PALEOMAGNETISM OF THE PEACH SPRINGS TUFF REVISITED

The widely distributed 18.5 Ma Peach Springs Tuff (PST) is an important marker for correlation between isolated and deformed Tertiary sections throughout the Colorado River extensional corridor (CREC) of the Basin & Range Province. The suggestion that this tuff contains a distinctive characteristic magnetic remanence “reference” direction (PSTREF=033.0/33.6; Wells and Hillhouse, 1989) has led to use of the PSTREF as not only a correlation tool but as a marker to assess structural rotations. Our studies of isolated PST sections has, however, revealed a spectrum of stable remanence directions between PSTREF and mid-Miocene geographic north (MGN). To explore this discrepancy, we have studied the PST in a number of well-exposed sections where independent control on potential structural rotations is provided by time-averaged remanence directions in enclosing basalt flows and/or the presence of the underlying Cook Canyon tuff (CCT) which appears to possess a reasonably well-defined paleomagnetic reference position (CCTREF) of ~356/51. Sites studied to date include the eastern Newberry Mtns. (NV), the Piute Range (CA), the Aquarius Mtns. (AZ), Cook Canyon (AZ), the southern, central and eastern Black Mtns. (AZ), and Castle Rocks (AZ). At all sites where present, tilt-corrected CCT remanence directions lie close to the CCTREF and time-averaged directions for enclosing basalts lie near MGN. However, few locations contain PST remanence directions near the previously defined PSTREF; most have directions well-removed from the PSTREF and close to the MGN. For example, in the Piute Range section, only the uppermost part of the thick (>110 m) PST contains the PSTREF while the lower 100 m has directions intermediate between the PSTREF and MGN. In a 90 m-thick PST section near Davis Dam, all PST remanence directions lie near the MGN and not the PSTREF. We suggest that the PST erupted and cooled during a rapid geomagnetic excursion.. Thick, more highly welded sections in the CREC are more likely to contain non-PSTREF positions than are more distal sections to the east where the PSTREF was defined.