Measuring late-stage fold flattening by brittle faults at the Whaleback anticline using 3D models and digital simulation tools

Mattathias D. Needle, University of Washington
Juliet G. Crider, University of Washington
Arlo B. Weil, Bryn Mawr College
Mary Beth Gray, Bucknell University

Abstract

Idealized buckle folds evolve through three stages: nucleation, amplification, and kinematic growth. Each stage is characterized by a distinct distribution of strain within the folding layer(s). In the nucleation stage, layer-parallel shortening dominates; in the amplification stage, internal strain must accommodate bending; and in the last stage of buckling, the fold may flatten with layer-parallel extension (LPE) in the limbs. Here, we use a high-resolution digital model of the Whaleback Anticline (Shamokin, Pennsylvania, USA) to quantify LPE accommodated by meso-scale faults on the north limb of this exceptionally-well exposed fold. Using 3D outcrop models and the digital field simulation tools of the Structural Geology Query Toolkit (Needle et al. 2022), we measured fault normal extension on eight transects perpendicular to the fold axis and twelve transects parallel to the fold axis to calculate mesoscale extensional strain. We find that axis-parallel extension averages about 2% and axis-perpendicular extension averages 3%. Compared to field-measured transects (Weil et al. in prep.), the digital models record less strain, possibly because the smallest faults (sub-cm) are not resolvable in the digital model. The digital surveys access more of the outcrop, however, permitting a higher resolution of the spatial distribution of strain. These findings advance buckle-fold theory models by informing future estimations of bedding thickness and length change, dimensions required for exploring the role of rheology in folding. The extension values at the mesoscale are broadly consistent with field data collected at the Whaleback, demonstrating the utility of this digital data-collection approach in potentially difficult to access outcrops.

Session

Structural geology techniques