Shear structures in the Serra do Azeite shear zone, southeastern Brazil: Transtensional deformation during regional transpression in the central Mantiqueira province (Ribeira belt)

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Journal of South American Earth Sciences 23 (2007) 176–192

Shear structures in the Serra do Azeite shear zone, southeastern Brazil:Transtensional deformation during regional transpression in the central

Mantiqueira province (Ribeira belt)

Nolan Maia Dehler a,*, Romulo Machado b, Elvo Fassbinder c

a PETROBRAS – E & P EXP/GEO/GEAT, Av. Republica do Chile, 65, Rio de Janeiro – RJ, 13� andar, CEP 20031-912, Brazilb Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociencias da Universidade de Sao Paulo, Rua do Lago 562,

Cidade Universitaria, Sao Paulo – SP, CEP 05508-080, Brazilc Departamento de Geologia, Centro Politecnico, Universidade Federal do Parana, Curitiba – PR, CEP 81531-900, Brazil

Received 1 December 2003; accepted 1 June 2006

Abstract

The Serra do Azeite shear zone (SASZ) is a northeast-trending regional structure in southeastern Brazil. Kinematic analysis carriedout in the SASZ suggests a ductile sinistral transtensional regime in amphibolite facies conditions. Constrictional, flattened, and simple-shear strain domains occur in mylonites with stretching lineation plunging to the east–southeast. Kinematic indicators suggest obliquetop–down-to-the-east–southeast and sinistral strike-slip components along variably oriented shear planes. Results of the simple geomet-rical construction applied to the shear-zone pattern, coupled with field data and kinematic analysis, suggest that sinistral transtensionalshearing resulted from east–northeast-directed crustal extension and sinistral strike-slip displacement, accompanied by north–northeast/south–southwest contraction and vertical thinning. The K/Ar and Ar/Ar cooling ages match the proposed interval for crustal extensionin the central Mantiqueira province (0.6–0.58/0.57 Ga) based on ages of alkaline granitoids and volcanic rocks. These data indicatetypes-I and -S granite magmatism, as well as metamorphism and dextral transpressional deformation along the Ribeira belt. Therefore,we interpret the transtensional regime as a result of southwest-directed lateral extrusion and uplift crustal slices (overall oblique extru-sion) during an orogenic-scale partitioned transpressional regime. Our results suggest this regime was coeval with a phase of regionalstretching subparallel to the Ribeira belt, which would explain the coexistence of extensional and compressional structures during overallplate convergence.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Transtension; Transpression; Extrusion; Uplift

1. Introduction

Extensional strains tend to have a greater effect on thethicker and hotter portions of the continental lithosphere,and extensional shearing is therefore common in orogenicbelts (England, 1983; Glazner and Bartley, 1985; Molnarand Lyon-Caen, 1988; Teyssier and Vanderhaeghe, 2001).Because continental crust is weaker than mantle material

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doi:10.1016/j.jsames.2006.09.017

* Corresponding author. Tel.: +55 11 3258 4744; fax: +55 11 3256 8430.E-mail addresses: [email protected] (N.M. Dehler), rmachado

@usp.br (R. Machado), [email protected] (E. Fassbinder).

(Brace and Kohlstedt, 1980), the lithospheric column isweaker where a high proportion of continental crust exists(Kuznir and Park, 1987). Consequently, areas of shorten-ing and thickening of the continental crust, which areexpected in subduction and collision environments, are nat-ural sites of extensional deformation of the continental lith-osphere (Dewey, 1988). In regions of overthickenedcontinental crust, buoyancy forces cause uplift and lateralspreading, particularly for the upper continental crust(Artyushkov, 1973; Molnar and Chen, 1983; Royden andBurchfield, 1987). Authors have proposed that supplantingof the colder, shorter lithospheric mantle by hot astheno-

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N.M. Dehler et al. / Journal of South American Earth Sciences 23 (2007) 176–192 177

spheric material is responsible for the large-scale uplift andaccumulation of potential energy in orogenic belts (Eng-land and Houseman, 1989; Platt and Vissers, 1989; Ander-sen et al., 1991). The driving forces of extension are eitherbody forces of unstable, thick, continental lithosphere orexternal forces linked to plate dynamics during plate con-vergence deformation (Lister et al., 1984; Norton, 1986).The extension of thick, shortened crust is known generical-ly as the extensional collapse of orogens (Dewey, 1988).

Although extensional deformation in orogenic systemsmay be largely late- to postcollision events (Coney andHarms, 1984; McClay et al., 1986; Malavieille, 1993), thekinematics and temporal relationship to compressivetectonics varies depending on the external boundaryconditions of the orogenic deformation (Molnar andLyon-Caen, 1988; Doglioni, 1996). In many orogenic belts,extensional deformation, described as genetically linked topop-up/lateral extrusion tectonics (Royden and Burchfield,1987; Ratschbacher et al., 1991a,b; Mancktelow, 1992),and strike-slip faulting (England and Houseman, 1985;Pecher, 1991) may be coeval with regional shortening andexhumation (Ave Lallemant and Guth, 1990; Harz et al.,2001). In some orogenic belts, lateral movements andextrusion processes tentatively have been associated withgravitational instabilities during plate convergence(Ratschbacher et al., 1991b; Avigad et al., 2001; Braathenet al., 2002).

Heilbron et al. (1995), Cunningham et al. (1996), Nalini(1997), Endo (1997), Dehler et al. (1999, 2000), Machadoet al. (2001), and Alkmin et al. (2002) describe extensionalshear zones in the Neoproterozoic mobile belts of the cen-tral and northern portions of the Mantiqueira province insoutheastern Brazil (Almeida and Hasui, 1984). Althoughregional references regarding an extensional setting in theprovince can be found in reviews by Pedrosa-Soares andWiedemann-Leonardos (2000) and Trouw et al. (2000), todate, no detailed characterization or kinematic analysis ofthese structures exist. Geodynamic models suggest a stageof orogenic collapse largely postdating regional compres-sive strain. In the southern segment of the province, indi-rect evidence indicates that an extensional settingoccurred at 0.61–0.57 Ga (Siga Jr. et al., 1999; Baseiet al., 2000). In the central and northern parts, the exten-sional setting would have occurred at 0.52–0.48 Ma accord-ing to Trouw et al. (2000) and Heilbron et al. (2004).

