Sequence and mechanisms of deformation around the Hellas and Isidis Impact Basins on Mars

Abstract

The distribution and age of early volcanic and tectonic features surrounding the Hellas and Isidis impact basins are shown to fit a four‐stage sequence. First, concentric “canyons” form outside the basin boundary scarp at or near the time of basin formation, followed shortly thereafter by radial troughs extending beyond the boundary scarp. After a hiatus, concentric graben develop inside the basin within the massif ring. Finally, soon after graben formation, widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin. Three models of basin‐centered deformation are compared to these observed deformation events: elastic flexure due to loading, elastic flexure due to uplift, and impact fracture. A central load of basin fill can account for concentric fractures by elastic flexure around the load. Formation of the distant concentric canyons, however, requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on Mars and the Moon. Although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values, flexure still fails to account for the large canyon widths observed. Impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse, however, is consistent with the three key observational constraints for the distant canyons: their age, width, and distance from the basin. As the basin then undergoes isostatic uplift soon after impact, elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs. Later, after a long period of basin infilling, elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring. The localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to Mars.

The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains

“…Based on the craterdiameter scaling relationships of Grieve and Cintala (1992), Clifford (1993) has estimated that the formation of a ∼1.5 × 10 3 km diameter impact basin will produce ∼10 7 km 3 of impact melt, the majority of which is expected to have pooled in the basin's interior. In addition, the intense fracturing of the underlying crust is likely to have focused considerable postimpact igneous activity in these locations (Schultz and Glicken 1979, Wichman and Schultz 1989, Tanaka and Leonard 1995. As a result, the lithology of the basin floors, as well as much of the surrounding terrain, may well be dominated by a combination of coherent igneous rock and rewelded breccias.…”

A 3 Ga old polythermal ice sheet in Isidis Planitia, Mars: Dynamics and thermal regime inferred from numerical modeling

“…It formed approximately 4 Ga ago near the end of the Early Noachian (Wilhelms, 1973;Wichman and Schultz, 1989;Schultz and Frey, 1990;Ivanov et al, 2012). Gravimetry measurements suggest that the impact that formed Isidis Planitia may have thinned the underlying crust to as little as 3 km (Zuber et al, 2000;Neumann et al, 2004).…”

Generic identification and classification of morphostructures in the Noachis-Sabaea region, southern highlands of Mars

“…Our interpretation is consistent with that of Melosh (1976), who described grabens generated by impact-related stress relaxation. In addition, Wichman and Schultz (1989) reported that some of these grabens are Hellas impact-induced, near-basin concentric canyons circular to the giant impact basin. These grabens are well defined with flat floors, and their western walls occur at higher elevations than their eastern walls (Figure 2 (a,b)).…”