Orogenic continental-margin history
Overlain by pillow basalt, oceanic ophiolite generated at the ocean ridges with local additions through plume volcanoes and transform faults, rides on convecting mantle to the converging plate margin, where it is subducted along the Benioff plane of seismic foci. Where a continent overlies a subduction zone (e.g., North America), orogenes form; but subduction zones are known in the Pacific Basin associated only with island arcs. How far the Benioff subduction zone extends beneath the continent is questionable, but mantle-derived taphrogenic intrusives suggest that it may extend past orogenic forelands (Figs. 4, 21).
Juvenile fluids and volatiles, mantle-derived silicate and metallic minerals including thorium and uranium, and ocean water entrained and fixed in oceanic crust all migrate in the seafloor to the orogene, where they can be remobilized. Some portion of these materials probably enters the ocean, where they remain stabilized unless selectively extracted by chemical sedimentation. A large portion of fluids and metals may remain fixed in oceanic crust as it moves down the subduction plane, and the movable elements may not be mobilized, to penetrate the overlying crust, before the ophiolite reenters the mantle. The relative portions of the mobile elements added to the orogene root or returned to the mantle are speculative.
Figures & Tables
Migration of Uranium and Thorium-Exploration Significance
The uranium resource industry since the late 1960s has presented a paradox to those concerned with the growing energy shortage and the relative ability of uranium resources to respond to the need on a timely basis. This publication reviews the possible ways that uranium in the earth might be concentrated into economic deposits, and considers what industry should be able to expect from an exploration effort. Some of the chapters in this volume include: Fundamental sources of uranium and thorium; Mechanisms of uranium and thorium transfer to the crust; Shallow uranium mobilization processes; Geochemical distinction of uranium moneralization processes; and Oceanic migration history of uranium and thorium.