Amounts of crustal stretching in Valles Marineris, Mars

Abstract

Terrestrial grabens and continental rifts are compared with the Valles Marineris system, in order to define the mechanisms which could be responsible for its geometry and formation. Simple shear/pure shear mechanisms, symmetric/asymmetric grabens and faults, high/low dip angles, block tilting/no block tilting, shouldering or not, lithospheric layering, and amount of sedimentation on chasma floors are discussed. Amounts of stretching on several transverse topographic profiles are then calculated, assuming either block tilting or no block tilting. On each profile initial dip angles ranging from 40 to 90, and sediment thicknesses ranging from 0 to 3 km are considered, as well as symmetric and asymmetric border fault dips. The case of Ius Chasma, the southwestern trough of the Valles Marineris system, is first considered, then the entire graben system. Considering a constant 660 km profile length, stretching increases eastward in Ius Chasma from the Noctis Labyrinthus boundary (stretching factor β = 1.01–1.02, assuming initial 60 dipping faults) to the middle part of this trough (β = 1.04–1.06). Then Ius Chasma stretching decreases, but is partly taken over by stretching in Candor and Ophir chasmata. Stretching decreases from the eastern part of Melas Chasma (β = 1.05–1.06) to almost the eastern end of Coprates Chasma (β = 1.01). Then stretching increases again eastward (β = 1.02–1.04) at the longitude Coprates Chasma splits around a horst and Gangis Chasma opens northward. A striking feature is that a low peak of extension in the centre of the Valles Marineris troughs is observed (β = 1.03–1.04). The low stretching peak in the central part of Valles Marineris may originate from the existence of buried structures in the grabens, and/or along-strike variations in sediment thickness. According to the profiles and to the hypotheses, some 9–26 km of additional normal movements along faults with dip angles equivalent to the dip angles of the walls would be expected within the central Valles Marineris grabens to get constant stretching from Ius to Coprates chasmata. The amount of this lacking offset may be partly explained by a few km subsidence of Ophir Planum, and the shallow inter-chasmata grabens. Part of the remaining lacking normal movements are best explained by buried structures (possibly shallow horst and graben alternances or other structures) in the major canyons: Melas, Candor, and/or Ophir chasmata—probably mainly in Melas Chasma. Alternatively, along-strike variations of sediment thickness (about 3 km) without taking such buried structures into account, can explain the results as well. Observation of images shows up that the most realistic structural model of Valles Marineris should probably consider both variations of sediment thicknesses and existence of buried structures. Models for the origin of Valles Marineris stretching are discussed. The role of passive rifting in crust weakened by hot spot is emphasized, although extensional stresses due to the Tharsis load should have also contributed to stretching.