Jökull - 01.01.2012, Page 177

Structural evolution of the 1890 Brúarjökull end moraine
Section 2a, the backslope.
This section is almost 3 m high and 12 m wide
and covers the backslope of the moraine at this site
(Figure 5). It is situated opposite to section 2b in the
abovementioned trench that penetrates the moraine.
A prominent listric thrust fault characterizes this sec-
tion and cross-cuts folded layers. The layers of the
hanging wall, outlined by 3–4 prominent tephra mark-
ers, dip 14–40◦ towards the south. Several small-
scale inclined and recumbent overturned folds with
vergence to the north occur immediately above the
thrust and occasional boudinage structures are also
found in the tephra layers. Higher with the section,
the deformation is less, as exemplified by the con-
tinuous and undisturbed white Öræfajökull AD 1362
tephra marker horizon near the surface. At ca. 3–8
m in the footwall of a thrust fault, the deformation is
similar to that of the lower part of the hanging wall
with small scale folds. A thick layer of black tephra
is observed in the footwall near the surface at 6–7 m.
Most likely, this tephra layer correlates with the black
tephra layers in the hanging wall, thereby giving a dis-
placement along the thrust fault of ca. 1 m. At 5–8
m in the footwall of the thrust, a linear structure was
observed which could possibly indicate another thrust
fault. However, this is difficult to determine due to
indistinct bedding in this area of the section and the
thrust’s unexposed footwall. At around 10 m, boudins
in the white Öræfajökull AD 1362 tephra marker and
LPT bedding indicate a synclinal structure, which po-
tentially relates to an overturned fold that connects to
the footwall of the aforementioned prominent thrust
fault (Figure 5).
Section 2b, the proximal part, 0–11 m.
The upper part is characterized by slightly de-
formed LPT and tephra layers forming the hanging
wall of a low-angle normal fault at ca. 2–6 m (Fig-
ure 6). The footwall of this normal fault is dominated
by compressive deformation as indicated by small-
scale open, inclined, and recumbent folds and back-
thrusts. Normal faults also occur and relate either
to the shearing of the upper strata across the lower
part, or to settling when compression ceased. Two
Figure 5. Section 2a. A) Photograph of the section in the proximal part of the end moraine. B) Drawing of the
section with structural data. Ice flow from right to left. – A) Ljósmynd af sniði 2a í innri hluta garðsins. B)
Teikning af sniði 2a. Grafið sýnir þrívíða legu setlaga og eins fellingaráss í sniðinu, sjá skýringar við 3. mynd.
Þrýstingur jökuls frá hægri til vinstri.
JÖKULL No. 62, 2012 175