Pre- and syn-orogenic tectonic evolution of the transverse zones dissecting the Central Southern Alps (Lombardy, Italy)

Along-strike variations in deformation and structural build-up within fold-and-thrust belts are often controlled by pre-orogenic inheritance (e.g. Krabbendam & Leslie, 2010). This is the case of the south-verging Central Southern Alps in the Lecco area, where the E-W elongated belt is segmented along its strike by N-S oriented transverse zones, formed by the reactivation of early Mesozoic rift-related normal faults (Schönborn, 1992). These normal faults displaced the pre-rift sedimentary succession and controlled the facies distribution and thickness variation of syn- and post-rift Mesozoic carbonates. This led to the lateral juxtaposition of rocks with different rheological properties, which prompted the compartmentalization of the thrust system and the complex along-strike repartition of shortening across variable numbers of thrusts.

In this study, we reconstruct the early Mesozoic rift-related structures of the Lecco area and analyse their influence on the Alpine thrust system. Mesostructural analysis, geological cross-sections, burial history provided by the analysis of inorganic paleothermal indicators from clay-rich layers, and U-Pb dating of syn-tectonic carbonates have been integrated to investigate the role of inherited pre-orogenic structures within the Alpine orogenic context.

Three major tectonic phases were identified in the Early Mesozoic rifting processes by meso-structural analysis and U-Pb dating of syn-tectonic carbonates. N-S and E-W striking normal faults started to develop during the Ladinian marking the transition from isolated carbonate platform units to the basin successions. A second extensional pulse in Norian led to the formation of euxinic intra-platform basins within the massive Dolomia Principale carbonate platform, bounded by N-S and E-W striking normal faults. Finally, during the Early Jurassic, E-W and major N-S striking faults developed coevally with crustal thinning and the drowning of the carbonate platforms, leading to a generalized basinal sedimentation.

During S-verging thrusting and folding, the E-W striking faults were either passively translated and rotated or partially positively inverted. Some N-S striking inherited faults were also passively translated, while others underwent strike-slip reactivation; the latter are particularly evident within the N-S striking transverse zones, which exhibit complex tectonic settings with superimposed structures originated throughout different tectonic phases.

U-Pb dating of syn-tectonic carbonates from S-verging thrusts returns us Lower and Upper Cretaceous ages for the more internal structures, and Oligocene to Upper Miocene ages for the external thrusts and related folds. The latters suggest the reactivation of the Cretaceous orogenic structures. U-Pb dating of syn-tectonic carbonates along N-S striking transverse zones, instead, span unevenly from the Early Cretaceous to the Late Miocene, suggesting that these structures acted as long-lasting structural elements that remained active throughout all the stages of the orogenic build-up.

 

Krabbendam, M., & Leslie, A. G. (2010). Lateral variations and linkages in thrust geometry: the Traligill Transverse Zone, Assynt Culmination, Moine Thrust Belt, NW Scotland. Geological Society, London, Special Publications, 335 (1), 335–357

Schönborn, G. (1992). Alpine tectonics and kinematic models of the central southern alps. Memorie Di Scienze Geologiche, 44, 229–393