DAVIDROBERTS&ARNE MYRVANG N GU -B ULL 442 , 200 4 - PA G E 5 3

Contemporary stress orientation features in bedrock,Trondelaq, central Norway, and some regionalimplications

DAVID ROBERTS & ARNE MYRVANG

Rob ert s, D.& Myr vang, A.2004:Contemporary st resso rientat ion features in bedrock,Tron delag, cent ral Nor way, and

some regi onal implicati on s.NorgesgeologiskeundersekelseBulletin 442, 53-63.

Based on in situ roc k st ress measureme nts and conte m po rary st ress orientat ion st ructures observed at d iverse sitesin Trondelaq, it can be show n that the Mo re-Tronde lag Fault Complex mar ks an im portant struct ural d ivide separa­

tin g crusta I b lock s w it h disparate, present -day st ress field s.This support s earl ier prop osals reached by both field andnumerical modelling st ud ies; and, in one case, our data con fir m published predicti on s that contrasting con tempo­

rary st ress field s sho uld, theoreti cally, characte rise th e foot wall and hangingwall blocks of th is major faul t zone.

The prevalent NW-SE hori zontal compression recorded in coastal areas of centr al Nor way north west of t he Mo re­

Trondelag Fault Com plex acco rds wi th bo reho le breakout and earthqua ke focal mechanism solut ion data acqu ired

offshore, indicating th at th is pat te rn is likely to relate to a distr ibuted ridge-pu sh force arising fro m divergent sprea­d ing along the act ive axial rid ge of th e North At lant ic Ocean.Taken as a who le, th e co mbinatio n of in situ rock st ressmeasureme nts and field observat ions of dr illho le reverse-slip off set s and com plem entary axial fractures is seen toprov ide a reliable ind icato r of the contemporary stress patterns existing in exposed bedrock.

David Roberts, Norgesgeologiskeundersekelse, N-7491Trondheim, Norway.Arne Myrvang, Professor emeritus, Department of Geology and Mineral ResourcesEngineering,NTNU, N-7491Trondheim,Norway.

IntroductionRecent research in the field of neot ecton ics in Norway hasrevealed growing evidence th at the near-surface continen­tal crust of this western part of the Baltic Shield is not asinherently stable as geologi sts had earlier believed.Observations of Late Quaternary faults, notably in northe rnparts of Norway and Sweden (Laqerback 1979, 1990 , Olesen1988, Olesen et al. 1992, Mbrner 2004) but also in somesouthern areas (Anda et al. 2002), coupled with seismotec­tonic investi gat ions, some scattered st ress measurement sand information gained from drill ing th rough fault s(Myrvang 1988, 1993, Bungum 1989, Bungum & Lindholm1997 , Roberts et al. 1997 ) have all indic ated the importanceof neotectonic crustaI movements. For a review of this topicand an assessment of th e reported evidence of neotecton­ics, with an extensive bibl iography, th e reader is referred toOlesen et al. (2004); and to the neotectonic map of Norwayand adjacent areas compiled by Dehls et al. (2000).

As well as the se larger-scale manifestations of crustaIinstabi lity, studies of contemporary, small-scale th rust fault­ing and other structura l features revealed by dril lholes inartificia l road-cuts or quarry faces have provided additiona linformation on the current crustaI-surface stress regime,notably in Finnmark, northern Norway (Roberts 1991,2000,Pascal et al. in press).These ind icator s of neotectonic st ressorientat ion can then be compared wi th direct determ ina­tions of in situ rock st ress made at various locations. InFinnmark county, although such rock-stress measurementsare compara tive ly few (Myrvang 1993 ), the drillho le data do

confo rm reasonably well with the contemporary horizontalst ressregime (Roberts 2000).

In th is cont ribution we present data from str uctural fea­ture s revealed by road-cut drillholes in part s of Trondelaq.central Norway,and compare the results and interpretationswith in situ rock-st ress measurements recorded in th isregion over the last 2-3 decades. In centr al and south ernNorway as a whole, the rock stressdatabase is more compre­hensive than in northern areas, thu s allowi ng for more rigor ­ous comparison between observat ion and measurement.

