THE ROLE OF DEFORMATION BANDS IN THE COLLAPSE OF SUBGLACIAL HYALOCLASTITE RIDGES: AN EXAMPLE FROM VALAHNUKAR, ICELAND

Deformation band zones (DBZs) are common in subglacially erupted basaltic tuffs in Iceland and form resistant ribs and fins cutting primary layering. The majority are compactional shear bands formed by cataclasis and granular flow, although rare dilation bands occur. Field and microscopic evidence indicates that DBZs formed while hyalotuffs were unconsolidated, prior to substantial palagonitization.

The 10 km-long tindar at Valahnúkar displays DBZs with three general geometries. Geometry 1: the most common deformation bands occur in dominantly steeply dipping clusters. Ladder and conjugate geometries are consistent in most places with dominantly normal displacement, with vectors raking at least 65-70° in the DBZ plane. DBZs occur in sets that have local orientations related to tindar geometry. Cross-cutting relationships among multiple sets are locally consistent and indicate local changes over time in conditions promoting deformation band formation. Geometry 2: DBZs also occur as curved bounding surfaces separating underlying hyalotuff from tongues and masses of pillow breccia and hyaloclastite. These DBZs commonly truncate older geometry 1 DBZs in the underlying hyalotuff. Geometry 3: rare DBZs occur in circular to oval concentric sets of outwardly dipping, paired cataclastic shear bands and dilation bands. These concentric dbzs are commonly cored by disrupted hyalotuff or are associated with disrupted layering in adjacent hyalotuff and are cut by geometry 1 DBZ sets, including rare sets with a larger strike slip component of motion.

DBZ geometry 1 formed as hyalotuffs accumulated and shifted in response to changes in size and shape of the enclosing sub-ice vault. Different sets developed in succession as local stress conditions changed. Geometries of the youngest sets in many places are consistent with collapse parallel to the local trend of the tindar. DBZ geometry 2 postdates many of the sets with geometry 1 and bounds the lower surfaces of slide and slump masses higher in the tindar. DBZ geometry 3 occurs only in SW Valahnúkar, where the tindar splays into a wide foot. These concentric DBZs and related disrupted layering may have formed in an early, transient, high pore fluid pressure event involving diapiric rise and disruption of original layering perhaps related to collapse of the SW end of the tindar.