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

DAVIDROBERTS&ARNE MYRVANG NGU-BULL 442,2004 - PAG E53

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

implications

DAVID R OBERTS & ARNE MYRVANG

Rob erts,D. &Myr vang,A.2004:Contempor ary stressorientation featuresinbedrock,Tron delag, cent ral Nor w ay, and someregi on alimplicati ons.NorgesgeologiskeundersekelseBulletin 442,53-63.

Based oninsiturockst ress measurementsandco ntem poraryst ressorient at io nst ruct uresobserve datdiversesites inTrondelaq,itcan beshownthatthe More-TrondelagFaultComplexmarksan important struct uraldividesepa ra- ting crusta Iblockswit h disparat e,present-dayst ressfield s.This sup portsearlier prop osalsreachedbybothfield and numericalmod elling st udies;and,inone case, ourdataconfir m publishedpred icti onsthat contrastin g contempo- rary stressfieldssho uld,theor eti call y, characte rise thefoot w alland han g in gw all blocks ofthis majorfaul t zo ne.

TheprevalentNW-SEhori zontalcompression recordedincoast al areas of centralNor w ayno rth w est of the More- TrondelagFault Com p lexaccordswit hboreho lebreakoutandeart hqua kefocal mechanism solutiondata acquired offshore,indicatingthatthispat te rn islikelytorelat etoadistr ibutedrid g e-pu shforce arisingfro mdiverg en tsprea- dingalong the act iveaxial ridge of theNorth At lant icOcean.Takenas a who le,the com b inatio nofinsitu rockst ress measurementsandfieldobservationsof drillholereverse-slipoffset s and comp lem entaryaxial fracturesis seen to provideareli ab leind icat o rofthe contemporary stresspatternsexistingin exposed bedrock.

David Roberts,Norgesgeologiskeundersekelse,N-7491Trondheim,Norway.

ArneMyrvang,Professor emeritus,Department of Geologyand MineralResourcesEngineering,NTNU,N-7491Trondheim, Norway.

Introduction

R ecent research in the field of neot ecton ics in Norway has revealed growing evidence that the near-surf ace continen - tal crust of this western part of the Baltic Shield is no

t

as

inherentl y

stable as geologi sts

had

earlier believed.

Observations of

Late Quaternary

faults, notably in northe rn parts of Norway and Sweden (Laqerback 1979, 1990 , Olesen 1988, Olesen et al. 1992, Mbrner 2004) but also in some southern areas (Anda et al. 2002), coupled with seismotec- tonic investi gat ions, some scatte red st ress measurement s and information gain ed from drill ing th rough fault s (Myrvang 1988, 1993, Bungum 1989, Bungum & Lindholm 1997 , Roberts et al. 1997 ) have all indic ated

the importance

of neotectonic crustaI movements. For a review of

this topic

and an assessment of

th

e reported evidence of neot ecton- ics , wit h an ext ensive bibl iography, th e reader is referred to Olesen et al. (2004); and to the neot ectonic map of Norway and adjacent areas compiled by Dehls et al. (2000).

As well as the se larger-scale manife stations of crustaI instabi

lity,

studies of contemporary, small-scale th rust fault- ing and other structural features revealed by dril

lholes

in artificia

l

road-cuts or qu arry faces have provided additiona

l

information on the current cru staI-surface st ress regime, notably in F innmark, north ern Norway (Roberts 1991 ,2000, P ascal et al. in press).These ind icator s of neotecto nic st ress orientat ion can then be compared wi t h direct determ ina- tions of in situ rock st ress made at variou s locations. In Finnmark county, although such rock-stress measureme

nts

are comparative ly few (Myrvang 1993 ), the d

rillho le

data do

conform reasonab

ly

w ell wit h the conte mporary horizont al st ress regime (Rober ts 2000).

In th is cont ributi on w e present data from str uctural fea- ture s revealed by road-cut drill holes in part s of Trond elaq.

central Norway,and compare th e results and interpr etations with in situ

rock-st ress measurements

record ed in th is region over the last 2-3 decade s.

In centr

al and southern Norway as a whole, the rock stress database is more compr e- hensive than in northern areas,thu s allowi ng fo r more rigor

-

ous comparison bet ween observat ion and measurement.

