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The record of impact cratering on the great volcanic shields of the Tharsis region of Mars

Blasius, Karl R. (1976) The record of impact cratering on the great volcanic shields of the Tharsis region of Mars. Icarus, 29 (3). pp. 343-361. ISSN 0019-1035. doi:10.1016/0019-1035(76)90138-x.

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Mariner 9 images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from 100mm to 20 km. Previous attempts to date the volcanoes from their apparent impact crater densities yielded a range of results. The principal difficulty is sorting volcanic from impact craters for diameters ⪆1km. 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. Craters of apparent volcanic origin are 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 10⁶km². 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 Mon's 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 surface, 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 1.2b.y. and 0.07 to 0.2b.y. Areas which are almost certainly younger, less than 0.5 or 0.07b.y., 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.

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Additional Information:© 1976 by Academic Press, Inc. Received September 10, 1975; revised March 2, 1976. E. M. Shoemaker and B. C. Murray of the California Institute of Technology provided substantial guidance to the work reported in this paper. A number of others contributed by aiding in the preparation of high-quality photographic renditions of Mariner 9 images. They are Donald Lynn, James Soha, Jean Lorre, Alan Gilespie, Arnold Schwartz, Reuben Ruiz, and William Green of the Image Processing Laboratory, Jet Propulsion Lab, Pasadena, California. This study was supported by NASA Grant NGR 05-002-302.
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NASANGR 05-002-302
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Caltech Division of Geological and Planetary Sciences2659
Issue or Number:3
Record Number:CaltechAUTHORS:20191028-073247060
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Official Citation:Karl R. Blasius, The record of impact cratering on the great volcanic shields of the Tharsis region of Mars, Icarus, Volume 29, Issue 3, 1976, Pages 343-361, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99477
Deposited By: Tony Diaz
Deposited On:28 Oct 2019 17:10
Last Modified:16 Nov 2021 17:46

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