Gravity synclines

The Scurano-Vetto-Carpineti syncline in the northern Apennines (Italy) is a small, isolated, bowl-shaped syncline made of arenaceous rocks resting on a clayey substratum. This syncline was thought to have formed during lower Pliocene. Our work shows instead that it is much younger and that gravitati onal;tectonics is important in its formation. The syncline forms a mesa-lik e relief, with the top surface shaped as a "bowl". The outer slopes of this "mesa" are very steep and are cut by strata dipping inward. All around the mesa is a zone of maximum tectonic deformation in the clayey substratum. W e rule out regional tectonics in the generation of the syncline because (1) the structures follow the mesa boundary instead of the regional trends; (2 ) there are tectonic contacts that prove the existence of substratum clay d iapirs around the mesa; and (3) there are marine abrasion planes predating the deformation. Our deductions are supported by modeling: fluid-mechanical analysis shows that when there is a brittle plate overlying a ductile subs tratum, gravitational deformation is inevitable, leading to a synformal str ucture. In addition, a silicone-and-sand experiment shows that the deformat ion generates a complex syncline with a horst-and-graben structure in the s and plate and with diapirs in the silicon substratum. Thus our syncline for ms in a local extensional tectonic regime dominated by gravitational proces ses, in contrast to "common" synclines, which are the result of regional co mpressive tectonics. From the order-of-magnitude strain-rate of the synclin e we estimate the viscosity of the ductile Substratum to be 10(18) Pa s.