Magnetic susceptibility of rocks in the Nutbush Creek ductile shear zone, North Carolina

Tecronophysics, 178 (1990) 207-223 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

Magnetic

susceptibility of rocks in the Nutbush shear zone, North Carolina

CHARLES V.H. MIMS I, CHRISTINE

207

Creek ductile

A. POWELL ’ and BROOKS B. ELLWOOD 2

’ Department of Geologv, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3315 (U.S.A.) 2 Department of Geology, University of Texas at Arlington, Arlington, TX 76019 (U.S.A.)

(Received April 13,1989; revised version accepted November 22,1989)

Abstract Mims, Ch. V.M., Powell, C.A. and Ellwood, B.B., 1990. Magnetic susecptibility anisotropy of rocks in the Nutbush Creek ductile shear zone, North Carolina. Tectonophysics, 178: 207-223. Applicability of anisotropy of magnetic susceptibility (AMS) as a strain indicator is investigated for the Falls lineated gneiss, a strongly lineated, weakly foliated tectonite lying within the Nutbush Creek fault zone (NCFZ). The NCFZ is a major Alleghanian ductile shear zone within the southern Appalachian Piedmont and is characterized along most of its length by a pronounced, linear trend on aeromagnetic maps. Sampling was conducted along a traverse near Raleigh, N.C., that extended through the lineated gneiss, across the eastern boundary of the NCFZ and into adjacent, supposedly unsheared Raleigh-belt gneiss. Petrofabric element orientations are similar within the Falls lineated gneiss and the Raleigh-belt gneiss but foliation is better developed than lineation in the Raleigh-belt gneiss. Mineral lineation parallels the strike of the NCFZ. Results of the AMS analysis are remarkably consistent along the entire traverse. Mean susceptibilty ellipsoids are uniformly prolate and display little variation in spatial orientation. In both the Falls lineated gneiss and the Raleigh-belt gneiss, mean maximum susceptibility axes are oriented along the trend of mineral lineation, and mean minimum susceptibility axes coincide with poles to foliation. Magnetic fabric in both lithologies is dominated by the alignment of magnetite grains in the direction of silicate-mineral lineation; magnetic lineation is better developed than magnetic foliation even in Raleigh-belt rocks. Similarity of the magnetic fabrics in the lineated gneiss and the Raleigh-belt gneiss suggests that both fabrics were produced by the same deformational process. The possibility that the Raleigh-belt rocks are actually part of the NCFZ is suggested by alignment of petrofabric elements along the strike of the fault zone. The presently mapped eastern boundary of the NCFZ which was defined on the basis of lithologic and mesoscopic rock fabric differences, may separate two distinct units within the fault zone. AMS results for Raleigh-belt rocks located north of the study area suggest that deformation associated with the fault zone may extend several kilometers east of its presently mapped boundary at certain locations. AMS results provide insight into the size of the strain field associated with the NCFZ but cannot be used to estimate either the strain history or the magnitude of strain experienced by rocks within the fault zone.

Introduction

Documentation of strain intensity and history is particularly difficult in highly deformed tectonites because high metamorphic grade, coupled with extensive recrystallization, generally obliterates conventional strain markers that may have oo40-1951/90/$03.50

0 1990 - Elsevier Science Publishers B.V.

been present in a protolith (Goldstein and Brown, 1988). Standard approaches to the analysis of strain in these tectonites employ petrographic methods such as quartz and feldspar fabric analysis (e.g., Lister and Price, 1978, Mawer, 1983). These methods produce reliable results, but are prohibitively time-consuming for a large number

208

C.V.H.

of samples. A faster petrofabric could produce results similar

technique that to petrographic

minerals

methods

strain and petro-

et al., 1987).

fabric

would obviously studies

analysis

of highly

benefit deformed

of anisotropy

tectonites.

of magnetic

(AMS) is such a promising AMS is a fundamental

The

formed

of any aniso-

involved

(1954)

magnetic

susceptibility

recognized

that

this variation

would be potentially

in

useful

to produce

of the

fabric

is a relatively

method,

and

mylonites.

as a strain

the mean anisotropy

and Daly, 1983; Borradaile

magnetic

tectonites

the AMS

tropic crystalline medium and is a measure of the directional variation of induced magnetization. Graham

Study

susceptibility

alternative. property

combine

of a rock (e.g., Henry

of highly

de-

new application

of

all studies

The possible

indicator

MIMS ET AL.

in strongly

to date

have

use of the method foliated

mylonites

is suggested

by the observed

coincidence

of mag-

netic

fabric

with mesoscopic

rock fabric.

In gen-

eral,

the direction

of maximum

susceptibility

is

as a petrofabric indicator for solving problems in structural geology. His observations of AMS for and sedimentary various igneous, metamorphic,

parallel to silicate-mineral lineation, and the direction of minimum magnetic susceptibility is perpendicular to the plane of foliation (Goldstein,

rocks led him to conclude that a direct relation exists between “final shaped distortions” in a rock

1980; Goldstein 1988). However,

and preferred

reliable kinematic indicators for shear zones (Borradaile and Tarling, 1984; Borradaile and Alford,

orientation

of magnetic

minerals

in

the rock. Subsequent to Graham’s investigations, the AMS method has become an increasingly important

tool in structural

geology

and

tectonics.

