Failed rifts are inactive mid-ocean ridge segments abandoned during processes that change the spreading geometry of lithospheric plates (Table 1). Morphologically, they resemble active ridge crests, and it seems in some cases that their failure or abandonment was closely coupled to the propagation of a neighboring active rift. In such cases, the propagating rift advances into old lithosphere formerly created by the dying rift, while the dying or abandoned rift ceases to function as a spreading center. A transient and complex transform-type plate boundary forms between the advancing rift and the dying one, as discussed by Hey and others (1986).
For propagating/retreating rifts (PRRs) (discussed fully in Hey and others, this volume) and migrating overlapping spreading centers (OSCs), also called nontransform offsets, (Rea, 1978; Lonsdale, 1983; MacDonald and Fox, 1983; Macdonald, this volume; Fox and Gallo, this volume), the distance separating the coupled propagating and failing rifts is usually less than or about 50 km. It is possible that this relatively small distance between the advancing and retreating rifts leads to close coupling of ridge behavior and ease in establishing a transform-type boundary between them. If even smaller rift offsets such as small non-overlapping offsets (SNOOs) (Batiza and Margolis, 1986) and deviations from axial linearity (DevALs) (Langmuir and others, 1986) migrate, it would be expected that close coupling of advancing and dying rifts and the ease of creating a transient plate boundary between them would be correspondingly enhanced due to the close proximity (<3 km) of the two rifts and the hot, thin, weak lithosphere between them
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.