This article deals with the shear-related structures andflow kinematics in the transtensional Serra do Azeite shearzone (SASZ) and the strain regime and flow type in theSASZ. To determine the orientation of the principal axisof strain of the incremental strain ellipsoid, we use the geo-metrically simple construction proposed by McCoss (1986).We analyze the results in relation to the kinematics of finitestructures observed in the field and discuss the structuraldata in view of geochronological SASZ data, as well asdata obtained from models of extensional deformation inorogenic belts, with particular emphasis on the spatialand temporal relationship between extensional and com-

pressional strain. Finally, we propose a model in whichtranstensional deformation in the SASZ may be accompa-nied by extrusion processes, coeval with oblique regionalconvergence in the central Mantiqueira province.

2. Regional geological setting

The Neoproterozoic northeast-trending Ribeira andnorth-trending Aracuaı belts (Almeida et al., 1973) arelocated, respectively, in the central and northern regionsof Mantiqueira province (Almeida and Hasui, 1984)(Fig. 1A). In the southwestern portion of the centralregion, the Ribeira belt is known as the Apiaı belt (APB)(Hasui et al., 1975). Southward of the APB, medium- tohigh-grade basement rocks occur, grouped either in aunique tectonic unit, which Kaul (1979, 1980) dubs theLuis Alves Craton, or into two major tectonic units (Heil-bron et al., 2004): the Luis Alves and Curitiba terranes (seealso Basei et al., 1992, 1998, 2000). However, semidetailedgeological mapping has created a dispute regarding theproposed subdivision (Vasconcelos et al., 1999). Therefore,we consider all lithostratigraphic units south of the APB aunique tectonic unit called the Luis Alves Terrane (follow-ing Kaul, 1980; Heilbron et al., 2004) (Fig. 1B). The SASZis a northeast-trending shear zone located in the Luis AlvesTerrane near the southern edge of the APB (Fig. 1C).

The APB is composed of low- to medium-grade metase-diments (marble, metarenite, and metapelite) and metavol-canics of the Acungui Group (Marini et al., 1967), intrudedby batholiths (ranging from granodioritic to granitic) andsmaller plutons. The calc-alkaline granitic batholiths in thisdomain have been formed by melting of an older crust at0.61–0.57 Ga (Guimenez Filho et al., 2000; Janasi et al.,2001). The emplacement of these linear batholiths occurredduring an overall convergent deformation (Basei et al.,2000). However, Hackspacher et al. (2000) suggest thatthe deposition of the Acungui group occurred in the inter-val between 0.62–0.61 and 0.60 Ga, contradicting the agesattributed to convergence-related magmatism (see alsoBasei et al., 1992). Consequently, tectonic evolution ofthe Acungui Basin is controversial and beyond the scopeof this article. Syntheses of Acungui stratigraphy, thenomenclature of units, and controversial evolutionarymodels appear in Fiori (1990), Fassbinder (1996), andCampanha and Sadowski (1999), among many others.

Many researchers have focused their attention on thestructure of the APB in this region (Hasui, 1986; Campa-nha, 1991; Silva et al., 1998; Yamato, 1999). Some authorssuggest that northwest–southeast shortening, with top-to-the-SE-verging thrusts, predates the regional, dextral trans-pressional strike-slip deformation (Campanha, 1991; Fiori,1992; Campanha and Sadowski, 2002). Other models envis-age dextral transpression as an explanation for regional,low- and high-angle contractional shear zones array (Ebertand Hasui, 1988; Fassbinder, 1996). The transpressionalmodel has been discussed in relation to other segments ofthe belt (Ebert et al., 1993; Machado and Endo, 1993;

Fig. 1. (A) Central and northern Mantiqueira province in southeastern Brazil. (B) Main lithotectonic units (Apiaı belt and Luis Alves Terrane) in southernRibeira belt (modified from Basei et al., 2000). (C) Main lithostratigraphic units, the regional context of the SASZ in the Luis Alves Terrane, and thestudied area (modified from Campanha, 1991).

178 N.M. Dehler et al. / Journal of South American Earth Sciences 23 (2007) 176–192

Ebert and Hasui, 1998; Dehler, 2002). According to themodel, east–west regional convergence (Wernick et al.,1981; Hasui et al., 1990) might explain the regional strainpartitioning in the APB, which presents contraction at ahigh angle to the belt (thrust movements and folds) andlarge-scale dextral strike-slip motion parallel to the orogen-ic trend during oblique collision (Ebert and Hasui, 1998).

The Luis Alves Terrane is primarily composed of medi-um- to high-grade basement rocks located south of theAPB (Fig. 1B). In this terrane (also known as a microplate;Basei et al., 2000), medium- to low-grade supracrustalrocks overlie high-grade basement rocks (mostly ortho-granulite with some kinzigite) (Basei et al., 1998). Nearthe border with the APB exists an area of banded and mig-


matitic orthogneisses and deformed granitoids known asthe Atuba Complex (Siga Jr., 1995). In the complex, isolat-ed charno-enderbitic nuclei of granulitic rocks also occur,occasionally accompanied by kinzigites and migmatiticorthogneisses. The U–Pb ages of the granulitic rocks areArquean and Paleoproterozoic, concentrated at 2.1 Gaand varying to 1.8 Ga, reworked during Brasiliano/Panaf-rican times (Picanco et al., 1998). The SHRIMP and Pb/Pbzircon data also suggest Arquean ages (Kaulfuss, 2001).Paragneisses, schist, and marble in amphibolite faciesmetamorphism also occur (Turvo-Cajati sequence; Silvaand Algarte, 1981a,b). Lower-grade supracrustals (phyl-lites, quartzites, and dolomitic marbles) make up thedeformed platformal Capiru Formation. In the zone ofcontact with the basement orthogneisses and higher-grademetasediments, transcurrent and oblique thrust faultsoccur (Yamato, 1999).

The available geochronological and geological datarelated to the southern tip of the APB have been interpret-ed in two ways: (1) the Neoproterozoic data would reflectreworking of the plate edge during the Brasiliano/Panafri-can orogeny (Kaul, 1980; Siga Jr., 1995) or (2) the Neopro-terozoic data relate to the subduction and collision of threedifferent plates. In the former model, the Curitiba Terranewould be the reworked plate edge formed during the colli-sion of the Luis Alves Terrane with the APB. In the latter,the Luis Alves Terrane would be primarily composed ofgranulites and orthogneisses of Arquean–Paleoproterozoicage, preserved by Neoproterozoic tectonothermal activity(Siga Jr. et al., 1990); because it is predominantly Arquean,the Luis Alves Terrane would differ in age from the orthog-neissic rocks in the Curitiba Terrane (Paleoproterozoic)(see Basei et al., 1992, 1998).

