NEOGENE STRIKE-SLIP BASINS AND THE WEAK FAULT CONCEPT FOR THE COLLISIONAL SUTURES OF ECUADOR AND COLOMBIA

The accretion of exotic terranes to the south American margin during Mesozoic and Cenozoic time characterizes Northern Andes. In Ecuador and Colombia several allochtonous terranes (or blocks) limited by collisional sutures have been identified (Fig. 1; MC Court et al., 1984; Mtgard, 1989; DuqueCaro, 1990; Van Thournout et al. , 1992; Aspden & Litherland, 1992, and references therein). In northwestern Colombia the Choc6 block is bounded to the east by the Cordillera occidental and to the south by the Istmina fault zone (IFZ). The coastal plain with the Cordillera occidental of Colombia and Ecuador form the oceanic Coastal Terrane. The Calacall-Pallatanga Suture (CPF) and the Cauca Fault (CF) limit the latter from the Chaucha-Amaime Terrane which accreted along the Peltetec (PF) and Romeral (RS) Sutures. In northern Peru, the Amotape-Tahuin Block (ATB) is bounded to the north by the Raspas Fault (RF) and to the east by the Las Aradas Fault (LAF). Strike-slip motion, related to increased subduction rate, occured along the sutures triggering basins formation during the Neogene (Baudino, 1995). The study of two Neogene strike-slip basins of Ecuador, structurally linked to the CPF and PF sutures, revealed that their evolution is however incompatible with classical pull-apart models.