Abstract Alternating subduction polarity along suture zones has been documented in several orogenic systems. Yet the mechanisms leading to this geometric inversion and the subsequent interplay between the contra-dipping slabs have been little studied. To explore such mechanisms, 3D numerical modelling of the Wilson cycle was conducted from continental rifting, breakup and oceanic spreading to convergence and self-consistent subduction initiation. In the resulting models, near-ridge subduction initiating with the formation of contra-dipping slab segments is an intrinsically 3D process controlled by earlier convergence-induced ridge swelling. The width of the slab segments is delimited by transform faults inherited from the rifting and ocean floor spreading stages. The models show that the number of contra-dipping slab segments depends mainly on the size of the oceanic basin, the asymmetry of the ridge and variations in kinematic inversion from divergence to convergence. Convergence velocity has been identified as a second-order parameter. The geometry of the linking zone between contra-dipping slab segments varies between two end-members governed by the lateral coupling between the adjacent slab segments: (1) coupled slabs generate wide, arcuate linking zones holding two-sided subduction; and (2) decoupled slabs generate narrow transform fault zones against which one-sided, contra-dipping slabs abut.

Petrological and fluid-inclusion data of high-grade metapelitic gneisses that occur as enclaves and in the immediate surroundings of the 2.612 Ga old Bulai granitoid intrusive are presented in this chapter. The Bulai intrusive is an important time marker in the tectono-metamorphic evolution of the Central Zone of the Limpopo Complex. The host-rock gneisses show one generation of garnet, cordierite, and sillimanite, whereas the enclave gneisses show two different generations of garnet (Grt 1,2 ), cordierite (Crd 1,2 ), and sillimanite (Sil 1,2 ). The first generation defines a gneissic texture, whereas the second generation shows a random mineral orientation. Grt 1 and Crd 1 show a higher Mg content compared with Grt 2 and Crd 2 . Host rock garnet and Grt 1 show K-feldspar micro-veins at the contact with quartz as a result of high-temperature metasomatism. Host rock garnet, Grt 1 , and Grt 2 are zoned and participate in two simultaneously operating reactions: sillimanite + garnet + quartz = cordierite and garnet + K-feldspar + H 2 O = biotite + sillimanite + quartz. The combination of petrographic, geothermobarometric, and fluid-inclusion results shows evidence of two different pressure-temperature (P-T) paths in the enclave and a single P-T path in the host rocks. The decompressional cooling P-T path in the host rock is typical of the country rocks throughout the Central Zone. The high-pressure part of the host-rock P-T path overlaps with the Grt 1 -Crd 1 -Sil 1 P-T path found in the enclave rocks. The second P-T path is calculated from the Grt 2 -Crd 2 -Sil 2 assemblage and is found only in the enclave rocks. The two P-T paths in the enclave rocks can be connected by a sub-isobaric heating event of ~50 °C at 5.5 kbar. This increase in temperature is followed by decompressional cooling but with a lower P-T gradient compared with that of the country rocks caused by the emplacement of the Bulai Pluton. Fluids present during granulite metamorphism include CO 2 and brines. Retrograde infiltration of water in graphite-bearing country rocks under relatively reduced conditions resulted in the formation of a methane-rich fluid.

This paper proposes a revision of the gravitational redistribution model suggested by Leonid Perchuk for the formation, evolution, and exhumation of Precambrian high-grade terranes (HGTs) located between granite-greenstone cratons. Such HGTs are separated from greenstone belts by crustal-scale shear zones up to 10 km wide and several hundred kilometers long. Pelite samples far (>~50 km) from the bounding shear zones show coronitic and symplectitic textures that reflect a decompression-cooling (DC) pressure-temperature (P-T) path. On the other hand, samples from within ~50 km of the bounding shear zones are characterized by textures that reflect an isobaric or near-isobaric cooling (IC) path. Local mineral equilibria in the schists from the shear zones record hairpin-shaped clockwise P-T loops. The results of a numerical test of the gravitational redistribution model show the following plausible scenario: The diapiric rise of low-density, hot granulite upward in the crust causes the relatively high-density, predominantly mafic upper crust in the adjacent greenstone belt, consisting of metabasalt and komatiite, to move downward (subducted), cooling the base of the granulites along the intervening syntectonic shear zone. This causes (1) the formation of local convection cells that control the movement of some of the ascending granulite blocks near the contact with the cratonic rocks, and (2) near-isobaric cooling (IC) of the granulite blocks in the vicinity of the boundary with the colder wall rocks. Cooling of granulite blocks farther away from the contact is not arrested, and they ascend to the Earth's surface, recording DC P-T paths. In general, the results of numerical modeling provide support for the buoyant exhumation mechanism of granulites owing to gravitational redistribution within the metastable relatively hot and soft early Precambrian crust that was subjected to high-temperature (HT) and ultrahigh-temperature (UHT) metamorphism.

