TECTONOPHYSICS

ELSEVIER

Waves of the future: Superior inferences from collocated seismic and electromagnetic experiments

Alan G. Jones'

(;cn/og/( ul SUlTn or CIII'",/a, 1 O/Jlerl'UlOn Crc.\ccm. Of/all·a. Om. K 1,\ ()Y.i. C(lI/(/,/"

Rccciv,·.! 11 Fchruary I <J'!7: acccptcd 10 Juh 1997

Abstract

The advent of high-quality seismological studies of the Earth's continental lithosphere ha, heen paralleled hy an explosion in both the quality and quantity ot concomitant high-resolution electromagnetic studie,. The latter were inspired by technological and intellectual advances during the last decade in the acquisition. processing. ll111delling and inversion of particularly natural-source magnetotelluric (\1TJ data. The complementary nature of seismic., and MT leads to rejection of hypotheses that may be tenable if only one of them is applied. Equally, inferences supported hy hoth have stronger conviction. Perhaps most useful is when apparent incompatibilities must he reconciled by re-exam:nation of both data,ets. This is demonstrated through examples of magnelotelluric and seismic reflection studies undertaken in the last decade in many tectonic environs. from Palaeoproter%ic collision zones to passive margins to active collision IOnes. Some aspects of MT are explained. particularly the method's sensitivity and resolution of geoelectric directionality and dimensionality. New directions are proposed whereby gre.lICr uti lit) of the joint datasets can occur, both at the outset during data acquisition, and in the interpretation phase in modelling and inversion. Also. laboratory measuremeilh of seismic. electrical and rheological properties nf the same rod 'ample will make integrated interpretation more tellahle. ~) 1l)9H Elsevier Science B.Y. All rights rescned.

Kenmrds: seismic surveys: clectromagnctil' ,urvcys: phYSical properties: continental crust

1. Introduction

Imaging structures within, and determining the

state and properties of. the continental lithosphere

continue to be important milestones in our attempt to understand the processes that created ,1I1d modified

the lithosphere. Many of the world's geoscientific ef­

forts in this endeavour are spearheaded bv the innova­

tive use in crystalline terranes of ,seismic rdlection technologies adapted from petroleum exploration.

'Corresponding author. hl\: 1-1 61.' 992 xX36 L-Illaii: Jonc, (alcf·nrcan.gc.cd

Coupled with seismic acqui,ition, some national

and international programs also include non-seismic

methods, for example the Canadian Lithoprobe pro­

gramme (C[owes et aL ISlYl.\, the U.S. National Sci­

ence Foundation\ Continental Dynamics programme (CD/2020, Phinney et al.. l'.)qJ), the European Euro­

prohe programme, and the German Continental Deep

Drilling programme (Kontll1entales Tiethohrpro­gramm, KTB) (Emmerman -1lld Lauterjung. 19'.)7).

Unequivocally, these ancillary ll1ultidisciplinary stud­ies add value to the imerprct;Jt ion of the seismic data

by providing supporting eVidence or, on occaston, suggesting alternative, viahle i IIterpretations.

()04()-1951/98/~19.0() 1<J9S Ehcvier SL'icl1ce B \ All righh rl'scncd. PII S004()-1951 (Y7 )()()27(1-Y

274 ,\.U J'''"',I/n'C/OIlOp/,'\,I/C".I 2Ho (I'NfI) 273-298

Table I

Collocated seismic and MT experiments comluctc(' dUring the lasl decade

Conlinent/Region Localion G'ologieal problem Project -------------------------------

Africa Ken) a

Antarctic Byrd BaSin

Atlantic Ocean

Europe

Europe

Europe

Europe

Europe

Europe

Europe

Europe

Europe

Europe

New Zealand

North America

North America

North America

North America

North America

North America

North America

)\;orth America

North America

North AllanliL'

Finland

SwedL'n and Norway

Finland, NOr\lay and

Sweden

CiermClI1\

Germany

Hungary Hungary

Spain

France

Irdand

South [si and

California

Saskalchewan Manitoha

and

North Dakol a

Southern BI itish

Columhia

California

Alherta

Ontano

QueheL' and Ontario

\Vc .... t ('oa~t

Ncw Mexico

Alilc rift A.live rift 1')1

C, Inlinent-contincnt collision

Illlllia-Asial

Uplifting core complex (Nanga

Parhat)

SI,reading ridge (Rc.' kj'lIles

Rld~el

1'"l:!eoprotef07oic granulite helt

(Lapland Granulit.: Bdt 1

\'1.)unt,lin helt evolution

I ( akdonidcs I

AI Lhaean Shield (Fennoscandia I

Rlienish \la"i f

Sill' characlt:ri/ati~)J1

B, ,hemian Massi t

D"L'p structure of ,:xtcnsional

haslns (Pannonian Basin)

Blli" (';'hll 1ll1i all on ).t!en~

V,ll"i .... I,.:llk" S"ulirern Alps

Palacorrolerol:oic ofllgen

(Trans-Hudson 1

Canadian Cordillcra

ALtne strike-slip taull (San

Andrcas I

Ba-.clllenl ... lrll! .. .'tllrl~~

l'ldiltcd middle LTuswl hlock

(Kdpuskasing Lplifl)

()c,:anic suhductioll (Juan lk

F u,:" plalC 1

COJll1l1cnlal volcano

KRISP

Indepth

RAM ASSES

EGT

HiT !lABEL

DEKORP

KTB

PA:-.JCARDI

(I'GT)

ESCI

(,eoFranedD

VARNET S[(,HT

sseD

Lithoprohe

Cocorp

Lilhoprohe

Lithoprohe

Lithoprohc

Lithol'rohc

E'VISLAB l'SC,S

DeS JTEX

SIITlI'" In ,'I al. (19961

Clark,' Cl al. ( 199X I:

Wanlldrnaker el al. (19961

Nclsllll el al. (19961: Chen d al.

( 19'1/)1

Mell/,"· Cl 011. 119961: Park and

Macki·, 1 19961

Slnlr.1 ,'I ,11 (ll)97 I: Conswhk L'I

al. ( Illl}7 r

K()I} I d "I 11"961

Hurrc!1 ct al. 119X91: Gharihl

alld ""rja ( 19961: Charihi cl al. ( ILJC,,' 1

Kml" ,'I al. ( IOLJ31

Frallf.c cl al. (ILJ901: Volhcrs Cl al. ( Il)l)1I1

Haak L'l al. 119LJ7 I: Harjes cl ,11,

( 19lJ'l

Aric cl "I. (199XI

PO'~,,, L'I al. (llJ96): Adall1 e(

al. I 1')C)6 I: Ad<Iln ( 1997)

Pous Cl al. ( 19961

C;rmq),: .'\RMOR (199X)

[nl,!hdlll (1996): Jiracck et al.

( 199"h I: Holhrook et al. ( 1996,:

D.1\1'\ ,'I al. ( I cJ9X): Stern el :11, ,19%;

FliL'd'''T et al. (1996,: Park ami

Madil, (1996), Wcrnickc ct al. ( 199(11

LlIC,I. d al. I I <19" I: .Ioncs ,'I "I. 11')1)\1 .

N,'ls»1I Cl al. (1993): Hookcr cl

al. 1 I )'171

Cooh . 9(1) I: Jonc, and (Joll1'h

( 19 l ).'

