The frontal portion of the Cordilleran thrust belt at latitude 37°N is characterized by a major, east-directed décollement-style thrust system developed within Paleozoic and Mesozoic strata intermediate in thickness between the craton to the east and the Cordilleran miogeocline to the west. Large-magnitude Neogene extension dismembered the thrust system, resulting in unusually complete exposures of it, both along and across strike, and providing an ideal setting in which to study the influence of older thrust structure on extensional faults. A shallow thrust flat was reactivated as a low-angle normal fault over a large area, but the thrust ramp was not. Due to structural duplication by the thrust system, Tertiary normal faults commonly place older rocks on younger, having excised or reactivated thrusts. The hanging walls of large normal faults are commonly brecciated for 10 to 100 m above the fault, whereas thrusts caused little disruption via brecciation, even within centimeters of the fault plane. This criterion was found to be more useful than stratigraphic juxtaposition for distinguishing thrust faults from normal faults. Detailed mapping has allowed identification and correlation both along and across strike of the following Mesozoic structural sequence: (1) autochthonous crystalline basement and depositionally overlying Phanerozoic cover (bottom), (2) subregionally developed duplexes torn from footwall ramps, (3) a regional décollement thrust that generally carries Middle Cambrian dolostone in its hanging wall, (4) an internally imbricated duplex composed of Middle and Upper Cambrian strata, and (5) an overlying roof thrust that detached at the same horizon as level (4) and carried a comparatively undisrupted sequence as young as Mississippian-Permian. The previously identified Glendale, Mormon, and Tule Springs thrusts are herein correlated and correspond to the thrust at the base of (3). The 50-km-long Weiser syncline (Longwell, 1949) formed as an inclined footwall syncline and was rotated into its recumbent attitude during Tertiary extension. Extension in the central Mormon Mountains-East Mormon Mountains-Tule Springs Hills transect was controlled by two major west-dipping detachment faults: the older and structurally higher Mormon Peak detachment and the younger, structurally lower Tule Springs detachment. The Mormon Peak detachment cut gradually (5 to 25°) down to the west in its initial trajectory from structurally high levels of the Tule Springs thrust plate in the east to autochthonous crystalline basement in the west. By contrast, the Tule Springs detachment followed the Jurassic footwall flat of the Tule Springs thrust in the Tule Springs Hills, then ramped downward through the Mesozoic autochthon into crystalline basement, probably flattening again at depth beneath the Mormon Mountains. Much of the uplift and eastward tilting of the East Mormon Mountains was probably caused by isostatic response to differential tectonic unloading in this extensional ramp zone. The entire Mormon Mountains-Tule Springs Hills area was translated westward, with only minor internal disruption, on the younger Castle Cliff detachment exposed along the edge of the Colorado Plateau to the east. The Mormon Peak detachment postdates ca. 14-Ma ignimbrites, the Tule Springs detachment probably predates the deposition of much or all of the Miocene-Pliocene Muddy Creek Formation, and the Castle Cliff detachment was active during Muddy Creek and more recent time. A broad east- to east-northeast-trending zone of Tertiary dextral normal oblique-slip faulting and oroflexure in the southern Mormon and East Mormon Mountains (the Moapa Peak shear zone) was active synchronously with detachment faulting. East-west-trending faults that separate differentially extended blocks to the north and south make up the north margin of the shear zone. Gravity, magnetic, and seismic reflection data suggest that the southern boundary is a south-southeast-facing bedrock scarp that forms the north edge of the Mormon Mesa basin. We interpret this scarp as the north edge of a scoop-shaped fault. The shear zone is apparently a transfer structure between areas of differential extension in the east-central and southern Mormon Mountains and southern East Mormon Mountains. The youngest faults in the area are steep, east- and west-dipping faults in and adjacent to the East Mormon Mountains and Tule Springs Hills. They may be related to movement on the Castle Cliff detachment. Locally, these faults cut Plio-Quaternary(?) pediment gravels, and they are associated with opening the Tule Desert basin and the basin east of the East Mormon Mountains.

