Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota

TECTONOPHYSICS ELSEVIER

Tectonophysics 249 (! 995) 109-124

Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota Weiwei Sun, Peter J. Hudleston, Mike Jackson Institute for Rock Magnetism and Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA

Received 9 August 1994; accepted 18 January 1995

Abstract

Two contrasting zones of deformation have been previously recognized in the Proterozoic Thomson Formation of east-central Minnesota. The northern zone consists of open, upright E-W-trending folds with a well-developed steeply south-dipping axial-planar cleavage. The southern zone is marked by a pervasive subhorizontal cleavage (S l), crenulated by a second cleavage that is vertical to steeply south dipping ($2). Paleomagnetic results and anisotropy of anhysteretic remanent magnetization (AARM) in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction: neither NRM nor AARM were much affected by tectonic deformation. By contrast, the low-field magnetic susceptibility, which is dominated by paramagnetic minerals (particularly chlorite), reflects the tectonic fabric of the rock. Anisotropy of magnetic susceptibility (AMS) is characterized by minima normal to the cleavage in the northern zone and to the S 1 cleavage in the southern zone. Thus, the S 2 deformation did not significantly affect the AMS fabric. "March strains" were obtained by measuring the (002) lattice preferred orientation of chlorite using an X-ray pole-figure goniometer. The principal orientations of March strain match well those of AMS, and AMS and March strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a Flinn diagram. Based on AMS signature, it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone. This could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north. In this scenario, the second deformation, the result of regional late-stage shortening, was only marked by a cleavage (S 2) in the southern zone due to the favorable orientation there of S j. 1. I n t r o d u c t i o n

A number o f studies has shown that the magnetic properties o f rocks can be used to help characterize the nature and degree o f deformation and decipher tectonic history. Anisotropy o f magnetic susceptibility ( A M S ) has been widely used as an efficient means o f quantifying the fabric o f deformed rocks (Hrouda, 1982; Borradaile, 1988). The principal orientations o f A M S correspond well to the principal axes o f strain in most studies, but magnitudes o f magnetic susceptibility anisotropy only correlate well

with strain magnitudes under certain favorable circumstances (Rathore, 1979; Cogn6 and Perroud, 1988; Borradaile, 1991; Hirt et al., 1993; Richter et al., 1993a). Borradaile (1991) considers that there may be a " w i n d o w " of strain in which valid correlations can be made, between a lower limit that excludes incompletely overprinted predeformational anisotropy and an upper limit that excludes saturated anisotropy. In addition to the effects of pre-existing anisotropy and saturation o f anisotropy, however, complications arise in establishing a correlation between strain and anisotropy because A M S is mea-

0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040- 195 1(95)00007-0

110

W. Sun et al./Tectonophysics 249 (1995) 109-124

sured in a low field and includes contributions from ferrimagnetic, paramagnetic and diamagnetic minerals, and thus has a strong dependence on mineralogical composition (Hrouda, 1982,1987; Henry and Daly, 1983; Borradaile, 1987,1988; Rochette, 1987; Johns and Jackson, 1991). A number of methods have been developed to assess the dependence of AMS on composition (Henry and Daly, 1983; McCabe et al., 1985; Stephenson et al., 1986; Rochette and Fillion, 1988; Ihmle et al., 1989; Jackson et al., 1989), a principal objective being to separate ferrimagnetic and paramagnetic fabrics. Because of their high susceptibility, ferrimagnetic particles will dominate the susceptibility of rock if they constitute more than 0.1% by volume of the total rock. If ferrimagnetic minerals are absent, paramagnetic minerals will dominate rock susceptibility if they exceed 1% of the rock (Tarling and Hrouda, 1993). During deformation all minerals can be affected, and new ferrimagnetic and paramagnetic minerals can be formed. Because anhysteretic remanent magnetization (ARM) is carried exclusively by ferrimagnetic minerals, the anisotropy of ARM (AARM) represents the fabric of ferrimagnetic particles. Earlier study of the rocks of the Thomson Formation in northern Minnesota (Johns et al., 1992) indicated a magnetic fabric, with both paramagnetic and ferrimagnetic components that could be related to the tectonic fabric. Differences were found in the relative contributions of the two components between and within two tectonic domains, a northern one affected by a single penetrative cleavage, and a southern one affected by two cleavages. Conflicting evidence was found regarding the timing of the two components of magnetic fabric with respect to the two deformational episodes. The purpose of the present study is to clarify the relationship between magnetic fabric and deformation in the Thomson Formation, and to consider the implications of the fabric for the tectonic history of the region.

2. Geological setting The interbedded slates and metagraywackes of the Thomson Formation form part of the middle Precambrian Animikie Group of the western Lake Superior

region of the North American continent. They are interpreted to have been deposited in a foreland basin of the Penokean orogen and deformed at a late stage of the same orogeny (Southwick et al., 1988), which is dated at 1.90-1.83 Ga (Van Schmus, 1976,1980; Sims et al., 1989). The foreland basin overlies Archean basement of the Superior Province and constitutes a distinctive lithologic and tectonic domain. To the south of this domain is a fold-andthrust belt (Southwick et al., 1988), and further to the east, separated from the western Lake Superior region by rocks of the late Precambrian Mid-Continental Rift, is a domain that consists of an assemblage of volcanic arc rocks of Penokean age (Sims et al., 1989). Two distinct structural zones can be identified within the Thomson Formation (Holst, 1985). In the northern zone, there is a single well-developed cleavage which is axial planar to folds of wavelengths from centimeters to kilometers. Most of the folds are open, upright and E - W trending, with axial surfaces that are vertical or steeply southward dipping. In the southern zone, there are two foliations. A pervasive, nearly bedding-parallel cleavage (S l) is affected by a spaced, steeply dipping crenulation cleavage ($2). The S t foliation, along with bedding, is folded into small-scale open upright folds that are similar in orientation to the folds in the northern area (Hoist, 1985). The shapes of the strain ellipsoids determined by Holst (1985) are oblate in the northern zone and weakly prolate in the southern zone. Based on the similarity in fold geometry and cleavage orientation, Holst (1985) suggested that the single cleavage in the northern zone is equivalent to the S 2 cleavage in the southern zone and that the associated strain is similar in magnitude and orientation in the two regions. He then calculated the strain of the early deformation in the southern zone by removing the strain of the northern zone from the cumulative strain measured in the south. The early strain so obtained was oblate, with the plane of flattening subparallel to bedding and to S~. Holst suggested further that a large-scale, N-directed, isoclinal, recumbent fold--probably a thrust nappe--developed in the southern area and was responsible for the subhorizontal cleavage and flattening strains. According to this model, the deformation resulting in the steeply dipping cleavage in both domains was

