Andréa Tommasi

Viscoplastic self-consistent and equilibrium-based modeling of olivine lattice preferred orientations : Implications for the upper mantle seismic anisotropy

Anisotropy of upper mantle physical properties results from lattice preferred orientation (LPO) of upper mantle minerals, in particular olivine. We use an anisotropic viscoplastic self-consistent (VPSC) and an equilibrium-based model to simulate the development of olivine LPO and, hence, of seismic anisotropy during deformation. Comparison of model predictions with olivine LPO of naturally and experimentally deformed peridotites shows that the best fit is obtained for VPSC models with relaxed strain compatibility. Slight differences between modeled and measured LPO may be ascribed to activation of dynamic recrystallization during experimental and natural deformation. In simple shear, for instance, experimental results suggest that dynamic re-crystallization results in further reorientation of the LPO leading to parallelism between the main (010)[100] slip system and the macroscopic shear. Thus modeled simple shear LPOs are slightly misoriented relative to LPOs measured in natural and experimentally sheared peridotites. This misorientation is higher for equilibrium-based models. Yet seismic properties calculated using LPO simulated using either anisotropic VPSC or equilibrium-based models are similar to those of naturally deformed peridotites; errors in the prediction of the polarization direction of the fast S wave and of the fast propagation direction for P waves are usually <15°. Moreover, overestimation of LPO intensities in equilibrium-based and VPSC simulations at high strains does not affect seismic anisotropy estimates, because these latter are weakly dependent on the LPO intensity once a distinct LPO pattern has been developed. Thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow. Two notes of caution have nevertheless to be observed in using these results: (1) the dilution effect of other upper mantle mineral phases, in particular enstatite, has to be taken into account in quantitative predictions of upper mantle seismic anisotropy, and (2) LPO patterns from a few naturally deformed peridotites cannot be reproduced in simulations. These abnormal LPOs represent a small percent of the measured natural LPOs, but the present sampling may not be representative of their abundance in the Earths upper mantle.

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.

Forward modeling of the development of seismic anisotropy in the upper mantle

Development of seismic anisotropy in response to upper mantle flow is approached through an integrated numerical model. This model allows to predict the splitting parameters for a shear wave propagating across an upper mantle which deformed in response to a given geodynamic process. It consists of (1) thermo-mechanical modeling of the finite strain field, (2) modeling olivine lattice-preferred orientation (LPO) generated by this strain field, (3) calculation of the 3-D elastic properties associated with this LPO, and (4) estimation of the shear-wave splitting parameters: the time lag between the fast and slow split shear wave arrivals .dt/ and the polarization azimuth of the fast wave ./. Modeled olivine LPO are constrained relative to LPO measured in naturally and experimentally deformed peridotites. Comparison of predicted shear-wave splitting parameters with seismological data allows us to quantify the possible contribution of the modeled upper mantle flow to the measured splitting values and, hence, to constrain the interpretation of shear-wave splitting data in terms of upper mantle flow. We use this forward model to investigate the seismic anisotropy generated in ocean basins by a velocity gradient between the plate and the deep mantle. Fast-shear wave polarizations calculated assuming a constant plate motion are in good agreement with both the SKS polarization and the fast propagation direction for P and Rayleigh waves observed in the Pacific and Indian oceans, suggesting that, away from mid-ocean ridges, seismic anisotropy in oceanic basins primarily results from asthenospheric deformation by resistive drag beneath the plate. Delay times are, however, overestimated. This may be ascribed to a stronger strain localization in nature or to partial erosion of the anisotropic layer by hotspots. Indeed, hotspot activity may explain the short length scale variations of dt in the southern Pacific. Finally, twolayer models that simulate a change in Pacific plate motion as suggested by the bend in the Hawaii‐Emperor chain fail to reproduce the observed shear-wave splitting. This is consistent with previous suggestions that the Emperor chain track may not faithfully record the Pacific plate absolute motion before 43 Ma.

