Crustal image of the Ionian basin and its Calabrian margins

Abstract

Previous seismic investigation of the crustal structure in the Ionian basin has been limited to shallow penetration seismics of the 1970's, characterized by inadequate source power and low fold. Earlier OBS and ESP seismic refraction experiments have not been able to firmly resolve one of the principal scientific problems for this region which is whether the Ionian basin is floored by oceanic crust or by highly attenuated continental crust. A second elusive problem is the nature of the transition of the boundaries between the Ionian basin and its margins. In this paper we describe and interpret new deep seismic reflection and wide-angle data collected in the western Ionian Sea and the Calabria region of Italy. One of the principal features of our multichannel reflection data beneath the Ionian basin is a band of ‘layered’ high-amplitude reflections near the base of the crust. This band shows a quasi-monochromatic (ca. 8 to 10 Hz) frequency and a traveltime thickness of 1 to 1.5 s. These images contrast with the well known reflection patterns of Mesozoic oceanic crust investigated in the Atlantic Ocean. There is evidence that the low-frequency band dips down towards the edge of the Malta Escarpment (ME), where landward-dipping reflectors separate continental and intermediate type crust in the central tract of the ME. The increased traveltimes of the lower-crustal reflectors and Moho, from the basin towards the southern and eastern margins of southern Calabria, could be partially due to the velocity pull-down effect of the sedimentary pile of the arc, although a true dip of 15 to 18%, over 60 km distance, can be substantiated. Moreover, the reflecting band maintains its reflectivity and thickness until its abrupt termination beneath the Ionian continuation of the Calabrian compressional crustal structures. The coincident acquisition of wide-angle seismic data and marine reflection seismic data provided a landward extension of the survey which will complement existing geologic information on the deep framework of the Ionian basin and its Calabrian margin.