Magnetic fabric in mid-Cambrian rocks of the Central Flinders Zone and implications for the regional tectonic history

165

Tectonophysics, 223 (1993) 165-176

Elsevier Science Publishers B.V., Amsterdam

Magnetic fabric in mid-Cambrian rocks of the Central Flinders Zone and implications for the regional tectonic history Z.X. Li and C.McA. Powell Depnrtment of Geology, The Uniuersity of Western Australia, Nedlands, WA. 6009, Australia

(Received March 19, 1992; revised version accepted January 11, 1993)

ABSTRACT

Magnetic fabric in the mid-Cambrian sedimental rocks in the Central Flinders Zone. South Australia, varies From a bedding-parailel foliation to a well-developed lineation. No cleavage is visible in sampled outcrops but an incipient disjunctive cleavage is developed elsewhere. Comparison of magnetic-fabric data from different positions on a soft-sediment fold suggests that the magnetic lineation is not of depositional origin. Rather, it is interpreted as the result of two interfering generations of magnetic fabric: one is a compactional bedding-parallel magnetic Foliation, and the other is a magnetic foliation of tectonic origin, defined by the girdle distribution of the site-mean lineation directions. This interpretation suggests that after bedding rotation during formation of the regional NNW-trending folds in the Central Flinders Zone, there was a phase of NNW-SSE-directed tectonic shortening during the Delamerian Orogeny. A model involving a southward-progressing orogeny could possibly account for the two phases of tectonic shortening, as well as for the development of the Nackara Arc in the Adelaide Fold Belt.

Magnetic fabric is defined as “the fabric of magnetic minerals determined through the magnetic anisotropy measurement” (Hrouda, 1978). Anisotropy of magnetic susceptibility (AMS) is commonly represented as an ellipsoid with three principal axes, K,,,, Kint and Kmin, where K,,, ’ Kint >

'rnin*

Although it is widely accepted that magneticfabric analysis is a quick and non-destructive technique for petrofabric investigations (e.g., MacDonald and Ellwood, 1987; Borradaile, 1988), there is not always a one-to-one correlation between the principal axes of the AMS ellipsoid and those of the strain ellipsoid. Borradaile and Tarling (1981) reported that in weakly deformed rocks, because the depositional (or compactional, see Kligfield et al., 1983; Li et al., 1988) magnetic fabric is very resistant to internal deformation, it could interfere with the superimposed tectonic fabric, and therefore prevent a one-to-one correHO-1951/93/$06.~

lation of the AMS ellipsoid with the finite tectonic strain ellipsoid. In this situation, the tectonic fabric can be isolated only by taking account of the primary fabric during data analysis. The reason for choosing the Central Flinders Zone (Rutland et al., 1981; Preiss, 1987, fig. 2) of the Adelaide Fold Belt, South Australia (Fig. I), for a magnetic-fabric investigation, is its palaeomagnetic interest. Palaeomagnetic studies of Late Precambrian-Cambrian rocks in the northern Flinders Ranges (~~~ijk, 1980; McWilliams and McElhinny, 1980) revealed an overprint remanence, which is generaily directed shallowly towards the northwest. Although the authors of both studies interpreted the remanence as a Cambro-Ordovician Delamerian Orogeny overprint, they differed on whether the overprint was acquired before or after the folding event. Furthermore, both studies reported a widespread within-sample inconsistency. The controversy about the relative age of the overprint and the within-sample inconsistent suggests that the

0 1993 - Elsevier Science Publishers B.V. All rights reserved

Z.X. LI AND

166

Geology and sampling The Adelaide Fold Belt is bounded by the Gawler Craton to the west, and by the Kanman1j7OE 0

139”E 100

200 ‘Km

a

I

,I

/’

Study area \ (

I’

---

Tasman Line

Q

I

,’

_‘_

MLR

.