Rock stress in general

In Scandinavia,

the early investigations of

Hast

(1958), mea- suring in situ st ress in bedrock, show ed th at horizontal str esses almo st always exceeded the theoretical horizon tal stressascribed to overburden

.Later

measurement s in many locations, also in Norway, have reproduced th is same gen- eral trend and, indeed, the m ajor prin cipal st ress is com- mon

ly

horizontal (e.g.,Stephansson et al. 1986). On th e con- trary, measured vertical stresses in most cases corr espond fairly we

ll

with the theoretical st ress calculated from the thickne ssof overburden,at

least do

wn to depths of

c.

500 m.

It has been found,too ,that th e horizontal stress field in most

locations is essentially anisotropic, w here one component

dominates, i.e.

,

the principal ho rizontal stress, S

Hmax

.1n pract i-

cal tunnelling and und erground excavation , high hori zont al

stresses normal to the tunne l axis have, in many cases, cre-

ated severe tec

hnical

prob

lems.This

high stress

is

concen-

NGU-BULL 442 ,2004 - PAGE 54 DAVID ROBERTS&ARNEMYRVANG

N

o

I

x

^,

, , , , ,

j,

, , , , ,

, ,

,

,

Bergen

,

x

x ,

-STOREN x

+ +

~

Beitstad fjord basin,

+ +

sediments ofMiddleJurassicage

[2J ^{. ..}

^Devoniansedimentary rocks

+

~

^{Granito id}mainly Ordovician^plutons,age

c=J

^{Middle,Upper&Uppermost}Alloc hthons

Caledonian napperock s,undifferentiated

,

c=J

^LowerMostly Prot erozoic^Allochthon rocks; ^I

BandedGneissComplexonFosen

c=J

Parautochthon

Proterozo ic crystallinerocks 64'N

--=::::::-

Major faults

9'E 11'E 13'E

Fig,1.Simplifiedgeologic al mapof thecentral Norwayregion.MTFC- More-Trondelag Fault Complex;HSF- Hit ra-Sna saFault;VF - VerranFault ;BF- Bie verd alen fault.

trated in the roo f and floor areas of t he tunnel, causing vio- lent shear failure of t he rock, a ph enom enon known as spallin g or rock burst. In several cases this has resulted in

fatal

accidents. This type of stress-related problem calls for comprehensive and expensive rock-support measures.

The same mechanism is also qui

te com

mon in vertic al pet roleum we

lls

offshore Norway, w here high horizontal stresses will be co

ncentrated intwo,

diametrically op

po

sed points on th e

hole peri

phery, leading

to

violent failure, known in th e petrol eum indu stry asborehol e breakouts.

In measuring in situ rock st resses, several technique s have been used in Norway over the years

,

includi ng the use of both biaxial and t riaxial gaug es, and also the hyd raulic fractu ring, rock-stre ss measuri ng method

(Myrvang 1993).

Most measurement s of stre ss magni tu des and ho rizo ntal st ress vectors have been done by the Rock Mechani cs Laborator y of t he Norwegian University of Science and Technology, NTNU, in more th an 200 locations, in mines, tun- nels and boreholes. The data have been compile d in map form

(e.g.,

Myrvang 1993

,

fig.2) which gives an immediate pictu re of the contemp orary hori zontal stress field operat- ing in diff erent part sof t he count ry, and in whi ch areasstr ess magnit ud es are high est. Comp arisons can then be made wi t h the general bedrock geology and maj or fault pattern s

occu rring in anyon e region,as has been don e

in

the case

of

Finnmark county

(Roberts 2000). In this

cont

rib ut ion we

consider neotec ton

ic

stress or

ientation indicators in relation

to the current horizontal stress fie

ld in the Mid Norway

region

.

On a larger scale, inversion of earthquake focal mecha-

nisms

may be used to reveal th e ratio between the pr

incipal

stresses at depth

,and

thus provide info rmation on th

e

stress regime

(Bungum

& Lindholm 1997,Hicks et al. 2000).

Apart from stress measurements and focal mechanism studies, in many cases there may be surface phenomena present indi cating high horizonta

l

stresses(Myrvang

1998):

(i)

S urface-parallel fractures

know n as exfo liat ion.These

are ext ensional fractures caused by high

hori zontal com-

pressive stresses. This is a near-surface phenomenon and exfolia tion joints are rarely seen deeper t

han 20-30 mbelow

th e present-da

y surface. Exfo liat ion

fracturing can be seen t hrou ghout Norway, part

icularly in competent Precamb rian

gn eissic rocks, but

itis also

recorded

in comp etent,

mult

ilay-

ered younge

r

rocks. In the

Mid-Norw ayregion,fine exam-

ples of exfol iation j oi nts are exposed in the R oan

district,

whi ch is also an area where reverse-slip d

isplacement s of

drillholes have been

recorded(see below,pp.

