Clay Smear Processes in Mechanically Layered Sequences ...friction angle and a shallower dip in the layers with a lower friction angle (Mandl, 1988; Peacock and Sanderson, 1992, Egholm

JOUR.GEOL.SOC.INDIA, VOL.75, JAN. 2010

74 JOYCE SCHMATZ AND OTHERS

Clay Smear Processes in Mechanically Layered Sequences – Resultsof Water-saturated Model Experiments with Free Top Surface

JOYCE SCHMATZ1, MARC HOLLAND

1,2, STEFFEN GIESE3, WOUTER VAN DER ZEE

1,4 and JANOS. L. URAI1

1Structural Geology, Tectonics and Geomechanics, Geological Institute, RWTH Aachen University,Lochnerstrasse 4-20, 52056 Aachen, Germany

2Present address: GeoMechanics International, Emmerich-Josef-Strasse 5, 55116 Mainz, Germany3Present address: Bilfinger Berger AG, Group Technology, Carl-Reiß-Platz 1-5, 68165 Mannheim, Germany

4Present address: GeoMechanics International, Parkweg 2, 2585 JJ, The Hague, The NetherlandsEmail: [email protected]

Abstract: The aim of this study is to improve our knowledge of the processes that lead to clay smear during faulting ofa layered sand-clay sequence in an analogue sandbox model. We carefully characterized mechanical properties of thematerials used by a series of geotechnical tests. Displacement field was quantified using PIV (Particle Image Velocimetry).The model is water-saturated to allow the deformation of wet clay and sand in one experiment comprising a sandpackage with a horizontal layer of clay above a predefined rigid basement fault. The thickness and rigidity of the claylayer are the parameters varied in this study. The model shows a range of structures that are related to competencecontrast between sand and different clay types. Results show ductile shearing of soft clay with a transition to brittlefracturing of stiff clay accompanied by the formation of rotating clay blocks in the fault zone. Localized deformation isobserved through time showing (i) the propagation of one active fault migrating laterally through the sediment package,and (ii) the formation of a stable prism between two or more active faults that gets progressively smaller with minorrotation of the hanging wall fault. Continuous clay smear is observed resulting from the lateral injection of clay as wellas from a reworked mixture of sand and clay.

Keywords: Clay smear, Fault seal, Mechanical heterogeneity, Normal fault, Analogue sandbox model.

et al. 1978; Lindsay et al. 1993; Vrolijk and van der Pluijm,1999; van der Zee and Urai, 2005; van der Zee et al. 2005;Bense and Person, 2006; Gudehus and Karcher, 2007;Egholm et al. 2008). Clay smear captures the incorporationof clay or shale in the fault zone of a layered sequence (e.g.Clausen and Gabrielsen, 2002; van der Zee, 2002). Themechanisms that control the geometry and kinematics ofclay smear are poorly understood. However, they are thekey issues to predict the amount and distribution of clayor shale in the fault gouge.

An important aspect of the development of a deformationband in a granular media is the first localization of strain.This is initiated at spatio-temporal fluctuations of porosityand grain-to-grain stresses (Nübel, 2002; Gudehus andNübel, 2004), in zones of lower density, where the rigidityof the particle aggregate is relatively small. Wolf et al.(2003) investigated the dependence of the developingdeformation band system on boundary conditions andmaterial properties. Their results show that a uniformlydistributed set of shear bands develops in a “homogeneous”

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.75, January 2010, pp.74-88

INTRODUCTION

In this study an analogue sandbox model (Fig. 1) is usedto study fault gouge evolution in a layered sand-claysequence, building on investigations by Richard andCobbold (1990), Richard and Krantz (1991), van der Zee(2002) and van der Zee and Urai (2005). Analogue modelinghas been widely used to investigate evolution of extensionalfault regimes. A major focus in most publications is thescaled simulation of large-scale tectonic structures and theevolution of structural style (e.g. Lindanger et al. 2004;Schreurs et al. 2006; Holland et al. 2006). Dry sand and wetclay have both been used to model faulting in the uppercrust (e.g. Cloos, 1930; Mandl, 1988, 2000; Lehner andPilaar, 1997; Fulljames et al. 1997; Clausen et al. 2003).

In our experiments we focus on smaller scale processesto get a better idea of the kinematics and the mechanicalprocesses during the evolution of fault gouge. In particularpredictions of clay smear in a fault gouge are used forpredictions of fluid transport and sealing capacity offaults and are of considerable economic interest (Weber

0016-7622/2010-75-1-74/$ 1.00 © GEOL. SOC. INDIA


CLAY SMEAR PROCESSES IN MECHANICALLY LAYERED SEQUENCES 75

sand that overlies a homogeneously stretching rubbermembrane at strains between 2 and 6%.

In a mechanically stratified sequence, fault localizationand propagation is also influenced by the contrast in strengthof the materials. In heterogeneous layered sequences withcompetence contrasts, e.g. clay and sand, the faulttheoretically assumes a steeper dip in layers with a highfriction angle and a shallower dip in the layers with a lower

friction angle (Mandl, 1988; Peacock and Sanderson, 1992,Egholm et al. 2008).

In a “real” sand-clay sequence this is more complicateddue to the segmented geometry of the fault zone, whereprocesses like lateral clay injection can be initiated (see alsovan der Zee et al. 2003). Analogue models to investigatefault zone evolution in mechanically stratified sequenceswere presented by Karakouzian and Hudyma, 2002;

Fig.1. (a) Sketch of “Underwater” Sandbox. (1) water saturated sandbox (inside), (2) waterproof container (outside), (3) rigid basementbottom with drain perforation, (4) motor driven fault offset, (5) basement fault dips 70°, (6) movable glass plate (inside),(7) movable glass plate (outside). (b) Block model showing the general statigraphy of a sandbox experiment, with a layer cake ofsand and clay (from bottom to top: 5 mm sand, 3-30 mm clay and 140 mm sand) overlying a rigid basement fault having 70° dip.