Outcropping Adelaide Fold Bel

Mt. Lofty Ranges

Flinders Ranges

Fig. 1. Tectonic

position

and regional

POWELL

too Fold Belt to the east (Fig. 1). To the west, it is floored by a late Archaean to mid-Proterozoic basement, which is composed of metamorphic complexes, and volcanic and sedimentary rocks. Covering the basement is a continuous sedimentary succession of the Late Proterozoic Adelaidean System and the succeeding Cambrian rocks. No Archaean to mid-Proterozoic rocks are known to the east, and the inferred basement to the Kanmantoo Fold Belt is oceanic (Veevers and Powell, 1984). In the Mt Lofty Ranges, high-grade sillimanite-bearing rocks from the Kanmantoo Fold Belt have been thrust westward over lowgrade parautochthonous rocks of the Adelaide Fold Belt (Clarke and Powell, 1989). The major tectonic movement in the Adelaide Fold Belt since the Late Proterozoic is the De-

overprinting/tectonic history of this region might have been more complicated than was previously thought. The primary aim of this study was to investigate the degree of internal strain of the midCambrian rocks in the Central Flinders Zone (Ranges) through magnetic-fabric study, and to use this information to interpret the deformational history, which would otherwise be difficult owing to the lack of any visible cleavage in outcrop. The palaeomagnetic results from these rocks are to be presented elsewhere.

35”E

C.McA.

structural

trends of the Adelaide

Fold Belt.

MAGNETIC

FABRIC

IN CENTRAL

FLINDERS

ZONE

ROCKS:

IMPLICATIONS

lamerian Orogeny (Thomson, 1969; Preiss, 1987). Although it is generally believed that the major phase of the Delamerian Orogeny occurred during the Late Cambrian-Early Ordovician, the younger age limit for the orogeny is not well constrained. During this orogeny, the whole fold

FOR

REGIONAL

TECTONIC

HISTORY

belt was folded and uplifted, and the Nackara Arc in the southern part of the fold belt was formed (Fig. I). In the Nackara Arc, and at the northern end of the fold belt, the cover rocks are commonly cleaved and weakly metamorphosed. In the Cen-

Reference

QuaternaryDeposits Grindstone Range Sst.

Billy Creek Fm.

Wilpena Gp. Um~ratana

/ Gp.

167

Faults

<

Bedding attitude indicating strike and dip-azimuth

Diapir

Fig. 2. Simplified geological maps of the studied areas and the sampling localities.

Z.X. Ll AND CMcA.

168

mid-Cambrian sedimentary Fiinders Zone (Figs. 1 and are 30-60”, though locally cent to faults, or shallower

tral Flinders Zone, the cover rocks are Iess deformed, but diapirs of carbonate-cemented breccia are very well developed (Webb, 1960, 1961). This study was carried out on the openly folded

(f) MF 07

(e) MF 05

POWELL

rocks in the Central 2). Dips in most areas can be steeper adjain the cores of larger

A

/----

l

S

Kmax

A

Kint

.

Kmin

i

Fig. 3. Some typical plots of the AMS data. Note that regardless of the shapes of the site-mean AMS ellipsoids, the K,,, axes always lie parallel to the bedding-planes (So). Equal-angle, lower-hemisphere stereographic projection. BC = Billy Creek Formation, MF = Moodlatana Formation, PRY = Pantapinna Sandstone.

MAGNETIC

FABRIC

IN CENTRAL

FLINDERS

ZONE

ROCKS:

IMPLICATIONS

regional folds. The principal folds trend NNW, but variation in regional bedding attitudes suggests that there is some interference with a regional ENE trend. No cleavage or other sign of ductile defo~ation was observed in the sampled outcrops, although a few other pelite outcrops in the region showed incipient disjunctive cleavage broadly parallel to the NNW-trending folds. Samples were collected along dry beds of Red Hill Creek (a tributary of the Wirrealpa Creek), Balcoracana Creek, Ten Mile Creek, and a minor creek 2 km north of the Brachina Gorge (Fig. 2). The sampled rock formations are, from oldest to youngest: the lowest Middle Cambrian Billy Creek Formation (6 sites), the Wirrealpa Limestone (7 sites), the Moodlatana Formation (8 sites), the Balcoracana Formation (1 site), and the Pantapinna Sandstone (6 sites), the last three formations forming part of the ?Upper Cambrian Lake Frome Group. Apart from samples from the Wirrealpa Limestone, all the rest were from redbeds. Each site includes a minimum of 8 samples; the samples were oriented using magnetic and sun compasses.