The internal structure of the northern tip of the LuisAlves Terrane is poorly understood. Campos Neto (1983)and Campagnoli (1996) suggest that it consists of north-westward ductile thrust-and-fold nappes. Other authorshave suggested regional thrusting with a top-to-the-SEsense of movement (Fiori, 1990, 1994; Campanha, 1991;Campanha and Sadowski, 1999). Vasconcelos et al.(1999) describe the Lancinha-Cubatao shear zone as amajor tectonic boundary. To the north, high-angle dextralshearing in a transpressional regime predominates, whereasto the south, the dip of the foliation progressively decreas-es, and northwest trends (overprinted by northeast-trend-ing sinistral shear zones) exist (Vasconcelos et al., 1999).Dehler et al. (2000) suggest a transtensional tectonic regimein these northeast-trending sinistral shear zones. In this ter-rane, a suite of granitoids with alkaline affinity is accompa-nied by volcanic rocks (Gois, 1995), interpreted as a recordof crustal extension during Neoproterozoic times (0.6–0.57Ga) (Siga Jr. et al., 1999).

3. Local geology

In the Cajati area, a section of south–southeast-dippingmylonitic rocks is exposed. These mylonites contain

orthogneisses, metasediments, and gabbroic protoliths.Concordant with the mylonitic foliation, foliated biotitegranite is also seen (Fig. 2). The orthogneiss is banded withlayers of white to gray rock, ranging from granitic togranodioritic in composition, with dark strips of amphibo-lite and diorite. This planar fabric is extremely well-devel-oped and locally folded. In addition, boudins andtectonic inclusions of amphibolites and meta-ultramaficrocks occur. The orthogneisses are probably correlatedwith the Atuba Complex. The metasediments comprisegarnet–kyanite–sillimanite–biotite–muscovite schist andgneisses, with layers of marble, amphibolites, and calcsili-cate rocks. Migmatitic granite and two-mica granite occurlocally. Silva and Algarte (1981a) name it the Turvo-Cajatisequence. Coarse-grained gabbroic rocks, mylonitic gab-broic rocks, and anorthosites occur at the top of the sectionin the southern portion of the studied area and may be cor-related with the Serra Negra Complex (Vasconcelos et al.,1999). Concordantly deformed granitic plutons also occur.The granitic bodies comprise porphyroclastic garnet–muscovite–biotite granite and monzogranites with micro-granular mafic enclaves (Fig. 2). The contacts betweenthese units are tectonic. A body of alkaline (Guarau massif)granite that postdates local deformation and metamor-phism crops out at the southeastern edge of the studiedarea (Fig. 2). Campagnoli (1996) obtains K–Ar coolingages of 565 ± 39 and 587 ± 21 Ma, respectively, for horn-blende and phlogopite in the mylonitic rocks of the SASZ,as well as a younger age (527 ± 26) for biotite.

4. Structures within the SASZ

Silva and Algarte (1981b), Campos Neto (1983), andCampagnoli (1996) have studied the structure of the SASZ.On the basis of geometric analyses of the structures, theseauthors suggest ductile thrusting with a top-to-the-NWsense of shear. In contrast, Dehler et al. (2000) argue thatprevious interpretations have failed to explain the sinistralsense of shear in horizontal planes described by Vasconce-los et al. (1999) and suggest finite oblique extension in asinistral transtensional tectonic regime. The authorsdescribe a mylonite belt with SSE-dipping foliation andan ESE–SE-plunging stretching lineation (Figs. 3 and4A). The map in Fig. 2 shows that, to the south, the folia-tion steepens and dips to the south–southeast (cross-sectionof SASZ, Fig. 3A). At the footwall, later movements over-print the main mylonitic fabric, whereas structural datashow that in the hangingwall, movements transition per-fectly from medium to higher dips (Figs. 3B and 4B andD), and SC mylonites developed in the gabbroic rocks.

Shear-sense indicators are consistent with oblique top-to-the-east–southeast shearing. The main ductile deforma-tion was partitioned into a horizontal sinistral strike-slipand a top–down-to-the-ESE component; these componentsworked together during the main ductile deformation(Figs. 5 and 6). This interpretation is consistent with the

Fig. 2. Lithostructural map of the SASZ near the northern edge of the Luis Alves Terrane, with locations of the lithostructural cross-sections of the SASZfrom Fig. 3. See text for a full description of structures, vergence, and lithostratigraphic units.

A

B

C

Fig. 3. Lithostructural cross-sections of the SASZ. See section locations in Fig. 2.


A

C D

B

Fig. 4. Stereographic plots (lower hemisphere) of the main structural elements of the SASZ; (A) stretching lineation, (B) poles to mylonitic foliation, (C)axis of intrafolial folds, (D) poles to the mylonitic foliation on the top of the mylonite belt.


sinistral pattern in horizontal planes south of the Cubataoshear zone.

Later structures deforming the ductile fabrics in theSASZ are poorly developed and play a minor role in thereorientation of the previous fabric element. These forma-tions include discrete contractional structures such as brit-tle-ductile thrust faults and (at the outcrop scale) openfolds with cylindrical profiles (Fig. 3C). Such structuresare not discussed herein.

5. Mylonitic foliation and stretching lineation

The SASZ is characterized by well-developed linearstructures, with or without well-developed mylonitic planarfabric (Fig. 5A). The morphology of foliation variesaccording to the rock type. In the schists, metagabbros,and metagranites, a coarse, anastomosing foliation devel-ops. Porphyroclastic textures and SC fabrics are common.In the orthogneisses, mylonitic foliation lies parallel to astrongly stretched compositional banding or lamination,defined by a monotonous sequence of alternating melano-cratic and leucocratic bands. Highly deformed bands andthe stretching lineation are well-developed (SL tectonites).

The leucocratic bands present a fine to medium grain andare granodioritic to granitic in composition. The melano-cratic bands or strips are composed of rocks rich in biotite,hornblende, or both.

