Kinematic and mechanic analysis of an exhumed case of Deep Water Fold-and-Thrust Belt: the Outer Tuscan Nappe, Italy

Fold-and-Thrust Belts occur worldwide in a variety of tectonic settings. Most of them develop in a deep-water environment (deep-water fold-and-thrust belts, DWFTBs), at both continental passive and active margins, mainly driven by gravity (near-field stresses) and/or tectonic forces (far-field stresses). Here we present a geological/geophysical study of the Outer Tuscan Nappe (OTN), an imbricate thrust system in the Northern Apennines of Italy, emplaced in Early Miocene times in a deep-water environment, as testified by the occurrence of syn-tectonic foreland basin deposits. The geometry and the kinematic evolution of the OTN were never reconstructed in detail. Furthermore, its total amount of shortening and its shortening rate, were never measured and calculated by restoration. The OTN involves a 2000 m thick, Late Cretaceous-Early Tertiary “Tuscan” Unit, consisting of thick succession of siliciclastic turbidites, deposited in the Oligocene-Early Miocene Tuscan foredeep overlying a complex succession of pelagic Eocene-Lower Oligocene marls and calcarenites, that form the major basal décollement of the imbricate system. Along this basal décollement, the OTN overthrusts eastward the younger Early-Middle Miocene turbidite units of the Umbrian foredeep. In this study we interpreted a set of 2D seismic reflection profiles calibrated with a deep borehole, crossing transversally (WSW-ENE) and longitudinally (NNW-SSE) the OTN. Three geological cross sections, based on detailed field mapping, were also constructed parallel to the transport direction (WSW-ENE) to better constrain the seismic interpretation. An integrated 33 km long geological cross section, sequentially restored, shows the internal geometry of the imbricate thrust system down to the main basal detachment. The latter shows thin-skinned deformation, where the reconstructed OTN tectonic wedge tapers progressively eastward, while the basal thrust becomes progressively shallower. The total measured shortening of the OTN imbricate thrust system is about 49 km, including 19 km of internal imbrication, 30 km of ENE-oriented transport along the basal décollement. Biostratigraphic data indicate that the OTN was emplaced during the Late Aquitanian – Late Burdigalian time interval with a shortening rate of about 10 mm/yr. The measured values of the wedge-taper angle, is about 8°. This relatively high value confirms that the OTN emplacement was driven by far-field stress, as indicated by the Fault Strength vs. Wedge Strength diagram (King and Morley, 2017). Our study helps to typify the OTN as an exhumed example of a deep-water contractional wedge, formed in response to deep-seated crustal shortening, during the early stage of collision. The study represents a first attempt of applying to fossil, exhumed fold-and-thrust belts, the concepts and classifications, recently proposed for modern DWFTBs.