Geol Rundsch (1997) 86, Suppl.:S34—S44 ( Springer-Verlag 1997

ORIGINAL PAPER

P. Bankwitz · E. Bankwitz

Fractographic features on joints in KTB drill cores as indicatorsof the contemporary stress orientation

Received: 9 September 1996 / Accepted: 25 November 1996

Abstract Fractographic features on joints in cores ofthe superdeep KTB drilling were used to estimate theorientation of the contemporary maximum horizontalstress (S

H). The unique opportunity to investigate cores

from a depth down to approximately 8085 m revealsinformation on in situ stress orientation with increasingdepth. We recognized on the cores the main normalstress axes which are reflected by common fracto-graphic pattern on coring-induced fractures, present inall sections. The analysis is based on the symmetry ofjoint characteristics: 3D (out-of-plane) shape of thejoint and surface features of joint planes. The symmetryaxes of these joint features are related to the orientationof principal stress, which permits the determination ofthe orientation of the contemporary maximum andminimum horizontal stress axes (S

HOS)), if the core

orientation is determined by other methods. For thatpurpose, the reorientated cores of the pilot borehole(VB) could be used down to 4000 m. The main borehole(HB) cores are generally not reoriented by several rea-sons. Some investigations were done by members of theKTB laboratory to determine the supposedly north-ward orientations of HB core sections by comparisonwith the foliation and of drilling-induced fractures atthe borehole wall (using FMI/Formation MicroImagerand FMS/Formation MicroScanner log data) andwithin the core. Independently, the maximum horizon-tal in situ stress orientation down to 7800 m was deter-mined from drilling-induced vertical joints, and thestrike and dip of the foliation from borehole measure-ments down to ca. 8600 m, carried out by the staffmembers at the KTB. Additionally, rock fragmentsfrom below 9050 m represent a palaeo-tectonic jointsystem developed at a depth of ca. 9 km.

P. Bankwitz · E. Bankwitz ( )GeoForschungsZentrum Potsdam, Telegrafenberg,D-14473 Potsdam, GermanyFax: #0331 8877524E-mail: ebank@gfz-potsdam.de

Key words KTB boreholes · Coring-induced joints ·Core disking · Fractography · In situ stressorientation

Introduction

The KTB (German Continental Deep Drilling Project)boreholes (pilot hole: VB; main borehole: HB) are situ-ated close to the southwestern border of the Bohemianmassif in the Zone of Erbendorf-Vohenstrauss (ZEV)which appears to be an allochthonous basement unit.The zone consists of paragneisses and metabasites withmedium-pressure/high-temperature metamorphism oflower Palaeozoic age. The structure contains stacks oflayered metamorphic rocks. The lithological units andtheir foliation are steeply inclined (Duyster et al. 1993;Harms et al. 1993; Hirschmann and Lapp 1994; de Wallet al. 1994). The drilling operation was finished ata depth of 9101 m (Hirschmann 1994), penetrating toa depth at 7500 m in a vertical position. Crystallinecores down to 8085 m could be used for our method ofjoint analysis and estimation of contemporary stressorientation. To the southwest the ZEV is bordered bythe Franconian lineament (FL) which dips to the north-east. Along this fault the Bohemian massif was thruston the South German Block with a main uplift inCretaeous and Neogen times. The FL was penetratedby the main drill hole at depths between 6680 and7260 m. In this area several deformation stages withchanges in stress orientation during post-Hercyniantimes have been described by Zulauf (1992) and Petereket al. (1994).

It is the first time that drill cores and cuttings downto a depth of 9050 m could be investigated fracto-graphically concerning variation in stress orientation.We have derived our results from the cored sections ofthe pilot borehole (VB) down to 4000 m, and from themain borehole (HB) between depths of 4000 and9050 m. Geophysical borehole measurements allow the

Fig. 1A–E Morphology of diskjoint surfaces in KTB cores.Arrows on surfaces propagationtraces of the growing joints;arrows at core wall direction ofdown-core. Parent joint centralplane of the fracture, with thepoint of origin and radiallytraces of joint propagation,limited by an arrest line. Fringemarginal zone in front of arrestlines, with continuingpropagation, mostly out ofplane. S

H, S

hSymmetry axes

reflecting the maximum (SH) and

minimum (Sh) horizontal stress

orientation. A Due to lowhorizontal stress and only weakstress anisotropy; B a preferreddirected propagation; C withstrong deviation of the fringezone. Both (B, C) were formedunder higher differential stressand superposition of additionaltectonic stresses. D The surfaceand E its profile represent anintermediate type which indicatean increase in differential stress

extrapolation into the uncored parts, similar to fieldobservations extrapolated between outcrops.

