Challenges in the interpretation of the metamorphic record in compositionally heterogeneous shear zones - insights from the Central Alps

Maino, Matteo and Schenker, Filippo and Casini, Leonardo and Corvò, Stefania and Perozzo, Michele and Langone, Antonio and Seno, Silvio (2022) Challenges in the interpretation of the metamorphic record in compositionally heterogeneous shear zones - insights from the Central Alps. In: Abstract Book, Congresso SGI 2022, Torino.

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Following the legacy of Emile Argand, several generations of geologists contributed to the reconstruction of the Alpine-Himalayan orogenic evolution. Throughout this long history of researches, a few key questions stand out, still challenging the geological community. One of the most important is the interpretation of contrasting metamorphic records preserved within closely spaced rocks. The reconstruction of metamorphic histories is based on the estimation of pressure and temperature conditions starting from the assumption that the mineral assemblages always reflect lithostatic pressure and near-equilibrium regional geothermal gradients. These PT paths represent a major tool for tectonic reconstruction as proxies of the burial and exhumation history of the rocks during subduction-exhumation phases. 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 back to the shallow crust are still highly debated. Scientists generally invoke either mechanical decoupling within a tectonic mélange or spatially variable metamorphic re-equilibration during the retrograde path. 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 (e.g., Wheeler, 2014; 2020; Moulas et al., 2019; Hess & Ague, 2021). In this contribution, we integrate structural, petrological and thermochronometric data from classic occurrences of ultra-high pressure and high temperature outcrops of the Adula and Cima Lunga nappes of the Central Alps. 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, potentially contradicting the assumption of a purely lithostatic gradient, as well as of a steady-state regional geothermal gradient. New findings provide arguments to discuss pros and cons of the tectonic models proposed to explain these contrasting metamorphic records.

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