Applicability of magnetic fabric analysis to problems in structural geology is discussed by Borradaile (1988). Numerous AMS field studies have demonstrated that magnetic fabrics are generally coaxial with other petrofabrics (e.g., Balsley and Buddington, 1960; Rathore et al., 1983; Lamarche and Rochette, 1987). AMS also has been used to study the evolution of progressive deformation (Hrouda and Janak, 1976). Furthermore, AMS-strain correlations from field studies of folded rocks have shown that AMS may be applicable tive measure

of principal

finite

as a quantita-

strain

ratios,

pro-

vided that some conventional strain markers are present and can provide calibration for the AMS data (Rathore, 1979; Kligfield et al., 1982; Hirt et al., 1988; Ruf et al., 1988). Results from laboratory experiments on synthetic samples (Borradaile and Alford, 1987) and from mathematical analyses (e.g., Henry and Daly, 1983) tend to support the findings of these field investigations. However, there are theoretical limitations to the AMS method. These include: (1) an inability of magnetic anisotropy to record accurately the effects of extremely high strains, because AMS apparently has a limiting finite magnitude (Hrouda, 1982; Lowrie et al., 1986); and (2) an incomplete understanding of how anisotropies of different magnetic

and Brown, 1988; Ruf et al., magnetic fabrics are not always

1989) and, as pointed

out by Borradaile

(1988)

it

is not possible to predict which natural situations will yield susceptibility data that faithfully represent principal strain directions or magnitudes, Further studies testing the applicability of magnetic fabrics for strain analysis are necessary. The present investigation is the first to test the applicability of the AMS method as a strain indicator for a tectonite displaying strong lineation without strong foliation, an L-tectonite. The Ltectonite studied is the Falls lineated gneiss, which occurs as a distinct unit within the Nutbush Creek fault zone (NCFZ). The NCFZ is a major current fault of the southern Appalachian

transPied-

mont

for a

(Fig.

1). AMS

results

are presented

traverse that extends across the Falls lineated gneiss, exposed in the emergency spillway of the Falls Lake dam, and into Raleigh-belt gneisses, exposed along the Neuse River (Fig. 2). The emergency spillway provides fresh exposure for nearly continuous sampling of the lineated gneiss, which Wylie (1984) mapped as almost spanning the NCFZ in this area. The presence of a relatively homogeneous lithology within the fault zone reduces the problem of distinguishing between variations in magnetic fabric caused by strain and those caused by diverse compositional differences. Good exposure of Raleigh-belt gneisses allows sampling to extend beyond the eastern boundary of the NCFZ into rocks that supposedly have

MAGNETIC

SUSCEPTlBlLlTY

ANISOTROPY,

NUTBUSH

escaped the intense ductile deformation to the NCFZ.

CREEK

SHEAR

attributed

Regional geology

Nutbush Creek fault zone The NCFZ is a major ductile shear zone within the southern Appalachians that strikes northnortheast through the Piedmont of North Carolina and southern Virginia (Fig. 1). Rock types within the fault zone include phyllonites, mylonites, and lineated tectonites that exhibit subvertical folia:

ZONE.

N.C.

209

tions and/or subhorizontal lineations (Druhan, 1983; Wylie, 1984; Blake, 1986). Displacement along the NCFZ is estimated to be 160 km based upon offset lithologic contacts (Stoddard et al., 1988; Druhan et al., 1988). Sense of displacement is consistently right-lateral as determined by S-C mylonites, shear bands, rotation of porphyroclasts, and minor fold vergence (Bartley et al., 1984; Stoddard et al., 1988; Druhan et al., 1988). The age of the fault is constrained presently by radiometric dates for the syn-tectonic (or pretectonic) sheared Buggs Island pluton (313 _t 8

N

Fig. 1. Generalized geologic map of the Nutbush Creek fault zone and adjacent lithologic features. The star marks the location of sampling sites 3-l and 3-2 referred to in the text.

C.V.H. MIMS ET AL.

210

Ma; Kish and Fullagar, 1978) and the undeformed Wilton quartz monzonite (285 + 10 Ma; Fullagar

shearing overprints all previous structural fabric in the protoliths found within the fault zone. Meso-

and Butler, margin

1979)

Bobyarchick the Eastern ted as one

the NCFZ

to Virginia.

The NCFZ tectonic

resulted

ing in the vicinity

of the NCFZ

into

deformation.

is interprefeatures

oblique

convergence

during

Permo-Carboniferous

1981; Druhan,

The most recent

zoic deformation

and

fault system, which extends

of the major

America

the western

et al. (1977)

(1981) incorporated

accommodated (Bobyarchick,

Hatcher

Piedmont

from Alabama

North

which cuts across

of the fault.

Thus, shearing

the structural within

brittle

fault-

and no penetrative fabric produced

the NCFZ

has not been

overprinted.

that

of Africa and

Falls lineated gneiss

time

1983).

and most intense

by ductile

in limited,

penetrative

deformation to affect the western flank of the Raleigh belt is shearing along the NCFZ (Stoddard et al., 1988). Deformation associated with the

Fig. 2. Site and sample location

The present study is confined to one unit within the NCFZ, the Falls lineated gneiss. This unit occurs as a narrow strip along the western margin of the Raleigh Belt in North Carolina from the southern exposure of the NCFZ to Wilton (W in

map for the study area.

MAGNETIC

SUSCEPTIBILITY

ANISOTROPY.

NUTBUSH

CREEK

SHEAR

ZONE.