Mackl" cl al. (19961: l;ns\\onh

ct al. 1 19971

Ro" cl a\. (19951: Bocrncr cl al (I ')')) I

I.L"'I,,,' et al. (199'+1: Mar",c11al

cl ,11, . 199.+ I: Pcrcil,d and WC,I

(i'IC)-I1

fvLt rc",L'h a I d "I. ( (99) I:

)CnCL h,d l'l al. ( 1996 I: Ji l'I ,d

11')9('1

Wanll.lIllaf.Cr Cl al. 1 i9X9a,hl:

Trehl " ai, 1199.+ 1 LullL'1 ,'I,d, ( 199:;): .IiraCt:k el

al. ( I ')q()" I

A,C;, ./"'/('1/ Tl'CIOIIII/)/n',I/( S 2Xr, (/VVXj 273 -2VX 27.'

Tahle I (continucd)

Contincnt/Region ],()calion (;eologica] prohkm Proie-et -----

North America Southweslern LT,S, r-klamorphic core complex PACE Kkin 1991): McCarlhy el al.

( 1991l

North America

North America

North America

Nevada

Mexico

Mexico

Ruhy Mounlains

Chicxuiuh impacl craler

()c,'anic plale suhduclion ( 'Ul'O!-,)

l\SF CD

CJEOLlMEX

Slocr Id ,md Smilhsol1 ( l'J9(, 1

Ar/dll' cl al (19961

Ar/dlc el al. ( 19951

South Pacilic (Jarrett :-.epn1L'nL I\kll n)igratiol1 and origin

hl'n,'alh mid-oce'nl ridge (E,,,I

I',Lcilil' R.isl'l

MELT ['or" III anJ Cha\'c ( 19941

;t References given to the 1lli.lJur review or O\'Cf"ViL'\' paper Whl'I\? wriltL'n

This paper will descrihe the symhiotlc relation­ship that can exist between crustal-scale seismic experiments and collocated eleclroma~tletic (EM) studies, Whereas ten years ago there were few joint surveys (Jones, 1987), since then collocated seismic and magnetotelluric (MT) experimenb have heen conducted on almost all continents (Tahle I). The considerable power of the Earth's natural time-vary­ing electromagnetic field, and its almosl uhiquitous extent, make MT the ohvious choice lor inexpen­sive, and logistically simple, deep EM il1\ estigations, A search with the joint keywords sci.\/llic rcflcctio/l

and l/lagncto/eI/lIric reveab over 300 papers and ah­stracts published since 1')86, and the lar\!er projecls are listed in Table I. It is not the pUll lose of this paper to review that vast hOlly of literatUl\.'. hut rather to pick examples illustrating where superior infer­ences have resulted from combined interpretations. It should be noted that such joint interpl etations are only of value when the sei smic and electrical prop­erties are correlated in ~ome fashion. St'i~mil' inter­pretations from images of a high seismi, impedance contrast hetween rocks of equal electrical conduc­tivity arc not aided or further constra; !led hy the inclusion of EM data, and an example 01 ~uch a case is discussed helow.

The sensitivity of electrical conductl\ itl' 10 the presence of small amounts of intercOflllected fluid makes EM methods particularly suitabk lor studies in geologically 'young' areas, such w, Tibet and North America's Cordillera. Such studi('~ may aho address the rheological properties or till' crust (sec, e.g" Jiracek, 1985: (Jough, 1')86). Howevcr. in older areas, EM methods imagL' metasedimenh l'ontaining interconnected conducting phases, such a~ graphite,

sulphides and iron oxides (c ,g" Duba et aI., 19')4). These metascdiments can play an important role in defining the present geometry llf structures and can contrihute toward~ unravelling the tectonic history of the region.

Deducing the prohahle call~e of the ohscrwd en­hanced conductivity u-.ually cannot be undertaken without reference to other information and accord­ingly hypotheses from EM images, just like those from seismic images, arc strL'ng-thened when other information supports them, and weakened when they do not. Some studies have determined the most likely cause of enhanced currcIll !low, such as that of .lones et al. ( 1997) which delilles the i'\orth Ameri­can Central Plain~ (NACP) c(llldul,tivity anomal) a~ due to pyrite grains concentrated in the hinge~ of folds and connected along strike. However. interpre­tations from resiqivity m()dd~, alone arc certainly not unique.

Thi, paper is structured intll three sections. First some aspects of MT, perhaps 1101 widely known hy non-experts, are descrihed. ThL'sl' aspects pertain to the method's ability to sense geoelectric dimension­ality and directionality as potential proxies for struc­tural dlll1ensionality and direL'1ionality, hoth of which are important given the usual a~sumption made when interpreting seismic or EM data from a prolile in a two-dimensional (2D) manllL:r. Second. examples will he chosen from those ill Tahle I to illustrate how -.eismics and EM together ean give superior itlference~ on aspects of the crusl and mantle, These examples are drawn from material that [ am most familiar with, hut are representative of what can he accomplished. Finally, a sholl ~ection concludes on the proposed future of joint s1udies using- new tech-

276 il.(;. /ol1es/Tec/ollo/'/n lies 21\(} (/<)'JI\; 273-2<)1\

nologies becoming available, both in~tJ umental and numerical.

The reader should note that the seismological studies are taken not just from vertical incidence reflection profiling, but from the whole spectrum of both active and passive source techniques, such as wide-angle reliection/refraction, convent ional re frac­tion, microearthquakes, and analyses lIsing distant earthquake sources.

2. Magnetotelluric method

General information on the magnetotelluric method can be found in. for example. Vozotl ( 1986. 1991) and Jones (1992. 1993a). Those papers tend to dwell on the resolving power of the method. and on structural features of the models that can be de­rived, and the reader is directed to thclll for these points as they will not be repeated here. Aspects that have been somewhat less explained are the ability of MT data to sense the dimensionality of the subsur­face. and to define directionality indicators. Obtain­ing these estimates at various frequencies indicates their variation laterally and with depth. and so indi­cates whether the typical 2D assumptions made are valid for the whole volume of investigation. part of it. or are not appropriate at all.

This sensitivity to dilllcnsionality and directional­ity comes from the tensor nature of the observations obtained. Measurement is made at a location of the three time-varying components of the Earth's magnetic field (h,. h,. h,1 and the tWll horizontal components of the electric (telluric) field (e,. l',)

measured on the Earth's surface. In the frequency domain. the 2 x 2 complex MT impedance tensor. Z, relates the amplitude and phase of the horizontal components of the magnetic field (H I (f)· H \ ( f)) to those of the electric field (E , (/). EJ!) I. at a given frequency, f:

[E'] [z\\ ZII] [H'] E, - Z"I Zn H,

(I)

(dependence on frequency assumed for clarity) and this tensor can he analy~('d for directionality and dimensionality. From the complex impedance ele­ments. one can define a scaled magnitllde and the phase of the impedance element. For ("{ample. for

the Z.,en element. the X Y apparent resistivity and phase are given by:

p(/,,(fJ = IZ,,(/)1 2 /2IT/I!

cPII (j i = arctan[llIl( Z 1\ (/ i I'Re[ Z '"~ (f) I (Jl is the magnetic permeahi lit y. usually taken as the free space \'al ue) and for a llni form half-space the X Y and Y X apparent resisti\ ilics hecome freyuency independent and give the truc electrical resistivity (inverse of conductivity) oltlw half space. and their impedance phases are 45° at all freyuencies.

For a layered one-dimem,ional (I D) Earth. then Z" = Z,.,= O. and Z". = -/" (the minus sign ensures a consistent right-hand coordinate system). and the apparent resistivities and phases as a func­tion of frequency can be interpreted into variation of resistivity with depth. As Cl consequence of the Skin Depth phenomenon of EM fields. penetration into a medium is inversely proportional to the square root of the freyuency of the incident wave. So in­formation ahout the near surface is ohtained from the high-freyuency data. and about the deep structure from the low-frequency data. Phases> 45° indicate a transition to a less resistive /(lne with increasing depth. whereas phases <..:15" indicate Cl transition to a more resisti\'e zOlle.

