Structural geology of the upper plate of the Bullfrog Hills detachment fault system, southern Nevada

Abstract

An extremely distended terrane containing two detachment faults and an overlying complex of normal faults is exposed in the Bullfrog Hills, southern Nevada. Shallow crustal rocks have been extended along the detachment faults by listric and planar-rotational normal faults. The detachment faults define three structurally discordant plates. The lower detachment fault separates a lower plate of metamorphosed Late Proterozoic rocks from an overlying middle plate, composed of slivers of lower and middle Paleozoic clastic and carbonate rocks. The middle-plate rocks are brecciated and essentially unmetamorphosed, and the stratigraphic succession is incomplete and highly attenuated. The upper detachment fault separates the middle-plate rocks from an upper-plate succession of block-faulted Miocene volcanic, volcaniclastic, and sedimentary rocks. Miocene rocks of the upper plate dip at moderate to steep angles into the upper detachment fault, or, where the middle plate has been tectonically removed, into the lower detachment fault. The rocks are broken, tilted, and repeated in blocks bounded by normal faults that terminate against, or flatten and merge into, the upper detachment fault or, where the middle plate has been removed, the lower detachment fault. The normal faults in the upper plate are (1) planarrotational faults that form imbricate map patterns and (2) listric faults that are characterized by oval and horseshoe map patterns. These fault map patterns may be due to (1) curvilinear intersection of a listric or planar fault with an antithetic fault, (2) complex intersection of two or more faults of different ages, (3) rotated listric faults, (4) extremely rotated planar normal faults that resemble listric faults, (5) nearly flat-lying normal faults, or (6) topographic and erosional effects. Attenuation of the Late Proterozoic and Paleozoic strata indicates large movement on the detachment faults; the upper plate has been extended more than 100% and possibly more than 275% locally. The geometry of the normal faults and the repetition and dip direction of the Miocene rocks indicate that major extension, at least of the upper plate, was west-northwest-east-southeast and occurred mostly between about 10 and 8 Ma.