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.
变形带（变形包括体变和形变）deformation bands，这一概念是Aydin1978年首先提出的，是指砂岩或孔隙性砂岩地层中因局部发生应变而产生的一种板状构造。变形带构造可以分为三类：(1)具有明显剪切位移的，称之为形变带断层 (Mollema & Antonellini, 1996), (2) 发生局部孔隙减少且没有剪切位移的，称之为压实带 (3) 发生孔隙增大且没有宏观剪切位移的，称之为膨胀带(Du Bernard, Eichhübl & Aydin, 2002). 根据颗粒破碎程度和粘土含量，形变带断层又进一步划分为(Antonellini, Aydin & Pollard, 1994): (a)略微或没发生碎裂的, (b) 发生碎裂的 (c) 发生泥岩涂抹的.
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.
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