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Quebrada Belem, Catamarca, Argentina: sheared K-feldspar megacrysts




Roberto Weinberg and Tato Becchio,
School of Geosciences, Monash University, Australia
Universidad de Salta, Argentina




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:


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.



This page documents the internal fracturing and shearing of K-feldspar megacrysts in a shear zone that varies in shear sense at a scale of 10 to 50 cm between W-sied-up and E-side-up on very steep foliation with a down dip lineation. The protolith is a K-feldspar megacrystic granite with phenocrystic blue quartz. The shear zone outcrops in the Quebrada de Belen just before arriving at the town of Belen in Catamarca Province. Feldspars have internal fractures that accommodate either synthetic or antithetic movement in regions with both shear senses. We have not determined whether there is an overprinting relationship between the two shear senses, and in some sections there are both shear senses in the same area making shear sense determination ambiguous. There are also K-feldspar with fractures at right angles to the grain margins that are simply stretched in a brittle fashion, and others that have an hourglass shape with no evidence for fracturing. E-side-up shear sense is typically on planes at 70 to 85 / 80 to 65 (dip direction/dip) whereas West-side-up shear sense is on planes dipping 260/sub-vertical.


sheared blue quartz megacryst asymmetry
a) Sheared K-feldspar and blue quartz porphyritic granite. b) E-side up megacryst with asymmetric tails and a smaller one on the west (left) showing synthetic fractures. .
antithetic fracturing synthetic fracturing
c) E-side-up shear sense, and fractured and sheared Kfs megacryst with antithetic microfaults. d) E-side-up shear sense, and fractured and sheared Kfs megacryst with synthetic microfaults. Note presence of C' shear planes obliquely from lower right to upper left. .
antithetic fracturing synthetic fracturing
e) W-side-up (left up) on 260/sv. Sheared Kfs megacryst, antithetic microfaults in both megacrysts shown. . f) W-side-up (left up) on 260/sv. Sheared Kfs megacryst, synthetic microfaults.
antithetic fracturing Tensile cracks
g) E-side up, antithetic and synthetic microfaults in K-feldspar megacrysts. . . h) Tensile cracks in K-feldspar megacryst.
Hourglass megacryst Hourglass megacryst
i) Hourglass phenocrysts of K-feldspar with no evident internal fractures. j) Hourglass megacrysts of K-feldspar. E-side-up (right-side-up) shear sense and fractures in Kfs are synthetic.