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How did Valles Marineris form?

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Mars
Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

The formation of Valles Marineris on Mars is primarily attributed to tectonic processes, specifically crustal faulting, rather than the erosional forces that carved features like the Grand Canyon.1

The system originated due to stresses and deformation within the Martian crust caused by the Tharsis rise, an enormous volcanic bulge located to the northwest. As the crust fractured, faults radiated outward from this volcanic center, creating the deep, steep-walled canyons that define the system.23

While faulting was the primary mechanism, the canyons were subsequently enlarged by significant erosional processes. Evidence of this includes deep gullies cut into the canyon walls. Furthermore, the presence of thick, sulfate-rich sedimentary sequences suggests that the canyons may have once held lakes, which potentially drained in catastrophic floods toward the east.12