GEOLOGIC MAP OF THE EASTERN RAND THRUST COMPLEX IN THE MOJAVE DESERT PRESENTING DETAILED FAULT REACTIVATION AND BRITTLE FAULT OVERPRINTING

We present a 1:3000 scale geologic map of the Rand Thrust Complex (RTC) in the eastern Rand Mountains. Work in Sections 19-20, T30S, R40E of the Johannesburg 7.5 minute quadrangle is ongoing to deduce the mechanisms, extent and timing of fault displacements, and to better understand the regional geologic context. A GPS receiver refined the location of previously mapped geologic contacts and facilitated precise positioning of new outcrop observations and structural measurements.

The RTC consists of four tectonic plates separated by low angle faults. All plates are well exposed within the map area along the S-dipping flank of an E-trending antiform. The deepest unit is Rand Schist (Plate 1): greenschist facies metagraywacke, metachert and metabasalt. Plate 1 is structurally overlain by Johannesburg Gneiss (Plate 2): garnet-amphibolite grade granitic and dioritic orthogneiss, marble and calcsilicate. A Late Cretaceous (87 Ma) hornblende-biotite-sphene granodiorite (Plate 3; Atolia Granodiorite) is sandwiched between Plate 2 and structurally higher biotite alkali granite (Plate 4). The basal contact of Plate 4 is marked by a qtz-rich ultramylonite zone that locally truncates fault contacts between the lower plates.

Structural data collected from mylonitic and brittle fault fabrics documents multiple movements that post-date metamorphism of Rand Schist at ~80 Ma. The earliest shearing episode records transport of Plate 3 over Plate 1 with intervening Plate 2 retrograded to greenschist facies and mylonitized to produce NW-trending lineation. A later ductile shearing episode is recorded by shallow NE-dipping mylonites of Plate 4 that display consistently oriented lineation with associated microstructures and asymmetric folds indicating top-to-the-NNE shear. Younger brittle structures that crosscut these low-angle faults include a ENE-dipping normal fault associated with quartz breccia and limonitic striations and two WNW-dipping sinistral-oblique normal faults that displace the west end of the RTC and cause significant CW rotation of older lineations. The observed crosscutting structural pattern is consistent with elements of Mojave Desert tectonic history: Late Cretaceous crustal shortening, Miocene extension, then Late Cenozoic left-lateral faulting likely associated with the Garlock fault.