TECTONIC RELEVANCE OF LATEST D3 PHASE STRONG SHORTENING IN FOOTWALL ROCKS SE OF THE NEOPROTEROZOIC BRASÍLIA DOME, CENTRAL BRAZIL

The Brasília Dome (BD) lies in the centre of the External Zone of the Brasília Belt, just to the East of the oroclinal concavity characterized by the northern and southern segments (respectively trending NE and SE) and in the hanging wall of the W-dipping Rio Paranã Thrust (RPT) a poorly known fault despite its regional relevance. The BD is an elliptical-shaped N-trending antiformal structural window formed in consequence of regional shortening (D_3N) co-axially superposed upon D₁-D₂tectonites. It is cored by Paranoá slate, quartzite, and metacarbonate (proximal passive margin) and mantled by napes of Canastra phyllite (distal passive margin) plus Araxá mica-schist (back-arc basin).

The evidences of napes transport (D₂ event) remain regionally constant to ESE across the belt, regardless of the ~10-20 km-wide Pirineus Lineament, a zone of high strain which occupies the oroclinal axial surface and stretches ~300 Km WNW since a point close to the dome´s western margin. D₁-D_3N deformation in the BD records WNW-ESE direction of ductile flow and is also typical of the belt´s northern segment. However, the core and mantle D₁-D_3N tectonites in the dome´s SW corner and beyond underwent latest SW-NE regional shortening (D_3S) typical of the belt´s southern segment in which D_3Nshortening is absent.

The data imply: (1) - the BD is a litho-structurally continuous D₁-D_3N bridge across the interface between the belt’s northern and southern segments; and (2) - both the oroclinal and the Pirineus lineament are consequence of D_3N + D_3Scrossed shortenings which co-existence began only after the dome´s uplift.

A recent structural analysis in the area SE of the dome has revealed that rocks typical of the RPT´s footwall also underwent D₁-D_3N+D_3S deformation, but shortening D_3S is far more intense than elsewhere in the southern segment. The data strongly indicate the RPT as a D_3N+D_3S fault and reinforce the importance of underthrusting (D₃ event) in the construction of the Brasília Belt.