partial melting. This suggests a model of strain transfer at the scale of ... Piranhas basement domain to the north from low- to medium-grade meta- sedimentary ...
Strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt: The Patos-Seridó system, northeastern Brazil Michel Corsini Departement de Géologie, Université des Sciences et Techniques de Masuku Franceville, Gabon Alain Vauchez Laboratoire de Tectonophysique, Université des Sciences et Techniques du Languedoc 34095 Montpellier Cedex, France Carlos Archanjo,* Emanuel F. J. de Sa Departamento de Geologia, Universidade Federal do Rio Grande do Norte 59000 Natal (RN), Brazil
ABSTRACT During the Brasiliano-pan-African orogeny, a complex continental-scale pattern of east-west transcurrent shear zones and northeast-trending fold belts formed in the northern and central Borborema province of northeastern Brazil. The east-west shear zones have been usually regarded as slightly younger features, but the study of the most spectacular case of intersection between these two structures, the Patos shear zone and the Serido transpressional belt, leads to a different tectonic model. Satellite imagery and structural, petrological, and geophysical data support the interpretation that these structures (1) are in structural continuity and (2) formed simultaneously under amphibolite facies metamorphic conditions that led to partial melting. This suggests a model of strain transfer at the scale of the orogen: at the eastern end of the Patos east-west dextral shear zone, the strain that accommodated the relative motion of the northern block was transferred to the northeast-trending Serid6 belt, where it resulted in folding, strike-slip faulting, and stretching parallel to the strike of the belt.
INTRODUCTION During the Brasiliano-pan-African orogeny (700-500 Ma), the Borborema province in northeastern Brazil was extensively deformed, metamorphosed, and intruded by granites. Although the effects of this orogenic event are widespread, their intensity is heterogeneous. Deformation is localized in relatively narrow and linear zones that may be either fold belts dominated by supracrustal rocks of Proterozoic age or continental-scale ductile strike-slip faults. These zones delineate less deformed domains in which high-grade basement rocks of Archean to Transamazonian age are exposed. The Brasiliano belts form a complex pattern that includes eastwest-trending, orogen-scale, dextral ductile strike-slip faults and northeasttrending fold belts with associated northeast-trending ductile shear zones (Fig. 1A). Historically, this pattern was regarded as resulting from the imposition of late orogenic eastwest-trending shear zones over preexisting northeast-trending belts. However, satellite imagery and field observations
*Present address: Laboratoire de Pétrophysique et Tectonique, 38, Rue des Trente-Six Ponts, 31400 Toulouse, France. 586
strongly suggest a progressive transition from east-west-trending to northeast-trending structures. Thus, available data suggest a mechanical continuity between east-west transcurrent shear zones and northeasttrending transpressional fold belts. We present here preliminary results of a structural and kinematic study of an east-west transcurrent shear zone connected with a northeast-trending transpressional belt, the Patos shear zone-Serido transpression belt (Fig. IB). We propose a new tectonic model involving simultaneous activity in the east-west shear zone and the northeast-trending transpressional belt. Similar systems are well known on a small scale, but they have not yet been described, to our knowledge, at the scale of several hundred kilometres. GEOLOGIC SETTING The Patos shear zone marks the northern boundary of an east-west continental-scale transcurrent zone (about 250 km wide and 500 km long) limited to the south by the Pernambuco shear zone (Fig. 1A). The Patos zone is up to 30 km wide and a few hundreds of kilometres long—one of the largest shear zones in the world. The Patos shear zone separates the Rio Piranhas basement domain to the north from low- to medium-grade metasedimentary and metavolcanic rocks that belong to the Brasiliano Cachoeirinha-Salgueiro belt to the south. The Serido belt is between the Rio Piranhas basement domain to the west and the Caldas-Brandao basement domain to the east. The structural trend of the Serido belt is northnortheast to northeast. Southward, approaching the Patos shear zone, the belt changes abruptly to an east-west orientation; the tip of the belt penetrates the Patos zone and is disrupted. Both the Patos shear zone and the Serido belt have the same lithologic character—granitic to