The structural and tectonic evolution of a Nevadan shear zone: a record of the origin, emplacement, and exhumation of the enigmatic outer Condrey Mountain schist

Date

2023-08

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Abstract

Tectonic models for the Nevadan orogeny have been a contentious topic since the assembly of the North American Cordillera was placed in a plate tectonic context at its inception. “Classic” Nevadan structures throughout the western U.S. include NW-trending axial planar cleavage metamorphosed during regional deformation to lower greenschist facies. Workers have used these field observations, cross-cutting relationships, and metamorphic cooling and igneous crystallization ages to define the Nevadan orogeny as a Late Jurassic event (ca. 155-150 Ma). Within a subset of the Cordillera called the Klamath Mountains Province (KMP) exists a domed structural window centered about Condrey Mountain, CA. This window is defined by a shear zone that juxtaposes amphibolite to locally granulite facies Rattlesnake Creek terrane (RCt) above albite-epidote amphibolite rocks of the ca. 170 Ma outer Condrey Mountain schist (CMSo). New CA-ID-TIMS U/Pb ages on migmatites from this shear zone have been dated to ca. 157-155 Ma, indicating partial melting at high-grade occurred during the Nevadan orogeny. New structural data from these foliation-parallel migmatites has noted a strong L>S fabric developed within the hangingwall shear zone. A detachment fold and mélange unit called the chaotic zone also defines the base of the shear zone. This new age and structural data redefine “classic” Nevadan structures and introduce a process of antiformal stacking into the tectonic model for the Nevadan orogeny in the KMP. New geochemical analysis of bulk rock data from the KMP has indicated that the CMSo is correlative with an albite epidote amphibolite to greenschist unit to the west of the window, near Fort Goff, CA. This western subunit has been mapped in thrust contact above the Nevadan-aged Galice Formation and in thrust contact below the RCt. If this structural relationship and geochemical correlation holds true, then the CMSo is a Klamath terrane that may be a remnant of an earlier basin, similar to the Josephine/Galice but ca. 10 my older. Additional field evidence and metamorphic cooling ages from in and around the domed structural window suggest extending the length of the Nevadan orogeny to 135 Ma as there was continued deformation, regional metamorphism, and magmatism related to the event occurring in both the Klamath Mountains and Sierra Nevada through this time.


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Keywords

constrictional deformation, Nevadan orogeny, Klamath Mountains, Condrey Mountain schist, CA-ID-TIMS, bulk rock geochemistry

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