Marcos Egydio-Silva

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.

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.

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.

Proterozoic rifting and closure, SE border of the São Francisco Craton, Brazil

Abstract This article presents an overview of the present knowledge of the tectonic evolution of the transition zone between the Sao Francisco Craton and the AracuaiFold Belt in Minas Gerais State. Two extensional events with rift formation and sedimentation and one compressional orogenic event were identified. The first extensional event, dated at 1750–1700 Ma, is represented by the N–S Espinhaco basin formation along a continental rift. The lower part of the sedimentary infill (Lower Sequence of the Espinhaco Supergroup) is made up of acid to intermediate metavolcanics and fluvial metasediments, with strong fault-block activity related to the initial rift phase (with mechanical subsidence). The Middle Sequence is composed of quartzite with large-scale cross-bedding and the Upper Sequence consists of an alternation of quartzite and metapelitic rocks deposited in a shallow marine setting related to a subsequent flexural phase (with thermal subsidence). The second extensional phase corresponds to the Aracuairift formation (1000–900 Ma), accompanied by a mafic dike swarm intruded into the Espinhaco Supergroup. On the highest uplifted blocks, continental glaciogenic facies and a glacio-marine sequence (JequitaiFormation) were deposited. To the east, in the Aracuaibasin, they grade into marine deposits with gravitational sedimentation (debris-flows and turbidites) organized in a probably prism of passive continental margins. To the West on the craton, the younger BambuiGroup was deposited directly on the basement. No equivalents of this group were identified in the Aracuaibasin, so either they were not deposited or they were eroded. The compressional orogenic event (650–550 Ma) is characterized by zones with asymmetric folds showing westward vergence separated by thin belts of ductile shearing (Dp phase). An east dipping Sp cleavage with down-dip stretching lineation is consistent with a relative westward tectonic transport, towards the Sao Francisco Craton. Regional Barrovian metamorphism is associated with Dp, increasing from anchizone to amphibolite facies. During this orogenic event the Sao Francisco Craton and the Aracuaiand Paramirim Fold Belts were formed as individual geotectonic units.

High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil

Abstract The Neoproterozoic Ribeira belt is subdivided in two domains with contrasting tectonic characteristics. The northern domain is dominated by shallowly dipping foliations and orogen-normal thrust tectonics. The southern domain is characterized by a 1000-km-long network of anastomosing transcurrent shear zones parallel to the belt. This contrast is interpreted as reflecting continent–continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain. The central, transitional, domain of the Ribeira belt displays the northern termination of the transcurrent shear zone network: the Alem Paraiba–Padua shear zone system (APPSS). The 250-km-long Alem Paraiba–Padua system involves granulites facies mylonites deformed through transpression. A detailed study of the microstructure and lattice preferred orientation (LPO) of the rock-forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust. Plagioclase crystals are plastically deformed; they display curved twins and cleavages, mechanical twins, and evidence of dynamic recrystallization. LPO of plagioclase is consistent with activation of the (010) [100] and (010) [001] slip systems. LPO of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the (100) [001] slip system. Quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration. The LPO of quartz is therefore the result of a static transformation of an initial, syn-kinematic LPO, and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization.

Quartz plastic segregation and ribbon development in high-grade striped gneisses

Quartz microstructures and c-axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high-grade Alem Paraiba shear zone, in southeastern Brazil, are documented. Cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process, where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons. These ribbons are separated by feldspar-rich domains devoid of quartz. The stage at which individual, stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode, which is reflected by an abrupt increase in the smoothness of the ribbon boundaries. This change is interpreted to represent a strain-softening kink in the stress-strain-time path. Progressive ribboning is accompanied by strengthening of the c-axis fabric Z-maximum, indicative of continued plastic flow by basal glide. Operation of basal glide at these high-temperature conditions (680–700°C) is interpreted to be a consequence of relatively dry deformation conditions. A model is then proposed for development of straight quartz ribbons in high-grade striped gneisses, where scattered quartz grains are continuously stretched and segregated by crystal–plastic processes. The small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement. Pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons.

