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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 (500-600C 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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A review of paleomagnetic studies from Northern Alaska and Yukon: implications for terrane reconstructions
Here, we present a comprehensive overview of available paleomagnetic studies and datasets from northern Alaska, USA, and Yukon, Canada. Most studies in this region were conducted when our understanding of best-practices paleomagnetism was still developing; as a result, many do not meet modern standards of data quality and many interpretations of the paleomagnetic data—though valid at the time—are now also outdated. In this review, we assess what data are reliable, what interpretations have stood the test of time, and what the existing data can constrain about the tectonic history of this region. We find that although a middle to Late Cretaceous (Aptian–Turonian) overprint pervades much of this area, many sites still retain primary remanence directions, some dating as far back as the Neoproterozoic. Studies that found complete overprinting in the Cretaceous also typically analyzed poor lithologic recorders of paleomagnetic directions, such as carbonates. Based on our assessment of the most reliable data from the study region, relative motion between the examined outboard terranes and Laurentia was not yet complete by the early Late Cretaceous. We also find that “high latitude” dinosaur fossil sites were more northerly than today, confirming previous assumptions about the paleolatitudes at these sites. Finally, we discuss how the widely cited Jurassic–Cretaceous counterclockwise rotation hypothesis for the Arctic Alaska terrane is no longer supported by the existing paleomagnetic data, and the validity of this hypothesis should be critically re-examined.
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- PAR ID:
- 10616552
- Publisher / Repository:
- Canadian Journal of Earth Sciences
- Date Published:
- Journal Name:
- Canadian Journal of Earth Sciences
- ISSN:
- 0008-4077
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1139/cjes-2024-0125
