The Okavango Dyke Swarm is a giant dyke swarm of the Karoo Large Igneous Province in northeast Botswana, southern Africa. It consists of a group of Proterozoic and Jurassic dykes, trending east-southeast across Botswana, spanning a region nearly long and wide. The Jurassic dykes were formed approximately 179 million years ago, composed of mainly tholeiitic mafic rocks. The formation is related to the magmatism at the Karoo triple junction, induced by the plate tectonic break up of the Gondwana supercontinent in the early Jurassic.
Geological information
The Okavango Dyke Swarm overlaps with the Lebombo and Save-Limpopo Dyke Swarms at Karoo Triple junction, as a part of the Karoo Large Igneous Province. Intrusion of Okavango Dyke Swarm occurred in two distinct stages. Most of the dykes in the swarm are Karoo-age, and about 13% are Proterozoic age. The age range of Proterozoic components is 800–1600 Ma, while the dominant Karoo-age rocks are dated to 178–179 Ma. The mean dyke thickness is .
Geology of the country rock in Northeast Botswana
The dyke swarm cuts through Archean basement rock to the west in Shashe area, and Jurassic volcanic rocks and sedimentary sequences to the east in the Tuli Graben. The basement country rocks are Archean crystalline gneisses and granitoids of the Limpopo Belt. They were found to be deformed in the Paleoproterozoic period. The rock's fabrics show folding of a number of separate stages. They contain complex trending of shear zones and fracture networks. The overlying Jurassic volcanic rocks are mainly basaltic lava flows.
Karoo triple junction
Huge dyke swarms are formed in radial geometry. They include the N110° striking Okavango dyke swarm, the N70° striking Save-Limpopo Dyke Swarm, the Olifants River Dyke Swarm and the N-S Lebombo dyke swarm. The coverage of the area in northeast Botswana is called the Karoo Triple Junction.
Intrusive events and dyke swarm emplacement in Karoo Large Igneous Province occurred during 183–174 Ma in the Mesozoic era. Rocks produced by the Karoo triple junction are predominantly continental flood basalts, which are usually associated with huge magmatic events during the Phanerozoic eon. In the Karoo triple junction, rocks are dated to the 180 Ma Karoo magmatic event, which occurred due to the breakup of Gondwana and the opening of Indian Ocean. It consisted of mafic lava flow, dykes, and sills covering a paleo-surface of 3 × 106 km2.
Jurassic dykes found in the dyke swarms are mostly composed of dolerite, containing clinopyroxene, plagioclase, and olivine. Grain size is from fine to medium. Fresh mineral alteration has occurred as sericite, serpentine, and chlorite are found. In Proterozoic dykes of the Save-Limpopo dyke swarm, Lebombo dyke swarm and Olifants River dyke swarms, grain size is observed to be coarser with absence of olivine. Amphibolite, chlorite, and pyrite are found in a significant portion. Minerals in Protozoic rocks are more severely altered than in Jurassic dykes.
Mineralogy
The Okavango dyke swarm is mainly composed of mafic rocks, namely basalt, dolerite, gabbro and olivine-tholeiites. Minerals include 35–45% plagioclase, 20–35% clinopyroxene and Fe-Ti oxides. Both high-Ti oxides and low-Ti oxides varieties occur in the rocks.
Tectonic setting
The intrusion of the Okavango Dyke Swarm is a polyphase system, meaning dyke emplacement occurred in a number of stages. Proterozoic dykes were formed by intrusions before the Dyke swarm extend westwards due to the Karoo magmatism at approximately 182 Ma.
The Gondwana supercontinent consists of present Africa, Madagascar, India and parts of Antarctica. The separation and assembly of plates induced the Karoo volcanism intrusive event. As two adjacent continents Africa and Antarctica moved in strike-slip motion, earthquakes were common. A dyke can be formed in a single earthquake. Hence in a series of earthquakes, a whole swarm of similar dykes can be formed.
There are different hypothesis developed to account for the origin of the Okavango Dyke Swarm as well as the Karoo Triple Junction, namely: Inherited dyke-induced fractures Mantle plume hypothesis Passive melting of Gondwana supercontinent
Model 1: Inherited and dyke-induced fractures
A number of dykes in the Okavango dyke swarm are dated as Proterozoic age (mainly ~884 Ma), which indicates the orientation of the dyke swarm was determined before the occurrence of Karoo magmatism.