Timing of Formation of Deformation Bands In Subglacially Erupted Palagonitic Tuffs In Iceland

Deformation bands in subglacially erupted palagonitic tuffs in Iceland closely resemble the more familiar and better-studied deformation bands that form by localized pore space collapse and grain shattering in brittle, porous sandstones. Deformation bands in Icelandic palagonitic tuffs are lighter in color and more resistant to erosion than the surrounding host rock and form conspicuous ribs and fins, as they do in sandstones. Microscopic study reveals significant pore space reduction (by ~10%) in the band areas relative to the adjacent palagonitic tuff, and grain size distributions show a higher percentage of smaller, partially shattered grains in the deformation bands than in the adjacent tuff.

Despite the evidence for pore space collapse and grain size reduction, the cataclastic microfabrics that are typical of the finest grained portions of deformation bands in porous sandstones are absent. Instead, the matrix in the deformation bands appears under the microscope to be nearly featureless palagonite, rather than fine, shattered grains. SEM photomicrographs, however, reveal that the palagonitic matrix in the deformation bands is not featureless but consists of tiny ghost grains converted to and rimmed by palagonite. We suggest that palagonitization effectively obliterated the fine-grained cataclastic fabric so typical of deformation bands in sandstones. Increase in the surface area/volume ratio accompanying grain size reduction likely promoted more complete palagonitization in the deformation bands than in the adjacent tuff.

These tuffs also contain microfaults and fractures that postdate the deformation bands and that are less resistant, rather than more resistant, than the adjacent tuffs. Taken in total, the evidence indicates that the deformation bands formed very soon after subglacial accumulation of the tuffs, prior to or during the very early stages of palagonitization, and that post-palagonitization deformation in the tuffs was accommodated by brittle fracture and slip, rather than by formation of deformation bands.