Palaeomagnetism of late Archaean flood basalt terrains : implications for early Earth geodynamics and geomagnetism

Abstract

Palaeomagnetic studies are e.g. important for demonstrating and quantifying horizontal movement and rotation of pieces of the Earth's crust. The constant movement and recycling of plates, in other words plate tectonics, is an important mechanism for the Earth to lose its heat. It is generally accepted that plate tectonics is the dominant mechanism since 2 billion years ago, but for older periods in the Earth's history, this is still a topic of debate. Plate tectonics cause large-scale horizontal movement, while other mechanisms mainly cause vertical movement. My PhD research has focused on 2.8 to 2.7 billion year old rocks of the Pilbara Craton, Western Australia, and 2.9 to 2.7 billion year old rocks from the Kaapvaal Craton, South Africa. Over 3000 oriented rock cores were taken from geological formations that have formed within ca. 60 million years. Palaeomagnetic tests demonstrate that the magnetization preserved in the rocks is still that from the time of rock formation. This means that the recorded magnetic directions can be translated to a (palaeo)latitude, which is the latitude at which the rocks have formed. The research demonstrates that the latitude of formation of the rocks has changed through time. At ca. 2720 million years ago, the Pilbara Craton changed latitudinal position with a rate of ca. 50 cm per year and shifted some 1600 km in total. This demonstrates that significant horizontal movements occurred as early as 2.7 billion years ago, which supports the arguments of the advocates of the occurrence of plate tectonics in Earth's early history. Additionally, my research has demonstrated that the Earth's magnetic field has been stable since at least 2.8 billion years ago. The character of the ancient field appears to be much like the current field, including the occurrence of geomagnetic reversals, the magnetic intensity, and the amount of secular variation. Some researchers ascribe a stable geomagnetic field to the presence of a solid inner core. If this were true, based on my findings, the Earth's solid inner core would have to be at least 2.8 million years old.