Propagating rifts and spreading centers
Propagating rifts are extensional plate boundaries that progressively break through rigid lithosphere. If the rifting advances to the sea-floor spreading stage, propagating sea-floor spreading centers follow behind, gradually extending through the rifted lithosphere. The combination of sea-floor spreading and propagation produces a characteristic V-shaped wedge of lithosphere formed at the propagating spreading center, with progressively younger and longer isochrons abutting the pseudofaults that bound this wedge. Figure 1 shows several variations of this geometry.
Although conceptually simple, the propagating rift hypothesis has important implications for both large-scale and fine-scale plate tectonic evolution. It explains: (1) the existence of several classes of structures that are oblique to both relative and absolute plate motion and that previously seemed incompatible with plate tectonic theory; (2) why some continental margins are not parallel to sea-floor isochrons and why some continental drift reconstructions are inaccurate; and (3) the large-scale reorganization of some sea-floor spreading systems, including both the onset and termination of many fracture zones as well as the formation of some transient microplates. The hypothesis provides a mechanistic explanation for the way in which many (if not all) spreading center jumps occur and why they occur in systematic patterns, how spreading centers reorient when the direction of sea-floor spreading changes, and the origin of large areas of sea floor with high petrologic diversity, including the major abyssal ferrobasalt provinces.
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.