56-57).

(ii)

S urface shear failure caused by high

ho rizontal

DAVIDROBERTS&ARNEMYRVANG ^{NGU-BUL L442,20 04 -PAGE55}

stresses acti ng at the bed rock surface.This can be seen on eit her a larg e or

small

scale,and surface spalling and exfolia- ti on joints are com mon ly found in th e same area and eve n in the same bedrock exposu re.

Fig. 2.(a)Inferred originofaxial fractures inthe wallsof vertical boreho- les at thetimeof blast ing. Intheuppersketch(planviewoftherock body), gas pressure-ind ucedfract uresaregeneratedparalleltoSHmax.

In thelowersketc h,the samerockbod y is show nafte r blasting and removal of the rock mass.New joints(exposed breakagesurfaces)are common paralleltothe alig nmentofthe axialfractures.Modified from Bell&Eisbacher1996 and Rob erts2000}.

(b) Sim plified blockdiagram show ing the coevaldevelop mentof a reversefaultwith offset drillholesand,onthe orthogonalshadedwall, axial fractures (A.F.)in the concavedrillho les.The extensional,axialfrac- turespenetratetherock bodyandliewithintheplane of the maximum andintermediate st resses.

is indubitable, are tak en into acco unt. Isol ated o r less con- vin cing examples th at await future, detai led, systema tic inve stigat ion are th erefore not considered .

The struc t ural feat ures that are meaning ful in the con- text of contemporary s t ress orientatio n are reverse-slip off- sets of vertical to near-vertical drill holes (also called bore- holes) in road-cuts, and axial fractures developed in th e con- cave w alls of many such drillhol es (Fig. 2). Both these str uc- tures are regard ed as st ress-relief feat ures tha t formed insta nt aneously at th e time o f road -cut blasting , essentially a sudde n release of accumulate d st rain energy

(Bell &

Eisbacher 1996). The displacem ent vector of th e offset records the direction of reverse slip along a min or fault o r th rust surface and, in t rend

,

is align ed parallel to SH

max

or

0'1

(Fig. 2b). Axial fractures, on th e other hand, are gas-pressure in duced exte nsio nal fractu res that prop agated parallel t o

Geological setting

Almost all th e regi on con sidered here falls wi t hin th e realm of the Caledon ian fold belt, alt ho ug h th ere are eviden t dif- ferences in the character of the bedrock from area to area

(Fig.

1). Western distric ts, for exam ple, are dominate d by Precamb rian (Palaeopro terozo ic) gr anit ic gneisses, w hich we re st ro ng ly rew ork ed during th e termin al Caledon ian (Scandia n) orogen y,w hereas mu ch of th e inland regi on and areas north of Grong are und erlain by nap pes consisting mostly of Cambro-Silurian, metasedimenta ry and volcani c rocks wi th scat tered intr usio ns of gabbro, d ior it e or gran ite

(Sigmond et

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

Steph ens 2000).The youn gest units are sedimentary rock sof Devon ian age occurring on the sout hweste rn Fosen P en insula

(Siedlecka

1975, Serann e 1992) and on severa l islands in this coastal dist rict of centra l Norway

(Siedlecka&

Siedl ecki 1972, B0e & Sturt 1991).

A maj or featu re of th e geo logy of central Norw ay is th e ENE-WSW-tr ending Mere

-Trend elaq

Fault Compl ex (MTFC) (Fig. 1). This comp osite, mult iph ase st ruct ure cuts through and displaces m any of the Sc and ian napp es and thrust sheets, and has a histo ry of movement and reactivati on rangi ng from Devon ian to T erti ary tim e

(Gro

nlie & Robert s 1989, Grenlie & Tor svik 1989, Grenli e et al. 1991, Gabrielsen et al. 1999, Sherlock et al. 2004).Two maj or faults are recog- nised on land, th e Snasa-Hit ra and Verran F aults

,

and an infer red th ird major st ructure, beneath Trondheim sfj ord, is traceabl e to th e sout hwest as the Bze verda len fault (or linea- ment; R edfield et al. 2004).Lineam ent and fract ure/fault pat- terns within and o n eit her side of th e MTFC show sig nificant d ifferences (Rindstad & Grenlie 1986, Grenlie et al. 1991, Gab rielsen et al. 2002), so m uch so th at th e MTFC has b een considered as a fund am en tal crusta l stru cture (Gabrielsen et al. 1999, Pascal & Gabrielsen 2001) t hat, on a seism ic profile, can be traced do wn to depths of at least 15 km (Hur ich 1996, Hurich & Robert s 1997). Gravim etri c data also show th e MTFC to be a major crustal st ruct ure w it h deep roots in the subsurface

(Fichler et

al. 1998, Skilb rei et al. 2002). F ission track st udies have confi rmed a type of up per-crusta], fault segme ntat io n into elonga te blocks

(Grenlle et

al. 1994, Redfield et al. 2004),w it h the inland areas sho w ing evid ence of Neogene uplift (Redf ield et al. in press).

