The four tectonic events (D1 to D4) that affected the MCD are recorded in distinct ways. The D1 event affected the 2.36-2.30 Ga basement rocks of the Granja Massif and not others, and is therefore inferred to have developed during the Trasamazonian orogeny (2.0-2.2 Ga). The D1 fabric is preserved as a low-angle SE-dipping foliation, normally subparallel to the S2 foliation that developed during D2, but is folded by F2 (Figure 10C). The basement rocks also record all later deformational fabrics and the intense overprinting by D2 and D3 practically erased the record of D1 (Figure 10).
Figure 10. Deformation of early Paleoproterozoic basement rocks
Deformation of early Paleoproterozoic basement rocks. a-b) Photograph and line drawing showing the relationship between the gently dipping S1//S2 foliation cross-cut by sub-vertical S3 foliation in tonalite gneiss. c) Recumbent F2 fold defined by S1 where S2 is the axial planar foliation in the Granja Massif granulite. d) Subvertical S3 foliation crosscutting S1//S2 foliation in the migmatite located south of Granja town.
In the MCD, D2 is characterized by medium- to low-angle SE-dipping S2 foliation and gently-plunging NE or SE stretching lineation, recorded by sillimanite, kyanite, staurolite, and muscovite. This fabric developed under amphibolite facies. Fold axes have a NE-SW orientation and resulted from northwestward compression. There are intensely deformed bands within the quartzite, where it becomes intensely foliated and fine-grained forming mylonitic to ultramylonite textures. These mylonites define the shear zones on which northwest-directed imbricated thrust sheets were emplaced (Figure 11). Microtectonic and petrographic evidence indicates that D2 thrusting of the Granja Massif and Martinópole Group, were developed, respectively, under the following metamorphic conditions: medium pressure and high temperature (granulite facies), medium pressure and medium temperature (amphibolite to greenschist facies). In the Granja Massif, the continuous northeast-northwestward movement is responsible for the uplifting and cooling of granulite rocks during the Neoproterozoic.
Figure 11. D2 deformation at different scales
D2 deformation at different scales, with the development of imbricate fans: (a) mylonite texture in quartzites; (b) intense dynamic recrystallization of quartz; (c) photomicrograph showing intense recrystallization of quartz. Width of view 2 mm.
In the Ubajara Group, record of the D2 event is similar to that in the basement, and in the Martinópole Group D2, however developed at lower metamorphic conditions. It is characterized by a gentle, SE-dipping S2 slaty cleavage and millimetric schistosity, SE-trending stretching mineral lineation (L2, recorded by chlorite, muscovite, sericite), and NW verging asymmetric folds affecting bedding (S0) (Figure 12). It is noteworthy that the late Paleoproterozoic Saquinho Volcanic Sequence presents a deformation pattern similar to the rocks of the Neoproterozoic Martinópole Group, however at lower metamorphic conditions, similar to the rocks of Ubajara Group. This is important because it precludes the overprinting of the Mesoproterozoic deformation, which occurs to the south in the Borborema Province (Transversal Domain) during the Cariris Velhos event at 1.1-0.95 Ga (Brito Neves et al., 1995). It was this NW thrusting that was responsible for stacking, from base to top, the Granja massif, the Martinópole and Ubajara Groups, respectively with metamorphic conditions of granulite, amphibolite and greenschist facies.
The tangential D2 event evolves progressively to D3, which is characterized by strongly subvertical S3 foliation striking NE-SW to E-W (Figure 12). The nature of the penetrative S3 foliation depends on the rheology of the deformed rock package. This foliation is well marked in phyllites and schists from the Santa Terezinha and Covão Formation, whereas in quartzites of the São Joaquim Formation is more discrete. The large shear zone development during D3, whose kinematic indicators such as asymmetric feldspar, S-C structures, mica-fish and microfold indicate a consistently dextral shear sense.
The gently plunging NE-SW to E-W stretching mineral lineation (L3) is defined by amphibole, sillimanite and kyanite. This implies that whatever lineation that might have developed during D2 thrusting, with an expected NW or SE trend was reoriented or erased by D3 transcurrent tectonic movements into the NE-SW direction (Figure 12).
The F3 folds show variable styles and scales, from a few centimeters to hundreds of meters in wavelength. F3 macrofolds show a predominant NE-SW axis orientation, with minor E-trending folds. F3 axes are parallel to the stretching lineation, and the limbs are usually overturned with the axial plane plunging steeply SE or S. In D3, kyanite, staurolite and sillimanite show plastic behavior (Figure 6E-F). With progressive deformation, these portions of the crust were uplifted and cooled becoming progressively brittle-ductile in D4, when quartz was recrystallized and sericite-muscovite crystallization occurred along a spaced cleavage, orthogonal to S3 foliation.
D4 is a brittle-ductile deformation phase related to transpressional movements and is best documented in the Martinópole Group quartzite, where it is characterized by a S4 spaced fracture cleavage. The Saquinho Volcanic Sequence and the Ubajara Group do not record the D4 event. The evolution of the D3 transcurrent regime into the D4 transpressional regime is recorded essentially in the microtectonic features and quartz c-axis fabric study in quartzite of the Martinópole Group. This transition is registered mainly along several strike-slip shear zones of the Médio Coreaú Domain, developed during D3, exemplified here by the Campanário and São Joaquim Shear Zones (Figure 13). The analysis of crystallographic fabrics in porphyroclast and recrystallized grains of quartz, define to distinct directions. The porphyroclast grains show a flow direction parallel to the E-W strike-slip zone (Figure 13). This pattern denote that c-axis of porphyroclast quartz was developed at high to medium temperature (Bouchez et al., 1985; Mainprice et al., 1986; Law, 1990; Passchier and Trouw, 1996), with maximum parallel to the stretching lineation, compatible with D3 metamorphic conditions. The preferred c-axis orientation of recrystallized quartz grain shows an orthogonal orientation to those of porphyroclasts. Otherwise, the new recrystallized grains show a small circle girdle centered about Z-axis indicating a low-temperature deformation (Lister and Hobbs, 1980; Law, 1990; Passchier and Trouw, 1996). The c-axis fabric for these recrystallized quartz are asymmetrically disposed with respect to the S3 foliation denoting a dextral movement during D4, in a transpressional regime. This suggests a swap in the maximum shortening axis from horizontal in D3 to vertical in D4.
Figure 13. Tectonic sketch of the São Joaquim and Campanário Shear Zones
Tectonic sketch of the São Joaquim and Campanário Shear Zones, showing the arrangement of tectonic fabric (foliation and stretching lineation) and related c-axes lattice preferred orientations (LPO) [001] developed during the transpressional deformation D4. a) The São Joaquim S.Z. is characterized by pronounced asymmetric c-axis [001] at high angles to stretching lineation and oblique to the XY plane, resulting from dextral simple shear plastic deformation with dominance of basal slip; b) the Campanário Shear Zone showing c-axis fabric with well-developed maxima near Y-axis and a moderate fabric asymmetry (predominantly pure shear). Stereographic projections within the kinematic XZ section
The orientation of the foliations and lineation associated to the D2 and D3 events, indicate an evolutionary history with northwestward thrusting progressively changing to a NE-SW transcurrent tectonic regime (Figure 14), while the change from D3 to D4 represents a general uplift and cooling of the exposed rocks as suggested by the growth of low-grade metamorphic minerals.
Figure 14. Schematic 3D model of the tectonic configuration of the Médio Coreaú Domain
Schematic 3D model of the tectonic configuration of the Médio Coreaú Domain.