Dextral strike-slip tectonism and arc processes in the Sierra Nevada and Idaho batholiths

Basil Tikoff, University of Wisconsin Madison, Geoscience, Madison, United States, Ad Byerly, University of Wisconsin Madison, Madison, WI, United States, Richard M Gaschnig, University of Maryland College Park, Department of Geology, College Park, MD, United States, Jeffrey D Vervoort, Washington State University, School of the Environment, Pullman, WA, United States and Paul Robert Kelso, Lake Superior State University, School of Chemistry, Environmental, and Geosciences, Sault Sainte Marie, United States
Abstract:
Both the Sierra Nevada (CA) and Idaho batholiths display clear evidence for magmatism simultaneous with dextral strike-slip movement during the Cretaceous. There is, however, a significant difference between both the style and the timing of the deformation and magmatism in these two magmatic arcs. The Sierra Nevada displays dextral strike-slip tectonism active from 92-80 Ma, as evidenced by a series of dextral shear zones associated with the axis of active magmatism. The concept of the plutons intruding en echelon pull-apart structures in a dextral system has been supported by the recognition of widespread, syn-tectonic shear zones along the axis of the arc. Further, a modern analog can be found in El Salvador, where dextral translation of the forearc has resulted in en echelon dextral shearing in a magmatic arc dominated by arc-perpendicular normal faulting. In contrast, dextral tectonism in Idaho both starts (~103 Ma) and ceases earlier (~90 Ma) than strike-slip motion in the Sierra Nevada batholith. Further, the deformation is better developed, as recorded by the ~5 km thick, dextral transpressional western Idaho shear zone (WISZ). The WISZ deformation affects I-type magmas that are not typically considered part of the Idaho batholith. The main phase of the 85-70 Ma, peraluminous Idaho batholith (Atlanta lobe) only contains local evidence for dextral shearing, such as the NS-oriented Johnson Creek-Profile Gap shear zone. The reasons for this along-strike variation in deformation for Cretaceous magmatic arcs in North America may relate to the collisional and translation histories of offshore terranes.
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