RUPES RECTA AND THE GEOLOGIC HISTORY OF THE MARE NUBIUM REGION, THE MOON: INSIGHTS FROM FORWARD MECHANICAL MODELING OF THE STRAIGHT WALL

Individual normal faults are rare on the Moon and their presence likely implies an anomalous geologic history of the region where these faults occur. The best-known example is Rupes Recta; it is also known as the Straight Wall because it is remarkably linear over its ~120 km length. It is located in eastern Mare Nubium on the lunar near side. Age and cross-cutting relationships suggest that the maximum age of Rupes Recta is 3.2 Ga, which may make it the youngest large-scale normal fault on the Moon.

Although this fault has been known for centuries, little is known about how it formed. Understanding this can give insight into the geologic history of Mare Nubium and Nubium basin and to recent normal fault formation mechanisms on the Moon. Several hypotheses for the formation of Rupes Recta have been suggested, varying from fault reactivation to differential mare subsidence.

In order to investigate formation mechanisms for Rupes Recta, a detailed structural map was produced using high-resolution imagery and topography, permitting determination of the distribution of fault throw along strike. Forward mechanical modeling was used to infer pertinent characteristics of the fault at depth and to provide estimates of the cumulative amount of stress required to form Rupes Recta.

Fault nucleation is interpreted to have occurred near the fault center and fault propagated bi-directionally, growing northward and southward by segment linkage. Modeling of fault topography gives a best-fitting fault dip of approximately 85° and suggests that Rupes Recta accommodated ~400 m of maximum displacement and extends to a depth of ~40 km.

The cumulative driving stresses required to form Rupes Recta were estimated to be between 26 and 105 MPa. This range of stresses, similar to stresses required to form normal faults on Mars, suggests that the cumulative driving stress required to form Rupes Recta, and perhaps other lunar normal faults and graben of comparable scale, may have arisen from regional rather than local tectonic processes.