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The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fi ne sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. We briefl y review 87 Sr/ 86 Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. We also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identifi ed as the 4.83 Ma Lawlor Tuff derived from the San Francisco Bay region. We conclude, as have some others, that the Bouse Formation was deposited in lakes produced by fi rst-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that fi rst arrival of river water occurred ca. Ma. If this interpretation is correct, addition of BristolBasin to the Blythe Basin inundation area means that river discharge was suffi cient to fi ll and spill a lake with an area of ~10,000 km 2 . For spillover to occur, evaporation rates must have been signifi cantly less in early Pliocene time than modern rates of ~2-4 m/yr, and/or Colorado River discharge was signifi cantly greater than the current ~15 km 3 /yr. In this lacustrine interpretation, evaporation rates were suffi cient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.

Large-magnitude Miocene extension in west central Arizona occurred primarily along three imbricate, northeast dipping normal faults. The structurally highest of these faults, the gently dipping Buckskin-Rawhide detachment fault, accommodated approximately 66 km of crustal extension, whereas the two structurally lower faults accommodated a total of about 20 km extension. Due to this large-magnitude extension, an area at the Earth's surface that was 10 to 20 km wide is now over a 100 km wide, and crystalline rocks with mid-Tertiary mylonitic fabrics, uncovered by detachment faulting, are exposed over roughly 2000 km 2 in the Harcuvar metamorphic core complex. Most of the upper plate of the Buckskin-Rawhide detachment fault was largely undeformed by internal extension; only the thin, tapered end of the upper plate was highly extended. During extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered. Grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded, forming extensionparallel folds in the lower plate. Low flexural strength characterized the lower plate during denudation, and a highly mobile, low-viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere.Detachment faults have been considered to be the extensional analogs of thrust faults [Wernicke, 1981]. In detail, however, the mechanical behavior of the crust during large-magnitude extension may be quite different from that typically associated with crustal shortening. Specifically, the uplift and warping of large-displacement, low-angle normal faults (detachment faults) and their commonly mylonitic lower plates (metamorphic core complexes) during extension indicate that the flexural strength (resistance to bending) of detachment-fault lower plates was far less during extension than was the flexural strength of the lower plates of foreland fold and thrust belts during thrusting. Indeed, simplistic models of detachment faulting assuming essentially zero lower plate flexural strength yield fault geometries that are quite similar to some known geometries [Spencer, 1984].Several types of evidence indicate that the Buckskin-Rawhide detachment fault and its downdip extension as a ductile (e.g., plastic [see Rutter, 1986]) shear zone had a broadly listfie form and that the hanging wall largely maintained this form during extension. The lower plate therefore must have undergone major flexural deformation as it was displaced up and out from beneath the upper plate. A significant conclusion of this study is that, during extension, the low flexural strength of the lower plate of the Buckskin-Rawhide detachment fault and a highly mobile middle to lower crust allowed rapid warping and short-wavelength isostatic adjustment during extension. We thus support earlier proposals for styles of extensional deformation in which a migrating monocline in the lower plate fol...

The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of manfie lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low-angle subduction that ended in the middleCenozoic. Finite element heat flow calculations of low-angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200-km-thick lithosphere. This allows for partial preservation of North American mantle lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau. 13,595 13,596 SPENCER: COLORADO PLATEAU UPLIFTare found to be the major, poorly constrained variables that influence uplift magnitude. A reasonable range of those parameters is found to produce required uplift with preservation of several tons of kilometers of the uppermost mantle lithosphere. Regional Geology and GeophysicsThe Colorado Plateau is a tectonic and physiographic province within the CordiIloran oregon of western North America (Figure 1). It consists primarily of early to middle Protorozoic crust overlain by Paleozoic strata typically about 1-1.5 lcm thick and overlying Mesozoic strata that vary laterally in thickness and degree of preservation. The Colorado Plateau is distinctive because it largely escaped the Mesozoic and Cenozoic deformation and magmatism that affected surrounding areas and because of its high elevation. The Basin and Range tectonic and physiographic province is characterized by both Mesozoic crustal shortening and Cenozoic extension, whereas the Rock), Mountains province is characterized primarily by basement uplifts generally attributed to crustal shortening during the latest Cretaceous and early Tertiary Laramido orogony. 94, 7083-7104, 1989. Bird, P., Stress and temperature in subduerion shear zones: Tonga and Mariana, Geophys. J. R. Astron. Soc., 55, 411434, 1978. Bird, P., Continental delaminatio...

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