Abstract

The Northern Virgin Mountains of northwestern Arizona and adjacent Nevada are an arc-shaped anticlinal uplift that contains marine Paleozoic shelf sedimentary rocks flanking a central core of Precambrian metamorphic rocks. Mesozoic and Tertiary rocks, including Upper Cretaceous beds previously unknown in northwestern Arizona, are present in downfaulted blocks around the edges of the range. Except for marine Lower Triassic beds and Tertiary lacustrine limestones, they consist largely of clastics of continental origin.

Tertiary and Laramide uplift in the Virgin Range resulted in gravitative adjustments that took the form of major glide plates. Gravity structures of different styles originated from each of three major uplifts.

The oldest uplift, the asymmetric Virgin Mountain Anticline, formed during the Laramide orogenic interval by draping of the sedimentary cover over a basement horst that shows evidence of recurrent activity from Precambrian through Recent time. The northeast-trending anticline produced a structural and topographical high from which the Paleozoic cover slid, by gravity, south-eastward down a slope formed by bedding. By moving across successively younger beds of the southeastern limb, older rocks in the upper plate came to rest on younger. Compression generated during movement created tight folds in the overridden strata and within the glide plate itself. Most important of these is an overturned syncline beneath the glide plate that extends for a distance of 12 mi parallel to the axis of the Virgin Mountain Anticline.

Two younger uplifts were formed by the Virgin Mountain fault system on the west border of the range and by the State Line fault on the east border. The strikes of both faults apparently were partly controlled by the structural grain of the basement; the faults collectively produced the late Tertiary uplift of the Virgin Mountain Anticline as a wedge-shaped horst. Along the State Line fault, uplift and westward tilting created a structural high from which a 25-sq-mi plate of sedimentary cover became detached and moved westward down a slope, subparallel to bedding, thereby truncating all older structures in both upper and lower plates. A horst uplifted on two large faults belonging to the Virgin Mountain fault system formed the third structural high in the Virgin Range. Six glide plates, apparently originating from this high, moved westward on bowl- and scoop-shaped faults across steeply dipping strata of the northwestern limb of the Virgin Mountain Anticline. The glide plates both truncate and are cut by faults associated with the frontal Virgin Mountain fault, which suggests that sliding was concurrent with and in response to movements on them.

The main distinction between the Laramide and late Tertiary gliding is the nature of the causal uplift and degree to which older rocks were emplaced above younger ones. Gliding from the crest of the Virgin Mountain Anticline under considerable cover resulted in plastic flow, which allowed tight drag folding, the formation of a sharp glide surface, and the emplacement of older rocks above younger ones. Shallower gliding from late Tertiary fault blocks permitted brecciation, minor folding, and resulted mainly in the movement of younger rocks to positions above older ones.

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