PALEOMAGNETISM AND ROTATION CONSTRAINTS FOR THE MIDDLE MIOCENE SOUTHWESTERN NEVADA VOLCANIC FIELD

Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt within the Basin and Range province and thus provide an interesting opportunity to test for late Cenozoic vertical-axis rotation. Paleomagnetic data from individual ash flow sheets document no significant relative vertical-axis rotation among localities within central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time-averaged mean paleomagnetic direction (D = 351.4-degrees, I = 52.7-degrees, alpha95 = 4.5-degrees) calculated from data from numerous separate rock units suggests that the central SWNVF underwent minimal counterclockwise vertical-axis rotation (R = -7.1-degrees +/- 6.6-degrees) with respect to the North American craton. No clockwise vertical-axis rotation is found to support projection of dextral faults of the Walker Lane beneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. These clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central SWNVF block and more strongly extended areas to the south and southwest of the field. Negligible rotation of 11.45-Ma to 13.25-Ma tuffs relative to the central SWNVF was found at the southeast margin of the field where 90-degree4s clockwise rotation at the northwest termination of the Las Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate 13.25 Ma, and thus dextral shear within this part of the Walker lane belt was not synchronous or connected across the southern margin of the field. Small counterclockwise vertical-axis rotation relative to the craton, as found for the central SWNVF block, might be a regional feature in the western Greater Basin.