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1. The Black Belt Shear Zone records the earthquake cycle at the brittle-ductile transition: implications for the SCEC Community Rheology Model

   Elena Miranda, Joshua Schwartz (September 2023, Poster Presentation at 2023 SCEC Annual Meeting)

   Abstract. The inception of the Laramide Orogeny in Southern California is marked by a Late Cretaceous arc flare-up in the Southern California Batholith (SCB) that was temporally and spatially associated with syn-plutonic development of a regionally extensive, transpressional shear zone system. This ~200 km-long system is the best analog for the shear zones that extend into the middle crust beneath the major lithotectonic block-bounding faults of the San Andreas Fault system. We focus on the Black Belt Shear Zone, which preserves an ancient brittle-ductile transition (BDT), and is exposed in the SE corner of the San Gabriel lithotectonic block. The mid-crustal Black Belt Shear Zone forms a ~1.5-2 km thick zone of mylonites developed within hornblende and biotite tonalites and diorites. Mylonitic fabrics strike SW and dip moderately to the NW, and kinematic indicators from the Black Belt Shear Zone generally give oblique top-to-SW, sinistral thrust-sense motion (present-day geometry). U-Pb zircon ages of host rock to the Black Belt mylonites demonstrate crystallization at ~86 Ma and metamorphism at ~79 Ma at temperatures ~753 ¡C. Syn-kinematic, metamorphic titanite grains aligned with mylonitic foliation in the Black Belt Shear Zone give an age of ~83 Ma. These data indicate syn-magmatic sinistral-reverse, transpressional deformation. The BDT rocks in the Black Belt Shear Zone are characterized by a ~10 m-thick section of high strain mylonites interlayered with co-planar cataclasite and pseudotachylyte (pst) seams. Microstructural and electron backscatter diffraction (EBSD) analysis shows that the mylonites and cataclasites are mutually overprinted, and pst seams are overprinted by mylonitic fabric development. Pst survivor clasts show the same shear sense as the host mylonite, and this kinematic compatibility demonstrates a continuum between brittle and ductile deformation that is punctuated by high strain rate events resulting in the production of frictional melt. EBSD analysis reveals a decreasing content of hydrous maÞc mineral phases in host mylonite with increasing proximity to pst seams. This suggests that pst was generated by melting of hornblende and/or biotite, implying that coeval development of mid-crustal mylonites and pst does not require anhydrous melting conditions. Rather, the production of pst may liberate water, implying that BDT rock rheology is affected by transient pulses of water inßux and strain rate increases. 

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2. Making sense of shear zone fabrics that record multiple episodes of deformation: Electron backscatter diffraction–derived and crystallographic vorticity axis–enhanced petrochronology

   https://doi.org/10.1130/G50982.1  

   Miranda, Elena A.; Brown, Virginia; Schwartz, Joshua J.; Klepeis, Keith A. (April 2023, Geology)

   We present a new method of linking microstructures, electron backscatter diffraction (EBSD)–derived crystallographic vorticity axis (CVA) analysis, and titanite petrochronology to directly link fabric development to specific deformation events in shear zone rocks with complex histories. This approach is particularly useful where overprinting is incomplete, such that it is unknown which fabric is being dated by the petrochronometer. Here, we compared single-phase CVA patterns of fabric-forming minerals with those of synkinematic petrochronometers (e.g., titanite) to associate the timing of fabric development with deformational events in the middle crust of the George Sound shear zone, Fiordland, New Zealand. The host rocks to the George Sound shear zone include the Carboniferous Large Pluton, where titanite petrochronology demonstrates an unequivocally Cretaceous age of metamorphic titanite growth within mylonitic foliation. However, the host rocks show two distinct CVA patterns: a transtensional deformation event recorded by quartz and plagioclase, and a pure-shear–dominated transpressional deformation event recorded by biotite and titanite. Therefore, the transpressional CVA pattern of the titanite, coupled with its Cretaceous age, shows that it cannot be used to date the quartz and plagioclase fabric developed in response to an older transtensional deformation event. These results demonstrate the necessity of combining EBSD and CVA analysis with petrochronology to demonstrate that synkinematic accessory phase petrochronometers show the same kinematic deformation geometry (i.e., CVA pattern) as the fabric being dated. 

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3. Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone

   https://doi.org/10.1016/j.tecto.2021.229186  

   Harigane, Yumiko; Michibayashi, Katsuyoshi; Morishita, Tomoaki; Tamura, Akihiro; Hashimoto, Satoshi; Snow, Jonathan E (January 2022, Tectonophysics)

   Mantle deformation processes leading to sea oor spreading are often dif cult to infer due to the highly ser- pentinized and weathered state of most abyssal peridotites. We investigated the development of high- temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-T deformed samples and ve mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with at REE patterns likely underwent refertilization with a high in ux of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-T crystal-plastic deformation (most likely at temperatures exceeding 1000 ◦ C) was active in the peri- dotites of the high-T deformation group, accommodating mantle ow beneath the Gakkel Ridge. The identi ed melt refertilization process may have contributed to the formation of [010]- ber olivine fabrics in these peri- dotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 ◦ C) and probably accom- modated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with “dusty” zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The develop- ment of these deformation processes in Gakkel Ridge suggests a shift from ow in the uppermost mantle to shear zone formation in the rift valley walls. 

