The 2,7 Ga Pilbara Drilling Project, Western Autralia: Paleomagnetic Results Charles Poitou*, Jean Besse*, Jean Pierre Valet*, Pascal Philippot** *Équipe de paléomagnétisme, **Équipe géobiosphère actuelle et primitive contact:
[email protected]
T
wo distinct behaviour of the samples upon demagnetization: in the first case (A), the same components are isolated by thermal and alternative field demagnetization. In the second case (B), AF demagnetization fails to completely isolate the characteristic component.
em Th ag erm et a iz l at io n
Al D ter em na ag tiv et e F iz i at el io d n
Al D ter em na ag tiv et e F iz i at el io d n
D
em Th ag erm et a iz l at io n D
0
10
80
°C
0
270
90
90
S Down
0.0
0
W
180
S Down
90
Primary magnetization of Maddina basalts (Strik et al. 2003) 11 sites 77 samples
E
Directions of principal axes of susceptibility ellispoids are plotted on stereographic projections. Flinn diagramm indicate the shape of the ellipsoids. Percentage of anisotropy lies between 0,15% and 2,98% for the Maddina basaltic flows. Note that K1 and K2 axes are interverted between oriented samples. Percentage of anisotropy lies between 0,56% and 8,55% for the samples of the Tumbiana Formation.
After 180
Paleomagnetic directions obtained from oriented samples are in red.
Fine-grained basalt
Characteristic component of magnetization of Maddina basalts (black dots and red stars) and Tumbiana Formation (cross).
Because re-orientation using magnetic susceptibility anisotropy is successful, it suggest that it would be the expression of a major geological event. It coincide with the axes of crustal extension seen on tectonic and paleogeographic reconstructions during deposition of Kylena, Boongal, Tumbiana, Pyradie, Maddina and Bunjinah formations (after Thorne and Trendall, 2001)
Large gas cavities in basalt Large vesicles in basalt Doleritic basalt Eutaxitic breccia Wrinkly stratiform stromatolite Stromatolitic dolostone Ripples Shale Sandstone Conglomerate to pebbly sandstone Broken core Fault
Conclusion Mean paleolatitude : 34,2° ± 9,3° Strik et al., 2003 + our study
Mean paleolatitude : 56,1° ± 8,9° Strik et al., 2003
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∆age = 35,5 ± 6,5 Ma 30
Depth (m)
Chlorite alteration
Maddina Basalt
-Lower limit= 2727 ± 5 Ma U-Pb on zircon (Blake et al. 2004, Tumbiana formation)
-Lower limit= 2772±2 Ma U-Pb on zircon (Wingate 1999, Black Range Dolerite Suite)
Limonite staining/alteration
Crustal extension
k2/k3
Black/white dots correspond to unoriented sedimentary/basaltic material. Average tensor is red for oriented samples at 24,2 m depth (in green) and 42,2 m depth (in orange).
Low relief landmass
Tumbiana Formation
Pre sen t cr at
90 9 0
90
270
0
100 km
1,02
0
Coa stal and nea rsho re she Offs lf faci hore es she lf faci es Sub marine ultram ma afic fic and Sylvania Inlier lava Ma in rift axis
-9
120
n
-Upper limit= 2715 ± 2 Ma U-Pb on zircon Mean age : 2721,0 ± 5,4 Ma (Blake et al. 2004, Maddina formation)
-Upper limit= 2741±3 Ma U-Pb on zircon Mean age : 2756,5 ± 3,6 Ma (Blake et al. 2004, Kylena formation)
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I - The reversal of the Earth's magnetic field probably occured 2,7 Ga ago. II - VGP scatter seems to be similar to recent times. III - Our mean plate velocity of 7,9 ± 5,5 cm.yr-1 at 2,7 Ga is much lower than previously proposed (100 cm.yr-1) and comparable to today's highest plate velocity.
50
60 Rippled tuffaceous sandstone and shale
70
Rippled carbonate sandstone 15 cm-high stromatolites
∆latitude= 21,9° ± 12,9°
Finally, we are able to evaluate the speed of the tectonic plate at 2,7 Ga between two extreme cases: -1 - 2,4 cm.yr is obtained with the longer time and shorter displacement. -1 - 13,3 cm.yr is obtained with the shorter time and the longer displacement.
