Recognizing episodic lithospheric thinning along a convergent plate margin: The example of the Early Oligocene Alps
Introduction
The role of extensional tectonism during orogenesis is still a matter of debate, particularly in regard to its scale, duration and significance in the evolution of convergent plate margins (e.g. Ring et al., 1999). Although evidence for extensional deformation has been found in many orogenic belts (e.g. Burg & Chen, 1984, Lister et al., 1984, Froitzheim, 1992, Balanyá et al., 1997, Rawling & Lister, 1999, Collins, 2002, Wells & Hoisch, 2008), it is still unclear whether extension represents a local exception to the dominant shortening regime, related to lithospheric thickening (e.g. Burg et al., 1984; Burchfield & Royden, 1985, Froitzheim et al., 1994, Schmid et al., 1996, Wheeer et al., 2001), or whether it can occasionally represent a transient dominant deformation mode at the scale of the whole orogen, associated with episodic lithospheric thinning (e.g. Collins, 2002, Forster & Lister, 2005, Wells & Hoisch, 2008). In order to address this question, the lithospheric structure of convergent plate margins at the time of extension should be determined. Studies of the metamorphic and magmatic evolution of plate margins can give important insights on the thermal regime and, by inference, on the syn-extensional lithospheric structure. These data sets can be correlated with information from other fields, such as sedimentology, to gain insights on the evolution of peripheral basins and of the topography of the mountain belt, which are also connected to lithospheric evolution (Garzanti et al., 2007). Therefore, the detection of specific “tectonic associations”, meaning the sum of different geological features that formed along convergent margins at specific times, may help address the controversy on the role of extension during orogenesis (e.g. Collins, 2002, Dewey, 2005).
Studies conducted in the last two decades have shown that the activity of major extensional shear zones in orogenic belts may be related to a specific tectonic association that forms within a short time frame (usually < 5 Ma). This comprises the occurrence of (bimodal) magmatism, Barrovian metamorphism, which usually overprints earlier formed high pressure mineral assemblages, and rapid exhumation of gneiss domes and metamorphic core complexes through the activity of regional-scale extensional shear zones. This tectonic association is common in ancient (e.g. Sandiford & Powell, 1986, Handy et al., 1999, Dostal et al., 2006) and modern orogenic belts (e.g. Lister et al., 1984, Jolivet et al., 2003). In the Aegean Sea, for example, its occurrence has been related to lithospheric thinning inferred from seismic images (Makris, 1978). Geological, seismic and heat flux data from the Basin and Range Province indicate that the same processes are taking place in that area (e.g. Parsons, 1995). The detection of lithospheric attenuation in recently- or actively extending terranes is facilitated by direct observations of the lithospheric structure allowed by seismic images. Such techniques, however, cannot be used for areas where later events have obliterated the original lithospheric structure. In these circumstances, the existence of episodes of lithospheric thinning can be inferred only indirectly through the detection of the specific tectonic association compatible with attenuation of the lithosphere.
Barrovian metamorphism and magmatism contemporaneous with the activity of extensional shear zones that exhumed metamorphic units characterize well-defined stages in the evolution of segments of the Alpine–Himalayan orogen, such as the Central and Western Alps in the Early Oligocene (e.g. Laubscher, 1983; Fig. 1). This tectonic association, in the Alps, has generally been interpreted to result from processes radically different from the lithospheric stretching models that have been adopted for the Aegean area or for the Basin and Range Province, where this interpretation is supported by lithospheric imaging. In the Alps, extensional episodes are generally regarded as local exceptions to the dominant shortening regime (e.g. Schmid et al., 1996), and limited in extent to specific parts of the crust (Froitzheim et al., 1994, Wheeer et al., 2001), without affecting the entire lithosphere. Accordingly, currently accepted models interpret the 34–30 Ma evolution of the Central Alps as resulting from the progressive subduction and imbrication of various continental and oceanic fragments beneath Adria, a promontory of Africa, in a continuously shortening setting, culminating in the Adria-Europe collision at the Eocene–Oligocene boundary (cf. Pfiffner, 1992, Becker, 1993, Merle, 1994, von Blanckenburg & Davies, 1995, Schmid et al., 1996, Bousquet et al., 1997, Engi et al., 2001, Pfiffner et al., 2002, Brouwer et al., 2004, Burg & Gerya, 2005).
