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