What is the difference between a Zelda clone and a Metroidvania?

At their core, both genres are about an open world in which certain areas are locked off until you get the power or item to unlock them.
 
Is it solely a presentation thing? Is the sole defining factor of a Zelda clone its unrelenting copying of Ocarina of Time?
 
In 2D, one could argue that Zelda clones are top down while Metroidvanias are sidescrolling. Does this make Zelda II: The Adventure of Link a Metroidvania game?
 

Well you could draw parallels but I wouldn't say they are the same. Zelda is an open world with specific dungeons you enter, while a Metroidvania type game has no dungeons per-say, but one big world. Also I've never seen a zelda map grid paper.

...MIND FUCK.
 
I suppose that could be true, but Adventures of Link was also a top-down game. It gets confusing.

Zelda relies heavily on puzzle solving  and dungeon crawling while Metroidvania games are more about exploration. Those similarities are striking, but so are the differences.

I never thought about it before, but in that respect I guess they are similar. Obviously the main difference, as you say, is the way it's presented. So to fall under the specific umbrella of a Metroidvania I'd say it still has to be a side-scrolling affair. 
 
The Zelda titles also have the dungeons that split the games into distinct 'levels', whereas a Metroidvania is more a sprawling map that has less defined areas. A Metroidvania has boss battles you need to complete (usually, but not always, in a specific order) and it could be argued that is what creates its levels, but those games usually have the option to explore in a few directions at once.
 
Also Zelda II has the map bits and more of the wandering into towns, etc. It's an interesting point, but don't think it qualifies.
 
Wow, I typed way more than I probably should have there.

well zelda clones imitate zelda, metroidvania refers to only castlevania games that mimic metroid progress extructure for example shadow complex is not a metroidvania but a metroid clone (a very good one but a clone none the less)

Zelda clones are based around bosses which protect items. 
 
Metroidvania uses a map with a percentage complete thing, and a lot of upgrades and/or weapons. 
 
I think the key thing is that Zelda clones revolve around items, bosses and dungeons while the Metroidvanias revolve around upgrading/leveling up your current items (while you still do get new ones).

The two "genres" have the same basic concepts, but have a different structure. Zelda in a sense has levels, while Metroidvanias take place in a single large world that requires constant backtracking.
 
Plus, there's the obvious difference of 3D and topdown as opposed to side-scrolling.