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Pseudotachylite Related to Ultramylonites, Sierra de Quilmes, Argentina

 

generator plane

Roberto Weinberg, Monash University, Australia

 

 

 

 

This page illustrates the features of pseudotachylites developed in ultramylonites of the Pichao-Ovejeria Shear Zone in Sierra de Quilmes, NW Argentina. This is a Famatinian age shear zone (~470Ma) part of the Sierras Pampeanas. All photographs are from a region of 50 by 20m in the Ovejeria Pass close to Puesto Potrerillo. There are two features of the pseudotachylites at Potrerillos that make them interesting: ductile deformation related to pseudotachylite development, and melt loss in breccias indicated by fragment rotation but very little matrix.

 

Interpretation: many of the feature suggest that the pseudotachylite was a result of shortening leading to folding and thrusting. We have a few features that suggest foliding of the ultramylonite: a) thrust plane with pseudotachylite matrix on the plane and a drag fold on the hanging wall (0677), b) fragments that are bent around another one, apparently squashed (0632), c) a crenulated ultramylonite with pseudotachylite on axial plane but also folded (0684), d) pseudotachylite approximately on axial plane of fold (0695), e) pseudotachylite matrix accumulation on a fold hinge (0660 and 0628).

 

NB: Simpson et al. (2003 JSAmES) found that, like in our example, all the Pampean-age mylonitic shear zones (520Ma) in the Serras de Cordoba have pseudotachylites. "Almost all the mylonite zones in the Sierras de Cordoba are cross-cut by pseudotachylyte veins that are rarely found in unsheared rocks. A biotite fusion region of one of these veins from La Calera (Fig. 6(c)) was dated at 428 ^ 12 Ma (MSWD 0.71) by 40Ar/39Ar spot fusion analysis (Fig. 8, Northrup et al., 1998b)."

 

 


Generation and injection planes
Generator plane
Figure 1a) Single generation plane and co-seismic injection veins.
Generator plane
Figure 1b) Single generation plane and co-seismic injection veins.
injections with flow
Figure 1c) Pseudotachylyte injections.
flow fold
Figure 1d) Detail of previous, showing injection with a similar fold defined by color banding.

Folds associated with pseudotachylite
fold
Figure 2a) Pseudotachylyte accumulated at the hinge defined but the bent ultramylonite fragment above.
fold
Figure 2b) Close fold in ultramylonites with pseudotachylyte crossing the hinge zone. Notice accumulation under the coin.
fold
Figure 2c) Pseudotachylyte accumulated at the hinge zone of a fold, defined by rotated fragments. The question is whether the fold happened due to brecciation and pseudotachylyte formation.

 

Figure 2d) Breccia with rotated fragments with little interstital matrix. Fragments look to be squashed in between clasts. Notice fragment in the lower right that bends slightly around the corner of the fragment on the left. This may suggest a thrusting of one side over another with the bent fragments defining the thrust plane?

fold and squashed fragment
ultramylonite folded with pseudotachylite
Figure 2e) Folded, crenulated ultramylonite associated with pseudotachylites one of which seems to be axial planar. Blocks of ultramylonite are rotated but the present volume of matrix is small. There is a thin lens of pseudotachylite that is folded too.
thrust plane and drag fold
Figure 2f) Thrust plane with pseudotachylite matrix on the plane and a drag fold on the hanging wall
bent fragments
Figure 2g) Fold defined by rotated fragments separated by pseudotachylite matrix. Notice the bent hinge zone fragment.
bent fragments
Figure 2h) Rotated and bent fragments in breccia of ultramylonite separated by pseudotachylite matrix.

Breccias
breccia
Figure 3a) Pseudotachylyte breccias bounded by two parallel planes, possibly a result of the contemporaneous movement on two parallel generation planes. Fragments have been rotated in the pseudotachylite matrix.
Breccia
Figure 3b) Breccia in rusty matrix, possibly showing more than one generation of pseudotachylite matrix.
Pseudotachylyte breccia with rounded and rotated fragments
Figure 3c) Pseudotachylyte breccia with rounded and rotated fragments. Notice that some blocks preserve the process of disaggregation by the pseudotachylite matrix.