In the early days of the MT method. after it was propmed independently hy Tikhonov (1950) and Cagniard (1953), only a scalar description cxisted wherehy the Earth responses were derived from ra­tios of the perpendicular compunents of the EM held. e.g .. Z" = E,/H,. This led to resuits that were not reliable and gave models that were not in agreement with those ohtained from global spherical harmonic analy~es of the geomagnetic field (Lahiri and Price. I Y3Y; Migaux et a!., I 96()). A tensor relationship betwecn the held components. as in Ey. I, was first proposed by Neves ( 1957) and Berdichevsky ( 1(60). and such a tensor descrihes fully the electrical con­ductivity variation of the suh,urface. both vertically and laterally.

Over a I D Earth. the MT tellsor is rotatinnally invariant. and correctly indicates that the resistivity at a given depth. i.e .. for a given frequency. is in­dependent of the orientation angle. Resistivity varies with depth alone.

Respectively. over a 2D Earth. when the .\ and r axes arc parallel and perpendicular to geoeJectric

AJi . .I('II{·\/l~CI()II()J>II\·.\I'·.\ 286 (1<)<)8) 273-- ]<)8 277

strike. Z" = Z \\ = 0, hut Z \\ 'I -- Z, ,. In an arhitrary coordinate aCLjuisition system. not aligned with electrical strike. the diagonal term, are not zero. Accordingly. with noise-free data (lver a 2D Earth one can simply rotate the measured impedance

tensor until it becomes purely anti-dia).!onal. This tensor is rotationally variant, and indicates that the lateral resistivity at a given depth is depel1dent upon the orientation angle. In the presence (lj noise. an­alytical and numerieal methods were proposed to estimate. in some optimum manner. the .lppropriate strike direction. e.g .. hy numerically maxilllizing the magnitude-sLjuared of onc of the anti-diagonal ele­ments (Bostiek and Smith. 1962; Everett and Hynd­man. 1967) or analytically maximizing the sum of the magnitude-sLjuared of the two anti-diagonal el­ements (Swift. 1<)671. Geoe/ectric strike directions determined using these methods were not \ ery satis­fying. as they often varied wildly from hl'LjUency to freLjuency and fi'om site to site. and were Illlt consis­tent with surficial geological strike. Nott' that these older methods use the magnitude information in the impedance tensor elements. not the ph<lsl' relation between the electric and magnetic field c(IIl1ponents.

The full three-dimensional (3D) prohlcm ()ccurs when all elements of the impedance ten~()r contain information. MT modelling and inversion i, in its in­fancy for fully 3D data. and recently signiticant ad­vances have been made using rapid. second-order ap­proximations (e.g .. Smith and Booker. 1991: Habashy et aI., 1993). hut 3D intcrpretations are not routine. However. a pivotal advance was realized with the ap­preciation that much of the MT data re,,:orded. cs­pecially in crystalline terranes. exhihit the effects of distortions caused by galvanic charges on the bound­aries of local. near-surface inhomogeneities. These distortions affect primarily the electric Ikld and he­come frequency-independent, so cannot hc avoided by recording at lower and lower freLjuenL'ies. Thus, a physical model wa, developed of a di~torting 3D thin sheet overlying a 2D regional Earth 13D!2D).

and can he described mathematically as:

where Z"h, is the observed 1 x 2 complex impedance tensor. R is a 1 x 2 rotation matrix that mtates the true regional 2D response, Z~[). into the I,bservation reference frame, C is a 2 x: :2 matrix or real. fre-

Ljuency-independent factors that describe the distor­tions on the electric field, and T represents transpo,e. This distortion model. first presented in conceptual form by Bahr (1984). has received widespread ac­ceptance within the MT community after physical

and statistical procedures for its estimation were proposed by Groom and Bailey 11989. 199 I). and a meth()dology of its applicati(ln was described by Groom et al. ( 19(3). In colllrast to earlier methods. this model relies upon the phasl~ information within the impedance tensor. not thl: magnitude informa­tion. for deriving the strike dircction.

Csing the Groom--Bailey approach. onc can an­alyze a given magnetotelluric impedance tensor for its directionality and dimelNlH1ality properties. If the impedance tensor is full. 'iuch that all clements contain independent information. then none of the above Ihree modeb (ID, 2]), or 3DI2D) will lit the data to within the elTor tolerances. In this case the suhsurface at the frequellc\ hand of intercst is 3D. However. commonly the dala are well described by the 3D!2D model. at kast for part of the fre­Ljuency range of ohservatiolL and one ohtains the strike direction at that frequenl':-' (depth). and statis­tical confidence that the sub,urfacc can be descrihed by a 2)) model at that freLjuelh:Y. All extension of this approach that u,es multiple sites and multiple frequencies to determine the ll10st consistent ,trike direction for most of the data was presented reccntly hy McNeice and Jones ( 1(96).

An example of the utility 01 thc approach is given in Marquis et al. (1995). DircClionality analyses of

the MT data from south-centr~d British Columhia. Canada. showed that the gc()elcctric strike has a significant depth-dependency I. Fig. I). Thc upper­most crust (to 7 km). derived from 1\1T data in the freLjuency band 100-10 HI (Fig. I a). has an aver­age ,trike direction 01" N25"\V. lonsistent with slIrJi­cial ge(l]ogica] strike of the majllr morphogeologil'al belts v. ithin the Canadian C)Jdillera. In contra,t. most oj' the crust 0-34 kill I. exemplified by the MT data in the frequency hand 1-0.1 Hz (Fig. IhL display..; an average strike dir,'l'tion that i, very dif­ferent. generally 1"\20°1:::. and lhc uppermost mantle. determined from the frequenC\ ixmd (1.01--0.00 I HI (Fig. Ic:). shnws a different anglc again. N60"E. The former is consistent with the strike of '\JOrlh American rocb where exposL'd in the Purcell Ami-

278 A,(j IOlles I TeclOI1IJI,hl s;cs 2;\6 (/1)<);\) 273-2Y/i

121 120- 11) 118

(A): Upper Crust 50 30 50 :Jl'

"-

r\-J + l line 9

50 00 50 0:-

i '\ '---t t

49 30 km -\9

~~"-

1')() 50 0

(B): Mid- and Lower Y 50 30' Crust "

50 :!C

i f " .-;0/ ?"' t' /f t , t

Omenica Intermontane Belt

50 ~O' Belt

50 uo

t 1ft /;1 t 49 30- I 49 :-(' km

1(,0 50 0

---------(C): Upper mantle ..--. 50 30 --- 50 3('

'c:Lr--prJ~J 50 00 /" 50 0(,

/

//4 vi/A -49 30 km 49 3(-

1(0 50 0

1:'1 120 119' 118

A.(i . .folle.1 /li'c/ollol'hnics 2X6 (J')f.JX; 273-2f.JX 279

clinorium, whereas the latter is consistent with the Juan de Fuca plate subduction direction.

Recent analyses of the seismic data from the region for reflectivity as a function of azimuth deter­mined a large variability, with four persistent trends from the data in the range 0-12 s tW1)-way travel time (TWTT), at (I) 025-040°, (11) 08'i-·09'i°, !Ill) 115-125°, and (IV) 140-160° (320-34()") (Cook et aI., 1997). Trends III and IV are interpreted to be associated with the regional middle Tertiary exten­sional fabric stretching direction of about 1200

, and with the strike of regional contraction, respectively. Trend I was conjectured to be abo possibly asso­ciated with the extension, with development of the flattening fabric causing relkctivity perpendicular to lineation. Trend 11 is not related to any known struc­tural orientations in the region. The signilicance of trends IV and I being commensurate with the strike directions derived from the MT data for thc upper and lower crust respectively, needs to be evaluated further. However, of note is the fact thelt MT data. by their tensor nature, are implicitly sensitive to di­mensionality and directionality, whereas ,eismic data along a profile require both fortuitous crooked-lines and sophisticated processing to extract comparable information.

