FROM SUBDUCTION TO RIFTING: NEOPROTEROZOIC-CAMBRIAN EVOLUTION OF THE OSSA-MORENA ZONE (SW IBERIA)

The geologic record in the Ossa-Morena Zone during the late Neoproterozoic and the early Paleozoic includes a complex evolution involving:

1) Neoproterozoic growth of an arc-back arc system related to N-directed subduction beneath a continental block;

2) Neoproterozoic accretion of the Ossa-Morena arc to the Iberian Autochthon continental margin (North Gondwana) after closure by antithetic (S-directed) subduction of the back arc basin (Cadomian orogeny);

3) Neoproterozoic to earliest Cambrian continuation of arc development related to the main N-directed subduction process;

4) Early Cambrian cessation of subduction and onset of extensional deformation, geothermal upraisal, basin development and massive mantle derived bimodal magmatism.

This final evolution characterizes a rifting event that propagated diachronously across the Iberian Autochthon part of Gondwana from the Early Cambrian through to the Late Ordovician, and culminated in formation of a new ocean, probably a part of the Rheic ocean. Existence of an Early Ordovician breakup unconformity indicates that a significant tract of new oceanic lithosphere already existed by that time.

A model is proposed to account for the evolution outlined above. This involves collision of a preexisting MOR with the trench at the outer margin of Gondwana and its subsequent propagation across the continent, in a similar way to the overriding of the East Pacific Rise by the North American plate and the consequent rifting apart of Baja California.