Well... GOOD JOB on your first map and welcome aboard!
As for the chasm: Your mountain ridges set a specific view, in this case top down. This means, a chasm could follow the same principle and you just use an area with opposed lightning, making it 'down' instead of 'up'. You could also think about indicating lava in there, if this is the sort of thing you think about including. As for positioning, you should think about tectonic movement, if you want to go for realism. Where are the plates drifting, where would they break. There is a bay in the middle, stretching from there to the North-North-West along the East of the mountains seems a good place to me (and likely the place two plates would touch).

Quality: I'll be honest here and say this - it is very good for a first map, really! But (I guess you aren't doing this professionally or haven't learned Photoshop extensively, etc.) there is room for improvement - as always even among the best here. There is a big tutorial-section here, there you will find A LOT of stuff to help you. Maybe you should take this map over to the Work-In-Progress-Section, I'm sure the community is willing to help you step by step with your questions and with suggestions! =)

Best wishes and keep up the good work,
Steffen