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Triangle are data from 0-5 Ma lavas, circle shows results based on paleomagnetic data from and Pilbara Craton (Strik et al., 2003), red star is based on our results. A model modified from Constable and Parker (1988) is shown (solid line) with its 95% confidence limits (dashed lines) from Jonhson and Constable 1996. Figure after Smirnov and Tarduno (2004).
180
Inclination 0 18
24
gi
0
180
ar
The age of the Tumbiana Formation is indicated by U-Pb SHRIMP zircons geochronoly from a tuffaceous sandstone from the middle of the formation, 2715 ± 6 Ma (Arndt et al., 1991) and a volcaniclastic sandstone, 2719 ± 6 Ma (Nelson, 2001 and Black et al., 2004).
90
Oblate
m
A U-Pb SHRIMP zircon from a rhyolite date of 2717 ± 2 Ma (Nelson, 1998) for the Maddina Formation.
0 27
117
on
0
180
First characteristic magnetization of Maddina basalts (our study) ~9 lava flows 75 samples
Plate motion rate
Declination 0 18
Before
Stratigraphy
Latitude
mT
0
Scale: 1e-3 A/m
E
90
Second characteristic magnetization of Maddina basalts (our study) ~9 lava flows 55 samples
0.5
270
Scale: 1e-3 A/m
W
180
270
M/Mmax
0 20 0 18 0 16 0 14 0 12 0 10 80 60 40 20
0
0
0
0
0
0
0
70
50
E S Down
60
Scale: 1e-3 A/m
30
270
W
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Mean IGRF dipole
N Up
40
90
0.0 0
20
mT
0
180
90
Primary magnetization of Tumbiana Formation (our study) 66 samples and reversal test
sedimentary rock 56,3 m depth
Paleosecular variation
Latitudinal dependency of the virtual geomagnetic poles
0
180
Mmax = 6.39e-3 A/m
10
E S Down
40
20
0 50 0 40 0 30 0 20 0
10
M/Mmax
0.5
0 20 0 18 0 16 0 14 0 12 0 10 80 60 40 20
Tectonic and paleogeographic reconstructions
90
270
1,01
180
0
0
0
0
0
Prolate
1,00 1,00 1,01 1,02 1,03 1,04 1,05
0
0
Effect of using anisotropy reorientation
Flinn diagram
1,03
270
Cat. B 1.0
0
Comparison with “previous results” from surface oriented cores (Strik et al. 2003) Secondary magnetization of Maddina basalts (Strik et al. 2003) 11 sites 60 samples
S Down
Scale: 1e-3 A/m
N Up
0.0
0
Magnetostratigraphic Results
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90
Lapilli tuff 97,9 m depth
iven our poor knowledge of the Archean geomagnetic field, the interpretation of the paleomagnetic directions relies on a few assumptions. In particular, we have to assume that the Earth’s magnetic field behaved as a geocentric axial dipole (GAD), even though it is still a topic of debate.
50 km
1,05
270
50
W Scale: 1e-3 A/m
Results of anisotropy of magnetic susceptibility measurements
1,04
°C
0
270
23°00'
Mmax = 1.47e-3 A/m
0.5
40
0
mT
G
Basalt 20,3 m depth
1.0
30
Z3 Z4
he drill hole has a plunge angle of 75° toward 323°N, perpendiculary to the strike and dip of the strata. At least, three small cylindrical specimens (15 mm diameter, 13 mm long) have been sampled every meter downcore, perpendicularly to the axis of the core (ie parallel to the stratigraphy). Measurements of anisotropy of magnetic susceptibility were used to re-orient samples from unoriented segments of the core with respect to the oriented parts (only ~0,6 metre in the upper basaltic part). Both thermal and alternating field demagnetization have been conducted in order to check for the consistency of the paleodirections.
