2D AND 3D STRUCTURAL ANALYSIS OF A MESOSCOPIC FAULT-RELATED FOLD: IMPLICATIONS FOR USING CROSS-SECTION BALANCING TO UNDERSTAND LATERAL VARIATIONS IN FOLD GEOMETRY

Geologists routinely interpret fault-related folds from limited exposures in roadcuts or on 2D seismic data, often ascribing a model of deformation without the benefit of structural restorations or additional 3D information from maps or adjacent cross sections. Such interpretations may not only oversimplify the complexity of the deformation, but may also hinder development of key structural analysis skills.

We present a detailed analysis of a mesoscopic fault-related fold from the Paleoproterozoic Gauyuzhuang Formation in western Liaoning Province, northern China using high-resolution photographs of 3 serial sections, quick-look balancing techniques, cross-sectional restoration, and 3D visualization. On the first section through the complexly deformed limestone sample, a central fault-bend fold (FBF) appears to be sandwiched between a backlimb thrust and a wedge-shaped block that was displaced upwards along a forelimb backthrust from between two horses of a duplex. This extrusion produced intense shearing of stratigraphic cutoffs, resulting in a chaotic zone of cataclasis between the backlimb thrust and the backthrust. Later, a sub-vertical fault cross-cut and offset the pre-existing structures. On the adjacent second and third sections, the majority of the first-order features could be correlated between the 3 sections. However, on the second section, while the central structure still resembled a FBF on the right side of the sub-vertical fault, corresponding cutoffs and faults were not present on the left side (where the backlimb also exhibited boudinage). On the third section, the right side appeared similar to a FBF whose forelimb had been oversteepened by a fault and by mineral precipitation. The left side, however, differed from the first and second sections and could not be coherently restored to match the right side. Given these observations, we hypothesize that there was either substantial out-of-plane motion along the fault(s) and/or that structures on the left side were obliterated and annealed by recrystallization. This work suggests that rapid along-strike changes in fault-related fold geometry may occur at any scale and shows how detailed 2D and 3D analysis of complex fault-related folds can enhance a geologist’s critical-thinking skills for creating balanced cross sections.