Forward magnetic models across the southern Apennines: implications for the basement setting

We discuss here the forward magnetic modeling, along a southern Apennines transect, of the new magnetic map of Italy (CHIAPPINI et alii, 2000a). This map has been realized through an integrated digital magnetic anomaly database built for the Italian territory and the surrounding sea. For the first time, all the magnetic observations made onshore and offshore the Italian peninsula have been compiled into a new digital database, that gives a complete regional scale view of the magnetic anomaly crustal field over the area. The new magnetic map shows a low-amplitude (~20 nT), long-wavelength positive anomaly along the external southern Apennines, and negative anomalies over both the Tyrrhenian margin of southern Italy and the Apulian foreland. South of the Vulture volcano (~41° N), the positive magnetic anomaly is elongated and exactly parallel to the external belt. Furthermore, the eastern anomaly margin is exactly parallel to the belt front, and located few tens of kilometers southwest of it. The negative pattern of the Tyrrhenian Sea and margins is likely due to the high thermal flow, which significantly reduces the thickness of the susceptive crustal layer. A magnetic modeling was performed along a selected southern Apennines transect (located ca. 50 km south of the CROP04 profile), where the shallow crustal setting (down to 5-6 km depth) is well-known by seismic and drill-hole data. Seismic data and oil wells suggest that the top of the magnetic basement cannot be shallower in the belt than in the Apulian foreland, where it was reached at ~6 km depth by the Puglia 1 well. Therefore, the positive signature of the external southern Apennines requires a strongly magnetic body beneath the belt. This source must be definitely more susceptive than the basement of the Apulian foreland. Since the solution of magnetic modeling is not unique, we show here three different settings that are able to bring into coincidence the observed and modeled residuals. First, we speculate that, within the belt, the basement located beneath the Apulian carbonates (Apulian basement) is differentiated and contaminated at depth by susceptive magmas or mantle fluids. Then, we explore the possibility that a very magnetic composite internal crustal wedge is tectonically interposed between the Apulian carbonates and basement. Two possible settings of such composite wedge are modeled and discussed. The uniformity of the magnetic anomaly pattern along 250 km of the belt, the remarkable parallelism between the southern Apennine positive residuals and the superficial belt front, associated with the gravity data, the estimates of Moho depth, and the flexural behavior of the Apulian lithosphere beneath the belt, support the two latter models