TOPOGRAPHY OF CLOSED DEPRESSIONS, SCARPS, AND GRABENS IN THE NORTH THARSIS REGION OF MARS - IMPLICATIONS FOR SHALLOW CRUSTAL DISCONTINUITIES AND GRABEN FORMATION

Using Viking Orbiter images, detailed photoclinometric profiles were o btained across 10 irregular depressions, 32 fretted fractures, 49 trou ghs and pits, 124 solitary scarps, and 370 simple grabens in the north Tharsis region of Mars. These data allow inferences to be made on the shallow crustal structure of this region. The frequency modes of meas ured scarp heights correspond with previous general thickness estimate s of the heavily cratered and ridged plains units. The depths of the f lat-floored irregular depressions (55-175 m), fretted fractures (85-89 0 m), and troughs and pits (60-1620 m) are also similar to scarp heigh ts (thicknesses) of the geologic units in which these depressions occu r, which suggests that the depths of these flat-floored features were controlled by erosional base levels created by lithologic contacts. Al though the features have a similar age, both their depths and their ob served local structural control increase in the order listed above, wh ich suggests that the more advanced stages of associated fracturing fa cilitated the development of these depressions by increasing permeabil ity. If a ground-ice zone is a factor in development of these features , as has been suggested, our observation that the depths of these feat ures decrease with increasing latitude suggests that either the thickn ess of the ground-ice zone does not increase poleward or the depths of the depressions were controlled by the top of the ground-ice zone who se depth may decrease with latitude. Deeper discontinuities are inferr ed from fault-intersection depths of 370 simple grabens (assuming 60 d egrees dipping faults that initiate at a mechanical discontinuity) in Tempe Terra and Alba Patera and from the depths of the large, flat-flo ored troughs in Tempe Terra. The frequency distributions of these faul t-intersection and large trough depths show a concentration at 1.0-1.6 km depth, similar to data obtained for Syria, Sinai, and Lunae Plana. The consistency of these depth data over such a large region of weste rn Mars suggests that a discontinuity or a process that transcends loc al and regional geology is responsible for the formation of these feat ures. If this discontinuity is represented by the base of the cryosphe re, its uniform depth over 55 degrees of latitude suggests that the cr yosphere did not thicken poleward. Alternatively, the concentration of depths at 1.0-1.6 km may represent the upper level of noneruptive dik e ascent (lateral dike propagation) of Mars, which is controlled by gr avity and atmospheric pressure and magma and country-rock characterist ics, and was probably controlled, in part, by ground ice. Fault-inters ection depths in the north Tharsis region locally extend down to a dep th of 5-7 km. The depth data between 2 and 3 km are attributed to the discontinuity at the interface of megaregolith and basement or to the upper limit of noneruptive dike ascent of magma with a high volatile c ontent. Intersection depths greater than 3 km, which were found at Alb a Patera, may be due to the megaregolith-basement discontinuity, which was buried and depressed by volcanic loading, or to the upper level o f noneruptive dike ascent of magma with a low volatile content. The ne ar absence of narrow simple grabens with fault-initiation depths less than 0.6-1.0 km in this study area, as well as in most of western Mars , suggests that this depth represents the minimum depth that normal fa ults can initiate; at shallower depths tension cracks or joints would form instead. This hypothesis is supported by the application of the G riffith failure criterion to this minimum depth of normal fault initia tion, which suggests that shallow crustal materials have a tensile str ength of 2-4 MPa throughout most of western Mars, in close agreement w ith previous estimates of tensile strength of martian basaltic rock. ( C) 1995 Academic Press, Inc.