Medewedeff and Krantz, 2002; van der Zee and Urai, 2005;Schreurs et al. 2006. Clay gouge properties as a function ofnormal stress were examined by Clausen and Gabrielesen(2002) using a ring shear apparatus with sand and wet clayas model materials. Eisenstadt and Sims (2005) made acareful comparison of analogue models made of dry sandor moist sand to examine the differences in the fault patternand the applicability of the materials as an analogue to theupper crust. Dry sandbox-experiments are well known (e.g.Horsfield, 1977; Mandl et al. 1977). The behavior of wetclay has also been investigated in numerous experiments(e.g. Cloos, 1930; van der Zee, 2002; Eisenstadt and Sims,2005). However, it is not possible to run experiments withboth cohesionless dry sand and wet clay, as this leads towater absorption in the sand producing cohesion due tocapillary forces, which strongly influences the model. Inour study a sand-clay layer cake was deformed in a water-saturated sandbox (first described by van der Zee, 2002)(Fig.1).Adam et al. (2005) have reported on water-saturatedexperiments in a homogeneous, fully water-saturatedpackage of sand. Results were comparable to dry sandexperiments of Horsfield (1977) done with similar boundaryconditions. Deformation started with a curved, precursorfault (Mandl, 2000), which initiated close to the basement-controlled fault tip. With ongoing shearing this fault movedthrough the material towards the footwall until it reachedits kinematically preferred orientation and the curvature wasstraightened out progressively. In this study we build on theseexperiments, adding one horizontal clay layer to the sandpackage with variable rigidity and layer thickness (Fig.1b).

As described above in a range of experiments a majorfocus is laid on the careful scaling of the model materials toallow a direct application to natural settings. With our modelsetup we aim to understand the interplay of the modelmaterials at granular scale. We focus on the effect of thecompetence contrast on shear zone development and claysmear.

METHODS

Boundary Conditions

Clay smear is ideally investigated starting from a model,which would develop one single, planar deformation bandin homogeneous material. Here, throw is largest in the centerof the deformation band and decreases to zero at the faulttip (Fig.2). Experimental simulation of this geometry is notpractical however, because the desired displacements ofseveral cm would require a very large sandbox model.Therefore the design of our sandbox experiment incorporatestwo important boundary conditions. The basement is a stiff

plate acting as a rigid guide. The predefined basement faultdips at an angle of 70° in the direction of the movablehanging wall. The top of the sediment package is 2 cm belowthe water level. A rigid, faulted base and a free surface atthe top of the sandbox will always initiate a fault propagatingup and curving towards the surface (Patton and Fletcher,1995; Mandl, 2000). The water column is the same for allexperiments to provide comparable conditions to all setupsregarding the pore pressure at the interface of the clay layerto the sand. Focusing on the contrast in competence of thedifferent materials the clay is supposed to be impermeablewhile the sand in contrast is accounted permeable. Agradation of permeability in the sand due to its compactionincreasing with depth is of minor influence to the pore fluidflow when compared to the competence contrast of thedifferent materials and is therefore neglected in this study.

Setup

The dimension of the box allows deforming a layerpackage with a size of 20×40×20 cm to a displacement ofup to 60 mm, with the structures depending on both thelayering and the boundary conditions. The whole sandboxis watertight like an aquarium. The front of the box is closedwith a removable window sealed with rubber.

Small holes (2 mm) in the basement block are coveredwith filter paper to make them permeable for water but sealfor sand grains. While setting up the experiment, the waterlevel is raised simultaneously with the filling of sand andclay. The bottom aluminum plate is covered with a sandlayer of 5 mm thickness to ensure a reasonable permeabilityat the base of the model. Sand was put in the box by

Fig.2. Schematic diagram showing displacement along an isolatedfault with arrows indicating extension of the body: if a faultis isolated in a medium, throw is theoretically largest in thecenter of the fault plane and decreases to zero at the faulttip. The large offset within the fault zone needs to fadeoutwards in all three dimensions. Lower right corner: Mohrdiagram illustrating stress conditions and failure mode withnormal stress �N. and shear stress �.



dispersing into the box from a height of around 20 cm toreach the densest packing. The clay layer was put on top ofthe basal sand package with a distance of 4 cm to the sidewalls to ensure permeability of the model and hydrostaticpressure. After smoothing the surface it was loaded with140 mm of sand. Therefore, the effective vertical stress atthe top of the clay was 0.75 kPa in all experiments. Thesand included thin marker horizons consisting of sand dyedwith methylene blue (coloring does not change the materialproperties). The experiments were run by moving thepredefined basement fault to a total fault throw of 30 mmwith a rate of 0.2 mm per minute. Deformation was observedand documented with high-resolution digital images throughglass panels on the front- and backside of the model.

MATERIALS

Sand

We used washed quartz sand with a grain size rangingbetween 0.1 to 0.4 mm. It is well sorted and consists ofangular to well rounded grains. The angle of friction in drycondition is 28°. Quality and composition of the sand wasmaintained throughout all experiments. The material wasnot pre-compacted for the experiments. With a grain densityof 2.57 g/cm3 and a porosity of 41 % the bulk density of thewater saturated sand package was 1.52 g/cm3. Measurementsshow that no significant compaction took place after thematerial is placed in the experimental setup (Fig.3).

