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  1. 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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    Free, publicly-accessible full text available May 15, 2025
  2. 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. 
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    Free, publicly-accessible full text available March 18, 2025
  3. 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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    Free, publicly-accessible full text available October 18, 2024
  4. 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. 
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    Free, publicly-accessible full text available October 15, 2024
  5. 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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    Free, publicly-accessible full text available October 15, 2024
  6. The beginning of the Laramide orogeny is a pivotal time in the geological development of the western United States, but the driving mechanism responsible for mountain building, basin formation and ore mineralization is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith sector of western North America at ca. 88 Ma which caused the angle of subduction beneath the continent to shallow. This subhorizontal (flat) subduction is thought to have led to shut-down of the arc, crustal cooling, and the formation of deep, basement-involved thrust faults that penetrated far into the continental interior. In contrast to these predictions, we show that the Southern California Batholith experienced a magmatic surge from 90 to 70 Ma, the lower crust was hot (835-750°C) and partially molten, and cooling occurred after 75 Ma. These data contradict plateau underthrusting as the driving mechanism for early Laramide deformation at 90-80 Ma; therefore, the Laramide orogeny cannot have been initiated by flat-slab subduction. We propose that the Laramide orogeny is best explained as a two-stage orogeny consisting of: 1) an arc magmatic ‘flare-up’ phase associated with sinistral-reverse ductile shearing in the Southern California Batholith from at 90-75 Ma and coeval dextral-transpression north of the Garlock fault, and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma. Only that latter phase is linked to flat-slab subduction beneath the Southern California Batholith. 
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    Free, publicly-accessible full text available October 1, 2024
  7. Free, publicly-accessible full text available September 10, 2024
  8. Basil Tikoff, Stacia Gordon (Ed.)
    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 
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  9. The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau. 
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  10. On May 31, 2019, a landslide near Waterbury in central Vermont removed >200,000 m3 of glacial lake deposits from a hillside in the Mt. Mansfield State Forest and transported the material across Cotton Brook, creating a dam near its toe. In the months following the slide, the brook breached the toe, removing ≥54, 000 m3 of sediment and transporting it ~1.3 km downstream into the Waterbury reservoir where it formed a large sedimentary delta. The delta grew 243% by Fall, 2020 until it began to erode into the reservoir in 2021. A collaborative team from the Vermont Geological Survey, the Vermont Agency of Transportation, Norwich University, and the University of Vermont team began a yearly monitoring of these events in 2019 using field-based mapping of bedrock and surficial geology, photogrammetry using annual drone surveys, and two LiDAR data sets. The first LiDAR data set was collected in 2014 prior to the slide by the Vermont Center for Geographic Information and the second after the slide in 2021 by the U.S. Army Corp of Engineers. Time-lapse spatial differencing allowed us to (1) quantify changes in surface topography over time, (2) calculate sediment budgets from source (landslide) to sink (delta), and (3) determine how mechanisms of mass wasting changed over time. Through this study we have also documented the following: (a) a second slip event in 2020 that removed ~25, 000 m3 of additional material from the hillside and contributed to growth of the Waterbury reservoir delta, (b) bedrock basins defined by the intersection of bedrock foliation and orthogonal fracture sets that appear to control slip location and geometry, (c) bedrock structures that influence the subsurface hydrology of the slip, which is expressed by oxidized groundwater seeps and a preferential deepening of rills into gullies on one side, and (d) how horizontal variations in the type and thickness of glacial lake sediments influenced mass-wasting mechanisms, including catastrophic failure of the hillside, the slumping of landslide sidewalls, the formation of crescent-shaped earth fractures, channeling around slumps, and the removal of material in deepening gullies. This study shows how a large landslide evolves from a major failure phase through later erosional and colluvial adjustments and supplies sediment at an episodic rate to the surface water system. 
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