Petrographic and microtectonic descriptions suggestthat deformation began to occur in amphibolite facies con-ditions and progressed to greenschist facies, preserving awell-developed mylonitic fabric (Dehler et al., 2000). Inmylonitic metasediments, kyanite is unstable and reactsto the sillimanite that marks the lineation. Thus, deforma-tion began in amphibolite facies conditions, at highertemperatures, and in a clockwise PT path. In the metasedi-mentary sequence, leucosome formation is accompanied bysillimanite crystallization. The presence of kyanite and theaforementioned polymorphic transformation also suggest apreviously shortened crust affected by transtensional defor-mation. The mylonitic foliation contains stretched andfractured garnet porphyroclast. Aligned with the plane offoliation, aggregates of brownish biotite, muscovite,quartz, and sillimanite may occur. Thin sections show evi-dence of intense quartz dynamic recrystallization, withmantle and core texture and deformation bands in the por-phyroclasts (Passchier and Trouw, 1996). In the matrix, the

A B C

D E F

Fig. 5. Shear structures in the SASZ. (A) Strongly developed stretching lineation in orthogneiss mylonite; (B) r-type asymmetric porphyroclasts insubvertical shear zone, suggesting sinistral sense of shear; (C) SC fabrics in metabasic lenses of orthogneisse mylonite, with sinistral sense of shear; (D) SCfabrics in the upper basic unit, suggesting oblique top–down-to-SE sense of shear; (E) section parallel to the YZ plane of finite strain showing ‘‘eye’’structure interpreted as a sheath fold profile in orthogneisse mylonite and opposite asymmetry of folds; (F) folding and shearing of layers in bandedorthogneisse mylonite, with folded and disrupted layering, moved over itself in a low-angle decollement surface. Sense of shear is top–down-to-SE,consistent with another kinematic indicators. All shear-sense indicators are described in sections subparallel to the XZ plane of finite strain.

A B

C D

Fig. 6. Structures indicating heterogeneous stretching in the SASZ. (A) Pinch-and-swell structures in a tectonic inclusion of metabasic rock in theorthogneisse mylonite. Boudins in the same outcrop (see Figs. 5C and D) show evidence of higher finite extension. (B) Asymmetric extensional shear bandsdeveloped in pinch zones of pinch-and-swell structures in orthogneisse mylonite. Sense of shear is top–down-to-SE. (C) Asymmetric boudins in bandedorthogneisse mylonite. Sense of shear is top–down-to-SE. (D) Sigmoidal asymmetric boudins consistent with top–down-to-SE sense of motion. Allkinematic indicators described in sections parallel to the XZ plane of finite strain.


quartz grains present irregular boundaries, polygonization,and oblique foliation.

In orthogneiss mylonite, lineation can be clearly markedby aligned hornblende crystals and recrystallized feldspars(Fig. 4A). Aggregates of biotite and quartz can also markthe strong stretching lineation. Strips and bands of

amphibolite may present evidence of ductile, heteroge-neous stretching (boudinage process). Along the foliationplanes, biotite and plagioclase present some evidence of ret-rogressions to chlorite and epidote, respectively. In graniticmylonite, intense recrystallization of plagioclase, micro-cline, and quartz may occur. In such rock, L > S fabric


appears, and the finite stretching axis is marked by thestrong linear arrangement of feldspar, quartz, and biotite.In the metagabbro unit, brownish to pale yellow horn-blende and plagioclase occur along the foliation planes.At the core of the green hornblende in the mylonitic folia-tion, brownish amphibole can be found. Locally, theamphibole presents evidence of undulose extinction. Epi-dote, chlorite, and quartz can also occur, on either foliationplanes or fracture planes. The mineral assemblage andmicrostructure relationships suggest that ductile deforma-tion probably occurs during cooling of the mylonite belt.

The outcrop structure can also be controlled by localhigh-strain folds, with axes subparallel to the stretching lin-eation and surfaces axial to the mylonitic foliation(Fig. 4C). Locally, the mylonitic foliation can be parallelto crenulation foliation, the geometry of which may beeither extensional or constrictional. A stereographic plotof mylonitic foliation appears in Fig. 4B and D. The direc-tion of foliation varies (preferentially, east–northeast towest–southwest; locally, east–southeast to west–north-west), dipping preferentially to the south–southeast. Thegeometric relationships between the C-foliation planesand the stretching lineation, as well as the shear-sense indi-cators in outcrops with foliation dipping by varyingamounts, remain the same. The attitude of the myloniticfoliation can vary, but the stretching lineation alwayspoints to the east–southeast. Within the mylonitic folia-tion, the absence of late map-scale folds, together withreversals in dip direction, suggest that strain partitioningcould be responsible for this behavior. In every domain,mylonitic foliation is roughly subparallel to the C foliation.

A well-developed stretching lineation plunging east–southeast is characterized by the orientation of feldspar,hornblende, and sillimanite, as well as quartz and micaaggregates (Figs. 4A and 5A). Occasionally, tectonites inwhich L fabrics predominate over S fabrics are found, sug-gesting constrictional strains within these domains. Fig. 5Ashows an outcrop of orthogneiss mylonite with a strongstretching lineation. The geometrical relationship betweenmylonitic foliation and the stretching lineation suggeststhe predominance of oblique movements over strike-slipand dip-slip movements.

Shear-sense indicators, such as asymmetrical porphyro-clasts (Fig. 5B), SC foliations (Fig. 5C and D), boudins(Fig. 6C and D), and extensional shear bands (Fig. 6B),suggest a finite top–down-to-the-east–southeast sense ofshear. In the studied area, two types of surface move-ment-related foliations are observed: asymmetrical exten-sional shear bands (White et al., 1980; Platt and Vissers,1980; Behrmann, 1987) and C foliation in composite fab-rics (Berthe et al., 1979). The shear bands overprint themain mylonitic foliation at a low angle. One shear-bandset with movement synthetic to the bulk sense of shear isclearly observed in close proximity to pinch zones of bou-dins and mica-rich layers (Fig. 6B). The C-foliation planesare subparallel to the main foliation in the observed out-crops (Fig. 5C and D).

Relatively low-strain domains, characterized by folia-tions oblique to the shear-zone walls (S foliations), occurwithin bands in the outcrops observed. The angular rela-tionships between these domains also vary; they are oftenslightly oblique to the C-foliation plane, suggesting highshear strain. This relationship is also observed elsewherein the mylonite belt. Therefore, the assumption that thestretching lineation should be measured on a plane subpar-allel to the shear-zone wall in each domain seems reason-able. In a simple shear deformation model, these planes(C planes) are expected to record a strain gradient orthog-onal to the shear-zone walls (Lister and Snoke, 1984; Han-mer and Passchier, 1991). This relationship may be used toconstrain the shear-zone boundary, following the methodproposed by McCoss (1986), to determine the orientationof the principal axis of the infinitesimal strain ellipsoid.

