GEOMETRY OF A DETACHMENT FAULT IN THE MONTGOMERY MOUNTAINS, SOUTHERN NYE COUNTY, NEVADA, AND RELATION TO YOUNGER AND OLDER FAULTS

Based on new mapping in the hills northwest of Pahrump, Nevada, we recognize the "Six Mile thrust" of Burchfiel and others (1983) as a Miocene detachment fault and reinterpret the geometry of Mesozoic thrust faults of this range, which were originally interpreted as one east vergent thrust that postdated the Six Mile fault. The Six Mile fault is a subregional-scale, low-angle fault that places younger rocks over older rocks, and locally has garnet-bearing schists in its lower plate. The upper plate consists of predominantly non-metamorphosed versions of the same Paleozoic/Eocambrian sedimentary formations found in the lower plate. A syntectonic basin-fill sequence consisting of poorly consolidated alluvium, less abundant rock-avalanche breccias, and a few thin water-lain tuff beds was deposited on the upper plate. Pumice fragments from a tuff in the upper part of the sequence yielded an Ar⁴⁰/Ar³⁹ date of 9.15 Ma. To the north, the Six Mile fault is continuous with the Point of Rocks detachment fault (Abolins, 1999), which forms the northwest boundary of the Spring Mountains, and which is constrained by dated tuffs in pre- and post-detachment sediments as having moved between 7.5 and 13 Ma. Both segments of the detachment have a west to northwest, inferred transport direction. At its southern end, the Six Mile detachment merges into a west-northwest-trending right-lateral strike-slip fault. This high-angle fault was active during movement on the detachment and is an inactive splay off of the northwest-trending Pahrump-Stewart Valley fault system. The upper plate basin-fill sequence is apparently offset ~3.5 km in a right-lateral sense across an active strand of the Pahrump-Stewart Valley fault system and stranded by younger subsidence in Stewart and Pahrump Valleys.