W. Sun et al. / Tectonophysics 249 (1995) 109-124

imposed on both the nappe and its substrate after nappe emplacement. In the northern zone, earlier work suggested that both AMS and A A R M are related to the tectonic fabric, with strain and magnetic anisotropy ellipsoids both oblate and with minima perpendicular to cleavage (Hoist, 1985; Johns et al., 1992). In the southern zone, however, the magnetic ellipsoids do not mimic the strain ellipsoids. All AMS and most A A R M ellipsoids are oblate, whereas strain ellipsoids are prolate. The orientations o f the magnetic ellipsoids in the south tend to reflect the S 1 cleavage. The compositional control o f AMS was assessed by the method of Henry and Daly (1983), with significant changes in the paramagnetic/ferrimagnetic contributions both within and between the northern and southern zones. In this study, more extensive magnetic measurements were carried out to clarify the relationship between tectonic and magnetic fabrics in the Thomson Formation. We sampled ten sites in the northern

111

zone and seven sites in the southern zone (Fig. 1). For each site, at least ten samples were collected. A total of over 200 oriented cores were sampled either by using a gasoline-power rock drill or from hand samples which were then drilled in the laboratory. " M a r c h strain" was measured for selected samples in order to seek a correlation of strain and magnetic fabric. Based on our results, we suggest an alternative explanation for the deformational history of the Thomson Formation to that proposed by Hoist (1982,1985).

3. Paleomagnetic study and fold test The natural remanent magnetization (NRM) of both slates and graywackes was measured on a three-axis 2G Corp cryogenic magnetometer. Stability of NRM was established by stepwise demagnetization using a Schonstedt AF demagnetizer. N R M in

ap Location

N

Minnesota

Carlto n

N1,2,3,5,6 i::i"D~: •

/Jttle Otter

Creek

omsor

LEGEND

~$9,17,18

~ S15,16

BOUNDARY BE3WEEN SINGLE AND DOUBLE CLEAVAGE DOMAINS SAMPLE LOCAnO N N-NOR'n.-IERN ZONE 0 1 S-SOU'II'-IERN ZONE L - - - - . I SCALE

|

2KM I

1

46037'30"

92022'30" Fig. 1. Sample location map. In the region south of the dashed line two cleavages (S~ and S2) are developed; north of the dashed line there is only one.

112

W. Sun et a l . / Tectonophysics 249 (1995) 109-124

Sitel

N

Sitei

S

~

6

800

. . .

, . . . ,

. . .

, . . . ,

.

500 ~ Fig. 2. A sketch to show locations of sites 1, 5 and 6, respectively, on the hinge, northern limb and southern limb of a fold.

the slates proved to be unstable, whereas graywackes gave good results. A m o n g the graywackes, sites 1, 5, and 6 are located in a single layer and are from the hinge and limbs o f an outcrop-scale fold (Fig. 2). The northern limb dips 39 ° to the north; the southern limb dips 20 ° to the south. The cleavage dips from 53 to 69 ° southward with steeper dips in the northern limb (Fig. 2).

N,

Up

!

E

lOO 80( 70(

0.4 n'~m

10

NRM,

Fig. 3. Orthogonal vector plots (Zijderveld, 1967) showing the remanent behavior during AF demagnetization, for graywacke sample from site 6. Circles labeled with AF values in milliteslas represent projections of magnetization vector onto the vertical E-W-oriented plane; dots are projections on the horizontal plane.

400 e 300 ~

0

20

.

z

1.G ~ 1.4 1.z

~ ~

200 100 0

.

40 60 unfolding(%)

80

1 100

Fig. 4. Stereo plots of site-mean characteristic directions for sites l, 5 and 6 at 0, 70 and 100% unfolding, and variation in k (the precision parameter) and ~: (McFadden's fold test statistic) during stepwise unfolding. At 70% unfolding, the k value reaches the maximum. There are more than six samples at each site.

A Zijderveld plot of a sample from site 6 during stepwise A F demagnetization to a peak field of 100 mT is shown in Fig. 3. Thermal demagnetization and physical magnetic separation indicate magnetite and possibly pyrrhorite as the main ferrimagnetic source (Johns et al., 1992). The fold test gives a maximum value of the precision parameter k at 70% unfolding (Fig. 4). Such patterns may arise as a result of remagnetization during deformation (synfolding) or by modification o f a pre-folding remanence due to deformation (Van der Pluijm, 1987; Kodama, 1988). However, the fold test statistic of McFadden (1990), x, shows that the distribution at 70% unfolding is not statistically distinguishable at 95% significance level from the 100% unfolding distribution (Fig. 4). So the magnetization may predate the folding. The remanence at 70% unfolding has declination 235 ° and inclination 69 °, which gives a pole location at 19. I°N and 240. I°E. On Van der V o o ' s (1989) polar wander path, this pole position plots near 2 Ga, which is close to the age of formation o f the rock (Fig. 5). The pole position is inconsistent with acquisition of

