Challenges in the interpretation of the structural and metamorphic record in the Adula and Cima Lunga units (Central Alps)

The Adula and Cima Lunga units show the best preserved record of the deformation and metamorphic history of the Central Alps. Alpine studies lasting more than a hundred years documented a complex tectono-metamorphic evolution, including the presence of relicts of ultra-high pressure and high temperature metamorphism. Throughout this long history of researches, a few key questions stand out, still challenging the geological community. Major questions regard how to reconcile the structural pattern with the metamorphic path, as well as the timing relationships.

The occurrence of ultrahigh-pressure and/or high-temperature rocks embedded within significantly lower grade metamorphic rocks rises a major challenge for developing a consistent geodynamic model for exhumation of such deep-seated rocks. Subduction zones are, in fact, efficient player driving material from the surface down into the Earth's mantle. However, the mechanisms to exhume part of this material (and particularly the denser oceanic rocks) back to the shallow crust are still highly debated. Scientists generally invoke either mechanical decoupling within a tectonic mélange or variable metamorphic re-equilibration during the retrograde path.  These interpretations are based on the common assumption that the mineral assemblages form under lithostatic pressure and near-equilibrium regional geothermal gradients. Hence, the resulting metamorphic histories based on the estimation of the pressure and temperature conditions represent the major tool for tectonic reconstruction as proxies of the burial and exhumation history of the rocks during subduction-exhumation phases.

Alternative explanations highlight the role of deformation in promoting the coexistence of multiple local equilibria, which cease to correlate with lithostatic conditions and thus burial depths. In this view, the non-hydrostatic stress and the local temperature deviations are accounted as important components potentially modifying the metamorphic system.

In this contribution, we show new structural, petrological and thermochronometric data from the Adula and Cima Lunga units. The wide dataset comprises new field mapping covering the entire nappes extension (several hundred square kilometres) and structural-petrochronological analyses at the meso- to micro-scale. Our results show the highly variable pressure-temperature-time-deformation paths experienced by the compositionally heterogeneous rocks of the Cima Lunga and Adula nappes. We present evidence of contrasting metamorphic records among the rocks of these nappes, providing arguments to discuss pros and cons of the tectonic models proposed to explain these contrasting metamorphic records.