Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Fig. 2: Geological map of the study area (Gobierno de
Navarra, 1997a). The white and orange dots show the
location of the TL samples.
OBSERVED DEFORMATIONS
We have analyzed the geological and geomorphological
maps (Gobierno de Navarra, 1997a, b), as well as the
digital terrain model, and stereoscopic pairs of aerial
photos, identifying six terraces at the Irati River: T1
(+40m), T2 (+36m), T3 (+30m), T4 (+24m), T5 (+10m) and
T6 (+5m). Several topographic profiles have been made in
order to examine possible deformation relating with the
tectonic activity.
The profile shown in Fig. 3 runs along the highest terraces
T1 (+40m) at the right side of the Irati River to the north
of the anticline of the sierra. The slope of the terrace
surface has 0.2º. We have identified a deposit of fluvial
terrace located just over the folded sierra, at the place of
Biezcas, which is lifted with respect to the trend of the
slight slope of the others.
Fig. 3: Topographic profile along the terraces T1 (+40m) at
the right side of the Irati River to the north of the sierra.
The terrace located just over the folded sierra is lifted with
respect to the trend of the slight slope (0.2º) of the others.
This fact could be related to the grow of the anticline that
built up the sierra. The northern flank of the fold lifts the
terrace deposit up to 43m over the thalweg of the river,
3m higher than the highest terrace identifying at the area.
Unfortunately, the farming works has modified
remarkably the original deposit loosing the initial slope of
the terrace surface, and no tilting could be measurement.
Nevertheless, attending to the trend of the slope of the
surface of the highest terrace T1 (+40m) to the North, the
terrace of Biezcas is lifted 4 m as a minimum.
At the other side of the sierra, a second topographic
profile runs along different terraces (Fig. 4). The profile
has a stepped shape for comparing purposes. The first
segment of the profile runs along the terrace T1 (+40m)
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
61
showing an angle of slope of 2º to the ESE. This slope
value is remarkably higher than that of the terrace at the
other side of the sierra. The segment running through the
following terraces seems to be quite flat again. In this
case it is difficult to fit the deformation to the grow of the
anticline of the sierra because of the oblique orientation
of the slope. In any case, such a slope in a terrace have to
be related with a tectonic origin, like a minor fault which
accommodate the local deformation, in absence of other
possible mechanisms like halocinetic processes.
Fig. 4: Topographic profile along the terraces (+40m) of the
Irati River to the south of the sierra. The terrace located at
the right side is tilted up to 2º. Compare the slope with that
of the terraces located to the north of the sierra (See Fig.3).
THERMOLUMINISCENDE DATING
In order to determine the age of the deformation found
at the study area, eight samples were collected at
different units for dating by Thermoluminiscence
procedure (TL) (see Fig. 1 for location). Not only the
terraces involved in the deformation where considered,
but also other levels and coluvial material were taking
into account to acquire a better understanding of the
evolution of the structure and the landscape.
The TL dating procedure considers the light exposure
occurring during transport of the sediment grains in a
dispersed state, prior to their deposition (Forman, 1989).
The ages obtained by this method are presented in the
Fig. 5.
Fig. 5: TL age of the samples (white dot) with interval of
error. The samples are grouped by geological unit and put
into order from older to younger