Rock stress in generalIn Scandinavia, the early investigations of Hast (1958), mea­suring in situ stress in bedrock, showed that horizontalstresses almost always exceeded the theoretical horizon talstressascribed to overburden. Later measurement s in manylocations, also in Norway, have reproduced th is same gen­eral trend and, indeed, the major prin cipal st ress is com­mon ly horizontal (e.g.,Stephansson et al. 1986). On th e con­trary, measured vertical stresses in most cases correspondfairly well with the theoretica l st ress calculated from thethicknessof overburden,at least down to depths of c. 500 m.It has been found, too , that th e horizontal stress field in mostlocations is essentially anisotropic, where one componentdominates, i.e., the principal horizontal stress, SHmax.1npract i­cal tunnell ing and und erground excavation, high hori zont alstresses normal to the tunne l axis have, in many cases, cre­ated severe technical prob lems.This high stress is concen-

NGU -BULL 44 2, 20 04 - PAGE 54 DAVID ROBERTS & ARNEMYRVANG

N

oI

x , ,,,,,

j ,,

,,,,

,,

, Bergen,,

x

x ,- STOREN

x

+ +

~Beitstad fjord basin, + +sediments of Middle Jurassic age

+[2J. .. Devonian sedimentary rocks

~Granito id plutons,mainly Ordovician age

c=J Middle, Upper & Uppermost AllochthonsCaledonian nappe rocks, undifferentiated ,

c=J Lower AllochthonI

Mostly Proterozoic rocks;Banded Gneiss Complex on Fosen

c=J ParautochthonProterozoic crystall ine rocks

64' N --=::::::- Major faults

9' E 11' E 13' E

Fig, 1. Simplified geologic al map of t he central Norway region. MTFC- Mo re-Trondelag Fault Complex; HSF- Hit ra-Snasa Fault;VF - Verran Fault ; BF ­Bieverdalen faul t .

t rated in the roof and floor areas of the tunnel, causing vio­lent shear failure of the rock, a phenom enon known asspalling or rock burst. In several cases this has resulted infatal accidents. This type of stress-related pro blem calls forcomprehensive and expensive rock-support measures.

The same mechanism is also qui te com mon in verticalpet roleum wells offshore Norway, where high horizontalstresses w ill be concentrated in two, diametrica lly opposedpoints on th e hole peri phery, leading to violent failure,known in th e petrol eum indu stry as borehole breakouts.

In measuring in situ rock st resses, several techniqueshave been used in Norway over the years, includi ng the useof both biaxia l and triaxial gaug es, and also th e hydraulicfractu ring, rock-stress measuring method (Myrvang 1993).Most measurement s of stress magni tu des and horizontalstress vectors have been done by the Rock MechanicsLaboratory of the Norwegian University of Science andTechnology, NTNU, in more th an 200 locations, in mines, tun­nels and bore holes. The data have been compiled in mapform (e.g., Myrvang 1993, fig.2) which gives an immediatepictu re of the contemporary hori zontal stress field operat­ing in different part sof the count ry,and in whi ch areasstressmagnit udes are highest. Comp arisons can then be madewi th the general bedrock geology and major fault patterns

occurring in anyone region, as has been done in the case ofFinnmark county (Roberts 2000). In th is cont ribut ion weconsider neotecton ic stressor ientation indicators in relat ionto the current hor izontal stress fie ld in the Mid Norwayregion .

On a larger scale, inversion of earthquake focal mecha­nisms may be used to reveal th e ratio between the principalstresses at depth ,and thus provide info rmation on th e stressregime (Bungum & Lindholm 1997, Hicks et al. 2000).

Apart from stress measurements and focal mechanismstudies, in many cases there may be surface phenomenapresent indicating high horizontal stresses(Myrvang 1998):

(i) Surface-parallel fractures know n as exfoliat ion.Theseare extensional fractures caused by high horizontal com­pressive stresses.This is a near-surface phenomenon andexfolia t ion joints are rarely seen deeper than 20-30 m belowth e present-da y surface. Exfo liat ion fracturing can be seenthroughout Norway, part icularly in competent Precamb riangneissic rocks, but it is also recorded in comp etent, mult ilay­ered younge r rocks. In the Mid-Norway region, fine exam­ples of exfol iat ion joints are exposed in the Roan distr ict,whi ch is also an area where reverse-slip displacements ofdrillholes have been recorded (see belo w, pp.56-57).

