Timing, kinematics and cause of Aegean extension: a scenario based on a comparison with simple analogue experiments

Since the early 1970s, most geodynamic models proposed for the Eastern Medi terranean consider that the initiation of extension in the Aegean, during t he late Middle Miocene or later, was a direct consequence of the westward e xtrusion of Anatolia away from the Arabia-Eurasia collision front. In the l ate 1990s, a few models have taken into account the increasing evidence for an earlier initiation of Aegean extension. arising mostly from the analysi s of deformation within the various Alpine metamorphic complexes of the are a. Based largely on our own investigations, we first present a review of th e data that reliably constrain the minimum age for the initiation of extens ion in these complexes. It shows that regional-scale extension with a patte rn of stretching orientations similar to that of the Pliocene-Pleistocene w as already strongly active in the Aegean before the onset of Arabian indent ation into Eurasia (Lower Miocene vs. Middle or Upper Miocene), This implie s that the initiation of Aegean extension did not result from the lateral e xtrusion of Anatolia. Instead, extension may have started, owing to a proce ss of gravity spreading of the continental lithosphere that had previously been thickened during Alpine collision. The main arguments favouring this i nterpretation are presented, and a preliminary scenario is proposed for the Neogene evolution of the Eastern Mediterranean, in which lateral extrusion of Anatolia occurred lately during southward spreading of the Aegean litho sphere. In order to test this hypothesis further, physical experiments simulating t he horizontal spreading of a continental lithosphere toward a free boundary have been carried out. The analysis of various elements of the deformation field (displacements, strain, rotations, fault pattern and kinematics), bo th in the experiments and in the Aegean domain, reveals an excellent agreem ent between the two. Not only do early stages of Aegean extension appear to fit the gravity spreading hypothesis, but so does the recent to present in ternal deformation of the Aegean. Transverse (approximate to E-W) active sh ortening observed in the northern Aegean is commonly taken as one of the mo st striking pieces of evidence that Aegean extension results from the later al extrusion of Anatolia. The experiments indicate that such transverse sho rtening may equally develop as a result of pure gravity spreading, associat ed with a pattern of faults and rotations that compares well with that of t he northern Aegean. Pure dextral shearing along some strands of the North A egean Trough fault system, and the related development of an asymmetric pat tern of faults in the northern Aegean, are eventually the only features tha t evidently arise from the extrusion of Anatolia, since less than 3 Ma, Finally, we argue that during the early stages of Aegean extension. since a t least 21 Ma and until ca. II Ma, the distribution of extensional strain o n a regional scale has been controlled by the distribution of earlier thick ening. Post-thickening thermal weakening, through partial melting of deep l evels of the crust, probably accounts for the localization of extensional s train within the areas of thick crust. Restoring the Aegean in its most lik ely configuration at the onset of bulk crustal extension suggests that (1) the extensional strain rate remained roughly constant, or has eventually de creased. since the onset of Aegean extension, and (2) the amounts of extens ion before and after 16 Ma have been roughly equal. (C) 1999 Elsevier Scien ce B.V. All rights reserved.