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1. Intra-arc transpression and thrusting in the Southern California Batholith document moderate ‘Sierra-BC’ translation and collision with the conjugate Hess oceanic plateau

   Schwartz, J; Klepeis, K; Miranda, E; Mora_Klepeis, G; Bixler, J; Robles, F (April 2025, Geological Society of America)

   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. 

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2. 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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3. Accuracy of Structure-from-Motion/Multiview Stereo Terrain Models: A Practical Assessment for Applications in Field Geology

   https://doi.org/10.3390/geosciences13070217  

   Pavlis, Terry L.; Serpa, Laura F. (July 2023, Geosciences)

   We assess the accuracy of Structure-from-Motion/Multiview stereo (SM) terrain models acquired ad hoc or without high-resolution ground control to analyze their usage as a base for inexpensive 3D bedrock geologic mapping. Our focus is on techniques that can be utilized in field projects without the use of heavy and/or expensive equipment or the placement of ground control in logistically challenging sites (e.g., steep cliff faces or remote settings). We use a Terrestrial Light Detection and Ranging (LiDAR) survey as a basis for the comparison of two types of SM models: (1) models developed from images acquired in a chartered airplane flight with ground control referenced by natural objects located on Google Earth scenes; and (2) drone flights with a georeference established solely from camera positions located by conventional, differentially corrected Global Navigation Satellite systems (GNSS). We find that all our SM models are indistinguishable in scale from the LiDAR reference model. The SM models do, however, show rigid body translations and rotations, with translations generally within the 1–5 m size of the natural objects used for ground control, the resolution of the GNSS receivers, or both. The rigid body rotations can be attributed to a poor imaging plan, which can be avoided with survey planning. Analyses of point densities in various models show a limitation of Terrestrial LiDAR point clouds as a mapping base due to the rapid falloff of resolution with distance. In contrast, SM models are characterized by relatively uniform point densities controlled by camera optics, the numbers of images, and the distance from the target. This uniform density is the product of the Multiview stereo step in SM processing that fills areas between key points and is important for bedrock geologic mapping because it affords direct interpretation on a point cloud at a relatively uniform scale throughout a model. Our results indicate that these simple methods allow SM model construction to be accurate to the range of conventional GNSS with resolutions to the submeter, even cm, scale depending on data acquisition parameters. Thus, SM models can, and should, serve as a base for high-resolution geologic mapping, particularly in a steep terrain where conventional techniques fail. Our SM models appear to provide accurate visualizations of geologic features over km scales that allow detailed geologic mapping in 3D with a relative accuracy to the decimeter or centimeter level and absolute positioning in the 2–5 m precision of GNSS; a geometric precision that will allow unprecedented new studies of any geologic system where geometry is the fundamental data. 

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4. 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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5. Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith

   Klepeis, K; Schwartz, J; Miranda, E; Baskin, J; Castro_Mendez, A; Mora_Klepeis, G (April 2025, Geological Society of America bulletin)

   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. 

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