Alain Vauchez

Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties

Forward numerical models are used to investigate the effect of deformation regime on the development of olivine lattice-preferred orientations (LPO) and associated seismic anisotropy within continental deformation zones. LPO predicted to form by pure shear, simple shear, transpression, or transtension are compared to a database comprising ca. 200 olivine LPO from naturally deformed upper mantle rocks. This comparison suggests that simple shear or plane combinations of simple and pure shear are probably the dominant deformation regimes in the upper mantle. Seismic properties, calculated using the modeled olivine LPO, suggest that seismic anisotropy data may carry information on the deformation regimes active in the lithospheric mantle, although not all deformation regimes are characterized by a distinct seismic anisotropy signal. Transtensional deformation in continental rift systems should result in fast S-wave polarization and P-wave propagation directions oblique to the rift trend within the extended lithospheric mantle. Simple shear (wrench) or transpression in vertical deformation zones and pure shear (horizontal extension) result in similar seismic anisotropy. Simple shear or widening‐thinning shear may, however, induce obliquity between seismic and magnetotelluric electrical conductivity anisotropy data. Similarly, it is not possible to distinguish between simple shear or lengthening‐thinning shear (plane transpression) in horizontal deformation zones (thrusts) and pure shear (vertical contraction=horizontal extension). In all cases, the polarization direction of the fast split S-wave and the fast P-wave direction parallels the flow direction, but the anisotropy for both Pn- and S-waves is lower in horizontal structures than in vertical ones. Finally, several deformations show an isotropic response to SKS and=or Pn waves, suggesting that seismic isotropy does not necessarily imply absence (or heterogeneity) of deformation. There is a good agreement between model predictions and seismic anisotropy data in both transtensional and transpressional zones, suggesting coupled deformation of the crust and mantle. Oblique fast S-wave polarization directions in the East African rift, for instance, may result from an early transtensional deformation in the mantle lithosphere below the rift system. In contrast, most thrust belts display fast S-waves polarized parallel to the trend of the belt. One possible interpretation is that the upper mantle is decoupled from the crust in these areas.

Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle

Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strike^ slip deformation, in particular in the early rifting stage. Moreover, although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process, there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts. These observations suggest that the deformation of the lithosphere, especially during rifting, is controlled by its preexisting structure. On the other hand, (1) the plasticity anisotropy of olivine single crystal and aggregates, (2) the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs, and (3) seismic anisotropy data, which require a tectonic fabric in the upper mantle coherent over large areas, suggest that preservation within the lithospheric mantle of a lattice preferred orientation (LPO) of olivine crystals may induce a large-scale mechanical anisotropy of the lithospheric mantle. We use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process. We assess the deformation of an anisotropic continental lithosphere in response to an axi-symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head. Model results show that a LPO-induced mechanical anisotropy of the lithospheric mantle may result in directional softening, leading to heterogeneous deformation. During continental rifting, this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique (30^ 55‡) to the preexisting mantle fabric. This directional softening associated with olivine LPO frozen in the lithospheric mantle may also guide the propagation of the initial instability, that will follow the preexisting structural trend. The preexisting mantle fabric also controls the deformation regime, imposing a strong strike^slip shear component. A LPOinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting (structural inheritance), the plume^rift paradox, and the onset of transtension within continental rifts. fl 2001 Elsevier Science B.V. All rights reserved.

The Borborema shear zone system, NE Brazil

Abstract The Neoproterozoic evolution of the Borborema Province is characterized by the development of a continental-scale network of transcurrent shear zones. These shear zones form a kinematically consistent system over more than 200,000 km2. This shear zone system is coeval with a high-temperature, medium- to low-pressure metamorphism, partial melting of the crust, and synkinematic magmatism involving both crustal- and mantle-derived magmas. Preliminary geochronological data suggests that the deformation in the shear zones probably began around 570–600 Ma and continued under decreasing temperature to around 500 Ma. The Borborema shear zone system is subdivided in two domains, a western domain in which rectilinear NE-trending dextral strike-slip shear zones dominate, and an eastern domain characterized by sinuous, discontinuous EW-trending shear zones that terminate in NE-trending metasedimentary belts. The sinuous pattern of the EW-trending shear zones may be due to pre-existing lithospheric heterogeneities: basins or domains where crustal accretion occurred at different ages. Finally, it is suggested that the Borborema shear zone system developed within a heterogeneous continental plate to accommodate the deformation imposed by plate tectonic processes (oblique collision?) active at the margin.