Thoughtful use of seismIC and EM togcther is ad­vocated, not adoption of a general rule 'H'hither goes seismiCl', there must KO MT (or vicc versa I. and cer­tainly joint interpretation must be approached with care. Clearly. only in the case of seismic impedance and electrical conductivity variation both occurring at an interface does joint interpretation make sense. For example, Occam's razor interpretation of anoma­lies seen in both can lead to pitfalls. as demonstrated by Cook and Jones (1995). Data acqUi rcd on the Earth's surface in southea~tern British Columbia dis­played high reflectivity and high conduct i\ ity in the upper crust, and the alluring temptation i; to assume that these resul t from the same ~tructurl'l s) or pro-

cess(es). However. a horeholc demonstrated that the high reflectivity resulted pnmarily from the inter­faces between metasedimenlary units and gabhroic sills, whereas the conductivity came from layered sulphides within the interlayered metasedimentary units. Detailed examination proved that the suspected spatial correlation of the two was unfounded.

An additional problem is [hat depth penetration by E\1 fields is not always assured if the crustal section contains significant quantitie~ of conducting material. An example is the Purcell Anticlinorium in southeastern B.C. and northern Montana, where abundant highly conducting sulphides (resistivity 01 I S m or less) precluded penetration below a few kilometres, even for periods as high as 1000 s (Gupta and Jones, 1(95). A second example is from south­ern Tibet. where the conduding and thick crust prevented sensitivity to mantle structure even for EM waves of 3Cl.OOn s period (Chen el al.. 1(96).

3. Examples

3. j. E.\U/np/cs 1: imoKillg .fluid\

Electrical conductivity is \ery sensitive to the presence of an interconnected !luid phase, either par­tial melt or aqueous fluid. A dry gabbro or granite at 5ClCl"C exhibits a resistivity in the laboratory of > 100.000 Q m. whereas in the presence of saline fluid its resistivity drops fiVl' orders of magnitude to < 10 Q m (Olhoeft, 1l)81. Shankland and An­der. 1(83). In comparison. the effect of pressure on

electrical properties is small. with partially molten rocks showing conductivities <It pressures of 25 kbar approaching 7()l!r of the zero pres~ure values (Tybur­czy and Waft, 1983). Exploratit>l1 for interconnt:cted fluid phases lIsing tht: appropriiltl' electrical or elec­tromagnetic method is therefor~' very efficient. as the effect on the response functIon is ~o significant (e.g .. Gough, 1992).

Fig. I. Strike directions for frcquencies .salllplilrg dilTcrent depth ranges in south-central British C"luI!lhia. la) Strikes from the

high-frequency band I I DO--I () Hi) which samples I hl' upper LTUst I "' ~ km). (h) Stri~es from a mcdiurri-l'rcyucnc\ hand ( I -D.I HI i that

samples the middle and lower Cllhl 1i;-32 kml. tl! Strikes from thl' low-frequcncy hand (O.OI-D.()OI HI! ,ampling the upper mantle

I -32 kml. The length of the arr,w. denotes iww "ell a ,0/2D nlUde! is lit by the data. with the long alTO\\" II!ling acceptahly. 'lIld the

short arrows titting poorly. The latter infers either 'hat tbere arc 3D regional structures pre.sent. or that the c'Iimated data ermrs arc too

small resulting in poor model lit. The locations or '.elsmi, lines l) '''1d III arc sho\"1 in la!. ;lIld the Illorphotecl,'~i,' helh in Ihl. (Adapted from Manluis et al.. 1l)9'i.)

2RO /I.G Jlllles /l~cl()II(}l'h\siCl 286 (19<)8) 273291)

Aqueous fluids have been proposed a, the reason for the globally observed enhanced conductivity of the continental lower crust (see, e.g., lones, 1992: Hyndman et aI., 1993), although there are strong objections from some in the petrological commu­nity (Yardley and Valley, 1997). This paper will not discuss that topic, but point out that the conduct­ing continental lower crust is a major geoscientitic conundrum that should be addressed DY a coor­dinated and cooperative effort between geologists, geochemists and geophysicists.

Joint seismic and EM studies for fluids have not been restricted to land. Two recent experiment'>, one in the North Atlantic (RAMESSES, Tabh: I) and one in the South Pacific (MELT, Tablc I) haw as their objective imaging melts associated with spreading ridges.

3.1.1. Eramp/e 1.1: Garihllldi helt A seismic reflection profile along the side of one

of the Garibaldi volcanoes (Mount Ca~ Icy) in the Coast helt of British Columbia, a northward ex­tension of the Cascades in Washington, imaged an unusually high-amplitude 'bright spot' hl'tween 4.15 sand 4.55 s two-way travel time (TWTT). equivalent to 12.5 to 13 km in depth., and extending 8 km lat­erally. This reflective package was initially attributed to magma or metamorphic fluids in the crust (Varsek et al., 1993), but careful study of the frequency and amplitude characteristics of the bright SP(,t suggested to Hammer and Clowes (19%) that it mor'e likely re­sults from a fossil sill complex associakd with the volcanic development of Mount Cayley.

Analyzing MT data recorded over the Garibaldi belt. Jones and Dumas (1993) were able to im­age three conductive features inferred te: be associ­ated with different types of l1uid~. The ~hallowest anomaly (T in Fig. 2a). or 20-100 Q 111. is inter­preted as due to a fluid-dominated montmorillonite clay cap layer above the rl~sistive sericite geother­mal zone. A pervasive conducting lower nus! ('2' in Fig. 2b), of ~ 100 Q m begins at about 14 km, and i\ interpreted to be due to metamorphic waters released by devolatization reactions as the Juan de Fuca plate to the west subducts beneath northwe~tern :'-Jorth America. These trapped liuiJs have been i Illaged be­neath Vancouver 1"land both electricall, I Kuru ct al.. 1986. 19(0) and ~eismically (Cassid: and Ellis.

1991, 1993). and electrically beneath Oregon as part of the EMS LAB experiment (Wannamaker et al.. 1989a,b). The most dominant feature imaged is the laterally bounded high-conductivity zone within this region (' 3' in Fig. 2b), of ~ 10-20 Q m, beginning at a depth of ~ 14 km. This is thought to be the source magma chamber of the volcanic belt, and its depth to top correlates well with estimates of 8-15 km from geothermal studies (Blackwell et aL 1990). Its spatial location correlates precisely with the Mount Cayley bright spot.

Hammer and Clowes (1996) admit that the seis­mic data do not exclude the possibility of the re­flective package resulting from melt lenses or saline l1uids. A sill complex would not generate a region of enhanced conductivity, and so an integrated inter­pretation should hlvOur a fluid explanation, with the magma chamher alternative being most plausihle.

3.1.2. Ewmplt' 1.2: Tibet Another example of MT ~tudies that have im­

aged fluids is the recent work undertaken in the southern part of the Tibetan Plateau as part of the INDEPTH program of study (Nelson et at.. 1996). Seismic studies in the Lhasa block identified 'bright spot>' in the reflection prolile:-- along the Yangbaijan Graben at TWTT of 5 s to 6 , (Brown et al.. 1(97) which exhibited P-S conversions in the wide-angle data (Makovsky et al.. 1997). I\1T studies in the re­gion perpendicular to the graben i maged regions of high conductivity t <3 Q m). not only in the graben ('2'. beneath 'YG' in Fig. 3) Dut also outside the graben ('I' and '3' in Fig. 3), within a generally con­ductive middle crustal layer 114-200 Q m) (Chen et ai., 1996). These independent observations were interpreted as evidence for a partially molten mid­dle crust heneath southern Ti bet of regional extent (Nelson et al .. 1996).

