Transforms of the eastern central Pacific
The regional maps of the northern Pacific Basin by Menard and Dietz (1952) established that offsets in the California margin, first documented by Murray (1939) and Shepard and Emery (1941), could be traced westward as bands of grossly irregular topography for more than 2,000 km. Typically these swaths of disturbed terrain separated regions of different depths and were made up of troughs, ridges, escarpments, and seamounts. This distinctive terrain was called a fracture zone, and continued reconnaissance mapping efforts by Menard and colleagues identified a family of subparallel fracture zones that could be traced as lineaments for thousands of kilometers (Menard, 1955; Menard and Fisher, 1958). The straightness of these fractures, their continuity along strike, and the fact that they separated terrain of contrasting depths as well as distinctive and correlatable north-south-trending magnetic anomalies (Mason, 1958; Vacquier and others, 1961) suggested that fracture zones were the product of faulting, with the dominant displacement having been horizontal.
The recognition of a world-girdling and seismically active Mid-Oceanic Ridge system established that this feature was the major morphotectonic element of the ocean basins (Ewing and Heezen, 1956; Heezen, 1957). Models of ridge formation proposed that the ridge axis was the site of extension, stretching, and volcanism (Heezen, 1960; Menard, 1960; Hess, 1962). The long, straight scars of the fracture zones that frequently offset the axis of the Mid-Oceanic Ridge were thought to be large strike-slip faults that broke the ridge into large blocks with differential motion taking place along the length of the fault. Wilson (1965) proposed that major faults, orogenic belts, trenches, and the Mid-Oceanic Ridge were not isolated features but are part of a continuous network of mobile belts that encircle the Earth. In this model, the portion of a fracture zone that links the extensional regions of the two offset ridge axes was called a transform fault. The key prediction of the model was that the relative motion along the transform would be opposite to that predicted if the ridge axis was offset along a transcurrent fault. The kinematic character of fracture zones was established when first-motion studies of earthquakes along several ridge-transform-ridge plate boundaries were shown to have motions in agreement with Wilson’s model (Sykes, 1967).
Figures & Tables
The Eastern Pacific Ocean and Hawaii
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.