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

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.

Curved orogenic belts, back-arc basins, and obduction as consequences of collision at irregular continental margins

“…Many numerical models of continental collision have focused on linear passive margins (Schliffke et al, 2019;van Hunen and Allen, 2011) or oblique collision (Bottrill et al, 2014) to study slab break-off and exhumation of subducted continental crust. Compressional stresses and resulting topography during continental collision are controlled by plate coupling (Faccenda et al, 2009), rheological flow laws (Pusok et al, 2018), and buoyancy ratios and convergence velocities (Pusok and Kaus, 2015).…”

Numerical modeling of subduction: State of the art and future directions

“…This mechanism of continental-crust transformation is called relamination (Hacker et al, 2011), and the "relaminant" is formed generally by buoyant felsic metasedimentary or metaigneous rocks. Recently, relamination gained growing attention due to its potential significance for the origin and composition of modern and ancient continental convergence systems (Hacker et al, 2011;Maierová et al, 2018;Schliffke et al, 2019). Maierová et al (2018) modeled relamination during continental collision and distinguished three main types of evolution of the subducted continental crust: (1) return along the plate interface in a subduction channel or wedge;…”

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…Traditionally three metamorphic series with different P/T ratios have been identified from petro‐structural analysis of orogenic belts, and they are usually associated with different stages of evolution of a convergent margin (e.g., Ernst, 1976, 1977; Kornprobst, 2002; Spear, 1993; Yardley & Warren, 2020): Metamorphic series with high P/T ratios, deduced by regional scale distribution of dominant metamorphic imprints (e.g., Ernst, 1973; Miyashiro, 1961), are generally associated with Pressure‐Temperature (P‐T) conditions interpreted as peculiar of subduction and are referred to as Franciscan (or Sanbagawa) metamorphic sequences. Barrovian (or Dalradian) metamorphic series, which are characterized by intermediate P/T ratios, are traditionally interpreted as the effect of crustal thickening during continental collision both by mountain belt tectono‐metamorphic analyzes and by thermo‐mechanical modeling predictions (e.g., Barrow, 1912; Bohlen, 1987; England & Richardson, 1977; England & Thompson, 1984; Jamieson et al., 1998; Sandiford & Powell, 1991; Thompson, 1981; Thompson & England, 1984). Metamorphic facies series characterized by low P/T ratios (Abukuma or Buchan‐type metamorphism) have been generally associated with abnormally high geothermal gradients such as those of the island arc or ridge settings (Cloos, 1993; Fagan et al., 2001; Favier et al., 2019; Mevel et al., 1978; Oxburgh & Turcotte, 1970; Verati et al., 2018), slab rollback or increase of the slab dip after continental collision (Ji et al., 2019; Li et al., 2013; Menant et al., 2016; Schliffke et al., 2019; Sizova et al., 2019), post‐collisional extension (Carmignani & Kligfield, 1990; Vanderhaeghe, 2012), melt migration through the crust (Depine et al., 2008), local effects of contact metamorphism (Rothstein & Manning, 2003). …”

“…Metamorphic facies series characterized by low P/T ratios (Abukuma or Buchan‐type metamorphism) have been generally associated with abnormally high geothermal gradients such as those of the island arc or ridge settings (Cloos, 1993; Fagan et al., 2001; Favier et al., 2019; Mevel et al., 1978; Oxburgh & Turcotte, 1970; Verati et al., 2018), slab rollback or increase of the slab dip after continental collision (Ji et al., 2019; Li et al., 2013; Menant et al., 2016; Schliffke et al., 2019; Sizova et al., 2019), post‐collisional extension (Carmignani & Kligfield, 1990; Vanderhaeghe, 2012), melt migration through the crust (Depine et al., 2008), local effects of contact metamorphism (Rothstein & Manning, 2003).…”

“…have been generally associated with abnormally high geothermal gradients such as those of the island arc or ridge settings (Cloos, 1993;Fagan et al, 2001;Favier et al, 2019;Mevel et al, 1978;Oxburgh & Turcotte, 1970;Verati et al, 2018), slab rollback or increase of the slab dip after continental collision (Ji et al, 2019;Li et al, 2013;Menant et al, 2016;Schliffke et al, 2019;Sizova et al, 2019), post-collisional extension (Carmignani & Kligfield, 1990;Vanderhaeghe, 2012), melt migration through the crust (Depine et al, 2008), local effects of contact metamorphism (Rothstein & Manning, 2003). Miyashiro (1961Miyashiro ( , 1973 promoted the concept of paired metamorphic belts, revisited by Brown (2010), and based on the juxtaposition along orogenic crustal sections of tectonic complexes characterized by Franciscan-and Abukuma-type metamorphic facies series.…”