Egan Range Volcanic Complex, Nevada: geochronology, paleomagnetism and magnetic fabrics

Abstract

In a relatively brief interval near the Eocene-Oligocene boundary, trachydacite lavas were erupted over an area of about 300 km 2 to form the Egan Range Volcanic Complex, in the southern Butte Mountains and adjacent Egan Range of east-central Nevada. The mean radiometric age for this volcanic episode is 34.1 ± 0.2 Ma, based on whole-rock and biotite KAr ages. Detailed comparisons were made between anisotropy of magnetic susceptibility (AMS) and the macroscopic petrofabric at 31 sites. Great variation in flow structures is attributed to complex flow in viscous lava over a surface of considerable relief. The magnetic fabric, as expressed by AMS axes, agrees well with the flow petrofabric. Typically, flow lineations are parallel to K 1 and flow foliation is perpendicular to K 3, as expected. Extension cracks and joints are normal to K 1. Late-stage deformation of pipe vesicles also impressed a measurable magnetic fabric. Because of significant initial dip and complexity of flow fabric, paleohorizontal is not generally demonstrable. Paleomagnetic studies indicate an exclusively normal polarity. The mean in situ characteristic direction is relatively well grouped ( D = 350°, I = 52°, α 95 = 8°) for 28 selected sites and provides a mean pole at 124°E, 81°N, which is not significantly different from the coeval cratonic reference pole. These results indicate that large Oligocene or younger rotations have not affected this sector of the Basin and Range province. The combination of age and magnetic polarity suggests a correlation with a normal polarity subchron of chron 13 or 15.