Deformation of Continental Lithosphere: Studies in the Ural Mountains, the Adriatic Region, and the Western United States

Geophysical and geological observations from the Ural Mountains have been compiled to test whether the surficial similarities of the Appalachian and Ural orogenic belts extend to include deep lithospheric structure and compensation mechanism. 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. The model preferred in this study 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. Geometric reconstruction of foredeep basins overlying the continental Adriatic lithosphere shows its changing deflection beneath the Dinaride, Southern Alp, and Apennine thrust belts throughout Cenozoic time. Basin reconstructions are based on backstripping techniques using well log data, biostratigraphy, and seismic stratigraphy of the foredeep basin fill. Growth of the foredeep basins occurred coevally with the maximum convergent tectonic activity in adjacent thrust belts. Lithospheric deflections beneath the Apennine and Dinaride foredeep basins in the central Adriatic and beneath the Apennine and Southern Alp basins of northern Italy can be modeled as two-sided flexure of a thin elastic plate with uniform thickness. Basin reconstructions are compatible with an effective plate thickness of 10 to 20 kilometers (flexural rigidity of ??) that is roughly constant both during individual convergent events and between the Eocene development of the Dinaride foredeep basin and the Pliocene development of the Apennine foredeep basin. In particular, the data in the central Adriatic are incompatible with models for significant weakening of the lithosphere subsequent to the Dinaride convergent event. Reconstructions suggest that the basement beneath the Eocene-Oligocene Dinaride and Pliocene-Quaternary Apennine foredeep basins may have dipped more steeply toward the thrust belts at the time of the cessation of major tectonic activity than it does at present. The underlying Adriatic lithosphere may have 'unflexed' as rates of convergence diminished in these settings. Flexural modeling of the Adriatic foredeep suggests that erosion of topographic load, reduction in regional stresses, or lithospheric weakening in regions of highest plate curvature may each in themselves be insufficient to explain the inferred posttectonic 'unflexing'. The deflection data may be better explained by a combination of these processes with changes in forces acting at depth within the convergence zone. The conditions are examined under which ductile flow of lower crustal material is effective in explaining the small variations in gravity anomalies and elevations observed between neighboring areas that have undergone highly variable amounts of upper crustal extension. This study addresses in particular the transition zone between the unextended Colorado Plateau and the extended eastern Basin and Range province near Lake Mead, Nevada, where Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. In addition several proposed mechanisms for producing uniform crustal thickness across the transition zone, such as overthickening of subsequently extended areas and crustal thinning in the footwall of low-angle normal faults are inviable in the Lake Mead area. The viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are found to be highly dependent on the channel thickness and on the length scale of flow required. Finite element modeling shows that flow over 700 kilometers across the eastern Basin and Range and western Colorado Plateau under the extension rates estimated near Lake Mead requires effective viscosities less than 1018 to 1020 Pa-s for ductile channels 5 to 25 kilometers in thickness. Flow over shorter length scales appropriate to metamorphic core complexes may be accommodated with effective viscosities as high as 1021 Pa-s. These effective viscosities may be sustained by lower crustal material deforming at laboratoryextrapolated power law creep rates. The longer-scale flow may require elevated crustal temperatures (more than 700*C) depending on the composition assumed for the ductile channel. Thus ductile flow in the lower crust appears to be a viable mechanism for preventing large variations in total crustal thickness between highly extended and less extended regions, thereby explaining the relative uniformity in gravity and topography between such regions.