Clay

For all experiments the same clay composition was used.The clay granules #801 (Royal Mosa B.V. Maastricht) are amixture from different quarries. X-ray diffractometryshowed that it consists of quartz (46.8%) and kaolinite(42%) (+9% muscovite, 1.3% anatase, <0.5% rutile). Thewater content of clay was used to control its shear strengthand state of consolidation. In a pharmaceutical mixingvessel the clay granules with an initial water content of6 wt% were mixed to a firm, normal or soft rheology bycontrolling the content of water. The components were mixedup for several hours until a homogeneous clay paste wasproduced. Soft (~50 wt% water) and firm clay (~30 wt%water) represent underconsolidated and overconsolidatedend-members, respectively, when compared to the stress inthe experiment. The normal clay (~45 wt%) was inequilibrium with the effective overburden load of 140 mmsand (0.75 kPa). The classification of these three strengthclasses resulted from subsidence tests. Subsidence curves(Fig.3) from different experiments were measured overseveral days. A 30 mm layer of soft clay (52% water content)

was placed into a glass cylinder overlying a bottom layer of20 mm sand, and loaded with different thicknesses of sand.The sand load of 140 mm equals the setup used in thesandbox experiments. Subsidence measurements on a puresand column of 140 mm were made as reference.

We used a thin (2×2 cm) glass plate on top of the sandon which a LVDT (Linear Variable DisplacementTransducer) was placed to measure the vertical displacement.The data shows that sand does not consolidate. The softclay (30 mm thick) compacts for approximately 2.5 days.After four days this clay was sampled and its water contentwas measured. The water content decreased from 52 wt%to 46 wt%, which corresponds to a subsidence of 5.2 mm.This water content was used to define normally consolidatedclay in the sandbox experiments. A mixture that contains aslittle water as possible (<30 wt%) to produce a homogeneousclay paste in a mixer was defined as firm clay.

The three different rigidities correspond to differentfailure behaviour ranging from plastic to brittle fracturing.To measure the shear strength, the different clay typeswere tested in a direct shear cell described in van der Zee(2002). It consists of a lower table that moves laterallyand of an upper table by means of which the normalstress can is controlled. A motor and a piston are used

Fig.3. Diagram showing subsidence curves of soft clay. In a claysetting experiment clay layers with varying thicknesses (3,12 and 30 mm) of very soft clay (52% water content) wereloaded with 140, 80 and 20 mm sand. The subsidence ofthe column was measured over a period of more than 2days to see how fast the material settles and dewaters.After two days, the samples were retrieved to measure thewater content. The clays showed (4 measurements) watercontents of around 45 %. This value is considered as thewater content of normal consolidated clay with a top loadof 140 mm sand.

time [days]

subsid

ence [

mm

]

Subsidence for 30 mm clay (52 wt.% water)

140 mm sand load (1.8 kPa)



sand only

40.50 1 1.5 2 2.5 3 3.5 4.50

1

2

3

4

5

6

7



to drive the lower table. A load cell and a LVDT were usedto measure the shear load and the displacement of thelower table.

A layer of clay with a thickness between 9-19 mm wasplaced on the 150×150 mm wide lower table. The uppertable was loaded to a vertical stress of 300 Pa for firm clayand 225 Pa for normal and soft clay. This stress regime, aswell as the unconsolidated and undrained conditionswithin the apparatus, corresponded to the conditions in thesandbox. With constant shear rates between 7×10-5 s-1 and1.4×10-4 s-1 the force to move the table was measured. Whilethe difference in water content was 20 % units between the

soft and the firm clay, their peak stress differed by a factor10 (Fig.4).

ANALYSIS WITH PIV

Particle Image Velocimetry (PIV) is a non-intrusive,optical technique to observe movements and flows. ThePIV-analysis was originally developed and has beenwidely applied to visualize displacement vector fields ofgases and liquids (e.g. Hage and Werther, 1997; Newberyet al. 2004; Liu et al. 2005). Recently it has also beenapplied to granular materials (e.g. Wolf et al. 2003; Adamet al. 2005; Steingart and Evans, 2005). Our experimentswere processed with DaVis strain master 6.2, produced byLaVision GmbH, Göttingen. The analysis enables measuringof displacements of a granular structure on the particle scale.This produces detailed velocity fields. The analysis useshigh resolution digital photographs of the models that weretaken in regular time intervals. The numerical procedure isas follows:

The digital images are subdivided into smaller fields ofinvestigation (“interrogation windows”). The position ofthe interrogation cells in successive images is locatedusing statistic correlation algorithms (“cross correlation”)(Fig.5). The absolute spatial resolution (dx) of thedisplacement field data depends on the optical resolutionof the digital camera system and the precision of the crosscorrelation algorithm (Adam et al. 2005).

In this study the base of the PIV-analysis was made upof digital photos of the sandbox experiments with aresolution of 2 Mpx. For each experiment the photos weretaken in a 30 second interval. The size of an interrogationcell was about 20 sand grains in most analyses (e.g. Nübeland Weitbrecht, 2002). Sand and clay have differentsurface structures, and an interrogation window size of128 pixels squared decreasing to a window of 32 squarepixels with an overlap of 75 % turned out to be best choice

Fig.4. Stress-strain diagrams derived from direct shear tests onclays used in the sandbox experiments: setup with 300 Pa(firm clay) or 225 Pa (soft and normal clay) vertical stress.The strain rates depend on the clay thickness. Thereforethe magnitude ranges from 7×10-5 to 1.4×10-4 s-1. Withinthis range, the magnitude of the peak stress remains similarin the respective clay mixture. Please note: the small loadof 225 Pa allows for high amplitude in the graph for normalclay.

