Proposed Ice Flow, Given 200m and 400m Additional Ice in the Allan Hills Region, Antarctica: Implications for Meteorite Concentration

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The Allan Hills-David Glacier region contains some of the most highly populated meteorite stranding surfaces in Antarctica. Nearly 2000 meteorites have to date been collected from the icefields associated with the Allan Hills, and nearly 1500 from areas around Elephant Moraine. While much attention has been focused on the current geological and glaciological conditions of these stranding surfaces, less work has been done concerning what they may have looked like in the past, when ice thicknesses may have been greater. In this study, conjectural maps of the current Allan Hills area with 200 meters and 400 meters of additional ice cover each are analyzed for probable regional and local ice flow patterns. A dramatic decrease in ice thickness over a relatively brief period of time could result either from climatic change or a geologically rapid regional uplift. Delisle and Sievers (1991) noted that the valley between the Allan Hills Main Icefield and the Allan Hills resembles a half-graben resulting from east-west extensional tectonics, and that the mesa-like bedrock features associated with the Near Western and Mid Western Icefields resemble fault blocks. They concluded that the Allan Hills area icefields may have become active stranding surfaces as a result of a regional uplift within the past 1-2 million years, assuming a current rate of uplift in the Allan Hills region of ~100 meters/million years. Whether the cause was climatic or tectonic, generalized maps of current ice contours plus 400 and 200 meters ice may provide views of what the Allan Hills region looked like just before activation of the modern meteorite stranding surfaces (Figs. 1 and 2). At an ice thickness greater by 400 meters, ice could flow smoothly over the Allan Hills and would drain down to the Mawson Glacier via the Odell Glacier, east of the Allan Hills; down the Manhaul Bay depression between the east and west arms of Allan Hills; and down the half-graben discovered by Delisle and Sievers (1991) to the west of Allan Hills. As the ice thinned toward plus 200 meters, areas of net ablation may have developed at the east and west arms of the Allan Hills but flow would still be active enough to carry exposed meteorites away. At about plus 200 meters ice, the Allan Hills western arm would emerge as a partial barrier to ice flow and the top edge of the half-graben below the Allan Hills Main Icefield would become effective as a meteorite stranding surface. This circumstance would mark the beginning of the Allan Hills Main Icefield as we see it today. Subsequently, ice flow between the Main and Near Western Icefields would decrease in volume and velocity until an area of net ablation developed over the mesa beneath the Near Western Icefield. Meteorites would therefore begin to strand at the Near Western Icefield as well. Huss (1990) described the Allan Hills associated icefields as being progressively younger toward the west. The history suggested here is not in conflict with that interpretation. References: (1) Delisle & Sievers (1991) JGR, 96(E1), 15577-15587; (2) Huss (1990) Meteoritics, 25, 41-56. Figure 1, which in the hard copy appears here, shows a map of Allan Hills and proposed ice flow given an additional 400 meters ice cover. Solid areas are proposed rock exposures. Areas outlined by solid lines are current rock exposures. The current Allan Hills Main Icefield is outlined by a dashed line. Areas outlined by dotted lines are proposed ancient standing surfaces, assuming they were areas of net ablation. The direction of ice flow is shown by the arrows; they are not meant to represent vectors. Figure 2, which in the hard copy appears here, shows a map of Allan Hills and proposed ice flow given an additional 200 meters ice cover. Solid areas are proposed rock exposures. Areas outlined by solid lines are current rock exposures. The current Allan Hills Main Icefield is outlined by a dashed line. Areas outlined by dotted lines are proposed ancient standing surfaces, assuming they were areas of net ablation. The direction of ice flow is shown by the arrows; they are not meant to represent vectors.

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