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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)

   Gordon, SM; Miller, RB; Rusmore, ME; Tikoff, B (Ed.)

   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. Cretaceous Kinematic Evolution of Southern California and the Baja-BC Problem

   Schwartz, JJ'; Miranda, EA; Klepeis, KA; Mora-Klepeis, G; Akciz, SO; Kylander-Clark, ARC (April 2026, Geological Society of America)

   Paleomagnetic data from the Insular and related terranes in the western Canadian and northern U.S. Cordillera indicate large-magnitude (~4000 km) northward translations along or adjacent (offshore) to western North America in the Late Cretaceous. In some widely debated ‘Baja-BC’ models, the Insular terranes are hypothesized to have collided with southern California and northern Baja California before being translated northward along dextral faults from 85–55 Ma. However, a major unresolved problem with Baja-BC controversy is the absence of recognized dextral faults in southern California capable of accommodating such large-magnitude northward translations. Here we synthesize new data from all known Mesozoic shear zones and faults within the southern California region – specifically, the Sawmill, Tumamait, Alamo Mountain–Piru Creek, Black Belt, and Eastern Peninsular Ranges shear zones – along with previously published data from the Cuyamaca–Laguna Mountain shear zone. These shear zones represent a system of intra-arc ductile shear zones and thrust faults active during the middle to Late Cretaceous. Field observations, microstructures, electron backscatter diffraction–derived crystallographic vorticity axis analysis, and titanite petrochronology reveal two distinct deformation phases: an early phase (120–90 Ma) of mostly arc-normal shortening with a minor dextral component, and a later phase (90–70 Ma) of widespread sinistral transpressional deformation accompanied by high-flux arc magmatism. We find no evidence for large-magnitude, dextral shear zones that could have translated Insular terranes northward. Instead, our findings support “offshore” models in which the Insular terranes were translated northward along structures farther west with little to no interaction with the southern California margin. 

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3. Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith, 2022, Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith

   Schwartz, J.; Lackey, J.S.; Miranda, E.; Klepeis, K.; Mora-Klepeis, G.; Robles, F.; Bixler, J. (October 2022, Geological Society of America Abstracts with Programs)

   We present >90 new igneous and metamorphic zircon and titanite petrochronology ages from the eastern Transverse Ranges of the Southern California Batholith (SCB) to investigate magmatic and tectonic processes in the frontal arc during postulated initiation of Late Cretaceous shallow-slab subduction. Our data cover >4000 km2 in the eastern Transverse Ranges and include data from Mesozoic plutons in the Mt. Pinos, Alamo Mountain, San Gabriel Mountain blocks, and the Eastern Peninsular mylonite zone. Igneous zircon data reveal 4 discrete pulses of magmatism at 258-220 Ma, 160-142 Ma, 120-118 Ma, and 90-66 Ma. The latter pulse involved a widespread magmatic surge in the SCB and coincided with garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust (Cucamonga terrane, eastern San Gabriel Mountains). In this region, metamorphic zircons in gneisses, migmatites and calc-silicates record high-temperature metamorphism from 91 to 74 Ma at 9–7 kbars and 800–730°C. The Late Cretaceous arc flare-up was temporally and spatially associated with the development of a regionally extensive oblique sinistral-reverse shear system that includes from north to south (present-day) 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. Syn-kinematic, metamorphic titanite ages in the Tumamait shear zone range from 77–74 Ma at 720–700°C, titanites in the Black Belt mylonite zone give an age of 83 Ma, and those in the eastern Peninsular Ranges mylonite zone give ages of 89–86 Ma at 680–670°C. These data suggest a progressive northward younging of ductile shearing at amphibolite- to upper-amphibolite-facies conditions from 88 to 74 Ma, which overlaps with the timing of the Late Cretaceous arc flare-up event. Collectively, these data indicate that arc magmatism, high-temperature metamorphism, and intra-arc contraction were active in the SCB throughout the Late Cretaceous. These observations appear to contradict existing models for the termination of magmatism and refrigeration of the arc due to underthrusting of the conjugate Shatsky rise starting at ca. 88 Ma. We suggest that shallow-slab subduction likely postdates ca. 74 Ma when high-temperature metamorphism ceased in the SCB. 

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4. Dating deformation in the Alamo Mountain and Piru Creek shear zones: Insights into Late Cretaceous tectonics in southern California

   https://doi.org/10.1130/2025.2565(15)  

   Bixler, Jonathan D; Schwartz, Joshua J; Swanson, Brian J; Miranda, Elena A; Klepeis, Keith A (January 2026, Geological Society of America)

   ABSTRACT Paleomagnetic data from the Insular superterrane and related terranes in the western Canadian and northern U.S. Cordillera seem to indicate large-magnitude (~4000 km), northward translations along the western margin of the North American Cordillera in the Late Cretaceous (the Baja–British Columbia [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 to 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 explored 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 focused on the Alamo Mountain and Piru Creek shear zones, located within the central Transverse Ranges. The kinematics of these shear zones help to elucidate whether southern California was experiencing predominantly dextral or sinistral faulting during the time of postulated large-magnitude northward translation along the western margin of the North American Cordillera. This information in turn allows for assessment and refinement of the Baja-BC hypothesis. We report new field observations and 21 U-Pb laser ablation–inductively coupled plasma–mass spectrometry zircon ages from deformed and undeformed host rocks and dikes. We find that ductile shear zones in the Alamo Mountain region are localized at the boundary between Paleoproterozoic gneisses and Mesozoic plutons. High-strain rocks consist of quartzofeldspathic mylonites and phyllonites up to 750 m thick, and both shear zones record sinistral strike-slip to sinistral-normal motion with a predominantly top-to-the-NW sense of shear in their present-day orientations. Our data show that the Alamo Mountain and Piru Creek shear zones were active at ca. 76–72 Ma and possibly included an earlier phase of deformation. 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 midcrustal conditions (500–600 °C and 4 kbar). 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 others in the Southern California Batholith, including the Nacimiento fault, the Mill Canyon window, and the Black Belt and Cucamonga shear zones (eastern San Gabriel Mountains), and the Tumamait shear zone (Pine Mountain block). The presence of this regionally extensive, sinistral shear zone system and the absence of dextral shear zones require reevaluation of the Baja-BC hypothesis in southern California during the Late Cretaceous. 

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5. DATING DEFORMATION IN THE ALAMO MOUNTAIN AND PIRU CREEK SHEAR ZONES: INSIGHTS INTO LATE CRETACEOUS TECTONICS IN SOUTHERN CALIFORNIA

   https://doi.org/10.1130/abs/2024AM-402994  

   Bixler, Jonathan; Schwartz, Joshua; Swanson, Brian; Miranda, Elena; Klepeis, Keith A (January 2024, Geological Society of America)

   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. 

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