Scattered remnants of highly diverse stratigraphic sections of Tertiary lacustrine limestone, andesite flows, and 23.8–18.2 Ma regional ash-flow tuffs on the north flank of the Mormon Mountains record previously unrecognized deformation, which we interpret as pre–17 Ma uplift and possibly weak extension on the north flank of a growing dome. Directly to the north of the Mormon dome, 17–14 Ma ash-flow tuffs and rhyolite are interstratified with landslides, debris avalanches, debris flows, and alluvial-fan deposits that accumulated to a thickness of more than 2 km in an extension-parallel basin. The source for the landslides and debris avalanche deposits is unknown, but it was probably an adjacent scarp along a transverse fault bounding an early part of the Mormon dome. An average 45° of easterly tilt of the entire Tertiary basin-fill succession represents the major post–14 Ma deformation event in the region. We question the basis for the published estimate of 22 km of westerly displacement on the Mormon Peak detachment fault and, on the basis of landslides in the upper plate having a probable source in the adjacent Mormon dome, constrain the heave to ~4 km. We interpret the dome and basin as coupled strains similar to others in the region and suggest that these strains reflect a waveform pattern of extension-normal lateral midcrustal ductile flow. Previously, doming was interpreted as an isostatic response to tectonic unloading by large-displacement detachment faults or as pseudo-structural highs stranded by removal of middle crust from adjacent areas. Moreover, we argue that the strong thinning of upper-plate rock successions throughout the Mormon Mountains and Tule Springs Hills resulted from a loss of rock volume by protracted fluid flow, dissolution, and collapse, seriously limiting the usefulness of upper-plate strain in evaluating extension magnitude. We present a geohydrologic model that couples uplift driven by ductile inflow with dissolution driven by fluid infiltration, possibly augmented by mantle-derived CO 2 -rich fluids. Karsting in the uplands led to carbonate sedimentation in adjacent lowlands. Whether or not our downward revision of extension in the Mormon Mountains is valid, extension at that latitude is isolated from extension in the Lake Mead area by a low-strain corridor between the two areas. Recognition of the isolated and potentially diminished strain impacts estimates of maximum finite elongation of the Basin and Range Province because one of three vector paths used in those estimates passes through the Mormon Mountains.

Abstract This paper summarizes the results of completed and ongoing research in three areas of the Basin and Range Province of the western United States that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated. Given that these examples were influential in the development of ideas about low-angle normal faults, and particularly in making the case for frictional slip at dips of appreciably less than the 30° lock-up angle for μ ≈ 0.6 (where μ is the coefficient of friction), we advocate a critical re-examination of interpreted detachments elsewhere in the Basin and Range Province and in other extensional and passive margin settings. The Sevier Desert ‘detachment’ of west-central Utah is reinterpreted as a Palaeogene unconformity that has been traced to depth west of the northern Sevier Desert basin along an unrelated seismic reflection (most probably a splay of the Cretaceous-age Pavant thrust). The absence of evidence in well cuttings and cores for either brittle deformation (above) or ductile deformation (below) is inconsistent with the existence of a fault with as much as 40 km of displacement. The Pavant thrust and the structurally higher Canyon Range thrust are erosionally truncated at the western margin of the southern Sevier Desert basin, and are not offset by the ‘detachment’ in the manner assumed by those inferring large extension across the basin. The Mormon Peak detachment of SE Nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction. A particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation, with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks. The Castle Cliff detachment in the nearby Beaver Dam Mountains of SW Utah is similarly regarded as a surficial feature, as originally interpreted, and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin. The middle Miocene Eagle Mountain Formation of eastern California, interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ESE from a location close to the Jurassic-age Hunter Mountain batholith of the Cottonwood Mountains, is reinterpreted as having accumulated in a fluvial–lacustrine rather than alluvial fan–lacustrine setting, with no bearing on either the amount or direction of tectonic transport. The conglomeratic rocks upon which the provenance argument was based are pervasively channelized, with erosional relief of less than 1 m to as much as 15 m, fining-upwards successions at the same scale and abundant trough cross-stratification – all characteristic features of fluvial sedimentation and not of alluvial fans. The interpretation of the Eagle Mountain Formation as having been deposited within a few kilometres of the Hunter Mountain batholith, which depends strongly on assumptions about the dimensions of alluvial fans, is therefore not required. The result is important because the Eagle Mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of California, and for the existence of detachment faults of regional dimensions.