W. Sun et al. / Tectonophysics 249 (1995) 109-124

113

4. Magnetic fabric A M S w a s m e a s u r e d in a n a u t o m a t e d s u s c e p t i b i l ity b r i d g e w i t h 1 m T a p p l i e d m a g n e t i c field. M e a s u r e m e n t s w e r e m a d e at 2 ° i n t e r v a l s in t h r e e o r t h o g o n a l planes, y i e l d i n g a total o f 6 0 0 d i r e c t i o n a l susceptibility measurements per sample. The best-fit A M S t e n s o r for e a c h s a m p l e w a s c a l c u l a t e d b y a computer program based on the procedure of Girdler (1961). T h e m e a n o r i e n t a t i o n a n d m a g n i t u d e o f t h e p r i n c i p a l a x e s in e a c h site w e r e o b t a i n e d b y t e n s o r a v e r a g i n g o f A M S r e s u l t s o f the i n d i v i d u a l s a m p l e s . A A R M w a s m e a s u r e d f o l l o w i n g the m e t h o d o f M c C a b e et al. (1985). T h e A R M in e a c h d i r e c t i o n w a s a c q u i r e d in an A F o f 95 m T p e a k s t r e n g t h w i t h a bias D C field o f 0.1 m T . T a b l e 1 s h o w s site m e a n v a l u e s o f the A M S p r i n c i p a l axis o r i e n t a t i o n s a n d m a g n i t u d e s . In t h e n o r t h e r n structural z o n e , A M S p r i n c i p a l d i r e c t i o n s a n d m a g n i t u d e s are d o m i n a n t l y c o n t r o l l e d b y the s i n g l e c l e a v a g e . T h e s i t e - m e a n A M S m i n i m a (in situ) f r o m ten n o r t h e r n sites c l u s t e r very well in a N - S s u b h o r i z o n t a l d i r e c t i o n , n o r m a l to the c l e a v a g e plane, w h i l e the m a x i m a are n e a r l y vertical a n d

Fig. 5. The pole position (dot) of the Thomson Formation and the polar wander path of Van der Voo (1989).

N R M d u r i n g d e f o r m a t i o n (ca. 1 . 9 - 1 . 8 Ga). W e int e r p r e t the N R M as a p r i m a r y d e p o s i t i o n a l r e m a nence, modified somewhat by the deformation.

Table 1 Site mean values of anisotropy of magnetic susceptibility Site

Maximum

Intermediate

Minimum

X

D

I

at,t

al,2

X

D

1

~2,1

a2,2

X

D

1

a3. l

a3, 2

NI N2 N3 N4 N5 N6 NI 1 N12 N 13 N14

4.0616 3.9376 4.0416 4.0944 3.6141 3.3534 5.6454 5.5386 3.1963 3.3709

175 191 209 269 243 151 164 168 319 127

64 68 82 62 50 45 81 73 88 80

8.3 15.4 7.3 19.7 3.5 4.2 15.8 35.6 6.3 8.6

2.7 2.4 3.3 1.8 2.2 2.6 0.4 1.0 0.4 1.1

3.9833 3.8745 3.9068 4.0216 3.4816 3.2369 5.5653 5.4976 3.0571 3.2935

270 285 85 83 113 255 273 278 90 277

3 2 4 28 29 13 3 6 2 8

1.6 2.2 2.6 1.1 4.9 0.7 0.1 0.1 0.3 0.9

8.3 15.4 7.3 19.7 3.3 4.1 15.8 35.4 6.1 8.3

3.6848 3.6999 3.6410 3.6029 3.2280 2.8768 4.6160 4.5550 2.7627 3.1066

2 16 354 175 8 357 3 10 180 8

26 23 6 3 25 42 8 16 2 5

2.8 2.7 6.0 2.8 1.4 3.4 1.0 5.3 1.5 2.9

1.6 2.1 1.7 0.7 5.2 1.1 0.1 0.2 0.3 0.9

$7 $8 $9 S15 S16 S17 S18

3.4925 4.3861 3.2442 3.3898 5.8219 4.1870 4.4639

167 89 227 183 91 69 211

53 13 71 43 l0 10 81

10.8 19.5 6.8 3.0 4.6 2.8 5.6

i.2 1.5 0.4 7.2 3.2 1.0 4.4

3.4315 4.3211 3.1075 3.2810 5.6049 4.0731 4.3729

259 188 89 79 0 160 81

2 35 14 15 2 2 6

1.8 3.4 3.0 1.3 1.1 0.6 0.3

11.1 19.4 8.4 3.9 4.9 3.0 5.6

2.9980 3.6202 2.7065 2.9631 4.7514 3.4785 3.8678

350 342 356 335 257 261 350

37 53 12 44 80 80 7

1.2 2.3 0.2 7.1 3.7 0.9 4.5

3.0 3.6 5.9 0.6 1.8 1.7 0.2

N and S = northern and southern zone, respectively; X = susceptibility (10 - 4 SI; D = declination; 1 = inclination; %,1 and a~.2 = axes of 95% confidence ellipse. The same locations are shown on Fig. 1.

114

W. Sun e t al. / Tectonophysics 249 (1995) 109-124

perpendicular to the hinges o f the folds (Fig. 6a, b). The orientations of the A M S principal axes are consistent with those of the strain principal axes recorded by carbonate concretions (Hoist, 1985). In the southern zone, the site-mean m i n i m a of A M S lie in the N - S - o r i e n t e d vertical plane, r a n g i n g from nearly horizontal through northward p l u n g i n g to ver-

a)

Northern Zone AMS in situ

tical. The m a x i m a are either parallel to the intersection o f S l and S 2 or vertical (Fig. 6c). After S l correction (S l rotated to horizontal), the m i n i m a for all but one site are vertical (normal .to the S 1 cleavage), which suggests that A M S was controlled by the first deformation in the southern zone (Fig. 6d). The site-mean ellipsoids of A M S in both zones plot

C)

Southern Zone AMS in situ

N

1

°11i E

W

g b)

S

Northern Zone AIdS Tilted

d) Southern Zone AMS Sl-Corrected It

N

W

• "!