(ii) Surface shear failure caused by high horizontal

DAVID ROBERTS&ARNEMYRVANG NGU-B UL L 44 2, 20 0 4 - PAG E 55

stresses acti ng at th e bedrock surface.This can be seen oneit her a large or small scale,and surface spalling and exfolia­ti on jointsare common ly found in th e same area and even inth e same bedrock exposu re.

Fig.2. (a) Inferred origin of axial fractu res in th e walls of vertical boreho­les at t he t ime of blast ing. In th e upp er sketch (plan view of the rockbody), gas pressure-ind uced fractures are generated parallel to SHmax.In the lower sketch, th e same rock body is show n afte r blasting andremoval of the rock mass. New joi nts (exposed breakage surfaces) arecommon parallel to th e alignment of the axial fractures. Modified fromBell & Eisbacher 1996 and Roberts 2000}.(b) Simplified block diagram show ing th e coeval develop ment of areverse fault with offset drill holes and, on the orthogonal shaded wall,axial fractur es (A.F.) in th e concave drillholes.The extensional,axial frac­tur es penetrate the rock body and lie with in the plane of the maximumand intermediate stresses.

is indubitable, are taken into account. Isolated or less con­vin cing examples th at await future, detai led, systema ticinvestigat ion are th erefore not considered .

The struc tural features th at are meaningful in the con­text of contemporary st ress orientation are reverse-slip off­sets of vertical to near-vertical drill holes (also called bore­holes) in road-cuts,and axial fractures developed in th e con­cave walls of many such drillholes (Fig. 2). Both these str uc­tu res are regarded as st ress-relief feat ures tha t formedinstantaneously at th e time of road-cut blasting ,essentially asudden release of accumulated strain energy (Bell &

Eisbacher 1996). The displacement vector of th e offsetrecords the direction of reverse slip along a min or fault or

th rust surface and, in t rend, is aligned parallel to SHmax or 0' 1

(Fig. 2b). Axial fractures, on th e other hand, are gas-pressureinduced exte nsional f ractu res that propagated parallel to

Geological settingAlmost all th e region con sidered here falls wi t hin th e realmof the Caledon ian fo ld belt , alt ho ugh th ere are eviden t dif­ferences in the character of the bedrock from area to area(Fig. 1). Western distric ts, for exam ple, are dom inated byPrecamb rian (Palaeopro terozo ic) granit ic gneisses, w hichwe re st rong ly reworked during th e terminal Caledon ian(Scandian) orogen y, w hereas much of th e inland region andareas north of Grong are underlain by nappes consistingmostly of Cambro-Silurian, metasedimentary and volcanicrocks wi th scattered intr usions of gabbro, d ior it e or gran ite

(Sigmond et al. 1984, Nordgul en et al. 1993, Rob ert s &

Stephens 2000).The youn gest unitsare sedimentary rocksofDevon ian age occurring on the southweste rn FosenPeninsula (Siedlecka 1975, Seranne 1992) and on severalislands in this coastal dist rict of centra l Norway (Siedlecka &Siedlecki 1972, B0e & Sturt 1991).

A major featu re of th e geo logy of central Norway is th eENE-WSW-tr ending Mere -Trend elaq Fault Compl ex (MTFC)

(Fig. 1). This composite, mu lt iph ase st ructure cuts throughand displaces many of the Scand ian napp es and thrustsheets, and has a history of movement and reactiva ti onranging from Devon ian to Terti ary t im e (Gronlie & Robert s1989, Grenlie & Torsvik 1989, Grenli e et al. 1991, Gabrielsenet al. 1999, Sherlock et al. 2004).Two major faults are recog­nised on land, th e Snasa-Hit ra and Verran Faults, and aninfer red th ird major st ructure, beneath Trondheimsfjord, ist raceable to th e southwest as the Bzeverdalen fault (or linea­ment ; Redfield et al.2004).Lineament and fract ure/fault pat­terns within and on either side of th e MTFCshow significantdifferences (Rindstad & Grenlie 1986, Grenlie et al. 1991,Gabrielsen et al. 2002), so much so th at th e MTFC has beenconsidered as a fundamental crustal stru cture (Gabrielsen etal. 1999, Pascal & Gabrielsen 2001) that, on a seismic profile,can be traced do wn to depths of at least 15 km (Hur ich 1996,Hurich & Robert s 1997). Gravimetri c data also show th eMTFC to be a major crustal st ructure w it h deep roots in th esubsurface (Fichler et al. 1998, Skilb rei et al. 2002). Fissiont rack studies have confi rmed a type of up per-crusta], fault

segmentat ion into elonga te blocks (Grenlle et al. 1994,Redfield et al. 2004),w it h the inland areas showing evid enceof Neogene uplift (Redf ield et al. in press).