211

N.C.

pretations of whom

of Parker (1979) and Wylie (1984) both cited similarity and fabric gradations

between

the Falls lineated

foliated

felsic gneisses

Raleigh-belt

gneiss and the adjacent

and schists to the west.

gneisses

Raleigh-belt

gneisses

lie east of the Falls

eated gneiss in the sampling The

boundary

marks Fig. 3. Photograph developed

of the Falls

subhorizontal

lineated

lineations.

gneiss

North

showing

well

is to the left.

the eastern

rock is a fine-grained, equigranular quartz-microcline-plagioclase gneiss with subordinate biotite, muscovite and garnet. The rock matrix is characterized by the presence of recrystallized, strainfree grains of quartz and feldspar. Opaque minerals occur as minor phases, with modal percentages generally in the range 6-10% (Barifaijo, 1986). Mesoscopic structures in the Falls lineated gneiss are illustrated in Fig. 3. These include welldefined mineral lineations of quartz, micas which are subhorizontal and 20” E. A weaker,

discontinuous

feldspar, or trend NlO-

foliation

is de-

fined by planar alignment of phyllosilicates. This foliation generally strikes north-northeast and dips steeply

to the southeast.

is most according

The Falls lineated

gneiss

appropriately termed an L-tectonite to the classification scheme of Flinn

(1962). The protolith for the Falls lineated gneiss has not been clearly established. The possibility that the gneiss formed from more than one protolith is suggested by the presence of two basic species, a pink lineated gneiss and a grey lineated gneiss, with color variations attributed mainly to changes in the relative proportion of potassic feldspar to sodic feldspar (Carpenter, 1970; Parker, 1979). In the Falls Lake area, Blake (1986) implied a genetic relation between the Falls lineated gneiss and the Raleigh Belt gneisses east of the NCFZ, based upon similarities in microcline content and overall composition. This association contrasts with inter-

area (Figs.

these

boundary

units,

1 and 2).

which

also

of the NCFZ,

was

proposed by Wylie (1984) based upon a change in mesoscopic rock fabric from strongly lineated rock to more obviously foliated rock. Raleigh-belt gneisses

Fig. 1). The unit was sampled at its type locality, Falls Lake, North Carolina. At this location, the

between

lin-

consist

of layered

biotite

gneiss,

granitic

gneiss and minor amounts of hornblende gneiss. The gneisses display compositional layering and are strongly foliated, with foliation lying parallel to layering. Regional NCFZ

magnetic

signature

associated

with

the

The NCFZ is characterized by a positive, linear trend on aeromagnetic maps which persists despite juxtaposition of different rock suites on opposite sides of the fault zone (Casadevall, 1977; Bobyarchick, 1981). The aeromagnetic map covering the portion of the NCFZ that lies in North Carolina is shown in Fig. 4. Outside the fault zone, magnetic

contours

do not follow linear trends

except over dikes. Unsheared protolith rock suites can be distinguished on the basis of their magnetic signature in Fig. 4; in general, Carolina slate beltrocks and their intrusive equivalents are characterized by relatively high magnetic intensities and Raleigh-belt intensities.

rocks

Magnetic susceptibility

are characterized

by low

anisotropy

Magnitudes and orientations of principal magnetic susceptibility axes were determined using a low-field, alternating-current, torsion-balance magnetometer at the paleomagnetism laboratory, University of Texas at Arlington. A detailed explanation of the procedure followed is presented

C.V.H.

Fig. 4. Total

field magnetic

intensity

characterized

map

for the part

of North

by a linear trend, striking

elsewhere (Ellwood and Whitney, 1980). The procedure requires one susceptibility value be measured independently of the rest. In this study, the

Carolina

roughly

shown

in Fig. 1. The Nutbush

N20 o E. Contour

interval

Creek

MIMS

fault

ET AL.

zone is

is 100 mT.

susceptibility value parallel to the long axis of the cylindrical rock sample was determined using an alternating-field susceptibility bridge.

MAGNETIC

SUSCEF’TIBILIN

ANISOTROPY,

NUTBUSH

CREEK

SHEAR

Statistical procedure Mean susceptibility-axis orientations for a particular sampling location were computed using an eigenvalue method summarized by Mark (1973). This method is independent of the sense given to axial data and is, therefore, appropriate to use in the analysis of AMS data because each principal AMS axis defines two directions on a unit sphere. Consider a particular sampling location for which N rock samples have been collected for AMS analysis. Observations for this location will consist of the N independently determined susceptibility ellipsoids. To determine the mean orientation for one of the principal axes (e.g. the mean orientation of the maximum susceptibility axis), a matrix, A, is formed containing the sums of the products of the N orientations determined for this axis. Let a unit vector parallel to the i th orientation computed for this axis be designated as:

(1) then:

A=z&KT=x=

Xi’

xiY;

xjz;

x;Yi

Yi’

Y,Zi

x;zi

yjzi

1

(2)

ZONE,

N.C.