[] O

Maximum Minimum

Fig. 6. Equal-area projections of AMS principal directions. The maxima and minima in the northern zone cluster very well in a vertical and N-S horizontal direction, respectively, in the in-situ plot (a) and disperse after bedding correction (b), showing a close association with the single cleavage (b). In the southern zone, the minima lie in the N-S-oriented vertical plane (c), and S l correction shows that the minima are normal to the cleavage (d). Open symbols = slates; solid symbols = graywackes.

W. Sun et al. / Tectonophysics 249 (1995) 109-124 1125

.

.

.

l

.

.

.

.

l

.

.

•

.-~,~.~

I

1~o

•

= ~ , ~

m

1.15

( o

.

.

.

I

.

.

.

I

.

.

.

.

from vertical through northward plunging to horizontal, and the maxima lie approximately in a plane dipping to the south (Fig. 8a). After bedding correction, most minima of AARM cluster in a steeply northward-plunging direction. The remaining few have subhorizontal N - S directions (Fig. 8b). With S l rotated to horizontal, the minima of AARM are more scattered (Fig. 8c). The ellipsoids of AARM thus tend to reflect the strain associated with primary compaction in the northern zone, although the principal orientations and magnitudes have been modified by deformation. This modification is dependent on the intensity of strain in different localities. The AARM minima in slates have a stronger tendency to be normal to cleavage than those in graywackes. In the weakly deformed graywackes, the minima of AARM retain a compaction-controlled orientation, while in the more strongly deformed slates, the minima of AARM are tectonically controlled. AARM ellipsoid in-situ orientations in the southern zone are similar to those in the northern zone (Fig. 9). When the principal orientations are corrected by rotation about strike bringing either bedding or S l cleavage to horizontal, neither of the corrected minima cluster very well. However, the S 1

.

1.10

"," 1.00

,

,

,

, .00

i

.

." ,

, .

.

.

1.05

i

.

.

.

o'o o° .

i

1.10

1.15

,

,

,

,

i

.

.

1.20

.

.

1.25

Kint/Knfin Fig. 7. Ellipsoid shapes of AMS (site mean values) represented on a Flinn plot. Data from both zones plot in the flattening field.

predominantly in the flattening field in a Flinn-type graph (Fig. 7). They have the same ellipsoid shapes as the concretion strain ellipsoids in the northern zone, but in the southern zone the ellipsoids of the concretion strain are prolate whereas the AMS ellipsoids are oblate. The orientations of the AARM principal axes are somewhat different from those of the AMS. In the northern zone the site-mean minima of AARM range in Situ N

5o Corrected N

\ W

ml

•

$

•

Sl Corrected II

[]

o

I

.

!

/ (a)

115

y (b)

/

°

°

S

(c)

[] Maximum 0 Minimum Fig. 8. Equal-area projections of AARM principal directions in the northern zone--site mean values. (a) In situ, the minima range from vertical through north-plunging to horizontal. (b) After bedding correction, most minima plunge steeply to the north. (c) The minima are more scattered after cleavage correction. Open symbols = slates; solid symbols = graywackes.

W. Sun et al. / Tectonophysics 249 (1995) 109-124

116

N

plot in the flattening field of a Flinn graph, but they are less oblate than the AMS ellipsoids (Fig. 10).

Equal Area

/°

5. C o r r e l a t i o n o f A M S a n d A A R M p o s i t i o n on fold

W

E

•

O D

Maximum Minimum

[] O

Fig. 9. Equal-area projections of AARM principal directions in the southern zone. The in-situ minima are oriented similarly to those in the northern zone. Open symbols = slates; solid symbols = graywackes.

It is instructive to consider how AMS and AARM principal orientations and ellipsoid shapes vary with position around the fold used for the fold test. The principal orientations of AMS for individual specimens from the fold-test sites (1, 5, 6) cluster very well in situ, and after bedding correction the principal orientations are scattered (Fig. 1 ta, b). Therefore, the single cleavage strongly controls the AMS principal orientations. The ellipsoids of AMS from the hinge and both limbs lie predominantly in the flattening field (Fig. 1 lc). The oblate shape of the AMS ellipsoids reflects alignment of paramagnetic minerals parallel to cleavage. In contrast to the oblate AMS results from all sites, the ellipsoids of AARM

cleavage is more closely related to the AARM principal axes than is bedding. This is probably due to the strong deformation in the southern zone, and the subparallel orientation of bedding and S~ cleavage. The site-mean ellipsoids of AARM in both zones

1.20 1.20

.

.

.

.

j

•

.~

115

•

I

o

.

.

.

.

i

. . . .

~-~,-',~ n

N-s~,,-,

I

s - ~

I

.

.

.

N

N

(a)

(b)

.

i

.

.

.

.

i

.

.

.

.

,

.

.

.

.

. . . . 1.15

/

I

s.,~.,

,

fabrics w i t h

~ I.I0

j

1.10

.

1.05

: LO0

L05

1.I0

1.15

1.20

Kin/Kmin I.~

. 1.00

.

.

.

n 1.1)5

.

.

.

.

i

.

.

.

.

1.10

i 1,15

.

.

.

(c)

. 1.20

gant/Kmm Fig. 10. The AARM ellipsoids (site mean values) are oblate for all but two sites in a Flinn diagram in both zones, but less so than the AMS ellipsoids (cf. Fig. 6).

Fig. I I. Equal-area projections of A M S principal directions for sites I, 5 and 6 in situ (a) and after bedding correction (b). The AMS ellipsoids from the hinge, southern limb and northern limb of the fold lie predominantly in the flattening field of a Flinn plot

(c).

W. Sun et al. / Tectonophysics 249 (1995) 109-124 N Umb N

117

Hinge

S Limb N

D

i 1.20

|

|

!