Table 1. Overview of experiments described in this paper

Experiment Thickness Layer setup Clay # images Photo Offset Waterclay (sand/ clay/sand) type interval velocity content

(mm) (mm) (sec) (cm/min) (wt%)

ged01 10 5/10/140 soft 224 30 0.2 52

ged03 3 5/3/140 soft 268 30 0.2 52

ged04 10 5/10/140 firm 276 30 0.2 27

ged05 30 5/30/140 normal 290 30 0.2 44

ged06 10 5/10/140 normal 300 30 0.2 40

ged07 3 5/3/140 normal 298 30 0.2 44

ged08 30 5/30/140 soft 310 30 0.2 50

ged10 30 5/30/140 firm 296 30 0.2 28

ged11 3 5/3/140 firm 302 30 0.2 28

strain

shear

str

ess [

kPa]

Shear test diagram for different clay types

0 0.2 0.4 0.6 0.8 1 1.2

0.5

0

1.0

1.5

2.0

2.5firm clay

normal clay

soft clay



for the total experiment. This produced a final window sizeof 8×8 pixels, which corresponded to approximately 16sand grains.

RESULTS

A total of nine experiments have been carried out inthis study (Table 1). We varied only two parameters: thestrength of the clay and its layer thickness. Informationon fault activity and propagation in the sand package isnormally only provided by displacement of marker horizons.PIV allows calculation of the displacement field at the scaleof individual sand grains, and from this the calculation ofthe incremental stain tensor field. In all experiments,incipient displacement along the basement fault initiated a‘precursor’ fault (Mandl, 2000) in accordance with the stressfield above a rigid basement fault (Patton and Fletcher,1995). With progressive displacement of the basement fault,the fault migrates through the material into a kinematicallyfavourable position. This way a triangular zone of strongdeformation (Cardozo et al. 2003) or a less deformedzone bounded by two active faults were formed. Thedeformation zone reached its kinematically stable

configuration after about 1.5 cm (equals 75 min or 150 imagecaptures) displacement.

In our sandbox two principle developments of faultsystems are observed (Fig. 6). Figure 6a shows an initiallysteep fault that rotates progressively towards the footwall,whereas Fig.6b illustrates the fault zone architecture causedby two active faults, one initiating in steep orientation and asecond one in continuation of the rigid basement fault. Thelatter develops fault prisms. The results of the physicalmodels and the PIV calculations displaying the velocityvector field as well as the rotational component of z(Figs.7-9) are described below:

Soft Clay Experiments

Soft clay in a 3 mm thick layer (ged03, Table 1) hadonly minor effect on the localization of faults (Fig.7a, firstrow). The kinematics was comparable to those ofexperiments that contained only sand (compare with Adamet al. 2005). In this experiment the clay was sheared in thefault zone and worked into a mixture of sand, clay and water.The fault rotated from its initially steep position towardsthe footwall (Fig.7b, first row). The final fault orientationfollowed the kinematics of the basement fault (dashed line

Fig.5. Sketch illustrating the principal correlation routine of PIV (Particle Image Velocimetry). Measurements are performed by comparingsuccessive images of the area of intreset. In particular, the velocity field is calculated from the correlation of the particles (x, y -position) between two or more given time steps (t1, t2) (modified after Steingart and Evans, 2005).

t = t1, position [x(t

1), y(t

1)]

t = t2, position [x(t2), y(t2)]

velocity = [x(t2), y(t

2)] - [x(t

1), y(t

1)] / (t

2-t

1)



in Fig.7a) and remained there in a stable position. The clayband in fault gouge remained continuous over very largedisplacements. The effect of soft clay on the evolution oflocalization was much more pronounced in the experimentwith 10 mm thick layer (ged01, Table 1, Fig.7a, second row).An important feature of this experiment in comparison tothe soft clay experiment with a thin clay layer was theformation of a prism in between the two deformation bandswhich deformed very little (Fig.7b, second row). This madethe fault zone with clay much wider and - with furtherdisplacement - the clay in the fault zone formed a thick,

continuous clay smear. In the experiments with a 30 mmthick soft clay layer (ged08, Table 1) the stable fault prismin between the two active faults was even wider. (Fig.7a, b,third row). The initial fault evolved dipping approximately30° with respect to the basement fault in direction to thehanging wall and a second fault evolved in the plane of thebasement fault stabilizing a fault prism in between.Movement of this fault prism clay is very similar to the lateralclay injection by a squeezing block fault (van der Zee andUrai, 2005). As the clay was initially very thick thismovement resulted in a step-shaped clay gouge (Fig.7athird row).

Normal Clay Experiments

With incipient deformation, the 3 mm thick layer ofnormal clay (ged07, Table 1) was bent, followed by brittlefracturing. The part in the footwall formed an asperity, whichwas eroded by further movement until it was reworked intoclay gouge (Fig.8a, first row). The variety of processesoccurring at the clay layer resulted in complex kinematicsof the local deformation. However, PIV results also showsimilarities to the experiment carried out with soft clay witha layer thickness of 3 mm. The initially steep fault rotatesthrough the sand to its kinematically preferred plane (Fig.8b,first row). In the experiment with a 10 mm thick layer ofnormal clay (ged06, Table 1) minor brittle fracturing wasobserved, but the dominant deformation was plastic (Fig.8a,second row). The fault prism showed the same kinematicevolution as in the corresponding soft clay experiment witha layer thickness of 10 mm. The prism was stabilized inbetween two active faults (Fig. 8b, second row). In theexperiment with a 30 mm thick layer of normal clay (ged05,Table 1) deformation was even more complex. The firstfault’s dip was approximately vertical, curved towards thehanging wall. A second fault developed initiating from thelocation where the first fault intersected the top of the claylayer. These bounding faults remained more or less inposition. The sand flowed around the clay along the asperityin the fault trace. Asperities were eroded with ongoingdeformation (Fig.8a, third row). This complex interplay ofprocesses resulted in a wide zone of deformation at the prismbase, and discontinuous clay smear over prolonged timeperiods (Fig.8b, third row).

