Modeling the attenuation of structural uplift beneath large lunar craters

Introduction. Rebounding shock compressed target material produces structural uplifts in central peak and peak ring craters. For central peak lunar craters 20 to 100 km in diameter, [1] suggested maximum structural uplift decreases from one-tenth to one-fifth of the final crater diameter, respectively. Seismic studies and numerical models of terrestrial craters (e.g., Chicxulub [2-4]), however, suggest uplift beneath a crater attenuates with depth. Here, we quantify the attenuation of uplift beneath large complex craters and basins on the Moon and offer a new equation for estimating maximum structural uplift.

Methods. The 2D iSALE hydrocode [4] was used to model large crater-forming lunar impacts. An infinite half-space target was divided into crustal (40 and 60 km thick) and mantle layers, each with appropriate lunar-like properties. Three thermal profiles were used – two estimating a 4 Ga Moon for basin-forming impacts and one estimating a cooler 1-3 Ga Moon for large complex crater-forming impacts. Impactor diameter was varied between 20 and 120 km. Cell size (varying between 1 and 3 km) was adjusted as a function of impactor diameter to keep a constant number (40 for basin-forming impacts, 20 for complex crater-forming impacts) of cells across the impactor. Impact velocity was varied between 10 and 20 km/s. Due to the two-dimensional, axisymmetric nature of the hydrocode all impacts were vertical.

Results. Maximum structural uplift (Umax) increases as transient crater diameter (Dtc) increases, with model results from all three thermal profiles fit by a single equation (Umax = 0.075 Dtc1.27). Maximum uplift occurs at a depth equivalent to 20-30% of the crustal annulus radius (the radial distance to the thickest crust – a measure of crater size [5]). Beneath this depth, the amount of uplift relative to the maximum uplift attenuates linearly when depth is normalized by the crustal annulus radius.

Discussion. Our result produces similar uplift values to the equation of [1] for lunar complex craters 100 to 300 km in diameter. For larger basin-forming impacts (>300 km diameter), our result produces uplifts only one-third to one-half of those predicted by [1]’s equation. The deviation at larger crater diameters may reflect different target thermal conditions during basin and complex crater formation, not considered in [1]; thermal conditions were warmer during the basin-forming epoch ~4 Ga. Maximum uplift is not represented in the material exposed at the surface, occurring instead at a depth equivalent to 20-30% of the crustal annulus radius. This effect is caused by thinning of the top of the collapsing central uplift as it flows laterally over the basin floor, while deeper material is unaffected.

References. [1] Cintala, M.J. & Grieve, R.A.F. (1998) Meteorit. Planet. Sci., 33, 889-912 [2] Christeson, G.L. et al. (2001) J. Geophys. Res., 106, 21754-21769 [3] Christeson, G.L. et al (2009) Earth Planet. Sci. Lett., 284, 249-257 [4] Collins, G.S. et al (2002) Icarus, 157, 24-33 [5] Potter, R.W.K. et al (2012) Geophys. Res. Lett., 39, doi:10.1029/2012GL059281.

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Impact cratering
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Postdoctoral Researcher
Lunar and Planetary Institute
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Center for Lunar Science and Exploration
Lunar and Planetary Institute
3600 bay Area Boulevard, Houston, TX
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David A. Kring, Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, TX, USA
Gareth S. Collins, Dept. Earth Science and Engineering, Imperial College London, London, UK