Contemporary stress features

Dur ing the cour se of fieldwork in th is regio n over th e last few years

,

in proj ect s not in volving neotecton ics, th e fir st aut hor has recorded diverse examp les of ne otecton ic s t ress ori entat ion indi cators in differe nt t yp es of bedroc k. In th e sho rt descriptions that follow, only localit ies whe re several examp les of t he features can be seen,or where the evide nce

a

b

I

^I

",,-\

A.F.

NGU-BULL 442,2004 - PAGE56

Fig.3.A particularly well developed,continuou s axial fracture in a drill- hole penetrating micaschists.Localit y alo ng a newsectionof the E39 road,currentlyunder construction,1.5 kmeast ofBersa,so ut hw estof Trondheim.

DAVID ROBERTS&ARNEMYRVANG

SHmax'intheplane of ^()j

and the prin

cipal

vertical stress

,

in

mo

st

cases()3'An

almost

perfect examp le

of such a fract ure

in aconcavedrillh ole is shown

in

Fig. 3.

Reverse-slip offsets of drill holes

T

w

o

localit ies where clear,definit

ive examples

of reverse-slip

displacementof drillholes can be seen are near Roan,

on the

Fosen Peninsula,and along

the

E

6 road close to Snasavatn et (Fig.

1).

Roan: Along th

e

road leadin

g

to R

oan, south of Beske-

landsfj ord

en,

five separate examp lesof

displaced dri

llho les havebeen

recorded along a c. 150 m stretch of nearly con-

tinuousroad-cut.The bedrock is a comparatively

massive,

quar

tz-monzonit ic granulite gne

iss

of Pa

laeoproterozoic age.Theroad-cut here reaches

up

to 8 metresin height,

and all

the d

isplaced boreholes

measured occur in the low er accessibleparts of the road-cut.The surfaces

along

which reverse slip hastakenplace

all appear to be knife-sharp mas-

terjoints(Fig.4).Details

of

the separate

displacementsare as

follow s:-

(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°

joi nt

(slip) surface.

(3) Offset

of1.

0-1.5 cm towards 288° along a 033°/25

°

jo

int

(slip) surface.

(4)

Off

set of 1.5 cm

towards 283°

along a 038°/4

1° joi nt (slip)

surface.

(5) Off

set of c. 2.5 cm to

wards

268° along a

031°/

22°

join t (slip) surface.

As canbe seen,the displacement

vector of

thesesepa- rate thrust-faulted drillholesis reasonably consistent,

aver-

agi ng285°.This wouldindicatethattheSHmax

in

thisparticu-

Fig. 4 (a)Reverse-slip offsetof drillholesalongan east-dippi ngjointin granuli t icgneisses;from aroad -cut south of Beskeland sfj ord en,Roan,Fo sen Peninsula,centralNorway;lookin gsout h. (b) Clo se-up ofa similar,reverse-slipdisplacementofadrillh ol efrom the same localit y,looking south.The reverse-slip moti o n in these areas istowardswest-no rthw est.

DAVIDROBERTS&ARNEMYRVANG

Fig. 5. (a) Surface-parallel,exfoliation fractures,indicativeof high hori- zonta lcompressive st resses,arecommon in many areas.Thisexampleis fromaroad-cut closeto Beskeland,c.4 km northeastofRoan,Fosen Peninsula.(b) Small-scale surface spalling; from exposuresclose to Sumstad,5 km north east ofRoan.

NGU-BU L L442 ,2004 - PAGE 57

lar area

trends c. WNW-ES E. E

xfoliation joi

nting is also quite

prominently developed in the Roan district

(Fig.5).afeat ure

indicative of the current existence of

high horizontal st ressesin the surface and near-surface

bedrock.