In this contribution, through a review of the evolution of the Alps in the Late Eocene–Early Oligocene, we argue that extension and contemporaneous nappe exhumation, Barrovian metamorphism and magmatism are concomitant expressions of a lithosphere-scale extensional event. We show that this period of lithospheric thinning at ca. 34–30 Ma was preceded by thickening, responsible for the formation of (ultra-)high pressure mineral assemblages at ca. 38–34 Ma, and followed by renewed thickening. We then explore the different geodynamic scenarios that may account for the proposed transition in tectonic regime from overall lithospheric shortening to extensional thinning and vice versa, along the actively convergent Europe–Adria plate boundary.
Section snippets
Geological setting
The Alps are located along the convergent Europe–Africa plate boundary. Orogeny in the Central and Western Alps is associated with subduction and collisional processes driven by the convergence of Adria, a promontory of Africa, and Europe (Dewey et al., 1989, Rosenbaum et al., 2002). Starting from ca. 90 Ma several continental terranes and ophiolitic sutures were progressively accreted to the orogen (see Rosenbaum and Lister, 2005 for review). During the Oligocene, closure of the Mesozoic basins
The Alps at 38–34 Ma
Studies conducted in the last 15 years have shown that the ages of high pressure metamorphism of the Western and Central Alps appear to clustered into 3 main groups: 70–66 Ma, 54–44 Ma and 38–34 Ma (e.g. Inger et al., 1996, Duchêne et al., 1997, Gebauer et al., 1997, Rubatto et al., 1998, Rubatto & Hermann, 2001, Lapen et al., 2003, Hermann et al., 2006, Lapen et al., 2007, Bachmann et al., 2009, Gabudianu-Radulescu et al., 2009). In particular, younger ages seem to be characteristic of units that
The Alps at 34–30 Ma
The episode of lithospheric thickening that affected the Adria-Europe margin at 38–34 Ma ceased abruptly at ca. 34 Ma. At ca. 34–30 Ma the Alps experienced a short-lived tectonic event that led to the rapid exhumation of (U)HP units (e.g. Gebauer, 1999, Rubatto & Hermann, 2001, Hermann et al., 2006, Gabudianu-Radulescu et al., 2009) contemporaneous with widespread extensional deformation (e.g. Marquer et al., 1996, Avigad et al., 2003, Pleuger et al., 2003) and episodic magmatism (e.g. von
The Alps post-32/30 Ma
In the previous sections we have argued that the Alps, in the 34–30 Ma interval, were affected by a short-lived tectonic episode characterized by the rapid exhumation of rock units, activity of extensional shear zones, bimodal magmatism, local re-heating of rock units, migmatization and topographic collapse of the mountain belt. The formation of this tectonic association was episodic and ceased abruptly at ca. 32–30 Ma (Table 1).
Abundant evidence suggests that at ca. 32–30 Ma the Alpine orogen,
Lithosphere-scale extension in the Central and Western Alps
The structural, metamorphic, magmatic and sedimentological data discussed in the previous sections suggest that the Alps, in the 38–28 Ma time span, experienced three relatively well-defined tectonic phases (Table 1): a first stage, at 38–34 Ma, was characterized by (1) the occurrence of (U)HP metamorphism, (2) evidence of widespread shortening deformation and (3) generation of topographic relief (Fig. 8a); a second stage, bracketed between 34 and 32–30 Ma was characterized by (1) onset of
Lithospheric thinning
The attenuated geotherms that are typical of the Alps in the 35–30 Ma period are best explained by a short–lived episode of lithospheric thinning (Fig. 7). The existence of this episode, which had already been postulated by Laubscher, 1983, von Blanckenburg & Davies, 1995, mainly to account for the episodic magmatic evolution of the Alps, is now critically supported by a wealth of new structural, metamorphic and sedimentological data. Importantly, new geochronological data provide robust
Beyond slab breakoff?
