Taras V. Gerya

Rayleigh^Taylor instabilities from hydration and melting propel 'cold plumes' at subduction zones

Abstract It is commonly thought that hot diapiric flows prevail in the mantle wedge above the subducting slab. However, hydration and partial melting along the slab can create a situation in which a Rayleigh–Taylor instability can develop at the top of a cold subducting slab. We have numerically modeled this parodoxically interesting geological phenomenon, in which rising diapiric structures, colder than the asthenosphere by 300–400°C, are driven upward by compositional buoyancy, with a high-resolution two-dimensional regional model. These ‘cold plumes’ with a compositional, hydrous origin, launched from a depth of greater than 100 km, are lubricated by viscous heating, have an upward velocity in excess of 10 cm/yr, penetrate the relatively hot asthenosphere in the mantle wedge within a couple of million years and thus can cool the surroundings. These ‘cold plumes’ are fueled by partial melting of the hydrated mantle and subducted oceanic crust due to fluid release from dehydration reactions wihin the slab, including the decomposition of serpentine. There may be a spatial correlation between seismicity and the particular depth of cold-plume initiation.

Fault-induced seismic anisotropy by hydration in subducting oceanic plates

The variation of elastic-wave velocities as a function of the direction of propagation through the Earth’s interior is a widely documented phenomenon called seismic anisotropy. The geometry and amount of seismic anisotropy is generally estimated by measuring shear-wave splitting, which consists of determining the polarization direction of the fast shear-wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleseismic fast shear-wave component is oriented generally parallel to the strike of the trench, although a few exceptions have been reported (Cascadia and restricted areas of South America). The interpretation of shear-wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide. Here we show that the amount and the geometry of seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections whose spacing is several times smaller than teleseismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.

Why is terrestrial subduction one-sided?

Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically—the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[φ] 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes.

The numerical sandbox: comparison of model results for a shortening and an extension experiment

Abstract We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commercial and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment. Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.

Asymmetric three-dimensional topography over mantle plumes.

The role of mantle–lithosphere interactions in shaping surface topography has long been debated. In general, it is supposed that mantle plumes and vertical mantle flows result in axisymmetric, long-wavelength topography, which strongly differs from the generally asymmetric short-wavelength topography created by intraplate tectonic forces. However, identification of mantle-induced topography is difficult, especially in the continents. It can be argued therefore that complex brittle–ductile rheology and stratification of the continental lithosphere result in short-wavelength modulation and localization of deformation induced by mantle flow. This deformation should also be affected by far-field stresses and, hence, interplay with the ‘tectonic’ topography (for example, in the ‘active/passive’ rifting scenario). Testing these ideas requires fully coupled three-dimensional numerical modelling of mantle–lithosphere interactions, which so far has not been possible owing to the conceptual and technical limitations of earlier approaches. Here we present new, ultra-high-resolution, three-dimensional numerical experiments on topography over mantle plumes, incorporating a weakly pre-stressed (ultra-slow spreading), rheologically realistic lithosphere. The results show complex surface evolution, which is very different from the smooth, radially symmetric patterns usually assumed as the canonical surface signature of mantle upwellings. In particular, the topography exhibits strongly asymmetric, small-scale, three-dimensional features, which include narrow and wide rifts, flexural flank uplifts and fault structures. This suggests a dominant role for continental rheological structure and intra-plate stresses in controlling dynamic topography, mantle–lithosphere interactions, and continental break-up processes above mantle plumes.

Dynamical causes for incipient magma chambers above slabs

Using a regional upper-mantle model with an unprecedented spatial resolution of ∼100 m, we have investigated at multiple resolutions the character of incipient magma chambers forming under oceanic arcs. The magma chambers are formed from wave-like structures propagating upward along descending slabs and consist of compositionally buoyant, hydrated, partially molten subducted crustal and mantle material. These wave structures are 300–500 °C colder than the mantle wedge and may have an upward velocity of >1 m/yr. Inverted temperature structures and transitory bimodal magmatism are plausible consequences of finger-like penetration of relatively cold, hydrated material of the incipient magma chambers into the hot mantle wedge. Apart from forming and periodic feeding of the magma chambers, “cold” waves may also transport upward thousands of cubic kilometers of subducted material and may cause the rapid exhumation of ultrahigh-pressure rocks along slabs.

A free plate surface and weak oceanic crust produce single-sided subduction on Earth

[1] Earth’s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection. Subduction zones on present-day Earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle. While global self-consistent numerical models of mantle convection have reproduced some aspects of plate tectonics, the assumptions behind these models do not allow for realistic single-sided subduction. Here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates: (1) the presence of a free deformable upper surface and (2) the presence of weak hydrated crust atop subducting slabs. We show that assuming a free surface, rather than the conventional free-slip surface, allows the dynamical behavior at convergent plate boundaries to change from double-sided to single-sided. A weak crustal layer further improves the behavior towards steady single-sided subduction by acting as lubricating layer between the sinking and the overriding plate. This is a first order finding of the causes of single-sided subduction, which by its own produces important features like the arcuate curvature of subduction trenches. Citation: Crameri, F., P. J. Tackley, I. Meilick, T. V. Gerya, and B. J. P. Kaus (2012), A free plate surface and weak oceanic crust produce single-sided subduction on Earth, Geophys. Res. Lett., 39, L03306, doi:10.1029/2011GL050046.

