Divergence in subduction zones and exhumation of high pressure rocks (Eocene Western Alps)

https://doi.org/10.1016/j.epsl.2011.08.002Get rights and content

Abstract

Exhumation of high-pressure rocks has long remained a controversial issue in the Earth sciences. In this article, we analyze the tectono-metamorphic, stratigraphic and plate-motion constraints from the Western Alps region, providing new insights on exhumation mechanisms and tectonic evolution during the earliest orogenic stages. Eocene eclogites of the Western Alps form a 20–25 km wide belt on the upper-plate side of the orogen (Eclogite belt), exposed beneath extensional shear zones at the rear of a lower-pressure accretionary wedge. Units of the Eclogite belt show the youngest peak-pressure assemblages within the subduction zone, and experienced superfast tectonic exhumation since 45–40 Ma. The role of erosion was negligible during the whole of this stage. Eocene foreland basins remained starved, and the massive arrival of axial-belt detritus began well after exhumation was completed. Tectonic reconstructions based on fixed-boundaries exhumation models (e.g. channel flow), and/or implying fast erosion at the surface (e.g. slab breakoff), are thus not consistent with geological evidence. In the lack of erosion, exhumation through the overburden requires divergence within the subduction zone. We demonstrate that this was not attained by rollback of the lower plate (Europe), but was instead attained by NNEward motion of the upper plate (Adria-Africa) away from the Western Alps trench. Such motion induced localized extension within the weak portion of the upper plate, at the rear of the accretionary wedge, and allowed tectonic emplacement of the Eclogite belt in the upper crust at rates much faster than subduction rates. Tectonic exhumation ceased in the Oligocene, when oblique-divergence along the Western Alps traverse changed into oblique-convergence. The onset of slow erosional unroofing was synchronously recorded by pressure–temperature paths in all major tectonic units of the Western Alps, and by arrival of orogenic detritus in sedimentary basins. This work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones. Our exhumation model can be applied to other eclogite belts showing a similar exhumational record, including the Western Gneiss Region, the Dabie-Sulu, and eastern Papua New Guinea.

Highlights

► Upper-plate motion away from the trench triggers exhumation of high pressure rocks. ► Exhumation in the Tertiary Western Alps was much faster than subduction. ► Stratigraphic record provides invaluable constraints on exhumation mechanisms. ► Basins were starved during exhumation of Western Alps eclogites.

Introduction

The exhumation of high pressure (HP) rocks within collision orogens is still controversial and poorly understood. Several different mechanisms have been proposed to explain how HP units travel back to the surface (Platt, 1993). Assuming a stationary trench and fixed boundaries within the subduction zone, the possibility for deep-seated rocks to rise from mantle depth to the surface requires removal of the overlying rock pile by tectonics/erosion or forced circulation in a low-viscosity wedge (e.g. Beaumont et al., 2001, Chemenda et al., 1995, Gerya et al., 2002, Yamato et al., 2008). This latter case is limited by the strength and coherence of the exhumed nappe pile, especially for the case of continental-derived units (Jolivet et al., 2003). Alternatively, in the case of freely moving trenches, boundary divergence within the subduction zone can drive exhumation of rock units from great depths to shallow levels (Brun and Faccenna, 2008, Jolivet et al., 1994, Lister and Forster, 2009). Despite the large amount of petrological and structural evidence, and of sophisticated modeling exercises, a mechanism of general validity to explain the occurrence and distribution of HP belts has not been proposed yet.

The Western Alps are a spectacular example of collisional belt (Frey et al., 1999) where widespread HP assemblages have been described since the beginning of the 20th century (Compagnoni and Maffeo, 1973, Franchi, 1902) and major HP units, both of continental and oceanic origin, have been accurately mapped in the field (e.g. Dal Piaz et al., 2010, Polino et al., 2002). They thus represent an unsurpassed natural laboratory to investigate processes leading to exhumation of HP rocks.

Here, we analyze literature data concerning the tectono-metamorphic and stratigraphic record of HP-rocks exhumation in the classical Western Alps region. Results are discussed within the framework of plate-kinematics and seismic-tomography constraints, and finally compared to the geological record in other HP belts, providing new insights on exhumation mechanisms and tectonic evolution during early orogenic stages.

