The existence of ridge transform faults and the near absence of fan-shaped magnetic-anomaly patterns suggest that seafloor spreading is a spatially incremental process. This concept requires that spreading-velocity contrasts exist across ridge transform faults and that the fracture zones beyond ridge transform structures contain a small component of strike slip motion. Magnetic-anomaly patterns in the North Pacific confirm this motion and give 0.55 cm/year as its average measure; this value is in close agreement with theory. These patterns demonstrate further that the plate segments into which the oceanic crust is divided by the great fracture zones move with different angular velocities and erratic spreading directions. Topographic persistence of fracture zones, patterns of submarine volcanism, and the "branching lines," here interpreted as a consequence of a change in spreading direction on an array of active fracture zones, are further evidence of deformation within the plate interiors.
The low seismicity of the fracture zones is the result of high pore pressure, intense brecciation, and serpentinization that cause a drastic reduction in shear strength in the fracture zones. The oceanic crust is considered as a distinct mechanical and seismic province which is cut by thin zones of extremely low shear strength and in which major deformation can occur continuously and aseismically. Crustal rigidity in such a system is the adjusting rigidity of a logjam. The concept of rigid plates is thus only a useful approximation to a dynamically complex crust.
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Plate Tectonics—Assessments and Reassessments
The general theme of this publication is the assessment and reassessment of various data, observations, and ideas about the earth as they relate to the concept that has come to be known as plate tectonics. Much widely scattered material was brought together for this publication, and its 24 papers contain an abundance of worldwide references that are important in studying plate tectonics.