Microfabric diversity and grain shape analysis of fault rocks from the selected areas of the Western Tatra Mountains

Maciej Kania

doi:10.7306/gq.1129

Abstract

Fault rocks in brittle and brittle-ductile shear zones played a key role in the evolution of the Western Tatra Mountains crystalline rocks (Poland–Slovakia). Microfabrics of these rocks, including grain shape analyses, were investigated in the six areas of the Western Tatra Mountains. Based on studies of thin sections, 14 types of fault rock microfabric are distinguished, according to the following criteria: (a) the presence and abundance levels of a cataclastic matrix and (b) the presence and form of a preferred orientation features. General tendencies observed in these areas indicate southwards increasing non-coaxial deformation as well as the domination of ultracataclasites or ultramylonites to phyllonites in areas with negative relief (e.g., sedlo Zabrat’ Pass, Dziurawa Przełęcz Pass). A model of shear zone evolution embracing following three stages is proposed: (1) deformation partitioning and block-controlled cataclastic flow, (2) matrix-controlled cataclastic flow, (3) selective leaching and deposition of silica, leading to the formation of softened and hardened deformation domains respectively. These microstructural observations were supported by statistical analyses of the grain shape indicators (compactness, isometry, ellipticity, solidity, convexity). Two trends of relationships between compactness and convexity were noted: the first, horizontal on the correlation diagrams, was interpreted as an effect of rapid cataclasis and then sericitization, the second, with a strongly negative correlation coefficient, was considered as an effect of long-term cataclastic flow. The different microfabric data and microstructural interpretations described in this paper are consistent with a new model of the tectonic history of the Western Tatra Mountains evolution, with an important role for a non-coaxial deformation during Alpine orogeny in brittle and brittle-ductile conditions.

Highlights

The processes of non-coaxial shearing may induce structural, mineralogical and geochemical modifications in deformed rocks
The subdivision of the fault rocks applied in this paper is according to the classification by Woodcock and Mort (2008) where prefix protomeans a content up to 50% of matrix, mesomeans 50–90% of matrix and ultra- means more than 90% of matrix
Fault rocks occurring in the Western Tatra Mountains are characterized by strongly heterogeneous fabric and microfabric, which indicates different and changing conditions of pressure, temperature, mechanism and rate of strain during their development

Summary

INTRODUCTION

The processes of non-coaxial shearing may induce structural, mineralogical and geochemical modifications in deformed rocks. The main processes responsible for these changes are: cataclastic flow embracing development of intragranular fractures and following slips and rotations, according to the local sense of shear, as well as pressure solution and dynamic recrystallization (Passchier and Trouw, 2005) These grain modifications can be described using a grain shape analysis method, based on the measurements of shape indicators in a thin section of the rock. The Tatra Mountains crystalline core, together with its paraautochthonous sedimentary cover is included into the Tatricum tectono-facial unit (e.g., Plašienka, 2003) This unit was covered by sheared and overthrusted strata of the Fatricum and Hronicum basins, forming the Krížna and Choè nappes, known as Lower and Upper Subtatric nappes (Piotrowska, 2009; Uchman, 2009). The Variscan tectonic exhumation of the Tatra Massif occurred in conditions of top-to-the south thrusting and E–W extension (Jank et al, 1999)

METHODS

DISCUSSION

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CONCLUSIONS