Contemporary stress featuresDur ing th e cour se of fieldwork in th is regio n over th e lastfew years, in proj ects not involving neotecton ics, th e fir staut hor has recorded diverse examples of neotecton ic st ressorientat ion indicators in differe nt types of bedroc k. In th eshort descriptions that follow, only localit ies whe re several

examp les of the features can be seen,or where the evidence

a

b

I I

",,-\A.F.

NGU-BULL 442 , 2004 - PAG E 56

Fig. 3. A particularly well deve loped, continuous axial fractu re in a drill­hole pene trating mica schist s. Localit y along a new sect ion of the E39road, currently under construction, 1.5 km east of Bersa, southw est ofTrondheim.

DAVIDROBERTS & ARNEMYRVANG

S Hmax' in the plane of () j and the prin cipal vertical stress, inmost cases ()3 ' An almost perfect example of such a fracturein a concave drillhole is shown in Fig. 3.

Reverse-slip offsets of drillholesTwo localit ieswhere clear,definit ive examples of reverse-slipdisplacement of drillholescan be seen are near Roan, on theFosen Peninsula,and along the E6 road close to Snasavatnet(Fig. 1).

Roan: Along th e road leadin g to Roan, south of Beske­landsfjord en, five separate examp les of displaced dri llho leshave been recorded along a c. 150 m stretch of nearly con­t inuous road-cut. The bedrock is a comparatively massive,quar tz-monzonit ic granulite gneiss of Pa laeoproterozoicage.The road-cut here reaches up to 8 metres in height,andall the displaced boreholes measured occur in the loweraccessible parts of the road-cut. The surfaces along whichreverse slip has taken place all appear to be knife-sharp mas­ter jo ints (Fig.4).Detailsof the separate displacements are asfollows:-(1) Offset of 3.5-4.0 cm towards 295° along a 008°/28 ° mas­

ter joint (slip) surface.(2) Offset of c. 2.5 cm towards 292° along a 026°/ 16° joint

(slip) surface.(3) Offset of 1.0-1.5 cm towards 288° along a 033°/25 ° jo int

(slip) surface.(4) Offset of 1.5 cm towards 283° along a 038°/4 1° joint

(slip) surface.(5) Offset of c. 2.5 cm towards 268° along a 031°/ 22° join t

(slip) surface.As can be seen, the displacement vector of these sepa­

rate th rust-faulted drillho les is reasonably consistent, aver­aging 285°.This wo uld indicate that the SHmax in this part icu-

Fig. 4 (a) Reverse-slip offset of drillho les along an east-di ppi ng jo int in gran uli t ic gne isses; from a road-cut south of Beskeland sfj ord en, Roan, FosenPenin sula, central Norway; looking south. (b) Close-up of a similar, reverse-slip disp lacement of a drillh ole from th e same localit y, looking south .Thereverse-slip moti on in these areas is towards west-northw est.

DAVID ROBERTS&ARNEMYRVANG

Fig. 5. (a) Surface-parallel, exfolia tion fractures, ind icative of high hor i­zonta l compressive st resses,are common in many areas.This example isfrom a road-cut close to Beskeland, c. 4 km northeast of Roan, FosenPeninsula. (b) Small-scale surface spalling; from exposures close toSumstad,5 km northeast of Roan.

NGU -BU LL 44 2 , 2004 - PAGE 57

lar area t rends c. WNW-ES E. Exfoliation joi nting is also quiteprominently developed in the Roan district (Fig.5).a featureindicative of the current existence of high horizontalst resses in the surface and near-surface bedrock.

