Miocene extension and fault-related folding in the Highland Range, southern Nevada: A three-dimensional perspective

Abstract

The Highland Range of southern Nevada contains a major anticline and syncline that constitute the westernmost segments of the Black Mountains accommodation zone in the highly extended Colorado River extensional corridor. The folds are defined by thick tilted sections of Miocene volcanic and sedimentary strata that accumulated immediately prior to and during regional extension. The folds are generally symmetrical, with interlimb angles that exceed 90??, subhorizontal hingelines, and steeply inclined axial surfaces. East- and west-dipping normal faults dominate the west- and east-tilted limbs of the folds, respectively. The limbs of the folds are parts of major half grabens. Tilt fanning within these half grabens and 15 new 40Ar/39Ar dates bracket major extension between about 16.5 and 11 Ma. Tilting of the opposing fold limbs occurred simultaneously and was contemporaneous with extension. The anticline and syncline are therefore interpreted as fault-related extensional folds produced by the partial, along-strike overlap of oppositely dipping normal-fault systems and attendant tilt-block domains. The anticline developed between overlapping listric normal faults that dip toward one another, including the east-dipping McCullough Range and west-dipping Keyhole Canyon faults. Each limb of the anticline is a rollover fold developed in the hanging wall of the inwardly dipping listric normal faults. The syncline formed between overlapping outwardly dipping listric faults, as adjacent fault blocks were tilted toward one another. The dominant folding style was fault-bend folding, with drag-folding and displacement-gradient folding playing subsidiary roles. The anticline and syncline significantly affected depositional patterns, with synextensional units, including two major ash-flow tuffs, thinning toward the anticlinal hinge and thickening toward the synclinal hinge. The Black Mountains accommodation zone is largely composed of intersecting northwest-trending anticlines and northeast-trending synclines, both occurring at a variety of scales depending on the amount of overlap between the opposing normal-fault systems. This three-dimensional geometry contrasts with the typical style of folding in contractional settings and may characterize extensional accommodation zones that include the partial along-strike overlap of multiple, oppositely dipping normal faults. The folds in the Highland Range serve as well-exposed analogues of similar features in hydrocarbon-rich areas of submerged continental margins. ?? 2002 Elsevier Science Ltd. All rights reserved.