Deformation regime variations in an arcuate transpressional orogen (Ribeira belt, SE Brazil) imaged by anisotropy of magnetic susceptibility in granulites

Abstract

The Ribeira belt of southeastern Brazil displays an arcuate shape, with a structural trend that varies from ∼NS in the northern domain to ENE–WSW in the southern domain. This curvature is accompanied by a transition from contraction-dominated to transcurrent-dominated tectonics. The transition in deformation regime is accommodated in the central domain of the belt where granulitic rocks dominate and mineral-stretching lineations are commonly concealed by metamorphic recrystallization. We present anisotropy of magnetic susceptibility (AMS) data from 664 samples from 62 sites in high-temperature gneisses, granulites and migmatites of the transitional, central domain of the belt, with the aim of investigating: (1) how well AMS allows one to map the mineral-stretching lineation and foliations in domains displaying a complex kinematic framework and (2) to investigate the kinematic pattern at the transition between the thrusting dominated and a wrench-faulting dominated orogenic segments. The mean magnetic susceptibility is 7.54×10 −3 SI. The degree of anisotropy varies from 1.32 up to 4.31, with an average value of 1.53. The shape parameter T is generally >0 meaning that the AMS ellipsoid is dominantly oblate. Magnetic lineations and foliations form a consistent pattern correlated with the modification of the structural characteristics observed along the Ribeira belt. In the southern wrench-fault-dominated domain, the magnetic lineation is subhorizontal, parallel to the trend of the steeply dipping magnetic foliation. This correlation with the fabric observed in mylonites suggests that the magnetic fabric is a valid proxy of the tectonic fabric in granulites. Results from the northern domain show that it comprises two sub-domains both displaying a ∼NS-trending magnetic foliation. Eastward, over a broad area, the magnetic foliation is consistently steeply dipping and bears a shallowly to moderately plunging magnetic lineation. Westward, the dip of the foliation decreases progressively and the lineation rotates from nearly parallel to nearly orthogonal to the trend of the foliation, suggesting a progressive transition from transcurrent to contractional tectonics. At the junction between the northern and southern domains, the magnetic fabric is composite: the magnetic foliation displays a constant NNE–SSW orientation, but the magnetic lineation spreads within the foliation plane from parallel to normal to the foliation direction. The AMS pattern mapped in the Central Ribeira belt supports a scenario in which the transcurrent and contractional deformation regimes are coeval, with a transition between the two regimes accommodated progressively over a large transpressional area where both strain regimes were active.