Divergence in subduction zones and exhumation of high pressure rocks (Eocene Western Alps)
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:
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eclogite units are exposed on the upper-plate side of the orogen, behind a lower-pressure accretionary wedge;
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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.
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