Jökull - 01.01.2012, Blaðsíða 169

Reviewed research article
Polyphase structural evolution of a fine-grained, fold-dominated
end moraine, Brúarjökull surge-type glacier, Iceland
Ívar Örn Benediktsson1,2
1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík, Iceland
2Department of Geology, Lund University, Sölvegatan 12, S-223 62 Lund, Sweden
iob2@hi.is, ivar_orn.benediktsson@geol.lu.se
Abstract – The glaciotectonic architecture and structural evolution of a fine-grained end moraine formed by
the 1890 glacier surge of Brúarjökull is described from four excavated cross-sections. The end moraine ridge
is a morphological expression of a marginal sedimentary wedge formed during the last days of the surge. The
actual end-moraine ridge was formed on the last day of the 1890 surge when the glacier became coupled to
the bed and ploughed into the reverse slope of the marginal sediment wedge. Ductile deformation, favoured
by high porewater pressure, dominated the construction of the end moraine while brittle deformation was
induced when porewater pressure decreased, particularly at the end of the surge. Thus, the deformation was
polyphase, developing from open folding to multiple overfolding when porewater pressure was high and finally
to overthrusting, faulting and shearing at the very end of the surge when porewater pressure dropped severely
upon porewater blow-out in front of the moraine. This structural continuum is exhibited by the four cross-
sections. The glaciotectonic stress was absorbed within a relatively narrow zone due to high friction along
a basal décollement. A new model illustrates the structural evolution of a fine-grained, fold-dominated end
moraine and may serve as an analogue to similar end moraines in modern and Pleistocene environments.
INTRODUCTION
Glaciotectonic end moraines signify the process of
proglacial and submarginal sediment deformation.
Thus, the analysis and interpretation of structural fea-
tures (e.g. folds, faults, shear zones, tectonic fabrics)
within them provide important information on influ-
encing factors in the subglacial and ice-marginal en-
vironments, and on the thermal and hydrological con-
ditions of the foreland wedge which is deformed to
produce the end moraine (e.g. Croot, 1988; van der
Wateren, 1995; Boulton et al., 1999; Bennett, 2001;
McCarrol and Rijsdijk, 2003; Pedersen, 2005; Aber
and Ber, 2007; Phillips et al., 2008; Benediktsson,
2010; Benn and Evans, 2010; Brandes and Le Heron,
2010; Phillips et al., 2011). A number of studies have
analysed complex glaciotectonic sequences in Pleis-
tocene (e.g. Thomas, 1984; van der Wateren, 1987,
1995; Pedersen, 2005; Aber and Ber, 2007; Phillips
and Merritt, 2008; Parkes et al., 2009; Weaver and Ar-
naud, 2010) and modern (e.g. Humlum, 1985; Croot,
1987; Krüger, 1994; Hambrey and Huddart, 1995;
Boulton et al., 1999; Huddart and Hambrey, 1996;
Krüger et al., 2002, 2010; Motyka and Echelmeyer,
2003; Bennett et al., 2004; Kuriger et al., 2006; Bene-
diktsson et al., 2008, 2009, 2010; Roberts et al., 2009)
glacial environments in order to elucidate the tec-
tonic development and the glacier-induced processes
and stresses involved. This paper contributes a case
study to this growing literature based on a fine-grained
glaciotectonic end moraine formed by the 1890 surge
of Brúarjökull, Iceland. Benediktsson et al. (2008)
JÖKULL No. 62, 2012 167