Firm Clay Experiments

The experiment with firm clay in a very thin layer of3 mm (ged11, Table 1) was very similar to the experimentwithout clay (compare to Adam et al., 2005). The first faultwas vertical and rotated clockwise towards the plane of thebasement fault (dashed line in Fig.9a). The migration of the

Fig.6. Sketch showing principle developments of faults observedin the experiments: the free surface in a sandbox allowsthe propagating fault to curve towards the surface in asteeper orientation than the predefined basement fault.(a) The fault rotates clockwise into its kinematicallypreferred plane causing a highly deformed triangular faultzone. (b) A less deformed fault prism is stabilized inbetween two active fault strands.

rigid basement

a

rigid basementb



ged03

ged01

ged08

5 cm

soft clay experiments

ged03

ged01

ged08

3 mm

clay thickness

10 mm

30 mm

offset: 0.5 cm 1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm


0.5 cm 1.0 cm 1.8 cm

0.3 cm 0.6 cm 1.8 cm

3 mm

10 mm

30 mm

a

bmax.0

Fig.7. (a) Matrix summarizing the experimental results for soft clay. Columns show offsets of 0.5, 1.0, and 2.0 mm, respectively. Therows display results on the different soft clay layer thicknesses of 3, 10 and 30 mm. The inclined dashed line is the basement fault.(b) Matrix showing the respective PIV results of experiments in matrix (a). The z-component of the particle rotation is shown inthe background overlain by the incremental velocity vector field in the foreground. See text for further explanation.



Fig.8. (a) Matrix summarizing the experimental results for normal clay. Columns show offsets of 0.5, 1.0, and 2.0 mm, respectively. Therows display results of the different normal clay layer thicknesses of 3, 10 and 30 mm. The inclined dashed line is the basementfault. (b) Matrix showing the respective PIV results of experiments of matrix a The z-component of the particle rotations shownin the background overlain by the incremental velocity vector field in the foreground. See text for further explanation.

ged07

ged06

ged05

5 cm

normal clay experiments

offset: 0.5 cm

3 mm

clay thickness

10 mm

30 mm

1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm

ged07

ged06

ged05


0.2 cm 0.6 cm 1.8 cm

0.2 cm 0.4 cm 1.4 cm

3 mm

10 mm

30 mm

a

bmax.0



Fig.9. (a) Matrix summarizing the experimental results for firm clay. Columns show offsets of 0.5, 1.0, and 2.0 mm, respectively. Therows display results of the different firm clay layer thicknesses of 3, 10 and 30 mm. Please note: the gentle waves in the 3 mmthick clay layer are due to initial model imperfections. The inclined dashed line is the basement fault. (b) Matrix showing therespective PIV results of experiments of matrix a. The z-component of the particle rotation is shown in the background overlainby the incremental velocity vector field in the foreground. See text for further explanation.

ged11

ged04

ged10

5 cm

firm clay experiments

offset: 0.5 cm

3 mm

clay thickness

10 mm

30 mm

3 mm

10 mm

30 mm

1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm

0.5 cm 1.0 cm 2.0 cm

ged11

ged04

ged10


0.2 cm 0.4 cm 1.8 cm

0.3 cm 0.6 cm 1.8 cm

a

bmax.0



fault through the materials resulted in inclined markerhorizons at the base of the fault gouge (Fig. 9a, b, first rows).In the experiment with firm clay in a 10 mm thick layer(ged04, Table 1) this layer was bent during the first few mmof offset. Then it broke by outer arc bending (Fig.9a, secondrow). The initial fault formed and became inactive beforepassing the clay layer. It was followed by a fault initiating atthe basement fault with a strong curvature towards thehanging wall (Fig.9b, second row). Sand flowed alongthis fault but did not affect asperities of the clay layer (incontrast to experiments carried out with soft clay). The faultthen rotated into the basement fault plane. The upward-pointing fractures of the clay were filled with sand flowingunder gravity forces during progressive deformation. Thefragmentation and the block rotation as well as the asperitiesin the clay generated a complex flow behavior of the sand,which caused the development of a more complex fault zonecompared to those observed in the soft and normal clayexperiments, with discontinuous clay gouge. The experimentwith a 30 mm thick, firm clay layer (ged10, Table 1) showeda similar evolution. At the points of the strongest curvatureopen fractures developed, and were filled with sand flowingdownwards. The clay block overlying the rigid basementfault remained rigid, and rotated a few degrees before settingin a stable position. Some fragments were sheared off whenpassing the rigid footwall basement block and from these, athin, continuous band of clay was formed in the gouge zone(Fig. 9a, third row). Above the clay layer a trishear evolved,similar to the principle decribed by Cardozo et al. (2003)(Fig. 9b, third row).

DISCUSSION

In this study we tried to investigate which parametersinfluence the final shape and composition of the gouge zone.In particular the amount and distribution of clay smear inthe gouge zone was examined. For this purpose we carriedout a series of analogue sandbox experiments in which wefocused on the two variables: (i) rigidity of the clay and(ii) the clay layer thickness. The effective vertical stress onthe clay layer amounts to 0.75 kPa. Nine experiments werecarried out, and offered a multiplicity of structural features.

Clay Properties

The graph in Fig. 4 shows the stress-strain curves of thethree clay mixtures used in the experiments. Thosemeasurements in combination with geotechnicalmeasurements on the shear strength, the Atterberg limits andthe adjusted consistency according to German standard DIN18122 allows us to propose a modified Cam-Clay-Model

on the clay’s mechanical properties. In the sketch shown inFig.10 firm clay can be classified as over-consolidated(wfirm < wNCL), medium clay as normally consolidated(wnormal ~ wNCL) and soft clay as under-consolidated(wsoft > wNCL). Reconstitution of clay under drainedshearing therefore results in softening of firm clayaccompanied by water suction while soft clay releases theexcess water and subsequently behaves similar to normallyconsolidated material.

