Fig 1.
Simplified geological map of the Twyfelfontein area highlighting the distribution of Damaraland basement rocks and present-day topography (false-colored ASTER digital elevation model).
The trends and inferred ice flow directions of MSGLs are shown as white arrows in the inset circular histogram. The axial traces of selected large folds are shown in black. The location of images in Fig 2 are shown. Geology from the Digital Atlas of Namibia (http://www.uni-koeln.de/sfb389/e/e1/download/atlas_namibia/index_e.htm).
Fig 2.
Megalineations at Twyfelfontein.
(A) Side-on field photograph of an asymmetric megawhaleback at Twyfelfontein, inferred ice flow direction from right (SE) to left. The megawhaleback is approximately 15 m tall. (B) Along axis view of a whaleback and megawhaleback pair near Xaragu Camp, truck for scale. The axis of the whaleback in the middle ground is parallel to the strike of the southwest dipping beds. The axis of the neighboring megawhaleback is perpendicular to bedding at the fold hinge (extreme left above truck). Note loose flaggy regolith on whaleback surface in the foreground. (C) Oblique image of megawhalebacks in a large granite koppie between Burnt Mountain and the Ugab River; USGS EROS (Earth Resources Observatory and Science (EROS) Center) http://eros.usgs.gov/#; illustrative of copyrighted images used in this study. (D and E) Oblique images of the southern and northern closures, respectively, of a large fold in the Swakop Group; USGS EROS (Earth Resources Observatory and Science (EROS) Center) http://eros.usgs.gov/#; illustrative of copyrighted images used in this study.
Fig 3.
(A) Cross-plot of MSGLs axes from Twyfelfontein (blue), Cree Lake, Saskatchewan (orange), Amadjuak Lake, Baffin Island (green), and Ullapool, Scotland (red). (B) Cross-plot of long axis length and elongation ratio. Fields for (I) megaridges, (II) rock drumlins and whalebacks, and (III) megawhalebacks follow Krabbendam et al. [18]. (C) Violin plot of the distribution of megalineation long axes between Twyfelfontein and the three comparison sites. The white dot is the median value, the heavy black line is the interquartile range, and the ‘violin’ is a mirror-imaged kernel density plot. (D) Violin plot of the distribution of megalineation elongation ratios for Twyfelfontein and the three comparison sites.
Fig 4.
Paleogeographic reconstruction.
Reconstruction of the SE Brazilian and southern African margins during the LPIA, adapted from [3] and [27], and with data from [19] and [20]. Known paleocurrent directions preserved in the Itararé Group and the paleo-ice flow directions reported here are shown.