paretic-Faber

data

Two standard palaeomagnetic specimens from each sample were used for magnetic-fabric study. The bulk magnetic susceptibility of each specimen was measured using a Commonwealth Scientific and Industrial Research Organization (CSIRO) Balanced Transformer Susceptibility Bridge (Ridley and Brown, 19801, and its AMS measured using an upgraded Digico Anisotropy Delineator. The intensities of the bulk susceptibility in the Billy Creek Fo~ation are the highest, ranging predominantly between 1 x 10e4 to 4 X 10B4 SI, whereas those of the Wirrealpa Limestone are the lowest, mostly below 5 x lop5 SI. The directions of principal axes of the AMS ellipsoids from the Wirrealpa Limestone are scattered at the site level, which may be attributed to either an insufficient number of magnetic grains in the rocks (Stacey and Banerjee, 1974), or the sensitivity of the equipment. Only data from the other fo~ations are considered further.

FOR

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TECTONIC

169

HISTORY

Figure 3 plots some representative data from various rock formations at different localities. At specimen level, the orientations of all three axes of the AMS ellipsoids are very consistent within site in only a few places (e.g., Fig. 3a,b); more commonly, there is only one well-grouped axis (Fig. 3c-h). However, at site level, three types of AMS ellipsoids can be generalized, namely: (1) an oblate type with magnetic foliation (the plane containing K,,, and Kint axes) parallel to bedding, and well-grouped Kmin axis perpendicular to it (Fig. 3c-e), (2) a prolate type, with Kint and Kmin axes falling on a great circle, and well-grouped K,,, axes lying subparallel to the bedding plane (Fig. 3f-h), and (3) a triaxial type, with all the three axes we11 grouped (Fig. 3a,b). As discussed Iater, and shown in Figure 7, the third type is in a transitional stage between the first and the second types, but closer to the first one. Fabric interference, and their structural implications The oblate type of AMS ellipsoid (Fig. 3c-e) has commonly been found in sedimental rocks with little or no internal tectonic defo~ation. While some workers interpreted such a magnetic fabric as of depositional origin (e.g., Graham, 1967; Van den Ende, 1975), others interpreted it as of compactional origin (e.g., Kligfield et al., 1983; Li et al., 1988). If we accept either of these 4 N 2

0 0

20

40

Ao

60

80

Fig. 4. A histogram showing the minimum angular difference (A) between the palaeocurrent direction and K,, declination (+ 180”) at 13 sites where data permit such a comparison. N= number of sites for each 10” interval of angular difference. For more palaeocurrent data see Table 1.

Z.X. tl AND C.McA. POWELL

170

inteq sites have apart rian

xetations, it means that the rocks in five (MF03, MFO5, PPSOI, PPSO2 and PPSO3) undergone very little internal deformatiun, from tilting of the bed during the DelameOrogeny. An alternative interpretation is

that these rocks have been subjected to tectonic shortening perpendicular to bedding. The triaxial and proIate types of AMS ellipsoids have a common feature in that they all have well-grouped K,,, directions which lie close to

Fig. 5. (a) Photograph and fb) sketch of a small soft-sechment toid rn the toreset tredsot a farge-scale <-- 1.2 m) cross-t)ed set, together witl; the sample locations. (c) Sample-mean directions of K,,, axis in the cross-bedding reference both before (dots) and after (squares) unfolding the small fold. The palaeomagnetic samples were drilled roughly parallel to the smal1 fold axis. The cross-bedding attitude used as the reference plane in 63 is the same as where sample F was drilled. The dispersion of the K,,, directions during unfolding (samples G, H, J and K in particular) demonstrate the post-depositional origin of the welt-grouped magnetic foliation. Equal-angle stereographic projection. Solid/open symbols are projections on the lower/upper hemisphere.

