The geology of the Kimberley, a region of Western Australia, is a rock record of early Proterozoic plate collision, orogeny and suturing between the Kimberley Craton and the Northern Australia Craton, followed by sedimentary basin formation from Proterozoic to Phanerozoic.
The area formed in a slow tectonic plate collision during the Paleoproterozoic. The Kimberley Craton, which was moving south-eastwards, smashed into the North Australia Craton. The resulting series of deformations created the Hooper Complex and Lamboo Complex, which can be seen today along the southern margin of the Kimberley Craton. During the Proterozoic to Early Phanerozoic, the region had phases of mountain building (orogeny), faulting and sedimentary basin formation. Finally, the two cratons sutured, becoming one craton.
After the main phases of mountain-building, the shallow marine and river sediments were deposited on the Kimberley Craton. These sediments are called the Speewah and Kimberley Basins. Sediment deposition on these basins ended in the Late Paleozoic.
Other major Proterozoic events include the Yampi Orogeny (1400–1000 Ma) and Elatina Glaciation (~610 Ma). In the Paleozoic, there was the King Leopold Orogeny (~560 Ma), the Kalkarindji Continental Flood Basalt Province (~508 Ma), early Ordovician thermal subsidence, and the Canning Basin (Late Ordovician to Early Cretaceous ). During the Neogene, the region was bowed downwards as the Australian Plate met the Indian Plate.
General geography
The Kimberley is the northernmost of the nine regions of Western Australia, with an area of 423,517 square kilometres (163,521 sq mi), about three times the size of England. Some towns in Kimberley are Broome, Derby, and Kununurra. Approximately 40% of the region's population is of Aboriginal descent.
The Kimberley consists of steep mountain belts to the northern side, with sandstone and limestone gorges cutting through. The northwestern side borders the Indian Ocean. Southern parts of the Kimberley are much flatter, consisting of mainly flat arid grasslands. The Kimberley is one of the hottest and driest regions of Australia, with a climate unfavourable to agriculture.
Tectonic evolution
Pre-Hooper Orogeny (> 1900 Ma) Earlier models of the Kimberley region suggested that, since the region's granitic composition is similar to the Paleoproterozoic granites found in other parts of Northern Australia, the region consisted of a stable Archaean cratonic basement.
This argument has since been challenged, because of newer magnetic and gravity data, indicate that the regional lithologies incorporated recycled orogenic and magmatic arc materials. This incorporation suggests that, rather than having a stable Archaean cratonic basement, the Kimberley Craton was likely shaped and reshaped by stages of magmatic and mountain-building episodes.
Newer models postulate a pre-1900 Ma convergent margin at the southeastern end of the Kimberley Craton, with the North Australian Craton being subducted under it. Convergence of the two cratons resulted in the advancement and accumulation of an exotic terrane (a fragment of the Kimberley Craton), termed crustal accretion, which could have instigated crustal growth. Accretion of the terranes increased friction between the overlying and subducting plates, which probably impeded further slab subduction. This probably caused the deeper parts of the Northern Australian plate to break off (slab detachment and lithospheric delamination).
In the Western Zone, turbiditic sediments of the Marboo Formation began to be deposited around 1872 Ma, during post-collisional rifting of the Kimberley Craton and North Australian Craton.
Early Hooper Orogeny (1865-1856 Ma) Slab detachment removed the weight of the downgoing slab from the Northern Australian plate. This changed the lithospheric isostasy; the tendency of the plate to sink decreased. The loss of the descending slab also changed mantle flow traction. This caused a subduction polarity reversal; instead of the Northern Australian plate to the southeast subducting under the Kimberley plate to the northwest, as it had previously done, the Kimberley Craton began to subduct under the Northern Australian plate. The newly formed subduction zone began to form the Tickalara Oceanic Arc. Partial melting was triggered by upwelling of hot aesthenospheric mantle associated with slab detachment.