The influence of mechanical stratigraphy on the development of detachment folds and associated mesoscopic structures: an example from the Lisburne group carbonates, northeastern Brooks Range, Alaska

Abstract

The mechanical properties of individual stratigraphic layers in a multi-layer sequence of sedimentary rock influence the deformational response before, during, and after fold development. To demonstrate this, the mechanical character of stratigraphic layers and mesoscopic deformational structures within individual stratigraphic layers were documented in two well-exposed outcrop-scale detachment folds in the Lisburne Group carbonates, northeastern Brooks Range, Alaska. Fold geometry and fold-related mesoscopic structures indicate that flexural slip and flexural flow are the operative fold mechanisms until a critical interlimb angle of 90° is reached, after which homogenous flattening occurs. Changes in bed thicknesses due to homogenous flattening alter the overall fold geometry. Lithostratigraphic unit boundaries do not always coincide with mechanical unit boundaries. Thin shale layers lower the bedding interface strength and commonly form flexural slip horizons that define mechanical unit boundaries. As fold shortening progresses, slip horizon spacing is interpreted to decrease, causing mechanical unit thickness to decrease. Newly forming mechanical boundaries alter the conditions of deformation, which change the overall fold dynamics. Surveyed fracture sets reveal the influence of lithology, mechanical unit thickness, anisotropy, and structural position on fracture distribution within individual mechanical units. Fracture densities vary from set to set and unit to unit due to structural and stratigraphic controls within these folds.