Structure and evolution of the Magnitogorsk forearc basin:: Identifying upper crustal processes during arc-continent collision in the southern Urals

The southern Urals of Russia contain a well-preserved example of a Paleozoic are-continent collision in which the intraoceanic Magnitogorsk volcanic are and its foreare basin sediments accreted to the East European Craton during the Devonian. The Magnitogorsk are records the evolution from incipient intraoceanic subduction to a mature are, and by comparing its surface geological features with those in active arc-continent collision settings it is possible to identfy upper crustal processes that were active in the southern Urals. The are edifice can be divided into western and eastern volcanic fronts that were active during different stages of are evolution and for which two distinct phases of foreare basin development can be recognized. The late Lower to Middle Devonian Aktau Formation represents a remnant of the intraoceanic to collisional foreare basin to the Irendyk volcanic front, whereas the Middle Devonian to Lower Carboniferous Ulutau, Koltubanian, and Zilair Formations were deposited in a suture foreare basin to the east Magnitogorsk volcanic front. It was not until the Late Devonian that these two basins were joined. Structural mapping, combined with reflection seismic profiling, shows these basins to be affected by open, nonlinear, volcanic basement-cored synsedimentary folds. The Karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture foreare basin. The foreare region is affected by minor thrusting that involves the volcanic basement, although it is not clear if these thrusts reactivate preexisting trench-parallel faults. Synsedimentary deformation, slumping, and olistostrome development were common throughout the suture foreare basin history but were especially widespread during the Late Devonian, when the full thickness of the continental crust is interpreted as having arrived at the subduction zone.