Orogens and slabs vs. their direction of subduction

Subduction zones appear primarily controlled by the polarity of their direc tion, i.e., W-directed or E- to NNE-directed, probably due to the westward drift of the lithosphere relative to the asthenosphere. The decollement pla nes behave differently in the two end-members, In the W-directed subduction zone, the decollement of the plate to the east is warped and subducted, wh ereas in the E- to NNE-directed, it is ramping upward at the surface. There are W-directed subduction zones that work also in absence of active conver gence like the Carpathians or the Apennines. W-directed subduction zones ha ve shorter life (30-40 Ma) than E- or NE-directed subduction zones (even lo nger than 100 Ma). The different decollements in the two end-members of sub duction should control different PTt paths and, therefore, generate variabl e metamorphic assemblages in the associated accretionary wedges and orogens . These asymmetries also determine different topographic and structural evo lutions that are marked by low topography and a fast 'eastward' migrating s tructural wave along W-directed subduction zones, whereas the topography an d the structure are rapidly growing upward and expanding laterally along th e opposite subduction zones. The magmatic pair calc-alkaline and alkaline-t holeiitic volcanic products of the island are and the back-are basin charac terise the W-directed subduction zones. Magmatic rocks associated with E- o r NE-directed subduction zones have higher abundances of incompatible eleme nts, and mainly consist of calc-alkaline-shoshonitic suites, with large vol umes of batholithic intrusions and porphyry copper ore deposits. The subduc tion zones surrounding the Adriatic plate in the central Mediterranean conf irm the differences among subduction zones as primarily controlled by the g eographic polarity of the main direction of the slab. The western margin of the Adriatic plate contemporaneously overridden and underthrust Europe tow ard the 'west' to generate, respectively, the Alps and the Apennines, while the eastern margin subducted under the Dinarides-Hellenides. These belts c onfirm the characters of the end-members of subduction zones as a function of their geographic polarity similarly to the Pacific subduction zones. (C) 1999 Elsevier Science B.V. All rights reserved.