Thick-skinned tectonics in the external Apennines, Italy: New evidence from magnetic anomaly analysis

Abstract

We discuss the tectonic implications of a new residual magnetic map of the Apennine belt/Adriatic-Apulian foreland obtained by integrating ground and offshore data sets [Chiappini et al., 2000a]. Negative anomalies are documented over the Adriatic-Apulian foreland areas, whereas the external Apennine belt is characterized by a ubiquitous lowamplitude (<30 nT), long-wavelength positive anomaly. In the central northern Apennines, three 100 km wide more intense (100–200 nT) round-shaped anomalies are superimposed to the long-wavelength feature. Finally, in the Tyrrhenian Sea and margins, high-intensity, short-wavelength positive-negative couplets coincide with magmatic outcrops or bodies at shallow depth. The low-amplitude anomaly pattern over Italy suggests that the magnetic basement beneath the Triassic evaporites is ubiquitously incorporated in the external belt compressive fronts, implying a thick-skinned tectonic style for the external Apennines. The new residual magnetic map resolves the inconsistency between previous aeromagnetic data [AGIP SpA. Italia, 1981], which suggested a lack of basement involvement in the Apennine belt, and recent seismic data, which imaged deep reflectors penetrating the basement. Two magnetic models along NE-SW transects in the northern and southern Apennines suggest consistent structural styles. In the northern Apennines, positive anomalies roughly coincide with the external compressive fronts, although there are local second-order differences between the belt front and the edges of the anomaly. Here the magnetic data show that the basement rises southwestward along the thrust fronts from 6–7 km depth in the Adriatic foreland to 2–3 km depth in the axial belt, where some exploration wells have penetrated basement. Within the belt front, basement exhumation is inferred to occur along high-angle, low-displacement thrust faults inverting preexisting normal faults. In the southern Apennines, a remarkable positive magnetic anomaly is parallel with and tens of kilometers southwest of the belt front. Seismic data and oil wells show that the basement surface cannot be shallower in the belt than in the foreland. Therefore the observed magnetic anomaly is produced by strongly magnetic basement beneath the belt, likely an internal crustal wedge tectonically interposed between the Apulian carbonate sequences and basement.