Rate of outward growth of the Mediterranean ridge accretionary complex

Abstract

The position as a function time of the deformation front on the southwest flank of the Mediterranean Ridge accretionary complex is constrained as follows: 1. (a) the deformation front is now active; 2. (b) the site of core BAN84-05GC was still near the abyssal plain when displaced shallow water benthic foraminifera of inferred African provenance were redeposited within an upper Pliocene age unit; 3. (c) the site of core BAN84-05GC on the outer flank of the Mediterranean Ridge was already within the topographically rugged accretionary complex when a Pliocene debris flow was emplaced; 4. (d) DSDP Site 125 had already been uplifted into a topographically elevated position when lower Pliocene pelagic ooze was deposited; 5. (e) a gypsum-bearing breccia in DSDP Site 125 requires that the site was either on the abyssal plain or within the tectonically active outer perimeter of the accretionary complex during the Messinian salinity crisis; 6. (f) DSDP Site 377 had already been uplifted into a topographically elevated position when middle Miocene age pelagic marl was deposited; 7. (g) DSDP Site 377 was still on or near the abyssal plain when early to lower-middle Miocene age, smectite-bearing turbidites of inferred African provenance were deposited; and 8. (h) the Mediterranean Ridge began to grow by offscraping against a backstop formed by the Alpine nappes of the Hellenic Arc at the time that subduction began (> 33 Ma). Together, these constraints define a range of potential growth curves for the Mediterranean Ridge, with a rate of outward growth of approximately 0.5 to 2 cm/yr. This growth rate is faster than that inferred for most other modern accretionary prisms, both as an absolute value, and as a fraction of the subduction velocity. An unusually thick incoming section and/or an unusually weak (evaporitic) décollement may contribute to the rapid growth rate. The inferred age of accretion does not increase linearly with distance from the deformation front; rather, there is an apparent acceleration of the rate of outward growth through time. This observation may reflect a true acceleration of growth rate in response to the onset of back-arc extension in the overriding plate (Aegean Sea), and/or in response to the development of a weak basal décollement following the introduction of halite into the incoming stratigraphic section. Alternatively, the non-linearity of the age vs distance curve may indicate that shortening of the offscraped sediments continues across the breadth of the Mediterranean Ridge, rather than being confined to the outer flank.