Numerical Modelling of a Potential Oceanic Impact Structure

1. Introduction

Compared to other planetary bodies, terrestrial impact craters can be rather difficult to discover due to different geological processes, including sedimentation and erosion, that modify and eventually erase the original impact crater topography. Only a small number of marine impacts are known. The paucity of oceanic impact craters can be explained by the relatively young age of the oceanic crust compared to continental shields, the relatively fast sedimentation rates, the fact that only sufficiently big and thus less frequent impactors can penetrate the water column and form a crater at the ocean floor, and last but not least, the poor accessibility of the ocean floor making it more difficult to find and confirm the impact origin of suspicious structures (Grieve et al., 1995).

This study focuses on a new potential impact structure discovered during a marine seismic survey (Groth et al., in preparation). Figure 1 shows seismic reflectors depicting the structural deformations beneath the ocean floor as a function of depth. The structure shows typical features of complex impact craters such as a central uplift and concentric rings at a depth in the sediment layer corresponding to the ocean bottom at the time of potential impact. These features suggest an impact origin. However, the relatively small size of approximately 4.6 km is uncommon for a complex crater. At a depth of 0.64 km the reflector appears to be not affected by the potential impact, which also provides constraints on the size of the event.

Figure 1: Profiles derived from seismic data courtesy of TGS.

 

To unambiguously confirm the impact origin, shock metamorphic effects in sample material is required (French, 1998), but has not been found, yet. Drill cores are only available at some distance from the structure, which makes it difficult to correlate and find impact ejecta.

Therefore, we test the impact origin hypothesis by numerical modelling of crater formation. We use the given structure as reference and investigate what conditions are required to enable the formation of the given morphology. This is particularly challenging as a central peak and ring structures are unusual for the given structure dimensions and may be only explained by specific material properties and rheology.

 

2. Method

We performed a parametric study utilizing the iSALE shock physics code for 2D-simulations (Wünnemann et al., 2006) in order to simulate a formation caused by impact cratering and reproduce the given morphology. A simplified set-up was applied in all simulations: A projectile with a radius of 75m and a velocity of 12km/s is assumed to impact a one-layered target.

The parametric study comprises the application of different rheology models, among those are models for a Newtonian Fluid, competent rocks, and granular material. In the numerical models the controlling parameter for a Newtonian fluid is the viscosity, which was varied ranging from 10² to 10⁹ Pa*s. The rheology models for competent rocks and granular material are used with varying values for the cohesion ranging from 10² to 10⁶ Pa and the friction coefficient ranging from 0.1 to 1. The rock and granular material rheology is combined with the acoustic fluidization model (Melosh, 1979) to enable temporary weakening during crater formation. Acoustically fluidized material behaves according to a Bingham rheology with a Bingham viscosity which was varied between 10^-4 to 10². Another controlling parameter is the attenuation time of the acoustically fluidized state of the material.

 

3. Results

The different combinations of parameters lead to results with a wide range of possible crater morphologies.

Figure 2: Comparison of seismic profiles (left) and models (right) using rheology models for (a) competent rock and (b) granular material with acoustic fluidization. Late timesteps are depicted when material is completely settled. Black lines represent tracers for material movement.

 

Figure 2 shows a comparison of the seismic profiles (left) with the models generated during this work (right). Assuming a rheology model for competent rock (a), it is possible to reproduce a central uplift with steep flanks. While the upper layer can be observed to be overturned, the material in a depth of 0.64 km is only to a small degree affected by the impact. Using a rheology model for granular rock and applying the concept of acoustic fluidization (b) as a temporary weakening, it is possible to produce a broad central uplift with shallow flanks. Although still larger than depicted in the seismic data the height of the central peak is noticeably smaller than in the model using competent rock. The material in a depth of 0.64 km is significantly deformed by the impact event.

To verify the visual analysis a quantitative analysis is performed by parameterizing the central peak of both the seismic data and the simulation results. The most prominent central peaks in depths of 0.3 km and 0.4 km are selected and their height and width are calculated for comparison. The quantitative analysis confirms that the model with a granular material rheology plus acoustic fluidization reproduces the seismic profiles in the upper layers while the model assuming competent rock agrees better with the seismic data in the lower layer.

 

4. Conclusion

An impact origin of the discussed structure is possible as the study succeeds in reproducing its most prominent morphologic features. It was possible to constrain rheology models that explain the formation of the given structural deformations. This study shows the significance of modelling to estimate the possibility of an impact origin when there is little prospect to find shock-metamorphic evidence.

 

References

Grieve, R., Rupert, J., Smith, J., Therriault, A. (1995), The Record of Terrestrial Impact Cratering, GSA Today, Vol. 5, pp. 195-196

French, B. M. (1998), Traces of Catastrophe: A Handbook of Shock–Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar&Planetary Institute, Houston, p.31.

Wünnemann, K., Collins, G., Melosh, H. J. (2006), A strainbased porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets, Icarus, Vol. 180, pp. 514-527.

Melosh, H. J. (1979), Acoustic fluidization: A new geologic process? Geophys. Res., 84 (B13), pp. 7513– 7520.