Snasa vatnet: Along a relatively new stretch of the mainE6 road on the northwest side of Snasavatn et, near the farmHaugan, reverse-slip displacements of drillholes can be seenin a c. 5 m-high cutting on the north side of the road inMiddle Ordovician Snasa Limestone .The offsets, reaching upto 7.5 cm, occur along a prominent 10-13 cm-thick shear(fault) zone with a strike/dip of 204°/13 ° (Fig. 6); and thethrust direction is towards 105°. Within the narrow shearzone, which is composed of crushed and goug ed limestone,there are int ernal shear bands dipping northwest at c. 20°.The slip vector here denotes aWNW-ESEtrend for SHmax' as inthe caseof the observat ions from the Roan area.

Axial fracturesMillimetre-wide, near-vert ical fractu res along the axes ofcompa rably or iented dr illholes are of quite variable andirregular occurrence.Their presence is, in part,depe ndent onlit hology and struct ure, but in view of their inferred originthey are more likely to be developed in areaswhere the hor­izontal stressfield is strongly anisotropic. Another restrictionis that they are only likely to be present in a reasonably highpercentage of dr illholes in road-cuts or quarry walls alignednormal to SHmax' For this reason, meaningful and regularlydeveloped axial fractu res do not usually occur along road­cuts showing displaced dril lholes.

Although several long road-cuts have been examin ed inthe Mid Norway region, only two have been found so farwhich display abundant and thu s significant, axial fractures.

Fig.6. (a) Reverse-slip offset of drillholes in th e Snasa Limestone; road-cut along th e E6 near th e secondary road to Nordaunen, nort h of Snasavatnet:looking north. (b) Close-up of a displaced drillhole from the same locality showing the crushed and gouged limestone in the shear zone.The align­ment of th e pencil is parallel to th e internal shear fabric in the crush zone (see text).

NG U - BU LL 442 , 200 4 - PA G E 58 DAVIDROBERTSs ARNEMYRVANG

Fig.7. Sketch of segmen­ted axial fractu res in adr illhole in deformed pi l­low lavas, near HagaBridge, Stor en, S0r­Trendelaq. Note th at theterminat ions of th e indi ­vidual axial fractu re seg­ments are deflected intothe trend of t he fol iat ionin the host-rock metala­vas.Looking c. north east.The sketch covers a 2metre length of drillhole,but the drillhole diame­ter,actually 7 cm, is exag­gerated in the figure. a

••••••

..:N

N

• •

•

Steren : A c. 400 m-long road-cut in Cambrian-EarlyOrdovician metaba salts (locally called greenstone s) w ith pi l­low structures, is present just north of Haga Bridge on theeast side of th e E6road, 3 km north of Storen (Fig. 1).Vert icalor subvert ical dril lho les along this NW-SE-aligned, 6-10 m­high road-cut shows many examples of fairly cont inuousbut irregul arly developed axial fractures. Measurementswere taken of the trend s of axial fracture s which exceeded c.1 m in length , i.e., the fractures could be judged to be con­tin uous th rough the greenstone host rock fro m one lava pil­low to the next. In a few cases, th e axial fractu res weredeflected at th eir extremities into a strong, inter-pi llowschistosity (Fig. 7).

Measurement s recorded from separate, concave drill­ho le walls along this long road-cut show that the meantrend of these part icular axial fractures is quite close to NE­SW (Fig. 8a). This would signify that SHmax is also alignedapproximately NE-SWat this locat ion - qui te different fromthe situat ion at Roan and Snasavatn et, but comparable w iththe in situ st ressdata recorded in this same area by Myrvang(1988, 1993).

Rang/an: A c. 500 m-long stretch of road-cut along theE6 ju st 1 km southwest of Rongl an (Fig. 1) also providesmany good examples of axial fractures in dri llholes (Fig. 9).The bedrock here is a multi layered succession of greywackeand phyllite of inferred Mid to Late Ordovician age.Measurements of the axial fracture s show a fairly consistentWNW-ESE (290°) trend (Fig. 8b) indicat ing that SHmax' in th isparti cular area,w ill also have approximate ly this same trend.The significance of thi s, in a regiona l conte xt, will be dis­cussed later.

•

bFig. 8. Stereograms show ing drillho le axial fracture data from (a) theHaga Bridge locality, Steren, and (b) the long E6 road-cut locality nearRonglan.

