Geophysical Signatures of Adjoining Lithospheric Domains
Abstract:Lithospheres of different age have distinctly different characteristics regarding their composition, thermal and density structure. Major differences exist between cratons and the Phanerozoic domains and mobile belts. We here investigate how the lateral transition from one lithospheric domain to another is reflected in the geophysical signatures, the seismic velocities, gravity, topography and geoid. We combine geophysical-petrological forward modeling with a comparison to worldwide occurrences of adjoining lithospheric domains.
Three distinctly different mantle types (Archean, Proterozoic, Phanerozoic) are used to calculate the geophysical signatures of a range of possible lateral transition zones. The mantle types are characterized by their different elemental composition, from which stable mineral phases and bulk physical properties are derived. Usually, older SCLM (sub-lithospheric mantle) is more depleted in heavier minerals and thereby lighter, but this effect is mainly counterbalanced by the increased density caused by long-term thermal cooling. At the edges of cratons, changes in the thermal structure affect this balance. A range of models is tested for the effects of lateral variations in the crustal and SCLM structure (thickness, smoothness of thickness changes) and mantle compositions. Abrupt changes in composition and lithosphere thickness generally cause distinct topographic lows or ridges. In the real world, these may be offset by respective adjustments in Moho depth, crustal structure or sediment infill. Gradual variations in lithosphere thickness, however, only show minor geophysical signatures. A possible expression of adjoining lithospheric domains is the Scandinavian Mountain Belt in Norway at the edge of Proterozoic Baltica.
Although many of the present-day topographic features are unlikely to have existed since the Precambrian, the evolution of the cratons (rejuvenation of the craton edges) may have assisted in shaping the present-day sedimentary basins and regional elevations.