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