Geodynamic models involving lithospheric thinning along convergent plate margins ascribe such process either to the episodic retreat of the subduction zone hinge (‘slab rollback’) or to the catastrophic loss of the lithospheric roots (‘slab breakoff’). Subduction rollback is generated by the gravitational instability of the subducting slab, which results in retrograde motion of the hinge of the subduction zone relative to the overriding plate (Molnar and Atwater, 1978). The drop in horizontal
Slab rollback in the Western Tethys during the oligocene?
Studies conducted in several orogenic belts have shown that rapid exhumation and constant direction of extension over large areas can be acquired in response to variations in the buoyancy of the material reaching a subduction zone. More specifically, the episodic retreat of a subduction zone hinge, due to the negative buoyancy of subducting oceanic lithosphere, is capable of producing the tectonic association characteristic of the Early Oligocene Alps. Furthermore, cycles of lithospheric
Conclusions
In this paper we present a unique tectonic association that allows the recognition of episodes of lithospheric stretching in orogens that have subsequently undergone shortening deformation. The tectonic association comprises Barrovian metamorphism, usually overprinting high pressure mineral assemblages, bimodal magmatism, exhumation of metamorphic units through the activity of extensional shear zones and topographic collapse of the orogen.
We recognize such a tectonic association in the Late
Acknowledgments
J. Hermann, B. Lombardo, D. Varrone, D. Rubatto, W. Schellart, R. Compagnoni, J. Clulow, A. Aikman and P. Mosca are gratefully thanked for discussions. Detailed reviews by L. Jolivet, U. Ring, W. Collins and C. Basile considerably helped improving the manuscript. Research supported by an Australian Research Council grant.
References (175)
- et al.
Basement-cover relationships in the Tambo nappe (Central Alps, Switzerland): geometry, structure and kinematics
Journal of Structural Geology
(1993) - et al.
On the evolution of orogens: pressure cycles and deformation mode switches
Earth Planet. Sci. Lett.
(2007) - et al.
Deformation mode switches in the Penninic Units of the Western Alps
Journal of Structural Geology
(2008) - et al.
Dating microstructures by the 40Ar–39Ar step-heating technique: deformation–Pressure–Temperature–time history of the Penninic Unit of the Western Alps
Lithos
(2009) - et al.
(Ultra-) High-pressure metamorphism and orogenesis: an Alpine perspective
Gondwana Research
(2010) - et al.
Kinematic, thermal and petrological model of the Central Alps: lepontine metamorphism in the upper crust and eclogitisation of the lower crust
Tectonophysics
(1997) - et al.
Late-orogenic heating during exhumation: Alpine PTt trajectories and thermomechanical models
Earth and Planetary Science Letters
(2004) - et al.
Migration of compression and extension in the Tyrrhenian Sea, insights from 40Ar/39Ar ages on micas along a transect from Corsica to Tuscany
Tectonophysics
(2000) - et al.
Dating the evolution of C–S microstructures: a combined Ar40/Ar39 step-heating and UV laserprobe analysis of the Alpine Roffna shear zone
Chemical Geology
(2003) - et al.
