Multiple tectonic phases and fluids variability along transverse zones of the Central Southern Alps (Lombardy, Italy)

Fold-and-thrust belts are usually deformed by major transverse zones oriented orthogonally to the main thrust faults. These structural discontinuities represent ancestral fault zones originated mainly during rifting and subsequently reactivated during orogenic shortening (Zanchi et al., 2012). Sectors displaying thicknesses and sedimentary facies changes within the syn-rifting successions are juxtaposed along transverse zones (Thomas, 1990). This variability results in diverse deformation and shortening styles, as thrusts exhibiting flat (detachment levels) or ramp (more competent lithologies) geometry occur at different depths.

Limited data exist on the origin of fluids circulating along transverse zones and how they control fluid flow at the regional scale. In this study, we analyse transverse zones in the Lecco area of the Central Southern Alps (Lombardy, Italy). This belt features N-S trending, km-scale transverse zones initiated as normal faults during Ladinian, Norian-Rhaetian and Early Jurassic rifting phases and subsequently reactivated as strike-slip faults and/or lateral ramps during the Alpine orogeny (Schönborn, 1992). Two transverse zones are foci of this work: the Lecco Line to the west, running along the southeastern branch of Como Lake, and the Faggio Line to the east.

Geological mapping and mesostructural analysis were conducted and geological cross sections were built to constrain the geometry and kinematics of such transverse zones. Inorganic thermal indicators were used to constrain the eroded overburden and the exhumation depth of the analysed fault zones. U-Pb radiometric dating on syntectonic calcite mineralizations allowed us to constrain the age of tectonic activity. Syntectonic calcite veins precipitated either during the Early Jurassic rifting phase or younger Alpine orogenic phases, testifying the complex and long-lasting tectonic history of these transverse zones. C and O stable isotopes analysis allowed us to assess the origin of fluids circulating within transverse zones and their degree of interaction with host rocks. Two significant findings emerged from these analyses: 1) calcite mineralizations with variable δ¹³C and δ¹⁸O values characterize these fault zones, pointing out different degrees of fluid-rock interactions and/or different origin of fluids circulating during rifting and orogenic shortening; 2) transverse zones and related structures bound sectors where mineralizations occasionally exhibit values within narrow ranges of δ¹³C and δ¹⁸O; conversely, in other sectors, isotopic values display significantly wider ranges. In conclusion, transverse zones in the Southern Alps are structural features with a complex tectonic and fluid flow history, as testified by the large variability of age and C-O stable isotopes values. Furthermore, they possibly exert a partial compartmentalization of fluid flow at the regional scale.

References:

- Schönborn, G. (1992). Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229–393.

- Thomas, W. A. (1990). Controls on locations of transverse zones in thrust belts. Eclogae Geologicae Helvetiae, 83(3), 727-744.

- Zanchi, A., D’Adda, P., Zanchetta, S., & Berra, F. (2012). Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy). Swiss Journal of Geosciences, 105(1), 19-38.