INTERACTION OF BASALTIC DIKES AND WET SEDIMENT AT GLACIOVOLCANIC CENTERS: EXAMPLES FROM ICELAND AND MARS

Basaltic dike intrusions into phreatomagmatic tephra deposits were investigated at a glaciovolcanic fissure-fed complex (Sveifluháls, Reykjanes Peninsula SW Iceland). Such dikes serve as excellent terrestrial analogs of high-level dike-cryosphere interactions on Mars. Field and photo-interpretation of the dikes revealed several characteristics which would aid the identification and interpretation of such dikes on Mars. The Sveifluháls dikes have eroded to form a flat reddish-orange plateau that runs parallel to the main complex at a lower altitude. The dikes have a gross tabular morphology, but include finger-like projections up to 5m in length into the host tephra. The upper surface of the dikes (dike tips) were preserved in several places, where forms resembling pillow-like haystacks are present, and a dike feeding a subaqueous pillow lava flow can be seen. The dikes typically have narrow (<1.5m wide) coherent cores, and include pillowed and non-pillowed types. 1-3m wide peperitic zones comprising both block and fluidal (pillow-like) types formed along both margins. Dike margins are also characterized by drag folded domains (1-3m wide) of bedded tephra immediately along the dike-host contact, and larger more distal rotated blocks of tephra (few meters to 30m wide). The tephra has also undergone a distinct reddening along the dike margins, in a zone up to 20m wide on either side of the dike. Dikes that have interacted with the cryosphere on Mars should also preserve marginal peperite and pillows, other irregular finger-like margins, contact areas of dragged and rotated host, distinct hydrothermal alteration zones, and possibly pillow haystack tops. One important implication of this study is that coherent dike width in such wet settings is difficult to estimate, but is typically at least an order of magnitude less than its geomorphic expression (ridge, plateau or trough).