Quaternary slip-rates probabilistic estimation for the Northern Apennines frontal thrust in the Po Plain (Northern Italy) by integrating surface and subsurface data

Abstract

The Northern Apennines thrust front in the Po basin exhibits active blind thrusts and associated anticlines, with some anticline crests either emerging or shallowly buried beneath late Pleistocene continental deposits. This study focuses on the outcropping San Colombano Structure and its buried neighbouring Casalpusterlengo-Zorlesco Structure, representing thrust-controlled anticlines in the central part of the Po basin. We reconstruct the Pleistocene evolution of these anticlines by integrating previously published surface geological maps and subsurface geological constraints from geophysical data and boreholes. We performed a trishear inversion of the deformation observed after the decompaction of the sediments. We used the solutions of the trishear inversion to compute the probabilistic distribution of slip rates over distinct time intervals. Our findings align with previous estimations of long-term slip rates in the Po Plain during the Quaternary, revealing rates of approximately 0.63 mm/yr and 0.53 mm/yr over the past 2.4 Myr for the San Colombano and Casalpusterlengo-Zorlesco Structures, respectively. The analysis of stratigraphic markers unveils a general decrease in faults activity during the Pleistocene, with slip rates around 0.2–0.3 mm/yr in the last 0.3 Myr, along with a diverse evolution of the thrust faults governing the two anticlines. Specifically, the activity rates of the San Colombano Structure supersede that of the Casalpusterlengo-Zorlesco Structure during the Middle to Late Pleistocene, implying an out-of-sequence propagation of the San Colombano ramp-anticline in the Late Pleistocene along an oblique right-lateral transfer zone. Incorporating a probabilistic approach in slip rates calculation provides a more comprehensive handling of uncertainties. This attribute is pivotal in seismic hazard assessment analyses and understanding complex fault systems' tectonic evolution.