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Abstract We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, an ~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 U-Pb detrital zircon geochronology of four quartzites and one metatexite migmatite to investigate the origin of the lower-crustal Cucamonga metasedimentary sequence, and U-Pb zircon petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga metasedimentary sequence shares 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 marginal to the Southern California Batholith. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750 °C), metamorphic events at ca. 124 Ma and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 Ma to 75 Ma and culminated in a magmatic surge from ca. 90 Ma to 75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by the 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. 

Varias zonas de fallas continentales registran la influencia de estructuras preexistentes en su desarrollo y comportamiento. Aquí usamos ejemplos de Nueva Zelanda y California del sur para mostrar cómo las fallas normales antiguas localizan la deformación e influyen el desarrollo de fallas activas en dos límites de placas transpresivas. La falla Alpina en Nueva Zelanda es una falla transforme transpresiva dextral que acomoda 60-90% del movimiento relativo entre las placas Australiana y del Pacífico. Su segmento más meridional es inusual porque está particionado, con movimiento de deslizamiento lateral acomodado en la falla principal y el acortamiento acomodado en fallas inversas separadas. La partición está controlada por la reactivación de dos conjuntos de fallas normales preexistentes. El primer conjunto se formó durante el Cretácico Superior cuando Nueva Zelanda se separó de Australia y Antártica. La segunda ocurrió durante el Eoceno-Oligoceno, cuando se formaron una serie de cuencas transtensionales en Fiordland. Después de que comenzó la transpresión en el Mioceno, dos orientaciones de fallas normales (rumbo NE y N) se reactivaron como fallas de deslizamiento lateral e inversas, respectivamente. Una consecuencia de esta reactivación fue la exposición de la sección más profunda a nivel mundial de la corteza de un arco continental. En California del sur, las montañas de San Gabriel se encuentran entre las trazas activas de la Falla de San Andrés (SAF) y la falla de Sierra Madre-Cucamonga (SCFZ). SAF es parte de un sistema transpresivo en que las fallas de deslizamiento lateral dextrales e inversas acomodan el movimiento relativo entre las placas del Pacífico y Norteamericana. SCFZ muestra movimientos inversos y oblicuos. Su pared colgante conserva fallas normales del Mioceno temprano que se reactivaron como fallas sinistrales y oblicuas-inversas después del inicio de la transpresión en el Mioceno medio. La reactivación de las fallas normales explica por qué los terremotos en la región exhiben movimientos complejos, incluyendo aquellos que son antitéticos al movimiento en SAF. Los resultados de este estudio nos permiten determinar el origen de patrones complejos de movimientos de fallas en dos límites de placas transformes continentales y explican por qué algunas fallas normales preexistentes se reactivaron en transpresión mientras que otras no. 

We investigate the deformation conditions of coeval mylonites and pseudotachylytes (pst) exposed in the brittle-ductile transition (BDT) in the Black Belt Shear Zone (BBSZ) in the Southern California Batholith using SEM (Scanning Electron Microscope) imaging, and Electron Backscatter Diffraction (EBSD) analysis. We selected four representative samples along a strain gradient of the BBSZ. The BBSZ is a transpressional shear zone developed within hornblende and biotite tonalites and diorites. The shear zone is discontinuous over a ~ 1.5 - 2 km wide zone, and kinematic indicators show oblique top-to-SW, sinistral-reverse to thrust-sense motion. Metamorphic titanite grains aligned within the mylonitic fabric date the deformation to ~ 83 Ma. SEM and EBSD data show mm-thick seams of pst contained within and parallel to mylonitic foliation, and mutually overprinting relationships between brittle and plastic deformation. We observe a brittle overprint of mylonitic fabric in sample 46 and fractured porphyroclasts reworked into mylonitic fabric in samples 45 and 47. EBSD maps from sample 45 and 47 show decreasing modal percentages of hydrous mafic minerals (biotite and hornblende) in the mylonites with proximity to pst seams, suggesting these melted to form pst. In pst seams, there are embayed and rounded/elliptical plagioclase survivor clasts and acicular and aligned biotite microlites parallel to mylonitic fabric (45 & 47). EBSD maps show pst survivor clasts with the same shear sense as the mylonitic fabric, suggesting co-development. Pole figures show weak CPO in hornblende and plagioclase of sample 46. Samples 45 and 47 have no CPO present in plagioclase, however samples 45, 46, and 47 show strong CPO patterns for quartz that are consistent with prism slip. We interpret dislocation creep as the deformation mechanism accommodating plastic deformation in host mylonites. Quartz CPO patterns provide evidence of mylonitic deformation at temperatures ~ 600o C, and the presence of plagioclase survivor clasts as evidence of pst temperatures of ~1100oC. The kinematically consistent sense of shear between pst and host mylonitic fabrics suggests coeval development that indicate shifts from brittle to ductile deformation. Our results suggest periodic pst-generating events involving melting of hydrous mafic minerals aided the development of coeval mylonites and pst in the BDT. 

