Tectonic evolution of the internal sector of Central Apennines (Italy)

A wide sector of the internal portion of the CentralApennines, which comprises the southern Lepini Mtns up to the northern Simbruini Mtns has been investigated through detailed field mapping and integrated with structural analyses. A few small productive oil fields and a large number of hydrocarbon seeps and oil impregnations are located in this sector. This area offers good opportunities for testing the use of structural fieldwork methodologies in order to highlight oil migrating paths, from Triassic source rocks, and prospecting chances for oil field exploitation. The main stages of the structural evolution of the area took place after deposition of the foredeep sediments (Frosinone Fm.), i.e. after Late Tortonian, under a stress field characterised by a NE–SW trending σ1, which was responsible for the early emplacement of major thrust faults present in the area. The Messinian-Early Pliocene thrust-top basin deposits allowed the reconstruction of an in-sequence evolution of the thrust system. The development of out-of-sequence thrusting post-dates these structures leading to a further strong shortening phase in the area during the Pliocene. This phase is characterised by a roughly NNE–SSW trending σ1. Some peculiar tectonic features evidenced by thrust faults with younger-over-older relationships and an inversion of the original stacking of thrust sheets developed during this phase. Successively, a block-faulting tectonic, mainly with NE–SW extension stress field, occurred and dismembered the compressive tectonic edifice. Later on up to the Middle Pleistocene, N–S to NNE–SSW trending dextral strike-slip faults also acted in the area. Associated to the strike-slip tectonics are local volcanic centres as well as necks, whose compositions show a mantle origin, thus indicating deep seating and a possible lithospheric significance of these structures. In the light of this study, the reduced extension of the productive oil area as well as the spotting of oil seeps, may indicate that the migration conditions are not tied to well defined structures but that likely the cross-cutting points among structures facilitate the conditions for an upwards rising of oil. These conditions in particular are achieved at least in two cases: (1) where the Late Triassic source rocks do not have great depth due to normal or reverse faults, or (2) at a major depth when encountered by transcurrent-oblique roughly N–S trending faults—in both cases oil can easily migrate along the damage zone associated to the fault plane.