6. Folds

Folds associated to the main extensional shearing arerare (restricted to some outcrops), tight to isoclinal, andoccasionally rootless. The axial surfaces of these foldsrun parallel to the mylonitic foliation, and their axes liesubparallel to the stretching lineation (Fig. 4C). Locally,such folds are noncylindrical, presenting curved axes, andform sheath folds, with eye structures and double vergencein the ZY section of the finite strain ellipsoid (Fig. 5E). Insome locations, folds with axes oblique to the lineation canbe accompanied by decollement parallel to the axial sur-face. In this situation, the layer can be disrupted and shearover itself with the same sense of motion suggested by othershear sense indicators (Fig. 5F).

The fold data collected are consistent with high finitestrains in the ductile deformation, where sheath foldsand folds with axes parallel to the finite stretching occur.In simple-shear deformation, folds with axes subparallelto the stretching lineation can be interpreted as rotatedfolds, formed at a high angle to and progressively rotatingtoward the shear direction with increasing strain (Bryantand Reed, 1969; Escher and Watterson, 1974), or as obli-que folds, with the axis of the growing structure lying obli-que to the shear direction (Fossen and Rikkelid, 1990).The local presence of sheath folds, interpreted in the liter-ature as a record of high finite strain by amplification ofsmall noncylindrical perturbations on the flow plane in anoncoaxial flow (Cobbold and Quinquis, 1980; Alsop,1992), suggests that, at least locally, rotation of the foldaxis toward the shear direction occurs during mylonitiza-tion. In these sections, the fold axes lying parallel to thestretching lineation can be interpreted as indicators of highfinite shear strain (Escher and Watterson, 1974; Skjernaa,1989).

7. Boudins

In the studied area, boudins and pinch-and-swell struc-tures occur in mylonitic orthogneiss (Fig. 6). The mylonitic


orthogneiss contains layers of amphibolite and amphiboliteinclusions that can show evidence of heterogeneous stretch-ing (Fig. 6A and B). Homogeneous stretched layers canalso be seen, generating the previously described banded/laminated structure. In the mylonites, trails of boudinsand pinch-and-swell structures run parallel to the surfaceof the mylonitic envelope. However, along some trails,the long axes of the inclusions/swell zones are slightlyback-rotated in relation to the envelope surface. Maficinclusions and bands present pinch-and-swell structuresin varying stages of development (Fig. 6A and C). Thisphenomenon appears only in sections parallel to thestretching lineation (XZ plane of finite strain), suggestingplane strain deformation.

The pinch-and-swell structures and boudins are fre-quently accompanied by asymmetrical extensional shearbands in the softened region (pinch and constricted zones)(Fig. 6B). The resulting structures are known as type IIpull-apart (Hanmer, 1986) and asymmetric boudins (Gold-stein, 1988), and the shear sense deduced from its asymme-try is consistent with that obtained using other shear-senseindicators (Fig. 6C and D). In the field, these small shearplanes (shear bands) are synthetic to the overall flow andmay present various dips among neighboring planes inrelation to the mylonitic foliation. This pattern is attribut-ed to a component of coaxial flow and antithetical rotationof planes in the appropriate orientation, such as in theshear bands (Gosh and Ramberg, 1974; Platt and Vissers,1980).

Boudinage and pinch-and-swell structures have beeninterpreted as resulting from normal layer shortening ina layered system with viscous contrasts (Price and Cos-grove, 1990). Some authors argue that general noncoax-ial strains may be present during boudinage (Malavieilleand Lacassin, 1988). In the studied area, certain indica-tors are important to interpreting the flow type in whichthese structures form. Within the same outcrop, pinch-and-swell structures present varying degrees of develop-ment. Boudins, occasionally sigmoidal and stronglystretched, occur side by side with poorly developedpinch-and-swell structures (Fig. 6A and B vs. Fig. 6Cand D). Therefore, we infer that incremental shorteningin this outcrop was at a high angle to the mylonitic foli-ation, at least for the last incremental history of ductiledeformation. These structures are also frequently accom-panied by a set of asymmetrical extensional shear bands(Fig. 6B), the movement on which provides an additionalrecord of the extension parallel to the foliation in a non-coaxial deformation regime (White et al., 1980; Harrisand Cobbold, 1984). The finite structure is markedlyasymmetric, also suggesting noncoaxial shearing (Chou-kroune et al., 1987). We attribute this characteristic togeneral noncoaxial strain during the boudinage process.In addition, because boudinage is not observed in thebanded outcrops of sections parallel to the ZY sectionof the finite strain ellipsoid, we envisage it as planestrain.

8. Qualitative analysis of infinitesimal strain axis orientation

8.1. Method

The method introduced by McCoss (1986) determinesthe orientations of the principal axis of the infinitesimalstrain ellipsoid by means of a simple graphic solution andfacilitates determination of the shape of the ellipsoid, tak-ing into consideration the boundary conditions for trans-pressional deformation suggested by Sanderson andMarchini (1984). Because the assumptions of the modelused herein specify an oblate shape for transpression anda prolate shape for transtension, only the orientation ofthe principal directions of strain are discussed. Our data(and data in the literature) suggest that the shape of thestrain ellipsoid can vary within the same tectonic regimeaccording to the boundary conditions during deformation(see Dias and Ribeiro, 1994).

To make the appropriate calculations, we must knowthe zone boundary and direction of zone boundary dis-placement. In natural, broad shear zones, determining zoneboundaries is problematic because contact between straindomains is frequently gradational and not well marked,as theoretical models note. In addition, in natural shearzones, shear-zone walls may be irregular. For our purpos-es, we consider the orientation of the C-foliation planesrepresentative of the local shear-zone walls (see Listerand Snoke, 1984).

Determining the displacement direction may be equallyproblematic in natural shear zones. Physical markers arefrequently absent, precisely constraining the shear-zonewalls is difficult, and material indicators of the displace-ment direction (e.g., stretching lineation) may be obliqueto the shear direction at low finite strain (Escher and Watt-erson, 1974). We consider the measurement of the stretch-ing lineation on C surfaces the best approximation of thisdirection, and we use those measurements to constrainthe direction of zone boundary displacement. We use themodel proposed by McCoss (1986), assuming no volumeloss in the mylonite zone, and adopt the boundary condi-tions for transtensional deformation proposed by Sander-son and Marchini (1984). We also assume that C surfaceson the field are subparallel to the local zone boundaryand (to deal with the high strain indicators) that thestretching lineation on C planes tracks the approximatedirection of the relative displacement vector between theshear-zone walls.

