MIOCENE DEFORMATION PATTERNS IN THE LAKE MEAD REGION, BASIN AND RANGE: TECTONIC WEDGES AND BLOCK ROTATIONS IN A TRANSTENSIONAL SETTING

Miocene extensional tectonics in the Lake Mead region is includes a complex array of detachment, normal, and strike-slip faults in an overall transtensional setting. Contractional structures are locally important and are related to strike-slip faulting and a poorly understood late-stage deformation. The main phase of transtensional faulting occurred from 17 to 8 Ma, when the rate of faulting decreased greatly. The pattern of Miocene faulting, and the importance of various types of faulting, have been debated for years. Our research has found these fault patterns, many of which are refinements of past worker’s interpretations. (i) Detachment faulting in eastern Lake Mead dominated from 17 – 15 Ma, then overlapped in time with an ever more dominant transtensional fault setting (Lake Mead fault system) that progressed westward. (ii) An inherited NE – SW tectonic boundary promoted strike-slip faulting in that orientation, while extension was dominantly west directed, and therefore the overall faulting pattern was transtensional. (iii) Wedge tectonics may have dominated the earlier transtensional faulting to the east from 15.5 – 13 Ma, but those fault patterns are less well known because of overprinting. (iv) Faulting in western Lake Mead was dominated from 13 to 8 Ma by a west-opening wedge of strike-slip faults of opposing sense of slip; we link the Saddle Island fault east to the Bitter Spring Valley fault, both with ~6 km offset, on the south side of the wedge. (v) The Frenchman Mountain block deformed within the fault wedge as three structural domains along the right-lateral Las Vegas Valley shear zone (LVVSZ, northern side of wedge): the Sunrise domain rotated internally in a broken-block style; the central block had bookshelf-style clockwise rotations along left lateral faults; the southern block tilted east and did not rotate. Deformation began at 15.5 – 15 Ma, but was likely most active after 14 Ma. Virtually all faults and folds in the Frenchman Mountain block south of the Boulevard fault can be explained by this rotation + translation model along the LVVSZ; our model agrees with previous paleomagnetic results. The fault patterns in the Lake Mead domain are similar to faults in the Death Valley region to the west, which were active beginning at the same time as peak transtensional faulting in the Lake Mead domain, 14 – 12 Ma.