The petrologic and geochemical characteristics of the East Pacific Rise and other actively spreading ridges provide important direct evidence concerning the composition of the Earth’s upper mantle because mid-ocean ridge basalts are thought to represent partial melts of upper mantle material rising beneath mid-ocean ridges. Since the characteristics of the upper mantle can be used to constrain models for the accretion and differentiation history of the Earth as well as the nature of physical and chemical processes taking place in the mantle, MORB (midocean ridge basalt) and other oceanic basalts play an important role in constraining models of mantle convection, continent formation, and related questions.
This chapter describes the petrologic and geochemical characteristics of lavas from the axes of East Pacific spreading ridges, primarily the East Pacific Rise (EPR) between the Tamayo Fracture Zone and the equator, and the Galapagos spreading center (GSC). The Juan de Fuca Ridge and the Gulf of California are considered in other chapters of this book (Johnson and Holmes, this volume; Lonsdale, this volume).
Figure 1 shows locations of the principal sampling areas along the EPR and the GSC and Figure 2 shows the distribution of dredge hauls in the eastern Pacific. Until very recently, the spreading centers of the eastern Pacific were very poorly sampled compared to the Mid-Atlantic Ridge. The Juan de Fuca and Gorda Ridges are reasonably well sampled (Melson and others, 1976; Delaney and others, 1982; Davis and Clague, 1987; Dixon and Clague, 1986). The Gulf of California is mostly covered with sediment, but basalt has been recovered by drilling (Saunders and others, 1982; Lonsdale, this volume). The data base for the GSC includes analyses from Melson and others (1976), Schilling and others (1976), Fornari and others (1983), Christie and Sinton (1981), White and others (1987), and references therein. For the EPR, detailed studies have been conducted at 23°N near the Tamayo fracture zone (Bender and others, 1984), 21°N (Moore and others, 1977; Juteau and others, 1980; Hawkins and Melchior, 1980), 12° to 13°N (Hekinian and others, 1983; Batiza and Vanko, 1984; Macdougall and Lugmair, 1986), and 8° to 9°N (Batiza and others, 1977; Batiza and Johnson, 1980; Natland and Melson, 1980; Morel and Hekinian, 1980). Many of these papers include analyses of volcanic rocks from the EPR axis, as do Engel and Engel (1964), Engel and others (1965), Bonatti (1967), Kay and others (1970), and Sun and others (1979). Recent expeditions have greatly increased the number of dredge hauls of the EPR axis at 10° to 12°N (Thompson and others, 1985), 5°30′ to 14°30′N (Langmuir and others, 1986) and overlapping spreading centers (OSCs) at 12°54′N, 9°03′N, 5°30′N, and 3°57′N (Natland and others, 1986), but these studies are not yet fully complete.
Figures & Tables
This new synthesis includes a section on plate kinematics, documenting the basis for a new interpretation of the magnetic anomaly patterns. It also includes: six chapters on various aspects of tectonics, petrologic characteristics, and hydrothermal processes of active ridges from the Galapagos Rift to the Juan de Fuca Ridge; a section on mid-plate volcanism, including the Hawaii-Emperor chain; five chapters on various aspects of northeastern Pacific sedimentary regimes; and nine chapters on the geology of the Pacific continental margin from the Aleutians to Guatemala, seen from the perspective of marine geology. Three separate oversize plates illustrate the bathymetry of the northeast Pacific; two more on the same base show distribution of sediment samples and types and magnetic anomaly data and tectonic interpretations; and others include a synthesis of the geology and bathymetry of the Hawaiian Islands, details of bathymetry along parts of the East Pacific Rise, and a major seismic profile across the Pacific margin of Guatemala.