Summary

Mars is a key intermediate-sized terrestrial planet that has maintained tectonic (and overall geologic) activity throughout its history, and preserved a record in rocks and terrains exposed at the surface. Among the earliest recorded major geologic events was lowering of the northern plains, relative to the southern highlands, possibly by a giant, oblique impact (or endogenic process) that left an elliptical basin with a thinned crust. Sitting on the edge of this global crustal dichotomy is Tharsis, an enormous elevated volcanic and tectonic bulge that rises ~10 km above the datum. It is topped by four giant shield volcanoes, and is surrounded by radial extensional grabens and rifts and concentric compressional wrinkle ridges that together deform the entire western hemisphere and northern plains. Deformation in the eastern hemisphere is more localized in and around large impact basins and volcanic provinces. Extensional structures are dominantly narrow grabens (several kilometers wide) that individually record of order 100 m extension, although larger (100 km wide), deeper rifts are also present. Compressional structures are dominated by wrinkle ridges, interpreted to be folds overlying blind thrust faults that individually record shortening of order 100 m, although larger compressional ridges and lobate scarps (thrust fault scarps) have also been identified. Strike-slip faults are relatively rare and typically form in association with wrinkle ridges or grabens.