[alnitak]

There are no crustal plates floating on a magma sea on Mars, so volcanoes stay over their hotspots and grow to colossal sizes (Olympus Mons is about the size of France).

You don't need tectonics to have vulcanism. Vulcanism can be thought of as a gradient heat-flow process, whereby heat left over from the planet's formation in the interior is "flowing downhill" to the cold of outer space. On a planet like earth that results in convection currents in the magma which push the continental plates around, and the rising plumes create "hotspots" like Hawaii and Yellowstone. Mars has such hotspots, but no tectonics that we know of.

You can have faults too, if the crust is pushed up enough it will split and rupture, like Vallis Marineris.

I am not a geologist, this is just a layman's understanding.