The regional meaning of the Bouguer gravity anomalies in the Mediterranean

Abstract

In the qualitative interpretation of gravity anomalies, the first rule is to try to separate the long-wave anomalies from the short-wave ones: in a relative sense, the former are due to deeper mass variations, the latter to shallower ones. Main emphasis will be given here to long-wave phenomena: that is, to the regional anomalies. In the Mediterranean Sea, the regional Bouguer anomalies are almost everywhere positive, so the relative differences will be in the “intensity” of the anomalies. It can be observed immediately that the main basins are all characterized by very strong positive anomalies: Provencal: +210 mGal; Tyrrhenian: +250 mGal; Levantine: +200 mGal; Jonian: +310 mGal; Aegean: +170 mGal. The left column indicates the areas of oceanic crust, with a thickness of a few kms and denser mantle very near to the surface. In the Western Mediterranean, this crust is relatively young, since it was formed during different stages of the late Tertiary and Quaternary (35-20 Ma in the Provencal basin, 8–2 Ma in the Tyrrhenian). Reduced thickness of the crust, due to extensional phenomena, can be deduced from the pronounced positive Bouguer anomalies in the Valencia Basin, in the Ligurian Sea, in the Sicily Channel, and, on land, in the Tuscany and Latium-Campania provinces in Italy. The strong gradients in gravity all around the Western Mediterranean coasts are connected with the very strong and extended fault systems which characterized the (intense) subsidence and formation of the present basins from the Miocene to the present time (as indicated by seismicity). The extended peri-Apenninic external negative anomalies, mainly on land but continuing also into the sea, from Piemonte to Sicily, are caused by thick sedimentary basins; smaller negative anomalies indicate sedimentary basins all around the Tyrrhenian Sea, between Corsica and Elba, etc. The strongest positive anomaly of all in the Mediterranean Sea region is the Ionian area (+310 mGal), probably indicative of a paleo-Tethyan oceanic domain, at least in its northern part. The rest of the Eastern Mediterranean—at least as it results from the deep seismic reflection and refraction profiles—is composed of a thin continental (African) crust, covered by a very thick sedimentary series (that could be as old as Paleozoic). The Aegean Sea positive anomaly is due to thinning and rifting (in the South) of the continental (European) crust: a situation similar to the Tyrrhenian evolution, but younger.