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All normal-fault systems must terminate both along and orthogonal to strike. As many as four terminations may be associated with a single culmination. Most normalfault systems terminate in either transfer zones or accommodation zones. In the nongenetic classification proposed here, transfer zones are defined as discrete zones of strike-slip and oblique-slip faulting that generally trend parallel to the extension direction and typically facilitate a transfer of strain between extended domains arranged in an en echelon pattern. Accommodation zones are belts of overlapping fault terminations and can separate either systems of uniformly dipping normal faults or adjacent domains of oppositely dipping normal faults. They can trend parallel, perpendicular, or oblique to the extension direction. A review of variously extended continental provinces and passive continental margins reveals that the style of deformation within transfer and accommodation zones is independent of the magnitude of extension.Strike-slip and oblique-slip faults within transfer zones are closely linked kinematically with major normal faults within the extended terranes. Transfer zones linking spatially separated loci of extension display along-strike variations in both magnitude and sense of motion, whereas local normal and reverse faults may develop in the vicinity of releasing and restraining fault bends. Strain within accommodation zones is transmitted directly between normal-fault systems, geometries being controlled by the amount of overlap between and relative dip direction of competing sets of normal faults. Antithetic accommodation zones develop between oppositely dipping normal-fault systems, whereas synthetic accommodation zones occur between similarly dipping systems. In the synthetic zone, relay ramps commonly connect the hanging wall of one fault to the footwall of another fault and trend obliquely in zones that have significantly overlapping normal faults but transversely (parallel to extension direction) in zones that have minimal overlap. Antithetic zones exhibit a wider variety of geometries, including (1) strike-parallel (parallel to trend of rift) anticlines and synclines between normal-fault systems having complete overlap that dip toward and away from one another, respectively, (2) obliquely trending anticlines and synclines in areas of partial fault overlap, and (3) transverse zones between minimally overlapping fault systems. The distinction between the strike-parallel and transverse accommodation zones is scale dependent; large strike-parallel segments are characterized by finescale offsets of hingelines along small transverse segments, and large transverse zones Faulds, J. E., and Varga, R. J., 1998, The role of accommodation zones and transfer zones in the regional segmentation of extended terranes, in Faulds, J. E., and Stewart, J. H., eds., Accommodation Zones and Transfer Zones: The Regional Segmentation of the Basin and Range Province: Boulder, Colorado, Geological Society of America Special Paper 323. 1 on March 24, 2...

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.

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We report strontium isotopic results for the late Miocene Hualapai Limestone of the Lake Mead area (Arizona-Nevada) and the latest Miocene to early Pliocene Bouse Formation and related units of the lower Colorado River trough (Arizona-CaliforniaNevada), together with parallel oxygen and carbon isotopic analyses of Bouse samples, to constrain the lake-overfl ow model for integration of the Colorado River. Sr iso topic analyses on the basal 1-5 cm of marl, in particular along a transect over a range of altitude in the lowest-altitude basin that contains freshwater, brackish, and marine fossils, document the 87 Sr/ 86 Sr of fi rst-arriving Bouse waters. Results reinforce the similarity between the 87 Sr/ 86 Sr of Bouse Formation carbonates and present-day Colorado River water, and the systematic distinction of these values from Neogene marine Sr. Basal Bouse samples show that 87 Sr/ 86 Sr decreased from 0.7111 to values in the range 0.7107-0.7109 during early basin fi lling. 87 Sr/ 86 Sr values from a recently identifi ed marl in the Las Vegas area are within the range of Bouse Sr ratios. 87 Sr/ 86 Sr values from the Hualapai Limestone decrease upsection from 0.7195 to 0.7137, in the approach to a time soon after 6 Ma when Hualapai deposition ceased and the Colorado River became established through the Lake Mead area. Bouse Formation δ 18 O values range from -12.9‰ to +1.0‰ Vienna Pee Dee belemnite (VPDB), and δ 13 C between -6.5‰ and +3.4‰ VPDB. Negative δ 18 O values appear to require a continental origin for waters, and the trend to higher δ 18 O suggests evaporation in lake waters.Sr and stable isotopic results for sectioned barnacle shells and from bedding planes of the marine fi sh fossil Colpichthys regis demonstrate that these animals lived in saline freshwater, and that there is no evidence for incursions of marine water, either long-lived or brief in duration. Lack of correlation of Sr and O isotopic variations in the same samples also argue strongly against systematic replacement of Sr in Bouse carbonates after deposition. Our results reinforce the conclusion that the Bouse Formation was deposited in a descending series of basins connected by overfl ow of Colorado River water. The Hualapai Limestone records a separate and earlier lake that may have been progressively infl uenced by Colorado River water as the time of river integration approached.

