I think this is actually a pale ('fulvescens'?) juvenile Greater Spotted Eagle. The extensive spotting on the wing and extremely short tail (I believe I saw the two wingtips projecting well beyond the tail both favour Spotted over Imperial.
Did you see the bird in flight? a pale white 'U' on the rump, with a characteristic shape of broad wings and short tail would be diagnostic of Spotted.
Also for circumstantial evidence - this pale juv. Greater Spotted has been reported hanging around Maipo for a month or so ^^".
