Paper No. 3
Presentation Time: 9:30 AM


DUNHAM, Jeffery and OLDOW, John S., Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080,

North-central Nevada is characterized by a complex array of extensional basins bounded by high-angle faults with three dominant orientations of east-west, north-northwest, and northeast. Deposition of Eocene to Holocene volcanic and sedimentary rocks was preferentially and successively localized in the basins of different orientation. Extension on east-west trending faults controlled the regionally extensive development of volcanic-tectonic troughs during the Eocene-Oligocene, north-northwest trending faults and a parallel dike complex record early to mid-Miocene extension during the growth of the Northern Nevada Rift, and northeast fault-systems controlled late Miocene to Holocene formation of basin and range structures. Fault-slip inversion on faults of all three orientations in the region centered on Crescent Valley of north-central Nevada documents three separate and superposed axes of extensional strain. With decreasing age, regional extension-axes rotated from north-south to east-northeast followed by a change to west-northwest. North-south extension initiated during the Eocene and resulted in large-magnitude dip-slip motion on east-west trending faults. Fault slip changed at ~19 Ma and dip-slip was localized on north-northwest faults. At ~10 Ma, fault slip again was reorganized and dip-slip displacement was dominant on northeast-striking faults in a direction that coincides with contemporary deformation recorded by GPS geodesy and earthquake focal mechanisms. The record of superposed slip on faults of all orientations indicates that the three primary orientations of faults has persevered since the Eocene and did not develop progressively as the extensional strain-axis changed through time. The implication is that the complex fault geometry of the region is not solely a consequence of a progressive change in the state of strain, but rather formed in response to a pre-existing structural grain imparted by a history of late Precambrian rifting and subsequent mid-Paleozoic to Mesozoic contractional deformation.