213

range from 1.0 for a cluster of identical axial orientations (no scatter) to 0.333 for a random scatter of orientations. Thus, 9 can range from 0 ’ to about 70”. The quantity + is used as an estimate of scatter because it is somewhat analogous to the a95 statistic (Fisher, 1953), routinely used and understood by paleomagnetists. However, #I is less sensitive to variations in N and can be used to contrast scatter between sites containing differing numbers of samples (see Schmidt et al. (1988) for a discussion). Sample collection

Oriented hand samples and rock cores for AMS analysis were collected along a traverse that cuts across the Falls lineated gneiss and extends into foliated Raleigh-belt gneisses. The lineated gneiss is exposed as fresh outcrop in the emergency spillway of the Falls Lake dam, and Raleigh-belt gneisses outcrop along the Neuse River, east of the lineated gneiss (Fig. 2). Four additional oriented hand samples for AMS analysis were collected near Wilton, North Carolina. Two of these samples were taken from Raleigh-belt gneisses located 3 km from the mapped eastern boundary of the NCFZ and two were taken from the unsheared Wilton quartz monzonite which partially crosscuts the shear zone (see Fig. 1).

z,f Collection of AMS samples

where XiT is the transpose of Xi. If yi 2 y2 2 y3 are the eigenvalues of A, the mean axis will correspond to the eigenvector associated with yi (Mark, 1973). The eigenvector means calculated in this manner may not be orthogonal. Mean susceptibility values calculated for many different samples from a particular site can not be expressed as second rank tensor due to large magnetic fabric variations that may exist between individual samples. The principal component method of Mark (1973) allows realistic variations between samples rather than constraining the data to conform to an orthogonal set of axes. Scatter about the mean axis can be characterized by the angle $B= arccos (S,) where S, = yJ(y, + y2 + y3) (Schmidt et al., 1988). S, can

A total of 110 cores from 18 locations along the traverse (Fig. 2) were collected for AMS analysis using a gasoline-powered portable drill. Sampling locations were surveyed in the field to maintain a separation of approximately 30 m. This interval was sufficient to allow adequate sampling of slight variations in composition or grain size that were apparent at the outcrop scale. Six to seven oriented cores were collected at each location, and each core was separated from the rest by 2 to 3 m. Sampling locations in the spillway lay roughly along a line parallel to the spillway walls. Along the Neuse River, locations were distributed over relatively unweathered outcrops that were accessible along the river banks.

214

C.V.H. MIMS

Collection of oriented hand samples and structural data

include orientations of principal susceptibility and, Graham’s (1966) V parameter. V is indicative

Oriented tions

hand

samples

3, 6, 12, and

within

the

lineated

Raleigh-belt chosen

were collected

9. Locations gneiss

at loca-

3, 6 and

and

9 lies

12 lie within

gneisses (Fig. 2). These samples

as representative

of variations

were

in composi-

tion and fabric of the rocks along the traverse, as sources

for oriented

of six measurements mesoscopic at all drilling the outcrops enabled

thin sections. and foliation

where

cancels

were recorded

to be taken.

Preparation of samples for petrographic

observa-

K,

principal

respectively. the

E. Following

(3)

are the maximum,

minimum

between

late ellipsoid

- K3) K,

and

and

magnitudes,

of both

by

(K, - K,)/(K, K,,

axes

of the shape of the susceptibility

and is defined

mediate,

An average

locations, provided that geometry of and visibility of structural fabric

these measurements

sin2V=

ference

of the orientation

lineation

and

ellipsoid

ET AL.

Note that taking susceptibility

standard

the dif-

magnitudes

terminology,

will be characterized

inter-

susceptibility

an ob-

by K, > K, B

K, and a prolate ellipsoid will be characterized by K, x==K, > K,. For V < 45 ‘, the shape of the ellipsoid is prolate. As I/ approaches 0 O, the magnitude of K, approaches that of K, and the shape of the ellipsoid approaches a prolate spheroid. For

Three oriented thin sections were prepared from each of the four hand samples collected. Thin sections for each sample were oriented to be mut-

V > 45 O, the shape is oblate and, as V approaches 90 O, the magnitude of K, approaches that of K, and the shape of the ellipsoid approaches an oblate spheroid. For V = 45 O, the magnitude of K2 is equal to the magnitude of the mean principal

ually perpendicular

susceptibility,

tions

and to approximate

planes

in

the rocks that are respectively: (1) parallel to foliation; (2) perpendicular to lineation and foliation; and (3) parallel to lineation and perpendicular to foliation. The direction of mineral lineation was apparent for the rock sample taken within the Raleigh-belt gneisses (location 9) when this sample was cut for thin section preparation. Thin sections were studied in transmitted light in order to observe the orientation, size range, and morphology

of the opaque

minerals.

defined

as:

K=(K,+K,+K,)/3

(4)

Over 93% of I/ values computed for individual samples are 45 o or less and all mean I/ values are less than 45 “. Thus, all mean susceptibility ellipsoids are prolate. Mean I/ is plotted in Fig. 5 as a function of sample location along the traverse. These values do not display a dependence upon distance into the fault zone, indicating that there is no obvious fabric trend associated with the lineated gneiss.

Results AMS and structural measurements for the sampling traverse Symbols used to express results of the AMS analysis conform to the nomenclature standards suggested by Ellwood et al. (1988). Uncertainty in the independently measured susceptibility term adds an unknown constant, e, to the computed principal susceptibility magnitudes. Consequently, the most reliable AMS parameters are those which remain invariant under a change in the independently measured term. These robust parameters

Fig. 5. Mean values off V plotted against sample location. values lie below 45” ellipsoid.

Samples

indicating

All

a prolate AMS susceptibility

20, 21, and 9 are located gneisses.

in Raleigh-belt

MAGNETIC

SUSCEPTIBILITY

ANISOTROPY,

NUTBUSH

CREEK

SHEAR

ZONE.

