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Palaeomagnetism of flood basalts in the Pilbara Craton, Western
Australia: Late Archaean continental drift and the oldest known
reversal of the geomagnetic field
Geert Strik,
1
Tim S. Blake,
2
Tanja E. Zegers,
1
Stan H. White,
3
and Cor G. Langereis
1
Received 27 February 2003; revised 29 May 2003; accepted 14 July 2003; published 3 December 2003.
[1]A late Archaean (circa 2775–2715 Ma) succession of terrestrial continental flood
basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks in the Nullagine
Synclinorium (and Meentheena Centrocline) of the East Pilbara Basin, Western Australia,
has been sampled for a palaeomagnetic study. Over 500 oriented, mostly basalt, drill cores
were collected from the supracrustal succession and associated dykes. Thermal and
alternating field demagnetization revealed two distinct components. Positive fold,
conglomerate, and reversal tests confirm that the primary natural remanent magnetization
(NRM) is still preserved. The secondary component is interpreted as the record of
remagnetization during a major thermal event, possibly in the Early Proterozoic. Analysis
of the primary NRM directions results in a magnetostratigraphy and an apparent polar
wander path (APWP) for the 60 Myr interval covered by the sampled succession.
Assuming a geocentric axial dipole during this time interval, the APWP shows that the
Pilbara Craton was drifting during the late Archaean and that drift rates probably varied
significantly. In particular, a mean 27.2shift in palaeolatitude is recorded across an
unconformity that represents a relatively short time period and that marks a significant
change in basalt geochemistry. This study suggests that continents moved horizontally
during the late Archaean and that the rates of movement were significantly faster than in
the Phanerozoic. In addition, a reversed polarity interval, with a positive reversal test, is
recorded. We argue that it documents the oldest known geomagnetic reversals of the
geomagnetic field. INDEX TERMS:1525 Geomagnetism and Paleomagnetism: Paleomagnetism
applied to tectonics (regional, global); 1535 Geomagnetism and Paleomagnetism: Reversals (process,
timescale, magnetostratigraphy); 8125 Tectonophysics: Evolution of the Earth; 8157 Tectonophysics: Plate
motions—past (3040); 9619 Information Related to Geologic Time: Precambrian; KEYWORDS:
paleomagnetism, Archean, Pilbara, drift rates, flood basalts, reversals
Citation: Strik, G., T. S. Blake, T. E. Zegers, S. H. White, and C. G. Langereis, Palaeomagnetism of flood basalts in the Pilbara
Craton, Western Australia: Late Archaean continental drift and the oldest known reversal of the geomagnetic field, J. Geophys. Res.,
108(B12), 2551, doi:10.1029/2003JB002475, 2003.
1. Introduction
[2] The nature of Archaean (4.02.5 Ga) tectonic pro-
cesses is largely unknown and often controversial. Schools
of thought still vary widely despite decades of research on
all continents [e.g., Hamilton, 1998; de Wit, 1998]. Central
to all controversies concerning larger-scale tectonic processes
is the question to which degree Archaean continental crust
moved horizontally across the Earth’s surface, in other
words, whether it was dominated by horizontal or vertical
tectonics.
[3] Palaeomagnetism, in conjunction with high-resolution
isotopic and biostratigraphic age control, has proven crucial
to quantifying both the rotation and latitudinal displacement
of the Earth’s plates during the Phanerozoic. However, in
rocks older than circa 180 Ma the oceanic polarity record is
not available, and in Precambrian rocks there is poor
biostratigraphic control and commonly only widely spaced
isotopic ages. In addition, Precambrian rocks are commonly
deformed and metamorphosed, rendering them unsuitable
for palaeomagnetic studies. Therefore with increasing age, it
becomes more difficult to derive and interpret palaeomag-
netic data.
[4] In Archaean rocks, palaeomagnetic studies are even
more problematic because there is an even greater paucity
of well-exposed, unaltered, and undeformed rocks that are
suitable for palaeomagnetic studies. Further, Archaean tec-
tonic processes are still poorly understood, which makes
interpreting the meaning of any palaeomagnetic results
problematic. While there is a broad consensus that the
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B12, 2551, doi:10.1029/2003JB002475, 2003
1
Palaeomagnetic Laboratory Fort Hoofddijk, Utrecht University,
Utrecht, Netherlands.
2
Chidlow, Western Australia, Australia.
3
Department of Earth Sciences, Utrecht University, Utrecht, Netherlands.
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2003JB002475$09.00
EPM 2 -1
Archaean mantle was substantially hotter than today [e.g.,
Pollack, 1997], there is little consensus concerning larger-
scaletectonicprocesses.Forexample,Hamilton [1998]
argues that plate tectonic processes such as those evoked
for the Phanerozoic did not occur until at least 2.6 Ga and
concludes that no observational evidence for rifting, rotation,
and continental plate reassembly has been found to support
plate tectonic activity. In contrast, de Wit [1998] argues that
the evidence for plate tectonics in the late Archaean is strong
and that the process appears to dominate this time period.
[5] Palaeomagnetic studies can make a significant con-
tribution to this debate, provided they have sufficient
geological and geochronological control and provided that
they demonstrate a primary natural remanent magnetization
(NRM). However, little is known about the Archaean
geomagnetic field, and certain assumptions have to be made
when interpreting Archaean NRM. In particular, we have to
assume that the Earth’s magnetic field behaved as a geo-
centric axial dipole (GAD), even though it is still a topic of
debate. For example, Kent and Smethurst [1998] conclude
that a 25% octupole contribution to the axial dipole field can
explain the anomalous inclination distribution for the Pre-
cambrian and Palaeozoic, based on the bias toward low
inclinations of known data from the Global Palaeomagnetic
Database (GPMDB). However, McElhinny and McFadden
[2000] claim that it is more probable that continental
lithosphere had the tendency to be cycled into the equatorial
belt and that the database used by Kent and Smethurst
[1998] does not allow a sufficiently random sampling of the
whole Earth throughout the Precambrian and Palaeozoic.
[6] Compared with Phanerozoic rocks, the palaeomag-
netic database of Archaean rocks is very small. The Ar-
chaean shields of Canada, particularly the Superior
Province, are the most extensively studied with 40 entries
in the GPMDB, [Zhai et al., 1994; McElhinny and Lock,
1996; Buchan et al., 1998], followed by Africa, Europe, and
Australia (16, 11, and 8 entries in the GPMDB, respectively).
Only a few studies convincingly demonstrate the timing of
magnetization through several field tests. For instance,
Meert et al. [1994] show by means of a positive fold,
reversal, and conglomerate tests that a succession of the
Nyanzian System (Tanzania Craton), carries an NRM of
2680 ± 10 Ma. However, many studies lack palaeomagnetic
field tests and/or precise age control. Previous palaeomag-
netic studies of Archaean rocks were mostly focused on a
single rock formation within a specific area and resulted in
one palaeopole for that area [e.g., Morimoto et al., 1997;
Wingate, 1998]. When combined, results from these studies
determine an apparent polar wander path (APWP) that is
poorly constrained, with average time gaps of circa 60 Myr
between following palaeopoles [Idnurm and Giddings,
1988]. Compared to many Phanerozoic APWPs (e.g.,
Laurentia, one key pole position per 20.8 Myr from the
Early Cambrian to present, based on more than 3000 pole
positions [cf. van der Voo, 1990; McFadden and McElhinny,
1995; McElhinny and McFadden, 2000]) these Archaean
APWPs are extremely crude and would be deemed mean-
ingless in a Phanerozoic context. Therefore to obtain mean-
ingful Archaean APWPs, it is essential to study precisely
dated Archaean successions within one geologically coher-
ent terrain and preferably within a coherent stratigraphic
section.
[7] Over the last 20 years the sensitive high-resolution
ion microprobe (SHRIMP) has revolutionized Precambrian
geochronology by providing ages commonly with preci-
sions of only a few million years. When these ages are
integrated into detailed geological and geophysical studies
of Archaean terrains they provide a framework that is
becoming suitable for high-precision palaeomagnetic stud-
ies, as we illustrate in this study.
[8] The circa 27752715 Ma Nullagine and Mount Jope
Supersequences [Blake, 1993, 2001] in the Nullagine
Synclinorium in the Pilbara Craton of Western Australia
(Figures 1 and 2) were chosen for this study because they
comprise a well-exposed, relatively undeformed and unal-
tered late Archaean succession that contains an abundance
of flood basalts. In addition, the succession in the study area
(Figures 1 and 2) has recently been divided (stratigraphi-
cally) in detail and has been precisely dated (Figure 3)
[Blake, 2001; T. S. Blake et al., Stratigraphic geochronology
of a late Archaean flood basalt province in the Pilbara
Craton, Australia: Constraints on basin evolution, mafic and
felsic volcanism and continental drift rates, submitted to
Precambrian Research, 2003; hereinafter referred to as
Blake et al., submitted manuscript, 2003].
