The isostatic state of the southern Urals crust

The Uralide orogen, in Central Russia, is the focus of intense geoscientifi c investigations during recent years. The international research is motivat ed by some unusual lithospheric features compared with other collisional be lts including the preservation of (a) a collisional architecture with an or ogenic root and a crustal thickness of 55-58 km, and (b) large volumes of v ery low-grade and non-metamorphic oceanic crust and island are rocks in the upper crust of a low-relief mountain belt. The latter cause anomalous grav ity highs along the thickened crust and the isostatic equilibrium inside th e Uralides lithosphere as well as the overthrust high-metamorphic rocks. Th e integrated URSEIS '95 seismic experiment provides fundamentally new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen that allows the construction of density models. In the Urals' lithos phere different velocity structures resolved by wide-angle seismic experime nts along both the URSEIS '95- and the Troitsk profile. They can be used to constrain lithospheric density models: a first model consists of a deep su bducted continental lower crust which has been highly eclogitized at depths of 60-90 km to a density of 3550 kg/m(3). The second model shows a slightl y eclogitized lower crust underlying the Uralide orogen with a crustal thic kness of 60 km. The eclogitized lower crust causes a too-small impedance co ntrast to the lithospheric mantle resulting in a lack of reflectors in the area of the largest crustal thickness. Both models fit the measured gravity field. Analyzing the isostatic state of the southern Urals' lithosphere, b oth density models are in isostatic equilibrium.