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 crystal 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°, 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.