Barnes, J., Kattenhorn, S.A. (2010)


Deformation bands in subglacially erupted hyaloclastite ridges, Reykjanes Peninsula, Iceland

Eos, Transactions of the American Geophysical Union 91, Fall Meeting Supplement, Abstract T41B-2126.

Pleistocene glaciation across the Reykjanes Peninsula, where the mid-Atlantic spreading ridge comes onshore in SW Iceland, created an environment for subglacial fissure eruptions along the plate boundary. The addition of meltwater to the eruption process resulted in fragmentation of magma and the creation of hyaloclastites and hyalotuff: subaqueous hydroclastic deposits, which are inherently weak and poorly consolidated. The resultant hyaloclastite ridges (also called mobergs or tindar) form linear hills (generally <300 m high) oriented NE-SW in response to sinistral-oblique spreading across the ENE-WSW plate boundary. Faults, fissures, and joints within Holocene lavas in low-lying areas around the hyaloclastite ridges show consistent patterns that can be related to theoretical stress fields associated with oblique spreading. However, deformation features within the hyaloclastite ridges show fracture orientations commonly inconsistent with those in the intervening lava plains, suggesting a more complex deformation history. We focus on prevalent deformation bands (DBs), which are strain localization features (tabular bands of cataclasis and porosity reduction) predominately studied in porous siliciclastic rocks. We advocate that DBs in the hyaloclastite and hyalotuff sequences of these mobergs play a critical role in their deformation in response to tectonic, magmatic, and gravitational influences. DBs forming in hyaloclastite and hyalotuff are unique in character compared to analogous features in porous granular sedimentary rock. For example, DBs in sandstone generally result in at most a few millimeters of offset along a single DB. However, offsets of up to several meters occur along DBs of comparable thickness (<1 cm) within the hyaloclastite and hyalotuff. This distinct characteristic of these bands indicates a fundamentally different evolutionary process than occurs in sedimentary rock, likely in response to the breakdown of weak glassy fragments (sideromelane) in poorly consolidated material under low confinement. We have also identified variable DB styles within mobergs that we ascribe to different driving mechanisms. We differentiate tectonic DBs, which form in response to plate boundary tectonic stresses or local stresses around intruding dikes, from non-tectonic DBs, which form in response to early post-eruptive gravitational collapse. The DB orientations may differ from those expected from plate boundary stresses in response to local processes. Our analysis of DB orientations, dimensions, age relationships, petrographic characteristics, and the cataclasis and porosity reduction process, will be used to define a classification scheme for the different DB styles in subglacially erupted materials. This work emphasizes a unique style of DBs that develop rapidly (compared to DBs in siliciclastics) within hyaloclastite and hyalotuff, but with a range of styles that vary as a function of driving mechanism and extent of consolidation by palagonitization.

External link: AGU database



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