Vertical movement in mare basins: Relation to mare emplacement, basin tectonics, and lunar thermal history

Abstract

The spatial and temporal relationships of linear rilles and mare ridges in the Serenitatis basin region of the moon are explained by a combination of lithospheric flexure in response to basin loading by basalt fill and a time‐dependent global stress due to the thermal evolution of the lunar interior. The pertinent tectonic observations are the radial distance of basin concentric rilles or graben from the mare center; the location and orientation of mare ridges, interpreted as compressive features; and the restriction of graben formation to times older than 3.6±0.2 b.y. ago, while ridge formation continued after emplacement of the youngest mare basalt unit (∼3 b.y. ago). The locations of the graben are consistent with the geometry of the mare basalt load expected from the dimensions of multiring basins for values of the thickness of the elastic lithosphere beneath Serenitatis in the range 25–50 km at 3.6–3.8 b.y. ago. The locations and orientations of mare ridges are consistent with the load inferred from surface mapping and subsurface radar reflections for values of the elastic lithosphere thickness near 100 km at 3.0–3.4 b.y. ago. The thickening of the lithosphere beneath a major basin during the evolution of mare volcanism is thus clearly evident in the tectonics. The cessation of rille formation and the prolonged period of ridge formation are attributed to a change in the global horizontal thermal stress from extension to compression as the moon shifted from net expansion to overall cooling and contraction. Restricting the time of peak lunar volume to 3.6 b.y. or earlier, together with the constraint that the accumulated thermal stress since the end of heavy bombardment did not reach levels capable of causing global‐scale lithospheric failure, greatly limits the range of possible lunar thermal histories. The zone of horizontal extensional stresses peripheral to mare loads favors the edge of mare basins as the preferred sites for mare basalt magma eruption in the later stages of mare fill, although subsidence may lead to accumulation of such young lavas in basin centers.