Structural architecture, tectonic stacking and extensional reactivation of nappe contacts in the southern Middle Tuscan Ridge, Inner Northern Apennines orogenic belt (Italy)

Abstract

Interpreting tectonic stacking patterns and the nature of contacts between tectonic units is a major challenge in orogenic belts, particularly when mafic rock slices are embedded within the tectonic stack. These occurrences raise fundamental questions about their origin and the tectonic processes responsible for their emplacement. Such complexities, which characterize several orogenic belts worldwide, are especially prominent in the inner Northern Apennines orogenic belt (Italy). This belt is characterized by abrupt metamorphic discontinuities, local omissions of portions of the tectonic stack, significant stratigraphic gaps within individual tectonic units and contains discontinuous mafic bodies within the nappe pile. These features point to a polyphase tectonic evolution, which over the last decades has led to a range of hypotheses concerning the Neogene evolution of the inner Northern Apennines. Testing these models requires integrated structural, petrological, and stratigraphic investigations. This study presents structural, kinematic and stratigraphic data on detailed field mapping in the southern Middle Tuscan Ridge (southern Tuscany), a key area within the inner Northern Apennines. Our results document a polyphase extensional evolution characterised by the sequential development of normal faults during post-orogenic deformation (∼19–20 Ma). Based on this evidence, we propose an updated tectonic model that integrates the geometry, timing, and evolution of extensional structures in this part of the chain. The model refines the regional tectonic framework and provides a basis for further investigation into the metamorphic evolution, the chronology of tectonic events, and the origin of mafic bodies within continental units. On a broader scale, it offers a reference for interpreting post-collisional extensional settings in other orogenic belts worldwide.