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Lunar and Planetary Science XXXII (2001) 1089.pdf

AMAZONIS PLANITIA: DETECTION OF PRE-AUREOLE FLOWS AND THE ORIGIN OF THE SMOOTHEST ON . J. W. Head1, M. A. Kreslavsky1,2 , S. Pratt1, and E. R. Fuller 1, 1Dept. Geol. Sci., Brown Univ, Providence, RI 02912, USA, 2Astronomical Observatory, 35 Sumska, Kharkov, 61022, Ukraine, [email protected].

Introduction: The northern lowlands of Mars contain Topography and surface roughness: MOLA data pro- some of the smoothest terrain on the planet [1]. Detailed vide high-resolution topography of this region and show a analysis of MOLA altimetry data show that the northern low- general NNE slope from eastern Elysium into Amazonis, a lands are smooth at virtually all scale lengths [2,3]. When sharp demarcation at the edge of the Olympus Mons aureole compared to topography on several bodies in the solar system and , a broad rise toward Alba Patera, and a [4], some of the terrain in the northern lowlands (Amazonis generally flat in Amazonis and southern Arcadia. To a Planitia) 'closely resembles in its smoothness only the heavily first order, geologic units correlate with topography; HNu sedimented surfaces on the Earth, i.e., oceanic abyssal plains forms hummocky terrain in the SW and outliers to the N, rela- and basins filled by fluvial deposition processes' ..... 'volcani- tively high-standing Hr occurs in the SW, and Hf and Aa oc- cally resurfaced terrain is markedly rougher on the Moon, on cur in uplands to the E. Two rectangular rimmed depressions Venus, and on Mars, than the peculiar Amazonis deposits.' In are seen just NW of the Olympus Mons aureole, and a WNW- order to analyze the terrain of the northern lowlands and assess trending subtle positive topographic element is seen emerging its mode of origin, we have used MOLA data to define at the boundary between Acheron Fossae and the northern part subunits [2,3] and detrended MOLA altimetry data [5] to of the Olympus Mons aureole lobe, extending completely characterize these units and their structure. In [3] we showed across the map area between the flanks of Alba Patera and the that the units in Amazonis Planitia were highly distinctive in general NNE-dipping slope of Amazonis. The detrended to- km-scale roughness. In this contribution we use these com- pographic map (Figure 1a) emphasizes the local topography bined data sets to analyze the sequence of events in this region and reveals important details of the -aged units. and their origin. First, the pervasive system of wrinkle ridges in the northern Stratigraphy and geological history: Amazonis Planitia lowlands [5] is readily seen, with those in Hr the most promi-

covers an area in the northern lowlands between and nent. Wrinkle ridges in Aa1 and Aa 2, the oldest of the Arcadia Elysium just of Arcadia Planitia (Figure 1). Geologic Formation subunits, appear modified and flooded by flows,

units that have been mapped there and their interpretation [6] and those in the younger Aa3 and Aa4 appear much more include, from oldest to youngest: 1) Nf, fractured material of flooded and modified. If the wrinkle ridges were formed in Hr Acheron Fossae. 2) HNu, undivided material: Forms hills and and were modified by later events [e.g., as in 5], then the small knobs near the highland-lowland boundary scarp; ero- stratigraphic relationships of [6] for the Arcadia Formation, sional remnants. 3) Hr, ridged plains: West-central and south and the relative modification of the ridges, are both consistent part of area, flow lobes and wrinkle ridges; volcanic origin. 4) with this. The kilometer-scale surface roughness map [3] Hv, the Formation, so prominent shows the plains to be extremely smooth, especially at long throughout the rest of the northern lowlands, is absent in this baselines and reveals two broad subunits in the plains. region. 5) Aa, Arcadia Formation: Forms low-lying plains Origin of units and their stratigraphic relationships: with lobate flow lobes in Arcadia and Amazonis Planitiae; The array of new MOLA-derived data illustrates a number of

mostly volcanic plains. Consists of five members; Aa1, the characteristics and relationships of this region that are impor- oldest, has wrinkle ridges; Aa 2 has many small hills and flows; tant in issues related to its geological history: 1) Lava flows Aa3 and Aa4 have flow fronts and Aa5 is dark with few super- from Elysium Planitia: The detrended data (Figure 1) clearly posed craters; subunits also distinguished locally by strati- show that lobate lava flows extend down and graphic relationships. 6) Ao, Olympus Mons Formation: Con- spread out along the regional slope, flowing around craters;

sists of aureole members Aoa1-4 (contemporaneous with Aa1 this unit corresponds closely with the Ach/Achp and Aa4 and Aa 2, Early Amazonian), and Aos, shield member of com- units. 2) Older Arcadia Formation: The Aa1 and Aa2 occur-

plex flows (contemporaneous with Aa4, Late Amazonian). 7) rence in the NE and the Aa1 occurrence in the S appear to be Ae, eolian deposits (contemporaneous with Aa5 and Ach, modified Hr (wrinkle ridged plains); lava flows from Alba Achp, Late Amazonian). 8) Ach, younger channel, and Achp, contribute to the modification in the NE, while flows from younger flood plain material, deposited in southern Tharsis contribute to the unit in the south. 3) Pre-

and on flood plains (contemporaneous with Aa5, Late aureole Olympus Mons flows: The WNW topographic trend in Amazonian). the northern part of the area is seen in the detrended data to According to this sequence, the geological history is consist of a multitude of flows in a unit extending from the dominated by breakup and deformation of ancient cratered NW part of the aureole across the mapped region, embaying

