MARTIAN PLATE-TECTONICS

The northern lowlands of Mars may have been produced by plate tectonic s. Preexisting old thick highland crust was subducted, while seafloor spreading produced thin lowland crust during Late Noachian and Early H esperian time. In the preferred reconstruction, a breakup margin exten ded north of Cimmeria Terra between Daedalia Planum and Isidis Planiti a where the highland-lowland transition is relatively simple. South di pping subduction occurred beneath Arabia Terra and east dipping subduc tion beneath Tharsis Montes and Tempe Terra. Lineations associated wit h Gordii Dorsum are attributed to ridge-parallel structures, while Phe legra Montes and Scandia Colles are interpreted as transform-parallel structures or ridge-fault-fault triple junction tracks. Other than for these few features, there is little topographic roughness in the lowl ands. Seafloor spreading, if it occurred, must have been relatively ra pid. Quantitative estimates of spreading rate are obtained by consider ing the physics of seafloor spreading in the lower (approximately 0.4 g) gravity of Mars, the absence of vertical scarps from age difference s across fracture zones, and the smooth axial topography. To the first order, the height of vertical scarps across fracture zones does not i nvolve gravity. Crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity. Thus, the velo city of the rough-smooth transition for axial topography also scales i nversely with gravity. Plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age. Possibl e plate reorganizations, for example, the end of spreading through Alb a Patera, occur when the ridge axis is far from the trench. That is, r apid plate motions are inferred to have placed young oceanic crust far from the ridge axis. The preferred full spreading rate 90-degrees fro m the plate pole is 80 mm yr-1 Plate tectonics, if it occurred, domina ted the thermal and stress history of the planet. A geochemical implic ation is that the lower gravity of Mars allows deeper hydrothermal cir culation through cracks and hence more hydration of oceanic crust so t hat more water is easily subducted than on the Earth. Age and structur al relationships from photogeology as well as median wavelength gravit y anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about Mars plate tecto nics.