Other observationsPreliminary inspect ions of new road-cuts southwest ofTrondhe im and sout h of Storen have also revealed evidenceof bot h dril lhole offsets and axial fractu res. About 5 kmsouth of Storen, along the E6, one definite reverse-slip offsetof c.3 cm is directed toward SSW, a trend which is similar tothat of SHmaxat the Haga Bridge locality near Storen. Other,recent ly blasted road-cuts, 1.5 km east of Borsa along thenew E39 road c. 20 km southwest of Trondheim, show sev-

DAVID ROBERTSs ARNEMYRVANG

Fig.9. Photo of adrillholeshowing asubvertical axial fracture with smallkinks,E6 road-cut,near Ronglan; looking west-northwest.

NGU-BULL 44 2, 2004 - PAGE 59

eral axial fractures in drill holes (Fig. 2).These tr end between

NNE-SSW and NE-SW, an ali gnment w hich corresponds

broadly with the trend of SHmax recorded near Storen.

It mu st be st ressed that th ese observa t ions are merely

preliminary and sporad ic. After complet ion of th ese new,

major road con stru cti on s, a mor e syste mat ic st udy of th e

road-cuts w ill be undertaken .

DiscussionOn e of th e most st riking features of th is co m pilat io n of co n­

temporary st ress orientat io n st ruct ures and in situ rock­

st ress measurements is th at th e Mor e-Trondelag Fault

Complex marks an important di vid e, separat ing major

crustaI blocks w it h apparentl y d isparate st ress fi eld s. Both

w it hin and to th e northwest of th e MTFC, th e princip al hori ­

zonta l st ress tr ends betwe en E-W and NW-SE, w he reas

in land, sout h ofTrondhei m, th e t rend of SHmax is close to NE­

SW (Fig. 10). The axial fracture data f rom Ronglan mi ght, at

fir st sig ht, appear to provid e an except ion to thi s otherwise

clear picture. Howe ver, sate lli t e ima ge ry clearly shows th at

thi s particular area has a lineament pattern w hich is com pa­

rable to that w it hi n th e MTFC along th e northwestern side

ofTrondheimsfjord (Rindstad & Gronl ie 1986).

As noted earlier, gravimetri c, seismic and apat ite fi ssion ­

tr ack data have all indicated that th e MTFC is a fundam ental,

deep-seated, cru stal st ruct ure. Even th e elongate blocks or

slivers of cru st within th e fault comple x have now yie lde d

Vikna

N

A +-1'1+ .::tT;

tt:- ....

-~:....~ Snasavatnet

Horizontal stresses, with centralmeasuring site , after Myrvang (1988)

Offset drill holes showing directionof thrust displacement

Trend of axial fracturesin road-cut drillholes

Borehole breakouts,trend of SHmax

Major faults , includingMme-Trondelag Fault Complex

Thrust vector, Berill Fault(Anda et al. 2002)

Fig.10. Outline map showing thediverse rock-stressorientation data from central Norwayand theTrondelag Platform.The small rosediagram (inset,top left) is from Hickset al.(2000, p.243) and depicts the trendsof maximum horizontal compressive stress as derived from earthquake focal mecha­nism solutions in thearea of offshoreMid Norway (period 1980-1 999).

N G U-BUL L 4 4 2, 2004 - PA GE 60 DAVIDROBERTSs ARNEMYRVANG

Fig. 11. Seismicit y of central Norway, and theTrendelaq Platform and Halten Terrace, inclu­ding the eastern margin of the overlappingMore and Vorinq Basins in th e far west; basedon Byrkjeland et al. (2000). The compilationtakes in all events recorded since 1880.lt shouldbe noted , however, that some of the lowestmagnitude events on land could possiblyrepresent blasting from road or tunnel con­struct ions. The major faults (cont inuous lines)are mostl y taken from Blystad et al. (1995, Plate1).For geograph ic locations onshore,seeFig. 1.

""'=--

Mag nitude

0 4

0 30 20 1

oo

o0 \;;;1=====;;;;;;;;;j! 12'

eviden ce of d ifferences in exhuma t io n histo ries; and the

inland, foo twa ll blo ck, southeast of the Beeverda len faul t,

exp erienced it s latest major up lift, by up to 2 km , in Neogene

t im e between 40 and 20 Ma ago (Redfield et al. in p ress).

