One kilometre-thick ultramylonite, Sierra de Quilmes, Sierras Pampeanas, NW Argentina

Finch, M.A., Weinberg, R.F., Fuentes, M.G., Hasalová, P., and Becchio, R. (2015) One kilometre-thick ultramylonite, Sierra de Quilmes, Sierras Pampeanas, NW Argentina. Journal of Structural Geology, 72. pp. 33-54.

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We describe a 1 km-thick ultramylonite forming the high strain base of the >3.5 km-thick El Pichao shear zone in the Sierra de Quilmes. This shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 Ma Famatinian orogeny. Strain grades upwards from ultramylonites to weakly sheared migmatites across the 3.5 km-zone and the mylonitic rocks define a geochemical field narrower than the protolith, suggesting they underwent mixing and homogenization through shearing. Ultramylonites this thick are uncommon. The width of a shear zone, in the absence of significant compositional rheological contrasts controlling strain localization, is controlled by the balance between shear heat generation and diffusion. Under typical crustal conditions a strain rate of 10−12 s−1 is required to form a 1 km-thick ultramylonite, and this is achieved when large movement velocities are imposed across the shear zone. We postulate that the El Pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny, and that the wide mylonitic shear zones characteristic of the Cambrian–Ordovician deformation of the Sierras Pampeanas result from the convergent movement being taken up by only a few active major shear zones.

Item ID: 73540
Item Type: Article (Research - C1)
ISSN: 1873-1201
Copyright Information: © 2015 Elsevier Ltd. All rights reserved.
Funders: Australian Research Council (ARC)
Projects and Grants: ARC DP110102543
Date Deposited: 29 Nov 2022 23:45
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 75%
37 EARTH SCIENCES > 3705 Geology > 370503 Igneous and metamorphic petrology @ 25%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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