Abstract

Basin and Range normal faulting in west-central Idaho reactivates preexisting Cretaceous structures, resulting in a series of normal fault–bound basins. The most intense extensional deformation is concentrated in the Late Cretaceous western Idaho shear zone, resulting in the development of the Long Valley basin. The area affected by the western Idaho shear zone displays two orientations of steep faults: one set of normal faults strikes north-south and is parallel to fabrics within the western Idaho shear zone; the other set strikes east-west and accommodates components of both normal and strike-slip movement. Areas within the Idaho Batholith that do not have strong Cretaceous fabrics (i.e., outside of the western Idaho shear zone) are characterized by a strong preferred north-northeast orientation of faults. From this preferred orientation we infer that the maximum infinitesimal stretch is oriented at 110/290° in this part of the Idaho Batholith. Gravity inversion indicates that the north end of Long Valley is an asymmetric basin about one kilometer deep, with the largest basin-bounding normal fault on the west side of the Long Valley. Paleomagnetic analysis of the Columbia River basalts indicates that the north-south elongate fault blocks within the western Idaho shear zone have not rotated. One block, located just to the west of the western Idaho shear zone, may have rotated counterclockwise. The lack of rotation of north-south oriented fault blocks, in combination with the fault orientations of the Idaho Batholith, indicate that the regional neotectonic deformation within and east of the western Idaho shear zone is characterized by dextral transtension with a divergence vector oriented 130/310°.

The extensional reactivation of the western Idaho shear zone demonstrates the effect of material anisotropy at local and regional scales. On a local scale, the mylonitic foliation of the western Idaho shear zone is reactivated as normal faults, even though the regional flow field is oblique to the foliation. On a regional scale, the possible counterclockwise fault block rotation recorded west of the western Idaho shear zone is inconsistent with dextral transtension, suggesting that extensional deformation has reactivated the western edge of the arc-craton boundary as a kinematic domain boundary. We conclude that the preservation of initial features in vertical shear zones and/or plate boundaries is unlikely, due the tendency for well-developed, subvertical fabrics to reactivate, particularly in extension.

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