[9] There are few previous palaeomagnetic studies of the
Nullagine and Mount Jope Supersequences and associated
igneous intrusions. Embleton [1978] presented the first
palaeopole for the Black Range Dyke (Figure 1) and the
Cajuput Dyke, (SCD in Figure 2) which are part of the
Black Range Suite that has now been dated at 2772 ± 2 Ma
by Wingate [1999], SHRIMP U-Pb zircon). Embleton
[1978] interpreted the NRM as primary based on a positive
baked contact test. Schmidt and Embleton [1985] presented
palaeomagnetic data for the Mount Roe basalt (Figure 4)
and the Mount Jope Volcanics (broad equivalents of the
Kylena basalt to Maddina basalt in Figure 4), as well as for
the older Millindinna Complex (Figure 1) that was dated at
2860 ± 20 Ma by Gulson and Korsch [1983]. Schmidt and
Embleton [1985] interpreted the NRM preserved in the
rocks as primary based on positive fold tests. Schmidt and
Clark [1994] carried out a palaeomagnetic study of the
banded-iron formations (BIFs) of the Hamersley Range
Megasequence, which overlies the Chichester Range Mega-
sequence. The rocks studied are dated between 2479 ± 3 Ma
(SHRIMP U-Pb, zircon [Nelson, 1999]) and 2449 ± 3 Ma
(SHRIMP U-Pb, zircon [Barley et al., 1997]). They found
remagnetized pole positions, which they interpreted to have
been caused by burial metamorphism. The age of remagne-
tization of the BIFs was estimated by Schmidt and Clark
[1994] at 2200 ± 100 Ma based on a positive fold test on
folds formed during the Ophthalmia Orogeny. Sumita et al.
[2001] presented the results of a study of BIF of the
Hamersley Province (Beasley River region; Figure 1) and
basalts of the Mount Jope Supersequence in the southern
Pilbara. They found that both the BIF and their basalt
samples are remagnetized and have not preserved any
primary NRM directions.
[10] The most extensive relevant work in the Pilbara
recently is by Li et al. [2000]. This work has focused
mostly on the BIFs of the Hamersley Province, though
some basalts of the Mount Jope Supersequence were also
sampled. Li et al. [2000] distinguished five phases of
remanent magnetization. The bulk of their samples revealed
EPM 2 -2 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
overprinted directions, possibly obtained during either the
Ophthalmian (<2.45-circa 2.2 Ga) or the Ashburton (1.8 –
1.65 Ga) Orogenies. Li et al. [2000] note that separating
Ophthalmian-related structures from Ashburton-related
structures is difficult. Thus there is a large degree of
uncertainty as to the ages of their magnetic overprint
directions. Li et al. [2000], however, did record a possible
primary NRM direction in basalt samples from the Chi-
chester Range Megasequence, but this was not tested.
[11] To summarize, palaeomagnetic studies of the
Hamersley Range Megasequence have revealed only
remagnetized NRM directions, whereas the Chichester
Range Megasequence and associated dolerite dykes appears
to have preserved a primary NRM.
2. Aims of the Study
[12] Important aims of our palaeomagnetic study are
summarized as follows: (1) to demonstrate that there is a
primary NRM in the succession; (2) to test the proposed
correlations by Blake [2001] between mafic dykes and flood
basalts in the area; (3) to establish a magnetostratigraphy for
a late Archaean flood basalt succession; (4) to look for
evidence of geomagnetic reversals in the succession; and
(5) to establish a series of palaeomagnetic poles within a
single, coherent stratigraphic succession for the period
27752715 Ma and thus to construct a high-precision
APWP for the Pilbara Craton for this time period.
3. Geological Setting
[13] The Mount Bruce Supergroup is a late Archaean to
Early Proterozoic succession of volcanic and sedimentary
rocks, unconformably overlying the circa 3.5 – 2.85 Ga
‘‘granite-greenstone’’ terrain of the Pilbara Craton. The
Supergroup has been divided into three groups, which, from
old to young, are the Fortescue Group, the Hamersley
Group, and the Turee Creek Group [e.g., MacLeod et al.,
1963; Trendall, 1979; Thorne and Trendall, 2001]. An
unconformity-based framework for part of the lower succes-
sion of the Hamersley Province (i.e., most of the Fortescue
Group or the Nullagine and Mount Jope Supersequences;
Figures 1 and 4) was initiated in the 1980s to early 1990s
[see Blake, 1993]. More recently, Blake [2001] extended the
unconformity-based divisions in the Nullagine Synclinorium
of the east Pilbara and this framework is followed herein
(Figures 2, 3, and 4; see Blake [1993, 2001] for details).
[14] The regional geotectonic evolution of the Nullagine
and Mount Jope Supersequences is presented in detail by
Blake [1993, 2001] and is summarized as follows. The two
supersequences are interpreted as the rock record of a two-
phase continental breakup in the late Archaean. The first
phase, represented by the Nullagine Supersequence, was
dominated by WNW-ESE directed extension (in present
coordinates) that resulted in the formation of large volumes
of mostly subaerial basalt, followed by the development of
extensional intracontinental sedimentary basins with asso-
ciated felsic and mafic volcanics, and an ocean may have
formed to the west of the present craton. The second phase
of breakup, represented by the Mount Jope Supersequence,
involved rifting of the southern Pilbara Craton margin,
possibly along an earlier transfer fault, resulting in the
eruption of large volumes of basalt, with minor felsic
volcanism. In the north, the craton was buried beneath
subaerial flood basalts and mafic tuff piles, whereas in the
Figure 1. Simplified geological map of the Pilbara, showing the locations of the Beasley River Region,
the Black Range Dyke, the Meentheena Centrocline, the Millindinna Complex, the Nullagine
Synclinorium, and the approximate location of Figure 2. The Hamersley Province comprises most of
the southwestern half of the map.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -3
south it was buried beneath a dominantly submarine mafic
succession.
[15] Figures 2 and 3 summarize the geology of the
Nullagine and Mount Jope Supersequences (and part of
the Marra Mamba Supersequence Package) in the Nullagine
Synclinorium. The succession comprises basalt, porphyritic
felsic lava domes (?), mafic and felsic tuffs, and terrigenous
clastic sedimentary rock. It is metamorphosed to very low
grade (prehnite-pumpellyite-epidote zone (<200 mPa,
<300C) [Smith et al., 1982]), gently folded by two gen-
Figure 2. Summary geological map of the Nullagine Synclinorium showing dominant lithologies,
related mafic dykes [after Blake, 2001], and sample locations (see Table 1).
EPM 2 -4 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
Figure 3. Summary stratigraphic column (cumulative maximum stratigraphic thickness) of the
Nullagine and Mount Jope Supersequences and the lower Marra Mamba Supersequence package in the
Nullagine Synclinorium [after Blake, 2001]. The figure includes (from left to right) the approximate
stratigraphic position of the Meentheena Centrocline Package 1 samples (GSP61-67), the stratigraphic
position of palaeomagnetic sample sites in the Nullagine Synclinorium, dominant lithologies, package
numbers, summary rock ages, second-order divisions, and comagmatic dyke and growth-fault
orientations (with palaeomagnetic sample site numbers). Correlations with the lithostratigraphic scheme
and a regional stratigraphic context are given in Figure 4. Each basalt sample number tie line represents a
single flow unit; two basalt sample sites with a single tie line represent two sample sites in the same flow
unit. The exact stratigraphic positions of Sites GSP 34 –37 were not recorded but they are shown in their
correct relative stratigraphic positions. Positions of samples GSP50, GSP 173, GSP183, and GSP 184 are
estimates of their positions within the stratigraphic section of the dacite porphyry. BRS, Black Range
Suite; CCS, Castle Creek Suite; FMCS, Five Mile Creek Suite; MC, Meentheena Centrocline; MM,
Marra Mamba Supersequence Package; MMS, Mount Maggie Suite.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -5
Figure 4. A comparison of stratigraphic subdivisions of the lower succession of the Hamersley Province in the Nullagine
Synclinorium [after Blake, 2001]. The right two columns are after Hickman and Lipple [1978], Thom et al. [1979], and
Thorne and Tyler [1996]. Sequence stratigraphic/lithostratigraphic correlations are simplified.
