New Mexico Geological Society Annual Spring Meeting — Abstracts

Paleomagnetic studies and 40Ar/ 39Ar dating are powerful, reliable techniques for correlation of Oligocene ignimbrites (ash-flow tuffs) in the Mogollon-Datil volcanic field. Accurate correlation of these regional ignimbrites has previously been hampered by lithologic variations within units, by the imprecision of conventional K-Ar dating, and by the discontinuous pattern of outcrops produced by Basin-and-Range faulting.

High-precision 40Ar/39Ar ages of sanidine separates from 21 Mogollon-Datil ignimbrites range in age from 35.6 to 24.2 Ma and define four brief eruptive intervals (35.6 to 33.5 Ma, 32.0 to 31.6 Ma, 29.0 to 27.4 Ma, and 24.2 Ma). These ages are sufficiently precise (relative error +/-0.15 Ma) to uniquely distinguish between units erupted at least 0.3 m.y. apart.

The outflow sheets of Mogollon-Datil ignimbrites typically exhibit uniform thermoremanent magnetism (TRM) directions, many of which are sufficiently distinctive to unambiguously identify the unit. Thin, unwelded distal fringes of some regional ignimbrites have been identified paleomagnetically, revealing previously unknown overlapping relationships between units fromI the Mogollon and Socorro eruptive centers. Magnetic polarity zone boundaries provide valuable time-stratigraphic marker horizons, some of which can be traced throughout much of the volcanic field.

Uncertainty of structural correction has been the largest obstacle to paleomagnetic correlation of Mogollon-Datil ignimbrite outflow sheets, because TRM vectors can be rotated by tectonic movements. Alternatively, where units are identified by indepecdent means, the TRM directions can provide a sensitive, reliable measure of local tectonic rotations in highly extended terrains.

The intracauldron facies of Mogollon-Datil ignimbrites commonly show non-uniform magnetizations and cannot be easily correlated with outflow facies using paleomagnetism.