1 .Z0

1.15

i

__~ 1.10 X t~ ~E 1.05 1.00 1.0(

I

1.05

I 1.10

I 1.15

1.20

|

i

|

1.20

,

1.15

1.15

1.10

1.10

1.05

1o5/

1.00 1.00

I

I

,0

1.00 /

'

,O

1.os

Int/Min

Int/Min

Ca)

(b)

Cc)

1.10

1.15

1.ZO

,

/

°

1.1o Int/Min

1.05

t.oo

,

1.15

1.zo

Fig. 12. (a) The AARM maxima in the northern limb (site 5) cluster very well. parallel to the intersection of bedding and cleavage, and the AARM ellipsoids are prolate. (b) In the hinge (site 1), the AARM minima are normal to the cleavage, and the maxima are parallel to the intersection of bedding and cleavage. (c) In the southern limb (site 6), the AARM minima are normal to cleavage, while the maxima lie in the cleavage plane. The AARM ellipsoids are oblate in this location.

are of different symmetry among the three sites. Both site-mean and individual specimen ellipsoids of the northern limb (site 5) of the fold are prolate, the maxima clustering very well and lying parallel to the intersection of bedding and cleavage (Fig. 12a). Both site-mean and individual specimen ellipsoids of the southern limb (site 6) are oblate, and the minima cluster very well and are perpendicular to the cleavage, while the maxima lie in the cleavage plane (Fig. 12c). In the hinge (site 1), the minima are normal to the cleavage, and the maxima are parallel to the intersection of bedding and cleavage. The site-mean ellipsoid is more prolate than in the southern limb (Fig. 12b). The variation in shape o f the A A R M ellipsoid around the fold indicates the influence of deformation on a primary ferrimagnetic ellipsoid that was oblate with minimum normal to bedding. In the northern limb, the X Y plane of the strain ellipsoid is

superimposed sub-orthogonally on the X Y plane of the compactional strain, so the A A R M ellipsoid is prolate with maxima parallel to the intersection of bedding and cleavage. In the southern limb, both bedding and cleavage dip south, and the shortening direction during deformation was closer to the primary compaction direction. Thus the shape of the A A R M ellipsoid is more oblate. The variation in shape of the A A R M ellipsoids from the northern limb to the southern limb suggests that new ferrimagnetic particles were not formed during deformation.

6. Correlation of March strain and AMS Strain can significantly affect magnetic fabric by a variety of mechanisms, including reorientation of ferrimagnetic and paramagnetic particles in rocks

W. Sun et al. / Tectonophysics 249 (1995) 109-124

118

Equal Area

N

(a)

{b)

AMS

1.Z 1.15 ¢-

4.m ° m

x

m °6

E

17~

18

1.1

1.05

zz / ,

W

1

y

1.0s

1.1 int/min

,

.

1.1s

Strain

=

~.z (e)

s.O

graywackes slates [] Hoist (198.5)

&

4.0 eD

x 3.0 m

S Lower Hemisphere • • [] O

2.o

March Strain Maximim March Strain Minimum AMS Maximum AMS Minimum

N11 ZX

NorthernZone

S17

1.0

1.0

z.o

3.0

4.0

5.0

Int/Min Fig. 13. Equal-area projection showing that the principal orientations of March strain are nearly parallel to those of AMS (a). The ellipsoids of both AMS (b) and March strain (c) lie in the flattening field of a Flinn diagram. Open triangles = slates; solid triangles = graywackes.

( B o r r a d a i l e , 1 9 8 8 , 1 9 9 1 ; J a c k s o n et al., 1993). T h u s it is o f i n t e r e s t to i n v e s t i g a t e t h e r e l a t i o n s h i p b e t w e e n m a g n e t i c f a b r i c a n d f i n i t e strain. D u e to d i f f e r e n c e s in c o m p o s i t i o n , d e f o r m a t i o n a n d m e t a m o r p h i c history for d i f f e r e n t localities, o n e c a n e x p e c t n o uni-

versal e q u a t i o n r e l a t i n g strain m a g n i t u d e d i r e c t l y to AMS. M a g n e t i c fabric in the T h o m s o n F o r m a t i o n is d o m i n a t e d by p a r a m a g n e t i c m i n e r a l s , p r i n c i p a l l y c h l o r i t e ( J a c k s o n et al., 1995). T h e m i n i m u m s u s c e p -

Table 2 Principal natural March strains (Ei) and magnetic susceptibilities (Xi) for five samples Sample

e~

E2

~3

Xl

X2

X3

Xmean

In( XI/Xm)

In( X2/Xm)

In(X3/Xm)

5-9a 6-8a 11-5 17-5a 18-3

0.34359 0.41871 0.71295 0.45108 0.34359

0.02956 0.14842 0.30748 0.16551 0.08618

-0.43078 -0.56212 - 1.02170 -0.61619 --0.43078

3.6050 3.0390 5.8410 3.9810 4.3710

3.4880 2.9360 5.7530 3.8990 4.2630

3.2380 2.5820 4.8380 3.3100 3.7850

3.4437 2.8523 5.4773 3.7300 4.1397

0.045785 0.063391 0.064284 0.065125 0.054377

0.012792 0.028911 0.049103 0.044312 0.029358

-0.06158 -0.09957 -0.12412 -0.11946 -0.08957

Subscript I = maximum extension and susceptibility direction; subscript 2 = intermediate strain and susceptibility direction; subscript 3 = minimum extension and susceptibility direction.

W. Sun et al. / Tectonophysics 249 (1995) 109-124

tibility axis of chlorite is parallel to the crystallographic c-axis, while the maximum and intermediate axes lie in the basal plane. The preferred orientations of chlorite in (002) lattice direction were measured for selected samples with an X-ray pole-figure goniometer in transmission mode (see Oertel, 1983). Two orthogonal sections with a thickness of about 100 mm were cut perpendicular to the single cleavage in the northern zone and to S 1 in the southern zone. Strain was quantified from chlorite-preferred orientation according to the March (1932) model: = (p-,/3)

_ 1

(1)

where Pi is a principal normalized pole density, and e i is a principal extensional strain (which is defined as the change in length divided by the original length). The resulting "March strain" is a measure of the intensity of the degree of chlorite-preferred orientation that has been shown to correspond to true strain in a number of studies (see Oertel, 1983). A computer program was used to calculate the March strain from the pole density contour diagram. Five samples were selected, respectively, from sites 5, 6 and 11 of the northern zone and sites 17 and 18 of the southern zone. The site 11 sample is a slate with a strong foliation. Samples from sites 5 and 6 are graywackes from the northern and the southern limb of the fold, respectively. At site 17, the samples are slates and S, is perpendicular to S 2, while at site 18, the samples are graywackes and S l is parallel to S 2.

