Recognizing episodic lithospheric thinning along a convergent plate margin: The example of the Early Oligocene Alps

Abstract

Large scale tectonic mode switches from lithospheric shortening to lithospheric extension may play a fundamental role in the tectonometamorphic evolution of orogens located along convergent plate margins. Short-lived episodes of lithospheric extension can be inferred from the concomitant occurrence of bimodal magmatism, rapid exhumation of rock units, activity of extensional shear zones, steep geothermal gradients and, possibly, topographic collapse of the orogenic belt. Here we provide a synthesis of existing data from the European Alps, supporting the hypothesis on tectonic mode switches. Data are interpreted to suggest that at ca. 38–34 Ma a period of N–S (orogen perpendicular) shortening led to the formation of medium to ultra-high-pressure mineral assemblages by lithosphere-scale thrust imbrication. A tectonic mode switch took place at ca. 34 Ma, marked by the onset of E–W extension. During this extensional event, internal units of the Western and Central Alps were rapidly exhumed to mid-crustal level in the footwall of regional-scale extensional shear zones. Tectonic thinning was also accomplished through pervasive E–W stretching of the nappe pile. Lithospheric stretching resulted in the onset of an anomalous thermal regime, which was responsible for the overprint of the older medium- to ultra-high pressure mineral assemblages by Barrovian-type mineral assemblages and by migmatization. At the same time, attenuation of the isotherms led to partial melting of mantle lithosphere and to bimodal magmatism. Contemporaneous topographic collapse of the orogen is suggested by the deposition in the foreland of flysch units rich in minerals that were probably derived from erosion of the Early Oligocene calc-alkaline volcanics erupted in the hinterland. This short-lived episode of lithospheric stretching took place in no more than 4 myr, from ca. 34 Ma to ca. 32–30 Ma. Renewed lithospheric thickening, which is observed in seismic tomography studies, resulted in large-scale folding of the extensional structures and the in rapid generation of topographic relief, feeding abundant clastic sedimentation in the peripheral basins.

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.