Geologie

Deformation mechanism in porous sediments

Introduction

Deformation bands (Aydin 1978) are one kind of frictional deformation structures in the uppermost Earth´s crust. Deformation bands, which were first described by Aydin (1978), may be defined as tabular structures of finite width resulting from strain localization commonly found in sand and porous sandstone. Three end members cases of these structures are distinguished: (1) deformation bands with clear shear offset, which have been termed deformation band faults (Mollema & Antonellini, 1996), (2) compaction bands that refer to tabular bands of localized porosity reduction that lack shear offset, which have been termed compaction bands and (3) tabular bands of localized increase in porosity that lack a macroscopic shear offset, which have been termed dilatation bands (Du Bernard, Eichhübl & Aydin, 2002). According to the degree of grain fragmentation and clay content, deformation band faults are further divided into three groups (Antonellini, Aydin & Pollard, 1994): (a) with little or no cataclasis, (b) with cataclasis and (c) with clay smearing.

 

Deformation band faults (for recent reviews, see Mair, Main & Elphick, 2000; Main et al. 2001) are typically about 1 mm wide, roughly planar deformation structures that show shear deformation in the range of millimetres to a few centimetres. The slip-to fault-length ratios are low compared to ordinary faults with slip surfaces in dense lithologies (Fossen & Hesthammer, 1997). Single cataclastic deformation band faults accommodate deformation across the entire band width by collapse or increase of porosity, grain fracturing, grain size reduction and cataclastic flow that lacks a discrete discontinuity surface. In high-porosity rocks under low stresses, grain fracturing may be absent and deformation is accommodated also by grain rotation, grain sliding and porosity reduction. Deformation band faults are solely the result of a displacement gradient (over a narrow tabular zone), in contrast to faults with slip surface which must include a displacement discontinuity.

 

Generally, deformation band faults may occur alone, but usually they group multiple, sub-parallel, closely spaced zones. The formation of zones of deformation band faults each of which have limited slip, is explained by repeatedly shifts of deformation to form new bands in order to accommodate bulk strain. The zones of deformation band faults are thought to grow by addition of new deformation band faults, side by side, thus the thickness of the zone depends on the number and spacing of individual deformation band faults.

 


Theoretical diagram demonstrating the difference between “ordinary” faults and deformation band faults. (a) Faults usually form discrete slip surfaces in fully cemented rocks. Once a slip surface is formed, subsequent strain is focused on this surface because of strain localisation mechanisms of this kind of deformation. (b) Deformation band faults are typical for deformation in porous or even completely uncemented granular material, where strain is accommodated by the formation of slightly undulating deformation surfaces. Porosity reduction, grain rotation and grain fracturing result in an overall strain hardening type of deformation.

The shifting of deformation is thought to be due to strain hardening via increased grain friction in the bands during grain breaking and porosity reduction processes. There are two models explaining the sequential groth of zones of deformation band faults. In the model of Aydin & Johnson (1978) the strain hardening mechanisms result in a sequential growth of deformation band faults with deformation widths of few millimetres into zones of deformation band faults with deformation widths of up to several tens of centimetres and finally into zones of deformation band faults with a slip surface on either side with offset of up to several tens of meters. Shipton & Cowie (2001) expand this classical model and mention that small slip-surfaces may already nucleate at a relatively early stage in the evolution of a zone of deformation band faults. With increasing strain the slip-surfaces propagate and link within a growing zone of deformation band faults. In the Shipton & Cowie (2003) model, both types of faults grow contemporaneously and interdependently from each other, controlled by the transition from strain hardening to strain softening and strain localisation.

Case study

Tectonic evolution studies of the Himalayan mainly focus on the Tertiary deformation and kinematics resulting from the Indo-Asian collision. Due to the magnitude and intensity of Himalayan tectonics in the rocks that comprise the Himalayan orogen, any pre-existing, older structures deformed by pre-Himalayan events are obscured or only partly preserved. As a result, pre-Himalayan deformation episodes are poorly documented and mainly inferred from lithostratigraphic anomalies such as tectonic unconformities; reports on pre-Himalayan structural field data are extremely rare. In recent years, increasing interest has been focused on possible pre-Himalayan deformation structures and their attendant influence on the Tertiary kinematic evolution of the Himalayas. We investigate deformation band faults in quartzites of the Lower Devonian Muth Formation (Pin Valley, NW Himalaya). The purpose of this investigation is to constrain the orientations, kinematics and microstructural characteristics of these deformation band faults. Based upon a clear separation of these structures from later faults in the same formation that have clearly different orientations as well as deformation mechanism a pre-Himalayan age for the deformation bands can concluded.

 

 


Geological map of the Pin Valley modified after Fuchs (1982). (b) Stereo plots (equal area projections; lower hemisphere; contours at 3-times and 5-times the random distribution) of deformation band faults (filled circles) and zones of deformation band faults (filled triangles) in the Muth Formation, at locations 1 to 3, southeast of Mikkim. Deformation band faults have been rotated to account for Eocene folding. Faults have been rotated to account for Eocene folding.

Left: Detail of a deformation band fault with well-developed cataclasis but absence of any internal foliation. Note the increase of grain size distribution due to refinement of some grains and porosity decrease compared to undeformed quartzite.
Right: Photomicrograph of area of the left Figure under sensitive tint plate shows a great variety of colours of the fragmented quartz grains suggesting rotation and translation of the quartz fragments.

Left: Photomicrograph of thin section of a protocataclasite in the Muth Formation that formed clearly after cementation of the quartzite. Microfaults cross cut the quartzite displacing aggregates of grains (open arrows) (crossed polarized light).
Right: Photomicrograph of area of left Figure but under sensitive tint plate showing crystal preferred orientation indicative for intracrystalline deformation.

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