Border between the central alps and the southern limestone alps?

Periadriatic Seam

Explanation:

The Periadriatic  fault is a distinct geologic fault in Southern Europe, running S-shaped about 1000 km from the Tyrrhenian Sea through the whole Southern Alps as far as Hungary. It forms the division between the Adriatic plate and the European plate.The term "Insubric line" is sometimes used to address the whole Periadriatic Seam, but it is mostly used to mean just a western part of it.

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