Magnetic properties of mid-Tertiary volcanic rocks west of Jiggs in northeast Nevada were investigated for the purposes of interpreting igneous, structural, and tectonic processes in this part of the Basin and Range province. Anisotropy of magnetic susceptibility (AMS) patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs. Paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north. New stratigraphic, chemical and K-Ar isotopic data support these interpretations. Dacite to rhyolite ash flow tuffs of the Indian Well Formation were subdivided into two main units: the lower and predominant tuff of Jiggs (35.8--37.0 Ma) and the unconformably overlying but areally restricted tuff of Hackwood (30.8 Ma). The Jiggs unit has two polarities whereas the Hackwood has only a reversed polarity remanence. Together these units with tilt correction define a pole (92¿E, 87¿N, dp=6¿, dm=8¿) for approximately 30--37 Ma. This pole is concordant with coeval North American reference poles, indicating that this zone of approximately 30 km NS extent has not undergone significant vertical axis rotation relative to the North American reference. Andesite lavas of normal magnetic polarity and of 32.5-Ma age characterize the Diamond Hills immediately to the south.

We interpret this region, from both structural evidence and discordant paleomagnetic direction, to have rotated approximately 25¿ counterclockwise relative to the Indian Well volcanic units to the north. The apparent rotation of the Diamond Hills is possibly the result of drag on the left-lateral Garcia fault which limits the Diamond Hills on the southwest. Analysis of AMS data suggests, by patterns of the K1 axes, two distinct sources for the Jiggs unit: a northern buried source and a central partially buried source. Lithologic evidence consistent with proximal vent facies is found near the latter source. An unusual pattern in the AMS K3 axes, suggested as resulting from the tilt correction applied to initial dip, is consistent with the interpreted flow directions. Tight age constraints can be placed on the deformation of the main ash flows of the Jiggs unit, in the 35.8- to 30.8-Ma interval. Because the Diamond Hills unit is slightly younger and appears to have been affected by the same deformative episode as the Jiggs, the regional deformation affecting both units can be more narrowly confined to the 32.5--30.8 Ma interval. This deformation is inferred to coincide with the major early Tertiary episode of crustal extension in this region of the Basin and Range. ¿1991 American Geophysical Union