Paper No. 127-3
Presentation Time: 9:00 AM-6:30 PM
AGE CONSTRAINTS ON INTERACTING SHEAR ZONES IN SHEAR ZONE NETWORK: A CASE STUDY FROM THE BORBOREMA PROVINCE, NE BRAZIL
Interacting strike-slip shear zones often follow or accompany continent collision and are associated with extrusion and exhumation in orogenic belts. Understanding branching and interacting shear zones requires knowledge of timing, deformation kinematics and rheology of the deforming rocks, which have implications for transport of crustal masses. The Senador Pompeu and Taua strike-slip shear zones of the Borborema Province (NE Brazil) have a central role and location in the Neoproterozoic assembly of Gondwana, and provide a means to understand shear zone dynamics. We apply (i) U-Pb SHRIMP analysis of zircons to constrain the timing of shearing, (ii) 40Ar/39Ar step-heating on amphibole and biotite from syn-tectonic felsic magmas to constrain cooling, and (ii) vorticity and strain determinations on pluton’s megacrystic facies deformed in the magmatic state and in final stages of crystallization. Zircon ages of the deformed plutons show that the shear zone pair was active under high-temperature at 585–581 Ma while felsic dykes were synchronously emplaced in brittle regime, emphasizing the role of lateral rheological heterogeneities. 40Ar/39Ar plateau-ages, furthermore, indicate that temperatures down to ca. 300 °C were reached fastly after the crystalization of the sin-kinematic magmas. Vorticity and strain estimates indicate although a simple shear component is dominant, the shear zones accomodate an important component of pure shear. The strain also record up to 45% of thinning and, notably in the Senador Pompeu shear zone, up to ca. 15% of thickening. Despite the bulk transpressional character, the dispersion of vorticity values for the Senador Pompeu shear zone highlights variations of offset rate for the interacting shear zone that leads to localized transtension for batholith emplacement. The collisional setting of the northern Borborema Province involved therefore heterogeneous ductile extrusion ruled by conjugate pairs of shear zones, magmatic thickening and fast cooling of the intervening crustal blocks.