The Ionian Sea: The oldest in situ ocean fragment of the world?

Abstract

.[1] It is well known that the Ionian Sea is characterized by thin (8-11 km) crystalline crust, thick (5-7 km) sedimentary cover, and low heat flow, typical for a Mesozoic (at least) basin. Yet seismic data have not yielded univocal interpretations, and a debate has developed on the oceanic versus "thinned continental" nature of the Ionian basin. Here we analyze the magnetic anomaly pattern of the Ionian Sea and compare it to synthetic fields produced by a geopotential field generator, considering realistic crust geometry. The Ionian basin is mostly characterized by slightly negative magnetic residuals and by a prominent positive (150 nT at sea level) "B" anomaly at the northwestern basin margin. We first test continental crust models, considering a homogeneous crystalline crust with K = 1 Â 10 À3, then a 5 km thick deep crustal layer of serpentinite (K = 1 Â 10 À1). The first model yields insignificant anomalies, while the second gives an anomaly pattern anticorrelated with the observed residuals. We subsequently test oceanic crust models, considering a 2 km thick 2A basaltic layer with K = 5 Â 10 À3 , magnetic remanence of 5 A/m, and a unique magnetic polarity (no typical oceanic magnetic anomaly stripes are apparent in the observed data set). Magnetic remanence directions were derived from Pangean-African paleopoles in the 290-190 Ma age window. Only reverse polarity models reproduce the B anomaly, and among them the 220-230 Ma models best approximate magnetic features observed on the abyssal plain and at the western basin boundary. The Ionian Sea turns out to be the oldest preserved oceanic floor known so far.

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

“…Another few of those small subduction zones are found in the Mediterranean (e.g. Royden, 1993;Malinverno and Ryan, 1986), but the Aegean slab is probably the last that remains powerful enough to be considered an active tectonic system, for the subducting Ionian lithosphere could be the oldest seafloor on Earth (Speranza et al, 2012). In the general framework of the now very slow AfricanEurasian collision, the subduction of the Aegean slab, that is attached to Africa, resembles the fixed mode of subduction of our models.…”

“…Carter et al, 1976;Hamilton, 1979;Bowin et al, 1980). Similarly, the Adriatic plate subducts underneath the Apennines; to the south, in the Ionian Sea, the subducting plate is oceanic, possibly the oldest worldwide (Speranza et al, 2012). To the north, the bathymetry is much shallower, which is diagnostic of a continental fragment, less dense than the underlying mantle.…”

Did Adria rotate relative to Africa?

“…Different approaches to this end, however, led to contrasting results. The Ionian Basin's sea floor is widely regarded as Mesozoic (e.g., Catalano et al, 2001; Frizon de Lamotte et al, 2011;Gallais et al, 2011;Schettino and Turco, 2011;Speranza et al, 2012), implying a semirigid connection between Adria and Africa since that time. Eastward increasing Neogene shortening in the Alps (Schön-born, 1999;Schmid et al, 2013), however, has been used to infer a Neogene ∼ 20 • counterclockwise (ccw) rotation of Adria relative to Eurasia , but only ∼ 2 • of which can be accounted for by Africa-Europe plate motion.…”

“…1). To the south, Adria is separated from the North African passive continental margin by oceanic lithosphere of the Ionian Basin (Catalano et al, 2001; Frizon de Lamotte et al, 2011;Gallais et al, 2011;Speranza et al, 2012).…”

“…1). Adria's conjugate margin across the Ionian Basin is likely the Hyblean Plateau of Sicily bounded to the east by the Malta escarpment (Catalano et al, 2001;Chamot-Rooke and Rangin, 2005;Speranza et al, 2012).…”