Based on numerical modell ing of Cenozoic st ress patterns

offshore Mid Norway, Pascal & Gabrielsen (2001) pro posed

that t he MTFC was a me chan ically weak, composite fracture

zone wi t h an in herently high potential for having inf luenced

both reg ional and local st ress fields from Devo nian t im e to

th e present day. Despite this, and no twithstan d ing its mu lt i­

ple reacti vati on histo ry, th e MTFC, on shore at least, is devoid

of any signif icant, recent or co nte mporary, seismic act iv it y

(Hicks et al. 2000), i.e., over the last two dec ades. Look ing

back over the last century or more, ho wever, t here is a defi­

nite concent rat ion of events above magnitude 2.0 in a crude

linear zone t rending c. NE-SW along and adjacent to th e

More-Trondelag Fault Complex (Byrkjeland et al. 2000, Plate

2).There is also a weak NE-SW t rend ing alig nme nt of events

passing through Storen.A version of t his compilation of seis­

mological data, based on Byrkj eland et al. (2000, Plate 2 and

chapte r on data sources,pp.6224-5), is sho wn here as Fig. 11.

Coastal areas of central Norw ay, on t he other hand , have

clearly been more act ive in terms of catalogued seism ic

events (Byrkjel and et al. 2000, Hicks et al. 2000), and t his is

refl ected in d iverse surface ph enomena. In one area c.30 km

northeast of Roan (Fig. 1), for example, a curious ort hogonal

mosaic of pebble-l ike agg regates of sand in mu lt ilayered

Quaternary sedi ments (Bargel et al. 1994) has bee n inter­

preted as liquefaction features arising from eart hq uake

activity during late-Weichselian crusta l up lift . Some 200 km

far ther south west, the area of coastal More is on e of the

most seism icall y act ive regions of mainland Norway (Dehls

et al. 2000 , Hicks et al. 2000) (Fig. 11).This area also reco rds

th e highest concentration of large-scale roc k slide s and

allied roc k-slope failures in Norwa y (Blikra et al. 2002, in

press), feature s that have bee n d irectly related to increased

ear thquake act iv ity in historic t im es. The post-g lacial Berill

Fault (Fig. 10) (Anda et al. 2002) also occurs in th is same

reg io n.Summing up, even thoug h Mid Nor way as a w hole is,

in relat ive terms, a seismi cally 'quiet' reg ion today, it is no te­

wort hy t hat recor ded act ivi ty above magnitude 1.0 dimin­

ishes rap idly southeast of the Bzeverdalen fault and its

extension into Trond heimsfjo rd (Byrkjeland et al. 2000, Deh ls

et al. 2000, Hicks et al. 2000). The inn er part of Trond elag is

thus one of the most seismical ly stab le part s of th e presen t­

day Balt ic Shield.

The present-day st ress orientat ion data onshore in

Trondelag and Nor d-More, and in par t icular th e prevalent

NW-SE to WNW-ESE t rend of SHmax along and to th e no rth­

w est of the MTFC,is also reflec ted in borehole brea kout and

eart hquake focal mechanism data (Fig. 10) from the offshor e

areas as far west as th e marg in to t he Vor ing Basin (Bungum

et al. 1991, Gregersen 1992, Gi:ilke & Brud y 1996, Byrkje land

et al. 2000, Hicks et al. 2000).Ther e are also substantial seis­

m ic-reflection and w ell data from the Trondelag Platform,

Halten Terrace and shelf marg in pointing to the presence of

wi despread compressiona l st ructures from Eocene to the

present day (Dare & Lundin 1996, Langa ker 1998, Vaqnes et

DAVID ROBERTS & ARNEMYRVANG

al. 1998).These st ructures, including reverse fau lts inverting

earlier normal faults, and conti nuous ly growing elonga tedo mal structures,indicate the existence of a NW-SE-orientedcomp ressional stress fie ld along th is Mid-Norwegian margin(Fig. 10),w it h an extrapo lated maximum bu lk shortening ofc.3% (Vagnes et al. 1998).