119

The principal orientations of March strain and AMS match very well for all five samples (Fig. 13a). This is because chlorite-preferred orientation controis both March strain and AMS. The ellipsoids from both March strain and AMS plot predominantly in the flattening field in a Flinn diagram (Fig. 13b, c). Table 2 gives the magnitudes of the principal natural March strains and magnetic susceptibilities. A correlation of the magnitude of natural March strains and natural logarithm of normalized susceptibilities is shown in Fig. 14. Because the samples used for the March strain analysis are of different lithology and deformational history, we cannot directly correlate March strain and AMS with regression analysis.

7. Petrofabric observations in thin sections

In thin section, it is seen that the single cleavage in the northern zone is delineated by the alignment of flaky minerals (mainly chlorite; Fig. 15a). In the southern zone, the foliation defined by alignment of flaky minerals is crenulated, with the axial surfaces of the crenulations corresponding to S 2 (Fig. 15b). Despite the crenulation cleavage, most chlorite particles are still parallel to S 1 (Fig. 16), so both AMS and March strain in the southern zone basically reflect the first deformation; the second deformation contributes very little to either.

8. Discussion 0.15

....

i . . . .

i . . . .

t .... I

i . . . .

i ....

8.1. Formation of magnetic carriers

I 0.10

o t~ o

maximum iamma~ minimum

I I tl~

0.05

I

ta

~'

itu~

-0.05

o o

o

-0.|0 o

-0.15 -1.50

....

o t . . . .

-1.00

~ . . . . . . . . -0.50

0.00 Natural

~ . . . . . . . . 0.50

1.00

1,5,0

Strain

Fig. 14. Graphical representation of the correlation between natural March strain and natural normalized susceptibility for each of the three principal axes.

The characteristics Of NRM and AARM indicate that the orientation of ferrimagnetic particles is dominantly controlled by primary compaction, modified somewhat by tectonic deformation. The formation of the remanence-carrying ferrimagnetic particles (magnetite or titanomagnetite, Johns et al., 1992) must therefore predate deformation. The NRM direction and strong relationship between AARM fabric and primary compaction rules out a postdeformational or even synfolding origin of the ferrimagnetic particles. The degree of modification of the primary AARM by later deformation depends on location and defor-

120

W. Sun et al. / Tectonophysics 249 (1995) 109-124

(a)

(b) Fig. 15. Photomicrographs of the rock in thin section. (a) A sample (from site 11) from the northern zone shows only one cleavage delineated by the alignment of phyllosilicates. (b) In the southern zone, the S I cleavage is crenulated by S 2 (from site 17). Scale bars are 1 mm in each photograph.

W. Sun et al./ Tectonophysics 249 (1995) 109-124

Chlorite Orientation i

~1'

I ' I ' i ' I ' I ' I '~

15

0

10

r..)

0

i -70-60-50-40-30-20-10

10203O4050607080

Degree Fig. 16. The distribution of chlorite flake orientation relative to the S 1 cleavage based on measurement made in thin section. Most of the chlorite particles are parallel to S~.

mation intensity. The primary NRM orientation must also have been rotated by later deformation. Strictly speaking, we should correct both NRM and bedding directions in Fig. 4 by unstraining before performing the fold test. Since the NRM vector lies close to the plane of cleavage, strain variation within the plane is small, and the strains are modest, the strain correction will be slight, and we have not done this.

8. 2. Magnetic fabric and natural strain As indicated above, AARM represents a fabric of primary compaction modified by later deformation. The AARM magnetic fabric cannot, therefore, be used directly as an indicator of deformation in the Thomson Formation. In contrast, the AMS fabric is closely related to deformation in both structural zones. Because the AMS fabric in the Thomson Formation is dominated by paramagnetic susceptibility, it reflects the preferred orientation of the crystallographic lattice planes of chlorite, as does March strain. In studying the Helvetic zone of the central Alps, Richter et al. (1993b) and Hirt et al. (1993) established empirical correlation equations between AMS, March strain and finite strain (from reduction spots), respectively. In this situation, mica-preferred orientation and AMS can be used to predict the strain geometry. Such results and those of a number of earlier studies (see Borradaile, 1991) suggest that the March strains may represent reasonably well the finite strains in the northern zone of the Thomson Formation. In the southern zone, however, the situa-

121

tion is more complex. The first cleavage is delineated by flaky mineral alignment, as is the single cleavage in the northern zone. S 2 is a crenulation cleavage, the formation of which may have involved pressure solution. It is well known that pressure solution can remove substantial amounts of host rock (e.g., Gray and Durney, 1979), leaving little physical evidence besides thin seams of insoluble residue that define a spaced cleavage, which is often associated with crenulations. In the samples we have examined, there are few flaky minerals parallel t o S 2 in the S 2 cleavage domains, and this may indicate a relatively small loss of material by pressure solution, and thus a weak D 2. If there has been no loss of material by pressure solution, the March strain and AMS reflect the total deformation, which is dominated by D 1. If, however, during D 2 there was significant removal of material by pressure solution, this deformation would be "invisible" to the March strain and AMS. This may be the reason for the difference between the oblate strains reported here, based on the March method and AMS, and the prolate strains reported earlier by Holst (1985), based on clast shapes, which would have recorded the total strain (provided that clast size is greater than the spacing of the cleavage domains). It should also be noted that our samples come from a restricted part of the southern zone, near its northern edge, whereas Holst's samples come from a broader region. It should be pointed out that in some instances growth of magnetic minerals in secondary spaced cleavage domains results in a magnetic fabric (Borradaile and Tarling, 1984), in contrast to the situation in the Thomson Formation.