gneissic basement (Caico complex) and a Proterozoic metamorphic cover (Jucurutu, Equador, and Serido formations) that includes paragneiss, amphibolites, quartzites, polymictic conglomerate, mica schist with calc-silicates lenses, and metavolcanic rocks. However, in the Patos zone, both basement rocks and Brasiliano granites preponderate over Proterozoic metasedimentary rocks, remnants of which are scattered through the zone. STRUCTURE AND KINEMATICS OF THE PATOS SHEAR ZONE Various types of faulted rocks are exposed in the Patos zone, suggesting that it was a long-lived and mechanically complex movement zone. To the south, the Patos zone is separated from the low-grade CachoeirinhaGEOLOGY, v. 19, p. 586-589, June 1991
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MIGMATITES
CACHOEIRUVHA LOW TO MEDIUM GRADE METAS EDIMENTS SERIDO MEDIUM TO HIGH GRADE METASEDIMENTS
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LOW-T MXLONITES
HIGH-T MYLONITES
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GRANITES
Figure 1. Junction of Patos shear zone and Seridô transpressional belt. A: Location map; N = Natal, R = Recife. Paleozoic to Phanerozoic basins are Chapada do Araripe (CA), Jatoba (JB), and Maranâo (MB). B: Schematic geologic map. Arrows indicate mineral stretching lineation. PSZ = Patos shear zone, CBD = Caldas Brandâo domain, CSB = Cachoeirinha-Salgueiro belt. Numbers give dip of foliation and plunge of mineral stretching lineation.
Salgueiro belt by 2-4-km-thick greenschist fades mylonites to ultramylonites, which indicate reworking of the Cachoeirinha metasedimentary rocks, the high-temperature (T) mylonites, migmatites from the Patos shear zone, and post-peak-metamorphism granites. Observation of a lowgrade vertical mylonitic foliation bearing a horizontal mineral stretching lineation and of various shear-sense indicators, including extensional shear bands, oblique shape fabric of quartz grains, asymmetrical porphyroclast systems, and sigmoidal lenses, leads us to suggest that the low-rmylonitization accommodated dextral strike-slip faulting. In places, these low-r mylonites are in turn reworked by cataclasites, the fabric of which suggests that they have undergone similar kinematics, but under lower pressure (P) and temperature conditions. North of this low-T shear zone, mylonites, which form most of the Patos shear zone, display higher grade microstructures and metamorphic assemblages, even grading into migmatites. The mylonitic foliation trends nearly east-west and dips steeply in most places (locally moderately) to the south or to the north. A subhorizontal mineral stretching lineation is observed in all kinds of rock. Stretching is marked by the elongation of pebbles in conglomerates, by quartzo-feldspathic aggregates in mylonitic gneiss and metasedimentary rocks, and by the elongated shape of sheared granite bodies and xenoliths within these granites. Ductile boudinage of the foliation in both mylonites and migmatites and the presence of sheath folds also support an east-west extension. The stretching lineation is associated with a mineral lineation defined by the alignment of synkinematic mica flakes, amphiboles (hornblende and sodic amphibole), sillimanite, and cordierite, which commonly displays internal foliation and asymmetrical micaceous pressure shadows. GEOLOGY, June 1991
A large variety of kinematic indicators have been observed across the Patos shear zone in the high-71 mylonites derived from different rock types. Feldspar porphyroclasts displaying asymmetrical recrystallized tails are commonly wrapped by red-brown biotite or hornblende. Rolling structures (e.g., Van Den Driessche, 1986), and drag folds are well developed in migmatitic mylonites and in layered migmatites. S-shaped quartz veinlets, synthetic shear bands, and S-C planar fabric are more frequent in mylonitized metasedimentary rocks. Although garnet is usually late- to post-tectonic, it may display sigmoidal (even double spiraled) inclusion trails. Cordierite may also have a sigmoidal internal foliation. Asymmetrical pressure shadows are present at the tips of staurolite porphyroblasts having a straight internal foliation. Systematic analysis of these kinematic indicators both in the field and in thin section consistently suggests a synmetamorphic dextral sense of shear. The metamorphic zonation in the Patos shear zone is poorly known. However, preliminary results suggest that mylonitization occurred during the main amphibolite faciès metamorphism. (1) Mylonites systematically display a high-T microstructure (e.g., coarse grain size, incipient plastic deformation of feldspar, exsolution of plagioclase along the boundary of K-feldspar, and sigmoidal porphyroclast systems wrapped by high- T biotite). (2) Mylonitic metasedimentary rocks contain synkinematic to postkinematic biotite and garnet ± cordierite ± staurolite ± sillimanite in the eastern part close to migmatites. (3) Amphibolite lenses and some sheared amphibole-bearing granites contain a synkinematic dark-green hornblende without any trace of retrogression. (4) Some mylonitic granites, probably of peralkalic composition, contain pyroxene relicts partially retrogressed into a blue amphibole. (5) Toward the center of the Patos shear zone and 587
