A DEEPER LOOK INTO OROGENIC CURVATURE: ANALOG MODELS IN CROSS SECTION

Field studies of curved orogens have focused on timing relationships between thrusting and vertical axis rotation to distinguish systems that initiate in an arc shape (primary curvature) from those that develop curvature during deformation (progressive curvature). In the current study, physical analog models of primary and progressive curvature are examined in cross section to identify characteristics of fault displacement and orientation that may help distinguish between these curvature processes in the field.

Construction of analog models involved layering colored sand on top of thin, rigid, plastic sheets. Three model designs were used: (1) experimental control models contained straight footwall ramps perpendicular to the shortening direction; (2) primary curve models contained an arc-shaped footwall ramp over which hanging wall layers translated; (3) in progressive curvature models, movable plastic strips below the hanging wall sand layers allowed for differential displacement over a straight footwall ramp, with displacement beginning in the center of the salient and progressing symmetrically out toward salient corners. Following deformation, models were dissected parallel to the imposed shortening direction to reveal serial cross sections in the deformed sand.

Cross sections demonstrated uniform vertical displacement along the primary arc and control models, and a systematic decrease in vertical displacement from the progressive arc center to the arc corners. The number of backthrusts remained constant throughout primary arc and control cross sections, whereas backthrusts decreased in quantity from the arc center to arc corners in the progressive curve model. These spatial variations in progressive curvature of a curved orogen mirror the temporal development of an individual thrust sheet because the total horizontal displacement decreases from the center to the corners of the progressive curve. Horizontal displacement is constant across the entire length of the primary arc and the control models. These distinguishing fault characteristics reflect more spatially homogeneous strain in primary arcs and more heterogeneous strain along the axes of progressive arcs.