Geology of the Isidis Basin, Mars

Introduction: Building on Bridges et al. [2003], we are currently studying the general geologic history and evolution of the Isidis basin based on topographic and imaging data obtained by orbiting spacecraft such as Mars Global Surveyor (MGS) and Mars Odyssey. This study complements our recently completed analyses on Syrtis Major to the west [Hiesinger and Head, 2003] and the transition between Syrtis Major and Isidis [Ivanov and Head, 2002]. The new data allow one to get a close view of the Isidis basin, its structure, stratigraphy, geologic history and its evolution. We are interested in a number of scientific questions, for example, what are the characteristics of the Isidis rim and what caused its present morphology? What is the role and fate of volatiles in the Isidis basin and what are the characteristics of the uppermost surface layer? Does the floor of the Isidis basin primarily consist of volcanic plains as indicated by wrinkle ridges and cone-like features, material deposited by a catastrophic collapse of the rim as proposed by Tanaka et al. [2000], or of sediments deposited in an ocean as suggested by Parker et al. [1989, 1993]? What is the stratigraphy of the deposits within the Isidis basin and what processes were responsible for its present appearance? Results: For our study we made use of the latest MOLA topography data with a spatial resolution of 128 pixel/deg. These data indicate that the basin floor is tilted towards the southwest. Southwestern parts are at about -3880 m elevation; northeastern parts of the floor are at about -3660 m elevation. This results in a slope of ~0.015 degree. A prominent rim of the Isidis basin is absent in several locations such as the passage to the northeast into Utopia/Elysium Planitia (Figure 1). A sharp, well-defined rim is also missing where Syrtis Major lavas flowed into the Isidis basin. However, the rim is well exposed along the southern edge of the basin as well as in the north of Isidis and Syrtis Major. The topography along the rim of Isidis is relatively smooth and only the Libya Montes exhibit a rough topography with high isolated peaks that are separated by deep valleys (Figure 2). Based on gravity and topography data Zuber et al. [2000] concluded that the basin is filled with sediments and/or lavas and that the crust underneath the basin is thin. In the geologic map of Greeley and Guest [1987] the southern rim consists of rough, hilly, fractured material (unit Nplh) of moderately high relief which they interpreted as ancient highland rocks and impact breccias that were formed during the period of heavy bombardment. The units that are exposed at the north rim (Nple, Npl2) are younger than the units of the south rim and are characterized by eolian dissection, collapse of ground ice, minor fluvial activity, and thin lava flows or sediments that partly bury underlying rocks [Greeley and Guest, 1987]. The obvious variations in elevation of the Isidis rim of almost 8000 m (Figure 2) can have several causes such as initial heterogeneities, erosion of parts of the rim, tectonic deformation or burial with Syrtis Major lavas [e.g., Wichmann and Schultz, 1988; Tanaka et al., 2002]. Despite the wide range of possible causes, the absence of the western Isidis rim has been used as an argument for the emplacement of thick (1-2.5 km) Syrtis Major lavas that covered the rim [e.g., Wichmann and Schultz, 1988].