Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

Crustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and con- verged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P ) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

Experimental investigation of antigorite dehydration fabrics at high pressure and high temperature

<p>Antigorite dehydration is well known as a key process in convergent boundaries for the genesis of mantle wedge partial melting and intermediate-depth earthquakes. However, the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and its effects on seismic anisotropy of subducting slabs remain ambiguous and controversial. Here we report hydrostatic dehydration experiments on foliated serpentinized peridotite at pressures of 0.3-6 GPa and temperatures of 700-900 °C. Our results show that the orientations of prograde olivine inherit orientations from adjacent olivine grains in the olivine-rich layer by epitaxial growth. In contrast, olivine CPOs evolved with the grain size from fiber-[001] featuring clear [100] point maxima and [001] girdles for fine-grained olivine to orthorhombic patterns characterized by clear [100] and [001] point maxima for coarse-grained olivine, i.e., type-C CPO. We propose that the fine-grained fiber-[001] CPO is developed by topotactic growth at the onset of dehydration, while the orthorhombic type-C CPO for the coarse-grained olivine, especially the [001] point maximum along the lineation, is mainly developed by anisotropic growth resulting from anisotropic fluid flow during the dehydration. The developed olivine type-C CPO in the antigorite-rich layer after antigorite dehydration could explain the trench or strike parallel seismic anisotropy observed at convergent plate boundaries.</p>

Overriding-plate deformation during micro-continent accretion

<p>Both divergent and convergent plate boundaries had been studied extensively throughout the last five decades. Among a host of other aspects came the realization, that given the right circumstances, a broad extensional basin can form behind a convergent plate boundary. The exact mechanisms triggering back-arc extension and why they are episodic, lasting only for tens of millions of years is still debated. The absolute and relative velocities of the plates, the age of the subducting oceanic plate and the inherited rheological properties of the back-arc lithosphere are all thought to be key players, shaping the dynamics of the fore-arc - back-arc systems.</p><p>Here we use 2D mantle scale plane-strain thermo-mechanical model experiments to investigate how the accretion of small continental crustal terrains onto the overriding plate affect the dynamics of the subducting slab and the deformation of the overriding plate.</p><p>Our results suggest that slab-retreat and back-arc extension can be achieved through the combination of slow convergence and micro-continent accretion. Back-arc extension during fast convergence is also possible through the subsequent accretion of more than one micro-continental terrain. Moreover, even the accretion of one such terrain can produce short (1-5 My) episodes of extension-contraction-quiescence in the overriding plate. These episodes are connected to slab break-off events, slab-interaction with upper mantle phase-change boundaries and variations in slab-pull due varying slab thickness.</p><p>Our model experiments also result in complex structures in the overriding plate where discrete outcrops from a single oceanic basin are preserved on the surface hundreds of kilometres apart. This indicates that in nature a simple accretion scenario could produce a surface geological record that is difficult to decipher. Our results compare favourably to observations from the Aegean back-arc basin.</p>

Melting processes at convergent plate boundaries: from melt segregation, extraction to the formation of crustal magmatic systems

<p><strong>Melting at convergent plate boundaries</strong></p><p>At divergent plate boundaries hot mantle upwelling is associated with abundant melt generation and volcanism. At convergent plate boundaries such as subduction zones and continental collision zones thick and cold plates feed mantle downwellings. Yet these "cold" regions also show abundant volcanic activity with mean volcanic output rates of almost similar order of magnitudes (White et al., 2006, G-cubed). Responsible melt generation mechanisms are addressed including a) volatile driven decrease of the solidus temperature, b) decompressional melting in the mantle wedge or in shallow asthenosphere associated with delamination, or c) increased radiogenic heating within thickened continental crust.     </p><p><strong>Melt transport mechanisms</strong></p><p>The above processes form partially molten regions. By which mechanism(s) does the melt segregate out of the melt source region and rise through the mantle or crust. The basic mechanism is two-phase flow, i.e. a liquid phase percolates through a solid, viscously deforming matrix. The corresponding equations and related issues such as compaction or effective matrix rheology are addressed. Beside simple Darcy flow, special solutions of the equations are addressed such as solitary porosity waves. Depending on the bulk to shear viscosity ratio of the matrix and the non-dimensional size of these waves, they show a variety of features: they may transport melt over large distances, or they show transitions from rising porosity waves to diapiric rise or to fingering. Other solutions of the equations lead to channeling, either mechanically or chemically driven. One open question is how do such channels transform into dykes which have the potential of rising through sub-solidus overburden. A recent hypothesis addresses the possibility that rapid melt percolation may reach the thermal non-equilibrium regime, i.e. the local temperature of matrix and melt may evolve differently.  Once dykes have been formed they may propagate upwards driven by melt buoyancy and controlled by the ambient stress field. As another magma ascent mechanism diapirism is addressed.  </p><p><strong>Modelling magmatic systems in thickened continental crust </strong></p><p>Once basaltic melts rise from subducting slabs, they may underplate continental crust and generate silicic melts. Early dynamic models (Bittner and Schmeling, 1995, Geophys. J. Int.) showed that such silicic magma bodies may rise to mid-crustal depth by diapirism. More recent approaches (e.g. Blundy and Annan, 2016, Elements) emplace sill intrusions into the crust at various levels and calculate the thermal and melting effects responsible for the formation of mush zones. Recently Schmeling et al. (2019, Geophys. J. Int.) self-consistently modelled the formation of crustal magmatic systems, mush zones and magma bodies by including two-phase flow, melting/solidification and effective power-law rheology. In these models melt is found to rise to mid-crustal depths by a combination of compaction/decompaction assisted two-phase flow, sometimes including solitary porosity waves, and diapirism. An open question in these models is whether or how dykes may self-consistently form to transport the melts to shallower depth. First models which combine the two-phase flow crustal models with elastic dyke-propagations models (Maccaferri et al., 2019, G-cubed) are promising.      </p><p>      </p>