Burial and thermal evolution of the Sicilian fold-and-thrust belt: preliminary results from the Scillato wedge top basin (central-northern Sicily, Italy)

Abstract

Wedge top basins are key elements for unravelling the tectonic evolution of fold-and-thrust belts. In detail, their thermal signature and sedimentary fill records modes and time of exhumation of their edges. The Scillato basin is a wedge-top basin located in the central-northern sector of the Sicilian fold-and-thrust belt (western sector of the Madonie Mts). Upper Serravallian-upper Tortonian succession composed by up to 1,200 m thick delta-river to open marine siliciclastic sediments, fills the basin. This succession lies on a deformed substrate made up of thrust sheets composed of Numidian Flysch, Sicilidi and Imerese units stacked with a SW tectonic transport. The basin fill records a polyphase tectonic evolution with two non-coaxial compressional to transpressional tectonic events since the middle-late Miocene that caused changes in basin geometry, uplift of local structural highs and creation of new source areas for sediments. Organic matter optical analysis and X-ray diffraction of clay minerals have been performed from the basin fill and the substrate to unravel source to sink evolution. The organic matter shows two separate clusters of vitrinite reflectance (Ro%) in the basin fill. The first one has values of 0.4-0.5 Ro% with an increase of thermal maturity with depth. The second one has values of 0.7-0.8 Ro%, indicating reworked, more mature kerogen. The substrate shows higher Ro% values: from 0.6% in Numidian Flysch up to 0.9 % at the base of the Imerese unit. XRD on clay minerals presents, among the other phases, two population of mixed layers Illite-Smectite (R0 and R1 stacking order) in the wedge top basin. The first population, R0, indicates thermal maturity levels in agreement with the Ro% and is authigenic; the second one, R1, has detrital origin. The substrate shows R3 in the older formations of the Imerese unit and R1 in younger formations of the Imerese unit and Numidian Flysch. Coupling the results from the organic and inorganic fine fractions of the studied sedimentary successions, it has been possible to: 1. quantify the maximum sedimentary/tectonic loads and exhumation the wedge top and the substrate underwent; 2. identify, at least, two distinct source areas for the basin fill changing through time: the Numidian flysch in the initial stage of the basin development and the Imerese unit in the final stages of basin filling.