3.2. EWlI1ples 2: ill1agillgfollil.l

MT and seismics have been used successfully to­gether to image the geometrics of crustal structures. This is particularly true of fault~, given that sub­vertical faults cannot be directly imaged seismically. Theoretical model studies demonstrate the advan­tages of acquiring and interpreting varied seismic and electrical data. particularh in the case of saline-

A.G . .Id/Il'l I Tecllll1l1l'ilr.Ilc.1 2Xr, (19')81273 298 2XI

A: shallow vv w VVW VVVW v

0

E 1000 --.r::. ..... 2000 Q.. ID 0

3000

- -<20 20-200 200-1,000 1,000-10,000 >10,000

B: crustal Resistivity (Q.m)

vv v w vvw vvvw v v v v

0

E .:::tt:. 10

.r::. ..... Q.. 20

2 ID 2 0 3

30

WEST km EAST j 15 ~5 ::0

Fig. 2. Resistivity l1lodel ootained oy 2D rom"rci l11odcllin(2 "-·n data eolleclcd across the (iarioaldi \dkcmie hell. the nortlmard

continuation of the Cascades. III 't,uthweskrn Illitish Colul11hia The upper figure is of tht' shallow SCl'll 'I] I 3.3 kllll. whe!,,'as Ihe

lower figure is or the whole crust (to 33 kIll). Sitl locations sho\\n h) inverted triangles; I = shallow condll"lin;! region (]O-IOO ~2.111) associated with l11ontlllorillonltc clay /Onc ,:ap I'IH h:.2 =, per'asl\,' lo\\-resisti\'lt\ 1~100 Q 1111 lower crust illkrl'reted 10 he caused h\

l11etal11orphie saline fluids; 3 ,,= high-condul·ti\it\ c~ion ('IO-lI' ~2.1ll1 though I to he tlw source magma CiLullh,'r 1'01 the \'olcanic hell. IAdapted from jones "md Dum"s. 1993.)

fluid-filled fault zones (Eberhart-Phillips cl al., 1995), Faults can be imaged electromagnetically if they (a) contain saline fluid>;. (b) contain conducting metasediments, or (C) juxtapose roeks o!' dilTeri ng re­sistivity. Below is given an example of e,lch or these.

3.2./. EWlIlpfe 2./: S'on And/(,lIs/uull

The San Andreas fault is the hest known C,)J1-

tinental strike-slip fault, and has heen the suhject of intense geophysical studic~ lor over half" cen­tury. Suhse4uent to the 19)N rupture, the LOI11a

2S2 A.C, JIIlIl'.I !7i'('II!II<i/!/I\'.li!'.l 211(, (J!.J!.J8) 27J-2!.J8

3.7 14 52 200 720 2700 10000

SE VG T NW

0

- 10 E ~

20 -.c ... Q. 30 CD

Q 40

50

50 o 50 100

Distance from Damxung (km) Fig, 3, Resistivity model l'roll1 the MT profile cn""llg the Yadollg··Ciulu rift north of the Zangho suture. AIIt)lllalies I-.J imaged hv MT

Anomaly 2 correlates spatiall) \I ith a hright ,po 1 ill the Yanghaijan ('rahcn (} (;1. Anomalies J and 3 lie .11 [he same depth hut outside

the grahcn. Anomaly 4 is a shallow anomaly fronl the t,,\Vn of Thak (7'1 dippinp to the ,outh ending on the ,,,utkrn Hank of the grahcll.

(Adapted from Chen et al .. 1\196.1

Prieta segment was studied using seismic and MT experiments, and a spatially collocated low velocity and low resistivity « 10 Q m) zone, extending to depths >5 km, was imaged (Eberhart-Phillip~ et al., 1995), At Parkfield, a region of low rc'i~tivity and low velocity was also imaged (Eberharl-Phillips and Michael, 1993; Eberhart-Phillips et al.. 1995), with

the suggestion that the conductive fault DlIle extends through the whole crust (Park et al.. 199 I ). These re­gional MT studies, with a wide MT ~tati()J1 spacing, could not resolve a detailed fault zonc sll"ll'ture,

Recently, a continuous protiling MT ,'\.periment, with sites 100 m apart rather than many ki lometres as in the previous surveys. conducted acrms the fault at Parktield, demonstrated the resolving power of MT for near-vertical structures (L:nswurth et al.. 19(7).

The resistivity model ohtained (Fig . ..J.) shows the San Andreas fault zone to he an ·~500 m widc Iow-resis­tivity vertical zone down to 4 km. and i, attrihuted to saline fluids present in the highly fractured fault lone. Observed microearthquakes and crecp in the low-resistivity zone support the suggestion that seis­micity at Parklleld is fluid driven.

3.2.2. Example 2.2: Fraserj(1II11 The Fraser fault system (FF) in southwestern

British Columbia is a Late Eocene strike-slip fault dextrally ofj\etting units hy -··100 km (Coleman and Parrish. 1991 J. Together with I IS \outherly extension in the state of Washington. tbe Straight Creek 1~IUIt,

it extend, in a north-wuth direclton for at least 500 km. and has heen proposed as I he southern segment of the 2000-km-long intracratonic Tintina-northern Rocky Mountain trench transf, II'm fault syqem (Price and Carmichael, 1(86). The fault ollsets the carlier Yalakot11-Ho,lameen nlLllt ('1'1, and HF in Fig. 5). Between the YF and FF to thl' north lies the Stikine terranc. one of the largest of the l'xotic terranes that compri se North America's cordiliera.

Data from a Lithoprohe seismic reflection profile (line Ii-;. Fig. 5) were interpreted as suggesting three possihle geometries for the PriNT I'<lult (Varsek et ai., 1993): a Iistric west-dipping gcot11etry that soled into the mid-crust at ahout 6.3 s (triqel'1ory FFA, Fig. 6): a hroad zone of west-dipping rdlectors tbat are lo­cated ill tbe lower crust and p()ssthly upper mantle (trajectory FFB, Fig. 6); and d bigh-angle crustal-

A.C. ).111<'\ I Tecfllnopil.r.lics 286 (19<)8) 273--2'J8 28.,

SOUTHWEST (A) NORrHEAST C T

Okm 300

"' ~ ;:; 100 E , km

E .t: £. 30

z: 2km ~ 10 ;;; .;;; Cl> a:

3km

~ E ~ ~ ~ '" 0 '" M '" C T

(8) Okm

, km

2km

3km

Fig. 4. Resistivity model of the San Andr\.'as fault at Parktield (top) and its cartoon interpretation (bottom): C = l rcst of 'vliddle

:vlountain; T = creeping trace: KK = Salinian .• :ranit\.': K/l=

Franciscan formation: T'T = Coast Rang,· sedillwntary Llnih: T;;l' = Great Valley sedimentary units: j)f = fill id-saturated

damaged zone. (Adapted from Unsworth et "I.. 19'n.)

penetrating fault with west-side-down Mnho offset (trajectory FFC, Fig. 6). Preference wa~ expressed by Varsek et al. (1993) for geometry FFA. based on the apparent through-going reflective sequence at 6.3 s and from consideration of geological models for oblique deformation.

