Impact-Basin Control of Channels and Valleys on Mars

IMPACT-BASIN CONTROL OF CHANNELS AND VALLEYS ON MARS. Peter H. Schultz, Lunar and Planetary Institute, Houston, TX., John L. Rogers, American Petrofina, Houston, TX., and R. A. Schul tz, Arizona State University, Tempe,AZ. The number of large (>ZOO km) basins on Mars generally has been regarded as deficient relative to the Moon and Mercury (1,2,3). However, a new survey (4) identifies nearly 20 additional mu1 ti-ring structures ranging in size from 200 km to 1200 km in diameter (outer ring). These relict basins are recog- nized by the concentric arrangement of massifs, scarps, and plains units accentuated by a systematic pattern of differential erosion and reactivation of ring faults. These ancient impact basins provide important. information for the evolution of the early martian crust. First, the effective masking and erasure of impact basins indicate the extensive erosional and depositional processes characterizing early martian history. Second, the reactivation of buried basin structures reveals the long-lasting and deep-seated imprint from early basin-forming impacts. Third, the style and pattern of basin . reactivation provides important clues for the origin of basin ring plans. Fourth, basin distribution and interassociation with regional volcanism (Isidis) and tectonism (Val 1is Marineris) underscore the underestimated role of basins on later geologic evolution of the crust. And fifth, ancient impact basins appear to control the distribution and arrangement of martian channels and valleys. This last observation has implications for the evolution and recycling of martian volatiles and is further considered below. Outflow Channels: Aram (21. "SW, 2. "7N) and Ladon (2g0W, 18"s) basins best illustrate the association of large outflow channels and multi-ring structure. Aram basin exhibits four concentric rings with an outer ring diameter of 550 km. Ares Valles originated in Iani Chaos ( a chaotic terrain along an outer ring of Aram basin) and in Aram Chaos, which forms the central portion of Aram basin. Ares Valles was topographically and structurally controlled by Aram Basin until it breached the outer ring to the north and continued toward Chrysie. Similarly, Hydaspis Chaos is a region of chaotic terrain along the same outer ring coincident with Iani Chaos and is a source region for two outflow channels, the northeast branch following the outer ring plan and joining Ares Valles at the northern breach. Ladon Basin has a major outflow channel that originated along an inner 580 km diameter ring, ponded at the base of the outer975 km ring, and enlarged as it breached this ring and proceeded northeastward. Ladon Val les (28"W, 23"s) originated along the base of an inner 260 km diameter ring of a multi-ring structure that overlaps Ladon Basin. Other outflow channels, many of which are un-named, originated a1 ong the rings of heavily degraded impact basins. Al-qahira Vallis and Ma'adim Vallis began along an outer ring of a 1200 km diameter multi-ring structure and their courses were partly controlled by the ring pattern. Well-known basins such as Isidis, Hellas, and Argyre have associated outflow channels. The concentric arrangement of channel source regions around Chrysie suggest an analogous role for this degraded and buried structure. The location of channel source regions along rings of old impact basins is similar to the source regions of sinuous rilles (lava channe1s)on the Moon. The morphology of these structures, however, are obviously different and suggestive of water erosion (5,6). Consequently, it is suggested that old multi-ring impact basins provided traps for water during the early history of Mars. Renewed periods of regional volcanism/tectonism were concentrated

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Schultz, P. H. et. al. along reactivated ring faults in a manner similar to lunar mare volcanism, thereby resulting in hydrothermal melting of trapped ices. Such a process would have resulted in repeated ponding and catastrophic release of material. Narrow Valleys: Ancient impact basins also exhibit extensive networks of narrow valleys, such as those described in (7). They are most numerous in regions corresponding to basin ejecta facies and around massifs of older basins. Maps showing the occurrence of narrow valley networks and the style of massif mass wasting provide clues for the origin of such valleys. The oldest structures (e.g. , a 1000 km-diameter basin near Copernicus; 166. "SW, 43.O5N) exhibit massifs that have been heavily furrowed and channeled. Many of these massifs have been labeled voTcanoes due to their unique appearance (8). The younger basins (e.g., Ladon and Argyre) exhibit faceted and scalloped massifs with furrowing and valley networks restricted to the outer ring regions where they occur in localized areas, e.g., along scarps. The selective occurrence of valleys around younger basins suggests an origin related to local sapping and run-off. The pervasive distribution of narrow valley networks in the ejecta facies and the heavy furrowing and channeling of basin massifs may require an early period of global warming. SUMMARY: Ancient impact basins are believed to play an important, but largely unappreciated role in the cycle of martian volatiles throughout geologic time. Narrow valley systems suggest an early period of trapping water-ri ch material in basi n-control led topographic 1ows and pemeabl e ejecta facies. Outf 1ow channels may represent episodic re-re1 ease of these volatiles through basin-control led intrusions related to regional volcanic/ tectonic processes.

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