CRITICAL TAPER WEDGE MECHANICS OF FOLD-AND-THRUST BELTS ON VENUS - INITIAL RESULTS FROM MAGELLAN

Fold-and-thrust belts exist on Venus at the margins of crustal blocks such as plateaus, tesserae, and coronae and as ridge belts within the plains. These fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics, a mechanics that is well known for fold-and-thrust belts and accretionary wedges on Earth. For example, an analysis of fold geometry at the toe of the Artemis Chasma fold belt indicates fault-bend folding above a regionally extensive decollement horizon at a depth of about 1.5 km. Near-surface deformation on Venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1-2 km. Critical taper wedge mechanics under anticipated Venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10-20-degrees, which is similar to some estimated slopes in the steep parts of the fold belts. The low taper toes of fold belts may be cohesion-dominated toes on either brittle or plastic decollement horizons. Once the base of the wedge undergoes the brittle-plastic transition, the surface slope is expected to flatten to near horizontal, in qualitative agreement with many topographic profiles of fold-and-thrust belts on Venus. The estimated depth of the brittle-plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric-temperature gradient. The relief of fold belts (measured between the toe of the wedge and the flat crest) displays a remarkable linear dependence on absolute elevation (Figure 15), ranging from 6 km for Maxwell Montes at an elevation of +10 km to a few hundred meters at the lowest planetary elevations (0 to -2 km). This remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle-plastic transition, possibly controlled by an isostatic coupling of elevation, lithospheric thickness, and geothermal gradient.