k1/k2
90
S Down
Scale: 1e-3 A/m
0
E
E 270
90
N Up
T
0
10
0.0
Z2
Z3
80
Pilbara Craton
Metamorphic zone and zone boundary of Smith et al. (1982)
k1 (maximum) k2 (intermediate) k3 (minimum)
M/Mmax
1.0
N Up
0.5
Zone 1: prehnite-pumpellyitte (0,5-1kb and 100-300°C) Zone 2: prehnite-pumpellyitte-epidote Zone 3: prehnite-pumpellyitte-epidote-actinolite Zone 4: actinolite (about 1,5 kb and 300-360°C)
0.0
W
0
Zijderveld diagrams are tilt corrected and re-oriented
Mmax = 1.25e-3 A/m
M/Mmax
Z1
20
Z1
0
180
Cat. A 10
Granitoid
0
180
Tumbiana and Pyradie Formations Greenstone
0.5
°C
180
120°00'
Fortescue Group excluding Tumbiana and Pyradie Formations
0.0
1.0
N Up
0.5
W
270
M/Mmax Mmax = 2.91e-3 A/m
Mmax = 2.70e-3 A/m
1.0
S Down
N
Gidley Granophyre
mT
Scale: 1e-2 A/m
Post-Hamersley Group rocks Hamersley Group
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90
40
118°00'
0
Basalt 42,2 m depth
0
270
S Down
0.0
°C
M/Mmax
N Up
20
0 60 0 50 0 40 0 30 0 20 0
0
Scale: 1e-2 A/m
116°00'
Mmax = 1.43e-2 A/m
1.0
E 0.5
10
Modified from Thorne and Trendall (2001)
M/Mmax
N Up W
0.5
0.0
Geologic map of the Northern Pilbara Craton
Mmax = 2.01e-2 A/m
1.0
E
Al D ter em na ag tiv et e F iz i at el io d n
M/Mmax
D T em h ag erm et a iz l at io n
N Up W
Al D ter em na ag tiv et e F iz i at el io d n
aleomagnetic studies of Precambrian rocks provide important informations on early Earth geomagnetic field and tectonic regime. However Precambrian rocks sequences can be affected by weathering, alteration and/or metamorphism, which may considerably hamper the determination of the paleomagnetic field. The one hundred metre core drilled in the Tumbiana Formation (Fortescue Group) in northwestern Australia provides an exceptionnal opportunity to study unaltered and relatively undeformed Late Archean material (2,7 Ga).
D T em h ag erm et a iz l at io n
P
Paper N° : GP43A-0923 Abstract ref N° : 4139
VGP angular dispersion
Ecole Dosctorale des Sciences de la Terre de l'Institut de Physique du Globe de Paris
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90 Sandstone, shale, and accretionary lapilli tuff
100 104
Oriented samples are red Cat. A samples are black Cat B. samples are green
References :
Blake, T. S., Buick, R., Brown, S. J. A., and Barley, M. E., 2004. Geochronology of a late archaean flood basalt province in the pilbara craton, australia: constraints on basin evolution, volcanic and sedimentary accumulation, and continental drift rates. Precambrian Research 133, 143–173. Constable, C. G. and Parker, R. L., 1988. Statisticsof the geomagnetic secular variation for the past five millions years. Journal of Geophysical Research, 93, 11569-11581. Nelson, D. R., 1998. Compilation of SHRIMP U-Pb zircon geochronology data. Western Australia Geological Survey, Record. Nelson, D. R., 2001. Compilation of geochronology data, 2000. Tech. rep., Geological Survey of Western Australia. Smith, R. E., Perdrix, J. L., Parks, T. C., 1982. Burial metamophism in the Hamersley Basin, Western Australia. Journal of Petrology, 23, 75-102. Smirnov, A. V. and Tarduno, J. A., 2004. Secular variation of the Late Archean-Early Proterozoic geodynamo. Geophysical Research Letters, vol. 31, L16607, doi:10.1029/2004GL020333. Strik, G., Blake, T. S., Zegers, T. E., White, S. H. and Langereis, C. G., 2003. Palaeomangetism of flood basalts in the pilbara craton, western australia: Late archean continental drift and the oldest known reversal of the geomagnetic field. Journal of Geophysical Research 108, B12. Johnson, C. L. and Constable C. G., 1996. Palaeosecular variation recorded by lava flows over the past five million years. Philos. Trans. R. Soc. London, Ser. A, 354, 89-114. Thorne, A. M., Trendall, A. F., 2001. Geology of the Fortescue Group, Pilbara Craton, Western Australia. Western Australia Geological Survey Bulletin 144. 249 pp. Wingate, M.T.D., 1999. Ion micropobe baddeleyite and zircon ages for Late Archean mafic dykes of the Pilbara Craton, Western Australia. Aust. J. Earth Sci., 46: 493-500. Van Kranendonk, M. J., Philippot, P. and Lepot, K., 2006. The Pilbara drilling project : c. 2.72 ga Tumbiana formation and c. 3.49 ga Dresser formation, Pilbara Craton, Western Australia. Record 2006/14, WAGS.
