Palaeomagnetic reversals and cooling in the Bushveld Complex, South Africa
Abstract
Magnetostratigraphy of igneous rocks is based on the principle that lava flows are sequentially erupted and get progressively younger upward. Is this also the case for large intrusions? In the South African Bushveld Complex, Letts et al., (2009, Geophys. J. Int. 179, 850-872) determined palaeomagnetic poles for 100 sites throughout the 8 km of stratigraphy. They document 7 magnetic field reversals. At the bottom in the Critical Zone, they record reversed polarities, followed upwards by a zone of normal polarity. In the overlying section (Main Zone), there are four reversals and in the Upper Zone there are two. All palaeomagnetic data, including both normal and reversed samples, show a tightly clustered paleopole position at latitude = 19.2°N, longitude = 030.8°E, A95 = 5.8°, for the 2054 Ma emplacement age of the complex. We reinterpret these reversals, based on a consideration of the cooling of the complex, and modify the magnetostratigraphy such that the oldest pole positions are at the top of the complex and the youngest are near the center. Cooling calculations for the mafic magmas of the Bushveld Complex demonstrate that it took ~180,000 years to solidify after emplacement to a temperature of 900°C. A further ~500,000 years was required for the entire complex to cool below the Curie temperature of 580°C. However, this cooling did not take place from the bottom upward as is the case in lavas. Because of the extreme thickness of the complex, cooling began at the top and progressed inward. The top of the complex cooled below the Curie temperature first and records the oldest magnetic field direction. As it continued cooling, it recorded younger reversals toward the center. After ~350,000 years of solidification the bottom also cooled below the Curie temperature, recording the magnetic field at that time. From our cooling calculations, the second reversal recorded at the top of the complex is the same as recorded at the bottom. Instead of recording a series of independent magnetic reversals from the bottom up, only 3 reversals are recorded from the top down with this interpretation. The last two reversals are also recorded at the base of the mafic sequence in reverse order. The most important parameter in the cooling calculations is the thickness of the complex, as thinner sections cooled more quickly. Where the complex is thinner, the magnetic reversals may crosscut the layering compared to the thicker sections. There is evidence for this effect in the south of the eastern limb where the stratigraphic location of the reversal does not correlate with the stratigraphic location in thicker sections to the north and in the west. Thus the reversals provide a marker horizon for cooling in the complex and may provide a proxy for thickness. Our cooling calculations show that for thick intrusions it is essential to consider cooling as the oldest magnetic field will be preserved at the top and the youngest towards the center. Thus palaeomagnetic results from all thick intrusions need to be interpreted in conjunction with cooling calculations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMGP43A1125C
- Keywords:
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- 1520 GEOMAGNETISM AND PALEOMAGNETISM / Magnetostratigraphy;
- 1527 GEOMAGNETISM AND PALEOMAGNETISM / Paleomagnetism applied to geologic processes;
- 3642 MINERALOGY AND PETROLOGY / Intrusive structures and rocks;
- 5134 PHYSICAL PROPERTIES OF ROCKS / Thermal properties