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Cross-Cutting Shear Zones/Ultramylonites in the Teixeira Pluton, Borborema Province

 

Roberto Weinberg, Monash University, Australia

Gorki Mariano, Universidade Federal de Pernambuco

Carlos Archanjo, University of Sao Paulo, Brazil

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

The Teixeira Pluton is cut by NE-trending sinistral shear zones and E-W trending dextral shear zones, typical of the large scale pattern of the Transversal Zone of the Borborema Province. These shear zones in the Teixeira Pluton can be inferred from lineament patterns on satelite images and produce deep valleys and controls the northern boundary of the pluton. In this page we document field relationships at an outcrop close to the northern margin of the pluton at Tendo, along the road from Patos to Teixeira (24M 0692226E 9203522N). There are three important aspects of the shear zones revealed here: a) overprinting relationships between the NE-trending sinistral and E-W trending dextral shear zones; b) most beautifully illustrated here, the ability of ultramylonites to effectively cross-cut the surrounding rocks; and c) the contemporaneous block movement of the country rock and internal folding of ultramylonites.

 

 

Steep, fault bounded, E-W trending. margin of the Teixeira Pluton in the State of Paraiba (close to the town of Patos). Ultramylonite band splaying into a tail of shear zones (the scale marks the limit of the splays). This is the same band depicted in sections b-c below. NNE is to the right of the photograph.

 

 

a) Overprinting shear zones

 

The photographs depict the overprinting of different shear zone orientation. Partly there are sinistral shear zones of different NE-trending orientations, cross-cutting one another, partly there are sinistral and dextral shear zones cross-cutting and overprinting each other.

 

Figure 1a: Overprinting sinistral shear zones, rotated from their main NE-trend to N70E. Figure 1b. Zooming in on a. Figure 1c. Further zoom.

 

Figure 2. Sinistral ultramylonite trending N60E/45N (well-developed C') cutting across a dextrally sheared granite with C planes trending 120. Figure 3. Cross-cutting shear planes on granite (diagonal lines across the photograph). Figure 4. Same as Fig. 3.

 

 

b-c) Ultramylonites cross-cutting country rock, and contemporaneous folding and block movement

 

>The following photographs depict the ability of mylonites/ultramylonites, once formed, to behave as a different rock type and in a different manner when compared to the undeformed or weakly deformed protolith (granite). Here, not only is shearing localized, but also the mylonite effectively truncates structures in the surrounding rocks.

 

The structures also record the shear folding of the ultramylonite to fill up the space created by rotation of blocks in the country rock.

 

Figure 5a. Irregular sinistral ultramylonite band (025/ 75E-90) in coarse granite. The banding and orientation of the ultramylonite generally cuts abruptly across the granite, with no transition. Only locally there is grading in grain size and in deformation to the granite (Fig. 8), which demonstrates that ultramylonite developed from a granite, as opposed to being made of a different rock type. NNE is to the right of the photograph. Figure 5b. Irregular margin of ultramylonite, depicting shear fold into a space between granite blocks bound by narrow semi-brittle faults. Arrow on scale indicates N. Figure 5c. Zoom into the folded area. We interpret the structure to indicate that the fold developed as a result of continued ductile shearing in the ultramylonite during block rotation of the surrounding granite. As the blocks rotated (bookshelf faulting), space opens up and the ultramylonitic material flows into it. This points to contemporaneous contrasting behaviour between granite (block faulting) and ultramylonite (flow).

 

Figure 6. Irregular boundary of the same ultramylonite band of Fig. 5, showing again faults in the granite affecting mylonite flow. NNE is to the right of the photograph. Figure 7. Same band, shoing local faulting in granite disturbing flow in ultramylonite. NNE is to the right of the photograph. Figure 8.. The same ultarmylonite band here shows a gradation to granite close to the margins, and the sheared granite defines asymmetric shear folds indicating sinistral shearing. Unlike other parts of this shear bands, the structures are continuous between the sheared band and the country rock. NNE is to the right of the photograph.

 


Figure 9. Similar feature from Fengshan pluton in North China Craton, ultramylontie shear zone with very sharp boundary to gneissic granodiorite
Figure 10. Also from the Fengshan pluton, a shear zone in granodiorite where on the one side the boundary is sharp from a granodiorite to an ultramylonite, whereas on the other side the strain gradient gradually changes.