• Abstract and Figures
• Public Full-text

Journal offhe Geological Society, London, Vol. 148, 1991, pp. 1101-1113, 13figs. Printed in Northern Ireland Structural evolution of a subducted continental sliver: the northern Dora Maira massif, Italian Alps J. WHEELER Department of Earth Sciences, Liverpool University, Brownlow Street, Liverpool L69 3BX, UK Abstract: Continental crust of the Dora Maira massif was subducted, in part, to pressures of 30 kbar during early stages of the Alpine orogeny. A new investigation in a transect across the north of the massif has shown that much of the dominant foliation in the upper Dora Maira (Pellice) unit was formed at high pressures of the order of 11 kbar, but at lower pressure than the locally preserved relicts of eclogite-facies metamorphism. This dominant deformation phase produced large recumbent isoclinal folds with intensely flattened limbs in the Pellice unit basement and the overlying Permian- Mesozoic cover (now white augen gneisses and calcschists). The rocks were thus part of the upper crustprior to their subduction, unlessan unrealistically large thickness of Mesozoic rocks (now removed) is hypothesized. The Pellice unit was later emplaced over the Chisone unit in greenschist facies conditions. All shearsense indicators are top-to-west,implying a simple but prolongedhistory in which high-pressure units were emplaced westwards over lower pressure ones whilst being exhumed. The intense horizontal stretching recorded in the basement gneisses may have helped bring them closer to the surface by thinning. Indirect arguments suggest that such tectonic thinning is not, on its own, sufficient to explain the present setting of the eclogite units. One or more sheets of eclogite material mayhave been extruded from the Eoalpine subduction zone in addition. Suchslabs would be bounded below by west-directed, and above by east-directed, shears. As the Dora Maira massif forms the structurally lowest Eoalpine eclogite unit in the Alps, the observed history is not incompatible withthis model. East-or southeast-directed structures in the Alps are commonlyidentified as backthrusts, but an origin for some as the extensional shears predicted by this model should not be excluded. A remarkable feature of some Phanerozoic orogenies is the addressedwithout a knowledge of the geometry andthe burial of continental material to great depths, in excess of pressure-temperature-time history of particular eclogite 50 km and sometimes to 100 km (Chopin 1984), followed by terrains. its rapid returnto the surface. The most likely burial This paper presents a structural interpretation of part of mechanism is subduction (Ernst & Dal Piaz 1978), implying the Dora Maira massif in the Italian Western Alps, as an anomalous conditions in which buoyant continental rocks, element of an ongoing study including metamorphic and ratherthan being incorporatedinto the hangingwall of a geochronological work with the aim of clarifying the uplift subduction zone as they approach it, remain attached to the history of the massif. The massif was chosen primarily downgoing slab.This must imply an unusually strong because of the exceptional burial to 100 km of rocks in its linkage betweensubducting crust and mantle. The southern part (Chopin 1984) during the early, Cretaceous, preservation of eclogite-facies mineral assemblages in such part of the Alpine orogeny (Paquette et al. 1989). Such early rocks, now seen at the surface in the Alps (Compagnoni et eclogite-facies metamorphism, termed ‘Eoalpine’, is of al. 1977, Chopin 1984) andthe Norwegian Caledonides regional significance in the internal Alps and is recorded in (Griffin et al. 1985), is interpreted as a result of rapid uplift both continental and oceanic units (Droop et al. 1990). In precluding both time for retrogression on a decreasing addition, the Dora Maira massif forms part of an orogen for temperaturepath, and time for heating andthermal which much structural and petrological data are available, equilibration with surrounding rocks which would result in andtherefore its regional setting is comparatively well high-pressure granulite facies conditions. In many orogenic constrained. Inthe last part of this paperthe structural terrains erosion is the main process of exhumation, and is evolution of this massifis discussed in relation to overall triggered by several uplift processes including shortening models for the subduction and exhumation of continental and isostatic rebound. However, it is difficult to conceive of rocks. erosion being a fast enough process to uncover such deeply The Dora Maira massif (Vialon 1966, 1967) lies in the buried eclogites before they are re-equilibrated under hotter internal(eastern) parts of the WesternAlpine arc, conditions, and, in addition,the