Chapter 4: The Saddle Island detachment; An evolving shear zone in the Lake Mead area, Nevada

The Saddle Island detachment fault is a major, and possibly the dominant, structural feature in the Lake Mead area, Nevada. Exposures on Saddle Island contain many of the characteristic elements of metamorphic core complexes of the Colorado River trough detachment terrane, including lower-plate mylonitic rocks, a detachment fault marked by chloritic phyllonite and microbreccia, and a brittlely deformed upper plate. The Saddle Island detachment is regionally important because it is the only well-documented exposure in the Lake Mead region of a deep-level, crystalline lower plate. The upper plate of the detachment is divided into three lithologically distinct, lens-shaped domains bounded by low-angle normal faults. The domains are lithologically heterogeneous and contain Precambrian crystalline rocks, lower Paleozoic sedimentary rocks, sedimentary rocks of the Tertiary Horse Spring Formation, and Tertiary intrusive rocks. The lower plate of the detachment consists principally of variably mylonitized Precambrian amphibolite and quartzofeldspathic gneiss. Subhorizontal mylonitic foliation broadly parallels the detachment. Contacts between variably mylonitized rock are typically low-angle chlorite phyllonite fault zones. The lens-like character of lithologic packages in the lower plate resembles large-scale structure within the upper plate. Kinematic analysis indicates top-to-the-west shear for mylonites and all low-angle faults within the complex. Systematic changes in retrograde mineralogy that accompany textural changes in fault rocks suggest that structures associated with the detachment represent a continuum of deformation at progressively lower temperatures (i.e., shallower crustal levels). The similarity in orientation and inferred shear sense of all fault zones suggests that they developed during a single deformational event. These geologic relations are best explained by single-stage deformation associated with an evolving, crustal-scale, normal-sense-displacement simple-shear zone. Regional geochemical correlations allow reconstruction of structurally disrupted mid-Miocene volcanic-plutonic complexes in the Lake Mead area. The sense of movement required for restoration of these complexes is compatible with kinematic indicators within the Saddle Island detachment zone. These combined data suggest 20 km of westward translation of upper-plate rocks along the detachment. The inferred 20-km displacement along the detachment argues against in situ crustal extension (pure-shear models). Available age data suggest that the Saddle Island detachment is younger than 13.5 Ma; the major period of movement along the detachment must have occurred prior to deposition of the Muddy Creek Formation (5 to 9 Ma). Low-angle faults exposed east of Saddle Island between Lake Mead and Detrital Wash may be correlative with the Saddle Island detachment. The lack of mylonitic fabric in lower-plate rocks to the east can be explained by the eastward-shallowing of the regional detachment structure.