Method

The surface morphology of joints reflects the fractureforming process consisting of initiation, propagation

and arrest. In the case of slow fracture growth undercontinuous loading, several cycles of propagation areindicated by various arrest lines. In the case of jointpropagation under higher energy, traced out by bifur-cation close to the point of origin, the development ofarrest lines will be restrained and the fracturing con-tinues without stopping during propagation. Four as-pects of joint morphology (Fig. 1) give rise to kinematic

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and dynamic interpretations of the (a) surface morpho-logy, (b) type of terminal fringes, (c) non-planar shapeof joints and (d) symmetry of all patterns of (a)—(c)(Fig. 1B—D).

Surface morphology and fringes

The polymineralic rocks are inhomogeneous and there-fore the elastic properties vary from point to point.Consequently, the orientation of the maximum tensilestress varies in front of a crack tip. Assuming that cracksadvance normal to the local maximum tensile stress, thecrack tip will tend to leave the plane of the initial parentcrack (Engelder et al. 1993). This out-of-plane propaga-tion is common and gives all KTB joints in the coresa rough surface with that forming fractographic patterns(centre of Fig. 1A and C). That characterizes the generalbehaviour of fracturing rocks on a microscopic and ona mesoscopic scale as well, but in the latter case depen-dent on the grain size of the rocks. The grain-size/joint-size relationship defines the grade of identifiability ofsuch roughness pattern. Only segments of joints appearplanar. Sometimes the roughness is so low comparedwith the size of the joint that the fracture appears planaras well. The quasi-planar type gives evidence for a ho-mogeneous remote stress field.

The roughness at the surfaces is not randomly dis-tributed. The joint roughness contains regular patterns(e.g. plumose or feather-like, or small increasing steps;Fig. 1B) that indicate the propagation direction, theconditions of fracture growth (higher energy/continu-ously; lower energy/arrest lines), and in many cases thetype of origin (subsidence, tectonics or upheavel re-lated; Bahat 1991; Bankwitz and Bankwitz 1994). Thesurface pattern is caused by the anisotropy of the stressfield and by the influence of the regional stress field.The out-of-plane propagation frequently terminates infringe zones with a combination of twist and tilt of theinitial parent joint plane. A twist causes the fracture tosplit into several rotated (mostly smaller) segments withincreasing steps in between (Fig. 2E). It is a reactiondue to the rotation of the local maximum tensile stressnormal to the crack propagation. A tilt causes rotationof the growing plane parallel to the propagation direc-tion. In this case, if the premise that the fracture grewradially from the initial area is realized, two inversionpoints on the fringe commonly exist (Fig. 1C). Thesepoints mark one of the possible symmetry axes of thejoint surface pattern and of the fracturing process itself.In Fig. 2D the inversion points correlate with the lowpoints of core disking.

Joint surface symmetry

One important aspect of fracture forming processes isthe symmetry of fractographic pattern (Bankwitz and

Bankwitz 1984). The symmetry reflects the related mainnormal stress axes (Fig. 1C). It is given by inversionpoints of tilted fringes, by the axes of diverging plumes,by the change of overlap in the case of twisted marginalsegments (Fig. 3) and by the more or less curved shapeof the joint (out-of-plane propagation; Fig. 1C—E).Curving of fracture paths is the manifestation of localrotation of stress field as the consequence of in-homogeneous remote stresses. The curving path pro-duces symmetry axes which are the prerequisite for theestimation of stress orientation.

These facts form the basis of our investigations ofjoints in the drill cores.

Suitability of joints for stress orientation determination

In the medium-grained cores of the KTB, with a dia-meter of 10 cm from depths between 4000 and 6000 m,well-developed fracture planes occur, but often withoutdistinctly formed fractographic features, as known fromsedimentary rocks. Additionally, veins and veinlets dis-rupt the propagation and cause irregular fracture sur-face structures. The varying grain size of the crystallinerocks defines the quality of fractographic features.Steeply to intermediately dipping foliation planes donot influence fracture propagation because they main-tain cohesion. Despite sometimes roughly developedpattern in the 10-cm cores, the overall shape of thesejoints reflects the origin, propagation direction andpartly arrest and termination of the fracture process.From depths below 6000 m down to 7400 m large dia-meter coring system (LDCS) cores with a diameter of23.5 cm were taken. Due to the large diameter, thefracture surface pattern is frequently better developed.