Snasa va tnet: Along a relatively new s t retch of th

e

main E

6

road on the north

west

side of Snasavatn et, near the farm Haugan, reverse-slip displacements of drill holes can be

seen

in a c. 5 m-high cutting on the north

side of the road in

Middle Ordovician Snasa Limestone

.The

offsets, reaching up to 7.5 cm, occu

r

along a prominent 10-13 cm-thick shear

(fault) zone wit h a strike/dip

of 204 °/13 °

(Fig.6); and the thrust direction

is towards 105°. Within the narro

w

shear zone, which is composed of crushed and gouged limestone, there are int

ernal

shear bands dipping northwest at c.

20°.

The slip

vector here denotes a WNW-ES Etrend

for SHmax' as in

the case of

the observat ions

from the Roan area.

Axial fractures

Millimetre-wide, near-vert ical

fractu res along th

e axes of compa rably or

iented

dr

illholes

are of quite variable and irregular occurrence.Their presence is, in part, dependen t on lit

hology

and struct ure, but in view of their

inferred origin

they are more

likely to be developed

in

areaswhere the hor-

izonta l

st ressfield is strongly

anisotropic. Anoth

er

restriction is t

hat

they are onl

y

likely to be present

ina

reasonably high percentage of drillholes in road-cuts or quarry walls

aligned

normal to SHm

ax'F

or this reason, meaningful and

regularly

developed axial fractu

res

do not usually occur along

road-

cuts showing displaced dril

lholes.

Alt houghseveral long road-cutshave been examin ed

in the Mid Norway region, only t

wo

have been found

so far

which display abundant and thu

s significant, axialfract ures.

Fig.6.(a) Reverse-slipoffsetof drillh oles in the Snasa Limeston e;road-cut along theE6nearthe secondary road toNord aunen,nort h of Snasavatnet:

lookin gnorth. (b)Close-up ofadisplaceddrillholefromthe same localityshowing thecrushed andgouged limestone in theshear zone.Thealign- mentof the pencil isparalleltotheinternal shear fabricin the crush zone(seetext ).

NG U-BU L L442,2004 -PAGE58 DAVIDROBERTS

s

ARNEMYRVANG

Fig.7.Sketchof segmen- ted axial fractures in a drillholeindeformed pil- low lavas, near Haga Bridge, Storen, S0r- Tre ndelaq. Notethat the terminations oftheindi - vidualaxialfractu reseg- ments aredeflected into thetrend of thefol iation inthehost-rockmeta la- vas.Lookingc.northeast.

The sketch covers a 2 metrelengt hofdrillhole, but thedrillholediame- ter,actually 7cm,is exag- geratedinthefigure.

a

•• ••

••

.. ^:

N

N

• •

•

Steren : A c. 400 m-long road-cut in Cambrian-Early Ordovician metaba salts (locally called greenstone s)

w

ith pil-

low struct u

res , is present just nor

t h

of Haga Bridge on th e east side of

th

e E6 road,3 km nor

t h

of Storen

(F

ig. 1).Vertical or subvert ical dril

lho les along this NW-SE-aligned,

6-10 m- high road-cut show

s

many examples of fairly cont inuous but irregul arly developed axial

fract ures.

Measurement

s

we re

taken

of

t

he trend

s

of axial fracture s

which exceeded

c.

1 m

in

length,

i.e., the fract ures could be j udged to be con- tin

uousth

rough the greenstone host rock fro

m

one lava pil- low to t

he

next.

In a few

cases

,

th e axial fractu res were deflec

ted

at

th

eir extre

mities into

a

st rong, int

er-pi llow schistosity

(Fig.

7).

Measurem ent s record ed from

separate, concave

drill- ho

le walls along this long road-cut

show th at the mean trend of

these

part icular axial fractures

is

quite close

to NE-

SW

(F

ig. 8a).

This

wo

uld

signify that S

Hmax

is also aligned approximately NE

-SWat

this locat ion - qui

te

different from the situation at Ro an and Sn asavatn et, but comparable w

it

h th e in situ

st ress

data

recorded inthis same area by

Myrvang

(1988,

1993).

Rang/an:

A c. 500 m-long stretch of road-cut along the

E

6 ju st

1

km sout hwest of R ongl an

(Fig.1)

also provides many good examples of axial fract ures in drillholes

(Fig.

9).

The

bedrock

here

is a mu

lti layered succession of

greywa cke and phyllit e of

inferred

Mid to

Late Ordovician age.

Measurements of the axial fracture s show a fairly consistent WNW-ES E(290°) trend

(Fig. 8b)indicat ing th

at S

Hmax'inth

is parti cular area,w ill also have approximate

ly

this same trend.

The

significance

of thi s, in a regiona

l

conte

xt,

will be dis- cussed

later.