215

NC

Orientations of K, and K, axes for individual samples are shown in Figs. 6a and 7a, respectively. Mean values for each location, computed using the eigenvalue method discussed previously, are given in Table 1. No distinction can be made between the lineated gneiss and the Raleigh belt gneisses based upon orientations of either the K, or the K, axes. Susceptibility ellipsoids are prolate in both rock units, and are oriented in similar directions. Orientations of lineations and poles to foliation measured in the field at each sampling location are shown in Figs. 6b and 7b, respectively. Mean values for each location are given in Table 1. Poor exposure of Raleigh-belt rocks did not permit lineations to be measured, and poles to foliation could be measured only at location 9. Poles to poles to foliation

(a)

Fig. 7. a. Equal-area,

. _(

susceptibility

t

kl susceptibility axes

represents

lower-hemisphere

axes. Cylindrical

the partial girdle containing

Same type of plot as in (a), but containing

lineations

i

Fig. 6. a. Equal-area, lower-hemisphere K, susceptibility

plot containing

all 110

axes. The filled square is the best-fit vector

forthedata,withD=192°,Z=10.b.Sametypeofplotasin (a), but containg 63 measured lineations. fitvectorfordata,witb

D=20°,

Filled square is best Z=O“.

Z = 12O. b.

75 measured poles

Filled square is the best-fit vector for the data, with D=289O,

1

110 K,

K, axes. Filled square

is the best-fit vector to the data, with D = 281°, to foliation.

(W

plot containing

best-fit plane (N81 o W, 79 o N)

Z=34O.

foliation measured at this location are similar to those measured in the lineated gneiss. Orientation of the mean K, axis is very similar to the orientation of the mean lineation axis (Fig. 6); each lies along a NE-SW azimuth and plunges near-horizontal. Whereas poles to foliation are tightly clustered in the northwest quadrant (Fig. 7), the K, axes are distributed along a partial girdle, for which the best-fit great circle strikes N 81° W, and dips 79“ N. Although there is greater scatter in the K, axial data than in poles to foliation, mean axes for each data set have similar orientations. Statistical parameters S, and + = arccos (S,) are listed in Table 2 for each sampling location. The angle C#Jcharacterizes the angular departure from each axial mean direction. Small values of $J

216

C.V.H.

for most locations tightly

clustered

ceptibility location. mean

indicate

that the axial data are

and that orientations

ellipsoids

do not vary greatly

Maximum

angular

K, axis orientation

1, are spread Fig. 7a. With tions,

the K,

at each

departures

from

occur at locations

K, axial orientations

6, and 18; computed of these locations,

of the sus-

with the exception

the

1, 12, at each

of location

along the great circle path noted the exception axes are tightly

tion 1, orientations

of these three clustered.

of the K, and

K,

in

loca-

At locaaxes were

rocks,

and

gneiss

(locations

location

thin

9 contain

plagioclase. oped,

sections larger

Foliation,

is better

oped to nonexistent The abundance

not strongly

than

for

and more

lineation

develat loca-

lineation

and weaky-devel-

foliation. of opaques

relative

to other

at all locations;

opaques

which, in turn, greatly outnum-

interchanged for one sample, giving rise to large values of cp for both mean axis orientations. The statistical analysis indicates that axial mean direc-

bers

tions determined for the Raleigh-belt gneisses just as tightly clustered as those determined

similar at all sites. In the plane of foliation (Fig. 8a), which contains the maximum and intermediate principal susceptibilities, individual o-

the Falls lineated

are for

gneiss.

and

lineated

sections

3, 6 and 12 are characterized

is similar

biotite

chlorite

Falls

grains

although

tion 9 while locations

outnumber

the

biotite

developed

by strongly-developed

dark minerals

for

3, 6, and 12). Thin

ET AL.

MIMS

all other

dark

minerals.

Iron-

bearing minerals form 6 to 10% of the total mineral assemblage. The distribution of opaques is also

paque grains range in size from less than 25 pm to Petrograpy

greater than to anhedral,

Distinct differences sections for location

were noted between thin 9, located in Raleigh-belt

550 pm. Grain shapes are subhedral and the subhedral grains occur as

roughly square or rectangular gate grains and approximately

forms. Both elonequidimensional

TABLE 1 Mean susceptibility ellipsoid shape parameters and, comparison between mean values of corresponding magnetic fabric elements and mesoscopic fabric elements Location

Lineation

Foliation poles

V

K,

(“)

D

I

D

I

D

I

D

I

(“)

(“)

(“)

(“)

(“)

(“)

(“)

(“)

K3

1

42

19

2

23

3

288

22

290

36

3

35

12

7

199

1

279

23

290

36

10

33

11

I

19

5

216

31

284

37

11

35

193

8

197

4

102

2

290

32

4

40

351

8

20

5

266

9

287

32

12

29

2

1

15

2

86

14

284

34

6

38

188

4

205

8

281

10

290

33

13

36

202

6

17

1

298

46

28.5

7

14

31

197

5

24

5

287

3

289

35

15

31

193

5

19

0

284

13

289

27

8 16

31 36

194 203

8 5

203 *

3 *

101 290

3 4

291 *

38 *

17 18

32 36

196 181

6 6

_ _

_ _

287 95

7 5

_~

_

19

31

194

1

_

_

284

5

_

_

20

33

190

1

_

_

281

14

_

_

21

39

12

3

_

281

13

9

31

189

2

_

279

23

291

50

D = declination, I = inclination. * Data set too small for calculation of mean value. _ No data available.

_

MAGNETIC

TABLE

SUSCEPTIBILITY

ANISOTROPY,

NUTBUSH

CREEK

SHEAR

ZONE,

217

N.C.

