The Dynamics of Allochthonous Terranes in the Pangean Suture Zone of Southern Iberia

Abstract

Most researchers contend that the destruction of the Rheic Ocean culminated in the formation of the supercontinent Pangea. However, despite the importance of this ocean, there are major uncertainties in the identification of its margins, mechanisms and timing of its formation, and the geodynamics of its closure. Rocks recording the evolution of the Rheic are excellently preserved in the southern Iberian peninsula of Western Europe. Here, the Ossa Morena (OMZ) is separated from the South Portuguese (SPZ) zone by a sequence of polydeformed rocks know as the Pulo do lobo Zone (PDLZ). The PDLZ isinterpreted as a late Paleozoic accretionary prism, which contains potential vestiges of the ancient Rheic Ocean (ophiolites). The objective of this study is to better understand the processes associated with the formation of Pangea by determining the lithotectonic histories of both the PDLZ and SPZ. New field, geochronological and geochemical data are used to test and further constrain current models for the evolution of Pangea as recorded in the Variscan orogen. Fieldwork and geochronological data indicate that the PDLZ was derived from neither the OMZ (Gondwana) nor the SPZ suggesting that if the PDLZ is an accretionary prism it was not derived from the upper or lower plate. This apparent conundrum can be reconciled by a model involving excision of a crustal fragment during collision between an Iberian indenter (Gondwana) with Laurussia during the formation of Pangea. Geochronological and Geochemical data from the SPZ indicate that the lower crust isnot compositionally similar to the overlying Devonian-Carboniferous continental detritus. This unusual relationship is similar to the relationship between the relatively juvenile basement and ancient upper crust documented in the exposed portion of the Meguma terrane in the northern Appalachians, which paleogeographic reconstructions show was immediately outboard of southern Iberia in the Late Devonian. Taken together with the suggested complex tectonic history of the PDLZ the results of this thesis provide important insight into the geometry and timing of the formation of Pangea and indicate that re-interpretation may be required for what is known concerning the tectonic evolution of both the Variscan and Appalachian orogens.