8.3. Deformational history In the northern zone, where there has been a single deformation, the March strain and AMS match well the oblateness of the finite strain determined using concretions (Hoist, 1985). In the southern zone, with its two cleavages, the interpretation of the deformational history and the reconciliation of the March strain and AMS with the strains determined by Holst depends on the extent of removal of material by pressure solution during D 2. The existence of open folds and crenulations and the continuity of S 1 across the S 2 cleavage domains, as seen in thin section (Fig. 15b), suggest that D 2 may have been

122

W. Sun et al./Tectonophysics 249 (1995) 109-124

E

Northern h~'%~'Zne ~ ~ Southern

~ ~ ~

;

Lower Green Schist

~

~

~

Lower Arnphibolite

Facies

(a)

~~~her. 7

Northern

E

v

Lower amphibolite

" ' I~"~-~=~

Facies

(b)

Fig. 17. A possible explanation of the deformational history of the Thomson formation. (a) N - S shortening and N-directed (upper plate) shear of the Thomson Formation during the Penokean orogeny. Cleavage subparallel to bedding was formed in the southern zone where deformation and translation was greatest, while upright open folds with axial plane cleavage were formed in the northern zone. (b) Late-stage regional shortening continued to affect the Thomson Formation. Crenulation cleavage was formed only in the southern zone due to the high angle between S I and

the direction of shortening.

weak in that part of the southern zone studied by us. In contrast, at least locally in the southern zone, the S 2 fabric is much more intense (Holst, 1985, fig. 2b and c), involving much more pronounced pressure solution films. It is possible that the rather similar chlorite fabrics - - a s indicated by March strains and A M S - - i n the northern and southern zones developed under lowgrade metamorphic conditions at the same time, in which case the single cleavage in the north correlates with S 1 in the south, rather than with S z as proposed by Holst (1985). Some explanation for the difference in orientation of the chlorite fabric in the two zones is then required. It is a common observation that

inclined or recumbent structures give way to upright structures towards the foreland of orogens (e.g., Sanderson, 1979), and this may be the case in the Thomson Formation. In tectonic deformation of a sheet or wedge of rock, in which gravity plays an important role, there will be a predominance of simple shear near the base of the sheet or wedge that gives way to pure shear and horizontal compression near the margin (e.g., Hudleston, 1992). Such a scenario is illustrated in Fig. 17. Cleavage would have formed at a high angle to bedding in the northern distal zone and subparallel to bedding in the southern proximal zone. This deformation would have occurred during the closure of the Animikie basin during the final stages of subduction and development of the Penokean orogeny. Late-stage regional shortening would have been responsible for the S 2 cleavage in the southern zone, and such a cleavage would not have been manifest in the northern zone because of the unfavorable orientation of the pre-existing foliation there. The interpretation proposed here also helps explain the lack of downward structural facing noted by Holst (1985), such facing being required at least locally for the nappe model he proposed. Unfortunately, the transition zone or tectonic contact between the two domains does not crop out, and so direct observation of the relationship between the two zones is not possible.

9. Conclusions

(1) Paleomagnetic results indicate that the magnetic remanence of the Thomson Formation recorded the field during rock formation. Deformation had a minor effect on the N R M direction. (2) The relationship between the A A R M principal orientations and structural fabric can be used to verify the timing of formation of ferrimagnetic particles. Anisotropy of ARM reflects primary compaction, modified by tectonic deformation. (3) AMS and March strain are controlled by the preferred orientation of chlorite, which dominates the magnetic susceptibility. AMS strongly correlates with March strain in orientation and in ellipsoid shape. (4) The first deformation in the southern zone possibly occurred at the same time as the single deformation in the northern zone. The lack of a

w. Sun et al. / Tectonophysics 249 (1995) 109-124

s e c o n d c l e a v a g e in t h e n o r t h r e f l e c t s t h e u n f a v o r a b l e orientation there of the pre-existing cleavage.

Acknowledgements This study was supported by the National Science F o u n d a t i o n g r a n t E A R - 9 2 1 9 6 1 6 . Drs. A. Y i n a n d G. O e r t e l are t h a n k e d for l e t t i n g W e i w e i S u n u s e t h e i r X - r a y t e x t u r e g o n i o m e t e r facility a n d f o r h e l p i n g h i m m e a s u r e s a m p l e s a n d c a l c u l a t e M a r c h strains b y u s i n g t h e c o m p u t e r p r o g r a m w r i t t e n b y R. C h e n . Graham Borradaile, Stephen Marshak and editor T e r r y E n g e l d e r are t h a n k e d for t h e i r c o m m e n t s o n the m a n u s c r i p t . T h e I R M is f u n d e d b y the K e c k Foundation, the National Science Foundation, and the U n i v e r s i t y o f M i n n e s o t a . I n s t i t u t e for R o c k M a g netism (IRM) contribution number 9407.