implied volumes of structurally beneath calcschists and ophiolites of the sediment are not apparent(Platt 1986). An alternative Piemonte nappe which are interpreted as oceanic (Fig. 1). process involves unroofing by extension along faults and On its eastern side it is directly onlapped by unmetamor- shear zones. Therefore there are two fundamental questions phosed upper Tertiary sediments of the PO basin. The three concerning these processes: first,what mechanical condi- massifs of Dora Maira, Gran Paradiso (Massonne & Chopin tions prevailed to allow subduction of continental crust, and 1989) and Monte Rosa (Ellis et al. 1989) all contain second, what were the kinematics and mechanics of the eclogite-facies continentalrocks and structurallyunderlie exhumation process? Neither of these issues can be the Piemonte nappe,and are interpreted to havebeen 1101 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/6/1101/4891944/gsjgs.148.6.1101.pdf by guest on 01 October 2021 1102 J. WHEELER BHelminthoid flysch nappe Internal basementmassif Austroalpinenappes mAustroalpine nappes m 0 km 50 I 0.... Brinconnais Zone Fig. 1. Main tectonic units in the Western Alps, showing the location of the Dora Maira massif. subducted beneath these oceanic rocks during the Eoalpine Fig. 2. The three main units within the Dora Maira massif, event.This term is used to describe ‘early’ Alpineevents distinguished on lithological and metamorphic criteria. Inset shows which involved eclogite-facies metamorphismfrom the position of Fig. 4. mid-Cretaceous onwards, of which the structural setting is strongly modified by later, Tertiary, Alpine shortening. In theMonte Rosa massif, this modification involved major pervasive backfolding and backthrusting (SE directed), and similar phenomena have been recorded at the NW margin of Lithological summary the Gran Paradiso massif (Caby et al. 1978; Butler 1987) and TheDora Maira massif consists of a variety of gneisses, within theDora Maira massif (Vialon 1966). Thus,the often showing gradationsbetween different texturesand so-called ‘massifs’ are not single tectonic units, but stacks of compositions, mostly of granitic or pelitic composition with distinct sheets (Ellis et al. 1989). For consistency the term some basic bodies. In this study, informal unit names were ‘massif will continue to be used here for the main outcrop used (Fig. 3) which correspond roughly to those used in the regions of continental basement, butwithout implication previous studies of Vialon (1966) and Borghi et al. (1984) as that those regions are structurally coherent. In the traverse shown in Table 1. The descriptions given refer tothe through the Dora Maira massif presented here, no evidence particular areas studied hereand may notcorrespond for backthrusting was found, and it is possible that the upper exactly tothe rocks types encounteredelsewhere in the margin of this massif was comparatively less affected by late Dora Maira massif. Alpine events. Nevertheless sheets of calcschist resembling a similar lithology in thePiemonte nappe occur within the Dora Maira gneisses and might suggest modification of the Calcschist original Piemonte nappe contact. The E-W traverse across Grey-brown,soft-weathering calcareous micaschists occur the Dora Maira massif mapped in this study was chosen to within the Piemonte nappe and also as sheets within Dora include one of the larger outcrops of such sheets, north of Maira basement rocks. Quartz,carbonate, phengite and Val Pellice (Figs 2 and 4), to allow investigation of their chlorite are the common minerals, and are of limited value origin. Thistraverse willnow bedescribed in detail, fordetermining grade. Phengite defines astrong shape beginning with adescription of the mappable lithologies fabric. Vialon (1966) does not distinguish the calcschist within and around the massif. Only the key metamorphic lithologies within the two settings, and it could be inferred aspects are mentioned:a more detailed study of the that they were all oceanic. However, more than one original metamorphism will be published elsewhere. setting is possible for these rocks, as argued below. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/6/1101/4891944/gsjgs.148.6.1101.pdf by guest on 01 October 2021 STRUCTURALMASSIFEVOLUTION MAIRA DORA OF 1103 Quaternary Ophiolitic rocks Atthe west end of thetraverse, ophiolitic rocks of the Piemontenappe include serpentinites,rodingites, heavily Mesozoic retrogressed eclogitized gabbros and albitic greenstones (‘prasinites’). Typically the metagabbros show partly BCalcschist alteredomphacite blasts surrounded and slightly wrapped Ophiolite rocks by a chlorite fabric. This fabric is pervasively crosscut by epidote, albite and random laths of tremolite, indicating a Marble

Load more

  13

Copy Link