Abstract

Although mid-Tertiary calc-alkalic volcanics equal in volume and composition to major batholiths played a crucial role in the evolution of western North America, interpretations are hampered by Basin and Range faulting and subsequent erosion and sedimentation which obscure primary volcano-tectonic features. To reconstruct them, criteria for tracing volcanic formations to their source are needed.

Seventy-one oriented samples were collected from the Pleistocene Bandelier Rhyolite ash-flow tuff and Battleship Rock welded tuff, which erupted from known centers in the Jemez Mountains, New Mexico. In each sample, orientation of elongated shards, pumice fragments and crystals was measured in thin sections cut parallel to primary layering. Lineation was pronounced and indicates a preferred orientation of microscopic or megascopic components, resulting from primary flowage. The statistical significance of results is documented by the Tukey Chi-square test and the vector method. Statistical parameters for these two techniques were calculated by Fortran IV program. Only seven samples (9 percent) indicated Chi-square values below the 90 percent confidence limit.

Flow azimuth, which indicates the absolute direction of movement at any point on a flow, is determined by observing objective textural criteria in thin sections cut parallel to the dip, or in vertical sections cut parallel to the predetermined flow-lineation direction.

The orientations of both equidimensional and non-equidimensional fork-shaped shards and penetration effects were found to be reliable objective criteria for determination of flow azimuth in dip-parallel sections. Imbrication, blocking effects, and orientation of spindle-shaped objects were found to be reliable objective criteria for flow azimuth determination in vertical sections. The plot of flow lineations and flow azimuths of the Bandelier Rhyolite tuff indicate flowage radially away from the Valles Caldera. Deviations from this pattern probably can be explained by influence of pre-flow topography.

When applying these techniques to a region where the source of ash-flow tuffs is unknown, sampling should be carried out over a large area. More than one sample should be collected at each station to assure a flow pattern relatively free from small-scale variation.

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