The Pliocene-Quaternary tecto-sedimentary evolution of the Larissa Plain (Eastern Thessaly, Greece)

Abstract

In order to understand the present-day morphological, geographical and environmental patterns of the Larissa Plain, a large amount of historical, archaeological, sedimentologi-cal, stratigraphic, tectonic and seismic data have been collected and analysed. The collaboration of different specialists was essential for these aims. The Larissa Plain is an inter-mountain basin formed during the Pliocene as a result of the post-orogenic collapse of the Hellenides. From the Middle Pleistocene, the region has been affected by a new tectonic phase which caused an important palaeogeo-graphic change and fragmented the study area into three distinct geographical domains characterised by different sedimentary conditions: the Pinios alluvial plain to the North, the Karla Lake to the South, and the Chasambali Bulge in between. The first sector corresponds to the active Tyrnavos Basin, and the second to the southern part of the inherited Pliocene Larissa Basin. The Chasambali Bulge has been differentiated by a system of northward down-stepping normal faults thus creating a temporary hydrographic divide between the two major domains. Recent dikes have disconnected the present physiography of the plain from the hazards of the recurrent floods of the Pinios River over the alluvial plain and discharged the Asmaki River of its past function of an overflow convector from the northern drainage system (Tyrnavos Basin) towards the southern one (Karla Lake). A four-step model of the tecto-sedimentary evolution of the Tyrnavos Basin is proposed. a) The profile of the Pinios River is in equilibrium. b) The area is affected by a tectonic paroxysm that produces a partition of the plain. c) Local erosion as well as a distributed fluvio-lacustrine sedimentation occures until the gap is filled up. d) Once widespread sedimentation halts, the abandoned alluvial plain suffers a diffuse pedogenesis. Several secondary factors (prolonged morphogenic activity, compactioninduced subsidence, differential sedimentary compaction, erosional phenomena outside the basin, climate variations and anthropic activity) that may have disturbed and changed this simple cyclic evolution, locally or temporarily, have been considered and analysed in detail. The paradox of the Larissa Plain, where the Pinios River flows 20 to 40 m higher than the Karla Lake and exits the area crossing the Palaeozoic bedrock along the Rodia Narrow, has been satisfactorily explained. Probably during the last few thousand years, this palaeogeographic pattern being characterised by an alluvial plain in the Tyrnavos Basin and an independent large lake in the southern Larissa Plain, since Middle Pleistocene, became unstable due to one or more concomitant and possibly inter-dependent causes which are discussed here in detail. The build up of an artificial embankment along the Pinios River North of Larissa, definitely fossilised the present-day geography. © 1994 Masson, Paris.