Effects of gravity in folding
Abstract
Effects of gravity on buckle folding are studied using a Newtonian fluid finite element model of a single layer embedded between two thicker less viscous layers. The methods allow arbitrary density jumps, surface tension coefficients, resistance to slip at the interfaces, and tracking of fold growth to a large amplitudes. When density increases downward in two equal jumps, a layer buckles less and thickens more than with uniform density. When density increases upward in two equal jumps, it buckles more and thickens less. A low density layer with periodic thickness variations buckles more, sometimes explosively. Thickness variations form, even if not present initially. These effects are greater with; smaller viscosities, larger density jump, larger length scale, and slower shortening rate. They also depend on wavelength and amplitude, and these dependencies are described in detail. The model is applied to the explosive growth of the salt anticlines of the Paradox Basin, Colorado and Utah. There, shale (higher density) overlies salt (lower density). Methods for simulating realistic earth surface erosion and deposition conditions are introduced. Growth rates increase both with ease of slip at the saltshale interface, and when earth surface relief stays low due to erosion and deposition. Model anticlines grow explosively, attaining growth rates and amplitudes close to those of the field examples. Fastest growing wavelengths are the same as seen in the field. It is concluded that a combination of partialslip at the saltshale interface, with reasonable earth surface conditions, promotes sufficiently fast buckling of the saltshale interface due to density inversion alone. Neither basement faulting, nor tectonic shortening is required to account for the observed structures. Of fundamental importance is the strong tendency of gravity to promote buckling in low density layers with thickness variations. These develop, even if not present initially. <Because of this, folds both initiate faster, and grow to much higher amplitude than if density is uniform. &Low density layers are ubiquitous in the crust, so these results shed considerable new light on crustal dynamics.
 Publication:

Ph.D. Thesis
 Pub Date:
 1991
 Bibcode:
 1991PhDT........35M
 Keywords:

 Anticlines;
 Buckling;
 Earth Crust;
 Finite Element Method;
 Folding;
 Geodynamics;
 Gravitational Effects;
 Mathematical Models;
 Newtonian Fluids;
 Earth Surface;
 Erosion;
 Inversions;
 Simulation;
 Geophysics