Flexural Response to Sediment Erosion and Unloading at Valles Marineris,

Introduction: The origin and evolution of the Valles Marineris canyon system and its stacks of interior layered deposits (ILD) on Mars remains as one of the most debated topics in Martian tectonics. The subtle broad topographic uplift around the periphery of the canyon system conflicts with the uplifted flanks predicted by models of normal faulting [1]. This uplift is obscured in the east and west by the Thaumasia ridge and Noctis Labyrinthus, respectively, but is clear north and south of the central chasmata. The troughs and their surroundings also have a distinctive signature in the gravity field. The troughs themselves exhibit a pronounced negative gravity anomaly, indicating that the deep trough depressions are not isostatically compensated, but rather are flexurally supported as a negative load on the lithosphere. The trough flanks exhibit positive gravity anomalies. Large-scale extensional tectonic provinces such as rift valleys on Earth form in a state of approximate vertical isostasy [2]. It is suggested that the depths of the troughs was controlled by isostatic balance with the sedimentary infill, with the troughs at one time being filled with sediment in a state of near isostasy [3]. Subsequent erosion of these sediments would have led to regional flexural uplift. Several studies have suggested that the present-day ILD’s are an erosional remnant of a once more extensive set of deposits [4-6]. In this study we show that much of the topography and gravity surrounding the troughs can be explained by flexural uplift due to the removal of these sediments. Methods: We used a thin-shell, elastic lithospheric loading model [7, 8] in order to determine the flexural response of the lithosphere due to the removal of a sediment load. The thickness of the erosional load (the sediments removed from the troughs) was estimated using the difference between the present-day surface topography within the troughs and 4000 m, where 4000 m was chosen as a representative elevation of the trough margins. Using this method we calculated a void space volume of 5.02x10 km, consistent with previous studies [9]. The density of the sediments was assumed to be 2500 kg/m, representative of an equal mixture of basalt and kieserite, with a 10% porosity. The models are sensitive to the elastic thickness of the lithosphere (Te) which is poorly constrained. Previous lithosphere thickness estimates for Valles Marineris range from 60-200 km [10]. As a result, the flexure was modeled using a range of lithosphere thicknesses. Figures 1a-c shows the results for Te = 50 km, 100 km and 150 km. Figure 1. Model Results. (a) MOLA topography. (b) Flexural uplift when Te = 50 km. (c) Te = 100 km. (d) Te = 50 km. All panels have the same north-south and east-west extents.