The Role of Crustal Buoyancy in the Generation and Emplacement of Magmatism During Continental Collision.

Nicholas Schliffke,Valentina Magni,Mark B Allen,Jeroen Van Hunen

doi:10.1029/2019gc008590

Nicholas Schliffke, Valentina Magni + Show 2 more

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https://doi.org/10.1029/2019gc008590

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Abstract

During continental collision, considerable amounts of buoyant continental crust subduct to depth and subsequently exhume. Whether various exhumation paths contribute to contrasting styles of magmatism across modern collision zones is unclear. Here we present 2D thermomechanical models of continental collision combined with petrological databases to investigate the effect of the main contrasting buoyancy forces, in the form of continental crustal buoyancy versus oceanic slab age (i.e., its thickness). We specifically focus on the consequences for crustal exhumation mechanisms and magmatism. Results indicate that it is mainly crustal density that determines the degree of steepening of the subducting continent and separates the models' parameter space into two regimes. In the first regime, high buoyancy values (∆ρ > 500 kg/m3) steepen the slab most rapidly (to 45–58°), leading to opening of a gap in the subduction channel through which the subducted crust exhumes (“subduction channel crustal exhumation”). A shift to a second regime (“underplating”) occurs when the density contrast is reduced by 50 kg/m3. In this scenario, the slab steepens less (to 37–50°), forcing subducted crust to be placed below the overriding plate. Importantly, the magmatism changes in the two cases: Crustal exhumation through the subduction channel is mainly accompanied by a narrow band of mantle melts, while underplating leads to widespread melting of mixed sources. Finally, we suggest that the amount (or density) of subducted continental crust, and the resulting buoyancy forces, could contribute to contrasting collision styles and magmatism in the Alps and Himalayas/Tibet.

Highlights

The Alps and Tibet/Himalayas are two major active continental collision zones that are strikingly different in many respects and can be seen as end‐members in a spectrum: There are strong variations in the extent of deformation, the amount, location and exhumation mechanisms of subducted continental crust, and the distribution, source, and degree of magmatism
By varying the crustal density as well as the oceanic slab age, we find three main types of behavior that differ in collision dynamics, final emplacement of subducted continental crust, and their magmatism
Our models address the influence of buoyancy forces on continental collision dynamics, with a specific focus on the continental subduction angle

Summary

Introduction

The Alps and Tibet/Himalayas are two major active continental collision zones that are strikingly different in many respects and can be seen as end‐members in a spectrum: There are strong variations in the extent of deformation, the amount, location and exhumation mechanisms of subducted continental crust, and the distribution, source, and degree of magmatism. Capitanio and Replumaz (2013) showed how the density contrast between subducting oceanic and continental domains during collision can control ongoing subduction, and partial or complete slab breakoff. Contrast in buoyancy forces from the continent and previously subducting oceanic slab lead to an alignment (i.e., continuous steepening) of the partially subducted continent (e.g., Cloos, 1993; Duretz et al, 2012; van Hunen & Allen, 2011). To understand the diversity in processes during continental collision, the slab steepening process, we focus on the effects of buoyancy on collisional dynamics, exhumation and location of subducting crust, and resulting magmatism

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