PALEOMAGNETIC EVIDENCE FOR THE AGE AND EXTENT OF MIDDLE TERTIARY COUNTERCLOCKWISE ROTATION, DIXIE VALLEY REGION, WEST CENTRAL NEVADA

Paleomagnetic data obtained from Oligocene to lower Miocene igneous rocks and middle Miocene basaltic rocks at fifteen localities from a region surrounding Dixie Valley in west central Nevada indicate that parts of the area experienced counterclockwise vertical-axis rotation, and these data provide constraints on the extent and timing of rotation. Counterclockwise vertical-axis rotation probably exceeding 30-degrees is indicated for Oligocene to lower Miocene rocks in the central part of the study area. Calculated rotations increase with increasing age of the Oligocene to lower Miocene rocks, indicating that vertical-axis rotation was concurrent with ash-flow deposition (about 23-33 Ma). Paleomagnetic data indicate that middle Miocene basaltic rocks (about 10-17 Ma) postdate significant counterclockwise vertical-axis rotation, although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation. Counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area. The total amount of crustal rotation in Tertiary time in the area is poorly known. Rotation estimates calculated from time-averaged mean directions that incorporate data from a broad range of the Oligocene to lower Miocene units probably underestimate total Tertiary rotation because the mean directions include data from units that postdate much of the rotation. For example, a rotation estimate for a composite sequence from the central part of the study area is -23-degrees +/- 15-degrees, whereas estimates from older and younger halves of the sequence are -37-degrees +/- 21-degrees and -11-degrees +/- 16-degrees, respectively. Paleomagnetic data indicate that Oligocene to lower Miocene rocks at some localities in the northern and southern parts of the study area (e.g., the Golconda Canyon locality) probably did not experience significant Tertiary counterclockwise rotation. Although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to Tertiary counterclockwise rotation, the data suggest that a discrete subregion was affected rather than the entire Basin and Range province.