I would really like to find another pair of trousers like these but I don't know what this fabric is called. It is heavy cotton, and there is a slanted pattern to it. In the picture, I've put a pencil to give a sense of scale.
It's a woven rib twill (diagonal pattern) like denim except that in denim the twill pattern is tighter and smaller in scale and the blue yarns in the rib cover the white yarns. In this fabric the brown and the tan yarns both show.
Chinos (khakis) are also a woven twill, although tighter and not as bold as this twill, and both yarns are usually the same pale tan color.
Fun fact: the reason jeans look whiter over time at high-wear points like knees and hems and the edges of pockets is that as the blue yarns are rubbed away with wear and tear, the white yarns underneath begin to show through.
Probably the best way to describe this fabric when looking for another pair is that it's a heavy twill in which there are two colors in the diagonal. Best yet is to take these with you if you shop in physical stores.
Good luck, this is a nice fabric.