Horizontal extension in the Hinterland of a thin-skinned thrust belt has been proposed as a natural consequence of crustal shortening and thickening and resulting topographic uplift. Most examples are known from modern orogens and ancient belts have yielded little evidence. This study will explore a suggestion of extension in the enigmatic, younger-over-older, top-to-the-west faults in southern Idaho that appear to be contemporaneous with Late-Cretaceous Laramide compression. The fault motion and timing will be addressed by 39Ar/40Ar dating and by reconstructing the thermal regime by illite crystallinity and conodont CAI. Results are expected to define the timing and kinematics of extensional tectonics in this region, and will bear on the relations among crustal shortening, thickening, topographic uplift, erosion and extension in the hinterland of thrust belts.