Deformation patterns in the southern Brazilian branch of the Dom Feliciano Belt: A reappraisal

Abstract Structural mapping of key areas linked by reconnaissance investigations in the southern Brazilian portion of the Brasiliano/Pan-African Dom Feliciano Belt has shown the importance of large-scale flat-lying and strike-slip shear zones in the tectonic evolution of this belt. These zones of intense deformation form km-thick mylonitic belts in Brasiliano granites and supracrustal rocks as well as their Transamazonian/Eburnian basement. Structures developed within flat-lying shear zones under metamorphic conditions of amphibolite facies are interpreted as having formed during oblique collision between crustal blocks — the Kalahari Craton and the Magmatic Arc I — in which the direction of tectonic transport was NW-SE. Further collision between this assemblage and the Rio de La Plata Craton gave rise to a transpressive regime whose tectonic transport direction is indicated by regionally consistent NE/SW-oriented stretching lineations, parallel to the belts length.

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.

Plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites

Abstract We use an anisotropic viscoplastic self-consistent (VPSC) model to simulate the development of omphacite lattice preferred orientations (LPOs) in response to deformation by dislocation glide. In these simulations, we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside. Simulated LPOs reproduce very well the characteristic omphacite LPO pattern in naturally deformed eclogites: a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles sub-perpendicular to the foliation. These models reconcile the interpretation of omphacite LPOs in eclogites and TEM observations of naturally deformed omphacite, since they show that omphacite LPOs in naturally deformed eclogites may develop by dislocation glide on 1/2〈110〉{110}, [001]{110} and [001](100). We also investigate the effect of the strain regime on the omphacite LPO development. The simulations show that changes in deformation regime lead to second-order variations in omphacite LPO patterns similar to those observed in eclogites, such as an asymmetry of the LPO relative to the structural frame, a stronger concentration of the [001]-axes relative to the (010)-pole concentration, or a dispersion of [001]-axes in the foliation plane. This suggests that omphacite LPO patterns may carry information on the deformation regime (simple shear, transtension…) active during the high-pressure events.

Self-indentation of a heterogeneous continental lithosphere

The Neoproterozoic Ribeira-Aracuai belt stretches along the southeastern edge of the Sao Francisco craton of southeastern Brazil and extends south of the cratonic domain for >1000 km. The termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt: (1) the structural trend bends from due north to northeast, (2) the dominant tectonic flow shifts from orogen-transverse to orogen-normal, and (3) the metamorphic conditions of deformation decrease southwestward from high to medium grade. Finite-element modeling suggests that the presence of a craton within a continent favors strain localization, initiation of continental-scale shear zones, and differential vertical deformations. The southward termination of the Sao Francisco craton may have triggered the development of the complex deformation pattern that characterizes the Ribeira-Aracuai belt.

Numerical simulations of depth‐dependent anisotropy and frequency‐dependent wave propagation effects

A numerical investigation of the effects of shear wave splitting for vertical propagation in a smoothly varying anisotropic medium is presented. Through forward modeling, we predict the olivine lattice preferred orientation (LPO) developed in the oceanic upper mantle in response to the absolute plate motion (APM). We consider the effect of a change in APM similar to the one that presumably caused the kink in the Emperor-Hawaii seamount island chain in the north Pacific. This results in an oblique orientation between lithospheric and asthenospheric anisotropy. Numerical simulations of shear wave propagation are used to estimate the characteristics of shear-wave splitting. Ray theory does not account for coupling between shear waves in the depth-dependent anisotropic medium due to the implicit assumption of high frequency. A forward propagator technique for calculating waveforms and splitting parameters is used to assess frequency-dependent effects. The results show that ray theory is valid for estimating the splitting only for frequencies above 1 Hz. At frequencies more realistic for SKS propagation, apparent splitting parameters exhibit a π/2 dependence on the incoming shear wave polarization (back azimuth). For certain back azimuth ranges, shear wave splitting is very frequency dependent with apparent delay times ranging from 1 to 4 s and apparent fast polarization directions changing rapidly by up to 80°. Thus stacking of shear wave splitting measurements for largely different initial polarizations and frequencies should be avoided. Depth-dependent anisotropy implies that shear wave splitting analyses will be sensitive to filtering. Anisotropic depth variations cannot be resolved unambiguously from splitting observations at relatively long periods (>5 s). It is not possible, for instance, to discriminate between smooth and abrupt transitions separating the anisotropic regions. Shorter-period waveforms provide further information on the fine structure of anisotropic depth variations. A comparison between splitting calculations and observations from Hawaii suggests a divergent past APM direction or may indicate an alternative mechanism responsible for the lithospheric anisotropy.