KINEMATICS AND TECTONIC SIGNIFICANCE OF I NTRACONTINENTAL DUCTILE STRIKE-SLIP SHEAR ZONE SYSTEMS IN THE BORBOREMA PROVINCE (NE BRAZIL)

During the Brasiliano (Panafrican) orogeny, the Borborema Province was structured by a complex network of continental-scale transcurrent she ar zones coeval with high temperature metamorphism, partial melting an d synkinematic magmatism. The Province is essentially composed of rewo rked basement rocks mostly Transamazonian in age, but containing some limited Archean domains, and of Proterozoic supracrustal rocks. A nort h-western domain characterised by nappes and rectilinear NE-trending s hear zones can be distinguished from a central and southern domain in which sinuous, discontinuous EW-trending shear zones are associated wi th N- to NE-trending transpression belts. The aim of this paper is to present results of recent studies performed on the Patos-Serido and Ca mpina Crande shear belts located in the central part of the Borborema Province and to propose a geodynamic model for this large-scale intrac ontinental ductile transcurrent fault system. The E-W trending Patos s hear zone, up to 30 km wide and a few hundreds of kilometres long, com prises mylonites, migmatites and synkinematic plutons and dykes. Westw ard the Patos shear zone progressively curves to a NE direction and th e bending is accommodated by a strike-slip duplex structure. Eastward, the shear zone merges into the NE-trending Serido transpressional bel t. East of the Patos shear zone-Serido belt system, the Campina Grande shear zone trends E-W parallel to the Patos shear zone in an en-echel on pattern and displays similar structural and metamorphic characteris tics. In the shear zones the mylonitic foliation is steeply dipping an d bears a subhorizontal mineral stretching lineation. Kinematic indica tors such as asymmetric tails around porphyroclast, rolling structures , C-S mylonitic fabric and sigmoidal lenses are consistent with dextra l shearing. Stretching is marked by boudinage of more competent layers in gneisses, elongation of pebbles in conglomerates and elongated sha pe of xenoliths in granites. Lineation is underlined by alignment of m etamorphic minerals. The amount of motion along the shear zone is prob ably in the range of few hundred kilometer. The shear zone systems pre sent an inhomogeneous finite strain distribution, resulting from nearl y simple shear in the EW-trending branch and transpressional deformati on in the NE-trending branch. Such a variation in the deformation patt ern is consistent with a change in orientation of the shear zone. In t he shear belts the deformation is coeval with high temperature-low to medium pressure metamorphism and local partial melting. Coarse grained mylonites formed under amphibolite facies metamorphism, which is supp orted by dislocation creep evidences in K-feldspars and large quartz r ibbons. Transition from high-temperature mylonites to migmatites is pr ogressive and marked by layered mylonitic fabric grading into nebuliti c fabric. Synkinematic partial melting results in melt collected in ax ial planes of asymmetric fold or in synthetic shear bands. Fine graine d mylonites formed under retrograde green-schist facies metamorphism d eveloped in narrow zones within the high- temperature mylonites belts. The shear zones were active during the main metamorphic event and rea ctived after the peak of metamorphism under retromorphic conditions su ggesting that shearing was long lasting and occurred al different crus tal levels during uplift of the area. Many syn- to late kinematic intr usions are represented by calc-alkaline to high-potassium calc-alkalin e granitoid resulting from melting of the lower crust. Involvement of the upper mantle in magma genesis is suspected in the whole Borborema Province from the systematic mingling of calc-alkalic magma with a mor e mafic magma, suggesting that the entire lithosphere: is implied in t he shearing processes. Their emplacement is coeval with both the trans current shear zones motion and the development of a flat-lying foliati on associated with flattening strain and probably related with crustal extension. Single grain Ar-40/Ar-39 and U-Pb on zircon analysis of ro cks collected in the shear zones and in the Serido belt support a sync hronous deformation in the different branches of the system. Thermochr onology also suggests a relatively slow cooling from 600 to 520 Ma, th en an exhumation and fast cooling around 500 Ma, coeval with evolution of mollasic basins. A comparison between the NE Brasiliano, West Afri can and Central African Late Proterozoic belts is made to better under stand the evolution of the Brasiliano/Pan-African chain in a plate tec tonic context. The north-western part of the Borborema province, the P an-African belt of West Africa and the Central African belt display hi gh-temperature metamorphism related to crustal thickening located at t he margins of cratons (respectively the West African and Congo craton) . The central part of the Borborema Province and the Nigerian Province are marked by intraplate deformation associated with high-T metamorph ism, abundant syn- to late magmatism and large-scale transcurrent shea r zones. The lithospheric transcurrent shear zones system of the Borbo rema Province may developed in a context of lateral escape of continen tal blocks and post-thickening crustal extension during the convergenc e between the West-African/Sao Luis, Amazonian, Sao Francisco and Cong o cratons.