toward its junction with the Serido belt, high-J mylonites grade into migmatites. The observed transition is from high- T mylonites intruded by leucosome veins to layered and boudinage migmatite with planar and linear fabric and shear criteria similar to those in mylonites, and finally to nebulitic migmatite. These data support a contemporaneity of mylonitization, amphibolite facies metamorphism, and partial melting. From these observations we interpret the Patos zone as the deep part of a continentalscale dextral strike-slip fault, the main activity of which took place during the high-7-, medium- to low-P regional metamorphism. This activity was contemporaneous with crustal melting, which triggered synkinematic intrusion of granites. In the northeastern part of the Patos shear zone, close to the junction with the Serido belt, a large volume of tonalitic to granodioritic magma has been emplaced parallel to, or, less commonly, slightly oblique to the mylonitic foliation. Most of these dikes have undergone solid-state, high- T shearing. SERIDO TRANSPRESSIONAL BELT Although the pre-Late Proterozoic history of the Serido belt is still a subject of conflicting interpretations (e.g., de Sa et al., 1987,1988; Caby, 1989; Bertrand and de Sa, 1991; Caby et al., 1991), the data already collected allow us to define an accurate frame for its Brasiliano evolution. Late Proterozoic deformation is characterized in this area by the following. (1) Polyphase folding—successive generations of folds have the same north-northeast orientation. The first and main phase formed upright to inclined, open to subisoclinal folds; simultaneously, a steeply to gently dipping foliation with common fanlike appearance developed (e.g., de Sa, 1988). Subsequent folding was more localized and produced upright folds. (2) Northeast-trending ductile strike-slip faults, which are consistently dextral and may reach a few kilometres in width. They are usually subvertical, but in places they display a progressive decrease in dip, and become low-angle strike-slip faults, resulting in a flowerlike structure (Archanjo, 1989). Approaching these shear zones, the degree of strain increases and the metamorphic foliation grades into a typical mylonitic foliation developed under amphibolite facies conditions. (3) Stretching consistently parallel to the trend of the belt. Computation of finite strain (Archanjo, 1989) using shape ratio of pebbles from polymictic conglomerate has shown that in the central domain of the belt, finite elongation may reach 300%-400%, suggesting constrictional strain. In the western domain, pebbles locally display a pancake-like shape, suggesting pure flattening. However, pre- or early-Brasiliano deformations (e.g., de Sa, 1988) may have contributed to finite strain; therefore, it remains difficult to derive an accurate interpretation of strain data. Both field and thin section observations show that the different types of structures were formed during a single tectonothermal event. The metamorphic conditions under which this deformation occurred usually reached the amphibolite facies (biotite, garnet, cordierite, andalusite, or sillimanite in mica schist; hornblende in amphibolites). The metamorphic zonation is clearly linked with the intrusion of synkinematic granites, which produced contact-metamorphic andalusite, cordierite, and sillimanite, and partial melting; the degree of regional metamorphism increases drastically approaching the granitic bodies (de Sa et al., 1987; de Sa, 1988). A link between deformation and melting is also suggested by the occurrence in most granites of a well-developed magmatic fabric similar to the tectonic fabric within the country rocks. This magmatic fabric is characterized by a steeply dipping magmatic foliation in granite bodies emplaced in regional-scale strike-slip fault zones, and by a gently dipping magmatic foliation in domains dominated by a flat-lying tectonic fabric (Archanjo et al., 1990). Involvement of the upper mantle in magma genesis is suspected in this area, as elsewhere in the Borborema province (Sial, 1987), from the systematic mingling of calc-alkalic magma with a more mafic, usually dioritic magma. 588
