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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. 

The Late Cretaceous paleogeography of Southern California potentially plays a central role in resolving conflicting models for postulated large-magnitude dextral translations along the western margin of North America (the Baja-BC hypothesis) and the beginning of the Laramide orogeny. The Mt. Pinos sector of the Southern California Batholith provides a unique window into this time because it preserves evidence for a kinematically and temporally partitioned fault system that includes a ductile shear zone (the Tumamait shear zone) and a ductile-to-brittle thrust fault (the Sawmill thrust). These two structures accommodated intra-arc strain during the Late Cretaceous to Paleocene during three phases of deformation (D3-D5) that are superimposed on older (D1 and D2) structures. D1 structures only occur in Pre-Mesozoic rocks and provide a reference frame for understanding subsequent deformation phases. D2 structures form part of a previously unmapped dextral-normal shear zone that predates the Tumamait shear zone. The initiation of displacements within the Tumamait shear zone is recorded by the formation of D3 mylonites which everywhere record reverse-sinistral movement. Petrochronology of syn- D3 titanites give lower-intercept 206Pb/238U dates ranging from 77.0 to 74.0 Ma and upper amphibolite-facies temperatures ranging from 699 to 718°C. Subsequent folding of the D3 mylonites during D4 was synchronous with late-stage, peraluminous magmatism at ca. 70 Ma. Near the Sawmill thrust, the D4 event resulted in a S4 crenulation cleavage and asymmetric, overturned folds that record top-to-the-NE tectonic displacements. NE-directed thrusting along the Sawmill thrust occurred at 67-66 Ma is interpreted to have been kinematically linked to D4 deformation. This thrust placed upper plate rocks of the Southern California Batholith above the Late Cretaceous Pelona schist. We interpret deformational fabrics in the Mt. Pinos area to record a kinematically partitioned, transpressional system that involved sinistral-reverse shearing (D3) closely followed by folding and arc-directed thrusting (D4-D5). We speculate that D3 structures developed in response to opening of the Kula-Farallon plate boundary and we hypothesize that the Kula-Farallon-North American plate triple junction was located at the present-day location of the Garlock Fault at ca. 85 Ma thereby segmenting the arc at this location. This geometry resulted in in dextral shearing in the Sierra Nevada Batholith (and northward) and sinistral shearing in the Southern California Batholith and Baja California. Continued subduction of the Farallon plate beneath the Southern California Batholith led to a major arc flare-up event from 90-70 Ma which was associated with D3 sinistral transpression. We interpret D3-D5 structures to record oblique convergence and the underthrusting of the Hess oceanic plateau beneath the Southern California Batholith at ca. 70-66 Ma. Our model for the segmentation of the California arc is compatible with a moderate (1000-1600 km), ‘Sierra-BC’ translation model in which the Insular superterrane was located north of the Southern California Batholith in the Late Cretaceous. 

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. 

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. 

The Cotton Brook landslide, located in Mt. Mansfield State Forest near Waterbury, Vermont is the state’s largest documented landslide. The site’s stratigraphy is characterized by glaciolacustrine sediment overlying glacial till and bedrock. When the hillslope initially failed in 2019, it mobilized up to 200,000 m3 of surficial material downstream toward the Waterbury reservoir. This study spans from 2014 to 2023 and integrates field-based and UAS-derived data to 1) identify the mechanisms of continued mass wasting following the 2019 slip and 2) develop a workflow that allows us to estimate the magnitudes and rates of topographic change linked to diverse styles of earthflow. We utilized ArcGIS, Metashape Pro and CloudCompare softwares to conduct topographic differencing techniques with DEMs and 3-dimensional point clouds. We compared their outcomes to refine the workflow and quantify uncertainty. Vertical change measurements derived from DEMs over-estimated topographic change by up to ~10% when compared to values from 3-D point cloud results. We attribute this discrepancy to errors introduced by georeferencing and interpolation of elevation values. The latest volumetric estimates detail material redistributed from the hillside to the surrounding watershed. For instance, volumes extrapolated from ArcGIS and CloudCompare for material accumulated at the toe are approximately 135,000 m3 and 126,000 m3, respectively. Calculated uncertainties ranging from 1 cm – ~50 cm from CloudCompare were mapped spatially. To ground truth our geospatial analysis results, we mapped the main active earthflow processes driving sediment movement. The predominant mechanisms contributing to mass wasting include the collapse of thick piles of glacial lake sediment bordering the main slip and deepening gullies on the slip surface. Our quantitative analyses suggest the collapse of glacial material is accelerating, in part due to recent historic flooding. Gully features began as shallow rills and have evolved to reach depths of up to 1.5 m and are responsible for channelizing sediment into Cotton Brook. Our findings provide an opportunity to quantify material displaced and make predictions about how the sediment budget in the watershed and the Waterbury reservoir is impacted by the Cotton Brook landslide. 

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. 

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. 

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. 

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 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.