3rd Planetary Data Workshop 2017 (LPI Contrib. No. 1986) 7080.pdf

Mapping Hrad Vallis, Mars. P. J. Mouginis-Mark1 and C. W. Hamilton2; 1. Hawaii Institute Geophysics Planetology, University of Hawaii, Honolulu, HI 96822 ([email protected]); 2. Lunar Planetary Lab., Univ. Arizona, Tucson, AZ, 85721.

Introduction: Hrad Vallis is a geologically past three years, we have been mapping this area young (Amazonian-age) outflow channel located at a scale of 1:175K, as well as using numerical within the Elysium volcanic province on Mars. simulations to assess whether Hrad Vallis formed The determination of its emplacement history through catastrophic aqueous flooding, effusive provides fundamental information about the plan- volcanism, or a combination of both processes. et’s hydrologic and thermal evolution. For the Our current map is shown in Fig. 1.

Fig. 1: Current version of our geologic map of the Hrad Vallis region, Mars. Hrad Fossae, the source region for Hrad Vallis, forms NW-SE trending structures in the southern portion of the map that is sur- rounded by the ridge and trough material (Aelt) which feeds into flood plain materials (Achp). Unit Apf is the unit interpreted to be an inflated lava flow. Galaxias Mons is unit Aelr. Insert at top right shows the location of our study area. Elysium Mons volcano is at lower right. 3rd Planetary Data Workshop 2017 (LPI Contrib. No. 1986) 7080.pdf

Key conclusions from our mapping DOMU are also well underway. Our plan there- Our geologic mapping reveals that the oldest fore is to complete the draft map and submit it exposed units near Hrad Vallis include assem- for review this summer. We also have a manu- blages of eroded knolls, plateaus, and flows, script submitted for peer review [11], and this is which form the high-standing rugged material focused on the sequence of the aqueous flooding (Aelc), and a series of lava flows comprising episodes and volcanic activity, and our interpre- plains-forming materilas (Ael). Amazonian-age tation of flow as an inflated pahoehoe-like lava flow units near Hrad Vallis are attributed to a flow. multi-stage emplacement history, including: (1) an intitial aqueous flood deposit (Aps) triggered What is clear is that the western segment of by the intrusion of a large sill beneath Hrad Fos- unit Apf extends well to the west of our map sae [1]; (2) an episode of effusive volcanism that area. The central segment of Apf also extends produced a lava-rise (Apf) with lava-rise pits far to the north, and approaches some of the distributed throughout its interior; and (3) a se- fractures associated with Galaxias Fossae ries of flow units (Aelt, Achp, and Ach) that (37.5oN, 217.5oE). The small segment of Apf were associated with the formation of Hrad Val- on the eastern edge of our map area is but a lis itself. These flow units are interpreted to be small segment of the flows identified by [8] and the product of another episode of magmatic in- mapped as part of a fluvial outflow system by trusion and aqueous flooding. This pattern of [7]. Thus our current map area really should to alternating aqueous flooding and effusive vol- be extended to fully understand the spatial ex- canism is consistent with observations of other tent of these inflated flows. Indeed, the entire outflow channel systems on Mars, such as Marte area between 33.0 – 37.5oN, 139.0 – 145.0oW is Vallis [2, 3], Athabasca Vallis [4, 5], and Man- a fascinating area with a complex interplay be- gala Valles [6]. This suggests that Amazonian- tween ice, groundwater, both effusive and ex- age outflow channels were not formed during a plosive volcanism. It therefore warrents addi- single event, but rather are the products of mul- tional attention by the planetary geology com- tiple stages of activity, alternating between munity. aqueous flooding and lava flow emplacement. References We believe that the intrusions and the lava [1] Wilson, L. and P. J. Mouginis-Mark (2003). flows near Hrad Vallis may have interacted with J. Geophys. Res. 108, doi: 10.1029/ surficial ice deposits, resulting in flow confine- 2002JE001927. [2] Burr, D. M. et al. (2002). ment and the development of explosive lava- Geophys. Res. Lttrs. 29(1), 1013. [3] Burr, D. water interactions (e.g., Galaxias Mons, Aelr, M., L. Wilson and A. S. Bargery (2009). In: [7]). Our preferred origin of unit Apf as a lava Megaflooding on Earth and Mars, Cambridge flow differs from previous interpretations [1, 8], Univ. Press, pp. 194 - 208. [4] Jaeger, W. L. et because it is not now believed to be a mudflow. al. (2007). Science 317, 1709 – 1711. [5] Jae- But, if this interpretation is correct, it may be ger, W. L. et al. (2010). Icarus 205(1), 230 – one of the best examples of an inflated pa- 243. [6] Keske, A. L. et al. (2015). Icarus 245, hoehoe-like flow on Mars. Such an interpreta- 333 – 347. [7] Mouginis-Mark, P. J. and L. tion would also provide an explanation for the Wilson (2016). Icarus 267, 68 - 85, 2016. [8] origin of the thermally anomalous craters first De Hon, R. et al. (1999). USGS Misc. Map I- identified by Morris and Mouginis-Mark [9]; if 2579. [9] Morris, A. R. and P. J. Mouginis- unit Apf is an inflated lava flow, then these Mark (2006). Icarus 180, 335 – 347. [10] anomalous craters can be interpreted as lava pits, Walker, G. P. L. (1991). Bull. Volcanol. 53: 546 similar to those identified in Hawaii [10]. – 558. [11] Hamilton, C. W. et al. (2017). Epi- sodes of aqueous flooding and volcanism from Next Steps geologicall recent outflow channels on Mars. As Fig. 1 illustrates, we have almost finished the Submitted to Earth Planet. Sci. Lttrs. linework for our map. The COMU and the