CENOZOIC EXTENSION AND STRIKE SLIP TECTONICS IN THE MOUNT IRISH RANGE, NEVADA; CHANGES IN STRESS FIELDS THROUGH TIME

Multiple faulting events have shaped the geology of the Western Mount Irish Range in Southern Nevada. Our purpose is to document and analyze the exposed Cenozoic faults in the area. We discovered a complex series of normal and transverse faults. Timing and age relationship between north-striking normal faults and east-striking transverse faults are well documented through 1:24000 scale mapping. Cross-cutting relationship indicate the transverse faults are younger than the normal faults. The youngest rocks cut are Oligocene-Miocene volcanic rocks, but the faults are poorly exposed in these units. The fault exposures in the Paleozoic section are excellent, and show the strike slip lineations on the surfaces of the transverse faults. Normal Faults also have lineations showing a relative direction of movement. Data shows that the transverse faults show a down to SW (240-250°) or down to NE (30-40°) movement, suggesting a dextral rotation about a vertical axis of down-dropped blocks. The normal faults generally dip to the east at 60-65°, while the transverse faults have much more erratic dip, suggesting a more scalloped surface associated with an anastomosing system of faults. Quaternary/Tertiary fans cover many structures and are not faulted, but Quaternary faults have been observed in the area.

These distinct normal faulting and strike slip events in the area may be a result of 2 or more periods of tectonism or could possibly be a result of slip being transferred onto older fault surfaces. Fault systems with different ages and that differ drastically in strike (to 90°) indicate a change in the orientation of the stress fields over time. We conclude that changing stress fields, and/or multiple extensional periods, may have produced faults that have either formed individually or reactivated preexisting fault surfaces. This interpretation is consistent with documented fault sets ~ 50km to the east in the Hiko Range, and ~15 km to west in the Timpahute Range. This consistency suggests a regional pattern of normal faulting followed by strike slip tectonism.