- to mid-crustal cross section of the arc allows us to investigate: 1) the timing and rates of Mesozoic arc construction, 2) mechanisms of sediment incorporation into the lower crust, and 3) the interplay between mantle input and crustal recycling during arc magmatic surges. We use detrital zircon geochronology of 4 quartzites and paragneisses to investigate the origin of the lower-crustal Cucamonga paragneiss sequence, and U-Pb petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga paragneisses share broad similarities to Sur Series metasedimentary rocks in the Salinia terrane, suggesting that both were deposited in a Late Paleozoic to Early Mesozoic forearc or intra-arc basin. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750°C) migmatization events at ca. 124 and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 to 75 Ma and culminated in a magmatic surge from ca. 90–75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by emplacement of voluminous, mid-crustal tonalites and granodiorites. Partial melting of the metasedimentary rocks played a subsidiary role relative to mantle input in driving the Late Cretaceous magmatic flare-up event. Our observations demonstrate that tectonic incorporation of sediments into the lower crust led to structural, compositional and rheological changes in the architecture of the arc including vertical thickening. These structural changes created weak zones that preferentially focused deformation and promoted present-day reactivation along the Cucamonga thrust fault. 

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