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Authors: Basili, R. 
Title: La componente verticale della tettonica plio-quaternaria dell'Appennino Centrale
Issue Date: 1999
Keywords: tettonica compressiva
interpretazione sismotettonica
Subject Classification04. Solid Earth::04.07. Tectonophysics::04.07.07. Tectonics 
Abstract: This study combines different approaches in order to evaluate the vertical component of tectonic movements of the upper crust in regions of active mountain building. The study area covers a NE oriented, 40 km wide, and 220 km long strip of land which crosses the Central Apennines, from the Tyrrhenian to the Adriatic shorelines, and is aligned between the cities of Rome and S. Benedetto del Tronto. The Central Apennine is an East-verging fold-and-thrust mountain belt overriding and accreting on the subducting Adria plate. Compressional tectonics originated in Late Oligocene (25-30 Ma) and progressively migrated northeastward affecting Meso-Cenozoic marine carbonate sequences and Neogene fore-deep siliciclastic units. Since Late Miocene (ca. 6 Ma) the emerged portion of the mountain belt also experienced contemporaneous compression at the thrust leading edge and back-arc extension, all combined with crustal uplift. In this study observations were aimed at identifying geomorphic, geological, and structural features which could have better characterised either crustal blocks or faults and fault-systems. As such, a wide range of methodologies have been applied, and on the whole, deformations have been detected at different spatial scales (103-10-2 m) and over a wide time-window (106-103 years). An outline of the topography at regional scale was done by using a Digital Elevation Model, which allowed to calculate several geomorphic parameters. Geomorphologic features providing information about crustal uplift have also been identified and detected with the aid of aerial-photo analysis and field surveys. On the basis of these analyses the mountain belt shows to be remarkably asymmetric and to be characterised by five crustal blocks. The five blocks not only show different geomorphic parameters but also express the signature of different geological evolutions. They have in common a step-like topography but with different spacing which probably reflects the different vertical displacement they have undergone over the last few million years. Uplift rates relative to sea-level variations have been determined in coastal areas by analysing marine terraces. Whereas the analysis of remnant land-surfaces, in the interiors, helped evaluate the maximum expected rate of uplift, relative to the adjacent blocks, and the style of crustal deformation in correspondence of the main faults. From the study of remnant land-surfaces it is also evident that the intermontane basins, which are typical half-grabens controlled by normal faults on the eastern side, represent independent minor down-thrown areas with respect to the uplifting surrounding ranges. Crustal uplift rates, averaged over ca. 1 Ma, seems to be very slow (<1 mm a-1) in comparison to other mountain ranges. Net uplift rate in the Adriatic side is faster then that in the Tyrrhenian side, in accordance with the asymmetric shape of the range. Major fault-systems separate the five crustal blocks, over-thrusting affects the NE side of the mountain range whereas 3 main normal fault-systems displace the SW side. The normal faults commonly strike NW-SE, plunge SW, and have a dip-slip kinematics which clearly evidences a deformation field with NE-SW trending principal extension axis. Their age spans over the Plio- Quaternary and are progressively younger from SW toward NE. A detailed geo-structural study was carried on in the narrow belt affected by the easternmost normal fault-system by analysing meso- and micro-scale structural features, displaced land-surfaces, and displaced continental deposits at several outcrops and in trenches dug across the fault-traces. 14C dating confirmed that the fault belonging to this system have been active in Late Pleistocene - Holocene time with an average slip-rate of about 0.2-0.5 mm a-1. On the whole, the above observations could be summarised in a model which point at defining a complex geo-dynamic framework that includes an active W plunging subduction driving an East oriented orogenic wave with a consequent crustal extension in the rear that dominates the tectonic style of the back-arc. If this process is still active and carries on the model predicts that the next generation of normal faults will eventually be located to the East of the present ones. The zone affected by the 1997 Umbria-Marche earthquakes, located about 60 km NW of the above mentioned study area, has also been investigated to contribute at understanding the relationships between moderate earthquakes, the faults which are recognisable in the geological record, and the vertical deformation they are able to produce at the surface. By analysing geological and seismological data related to the earthquake swarm, it was found that the mainshocks occurred on September 26, 1997 (ML=5.6 and ML=5.9) originated on the same structure reactivating at depth a listric normal fault and did not break the surface. The largest event produced surface warping similar to those occurring in extensional forced-folds. This deformation was measured by repeated levelling surveys which allowed to detect a maximum of a few decimetre coseismic warping at 2 km distance from the fault-trace in the hanging-wall and a 20% post-seismic rebound. The earthquake occurred on October 14, 1997 (ML=5.5) originated on another fault-branch and produced surface ruptures, with an offset of a few centimetre, in an area where no faults were previously mapped. The analysis of the geological structures suggests a possible correlation between the long-term (Middle Pleistocene) cumulative effects of the Colfiorito Fault System (CFFS) and the short-term behaviour of the fault planes observed during this earthquake swarm, favouring the idea of a seismogenic source producing clustered moderate-size earthquakes rather than large events scattered in time. These results also suggest that the integration of geological and seismological observations can constrain the seismotectonic interpretation in regions of low level seismicity and that this integration can pose a reliable basis in the seismic hazard assessment.
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