GEOMAGNETIC EXCURSION DURING ERUPTION OF THE PEACH SPRINGS TUFF

Peach Springs Tuff (PST) erupted 18.5 Ma covering a wide region from the Mojave to the Colorado Plateau. PST is, in part, distinguished by its paleomagnetic remanence which, in many ranges, defines a shallow, NE direction, well-removed from the mid-Miocene dipole direction (MMD). This PST “reference” direction (PSTREF) has been used for both correlation and to assess structural rotations.

Discovery of the Silver Creek caldera (SCC), source of the PST in the western Black Mountains (BM), AZ (Ferguson, 2008) permits exposures of PST to be placed into context. Sites defining the PSTREF (Wells & Hillhouse, 1989) are mostly distant (>50 km) from the PST source and display low to moderate welding. We have concentrated on thick, near-source (<50 km) PST with moderate to high degrees of welding. To assess secular variation, our sites include the PST as well as stratigraphically enclosing volcanics. These exposures have tilt-corrected remanence that either lie wholly near the MMD or define great-circle positions between the PSTREF and MMD. For example, remanence directions within the PST in the Grasshopper Junction area lie at and intermediate to the MMD and PSTREF. In the Paiute Range, all directions lie close to the MMD. Significantly, in the thick PST section at Warm Springs (WS) in the southern BM, all directions lie close to the MMD except for sites within vitrophyre near the top of the section that have PSTREF directions. The WS vitrophyre likely represents the chilled glass surviving cooling-related crystallization. This suggests that the thick, near-source ignimbrite at WS chilled as glass, locking in the PSTREF, and later largely crystallized during cooling resetting remanence directions to the MMD.

Remanence data from near-source sections of PST suggest that eruption occurred during a geomagnetic excursion to the PSTREF. Quickly cooled, or quenched portions of the tuff (such as distal exposures or the WS vitrophyre) record the position of the excursion while crystallized portions of the tuff record various intermediate directions capturing the return of the dipole direction to the MMD. Supporting this hypothesis are three new sites in the kms-thick PST within the SCC. These sites have remanence directions that lie close to the MMD, consistent with slow-cooling overprinting of the PSTREF during crystallization.