Dynamical instability produces transform faults at mid-ocean ridges.

Cracking Up Transform faults perpendicular to mid-ocean ridges are some of the most prominently visual features on the sea floor. Because they form slowly over thousands of years, the lack of observational data means their mechanism of formation has remained controversial. Taking a numerical modeling approach, Gerya (p. 1047) suggests that due to asymmetric growth of the plate boundary, sections of the mid-ocean ridge become unstable and eventually rotate 90°. As the ridge continues to grow, the transform faults continue to develop long after their initiation. This mechanism also explains how offsets of these transform faults occur discontinuously as a result of new fractures at the ridge. Transform faults on the sea floor are rotated and stretched sections of the mid-ocean ridge. Transform faults at mid-ocean ridges—one of the most striking, yet enigmatic features of terrestrial plate tectonics—are considered to be the inherited product of preexisting fault structures. Ridge offsets along these faults therefore should remain constant with time. Here, numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries, irrespective of previous structure. Boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections; the resultant curved ridges become transform faults within a few million years. Fracture-related rheological weakening stabilizes ridge-parallel detachment faults. Offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps.

Dynamic effects of aseismic ridge subduction: numerical modelling

The subduction of oceanic aseismic ridges, oceanic plateaus and seamount chains is a common process that takes place in a variety of tectonic settings and seems to coincide spatially and temporally with a gap of volcanic activity, shallow or even horizontal slab angles, enhanced seismic activity and various topographic features. In the present study we focus on these dynamic effects on the basis of 2D thermomechanical modelling incorporating effects of slab dehydration, mantle-wedge melting and surface topography development. In order to ascertain the impact of a moderate-size (200 × 18 km) aseismic ridge, 12 pairs of experiments (one for the case with a ridge, the other without) were carried out varying slab density and subducting- and overriding-plate velocities. By analysing pairs of experiments we conclude that subduction of a moderate-sized ridge does not typically result in strong slab flattening and related decrease of magmatic activity. This, in turn, suggests that, when slab flattening is indeed associated with the ridge subduction in nature, the slab itself should be in a nearly critical ( i.e ., transient from inclined to flat) state so that any local addition of positive buoyancy may strongly affect overall slab dynamics. Therefore, subducting ridges may serve as indicators of transient slab states in nature. Another important result from our study is the numerical quantification of strongly decreased magma production associated with flat slabs that may explain gaps in recent active volcanism at low-angle subduction margins. Lowering of magmatic rock production is caused by the absence of a hot mantle wedge above the flat slabs and does not directly depend on the mechanism responsible for the triggering of slab flattening. Finally we document several very distinct surface effects associated with the moderate-size ridge subduction such as local increase in elevation of overriding margin, enhancement of subduction erosion and landward trench displacement. Surface uplift may exceed the original ridge height due to additional uplift resulting from the overriding plate shortening. Topographic perturbations within the accretionary wedge domain are transient and have a tendency to relax after the ridge passes the trench. In contrast, the topographic high created in the continental portion of the overriding plate relaxes more slowly and may even be sustained for several millions of year after the ridge subduction.

Numerical modelling of PT-paths related to rapid exhumation of high-pressure rocks from the crustal root in the Variscan Erzgebirge Dome (Saxony/Germany)

Abstract The Bohemian Massif in the Central European Variscides contains many crustal slices with (ultra-)high-pressure rocks related to continent-continent collision. After closure of pre-existing oceans during the Devonian, excess crustal thickness was maintained for about 50 Ma until at around 340 Ma large volumes of high-pressure rocks from the crustal root were exhumed within a few million years. We relate this event to delamination and complete detachment of the lithospheric mantle, causing a crustal-scale isostatic instability. In the Erzgebirge dome, a model region in the northern part of the massif, an array of interrelated PTtd-paths with “decompression/cooling” and “decompression/heating” trajectories in juxtaposed tectonometamorphic units has been established. Numerical 2D-experiments using a rheologically, thermally and dynamically consistent convection technique show three stages of the crustal evolution related to delamination and detachment of mantle lithosphere under the crustal root: (1) During delamination a rapid overthickening of the crust can occur with the crust penetrating down to >160 km depth. (2) After detachment extensional crustal thinning controlling exhumation occurs with escape of rocks from the crustal root towards the margins of the orogen through tectonically weak zones. Horizontal displacement exceeds vertical by a factor of ∼3. (3) Forced circulation in the weak zones follows and upward flow of lower crustal rocks is compensated by subduction of upper crustal rocks in the footwall of these zones. One-dimensional modelling was used in order to further understand basic processes and to simulate the rock record in detail. According to 2D and 1D modelling, strongly decelerating exhumation rates with decreasing overburden and a late increase in the geothermal gradient due to upward heat transfer are necessary corollaries of this scenario, in keeping with observations from the Erzgebirge dome. Exhumation PT-paths do not conform to one single uniform exhumation trajectory; rather, assemblages of interrelated PTtd-paths are characteristic.