Gravity synclines

Abstract

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 gravitational tectonics is important in its formation. The syncline forms a mesa-like 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. We 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 diapirs 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 substratum, gravitational deformation is inevitable, leading to a synformal structure. In addition, a silicone-and-sand experiment shows that the deformation generates a complex syncline with a horst-and-graben structure in the sand plate and with diapirs in the silicon substratum. Thus our syncline forms in a local extensional tectonic regime dominated by gravitational processes, in contrast to "common" synclines, which are the result of regional compressive tectonics. From the order-of-magnitude strain-rate of the syncline we estimate the viscosity of the ductile substratum to be 1018 Pa s. Copyright 2000 by the American Geophysical Union.