Jökull - 01.01.2012, Síða 184

Í. Ö. Benediktsson
sections, as exemplified by ductile and brittle defor-
mation structures and horizontal shortening ranging
from 20% to 76%.
Ductile deformation was favoured by high pore-
water pressure and dominated the formation of the
1890 end moraine, developing from open folding to
multiple overfolding. A substantial drop in porewater
pressure at the very end of the surge induced brittle
deformation as the final phase of the end-moraine for-
mation.
Folds are generally rooted indicating high friction
along a basal décollement, which lies across bedrock
or outwash. This suggests that the applied glacio-
tectonic stress was absorbed within a narrow zone,
as also suggested by steeply dipping faults and axial
planes of folds.
Orientation of faults and axial planes of folds pri-
marily reflects local and marginal ice-flow direction
(e.g. in an outward-spreading surge lobe) which is
perpendicular to the moraine strike and not necessar-
ily coinciding exactly with the main ice-flow direction
of the 1890 surge. In general, though, structural data
coincide with the ice flow from southerly directions,
as expected.
Acknowledgements
Jaap van der Meer is thanked for collaboration on
the 1890 end moraine in Kringilsárrani. Thanks are
furthermore due to other members of the Brúarjökull
field campaigns for great company and collaboration.
Constructive comments by reviewers Jonathan R. Lee
and Jón Eiríksson and editors Helgi Björnsson, Bryn-
dís Brandsdóttir and Leó Kristjánsson, improved the
manuscript and are gratefully acknowledged.
ÁGRIP
Rannsóknir á innri gerð jökulgarða (Hrauka) sem
mynduðust við aflögun fínkornóttra setlaga í fram-
hlaupi Brúarjökuls 1890 sýna að garðarnir urðu til á
síðasta degi framhlaupsins, þegar jökullinn ýtti fram
efsta og ysta hluta setfleygs sem myndast hafði und-
ir jökulsporðinum dagana á undan. Vatnsþrýstingur í
setlögunum undir og framan við jökulsporðinn var hár
er þau aflögðuðust undan fargi hans; fyrst í opnar fell-
ingar sem síðan steyptust yfir sig við aukna framrás
jökulsins. Undir lok framhlaupsins þrýstist vatn út úr
setlögunum og þau stífnuðu. Fellingarnar brotnuðu þá
um skerfleti og þrýstimisgengi uns álaginu frá jöklin-
um var létt. Þetta ferli einkennir aflögun fínkornóttra
setlaga við framhlaup jökla og varpar ljósi á myndun
margra fornra jökulgarða.
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182 JÖKULL No. 62, 2012