STRUCTURES, INTERFERENCE PATTERNS, AND STRAIN REGIME DURING MIDCRUSTAL DEFORMATION IN THE ALPI APUANE (NORTHERN APENNINES, ITALY)

Abstract

In this paper, we present results of new investigations and structural analyses on the geometries, strain patterns, and kinematics of folds in the Alpi Apuane, northwest Tuscany, Italy. The analyzed structures were formed in mid-upper crustal conditions (temperature and pressure range between 450 and 300°C and 0.6 and 0.4 GPa) during underthrusting and early exhumation of a slice of Adria-derived continental crust involved in the continental subduction process. In particular, we focused our attention on the relationships between folds, localized zones of deformation, and superimposed strain patterns. Kilometer-scale fold-nappes with strongly noncylindric 3D geometries developed during the early regional scale deformation event D1, whereas later largescale noncylindric folds associated with a subhorizontal crenulation axial planar foliation refold previous structures during D2. Although the early deformation D1 can be related to an important component of noncoaxial deformation, the following D2 was instead associated with a vertical shortening and a bulk coaxial strain. The above described D1 and D2 structures developed during crustal underplating and syncontraction exhumation record strain regime partitioned through time with an overall decrease in the noncoaxiality of deformation. This possibly reflects the movement of the unit across the deeper part of the Late Oligocene-Miocene Apenninic wedge, with a progressive increase of gravity-induced sagging in the rear part of the orogen. © 2006 Geological Society of America.