Section snippets

Tectonic setting

The Western Alps are located at the junction between the Adria-Africa and European plates, which is marked by a ~ 250 km wide deformation zone extending from the Po Plain to the Jura mountains (Fig. 1). Tertiary-age metamorphic units are exposed in the axial part of the belt, between the Insubric Fault and the Frontal Pennine Fault (Polino et al., 1990, Schmid and Kissling, 2000). These units record subduction of attenuated European continental-margin crust, and associated continental-ocean

Metamorphic record

Petrological and geochronological studies have largely enhanced our understanding of orogenic belts (e.g. Carswell et al., 2003, Liou et al., 2009). Extracting segments of pressure–temperature–time paths from rocks is not straightforward. It requires attainment of equilibrium at some point in the rock history, and unambiguous linkage of mineral ages with petrological and structural data (Vance et al., 2003). The low-temperature segment of exhumation paths is generally unconstrained, and

Exhumation without erosion

Exhumation of deep-seated rocks requires either removal of the overburden, or transport of rocks through the overburden (Ring et al., 1999). Starting from this basic consideration, occurrence of eclogitic rocks at the surface has been tentatively explained, since the early 1980s, by a number of exhumation models generally conceived for fixed-boundaries subduction systems (e.g. Beaumont et al., 2001, Cloos, 1982, Gerya et al., 2002). According to these models, uplift of HP rocks may be driven

Divergence in subduction zones

Two independent mechanisms may explain divergence in subduction zones (Fig. 6). They are (i) the retreat of the subduction hinge (i.e., trench rollback), and (ii) the motion of the upper plate away from the trench (Dewey, 1980). Rollback is described in Mediterranean subduction zones associated with the Apennines or the Carpathians (Doglioni et al., 1999, Jolivet et al., 2003), and has been proposed as a possible mechanism for the exhumation of blueschist-facies units during backarc extension

Tertiary evolution of Alpine subduction

A possible evolution of the Western Alps subduction zone, coherent with available geological constraints on HP-rocks exhumation, is here illustrated in three time intervals (i.e., before, during, and after exhumation; Fig. 7). In our reconstruction, the rotation poles of Dewey et al. (1989) were adopted to estimate the relative motion of Eurasia and Adria. We considered coherent motion of Adria with Africa during most of the Cenozoic, as constrained by paleomagnetic data and paleogeographic

General validity of the Western Alps exhumation model

The Western Alps are the orogenic belt where tectono-metamorphic, stratigraphic and plate-motion constraints on HP-rocks exhumation are best preserved and documented. Eocene exhumation by upper-plate motion away from the trench is collectively indicated, in this belt, by the following lines of evidence:

  • eclogite units are exposed on the upper-plate side of the orogen, behind a lower-pressure accretionary wedge;

  • eclogite units are bounded at the top by opposite-dipping extensional shear zones that

Conclusions

An original solution is proposed here to the long controversy concerning mechanisms for exhumation of HP rocks. We considered first that fixed-boundaries models (e.g., channel-flow), can explain unroofing of large and coherent eclogite units only in the presence of superfast erosion, whereas stratigraphic evidence argues against major syn-exhumational erosion in the Western Alps. We considered next that, in the lack of erosion, exhumation of eclogite units requires divergence within the

Acknowledgments

Fruitful discussions with T. Andersen, M. Beltrando, A. Borghi, J.P. Brun, A. Cerrina Feroni, R. Compagnoni, L. Crispini, J. Dewey, C. Doglioni, L. Federico, E. Kissling, B. Lombardo, M. Lustrino, N. Malaspina, G. Molli, G. Ottria, C. Piromallo, C. Rosenberg, F. Rossetti, D. Rubatto, P. Tricart, G. Vignaroli, I. Villa, and constructive comments by M. Konrad-Schmolke and an anonymous reviewer, helped substantially in conceiving and improving this work.

References (111)

  • J. Gattacceca et al.

    Miocene rotation of Sardinia: new paleomagnetic and geochronological constraints and geodynamic implications

    Earth Planet. Sci. Lett.

    (2007)
  • B.R. Hacker et al.

    High-temperature deformation during continental-margin subduction and exhumation: the ultrahigh-pressure Western Gneiss Region of Norway

    Tectonophysics

    (2010)
  • L. Jolivet et al.

    Exhumation of deep crustal metamorphic rocks and crustal extension in arc and back-arc regions

    Lithos

    (1994)
  • S. Li et al.

    Two-stage Triassic exhumation of HP–UHP terranes in the western Dabie orogen of China: constraints from structural geology

    Tectonophysics

    (2010)
  • J.C. Lihou et al.

    Provenance of the Sardona Flysch, eastern Swiss Alps: example of high-resolution heavy mineral analysis applied to an ultrastable assemblage

    Sediment. Geol.

    (1996)
  • J.G. Liou et al.

    Ultrahigh-pressure minerals and metamorphic terranes—the view from China

    J. Asian Earth Sci.

    (2009)
  • G. Lister et al.

    Tectonic mode switches and the nature of orogenesis

    Lithos

    (2009)
  • W.Y. Lus et al.

    Age of the metamorphic sole of the Papuan Ultramafic Belt ophiolite, Papua New Guinea

    Tectonophysics

    (2004)
  • M.G. Malusà et al.

    Late stages of exhumation constrained by structural, fluid inclusion and fission track analyses (Sesia-Lanzo unit, Western European Alps)

    Earth Planet. Sci. Lett.

    (2006)
  • M.G. Malusà et al.

    Detrital geochronology of unroofing magmatic complexes and the slow erosion of Oligocene volcanoes in the Alps

    Earth Planet. Sci. Lett.