The MT data recorded over the Ft suggest a ditferent geometry (Jolles et aI., 1992). Phases at a period that sample the lower crust (I 0 ~) show a distinct difference on either side of the fault to the south, the phases are higher (~72°) to the west and are lower (~6(n to the east (Fig. 5). To the north, the lower phases appear to follow the t rend of the

YE rather than the FF. Thus. from the data alone, without sophisticated analyses or modelling, one can already draw significant conclusions about the likely geometry of the fault at depth. Inversion of the MT data from along the seismic reflection line gave a model with two orders of magnitude difference in resistivity in the lower crust on either side of the surface trace of the FF (Fig. 7), and imaged a zone of low resistivity in the fault zone within the mid-crust. The highly resistive Coast belt plutons west of the FF were also well imaged, with a depth extent of ~·15 km. From the 8D and 8u C values recorded in the vicinity of the FF (Nesbitt and Muehlenbachs, 1991), the mid-crustal zone was interpreted as a region of enhanced organic carbon or graphite deposited dur­ing upwelling in the fault zone of deeply penetrating meteoric waters. Current concentration in this zone was mapped by the induction arrow responses at 3.5 s (Fig. 8), equivalent to depths of approximately a few kilometres, and are consi,tent with the surface trace of the FF and YF. Such current concentration has also been observed and mapped in another major strike-~lip fault system, the crustal-penetrating Great Glen fault of Scotland (Kirkwood et aI., 1981).

Interpretation of seismic refraction data recorded across the fault indicates a structural discontinuity in the wide-angle reflecting horizons of the lower crust beneath the surface location of the Fraser fault (McLean, 1995), consistent with the MT interpre­tation, although the lower crustal velocity does not appear to change from one side to the other (Clowes et aI., 1995). Taking these MT results into consid­eration, the seismic reflection data were reprocessed using unconventional proces~ing techniques to ac­count for the severe crookedness of the line (see Fig. 5 I. One significant aspect of the reprocessed dataset was evidence of deep crustal extent, or pos­sibly crustal penetration, for the Fraser fault (pcrz, 1993).

The final integrated interprdation of the avail­able seismological and MT data is of a near-verti­cal, deep-penetrating fault geometry (Fig. 6) (Cook, 1995).

3.2.3. Ewmple 2.3: Slocan Lukefault The Slocan Lake fault (SLF) in southeastern

British Columbia is an Eoccne extensional normal fault that accommodated crustal thinning and ex-

. t(; . .folies I TI'CIOI1()I'/1Isic,l 286 (}l!I)Xj 273 -21)8

237" 238'

FF 51" 30' Stikinia

51' 00'

239'

I ntermontane Belt

240'

75

70

51' 30'

51' 00' , ••

50' 30' - -. 50° 30'

• 65

line 18 50' 00' 50' 00'

Coast Belt

) " ......

49' 30'

60 (

I

49° 30'

55

49' 00' 49' 00' 237' 238' 239

0

Fig. 5. Trace of the ll1id-l::oc~n,' i'ra,er faull 11-1, and ~arlicr Yalakolll (YFI anti Hozamecn (Hr) faulh. IOi":lher wilh the location 01 Ihe

MT sill'S I sLJuaresl and Ihe SCiSllllC reflectioll pI'< ,llie Iline I X J. The sljuare, arc' grey-shaded 10 indicate Ill\' efkclivc ph'bCs at a period

sampling the lowcr Gusl 110 si ]\ote thal 10 Ih,' Ill'sl of the 1+ and YF all I,h",es ar~ >70'. whereas (,) Ih,' ca,t they ar~ all _60"­(Adapted frol11 jone, et "I .. 19') ~ I

posed the Valhalla gneiss metamorphic core com­plex in its footwall. At this location. it defines the boundary between two or British Colurnbia's mor­phogeological helts. the Omineca belt (OB) to the west and the Foreland belt (FB) to the east. The SLF is east-dipping, and is thought to he possibly crustal­penetrating and to cut and offset the Moho (Fig. l)).

The reflection interpretation is that Nonh American

basemcnt underlies the rcgiull in its entircty. and that the fault only onsets the basement (Cook et al .. 1988. 1992).

Thi s region has been investigated for over three decades using EM methods hee lones. 1993h). as it was recognised by Hyndman (1963) in the early­I %Os that a significant houndary III conductivity must exist to explain the differences in the ohs;cned

+-' en co UJ

LL LL

o

o

I ,I , I

I,

i

" i I \

'.' ~.'

~., \,:, \ . ~\:'

'\ ,

(s) 9W!lI9ABJl ABM-OMl

<Xl ...

,-­,--

z

" c..

s ~ 1 :: '...J

:.;

2 ~ -:S '-,

~ ~ Ir, ~

J:J2 :x: .-

(.I ' ....

. ::: E..

~ - '0

; -::: '-,

.~ -~ ':.I

,.;,: '2 :.-

~~ ~ U"; rJ:'~

'" 0

286 A.G. '(i/In /TeC/ollol'i1ni<".l 2R6 (1')<')R) 273-·2YR

0

5

10 .-.. E 15

..::£ --..c 20 -Q.. Q) 25 0

30

35

40

FF

p [0 m]

>4,000

250 - 4,000 100 - 250

15 - 100 < 15

Fig. 7. Resistivity model ohtained from the MT ,ires crossing the Fraser fault (FF) along line 18 (Fig "J. Thl' three trajectories from

Fig. (, are also imposed. as well as the suggested MT rmjcctor) (white lines) Vertical exaggeration I I '/I..dapted from Jom:, l'l al.. 1992.)

ratios of the vertical to horizontal magnetic field am­plitudes between sites in southern Alberta compared to southern British Columbia, Canada. Subsequent studies by Caner et aL (1967) confirmed Hyndman's observation. However, thi~ 20 picture wa, shown to be too simplistic by a denser network of magnetome­ter sites around Kootenay Lake, southea,tern British Columbia (Lajoie and Caner, 1970). The responses were modelled in terms of a three-dimen,ional lower crustal transition zone, with a resistive 1250-1000 Q m) lower crust to the east of Kootenay Lakc. and a conductive (~5 Q m) lower crust to the wcst and south. The Lithoprobe MT data from along the seismic lines display a strong change ill impedance phase at periods sampling the lower cnIst (10-100 s), with high phases (60°-75°) to the west. and lower phases (30°-45°) to the east (Fig. 10). lI1dicating a ditference in the resistivity of the lower crust for the two regions (lones et aL, 1988). The resistivity model of the MT data has a change of hal r an order of magnitude in the resistivity of the lower crust at approximately the location of the Sl.F (Fig. 11) (lones et aL, 1993a).

This change in resistiVity is accompanied by a change in seismic velocity. Zelt and White (1995) model the Lithoprobe seismic refractioll data from

the region, and show a change on either side of the SLF with faster P-wave velocnies to the west beneath the OB (6.6-6.7 krnls) compared to the east beneath the FB (6.2-6.5 km/s) (Fig. 9). The surprisingly low velocities beneath the FB are difficult to explain, and a more fclsic lower crust wa~ suggested hy Zelt and White ( 1995). The corre~pondingly more mafic lower crust beneath the OB does not explain the observed resistivity decrease.

Clearly, if 'North American' basement continues to the west of the SLF, it is ll10dified by reworking, possibly as a consequencc of underplating. An alter­native explanation is that thl' basement west of the SLF i, not North American, hut was attached to the exotic terranes that docked with :--.Jorth America.

