Reconstruction of burial and exhumation history of the apenninic-Maghrebian fold-and-thrust belt in eastern Sicily by means of integrated studies of thermal and thermochronological constraints

Abstract

The purpose of this study has been to decipher the maximum paleo-temperatures and the exhumation rates of the sedimentary successions constituting the Eastern Sicily fold-and-thrust belt through a multidisciplinary approach based on the integration of different thermal and thermo-chronological methodologies. In particular, three sectors of the Eastern Sicily fold-and-thrust belt were investigated: the Internal Zone of the Peloritani Mts., the frontal part of the thrust belt in the Mt. Judica area and the outcropping area of the Numidian Flysch. In the Peloritani Mts., the integration of Vitrinite reflectance and mixed-layered clay minerals with published apatite fission-track and (U-Th)/He ages allowed to reconstruct the paleo-geothermal gradient of the Stilo-Capo d’Orlando basin in Oligocene-Miocene times, to constrain its burial evolution and discriminate between areas where it has been affected by sedimentary and/or tectonic load. In the southern area of the basin, organic and inorganic thermal parameters increase as function of depth suggesting that their evolution is ruled by sedimentary burial. They recorded a decrease in paleo-geothermal gradient values which marked the evolution of the basin from a fore-arc to a thrust-top setting during the convergence-collision process between the Calabria-Peloritani Arc and the African plate. Differently, in the northern edge of the basin, high vitrinite reflectance values (0.46-0.58%) indicate that the thermal evolution of this area was controlled by tectonic burial related to a late Langhian-early Serravallian out-of-sequence thrusting. The tectonic overburden has been totally removed by extensional tectonics and/or erosion since Late Miocene. The short time span at maximum temperature (<2 Ma) elapsing between thrust stack emplacement and the beginning of its removal have allowed only vitrinite reflectance and thermo-chronological indicators to record this compressive reactivation. In the Mt. Judica area, the integration of X-ray diffraction (XRD) data concerning the illite content in mixed layer illite-smectite with data derived from Fourier Transform Infrared spectroscopy (FTIR) on H-rich organic matter allowed to constrain the burial- exhumation path of the Mt. Judica sedimentary succession cropping out in tectonic window in Eastern Sicily. Thermal constraints showed that the Mt. Judica succession experienced paleo-temperatures in the range of 100-130 °C in late diagenetic conditions and early mature stage of hydrocarbon generation with a general depth-dependent thermal maturity feature. Specifically, the illite content in mixed layer I-S increases from 50 to 76% and FTIR-derived indexes suggest a thermal maturity equivalent to VRo values of at least 0.5-0.7%. As a whole, the Mt. Judica succession experienced maximum tectonic burial (ranging between 2.4 and 3.2 km) during the Middle Miocene as a result of the emplacement of the Allochthonous Units atop it. The subsequent breaching phase characterized by up-thrust geometries, and erosion during Pliocene times ruled out the Mt. Judica exhumation. This last tectonic phase did not overprint thermal maturity because the extent of overthrusting was negligible when compared with the magnitude of vertical movements. Restoration of balanced cross-sections revealed an increase of shortening from the salient to the Mt. Judica recess with values from 12.3 to 23.9 km, consistent with the increase of tectonic thickening of the fold-and-thrust belt. Integration of maximum burial and shortening values along the strike of the Sicilian fold-and-thrust belt allowed to reconstruct the wedge paleo-geometry in the Mt. Judica recess area, to investigate the along-strike variations of the tectonic overburden, and to discuss the geodynamic causes of these changes. These results were compared with theoretical models of wedge dynamics. For the Numidian Flysch thrust stack an increasing level of diagenesis from the uppermost to the lowermost tectono-stratigraphic units has been observed. From the top to the bottom of the tectonic pile, the Nicosia Unit showed the lowest % I values in I-S mixed layers in the range of 20-50% and vitrinite reflectance data between 0.36-0.42% in the immature stages of hydrocarbon generation. The Mt. Salici Unit displayed random ordered mixed layers I-S with an illite content of 50-55%. These data correlated to slightly higher levels of diagenesis than those recorded in the Nicosia Unit, suggesting that sedimentary burial is the main factor affecting the thermal maturity of both units. The lowermost of the Numidian tectonic pile (Maragone Unit) revealed mixed-layered ordered structures with % of I in I-S between 66 and 79% indicating the highest levels of thermal maturity for the Numidian Flysch. The integration of organic and inorganic thermal parameters with thermo-chronological data performed for my PhD project on the main outcropping tectono-stratigraphic Units in Eastern Sicily, contributed to the reconstruction of the burial and exhumation history of the Peloritani Mts. and the Apenninic-Maghrebian fold-and-thrust belt between the Nebrodi Mts. and the Hyblean Foreland.