David Weiss GEOL 2810 2/23/2015

Summary:Wünnemann, K., J. V. Morgan, and H. Jödicke (2005), Is Ries crater typical for its size? An

analysis based upon old and new geophysical data and numerical modeling, Geological Society of America Special Papers, 384, 67–83.

This paper treats the Ries crater in Germany as the archetypical mid-sized complex terrestrial impact crater, and investigates the subsurface structure, target mechanical properties upon formation, and relationship to other putatively similarly sized terrestrial impact craters.

The authors use seismic and magnetotelluric data to investigate the subsurface porosity structure of Ries. Three seismic reflection profiles across the crater show increasing seismic velocity with distance from the crater, and that the seismic velocity reaches ~6 km/s ~2 km below the crater center. Given that the granitic basement rock seismic velocities are ~6 m/s, these results indicate that the Ries crater exhibits a zone of concentrated fractures within the crater interior down to a depth of ~ 2 km. This result is generally corroborated by the magnetotelluric depth-sounding investigation conducted by the authors. They find that the inner ring hosts a 2-5 km deep zone of anomalously high conductivity present within the inner ring (though offset slightly NW by 2-3 km), whose location is coincident with a Bouger gravity low. The authors interpret this result to indicate that the interior crater region hosts a brine-filled fractured zone within the upper few kilometers of crust.

The authors then conduct a series of SALE-3MAT hydrocode models of a wet tuff (sediment layer) overlying granitic basement in order to evaluate the influence of target mechanical properties on final crater morphology. The authors vary the final crater morphology by altering the thickness of the overlying sediment layer, its cohesion, and its friction coefficient, as well as the acoustic fluidization parameters which define the impact-induced target weakening during crater formation: viscosity and the decay timescale. Fig. 1 (A1) shows the model in a high strength case, while Fig. 1 (A2 and A3) show the model run for progressively lower strengths of the upper sediment layer. Fig. 2 (A2 and A3) show the model run for progressively weaker acoustic fluidization parameters. These results illustrate how the mechanical properties of the target can drastically effect the final crater morphology. Decreasing the strength of the target allows a more subdued final crater morphology, while stronger targets preserve a pronounced transient cavity rim (which potentially corresponds to the inner ring). As the fluidization weakening parameters decrease in viscosity, the transient cavity becomes progressively less pronounced, until it is entirely unobservable, and the height of central structural uplift increases so that central peaks begin to form. At even lower weakening viscosities, the central peak may collapse and spread outwards; the limit of this collapsed material could define the inner ring of Ries.

The authors then compare the Ries crater to two other terrestrial craters: the Bosumtwi and Zhamanshin impact craters. The authors suggest that the observed diameters for these craters are analogous to the inner ring of Ries crater, making these craters comparable in size. The authors find that these craters have similar depth/diameter ratios after accounting for target properties. They propose that the morphological differences between Ries and the relatively flatter Zhamansin are due to enhanced faulting at the Ries (weaker upper sediment layer), whereas Zhamanshin may have been a water-saturated target, promoting a smoother final crater morphology (e.g., Fig. 2 (A3)).

David Weiss GEOL 2810 2/23/2015

Questions for consideration:

1) Is the mass deficit in the central region of the crater a shallow (~2 km) fragmented zone (with interstitial brines) as proposed, or due to a deeper (~5 km) hemispherical fracture pattern? The magnetotelluric data could support either scenario, but the gravity data favors the shallow fragmented zone. What does this mean for our assumptions of impact-induced deep bedrock fragmentation on all of the planets ? What does GRAIL data tell us about crustal fracturing on the moon? Is Ries the exception or the rule?

2) Perhaps of less importance (but no less curious) what is causing the in-flight "clumped" ejecta pattern? Most ejecta curtains in numerical models appear to be more contiguous. Obviously cell resolution is a culprit, but what about the ejecta separated from the curtain? Is this from deceleration (I doubt it) or from variable excavation angles? Is this unique to the code used?

3) The numerical models in this study show that the inner ring can be formed in both high-strength (the transient crate rim) and low strength (central peak collapse) modes. Why then in the literature is the central peak collapse the dominant mechanism discussed? The structural relationships for the Ries inner ring do not point to "fluidization": the inner ring is composed of crystalline basement rock. Could the Ries crater inner ring correspond to peak rings on other planets, or is a unique Ries a case of exceptional wall collapse?

4) Why does Ries not have a central peak? Is the lack of central peak consistent with the diameters of peak-ring craters on other planets (after accounting for the higher gravity)? Do larger craters exist on the Earth that do have central peaks?

5) The authors suggest that the apparent crater diameters of the Bosumtwi and Zhamanshin impact craters correspond to the inner ring of the Ries crater. They note that because the inner ring is approximately equal to the transient crater diameter, all 3 craters are approximately the same size. If target properties (mainly assumptions on strength) have a huge effect on the formation of the inner ring, as the authors showed, is this assumption valid? Principally, whether the inner ring corresponds to the transient cavity is heavily influenced by the target strength chosen: this assumption holds for high strength, but if the inner ring is caused by central peak collapse, this relationship has no bearing.

6) What is the basis for assuming the apparent crater diameters of the Bosumtwi and Zhamanshin craters correspond to the structural inner ring? Are there any geophysical/structural/morphometric techniques that can be used to determine whether a ring is the outer (tectonic) ring, or the inner ring?

David Weiss GEOL 2810 2/23/2015

Figure 1. Figure 2.