L. Martelli

Factors controlling foredeep turbidite deposition: the case of Northern Apennines (Oligocene-Miocene, Italy)

Abstract Three major controlling factors affect turbidite deposition in foredeep basins: tectonics in the source area, tectonics in the belt-basin system, and variations of sea-level (local or global). These factors are expected to have different effects on the volume, grain size, provenance and distribution of clastic sediments during the evolution of the basin. The interplay of these factors is investigated for the latest Oligocene-Middle Miocene Northern Appennines Foredeep turbidite wedges by means of turbidite-system-based lithostratigraphy and field mapping, integrated with nannoplankton biostratigraphy and sedimentary petrography. Almost all recognized turbidite systems, unless tectonically truncated, show an overall stacking pattern formed by a lower sand-rich, thickly bedded stage (depocentre stage) passing upward into mud-rich, thinly bedded stages, eventually grading up to mostly mudstone units (abandonment stage). This rhythmically repeated pattern is interpreted as the result of the abrupt switching on and off of coarse-grained input, coupled with an alternating increase/decrease of depositional rate recorded in all detected systems. Biostratigraphy makes it possible to distinguish the switching-off of turbidite systems due to depocentre migration (a new system is switched on basinward) from that due to a regional decrease of clastic input. Sandstone petrography records the compositional variation related to tectonically induced source reorganization. In the latest Oligocene-Middle Miocene NAF foredeep wedges, this integrated dataset allows us to recognize: two different phases of source tectonics in the latest Oligocene and the middle Burdigalian; two major episodes of basin tectonics and related depocentre shift in the latest Oligocene and the Langhian, plus a minor middle Aquitanian phase; and three intervals of reduced regional turbidite deposition during the Late Aquitanian, Middle Burdigalian and Early Serravallian, possibly linked to sea-level rises.

Oligocene-Early Miocene tectonic evolution of the northern Apennines (northwestern Italy) traced through provenance of piggy-back basin fill successions

Abstract The provenance history of sediments deposited in the piggy-back basins of the Northern Apennines has been drawn by means of a petrographic study of nearly 200 sandstone samples collected over 250 km of the belt; it allows the evolution of the eroded part of the belt in Oligocene-Early Miocene times to be determined in detail, with special emphasis on the age of the exhumation and the onset of erosion of the high-pressure/low-temperature Pennine metamorphic units of the Ligurian Alps and Corsica that form the innermost part of the chain. Five petrofacies were distinguished, representing three sources that were active separately (three ‘pure’ petrofacies) or together (two ‘mixed’ petrofacies). The resultion sandstone composition reflects the erosion of different source units, changing through time and space along the belt. The stratigraphic distribution of petrofacies records a change in the main clastic source from Ligurian calcareous units to Penninic units. This change occurred over most of the study area, reflecting the complete exhumation of the Penninic metamorphic units within the innermost part of the belt. It occurred at different times along the chain, migrating from northwest to southeast from Late Rupelian to Aquitanian. This time shift is interpreted to be related to the obliquity of the Northern Apennines convergent system.

Seismic Stability Analyses of the Po River Banks

The Po River is the major Italian watercourse. Over half its length is controlled with embankments as protection measures against heavy floodings. Recently, the Italian Government has funded a project for the evaluation of the seismic stability of about 90 km of embankments of the Po River. The project mainly aims at the seismic stability analyses of the river banks, with assessment of local site response and evaluation of the liquefaction potential. Hundreds of geotechnical investigations within the study area were performed and the water level variations in the embankment and subsoil were investigated using piezometers. This paper describes the methodology and the main results of the analyses. The safety of 43 significant sections in static and seismic conditions was investigated using limit equilibrium analyses. Dynamic effects in the seismic condition were considered using the pseudostatic method. Local seismic hazard and effects of site conditions on the ground motion are taken into account in the definition of the expected seismic action. Eventually, the analysis results are summarized in a static and seismic stability map of the investigated area, a useful tool for the local Authority in the prevention and mitigation.

Seismic hazard of the Northern Apennines based on 3D seismic sources

Seismic hazard has been computed for the Northern Apennines in northern Italy based on a new seismogenic zonation. This zonation considers inclined (dipping) planes as seismogenic sources, defined on the basis of all the seismotectonic information available so far. Although these geometries are extremely rough because they simplify with a few inclined elements the totality of faults constituting a source, this model mimics the tectonic style better than that based on horizontal planes. Nevertheless, for a comparison between the new ground motions obtained and those available in the literature, the plane version of the zonation has been developed, where horizontal areas (the standard seismogenic zones), representing the surficial projection of the inclined planes, are used as seismogenic sources.