DEFORMATION BANDS IN SUBGLACIALLY-ERUPTED PALAGONITIC TUFFS IN ICELAND

Deformation bands form by localized pore space collapse and grain shattering in brittle, porous rocks subjected to stress. Deformation bands and zones of deformation bands have been studied in sedimentary rocks, most commonly in sandstones (e.g., Davis, 1999). However, deformation bands do occur in other types of porous rocks, most notably in non-welded volcanic tuffs. Only a few studies have been done of deformation bands in volcanic rocks, and none have been done in Iceland. This study examined deformation bands in subglacially-erupted, palagonitized hyaloclastite tuffs in Iceland.

In outcrop, the palagonitic tuffs are light yellow-brown rocks and consist of sideromelane fragments, scoria clasts, and a palagonite matrix. Some of the palagonitic tuffs are matrix-supported, whereas others are grain supported. The deformation bands are lighter in color and more resistant to erosion than the surrounding host rock due to reduction in pore space and grain size. As a result, the deformation bands project in relief and are easily identified in outcrop.

Initial results that suggest that these structures are deformation bands include 1) significant pore space reduction (by approximately 10%) in the band areas relative to adjacent palagonitic tuff; and 2) grain size distribution that suggests a higher percentage of smaller, partially shattered grains in the deformation bands than in the adjacent palagonitic tuff. Despite the evidence for pore space collapse and grain size reduction, the cataclastic microfabrics typical of the finest grained portions of deformation bands in porous sandstones are absent. Instead, the matrix in the deformation bands in these palagonitic tuffs consists of nearly featureless palagonite, rather than fine, shattered grains. We propose for these rocks that grain size and pore space reduction occurred before palagonitization of the rocks was complete and that the large surface area/volume ratio of the fine grains in the deformation bands made them particularly susceptible to alteration to palagonite.

The deformation bands may have been caused by gravitational collapse of unstable subglacial piles shortly after eruption, by stresses induced during ice flow after eruption of the hyaloclastites, or by tectonic stresses associated with faulting along this portion of the Mid-Atlantic Ridge.