GEOLOGICAL REMARKS ON THE RELATIONSHIPS BETWEEN EXTENSION AND CONVERGENT GEODYNAMIC SETTINGS

In convergent geodynamic settings, extensional and compressional tecto nics occur contemporaneously or in different times and places, but the y are deeply linked to each other in a number of different ways. The m ain types of extensional regimes here considered are for west-dipping subduction zones: (1) back-are extension (e.g., Tyrrhenian Sea); (2) u plift and related extension due to asthenospheric wedging (e.g., Apenn ines); (3) extension at the subduction hinge (e.g., Marianas); (4) ext ension due to increasing length of arcs (e.g., Apennines). For east-, northeast- or north-northeast-dipping subduction zones: (5) extension induced by collapse of orogens (e.g., Alps, Himalayas); (6) extension induced by differential lithosphere-asthenosphere coupling in the hang ing wall of a subduction zone (e.g., Aegean rift); (7) rifting induced by orogenic roots anchored into the mantle (e.g., Atlantic and Tethys ). Other types may be considered: (8) inversion of plate motion, from convergent to divergent relative plate vectors due to external velocit y fields (e.g., Basin and Range); (9) transtension may occur along obl ique ramps of thrust area due to differential forward propagation of t hrusts planes (e.g., southern arm of the Barbados accretionary prism?) ; transtension and transpression also occur contemporaneously along an undulate strike-slip setting, or extension can accompany foreland dip ping duplexes; (10) apparent extension due to down-section cut of thru st planes along subduction zones. Each type of extension generates dif ferent shape, depth and relationships between hanging wall and footwal l of the associated normal faults and shear zones.