Structural Uplift Field

To understand how the addition of a sedimentary sequence influences structural uplift, we begin by analyzing the structural uplift field and target deformation near the centre of the crater 195 s after impact (i.e., once the dimensions of the crater remain unchanging and crater formation has largely ceased; Figs. 4.3, 4.4 for the large and small impactor simulations, respectively). For simplicity, we focus on five target geometries: a purely crystalline target (Figs. 4.3a, 4.4a), and mixed targets with sedimentary layers between 0.75 km and 3.0 km thick in increments of 0.75 km (Figs. 4.3b–e, 4.4b–e). We first focus on the large impactor simulations before discussing the small impactor suite.

Figure 4.3: A comparison of the subsurface deformation in the a) crystalline target, and mixed targets with b) 0.75 km, c) 1.5 km, d) 2.25 km, and e) 3.0 km sedimentary layers for the large impactor simulations. The left panels in each plot show the sedimentary material plotted in dark grey and crystalline material plotted in light grey. Target deformation is visualized using a grid of Lagrangian tracer lines (spaced every 5 lines or 250 m). The right panels in each plot show the stratigraphic uplift field. Reds indicate highly uplifted regions in the target while blues indicate material that either hasn’t experienced any uplift or that has been depressed during crater formation.

Figure 4.4: A comparison of the subsurface deformation in the a) crystalline target, and mixed targets with b) 0.75 km, c) 1.5 km, d) 2.25 km, and e) 3.0 km sedimentary layers for the small impactor simulations. The left panels in each plot show the sedimentary material plotted in dark grey and crystalline material plotted in light grey. Target deformation is visualized using a grid of Lagrangian tracer lines (spaced every 5 lines or 125 m). The right panels in each plot show the stratigraphic uplift field. Reds indicate highly uplift regions in the target while blues indicate material that either hasn’t experienced any uplift (or that has been depressed during crater formation).

In our simulations of a large impact into a purely crystalline target, there is a region of highly uplifted material up to ~1.0 km depth directly below the point of impact (Fig. 4.3a). According to the structural uplift field, this region has experienced a vertical displacement in excess of 1.3 km. The amount of uplift attenuates gradually with increasing depth and radius, approaching 0 km at a radial distance of ~5.0 km from the point of impact. The addition of a thin sedimentary layer results in a slight decrease in structural uplift when compared against the crystalline case (Fig. 4.4b–c). Observing the left panels in these plots, an apparent difference compared to the purely crystalline case is the disruption of tracers in this central region. It is notable that the sedimentary infill does not completely obscure the basement layer in these simulations (refer also to Fig. 3.2 in 3.3.1.1). When a 1.5 km thick sedimentary layer is modelled, there is a region of increased structural uplift at ~2.0 km depth that is noticeable and not present for thinner sedimentary layers.

For impacts into mixed targets with thicker sedimentary sequences (2.25 km and 3.0 km; Fig. 4.3d–e), the structural uplift field is noticeably different compared to the crystalline case or the mixed target cases with relatively thin sedimentary sequences discussed above. First, the sedimentary target rocks entirely obscure the basement. This region is heavily deformed and highly uplifted according to the structural uplift field. The area of highly uplifted material at a radial distance of ~2.0 km that was observed in the 1.5 km mixed target case is even more pronounced for the 2.25 km and 3.0 km cases. Focusing on the region directly below the point of impact at a depth of ~2.0–4.0 km, the basement layer seems to have experienced less uplift compared to basement material at similar depths in the crystalline target.

The structural uplift field in our small impactor suite of simulations (Fig. 4.4) differs significantly compared to the large impactor suite. In the purely crystalline case, or when the sedimentary layer is relatively thin (0 km and 0.75 km sedimentary layers; Fig. 4.4a– b), there is a small zone of uplifted material on the floor of the crater near the centre of the crater. The floor of the crater is completely flat in these two simulations, and when a 0.75 km sedimentary layer is modelled in the target, no sediments are present in the cavity following crater formation. Material in this region has experienced a maximum vertical displacement of ~400–500 m in the crystalline case, and ~200–300 m in the 0.75 km

sedimentary layer case. The uplifted region does not extend deep into the target and attenuates quickly with increasing radial distance from the impact site. Interestingly, there is a region of relatively highly uplifted material in the walls of the crater formed in the crystalline target.

For thicker sedimentary sequences (1.5–3.0 km; Fig. 4.4c–e), the uplift field changes dramatically relative to the thin sedimentary layer and crystalline cases. There is an extended region of highly uplifted material (vertical displacement in excess of 700 m) at the surface near the centre of the crater in all three of these simulations, which corresponds to sediments that have collapsed back towards the centre of the crater, entirely covering the uplifted basement. There is highly uplifted material that appears to have spread across the floor of the crater. The structural uplift field in the 2.25 km and 3.0 km large impactor cases and the 1.5 km and 2.25 km small impactor cases (Figs. 4.3d–e, 4.4c–d) show similar patterns. Specifically, there is a highly uplifted zone away from the centre of the crater (at a radial distance of ~150 m in the 1.5 km small impactor case and ~400 m in the 2.25 km case) which is not present for thinner sedimentary sequences. Interestingly, this trend does not continue for the 3.0 km thick sedimentary sequence (Fig. 4.4e). The structural uplift field plotted for that simulation shows substantial uplift up to ~3.0 km depth. In this simulation the structural uplift attenuates gradually with increasing radial distance from the point of impact; the most uplifted material is primarily located near the centre of the crater directly below the point of impact.

While the structural uplift field provides valuable insight into the influence of a sedimentary layer on structural uplift, it is difficult to draw quantitative conclusions from these visualizations alone. We now shift our focus to the motion of tracer particles within the subsurface so that we can provide a more thorough explanation of the processes that result in the uplift field plotted in Figures 4.3 and 4.4. Specifically, we hope to show that both methods (the structural uplift field and tracer trajectory tracking methods) produce consistent results.