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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Mesozoic Tectonic Evolution in the Kurgovat‐Vanch Complex, NW Pamir
Different crustal deformation histories between Tibet and the Pamir reflect along‐strike variations in geodynamics of the Tethys orogen. To investigate the less well‐documented deformation history of the Pamir, which has been a barrier in understanding the nature of these differences, we conducted an integrated study in the Kurgovat‐Vanch region, NW Pamir. The lithologies are primarily Ediacaran‐to‐Carboniferous metasedimentary rocks intruded by Carboniferous plutons, which then experienced Late Triassic to Early Jurassic regional metamorphism. Structural mapping and analyses document a low‐angle NW‐directed thrust fault, the Poshkharv thrust, separating the overlying upper‐greenschist facies Poshkharv complex from the underlying amphibolite facies Kurgovat complex. Regional geologic maps indicate the Poshkharv thrust continues for ∼300 km across the NW Pamir. Our study also documents another regional thrust fault, the top‐to‐the‐SE Vanch thrust that juxtaposes the Southern Kurgovat complex above the lower‐grade Vanch complex in the south. Biotite40Ar/39Ar thermochronology indicates Early Cretaceous movement on all structures with ∼135–125 Ma exhumation along the NW‐directed Poshkharv thrust and ∼125–115 Ma exhumation along the SE‐directed Vanch thrust. Regional crustal deformation in the Northern Pamir was formed in a Cretaceous retro‐arc setting, unrelated to the Cenozoic India‐Asia collision. Cretaceous deformation in the NW Pamir was broadly coeval with the NE Pamir, but preceded Cretaceous shortening and coeval arc magmatism in the Southern Pamir. We interpret Early Cretaceous thrusting and crustal thickening followed by southward migration of shortening and magmatic flare‐up in the Pamir to have resulted from a transition of Neotethys subduction from northward flat‐slab advancing to southward retreating.
more » « less- NSF-PAR ID:
- 10377164
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 41
- Issue:
- 10
- ISSN:
- 0278-7407
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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