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Buoyancy tectonic models of uplift and subsidence along high-angle faults in extensional and compressional tectonic regimes

Publication Date
DOI: 10.1016/0040-1951(95)00015-f
  • Earth Science


Abstract This study models the potential uplift and subsidence of brittle, upper-crustal trapezoidal blocks which move along pre-existing high-angle faults in extensional tectonic regimes; similar models for uplift in compressional tectonic regimes are also presented. A force balance is assumed to exist between the weight of the trapezoidal block, the buoyancy force derived from the lower-ductile crust, the supporting forces from the neighboring blocks, and the frictional resistance to motion across the fault. Generally, the weight of the brittle crustal block is effectively opposed by the buoyancy force, making vertical movement quite dependent on tectonic forces redirected along the high-angle bounding faults. The analysis relates the state of stress in the brittle-upper crust (and other critical parameters such as structure width, thickness of the brittle-upper crust, frictional resistance along faults, mobility of the brittle-ductile transition which is related to slow and fast geologic movement, different lower crustal densities, and deposition and erosion) to possible uplift or subsidence. Greater vertical displacement is generally compatible with narrower crustal blocks, more deviatoric (tectonic) stress, lower fault friction, a thicker brittle crust, and similar upper and lower crustal densities. Without deposition or erosion, approximately equal amounts of vertical movement appear possible for trapezoidal blocks with similar conditions; however, deposition can typically induce far more subsidence than erosion can induce uplift. Although large amounts of uplift appear possible in both extensional and compressional regimes, the effects of erosion and the higher stresses possible with tectonic compression should produce larger amounts of uplift in compressional vs. extensional regimes. Slow or fast geologic movement can be defined as whether the brittle-ductile transition remains at a constant depth or is displaced; if the brittle-ductile transition remains at a constant depth (slow movement), uplift is enhanced in both extensional and compressional regimes. Low resistance to motion along faults is necessary for vertical movement and is consistent with several other recent studies. The models suggest that much of the observed relief on the earth's surface (up to several tens of kilometers width) can be caused by the dynamics in the earth's crust, however related to deeper earth processes; i.e. much topography can be analyzed in terms of stresses in the brittle crust and the buoyancy effect of the lower crust, however the stresses in the brittle crust may be generated by deeper and more global processes.

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