SPATIAL ISOLATION OF EXTENDED AREAS IN THE CENTRAL BASIN AND RANGE

The commonly assumed interconnectedness of extended areas of the central Basin and Range is largely invalid, and published province-wide summations of strain need reconsideration. 1) In Utah, the 40+ km of west-directed elongation in the Sevier Desert/Mineral Mountains area (SDM) can not be projected southward through the Black Mountains/Escalante Desert area where geologic, drill-hole, and geophysical data show weakly faulted, gently dipping, pre-extension Tertiary strata and no breakaway against the Colorado Plateau. Published models that link the SDM strain to extension along the Beaver Dam Mountains/Mormon Mountains transect (BDM) in Utah and Nevada via the large-magnitude clockwise rotational strain in the intervening Clover Mountains are, accordingly, invalid, leaving the large rotational strain open for reinterpretation. 2) The contested 50+ km (ca 200%) of west-directed BDM elongation does not link southward with the well-documented 65+ km of elongation in the Lake Mead area (LMA) in Arizona and Nevada because detailed structural studies and regional cross sections show only a few percent extension in the intervening E-W corridor at the latitude of Overton, NV and no breakaway against the Colorado Plateau. 3) The large-magnitude west-directed elongation in the LMA has been modeled as coupled to elongation in the Nevada Test Site area (NTS) via the Las Vegas Valley shear zone despite a) 25°-30° misalignment of the shear zone with the elongation directions and b) tomographic modeling showing that the pre- and syn-extension calderas in the NTS area are located above their crustal-penetrating roots, precluding major elongation there. To accommodate these isolationist aspects of the extended central Basin and Range, diapiric models that integrate multidirectional flow capable of producing both the extreme vertical lumpiness and localized large lateral displacements need to be developed. The suggestion made by Snow and Wernicke (2000) that province-wide mass balance can be achieved by “borrowing” crust from adjacent provinces to “pay” for deficiencies in the extensional province needs to be applied to balancing localized lumpiness within the context of synmagmatic events and greatly reduced regional elongation.