Abstract Slab detachment or breakoff is directly associated with phenomena like morphological orogenesis, occurrence of earthquakes and magmatism. At depth the detachment process is slow and characterized by viscous rheolgy, whereas closer to the surface the process is relatively fast and plastic. Using a 2D mantle model 1500 km deep and 4000 km wide we investigated, with finite-difference and marker-in-cell numerical techniques, the impact of slab age, convergence rate and phase transitions on the viscous mode of slab detachment. In contrast to previous studies exploring simplified breakoff models in which the blockage responsible for inducing breakoff is kinematically prescribed, we constructed a fully dynamic coupled petrological–thermomechanical model of viscous slab breakoff. In this model, forced subduction of a 700 km-long oceanic plate was followed by collision of two continental plates and spontaneous slab blocking resulting from the buoyancy of the continental crust once it had been subducted to a depth of 100–124 km. Typically, five phases of model development can be distinguished: (a) oceanic slab subduction and bending; (b) continental collision initiation followed by the spontaneous slab blocking, thermal relaxation and unbending – in experiments with old oceanic plates in this phase slab roll-back occurs; (c) slab stretching and necking; (d) slab breakoff and accelerated sinking; and (e) post-breakoff relaxation. Our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models. The results also demonstrate a non-linear dependence of the duration of the breakoff event on slab age: a positive correlation being characteristic of young (<50 Ma) slabs while for older slabs the correlation is negative. The increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness, which increases both the slab thermal relaxation time and duration of the necking process. In older slabs this tendency is counteracted by negative slab buoyancy, which generate higher stresses that facilitate slab necking and breakoff. A prediction from our breakoff models is that the olivine–wadsleyite transition plays an important role in localizing viscous slab breakoff at depths of 410–510 km due to the buoyancy effects of the transition.

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 commereiai 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.

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous , multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.

Diapir and dome structures on a scale of meters to kilometers are widespread in Earth's continental crust and represent an important tectonic element of cratons, orogenic belts, and sedimentary basins. These structures advect heat from lower to higher crustal levels, often producing pronounced prograde contact metamorphic aureoles. This standard thermal situation is violated by the up to 8 km in diameter migmatitic domes and diapirs of metasedimentary rocks that penetrate the world's largest layered intrusion, the Bushveld Complex. These domes and diapirs rose with an average rate of about 8 mm/yr and were characterized by an unusual inverted thermal structure, with the cores of the structures 200–300 °C colder than the rims. Numerical modeling supports the interpretation that the process was triggered by the emplacement of an 8-km-thick, hot, dense mafic magma over a cold, less dense sedimentary succession, resulting in a dramatic lowering of the viscosity of the sediments during contact metamorphism and partial melting. Dome and diapir nucleation is interpreted to have been defined by the formation of initial anticline-shaped disturbances related to fingered lateral injection of the Lower Zone of the Bushveld intrusion between the felsic roof and sedimentary floor sequence. The partially molten, mobile, but relatively cold domes and diapirs promoted cooling of the giant magma chamber, rapidly bringing cooler material into higher crustal levels, and freezing the surrounding magmas. We argue that our work has a more general significance as similar thermal structures should be a widespread feature associated with partially molten mantle diapirs (“cold plumes”) generated in the proximity of subducting slabs. These structures are likely responsible for rapid upward melt transport above subduction zones and for the associated volcanic activity. The exposed structures observed in the Bushveld Complex provide a unique opportunity to study the “cold” diapir/plume phenomenon, thus leading to a broader recognition and understanding of this geological process.