Synmagmatic folding of the base of the Bergell pluton, Central Alps
Tectonophysics
(1996)
Evidence for the granulite–granite connection: penecontemporaneous high-grade metamorphism, granite magmatism and core complex development in the Liscomb Complex, Nova Scotia, Canada
Lithos
The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: a petrological and geochronological study of “silvery micaschists”
Lithos
Fold interference pattern at the top of basement domes and apparent vertical extrusion of HP rocks (Ambin and South Vanoise massifs, Western Alps)
Journal of Structural Geology
The Oligocene Alps: domal unroofing and drainage development during early orogenic growth
Earth and Planetary Science Letters
35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif, Western Alps
Lithos
Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif
Lithos
The relief of the Swiss Alps and adjacent areas and its relation to lithology and structure: topographic analysis from a 250 m DEM
Geomorphology
Burial rates during prograde metamorphism of ultra-high-pressure terrane: an example from Lago di Cignana, Western Alps, Italy
Earth and Planetary Science Letters
Exhumation of the Schistes Lustés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps
Journal of Metamorphic Geology
The inception and early evolution of the North Alpine Foreland Basin, Switzerland
Basin Research
Thrusting and extension in the Southern Dora Maira ultra-high pressure massif (Western Alps): view from below the coesite-bearing unit
Journal of Geology
Abandonment of the South Penninic_Austroalpine palaeosubduction zone, Central Geochronology Alps, and shift from subduction erosion to accretion: constraints from Rb/Sr
Journal of the Geological Society
Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betic, Gibraltar Arc)
Tectonics
Metamorphism and deformation in the Tambo nappe (Swiss Central Alps): evolution of the phengite substitution during Alpine deformation
Schweizerische Mineralogische und Petrographische Mitteilungen
Garnet peridotite and eclogite Sm–Nd mineral ages from the Lepontine dome (Swiss Alps): new evidence for Eocene high-pressure metamorphism in the central Alps
Geology
Le magmatisme périmediterranéen occidental. Essai de synthèse
Bulletin de la Société géologique de France
Was the Valaisan basin floored by oceanic crust? Evidence of Permian magmatism in the Veroyen Unit (Valaisan Domain, Western Alps)
Ofioliti
Correlative chart of the European Oligocene and Miocene- Application to the Swiss Molasse Basin
Eclogae Geologicae Helvetiae
Upper-Cretaceous deep-water sediments near Prella (Southern Alps, Mendrisiotto, Switzerland)
Memorie della Società Geologica
Composite P–T paths in the Internal Penninic massifs of the Western Alps: petrological constraints to their thermo-mechanical evolution
Eclogae Geologicae Helvetiae
The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: new constraints on the evolution of the Alps
Geological Society of America Bulletin
Metamorphism of metasediments in the scale of an orogen: a key to the Tertiary geodynamic evolution of the Alps
Sources of the andesitic components in the Taveyannaz sandstones and Champsaur sandstones: implications for the Paleogene geodynamic evolution of the Alps
Bulletin de la Société géologique de France
Clinoenstatite in Alpe Arami peridotite: additional evidence of very high pressure
Science
Structure and metamorphism of the Gran Paradiso massif, Western Alps, Italy
Contributions to Mineralogy and Petrology
North-south extension within the convergent Himalayan region
Geology
Tectonics and structural zonation of southern Tibet, China
Nature
The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps
Journal of Metamorphic Geology
Low-angle extrusion of high-pressure rocks and the balance between outward and inward displacement of Middle Penninic units in the western Alps
Eclogae Geologicae Helvetiae
Pianura Padana, interpretazione integrata di dati geofisici e geologici
Syn-extension leucogranite deformation during convergence in the Eastern Central Alps: example of the Novate intrusion
Terra Nova
Hot orogens, tectonic switching, and creation of continental crust
Geology
Ultra-high pressure metamorphic rocks in the Western Alps
Alpine and late-Hercynian geochronological constraints in the Argentera Massif (Western Alps)
Eclogae Geologicae Helvetiae
A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps): evidence for fluid flow and constraints on nappe kinematics
Geological Society of America Bulletin
Thermal model of continental lithosphere
Journal of Geophysical Research
Tertiary volcanic Turbidites of taveyanne (Savoye, France): a riddle for Alpine kinematics
Calc-alkaline to ultra potassic post-collisional volcanic activity in the internal North Western Alps
Memorie della Società Geologica Università di Padova
Geological outline of the Alps
Episodes
Cited by (44)
Post-collisional orogen-parallel extension in the Trans-North China Orogen: Evidence from syn-kinematic pegmatite dikes
2022, Precambrian ResearchCitation Excerpt :In contrast, orogen-parallel extension (D2) in the TNCO postdated the syn-collisional shortening (D1) and took place in a post-collisional setting, unlike the tectonic extrusion or escape model. Orogen-parallel extension is common in collisional orogens, but also takes place in post-collisional settings (Davis and Maidens, 2003; Beltrando et al., 2010; Xu et al., 2013; Pascual et al., 2016; Gerbault et al., 2018). The evolution of the TNCO is a good example of post-collisional orogen-parallel extension.