Reorganization and superposition of late Eocene to Holocene extension and late Cenozoic displacement field conservation on a kinematically linked array of basin-bounding faults, north-central Great Basin.

摘要

Three superposed directions of north-south, east-northeast and west-northwest extension were accommodated during Cenozoic displacement on a complex array of high-angle faults in the Crescent Valley region of north-central Nevada. Contemporary N66°W extension was initiated at about 9 Ma and is responsible for the accumulation of 1.5 to 2.5 km of sedimentary and volcanic rocks in spatially restricted half-grabens during the development of present mountain ranges. Earlier N75°E extension commenced at about 17 Ma and was accompanied by deposition of volcanic and sedimentary sequences that range up to several hundred meters thick. These rocks show only minor offset on syndepositional faults and minor thickness variations across the region. The earliest phase of extension, N10°E, started at or before 34 Ma during localized accumulation of 1.5 to as much as 4 km of volcanic tuff and sedimentary rocks in a west-northwest-trending fault-bounded volcanic center. Outside of the structural depocenter, coeval volcanic and sedimentary rocks are only hundreds of meters thick, with only minor lateral variations. All structures in the fault array were active during each period of extensional deformation. Prominent basin and range structures and topography did not evolve until late Miocene to Holocene extension, suggesting that earlier magnitudes of extension were modest and not associated with high strain-rates. Three-dimensional fault displacement models illustrate that the extensional faults of the Crescent Valley fault system are kinematically coordinated over length scales of kilometers to tens of kilometers. The fault network, consisting of northwest, north-northwest, north-south, north-northeast, and northeast oriented normal and oblique-slip structures, displays substantial fluctuations in the along-strike displacement on individual faults. Cross sections, constructed using geologic maps, gravity inversion depth to basement models, a seismic reflection profile, and recent west-northwest extension direction, indicate that more than 90 percent of the along-strike variations in the magnitude of fault offset is accommodated by displacement transfer within the fault system. The remaining deformation is likely attributed to uncertainty in the model, but may represent non-recoverable strain accumulation within fault blocks. Our analysis indicates that in order to completely conserve the fault displacement field, fault systems of length scales greater than 50 km are needed.