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 Paleozoi c are-continent collision in which the intraoceanic Magnitogorsk volcanic a re and its foreare basin sediments accreted to the East European Craton dur ing the Devonian. The Magnitogorsk are records the evolution from incipient intraoceanic subduction to a mature are, and by comparing its surface geol ogical features with those in active arc-continent collision settings it is possible to identfy upper crustal processes that were active in the southe rn Urals. The are edifice can be divided into western and eastern volcanic fronts that were active during different stages of are evolution and for wh ich 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, wh ereas the Middle Devonian to Lower Carboniferous Ulutau, Koltubanian, and Z ilair Formations were deposited in a suture foreare basin to the east Magni togorsk volcanic front. It was not until the Late Devonian that these two b asins were joined. Structural mapping, combined with reflection seismic pro filing, shows these basins to be affected by open, nonlinear, volcanic base ment-cored synsedimentary folds. The Karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments i n the suture foreare basin. The foreare region is affected by minor thrusti ng that involves the volcanic basement, although it is not clear if these t hrusts reactivate preexisting trench-parallel faults. Synsedimentary deform ation, slumping, and olistostrome development were common throughout the su ture foreare basin history but were especially widespread during the Late D evonian, when the full thickness of the continental crust is interpreted as having arrived at the subduction zone.