THRUST FAULT NUCLEATION AND PROPAGATION AND THE IMPORTANCE OF FOOTWALL SYNCLINES

Analysis of small scale examples of thrust faults and related folding in outcrops of the Cretaceous Boquillas Formation within Big Bend National Park in west Texas provide the basis for reexamining current understanding of fault nucleation and propagation. Thirty-four thrust faults in the study area have maximum displacements ranging from 0.5 cm to 9 cm within competent limestone beds. These displacements abruptly diminish upward and downward within the interbedded, weaker mudrock layers. Faults that continue to propagate into the mudrock result in fault tip folding with hanging wall anticlines above upwardly propagating fault tips, and footwall synclines beneath downwardly propagating fault tips. Close inspection of the faults reveals no evidence of discrete layer-parallel faults, sliding surfaces, or shear zones within the mudrock layers as predicted by other models. We find that the maximum displacements on the thrust faults are consistently localized in the competent beds and diminish in the weaker bounding mudrocks, suggesting that thrust faults preferentially nucleate in competent layers rather than forming as splays from preexisting layer-parallel or low-angle detachments. Footwall synclines that formed by this mechanism are evidence of the location of fault nucleation and downward fault propagation rather than the fault propagating or “ramping” upward from a low angle detachment. The observed structural style may be to some extent scale independent and provide insights in the nucleation of faults at the formation scale and structural development at the mountain-range scale. Décollement or detachment layers may be a consequence rather than cause of thrust ramps through competent units and could be over interpreted from seismic data.