Evoluzione geologica della Piana del Fucino (Abruzzo) negli ultimi 30.000 anni

Abstract

Fucino Plain comprises the bed of the homonymous lake, drained completely in 1875 and some terraces surrounding it. Geological studies carried out in this area have allowed us to point out the presence of terraces, soft-rock pediments and rockcut terraces, either fluvial or lacustrine, dated between Upper Pleistocene and the present day, as well as many indications of surficial faulting and recent tectonic movements. Many archeological data, tephra levels and radiometric 14 C dating have been used to date the above mentioned features and sediments. On the basis of the ages of the sediments changes of the Fucino lake levels have been recognized; they have taken place in the last 30,000 years and date the tectonic activity. Such changes can be summarized as follows: general increase of level between 33,000 and 18-20,000 years B.P., general decrease between 18-20,000 and 7,500-6,500 years B.P., increase between 6,000-5,000 years B.P., decrease between 5,000-2,800 years B.P., increase between 2,800-2,300 B.P., decrease between 2,300-1,800 B.P., low levels in the 16th century, increase during the 1750 -1861 period of the so-called Little Ice Age. The increases of lake level were, generally, contemporaneous with glacier advances. There is an evident relationship between oscillation in level, seasonal changes of insolation produced by the orbital movement of the Earth (precessional of the equinoxes and obliquity of the ecliptic), average annual rainfall and summer evaporation. The oscillations of lake levels do not seem to have been influenced significantly by tectonic movement which have taken place in the Fucino Plain, or by small karst swallow hole present at its borders. Faults, fault scarps and linear boundaries among areas covered by soils with different lithology and moisture, are the geological and morphological consequences of recent tectonic activity in the area. Moreover Fucino Plain is surrounded by fault-line scarps. Some faults have been active also in historical times, and a few gave rise to surficial faulting during the 1915 Fucino earthquake. Some faults have been called "primary" for their morphological evidence, and others "secondary", for their smaller extent. Some sets of secondary faults are located within the angles of primary fault crossings; this suggests a structural pattern related to a stress field controlled by right shear of NW-SE strike-slip fault. The primary faults, even if they show mainly vertical displacement, could correspond to synthetic and antithetic faults caused by strike-slip faults without direct display on the topographic surface. On the ground of its structural pattern or the features of its boundaries, Fucino Plain could be interpreted as a "pull apart basin". Morphological investigations on fault scarps allow to make the hypothesis that they have been produced by surficial faulting connected with ancient earthquakes, even larger than those that took place during the 1915 earthquake. The occurrence of some fault scarps parallel to each others could be caused by the gradual erosion of scarps produced by the same fault during different earthquakes. The datations of the fault scarps, and consequently of the main ancient earthquakes, are provided by geological data; the age of some earthquakes has been determined with a better approximation because the events could be correlate to rockfalls in some caves with well known archeological settlements; furthermore other archeological informations, especially the position of the roman drainage channel are available. The seismic history herein reconstructed consists of at least five large earthquakes, which occurred between 18-20,000 and 13-14,000 years ago, about 5,000-5,500 years ago, about 3,100 years ago, in late-Roman Age or in the Middle Ages, and finally in 1915a.D.