Restoration of Tertiary deformation in the Lake Mead region, southern Nevada: The role of strike-slip transfer faults

The Lake Mead area of Nevada and Arizona is characterized by a complex three-dimensional strain field and contains some of the largest and best-exposed examples of extension-related strike-slip faults in the world. Faults of the Lake Mead fault system and the Las Vegas Valley shear zone form a kinematically linked array of structures that accommodated both large-magnitude east-west extension and significant north-south shortening in the middle Miocene. Collectively, these linked structures define a regional-scale complex transfer zone that marks the transition between the northern and southern Basin and Range provinces. A reconstruction of extensional tectonism in the Lake Mead area underscores the importance of recognizing kinematic linkages between normal and transverse strike-slip faults in accommodating regional extension. Confirming this link are the following observations: (1) several strike-slip faults curve into or merge with normal faults; (2) strike-slip faults locally bound areas that have undergone differential extension; and (3) displacement magnitude varies along some strike-slip faults, requiring that these faults must be coupled to other structures to maintain regional strain compatibility. In the reconstruction, tectonic elements are restored sequentially in five time intervals along specific structures, the timing and magnitude of displacement of which are either known or can be reasonably inferred. (1) Post-10 Ma: Restoring 20 km of left-slip movement along the Hamblin Bay fault and 8 km of associated north-south shortening near the western lobe of Lake Mead places Frenchman Mountain near the present Gale Hills and the River Mountains near Callville Bay. (2) 12.5 to 10 Ma: The River Mountains stratovolcano complex is moved a minimum of 7 km east along the Las Vegas Valley shear zone and a combined proto-Hamblin Bay-Saddle Island fault system to bring it into contact with the cogenetic Wilson Ridge pluton. (3) 13.5 to 12.5 Ma: We restore 15 km extension south of Lake Mead that was associated with dikeon- dike emplacement of the northern Wilson Ridge pluton. North of Lake Mead, extensional strain was accommodated by a system of strike-slip and normal faults, including the Lakeside Mine fault. (4) 14.0 to 13.5 Ma: Restoration of about 20 km of left-slip movement on the Lime Ridge fault places Frenchman Mountain about 10 km north of Gold Butte. (5) Pre-14 Ma: To align paleogeographic features, we suggest that the Virgin Mountains block was probably 14 to 22 km northeast of its present position prior to 14 Ma. The net-transport vector of Frenchman Mountain relative to an original position near the Colorado Plateau is 60-80 km in a direction of 250°. This vector resolves into a 56-75 km east-west component of extension and a 20-27 km component of north-south shortening. The majority of stratal tilts in the Lake Mead region are to the east. This observation is in accord with inferred westward motion of shallow crustal blocks relative to the stable Colorado Plateau and is most consistent with unidirectional flow of upper crustal material on a fluid, midcrustal layer. Normal and strike-slip faults acted in concert to accommodate this west-southwestward flow of material. The role of strike-slip transfer zones cannot be ignored in reconstructions of extended terranes.