Geology and Geomorphology in Panagopoula
Research Studies by T. Lebourg and S. El Bedoui, Geoazur, UNS, Nice, France.

Friday 4 December 2009 by Thomas Lebourg

traduction [English ] [français ]

The Panagopoula area is affected by three major fault families: (i) N080-N100, (ii) N040 and (iii) N140. N080-N100 faults structured several major grabens (Gulf of Corinth and Patra Gulf) connected by N040 and N140 transfer faults. Elevations of the area are between 0 m and 500 m.

The active normal faulting along the coast shaped the landscape in E-W oriented high slopes (30°-35°) reaching high elevation close to the sea. The slope was affected by a "shallow" landslide during 70s. A 1 million cubic meters complex landslide (translational in the upper part, earthflow at the foot of the slope) was triggered by human made structure (highway construction) on a 300 m length. Inclinometer data showed a failure 25 m deep and cumulative movements of 40 mm over 1 year. The stabilization of the slope was based on two drainage tunnels and concrete trenches on the slope (water drainage, control of erosion).

The slope is made of two sedimentary geological units: (i) Cretaceous deposits, purple and grey limestone (the Pindus unit) related to the Hellenides compression phase and (ii) the Pliocene synrift deposits (conglomerates) (T. Doutsos and D.J.W. Piper, 1990 ; H. Cornet et al, 2004 ; N. Flotté et al, 2005). The limestone substratum is affected by a global tilt toward the south, observed at the regional scale, due to the normal faulting. This geometry is closed by Quaternary conglomerates (syn-rift deposits) reaching a thickness of about several hundred meters (N. Flotté et al, 2005).

On the field, limestone outcrops show an oriented N090-110 0-30 S stratigraphic surface. These outcrops show low wave length folds (around several meters) with N150 axial planes.

The grey limestone gives massive outcrops, in particular in the western side of the site, and is characterized by a local high fractured aspect. The purple limestone is more affected by alteration, giving some gully forms observed on the field.

The conglomerate outcrops appear in the southern part (250 m elevation) with N090 35 S stratigraphic planes. They are constituted by cobbles cemented by a yellow sandy matrix. Here and there, they show a high cohesive aspect. Their thickness increases toward the south, reaching several hundred meters and forming high elevation cliffs clearly observed in the N160 oriented thalwegs.

The area is affected by two fault families: N140-160 vertical and not clearly expressed on the field, and N080-100 60 N and currently active. The ENE-WSW faults mapped in the area constituted a major fault zone: the "Psathopyrgos" fault zone, described by previous works at a larger scale (I.K. Koukouvelas and T. Doutsos, 1997 ; T. Doutsos and S. Kokkalas, 2001 ; N. Flotté et al, 2005). The geometry of these E-W faults is constrained by observations made in N160 oriented thalwegs, in the eastern part of the studied zone, showing a listric geometry. The vertical scarp associated is about 10 m with a normal movement and a local sinistral component.

Geomorphologic investigations show that the slope is currently affected by surface deformations. Six recently active moving masses are distinguished and four of them are limited by E-W faults. The major morphological signs of gravitational deformations appear in the central part of the area. Important scarps (vertical displacements of about 2 to 3 m) occur in the middle of the slope. The structures of surface drain waters are sheared, and shifted by a few centimetres, illustrating a still active recent slip. The upper part is characterized by an important tilt of trees and shearing in the draining channels. In the western part of the slope, a 3 m high N-S scarp bounds the most active zone.

Deformations are also observed along the highway at the foot of the slope, indicating an average movement of around several mm. A large volume of colluviums is mapped at the western side of the area bounded at its upper part by an E-W normal fault. Field observations show that this colluvium has recently moved.

Field investigation and aerial photography analyses highlight two main facts: (i) gravitational deformations currently affect the slope and (ii) instability signs like scarps correspond with normal faults mapped in the field. By comparison with catastrophic landslide occurred in 70s, field approach highlights a current deformation process characterized by a larger extension and a more diffuse aspect without mass movement individualized by a failure surface. Geometrical relations between faults and scarps mapped on the field indicate a high level of correlation.

These surface observations were completed by geometrical constraints (fault and contact) at depth acquired by geophysical investigations.

Cross Section and Seismic Tomography of Panagopoula