Compensation of Paleozoic orogens: a comparison of the Urals to the Appalachians

Abstract

The Urals and Appalachians are two Paleozoic mountain belts with similar geologic and morphologic features. We combine geophysical (Bouguer gravity and deep seismic sounding) and geological observations from the Ural Mountains to test whether the surficial similarities of these Paleozoic orogens extend to include deep lithospheric structure and compensation mechanism. We find the combined data suggest support of the mountains by a stiff continental slab which is depressed by an effective subcrustal load in addition to the topography. As has been recently proposed for the Appalachians, our preferred model, which fits the geophysical data and predicts the presence and shape of the Ural foredeep to the west of the mountains requires eastward-directed underthrusting of the Russian platform behaving as an elastic plate more than 50-km thick. It is unlikely that an elastic plate this stiff could have resulted from conductive cooling of a thermal plate only 125-km thick. The weight of the existing mountains on such an elastic plate is insufficient to produce the deflection of the Moho inferred from seismic and gravity data. An additional load on the elastic plate may be provided by a buried mass of similar origin to surface outcrops of dense mafic and ultramafic rocks in the eastern Urals. Alternatively, the load may represent the effect of a shallowing of the Moho east of the mountains which is a remnant of a buried passive continental margin. Seismic data and Bouguer gravity anomalies suggest that the effect of the subsurface mass anomaly is at least as great as the present-day topography and that the Russian plate flexurally underthrusts the Urals for more than 100 km. We speculate that similar effective subsurface loads may exist at younger continental orogens but are obscured by the topographic load in less eroded mountain belts.