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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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Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith
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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- Award ID(s):
- 2138734
- PAR ID:
- 10583834
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America bulletin
- ISSN:
- 0016-7606
- Subject(s) / Keyword(s):
- Transpression, lower continental crust, fault, shear zone, Southern California batholith
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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