Complex sentence: The orogen consists of a stack of detached nappes that involves mostly the Mesozoic carbonate, which had been folded subsequently into a syncline, underlying a broad anticline of the Caltanissetta area.
Split into simpler sentences: The orogen consists of a stack of detached nappes that involves mostly the Mesozoic carbonate. The stack of detachments had been folded subsequently into a syncline, underlying a broad anticline of the Caltanissetta area.