MAGNETIC

FABRICINCENTRALFLINDERS

ZONEROCKS:IMPLICATIONS

the bedding plane (Fig. 3a,b,f-h). This feature is most unlikely to be of depositional origin, as has been reported by many workers in deep-sea deposits (see summary in MacDonald and Ellwood, 1987). The interpretation is based on the following arguments. First, the rocks in the Central Flinders Zone are all fluvial to shallow-marine deposits. Secondly, as shown in Figure 4, there is no systematic relationship between the site-mean K max directions and local palaeocurrent direction (see also Table 1). Thirdly, and most conclusively, a fold test on the K,, directions from a small soft-sediment fold (Fig. 5a,b) shows that the well-grouped in situ K,,, directions become much more dispersed (K decreases from 73.0 to 14.5) by unfolding the soft-sediment fold (Fig. 5~1, implying that the well-grouped K,,, axes are of post-depositional origin. When all the well-grouped site-mean K,, directions are plotted on a stereonet, an interesting pattern emerges (Fig. 6). Apart from a few sites which deviate slightly, the majority of the K,,, directions form a great circle (Fig. 6a). The girdle distribution and the rough bedding-parallelism of the Km,, directions leads to the conclusion that

FOR REOIONALTE~ONICHISTORY

there are two distinct generations of magnetic fabric in the rocks. The first one is the compactional magnetic fabric in which the AMS ellipsoids have oblate shape and the magnetic foliation is parallel to the bedding. The second generation of magnetic fabric is also of ablate shape, with a foliation plane defined by the girdle of K max directions (Fig. 6a). This second generation of magnetic foliation is of tectonic origin, and is roughly perpendicular to the first one. It occurs mainly in rocks with the second and third types of AMS ellipsoids. The intersection of the two foliations defines the magnetic lineation corresponding to the well-grouped K,,, directions. This is similar to the interference-fabric model previously proposed by Borradaile and Tarling (1984) to explain magnetic lineations. The pole to the tectonic magnetic foliation is directed towards Y/350”. This implies that in addition to the phase of ENE-WSW shortening, which produced the NNW-trending regional folds, there was another phase of tectonic shortening roughly perpendicular to it (directed subhorizontally NNW-SSE). The reason that the first phase of tectonic shortening did not significantly affect

TABLE 1 Comparison of the site-mean K,,,

directions and palaeocurrent directions Angular difference

Site

Decl. of bedding-corrected K maxdirection

Mean palaeocurrent direction

BFOl

115*

No data available

PPS06 PPSOS PPSO4 PPSOZ

083 f 4” 067 f 6” 062 f 8 073 f 10”

048” unimodal 048” unimodal 065” unimodal 064” weak, unimodal

35” 19” 3 9

MF08 MF07 MF06 MF04 MF02 MFOl

085 + 5” 081+_ 4” 268 + 10 114+ 8 085 k 9 096 f 12”

138”weak, unimodal 166” weak, unimodal 340” weak, unimodal No data available 028” bimodal to weak unimodal 088” weak, unimodal

53 85 72

BC06 BC05 BC04 BC03 BC02 BCOI

228 + 10” 049 _+ 6” 129 f 6” 266 + 4” 271 f 7” 264 + 6”

288” unimodal 325” unimodal No data available 292” unimodal 279” unimodal 280” unimodal

60 84”

9

171

57 8

26 8 16

Z.X.

172

(4

LI AND

C.McA.

POWELL

(W

W ‘MF

07-

-’ 43-

n BFO1

I i

AMFO4

directions from sites where the K,,, axes are well-grouped. Fig. 6. (a) Equal-angle stereographic projection of the mean K,, Triangles are data from places where the structural trends differ from the regional NNW-trend. c3 is the pole to the great circle of the K,,, directions, and is also the direction of the proposed maximum shortening during the second phase of deformation. (b) Schematic diagram showing the possible local rotations at sites BCO4-06 and MF04 during the second phase of deformation, as suggested by data in (a). (c) The two directions of folds in the studied region as proposed by Richert (1976) based on regional structural analysis.