On the undisturbed KTB cores the orientation ismarked by double reference lines. For some core sec-tions the northward reorientation was done by the staffmembers at the KTB (J. Duyster, pers. commun.; Th.Rockel, pers. commun.) who have used indirectmethods described by Rockel et al. (1992). That led, forexample, to the structural scheme for the mean strikeand dip of the foliation (Hirschmann 1994). We haveused the results of the investigations of Rockel and, onthe other hand, the mean values of the foliation dipcarried out by the staff. Up to now, the cores of themain hole are not reoriented, with the exception ofsingle parts. The varying mean strike and dip of thefoliation allows only estimations.

The offset of 90° between the orientation of thedrilling-induced vertical joints and the orientation ofthe breakouts proves that the vertical fractures area reliable stress indicator as well (Brudy et al. 1993).Using centre-line fractures or the S

Hmax orientation of

core disking joints, these joints are suitable for thereorientation of the drill cores from the KTB VB andKTB HB below 4000 m as well. The comparison of

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Fig. 2 Core disking (CD)fractures in core sections fromthe main borehole (A, C–E) andthe pilot hole (B). A Thereference lines mark the lowpoint regions of the typicalsaddle and trough structure. Thelow point area of CD reflects themaximum horizontal stressdirection (S

H). B Small

variations of the SH

orientationwithin the same core. The disksurfaces (C, D) give evidence thatCD fracture development isdifferent from the conceptsdescribed in previous literature.The CDS do not originate athigh points and they do not runto the centre of the surface. CDSare comparable with the jointtypes of Fig. 1. Arrows on thesurfaces point in thepropagation direction. B CDSwith bifurcations indicatefracturing due to higher energy;C with origin at a low point anda preferred directed propagationthrough the core indicatingintermediate stress; D, E withorigin in the centre give evidenceof decreasing stress. ¸P lowpoints; O point of origin; a arrestline; f fringe zone

specific structures (e.g. foliation, layering, verticalfractures) on the orientated borehole wall images(gained by geophysical logging tools) with the coresections allows a northward reorientation in the caseof optimal conditions. (But the realization is difficultwithout corrections of the difference between core dataand log data.) Unfortunately, in the main borehole(KTB HB) the coring was discontinuous with poor corerecovery and a poorer log quality with regard to thepilot hole because of smaller coverage (Rockel et al.1992). But numerous drilling-induced fractures wereidentified and their maximum horizontal stress orienta-tion down to 7800 m was determined (Rockel 1995).The correlation of this known maximum horizontal

stress orientation with the identified maximum hori-zontal stress axes of the fractographic features on diskjoints allows at least the attempt of reorientation ofsome core segments.

Joint types of the KTB cores

Palaeo-tectonic joints

Tectonic joints even at a depth of approximately9050 m (core H035) are preserved on rock segments,but only on back-falling ones (J. Duyster, pers.

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Fig. 3 Disk joint surface of a KTB core (7401 m) with rough radialpropagation traces and small twist hackles indicating the naturalend of the joint. C¼ clockwise; CC¼ counterclockwise step ofoverlap. Diameter: 23.5 cm

commun.). Nevertheless, the fragments are shaped likecore pieces, subdivided into slices approximately 5 cmthick. Their primary orientation is unknown. The frag-ments are bounded by an older system of non-ortho-gonal joints with angles of approximately 110°. A pre-drilling origin of these joints is identified by iron hy-droxide films. The well mineralized coating of the jointsurfaces is not due to the drilling fluid. The formation ofthese joints due to a tectonic event is evidenced byfractographic surface features with bifurcation of thejoint (twist hackles) close to the point of origin andwithout arrest lines. These patterns indicate higherenergy during joint formation. These pre-drilling jointsare larger than the core and represent only parts of thecomplete fracture. The joints occurred in fine-grainedlaminated gneiss with a supposed intermediate dip offoliation. It should be noted that these brittle fractureswere formed in the lower level of the upper crust ata depth of 9 km.