3.3. E.ramp/(' 3: imaging omgel1.1

The Palaeoproterozoic Trans-Hudson orogen (THO) in the U.S. Dakotas, and Saskatchewan and Manitoba in Canada welded together the Superior, Wyoming and Rae/Hearne ,.-ratons (Fig. 12). EM studie, played a large part in the discovery of its continental scale and, in facl, Camtield and Gough ( 1<,)77) were the tirst to propose that a suture zone lay heneath the thick sediments i 11 the mid-colltinent

A.C. j·)IWI / TeCfoflol'i1r.lic.I 286 ( 19<i1i) 273 ·298

N •

\ ~\

One Interval= 25 km

Period: 3.5 s

Real (reversed), unit magnitude

line 18 \

FF

Fig. X. Real induction arrow, (rner,ed) at 3.5 , period. Note the rever,,,1 in thl arrow, where the fault, df"l". IIldlcating the prescnce of

currents concenlraled in the fault lones. (Adapted !"rom .Iones cl al.. IYY2.1

of North America. This was based on magnetome­ter array studies through the 1960s and 1970s that identified an elongated and narrow LOne of electric current concentration running from the Black Hills of southern South Dakota to northern Saskatchewan.

named the North American Central Plains (~ACP) conductivity anomaly (see references in Camtield and Gough, 1977). This current must flow in an anomalous body of enhanced conductivity. Subse­quent profiles in northern Canada demonstrated that

kG . ./0111'.1 I TeCI{)/lophnics 2116 (1'N1l) 273 -291l

OMINECA BELT WEST

Valhalla Gneiss Complex SLF

o

Nelson Batholith

FORELAND BELT

Puree'11 ,Anticlinorium

100 km

Rocky Mtn Trench

EAST

Fig, 9, Geometry of structures in s()utheao,tcrn Brllish ('olumhia '1,,,ed on the seismic reflection results of ('tl()~ et al (19Xg I. Velocities

for the lower crust determined by Zelt and White 11995). SLF = Slocan Lake fault. VR = Valhalla reflect,,!. tAdapted from .iones.

1993h.)

VC NB KA PA 0.01

w ... ... .. ... ................... w .....

0.1

~ 1 "'0

0 ·c

10 Q)

0...

100

1,000 WEST EAST

0 100 km Fig. 10. Impedance phase pseud,>section from slles Cf'o"ing the structures in Fig. 9: VC = Valhalla gn,'", complex: NB = Nelson

batholith: KA = Kootenay Arc: I'A = Purccll Anliclinoriulll: .lIt = Slocan Lakl' fault. High phases (>4:,>° 1 ·,if!nify decreasing rcsistivity

with depth, and low phases the rl~versc. Note the change in phases sampling the lower crust !lO-IOO si 011 l'ithcr side of the Kootenay

Arc (KA). Also. the mantle must be electrically anisotropic to exphlin the observations. (Adapted from Jone'. l't al.. 1993a.1

this anomaly turned from north-south to east-west to enter Hudson Bay (Gupta et aI.. 1985) Correlation of the location of the current with exposed geology in northern Saskatchewan and Manitoba showed that the anomalous region lay in the we<.,tern part of the internides of the THO. in particular the maximum response was in the La Ronge domain. a volcano-

plutonic arc domain containing metasedimentary se­quences (Handa and Camtlcld. Il)R4).

Extcnsive MT studies have heen undertaken of the NACP since the rnid-Il)8()~ in Saskatchewan and the Dakotas as part of both Lithoprobe and Cocorp efforts to understand the THO. However. the lifst systcmatic study was undertaken by PanCanadian

A.c;. lolles I Teclol1oj,h\.\ics 2!16 (j'}<)!1) 273-29!1 2X'J

..c +-' 0... <D

o 10

20

vc slf NB KA PA

o 40 1,000 (}m N30W 110 (}m N60E

VE 1:1

EAST 50 WEST o 10 20 30 40 50 60 km

Fig, 11. Resistivity model for southeastern British Columbia: VC = Valhalla gneiss complex: NB = Ncl"'1l batholith: KA = K(lotcna)

Arc: p.4. = Pureell Anticlinorium, Note the change in lower crustal resistivity on either side of the Kootcflay Arc I KA) to explain tIll' phase change in Fig, 10. (Adapted from .Tones et ,d" 1993a.)

Oil Co,. who were interested in the possible control of sedimentary structures in the Williston Basin by basement features (profile S in Fig, 12) This profile imaged the NACP as an arcuate. mid-C1mtal feature (lones and Savage, 1986; Jones and Craven. 1(90). which was later showed to drape over 11l Archaean microcontinent of unknown affinity (;\elson et aL. 1993). The Lithoprobe seismic amJ MT studies in northern Saskatchewan (profiles L in hg, 12) gave essentially the same result (lanes et aL. 1993h; Lucas et aL, 19(3). and the model for the we.;tern part of profile L is shown in Fig, 13, An 0\ ('rlay of the seismic section on this nmdel is given ill .Iones et al. (1993b), and the main identified structural features in the seismic data are shown as solid lines in Fig, 13, Spatial correlation confirmed the La Ronge metascdiments as responsihle for the NACP. and recent lahoratory studies show that thi' anomalous conductivity is caused hy pyrite grains connected along strike that have migrated to the hinges of folds (lones et aL. 1(97).

The MT data image the conducting metasedimen­tary sequences of the La Range. and inl"er that they underlie the Rottenston~ domain (RD), presumed shelf and slope-rise sequences. and the Andean­like Wathaman batholith (WB), These ~edimenh lie structurally ahove a mapped late west-dipping com­pressional feature. the Guncoat thrust (GeT). and arc

interpreted to end at ahout the location of the sur­face trace of the Needles Falls shear zone (NFSZ). a late dextral fault along WhlCh tens of kilometres of displacement occurred, These sediments arc JUX­

taposed against highly resiqive rocks to the 'Aest. which probably represent the Archaean hinterland,

Tectonic models of the region invoke initial casl­ward(?)-directed subduction as the La Ronge arc and the Rae/Hearne cTalOn collided, followed hy a tectonic reversal and suhsequent west-northwest subduction of the oceanic Ilthosphere betwecn this arc-continent margin and th\~ approaching SuperiOJ continental plate. with an i ntlTvening Archaean hody of unknown derivation (LucCls et al.. 1993 J. How­ever, such a geometry for the initial phase wuuld not place the metasedimentilry arc sequences wcq­ward-dipping beneath the RI), hut rather ea"tward­dipping, Accordingly. the MT model is more consis­tent with continuous west'A ard-dipping suhduction on this western houndary r(lr the whole orog~nesis, ln addition, the model i mpl it'S that the GCT IS a reactivated subduction-related feature,

3.4, Ewmpl(' 4: illlaging filhmpheric mall tie strllctllres

Seismic reflection studies of the subcrustal con­tinental lithosphere have heen relatively rare, hut.

290 A. G. IUl1e,1 / TeC/()/loJ!hnics 2i'io ( I fJ9i'i) 273-29i'i

100

90

---0

-

Hudson Bay

Archean Provinces

Archean Platform

Deformed Continental Margins

Trans - Hudson Intemides

NACP from GDS & MT

90

Fig. 12. Map showing trace of the North American Central Plains (NAC?) conductivity anomaly and the 'r ran,·Hlldson orogen. MT

profiles S. M. N. and L. and the North Dakota (NO[)) survey. iden! iticli. (Adapted from ]ones L·t al.. 1993h.)

when successful, they image enigmatic features that are often considered relics of subduction zone pro­cesses. e.g., the F1annan reflector north of the British Isles (Snyder and Flack, 1990) and the Opatica re­flector in the Superior Province of Canada (Calvert et aL. 1995). Passive seismic method~ are suited to regional mapping of the mantle in a cost-effec­tive manner, albeit with far lower resolution. MT studies have an advantage over scismolugical ones since the same acquisition. analytical and numerical methods can be applied for both crust,d and man-

tIe studies (ct'. retlection proti I mg for the crust and teleseismics for the mantle); onc merely needs to use different sensors for the maglletic field to ohserve the lower frequencies (longer periods) and to worry about possible source-field contamination (Garcia et al.. 1997).

