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

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.