Consolidation time for clay layers of various thicknessescan be determined from the subsidence curves (Fig. 3) ofthe clay layers in the sandbox experiments. According tothe measurements, consolidation of a 30 mm clay layer tookapproximately 75 hours. Assuming a quadratic relationshipof consolidation time to layer thickness the consolidationtime for a soft clay layer with a thickness of 3 mm can beestimated to be 45 minutes. Thus the thin layers did reach anormally consolidation stage before the sandboxexperiments were completed (150 minutes). This explainswhy experiments with soft and normal clay in a thin layerset up showed a similar shearing behaviour.

Clay Smear

Fulljames et al. (1997) summarize clay smear as a resultof a ductile flow of a clay source bed whereby the clay issqueezed into and smeared along the fault between the upand down thrown beds. The processes they describe are

Fig.10. Modified Cam-Clay model illustrating the mechanicalproperties of the different clay types, with w = watercontent,�N = normal stress, � = shear stress, NCL = normalconsolidation line and CSL = critical state line. Dashedlines record consolidation, and dewatering processes ofthe under-consolidated “soft” clay within the experimentalsetup. The over-consolidated “firm” clay as well as thenormal consolidated “normal” clay show softeningbehaviour incorporating water absorption. See text fordetails.



similar to those observed in the soft clay models. As shownin Fig. 11a in those experiments clay smear is derived fromductile flow of the source bed. Whereas models that contain

normal and firm clay continuous clay smear can developalong the fault plane. However, this is not the case over thefull duration of the experiment. The monoclinal flexuringof the clay layer causes tensile failure, which leads to brittlefracturing, and the formation of the fault (Fig. 11b) (Schöpferet al. 2006). Fracturing occurred simultaneously in regionswhere the clay layer is in tension and from a very early stageonwards the main fault develops along the right outer-arcextension crack where consequently shear displacementconcentrates. The water-saturated conditions eliminate thesurface tension in the clay (Cloos, 1955) and therefore thetensile strength of the material is reduced. Hence, sheardisplacement results in tension gashes. Fracturing leads tothe asperities within the clay, which are competent enoughto resist the erosion forces of flowing sand grains. Thereworking and mixing of clay asperities and clay fragmentswith sand cause the thickening of the clay bands.

Fault Evolution

First we define some geometric parameters that help usto summarize and evaluate the results. As listed in Table 2,clay smear that evolves continuously or with irregularitiesthrough time is a feature observed in all experiments.Fracturing and block rotation within a deformed clay layerare phenomena that are only observed in experiments set upwith normal or firm clay. The formation of a step-likegeometry in the clay that stabilizes a fault prism wasobserved in experiments that include a clay layer of 10 or30 mm thickness. Shape and stability of the fault prism maybe affected by asperities within the clay.

In experiments carried out with thin clay layers (3 mm),the offset is accommodated by a clockwise rotating fault.This mechanism is the same as in fault propagation observedin a homogeneous sand cake (e.g. Horsfield, 1977; Adam etal. 2005). Cardozo et al. (2003) describe a kinematic modelfor trishear in an elasto-plastic material in which the zoneof deformation is localized asymmetrically to the fault that

Fig.11. Sketch showing end members of clay strengths in theirreaction to the basement fault: (a) dominant lateral flowof the soft clay, buffering the deformation in the overburdensand, (b) brittle failure occurs in the sand below the firmclay while the top sand layer will show a wide monocline.With a high strength the clay will bend resisting the locali-zation of the basement fault. Outer arc fracturing (indicatedby grey arrows) will occur and voids may evolve but thebrittle failure of the clay is delayed. Until then, the bentclay will buffer the deformation of the sand overburden.

Table 2. Overview of experimental results

Experiment Clay Clay Clay Continuous Step Block Fault Fault Voidsthickness type smear rotation rotation prism

[mm]

ged01 10 soft � � � �

ged03 3 soft � � �

ged04 10 firm � � � � �

ged05 30 normal � � � �

ged06 10 normal � � � � �

ged07 3 normal � � � �

ged08 30 soft � � � �

ged10 30 firm � � �

ged11 3 firm � � � �

a

b

rigid basement

sand

sand

clay

rigid basement

sand

sand

clay



migrates through the material resulting in a highly deformedtriangular zone that provides a gradient in the velocity vectorfield as it decreases towards the footwall (see also Fig. 6a).Interestingly we observe a similar damage zone evolutionin the sandbox experiments with a 3 mm thick layer of clay(ged03, ged07, ged11, Table 2). However, this model doesnot explain the fault kinematics observed in the experimentswith layer thickness > 3mm (ged01, ged08, ged06, ged05,Table 2), in which we observe the formation of a triangularzone with a constant velocity field stabilized between twoactive faults (see also Fig.6b).

Contrast in competence of sand and clay governs theinitial fault evolution of soft and normal clay experimentswith clay layer thicknesses of 10 and 30 mm. Those analoguemodels form much thicker clay smear in a wider and morecomplex fault zone. The width of the zone in which earlylateral strains localize in the clay increases with layerthickness (van Mier, 1986). Thus, the strain distribution inthe overlying sand package is influenced as well, resultingin a prism-shaped squeezing block in the shape of a prismbordered by two active faults.

The fault zone evolution of the firm clay experimentswith layer thicknesses of 10 and 30 mm (ged04, ged10,Table 2) shows brittle fracturing and discontinuous clay.Fracturing that follows initial bending of the clay leads toasperities that influence the flow of sand and therefore tofault evolution. Bending and minor fracturing alsoaccommodate initial strain in firm, 3 mm thick clay, but thethin layer behaves more ductile although the mechanicalstrength is the same (van Mier, 1986). Therefore mechanicalcontrast with respect to the sand is decreased in the residualstage, enabling the precursor fault to propagate towards itskinematically favorable position.

