EVALUATING TECTONIC RECONSTRUCTIONS FOR THE LAKE MEAD REGION, NEVADA-ARIZONA: INSIGHTS FROM STRATIGRAPHIC STUDIES

Ernie Anderson's pioneer work in the Lake Mead region (USGS PP 794, 1973) recognized 20 km of left-lateral offset of the Miocene Hamblin-Cleopatra volcano and suggested that Frenchman Mountain (FM) had been translated about 60 km westward from its original position. This led to his ideas of tectonic rafting in this part of the Basin and Range Province. His interpretation was based on landslide blocks and breccias, found within the Miocene Thumb Member of the Horse Spring Formation at FM, that were derived from Gold Butte Granite exposed 60 km to the east in the Gold Butte Block of the South Virgin Mountains (SVM). Since then, many proposed reconstructions have placed FM in various positions relative to Gold Butte. The most recent proposal by Fryxell and Duebendorfer, 2005, supports two possible starting positions: 1) a preferred position above the Gold Butte block in the hanging wall of the Lakeside Mine fault and 2) directly north of the Gold Butte fault as proposed by Rowland and others, 1990.

We examine the Fryxell/Duebendorfer reconstruction using stratigraphic data, Ar-Ar ages from the Horse Spring Formation north of Gold Butte, and new data from the Echo Wash area. During lower Thumb time (about 16- 15.2 Ma) in the SVM, a syn-extensional lacustrine basin extended from south of the Gold Butte fault northward to the central Virgin Mountains. By 15.2 Ma, the basin was partitioned by normal and oblique left-normal-slip faults, and coarse conglomerate and megabreccia deposits were shed across the faults into the deforming basin. The thickest and coarsest deposits contain Proterozoic clasts derived from the Gold Butte block and were shed northward from before 14.4 Ma to at least 13.83 +/- .08 Ma (Kunk, written commun, 2000). Younger sandstone and conglomerate deposits containing a 13.28 +/- 0.07 Ma tuff (Kunk, 2000) are exposed in the hanging wall of the Lakeside Mine fault west of Gold Butte and must postdate westward translation of FM. Detailed work by Martin (2005) at Echo Wash suggests that the northwestern part of the original Thumb basin was also translated westward to its present position. If FM originated in the SVM area it must also carry part of the same lower Thumb basin. Detailed studies comparing the Tertiary stratigraphy between Frenchman Mountain and the SVM are needed to further resolve the starting position of FM.