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4. LATE CRETACEOUS INTRA-ARC TRANSPRESSION AND THRUSTING IN THE SOUTHERN CALIFORNIA BATHOLITH

   https://doi.org/10.1130/abs/2023AM-394372  

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

   The Late Cretaceous arc flare-up event from 90 to 70 in the Transverse Ranges of the Southern California Batholith was temporally and spatially associated with the development of a large contractional shear system that includes discontinuous segments of the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. The age and kinematics of these shear zones inform the tectonic setting of the continental arc in Southern California during the beginning of the Laramide orogeny and during postulated large-magnitude dextral translations along the margin (the Baja-BC hypothesis). The Mt. Pinos sector of the Southern California Batholith preserves the intra-arc, transpressional Tumamait shear zone and the ductile-to-brittle Sawmill thrust, both of which record Late Cretaceous deformation. The batholith and shear zone are hosted by Mesoproterozoic biotite gneisses and migmatites (1750-1760 Ma), Neoproterozoic biotite granites (660 Ma), Permo-Triassic granitic gneisses and amphibolite (260-250 Ma), and Late Jurassic granites and gneisses (160-140 Ma). Late Cretaceous rocks are variably deformed and include porphyritic granodiorite gneisses and peraluminous granites emplaced at 86 to 70 Ma. Mylonites of the Tumamait shear zone affect all rocks in the area and generally strike NW-SE and dip moderately to the NE and SW. Mineral stretching lineations plunge shallowly to the SE. Mylonitic fabrics are folded into a regional, SE-plunging synform that results in alternating bands of sinistral and dextral shear fabrics. Syn-kinematic titanites from 5 mylonitic samples give a 720-700°C temperature range, and lower-intercept 206Pb/238U dates of 77.0 Ma, 76.8 Ma, 75.1 Ma, 74.2 Ma, and 74.0 Ma. Subsequent folding of the mylonite is linked to N-directed motion on the Sawmill thrust. 40Ar-39Ar thermochronology ages of 67-66 Ma and onlapping Eocene shales indicate Latest Cretaceous activity on the thrust, prior to Eocene arc collapse. Based on the age of the Tumamait shear zone, we speculate that it is related to sinistral deformation observed in the nearby Alamo Mountain-Piru Creek and the Black Belt shear zones. We attribute the younger Sawmill thrust to collision of the Hess oceanic plateau with the Southern California Batholith after 70 Ma. 

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5. Superplasticity in Amphibole and its Implications for the Rheology of the Lower Crust

   https://doi.org/10.1130/abs/2025AM-10533  

   Miranda, Elena; Schwartz, Joshua; Klepeis, Keith; Rhea, Gillian (January 2025, Geological Society of America)

   Superplastic behavior is observed in metals, ceramics, and some geologic materials where fine grained materials deform to large strains (>200%) without failure. While somewhat common in metals and ceramics, superplasticity is less studied in geologic materials, and tends to be restricted to olivine deformation experiments that are used to infer mantle rheology. Here we show unequivocal evidence for the operation of DisGBS-mediated superplasticity in naturally deformed amphibole, an abundant mineral in deep continental crust, which has not been previously shown to deform by high-temperature grain size sensitive processes. We sampled hornblendites from the base of Cordilleran-style continental arc crust exposed in Fiordland, New Zealand. The samples are from the Hawes Head ultramafic complex at the base of the Misty Pluton, which is composed of a hornblende diorite emplaced at 120-115 Ma at depths of 25-40 km. The hornblendites have an anastomosing mylonitic foliation which formed within the long-lived Straight River Shear Zone at the western edge of the Misty Pluton when it was reactivated in the Cretaceous (~126-110 Ma) during a pulse of arc magmatism. We place maximum P and T estimates on mylonitic fabric development in our samples based on amphibole crystallization pressures of 8.5 -10 kbar and temperatures of 940-1000ºC derived from electron microprobe analyses of an adjacent undeformed hornblendite. We use electron backscatter diffraction to investigate deformation mechanisms of naturally deformed amphibole in the hornblendite. Microstructural, pole figure, and misorientation data indicate that dynamic recrystallization (DRX) and grain size reduction led to the transition from dislocation creep to the onset of DisGBS-mediated superplasticity in the mylonites. Our data are consistent with a textural evolution in which easy slip on (100)[001] initially drives subgrain rotation DRX and CPO development in porphyroclasts during dislocation creep, producing dislocations that quickly climb out of the grains, leaving behind dislocations indicative of a harder slip system such as (010)[100] or {110}1/2<110>. Matrix grains show textural traits of DisGBS-mediated superplasticity including four grain junctions, aspect ratios >1.5, a paucity of subgrains, and the operation of a single hard slip system. This discovery of superplasticity in a natural example of amphibole-rich lower crust has profound implications for the ease of strain localization leading to the creation of crustal-scale shear zones that reactivate and drive plate tectonic boundary development. 

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