Valles - a world of deep canyons, could it work?

Question

I’m developing Valles - a world of deep canyons, and I need some opinion on geology, hydrology and climate.

Valles has earthlike mass and density (for simplicity sake). Planet’s surface is a desert. Thin atmosphere, lack of UV protection, big craters from geologically recent asteroid bombardment, some big mountains and high daily temperature differences. Basically it is a more hospitable Mars.

Humans could survive there with oxygen bottle, water and a lot of sunscreen. However they don’t have to, because there is a lot of very deep and narrow canyons crisscrossing the planet’s surface where water and air concentrate. Those canyons are from a few to hundred kilometres wide, around ten - fifteen kilometres deep.

How do I justify existence of such environment?

Geology

I initially assumed that canyons can be rift valleys, like Valles Marineris on Mars (hence the name). But that would suggest that planet has to have tectonic plates. It’s not bad, tectonic activity is probably necessary for life but it also gives rise to a few questions. Does presence of plate tectonics suggest that Valles has to have basaltic basins created when plates move? They would be an equivalent of Earths ocean floors.

Oceans give me a place for sediments to go, which is good. However oceanic bottoms would be lower than canyons, and so most of air and water would probably move there. A few small seas and green plains are good for my project, but too much would be distracting.

How fast would such deep canyons would be filling with sediments?

If rifts exist on a long time scale that means that they must be created very fast or there is a strong mechanism to transport sediments somewhere else (see: hydrology below). Fast rift creation means active volcanism, which has its advantages. A waterfall of lava (lavafall?) several kilometers long, falling into river canyon and filling it with deadly hot scalding steam looks nice.

Hydrology

Is there a planetary hydrological cycle possible on such planet (with things like: evaporation from lakes and small seas on the bottom of the rift valleys, polar icecaps thawing, big rivers flowing through canyons) or water is stuck forever at the lowest level?

Edited: some questions removed - about climate, commentary bombardment, hydrology left because it is strongly tied to the main question.

The main question I’m looking for an answer to is:

How to justify canyon rich environment? Rift valleys or something else? And if rift valleys does this imply that I must also have ocean basins?

Answer 1

Rift valleys seem reasonable- Valles Marineris, the deepest canyon in our solar system, is one. If this planet has ongoing tectonic activity, then there's likely to be volcanic fissures at the bottom of these canyons- see Iceland for a real-world example of this. Said volcanic/geothermal activity could draw various forms of life, ranging from extremophile bacteria to sapient beings wanting to build a power plant. Once you have these massive canyons, auxiliary gorges branching off of them could form from water draining towards the lowest point in the landscape, creating channels that are more like the Grand Canyon. You could have an Earth-like hydrologic cycle in such an environment, as long as some areas outside the canyons have thick enough atmosphere-coverage for liquid water to exist.