Titanohematite lattice-preferred orientation and magnetic anisotropy in high-temperature mylonites

Magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km-wide strike-slip shear zone in the Ribeira Belt (southeastern Brazil). In these mylonites, a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite, amphibole and orthopyroxene crystals defines a mineral stretching lineation. Magnetic fabric deduced from anisotropy of magnetic susceptibility (AMS) measurements and tectonic fabric compare favorably. Rock-magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of AMS. The anisotropy of isothermal remanent magnetization, which is due to the shape-preferred orientation of magnetite grains, is coaxial with AMS. Lattice-preferred orientation (LPO) measurements using the electron backscattered diffraction technique show that orthopyroxene, amphibole, biotite and titanohematite have a strong LPO tightly related to the tectonic fabric. Among these four minerals, titanohematite is the only mineral present in relatively large proportions (s 1.5%) in all studied samples. Titanohematite LPO is characterized by a strong concentration of (0001) poles (c-axes) sub-perpendicular to the foliation and by a distribution of the poles of the (2110) and (1010) prism planes within the foliation with a maximum close to the lineation. This characteristic LPO is interpreted as resulting from dislocation creep during the mylonitization. Magnetic fabrics and titanohematite LPO fit well: the axis of minimum susceptibility is aligned with the c-axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes. Titanohematite LPO may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics. fl 2002 Elsevier Science B.V. All rights reserved.

Shear wave splitting in SE Brazil: an effect of active or fossil upper mantle flow, or both?☆

We investigated the structure of the upper mantle beneath southeastern Brazil using teleseismic shear wave splitting measurements. Measurements were performed on seismic data recorded in the Ribeira and Brasilia Neoproterozoic belts, which wrap around the southern termination of the Sa ‹o Francisco craton and disappear westward under the Paranabasin. In the northern Ribeira belt, dominated by thrust tectonics, the fast shear wave polarization planes trend on average N080‡E, whereas in the central domain, dominated by strike-slip tectonics, fast shear waves are polarized parallel to the structural trend (N065‡E). Stations located above the main transcurrent fault display large delay times (s 2.5 s). Such values, among the largest in the world, require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. Within the southern Brasilia belt, fast split shear waves are polarized parallel to the structural trend of the belt, at a high angle from the APM. Although part of our data set strongly favors an origin of anisotropy related to a fabric frozen in the lithospheric mantle since the Neoproterozoic, a contribution of the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. > 2003 Elsevier Science B.V. All rights reserved.

New polygonal grains formed by dissolution-redeposition in quartz mylonite

Abstract Arrays of polygonal grains are common along original grain margins, subgrain boundaries and intragranular fractures of deformed quartz porphyroclasts. We investigated these internal polygonal grains within quartz porphyroclasts of a partially recrystallized quartz mylonite deformed in the presence of fluid in a greenschist facies shear zone from Quadrilatero Ferrifero, southeastern Brazil. Optical observation showed profuse fluid inclusions and muscovite flakes inside the deformed porphyroclasts. Observation by SEM revealed the presence of crystal faces on the internal polygonal grains. Domainal c-axis analysis showed that the internal polygonal grains have a preferred orientation similar to the host porphyroclasts (c-axes at low angles to the stretching lineation), but different from the fabric of recrystallized matrix grains (type-1 cross girdle). Based on these observations, we suggest a mechanism of in situ solution-reprecipitation, where the new grains are nucleated in the intergranular fluid-filled pores present along the internal discontinuities of the porphyroclasts. The grains grow with their boundaries largely in contact with fluid, allowing the development of crystal faces, at least during the early growth stages. After the new grain reaches the opposite pore wall, further grain growth should occur through ordinary grain boundary migration mechanisms driven by the strain energy difference between the new grain and the host porphyroclast. We conclude that in these rocks, in situ solution-reprecipitation operated in conjunction with crystal-plastic processes, accounting for the nucleation of new grains during deformation/recrystallization in the presence of fluid under low grade metamorphic conditions.

Determination of stress directions from plagioclase fabrics in high grade deformed rocks (Alem Paraiba shear zone, Ribeira fold belt, southeastern Brazil)

Abstract A new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high-temperature mylonitic rocks from the Alem Paraiba shear zone, Ribeira fold belt, southeastern Brazil. We have measured the lattice-preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations. The maximum compressive stress direction ( σ 1 ), determined for all studied samples, is a function of the mechanical twin orientations of a number of distinct plagioclase populations. The σ 1 direction is generally subperpendicular to the (010) plane. The statistical treatment for most of the plagioclase grains examined for each sample shows that σ 1 is almost perpendicular to the foliation plane, suggesting a significant coaxial component in the deformation process of these rocks.