The geology and structural significance of the Arch Mountain area, northern Black Mountains, Mohave County, Arizona

Abstract

The Arch Mountain area of northwestern Mohave County, Arizona, is typified by rocks that range from Precambrian to Quaternary 1n age. Arch mountain is a north-trending horst bounded on the west by the Petroglyph Wash graben and on the east by the V1rg1n Basin. A lowangle normal fault, herein named the Arch Mountain detachment fault, is exposed for 6 km along strike just below the ridge crest of Arch Mountain, and 1t Is typified by a 2-20 em thick zone of hematiteImpregnated breccia and cataclasite. The detachment fault places an upper plate of Precambrian gneiss, Paleozoic quartzite, shale and dolomite, and Tertiary intrusive rock on a lower plate composed of quartz monzonite that is geochemically similar to quartz monzonite of the Wilson Ridge pluton. The western margin of the horst is cut by en echelon, west-dipping, high-angle normal faults that step the detachment fault to the west and rotate it to the east. The Ransome fault forms the western boundary of the horst and separates Arch Mountain from Precambrian gneiss and Tertiary volcanics. The eastern margin of the horst is cut by en echelon, east-dipping, highangle normal faults that step the detachment fault to the east and rotate 1t to the west. The Boulder wash fault forms the eastern bOundary of the hOrst and separates Arch Mountain from Tertiary Muddy Creek sediments in the Virgin Basin-Detrital Wash area. ii j The Ransome and Boulder Wash faults may be genetically related to and may have formed synchronously with the left-lateral Lake Mead fault system at approximately 12 Ma. Crosscutting relationships indicate that the Arch Mountain detachment fault is older than the Lake Mead fault system. The Arch Mountain detachment fault is possibly related to the development of the Las Vegas Valley shear zone, and 1t is interpreted to be correlat1Ve w1th the Saddle Island detachment fault. I propose that the Arch mountain detachment fault represents the brittlely deformed, upper crustal part of a regional detachment fault system that has a breakaway zone adjacent to the Grand Wash Cliffs. I interpret the detachment fault and lower plate mylonites at Saddle Island to be the plastically deformed, deep level part of the regional detachment fault system that became exposed by progressive structural denudation, isostatic uplift and erosion. The fault history of the regional detachment system is compltcated and poorly constrained but I suggest the following scenario: ( 1) regional uplift and arch formation south of Lake Mead during the late-Cretaceous to early Tertiary; (2) early to mid Tertiary detachment faulting, development of the Las Vegas Valley shear zone and westward transport of the Paleozoic and Mesozoic rocks represented at Petroglyph Wash, Boulder Canyon, the Frenchman Mountain and Sheep Mountain structural blocks and at the Spring Mountains; (3) continued detachment faulting and westward transport of the River Mountains volcanics, and the development of the Lake Mead fault system wtth related high-angle normal faults. iv INTRODUCTION Location Previous Work TABLE OF CONTENTS