EPM 2 -6 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
Table 1. Summary of High Temperature Results From the Nullagine Synclinorium and the Meentheena Centrocline
a
Area Package/Dyke Site Number Lithology AMG Coordinates (AGD 84) NnDec Inc Ka95 R Plat VGP Lat VGP Long
NS 10 U904 basalt 50K 0803892-7529581 10 10 152.6 46.7 52.3 6.7 9.828 27.9 64.6 191.5
NS 10 U907 basalt 50K 0794956-7533268 10 8 147.2 48.6 49.4 8.0 7.858 29.6 59.7 189.0
NS 10 U905 basalt 50K 0800384-7528231 12 8 158.9 55.9 77.0 6.4 7.909 36.4 66.9 167.5
NS 10 mean 3 26 153.3 52.1 243.5 7.9 2.992 31.3 64.1 183.5
NS 9 U908 basalt 50K 0790365-7538167 14 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 9 U909 basalt 50K 0790358-7538230 15 4 149.0 39.3 19.7 21.2 3.848 22.3 61.4 203.9
NS 9 mean 1 4 149.0 39.3 n/a n/a n/a 22.3 61.4 203.9
KB CCS U912 dolerite 51K 0211344-7562821 27 14 127.0 53.5 102.9 3.9 13.874 34.0 42.2 183.6
KB CCS mean 1 14 127.0 53.5 n/a n/a n/a 34.0 42.2 183.6
NS 8 U902 basalt 50K 0801760-7550680 11 2 147.2 43.4 n/a n/a 1.997 25.3 59.9 197.3
NS 8 U901 basalt 50K 0783579-7542746 10 7 141.5 50.0 342.6 3.3 6.982 30.8 54.7 187.4
NS 8 U903 basalt 50K 0804763-7554167 15 10 142.2 50.5 80.6 5.4 9.888 31.2 55.2 186.6
NS 8 mean 3 19 143.2 48.0 292.1 7.2 2.993 29.1 56.7 190.1
KB FMCS U913 mafic-rich dolerite 51K 0213166-7563166 36 19 155.8 58.0 40.2 4.8 18.526 38.7 63.4 165.9
KB FMCS mean 1 19 155.8 58.0 n/a n/a n/a 38.7 63.4 165.9
NS 7 U900 basalt 50K 0777731-7542868 10 10 175.9 77.2 884.4 1.6 9.990 65.6 46.6 122.2
NS 7 U899 basalt 50K 0778502-7544894 11 8 132.0 75.7 564.4 2.3 7.988 63.0 38.2 145.1
NS 7 U896 basalt 51K 0190962-7558533 10 3 164.4 57.9 47.8 18.0 2.958 38.6 68.8 155.5
NS 7 U898 mafic tuff 50K 0792063-7561302 10 7 181.3 72.6 62.6 7.7 6.904 57.9 54.1 118.7
NS 7 mean 4 28 160.8 71.7 56.3 12.3 3.947 56.3 52.8 133.7
NS 6 U897 basalt 50K 0809522-7566847 10 5 104.6 67.0 49.6 11.0 4.919 49.7 25.9 164.1
NS 6 U888 basalt 51K 0197002-7571844 10 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 6 U887 basalt 51K 0197465-7572443 10 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 6 U886 basalt 51K 0197449-7572544 10 4 165.9 61.7 96.8 9.4 3.969 42.9 66.0 146.1
NS 6 U884 basalt 51K 0197264-7572677 10 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 6 U889 basalt 51K 0198233-7572870 11 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 5 U891 basalt 51K 0198573-7573091 10 2 170.7 77.4 n/a n/a n/a 65.9 45.6 125.5
NS 5 & 6 mean 3 11 145.6 71.4 28.9 23.4 2.931 52.8 47.1 147.1
NS 4 U915 basalt 50K 0796304-7572341 26 7 167.6 75.1 120.4 5.5 6.950 62.0 49.1 128.7
NS 4 U916 basalt 50K 0796058-7572405 5 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 4 GSP52 mafic tuff 51K 0199348-7577225 9 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 4 mean 1 7 167.6 75.1 n/a n/a n/a 62.0 49.1 128.7
NS 3 GSP51 terrigenous mudrock 51K 0196019-7584831 4 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 3 mean 00
NS 2 GSP50 massive dacite porphyry 51K 0198794-7589321 14 10 332.4 70.1 29.2 9.1 9.692 54.1 51.5 326.0
NS 2 GSP173 massive dacite porphyry 51K 0201271-7588274 7 7 318.4 74.7 78.4 6.9 6.923 61.3 41.2 325.2
NS 2 GSP184 Massive dacite porphyry 51K 0201617-7588455 7 1 313.6 63.7 n/a n/a n/a 45.3 45.6 346.8
NS 2 GSP183 Massive dacite porphyry 51K 0201729-7588543 7 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 2 mean 3 18 320.9 69.7 153.7 10.0 2.987 53.6 46.5 332.7
NS 2 GSP33 conglomerate containing Package 1 basalt clasts 51K 0192952-7608690 40 26 n/a n/a n/a n/a n/a n/a n/a n/a
NS 2 U895 conglomerate containing Package 1 basalt clasts 51K 0192871-7608499 10 0 n/a n/a n/a n/a n/a n/a n/a n/a
MC 1 GSP67 basalt 51K 0212666-7628329* 7 6 130.1 73.0 255.9 4.2 5.980 58.6 38.6 151.0
MC 1 GSP66 basalt 51K 0212666-7628329* 7 7 142.1 71.5 4898.0 0.9 6.999 56.2 45.4 149.4
MC 1 GSP65 basalt 51K 0212666-7628329* 7 6 136.6 67.2 748.0 2.5 5.993 49.9 45.6 159.5
MC 1 GSP64 basalt 51K 0212666-7628329* 7 5 137.9 70.7 117.2 7.1 4.966 55.0 44.1 152.6
MC 1 GSP63 basalt 51K 0212600-7628194* 7 7 129.1 70.5 161.5 4.8 6.963 54.7 39.6 155.8
MC 1 GSP62 basalt 51K 0212600-7628194* 7 6 132.9 70.4 175.6 5.1 5.972 54.5 41.7 154.9
MC 1 GSP61 basalt 51K 0212600-7628194* 7 7 129.4 66.5 1014.7 1.9 6.994 49.0 41.6 162.9
NS 1 GSP37 basalt 51K 0194857-7608645* 5 4 124.8 60.2 481.8 4.2 3.994 41.1 39.9 173.8
NS 1 GSP36 basalt 51K 0194857-7608645* 5 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS 1 GSP35 basalt 51K 0194857-7608645* 5 0 n/a n/a n/a n/a n/a n/a n/a n/a
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -7
erations of folds, and cut by a complex array of normal
faults [Blake, 2001].
[16] The approximate 6-km-thick (cumulative maximum
thickness) succession has been divided into 11 unconformity-
bound packages and an upper package with a basal uncon-
formity (Figure 3). The packages have been numbered
sequentially from oldest to youngest. Each package is inter-
preted as a cogenetic unit, but, importantly, packages are not
hierarchically identical. Package 1 is dominated by subaerial
basalt, Package 2 is dominated by clastic sedimentary rock
and felsic porphyry, and Packages 3 and 4 are dominated by
clastic sedimentary rock with minor subaerial basalt near the
top of Package 4 [Blake, 1993, 2001]. Packages 5 – 10 are
dominated by basalt and mafic tuff but also contain minor
felsic tuff bands. Package 11 is a marine succession that
comprises mostly laminated mudrock with subordinate
reworked crystal-rich felsic tuff, carbonated mafic volcaren-
ite, and chert. Package 12 is a marine succession that
comprises laminated carbonaceous mudrock that passes
gradationally upward into ferruginous laminated chert.
[17] In addition to extrusive mafic rocks, there are four
physically and geochemically distinct mafic dyke suites in
the area, each with a characteristic orientation (Figure 2).
These are the Black Range Suite [Blake, 1993] and the
Mount Maggie, Five Mile Creek, and Castle Creek Suites
[Blake, 2001]. Each dyke suite has been matched geochem-
ically to basalts within the Nullagine or Mount Jope Super-
sequences in the Nullagine Synclinorium by Blake [2001].
[18] Initial dating of the Hamersley Province succession
was dominated by the use of the Rb-Sr whole rock and
mineral isochron methods, which yielded mostly mean ages
of 2300–2000 Ma [see summaries by Trendall, 1983; Blake
and McNaughton, 1984; see also Nelson et al., 1992].
Pidgeon [1984] published the first conventional zircon
U-Pb age of 2768 ± 16 Ma for the felsic porphyry in
Package 2 in the Nullagine Synclinorium (Figure 3), and
Arndt et al. [1991] provided the first SHRIMP zircon U-Pb
ages for the Fortescue Group. These zircon U-Pb ages
established a broad geochronological framework and
showed that Rb-Sr and Pb-Pb isochrons obtained from
igneous rocks probably date younger hydrothermal and/or
metamorphic events rather than the age of magmatism
[Blake and McNaughton, 1984; Nelson et al., 1992].
[19]Wingate [1999] determined a mean ion microprobe
baddeleyite
207
Pb/
206
Pbageof2772±2Mafromfour
samples, each from a different dyke of the Black Range Suite
including the Black Range Dyke itself. Geochemical and
geological correlations [Blake, 2001] suggest that Package 1
basalt in the Nullagine Synclinorium is comagmatic with the
Black Range Suite and hence is of the same age (Figure 3).
Blake et al. (submitted manuscript, 2003) [see Blake, 2001]
have obtained 11 high-precision SHRIMP zircon U-Pb ages
from rocks within the Nullagine Synclinorium covering
Packages 2–11. Package 2 is dated at circa 2766 Ma [cf.
Pidgeon, 1984], the upper part of Package 4 is dated at circa
2752 Ma, Package 5 is dated at circa 2741 Ma, and Packages
7–10 were deposited between circa 2725 and 2715 Ma
(Figure 3).
4. Sampling
[20] Four hundred and seventy-one oriented drill cores
(25mmdiameter,5–6cmlong)weretakenfromthe
Area Package/Dyke Site Number Lithology AMG Coordinates (AGD 84) Nn Dec Inc Ka95 R Plat VGP Lat VGP Long
NS 1 GSP34 basalt 51K 0194857-7608645* 5 2 126.3 71.4 n/a n/a 1.973 56.1 37.8 154.8
NS 1 U894 basalt 51K 0194425-7608523 10 7 125.7 63.3 139.1 5.1 6.957 44.8 40.1 168.9
NS 1 U892 basalt 51K 0194045-7608483 10 7 120.2 58.3 327.0 3.3 6.982 39.0 36.5 176.8
NS 1 U893 basalt 51K 0193839-7608409 10 10 129.1 70.0 176.0 3.7 9.949 53.9 40.0 156.6
NS + MC 1 mean 12 74 129.6 67.9 231.0 2.9 11.952 51.1 41.2 159.9
NS BRS U914 gabbro 51K 0201215-7570357 5 0 n/a n/a n/a n/a n/a n/a n/a n/a
NS BRS U917 dolerite 51K 0199583-7561500 19 3 120.9 69.7 190.8 8.9 2.990 53.5 35.8 159.1
NS BRS mean 1 3 120.9 69.7 n/a n/a n/a 53.5 35.8 159.1
a
Results are in stratigraphic order, with each comagmatic dyke placed at the base of its associated package. Meentheena Package 1 data are placed above the Nullagine data (in stratigraphic order) for convenience
only; exact correlations are uncertain. Correlation of Meentheena Package 1 and Nullagine Package 1 basalts is based on relative stratigraphic position, flow morphologies, and petrographic characteristics. Bold type
indicates means. Abbreviations: AMG, Australian Map Grid; N, number of specimens from each site and number of sites contributing to the mean; n, number of specimens accepted and the number of specimens
contributing to the mean; dec, declination; inc, inclination; Plat, palaeolatitude; VGP lat (long), latitude (longitude) of VGP; n/a, not applicable; KB, Kurrana Batholith; MC, Meentheena Centrocline; NS, Nullagine
Synclinorium; CCS, Castle Creek Dyke Suite; FMCS, Five Mile Creek Suite; BRS, Black Range Suite. Identical AMGs with asterisk indicate sequences of sites in basalt flows directly on top of each other. Dec, Inc,
a95, Plat, VGP Lat and VGP Long in degrees.
