The Southwest Tilt of

Introduction: One of the most prominent features of Mars is the global south to north slope of ~0.036° [1] discovered by analysis of data from the Mars Orbiter Laser Altimeter (MOLA) aboard Mars Global Surveyor. In contrast, Isidis Planitia, the floor of Isidis basin, slopes southwest at an angle of ~0.02° [2]. It has been suggested [3] that deflection of the lithosphere under a load located in Utopia Planitia (NE and adjacent to Isidis basin) could be the cause of this anomalous tilt. In support of this theory, more quantitative models [4,5,6] suggest that in the Terra Cimmeria region, southeast of Isidis basin, a flexural bulge exists in the highlands adjacent to the dichotomy boundary. This bulge is explained by the downward flexure of the lowlands in response to a line load on the lowland side of the dichotomy boundary that has broken the lithosphere. These models compare the current topography of the northern Terra Cimmeria dichotomy boundary area with the universal flexure curve for a line load with a broken plate [7]. However, the source of the inferred line load along the dichotomy boundary is not clear. Geology: Isidis basin, centered at 87.3° E, 12.5° N, is an ancient multi-ringed impact basin [8,9] straddling the dichotomy boundary. Frey et al., [9] identified two rings: an inner 1100km diameter ring and an outer 1500km diameter ring. Isidis Planitia is entirely within the inner ring. The western section of the rim of Isidis basin appears to be covered either by lava flows from Syrtis Major [10], or cryogenic debris flows [3]. The northeast section of the rim is lower then the rest of the rim, and is continuous with the floor of the basin in profile view (Figure1). Utopia basin is also an ancient impact basin, now buried beneath Utopia Planitia, which was originally identified by McGill [11] based on geological arguments. Its existence has been verified from MOLA altimetry [1]. The apparent center of Utopia basin is at 40° N latitude and 110° E longitude, ~1800km northeast of the rim of Isidis basin. Isidis Planitia is covered with the Isidis Planitia unit, the southwest section of Utopia Planitia is covered by the Vastitas Borealis interior unit and the saddle between Isidis Planitia and Utopia Planitia is covered by the Utopia Planitia 2 unit [12]. Methodology and results: In order for the floor of Isidis basin to be tilted as the …