terrain to form HNu and Nf, emplacement of Hesperian ridged Aa1 and Aa 2. This unit appears to cut across boundaries of Aa3 plains, and the emplacement of a series of Amazonian-aged and Aa 4. On the basis of structure, regional slope, and topog- lava flow units, during the early part of which the Olympus raphy, this unit appears to be composed of lava flows that Mons aureole formed. Among the latest units are channel de- extend radially from Olympus Mons that originated prior to posits on the floor of Marte Valles from Elysium and eolian the formation of the aureole and which were broadly chan- deposits and lava flows at the base of Olympus Mons. In this neled into the low between the regional Alba and Amazonis area and Elysium Planitia to the west, debate has centered on slopes (Figure 1). 4) Elongate lobate flows from southern the relative role of fluvial channels and lakes [7], and volcanic Tharsis: In the south central part of the area, several wide flows [8,9] in forming and modifying the surface. Recent data elongate lobate flows extend down-slope into Amazonis, but [9] confirm earlier work [8] suggesting that lava flows fill lose their identity at about 23N. These appear to be lobate lava older fluvial channels. flows that extend from southern Tharsis and which contributed to the resurfacing of Amazonis. They are mapped as part of Lunar and Planetary Science XXXII (2001) 1089.pdf

AMAZONIS PLANITIA, MARS: J. W. Head et al.

Aa3 [6], but clearly some material is superposed on them (Fig- of sedimentary deposition from outflow channels and lava ure 1). 5) Central Amazonis Planitia: The central part of flows.

Amazonis Planitia is composed predominantly of Aa3 but the Geological history of Amazonis Planitia: On the basis of detrended topography and km-scale roughness maps show these new data, we find that the major events in the history of significant texture and details (see discussion in [3]). The km- Amazonis Planitia are: 1) Hesperian resurfacing by ridged scale roughness map [3] shows that the terrain, as in Elysium plains [8, 3]; if early Hesperian channels existed here [12] Planitia, consists of two basic units, a very smooth unit (black) evidence for them has been covered; 2) emplacement of pre- and a slightly less smooth unit (bluish in the roughness map aureole flows from Olympus Mons (and Alba Patera); 3) for- [3]). The detrended map reveals that very smooth terrain ap- mation of the Olympus Mons aureole; 4) emplacement of pears to embay the aureole and the flows from southern flows from southern Tharsis representing flow around the Tharsis, and to abut against the pre-aureole linear flow deposit aureole; 5) emplacement of the Mid-Late Amazonian very and the rectangular ridged depression. Within this unit are smooth central Amazonis unit largely through Marte Vallis; seen a variety of curving channel-like features that tend to the origin of this unit is interpreted to be due to both outflow follow the regional slope (Figure 1). There appears to be a channel activity from Elysium Planitia through Marte Valles strong correlation of these channels with the mouth of Marte (covering the veneer of pre-aureole flows from Olympus Mons Vallis, and in three cases, flow lobes of the younger Marte and post-aureole lava flows from southern Tharsis), and Valles flows extend a few km into the channels. We thus in- middle Amazonian lava flows from the same region; 6) the terpret this unit (Figure 1) to predate the Marte Vallis volcanic emplacement of the Late Amazonian (perhaps as young as flows. What is the origin of this unit? One possibility is that it several tens of millions of years [10]) lava flows [8, 9] into the represents additional lava flows similar to those from western edge of Amazonis Planitia. We conclude that the ex- Olympus, southern Tharsis, and Marte Vallis [9]. Another is treme smoothness of Amazonis Planitia can be accounted for that it represents outflow channel deposits from channels by the smoothing of the underlying Hesperian ridged plains forming Marte Vallis [7], predating the very youngest lava unit by lava flows from Olympus Mons and further smoothing flows [8, 9]. by sedimentary deposits from outflow channels entering To address this question, we analyzed all released MOC Amazonis from Elysium Planitia through Marte Vallis. The images from this unit in Amazonis Planitia and found three relation of these units to the Amazonian-aged Medusae Fossae terrain types at MOC resolution: 1) lava flow-like fea- Formation to the south is presently under study. tures, rough-textured at the small scale, and similar to those observed in the younger Marte Vallis lava flow deposits [9, References: 1) D. et al., Science, 279, 1686, 1998; 2) M. Kreslavsky and J. Head, JGR, 104, 21911, 1999; 3) M. Kreslavsky 10] but slightly more degraded; 2) very smooth and pitted and J. Head, JGR, 105, 26695, 2000; 4) O. Aharonson et al., GRL 25, deposits, which show abundant, largely rimless pits but show 4413, 1998; 5) J. Head et al., JGR, in review, 2000; J. Head et al., little evidence of lava flow-like features; dunes and eolian LPSC 32, #1063, 2001; 6) D. Scott and K. Tanaka, USGS Map I-1802- features are relatively common; 3) a combination of these in A, 1986; R. and J. Guest, USGS Map I-1802-B, 1987 ; 7) D. which flow-like features overlie very smooth fragmental mate- Scott and M. Chapman, PLPSC 21, 669, 1991; K. Tanaka and D. rial, and vice versa. In eastern Elysium Planitia, detrended data Scott, NASA TM-88383 , 403, 1986; D. Scott et al., USGS Map I-2461 , [11] show evidence for an early stage of fluvial outflow 1995; 8) J. Plescia, Icarus, 88, 465, 1990; 9) L. Keszthelyi et al., JGR, channels and a later stage of lava flows, as also proposed on 105, 15027, 2000; 10) W. Hartmann and D. Berman, JGR, 105, the basis of Viking [8] and MOC data [9], both flowing into 15011, 2000; 11) J. Head and M. Kreslavsky, LPSC 32 #1002, 2000; 12) J. Dohm et al., GRL, 27, 3559, 2000. Amazonis Planitia. We interpret these data to mean that the central Amazonis Planitia unit is formed from a combination

Figure 1. a) Detrended MOLA altimetry map. b) Geologic sketch map of the units and features in the detrended map.