What term is given to the border between the central alps and the southern limestone alps?

The Periadriatic Seam

Periadriatic Seam.

Explanation:

The Periadriatic Seam (or 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 more commonly used to mean just a western part of it.

because the center-south is called the heart of the country