the compactional fabric could be that the first tectonic shortening was taken up mostly by bedding-parallel slip during open flexural-slip folding. The relative ages of the two phases of shortening can not be identified confidently from the magnetic-fabric data alone. However, there are some clues: as shown in Figure 6a, the K,, directions from four of the five sites where the structural trends differ from the regional NNW trend (Sites BC04-06, MF04 and MF06 in Figs. 2 and 6a), deviate slightly from the great circle of K max directions. If a semi-vertical rotation axis lying in the great circle is chosen, the K,, directions of the four aberrant sites can be aligned with the great circle by applying small-circle rota-

tions (Fig. 6a). Applying the same rotations to the structural trends in these regions aligns them closer to the regional trend (Fig. 6b). The sense of these rotations is consistent with a sub-horizontal NNW-SSE shortening. This implies that the NNW-SSW tectonic shortening postdates the formation of the NNW-trending regional folds. The tectonic magnetic fabric and the local rotations probably occurred during the same phase of NNW-SSE tectonic shortening, with the local rotations slightly later than the magnetic fabric. The two-phase deformation model suggested above, based on magnetic-fabric study, supports Richert’s (1976) model for the Central Flinders Zone based on regional structural analysis (Fig. 6~).

MAGNETIC

FABRIC

IN CENTRAL

FLiNDERS

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~MFL~CATIGNS

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TECTONiC

compactional petrofabric. In their model, the compactional strain eliipsoid was oblate parallel to the bedding, and the shortening direction of the progressive deformation was paraliel to the bedding. It was shown that the shape of the bulk strain ellipsoid in the rocks changed from a bedding-paraIle1 obIate stage to a prolate stage, and then back to a oblate ellipsoid parahel to the tectonic fabric, finally ending up with a triaxial oblate shape (Fig. 7a). Deformation paths similar to this model have been reported in the Permian rocks of apes-Maritimes (Graham, 1978). ICI& field et af. (1981) afso demonstrated that the magnetic su~eptibi~ity path of Permian rocks of

Site MI?06 is anomalous in that although the bedding strike there is significantly different from that of the regional bedding, its mean K,,, direction falls on the great circle (Fig. ha). The tectonic magnetic fabric at this site could have been superimposed after any local rotation. Degree of internal strain in the rocks The difference in shape of the AMS ellipsoids from site to site indicates variation in the degree of ductile deformation, Ramsay and Wood (I9731 modelled the shapes of a strain ehipsoid during progressive ductiie deformation in rocks with a

@I

/

Billy Creek Formation

Oblate

/

c E2

I73

f+iSTORY

- Es I>O) Or

Qntl K,in (~1)

0

I .02

104

1.06 Kint /

Kint’ Kmin

108

Kmin

Kintf Kmm

Fig. 7. Deformation paths of the studied rocks, (al A schematic diagram &awing the general deformation path followed by the shapes of AMS ellipsoids and strain ellipsoids in sedimentary rocks. (b-d) show the magnetic fabric data from three of the studied formations. Each envelope represents the distribution of the AMS data within a site. Arrows indicate the general trends of these data on the deformation path, and numbers on arrows are site numbers,

Z.X. LI AND

174

Conclusions

and implications

0 L

Outcropping Adelaide Fold Belt

100 km

Shear zones proposed by Glen et al. (1977)

/ Regional fold trend / /

Direction of the regional/local

tectonic compression

H Fig. 8. A two-phase

tec-

Magnetic-fabric information has proven to be very useful in detecting feeble deformation fabrics in non-cleaved rocks. Results from the Central Flinders Zone indicate that the degree of tectonic strain in the studied rocks is low, with a maximum level equivalent to the compactional shortening. Such a low degree of deformation is barely observable on outcrops. It also suggests that there were two phases of deformation in the Central Flinders Zone of the Adelaide Fold Belt

(W

/’

for regional

tonic history

(4

,’

POWELL

strained, with the maximum tectonic shortening level similar to that of compactional shortening.

the Alpes-Maritimes roughly resembles this deformation path. In the Central Flinders Zone the shapes of the average AMS at site level vary from compactional (bedding-parallel) oblate to prolate. Their corresponding positions on the deformation path is determined by both the shape of the AMS ellipsoids and their orientational relationships with the bedding and the interpreted tectonic magnetic fabric. Figure 7b-d shows the distribution contours of the AMS data from each site for three of the elastic rock formations. The data indicate that the distribution of ellipsoids resembles the first two stages of the deformational path shown in Figure 7a. One conclusion from these data is that these rocks have not been strongly

0

C.McA.

model for the development

of the Delamerian

Orogeny.

