Ah, I had a similar experience at the same location my first time through
Actually I don't think the BG1 manual mentioned how to disarm traps, which is rather essential information in my opinion. First your thief must have spotted the trap with his trap disarm ability (warning: a thief will stop looking for traps if you have him perform any action other than walking). Now have your thief walk near the trap and use his thieving ability on the red highlighted trap just as you would unlock a locked chest. If all goes well then the trap will be disabled and your party will be free to pass. It is of course a good idea to save the game before hand though, in case something goes awry
Alternately, you could have a worthless NPC that you don't much care for and that is easily replaced (a certain half-elf/half-chicken comes to mind...) join your party for the sake of setting off traps. If he gets killed, no big loss!
-Darkman