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The impact of body forces on Archaean orogenic processes
Goldschmidt Conference Abstracts 2006
The impact of body forces on Archaean orogenic processes
Mid-Archaean crustal geodynamics: Pilbara controversy
P.F. REY1, G. HOUSEMAN2
P.F. REY1, N. THEBAUD1,2, P. PHILIPPOT2
A527
1
School of Geoscience, Bldg. H11, The University of Sydney, NSW 2006, Australia ([email protected]) 2 School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK ([email protected])
1
In the Archaean, the combination of warmer continental geotherm with a lighter subcontinental lithospheric mantle suggests that gravitational forces played a more significant role in continental lithospheric deformation. To test this hypothesis, we compare the evolution of the deformation and the regional state of stress in ‘‘Archaean-like’’ and ‘‘Phanaerozoic-like’’ lithospheres submitted to the same boundary conditions in a triaxial stressfield with imposed convergence in one direction. For plausible physical parameters, thickening of normal to cold Phanaerozoic lithospheres produces relatively weak buoyancy forces either extensional or compressional. In contrast, for Archaean continental lithospheres, or for anomalously warm Phanaerozoic lithospheres, lateral gravitationally driven flow prevents significant thickening. This conclusion is broadly consistent with: (1) the relative homogeneity of the erosional level now exposed at the surface of Archaean cratons, (2) the sub-aerial conditions that prevailed during the emplacement of up to 20 km of greenstone cover, (3) the relatively rare occurrence in the Archaean record of voluminous detrital sediments, (4) the near absence of significant tectonic, metamorphic and magmatic age gradients across Archaean cratons, (5) the relative homogeneity of strain across large areas, and (6) the ubiquitous presence of crustal-scale strike slip faults in many Late Archaean cratons.
The analysis of the geomorphology and finite strain field characteristics suggests that the Mt Edgar granitic dome in the Pilbara craton extends further west under the Marble Bar belt. This interpretation eliminates the two main arguments put forward against the vertical tectonics model in the East Pilbara. The strong subvertical shearing that affects the southern margin the Mt Edgar dome disappears further west because the granite-greenstone contact flattens out under the flat lying Marble Bar belt. The shear zone along the Mt Edgar southern margin is therefore not a detachment fault, but rather a de´collement zone that keeps parallel to the granite-greenstone boundary and whose strain characteristics depend on its dip. In addition, when one takes into account the position of the granite-greenstone boundary under the Marble Bar belt, stretching lineations along the Mt Edgar southern margin and in the Marble Bar belt organize themselves into a regular radial pattern as expected along a de´collement zone. We also bring into the debate a new observation. There is in the Warrawoona syncline a family of quartz vein whose pole remains sub-parallel to the stretching lineation that rotates to become vertical where the strain is constrictional. This suggests that the axes of the finite and incremental strain ellipsoids remain parallel despite the rotation of the finite strain ellipsoid. Coaxial yet rotational deformation is highly unusual in deformation driven by far-field plate boundary stresses. However, the rotation of the stress ellipsoid in step with the rotation of the finite strain ellipsoid is compatible from the gravitational pull from the sagducting greenstone. Finally, we have conducted numerical thermo-mechanical experiments in order to test the gravitational instability model. The results of these experiments show that for realistic density contrasts, greenstone thicknesses and viscosities, the gravitational instability model accounts for the time-scale, and the length-scale of the dome and basin pattern of the EPGGT. Furthermore, the temperature evolution recorded in the upper crustal levels including Ky-bearing assemblages is remarkably compatible with the temperature evolution in our numerical experiments.
doi:10.1016/j.gca.2006.06.970
School of Geoscience, Bldg. H11, The University of Sydney, NSW 2006, Australia ([email protected]) 2 Laboratory of Bioge´oscience, UMS 834, IPGP, Paris 75252, France ([email protected])
doi:10.1016/j.gca.2006.06.971
The impact of body forces on Archaean orogenic processes
Mid-Archaean crustal geodynamics: Pilbara controversy
P.F. REY1, G. HOUSEMAN2
P.F. REY1, N. THEBAUD1,2, P. PHILIPPOT2
A527
1
School of Geoscience, Bldg. H11, The University of Sydney, NSW 2006, Australia ([email protected]) 2 School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK ([email protected])
1
In the Archaean, the combination of warmer continental geotherm with a lighter subcontinental lithospheric mantle suggests that gravitational forces played a more significant role in continental lithospheric deformation. To test this hypothesis, we compare the evolution of the deformation and the regional state of stress in ‘‘Archaean-like’’ and ‘‘Phanaerozoic-like’’ lithospheres submitted to the same boundary conditions in a triaxial stressfield with imposed convergence in one direction. For plausible physical parameters, thickening of normal to cold Phanaerozoic lithospheres produces relatively weak buoyancy forces either extensional or compressional. In contrast, for Archaean continental lithospheres, or for anomalously warm Phanaerozoic lithospheres, lateral gravitationally driven flow prevents significant thickening. This conclusion is broadly consistent with: (1) the relative homogeneity of the erosional level now exposed at the surface of Archaean cratons, (2) the sub-aerial conditions that prevailed during the emplacement of up to 20 km of greenstone cover, (3) the relatively rare occurrence in the Archaean record of voluminous detrital sediments, (4) the near absence of significant tectonic, metamorphic and magmatic age gradients across Archaean cratons, (5) the relative homogeneity of strain across large areas, and (6) the ubiquitous presence of crustal-scale strike slip faults in many Late Archaean cratons.
The analysis of the geomorphology and finite strain field characteristics suggests that the Mt Edgar granitic dome in the Pilbara craton extends further west under the Marble Bar belt. This interpretation eliminates the two main arguments put forward against the vertical tectonics model in the East Pilbara. The strong subvertical shearing that affects the southern margin the Mt Edgar dome disappears further west because the granite-greenstone contact flattens out under the flat lying Marble Bar belt. The shear zone along the Mt Edgar southern margin is therefore not a detachment fault, but rather a de´collement zone that keeps parallel to the granite-greenstone boundary and whose strain characteristics depend on its dip. In addition, when one takes into account the position of the granite-greenstone boundary under the Marble Bar belt, stretching lineations along the Mt Edgar southern margin and in the Marble Bar belt organize themselves into a regular radial pattern as expected along a de´collement zone. We also bring into the debate a new observation. There is in the Warrawoona syncline a family of quartz vein whose pole remains sub-parallel to the stretching lineation that rotates to become vertical where the strain is constrictional. This suggests that the axes of the finite and incremental strain ellipsoids remain parallel despite the rotation of the finite strain ellipsoid. Coaxial yet rotational deformation is highly unusual in deformation driven by far-field plate boundary stresses. However, the rotation of the stress ellipsoid in step with the rotation of the finite strain ellipsoid is compatible from the gravitational pull from the sagducting greenstone. Finally, we have conducted numerical thermo-mechanical experiments in order to test the gravitational instability model. The results of these experiments show that for realistic density contrasts, greenstone thicknesses and viscosities, the gravitational instability model accounts for the time-scale, and the length-scale of the dome and basin pattern of the EPGGT. Furthermore, the temperature evolution recorded in the upper crustal levels including Ky-bearing assemblages is remarkably compatible with the temperature evolution in our numerical experiments.
doi:10.1016/j.gca.2006.06.970
School of Geoscience, Bldg. H11, The University of Sydney, NSW 2006, Australia ([email protected]) 2 Laboratory of Bioge´oscience, UMS 834, IPGP, Paris 75252, France ([email protected])
doi:10.1016/j.gca.2006.06.971