FRAGILE GLASS: DEFORMATION BAND FORMATION IN UNCONSOLIDATED HYALOTUFF, VALAHNúKAR, ICELAND

Deformation bands in subglacially erupted hyalotuffs at Valahnúkar tindar in Iceland are lighter in color and more resistant to erosion than the surrounding host rock and cluster in deformation band zones (DBZs) to form ribs and fins. The majority show significant pore space and grain size reduction, grain shape change to more equant and less vesicular grains, and offsets of primary layering, indicating that most are compactional shear bands formed by cataclasis and granular flow. Microscopic evidence indicates that deformation band formation occurred while the hyalotuffs were unconsolidated prior to substantial palagonitization.

Valahnúkar tuffs that contain the most well-developed deformation bands are dominated by large, irregular, vesicular grains. During granular flow, these large, fragile glass grains impinged on each other and shattered, breaking at weak bubble necks into more equant, non-vesicular pieces. Lithologies that initially contained a fine matrix or that consisted of porous hyaloclastite with blockier, less vesicular grains, contain fewer deformation bands or no deformation bands at all.

We suggest that Valahnúkar deformation bands formed primarily as a result of local stresses in the tindar edifice as pyroclastic deposits adjusted and collapsed during and shortly after emplacement. This model is consistent with 1) slump masses of pillow breccias separated from underlying hyalotuffs by DBZs that dip away from the tindar crest and truncate swarms of older DBZs in the underlying hyalotuff; 2) results of detailed mapping that fail to reveal any obvious correlation with regional tectonic features in the Valahnúkar area; 3) petrographic evidence that Valahnúkar deformation bands formed prior to consolidation of the tuffs by palagonitization; 4) work by others at Surtsey and Gjálp showing that palagonitization of subglacial tuffs occurs within 1-2 years of eruption, and 5) recent work by others that demonstrates that deformation bands can form in unconsolidated materials subjected to stress. We argue that the fragile nature of large, unsupported vesicular glass grains made Valahnúkar hyalotuffs susceptible to deformation band formation even though they were unconsolidated. This ongoing work represents the first documentation of cataclastic deformation bands in subglacial hyalotuffs.