Tectonic evolution of the Cocos-Nazca spreading center

Abstract

Magnetic and bathymetric data from the eastern Pacific have been analyzed and a model for the evolution of the Galapagos region developed. The Farallon plate appears to have broken apart along a pre-existing Pacific-Farallon fracture zone, possibly the Marquesas fracture zone, at about 25 m.y. B.P. to form the Cocos and Nazca plates. This break is marked on the Nazca plate topographically by the Grijalva scarp and magnetically by a rough-smooth boundary coincident with the scarp. The oldest Cocos-Nazca magnetic anomalies parallel this boundary, implying that the early Cocos-Nazca spreading center trended east-northeast. This system soon reorganized into an approximately east-west rise–north-south transform configuration, which has persisted until the present, and the Pacific-Cocos-Nazca triple junction has since migrated north from its original location near lat 5°S. If correct, the combination of these simple geometric constraints produced the “enigmatic” east-trending anomalies south of the Carnegie Ridge. The axes of the Cocos-Nazca spreading center and the Carnegie Ridge are essentially parallel; this can lead to paradoxical conclusions about interpretation of the Cocos and Carnegie Ridges as hotspot tracks. Hey and others (1977) have shown that recent accretion on the Cocos-Nazca spreading center has been asymmetric, resulting at least in part from small discrete jumps of the rise axis. I show here that the geometric objections to both the “hot-spot” and “ancestral-ridge” hypotheses on the origin of the Cocos and Carnegie Ridges can be resolved with an asymmetric-accretion model. However, all forms of the ancestral-ridge hypothesis encounter more severe geometric difficulties, and these results support the hotspot hypothesis. After further elaboration of the hotspot hypothesis by Johnson and Lowrie (1972) and Hey and others (1973), Sclater and Klitgord (1973) examined both the hotspot and ancestral-ridge hypotheses and decided that both should be rejected, concluding that the Cocos and Carnegie Ridges “are not tectonically related” (p. 6973). The difficulty in the Galapagos area arises primarily because the older magnetic anomalies have proven extremely difficult to correlate, surprisingly so considering the high data density and the ease with which the very young anomalies are correlated. An important problem is the reason for this difficulty in correlating older anomalies — either clearly recognizable anomalies were never formed here, or some mechanism has acted in this area to destroy them after they were formed. On the basis of all available evidence, including new data presented here, I conclude that a model based on the hotspot hypothesis, with the modification of asymmetric accretion resulting at least in part from discrete jumps of the rise axis as discussed by Hey and others (1973) and demonstrated by Hey and others (1977), successfully meets the objection of Sclater and Klitgord (1973) and allows us to outline the history of the area from the break-up of the Farallon plate and birth of the Cocos-Nazca spreading center to the present. The “instantaneous” (a term Hey and others, 1977, have examined) configuration of plate boundaries and motions (Fig. 1) has generated a wedge of crust spread from the Cocos-Nazca spreading center, which is characterized by a slow spreading rate, rough topography, and strong magnetic anomalies. This wedge, termed the Galapagos gore by Holden and Dietz (1972) and discussed in detail by Hey and others (1977), is surrounded by crust spread from the Pacific-Cocos and Pacific-Nazca spreading centers which has the smooth morphology common to fast-spreading rises and low-amplitude magnetic anomalies, as both these segments of the East Pacific Rise are oriented nearly parallel to the Earth's magnetic field vector. My model explains the location and orientation of the magnetic and bathymetric rough-smooth boundaries that thus bound the gore and implies that there are two genetically different magnetic and bathymetric boundaries in the area.