Table 1. (continued)
EPM 2 -8 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
supracrustal succession in the Nullagine Synclinorium and
associated comagmatic dykes in the area (Figure 3 and
Table 1). Mafic dyke samples were taken from the Black
Range, Five Mile Creek, and Castle Creek Suites (Figure 3)
[Blake, 2001]. The Mount Maggie Suite (Figure 3) [Blake,
2001] was not sampled because it is poorly exposed.
Samples were collected using a petrol drill, with an average
of nine cores per site for the supracrustal succession. Four
oriented hand samples were collected (Site GSP51) and
cored in the laboratory (two cores per hand sample). In
addition, to enable a fold test for Package 1, 49 cores were
collected from 7 sites from basalts of a stratigraphic
equivalent to Package 1 in the Meentheena Centrocline
(Figures 1, 3, and 4).
[21] Sample sites were chosen carefully to allow collection
of fresh in situ rock in areas least likely to have been struck by
lightning. Most supracrustal samples are of flood basalts,
with fewer samples from massive dacite porphyry, terrige-
nous mudrock, mafic tuff, and conglomerate (Table 1).
Packages 5 and 6 were sampled as one unit because the
Figure 5. Examples of characteristic Zijderveld diagrams from each package and sampled comagmatic
dykes. Key temperature steps are shown. Specimen numbers indicate location (Figure 2 and Table 1)
followed by sample number. Figures 5a, 5b, 5f, 5h, 5j, 5k, and 5l show two components of magnetization,
and Figures 5c, 5d, 5e, 5g, and 5i show three components.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -9
distinction between these packages had not been made at the
time of sampling.
5. Methods
[22] In the field, a susceptibility meter (Scintrex K2) was
used as an indication for magnetic intensity of the sampled
rocks in the field. For low values, a magnetic compass was
used to orient the samples, and for higher values, as a
precaution a sun compass was used in addition to the
magnetic compass. No significant difference was recorded
between magnetic and sun compass directions (between 0
and 2in all cases).
[23] In the laboratory, the cores were cut to standard
specimens of 22-mm length. The anisotropy of magnetic
susceptibility was measured on an AGICO KLY 3 suscep-
Figure 6. Equal-area plots of results from all studied packages and dykes. Figures 6a, 6b, 6d, 6e, 6f, 6g,
and 6h represent equal-area plots of multiple sites. N, total number of sites with accepted data, small
symbols are site means, larger symbols are the mean of site means with the corresponding a95 circles
(where applicable). Figures 6c, 6i, 6j, and 6k represent single sites, n, total number of specimens, larger
symbols are the sample means showing error circles. Open (solid) symbols indicate negative (positive)
inclinations, circles are high-temperature directions, squares are medium-temperature directions, and
triangles are low-temperature directions. Data are shown in Tables 1 and 2.
EPM 2 -10 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
tibility bridge, which showed that the samples are not
significantly anisotropic (range from 0.3 to 3.4%). At least
one specimen and occasionally two specimens of each drill
core were thermally demagnetized in a magnetically shielded,
laboratory-built furnace. The NRM was measured on a 2G
Enterprises DC superconducting quantum interference de-
vice (DC-SQUID) cryogenic magnetometer. Eleven speci-
mens were demagnetized using the alternating field (AF)
technique, where demagnetization was performed in a labo-
ratory-built AF demagnetizer up to a peak field of 200 mT.
[24] NRM directions were analyzed and interpreted with
Zijderveld diagrams [Zijderveld, 1967]. Principal compo-
nent analysis [Kirschvink, 1980] was used to determine the
directions of the various NRM components. Mostly,
no directions with a maximum angular deviation (MAD)
greater than 10were used, except for very weak samples.
6. Results
6.1. Thermal Demagnetization
[25] The NRM of all rock units shows either two or three
components. A low-temperature (LT) component was
recorded in the temperature range of 20to 300– 350C;
this component was well developed in Packages 2, 7, 9, and
10 and in the Five Mile Creek Suite. A medium-temperature
(MT) component was recorded from 350C to approximately
450C; this component is well developed in specimens
from Package 7 upward and in the Five Mile Creek Suite.
A high-temperature (HT) component was recorded from
450to 580C; this component is well developed in almost
all rock units and rock types, except in Packages 3 and 9.
[26] The LT component records the present-day field
direction (Figures 5 and 6), which is generally only apparent
in samples with a weak magnetic intensity of the HT
component. All samples are of fresh rock, so the LT
present-day field component is unlikely to be caused by
weathering. Possibly, a (small) part of the NRM has been
reset in the ambient field by high surface rock temperatures
(air temperatures commonly exceed 40C in summer) and is
recorded by low coercivity or low unblocking temperature
magnetite. For example, the normalized intensity versus
temperature plot of sample U892.2A (Figure 7c) shows a
two-step intensity decay, with 20% intensity loss between
the 20and 300C steps, which is when the LT component
unblocks. Between 300and 500C only a gradual intensity
decrease is observed and from 500C the HT component
effectively starts unblocking in this sample.
[27] The following paragraphs describe the palaeomag-
netic results per package and dyke suite. Confer Table 1 for
statistical details.
[28] Package 1 has a well-developed HT component and
magnetizations of relatively high magnetic intensity (on
average 50 10
3
Am
1
, whereas most packages have
an average magnetic intensity of 5 – 10 10
3
Am
1
,
although compared to modern basalts, these intensities are
all exceptionally low). Except for Sites GSP35 and GSP36,
which have been affected by lightning, the distribution of
high-temperature directions from both the Nullagine Syn-
clinorium and Meentheena Centrocline samples is very well
clustered (Figure 6a). LT and MT components were poorly
developed or did not have consistent directions in this
package (e.g., Figure 5a).
[29] The Package 2 felsic porphyry showed an HT
component with a consistent direction, which is antipodal
to the HT components of other packages (Figures 5b and 6b).
The MT directions were poorly defined, and the LT
directions could rarely be determined.
[30] Most exposed rocks in Package 3 were unsuitable for
palaeomagnetic sampling. An attempt was made to acquire
data from a siltstone (GSP51) but no reliable HT compo-
nents were derived.
Figure 7. Examples illustrating the effects of lightning
strikes. (a) Zijderveld diagram of a basalt sample struck by
lightning, showing a characteristic steep intensity decrease
at low temperatures. (b) Zijderveld diagram of a basalt
sample not struck by lightning, showing a gradual intensity
decrease up to 490C followed by a steep decrease up to
580C. (c) Normalized intensity versus temperature of
selected basalt samples. The dashed lines show the intensity
decay of representative samples struck by lightning (speci-
mens U894.10, U896.6, U915.11, .15, .16, and .19). Solid
lines show the intensity decay of samples not struck by
lightning (specimens U892.2, .7A, and .7B, U893.1, .2, and
.5, and U899.5). Specimen U892.2 is pointed out to show
two-step unblocking: the LT component unblocks between
20and 300C and the HT component unblocks between
500and 580C. Conventions of Figures 7a and 7b as in
Figure 5, see Figure 2 for specimen locations.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -11
[31] In Package 4, a mafic tuff horizon (GSP52), and a
single basalt flow (U915, U916) near the top of this
succession were sampled. The mafic tuff revealed only
present-day field directions and was unsuitable for thermal
demagnetization (i.e., they exploded above 350– 400C).
Four specimens of the basalt flow revealed a low-temper-
ature present-day field direction (e.g., Figure 5c) and
only seven specimens revealed a consistent HT direction
(Figure 6c). The remaining samples from this package were
affected by lightning.
[32] No well-defined component could be derived from
basalt samples from Packages 5 to 6 because the HT
component is largely overprinted by an MT component
of inconsistent direction, resulting in an unreliable end
vector at high temperatures. Out of seven sites, with a total
of 71 samples (Table 1), only three sites, with a total of
11 samples, revealed meaningful results (e.g., Figure 6d).
[33] Most samples of Package 7 were taken from flood
basalts and one sample set was taken from a mafic tuff
(U898). Individual sites show a good clustering of the data
(e.g., Site U899, with k= 564, Table 1), but the package
mean direction has a larger spreading (k= 56, Table 1). In
the flood basalts, except for Site U896, all three NRM
components are developed (Figures 5e and 5f) and cluster
well (Figure 6e). While some mafic tuff samples are
affected by a present-day field overprint, the MT and HT
components are clearly present.
[34] In Package 8, the MT and HT directions form well-
defined clusters, and the present-day field overprint is
generally poorly developed, but some samples show all
three NRM components, as illustrated in Figure 5g. Data
from individual sites cluster well as does the package mean,
both for the HT and MT directions.
[35] Suitable fresh exposure of Package 9 basalt was
not found in the Nullagine Synclinorium. The basalt is
commonly strongly weathered because of its coarse grain
size [Blake, 2001, Table 6]. Despite this problem, the
few samples from a single basalt flow unit in Package 9
(Figure 3) yielded all three NRM components (e.g.,
Figures 5h and 6g). The MT component is well developed
at both sites, although the HT component is only consistent
at Site U909 (Figure 6g).
[36] In Package 10, all three components of magnetiza-
tion are recorded (e.g., Figure 5i) and their directions form
well-defined clusters (Figure 6h). Occasionally, the NRM
could not be completely removed, giving scattered direc-
tions at temperatures of 560C or higher, possibly because
of growth of new magnetic minerals during thermal
demagnetization.
