Syrtis major: A low‐relief volcanic shield

Abstract

Viking orbiter images and earth‐based radar elevation data have been used to document the volcanic geology of Syrtis Major and to reevaluate its physiographic setting. These data indicate that Syrtis Major is actually a very low relief, simple shield with a central caldera complex; arcuate segments of concentric grabens at the shield summit define the limits of a 280‐km‐diameter depression that is thought to have been created by foundering of a magma chamber. Lava flows with well‐defined scarps are associated with two summit calderas; fissures and lava tubes are recognized on the shield flanks. The lava flows were deposited on volcanic ridged plains that do not display flow scarps. Compressional stresses acting on the shield were apparently greatest following the deposition of the ridged‐plains unit but continued at a somewhat reduced rate after deposition of the younger lavas. The extremely low‐relief shield of the Syrtis Major as well as other volcanic plains of intermediate age in the eastern hemisphere of Mars is in striking contrast to the giant shields of Tharsis Montes. Such different types of volcanic landforms may result from changes with times in the composition, differentiation history, eruption rates, and eruption temperatures of surface igneous rocks, thus, directly reflecting the thermal and chemical evolution of the interior. The low relief on the shields may also reflect a substantially thinner crust in these regions relative to Tharsis. The physiographic evidence and radar elevation data confirm the fact that Syrtis Major is a ‘planum’ (plateau) rather than a ‘planitia’ (low plain), as published maps of Mars now portray the feature. In addition, present evidence does not support an earlier suggestion that an impact basin predated volcanism at Syrtis Major.

Minimum estimates of the amount and timing of gases released into the martian atmosphere from volcanic eruptions

“…For example, Olympus Mons, the Tharsis Montes, Apollinaris Patera, and the Elysium and Syrtis Major volcanoes resemble terrestrial shield volcanoes (Carr, 1973;Carr et al, 1977;Arvidson et al, 1980;Greeley and Spudis, 1981;Schaber, 1982;Plescia, 1990;Mouginis-Mark et al, 1992;Robinson et al, 1993). Leveed, channelized flows with lobate fronts also resemble basaltic lava flows.…”

Tectonics of Syrtis Major Planum on Mars

“…The central Syrtis Major Planum is totally covered by low-viscosity lavas (Schaber, 1982) but can be interpreted as being a low-relief volcanic shield associated with a large negative Bouguer anomaly thus being mostly compensated (Scott and McDonald, 1984). Thickness estimations for lavas vary from 0.5 km to 1-2 km with a maximum of 3 km (Schaber, 1982;Wichman and Schultz, 1988).…”

“…Thickness estimations for lavas vary from 0.5 km to 1-2 km with a maximum of 3 km (Schaber, 1982;Wichman and Schultz, 1988). All lavas were drawn from magma chambers below the planum where magma pipes led lavas to extrusion vents.…”

“…In the west, northwest and north the rim is at its most distinct as an arcuate horst. Outside the rim there is an arcuate fault-bounded trough or rille graben, possibly indicating the most recent tectonic activity cutting the surrounding lava planum in the west around Nili Patera (Schaber, 1982). The northeastern rim of the caldera has several rilles on the wall.…”

“…The distinct Syrtis Major Planum area is located on the eastern side of Isidis Planitia (Figure 1) just on and north of the equator between longitudes 280 ° and 300 ° (Meyer and Grolier, 1977;Schaber, 1982). This 1000 × 1000 km 2 in planum is bordered by Martian cratered highlands in the north, west and south.…”