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.
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.