Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p31a12h&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P31A-12
Other
5464 Remote Sensing, 5480 Volcanism (8450)
Scientific paper
The circular shape of Syrtis Major has been interpreted to be impact related [1] but the topography of the structure is more consistent with a low shield volcano [2, 3]. The low relief of Syrtis Major is very different from other Martian shield volcanos (e.g., Tharsis) and may be related to changes in composition, differentiation history, eruptive styles or differences in crustal thickness [2]. Individual lava flows of Syrtis Major are among the thinnest on Mars (25-30 m; [3]), are up to 120-150 km long [2], and ISM data suggest a SNC-like [4] composition. Slopes of northern Syrtis Major are ~0.13°, to the south slopes are ~0.02°, and to the west are on the order of 0.4°. We observe steeper slopes of ~0.5 to the east. According to Schaber [2] and Hodges and Moore [5] Syrtis Major is ~1100 km in diameter and has an estimated maximum height of only 0.5 km. E-W profiles based on MOLA data show that the height of the shield is ~0.5 km, consistent with previous estimates. However, MOLA N-S profiles indicate a significantly higher edifice of ~1 km. Nili Patera (caldera C1 of [2]) and Meroe Patera (caldera C2 of [2]) are located within a complex large N-S elongated depression [2, 6] and their floors are at an elevation of ~100-200 m and 180-250 m, respectively. The caldera floors are at about the same elevation as the cratered highlands immediately north of Syrtis Major and at significantly lower elevation than the cratered highlands west and south of Syrtis Major. The highest point of the shield is NW of Nili Patera at about 2300 m. The terrain southwest, west and north of the calderas is noticeably higher ( ~2000-2300 m) than east of the calderas ( ~1500-1700 m), hence forming a crescent-like summit with steeper slopes into the summit depression and gentler slopes away from the summit. A MOLA map of kilometer-scale surface roughness shows that the Syrtis Major Formation is rougher at all wavelengths (0.6-19.2 km) compared to other investigated Martian volcanic units (e.g., Hr, At4, At5, Aop) [7]. Syrtis Major can be clearly distinguished from the surrounding cratered highlands by its smoother surface. Isidis Planitia is significantly smoother than Syrtis Major and this might be related to sedimentation within the impact basin [8, 9]. A map derived from MGS data does not show significant differences in crustal thickness underneath the Syrtis Major volcanic complex compared to adjacent cratered highland plains [10]. Cratered highlands and Syrtis Major both have a crustal thickness on the order of ~45-60 km. However, the crust is significantly thinner compared to the Tharsis area (>60 km). The new data also indicate that there is no evidence for a gravity anomaly associated with Syrtis Major, hence supporting an origin independent of an impact [2]. [1] Meyer and Grolier, 1977, USGS I-995; [2] Schaber, 1982, J. Geophys. Res. 87, 9852-9866; [3] Head et al., 1998, LPSC 29, 1322; [4] Mustard et al., 1997, J. Geophys. Res. 102, 25605-25615; [5] Hodges and Moore, 1994, USGS Prof. Paper 1534, 152-153; [6] Crumpler et al., 1996, in: Volcano Instability on the Earth and other planets, 307-348; [7] Kreslavsky and Head, 2000, J. Geophys. Res. 105, 26695-26711; [8] Head and Bridges, 2001, LPSC XXXII, 1236; [9] Grizzaffi and Schultz, 1989, Icarus 77, 358-381; [10] Zuber et al., 2000, Science 287, 1788-1793.
Head James W.
Hiesinger Harald
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