Within the frame of a meso- and microstructural investigation on the metamorphic rocks of the Alpi Apuane, this contribution deals with an area located northeast of Carrara, the upper Colonnata valley. This area provides a wide range of structures and strain-related features formed during the compressive (D1) and the later (D2) events of the Tertiary tectono-metamorphic evolution of the Alpi Apuane complex, in the general frame proposed by Carmignani & Kligfield (1990, and references). Studies of the geometry of folds and related foliations, have been combined with a detailed microfabric analysis on the different types of marble tectonites. The analyses allowed to recognize structures related to the transition between the late stages of compressive phase and beginning of D2 evolution. The structures (two generations of successive folds and shear zones) we have focussed our attention on, appear to be related with a large scale strain partitioning that involves a component of coaxial deformation leading to a shortening in vertical direction. Thanks to microstructural evidence, we have been able to refer such deformational features to a retrograde thermal path in conjuction with the late development of the antiformal stack and at the beginning of D2 deformation.
The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U‐Th)/He and fission‐track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15–30 km and ∼400°C at about 20 Ma. Exhumation to 240°C and 9 km depth (below sea level) occurred at 10–13 Ma. By 5 Ma the Apuane rocks were exhumed to 70°C and ∼2 km. The Macigno and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at ∼15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (∼0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level.
The present contribution summarizes the first results of a study focusing on microstructures from Alpi Apuane marbles. Its aim is both an analysis of the evolution of the metamorphic complex recorded in marbles and the supply of basic material for process-oriented studies on calcite microstructures due to natural deformation. Quantitative analysis of the variations of statically recrystallized microstructures suggest a relationship with the peak metamorphic temperatures. Previously unrecognized post-thermal peak shear zones, showing overprint microstructures typical of grain-boundary migration and dynamic recrystallization, are described; they document the natural deformation of Carrara marble.