Drilling- or coring-induced fractures

Drilling- or coring-induced fractures are the basis forcontemporary stress field determination by means ofjoints. Nearly 40 years ago it was recognized that the insitu stress field influences the surface morphology of thedisk joints in drill cores (Strasser and Wolters 1963;Sangree 1969; Peterss 1980a, b). Stacey (1982) gave thetheoretical reasons for the core disking formation dur-

ing the coring process. Dyke (1988) could prove bynumerical modelling that the anisotropy of horizontalstresses causes the curved shape of core disking joints.The subvertical drilling-induced fractures at theborehole wall and their counterpart in the core (centre-line and petal fractures) are obviously created simulta-neously (Natau and Rockel 1993), of tensile origin andoriented in the direction of the maximum horizontalstress (sigma

.!9).

Drilling-induced centre-line fractures

Drilling-induced centre-line fractures (parallel or ob-lique to the core axis) exist in both boreholes (VB andHB; Rockel 1995; Brudy et al. 1994). In some cases thecentre-line fractures represent a fracture bifurcationclose to the point of origin and propagation in twoplanes with a distance of some millimetres in between.That implies a higher energy of fracture growth than incases without bifurcation and overlap. The strike direc-tion of such drilling-induced vertical joints correspondsto the orientation of pre-drilling vertical joints in thecores which were healed in many cases. At a few places,joints seem to have started as petal lips, but theirfurther propagation continued as core disking surfaces,not as petal-centre-line joints.

Coring-induced disk fractures

Coring-induced disk fractures with horizontal to sub-horizontal orientation which were initiated and termin-ated in the core are dominant in the KTB mainborehole. LDCS-cores with 23.5 cm diameter frombelow 6000 m down to 7400 m depth are intenselyfractured (Figs. 4A and 5). The disk joints allow theopportunity to analyse numerous surface patterns(Figs. 1, 2) or their forking traces on the core surfacewhich indicate twist hackles (Figs. 2B, 4B).

Two main groups of disk fractures are dominantoccurring nearly perpendicular to the core axis:1. Core disking surfaces (CDS; Fig. 2A—D), character-

ized by saddle and trough structures with two highpoints and two low points (Bankwitz and Bankwitz1995). These are restricted to the more fine-grainedisotropic rocks of the cores. The connecting linebetween the low points represents the maximumhorizontal stress orientation (S

H).

2. Disks with usual fractographic surface pattern(UFS), similar to those found on natural joint (with-out saddle and trough relief; Figs. 1, 4C and D, and5). These occur much more frequently than the CDS.Subgroups are determined according to the differingstress conditions (Bankwitz and Bankwitz 1995).The CDS on the KTB drill cores were investi-

gated by Rockel (1995). Besides the saddle andtrough relief we identified additional fractographicpatterns on CDS. These patterns can be used to infer

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Fig. 4A–E Different joint types. A Planar bifurcated disks (HBH031C, 7011 m); B bifurcated core disking traces (VB 940A1c,3840 m); C, D disk joints with fractographic features (HB H027,6360 m); E cutting of shatter cone type (7500 m, length 15 cm).Diameters of cores A, C and D: 23.5 cm; diameter of core B: 10 cm

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Fig. 5 Wall of a LDCS (large-diameter diamond coring system) corewith dense fracturing. Diameter of the core: 23.5 cm, hornblendegneiss (7011 m). Length: 80 cm

fracture-forming processes and the state of stress(Fig. 2C—E). The CDS type is equivalent to the basicsymmetry type of four-part fractographic surfaces ofnatural joints (Bankwitz and Bankwitz 1984, Fig. 1, p.307). The four-part morphology with two symmetrymirror planes is formed by fourfold changes of high andlow sections of the joint surface or of the fringe. Thetypical core disking (CDS) represents only one endmember of the varying types of the much more frequentusual disks (UFS). These common disks are equivalentto other basic symmetry types of joint surface struc-tures. The joint types are:1. Disks with weak radial patterns and uniformly ar-

ranged twist hackles. These joints are characterizedby a symmetry centre of inversion and give evidenceof reduced horizontal differential stress (Fig. 2E).

2. Disks with varying overlap of twist hackles (clock-wise/counterclockwise) reflecting one or two sym-metry mirror planes (Fig. 3).

