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Fault-Parallel Stretching and Evolution of Growth Extensional Faulting in the Poorly Lithified Sediments of the Tarquinia Basin, Italy


Balsamo, Fabrizio1, Fabrizio Storti1, Francesca Cifelli1, Barbara Piovano1, Francesco Salvini1, Claudio Lima2 (1) Università degli Studi Roma Tre, Rome, Italy (2) Cenpes, Petrobras, Rio de Janeiro, Brazil


Sub-seismic scale extensional faults affecting poorly lithified Pliocene sandstones are exposed in a quarry within the Tarquinia basin, north of Rome. Syn-sedimentary faulting is indicated by along- and across-strike sediment thickness variations in the fault hanging­walls. The structural architecture of the study area is dictated by 20-30 m long NW-SE fault segments which are partially overlapped and connected by relay ramps. Estimated displace­ments never exceed 15m. Kinematic analyses on fault indicate pure dip-slip motions. Subsidiary faults and deformation bands accommodate most displacement at the fault tips. On the other hand, well cemented master slip surfaces accommodate most displacement in the central sectors of the fault zones. Fault damage zones are 5-10m wide and include sub­sidiary synthetic and antithetic extensional faults, joints and deformation bands. Near verti­cal joints are frequent in the footwall damage zones, and are systematically orthogonal to the fault strike. The anisotropy of magnetic susceptibility (AMS) in shaly sediments along transects perpendicular to a major fault zone shows that K1 orientations are consistent with the stretching directions inferred from structural data. We propose a structural evolutionary model for normal fault growth in poorly lithified sediments which involves the occurrence of fault parallel stretching and consequent jointing near perpendicular to the fault strike. The proposed evolutionary pathway has been validated by numerical-analytical modelling (FRAPtre). The occurrence and timing of fault-parallel stretching and related deformation has a considerable impact on the permeability properties of fault zones and on the evolution of permeability anisotropy through time.