2

Statistical

parameters

Location

S, and 9 = arccos (St) for each mean principal AXiS

Sl

+

susceptibility

axis

Location

AXiS

Sl

15

Kl

0.980

11.6

cp (“1

(“)

1

3

10

11

4

12

6

13

14

S, ranges

from

Kl

0.813

35.6

K2

0.942

19.6

K2

0.901

25.7

K3

0.825

34.5

K3

0.914

24.0

8

K,

0.983

10.4

K,

0.966

14.9

K2

0.973

13.2

K2

0.842

32.6

K3

0.976

12.5

K3

0.857

31.0

K,

0.972

13.7

K,

0.936

20.5

K2

0.949

18.5

K2

0.859

30.8

K3

0.947

18.7

K3

0.876

28.8 19.2

16

17

Kl

0.992

7.0

KI

0.945

K2

0.933

21.2

K2

0.856

31.1

K3

0.933

21.1

K3

0.892

26.9 44.2

18

Kl

0.962

15.8

KI

0.717

K2

0.895

26.5

K2

0.900

25.8

K3

0.911

24.3

K3

0.696

45.9

Kl

0.826

34.3

Kl

0.949

18.4

K2

0.736

42.6

K2

0.899

26.0

K3

0.752

41.2

K3

0.916

23.7

Kl

0.955

17.3

KI

0.975

12.8

K2

0.773

39.3

K2

0.919

23.3

K3

0.752

41.2

K3

0.922

22.8

Kl

0.977

12.2

Kl

0.976

12.5

K2

0.959

16.4

K2

0.944

19.3

K3

0.956

17.1

K3

0.935

20.7

K,

0.961

16.0

Kl

0.974

13.1

K2

0.912

24.2

K2

0.932

21.3

K3

0.956

17.1

K3

0.933

21.1

1.0 (no scatter)

to 0.333 (random

scatter).

19

20

21

9

Angle

grains are common. Most opaques occur in distinct linear trains parallel to the direction of lineation defined by matrix-forming minerals. Isolated single grains, however, are not uncommon over the area of a thin section. Linear aggregates can be observed best in the plane perpendicular to foliation and parallel to lineation. In the plane perpendicular to lineation and foliation (Fig. 8b), which contains the intermediate and minimum susceptibilites, individual opaque mineral grains range in size from less than 25 pm to greater than 200 pm. Grain shapes range

cp characterizes

the angular

departure

from

each

axial

mean

between round to roughly square. Most opaque minerals occur as singular grains or as small, round clusters that are fairly evenly distributed throughout the thin section; more rarely, they occur in short, linear distributions, parallel to the trace of foliation. Magnetic

mineralogy

Opaque grains were shown to be magnetite by reflected light microscopy and electron rnicroprobe microanalysis; reflected light microscopy

218

C.V.H. MIMS

Fig. 8. a. Photomicrograph rock units sampled. approximately subhedral

Opaque matrix

grains

transparent

b. Photomicrograph lincation

minerals

of opaques

quartz

(darkest

regions

sampled.

as is evident

light showing Strike

best seen along

Grains

occurrence

in upper

of opaques

left corner

in the plane of foliation

oriented

equidimensional

in the upper

N20° E, 62”SE. to elongate

half of the photo.

grains.

Matrix

for the

Scale bar is Isolated,

minerals

are

perpendicular

to

feldspar.

transmitted

in photo,

feldspar.

symbol

in the lower half of the figure contains

are not uncommon,

for the rock units

and weathered

light of the typical

12; orientation

and weathered

in plane-polarized,

and foliation of quartz

transmitted

from location

1 mm. The linear array of opaques

to anhedral

predominantly

in plane-polarized,

Thin section

ET AL

are rounded

the typical

and dip symbol midline

occurrence oriented

parallel

to roughly

square,

plane of foliation.

of opaques

S71° E, 90°.

to long dimension and average

in the plane

Scale bar approximately

of photo)

occur

size is much smaller

dispersed

2 mm. through

in this view than in

MAGNETIC

SUSCEPTIBILITY

ANISOTROPY,

NUTBUSH

CREEK

SHEAR

TABLE 3 Susceptibility ellipsoid shape parameter V and orientations of maximum and minimum principal susceptibilities for sampling sites apart from the main traverse Sample

V

Kl

(“1

L)

I

D

(“)

(“)

(“)

(“1

10 8 310 293

16 15 29 26

108 106 80 61

21 29 49 52

31 31 64 61

3-1 3-2 2-l 2-2

D = declination;

K3

I

Z = inclination.

eliminated the presence of hematite while the microprobe microanalysis demonstrated that the opaques are not ilmenite. The dominance of magnetite is also suggested by the large magnitudes of initial mass susceptibilities measured for samples from both the Falls lineated gneiss and the Raleigh-belt gneisses as part of the AMS analysis. These values lie in the range 5 X low4 to 4 X 10e2 SI units/gram (average approximately 7 X 10e3 SI units/gram). Magnetite was also identified in mineral seperates obtained for the Falls lineated gneiss (Barifaijo, 1986). AMS

results for sampling

sites near Wilton

V values determined

for oriented hand samples collected near Wilton, N.C. are given in Table 3. Samples 3-l and 3-2 are Raleigh-belt rocks located approximately 3 km from the mapped eastern boundary of the NCFZ (Fig. 1). V for these samples indicates that the susceptibility ellipsoids are prolate with axes of maximum susceptibility oriented along the strike of the fault zone. Samples 2-l and 2-2 were taken from unsheared Wilton quartz monzonite (Fig. 1). Susceptibility ellipsoids for these samples are oblate with nearvertical minimum susceptibility axes. Discussion

The results of the AMS analysis are remarkably consistent along the entire traverse sampled; mean susceptibility ellipsoids are uniformly prolate and display little variation in spatial orientation. Mean

ZONE,

N.C.