8.2. Results

We presents the results of the geometrical constructionin the map shown in Fig. 7. The direction of maximumincremental stretch (k1) always presents a southeast–north-west orientation. In the field, a subhorizontal granite dikethat is boudined and sheared was found in a medium-angleshear zone dipping to the southeast, which suggests sub-horizontal incremental stretching plunging slowly to the

Fig. 7. Form surface map of the SASZ showing results obtained with McCoss’s (1986) method for the orientation of maximum and minimum axes ofincremental strain ellipse. k1 (maximum incremental stretch) always points to SE–ESE, oblique to the stretching lineation. Note theoretical relationbetween incremental and finite stretching axes and extension direction.


southeast/east–southeast, consistent with the asymmetry ofthe main mylonitic fabric. This observation denotes thesame vorticity suggested by the finite shear-sense indicatorsin the mylonitic foliation and agrees with the orientationpredicted in McCoss’s construction.

The orientation of k3 and k2 can vary according to thelocal tectonic regime and shear plane. In zones with sub-horizontal stretching lineation (with predominantly sinis-tral strike-slip), the likely orientation of the minimumprincipal incremental stretching is north–northeast tosouth–southwest, consistent with the determined sign ofvorticity. The orientation of the plunge could vary fromslightly to sharply to the northeast, according the shearplane and dip direction. In the northern portion of thestudied area, strike-slip shear may be accompanied by ver-tical (west–northwest-trending) C planes or subhorizontal

C planes dipping to the south (Figs. 2 and 7), which mayhave resulted from the sinistral simple-shear componentpartitioning in different shear-plane orientations, with sub-vertical and slowly-dipping planar fabric, observed in thestudied area. This partitioning also accounts for the orien-tation of the incremental k3 at a high angle to the myloniticfoliation in transpressional, obliquely diverging domains.In this situation, subvertical incremental shortening canbe attributed to vertical crustal thinning; this process gen-erates boudins in amphibolite layers parallel to the shearplanes (Fig. 6B).

In a simple shear-zone model with monoclinic symme-try, the same behavior could be expected for the k2 or axisof rotation. The rotation axis can deviate from near vertical(sinistral strike-slip component of the transtensional defor-mation), which plunges to the southwest. In the last orien-


tation and when stretching lineation is oblique or strike-parallel, it may accommodate the divergent movement ofthe zone and horizontal directional shear component.

The kinematic data and predicted orientations of theincremental principal axis of the strain ellipsoid for ‘‘homo-geneous’’ transtensional shear zones suggest an interestingand simple approach for the kinematics analysis. All datasuggest a transtensional tectonic regime, with southeastextension and directional sinistral components. The waysin which this deformation is achieved and partitionedreflect the heterogeneity of flow within a deforming shearzone. The lineation and sense of shear consistently indicatesinistral extension or a transtensional deformation regimein which the borders of the zone obliquely diverge (Har-land, 1971). Heterogeneities, mechanical behavior, andboundary conditions of the overall deformation constrainthe partitioning mechanisms.

9. Inferred extension direction

Inferring the extension direction, similar to any otherboundary conditions of natural deformations, appears tobe an extremely complex matter (see Fossen et al., 1994).Strain partitioning and unsteady flow are among the maincomplicating factors (Jiang and Williams, 1999), as well asrestrictions such as the impossibility of determining theexact flow apophysis based on finite and incremental straindata in obliquely convergent and divergent environments(Fossen and Tikoff, 1993, 1998). The McCoss (1986) con-structions have the additional restriction of constant vol-ume deformation (extrusion and intrusion of material isprohibited). Therefore, the challenge is to obtain, with anacceptable degree of confidence, an inferred extensiondirection. Taking into account that the orientation of thefinite strain ellipsoid is not strongly influenced by the ratioof vertical to lateral stretch (Jones et al., 1997) and that ourapproach is primarily qualitative, we believe our assump-tions are reasonable.

The structural data presented herein, together with thoserelated to the incremental strain axis orientation, facilitateconstruction of a kinematic picture during ductile deforma-tion. Experimental and mathematical modeling of transten-sion zones suggests that the direction of the externalsubhorizontal extension is consistently oblique to the finitelinear structures within the zone (Whithjack and Jamison,1986; Fossen et al., 1994). However, this obliquity dimin-ishes parallel to the increasing finite strain. In the studiedarea, structures suggest that the finite strain, at least local-ly, was high. Hence, we propose that the extension duringductile deformation is directed to the east/east–northeastand slightly oblique to the stretching lineation maxim (ste-reoplots in Figs. 3A and 7).

10. Discussion

The structural and kinematic behavior of myloniticstructures discussed herein appears coherent. These struc-

tures present finite stretching plunging to the southeast/east–southeast and oblique top–down-to-the-southeastkinematics. In the northern portion of the studied area,horizontal sinistral shear on horizontal to vertical planespredominates, whereas more oblique shear predominatesto the south. In the center of Figs. 2 and 7, a north–southfoliation trend can be seen in orthogneisses mylonites, withnear-frontal lineation plunging to the east and top–down-to-the-east kinematics. Although this structure has beeninterpreted as a discrete, late top-to-the-west ductile-ruptilethrust fault, the earlier, internal ductile fabric presentscoherent top–down-to-the-southeast movement (Fig. 3C).Therefore, the geometrical differences among shear planeorientations appear to us cinematically consistent with atranstensional tectonic model, with minor noise of laterstructures such as local folds and faults. Strain partitioningof transtensional deformation may explain the mainly geo-metric and kinematic differences observed in the describedmylonites.

Differences appear in the flow regimes of ductile defor-mation. In outcrops of mylonite structures, they may sug-gest constriction, simple shear, and overall noncoaxialflow. Stretching in the direction of lineation orientation(boudinage and pinch-and-swell in plane strain deforma-tion) appears to compensate for shortening at a high angleto the mylonitic foliation. This regime resembles a trans-pression model, in which stretching in one direction anda component of noncoaxial shear (simple shear componentorthogonal to the direction of ductile stretching) compen-sate for shortening at a high angle to the shear-zone bound-ary, consistent with the strain regime described by Jordan(1991) during the formation of pull-apart structures.

