Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

Abstract

Abstract. Slab breakoff is the buoyancy-driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low-degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike-slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high-pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55-40 Ma, followed by uplift at 40-35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high-pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

An Early Permian (ca. 280Ma) silicic igneous province in the Alxa Block, NW China: A magmatic flare-up triggered by a mantle-plume?

“…The wide distribution and lack of zonation exhibited by the ca. 280 Ma Alxa Block magmatic rocks also argue against a slab break-off model (e.g., von Blanckenburg and Davis, 1995;Whalen et al, 2006Whalen et al, , 2010. This model predicts a relatively narrow, linear zone of magmatism and uplift that is located along a suture zone.…”

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…When two continents collide, the tensile stresses between the buoyant continental lithosphere and previously subducted oceanic lithosphere lead to the separation of the oceanic slab from the continental lithosphere (e.g., Davies and von Blanckenburg, 1995;van Hunen and Allen, 2011;von Blanckenburg and Davis, 1995). Slab break-off typically occurs in the early stages of continental collisions (von Blanckenburg and Davis, 1995), and can place important constraints on associated collision dynamics (van Hunen and Allen, 2011). Timing and other details of the Neo-Tethyan slab break-off in the India-Asia collision belt have long been an issue of hot debate (e.g., Chung et al, 2005;Ji et al, 2009Ji et al, , 2012Lee et al, 2009Lee et al, , 2012Wen et al, 2008a;Xu et al, 2008;Zhu et al, 2013).…”

“…Second, the upwelling of asthenosphere during slab break-off can trigger the formation of a variety of magmas, especially tholeiitic basaltic magma (e.g., von Blanckenburg and Davis, 1995;Xu et al, 2008), which would contribute to the Cenozoic "flare-up" of mafic to felsic arc magmatic rocks in southern Tibet (e.g., Chung et al, 2005;Lee et al, 2009;Mo et al, 2007). For example, low-K tholeiitic basalts, generated by decompression melting of the asthenosphere, have been identified in the Pana Group (the upper Linzizong Formation, ca.…”

“…Here we use the slab break-off model to account for both the formation of the Napuri adakitic rocks and the associated magmatic "flare-up" event (e.g., Chung et al, 2005;Ji et al, 2009Ji et al, , 2012Lee et al, 2009Lee et al, , 2012Wen et al, 2008a;Zhu et al, 2011). When two continents collide, the tensile stresses between the buoyant continental lithosphere and previously subducted oceanic lithosphere lead to the separation of the oceanic slab from the continental lithosphere (e.g., Davies and von Blanckenburg, 1995;van Hunen and Allen, 2011;von Blanckenburg and Davis, 1995). Slab break-off typically occurs in the early stages of continental collisions (von Blanckenburg and Davis, 1995), and can place important constraints on associated collision dynamics (van Hunen and Allen, 2011).…”

Abrupt tectonics and rapid slab detachment with grain damage

“…Although highly speculative, ingestion of a plateau into the Aleutian trench at 50 Ma may have abruptly disrupted plate forces, causing the sudden change in plate motion associated with the Emperor-Hawaiian bend. Moreover, when thick continental crust is entrained into a subduction zone, the slab can also detach (13), and this is possibly associated with a rapid isostatic uplift of the overriding plate. This process is apparent in the correlation between the tectonic evolution of various margins with tomographic images of slabs (e.g., ref.…”