Strain analysis of a Northern Apennine shear zone using deformed marble breccias

Abstract

The Alpi Apuane region of the Northern Apennines appears to have been deformed within a large-scale, low-angle shear zone with an overthrust sense of movement. The presence of mineral stretching lineations, folds progressively rotated into the X strain direction, and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation. The strain ratios and orientations on two-dimensional sections have been determined from deformed marble breccias, reduction spots, and oncalites. Data from three or more non-perpendicular, non-principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone. The finite strains have been separated into components of simple shear (γ), longitudinal strain (λ), and volume change (Δ). Algebraic expressions have been derived and graphs constructed which enable components of γ, γ and Δ, and γ and λ to be determined directly from a knowledge of strain ratio ( R) within the shear zone and the angle (θ) between the principal strain direction and the shear zone boundary. The Alpi Apuane data indicate that neither simple shear alone, nor simple shear with volume change can satisfactorily explain the observed strains. Consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases, and the values of longitudinal strain change along a SW-NE profile across the zone. Integration of the resulting shear strain-distance curves gives a minimum displacement of 4 km within the shear zone. Combination of the finite strains with the total time of deformation known from K/Ar studies leads to average strain rates from 1.4 to 9.6 × 10 −15 sec −1. A characteristic flat-ramp-flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone, and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing. Estimates of 21% crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the Northern Apennines.