Olivier Vanderhaeghe

Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

Abstract The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±hogbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region. The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone ( P >8–10 kbar) to the sillimanite–cordierite zone ( P T ∼750 °C, with maximum recorded temperatures of ∼800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±hogbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks. Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.

Two-phase orogenic convergence in the external and internal SW Alps

The NW–SE-trending sector of the SW Alps includes the Dora Maira massif where Tertiary eclogites record ultrahigh pressures and rapid exhumation. Along a NE–SW crustal cross-section (Italy–France) compiled pressure–temperature–time data in internal zones are correlated with Tertiary stratigraphy in external zones to reconstruct orogen evolution, revealing a coherent two-phase convergence history. During the first, subduction–accretion phase (Eocene, 55–34 Ma) rapid north–south plate convergence caused the subduction and exhumation of high-pressure and ultrahigh-pressure (UHP) rocks in a steady-state subduction channel. This coincided with the north to NNW migration of an underfilled flexural basin across the European foreland. Nappe stacking within the subduction channel did not create significant relief, implying that primarily subduction forces generated this flexural basin. From 34 Ma onward, the second, collisional phase was characterized by slower NW–SE plate convergence. The internal units of the SW Alps underwent considerable anticlockwise rotation as they became involved in a NW–SE-oriented sinistral transpression zone between the European and Adriatic plates. To the north of the orogen the North Alpine Foreland Basin became overfilled as a result of high sediment supply from increasing orogen relief. In contrast, in SE France active flexure of the European plate appears to have ceased and sedimentation became limited to small thrust-sheet-top basins created by continuing gentle NE–SW shortening. Internal units were exhumed slowly from depths of c. 20 km, principally by erosion. In the SW Alps, the transition between these two phases was marked by the rapid subduction and exhumation of the Dora Maira UHP unit. Assuming lithostatic pressure, this unit would have been exhumed from 100 km depth, requiring a rate that exceeds that generated by plate convergence. Therefore, either exhumation was accelerated by additional stresses (locally generated by transpression, slab breakoff or high density contrasts) or, more controversially, the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure.

Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and 40Ar/39Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (δDms) of precisely dated muscovite attain values as low as −156‰ in the detachment mylonite, whereas footwall quartzite has a δDms value of −81‰. The very low δDms values in the detachment are best explained by infiltration of meteoric water, with maximum δD values of −135‰ ± 3‰, during extensional unroofing of the orogen at 49.0–47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 ± 250 m to 4320 ± 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse.

Crustal-scale rheological transitions during late-orogenic collapse

Abstract Orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust, culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late-orogenic collapse. Collapse of the Canadian Cordillera is expressed in the Early Tertiary Shuswap metamorphic core complex, British Columbia, which displays a three-layer crustal section separated by two fundamental rheological discontinuities: (1) the brittle–ductile transition, across which high-angle normal faults in the upper crust control basin formation merge into a low-angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low-temperature conditions; and (2) the metatexite–diatexite transition across which the rocks lose their solid framework and behave like a viscous magma. This transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late-orogenic gravitational collapse.

Flow of partially molten crust and origin of detachments during collapse of the Cordilleran Orogen

Abstract In metamorphic core complexes two types of detachments develop, coupled by flow of partially molten crust: a channel detachment and a rolling-hinge detachment. The channel detachment, on the hinterland side of the orogen, represents the long-lived interface that separates the partially molten crust flowing in a channel from the rigid upper crustal lid. On the foreland side of the core complex, a rolling-hinge detachment develops. This detachment dips toward the foreland, probably affects the whole crust, and its geometry is governed by strain localization at the critical interface between cold foreland and hot hinterland. Activation of the rolling-hinge detachment drives rapid decompression and melting, leading to the diapiric rise of migmatite domes in the footwall of the detachment. A kinematic hinge (switch in sense of shear) separates the two types of detachments. Structural, metamorphic and geo/thermochronological studies in the Shuswap core complex (North American Cordillera), combined with an anisotropy of magnetic susceptibility study of leucogranites concentrated in the detachments, suggest that this orogen collapsed rapidly through the development of channel and rolling-hinge detachments in the early Eocene. The kinematic hinge is currently located approximately 40 km west of the footwall in which it originated, corresponding to a mean exhumation rate of >5 km Ma−1, which explains the near-isothermal decompression recorded within the migmatite dome.

Pressure-Temperature-time Evolution of Metamorphic Rocks from Naxos (Cyclades, Greece): Constraints from Thermobarometry and Rb/Sr dating

The Pressure-Temperature-time paths of metapelites sampled on an east-west transect across the structural dome of Naxos (Greece) have been reconstructed on the basis of new geothermobarometric data and Rb/Sr dating, as well as previously published data. One sample from an intermediate structural level records pressure and temperature conditions of 10 kbar, 500°C, corresponding to its exhumation in a highpressure/low-temperature (HP/LT) setting. The corresponding Rb/Sr exhumation age is 29.3±1.3(2s) Ma. Toward the center of the dome, metamorphic assemblages record an increase in peak-temperature and corresponding pressure (from 500 to 700°C, and from 5 to 8 kbar), in a medium-pressure/medium-temperature (MP/MT) metamorphic field gradient. Whole-rock, muscovite and biotite on two samples from deep structural levels define ages of 5.2 and 7.0 Ma, whereas garnets fall outside the isochrons and retain earlier less radiogenic signatures. Rb/Sr data on these two samples demonstrate open system behaviour coeval with, or subsequent to MP/MT metamorphism. The interpretation of the Pressure-Temperature-time paths indicates a time span of 15-20 Ma for dome formation. It also suggests that the HP/LT to MP/MT transition is due to i) heating of deeper parts of the dome through magma injection or ii) either homogeneous (75 %) or localized thinning during dome formation.