3. Disks which are divided into two sections: one withdownwards and one with upwards out-of-plane pro-pagation (Fig. 1C), characterized by one mirror planeand one symmetry rotation axis (Bankwitz and Bank-witz 1984, Fig. 1, p. 307). This type of double-dividedplane is commonly formed under superposition of anadditional tectonic stress (higher differential stress).These joints are the manifestation of a rotation of thelocal stress field (Engelder et al. 1993). The CDS(Figs. 2 and 4B) appear mostly in fine-grained andquasi-isotropic rocks, whereas the UFS (Figs. 1 and4C, D) appear in cores of all kinds of rocks.Therefore our main topic was the investigation of

usual disks. These disks are often initiated in the centreof the core and grow in regular patterns with plumes,

hackles and arrest lines, terminating in twist-hacklefringes (Fig. 1).

Transitional forms

Transitional forms between CDS and UFS have beenrecognized. We have evidence that the CDS with theirsaddle-trough structure do not represent a special caseof disk development, and CDS do not initiate at thehigh-point areas of the core margin, propagating to-wards the centre of the core (i.e. Wolter et al. 1990;Rockel 1995). Frequently, on KTB core disking surfa-ces the initiation occurred at one low point propagat-ing radially through the core (Fig. 2C). From a physicalpoint of view, if the CDS initiate at two points of themargin, there should be some evidence of overlap in thecentre (T. Engelder, pers. commun.). Figure 6 of Wolteret al. (1990) depicts a bifurcating joint due to a higherenergy of propagation. These are not two independentoverlapping joint planes, as was supposed by Wolteret al.

Fractographic features, observed on core diskingsurfaces, and details of the shape of the core diskingfractures, yield additional information on the diskingprocess of KTB cores. Such fractographic features,superimposed on the saddle structures of the CDS, areoften weak and not easy to detect. In nearly all casesour observations indicate an initiation of the CDS ina low-point region of the growing fracture and not atthe marginal high points. The fracture grew from thissingle point through the entire core to the oppositemargin (Fig. 2C). We never have observed fracto-graphic structures indicating a propagation from twomarginal points towards the centre. This is in contrastto statements in the literature on the basis of computermodelling (Dyke 1988).

From our observations, the development of the CDSdoes not differ from the development of the UFS joints.The fractographic features superimposed on core

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disking saddle relief include fine hackle marks (rare)and small twist-hackle fringes (common; Fig. 2D). Thefringe zones (1—2 cm) become broader in the high-pointregion and may disappear in the low-point area. Oftencore disking is characterized by a triangular surfacestructure, caused by a ridge within the trough of thedown-core curved plane. In this case three low pointsare developed (Bankwitz and Bankwitz 1995). Also ofsignificance is a forking which is known from commontension fractures as well. The forking (bifurcation)marks the termination of core disking surfaces, if theyare not developed through the whole core. Concerningtheir shape and surface structures, we could definea range of joints that differ from both CDS and UFS.These are end members with many transitional forms.

Occurrence of tension and shear fractures

All investigated joints in the cores reveal a tensile origin(without shearing). There is evidence that even cuttingsfrom the borehole wall (of ca. 100 cm or more) are oftensile origin as in the case of Fig. 4E. This cutting froma depth of 7500 m is shaped like a shatter cone. Sevenfracture planes diverge like a fan. The cutting (onecurved side indicates that the primary position wasvertical, but top and bottom are unknown) originatedat the lowermost point in the reproduction of Fig. 4Eafter an early bifurcation close to the point of originrunning to the top of the figure. The fracture processwas rapid; the planes propagated in a distance of 2 upto 9 mm. They overlap each other up to 11 cm, al-though their overall shape is only 15 cm wide. Such flatcones provide information about the breakout behav-iour of the borehole. Similar results are given by Lichand Duyster (1993) who developed a method for cut-ting-shape measurements defining the 3D flatness ofcuttings. The highest degree of flatness occurs in sec-tions of undisturbed rocks. This flatness should becorrelated with ‘‘explosive’’ breakouts.

Occasionally, on disk joints (brittle tensile joints)slickenlines of a few centimetres length were recognizedon the initial plane area. The slickenlines suggest thatthe tensile joints may have propagated across the core,followed by a slight rotation of the local stress fieldwhich has permitted slippage and slickenline develop-ment on the smooth initial section of the fractures.Without any doubt, the very fine, regular slickenlineshave nothing to do with the much more rough, butpartly polished, striae of drilling.

