Structural model of the Trieste Gulf: A proposal

Abstract

Abstract The onshore geology of the Trieste Gulf is well known in Italian reliefs, as well as in Slovenian and Croatian ones (Istria peninsula). In the northeastern and southern part carbonate platforms outcrop mainly in shelf facies of the Cretaceous to the Paleogene ages and are covered by Eocene flysch deposits. In the northern part the Gulf is closed by the coastal line of the Friuli plain composed mainly of glacio-fluvial deposits of Quaternary age. From a structural point of view, overthrusts set in a typical Dinaric NW–SE direction dominate and where one can identifiy many tectonic units with a SW vergence, tending to overlap the Mesozoic–Paleogene carbonate units which lie over the Eocene terrigenous ones. In this general picture, some sub-vertical faults with an anti-Dinaric NE–SW orientation and a strike-slip activity, cut the previous overthrusts. On the other hand the offshore geological knowledge of the Gulf is scarce due also to the absence, in the inner part of the Gulf, of exploration wells. Only recent studies on some tectonic structures of the Trieste Gulf revealed by geophysical explorations made a few years ago, have suggested making a hypothesis on its formation that has never been studied before. The model proposed here starts from the hypothesis that all tectonic lines that come from the Italian, Slovene and Croatian coastline, continue offshore. This is validated by highlighting all the geometries and morphologic anomalies obtained by analyzing various documents that regard the buried subsoil and obtained through geophysical prospections. With this idea, the maps showing the isobaths of the top of the limestones, the depth and structures of the top of the flysch, the isobaths of the base of the Quaternary deposits and the morphology of the actual sea bed were compared. All these documents present many morphological elements, found repeatedly on maps with different finalities but which appear coherent with the general picture of the structural outcrop and with the starting hypothesis. Hence, the Gulf of Trieste, is situated on a graben defined by master faults in an anti-Dinaric NE–SE direction. These extend into the Gulf from two vertical faults documented in the onshore with a transtensional effect, and cut the NW–SE Dinaric thrusts that carry on length-wise into the offshore from the Istria coast. On the basis of the geometry of the top of the buried limestones and other pieces of evidence, we hypothesize also a new line, NE–SW oriented (Aquileia line) that could actually separate two buried sectors of limestone platform each with a different evolution.