Physics
Scientific paper
Dec 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..61..333h&link_type=abstract
Earth and Planetary Science Letters, Volume 61, Issue 2, p. 333-345.
Physics
8
Scientific paper
Resistance of modern oceanic tectosphere to the initiation of subduction can be evaluated by considering its flexural rigidity, shear resistance in the subduction zone, and the negative buoyancy force, after McKenzie [13]. These depend on the effective elastic thickness, the depth to which shear stresses are appreciable, and the tectospheric thickness respectively. Evaluation of these thicknesses and their variation with age allows development of a model in which oceanic tectosphere becomes progressively less stable with increasing age and spontaneously subducts at an age of 200 Ma. The same model applied to early Proterozoic oceanic tectosphere, distinguished only by having sub-tectosphere temperatures 100°C higher than in the modern, rquires that Proterozoic ocean tectosphere subduct spontaneously at ages of about 75 Ma.
The subduction of universally young oceanic tectosphere inhibits the production of juvenile magmas, due to the release of volatiles from the descending slab at low pressures, near the trench. The model therefore accounts for the scarcity of subduction-related magmas in many early Proterozoic orogens. It also predicts that ocean basins opening in the early Proterozoic should have generally limited width, that blueschist metamorphism should be rare and that early Proterozoic oceanic crust should rarely be preserved at the earth's surface.
With progressive cooling of the mantle the Proterozoic tectonic scheme may have changed gradually into the modern one by early Phanerozoic time, although parts of the pan-African system may exhibit the characteristics of still unstable oceanic tectosphere.
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