•

b

Fig. 8. Stereog ramsshow ingdrillho leaxial fract ure datafrom(a) the HagaBridgelocality,Steren,and(b)thelongE6 road-cut localitynear Ronglan.

Other observations

Prelimin ary

inspections

of new road-cut s sout hwest

of

Trondhe im

and sout h of

Storen

have also revealed

evidence

of bot h dril

lhole offsets

and

axial fractu res.

About 5

km

sout h of Storen, along t he E 6, one definit

e

reverse-slip offset

of c. 3 cm is directe d to

ward

S SW,a trend

w

hich

issimilar to

th at of S

Hmaxat

the Haga Bridg

e

locality near S toren. Other,

recently blasted

road-cuts,1.5 km east of

B

orsa along the

new E

39

road c.

20kmsout

hw est of Trondheim

,show sev-

DAVID ROBERTS

s

ARNEMYRVANG

Fig.9. Photo of adrillhole s howing a s ubvertical axial fracturewit h

small

kinks,E 6 road-cut, nearR onglan; looking west-northwe

st.

NGU

-BULL

44 2, 2004 - PA GE 59

eral axia l fractures in drill hol es (Fig.

2).These

tr end bet w een NNE-SSW and NE-SW , an alignment w h ich co rrespo nds broadly with the trend of SHm

ax

record ed near S to ren.

It mu st be st ressed that th ese o bse rva t io ns are merel y pr elimin ary and spo rad ic. After compl et ion of th ese new, m ajor road con stru cti on s, a m or e syste ma t ic st udy of th e road-cuts w ill be undertaken.

Discussion

On e of th e mo st st riking features of th is co m p ilat io n of con- t emporary st ress orie nta t io n st ruct u res and in situ rock- st ress m easur ement s is th at th e Mor e-Trond elag Fault Compl ex m arks an important di vid e, separat ing m aj o r crustaI blo cks w it h apparentl y disparat e st ress fi eld s. Both w it hin and t o th e northw est of th e MTFC, th e prin cip al hori - zo nta l st ress tr end s bet we en E-W and NW-SE, w he reas in land, so ut h ofTro nd hei m, th e t rend of SHmax is close to NE- SW (Fig. 10). The axi al fracture dat a from Ronglan mi ght, at fir st sig ht, appear to provid e an except io n to thi s ot herwise clear picture. Howe ver, sate lli t e ima ge ry clearl y shows th at this particular area h as a lineament pattern w h ich is com pa - rable to that wit hi n th e MTFC alo ng th e north w estern side ofTrondheimsfjord (Rindst ad & Gronl ie 1986).

As noted earlier, gravimetri c, seism ic and apa t 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 elo nga te bl ocks o r slivers of cru st within th e fault com ple x have no w yie lde d

Vikna

N

A +

-1'1+ _T; ^.::t

tt:- ....

-~:

....~ Snasava tnet

Horizontal stresses , with central measuring site, after Myrvang (1988) Offset drill holes showing direction of thrust displacement

Trend of axial fractures in road-cut drillholes Bo rehole breakouts, trend of SHmax Major faults , including

Mme-Trondelag Fault Comple x Thrust vector, Berill Fault (Anda et al. 2002)

F ig.1 0. Outline map showing the diverse rock-stress orientation data from central Norwayand theTrondelagPl atform.The s mall rosediagram

(inset,

top left) isfrom Hicks et

al.(2000, p.243)

and depicts the trends of ma

ximum horizontal compress

ive stress as derived from earthquake foca l mecha-

nism

solutions in the area

of offshore Mid Norway (period 1980-1 999).

NG U-BULL44 2,2004 - PA GE 60 DAVIDROBERTS

s

ARNEMYRVANG

Fig. 11. Seismicit y ofcentral Norw ay,and the Trendelaq Platform and Halt enTerrace,inclu- ding the eastern margin of the overlappin g More andVorinq Basins in thefar west;based on Byrkjeland et al. (2000).The compila tio n takesinalleventsrecordedsince1880.ltshould be noted,however,that some of the lowest magnitude events on land could possibl y represent blasting from road ortunnel con- st ructions. The major faults(cont inuous lines) aremostlytakenfromBlystad etal.(1995,Plate 1).Forgeograph iclocati ons onshore,seeFig.1.

""'=--

Mag nitude

0 ⁴

0 ³

0 2

0

1

o o

o

0 \;;;1

=====;;;;;;;;;j!

12'

eviden ce of d ifferen ces in exh uma t io n histo ries; and t he inland, foo t wa ll blo ck, southeast of t he Beeverda len f aul t, exp erienced it s lat est major up lift, by up to 2 km , in Neog en e t im e bet w een 40 and 20 Ma ago

(R

edf ield et al. in p ress).

