Mars: Troughed terrain

Abstract

An assemblage of huge parallel troughs, individually up to 200 km wide and hundreds of kilometers long, forms a belt 500 km wide extending for 2700 km E 15øS through the Tithonius Lacus-Coprates region of equatorial Mars. Some troughs are closed depressions more than 3 km deep, and the maximum trough depth may approach 6 km. These troughs, heading just southeast of the huge Tharsis volcanic ridge, are transitional westward into a festoon of U-shaped hollows and eastward into chaotic and fretted terrain. Parallel linear chains of rimless pits and shallow graben on the adjacent cratered upland suggest structural control of trough development by fractures in the Martian crust. Trough walls show scarring by slides, slumps, and U-shaped avalanche chutes. Some are dissected into narrow sharp anastomosing ridges reminiscent of badland topography. Dendritic tributaries extend 150 km into the bordering upland. These tributaries and the mass movements causing recession of trough walls are attributed to a sapping process possibly involving the evaporation or melting of exposed ground ice. Trough floors range from chaotically rough to rolling and subdued. They are not smoothly graded. In one instance layered materials, possibly 2 km thick, compose a dissected trough floor tableland. Most troughs may be partly filled by such deposits. The major problem of trough genesis involves disposal of about 2 X 10 ø km a of material. Running water, solution, deflation, and ground ice deterioration all appear to have significant limitations as the sole or principal agent of trough formation. Subsidence caused by magma withdrawal to supply the extensive nearby volcanic fields or the spreading of crustal plates are both quantitatively adequate. Crustal spreading would have significant implications concerning the conditions and the behavior of the Martian interior, and the lack of any obvious subduction zones would imply planetary expansion. In our ignorance concerning ........... magma withdrawal and crustal spreading merit continued consideration in respect to trough formation, with some favor to the former because of the obvious large-scale Martian volcanism. Ground ice may have played a significant subsidiary role through sapping to produce extensive recession of trough walls. One of the spectacular revelations of the Mariner 9 pictures was the assemblage of huge troughs in the Tithonius Lacus-Coprates region of equatorial Mars. These features represent a significant phase in the evolution of the planet, and their interpretation, although necessarily highly speculative, is justified by the potential for indicating something of the nature of sur-ficial and internal conditions and processes of Mars. Although the terms canyons [McCauley et al., 1972] and canyonland have been widely used for these features and this terrain, presumably because of a similarity to the deeply dissected high plateaus of the western United States, these designations do not appear particularly appropriate. Some of the canyons are actually closed depressions (Figure la); others, although joined together, do not compose a normally integrated trunk and tributary system, and their floors are not smoothly graded. The term canyon also implies a genesis by running water [Howell, 1957]. For these reasons, the individual features are herein termed troughs, and the area they inhabit is. called troughed terrain, terms designed to be purely descriptive without genetic implications. This is a brief and preliminary report on these features, which will undoubtedly be a subject of more detailed future treatment.