Origin and trajectory of the Frenchman Mountain block, an extensional allochthon in the Basin and Range province, southern Nevada

The central Basin and Range province has provided a natural laboratory for the study of continental extension since the pioneering studies of Chester R. Longwell. It is here that large-magnitude crustal extension along systems of normal faults was first documented by R. Ernest Anderson. Estimates of crustal extension in the Lake Mead extensional domain (eastern central Basin and Range province) have been based largely on restoration of displaced blocks, most notably the Frenchman Mountain block. The original position of Frenchman Mountain, an allochthonous block that has been translated more than 60 km westward during regional extension, has been controversial for more than 2 decades. We use correlation of mass wasting deposits with source areas, Paleozoic stratigraphic characteristics, and subunconformity pinchout relations to evaluate previously published models for the original position of the Frenchman Mountain block and use new evidence to propose a more tightly constrained preextension position for this block. Our work restricts the original position of the Frenchman Mountain block to ∼8 km or less from the Gold Butte block. We favor an interpretation in which Frenchman Mountain originated as the hanging wall of the tectonically denuded Gold Butte block and was therefore translated to its present position by movement along kinematically coupled detachment and strike-slip faults. Regional relations require multiple stages of movement along numerous structures from this preextension location, and we propose a new model for the trajectory of the Frenchman Mountain block to its present position.