[37] The Black Range Suite was sampled at two locations
(U914, U917). The Cajuput Dyke (Figure 2; Site U914) was
difficult to drill because it contains abundant quartz. The
few samples from this site revealed only a present-day field
direction. A smaller dyke from the same suite (Site U917)
is finer grained. The demagnetization diagrams for this
site generally show an HT and an LT component (e.g.,
Figure 5j), of which only a few consistent and reliable HT
directions could be derived (Figure 6i). The LT directions
do not cluster.
[38] The Five Mile Creek Suite was sampled at one
location (Site U913), and all three magnetization compo-
nents are present (e.g., Figure 5k). The HT component is
commonly present and its directions cluster reasonably
(k= 40), while also the LT and MT components form
acceptable clusters (Figure 6j).
[39] The Castle Creek Suite was sampled at one location
(Site U912) and shows MT and HT components of mag-
netization (Figure 5l). The MT component is poorly devel-
oped and does not cluster, whereas the HT component is
easily distinguished and its directions show clear clustering
(Figure 6).
6.2. Lightning
[40] Previous work has shown that exposed rocks in the
Pilbara have commonly been struck by lightning [Schmidt
and Embleton, 1985; Schmidt and Clark, 1994]. Lightning
strikes can destroy or partly reset NRM [Hallimond and
Herroun, 1933]. Lightning strikes have been shown to
disturb the remanent magnetization of a surface area by
as much as 25 m
2
[Graham, 1961] and can affect rocks
down to 25 m below the surface. However, the original
remanent magnetization can usually be detected at a depth
of 90–160 cm in areas struck by lightning. Since lightning
commonly strikes on high and/or open places, sampling
sites were chosen in narrow and deep-cutting gullies or on
steep rock faces.
[41] Following several pilot studies (T. van Hoof, per-
sonal communication, 1997), careful field procedures have
resulted in only 10% of the samples showing a lightning-
induced remanent magnetization. Lightning has the follow-
ing effects on NRM, which proved to be diagnostic in the
recognition of lightning-induced NRM.
[42] 1. The intensity of the magnetic signal is usually
much stronger (commonly by more than an order of
magnitude) than the intensity of the magnetic signal of the
same rock unaffected by lightning.
[43] 2. Samples affected by lightning generally reveal
only one component (e.g., compare Figure 7a with Figure 7b).
[44] 3. The direction of this one component compared to
other samples from the same site or location is random, so
sampling an area around a point which has been struck by
lightning should reveal a set of random directions [Graham,
1961].
[45] 4. The (normalized) magnetic intensity is observed
to decrease rapidly with increasing temperature, whereas
the magnetic intensity of (magnetite bearing) rocks unaf-
fected by lightning shows a gradual decrease until about
525–550C and then a steep decrease close to 580C
(Figure 7c).
6.3. Field Tests
[46] To help constrain the time at which the NRM was
acquired, three palaeomagnetic field tests were carried out,
namely a fold test, a reversal test, and a conglomerate test.
[47] A fold test could not be achieved in the Nullagine
Synclinorium because the succession dips too shallowly
(mostly <5) and is only gently folded. However, HT data
from Package 1 in the Nullagine Synclinorium can be
compared with HT data from a stratigraphic equivalent of
Package 1 in the Meentheena Centrocline (Figure 1) to
define a fold test. Directions before and after tilt correction
are shown in Figure 8a (data in Table 1). These data do not
pass the fold test according to definition 1 of McFadden
[1990] because statistically the two groups do not belong to
EPM 2 -12 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
the same population, but in an incremental fold test using an
eigenvector approach, they do give better results at 100%
tilt correction [Tauxe and Watson, 1994]; but the best result
is obtained at approximately 130 ± 30% unfolding. The
large error is caused by the small difference in bedding
between Nullagine and Meentheena. One problem in ap-
plying the McFadden fold test is that the precision param-
eter of the Meentheena samples (k
1
= 806) is much higher
than that of the Nullagine samples (k
2
= 182; so k
1
/k
2
= 4.4).
Ideally, the k
1
/k
2
ratio should not be larger than 2. A second
problem is determination of the exact dip of shallow dipping
basalt flows. Flow tops are commonly undulose at several
scales, and tectonic dips are difficult to estimate accurately.
This may explain the misfit of site means after tilt correction
in Figure 8a. However, because the incremental tilt correc-
tion gives better results at 100% unfolding, we conclude
that the HT magnetization was most probably acquired prior
to folding. The age of the folding is not exactly known.
Blake [2001] describes that NNE trending folds are thought
to have formed contemporaneously with the development of
the Nullagine and Mount Jope Supersequences. The age of
the SE trending folds is more problematic, but a Mount Jope
Supersequence age cannot be excluded.
[48] The discovery of magnetic reversals in the HT
component of the succession allowed for a reversal test,
which was carried out using two different approaches. First,
the HT site means of Package 1 (Nullagine Synclinorium
and Meentheena Centrocline data) were compared to those
of Package 2 and the reversal test of McFadden and
McElhinny [1990] was performed (Figure 8b, left). With
and without simulation the reversal test was found to be
positive (classification B), meaning that the antipodal angle
lies between 5and 10. The angle between the mean of the
normal and the mean of the reversed polarity set is 8.0,
which is smaller than the critical angle of 9.2. Second, a
reversal test was performed on all accepted individual HT
sample directions from Packages 1 to 6 (except for the
Package 2 conglomerate samples; Figure 8b, right). This
reversal test is also positive, with the angle between the
mean of the normal and the mean of the reverse polarity set
being 6.0, which is lower than the critical angle of
6.2(resulting in classification B). Although no data are
available for the sedimentary successions of Packages 3 and
4, the reversed interval is stratabound, which is additionally
suggestive of a primary magnetization.
[49] A conglomerate test was carried out on basalt clasts
(cobbles to boulders) in a conglomerate that overlies Pack-
age 1 basalts. Field relationships, petrographic data, and
unpublished geochemical data show that the basalt clasts
were derived from Package 1 basalts. Basalt clasts were
sampled (one core per clast) and thermally demagnetized
(Table 1). The succession above the conglomerate was also
Figure 8. (a) Equal-area plots of HT directions from the fold test before and after tilt correction.
Symbols represent site means. All directions are from Package 1, both from the Nullagine Synclinorium
and from the Meentheena Centrocline. The a95 circle of the Meentheena data are too small (1.9)tobe
noticed on the plot. (b) Equal-area plots of HT directions from the reversal test. The left plot shows the
directions of site averages of Packages 1 and 2 in the Nullagine Synclinorium; the right plot shows the
directions of individual samples of Packages 1, 2, 4, and 5 –6 from the Nullagine Synclinorium. (c) Equal-
area plots of HT directions from the conglomerate test, taken at three different levels. All directions are
from specimens of Package 2, Site GSP33 (Figure 2). Conventions as in Figure 6.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -13
sampled (Package 2 massive dacite porphyry to Package 10),
making this conglomerate test an intraformational con-
glomerate test [McElhinny and McFadden, 2000]. The test
was carried out at two sites approximately 100 m apart. The
characteristic remanent magnetization (ChRM) of speci-
mens from the first site (U895; Figures 2 and 3) shows no
preferred orientation. However, from the strong magnetic
intensity of these samples and from the intensity decay
curves we interpret these samples as having been struck by
lightning.
[50] The second site (GPS33; Figures 2 and 3) was
sampled at the surface and, following blasting, at depths
of 25 and 70 cm below the original surface (Figure 8c).
Samples from basalt clasts at this site mostly have much
lower magnetic intensities than the U895 basalt clast sam-
ples. In addition, the magnetic intensity of these basalt clasts
was mostly two orders of magnitude lower than the inten-
sities of the underlying Package 1 basalts, an observation
similar to that of Shipunov et al. [1998]. Of a total of 40
samples, 14 (35%) are interpreted as affected by lightning.
Of the 21 basalt clasts sampled at the surface, 8 (38%) are
affected by lightning. At 25 cm below the surface, 3 (30%)
of the 10 clast samples are affected by lightning. At 70 cm
below the surface, three (33%) of the nine clast samples are
affected by lightning. Hence blasting to a depth of 70 cm
made little difference to the percentage of samples affected
by lightning.
[51] The ChRM of the samples of GSP33 from the
surface that were not affected by lightning has no preferred
orientation (Figure 8c). The test for uniform randomness of
Watson [1956] requires the resultant (R)ofallNindividual
directions to be smaller than a critical value R
0
. In this case,
N= 13, R
0
= 5.75, and R= 3.150, so R<R
0
, meaning that
there is at least a 95% probability that the directions come
from a random population. Further, an alternative and more
sensitive conglomerate test by Shipunov et al. [1998], which
allows for the inclusion of a known secondary component
or reference vector for the studied area, is also positive, with
r
0
= 0.263 and r= 0.207, so r<r
0
. The reference vector
used in the calculations is 132.5/68.0, the average direc-
tion of NRM of Packages 1, 4, and 5 –6.
[52] The ChRMs from 25 cm below the surface give a
negative Watson test for uniform randomness and the test of
Shipunov was also negative, with N=7,R
0
= 4.18, R=
4.939, r
0
= 0.362, and r= 0.686 (Figure 8c). The ChRMs
from the 70 cm below the surface also did not pass the
Watson test for uniform randomness, with N=6,R
0
= 3.85,
and R= 4.211. However, the test of Shipunov with r
0
=
Figure 9. Aitoff projection of the apparent polar wander path (APWP) for the Pilbara from circa 2860
to circa 1700 (?) Ma showing approximate pole ages. Dashed part of the APWP is speculative. The
APWP is based on this study and published data. Square symbols are poles not included in the APWP.