200 1

MAGNETIC

FABRIC

IN CENTRAL

FLINDERS

ZONE

ROCKS:

IMPLICATIONS

during the Early Palaeozoic Delamerian Orogeny, an interpretation consistent with the model suggested by Richert (1976) from regional structural analysis. There are at feast two models as to how these two phases of deformation can be correlated with the regional deformation history, i.e., the evolution of the Delamerian Orogeny. One is that the two phases of deformation in the Central Flinders Zone exist throughout the whole Adelaide Fold Belt, as suggested by Preiss (1987). In this model, the first phase of deformation was responsible for almost all the folds in the Mount Lofty Ranges and the NNW- to NNE-trending folds in the Olary region and the Flinders Ranges, whereas the second phase was responsible for the ENEtrending folds in the fold belt, including the dominant folds in the Ofary region (Preiss, 1987, figs. 106, 107). A draw-back of this model is that it implies that the two wings of the Nackara Arc (Fig. 1) were formed in two distinct phases of deformation, in which case fold interference would be expected at least in the middle part of the arc. No such interference has been reported. To the contrary, on all geological maps of this region, the entire arc appears as a single continuous feature. An alternative regional model, which accommodates the contemporaneous formation of the whole Nackara Arc, as well as the two-phase compression in the Central Flinders Zone, is a modified version of Glen et al.% (1977) model (Fig. 8). In this model, the conjugate shear zones proposed by Glen et al. (1977) on the basis of basement domains dominated by particular trends of retrograde schist zones and later faults, are accepted. The regional compression direction during the whole Delamerian Orogeny would have been between NW-SE to WNW-ESE, as suggested by Coward (19761, Glen et al. (19771, Mancktelow (1989) and Clarke and Powell (1989). In order to interpret the two phases of perpendicularly-oriented deformation in the Central Flinders Zone, as well as in the Olary region (Berry et al., 19781, we suggest that there could have been a southward progression of the orogeny. During the early stage, the tectonic compression was concentrated in the northern

FOR

REGlONAL

TECTONIC

HISTORY

175

part of the Adelaide Fold Belt (Fig. 8al. A sinistral shear zone (shear zone 1 in Fig. 8) could have partitioned the regional WNW-ESE-directed shortening into shear parallel to the basement shear zone and a shortening directed roughly ENE-WSW in the region west of it ~including the Central Flinders Zone). This local shortening could have formed the dominant NNW-trending structures in the CentraI Ffinders Zone and the Olary region. With the locus of the orogeny gradually migrating to the southern Adelaide Fold Belt, shear zones 2 and 3 could have become active (Fig. 8b). The simultaneous activation of these two shear zones could have generated the Nackara Arc, as well as the second phase of deformation in the Central Flinders- Zone. One difficulty with this model is that there is no convincing evidence for a difference in the starting time of the Defamerian Orogeny within the Adelaide Fold Belt. Acknowledgements We thank C. Gatehouse for collaboration in the field, and in the collection of palaeocurrent and stratigraphic data, to be published elsewhere. Comments from D.A. Clark and P.W. Schmidt on the early drafts, and critical reviews by G.J. Borradaile and G.L. Clarke, have improved the paper. Z. Chen assisted during the field sampling and laboratory sample processing. This project was supported by ARC grants A38831488 and A39030719, and Macquarie University. Laboratory work was carried out in the Rock Magnetism Laboratory, CSIRO Division of Exploration Geoscience, North Ryde. References Berry, R.F., Flint, R.B. and Grady, A.E., 1978. Deformation history of the Outalpa area and its application to the Olary Province. Trans. R. Sot. South Amt., 102: 43-53. Borradaile, G.J., 1988. Magnetic susceptibility, petrofabrics and strain. Tectonophysics, 156: l-20. Borradaile, G.J. and Tarling, D.H., 1981. The influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks. Tectonophysics, 77: 151-168. Borradaile, G.J. and Tarling, D.H., 1984. Strain partitioning and magnetic fabrics in particulate flow. Can. J. Earth Sci., 21: 694-697.

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