Grabens, basin tectonics, and the maximum total expansion of the Moon

Abstract

The presence of grabens and wrinkle ridges indicates that stresses sufficient to deform or fault crustal rocks have existed for at least part of lunar history. These structures have been explained as due to global expansion and contraction, basin subsidence, or both. Even if the extension implied by all lunar grabens were due to global expansion, the total radius increase permissible after terminal bombardment would be much less than that predicted by the most conservative models of lunar thermal evolution. The obvious areal association of most grabens and wrinkle ridges with impact basins suggests that a genetic relationship exists also. But it is necessary to develop either kinematic or dynamic models to determine if basin subsidence alone can account for these structures. Kinematic models are the most useful because fewer assumptions regarding material properties are needed than for dynamic models. For both types, the grabens and wrinkle ridges provide the data that must be predicted by the models. Considerations of geometry, kinematics, and mechanics suggest that the simple grabens on the moon are reliable indicators of crustal extension. Two end‐member kinematic models of postimpact basin modification are derived: one assuming that subsidence is accomplished entirely by an increase in radius of curvature of the basin floor, the other assuming that subsidence is accomplished without any change in radius of curvature of the basin floor. Any combination of the two also would be possible. Both models predict enough extension to account completely for basin‐related grabens, and thus they indicate that virtually all of the extension implied by lunar grabens can be provided by modest amounts of basin subsidence. In addition, both models also predict total radial and hoop shortening of the basin floor of about the correct magnitude to explain wrinkle ridges if these features are structures resulting from compression. Consequently, almost all extension and shortening of the lunar crust implied by grabens and wrinkle ridges can be explained by basin subsidence alone; significant global expansion after ∼3.9 b.y. is unlikely, and significant global contraction is unnecessary.