219

maximum susceptibility axes are oriented NO-20 o and plunge subhorizontally either to the northeast or southwest. These axial orientations are similar to the strike of the NCFZ in the study area, which is about N25” E. Mean minimum susceptibility axes are oriented O-45” to the N60-95” W. In addition, very little variation is found in the degree of scatter of the principal axes about their mean azimuths; orientations of susceptibility ellipsoids are as tightly constrained in the Raleighbelt gneisses as they are in the Falls lineated gneiss. Within the Falls lineated gneiss, the magnetic fabric mimics the petrofabrics; mean maximum susceptibility axis orientations coincide with silicate mineral lineations, and mean minimum susceptibility axis orientations are roughly parallel to poles of foliation. The prolate shape of the ellipsoid indicates that magnetic lineation is developed to a greater extent than is magnetic foliation. Structural measurements made for the Raleigh-belt rocks sampled, although limited in number, suggest that mesoscopic rock fabric elements in these rocks are oriented in the same direction as corresponding fabric elements in the Falls lineated gneiss. This is not surprising as these rocks are located close to the eastern boundary of the NCFZ and this boundary was chosen by Wylie (1984) based upon a transition from strongly lineated rocks to more obviously foliated rocks. A structural similarity with the NCFZ is observed in Raleigh-belt gneisses located up to 3 km from the fault zone (see figures in Parker, 1978, 1979; Druhan, 1983; Carpenter, 1970). Elsewhere, the Raleigh-belt gneisses are complexly folded and foliation is generally subhorizontal (Parker, 1979; Druhan, 1983; Stoddard et al., 1988). Similarity of petrofabrics in the Falls lineated gneiss and the Raleigh-belt gneisses suggests that magnetic fabric elements are aligned parallel to corresponding rock fabric elements in the Raleigh-belt rocks sampled. This follows from the observed agreement of (1) magnetic fabric in both rock units and (2) magnetic fabric and petrofabric in the Falls lineated gneiss. The coincidence of magnetic fabric and petrofabr& in both rock units was not totally unexpected. The surprising result is that, although the

220

C.V H. MIMS

Raleigh-belt lineation

rocks are distinctly

is better

tion.

In most

rocks

in which

developed

AMS

ellipsoid

is

flattening 1980;

studies

mesoscopic

nates over lineation,

Hrouda,

magnetic

involving rock

suggesting and

1982;

deformed

foliation

domi-

apparent

finite

even in situations

(1989) found where

were slightly constricted. Reflected light microscopy

planar

magnetic

the petrofabrics

and microprobe

croanalysis have shown that the the Falls lineated gneiss and in gneisses are primarily magnetite. servations of Parker (1979), Blake

Bor-

mi-

opaque grains in the Raleigh-belt Petrographic ob(1986), Barifaijo

(1986) and this study indicate that the opaque grains are arranged in elongate aggregates oriented parallel to silicate mineral lineations. Stringing together of magnetite grains produces the same AMS as magnetite grain shape anisotropy (Banerjee and Stacey, 1967) and grain shape anisotropy has been shown to be the most important control on AMS measured with low-field instrumentation for rocks in which the sole or principal magnetic constituent is magnetite (Bhathal, 1971). The obof magnetite

rocks.

than In most

foliation, foliated

grains

in elon-

to mineral lineation probdevelopment of magnetic even in the Raleigh-belt rocks,

magnetic

minerals

are distributed more evenly in the plane of foliation and magnetic foliation is better developed than magnetic lineation (Hrouda and Jam& 1976; Ruf et al., 1988). Observed agreement between magnetic fabric and petrofabric in the Falls lineated gneiss suggests that aggregates of magnetite grains formed as the result of ductile deformation within the fault zone. This suggestion is supported by several studies that have shown that ductile deformation is a highly effective mechanism for orienting ferromagnetic minerals (e.g. Balsley and Buddington, 1960; Goldstein, 1980; Rathore, 1985; Ruf et al., 1988). Similarity of magnetic fabrics in the Falls lineated gneiss and the Raleigh-belt gneisses sug-

by the same

the Raleigh-belt

by ductile

shearing

with the NCFZ.

However,

magnetite

does not in itself re-

quire

these

rocks

tends

grains,

that

associated

zone; studies

magnetite

and

of aligned

Goldstein

and Alford

lineation

were affected

sampled

1985;

fabrics

arrangement

rocks

Rathore,

radaile

served

process

Goldstein, shear

were produced

deformational

1976;

inequant

gate aggregates parallel ably results in stronger

gests that both fabrics

Janak,

and Brown, 1988). In studies of experimental zones containing

folia-

the shape of the susceptibility

oblate,

(Hrouda

foliated,

than magnetic

ET AL.

