That's a good question. Given that parts of the Martian surface are 4 billion years old I would say that had Mars ever had plate tectonics we would see evidence of it in the gross landforms and composition. Large scale features like island arcs, mid ocean rifts, subduction zones, continental and oceanic crust, fold belts etc. should be preserved. There is little or no evidence for any of these.<br /><br />Rifts - as Bob points out, we have the Vallis Marineris system. But that is it.<br /><br />Island arcs - Apart from the arcuate mountain range of Thaumasia, there is nothing that looks like an island arc. Structurally Thaumasia is a fold belt, and lacks the andesitic volcanics of an island arc.<br /><br />Fold belts - Just the one,Thaumasia, as mentioned. This lies south of Vallis Marineris and may be where the crustal extension of that rift is taken up by matching crustal shortening.<br /><br />Continental/oceanic crustal dichotomy - there is the hemispheric dichotomy in elevation on Mars, which would match the eleveation dichotomy between oceanic and continental crust on Earth. But the composition is wrong. The Martian highlands are basaltic in composition, not granitic. The very few granites are local differentiates, not wide spread masses as on Earth.<br /><br />Subduction zones - no sign.<br /><br />Mid ocean ridges - no sign.<br /><br />Magnetic striping anomalies - very local and have alternate explanations like parallel dykes emplaced during periods of differing magnetic polarity (we see these on Earth as well).<br /><br />All in all I would say that at most Mars shows signs of having had incipient plate tectonics, but did not develop the full condition.<br /><br />As to why not, who knows? We don't really understand why Earth is the only planet with full blown plate tectnoncs, and there is still debate as to when it actually started here. <br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>