EXPERIMENTAL MODELING OF THE CONTROLS OF BASEMENT FAULTS ON STRUCTURAL GEOMETRY: 2. OFFSET AND RELAY STRUCTURES

A characteristic feature of foreland basement structures is that chains of structures commonly show offset or relay patterns among adjacent structures. Examples of these configurations include the Hamilton dome-Wagonhound anticline chain, the Maverick Springs-Circle Ridge chain, the Grass Creek-Walker dome trend, and the Piney Creek structural complex, in the Rocky Mountain foreland. These configurations are strongly suggestive of the control of pre-existing frontal or lateral discontinuities on the trends and locations of adjacent structures. Lateral discontinuities may either be pre-existing, controlling the lateral extent of structures, or they may be formed during deformation to accommodate the formation of structures formed along offset frontal faults. Experimental clay models were conducted to investigate the final structural geometries and secondary fault orientations resulting from various configurations. Laser scanning was used to develop maps of deformed horizons to enable a direct comparison with mapped natural structures. Two-layer models, with stiff clay representing the basement, and soft clay representing the sedimentary cover were used, with all preexisting discontinuities confined to the basement unit. The results of the modeling indicate that the spatial relation and lateral extent of multiple offset structures is controlled by the presence or absence of lateral weakness zones in the basement. A relay of reverse faults in the basement connected by buried tear faults results in surface structures with a large overlaps between them. On the other hand, tear faults that penetrate the entire basement have greater control on the lateral extremity of surface structures. Tear faults can connect initially offset reverse faults in the basement or they can connect hanging wall blocks with differential displacements along an initially continuous reverse fault. Natural examples can have a combination of both differential displacement and initial offset in the basement faults. Finally, a relay of reverse faults in the basement not connected by a lateral weakness zone may develop parallel or oblique structures with transfer zones between them.