Abstract
The Archaean era lasted for about one third of the Earth's history, from ca 4.0 until 2.5 billion years
ago. Because the Archaean spans such a long time, knowledge about this era is for understanding
the evolution of the Earth until the present day, especially because it is the time offormation
of much
... read more
of the Earth 's continental However, because it was a very long time ago, this
period is difficult to study; the geological record contains a limited amount of rocks that date from
this era.
Hutton's doctrine of uniformitarianism; present is the key to the past" may be in a
general sense. The Archaean Earth may have had a stmcture very much like it does today: it probably
had a core, a convecting mantle, a crust, oceans, and an atmosphere. However, on closer inspection
there are many differences between the present day and the Archaean Earth. During the
Archaean the earliest life on Earth evolved. Geodynamic processes may also have been significantly
different and they are the focus of this thesis.
There is still much controversy about whether modern-style plate tectonics, characterized
by rigid plates, spreading and subduction of ocean floor, recycling of oceanic crust, generation of
continental crust, and continental growth by lateral accretion ofterranes, were possible in the Archaean
(see papers in Coward and Ries, 1995; De Wit and Ashwal, 1997). Thermal and geodynamical
modeling studies have not provided conclusive answers, mainly because the temperature of the
interior of the Earth in the Archaean still is a major unknown. Geological field studies can potentially
provide the data necessary to constrain the thermal, tectonic and geodynamic regimes of the
early, mid and late Archaean.
Modern plate tectonics can be defined by several characteristic features, however, some of
these are very difficult to identify in ancient rocks. The main problem is that the ocean floor plays
tbe leading role in the plate tectonic process, but it is this same ocean floor which is continuously
recycled, and not preserved and accessible to field studies. We bave to look for features that were
preserved on the continents. Most diagnostic are those that are directly related to subduction, such
as ophiolites (obducted slices ofoceanic crust), accretionary tectonic melanges (formed at the leading
edge of the overriding plate), and the occurrence of high pressure-low temperature metamorphic
rocks at the surface (indicating tbal rocks have been taken down to great depths and subsequentlyexhumed).
Other characteristic features are tbe occurrence ofmagmatic products from subducted ocean floor and subduction-modified mantle; the widespread occurrence of intermediate magmatism, and
compression-extension tectonic cycles followed by major strike-slip events. These characteristic
features have been researched in the Archaean Pilbara Craton in Western Australia, in the context of
this PhD thesis focusing on the West and Central Pilbara. A temporally constrained structuralkinematic
framework supported by petrology and geochemistry is built, in order to assess possible
geodynamic regimes in the mid-Archaean.
show less