Discussion and conclusion

Determination of stress orientation

The fractographic features are the petrified documenta-tion of stress conditions. The resulting morphology of

joints give information about the horizontal stress ori-entation and its anisotropy (Bankwitz and Bankwitz1984). On the basis of our fractographic analysis, vari-ations of the stress orientation can be recognized. Themaximum horizontal stress axes S

Hderived from the

symmetry of coring-induced disk joints in the KTB VBand HB were estimated between 165 and 180°. Thisresult is in accordance with determinations by otherindirect methods based on geophysical measurements.Additional information and observed stress changesare mentioned below. Palaeo-stress directions in KTBcores have been deduced by Zulauf (1992) and Voll-brecht et al. (1994) using fault-slip data and micro-cracks, respectively.

Down to a depth of 7.8 km the mean value of themaximum horizontal stress orientation (Rockel1995) derived from drilling-induced vertical fracturesand centre-line fractures is 170$13°. The stressmagnitudes of S

H.!9, the relation to the magnitude

of S7%35*#!-

and S).*/

at the final depth of 9 kmwere calculated regarding different conditions. Themagnitude of S

7%35*#!-approaches 250 MPa at the

final depth of the drilling at 9100 m, whereas thevalue for S

).*/approaches approximately 160 MPa

(Rockel 1995). But the latter value is under discussion(Rummel and Zoback 1993; Brudy et al. 1993, 1994)and possibly higher stress differences also (Huengeset al. 1995).

Nature of joints

All analysed drilling- or coring-induced joints in KTBcores, and also the cuttings, give evidence for tensileorigin; the fractographic features show no evidence fororiginating as shear fractures.

Varied joint types with increasing depth

With increasing depth, at some levels along the core,special joint types are found. From results of fractogra-phy we know that the specific types of joint growth(propagation) and the resulting complete shapes (e.g.planar/out-of-plane, etc.) reflect their formation condi-tions. The more planar the surface, the more the remotestress field has dominated the local stress field. Theamount of out-of-plane propagation (e.g. four-partstructured joint surfaces; Fig. 1 in Bankwitz and Ban-kwitz 1984) indicates: (a) the nature of the post-initia-tion local stress field (homogeneous or rotating); (b) thepropagation path (arrest lines or not, bifurcationor not, angle and frequency of bifurcation; (c) thepresence of effective stress (two-part structured joints)and other information. Therefore, based on fracto-graphic pattern analysis, fault-dependent variations offracture types and related stress conditions could bedefined.

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High deformation zone

A high density of coring-induced disk fractures (UFStype) at great depth is a manifestation of superpositionof additional tectonic stress. These fractures of the coreH031 from a depth of 7011 m were formed by preferreddirected propagation, and with intense and multifoldbifurcation. The bifurcation divides the core sectioninto narrow slices (1 mm thick at the end of the disk;Fig. 5). The directed propagation and cascade-like bi-furcation of the joint surfaces are evident for a dynamicfracture process of high energy indicating tectonicstress. (Contrary to this type of fracturing, disks withradially and low out-of-plane propagation indicate lowhorizontal stress. In this case the origin probably de-pends only on tensile stress due to unloading.) In thissection of the borehole the local stress field is morehomogeneous and apparently influenced by the re-gional stress field. There the drilling penetrates theSE1 reflector which represents faults of the Franconianlineament (FL; Harjes and Janik 1994). These coresections allow an estimation of the northward reorien-tation by comparison of drilling-induced vertical frac-tures and the foliation in the core and at the boreholewall (de Wall et al. 1994). According to our joint analy-sis, the strongly preferred directed propagation of thesedisk joints is influenced by the main fault zone (SE1).The changed propagation of the disk joints fromnorth—northeast to the south reflects a maximum hori-zontal stress orientation of approximately 180—190°.According to Rockel (1995) the mean values of thedrilling-induced vertical wall fracture azimuths changefrom 167$11° above the SE1 to 180° within the faultzone and to 176$9° below the FL. This result corre-lates with the previously mentioned variation derivedfrom disk joints.

In the section where the SE1 reflector intersects themain borehole, the drilling-induced fractures changetheir orientation slightly from 170$9° to 182$21°(Brudy et al. 1994), but ‘‘without a significant rotationof the stress field in the area of the SE1 reflector’’ (p.109). This fact supports our results. Our observation ofdense horizontal fracturing at this depth is in accord-ance with the result of Brudy et al. (1994: Fig. 2, p.A110). The authors show the corresponding increasingfrequency of subvertical drilling-induced fractures justat a depth of approximately 7000 m, within the sectionof the SE1. Below 7160 m the temperature increasedbeyond 200°C. Fabric and mineralization indicate thatthe fault system was active several times, partly withductile behaviour (ca. 300—350°C), but also at semi-brittle conditions causing cataclasites (de Wall et al.1994). The fault zone pattern is evidence for compres-sive conditions (S. Lich, pers. commun.). According toour observations, the dense fracturing by UFS diskjoints should result from residual stresses related toprevious activities of the FL. On the other hand,a neotectonic activity of the FL up to recent times is

known from fission-track investigations (Coyle andWagner 1994) and from geological data (Peterek et al.1994). Also, open microcracks indicate existing in situstress (Vollbrecht et al. 1993).