Circular symbols are poles included in the APWP, of which the black solid circles are poles from this
study. Poles comprising the APWP are MC (Millindinna Complex [Schmidt and Embleton, 1985]), P1
(Nullagine and Meentheena Package 1 HT data and the Black Range Suite HT data), P2 (Package 2 HT
data), P4–7 (Packages 4–7 HT data), P8– 10 (Packages 8 – 10 HT data and Five Mile Creek Suite and
Castle Creek Suite HT data), MT7-10 (medium-temperature directions), TPA (Mt. Tom Price iron ore
[Schmidt and Clark, 1994]), and HP2 (Hamersley Province F3 pole [Li et al., 2000]). Each package pole
is a grouped mean of the mean site data, including comagmatic dykes (cf. Table 3). Poles not included in
the path are BR (Black Range Dyke) and CD (Cajuput Dyke [Embleton, 1978]), RB (Mount Roe basalt)
and JV (Mount Jope Volcanics, [Schmidt and Embleton, 1985]).
EPM 2 -14 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
0.392 and r= 0.258 is positive. When all accepted samples
from the surface, 25- and 70-cm depth are pooled, both the
test of Watson (N= 26, R
0
= 8.18, and R= 9.621) and the
test of Shipunov (r
0
= 0.186 and r= 0.348) are negative.
[53] The results of the conglomerate test after the blasting
are unclear, and it is possible that the ChRM has been
(partly) reset by the shock wave from the blasting (shock-
induced remagnetization?). Although the clasts from 70 cm
below the surface pass the test of Shipunov, the negative
results from 25 cm below the surface make the validity of
the data after the blasting uncertain and are thus disre-
garded. However, since the ChRM in the basalt clasts not
struck by lightning from the surface is random, the con-
glomerate test is interpreted as positive. Together with the
positive fold test and positive reversal test, we conclude that
the ChRM preserved in the rocks from Packages 1 to 10 is a
primary NRM.
[54] Thus the HT component of the accepted basalt,
dolerite, and dacite porphyry samples is interpreted to have
been acquired during or shortly after the cooling of the
rocks in the late Archaean and is a record of the geomag-
netic field at that time. The HT component of the accepted
mafic tuff samples (U898) is interpreted as the result of
lock-in of free magnetite grains during consolidation of the
tuff and is also a record of the geomagnetic field in the late
Archaean.
7. Discussion
7.1. Dyke to Package Correlations
[55] From earlier palaeomagnetic correlations it could be
deduced that the Black Range Suite (BR in Figure 9) was
possibly of similar age as the Chichester Range Megase-
quence. This follows from data presented by Schmidt and
Embleton [1985], although they used a different lithostrati-
graphic terminology. New geochemical evidence shows that
the Black Range Suite is most probably comagmatic with
Package 1 [Blake, 2001]. This is supported by the palae-
omagnetic data presented herein (Table 1). Further, the Five
Mile Creek Suite correlates geochemically to Package 8 and
the Castle Creek Suite to Package 9 or 10 [Blake, 2001].
From our palaeomagnetic data, these correlations are less
apparent (Table 1), but when the results of Packages 8–10
are averaged as one group (see below), the Five Mile Creek
and Castle Creek Suite results fall in this group and do not
contradict the proposed correlations. More sampling of the
dykes is needed, however, to convincingly confirm the
proposed correlations palaeomagnetically.
7.2. MT and HT Components
[56] The evaluation of all package means, including both
the HT and the MT directions (if applicable), with their
corresponding a95 made it apparent that there are five
distinct groups of data (Tables 1 and 2) which have
significantly different mean directions (Table 3). These
groups are (1) P1, which comprises the HT directions of
Package 1 samples plus the HT directions of Black Range
Dyke Suite samples; (2) P2, which comprises the HT
directions of Package 2 samples; (3) P4-7, which comprises
the HT directions of Packages 4–7 samples; (4) P8-10,
which comprises the HT directions of Packages 8– 10
samples plus the HT directions of the Five Mile Creek
Suite and the Castle Creek Suite samples; and (5) MT7-10,
which comprises the MT directions of Packages 7 – 10 plus
the MT directions of the Five Mile Creek Suite samples
(Table 2).
[57] On the basis of the positive field tests, the HT
component is interpreted as the primary NRM. It follows
therefore that the age of the HT component is the same as
the age of cooling of the basalt, dolerite, and dacite
porphyry samples. These ages have been determined by
high-precision geochronology and bracket the ages of
the four HT groups as follows. P1 magnetization is circa
2772 Ma, P2 is circa 2766 Ma, P4-7 is bracketed between
circa 2752 Ma and circa 2725 Ma, and P8-10 is bracketed
between circa 2718 Ma and circa 2715 Ma (Figure 9).
[58] The direction of the MT component is in all cases
reversed with respect to the direction of the HT component
and has a relatively shallow inclination, in contrast to the
relatively steep inclination of the HT components. Whereas
Table 2. Summary of Medium Temperature Results From the Nullagine Synclinorium
a
Package/Dyke Site Number Nn Dec Inc ka95 R Plat VGP Lat VGP Long
10 U904 10 6 315.7 25.5 24.9 13.7 5.800 13.4 47.1 34.1
10 U907 10 8 315.3 34.0 34.2 9.6 7.795 18.6 48.1 26.4
10 U905 12 3 307.7 18.4 19.0 29.1 2.895 9.4 38.3 35.6
10 mean 3 17 312.7 26.0 84.2 13.5 2.976 not calculated
9 U908 5 3 324.8 40.3 56.6 16.5 2.965 23.0 57.6 21.7
9 U909 5 3 314.9 43.9 345.8 6.6 2.994 25.7 48.8 15.8
9 mean 2 6 320.0 42.2 n/a n/a 1.995 not calculated
8 U902 11 6 326.9 38.0 103.0 6.6 5.951 21.3 59.3 25.1
8 U901 10 3 333.5 46.0 63.6 15.6 2.969 27.4 65.4 12.2
8 U903 15 8 320.3 31.6 97.1 5.6 7.928 17.1 52.4 30.3
8 Mean 3 17 327.3 39.8 64.6 15.5 2.969 not calculated
FMS U913 36 11 310.1 35.1 24.6 7.6 10.556 19.4 43.4 23.9
FMS Mean 1 11 310.1 35.1 n/a n/a n/a not calculated
7 U900 10 7 327.3 33.4 25.8 12.1 6.767 18.2 59.1 31.0
7 U899 11 7 321.3 52.6 23.2 12.8 6.742 33.2 54.2 3.3
7 U896 10 0 n/a n/a n/a n/a n/a n/a n/a n/a
7 U898 10 6 328.3 43.0 21.4 14.8 5.766 25.0 60.8 17.7
7 Mean 3 20 326.0 43.0 66.5 15.2 2.970 not calculated
Total MT 12 71 320.0 37.1 52.0 6.1 11.788 20.7 53.2 23.9
a
Consistent MT directions were found only in Packages 7, 8, 9, and 10 and the FMCS. Results are grouped by package and comagmatic dyke suite. See
Table 1 for additional sample data and explanation of abbreviations. Dec, Inc, a95, Plat, VGP Lat and VGP Long in degrees.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -15
the HT directions of Packages 7 and 8 are clearly different
(Table 1), the MT directions are similar, and therefore all
MT directions are grouped in MT7-10. Hence MT7-10 is
interpreted as the result of a (re)magnetization that is
younger than the HT magnetization. Remagnetization in
the Pilbara has previously been ascribed to either burial
metamorphism [e.g., Schmidt and Clark, 1994; Sumita et
al., 2001] and/or orogenic activity [e.g., Li et al., 2000].
However, if the MT remagnetization was caused by the
thermal effects of regional-scale burial metamorphism or a
regional-scale orogenesis, it should have been detected
throughout the succession, not in the upper four packages
only. Because no data were acquired higher than Package
10, the full stratigraphic distribution of the MT component
is unknown. While the MT component is interpreted as the
result of a ‘‘thermal’’ event, the exact nature and scale of the
event is unknown.
[59]Li et al. [2000] describe phases of remanence
acquired possibly at the end of the Ophthalmian Orogeny
(circa 2200 Ma) and during the late Ashburton Orogeny
(circa 1700 Ma). The MT component of this study is
different from both of these remagnetizations and hence
cannot readily be ascribed to either orogenic phase with any
certainty. The pole of the MT direction plots approximately
midway between the P8-10 pole (circa 2715 –2718 Ma) and
the circa 1700 Ma pole of the Hamersley BIFs (TPA and
HP2, Figure 9) [Schmidt and Clark, 1994; Li et al., 2000].
However, appropriate field tests should be done before the
age of the MT7-10 magnetization can be constrained more
accurately and more confidently.
7.3. Apparent Polar Wander
[60] Table 1 is an overview of the HT component data of
all sampled rock units. Table 2 summarizes the directions of
MT7-10. Virtual geomagnetic pole (VGP) positions have
been calculated for the HT and MT components, both from
the mean of each site and from the mean of each package
and dyke suite. The palaeomagnetic poles for the grouped
data (Table 3), which were calculated from the VGPs of the
site means, are plotted in Figure 9, which also includes
previously published relevant palaeomagnetic poles (cf. also
Table 3). Although P2 plots within the 95% confidence limit
of both P1 and P4-7, its average pole position plots halfway
between these groups.
[61] There is some overlap of the relevant part of the
previous APWP and the new APWP of this study within
their 95% confidence limit (Figure 9). The previous poles
plot on the APWP, but in an inconsistent order. For
instance, BR, CD, and RB in Figure 9 are all similar in
age (analogues to P1, 2772 ± 2 Ma) yet plot on three
distinctly different positions on the new APWP. BR and
CD have overlap with the P1 pole, but RB overlaps with
P8-10.
