William A. Rehrig, 1996. "A Field Trip Overview to Low-Angle Faults and Associated Mineralization", Tertiary Extension and Mineral Deposits, Southwestern U.S., William A. Rehrig, James J. Hardy
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The Cordillera of western North America may succinctly be described as an accreted continental margin consisting of obducted and stacked oceanic and miogeoclinal plates (Late Precambrian to Mesozoic), which has been overprinted by extraordinary Tertiary extension, particularly in middle North American. Both compressional and extensional processes have utilized gently dipping (low-angle) structural features, which include thrust faults, detachment faults, and rotated or listric normal faults. The thrusts commonly are mylonitic, indicating ductile shear at some depth. The detachments are marked by mylonitic and later brecciated footwall fabrics, suggesting deep and shallow deformation.
Such low-angle zones of shear, while not unique to the southwestern United States, are especially well exposed there. On this field trip we will have the pleasure of viewing key outcrops over a broad region of southern Nevada and western Arizona. These exposures are particularly instructive not only as examples of unique tectonic processes, but also with regard to mineral deposits hosted by such low-angle faults.
Figures & Tables
Starting in Las Vegas, we will traverse through many of the geometric elements and complexities of hanging wall deformation above the regional detachment systems of the Colorado River extensional terrane. We will study the interaction of normal faults as arranged in regional, crustal-scale mega-domains and the bounding structures that separate these tilt domains. As we progress through the classic Eldorado Mountains-Hoover Dam region, where many of the ideas of listric normal faulting were first popularized, we will see both the real rocks and the historic rationale for their deformation. By examining the listric versus domino models for normal faulting, we will utilize different geometric techniques for determining the depth to the detachment structures and percent extension. Continuing further south toward southernmost Nevada, we will cross the accommodation zone that separates the Lake Mead and Whipple dip domains and further descend to deeper structural levels to examine lower levels of the major normal faults and their tilting of upper-crustal blocks and associated offset along the regional detachment faults. Fluid flow within the shattered fault zones and its relationship to the 3-D geometries of the fault surfaces will be studied both along the faults and within the hydrothermally altered and mineralized wallrocks.
(From Las Vegas proper, drive east on Tropicana to Pecos, south to Russell, then continue east to Southern Nevada Vocational Technical Center. Follow signs to its entrance then drive around edge of campus to dirt road on northern edge, which provides elevated view toward the north to Frenchman Mountain. View is much better in the afternoon than in the morning.)
Stop 1-1: Frenchman Mountain Tilt Block
(Return to Russell Road, turn right to freeway (1.4 miles) and south along freeway to Boulder City. Take truck route, Buchanan Blvd., around Boulder City and turn off highway at “Lakeview Overlook,” about 5 miles pastBoulder City for view and discussion of Lake Mead shear zone.)
Stop 1-2: Lake Mead Shear Zone and Associated Alteration by Hydrothermal Fluids
(Return to Highway 93 and continue descent to Hoover Dam. Park in new parking structure and walk to gorge overlook about 100 meters up the road on the Nevada side for best view of structural features.)
Stop 1-3: Extension Within the Volcanic Rocks Exposed at Hoover Dam
(Return to Boulder City along Highway 93 and to turnoff to Highway 95 south toward Searchlight. Note extreme alteration at junction of Highways 93 and 95. Continue south