As a summary it can be stated that in sufficiently thicklayers, the stiffness of the clay has the strongest effect onfault gouge evolution.

CONCLUSIONS

1. The strength of the clay layer has major effect on theevolution of fault gouge.

2. The evolution of fault gouge in sand-clay sequences is

a complex interplay of kinematics and mechanics, withseveral feedback mechanisms.

3. Weak, undercompacted clay is more prone to beenriched in the fault zone and forms a continuous smear,while strong, overconsolidated clay first deforms in abrittle fashion but may be reworked to a soft gougewith ongoing fault movement.

4. Although only two parameters were changed in theexperiment series, the resulting geometries and featuresdiffer on a wide scale. Plastic or brittle behaviours arethe commonly observed end members, sometimesoccurring at different stages within the sameexperiment.

5. The early fault is at first steeper than the basement fault.This imposes a stepping geometry when the faultactivity switches to the kinematically preferred planein elongation to the rigid basement fault.

6. The formation of a stable fault prism without interiordeformation is dependent on the clay thickness. Asufficiently thick clay layer has the ability to decouplefault strands in the sand. A thin clay layer has minoreffect on the geometry in comparison to an experimentwith a homogeneous sediment package.

7. Fragments and blocks of clay that tend to form inrelatively rigid clay layers may be reworked withongoing deformation. This implies that a fault planethat is properly impregnated with clay smear may havehad less sealing qualities during an earlier stage ofdeformation. This important observation suggests thatthe sealing qualities of a fault cannot be made from thefinal gouge properties. Thus, the sealing capacities ofa fault zone within a layered sequence are stagedependent.

8.The main challenge for future work is to understand thefeedback between mechanical properties and fault zonearchitecture.

Acknowledgements: The project was funded by ShellInternational E&P under supervision of J. Konstanty andW.F. Dula. The authors gratefully acknowledge theconstructive and thorough review of N. Mandal and theeditorial help of M. A. Mamtani.

References

ADAM, J., URAI, J.L., WIENEKE, B., ONCKEN, O., PFEIFFER, K.,KUKOWSKI, N., LOHRMANN, J., HOTH, S., VAN DER ZEE, W., andSCHMATZ, J. (2005) Shear localisation and strain distributionduring tectonic faulting—new insights from granular-flowexperiments and high-resolution optical image correlation

techniques. Jour. Struc.Geol., v.27, pp.283-301.BENSE, V.F. and Person, M.A. (2006) Faults as conduit-barrier

systems to fluid flow in siliciclastic sedimentary aquifers, WaterResources Research, v.42, W05421.

CARDOZO, N., BHALLA, K., ZEHNDER, A. and ALLMENDINGER, R.



(2003) Mechanical models of fault propagation folds andcomparison to the trishear kinematic model. Jour. Struc.Geol., v.25, pp.1-18.

CLAUSEN, J.A. and GABRIELSEN, R.H. (2002) Parameters that controlthe development of clay smear at low stress states: anexperimental study using ring-shear apparatus. Jour. Struc.Geol., v.24, pp.1569-1586.

CLAUSEN, J.A., GABRIELSEN, R.H., JOHNSEN, E. and KORSTGÅRD, J.A.(2003) Fault architecture and clay smear distribution. Examplesfrom field studies and drained ring-shear experiments. NorskGeologisk Tidsskrift, v.83, pp.131-146.

CLOOS, E. (1955) Experimental analysis of fracture patterns. Geol.Soc. Amer. Bull., v.66, pp.241-253.

CLOOS, H. (1930) Zur experimentellen Tektonik - V. VergleichendeAnalyse dreier Verschiebungen. Geologische Rundschau, v.21,pp.353-367.

EGHOLM, D.G., CLAUSEN, O.R., SANDIFORD,, M., KRISTENSEN, M.B.and KORSTGÅRD, J.A. (2008) The mechanics of clay smearingalong faults. Geology v.36, pp.787-790.

EISENSTADT, G. and SIMS, D. (2005) Evaluating sand and claymodels; do rheological differences matter? Jour. Struc. Geol.,v.27, pp.1399-1412.

FULLJAMES, J.R., ZIJERVELD, L.J.J. and FRANSSEN, R.C.M.W. (1997)Fault seal processes: Systematic analysis of fault seals overgeological and production time scales. In Moeller-Pedersen,and A.G. Koester, Eds. Hydrocarbon Seals, 7, pp.51-59.

GUDEHUS, G. and NÜBEL, K. (2004) Evolution of shear bands insand. Geotechnique, v.54, pp.187-201.

GUDEHUS, G., and KARCHER, C. (2007) Hypoplastic simulation ofnormal faults without and with clay smears. Jour. Struc. Geol.,v.29, pp.530-540.

HAGE, B. and WERTHER, J. (1997) The guarded capacitance probe— a tool for the measurement of solids flow patterns inlaboratory and industrial fluidized bed combustors. PowderTechnology, v.93, pp.235-245.

HOLLAND, M., URAI, J.L. and MARTEL, S. (2006) The internalstructure of fault zones in basaltic sequences. Earth Planet.Sci. Lett., v.248, pp.286-300.

HORSFIELD, W.T. (1977) An experimental approach to basement-controlled faulting. Geologie en Mijnbouw, v.56, pp.363-370.

KARAKOUZIAN, M. and HUDYMA, N. (2002) A new apparatus foranalog modeling of clay smears. Jour. Struc. Geol., v.24,pp.905-912.

LEHNER, F.K. and PILAAR, W.F. (1997) The emplacement of claysmears in synsedimentary normal faults: inference from fieldobservations near Frechen, Germany. In: P. Pedersen (Ed.),Hydrocarbon seals: importance for petroleum exploration andproduction. Elsevier, pp.39-50.

