Bonini, M.

Del Ventisette, Chiara

Montanari, Domenico

Sani, Federico

English

Florence Research

We present the results of an integrated study betweenfield analyses and analogue modelling approach of theVena del Gesso Basin in Western Romagna (Italy). Thisbasin is one of the few areas in the Northern Apenninesforeland where widely crop out primary evaporites,related to the Messinian Salinity Crisis of the Mediter-ranean. These deposits give rise to an elongate and wellexposed gypsum ridge named «Vena del Gesso», whichattains a NE-dipping monoclinal attitude (e.g. MARABINI& VAI, 1985; ROVERI et alii, 2003) (fig. 1).Geological mapping together with the analysis ofstructures highlighted that the evaporites are completelydetached at the base and that back-thrusts repeatedlydoubled these deposits, strongly contrasting with theregional forelandward vergence of structures in the Apen-nines. The tectono-sedimentary architecture, like theangular unconformity exposed spectacularly at Brisi-ghella (e.g. MARABINI & VAI, 1985), allowed us to con-strain the timing of deformation.Analogue modelling techniques were applied to gain abetter understanding of the mechanisms responsible for thedevelopment of the structural pattern characterising thestudied area, specifically the factors favouring the back-thrusts development. Analogue models were performed atthe «Tectonic Modelling Laboratory» of the Department ofEarth Science and of the CNR-IGG in Florence.The experiments were geometrically, dynamicallyand cinematically scaled (e.g. WEIJERMARS & SCHME-LING, 1986) in order to simulate the geological boundaryconditions of the natural prototype. The models approx-imated the natural rheological multilayer and had initialdimensions of 24 cm × 25 cm × 2.8 cm. The sand-sili-cone models were built in a Plexiglas squeeze box andscaled with length ratio l*=2 10-5 (1 cm in the modelrepresents about a 0.5 km in nature). The models werethen shortened at the constant displacement rate of 0.6cm h-1 (in a normal gravity field) by a rigid wall drivenby an electric motor.Based on the comparison between analogue modelresults and field data, the overall features affecting thearea are interpreted to result from the deformation linkedto the sequential activation of an obliquely propagatingpassive-roof thrust (Banks and Warburton, 1986; andfig. 2) linked at depth to a NE-verging blind thrust anticlinein the underlying Marnoso-arenacea Formation. Thisthrust anticline is in fact almost completely overthrust bythe foreland-dipping back-thrust imbricate, which is com-posed of splays emanated from the main passive-roofthrust localised along the frontal anticline zone.Moreover, the experimental results evidence themajor role played by syntectonic erosion that dictated thedeformational style. This is illustrated by two models(VdG04 and VdG 05) sharing the same initial set-up,velocity of deformation and bulk shortening, but erosionwas applied to the internal sectors of model VdG-04 only.The comparison of top view photographs and cross sec-tions (fig. 3) between the two models highlights thatmodel VdG-04 developed a quite different structuralarchitecture in respect to the other (model VdG05), thusRend. Soc. Geol. It., 1 (2005), Nuova Serie, 144-145, 3 ff.Kinematic evolution of the Vena del Gesso (Romagna, Italy),analogue modelling of the interaction between erosion and tectonicsDOMENICO MONTANARI (*), CHIARA DEL VENTISETTE (*), MARCO BONINI (**) & FEDERICO SANI (*)(*) Dipartimento di Scienze della Terra, Università degli Studidi Firenze, via G. La Pira, 4, 50121 Florence, Italy. Tel. 055/2757528;montanari@geo.unifi.it(**) C.N.R., Istituto di Geoscienze e Georisorse, Sezione diFirenze, via G. La Pira, 4, 50121 Florence, Italy.Fig. 1 - Panoramic photograph showing the characteristic appearance of the Vena del Gesso monocline.KINEMATIC EVOLUTION OF THE VENA DEL GESSO (ROMAGNA, ITALY) 145confirming that erosion plays a key role controlling thedeformation style. Particularly, by eroding the internalpart of the model the deformation remained restricted tothe area near the moving wall, without propagating out-ward (compare the cross sections in fig. 2a and b). Themost noteworthy divergence is that the structures affect-ing the models exhibit an opposite sense of the overallvergence. In the model affected by erosion, propagationof shearing along the ductile silicone layer gives rise topassive back-thrusting of the roof sequence, this lattergliding passively over the underlying blind fore-thrust.Model VdG05 (without erosion) exhibits instead mostly aforeland vergence.In conclusion, we suggest that the dominance ofback-thrusting characterising the Vena del Gesso area isconsistent with the development of a thin-skinned passiveroof duplex, in which the erosion acted as one of themain driving factors.REFERENCESBANKS C.J. & WARBURTON J. (1986) - Journal of Structural Geology,8, 229-237.MARABINI S. & VAI G.B. (1985) - Bollettino della Società GeologicaItaliana, 104, 21-42.ROVERI M., MANZI V., RICCI LUCCHI F. & ROGLEDI S. (2003) - GSABullettin, 115, 387-405.WEIJERMARS, R. & SCHMELING H. (1986) - Phys. Earth Planet.Inter., 43, 316-330.Fig. 3 - Representative model longitudinal cross sections; the comparison between the cross sections highlights the contribution of sin-kine-matic erosion in promoting passive roof thrusting geometries.Fig. 2 - Schematic cartoon illustrating the passive-roof thrustingconcept.

Kinematic evolution of the Vena del Gesso (Romagna, Italy), analogue modelling of the interaction between erosion and tectonics

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Kinematic evolution of the Vena del Gesso (Romagna, Italy), analogue modelling of the interaction between erosion and tectonics

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