Paleomagnetic evidence for counterclockwise rotation in a broad sinistral shear zone, Basin and Range Province, southeastern Nevada and southwestern Utah

Geochronologic and paleomagnetic evidence defining the relationship between the Miocene Hiko and Racer Canyon tuffs, eccentric outflow lobes from the Caliente caldera complex, southeastern Great Basin, USA

“…The large scatter in all measured magnetic parameters is evident, and the angular standard deviation for the section is 137. This relatively high value may be compared with the more usual values of 57 and 77 measured in the upper and lower parts of a thick section of Hiko tuff in the South Pahroc range, Nevada (Hudson et al 1997). The average intensities of natural remanent magnetization are similar in the two formations, 0.7 to 0.9 A/m, although the Hiko tuff tends to be somewhat more strongly magnetized.…”

“…On the basis of typical historic rates of geomagnetic secular variation, the time elapsed between emplacement and cooling of the two units could be as little as one or two centuries. The paleomagnetic direction in the upper cooling unit is typical of that measured elsewhere in the Hiko tuff, but the direction in the lower cooling unit is divergent, implying that the upper unit is the most widespread and has by far the largest volume (Hudson et al 1997).…”

“…1). In most places outside the caldera complex the Hiko tuff appears to be a single cooling unit less than 50 m thick (Dolgoff 1963), but in the South Pahroc range it consists of two cooling units, is at least 250 m thick, and makes up most of the mountain range (Hudson et al 1997). Paleomagnetic sampling was done throughout this section by Hudson et al (1995Hudson et al ( , 1997 who report mean directions for the upper and lower cooling units that differ by 117 of arc.…”

“…Mackin (1960) was first to emphasize the importance of regionally extensive welded ashflow tuff sheets in analyzing the structural geometry of Tertiary extension in the Great Basin. Accurate knowledge of the volcanic stratigraphy is essential to this kind of analysis (Hudson et al 1997).…”

“…1). Interpretation of the paleomagnetic data is complicated by the existence of a broad zone characterized by counterclockwise verticalaxis rotation that has been termed the Caliente-Enterprise zone (CEZ) by Hudson et al (1997). The zone extends for at least 120 km from the general vicinity of Alamo, Nevada, on the west to just east of Newcastle, Utah, on the east (Fig.…”