Morphological analyses of six Martian volcanoes, Ceraunius Tholus, Hecates Tholus, Alba Patera, Hadriaca Patera, Apollinaris Patera, and Tyrrhena Patera, indicate that fluvial processes were the dominant influence in the initiation and subsequent development of many dissecting valleys. Lava processes and possibly volcanic density flows were also important as valley-forming processes. Fluvial valleys are especially well developed on Alba Patera, Ceraunius Tholus, and Hecates Tholus. These valleys are inset into the surrounding landscape. They formed in regions of subdued lava flow morphology, contain tributaries, and tend to widen slightly in the downstream direction. Lava channels on Alba Patera are located on the crest of lava flows and have a discontinuous, irregular surface morphology, and distributary patterns. These channels sometimes narrow toward their termini. Possible volcanic density flow channels are located on the northern flank of Ceraunius Tholus. Valleys dissecting Apollinaris Patera, Hadriaca Patera, and Tyrrhena Patera appear to have a complex evolution, probably a mixed fluvial and lava origin. They are inset into a subdued (possibly mantled) surface, lack tributaries, and either have fairly constant widths or widen slightly downvalley. Valleys surrounding the caldera of Apollinaris appear to have formed by fluvial and possibly by volcanic density flow processes, while those on the Apollinaris fan structure may have a mixed lava and fluvial origin.

Valleys on Tyrrhena have broad flat floors and theater heads, which have been extensively enlarged, probably by sapping. Hadriaca valleys have a trough-shaped form like those on the fan structure on Apollinaris but are more enlarged and have theater heads. Fluvial valleys on the Martian volcanoes geneally followed a two-stage evolutionary sequence similar to that of the Hawaiian volcano valleys, including (1) initial formation by surface runoff and (2) subsequent enlargement by groundwater sapping. Because basalt is highly permeable, valleys on Hawaii Island formed only in regions where lava flows were mangled by less permeable volcanic ash. Similarly, fluvial valleys on the Martian volcanoes, particularly those on Alba Patera, formed in regions where lava flow morphology is subdued, probably by ash mantling. Thus the presence or absence of this mantling may explain why fluvial valleys are present on some Martian volcanoes but not on others of the same age. The formation of fluvial or fluvial-modified valleys on the older volcanoes Ceraunius, Hecates, Tyrrhena, Apollinaris, and Hadriaca implies that surface and near-surface water activity was widespread during late Noachian to early Hesperian (period of heavy bombardment). However, the presence of anomalously young fluvial valleys on Alba Patera indicates that fluvial valley activity did not entirely cease near the end of lateheavy bombardment but rather persisted in isolated regions until mid to late Amazonian. ¿ American Geophysical Union 1990