TECTONICS, VOL. 13, NO. 2, PAGES 258-277, APRIL 1994 Paleomagnetismand rotation constraintsfor the middle Miocene southwestern Nevada volcanic field Mark R. Hudson andDavid A. Sawyer U.S. Geological Survey, Denver, Colorado Richard G. Warren LosAlamos NationalLaboratory, LosAlamos, New Mexico Abstract. Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt withinthe Basin andRange province and thus provide an interesting opportunity to test for late Cenozoic vertical-axis rotation. Palcomagnetic data from individual ash flow sheets document no significant relative vertical-axis rotationamong localitieswithin central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time-averaged mean paleomagnetic direction (D = 351.4 ø, I = 52.7 ø, ot95 = 4.5 ø) cal- culated from datafrom numerous separate rockunitssuggests that the central SWNVF underwent minimal counterclockwise vertical-axis rotation (R = -7.1 oñ 6.6 o)with respect to theNoah American craton. No clockwise vertical-axis rotation is found to support projection of dextral faults of the WalkerLanebeneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. Theseclockwise rotations probably reflectdextral shear strain developed at the interface between the little emended central SWNVF block and more strongly extended areas to the south andsouthwest 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 ø clockwise rotation at the northwest termination of theLas Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate13.25 Ma, and thus dextral shear withinthispartof the WalkerLanebelt wasnot synchronous or connected across the southern marginof the field. Small coun- terclockwise vertical-axis rotation relative to the craton, as found for the central SWNVF block, mightbe a regional feature in the western Great Basin. Introduction Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) [Byers et al., 1976, 1989] lie across the projec- tionof theWalkerLanezone withintheBasin andRange prov- ince (Figure 1) and thus providean opporttmity to constrain paleomagnetically whether the SWNVF underwent vertical-axis rotationduring late Cenozoic extension within the southern Great Basin. Recent palcomagnetic studies have revealed areas affected by vertical-axis rotation of known or presumed Ceno- zoic age within the province [Gillerr and Van Alstine, 1982; Hudson and Geissman, 1987, 1991; Nelson and Jones, 1987; Copyright 1994by theAmerican Geophysical Union. Papernumber93TC03189. 0278-7407/94/93TC-03189510.00 Hagstrum and Gans; 1989; Wells and Hillhouse, 1989; Li et al., 1990; Rosenbaum et al., 1991; Palmer et al., 1991]. The amount, areal extent,and age of such rotations are still incom- pletely constrained because published paleomagnetic studies pertain to smallparts of the GreatBasin,andbecause the geo- logicexpression of vertical-axis rotations is commonly cryptic. Consequently, the structural mechanisms responsible for such rotations are not well documented. Cenozoic volcanic rocks that were deposited within the province affordmanyopportunities for further paleomagnetic studies to address these uncertainties. Paleomagnetic data obtained from volcanic rocks are used in two ways to test for tectonic rotations. Reinanent directions from a number of sitesfrom a singlewidespread volcanic unit canbe compared to assess relative rotations if the unit acquired its magnetization quickly with respect to paleosecular variation of the geomagnetic field [Magi# et al., 1982; Wells and Hillhouse, 1989]. Such comparisons can yield precise estimates of rotations because they circumvent uncertainties produced by directional dispersion due to paleosecular variation. Without independent information, however, it is not possible to deter- mine which sites actually rotated with respect to the stable cra- ton. Moreover, such rotation estimates maybe strongly affected by anysite-specific errors such as unit misidentification or inac- curate tilt correction. An alternative method to assessrotation is to calculate a mean remanence direction from a number of tem- porally discrete volcanic units from a localityto average the paleosecular variation. This time-averaged directionis then compared to an expected direction calculated froman appropri- ate cratonic paleopole. Rotationestimates of this type com- monlyare not as precise as relativerotations determined from individual units unless theyincorporate datafroma large num- ber of volcanic unitsbecause they must average paleosecular variation. Rotations basedon time-averaged directions, how- ever, have the advantage of being "absolute" in senseand magnitude relativeto the craton and of beinglessaffected by random site-specific errors. For thisstudy, paleomagnetic datafromthe SWNVF areused to assess vertical-axis rotation using both of these methods. Emphasis is placedon data from the central,less deformed region of the volcanic field to calculate a time-averaged mean direction for comparison with an expected direction for the sta- ble North American craton. This comparison indicates that the central 2400 km 2 ofthe SWNVF experienced only minor verti- cal-axis rotation during thepast15 m.y. Directions of remanent magnetizations fromindividual ashflow cooling units fromthis central area are subsequently used to assess tectonic rotations in themarginal, more deformed parts of the volcanic field.