Fault zone / disk-type relation

Cone-like structures appear in the main boreholewithin the cores below 4000 m. The 3D shapes of thecone-like joints originated near the centre of the core.They propagated from the deepest point of the jointsurface radially upwards as a cone, but over a small ora broad distance at different sides. These UFS shapesare comparable with CDS types (saddle and troughstructure) where the high points are asymmetrical withregard to the core axis. In such a case one high pointdips clearly steeper than the other one. A steeply eastdip dominates above 4000 m in the section of the dril-led-through SE2 reflector (3200—4000 m). Dyke (1988)suggests that a deviation of the core axis from one ofthe main normal stress axes cause such asymmetricalcore disking. We suppose that a relationship to thesoutheast dipping fault system exists which is repre-sented by the SE2 seismic reflector. These faults couldhave caused a local rotation of the stress field. Similarconditions appear below 4000 m where an additionaldistinct reflector was recognized (Luschen et al. 1996).The reflector appears within a homogeneous section ofamphibolites and is not related to changing lithology.Open fissures and a high water influx were observed inthat core section. This hints at an open fault zonemarked by the reflector. The stress conditions differwithin and below these fault systems concerning thevarying disk joint symmetries and their nature. There isevidence for a higher amount of differential stress in thesection below the fault by the occurrence of specificdisks. Such disks are ‘‘transform’’ joints (core diskingjoints with fractographic patterns of a preferred di-rected propagation) and joints with complete and out-of-the-plane rotated fringes (Fig. 1). These joints wereonly developed in sections with important faults andprove the superposition of additional tectonic stressand local rotation of the stress field. This local rotationof the stress field correlates with the eastward dip ofdrilling-induced vertical joints (Rockel 1995). The esti-mated azimuth of the maximum horizontal stress axes(S

H) deviates from the commonly recognized orienta-

tion (165—180°). This deviation supports the followingassumption: In several cases the symmetry axes of thedisk joint propagation is oriented parallel to importantfaults of the FL system (northwest—southeast toeast—west). Such behaviour was found only again ata depth of 6300 m and clearly developed in the sectionwhere the SE1 reflector was drilled through (at ca.7000 m).

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Anisotropy of the stress field

The postulated decreasing anisotropy of the stress fieldwith increasing depth could not be proved by our jointanalysis. Joints with evidence for differential stress (e.g.rotated fringes or preferred directed propagation) occurdown to a depth of 7 km. At greater depth the effectivestress decreases, and the stress field becomes moreisotropic (Kulander et al. 1990; Engelder 1993). Theorigin of centre-line fractures is only possible undersignificantly anisotropic horizontal stress (Dyke 1988;Kulander et al. 1990). This correlates with the results ofRockel (1995). In both boreholes he found the firstcentre-line fractures below 3000 m, because above thislevel the horizontal anisotropy was too small. Theseaxial planes occur frequently between 7600 and 7890 m,but were absent below that depth. In the KTB mainborehole below 7 km the magnitude of the minimumhorizontal stress increases disproportionately. Thepore pressure increases as well. These facts, togetherwith the absence of drilling-induced fractures below7800 m, indicate decreasing stress anisotropy. The onlyjoint indication for lower anisotropy of the stress field isthe frequency increase of radially propagated diskjoints below 7 km depth.

The investigations provided additional knowledgeon the fracturing process in deep drills especially con-cerning the superposition of faults and the influence onstress orientation. Borehole measurements do not sub-stitute such observations, and vice versa; they supple-ment each other.

Acknowledgements This investigation has been financially sup-ported by ‘‘Deutsche Forschungsgemeinschaft’’ (DFG) since 1992under the grant Ba 1184/5—(1—4). T. Engelder and J. Duyster arethanked for their critical comments on the manuscript, and the teamof the KTB laboratory for discussion and technical support. Wethank P. Denny for improving the English.

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