DISCUSSION: STRAIN TRANSFER AT A CONTINENTAL SCALE Although geochronological data from the Serido belt are sparse and are totally lacking for the Patos shear zone, a comparison of the Brasiliano tectonothermal evolution shows that the main deformation in both zones is broadly contemporaneous with the same amphibolite facies metamorphism accompanied by crustal melting. Moreover, the connection of these orogenic zones is characterized by a complete structural continuity. Southward, the orientation of both the lithologic units and the tectonic structures (folds, shear zones, foliation, and stretching lineation) of the Serido belt displays a clockwise rotation; at the tip of the belt, the orientation is east-west. Satellite imagery (Landsat TM and SPOT) shows that the Patos-Santa Luzia area, where this bending occurs, has no transverse structures. The mylonitic foliation, conspicuous within the Patos shear zone, becomes more diffuse approaching the zone of junction with the Serido belt, and clearly does not intersect the thick sequence of quartzite exposed in a regional-scale open anticline on the eastern side of the belt. Southward, this anticline is rotated to east-west and penetrates the Patos shear zone. Moreira et al. (1989) have shown that the junction area is characterized by both gravity and magnetic anomalies that underline the arcuate shape of the belt and support the absence of transverse features. From these data, Moreira et al. (1989) have also derived a model of differentiated crust, including a dense and magnetic, probably mafic, lower crust, and a lighter felsic upper crust, mostly composed of granite containing magnetite xenocrysts and mafic lenses. At the junction of the Patos shear zone and the Serido belt from the west, it appears that a thick (up to 20 km) unit of high-J mylonites merges eastward into the Serido belt where the mylonites are progressively replaced by migmatites and anatectic granites formed by the melting of the Archean basement. The structural continuity between the Patos zone and the Serido belt suggests a mechanical continuity implying a certain conservation of strain from one branch of the system to the other. Considering the geometry of the system, this condition may be satisfied if it is assumed that both strike-slip faulting along the Patos zone and composite deformation (folding, longitudinal stretching, and strike-slip faulting) within the Serido belt accommodate the same eastward displacement of the Rio Piranhas domain relative to the terranes south and east of the system (Fig. 2). The lack of a major thrust fault in the Serido belt precludes interpreting this structure as a frontal ramp due to the displacement of a rigid block. A more realistic model should consider that (1) dextral shearing that represents the main deformation mode in the Patos shear zone is also a major component of deformation within the Serido belt; (2) the Rio Piranhas
Figure 2. Schematic tectonic model relating transcurrent shearing along Patos shear zone, together with folding, stretching, and shearing in Serido transpressional belt, to motion of more rigid Rio Piranhas domain. Long axis of ellipses represents direction of stretching. A and B represent two successive positions during displacement of Rio Piranhas domain; C is its final position. GEOLOGY, June 1991
domain did not behave like a rigid body, especially in that its eastern boundary is increasingly deformed approaching the Seridó belt and becomes deeply involved in the structures of the belt (both folds and strikeslip faults); and (3) a significant part of transverse shortening was accommodated within the Seridó belt through longitudinal stretching, suggesting that lateral extrusion may have occurred. Although thermomechanical modeling is needed to test the crustal behavior in such a system (which is quite uncommon, because of its size), we conjecture that the upper mantle was involved in the process, together with the crust. A deformation of the entire lithosphere may account for (1) the mechanical accommodation of strain transfer, including a possible crustal differentiation through extensive melting localized in the junction area; (2) the observed connections among deformation, metamorphism, crustal melting, and granite genesis, which suggest shear heating; (3) the systematic mingling of felsic (calc-alkalic) and mafic (dioritic) magmas, which probably represent crust and mantle contributions to magmatism; and (4) the geophysical anomalies that characterize the junction zone.