Seismic anisotropy in the eastern United States: Deep structure of a complex continental plate

We have analyzed shear wave splitting recorded by portable and permanent broadband and long-period stations located in the eastern United States. Teleseismic shear waves (SKS, SKKS, and PKS) were used to retrieve the splitting parameters: the orientation of the fast wave polarization plane ϕ and the delay time δt. In total, 120 seismic events were processed, allowing for more than 600 splitting measurements. Within the Appalachians, stations located in the western (external) part are characterized by δt≈1s and ϕ trending N50°– 70°E in the south and central regions and N30°–40°E in the north, closely following the trend of the orogenic belt in these areas. The transition region between north and central is characterized by δt≈1–1.3 s and by E-W trending ϕ that are at a high angle to the regional geologic trend. Measurements at two stations located in the eastern (internal) part of the belt indicate very weak anisotropy. The large-scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate. Splitting along the southern and eastern margins of the continent is consistent with that expected for Grenvillian deformation, an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out. Within the cratonic core, the correlation between δt and lithospheric thickness suggests a lithospheric anisotropy. Smaller-length-scale variations also argue for a significant contribution of lithospheric structures. The fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern United States, i.e., the Grenville and Appalachian orogenies and the subsequent rifting of the North Atlantic Ocean. Our observations in the western Appalachians suggest that the anisotropy may be preserved since the Grenvillian orogeny. The absence of detectable splitting in the two stations in the eastern Appalachians is attributed to the igneous intrusions related to the Atlantic rifting. The measurements in the transition between the northern and central southern Appalachians, constitute an intriguing anomaly, whose E-W ϕ have little obvious relation to the regional surface geology. We suggest two possible causes: (1) the local dominance of asthenospheric flow, motivated by the proximity of a pervasive low-velocity anomaly and (2) lithospheric deformation in a transcontinental strike-slip fault zone active during the Appalachian collision.

A simple parameterization of strain localization in the ductile regime due to grain size reduction: A case study for olivine

We propose a simple parameterization of the transition between dislocation creep and grain-size-sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine-rich rocks. Through numerical modeling and linear stability analysis, we determine the conditions under which this transition takes place and potentially leads to strain localization. We pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas. We conclude that at constant temperature, strain localization can only take place if the rheology of the material is nonlinearly related to grain size; that strain localization is facilitated by syndeformation cooling; that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place; and that grain growth inhibits strain localization at fast cooling rates but may lead to “grain growth localization” at low cooling rates. We draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the Earths surface.

Magma-assisted strain localization in an orogen-parallel transcurrent shear zone of southern Brazil

