Review of oceanic transform activity and development

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Abstract
Oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on Euler poles of relative plate motions. It is proposed that the special properties of transforms arise because they are inherently leaky, i.e. new plate area forms along them. As transforms have finite widths the adjacent ridge tips are at different distances from the Euler poles, and the plate boundaries along transforms are slightly oblique to the direction of plate motions, such that new plate area forms along them. However, along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process. Transforms are underlain by crust consisting of variable proportions of (a) material emplaced in the transform domains and (b) stretched ridge-generated crust. The transform structure is further modified by shearing, deformation related to irregularities of lateral faults and because of propagation of adjacent ridges. Transform leakiness is not considered to result merely from changes of plate motions (though such changes will influence transform structure), but it reflects the segmented structure of mid-oceanic ridges. The persistence of transforms and their migration in the absolute reference frame, with a component perpendicular to their strikes, shows that the ridge structure is decoupled from flow in deeper levels. Thermal anomalies along transforms, especially enhanced heat losses, stabilize the segmented ridge structure.