de Sa, E.F. Jardim, 1988, An update of the Precambrian geology of northeast Brazil, in International meeting on Proterozoic geology and tectonics of highgrade terrains: Ile-Ife, Nigeria, program and lecture series, 23 p. de Sa, E.F. Jardim, Macedo, M.H.F., Legrand, J.M., McReath, I., Galindo, A.C., and Martins Sâ, J., 1987, Proterozoic granitoids in a polycyclic setting; the Seridó region, NE Brazil [abs.], in International symposium on granites and associated mineralizations: Brasil Superior, Geologia e Recursos Mineralogia, p. 103-110. de Sâ, E.F. Jardim, Macedo, M.H.F., Falcao Torres, H.H., and Kawashita, K., 1988, Geochronology of metaplutonics and the evolution of supracrustal belts in the Borborema Province, NE Brazil: Anais VII, Congresso LatinoAmericano de Geologia, Belem, v. 1, p. 49-62. Moreira, J.A. de M., Medeiros, W.E. de, Ling, F.A.P., and Archanjo, C.J., 1989, Urna anomalia magnetica de caracter regional no Seridó (RN/PB) e discussao de sua origen (Brazilian Society of Geophysics, 1st Congress): Revista Brasileira de Geofisica, v. 7, p. 81. Sial, A.N., 1987, Granitic rocks of northeast Brazil [abs.], in International symposium on granites and associated mineralizations: Brasil Superior, Geologia e Recursos Mineralogia, p. 61-70. Van Den Driessche, J., 1986, Structures d'enroulement et sens de cisaillement. Exemples et modèles: Paris, Académie des Sciences Comptes Rendus, v. 300, p. 413-418.
REFERENCES CITED Archanjo, C., 1989, A deforma?ao constriccional nos conglomerado da faixa Seridó, in Proceedings, Congresso Brasileiro de Geología, 35th: Belem, Sociedade Brasileira de Geología, v. 5, p. 2240-2247. Archanjo, C., Olivier, Ph., Bouchez, J.L., and Vauchez, A., 1990, A estrutura interna dos principáis plutons graníticos brasilianos da faixa Seridó (RN-PB), Brasil: Resultados preliminares, in Abstracts, Congresso Brasileiro de Geología, 36th: Natal, Sociedade Brasileira de Geología, p. 240. Bertrand, J.M., and de Sá, E.F. Jardim, 1991, Oü sont les chalnes de collision Eburnéennes et Transamazoniennes?: Canadian Journal of Earth Sciences (in press). Caby, R., 1989, Precambrian terranes of Benin-Nigeria and northeast Brazil and the Late Proterozoic south Atlantic fit, in Dallmeyer, R.D., ed,, Terranes in the circum-Atlantic Paleozoic orogens: Geological Society of America Special Paper 230, p. 145-158. Caby, R., Síal, A.N., Arthaud, M., and Vauchez, A., 1991, Crustal evolution and the Brasiliano orogeny in northeast Brazil, in Dallmeyer, R.D., and Lécorché, J.P., eds., The West African orogen and circum-Atlantic correlatives: New York, Springer-Verlag (in press).
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ACKNOWLEDGMENTS Supported by European Economic Community Project CI1-0320-F (CD) and Institut National des Sciences de l'Univers-Dynamique et Bilans de la Terre Program "Croissance continentale." This paper is a contribution to International Geological Correlation Program 279, Terranes in Latin America. We thank R. Caby, I. Duncan, D. Mainprice, and A. Nicolas for helpful comments on the manuscript. Manuscript received May 21,1990 Revised manuscript received December 31,1990 Manuscript accepted February 28,1991
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