Contrasting mechanism of crustal growth Geodynamic evolution of the Paleoproterozoic granite-greenstone belts of French Guiana

Abstract The Paleoproterozoic granite–greenstone belts of French Guiana formed during a period of major crustal growth at the scale of the Earth. Two contrasting mechanisms of crustal growth have been identified by recent structural, metamorphic, geochronological and geochemical investigations in the Cayenne–Regina region, which is part of the northern granite–greenstone belt of French Guiana. Following the formation of oceanic crust, which is characterized by a tholeiitic volcanic sequence dated at 2174±7xa0Ma, the early stage of crustal generation was marked by successive building of calc-alkaline plutonic–volcanic complexes, with the formation of the Ile de Cayenne complex dated at 2144±6xa0Ma to the north, and of the Central Guiana complex dated at 2115±7xa0Ma to the south. This first period of crustal generation by accumulation of mantle-derived magmas was followed by a period of crustal recycling and tectonic accretion. Sediments derived from the granite–greenstone belts were deposited in large marginal basins, such as the Orapu basin, separating the plutonic–volcanic complexes. Convergence between the newly formed crustal blocks caused deformation during the Transamazonian orogeny of the Orapu marginal basin caught in between the Ile de Cayenne and Central Guiana complexes. Oblique convergence is further characterized by the development of en-echelon pull-apart basins along the North Guiana Trough sinistral strike-slip shear zones, affecting the edges of the Orapu marginal basin. The lithological succession in these basins is typical of foreland basins, and the metamorphic and structural evolution of the Kaw and Regina basins in the Cayenne–Regina region indicate burial to depth up to 20xa0km in a context of heterogeneous transpression. The geochemical signature of small syntectonic granitic plutons dated at 2093±8xa0Ma and 2083±8xa0Ma emplaced along the major strike-slip shear zones is consistent with crustal thickening and partial melting of an enriched crustal source. These features suggest that the last stage of the Transamazonian orogeny was marked by oblique convergence, collision and crustal thickening in a similar way to modern orogens.

Formation of the shuswap metamorphic core complex during late-orogenic collapse of the canadian cordillera: Role of ductile thinning and partial melting of the mid-to lower crust

AbstractThe early Tertiary evolution of the Shuswap metamorphic core complex is characterised by low-angle crustal detachments and nearly isothermal decompression followed by rapid cooling of rocks in the footwall of the detachments. Previous work as well as our own observations suggest that Paleogene late-orogenic extension produced the main tectonic features of the region. Furthermore, structural analysis of the migmatites and published geochronological data indicate that partial melting of the mid- to lower crust was coeval with extension in the upper crustal levels, suggesting that these two processes are linked genetically. Consequently, we propose that the formation of the Shuswap metamorphic core complex corresponds to late-orogenic gravitational collapse of the Canadian Cordillera accommodated by normal faulting of the brittle upper crust and by ductile thinning of the mid- to lower crust. The initiation and amplification of extension during the Paleocene in the Shuswap metamorphic core complex ar...

Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece

[1]xa0Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces, folds, and fabrics while units mantling the dome display a simpler geometry, typically formed by transposition during crustal extension. We use field observations and magnetic fabrics in the Naxos dome (Greece) to quantify the complex flow of anatectic crust beneath an extensional detachment system. The internal structure of the Naxos dome is characterized by second-order domes (subdomes), pinched synforms, and curved lineation trajectories, which suggest that buoyancy-driven flow participated in dome evolution. Subdomes broadly occur within two compartments that are separated by a steep, N-S oriented, high-strain zone. This pattern has been recognized in domes formed by polydiapirism and in models of isostasy-dominated flow. The preferred model involves a combination of buoyancy- and isostasy-driven processes: the Naxos dome may have been generated by regional N-S extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome. This pattern is complicated by gravitational instabilities and/or overturning of the high melt fraction crust leading to the growth of subdomes. As the migmatites within the Naxos dome reached a higher structural level, they were affected by regional top-to-the-NNE kinematics of the detachment system. Dome formation therefore occurred by a combination of coeval and coupled processes: upper crustal extension, deep crust contraction during convergent flow of anatectic crust, diapirism and/or density-driven crustal convection forming subdomes, and north directed detachment kinematics.

Structures and way-up criteria in migmatites, with application to the Velay dome (French Massif Central)

Abstract A detailed structural analysis has been applied to the migmatite sequence of the southern limb of the Velay dome, France, in an attempt to study the structural implications of the occurrence of metatexites and diatexites. Metatexites have a plastic behaviour presumably dominated by the solid skeleton which largely takes up the deformation. In contrast, diatexites have a predominantly viscous behaviour because the solid framework has been disrupted and they are dominated by the granitic melt rather than the solid component. Therefore, the metatexite to diatexite transition has a major rheological and tectonic importance because it separates regions that record plastic (stable) deformation from regions that record viscous (unstable) deformation where gravitational instabilities may develop. Mesoscopic, syn-migmatitic way-up criteria, which assume that melt migration is partly driven by gravity, give evidence for a southward overturning of the Velay dome. This overturning is supported by sedimentary and plutonic way-up criteria seen in schists and granitic plutons, respectively. The N-S asymmetry of the Velay dome is related to late-Variscan, asymmetric extension on a regional scale. Consistency of data provides further evidence that syn-migmatization way-up criteria are regionally reliable structural tools applicable to other migmatitic terrains.