[62] In contrast, the palaeopoles from this study follow a
consistent time path and have smaller errors (Table 3).
These differences may be attributed to the limited number
Table 3. Summary of Grouped Palaeolatitude and Palaeopole Positions From This Study and From Previously Published Data
a
Group NDec Inc Min Lat Max Lat Av Lat Latp Longp a95 dp dm
TPA 1 308.8 9.3 0.0 10.6 4.7 37.4 220.3 n/a 5.7 11.3
HP2 9 304.2 18.1 6.9 11.8 9.3 35.3 211.9 3 n/a n/a
MT7-10 12 320.0 37.1 16.7 25.2 20.7 53.2 203.9 5.3 n/a n/a
JV 5 n/a n/a n/a n/a n/a 40.5 128.7 n/a 19.9 20.8
P8-10 9 146.8 49.9 26.2 35.7 30.7 59.1 186.3 6.1 n/a n/a
P4-7 8 157.5 72.2 46.2 70.6 57.3 50.4 138.2 12.5 n/a n/a
P2 3 320.9 69.7 40.6 70.0 53.5 46.5 152.7 15.2 n/a n/a
P1 13 129.0 68.0 47.4 55.0 51.1 40.8 159.8 3.7 n/a n/a
CD 9 145.0 71.0 38.7 78.1 55.4 46.0 146.0 22.0 n/a n/a
BR 16 115.0 72.0 48.3 67.0 57.0 32.0 154.0 9.0 n/a n/a
RB 4(12) 320.0 53.8 28.5 41.2 34.3 52.4 178.0 n/a 6.4 9.1
MC 1(8) 265.0 65.1 40.8 54.4 47.1 11.9 161.3 n/a 6.8 8.4
a
Group name in bold type indicates positions summarized in this study; group name in nonbold type indicates positions summarized in previously
published data. Group definitions are given in the caption to Figure 9 and in the text. Site means contributing to each group mean are given in Tables 1
and 2, and each group is plotted in Figure 9. Nis the number of sites contributing to the grouped mean (site means given in Tables 1 and 2). Site
number in italic type is the number of localities, with number of sites in parentheses. Abbreviations: dec, declination; inc, inclination; min lat, minimum
palaeolatitude; max lat, maximum palaeolatitude; av lat, average palaeolatitude; latp, latitude of pole position; longp, longitude of pole position; a95,
confidence circle at the 95% level; dp and dm define the oval of 95% confidence about the mean pole. All units in degrees.
Figure 10. (opposite) Mean palaeolatitudes for the Nullagine and Mount Jope Supersequences in the Nullagine
Synclinorium and Package 1 of the Meentheena Centrocline. The lower right column shows site mean HT palaeolatitudes
(in degrees) linked to the stratigraphic column. Sample site numbers in grey boxes indicate dykes, and site numbers in bold
indicate Meentheena data. The upper right column shows site mean medium-temperature (MT) palaeolatitudes, with the site
numbers in italics. HT and MT site means are in stratigraphic order, with dyke sites at the base of their comagmatic
package. The exact position of the Meentheena data in this column is uncertain and has been placed above the Nullagine
Package 1 data for convenience only. Site mean palaeolatitudes are represented by a circle with a horizontal bar
representing the a95 [after Fisher, 1953]. Closed (open) symbols represent normal (reversed) intervals. Five groups of
palaeolatitudes (four HT means, one MT mean) have been distinguished, and their means are the means of their constituent
sites (see Table 3). Groups are defined in the text and in the caption to Figure 9. Group means are given by thick black
vertical lines, and their a95 is given by the grey boxes. The five groups have been linked to the stratigraphic column by
dashed lines. N indicates normal polarity, R indicates reversed polarity.
EPM 2 -16 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
of localities, sites, or samples per site in previous studies,
which resulted in larger errors on the mean average
magnetic directions, and hence in less precise pole deter-
minations. From the new APWP (Figure 9), it becomes
apparent that there has been considerable rotation, from at
least Package 1 to Package 7. The distance between P4-7
and P8-10 on the APWP resembles a phase of latitudinal
drift.
7.4. Magnetostratigraphy
[63] Magnetostratigraphic schemes have been developed
for many Phanerozoic flood basalt provinces and have
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -17
proven useful in stratigraphic divisions and correlations
[e.g., Reidel and Hooper, 1989; Marsh et al., 1997;
Saunders et al., 1997; Westphal et al., 1998]. This paper
presents the first magnetostratigraphic scheme for an
Archaean flood basalt succession (Figure 10). Two partic-
ular components of this scheme may prove useful in
assisting stratigraphic correlations within the Nullagine
and Mount Jope Supersequences across the Pilbara Craton.
These are (1) the normal-reverse-normal primary NRM up-
stratigraphy (Figure 10) and (2) the major shift in palae-
olatitude across the Package 7/8 boundary (Figure 10, see
below).
7.5. Implications of Changes in Palaeolatitude
[64] The palaeolatitudes of individual site means and
grouped means are plotted from oldest to youngest in
Figure 10. Although the mean pole positions of groups
P1, P2, and P4-7 differ, they have average palaeolatitudes
that have overlapping errors, which implies that either no
(significant) latitudinal movement has taken place or that it
cannot be resolved with the current amount of data.
[65] There is a significant change in palaeolatitude
(average 27.2) at the Package 7/8 boundary between
P4-7 and P8-10 (Figure 10). Geochemical data from the
Nullagine Synclinorium show that this boundary also
coincides with a major change in the composition of
mafic rocks. For this reason, Blake [2001] split the Mount
Jope Supersequence into two second-order packages in
the Nullagine Synclinorium (Figure 4). The difference in
mean SHRIMP zircon ages from above and below this
boundary is 3 Myr (95% confidence error range of
0–10 Myr, (Blake et al., submitted manuscript, 2003).
Assuming an Earth of current radius, the average shift is
3025 km, which in 3 million years gives a rate of shift of
palaeolatitude of 100 cm per year. However, taking into
account errors in average palaeolatitude and age, the
maximum rate of shift is instantaneous motion (if Pack-
ages 7 and 8 are of the same age) and the minimum
possible rate is a shift of 12 cm per year. Importantly,
these are all minimum estimates since longitudinal shift is
unconstrained.
[66] The geological meaning of this 27.2shift in palaeo-
latitude is unknown, and fundamentally different explan-
ations are possible depending on different assumptions.
These are summarized as follows.
7.5.1. Option 1
[67] The Earth did not have a purely GAD in the late
Archaean. Long-term contributions of nondipole fields and
the change of this contribution through time could generate
an apparent shift in palaeolatitude. For example, if an
octupole contribution to the dipole field changed from 0
to 40%, this would result in an apparent latitudinal shift of
25(cf. Figure 4 of van der Voo and Torsvik [2001]). The
likelihood of long-term nondipole fields through geological
time is a topic of debate [cf. van der Voo and Torsvik, 2001,
and references herein], but as an option it cannot be ruled
out. However, since the shift in palaeolatitude is sustained
in Packages 9 and 10, a sudden change in octupole
contribution (or other nondipole contribution) must have
remained stable for at least circa 3 Myr. This interpretation
has no precedent throughout geological history and is not
regarded as the most likely option.
7.5.2. Option 2
[68] The shift in palaeolatitude is the result of true polar
wander (TPW). The nature and validity of TPW is still a
topic of debate. Kirschvink et al. [1997] argue that fast
apparent motion of continents in the Cambrian was the
result of a 6Myr
1
shift of the Earth’s spin axis with
respect to the geographic reference frame. However, Meert
[1999] finds no compelling palaeomagnetic support for this
model, although the author states that a relatively small
contribution of TPW cannot entirely be ruled out. Courtillot
and Besse [1987] note that TPW is a continuous process and
argue that TPW only comes to a standstill during periods of
low reversal frequency and during episodes of continental
breakup. Aspects of our data are not consistent with TPW
being an explanation of the 27shift in palaeolatitude.
These include an apparent low reversal frequency (see
below) and the relatively fast and sudden shift across the
Package 7/8 boundary. While TPW cannot be discounted,
observations to date do not support it as an explanation for
the documented shift in palaeolatitude. As a special case,
inertial interchange (II) TPW could explain the sudden shift
in palaeolatitude. In this scenario, unbalance of the Earth’s
landmasses cause the lithosphere and mantle to rotate 90
with respect to the Earth’s spin axis [cf. e.g., Kirschvink et
al., 1997], which results in rapid TPW. (II)TPW is a topic of
considerable controversy, which so far has not even been
proven. Therefore we retain a conservative interpretation
and we feel that the burden of proof is with the advocates of
nondipole fields and/or (II)TPW.
7.5.3. Option 3
[69] The Earth had a GAD, and the latitudinal shift is the
result of plate movement. Possible drift rates across the
Package 7/8 boundary (as discussed above) range from fast
Phanerozoic drift rates [e.g., Meert et al., 1993] to drift rates
that are up to an order of magnitude faster than any known
Phanerozoic drift rate. In a Phanerozoic plate tectonic
setting, rapid drift rates are commonly associated with fast
spreading following continental rifting. Blake and Barley
[1992] and Blake [1993] proposed a broad two-stage rifting
to rifted setting for the development of the Nullagine and
Mount Jope Supersequences. Their proposed first stage,
represented by the Nullagine Supersequence, could not be
confirmed by our palaeomagnetic data in terms of latitudinal
drift because the palaeolatitude results of Packages 1 –4 are
not significantly different. However, there is evidence for
rotation during this phase (Figure 9), as is confirmed by a
change in declination from 129.0in P1 to 157.5in P4-7
(Table 3). However, their second stage of rifting is sup-
ported by our palaeomagnetic data. Indeed, the shift in
palaeolatitude between Packages 7 and 8 was used by Blake
[2001] to divide the Mount Jope Supersequence in the
Nullagine Synclinorium into two parts (Figure 3). The
recorded 3025 km of drift in a single event represents a
phase of rapid horizontal movement.