Based on numerical modelling of Cenozoic st ress patterns offshore Mid Norway, Pascal & Gabrielsen

(200 1)

pro posed t hat t he MTFC was a me chan ically weak, composite fra cture zone wi t h an in herently hig h potential for having influenced both regional and loc al st ress fie lds f rom Devo nian t im e t o th e present day. Despite t h is, and not withstan d ing it s mu lt i- pl e reacti vati on history, th e MTFC

,

on shore at least, is devoid of an y sig n ificant, recent or co nte mporary, seism ic act ivit y (Hicks et al. 2000), i.e., over the last t wo dec ad es. Look in g back ove

r t

he last cent u ry or mor e, ho w ever, t

here

is a defi- nite conce nt ration of event s above m agnitud e 2.0

in

a crude

linear zone

trending c. NE-SW alo ng and adja cent to th e More-Trond elag

Fau lt Complex(Byrkj eland

et al. 2000, Plate 2). There is also a weak NE -SW t rend in g alig n me nt of event s passing through Storen.A ve rsion of t his compila tion of seis- m olo gical data , based on Byrkj eland et al.

(2000,

Plat e 2 and chapte r on data so urces, pp. 6224-5), is shown here as Fig

.

11.

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

,

have clearly been more act ive in t erms of catalogued seism ic eve nt s (Byrkjel and et al. 2000, Hicks et al. 2000), and t his is refl ected in d iverse surface ph enom ena . In o ne area c. 30 km northeast of R oan

(Fig.

1), for example, a cu rio us o rt hogona l mosaic of pebble-like agg regates of sand in mu lt ilayered Qu atern ary sedi m ent s

(Bargel

et al. 1994) has bee n inter- pret ed as liquefaction features arising from earthq uake

activity during lat e-Weichselian crusta l up

lift

. S

ome

200 km far ther south

west,

t he area of coastal

More is

one of the most seismicall y act

ive

regions of main

land Norway(Dehls

et al. 2000

,

Hick s et al. 2000) (Fig .

11).Th is areaalsoreco rd s

th e highest concentration of

larg e-scale rock

slide s an d allied roc

k-slope

fail ures in Norwa y

(Blikra et al.

2002, in press), feat ure s t hat have bee n d

irectly

related to

increased

eart hquake act

iv ity in historic

t

im es.

The post-g

lacial

Berill Fault

(Fig. 10) (And a et

al. 2002) also occurs

in this

same reg io n. Summing up, even t ho ug h M

id

Nor

wa y

as a

wholeis,

in rela t ive t erm s

,

a seismi cally

'q

u iet' reg

ion

t o d ay, it is note- wort hy t hat recor de d act ivi ty above magnitude 1.0 dimin - ishes rap idly so ut heast of t he Bzeverd alen fault and its ext ension into Tro nd heim sfj o 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 t he m o st seismical ly stab le part s of th e presen t- day Balt ic S hield.

The present-d ay st ress o rientation data onshore

in

Trondelag and Nord -M ore

,

and

in

par

t icular

th

e

prevalent

NW-SE t o WNW -ESE t rend of S

Hmax

al o ng

and to the

no

rth- w

est of t he MTFC,is also reflec te d in borehole brea

ko ut

and

eart hq uake fo cal mechanism data

(Fig.10)

from the

off shor e

areas as far

west

as th e m argin to t he Voring Basin (Bu

ng um

et al. 1991, Greg ersen 199 2, Gi:ilke

&

Brud y 1996, Byr kje land

et al. 2000, Hicks et al. 2000).Ther

e

are also substantial seis-

m ic-reflec tion and

w

ell data from the Trondelag Platform,

Halt en Terrace and

shelf mar

gin pointing to t he presence of

wi de sp read compressiona l st ructu res from E o cene t o the

present day

(Da re&

Lun din 1996, Langa ker 1998, Vaqnes et

DAVIDROBERTS&ARNEMYRVANG

al. 1998). These st ruct ures, including reverse fault s inverting earlier normal faults, and co nti nuous ly growing elonga te do mal struct ures,in dicate the existence of a NW-SE-oriented co mp ressional stress fie ld along th is Mid-Nor w egian margin (Fig. 10), w it h an extrapolated maximum bu lk shorte ning of c.3% (Vagnes et al. 1998).

