A GEOLOGIC MAP OF THE CALORIS BASIN, MERCURY

A geologic map of the Caloris basin has been created to synthesize the results of previous studies into a contextual framework for quickly viewing the thematic research that has been performed on this interesting region. The 1,550 km-diameter basin, the largest impact structure on Mercury, is a highly complex geologic landform. The basin is floored by volcanic light-toned plains, and multiple landforms, including volcanic vents and even a possible small shield volcano, have been identified. The basin floor shows a degree of tectonic diversity that is far greater and more complex than anywhere else on the planet. However, the nature of the annulus of dark-toned material that surrounds the basin remains unclear. While the hummocks are thought to be ejecta blocks, the smooth, dark, plains interfingering them have been interpreted to be younger than the light-toned plains within the Caloris basin. This would imply a second, plains emplacement event, possibly involving lower albedo volcanic material, which resurfaced the original ejecta deposit.

In Mercury’s mapping scheme, the Caloris basin crosses four quadrangles: H-3 Shakespeare (21º-66°N, 90º-180°W), H-4 Raditladi (21º-66°N, 180º-270°W), H-8 Tolstoj (21ºS -21°N, 144º-216°W) and H-9 Eminescu (21ºS- 21°N, 216°-288º W). Given the importance of Caloris, it is better served by having a basin-centric geologic map. The original extent of the map was 0º-60°N, 165º-230°W, but during mapping we realized that the map needed to extend from 160º- 235°W to cover the entire surrounding dark annulus. This current areal extent best displays the Caloris basin region.

Two Mercury quadrangle maps based on Mariner 10 data cover the eastern third of the Caloris basin: H-8 Tolstoj [1] and H-3 Shakespeare [2]. Several terrain units associated with the Caloris basin were identified and mapped. We have utilized higher resolution MESSENGER data to complete our Caloris basin map, although we have recognized several of the previously mapped geologic units, and continued their use. In addition, new geologic units were identified and mapped in the regions not previously covered by the H-3 and H-8 geologic maps.

References: [1] Schaber G. G. and McCauley J. F. (1980) U.S. Geol. Survey, Map I-1199. [2] Guest J. E. and Greeley R. (1983) U.S. Geol. Survey, Map I-1408.