ARC-CONTINENT COLLISION IN THE SOUTHERN URALS

The Southern Urals of Russia contain what is arguably one of the best-preserved examples of an arc-continent collision in any Paleozoic orogen. The arc-continent collision history recorded in the rocks of the Southern Urals began in the Early Devonian with the onset of intra-oceanic subduction and the formation of the Magnitogorsk Arc and ended with its collision with the margin of Laurussia during the Late Devonian. The Laurussia margin consisted of a basement that was composed predominantly of rocks of Archean and Proterozoic age that, by the time of arc-continent collision, was overlain by Cambrian, Ordovician, Silurian, and Devonian sediments interpreted to have been deposited in rift-related grabens on the continental slope and rise, and on the shallow marine platform. The Magnitogorsk Arc consists of Early to Late Devonian island arc volcanic rocks and overlying volcaniclastic sediments. Arc-continent collision led to the development of an accretionary complex that includes shallowly and deeply subducted continental margin rocks, ophiolite fragments, and sediments that were deposited in a foreland-basin setting. High-pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km (eclogite assemblages) to 120 km (micro diamonds). It is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the Laurussia margin by the accretion of the Magnitogorsk arc. The geochemistry of the Magnitogorsk Arc volcanic rocks, the structure of the arc-continent collision accretionary complex and the forearc, the high-pressure rocks beneath and along the suture zone, the mafic and ultra-mafic ophiolitic material, and the syn-tectonic sediments show that the Paleozoic tectonic processes recorded in the Southern Urals can be favorably compared with those in currently active settings such as the west Pacific.