MAGMATIC MAPPING: DISTILLING LIQUID MOMENTS IN TIME IN BASALTIC VOLCANIC FIELDS

The goal of the detailed mapping (at 1:24K) in the Springerville volcanic field in Arizona was to delineate the size, character, vent location and timing that each magmatic body was emplaced (its last liquid moment). Such detailed work is made possible by the youthful lavas (most < 3Ma), widely spaced vents (~400 distributed over ~3000 sq. km), mostly monogenetic character of the volcanism, and the arid climate. The result, published at 1:100K (USGS MI Map 2431 by Condit, Crumpler and Aubele, 1999) has allowed an analysis of structural controls from vent distribution (Connor and others 1992), of the spatial and temporal recurrence rates of basaltic volcanism (Condit and Connor, 1996), and of a cross section of a magma conduit system (Putirka and Condit, 2003).

Detailed mapping involved walking flow fronts, examining the lithology of the lavas. We recognized 11 distinct lithologies based on phenocryst content and size, and groundmass. Close attention was paid to elucidating not only the basalt stratigraphy, but also the freshness of the each flow's surface, and the thickness and development of soils on both the flow top, and where possible between flows. In cases where flow margins were indistinct and lithologies varied greatly, we chose to group the flow(s?) and define them as a composite unit. Once the linework was completed and the basalt stratigraphy correlated, a systematic “blob” geology pattern emerged. This pattern of flows from clusters of vents allowed localized correlations and defined geographic subdivisions, used along with the lithology to name the ~400 flow units.

Temporal relations were further refined by building stratigraphic columns, concentrating first within and then across geographic subdivisions. Boxes, representing the time a unit cooled from a liquid, were placed vertically (in time). Placement was controlled first by stratigraphic relations, that were anchored, where available, by radiometric ages and then by the magneopolarity of a unit. The polarity was fit into accepted magnetopolarity reversal time scales. These boxes were then further adjusted up or down in time, based on the degradation of the flow surface, the maturity of soils, and thickness of soil zones between flows, among numerous criteria.