Control of pre-orogenic inherited faults on Alpine thrusting and fluid circulation: an example from transverse zones of the Central Southern Alps (Lombardy, Italy)

Abstract

Multiple rifting phases can strongly influence the structural architecture and stratigraphic evolution of a developing passive margin. Some stratigraphic intervals can be characterised by distinct changes in thickness, lithology and facies controlled by synsedimentary faults. These features profoundly modify and alter the classic “layer-cake” model. The Central Southern Alps, and the Lecco area in particular, are a first-class example of interaction between inherited and contractional structures (Gaetani & Jadoul, 1987). A Ladinian rifting phase caused the coexistence of both deep- and shallow-water successions of Middle Triassic age, as well as considerable changes in their thickness across the study area. During the Early Jurassic, another rifting phase caused the drowning of the Late Triassic to Hettangian carbonate platforms, leading to the formation of intra-basin structural highs and lows as well as extreme lateral thickness variations within the syn-rifting succession. The most striking evidence of the role of inherited structures during Alpine contraction are N-S trending transverse zones parallel to the main orogenic transport direction (Schönborn, 1992). During orogenic build-up, rift-related faults were passively transported along thrusts, preserving part of the post-rifting, pre-orogenic framework within the same tectonic unit or were reactivated as large displacement transfer faults separating tectonic sectors with different shortening. These transverse zones and the inherited, pre-orogenic structural architecture strongly influenced thrust development: lateral ramps, oblique thrusting, younger-on-older-relationships and lateral transfer of displacement occur throughout the entire study area. Geological mapping and structural analysis have been conducted to reconstruct kinematics and geometries of fault zones. Several geological cross-sections have been realized to constrain the fold-and-thrust belt geometry and reconstruct the structural evolution of Central Southern Alps. The complex pre- and syn-orogenic tectonic history of fault activity, particularly of the main thrusts and transverse zones, has been constrained from in-situ U-Pb dating of syn-tectonic carbonates. Inorganic thermal indicators were used to constrain the eroded overburden and the exhumation depth of the faulted succession. Another goal of our work is to reveal how fluid circulation may change from the high-angle dipping, inherited and misoriented transverse zones to the low-angle thrust faults, from the internal to the frontal sectors of the belt. C-O stable isotopes and clumped isotopes analyses on syn-tectonic carbonates collected along thrusts and transverse zones have been performed to assess fluid-host rock chemical and thermal (dis)equilibrium. We compare compressional mineralizations with those exposed in transverse zones where fluids might circulate in an open to semi-open system, with the ingress of cold (meteoric) and/or hot (deep) fluids.Gaetani, M., and F. Jadoul. "Controllo ancestrale sui principali lineamenti strutturali delle Prealpi lombarde centrali." Rendiconti della Società geologica italiana 10 (1987): 21-24.Schönborn, G. (1992b). Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229–393.