ELYSIUM REGION, MARS: CHARACTERIZATION OF TECTONIC FEATURES. J. Lynn Hall and Sean C. Solomon, Dept. of Earth and Planetary Sciences, M.I.T., Cambridge, MA 02139; James W. Head, Dept. of Geological Sciences, Rrown University, Providence, 91 02912; and Peter J. Mougini s-Mark, Planetary Geosci ences Division, Hawai i Institute of Geophysics, University of Hawai i , Honoluluy HI 96882. Introduction. The Elysium region is the second largest volcanic center on Mars L1,2J. It consists of a broad dome, 2400 by 1700 km, on top of which sit three volcanoes: Elysium Mons, Hecates Tho1 us and A1 bor Tholus [2]. The Elysium province has both a topographic rise and a broad positive free-air gravity anomaly [3]. On the basis of crater densities, the surfaces of both the plains and the shields of Elysium are older than their Tharsis counter- parts [2,4], with the surfaces of the volcanoes being about 1 b.y. old. In this paper we identify and characterize the tectonic features of the Elysium region. Identification of features was made using USGS control led photomosaics (Elysium quadrangle, and portions of henthes and Cebrenia quadrangles) ; Viking Orbiter photographic data were used in indi vidual cases to assist in identification. The positions and orientations of tectonic features can then be used, in conjunction with estimates of the mass of the volcanic load obtained from gravity model1 ing, to constrain the thickness of the elastic lithosphere in the region. The principal physiographic features of the Elysium region are shown in Figure 1. These features can be grouped into five categories: (1) Elysium Mons concentric fractures. An extensive set of concentric graben almost enti rely enci rcles Elysium Mons at distances of approximately 100 to 450 km from the central caldera. The two most distant fractures cut across the shield of Albor Tholus, the southernmost shield of the group, and disappear into the volcanic plains that surround the shield. Some of the fractures are quite fresh in appearance, while others appear to have been mantled by subsequent volcanic or aeolian activity. Differences in appearance of the graben cannot be completely attributed to differences in ages of the fractures, but may also be due to the asymmetric geometry of the mantling material. The presence of concentric fractures indicates that there has been failure of the elastic lithosphere under the load imposed by the Elysium Mons shield [5], and their locations can be used to constrain the thickness of the lithosphere at the time of fracturing [6,71. The absence of concentric fractures around the other two shields is inconclusive, since both Hecates Tholus and Albor Tholus are surrounded by relatively young lava plains originating at or near Elysium Mons [8]. Any early episodes of fracturing would likely have been obscured by these later flows. (2) Narrow linear depressions. Most of these features have a NW-SE trend. Tmority of these features are interpreted as being fractures, and tectonic in origin. (3) Wide depressions. These also have a predominantly NW-SE trend. Although some of these features may be exclusively fluvial or erosional in origin, others, particularly the concentric depressions SE of Elysium Mons, are almost certainly tectonic in origin. If formation of graben was followed by volcanic activity (as was the case in Elysium, where fractures are in places partly covered by younger lava flows), erosional processes could have modified the graben to produce the features observed. (4) Sinuous depressions. These are found throughout Elysium, most notab- ly in the Elysium Fossae region NW of Elysium Mons. Many of these features originate in the wide linear depressions [9], which suggests some tectonic control of the volcanic history. These features closely resemble lunar sinu- ous rilles [9] and are thought to be the result of erosional processes [lo].

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Hall, J.L. et al.

(5) Ridges. There is a system of ridges on the eastern edge of Elysium Planitia, generally concentric to the center of the topographic rise. These ridges, which lie in the terrain between the Elysiuni and Tharsis regions, resemble in appearance both lunar mare ridges and the ridges of the east Tharsis system. Ridges such as these are taken to be evidence of horizontal compression of near-surface material [5,11,12]. Conclusions. The linear features trending NW-SE through the Elysium region indicate a regional stress field with least compressive stress oriented NE-SW. These features may be the result of large-scale lithospheric loading over Elysium Planitia, similar to that which led to radial fracturing in Tharsis [5,13,14]. The failure features around Elysium Mons are most prominent in locations where the local stress field adds constructively to the regional field. Future work will be directed towards determining the relative contribution to the regional stress field of large-scale loading in the Elysiurn and Tharsi s provinces. References: [I]M.H. Carr (1973) JGR, 78, 4049. [2] M.C. Malin (1977) BGSA, 88, 908. [3] W.L. Sjogren (1979)Tie=e, 203, 1006. [4] J.B. Plescia and R.S. Saunders (1979) PLPSC10, 2841. ~s.C.solomonand J.W. Head (1982) -JGR, 87, 9755. [6] S.C. Solomon et al. (1979) NASA TM 80339, 60. [7] R.P. ComerTt a1 . (1979) PLPSC10, 2441. [8] S.H. BrF(n80),npubl ished. [9] M.H. Carr (1981) The Surface of Mars, Yale. [lO] P.J. Mouginis-Mark and S.Y. Rrown (1981) LPS~,729. [nlx.Solomon and J.W. Head (1979) JGR, -84, 1667. [I21 S.=omon and J.W. Head (1980) RGSP, 18, 107. 1131 m. Willemann and D.L. Turcotte (1982) --JGR, 87, 9793. KB. Banerdt et al. (1982) --JGR, 87, 9723. 4 o0

Fig. 1. Physiographic features of the Elysium region. EM = Elysium Mons; AT = ~rTholus;HT = Hecates Tholus; HF = Hephaestus Fossae; EF = Elysium Fossae; CR = Cerberus Rupes. Solid line indicates EM concentric fractures; beaded line = narrow depression; closed line = wide depression; dashed line = sinuous depression; dotted line = edge of volcanic shield; notched line = ridge. Some detail has been lost in reduction. Prepared on base map 1-1321, using USGS photomosai cs and VO photographs.

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