In a lithospheric-scale, orogen-parallel transcurrent shear zone of the Pan-African Dom Feliciano belt of southern Brazil, two successive generations of magmas, an early calc-alkaline and a late peraluminous, have been emplaced during deformation. Microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions. Magmatic deformation is indicated by the coexistence of aligned K-feldspar, plagioclase, micas, and/or tourmaline with undeformed quartz. Submagmatic deformation is characterized by strain features, such as fractures, lattice bending, or replacement reactions affecting only the early crystallized phases. High-temperature solid-state deformation is characterized by extensive grain boundary migration in quartz, myrmekitic K-feldspar replacement, and dynamic recrystallization of both K-feldspar and plagioclase. Decreasing temperature during solid-state deformation is inferred from changes in quartz crystallographic fabrics, decrease in grain size of recrystallized feldspars, and lower Ti amount in recrystallized biotites. Final low-temperature deformation is characterized by feldspar replacement by micas. The geochemical evolution of the synkinematic magmatism, from calc-alkaline metaluminous granodiorites with intermediate 87Sr/86Sr initial ratio to peraluminous granites with very high 87Sr/86Sr initial ratio, suggests an early lower crustal source or a mixed mantle/crustal source, followed by a middle to upper crustal source for the melts. Shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle, but, whatever the process initiating partial melting, lithospheric transcurrent shear zones may collect melt at different depths. Because they enhance the vertical permeability of the crust, these zones may then act as heat conductors (by advection), promoting an upward propagation of partial melting in the crust. Synkinematic granitoids localize most, if not all, deformation in the studied shear zone. The regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies, consistently displaying similar plane and direction of flow, argue for accommodation of large amounts of orogen-parallel movement by viscous deformation of these magmas. Moreover, activation of high-temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and, consequently, contributed significantly to the localization of deformation. Finally, the small extent of the low-temperature deformation suggests that the strike-slip deformation ended approximately synchronously with the final cooling of the peraluminous granites. The evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone. Magma intrusion in an orogen-scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust. It triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated. Therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments.

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

Abstract This paper aims to present an overview on the influence of rheological heterogeneity and mechanical anisotropy on the deformation of continents. After briefly recapping the concept of rheological stratification of the lithosphere, we discuss two specific issues: (1) as supported by a growing body of geophysical and geological observations, crust/mantle mechanical coupling is usually efficient, especially beneath major transcurrent faults which probably crosscut the lithosphere and root within the sublithospheric mantle; and (2) in most geodynamic environments, mechanical properties of the mantle govern the tectonic behaviour of the lithosphere. Lateral rheological heterogeneity of the continental lithosphere may result from various sources, with variations in geothermal gradient being the principal one. The oldest domains of continents, the cratonic nuclei, are characterized by a relatively cold, thick, and consequently stiff lithosphere. On the other hand, rifting may also modify the thermal structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a stiff or a weak heterogeneity may develop. Observations from rift domains suggest that rifting usually results in a larger thinning of the lithospheric mantle than of the crust, and therefore tends to generate a weak heterogeneity. Numerical models show that during continental collision, the presence of both stiff and weak rheological heterogeneities significantly influences the large-scale deformation of the continental lithosphere. They especially favour the development of lithospheric-scale strike-slip faults, which allow strain to be transferred between the heterogeneities. An heterogeneous strain partition occurs: cratons largely escape deformation, and strain tends to localize within or at the boundary of the rift basins provided compressional deformation starts before the thermal heterogeneity induced by rifting are compensated. Seismic and electrical conductivity anisotropies consistently point towards the existence of a coherent fabric in the lithospheric mantle beneath continental domains. Analysis of naturally deformed peridotites, experimental deformations and numerical simulations suggest that this fabric is developed during orogenic events and subsequently frozen in the lithospheric mantle. Because the mechanical properties of single-crystal olivine are anisotropic, i.e. dependent on the orientation of the applied forces relative to the dominant slip systems, a pervasive fabric frozen in the mantle may induce a significant mechanical anisotropy of the whole lithospheric mantle. It is suggested that this mechanical anisotropy is the source of the so-called tectonic inheritance, i.e. the systematic reactivation of ancient tectonic directions; it may especially explain preferential rift propagation and continental break-up along pre-existing orogenic belts. Thus, the deformation of continents during orogenic events results from a trade-off between tectonic forces applied at plate boundaries, plate geometry, and the intrinsic properties (rheological heterogeneity and mechanical anisotropy) of the continental plates.

