Topical Studies of the Geology of the Tharsis Region of Mars

Citation

Blasius, Karl Richard (1976) Topical Studies of the Geology of the Tharsis Region of Mars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1AMY-CD94. https://resolver.caltech.edu/CaltechTHESIS:10252019-135222249

Abstract

I.

Mariner IX images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from one hundred meters to twenty kilometers. Previous attempts to date the volcanoes from their apparent impact crater densities yielded conflicting results. The principal difficulty is sorting volcanic from impact craters for diameters <lkm. Many of the observed craters are aligned in prominent linear and concentric patterns suggestive of volcanic origin. In this paper an attempt is made to date areas of shield surface, covered with high resolution images using only scattered small (≤ 1 km) craters of probable impact origin. In some cases a fraction of the visible craters, those of apparent volcanic origin, is systematically excluded from the dating counts.

The common measure of age, deduced for all surfaces studied, is a calculated "crater age", F', defined as the number of craters equal to or larger than 1 km in diameter per 10⁶ km². The conclusions reached from comparing surface ages and their geological settings are:
1. Lava flow terrain surfaces with ages, F', from 180 to 490 are seen on the four great volcanoes. Summit surfaces of similar ages, F'=360 to 420, occur on the rims of calderas of Arsia Mons, Pavonis Mons, and Olympus Mons. The summit of Ascraeus Mons is possibly younger; F' is calculated to be 180 for the single area which could be dated.
2. One considerably younger surface, F'≤110, is seen on the floor of Arsia Mons' summit caldera.
3. Nearly crater free lava flow terrain surfaces seen on Olympus Mons are estimated to be less than half the age of a summit surface. The summit caldera floor is similarly young.
4. The pattern of surface ages on the volcanoes suggests that their eruption patterns are similar to those of Hawaiian basaltic shields. The youngest surfaces seem concentrated on the mid-to-lower flanks and within the summit calderas.
5. The presently imaged sample of shield surfaces, though incomplete, clearly shows a broad range of ages on three volcanoes—Olympus, Arsia, and Pavonis Mons.

Estimated absolute ages of impact dated surfaces are obtained from two previously published estimates of the history of flux of impacting bodies on Mars. The estimated ranges of age for the observed crater populations are 0.5 to 1.2 billion years and 0.07 to 0.2 billion years. Areas which are almost certainly younger, less than 0.5 or 0.07 billion years, are also seen. The spans of surface age derived for the great shields are minimum estimates of their active lifetimes, apparently very long compared to those of terrestrial volcanoes.

II.

Three types of large-scale mass movements - rockslides, slumps, and a type of mass flowage have been tentatively identified in photographs of the basal escarpment of the Martian volcano Olympus Mons. The morphology of slide deposits suggests lubrication of their movements by cushions of compressed gas. Gas trapped from the present rarefied atmosphere is probably insufficient, but a denser atmosphere in the past or the release of gases adsorbed on surface materials may provide adequate slide lubrication. Slumps occur widely along the north and southeast reaches of basal scarp and are mantled in the southeast by some of the youngest lava flows observed on Olympus Mons. The initiation of mass flowages, confined to two areas along.the west reach of basal scarp, probably required a major change in the physical properties of surface materials. Chemical alteration of surface materials or the melting of ground ice, both possibly related to volcanism, might effect such a change.

Differences in escarpment morphology and mass movements suggest considerable recession of the west and north reaches of basal scarp and only slight recession, primarily by slumping, of the southeast scarp. Low scarps on the plains near the foot of the southeast basal scarp and the apparent tilt of grooved terrain surfaces suggest the southeast reach of basal scarp formed by subsidence or downwarping along the margin of the volcano.

III.

Many overlapping pictures, potential data for the construction of topographic maps, were obtained by the television cameras on Mariner 9. An analysis of the sources of error in photogrammetric determinations of relief from these pictures singles out photo resolution as the primary limiting factor. Topographic maps of several Martian surface features, derived by an original analytic scheme, are presented. The observed errors in relief determinations using this technique are in good agreement with the independent error analysis.

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