Extensional accommodation zones, or tilt-block domain boundaries, facilitate reversals in the dominant tilt direction of fault blocks and possibly inversions in the dip of regional detachment systems in rifted continental crust. The amount and direction of movement of the footwall (lower plate) and hanging wall (upper plate) of the detachment terrane dictate the deformational style along accommodation zones. Various models of extension can potentially be evaluated by defining modes of deformation along accommodation zones.A 40-km-long, east-west-trending, middle Miocene accommodation zone bisects the central Black Mountains, northwestern Arizona, and southern Eldorado Mountains, southern Nevada. The Black and Eldorado Mountains lie within the northern Colorado River extensional corridor, a 50-to 100-km-wide region of severely extended crust. The generally sublinear, 5-to 10-km-wide accommodation zone separates more than 5,000 km 2 of east-tilted fault blocks to the north from 25,000 km 2 of dominantly west-tilted fault blocks to the south. The zone may also mark the join between regionally extensive, oppositely dipping detachment systems.Transversely oriented segments (i.e., perpendicular to strike of tilted blocks) of the accommodation zone in the upper-plate rocks correspond to areas of intermeshing conjugate normal faults. East-and west-dipping normal faults dominate the west-and east-tilted domains, respectively, whereas east-and west-dipping faults are equally common in the axial part of the zone. Some of the major normal faults in the west-and east-tilted domains terminate in drag folds within the axial part of the zone. Fault-block tilting on either side of the accommodation zone commonly exceeds 60°. Tilting decreases progressively toward the axis of the zone, where transversely oriented, obliqueslip normal faults accommodate scissors-like torsional offset between gently tilted (10 to 35°) individual fault blocks of opposing polarity. Concomitant with the decrease in tilting, fault spacing decreases, and average fault dip increases. Fault blocks within the zone were periodically tilted in opposite directions during the same episode of extension.Minor amounts of open to tight folding characterize along-strike segments (i.e., parallel to strike of tilted blocks) of the accommodation zone. •Present address: Structural development of a major extensional accommodation zone in the Basin and Range Province, northwestern Arizona and southern Nevada; Implications for kinematic models of continental extension, in Wernicke, B. P., ed., Basin and Range extensional tectonics near the latitude of Las Vegas, Nevada: Boulder, Colorado, Geological Society of America Memoir 176. J. E. Faulds and OthersThe lack of strike-slip faulting along transversely oriented segments and only minor amounts of compression on along-strike segments of the accommodation zone indicate little relative movement between opposing tilt-block domains. The transversely oriented, oblique-slip normal faults in the zone facilitated torsional offset, a...

The South Virgin–White Hills detachment (SVWHD) in the central Basin and Range province with an along‐strike extent of ∼60 km is a major continental detachment fault system. Displacement on the SVWHD decreases north to south from ∼17 to <6 km. This is accompanied by a change in fault and footwall rock type from mylonite overprinted by cataclasite to chlorite cataclasite and then fault breccia reflecting decreasing fault displacement and footwall exhumation. Apatite fission track (AFT) thermochronology was applied both along‐strike and across‐strike to assess this displacement gradient. The overall thermal history reflects Laramide cooling (∼75 Ma) and then rapid cooling beginning in the late early Miocene. Age patterns reflect some complexity but extension along the SVWHD appears synchronous with rapid cooling initiated at ∼17 Ma due to tectonic exhumation. Slip rate is more rapid (∼8.6 km/Ma) in the north compared to ∼1 km/Ma in the south. The displacement gradient results from penecontemporaneous along‐strike motion and formation of the SVWHD by linkage of originally separate fault segments that have differential displacements and hence differential slip rates. East–west transverse structures likely play a role in linkage of different fault segments. The preextension paleogeothermal gradient is well constrained in the Gold Butte block as 18–20°C/km. We present a new thermochronologic approach to constrain fault dip during slip, treating the vertical exhumation rate and the slip as vectors, with the angle between them used to constrain fault dip during slip through the closure temperature of a particular thermochronometer. AFT data from the western rim of the Colorado Plateau constrain the initiation of timing of cooling associated with the Laramide Orogeny at ∼75 Ma, and a reheating event in the late Eocene/early Oligocene associated with burial by sediments (“rim gravels”) most likely shed from the Kingman High to the west of the plateau.