Origin of Periodically Spaced Wrinkle Ridges on the Tharsis Plateau of Mars

Ridged plains are a major geologic unit on Mars and are a predominant unit on the Tharsis Plateau, a region that has undergone uplift, extension, and extensive shield and flood volcanism. These units, probably flood volcanic in origin, are characterized by landforms classed as wrinkle ridges. Wrinkle ridges are interpreted to be folds, resulting from buckling followed by reverse to thrust faulting (flexure-fracture). A prominent characteristic of many of the wrinkle ridges on Mars, particularly those in the Tharsis ridge system, is the periodic nature of their spacing. The periodic spacing has been evaluated in six major provinces on the Tharsis Plateau with regions further divided into domains based largely on variation in ridge orientation. The average spacing of the Tharsis ridges, based on 2934 measurements, is 30 km. In an effort to account for the periodic nature of the wrinkle ridges, the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding. Free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows. The near-surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement. Buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith. ~ e c o u ~ l i n ~ is enhanced if the megaregolith were wateror ice-rich at the time of deformation. The rheolo~ic behavior of the ridged olains andmegaregolith is aooroximated bv a linear elastic and linear vis~o& material. The model; are examined for a range in (I j ihe strength cbntrast between the ridged plans material and the underlying megaregolith; (2) thickness of the ridged plains material; (3) thickness of the megaregolith; and (4) number of layers. The elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick (2 2400 m) with multiple layers ( n > 1) and relatively high Contrasts in Young's modulus (EIEo 2 100). The high EIEo required is possible only if the megaregolith was water-rich at the time of deformation. Over the same range in values of the parameters, viscous buckling is much less restricted. The viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness (250 m to several kilometers), megaregolith thickness, number of layers (n = I to 8) and contrasts in viscosity (rJqo 2 10). In addition, viscous buckling is viable if the megaregolith were dry, water-rich or ice-rich at the time of deformation. The absence of folds with a cross-sectional geometry in the shape of a sinusoid (anticline-syncline pairs) may be the result of initial deformation of the ridged plains material into low-amplitude folds (in infinitesimal strain) followed by plastic yielding in the cores of the anticlines (in finite strain). Initial elastic or viscous buckling, coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting, could account for the asymmetric fold geometry of many of the wrinkle ridges.