The Cocos and Carnegie Ridges are two prominent submarine aseismic ridges that dominate the basin morphology of the eastern Panama Basin (Fig. F1). The Cocos Ridge is an ~1000-km-long and up to 200-km-wide positive morphological feature on the ocean floor of the Cocos plate. It reaches elevations of <1000 m below sea level and is thus considerably shallower than the surrounding oceanic crust of the Cocos plate with water depths of >4000 m. Its northeast trend (45°) is almost normal to the strike of the Middle America Trench, along which it is being subducted off Costa Rica and Panama. The indentation into the Central America landbridge leads to strong uplift and exhumation of deep crustal stockworks (Graefe et al., 1997; Graefe, 1998; Meschede et al., 1999). A paleogeographic restoration juxtaposes the smaller Malpelo Ridge, today located east of the Panama Fracture Zone on the Nacza plate (Fig. F1), in prolongation of the Cocos Ridge (Hey, 1977; Lonsdale and Klitgord, 1978; Meschede et al., 1998). An ~250 km missing part of the once-continuous Cocos-Malpelo Ridge system has already been subducted beneath the Central America landbridge. The Coiba Ridge south of Panama has been suggested to have been formed not as a hotspot trace but rather by uplift beside a long meridional transform fault during the late Miocene and Pliocene (Lonsdale and Klitgord, 1978). Its origin, however, remains unclear because of the lack of data.

The Carnegie Ridge is an ~1350-km-long and up to 300-km-wide structure on the ocean floor of the northern Nazca plate (Fig. F1). It is separated into two elongated triangular-shaped parts and includes the Galápagos archipelago at its western end. Its east-west trend is almost normal to the strike of the Peru-Chile Trench, along which it is being subducted beneath the South American plate. Although in contrast to the Cocos Ridge, uplift and exhumation of the upper plate are not observed at the indentation front of the Carnegie Ridge; therefore, the indentation of the ridge into South America is interpreted to have started at ~2 Ma (Gutscher et al., 1999).

The Cocos, Malpelo, and Carnegie Ridges are interpreted to be hotspot traces that began to form when the Galápagos hotspot initiated at ~20-22 Ma (Hey, 1977; Lonsdale and Klitgord, 1978). Meschede et al. (1998) demonstrated that the products of hotspot volcanism overprinted a complex pattern of oceanic crust formed at three subsequently active and differently oriented spreading systems where, in contrast to older reconstructions (e.g., Hey, 1977; Lonsdale and Klitgord, 1978), symmetric spreading occurred. The identified extinct spreading systems represent precursors of the presently active Cocos-Nazca spreading center (Fig. F1).

During the late Oligocene, the Farallon plate split into the Cocos and Nazca plates as a result of global rearrangement of plate boundaries (e.g., Silver et al., 1998). Based on interpretations of magnetic anomalies, the oceanic crust of both the Cocos and Nazca plates was formed along the presently active Cocos-Nazca spreading center (CNS-3), its two precursors (CNS-2 and CNS-1) (Meschede et al., 1998), and the East Pacific Rise. The oldest magnetic anomalies, which belong to the CNS-1 system, are identified as Anomaly 6B, giving an age of 22.8 Ma (according to the geomagnetic polarity time scale of Cande and Kent, 1995), and thus dating the splitting of the Farallon plate to this time period. The first spreading system (CNS-1) was active until 19.5 Ma, when the orientation of the spreading axis changed from northwest-southeast to east-northeast-west-southwest. The second spreading system (CNS-2) was abandoned at 14.7 Ma, when the presently active east-west-oriented CNS-3 started its activity. Sharp and discordant contacts of magnetic anomalies (Barckhausen et al., 1998; Meschede et al., 1998) indicate abrupt changes in spreading direction. The boundary between oceanic crust formed at the Cocos-Nazca spreading center and the East Pacific Rise is marked by the rough/smooth boundary (RSB in Fig. F1).