Pas~ive seismic studies ha\'c led to an interpreta­tion of observed shear wave anisotropy. from shear wave phases leaving the crust·mantle boundary (pre­dominantly SKS, more rarel) PKS). in terms of aligned olivine crystaIlographic axis oriented in the

A.G .I01/{'\ I TeC/(l110l,h\"lics 21i6 (J'i'i8) 273-29;'; 291

WB RD LRB NFSZ GeT

.,..,. 'Y 'Y 'Y 'Y'Y W W 'Y'f"9''Y ""'" 'Y'Y 'Y 'Y 'Y 'Y 'Y 'Y 'Y'Y

..-E

.:::t:. --..c 2 ..... 0-m 0

WEST I I EAST

<10 100 1000 > 10,000 Resistivity (~2.m)

Fig. 13. Resistivity model of thL' western part 01 Ihe Trans-Hudslln orogcll (\crtical <:xaggeratloll I: 1,. Wllitl' regions have resistivity

< I () S m. Black regions have resistivity > I Q,O(lO S Ill. WII = Wathaman hatholith: lIf) = Rottenstolle domain: LlIB = La Ronge hell: NFSZ = Needle Falls shear ",me: GCI' = GUIKoat thrust. Solid lines arc from the interpretation of Ihe "'islllic data hy Luca, et al

( 19931. (Adapted from jones et ill.. I 993h. )

direction of fossil stress (Silver and (~han. 19(1). This interpretation has prompted a prolific rise in the number of such studies on almost all of the Earth\ continents.

Until recently. there werc no coincident ohserva­tions of mantle electrical anisotropy to compare with seismic ones. However. impedance phase diflerences in orthogonal directions llhserved hy M,m:schal el a!. (1995) across the Gremi lie Fronl were interpreted in terms of foliation direction (Fig. l-n A suhse­quent high-resolution SKS study hy Scnechal el a!. (1996) found an ohliquily between the seismic and MT-determined anisOlropy directions (hI:'. 1-1), with the fast direction estimated at N 1O.3"E (±5") and the most conductive direction at j'\XooE f±6°). This was interpreted hy Ji et a!. (1996) a' a potential indicator of fossil shearing direction. with the seis­mic and electrical anisolropies heing cDntrolled hy lattice-preferred and shape-preferred onentations of mantle minerals (mainly olivine), respl·clively. The inferred dextral shearing was shown t,) he consis­tent with major faults on Ihe surface and also with crystallographic anal yses on material fr')111 a manlle xenolith.

Although the causes of hoth seismic ,lIld electrical anisotropy remain 10 he placed on a lirmer foun­dation, clearly there is here an area [liat demands

future effort if we are to understand the nature of the subcrustal continental lithosphere and the role Ihat il played in the formation of the ,:onrinents.

4. New directions

MT data acquisition is currently undergoing a quantum leap with the development of 24-bit sys­lems that can acquire many tens to hundreds of chan­nels of data (Phoenix's V5-2000, EM!,s MT-24) These systems should further the use of continuous electric field acquisition such as the San Andreas faul! study (Unsworth et al.. 1(97). For longer pe­riods, the construction of highly sensitive. ring-core Iluxgate magneto meters has kd to the developmenl of relatively cheap MT syslcm~, such as the Geolog­ical Survey of Canada's LiMS (Long period Magne­IOtelluric System). which is resulling in a resurgence in projects to study the mant le. There are oppor­!Unities to design systems that record hoth seismic signals and electromagnetic unes for both crustal studie~ (reflection and continuous-dipole MT) and mantle sludies (hroadhand sel ~ll1ic and long-period MT). Such systems olfer siglliflcant logistical and linancial advantages over independent systems.

New methods for processing the time series, such as Chave and Tholllson (1')l)X) and Larsen el al.

292

N

A • TELESEISMIC STATION

~ :!3:t<O.5s

o "null direction"

• MT STATION

ELECTRICAL ANISOTROPY

~ 0 ij,x 1=i0 M'/' 0° 10° 20° I I I

SEISMIC ANISOTROPY

012 1 1 1

:!3:t (s)

o 50 i=:=~km---

L-_______ --':-________ --:-__________ ._

790W 7SoW

Fig. 1.:1. Seismic and MT anisotropy information from the Superior Province of Canada. Seismic SKS t,lllC ditlerences shown by

ehevroned arrows. with the average around 1.5 s anJ direction of + 110° (±5°) MT impedance phase differences shown by plain arrows. with an average of around ~OO and direction Pt' +X5' (±6"1. (Adapkd from Ji et al .. 1996.1

(1996). result in highly precise MT response func­tion estimates. and some of the technique~ are trans­ferrable to seismology (e.g .. Jones and Holliger. 1(97). Removal of near-surface 3D distortions. such as described in the \1T section. are no\\> routine. Two-dimensional inversion of the regionai responses for structure has become tht' norm. after ,ignilicant advances over the last decade with the advent of various codes (Constable ct al.. 1987: )mith and Booker. 1991: Agarwal et al.. 1993: Old('nburg and Ellis. 1(93). A comparison of various 20 forward and inverse modelling approaches applied to the same dataset can be found in Jones (199:\c 1. Three­dimensional modelling is becoming more tractable.

with fast approximate schemes such as advocated by Habashy et al. (1993). and 3D inversion will not be far behind. There have been equivalent advances in modelling and inverting seismic data. There is an untested potential for undertaking joint or coopera­tive modelling and inversion si udies of seismic and EM data.

Finally. an area that needs addressing is laboratory studies for electrical. seismic and rheological prop­erties 01' the same rock sample,. In particular. the correlatIon of anomalous conductivity zones to re­gions of enhanced strain in the lithosphere addresses important rheological issues.

A.G . .Idill'.I/Te("{()170pinlin 286 I 19'J81 273- 298

5. Conclusions

Science proceeds by testing hypothc;-.es. Obvi­ously, the more tests that can be applied tu a partic­ular hypothesis, the more convincing the hypothesis becomes. The more powerful consequence or a hy­pothesis failing a test is, unfortunately, considered less significant than it should be. After aiL in sci­entific discovery, while we cannot fir()\ e anything, we can falsif\' a hypothesis (Popper, 195')). Unfortu­nately, but too few 'failures' arc puhlisheJ.

The examples ahove serve to demon~trate that hypotheses built on seismological evidence can often also be tested by EM methods, and vice \'ersa. The conclusions drawn Can be profound. as ill tl1e case of the imaged partial melt /.one beneath soul hem Tihet.

EM methods in general, and MT ill particular, have now become sufficiently sophisticated that they can be used as a geological imaging to(ll within the arsenal at the disposal of the inquiring ;,:coscientist. Unequivocally, there is great beneht frull1 acquiring collocated seismic and EM data, and thl' number llf joint studies is on the increase.

Acknowledgements

I would like to acknowledge, with gratitude. the organisers of the Asilomar workshop for their invi­tation to give an update on MT ten years after the last presentation (jones. 1987). The malority of the examples I have presented come from the Canadian Lithoprobe program, and I wish to thank all my colleagues for their collaboration, in particular Fred Cook and Ron Clowes. Martyn Unsworth provided the San Andrea, fault example. Reviews hy Don White, David Boerner, Geurge Jiracck, and an un­known referee, plu~ the comments of "oth editors, Waiter Mooney and Simon Klemperel. are grate­fully acknowledged. Geological Surve) of Canada contrihution No. 1996497.

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