References Borradaile, G., 1987. Anisotropy of magnetic susceptibility: rock composition versus strain. Tectonopbysics, 138: 327-329. Borradaile, G., 1988. Magnetic susceptibility, petrofabrics and strain: a review. Tectonophysics, 156: 1-20. Borradaile, G., 1991. Correlation of strain with anisotropy of magnetic susceptibility (AMS). Pure Appl. Geophys., 135: 15-29. Borradaile, G.J. and Tarling, D., 1984. Strain partitioning and magnetic fabrics in particulate flow. Can. J. Earth Sci., 21(6): 694-697. Cognr, J. and Perroud, H., 1988. Anisotropy of magnetic susceptibility as a strain gauge in the Flamanville granite, NW France. Phys. Earth Planet. Inter., 51: 264-270. Girdler, R.W., 1961. The measurement and computation of anisotropy of magnetic susceptibility in rocks. Geophys. J. R. Astron. Soc., 5: 34-44. Gray, D.R. and Durney, D.W., 1979. Investigations on the mechanical significance of crenulation cleavage. Tectonophysics, 58: 35-79. Henry, B. and Daly, L., 1983. From qualitative to quantitative magnetic anisotropy analysis: the prospect of finite strain calibration. Tectonophysics, 98: 327-336. Hirt, A., Lowrie, W., Clendenen, W. and Kligfield, R., 1993. Correlation of strain and anisotropy of magnetic susceptibility in the Onaping Formation: evidence for a near-circular origin of the Sudbury Basin. Tectonophysics, 225: 231-254. Hoist, T.B., 1982. Evidence for multiple deformation during the Penokean Orogeny in the Middle Precambrian Thomson Formation, Minnesota. Can. J. Earth Sci., 19: 2043-2047. Hoist, T.B., 1985. Implication of a large flattening strain for

123

origin of a bedding-parallel foliation in the early Proterozoic Thomson Formation, Minnesota. J. Struct. Geol., 7: 375-383. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surv., 5: 37-82. Hrouda, F., 1987. Mathematical model relationship between the paramagnetic anisotropy and strain in slates. Tectonophysics, 142: 323-327. Hudleston, P.J., 1992. A comparison between glacial movement and thrust sheet or nappe emplacement and associated structures. In: S. Mitra and G.W. Fisher (Editors), Structural Geology of Fold and Thrust Belts. Johns Hopkins Univ. Press, Baltimore, MD, pp. 81-91. Ihmle, P.F., Hirt, A.M., Lowrie, W. and Dietrich, D., 1989. Inverse fabric in deformed limestones of the Morcles Nappe, Switzerland. Geophys. Res. Lett., 16: 1383-1386. Jackson, M., Sprowl, D. and Ellwood, B., 1989. Anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales: Grainsize-and compositional-dependent magnetic fabric. Geophys. Res. Lett., 16: 1063-1066. Jackson, M., Borradaile, G., Hudleston, P. and Banerjee, S., 1993. Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties and magnetic anisotropy. J. Geophys. Res., 98B1: 383401. Jackson, M., Sun, W.W. and Hudleston, P., 1994. Rock magnetic studies on the compositional control of magnetic anisotropy in the Thomson formation, Minnesota. (in prep.). Johns, M.K. and Jackson, M., 1991. Composition control of magnetic anisotropy in synthetic samples. Geophys. Res. Lett., 18: 1293-1296. Johns, M.K., Jackson, M.J. and Hudleston, P.J., 1992. Compositional control of magnetic anisotropy in the Thomson formation, east-central Minnesota. Tectonophysics, 210: 45-58. Kodama, K., 1988. Remanence rotation due to rock strain during folding and the stepwise application of the fold test. J. Geophys. Res., 93: 3357-3371. March, A., 1932. Mathematische Theorie der Regelung nach der Korugestalt bei affiner Deformation. Z. Kristallogr., 81: 285297. McCabe, C., Jackson, M. and Ellwood, B., 1985. magnetic anisotropy of the Trenton Limestone: results of a new technique, anisotropy of anhysteretic susceptibility. Geophys. Res. Lett., 12: 333-336. McFadden, P.L., 1990. A new fold test for palaeomagnetic studies. Geophys. J. Int., 103: 163-169. Oertel, G., 1983. The relationship of strain and preferred orientation of phyllosilicate grains in rocks: a review. Tectonophysics, 100: 413-447. Rathore, J.S., 1979. Magnetic susceptibility anisotropy in the Cambrian slate belt of North Wales and correction with strain. Tectonophysics, 53: 83-97. Richter, C., Van der Pluijm, B.A. and Housen, B.A., 1993a. The quantification of crystallographic preferred orientation using magnetic anisotropy. J. Struct. Geol., 15:113-116. Richter, C., Ratschbacher, L. and Frisch, W., 1993b. Magnetic fabrics, crystallographic preferred orientation and strain of progressively metamorphosed pelites in the Helvetic Zone of

124

W. Sun et al. / Tectonophysics 249 (1995) 109-124

the Central Alps (Quartenschiefer Formation). J. Geophys. Res., 98B6: 9557-9570. Rochette, P., 1987. Magnetic susceptibility of the rock matrix related to magnetic fabric studies. J. Struct. Geol., 9: 10151020. Rochette, P. and Fillion, G., 1988. Identification of multicomponent anisotropies in rocks using various field and temperature values in a cryogenic magnetometer. Phys. Earth Planet. Inter., 51 : 379-386. Sanderson, D.J., 1979. The transition from upright to recumbent folding in the Variscan fold belt of southwest England: a model based on the kinematics of simple shear. J. Struct. Geol., 1(3): 171-180. Sims, P.K., Van Schmus, W.R., Schulz, K.J. and Peterman, Z.E., 1989. Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Can. J. Earth Sci., 26: 2145-2158. Southwick, D.U, Morey, G.B. and McSwiggen, P.L., 1988. Geologic map of the Penokean Orogen, central and eastern Minnesota and accompanying text. Minn. Geol. Surv. Rep. Invest. 37. Stephenson, A., Sadikun, S. and Potter, D.K., 1986. A theoretical

and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals. Geophys. J. R. Astron. Soc., 84: 185-200. Tarling, D.H. and Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, 217 pp. Van der Plnijm, B.A., 1987. Grain scale deformation and the fold test--evaluation of synfolding remagnetization. Geophys. Res. Lett., 14: 155-157. Van der Voo, R., 1989. Paleomagnetism of North America; The craton, its margins and the Appalachian Belt. In: L.C. Pakiser and W.D. Mooney (Editors), Geophysical Framework of the Continental United States. Geol. Soc. Am. Mere, 172: 447470. Van Schmus, W.R., 1976. Early and Middle Proterozoic history of the Great Lake area, North America. Philos. Trans. R. Soc. London, 280: 605-628. Van Schmus, W.R., 1980. Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin. Geol. Soc. Am. Spec. Pap., 182: 159-168. Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: Analysis of results. In: D.W. Collinson et al. (Editors), Methods in Paleomagnetism. Elsevier Amsterdam, pp. 254-286.