HR: 09:45h
AN: T21D-07    [Abstracts]
TI: From detachment to transtensional faulting: A model for the Lake Mead extensional domain based on new ages and correlation of subbasins
AU: * Beard, L
EM: sbeard@usgs.gov
AF: USGS, Flagstaff Office, Flagstaff, AZ 86001, United States
AU: Umhoefer, P J
EM: paul.umhoefer@nau.edu
AF: Northern Arizona University, Geology 4099, Flagstaff, AZ 86011, United States
AU: Martin, K L
EM: Luke_Martin@SWN.COM
AF: Northern Arizona University, Geology 4099, Flagstaff, AZ 86011, United States
AU: Blythe, N
EM: nblythe@bluebottle.com
AF: Northern Arizona University, Geology 4099, Flagstaff, AZ 86011, United States
AB: New studies of selected basins in the Miocene extensional belt of the northern Lake Mead domain suggest a new model for the early extensional history of the region (lower Horse Spring Formation and correlative strata). Critical data are from (i) Longwell Ridges area west of Overton Arm and within the Lake Mead fault system, (ii) Salt Spring Wash basin in the hanging wall of the South Virgin-White Hills detachment (SVWHD) fault, and (iii) previously studied subbasins of the south Virgin Mountains in the Gold Butte step-over region. The basins and faulting patterns suggest two stages of basin development related to two distinct faulting episodes, an early period of detachment faulting followed by a switch to faulting mainly along the Lake Mead transtensional fault system while detachment faulting waned. Apatite fission track ages suggest the footwall block of the SVWHD was cooling at 18-17 Ma, but the only evidence for basin deposition at that time is in the Gold Butte step-over where slow rates of sedimentation and facies patterns make faulting on the north side of the Gold Butte block ambiguous. The first basin stage was ca. 16.5 to 15.5 Ma, during which there was slow to moderate faulting and subsidence in a basin along the SVWHD and north of Gold Butte block in the Gold Butte step-over basin; the step- over basin had complex fluvial and lacustrine facies and was synchronous with landslides and debris flows in front of the SVWHD. At ca. 15.5-14.5 Ma, there was a [dramatic] increase in sedimentation rate related to formation of the Gold Butte fault, a change from lacustrine to widespread fluvial, playa, and local landslide facies in the step-over basin, and the peak of exhumation and faulting rates on the SVWHD. The simple step-over basin broke up into numerous subbasins [at[ as initial faults of the Lake Mead fault system formed. From 14.5 to 14.0 Ma, there was completion of a major change from dominantly detachment faulting to dominantly transtensional faulting in the Lake Mead fault system as detachment faulting waned. The Lake Mead fault system began to propagate to the west and faults and subbasins north of Gold Butte died with major progradation of alluvial conglomerates over the step-over basin. The geometry of the SVWHD that dominated the early Lake Mead extension history fundamentally controlled patterns of faulting and magmatism throughout the rest of the extensional history. This process of detachment faulting changing to dominant high-angle faulting and possible lower crustal flow has been suggested previously and may be a general process.
DE: 8109 Continental tectonics: extensional (0905)
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting