Chapter 3: 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
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 km2 of east-tilted fault blocks to the north from 25,000 km2 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.