[70] The cause(s) of this possible event is/are unknown.
In a Phanerozoic-style plate tectonic setting the implied drift
distance is not unlikely but the implied great rapidity of drift
is unlike any known Phanerozoic setting. If this interpreta-
tion is correct, it has fundamental implications for models of
plate movement during the late Archaean. It suggests that at
least some late Archaean crust moved thousands of kilom-
eters across the Earth but at a rate incompatible with modern
EPM 2 -18 STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL
style plate tectonics. Whereas in Phanerozoic successions
flood basalt volcanism is associated with mantle plumes, in
the late Archaean, mantle plumes are not necessarily a
prerequisite for the formation of flood basalts [Blake,
1993]. With a hotter late Archaean mantle, it is conceivable
that a different style of tectonism took place.
[71] Finally, it is conceivable that the observed drift is a
result of a combination of the above mentioned options. To
summarize, the palaeomagnetic shift of 27.2across the
Package 7/8 boundary could be the result of apparent polar
wander and indicates a phase of rapid horizontal tectonic
movement between circa 2721 Ma and circa 2718 Ma,
although other explanations cannot be ruled out. Work in
progress, i.e., additional sampling of Packages 7 and 8 at
different locations, will aid in further constraining the drift
rate.
7.6. Geomagnetic Reversals
[72] The antipodal directions of Package 2 with respect to
Package 1 beneath and Package 4 above are interpreted as
the record of two geomagnetic reversals, one which
occurred between 2772 ± 2 Ma and circa 2766 Ma, and
one between circa 2766 Ma and circa 2741 Ma (Figures 3
and 10). Following the Package 2 reversal there is no record
of magnetic reversals in the section studied. This interval
without geomagnetic reversals has a minimum duration of
26 Myr (from circa 2741 Ma to circa 2715 Ma). While
reversals may have occurred during this period and have not
been recorded, this period of single polarity is comparable
in duration with the Cretaceous Normal Superchron (CNS,
circa 30 Myr [Cande and Kent, 1995]). A discussion about
the occurrence and importance of Superchrons during the
Archaean, however, is beyond the scope of this paper.
[73] The oldest Archaean geomagnetic reversal reported
to date is from the Kaap Valley pluton of South Africa
[Layer et al., 1996]. The authors sampled a variety of rock
types of different ages and argued that if all end vectors
were plotted together and contoured, a pattern of higher-
density distributions of directions was found, but a reversal
test was not carried out. The exact age of the Kaap Valley
pluton reversal could not be further constrained as being
younger than 3224 Ma and older than circa 2000 Ma.
[74] The magnetic reversals in the Nullagine Superse-
quence are, however, the oldest unambiguous and precisely
dated Archaean reversals, and importantly, they are strata-
bound and recorded in a single continuous stratigraphic
section. This implies that there is no uncertainty about the
relative age of the rocks and the normal-reverse-normal
sequence. In addition, the data presented here pass the
reversal test and the encountered NRM is demonstratively
primary. The next younger reported reversal occurs at
2680 ± 10 Ma in the Tanzania Craton [Meert et al., 1994].
[75] It is also likely that a period of reversed polarity
occurred between circa 2700 and circa 1700 Myr since the
MT directions have all reverse directions. This is consistent
with palaeomagnetic data from the circa 2500 Ma Hamers-
ley BIFs [e.g., Schmidt and Clark, 1994; Li et al., 2000],
which have reverse directions as well.
7.7. Implications for the Vaalbara Hypothesis
[76] Recently, Cheney [1996] put forward the hypothesis
that between 2.7 and 2.1 Ga the Pilbara and Kaapvaal were
part of the same continent, named ‘‘Vaalbara.’’ Zegers et al.
[1998] made a palaeomagnetic reconstruction showing that
the Vaalbara supercontinent may have existed between 3.1
and 2.7 Ga. Wingate [1998] compared the mean palaeo-
latitude of 34.3±6.4from the interpreted 2772 ± 2 Ma
Mount Roe basalt (Package 1 is part of the Mount Roe
basalt; Figure 4), as published by Schmidt and Embleton
[1985], with the slightly older palaeolatitude of the 2782 ±
5 Ma (concordant SHRIMP U-Pb, zircon [Wingate, 1999])
Derdepoort basalts, Kaapvaal Craton. Wingate [1998] found
a mean palaeolatitude of 64.5± 17.5(95% confidence
limit) for the Derdepoort basalts and concluded that it is
unlikely that the Kaapvaal Craton and the Pilbara Craton
were part of the same Vaalbara continent at 2.77–2.78 Ga
since there is a gap of 6.3between the minimum palae-
olatitude of the Derdepoort basalts and the maximum palae-
olatitude of the Mount Roe basalt (Figure 11).
[77] Now, with better constrained data from this study, a
similar palaeolatitude reconstruction has been made. With a
mean palaeolatitude of 51.1±3.9for Package 1, the data
fall well within the range of 64.5± 17.5for the palae-
olatitude of the Derdepoort basalts (Figure 11). Earlier
studies of the Black Range and Cajuput Dykes [Embleton,
1978] gave palaeolatitudes similar to those found in this
study but were not included in the reconstructions of
Figure 11. The new data imply that the Vaalbara hypothesis
cannot be rejected at 2.78–2.77 Ga; however, since longi-
tude is not constrained in palaeolatitude reconstructions, the
Pilbara and Kaapvaal Cratons could still have been situated
Figure 11. Palaeolatitude reconstructions for the Pilbara
and Kaapvaal Cratons at 2.78–2.77 Ga after Wingate [1998]
(lower globe) compared to this study (upper globe).
Reconstructions are based on average palaeolatitudes,
which are indicated by stars and the corresponding 95%
confidence limits, which are indicated by grey areas). The
possible range of Kaapvaal Craton palaeolatitudes in
the reconstruction of Wingate [1998] compared with the
possible range of Pilbara Craton palaeolatitudes from
Schmidt and Embleton [1985] do not overlap. However,
the possible range of Kaapvaal Craton palaeolatitudes does
overlap with the possible range of Package 1 palaeolatitudes
from this study.
STRIK ET AL.: LATE ARCHAEAN DRIFT AND GEOMAGNETIC REVERSAL EPM 2 -19
up to 180away from each other. Further, because of a
minimum age difference of 3 Myr and an average age
difference of 10 Myr between the Kaapvaal and Pilbara
basalts, matching palaeolatitudes do not prove the Vaalbara
hypothesis at 2.78–2.77 Ga, since this study shows that
large amounts of drift may occur within a few million years
in the late Archaean. Finally, the errors on the Kaapvaal
Craton palaeolatitude determinations are still significant
and therefore more sampling is needed. Nevertheless, we
conclude that with the data available at this moment, the
Vaalbara hypothesis cannot be rejected.
8. Conclusions
[78] This study has demonstrated that a meaningful
APWP can be constructed for a late Archaean succession.
Importantly, there is no ambiguity about the relative ages of
the obtained pole positions since the data come from a
single stratigraphic succession. Further, the availability of
precise, robust geochronology within the succession ena-
bles the determination of absolute ages for the palaeomag-
netic poles. The results of this study are summarized as
follows:
[79] 1. On the basis of positive field tests and the ability
to distinguish between lightning affected directions, over-
print directions, and high-temperature directions, we con-
clude that the high-temperature NRM component is of
primary origin, acquired at the time of cooling.
[80] 2. Late Archaean-aged dyke suites can be palae-
omagnetically correlated to flood basalt packages, providing
additional evidence for comagmatic relationships.
[81] 3. Contrary to the suggestion of Wingate [1998],
the Vaalbara hypothesis cannot be rejected for the
2.782.77 Ga interval since the palaeolatitude of Package
1 lies well within the 95% confidence limits of the
palaeolatitude of the Derdepoort basalts.
[82] 4. Within a circa 60 Myr succession, the geomag-
netic field has reversed at least twice. It appears that the
geomagnetic field in the late Archaean was mostly of single
polarity. However, there are significant time intervals that
have not been sampled (e.g., Package 3 and most of
Package 4, circa 2766–<2752 Ma; Figure 3). These may
contain additional geomagnetic field reversals.
[83] 5. It is possible to generate meaningful palaeopole
positions for almost all packages within the Nullagine and
Mount Jope Supersequences and to construct a consistent
APWP.
[84] 6. A major palaeomagnetic break occurs at the
Package 7/8 boundary, demonstrating the unconformable
nature of this boundary. Assuming that this break reflects
plate movement, this result supports the tectonic model for
rifting of the Mount Jope Supersequence [Blake and Barley,
1992; Blake, 1993, 2001]. The results indicate a possible
average drift rate as high as 100 cm per year, which is an
order of magnitude faster than the modern plate tectonic
drift rate.
[85]Acknowledgments. This work was conducted under the
programme of the Vening Meinesz Research School of Geodynamics
(VMSG). We thank Alkane Exploration N. L., in particular D. I. Chalmers
(Exploration Manager), for providing logistical support and allowing access
to their tenements. Peter Selkin and an anonymous reviewer are thanked for
their thorough reviews, which have improved the manuscript. Thanks are
also due to Mark Barley, Mark Dekkers, Carmen Go´ mez Portilla, Dave
Heslop, Ton van Hoof, Armelle Kloppenburg, Henk Meijer, Tom
Mullender, and Dave ‘‘dynamite’’ Taylor. The Schu¨ rmann Fonds is thanked
for additional funding for fieldwork, grants 1997, 1999, and 2001/14.
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