Paleomagnetic data from the Insular superterrane and related terranes in the western Canadian and northern US Cordillera argue for large-magnitude (~4000 km), northward translations along the western margin of the North American Cordillera in the Late Cretaceous (the Baja-BC hypothesis). This model postulates that initial collision of the Insular superterrane occurred in southern California and/or northern Baja Mexico prior to dextral translation along the western North American margin from 85-55 Ma. A major unresolved problem with the Baja-BC hypothesis is that faults that could have accommodated large-magnitude translation are missing or obscured by later Cenozoic faulting and/or sedimentary cover. Here, we investigate the deformation record of Late Cretaceous ductile shear zones in southern California with the goal of understanding the timing and kinematics of deformation at this time. We focus on the Alamo Mountain and Piru Creek shear zones, located within the central Transverse Ranges. We report new field observations and twenty-one U-Pb LA-ICPMS zircon ages from deformed and undeformed host rocks and dikes with the goal of documenting the timing of deformation. Our data show that the Alamo Mountain and Piru Creek shear zones were active at ~76-72 Ma and possibly included an earlier phase of deformation. Both shear zones record sinistral strike-slip to sinistral-normal motion in their present-day orientations. When Cenozoic block rotations are restored, we find that the Alamo Mountain and Piru Creek shear zones originated as NNW-SSE striking, moderately ENE dipping shear zones that formed at mid-crustal conditions (500-600C and 4 kbars). Structural analysis of the shear zones indicates that the dominant component of motion was sinistral strike-slip and that the dip-slip component of motion was minor. The timing and kinematics of deformation in the Alamo Mountain and Piru Creek shear zones are similar to other Late Cretaceous shear zones in the Southern California Batholith. When palinspastic reconstructions are considered, these shear zones comprise a regionally extensive shear zone system over 200 km long. The presence of this regionally extensive, sinistral shear zone system and the absence of dextral shear zones requires reevaluation of the Baja-BC hypothesis in southern California during the Late Cretaceous. 

Penrose Meeting, Developing a New Paradigm for the Late Cretaceous to Eocene North American Cordillera: A Dominantly Oblique Plate Boundary, Convened by Basil Tikoff, Stacia Gordon, William A. Matthews, Elena Centeno-Garcia, 18-25 August, McCall and Riggins, Idaho, USA 

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. 

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. 

The Southern California Batholith is a ~500-km-wide segment of the Mesozoic California arc that lies between the northern Peninsular Ranges and the southern Sierra Nevada mountains. We use structural data and U-Pb zircon analyses from the eastern San Gabriel mountains to examine how the batholith responded to the onset of the Laramide orogeny during the Late Cretaceous. Zircon analyses show that the middle and lower crust of the batholith was hot and records a magmatic flareup from 90-77 Ma. From 90 to 86 Ma, tonalite of the San Sevaine Lookout intruded a thick package of metasedimentary rock that records a history of reverse displacements, crustal imbrication, and granulite metamorphism prior to tonalite intrusion. During the early stages of the magmatic flare-up, granodiorite dikes were emplaced and soon became tightly folded and disaggregated as younger sheets of comagmatic tonalite intruded. Deformation accompanied the magmatism, forming two parallel shear zones several 100 m thick. These two shear zones, which include the Black Belt Mylonite, are composed of thin (≤10 m) high-strain zones spaced several tens of meters apart. Each discrete high-strain zone contains subparallel layers of mylonite, ultramylonite, cataclasite and pseudotachylyte, all recording oblique sinistral-reverse displacements on gently and moderately dipping surfaces. This architecture, whereby individual high-strain zones are widely spaced and parallel the margins of intruding tonalite sheets, reveals the influence of magma emplacement on shear zone structure. U-Pb zircon geochronology on syn-tectonic dikes indicate that these different styles of deformation all formed within the same 89-85 Ma interval, suggesting that they reflect non-steady flow on deep seismogenic faults. Widespread (garnet) granulite-facies metamorphism and partial melting accompanied intrusion of the tonalites and sinistral- reverse displacements. The ages of undeformed dikes indicate that the deformation was over by 77-75 Ma. Together, these data show that arc magmatism and transpression within the Mesozoic California arc occurred from ~90 until ~75 Ma, implying that flat-slab subduction and the migration of the Laramide orogenic front into the North America interior occurred after ~75 Ma. 

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. 

The second day of the 2023 SCEC Rheology Workshop features a field trip to local exposures of shear zones that were exhumed by deformation along the San Andreas Fault System. We are headed to the Cucamonga mylonites (lower crust) and the Black Belt mylonites (middle crust) exposed in Cucamonga Canyon, near Rancho Cucamonga.