In some places, constrictive strain occurs. Strong linearfabrics can be seen in orthogneissic and granitic mylonitesat the base (low-angle) and upper structural levels (medi-um- to high-angle), respectively, of the mylonitic belt.Some transpression/transtension models predict constric-tion in transtension and oblate strains in transpressionaltectonic regimes (Sanderson and Marchini, 1984). Howev-er, natural deformations can present more complex pat-terns than theoretical models, and constriction may occuralso in transpressional regions when volume loss exists(Dias and Ribeiro, 1994). In the studied area, the localpresence of mylonites presenting evidence of transpressionduring deformation, in the structural sense of pure shearstrain simultaneously applied with noncoaxial shearing ina planar zone (Robin and Cruden, 1994), suggests thatthe shape of the strain ellipsoid could deviate from the pre-dictions of theoretical models in transtensional environ-ments, which in turn implies scale-dependent deformationboundary conditions in a deforming zone.

In some domains, however, structures such as SC fab-rics, asymmetrical extensional shear bands, and sheathfolds suggest prevalence of simple shear deformation (Bert-he et al., 1979; Lister and Snoke, 1984; Platt, 1984; Cob-bold and Quinquis, 1980), at least for the last incrementsof the movement history of the zone. These data also sug-


gest a well-developed monoclinic symmetry, consistentwith a simple shear model. Asymmetric structures in con-strictive and flattened domains, contiguous with that ofwell-developed simple shear fabrics, are observed in planesparallel to the XZ plane of finite strain, suggesting that therotation axis remained the same during ductile deforma-tion. These assumptions are reasonable for the kinematicanalysis in the studied area.

Taking into account the changes in shear plane orienta-tion within the field, the same stretching lineation trend fordifferently oriented shear planes, and the kinematicapproach, we conclude that, during the main transtension-al deformation, the incremental stretching direction wassubhorizontal to slightly southeast-plunging (resultingfrom an imposed oblique east/northeast-directed exten-sion), which explains the finite kinematic frame observedin the mylonites. These results in a counterclockwisestrike-slip component appear in the map in Fig. 7 and varyaccording to the rake of stretching lineation. Local generalshear resulted from strain partitioning within the deform-ing zone, likely attributable to a component of ductilestretching and thinning of the continental crust in a pureshear fashion.

We interpret sinistral transtensional shearing as a resultof differential relative motions subparallel to the belt ineast–northeast-trending shear zones along the northernedge of the Luis Alves Terrane. The terrane was expectedto move southwest in relation to the belt (dextral regionalmotion), but heterogeneous behavior due to extrusion nearthe border with the APB might explain the contemporane-ous sinistral extension at deeper structural levels. Fig. 8Ashows a schematic drawing of this interpretation. Differen-tial movement at the deeper level of the orogen (Luis AlvesTerrane with sinistral kinematics) and a required east-directed extension may explain the transtensional environ-ment in the studied area, coeval with syn- to late-collisionaltranspressional deformation in the northern shear belt andlateral extrusion of the northern tip of the Luis AlvesTerrane.

In the proposed model and upper crustal levels, dextralnoncoaxial ductile deformations concentrate in the softmetapelitic/metavolcanic unit of the APB and are com-posed of low-grade, carbon-rich phyllites and metasiltitethat easily accommodate deformation by distortion andslip on micaceous planes near the contact point betweenthe terranes (Fig. 8). Therefore, convergent ductile flowin the relatively higher crustal levels of APB concentratesin a kilometer-scale shear zone in the previously describedmetapelitic/metavolcanic unit, north of the studied area.The results of finite strain analysis (Campanha and Sadow-ski, 2002) agree with this interpretation. We depict this sug-gested situation in Fig. 8A and B, along with an interpretedheterogeneous displacement gradient. The increasingamount of displacement parallel to the orogenic trendwould be responsible for the reverse shear component atthe lower crustal levels in the Luis Alves Terrane. We alsocould argue that, together with some components of iso-

static rebound in which the tectonic denudation has beenrelatively high (see Wernicke and Axen, 1988), a deepercontractional strain at a high angle to the extension direc-tion may be responsible for the uplift of crustal slices in theLuis Alves Terrane near the contact point with the APBduring an overall transpressional regime (Fig. 8B).

11. Regional implications

The characterization of Neoproterozoic oblique exten-sional shears along the northern edge of the Luis AlvesTerrane adds new evidence to support the proposed occur-rence of crustal thinning. Alkaline magmatism and basinformation are thought to have occurred at 600–570 Ma(Siga Jr. et al., 1999). However, Hackspacher et al. (2000)suggest ages for Acungui Basin deposition of 618–605Ma. Therefore, the proposed tectonic scenario in thisregion suggests Neoproterozoic extension in at least twoperiods: an earlier one, during the opening of AcunguiBasin, and a younger one, with syn- to late-tectonic basininversion and granitoid emplacement.

Convergence-related deformation in the Apiaı domain isbelieved to have occurred at 0.610–0.60 Ga and would beactive in younger ages (0.59–0.565 Ga) in other regionsof the belt (Machado et al., 1996). Ages of 0.59 and 0.57Ga for convergence-related metamorphism and graniteemplacement, respectively, have been reported for otherlocations in the Mantiqueira province (Sollner et al.,1987; Tassinari et al., 1999; Heilbron et al., 2004). SigaJr. et al. (1999) argue that the extensional event in the adja-cent fold belts began just after the collision (at the end ofthe compressive regime) and was a response to the instabil-ity of a previous thickened crust. According to theseauthors, the primary collision-related deformation termi-nated at approximately 0.60 Ga. This conclusion conflictswith geochronological data and the proposed tectonicmodels, suggesting either plate convergence at 0.620–0.580 Ga or basin deposition slightly before 0.61–0.605Ga. We suggest that the proposed geochronological andstructural interpretations indicate inversion immediatelyafter deposition of the Acungui Group at 0.61–0.60 Ga.

The K/Ar cooling ages presented by Campagnoli (1996)for the mylonites of SASZ agree with the time span forextensional tectonics (0.60–0.57 Ga) proposed by Siga Jr.et al. (1999) for alkaline magmatism. These data also agreewith the Ar/Ar cooling ages in hornblende and muscoviteobtained by Machado (submitted), who suggests that theminimal interval of activity for the SASZ is 0.60–0.575Ga. The mylonites in the studied area are cut by the alka-line granitic (Guarau Massif) batholith and therefore post-date the main ductile movement phase. These data suggestthat extensional shearing predates alkaline magmaemplacement in the studied area and is probably relatedto the crustal thinning and melting that generated thesemagmas.

