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6EOL06IC HAPPING OF THE GRANICUS VALLES AREA, MARS; nary 6. Chapman, U.S. Geological Survey, Flagstaff, Ariz. 86001 Fluvi a1 channel s and vol uminous 1obate and rough-textured flow deposits emanate from the northwest-trending troughs and fractures of Elysium Fossae on the northwest flank of the Elysium rise, Mars. One of the fossae (at about lat 26.3" N., long 223") gave rise to the Granicus Val 1es, rare (because sinuous) channel s whose stream1 ined isl ands and mu1 tip1e stream levels indicate fluvi a1 erosion [1,2]. Geologic mapping at 1:500,000 scale of the Granicus Valles area, west of Elysium Mons (MTM quadrangles 30227 and 30222) indicates that (1) a Lower Amazonian basal scarp around the northwest flank of Elysium Mons may have triggered growth of Elysium Fossae and Granicus Valles; (2) several sources existed for 1ava flows in the map area, including 1inear chains of small volcanoes on the flank of Elysium Mons; (3) rough-textured deposits are the oldest materials released from the fossae, followed by water and lava; and (4) four stages of channel erosion carved Granicus. The photomosaic base of MTM 30222 shows evidence of a long scarp that trends northeast across the entire map area (as well as south of the area; see MTM 25222) and forms a general boundary between the shield of Elysium Mons and the varied materials that fill lower lying Utopia Planitia. The scarp appears as a break in slope where it is locally covered by mounds of rough-textured material interpreted to be either lahars [3,4] or erosional plains [2] and by lava flows (units Ael, [5] and Ael, [6]). This linear feature may be part of a basal scarp (west side down), formed by Early Amazonian faulting, around the northwest flank of Elysium Mons. This origin is suggested because Upper Hesperian lavas of the Elysium shield are cut by the scarp (between lat 28.1" N., long 223.1" and 1at 28.8" N., long 222.4"), but younger Amazonian shield 1avas partly bury it. The scarp is interpreted to be the expression of a hinge fault with inferred displacement of the downthrown block (hanging wall) increasing from zero (south of about lat 25" N.) to a maximum northward along strike in the map area. The displacement probably forms a significant change in slope, as most flow deposits trend northwest after- encountering the scarp, the Grani cus Val 1es are sharply deflected northward (parallel to the scarp) below the feature (between 1at 26.8" N., long 224.1" and lat 28.2" N., long 223.'5"), and a tongue of lava can be observed that broke out of the main flow where it encountered the scarp (at about lat 29.8" N., long 222.1"). The scarp may also be the key to the growth of Elysium Fossae northwest of the volcano, as the mouths of six major theater-headed fossae lie along the scarp as does the head of the Granicus Valles. Faulting along the scarp may be associated with extensional regional stress along a pre-existing zone of weakness, as the scarp 1ies along the trend of a major wrinkle ridge in Lower Hesperian ridged plains about 600 km southwest of the map area (at about lat 16"-19" N., long 230"; [5]). This fault may have breached a perched water source beneath the Elysium shield; re1ease of 1ahars and groundwater sapping along dikes or fissures above the scarp could then have caused the theater-headed fossae to form.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 222 LPSC XSIII

GRANICUS VALLES AREA, MARS; Chapman, Mary G.

Different sources exist for the many lava flows in the map area; flows can be traced to general shield sources, fossae, or 1inear chains of small domes. Shield lavas in the area are rootless flows whose sources (1 ikely buried by volcanics) lie somewhere on Elysium Mons. Fracture flows are lavas that can be traced to two fossae sources (east of MTM 30222), and dome flows can be traced to many small domes in linear chains that parallel the trend of Elysium Fossae. These domes are interpreted to be volcanoes that formed along fissures on the flank of Elysium Mons. Rough-textured flows that head at the fossae appear to have been the initial deposits emitted from these troughs, because the material is cut by channels and embayed by both shield and fossae lavas. However, these deposits may have buried any older materials. The rough-textured material contains unusual linear and sinuous ridges that bear no resemblance to mare-type ridges. If this material is indeed lahars, the linear and sinuous features may be medial ridges consisting of coarse blocks that accumulated in the center of the mudflows. In fact, the ridges and other high-standing parts of the deposits appear to have diverted some lavas, protecting Granicus Valles (to the south) from burial. Four stages of channel erosion can be observed at Granicus Val les. The first three stages consist of meandering and anastomosing channels that head at one of the fossae; third-stage channels dominate, as they are more deeply incised. The fourth stage of fluvial erosion is not related to emissions from the fossae, because it consists of small rectilinear channels cut back from third-stage meandering channels. These youngest channels probably formed by late-stage sapping of water-rich deposits along fractures or joints. The geologic mapping is focused on del ineation of material units and their stratigraphic ages. The maps will assist further research whose goals are to (1) compare estimated original volumes of erosional features with those of surrounding flow deposits to determine genetic re1ations, and (2) study meander reaches of Granicus Valles to provide information concerning bedrock 1i tho1 ogy and gradient. These additional goals wi11 be pursued with the use of photoclinometry and measurements of meander-length sinuosity.

References [l] Malin, M.C. (1976) JGR, 81, 4825-4845. [2] Mouginis-Mark, P.J., Wilson, L., Head, J.W., Brown, S.H., Hall, J.L., and Sullivan, K.D. (1984) Earth, Moon and Planets, 30, 149- 173. [3] Christiansen, E.H., and Greeley, R. (1981) LPSC XII, 138-140. [4] Christiansen, E.H. (1989) Geology, 17, 203-206. [5] Greeley, Ronald, and Guest, J.E. (1987) USGS Map I-1802-B. [6] Tanaka, K.L., Chapman, M.G., and Scott, D.H. (1991) USGSMap 1-2147.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System