Wrinkle ridge assemblages on the terrestrial planets

Abstract

The wrinkle ridge assemblage is a group of associated features that have been observed on the Moon, Mars, and Mercury. Members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first‐, second‐, or third‐order ridges on the basis of dimensions. The ridge assemblages described in this study are known to occur in mare basalts on the Moon and smooth plains units that may be volcanic in origin on Mars and Mercury. The structures that occur in the continental flood basalts of the Columbia Plateau in the northwestern United States are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets. The anticlinal ridges of the Columbia Plateau and many first‐order ridges on the Moon, Mars, and Mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling (flexure‐fracture) or buckling as a consequence of reverse or thrust faulting (fracture‐flexure). Estimates of the unit shortening across first‐order ridges, using simple geometric forms to approximate the ridge morphology, are lower than those determined for the anticlines of the Columbia Plateau, where displacements are the results of folding and reverse to thrust faulting. Evidence of layer‐parallel extension that often accompanies folds developed by fracture‐flexure mechanisms (i.e., drape or fault‐bend folds) is not found in the anticlinal ridges of the Columbia Plateau and has only been observed in one ridge on the Moon. A kinematic model involving buckling followed by reverse to thrust faulting (flexure‐fracture) is favored for the origin of the anticlinal ridges of the Columbia Plateau and many of the first‐order ridges on the Moon, Mars, and Mercury.