Numerical modell ing of st ress patterns in the Mid Norw ay region, both offshore and onshore,from th e Tertiary to t he present day has also defined a fundamental NW-SE- t rend ing, maximum horizonta l st ress (Golke & Cob lentz 1996, Golke et al. 1996, Pascal & Gabrie lsen 2001).One of th e models also predicts that co ntras ti ng, contem porary st ress patterns should, indeed, exist in the footwall and hanging- w all blocks of th e MTFC(Pascal & Gabrie lsen 2001). However, the precise trend s and m agnitud es, and degree of ani sotropy of th e horizonta l stre sses in th e inland block are difficult to model, since the stress field is very sensitive to local effects such as,for example, to pogra ph ic forces

,

which aug me nt exte nsio nal stresses and cause stress axis rot a- tio ns. An exp lanat ion for the NE -SW orie nta tion of SHm

ax

in t he S t eren di st rict,for examp le,may be soug ht in a combina- tion of st ress axis deflections, influence of topography, and distance from the mechanically weak MTFC, but far more stress data are requ ired from this inland region before we can sa y anyt hing defi nitive.

In ge neral terms, t he prevailin g NW-SEcom pression doc- ume nt ed in both the offshore and t he oute r onsho re d om ains is considere d by a large proportion of the geo- science community as relati ng t o a distributed ridge push force arising from mid -At lantic sea-floor spreadi ng (e.g., S t ein et al. 1989, Mul ler et al. 1992, Fejerskov & Lind holm 2000, Lindhol m et al. 2000).This coincides very we ll wi th th e pattern seen on the World S t ress Map, where stress data from bot h shallow depth s and mi dd le to low er crusta lleve ls from all over th e worl d have been com piled (Zobeck et al.

1989, Re inecker et al. 2004). The data that we have acquired on- land, in Trondelag, i.e., contemporary horizontal stress orientations and eviden ce for WNW-ESE-trending, thrust- fault displacements of roadsid e dr ill holes

,

support this gen- eral compressional model.

Wit h regard to t he genera l stress situa tio n, t he mechani- cally weak natu re of th e MTFC, and th e paradox of compara- tively insignif icant contemporary eart hquake activ ity alo ng this megastr ucture, it is not improbable that an expected accumulation of stresses may result in an eart hquake of moderate magnitude in this regi on at some stage in t he future. On the ot her hand, slip partitioning along the innu- merable fault s wi t hi n the MTFC system may be such that seism ic energy wo uld possibly be widely dissipa ted , th us pr eclud in g th e gene rat ion of sudden, major earthq uakes w it hi n this zone.

Conclusions

Based on in situ rock-stress measurements and contempo- rary stress orie nta tion st ructures at dive rse localities in Trondelag, it can be shown that th e More -Tronde lag F ault

NGU-BULL442 , 200 4 - PAGE 61

Compl ex marks an important st ruct ural lin e separat ing crustal blocks wit h disparate, curren t stress fields.This sup- po rts earlier pro posals reached by both f ield and nu m erical m od ellin g st udies, th ou gh in th ese cases pert ain in g mostl y to earlier periods of geo logical tim e. In one case, howev er, ou r data co nfirm predictions (Pascal & Gabrielsen 2001) that contrasting con temporary stress regimes should, theoreti- cally, charact erise the foo twall and hangingw all bloc ks of the MTFC.

Given this curre nt situatio n, it is t herefore not inconceiv- able that an earthq uake of moderate mag nitude may occur along t he MTFC, on land, at some fu ture date - effective ly marki ng a release of the st resses th at have been accum ulat- ing during the last few decade s of relat ive inactivity.

The pr evalent NW-SE hor izonta l compression recorded in the near-coasta l areas of Trond elag accord s wi t h data acquired offshore, including earthquake focal mech anism solut ions derived fro m deeper crust al levels, indicating th at th is pat te rn is likely to relate to a distr ib uted ridge -p ush force arising fro m divergent spreading along the axial ridg e of the North Atlan tic Ocean.

T aken as a w hole, the combination of in situ rock stress measurements and f ield obse rvat ion s of drill hol e reverse- slip offsets and compl ementary axial fractures is seen to pro- vide a reliable indicator of the contemporary stress patt ern s existing in exposed bedrock.

Ackno wledgem ents

We aregrateful tothe reviewers,ChristophePascal and OdleivOlesen, fortheir constr uctiveand useful comments,and forsuggestio ns which ledto improveme ntsinthe generaldiscussion.lreneLundquistassisted with prepara tio nofso me ofthe figures,andOdleivOlesen helped in produci ng theseismiceventsmap,Fig ure 11.

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