Although dominated by a strong late Tertiary extensional overprint, the turtlebacks of the Black Mountains of Death Valley display evidence for earlier, possibly Laramide-age, basement-involved thrust faulting. The thrust faults consist of now highly deformed contacts in which >200-m-thick bodies of basement gneiss structurally overlie upper Proterozoic metasedimentary rock. In addition, the faults contain locally preserved, east- directed mylonitic fabrics in their footwalls that are demonstrably early Tertiary or older. The association of these fabrics with Laramide-age pegmatite suggests that they may have formed concurrently with the intrusions. These basement thrust faults are significant because they formed in a region long considered to have exclusively thin-skinned deformation. Their presence also places new constraints on this part of the fold-and-thrust belt's preextension geometry by limiting the amount of late Tertiary extension. Specifically, these faults disallow the correlation of thin-skinned structures that currently exist to the east and west of the Black Mountains.

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