Topography of nearside mare graben: Implications for dike-induced or passive extension formation

Abstract

Graben are a common tectonic landform associated with lunar mare. These graben are hypothesized to have formed either due to subsidence induced flexure resulting from loading of the lithosphere by mare basalt (mascon tectonics) or by dikes related to intrusive volcanism. High-resolution topography derived from data from the Lunar Reconnaissance Orbiter (LRO) and SELenological and Engineering Explorer (SELENE) allow for the detailed analysis of flat-floored mare-radial and mare-concentric graben. Modeling of graben formation from dike-induced dilation predicts rises on graben flanks that decay in relief with distance from the central trough. Graben formation from passive extensional stresses are not expected to produce discernable flank rises. We take a systematic approach to assessing morphology of flat-floored graben in mascon and non-mascon mare settings. Modeling of dilation effects due to intrusion of dikes of realistic dimension and depth indicate that the predicted relief of flank rises are at or below the limits of detection using best available topography. However, it is concluded that the lack of evidence of flank rises indicates nearside graben formed by passive extension. The presence of graben in both mascon (e.g., Mare Humorum) and non-mascon settings (e.g., Oceanus Procellarum) suggests that flexure and extension are strongly influenced by the local thickness and/or strength of the mechanical lithosphere.