KINEMATICS AND SIGNIFICANCE OF MYLONITES IN THE LOWER PLATE OF THE NEWBERRY DETACHMENT FAULT, CLARK COUNTY, NV

The Newberry Mountains are a north-south trending range that lie within the highly extended Colorado River extensional corridor of the Basin and Range province in the southernmost portion of Clark County, Nevada. The Newberry Detachment Fault (NDF) is a Miocene low-angle normal fault that encircles the Newberry Mountains, and is inferred to be associated with a larger detachment fault that encompasses the El Dorado Mountains to the north, the Black Mountains to the east, the Dead Mountains and Sacramento Mountains to the south, and Homer Mountain to the west. This detachment fault and the associated metamorphic core complexes lie within the Whipple tilt-block domain, which exhibits top-to-the-northeast movement.

In the lower plate of the NDF west of the north-south stretch of Highway 163, three non-mylonitic biotite granite facies (fine-grained, coarse-grained, and megacrystic biotite granite) and a mylonitic biotite granite facies (fine-to medium-grained mylonitic biotite granite) have been established. The purpose of this study is to determine the kinematics of the mylonites present in the lower plate of the NDF and test two hypotheses for their origin: that (1) the mylonites are related to the brittle NDF and thus the mylonitic fabric formed during the formation of the local metamorphic core complex, representing extensional strain at deeper crustal levels than the detachment proper, or (2) the mylonitic fabric is unrelated to extension along the NDF and instead formed as a result of internal strains related to the emplacement of the Spirit Mountain pluton.

To determine the origin of these mylonites, geologic mapping of granite facies and zones of deformation intensity was carried out at a 1:6,000 scale, coupled with microstructural analyses. Preliminary observations of mesoscopic kinematic indicators show top-to-the-west movement, which is contradictory to the hypothesis that the mylonites are related to the detachment fault, as the NDF records top-to-the-northeast movement. However, the kinematic indicators included very weak asymmetric porphyroclasts and S-C fabrics and are not a sufficient base for interpretation; the results from the microstructural analyses are needed to reach any substantial conclusions on the origin of the mylonitic fabrics present in the lower plate of the Newberry detachment.