The kinematic data we discuss reveal a transtensionalregime, with ductile crustal stretching slightly oblique to

A

B

Fig. 8. Kinematic model proposed for transtension in the SASZ coeval with transpressional strain in the Apiaı belt. (A) Imposed transpressionaldisplacement field (a) was disturbed due to tectonic extrusion of the orogen (leading to higher finite strains in the upper, soft deep-marine facies of theAPB) (b); regional oblique extrusion accommodated by reverse shearing and uplift of rocks at deeper crustal levels in the northern edge of the Luis AlvesTerrane (c). (B) Uplift and lateral components (overall oblique displacement) of the northern edge of the Luis Alves Terrane.


the northeast-trending APB, accompanied by lateral massmovement along the belt (strike-slip component). Thematerial in the hangingwall obliquely escaped and col-lapsed to the east–southeast, whereas rocks from the foot-wall were unroofed and displaced to the west–southwest,subparallel to the Neoproterozoic orogenic trend. We sug-gest that sinistral and dextral shears, though active on dif-ferent crustal levels, are coeval and accommodate theoblique extrusion of crustal slices during orogenic trans-pression in the APB. Recent geobarometric data obtainedby Guimaraes (2000) suggest decompression during theemplacement of some granitic rocks in the Tres Corregosbatholith at 0.60–0.565 Ga (north of the studied area; seeGuimenez Filho et al., 2000; Janasi et al., 2001), as wellas the emplacement of magma during the exhumation ofthe APB. We interpret this exhumation as coeval withtranspression, and geochronological data suggest it is alsocoeval with ductile shearing throughout the SASZ.

The kinematic interpretation of flow during the defor-mation in the SASZ leads to significant constraints onthe models of extension in the Mantiqueira province. Theextensional collapse environment at 0.52–0.48 Ga inter-preted for the central and northern Mantiqueira province(see Trouw et al., 2000) seems too recent. Extensional col-lapse at 0.60–0.57 Ga, as proposed for the studied segment,

fails to explain the coeval convergent deformation in adja-cent areas. In addition, the installation of a late-orogenicextensional regime, as has been described for other loca-tions in the belt, assumes an early overthickened continen-tal crust. This scenario is possible and probable butremains to be proved in the northeast-trending Ribeirabelt.

In the SASZ, extensional shear was coeval with lateralmotion subparallel to the regional trend of APB and tookplace due to the influence of sinistral transtensional shearzones, in contrast with the major right-lateral transpres-sional regime described in the APB (see Fassbinder, 1996;Campanha and Sadowski, 1999). Lateral mass transportin collisional orogenic systems has been described in theAlps and Himalayas (Ratschbacher et al., 1989; Pecher,1991) and could be an important boundary condition forregional deformation patterns in transpressional belts(Jones et al., 1997). The previously discussed data suggestthat transtension played a role during a period of intensetectonic activity (lateral motion, transpression, metamor-phism, and magmatism) in the APB. Therefore, we proposea model in which the movement of material parallel to theregional trend occurs during convergence and unconfinedregional dextral transpression (Fig. 8). This interpretationimplies the coexistence of compressive environments (in a


northeast–southwest-trending shear belt, north of the stud-ied area), and transtensional environments (to the south)explain the similarities in age between alkaline granitoidsand syntectonic batholiths in the APB.

The kinematic results from the application of theMcCoss (1986) construction must be interpreted withcaution because it is valid only in volume-constant andwell-constrained boundary conditions (the Sanderson andMarchini (1984) model of transpression). These results alsomust be interpreted in conjunction with the mathematicalmodeling of transpression/transtension zones and the theo-ry of oblique rifting, as well as regional data related toextensional structures. Although detailed kinematic analy-sis of the extensional structures has not been performed forother regions, our regional interpretation agrees with otherstructural data collected elsewhere in the Mantiqueiraprovince (see Machado et al., 2001). Previous authors havesuggested Neoproterozoic top-to-the-east ductile exten-sion, with minor variations to the northeast and southeast,consistent with the results we present. Additionally, con-striction and orogen-parallel trending stretching lineationshave been described elsewhere in the belt.

12. Conclusions

The kinematic analysis of the SASZ suggests a sinistraltranstensional regime of ductile deformation that wouldexplain the heterogeneity of flow in different domains, withthe orientation of the vorticity vectors and shear planesvarying to accommodate two main components of overalldeformation: oblique crustal extension and subhorizontalsinistral wrenching. The study of finite and incremental ori-entations of the principal axis of the strain ellipsoid indi-cates an east/east–northeast-directed crustal extension(slightly oblique to k1), variable k2, variable k3, and subhor-izontal sinistral wrenching. Homogeneous transpressioncan also occur in this tectonic regime and may suggestthe escape of material, which contributes to the stretchinglineation in a plane strain deformation. The means of par-titioning would constrain the orientation and magnitude ofminimum and intermediate elongation axes, and transpres-sional and constrictional fabrics may occur.

The results of our kinematic analysis are interpreted interms of lateral mass transport and tectonic extrusion dur-ing transpression throughout the APB and along the north-ern edge of the Luis Alves Terrane. The lateral extrusiongenerates the east–northeast–directed crustal stretchingcomponent, slightly oblique to the trend of the belt, andthe sinistral transtensional shears developed through differ-ential movements subparallel to the belt. This regime caus-es lateral and upward motion of the footwall in relation tothe hangingwall and exhumation of the footwall from thepopping up of crustal slices.

Transtensional tectonic studies add new evidence, suchas the alkaline/peralkaline granitoids of the Serra do Marsuite and the correlative volcanic and basin formations,to support the idea that Neoproterozoic extensional

regimes developed in this portion of the crust. Therefore,we propose that this structure is probably rooted deep inthe crust or mantle. If so, transtensional tectonic environ-ments would be related to crustal thinning during the melt-ing of the lower crust and have a temporal relationshipwith dextral transpressional shear in the APB.

Acknowledgements

We thank the CPRM-SGB (Brazilian Geological Sur-vey) and the ‘‘Secretaria de Minas e Energia do Estadode Sao Paulo’’ for financial support of the fieldwork. Wealso thank Claudemir Severiano de Vasconcelos for fruitfuldiscussions in the field. The article was greatly improved byDr. Fernando Hongn and an anonymous reviewer. TheEnglish version was carefully reviewed by Dr. MarcusWaring di Valderano and Wilson Marson.

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Documents

Shear structures in the Serra do Azeite shear zone, southeastern Brazil: Transtensional deformation during regional transpression in the central Mantiqueira province (Ribeira belt)