AMOUNTS OF CRUSTAL STRETCHING IN VALLES-MARINERIS, MARS

Terrestrial grabens and continental rifts are compared with the Valles Marineris system, in order to define the mechanisms which could be re sponsible for its geometry and formation. Simple shear/pure shear mech anisms, symmetric/asymmetric grabens and faults, high/low dip angles, block tilting/no block tilting, shouldering or not, lithospheric layer ing, and amount of sedimentation on chasma floors are discussed. Amoun ts of stretching on several transverse topographic profiles are then c alculated, assuming either block tilting or no block tilting. On each profile initial dip angles ranging from 40 degrees to 90 degrees, 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, t he southwestern trough of the Valles Marineris system, is first consid ered, then the entire graben system. Considering a constant 660 km pro file length, stretching increases eastward in Ius Chasma from the Noct is Labyrinthus boundary (stretching factor beta = 1.01-1.02, assuming initial 60 degrees dipping faults) to the middle part of this trough ( beta = 1.04-1.06). Then Ius Chasma stretching decreases, but is partly taken over by stretching in Candor and Ophir chasmata. Stretching dec reases from the eastern part of Melas Chasma (beta = 1.05-1.06) to alm ost the eastern end of Coprates Chasma (beta = 1.01). Then stretching increases again eastward (beta = 1.02-1.04) at the longitude Coprates Chasma splits around a horst and Gangis Chasma opens northward. A stri king feature is that a low peak of extension in the centre of the Vall es Marineris troughs is observed (beta = 1.03-1.04). The low stretchin g peak in the central part of Valles Marineris may originate from the existence of buried structures in the grabens, and/or along-strike var iations in sediment thickness. According to the profiles and to the hy potheses, some 9-26 km of additional normal movements along faults wit h dip angles equivalent to the dip angles of the walls would be expect ed within the central Valles Marineris grabens to get constant stretch ing from Ius to Coprates chasmata. The amount of this lacking offset m ay 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 ho rst 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 th e results as well. Observation of images shows up that the most realis tic 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 emphasize d, although extensional stresses due to the Tharsis load should have a lsb contributed to stretching. Copyright (C) 1996 Elsevier Science Ltd