From subduction to collision in the Parautochthon and autochthon of the NW Variscan Iberian Massif

Abstract

A stacking of nappes forms the Parautochthonous Domain located beneath the Allochthonous Complexes of the NW Iberian Massif. Lower Paleozoic metasedimentary and felsic metavolcanic rocks, including some riebeckite gneiss, form the uppermost nappe of the Parautochthon. Micaschists of this nappe may contain albite porphyroblasts with aligned mineral microinclusions defining a relic internal schistosity. Thermobarometric estimates on the relic mineral assemblage 15 suggest an early M1 stage of High-Pressure, Low-Temperature (HP-LT) metamorphism (11-14 kbar; 450-500 ºC), probably related to the same continental subduction process that affected the overlying Lower Allochthon nappes or Basal Units. This uppermost nappe was emplaced onto its relative autochthon by a broad, ductile shear zone that shows top-to-the-E/ENE shear-sense and whose exposure has lateral continuity through to the basal thrust of the Lower Allochthons in the Bragança Complex. The HP-LT metamorphism and its hanging wall position relative to the basal thrust of the Allochthonous 20Complexes, suggest a re-interpretation of this uppermost parautochthonous nappe as another nappe of the Lower Allochthon.In turn, the other parautochthonous nappes comprise both, Lower Paleozoic pre-orogenic rocks, with no M1 relics, and Upper Paleozoic syn-orogenic rocks. Shear-sense criteria related to the current juxtaposition of the Parautochthon Domain onto the Central Iberian autochthon show contrasting top-to-the-S/SE kinematics. The whole orogenic section was affected by an M2 episode of recrystallization under intermediate P/T conditions during the first stages of collision. P-T estimates on 25 kyanite-bearing schists from the upper sections of the autochthon (9.0 kbar; 425-450 ºC) are consistent with a geothermal gradient somewhat lower than the classical Barrovian one at the beginning of collision. Thermobarometric calculations on schists from the lower parautochthonous nappes yield peak metamorphic conditions around 7.5 Kbar and 600-700 ºC, constraining the original thickness of the allochthonous/parautochthonous pile to about 22.5-27.0 km. Schists sampled from deeper sections of the autochthon yield M2 conditions around 11-12 kbar and 700-725 ºC, matching those registered by 30 correlative regions from Central Iberia. Subsequent M3 syn-collisional recrystallization under High-Temperature and Low-Pressure conditions is associated with top-to-the-N/NW and top-to-the-S/SE extensional flow. The early metamorphic 2 Geological setting The NW of the Variscan Iberian Massif is formed by an allochthonous/parautochthonous tectonic pile (Ries and Shackleton, 1971), the so-called Galicia -Trás-os-Montes Zone (GTMZ, Farias et al., 1987; Fig. 1), emplaced onto the autochthonous Central Iberian Zone (CIZ) in Early Carboniferous times (Dallmeyer et al., 1997). The Galicia -Trás-os-Montes Zone in overview 65The GTMZ is divided into the Domain of the Allochthonous Complexes, structurally above, and the parautochthonous Schistose Domain, below. The Allochthonous ComplexesThe All...

Reply to RC #1

“…Another possibility is that top-to-the-S and top-to-the-N could be conjugate movements of our E1 stage, as proposed for the Redondela-Beariz Detachment in Díez Fernández et al (2012b), and for Bande-Celanova Dome in this manuscript. This one is a more likely option, as top-to-the-south (sinistral) shearing in the eastern limb affects the Beariz granite as well (González Cuadra et al, 2006). Topto-the-north kinematic criteria are also found by Gloaguen et al, 2014 (and references therein) to the east of Forcarei, where the authors relate both, stretching lineations (our E1) and upright folds (our C3), with a post-D2 (main thrusting) stage D3.…”

Reply to RC #2

“…The works by Dias da Silva et al in the eastern area of Morais concluded that most of the previously considered Lower Allochthon (e.g. Ribeiro et al, 2006) is currently a part of the parautochthonous section. The lawsonite-bearing rocks (HP) described in that area by Schermerhörn and Kotsch (1984) -and cited in their work-, were then interpreted as belonging to a possible, thin, uppermost unit of the top of the Parautochthon (please see Fig.…”