THE HENDERSON FAULT AND TECTONIC ERRATICS AT LONE MOUNTAIN AND DEVILS GATE, EUREKA COUNTY, NEVADA

The Roberts Mountains region has undergone multiple deformational phases including the Paleozoic Antler Orogeny, which formed the Roberts Mountains thrust (RMT). Previous interpretations of the study area suggest that subsequent structural quiescence prevailed during Mesozoic time prior to emplacement of Jurassic plutons and sills. Later, during the Cenozoic, extensional faults segmented the RMT and formed the Basin and Range topography of the region. New data from the study area indicate that regional, Mesozoic faulting displaces the RMT and is oriented along a similar north-south strike. Finney et al. have defined this as the Henderson fault and mapped it throughout the Roberts Mountains and neighboring ranges (Finney, pers. comm., 2013). The Henderson fault is characterized by repetition of the RMT, a thick deformation zone and brecciated fault rock. A combination of the structurally repeated RMT and erosion of the RMT upper plate has resulted in many isolated carbonate outcrops. These carbonate outliers comprise the upper-plate of the Henderson fault and represent tectonic erractics. Cross-cutting relationships and preferentially oriented asymmetric folds, observed throughout the Roberts Mountains area, are evidence of Mesozoic eastward vergence of the Henderson fault. Previously, carbonate outliers that form Lone Mountain and Devils Gate, approximately 20 km south of the Roberts Mountains, were interpreted as tectonic windows through the Roberts Mountains allochthon. Instead, these two outcrops represent tectonic erratics transported in the upper-plate of the Henderson fault that were preferential eroded out of the upper-plate. The Henderson fault at Lone Mountain and Devils Gate is a sub-horizontal fault that displaces and repeats tectonostratigraphic units of the RMT. Here, carbonate outliers are underlain by a 20 to 200 meter thick deformation zone, the Henderson fault and the Roberts Mountains allochthon. This relationship also explains the shallow petroleum accumulation in the nearby Blackburn oil filed, 25 km north of the Roberts Mountains, where Henderson fault deformation has created suitable conditions for oil generation and entrapment.