Syrtis Major and Isidis Basin contact: Morphological and topographic characteristics of Syrtis Major lava flows and material of the Vastitas Borealis Formation

[1] The floor of Isidis Basin is covered by materials of the Vastitas Borealis Formation (VBF) that appear to be emplaced essentially as a single unit. Along its western boundary, Isidis Basin is in contact with volcanic flows from Syrtis Major Planum. The contact between the Isidis unit and volcanic flows from Syrtis Major is sharp to gradational and in places is characterized by a high ( 500 m) scarp or by a network of faults that separate pieces of lava plains off the main plateau of Syrtis. Clusters of knobs and mesas, sometimes arranged in flow-like features, are also typical features of the transition zone. Several important characteristics of the transition from Syrtis Major to Isidis Basin are documented. (1) The small-scale surface texture seen in MOC images appears to be the same for both the Syrtis lava plateau and the knobs and mesas that characterize the transition. (2) There is strong evidence for the breakup of the coherent surface of Syrtis Major where it is in contact with materials in Isidis Basin. (3) The plateau breakup (the knobby terrain) occurs basinward after the major break of slope of Syrtis Major where it enters the Isidis Basin. (4) There is no evidence for plateau breakup anywhere up on the slopes of Syrtis Major Planum. (5) The lavas of Syrtis remain morphologically intact where they are in contact with other units, such as the Noachian cratered terrain or where lava flows are stacked within Syrtis Major itself. These characteristic features of the transition zone from Syrtis to Isidis are readily explained if the zone of plateau breakup consists of relatively young lava flows that have been superimposed onto the surface of a volatile-rich substratum, such as the interior unit of Isidis Basin (the Vastitas Borealis Formation). Thus simple superposition of volcanic materials on top of volatile-bearing sediments can explain the key geological and topographic aspects of the transition zone from Syrtis Major to Isidis Basin. On the basis of our findings, we outline the following scenario for the evolution of this region. In the Early Hesperian, volcanic plains are emplaced in Syrtis Major (the lower part of the Syrtis Major Formation), and wrinkle ridges deform their surfaces soon thereafter. Concurrently, volcanic plains are emplaced on the floor of the Isidis Basin, and wrinkle ridges deform their surfaces soon thereafter. The apparent simultaneity of these units may mean that Syrtis Major was the source of many of the flows in the Isidis Basin. In the early part of the Upper Hesperian, subsequent to the formation of most of the wrinkle ridges, the Vastitas Borealis Formation was emplaced in the Isidis Basin and elsewhere in the northern lowlands. Following the emplacement of the Vastitas Borealis Formation, the upper part of the Syrtis Major Formation was emplaced, erupting from the eastern margins of Syrtis Major Planum and flowing down into the westernmost part of the Isidis Basin on top of the recently emplaced Vastitas Borealis Formation. Modification of the superposed lavas by degradation and evolution of the VBF formed the scarps and unusual morphology of the marginal areas. We found no compelling evidence for massive or sudden erosion from Syrtis Major to produce the plains currently on the surface of the floor of the Isidis Basin (the Vastitas Borealis JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E6, 5063, doi:10.1029/2002JE001994, 2003