Abstract: Physical Models of Fault-Bend Folding

Gomes, Caroline Janette Souza - Universidade Federal de Ouro Preto

Kinematic and geometric features of thrust-related folds have been largely simulated by sandbox and computer models. However, those models present the hanging wall flat at the top of the rock layers in opposition to the theoretical prediction that the hanging wall flat form in an interlayered incompetent rock layer.

This study presents the results of four sandbox series designed to investigate the mechanism of staircase trajectory thrusting with associated bending of the hanging wall. The experiments were performed in a glass-sided rectangular 60 x 20 x 15 cm (length, width, height) deformation box. In series M1 and M2, initial 40 cm long sandpack consisting of six 0.5 mm thick beds, were interlayered by a 1 cm thick incompetent analogue material (model M1, with a bed of silicone and model M2, with a bed of mica flakes). The progressive shortening, of the Ml model has produced thrusts in the sand layers below and above the silicone bed, but the viscous silicone has undergone thickening, especially near the moving wall of the experimental box. The resulting structure was a detachment fold. The bed of mica flakes, of the M2 model, behaves differently: strong bedding-parallel slip promotes strain along the entire layer length impeding both the formation of a detachment fold and the staircase trajectory of thrust fault. Series M3 repeat the latter M2 experiment, with some modifications. The base of the box was covered with sandpaper (A237 - 80), to produce high basal friction, and the end-wall constraints were removed. These new boundary conditions in the M3 model caused a simple shear deformation: the intermediate mica bed induces a strong slip of the above sand layers, into the foreland of experimental box, while the basal layers move just a little. Instead of, the basal layers undergo thickening and thrusting. This process has generated the staircase trajectory of thrusts, with a ramp in the basal sand layers and a flat in the mica bed (Fig. 1). The model M4 was set up with a 1.5 cm high, 30°-angle wooden wedge to simulate a high strength foot wall and avoid the formation of new forethrusts. In addition, the 0.5 cm thick mica bed of model M4 was removed and thin interlayers of mica flakes were introduced in the above sand pack which extend over the wedge. The thin mica interlayers bestow plasticity to the sand, a normally brittle analogue material. Shortening of the model M4, also characterized by high basal friction and no end constraints, produced in the hanging wall a rounded anticline by flexure-slip deformation (Fig. 2).

The physical experiments demonstrate that fault-bend folds form under special conditions: their nucleation requires always high basal friction. In addition, their formation depends either on the presence of an interlayered incompetent layer or the existence of slightly plastic rocks covering basal layers with increasing strength in the foreland. An interlayered incompetent layer promotes bedding parallel-slip producing a flat thrust, and the impediment of new thrust nucleation in the foreland forces the hanging wall over the thrust ramp generating an anticline.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil