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This view of the Overton Arm of Lake Mead is from the Stewarts Point area. Across the lake, incised alluvial fans extend into the Muddy River Valley (now reservoir) from the Virgin Mountains. The Virgin Mountains consist of complexly faulted and folded rocks of the same sequence exposed in the western Grand Canyon region (Precambrian granitic basement overlain by a mile-thick sequence of Paleozoic rocks). This same sequence also occurs in the Frenchman Mountains near Las Vegas. Current thought is that the Frenchman Mountains are a great block of crust that was rifted away from the southern end of the Virgin Mountains. Beginning about 16 million years ago, the Frenchman Mountains crustal block was transported westward along the Lake Mead fault and Las Vegas shear zones eventually to its present location (see a geologic map). This kind of tectonic movement was concurrently happening throughout the southern Basin and Range Province. This ongoing tectonic activity produced the faults, folds, and complex crustal features responsible for the mountain ranges and sediment-filled basins we see throughout the region today (Tingley and others, 2001, Beard and others, 2007). |