grains

to be located

have shown

to respond

the fault

that the magnetic

silicate

events

fabric (Hrouda,

Goldstein,

1980). For example,

noted

appearance

the

within

to deformational

than the mesoscopic

appearance

Goldstein

of a mylonitic

fabric faster 1982; (1980)

magnetic

fabric with the earliest precursors of mylonitization and suggested that the minerals carrying the magnetic fabric responded to deformation before production of a ductile mylonitic matrix. A similar phenomenon was noted by Ruf et al. (1988). However, the combination of AMS results and observed alignment of petrofabric elements along the strike of the fault zone suggests that the Raleighbelt rocks sampled are part of the NCFZ. The eastern boundary of the NCFZ postulated by Wylie (1984) probably separates two lithologitally distinct units within the fault zone. Identification of the actual eastern boundary of the fault will require a combined structural, petrographic, and AMS investigation along a traverse extending into Raleigh-belt rocks. The results of this study suggest that it is inappropriate to use the appearance of a distinct foliation as the only criterion for locating when the foliation fault. Evidence

the fault plane

boundary,

parallels

that deformation

particularly

the strike of the

associated

with the

NCFZ may have extended over a broad area is provided by the AMS results for two oriented hand samples taken from Raleigh-belt gneisses located 3 km from the mapped eastern boundary of the NCFZ (see Fig. 1 and samples 3-1 and 3-2 in Table 1). I’ for these samples indicates that the susceptibility ellipsoids are prolate, with maximum susceptibility axes oriented along the strike of the fault zone. Similarity of deformational history of these rocks with rocks in the fault zone is also suggested by the orientation of the foliation plane which parallels the strike of the NCFZ (J.R. Butler, pers. commun., 1988). AMS results for hand samples taken from the unsheared Wilton quartz monzonite (see Fig. 1

MAGNETIC

SUSCEPTIBILITY

ANISOTROPY,

NUTBUSH

CREEK

SHEAR

ZONE,

221

N.C.

and samples 2-1 and 2-2 in Table 1) suggest that

rofabric elements in the Falls lineated gneiss and

the magnetic

fabric

the same correspondence

through

Falls

the

Raleigh-belt

found

along

lineated

gneiss

the traverse and

gneiss was not superimposed

into upon

the region after faulting occurred. According Druhan (1983), the post-tectonic void of any deformation biotite-chlorite

to

monzonite is de-

other than a weak, local,

foliation that parallels the margin

the Raleigh-belt

is strongly suggested for

gneiss. Magnetic lineation in both

lithologies parallels the strike of the NCFZ. (2) Magnetic

fabric in both lithologies is pro-

duced by alignment of magnetite grains parallel to silicate

mineral

lineation.

Magnetic

Raleigh-belt

placement.

formed distinct foliation planes.

samples indicates

that the susceptibility ellipsoids are oblate and the axes of minimum

susceptibility

are steeply in-

clined. This magnetic fabric may result from crystal settling during cooling within an intrusive

is

better developed than magnetic foliation, even in

of the stock and is probably associated with emV for the Wilton

lineation

rocks where silicate

minerals

have

(3) Similarity of the magnetic fabric in the Falls lineated gneiss and the Raleigh-belt gneiss suggests that this fabric was produced by the same deformational process. Development of this magnetic fabric and the alignment of petrofabric ele-

body (Balsley and Buddington, 1960; Ellwood and Whitney, 1980) or some other essentially primary emplacement process, but does not reflect post-

ments along the strike of the fault strongly suggest that the Raleigh-belt rocks sampled lie within the

emplacement fabric changes. The AMS results provide insight into the spa-

fault zone. Thus, the presently mapped eastern boundary of the NCFZ in the study area probably

tial extent of the strain field associated with the NCFZ but cannot be used to estimate either strain history or strain magnitude. These estimates are

separates two lithologically the fault zone.

distinct units within

(4) Prolate susceptibility

ellipsoids determined

precluded by the similarity of the computed AMS parameters along the entire traverse, an unde-

for Raleigh-belt rocks located north of the sampling traverse suggest that deformation associated with the NCFZ may extend several kilometers

termined “pre-strain” magnetic fabric, and the lack of conventional strain markers that could be used for calibration. Even if natural strain markers were present, computation of strain magnitude from the AMS results would be hindered by uncertainty in the principal susceptibility magnitudes and by the observation that susceptibility anisotropy quickly approaches a finite, limiting value at low strains (Hrouda and Janak, 1976; Lowrie et al., 1986; Ruf et al., 1988). The results of this study suggest that ductile deformation associated with the NCFZ extended into adjacent Raleighbelt rocks. However, the AMS results cannot be used to determine if the strain intensity within these rocks was comparable to strain intensity within the Falls lineated rocks. Conclusions (1) Mean susceptibility ellipsoids are uniformly prolate and display little variation in spatial orientation along the entire sampling traverse. Excellent agreement is found between the orientation of magnetic fabric elements and corresponding pet-

from the mapped eastern boundary of the fault in certain areas. (5) AMS results provide insight into the size of the strain field associated with the NCFZ but cannot be used to estimate strain history or strain magnitude. Strain intensity within the Raleigh-belt rocks may not have been comparable to strain intensity within the Falls lineated gneiss. Acknowledgements The authors wish to express their gratitude to Bob Butler, Bob Druhan and Kevin Stewart for their assistance with the fieldwork, to Jim Meen for his help with the microprobe analysis and to many colleagues at UNC-CH for their stimulating discussions and constructive reviews of the manuscript. Partial financial support was provided by a 1987-1988 grant for geological studies in North Carolina from the North Carolina Geological Survey and by UNC-CH URC grant 5-43941. C.V.H. Mims received support from two AMOCO Master’s Fellowships in Geophysics.

222

C.V.H. MIMS

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