This is interesting, because the Chile/Argentina border is defined as the biggest peaks on the Andes that divide watersheds as per the 1881 treaty [1].
There is still an undefined portion of the border [2], between Mt Fitz Roy and Mt Murallon, because the land between is covered by the Southern Ice Field.
For one, Mapbox shows some kind of (unofficial) border. And lastly, this webapp might have solved a 200+ year issue between neighboring countries :)