EBSD-measured lattice-preferred orientations and seismic properties of eclogites

We investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities (Alps, Norway, Mali and eastern China) through the analysis of their microstructures and lattice-preferred orientations (LPO). These samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions. Omphacite and garnet LPO were determined from electron backscatter diffraction (EBSD) technique. Garnet appears to be almost randomly oriented whereas omphacite develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the (010)-poles concentrated sub-perpendicular to the foliation. In order to analyze the deformation mechanisms that produced such omphacite LPO, we compare our observations to LPO simulated by viscoplastic self-consistent numerical models. A good fit to the measured LPO is obtained for models in which the dominant slip systems are 1/2h110i{11 ¯ 0}, [001] {110} and [001] (100). Dominant activation of these slip systems is in agreement with TEM studies of naturally deformed omphacite. Seismic properties of eclogite are calculated by combining the measured LPO and the single crystal elastic constants of omphacite and garnet. Although eclogite seismic anisotropies are very weak (less than 3% for both P-and S-wave), they are generally characterized by a maximum P-wave velocity sub-parallel to the lineation and by a minimum velocity approximately normal to foliation. The mean P-and S-wave velocities are high (respectively, 8.6 and 4.9 km/s). The S-wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation. Calculated reflection coefficients show that an eclogite/crust interface is generally a good reflector (Rc > 0.1), whereas an eclogite body embedded in the upper mantle would be hardly detectable.

Successive: mixing and mingling of magmas in a plutonic complex of Northeast Brazil

Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300–500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: (1) coarse-grained granite; (2) porphyritic granite and (3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite. The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and majorelements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiO2 in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr. The diorites contain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Abundant enclaves of diorite in the porphyritic granite, despite their textural diversity, are typically devoid of chilled margins, and were therefore formed relatively early in the crystallisation history of the granite. They are interpreted as liquid droplets separated from the heterogeneous hybrid magma through convection currents and incorporated in the, crystallising granitic magma. Subsequently, during the crystallisation of the porphyritic granite, mafic magma supply to the batholith continued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of stromatitic-like structures, with alternating bands of mutually contaminated granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled finer-grained margins.

The Pombal granite pluton: Magnetic fabric, emplacement and relationships with the Brasiliano strike-slip setting of NE Brazil (Paraiba State)

Abstract The Pombal pluton (500 km 2 ), a suite of diorite, syenite and porphyritic granite bodies, is here used to constrain kinematics of Brasiliano-age tectonic episodes in northeast Brazil. The pluton intrudes high-grade to migmatitic gneiss forming the western basement of the Serido belt, and is located at the intersection between two sets of continental-scale dextral strike-slip shear zones. The northern set of shear zone strikes NE-SW and branches, southwards, into the E-W Patos mega-shear zone. A detailed microstructural and low-field magnetic susceptibility study was performed to unravel the relationships between solid-state deformation in the country rocks and magma emplacement. Porphyritic granite and syenite have quite high magnetic susceptibilities (10 −3 –10 −2 SI units) indicative of magnetite as the principal carrier of susceptibility. The magnetic fabric is remarkably homogeneous in orientation throughout the pluton. It is characterized by a shape-preferred alignment of magnetite, itself parallel to the shape fabric of mainly biotite (±amphibole), i.e. to the magmatic fabric. Even close to the contact with the high-temperature mylonites of the Patos shear zone, south of Pombal, no imprint of the E-W-trending structures is observed in the fabrics of either the granite or the host rocks. Granite emplacement and its internal fabric development is concluded to be independent of the movement of the Patos shear zone. In the southwestern border of the pluton, a low-dip foliation bearing a NE-SW-striking lineation is shared in both the magmatic fabric of the pluton and the solid-state fabric. Farther to the north, approaching the NE-SW strike-slip shear zone, the magmatic fabric is characterized by a steeply dipping NE-striking foliation carrying a subhorizontal lineation. Transition from low to steep dips of the planar fabrics is progressive. Two models are proposed for emplacement of the Pombal pluton. One considers magma injection during an early episode of tangential tectonics, responsible for the gently dipping foliations, evolving later to strike-slip deformation. The other model considers that the pluton was emplaced in a pull-apart domain developed in the overlapping sector of a right-hand en echelon system of a dextral shear zone. Compatibility of these models with the tectonic evolution of the Serido belt is discussed.