The role of Precambrian mylonitic belts and present-day stress field in the coseismic reactivation of the Pernambuco lineament, Brazil

The Pernambuco lineament is a steeply dipping continental-scale ductile shear zone rooted within the Precambrian lithosphere of intraplate northeastern Brazil. It was formed during the Brasiliano orogeny ~ 600 Ma and reactivated during the Pangea breakup in the Cretaceous, when it controlled fault propagation and sediment accumulation in several rift basins. The region is now under an ~ E–W-trending horizontal compression and ~N–S-trending extension, and faulting occurs predominantly in a strike-slip regime. We investigated two aftershock sequences and the preexisting tectonic fabrics along the Pernambuco lineament in order to evaluate the role of these fabrics in the coseismic reactivation of continental-scale structures. The lineaments consist of a main ENE–WSW-trending mylonitic belt about 2–3 km wide, and two secondary NE-trending mylonitic belts about 100 m wide. They both present steeply dipping mylonitic foliations and shallowing plunging stretching lineations. The mylonites present granitoid protoliths and mineral parageneses that range from amphibolite to greenschist facies. Brittle deformation overprints the ductile fabric in all mylonitic belts. In 1991, coseismic reactivation nucleated along the ENE–WSW-striking, ~ 3.3-5.6 km deep, normal fault of less than 1 km in length in the main mylonitic belt. In 2002, seismicity migrated to a NE-trending secondary mylonitic branch and moved as a right-lateral strike-slip, ~ 1.2 km long, 3.8–4.9 km deep fault plane. Both fault segments reactivated the mylonitic foliation and form part of a major system. We conclude that the interplay between the present-day stress field and preexisting fabrics controls seismogenic fault location, attitude, and kinematics. The Pernambuco lineament is an example of a long-lived continental-scale structure, where selective reactivation has occurred. Other shear zones in the region also show a long history of brittle reactivation and present similar orientation in relation to the present-day stress axes. They might be dormant structures prone for reactivation under the present-day stress field.