Synextensional Granitoids and Detachment Systems Within Cycladic Metamorphic Core Complexes (Aegean Sea, Greece): Toward a Regional Tectonomagmatic Model

Within deforming continental regions where metamorphic core complexes (MCCs) and synextensional granitoids are closely associated, deciphering the link between detachment faulting and magmatism often remains complex as (1) the rheological weakness of magma may stimulate mechanisms of strain localization, and conversely, (2) tectonic processes may open/close drains where magmas can intrude. Here we tackle this issue by focusing on the Cyclades with the comparison of five granitoid-cored MCCs (Tinos, Mykonos, Ikaria, Naxos, and Serifos) and their flanking detachment systems. In this region, granitoids were emplaced into the middle/upper crust over a relatively short time period (15-9Ma), while metamorphic domes were largely exhumed after more than 10Myr of extension. None of those intrusions thereby proves to be a real candidate for the genesis of MCCs but would rather be a consequence of a warmer regime during lithospheric thinning. However, all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated. Indeed, late evolution stages of MCCs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred, interfering with the sequential development of detachments. During their tectonically controlled emplacement, magmatic products intruded already formed detachments at depth, locally inhibiting their activity, associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities. The newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions, followed by cataclasis along detachments. Plain Language Summary Tectonics intensely deform continents forming mountains when the crust is thickened or low-standing regions where the sea can invade in the case of extension. Extension is less spectacular as deformation is hidden below sea level, but it involves intense deformations that can be studied when the deep crustal levels are exhumed below extensional faults with low-angle detachments. The portions of exhumed crust reveal deep deformation processes associated with emplacement of magmatic rocks because extension is coeval with heat advection toward the surface. One long pending question is whether or not the evolution of these detachments is influenced by growing granites. Based on a detailed field study of the deformation of five such examples in the Cycladic Archipelago, we show that the granites modify the evolution of the low-angle extensional faults leading to the sequential development and upward migration in the crust of a series of detachments through time. Granites indeed influence the evolution of detachments because they heat the crust in their vicinity and thus modify its mechanical resistance and introduce a mechanical heterogeneity during growth and cooling as the magma is intrinsically weak. The flow of magma in these granitic bodies is in turn influenced by tectonic forces. Key Points