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.