Abstract

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are parts of a 55-km-long, regional detachment fault that may form the principal breakaway for extensional allochthons at this latitude. This fault, herein named the South Virgin–White Hills detachment, comprises the Lakeside Mine, Salt Spring, and the Cyclopic Mine faults, and it extends from the South Virgin Mountains in southeastern Nevada to the central White Hills in northwestern Arizona.

The previously unstudied central segment, the Salt Spring fault, is a 50-m-thick, gently west-dipping cataclastic fault zone that separates Proterozoic crystalline rocks in its footwall from northeast-dipping Tertiary volcanic and sedimentary rocks, including footwall-derived megabreccia, in its hanging wall. The 40Ar/39Ar and K/Ar dates on volcanic rocks in the upper plate constrain the principal phase of extension to between 15.2 and 14.6 Ma, although lesser extension continued to about 11 Ma. Lower-plate crystalline rocks retrograded from granulite to greenschist facies and regional steep foliations were tilted to low dips as a result of Tertiary extension.

All three segments of the regional detachment record top-to-the-west displacement and were active principally during the time interval between 16 and 14 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum at the Gold Butte block in the north to a minimum at the Cyclopic Mine in the south. This along-strike, southward decrease in extensional strain along the South Virgin–White Hills detachment is accompanied by a change in fault and footwall rock type from mylonite along the Lakeside Mine fault (northern segment), to chlorite cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in footwall and fault rocks reflect decreasing exhumation of footwall rocks due to decreased extension from north to south.

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