Abstract
Abstract. High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr−1, and the exhumation path involved a cooling of ∼ 20 °C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe.
Highlights
The processes involved in the exhumation of high-pressure (HP) and ultra-high-pressure (UHP) rocks in subduction zones remain a hot topic in tectonics given the complexity of strain and displacement paths that rocks follow, from the surface to great depths and back to the surface (e.g. Gerya et al, 2008)
The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe
We present a detailed structural analysis of the high-grade tectonic sequence at the contact between the eclogitic mafic gneisses and the qz-fsp gneisses, on the eastern side of the Cabo Ortegal nappe (Fig. 2a)
Summary
The processes involved in the exhumation of high-pressure (HP) and ultra-high-pressure (UHP) rocks in subduction zones remain a hot topic in tectonics given the complexity of strain and displacement paths that rocks follow, from the surface to great depths and back to the surface (e.g. Gerya et al, 2008). We present a detailed structural analysis of the high-grade tectonic sequence at the contact between the eclogitic mafic gneisses and the qz-fsp gneisses, on the eastern side of the Cabo Ortegal nappe (Fig. 2a) These gneisses are quite exposed in clean exposures along the shores of the Masanteo peninsula, covering an area of 4.5 km. The integration of published geochronological data, metamorphic evolution and the structural development helps to constrain some characteristics of the process of exhumation of highgrade gneisses from eclogite conditions to greenschist facies In this contribution, we provide robust figures for the bulk exhumation rate and amount of cooling during exhumation of high-pressure rocks in the Cabo Ortegal nappe which may be regarded as representative for a larger lower crustal section
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