Numerical modell ing of st ress patterns in the MidNorway region, both offshore and onshore, f rom the Tertiaryto the present day has also defined a fundamental NW-SE­t rend ing, maximum horizonta l st ress (Golke & Coblentz1996,Golke et al. 1996, Pascal & Gabrie lsen 2001).One of th emodels also predicts tha t contrasti ng, contemporary st resspat terns should, indeed, exist in the footwall and hanging­wall blocksof th e MTFC(Pascal & Gabrie lsen 2001).However,

the precise t rend s and magnitudes, and degree ofanisotropy of th e horizontal stre sses in th e inland block aredifficult to model, since th e stress field is very sensitive tolocal effects such as,for example, to pogra ph ic forces, whichaug ment extensional stresses and cause stress axis rota­tions. An explanat ion for th e NE-SW orie ntation of SHmax inthe Steren dist rict, for examp le,may be soug ht in a combina­tion of st ress axis deflections, influence of topography, anddistance from the mechanically weak MTFC, but far morestress data are requ ired from this inland region before wecan say anything defi nitive.

In general te rms,t he prevailin g NW-SEcom pression doc­umented in both the offshore and the oute r onshoredom ains is considered by a large proportion of the geo­science community as relati ng to a distributed ridge pushforce arising from mid -At lantic sea-floor spreadi ng (e.g.,Stein et al. 1989, Mul ler et al. 1992, Fejerskov & Lind holm2000, Lindholm et al. 2000).This coincides very we ll wi th th epattern seen on the World St ress Map, where stress datafrom both shallow depth s and midd le to lower crusta llevelsfrom all over th e worl d have been com piled (Zobeck et al.1989, Reinecker et al. 2004). The data that we have acquiredon- land, in Trondelag, i.e., contemporary horizontal stressorientations and eviden ce for WNW-ESE-trending, thrust­fault displacements of roadsid e dr ill holes, support this gen­eral compressional model.

With regard to the genera l stress situa tion, t he mechani­cally weak natu re of th e MTFC,and th e paradox of compara­t ively insignif icant contemporary earthquake activ ity alongthis megastr ucture, it is not improbable tha t an expectedaccumulation of stresses may result in an eart hquake ofmoderate magnitude in this region at some stage in thefuture. On th e ot her hand, slip partitioning along the innu­merable faults wi thin the MTFC system may be such tha tseismic energy wo uld possibly be widely dissipated , th uspreclud ing th e gene rat ion of sudden, major earthquakeswit hin this zone.

ConclusionsBased on in situ rock-stress measurements and contempo­rary stress orie ntation st ructures at dive rse localities in

Trondelag, it can be shown that th e More -Tronde lag Fault

NGU -BULL 44 2 , 2 00 4 - PAGE 6 1

Compl ex marks an important st ructural line separat ing

crustal blocks wit h disparate, curren t stress fields.This sup­po rts earlier pro posals reached by both field and nu mericalmod ellin g studies, th ough in th ese cases pert ain ing mostl yto earlier periods of geo logical t ime. In one case, however,ou r data confirm predictions (Pascal & Gabrie lsen 2001) thatcontrasting con temporary stress regimes should, theoret i­cally, characterise the foo twall and hangingwall blocks of

the MTFC.Given this curre nt situation, it is t herefore not inconceiv­

able th at an earthquake of moderate magnitude may occu ralo ng the MTFC, on land, at some future dat e - effective ly

marki ng a release of the st resses th at have been accum ulat­ing during th e last few decade s of relat ive inactivity.

The prevalent NW-SE hor izonta l compression recorded

in the near-coastal areas of Trondelag accords wi t h dataacquired offshore, including earthquake focal mechanism

solut ions der ived fro m deeper crust al levels, indicating th atth is patte rn is likely to relate to a distr ibuted ridge -pushforce arising fro m divergent spreading along the axial ridg eof the North Atlan tic Ocean.

Taken as a whole, the combination of in situ rock stressmeasurements and field observat ion s of drill hole reverse­slip offsets and complemen tary axial fractures is seen to pro­vide a reliable indicator of the contemporary stress patt ernsexisting in exposed bedrock.

Ackno wledgem entsWe are grateful to th e rev iewers, Chri stop he Pascal and Od leiv Olesen,

for th eir constr uctive and useful comments, and for suggestions whichled to improveme nts in the general di scussion .lrene Lundquist assistedwi th prepara tio n of some of the figures, and Odleiv Olesen helped in

producing the seismic events map, Figure 11.

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