Giovanni Deiana

Structural styles, chronology rates of deformation, and time-space relationships in the Umbria-Marche thrust system (central Apennines, Italy)

Structural interpretation of geological and geophysical data available for the central Apennines (Italy) allowed us to draw, balance, and restore three geological profiles across the external zones of the Umbria-Marche thrust system and to evaluate the timing and rates of deformation in a roughly 4 Ma time window (from late Messinian to late Pleistocene time). From this data set we established a general correlation between spacing L and thickness D of the thrust sheets (which was used as a depth correction factor for deriving local versus regional depths to the sole thrust) and a time-space relationship between the closing time of activity of each thrust and the distance X from a reference point within the system.

Pre-orogenic tectonics in the Umbria-Marche sector of the Afro-Adriatic continental margin

Abstract The pre-orogenic deformation of the Afro-Adriatic paleomargin during Jurassic–Paleogene times is recorded in the Umbria–Marche regions of Central Italy by facies differences, lateral thickness variations and, most importantly, by faults. Structural data suggest the persistence of normal faulting not only during rifting (Late Trias–Bathonian) but also during drifting and more precisely in Late Cretaceous times. Syn- and post-rift extension caused the development of mainly NNW–SSE- and ENE–WSW-trending structures. Post break-up normal faults are found in several localities of the Apennines and generally coincide with older, syn-rift sites of stretching. One-dimensional numerical modelling of subsidence suggests that thinning during rifting was about 15%, compatible with the stretching factor reconstructed from geological sections. Late Cretaceous thinning was obviously less but still significant and estimated at ca 5%. The acquisition and analysis of the stratigraphic and structural data presented in this study allowed us to correlate the subsidence history of the Umbria–Marche basin with the tectonic events that occurred in pre-orogenic times, in this sector of the Apennines, thus deriving useful indications about the pre-Oligocene evolution of this segment of the perimediterranean mountain belt.

Stratigraphic and structural features of the Sibillini Mountains (Umbria-Marche Apennines, Italy)

In this paper we illustrate the stratigraphic and structural features of the Sibillini Mountains on the basis of a 1:40.000 geological map. Following the “Foglio 132 Norcia” (Geological Map of Italy at 1:100.000 scale; [Scarsella, 1941][1]), this new geological map is the first cartographic document that covers the whole area of the Sibillini Mts. This area is key for understanding the geological evolution of the external zones of the Apennine orogen, mostly owing to the pronounced structural elevation of the Apennine ridge at the Sibillini Mts. area. This allowed us to: a ) carry out stratigraphic and structural analyses for the Umbria-Marche sedimentary cover to the oldest units; b ) analyze the paleotectonic setting of this sector of the Afro-Adriatic continental margin and the behavior of pre-existing structures during the subsequent deformation events; c ) investigate in depth the major Apennine thrust front (“Sibillini Mountains Thrust”) exposed in several sites (Fiastrone, Ambro, Tenna and Tronto valley) and its relationship with the units of the adjacent Messinian foredeep. Inherited (pre-thrusting) structures played an important role on the tectonic evolution of the study area. In addition to those of Jurassic age, associated to thinning of the Adriatic continental margin (the most frequent), there are also some Cretaceous-Eocene extensional faults and others probably related to Miocene foreland deformation. These extensional faults show very limited, or none, reversal of slip during contractional neogenic deformation. Usually they were tilted and, subsequently, displaced by thrust faults; locally they were deformed by buttressing processes. The paleotectonic setting also influenced the development of minor folds, as is evident in the Mt. Bove anticlinorium. Shortening-related structures are mainly represented by asymmetric, northeast-verging thrust-related anticlines involving Mesozoic-Tertiary sedimentary successions. Among these structures, the most important and well-known is the Sibillini Mts. Thrust, which bounds the Apennine mountain front, separating it from the Marche-Abruzzi foothills. This major fault affects the eastern, vertical to overturned, limb of the arcuate shaped Mt. Fiegni-Mt. Vettore anticline, which shows several complications, mostly due to the occurrence of minor thrust splays forming isolated lens-shaped tectonic slices up to several kilometers-long. The geological cross sections show that in the southernmost part of the Sibillini Mts. Thrust the displacement is partly buried and forms a detachment located at the base of the Laga Fm. Much of the Neogene compressional structures has been dissected by NNW-SSE trending Quaternary normal and oblique-slip faults, some of which reactivated older extensional structures. These faults do not seem to crosscut the Sibillini Mts. Thrust. Some of them show fresh fault scarps in the substrate and/or affect recent continental deposits and are considered responsible for the intense seismic activity of the area. [1]: #ref-141

Ground effects and surface faulting in the September-October 1997 Umbria-Marche (Central Italy) seismic sequence

The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved useful for (1) defining the epicentral area and the location of the causative fault; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a better evaluation of the magnitude from intensity data. Of special interest was the observation of surface ruptures generated along segments of a system of normal faults already mapped as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm. Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.

Geological constraints for earthquake faulting studies in the Colfiorito area (central Italy)

On September 26, 1997, at 00.33 h(GMT), a Mw 5.7 earthquake occurred in the axial zone of theUmbria-Marche Apennines of central Italy, in the Colfiorito basin area. At09.40 h (GMT), a Mw 6.0 earthquake again struck the area withinthe Colfiorito basin, a major intramontane basin filled with Quaternarycontinental deposits. The two main shocks, and the associated aftershockswere within a roughly NNW-SSE trending zone of largest damage (Imax10), in which ground deformation has been observed. Along this trend,Cello et al. (1997a) had mapped a few capable faults, showingtranstensional to pure extensional kinematics. Field inspection of themapped faults, carried out after the main shocks, revealed that some ofthem were locally reactivated (for lengths of several hundreds metres andsurface slip in the range of 2–8 cm) during the September 26, 1997earthquakes.

Role and effects of pore fluid pressure in thrusting: The case history of the Umbria-Marche Apennines, central Italy

Structure, timing and rates of deformation, and the aspect ratio of the external zones of the Umbria-Marche thrust system (UMTS) have been investigated with the aim of comparing features and processes occurring during the growth of a tectonic wedge with those inferred from mechanically based model predictions. The results of the UMTS test seem to support the idea that most of the characteristic features of a thrust system may be considered as the result of the incremental, transient increase of the pore fluid pressure (Pf) as a consequence of the amount of shortening of a crustal section. The strong dependence of Pf on strain rates and on the thickness of the rock mass involved in the deformation may also help to explain local and/or regional variations in the aspect ratio of the deformed-state profiles of different fold and thrust belts.