Reading the marbles: Calcite decodes strain distribution during exhumation of high-pressure metamorphic rocks (Ios Island, Cyclades)

Abstract

We present new calcite microstructural, petrofabric, and grain size data from high-pressure marbles to investigate spatio-temporal variations of deformation within the exhumed Cycladic Blueschist Unit (CBU) on Ios Island. Early exhumation of the CBU, from eclogite-to blueschist-facies conditions, was accompanied by the development of the mesoscopic foliation and isoclinal folds. Our results indicate static recrystallization during blueschist-facies conditions, evidenced by coarse columnar calcite grains oriented at high angles to the pre-existing foliation. Continued exhumation involved top-to-the-SSE shearing, localized at the base of the unit, as indicated by the dynamic recrystallization of columnar calcite via subgrain rotation. A subsequent reversal to top-to-the-NNW shearing caused intense deformation across the CBU from blueschist-to greenschist-facies conditions, producing a wide range of recrystallized grain sizes (∼500–∼20 μm). During the final stages of NNW-directed shearing, deformation localized at the top of the CBU within foliation-parallel bands, with differential stresses from 26 to >64 MPa, forming fine-grained (∼30 μm), nearly uniform recrystallized calcite through subgrain rotation and bulging recrystallization. These findings underscore the previously underestimated significance of foliation-parallel, NNW-directed shearing during ductile exhumation of the CBU on Ios Island and highlight the critical role of high-pressure calcite marbles in unraveling these tectonic processes.