LINDANGER, R, ØYGAREN, M., GABRIELSEN, R.H., MJELDE,R.,RANDEN,T. and TJØSTHEIM, B.A., (2004) Analogue (plaster)modelling and synthetic seismic representation of hangingwallfault blocks above of rampflat ramp faults. First Break, v.22,pp.22-30.

LINDSAY, N.G., MURPHY, F.C., WALSH, J.J. and WATTERSON, J. (1993)

Outcrop studies of shale smears on fault surfaces. Spec. Publ.Internat. Assoc. Sedimentologists, v.15, pp.113-123.

LIU, Z., ZHENG, Y., JIA, L. and ZHANG, Q. (2005) Study of bubbleinduced flow structure using PIV. Chemical Engineering Sci.,v.60, pp.3537-3552.

MANDL, G. (1988) Mechanics of tectonic faulting; models and basicconcepts Developments in structural geology. Elsevier Sci.Publ., Amsterdam, Netherlands.

MANDL, G. (2000) Faulting in brittle rocks. Springer, London.MANDL, G., JONG, L.N.J.D. and MALTHA, A. (1977) Shear Zones in

Granular Material – An Experimental Study of their Structureand Mechanical Genesis. Rock Mechanics FelsmechanikMécanique des Roches, v.9, pp.95-144.

MEDWEDEFF, D.A. and KRANTZ, R.W. (2002) Kinematic and analogmodeling of 3-D extensional ramps: observations and anew 3-D deformation model. Jour. Struc. Geol., v.24, pp.763-772.

NEWBERY, A.P., RAYMENT, T. and GRANT, P.S. (2004) A particleimage velocimetry investigation of in-flight and depositionbehaviour of steel droplets during electric arc sprayforming.Materials Science and Engineering A, v.383, pp.137-145.

NÜBEL, K. (2002) Experimental and Numerical Investigation ofShear Localization in Granular Material, PhD, Karlsruhe,p.159.

NÜBEL, K. and WEITBRECHT, V. (2002) Visualization of Localizationin Grain Skeletons with Particle Image Velocimetry. Jour.Testing and Eval., v.30, pp.322-328.

PATTON, T.L. and FLETCHER, R.C. (1995) Mathematical block-motion model for deformation of a layer above a buried faultof arbitrary dip and sense of slip. Jour. Struc. Geol., v.17,pp.1455-1472.

PEACOCK, D.C.P and SANDERSON, D.J. (1992) Effects of layeringand anisotropy on fault geometry. Jour. Geol. Soc. London,v.149, pp.793-802.

RICHARD, P. and COBBOLD, P. (1990) Experimental insights intopartitioning of fault motions in continental convergent wrenchzones. Annales Tectonicae, v.4, pp.35-44.

RICHARD, P. and KRANTZ, R.W. (1991) Experiments on faultreactivation in strike-slip mode. Tectonophysics, v.188, pp.177-131.

SCHÖPFER, M.P.J., CHILDS, C. and WALSH, J.J. (2006) Localisationof normal faults in multilayer sequences. In: C. Childs andJ.J. Walsh (Eds.), Jour. Struc. Geol., v.28, pp.816-833.

SCHREURS, G. , BUITER, S.J.H., BOUTELIER, D., CORTI, G., COSTA, E.,CRUDEN,A.R., DANIEL, J.-M., HOTH, S., KOYI, H.A., KUKOWSKI,N., LOHRMANN, J., RAVAGLIA, A., SCHLISCHE, R.W., WITHJACK,M.O., YAMADA, Y., CAVOZZI, C., DEL VENTISETTE, C., ELDER

BRADY, J.A., HOFFMANN-ROTHE,A., MENGUS, J.-M., MONTANARI,D. and NILFOROUSHAN, F. (2006) Analogue benchmarks ofshortening and extension experiments. In: S.J.H. Buiter andG. Schreurs (Eds.), Analogue and numerical modeling ofcrustal-scale processes. Geol. Soc. Spec. Publ., v.253, pp.1-27.

STEINGART, D.A. and EVANS, J.W. (2005) Measurements of granularflows in two-dimensional hoppers by particle image



velocimetry. Part I: experimental method and results. ChemicalEngg. Sci., v.60, p.1043.

VAN DER ZEE, W. (2002) Dynamics of fault gouge development inLayers sand-clay sequences. Shaker Verlag, Aachen.

VAN DER ZEE, W. and URAI, J. (2005) Processes of normal faultevolution in a siliciclastic sequence: a case study from Miri,Sarawak, Malaysia. Jour. Struc. Geol., v.27(12), pp.2281-2300.

VAN DER ZEE, W., URAI, J.L. and RICHARD, P.D. (2003) Lateralclay injection into normal faults. GeoArabia, v.8(3), pp.501-522.

VAN MIER, J.G.M. (1986) Multiaxial strain-softening of concrete.

Materials and Structures, v.19, pp.179-190.VROLIJK, P. and VAN DER PLUIJM, B.A. (1999) Clay gouge. Jour.

Struc. Geol., v.21, pp.1039-1048.WEBER, K.J., MANDL, G., PILAAR, W.F., LEHNER, F.K. and PRECIOUS,

R.G. (1978) The role of faults in hydrocarbon migrationand trapping in Nigerian growth fault structures. Proc. 10thAnnual Offshore Technology Conference, Houston, Texas 4,pp.2643-2653.

WOLF, H., KÖNIG, D. and TRIANTAFYLLIDIS, T. (2003) ExperimentalInvestigation of shear band patterns in granular material. Jour.Struc. Geol., v.25, pp.1229-1240.

(Received: 30 November 2008; Revised form accepted: 3 April 2009)

Documents

Clay Smear Processes in Mechanically Layered Sequences ...friction angle and a shallower dip in the layers with a lower friction angle (Mandl, 1988; Peacock and Sanderson, 1992, Egholm