The Guatemalan Margin, the Nicoya Complex, and the Origin of the Caribbean Plate

Abstract

One of the most important aims of the DSDP Leg 67 investigation of the Middle America Trench off Guate- mala was to test for and, if possible, confirm the ex- istence of an accretionary prism in an active margin that constitutes a classic example of a Benioff zone and is characterized by a high rate of subduction (10 cm/yr.). We had planned a series of deep drill holes on the platform and continental slope along a transect perpen- dicular to the Trench axis, in depths ranging from 1600 to 5500 meters (Fig. 1A and B). We chose this area be- cause previous detailed geophysical work suggested that the continental platform is underlain by widespread Tertiary sediments overlying deformed older sequences, including sheets of basic and ultrabasic rocks (Fig. lC) (Ladd et al., 1978). This interpretation is supported by a strong magnetic anomaly bordering the continental platform (Fig. 2) and aligned with the Nicoya and San- ta Elena peninsulas of Costa Rica (Woodcock, 1975; Couch, 1976; Ladd et al., 1978), where Mesozoic vol- canic rocks and sediments (the Nicoya Complex) and ultrabasic rocks, thought to represent mainly oceanic crust, are exposed (Dengo, 1962b, 1973; Weyl, 1966; Henningsen and Weyl, 1967; Pichler et al., 1974; Sti- bane et al., 1977; de Boer, 1979; Galli, 1979; Schmidt- Effing, 1979; Azéma and Tournon, 1980; Kuypers, 1980). Unfortunately, the presence of abundant gas in the drill holes prevented accomplishment of the original aim —drilling at Sites 496, 497, and 498 had to be stopped. For reasons of safety drilling at the other planned sites on the slope and platform was also abandoned. Thus Leg 67 did not provide direct evidence for the presence of old oceanic sediments off Guatemala. The presence in this area of rocks equivalent to the Nicoya Complex is probable but remains a hypothesis.