Le piroclastiti medio-pleistoceniche di Massa Martana- Acquasparta (Umbria): Caratteri strutturali e vulcanologici

Abstract

The Martani Mts. fault zone (Umbria, central Italy) is an important Plio- Pleistocene extensional structure with a characteristic arcuate eastward concave shape and with a great structural relief. It is made up of a NNW-SSE trending northern segment that delimits eastward the graben of the Tiber river middle valley, and of a WNW-ESE trending southern segment delimiting northwards the Terni basin (Conca di Terni) graben. Independent of the direction of the fault zone, directions of slip vectors keep their SW-NE trend and the Plio-Pleistocene stress field is characterized by a triaxial stress ellipsoid, with a sub-horizontal NE-SW trending main tension axis. Middle Pleistocene pyrociastic deposits (390,000 years on the basis of ³⁹Ar/⁴⁰Ar age determinations) form a 10 km long continuous plateau extending from Massa Martana to Acquasparta along the fault system. Pyroclastic deposits in part cover the Mesozoic terrains forming the fault foot-wall and in part rest on the early Pleistocene continental deposits forming the fault hanging-wall. On the basis of morphology, attitude and sedimentologic features of the pyroclastic deposits, three major vent-areas were identified along the fault system. These areas are interpreted as the effect of the fault control on magma emplacement. Volcanic activity - mostly of phreato-magmatic origin - triggered debris-flows, pyroclastic flows and later produced surge and fall deposits. A detailed mapping and stratigraphy of the pyrociastic deposits and of the inferred associated volcanic centres are shown. Chromite-rich olivine suggests a possible mantle origin. In addition, sanidine, leucite, phlogopite and diopside in the mineral assemblage suggest a phonolitoid composition of the erupted magma. Mineralogy and grain-size distribution indicate that these pyroclastic deposits belong to the Umbria-Latium Ultra-alkaline District (ULUD). The pyrociastic products are affected by brittle deformations testifying that tectonic activity along the Martani Mts. fault continued after the volcanic events, probably developing up to late Pleistocene times. Two minor fault systems are recognized: one system, with a N100° to N160° trend, is extensional and is kinematically compatible with the extensional stress field which originated the Martani fault; the other system mainly consists of N-S±10° trending right-lateral strike-slip faults, and involves a sub-horizontal SW-NE stretching main compression axis. Space-time relationships between these two fault systems show that the strike-slip deformations are younger than the extensional ones, suggesting a very recent inversion of the ellipsoid stress axes.