    (2011)
  • N.S. Mancktelow

    Tectonic pressure: theoretical concepts and modelled examples

    Lithos

    (2008)
  • G. Muttoni et al.

    Motion of Africa and Adria since the Permian: paleomagnetic and paleoclimatic constraints from northern Libya

    Earth Planet. Sci. Lett.

    (2001)
  • S.M. Reddy et al.

    Kinematic reworking and exhumation within the convergent Alpine Orogen

    Tectonophysics

    (2003)
  • G. Rosenbaum et al.

    The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions

    Earth Sci. Rev.

    (2005)
  • G. Rosenbaum et al.

    Relative motions of Africa, Iberia and Europe during Alpine orogeny

    Tectonophysics

    (2002)
  • D. Rubatto et al.

    Dating of eclogite-facies zircons: the age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)

    Earth Planet. Sci. Lett.

    (1999)
  • L.A. Savostin et al.

    Kinematic evolution of the Tethys Belt from the Atlantic Ocean to the Pamirs since Triassic

    Tectonophysics

    (1986)
  • P. Agard et al.

    Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

    J. Metamorph. Geol.

    (2002)
  • T.B. Andersen et al.

    The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides

    J. Geol. Soc. London

    (1990)
  • J. Babist et al.

    Precollisional, multistage exhumation of subducted continental crust: the Sesia Zone, western Alps

    Tectonics

    (2006)
  • S.L. Baldwin et al.

    Late Miocene coesite–eclogite exhumed in the Woodlark Rift

    Geology

    (2008)
  • M. Ballèvre et al.

    Tertiary ductile normal faulting as a consequence of lithospheric stacking in the Western Alps

    Mém. Soc. Géol. Fr.

    (1990)
  • Barfety, J.C., Lemoine, M., Mercier, D., Polino, R., Nievergelt, P., Bertrand, J., Dumont, T., Amaudric du Chaffaut,...
  • A.C. Barnicoat et al.

    The timing of and nature of greenschist facies deformation and metamorphism in the upper Pennine Alps

    Tectonics

    (1995)
  • C. Beaumont et al.

    Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation

    Nature

    (2001)
  • A. Borghi et al.

    Composite P–T paths in the Internal Penninic massifs of the Western Alps: petrological constraints to their thermomechanical evolution

    Eclogae Geol. Helv.

    (1996)
  • S. Bucher et al.

    Late-stage deformation in a collisional orogen (Western Alps): nappe refolding, back-thrusting or normal faulting?

    Terra Nova

    (2003)
  • F.A. Capitanio et al.

    Mesozoic spreading kinematics: consequences for Cenozoic Central and Western Mediterranean subduction

    Geophys. J. Int.

    (2006)
  • D.A. Carswell et al.

    The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region

    J. Metamorph. Geol.

    (2003)
  • R. Catanzariti et al.

    Le marne dell'Oligocene–Miocene inferiore al limite tra dominio subligure e dominio toscano: dati biostratigrafici ed evoluzione spazio-temporale

    Atti Soc. Tosc. Sci. Nat. Mem.

    (1996)
  • C. Chopin et al.

    Geology and petrology of the coesite-bearing terrain, Dora Maira massif, Western Alps

    Eur. J. Mineral.

    (1991)
  • U. Cibin et al.

    Continental collision history from arenites of episutural basins in the Northern Apennines, Italy

    Geol. Soc. Am. Bull.

    (2001)
  • M. Cloos

    Flow melanges: numerical modelling and geologic constraints on their origin in the Franciscan subduction complex, California

    Bull. Geol. Soc. Am.

    (1982)
  • R. Compagnoni et al.

    Jadeite-bearing metagranites l.s. and related rocks in the Mount Mucrone area (Sesia-Lanzo Zone, Western Italian Alps)

    Schweiz. Mineral. Petrogr. Mitt.

    (1973)
  • Dal Piaz, G.V. (coord.) et alii., 2010. Carta Geologica d'Italia alla scala 1:50.000—foglio 91 Chatillon, Agenzia per...
  • H.L. Davies et al.

    Eclogites of the D'Entrecasteaux Islands

    Contrib. Mineral. Petrol.

    (1992)
  • J.F. Dewey

    Episodicity, sequence and style at convergent plate boundaries

    Geol. Assoc. Can. Spec. Pap.

    (1980)
  • J.F. Dewey et al.

    Changing Silurian–Devonian relative plate motion in the Caledonides: sinistral transpression to sinistral transtension

    J. Geol. Soc. London

    (2003)
  • J.F. Dewey et al.

    Kinematics of the western Mediterranean

  • A. Di Giulio et al.

    Middle Eocene to Early Miocene sedimentary evolution of the western segment of the South Alpine foredeep (Italy)

    Int. J. Earth Sci.

    (2001)
  • Cited by (106)

    View all citing articles on Scopus
    View full text