Goffredo Mariotti

The dip of the foreland monocline in the Alps and Apennines

The foreland monocline dips underneath thrust belts and accretionary wedges, both in oceanic and continental subduction zones. We present new data on the dip of the monocline in the frontal part of two orogens, the Alps and the Apennines. There is an overall difference between the dip of the relative monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0° in the remote foreland, to an average of 2–3° at the front of the thrust belt below the foredeep, to about 5° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. The regional dip of the monocline in the Apennines has an average of 4–5° at the front of the thrust belt below the foredeep, to about 10° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. There are areas where the dip exceeds 20°. The Apennines though topographically lower than the Alps present higher monocline dips and a deeper foredeep. Moreover, there are variations in the dip of the monocline moving along the strike of the two belts: the low values coincide with Permian–Mesozoic inherited horsts, whereas the steeper values correspond to basinal areas, and they usually match the salients of the thrust belt front. Within the salients the distance between thrust ramps increases. Therefore, there are two orders of mean values of the dip of the foreland monocline, the first at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100–200 km wavelength) within the single orogen. Lateral variations in the lithospheric buoyancy due to the inherited Mesozoic stretching may explain the second order variations in foreland dip, but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the Alpine and in the central-northern Apennines slabs.

Normal faulting vs regional subsidence and sedimentation rate

Abstract Normal faults occur in a variety of geodynamic environments, both in areas of subsidence and uplift. Normal faults may have slip rates faster or slower than regional subsidence or uplift rates. The total subsidence may be defined as the sum of the hangingwall subsidence generated by the normal fault and the regional subsidence or uplift rate. Positive total subsidence obviously increases the accommodation space (e.g., passive margins and back-arc basins), in contrast with negative total subsidence (e.g., orogens). Where the hangingwall subsidence rate is faster than the sedimentation rate in cases of both positive and negative total subsidence, the facies and thickness of the syntectonic stratigraphic package may vary from the hangingwall to the footwall. A hangingwall subsidence rate slower than sedimentation rate only results in a larger thickness of the strata growing in the hangingwall, with no facies changes and no morphological step at the surface. The isostatic footwall uplift is also proportional to the amount and density of the sediments filling the half-graben and therefore it should be more significant when the hangingwall subsidence rate is higher than sedimentation rate.

Mesozoic Syn- and Postrifting Evolution of the Central Apennines, Italy: The Role of Triassic Evaporites

Jurassic–Cretaceous syn- and postrift successions from the central Apennines were backstripped to gain information on the Mesozoic evolution of the passive margin of the Adriatic Plate. Early Jurassic rifting led to the development of a horst-and-graben paleogeography (the Latium-Abruzzi Carbonate Platform and the Sabina-Umbria-Marche Pelagic Basin). Subsidence curves were built for both carbonate platform and pelagic-basin domains from original and literature stratigraphic data. The paleodepositional depths of the deepwater sediments were reconstructed from field geology data, including new paleontological data. It is proposed that after the deposition of lower Hettangian shallow-water carbonates, an abrupt increase in paleowater depth, to 600–1000 m, occurred during the late Hettangian–Sinemurian synrift stage. The postrift stage was characterized by basin filling, with decreasing paleowater depths during the Jurassic, and by a new deepening during the Cretaceous. Our backstripping curves show, for the Sabina-Umbria-Marche Basin, a short period (<5 m.yr.) of rapid tectonic subsidence at the beginning of the Jurassic, followed by very slow (likely thermally controlled) or absent tectonic subsidence until the Cretaceous. The slight increase in subsidence observed from Cenomanian time is linked to a renewal of extensional tectonics. The Latium-Abruzzi Carbonate Platform shows variable subsidence rates in both place and time. Fast subsidence occurred in the Rhaetian–Hettangian, Toarcian, Berriasian, and Cenomanian and is linked with extensional or transtensional tectonic events. After the Early Jurassic rifting, subsidence rates (on average 30–40 m/m.yr.) affecting the Latium-Abruzzi Carbonate Platform were faster than those recorded by the Sabina-Umbria-Marche Basin. Faster postrift subsidence in carbonate platform areas is a geological paradox that is here explained by the lateral flow of upper Triassic evaporites toward the deepwater domains, as a result of higher sedimentary loading in the carbonate platform areas and the onset of a pressure gradient toward the pelagic basin at the depth of the evaporites.

Indicators of paleoseismicity in the lower to middle Miocene Guadagnolo Formation, Central Apennines, Italy

Convolute bedding—pillow horizons—of likely seismic origin are identified in a bioclastic carbonate succession, the Guadagnolo Formation, in the central Apennine Mountains of Italy. These sediments, which were deposited in a carbonate-ramp environment, are from Burdigalian to Langhian (Relizian) in age. The lower part, about 500 m thick, consists of marlstones, marly limestones, and calcarenites, representing cyclic, shallow-water, coarsening-upward sequences. The second part, about 100 m thick, is dominated by prograding bodies of calcarenites. The horizons containing the pillow beds are in the topmost of the lower part, about 30 m below the base of the overlying calcarenites. They are present at the same stratigraphic position from the Prenestini to the Ruffi Mountains across a distance of about 20 km. Pillows, 20 cm to more than 1 m thick, are present in all the deformed layers and consist of marly calcarenites, which differ texturally from the enclosing matrix. They are regarded as the product of deformation ensuing from liquefaction of a denser layer overlying a lighter, silty layer that is richer in clay. These structures are interpreted to reflect liquefaction processes induced by seismic shocks, and they correlate well with coeval Miocene tectonism in this sector of the Apennines. Mariotti, G., Corda, L., Brandano, M., and Civitelli, G., 2002, Indicators of paleoseismicity in the lower to middle Miocene Guadagnolo Formation, central Apennines, Italy, in Ettensohn, F.R., Rast, N., and Brett, C.E., eds., Ancient seismites: Boulder, Colorado, Geological Society of America Special Paper 359, p. 87–98.