Basin and range extension in west-central Idaho has created a set of Miocene-Present normal faults. The normal faults offset both Cretaceous aged granitoids and Miocene Columbia River basalts. These faults strike N-S and dip 70° E, parallel to foliation within the Late Cretaceous western Idaho shear zone. We utilize the basalt layers as markers for deciphering the effects of post mid-Miocene extension in the region.

Paleomagnetic analysis of Columbia River basalts were conducted on three tectonic blocks which are separated by normal faults. The results indicate that most samples had one, or sometimes two, well-resolved magnetic components that were isolated by either AF or thermal demagnetization techniques. Stepwise thermal demagnetization was used for most samples and they typically demagnetized between 250-500°C. The higher temperature component is used for the following analysis. Rock magnetic analysis suggest that the magnetic carrier is primarily single-domain to pseudosingle-domain titanomagnetite.

The eastern most block contains sites from 10 flows with a mean D=184.9, I=-41.6, alpha95%=6.9, kappa=49.9. The central block is constrained by sites from 6 flows with a mean D=203.8, I=-47.3, alpha95%=13.0, kappa=27.5. The western most block contains sites from 10 flows with a mean D=186.3, I=-65.5, alpha95%=6.5, kappa=56.1. The observed 20°+ variation in directions between sites suggest that the blocks have undergone substantial differential rotation and that this rotation is not always along a horizontal axis that is N-S, i.e., parallel to the strike of the normal faults. Reorientation of the Columbia River Basalts to their original position indicates that the present orientation of fabrics within the western Idaho shear zone are not their original, Cretaceous orientation. Using the paleomagnetic analysis, we reconstruct the pre-Miocene orientation of the shear zone.