The Okavango Dyke Swarm is a giant

dyke swarm A dike swarm (American spelling) or dyke swarm (British spelling) is a large geological structure consisting of a major group of parallel, linear, or radially oriented magmatic dikes intruded within continental crust or central volcanoes ...

of the Karoo

Large Igneous Province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...

in northeast

Botswana Botswana (, ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label= Setswana, ), is a landlocked country in Southern Africa. Botswana is topographically flat, with approximately 70 percent of its territory being the Kalaha ...

southern Africa Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number o ...

. It consists of a group of Proterozoic and

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

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 The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

rocks.Le Gall, Bernard, Gomotsang Tshoso, Jérôme Dyment, Ali Basira Kampunzu, Fred Jourdan, Gilbert Féraud, Hervé Bertrand, Charly Aubourg, and William Vétel. "The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province." Journal of structural geology 27, no. 12 (2005): 2234–2255. The formation is related to the

magmatism Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of ...

at the Karoo triple junction, induced by the plate tectonic break up of the Gondwana supercontinent in the early Jurassic. __TOC__

Geological information

The Okavango Dyke Swarm overlaps with the Lebombo and Save-

Limpopo Limpopo is the northernmost province of South Africa. It is named after the Limpopo River, which forms the province's western and northern borders. The capital and largest city in the province is Polokwane, while the provincial legislature is ...

Dyke Swarms at Karoo Triple junction, as a part of the Karoo

. 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 The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Arc ...

basement rock to the west in Shashe area, and Jurassic

volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

and

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

sequences to the east in the Tuli Graben. The basement country rocks are Archean crystalline

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

es and

granitoid A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...

s of the

Limpopo Belt The Limpopo Belt is located in South Africa and Zimbabwe, runs E-NE, and joins the Kaapvaal Craton to the south with the Zimbabwe Craton to the north. The belt is of high-grade metamorphic rocks that have undergone a long cycle of metamorphism and ...

. They were found to be deformed in the

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

period. The rock's

fabrics Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...

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 Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

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 The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

era. Rocks produced by the Karoo triple junction are predominantly

continental flood basalt A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...

s, 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 The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...

. 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 Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grain ...

, containing

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

, and

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

. Grain size is from fine to medium. Fresh

mineral alteration Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography. Mineral alteration is essentially governed by the laws of thermodynamics related to energy conservation, relevant to environ ...

has occurred as

sericite Sericite is the name given to very fine, ragged grains and aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or plagioclase feldspars in areas that hav ...

, 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 The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

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 Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

and

-tholeiites. Minerals include 35–45%

, 20–35%

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 Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

and parts of

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

. The separation and assembly of plates induced the Karoo

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

intrusive event.Reeves, C. "The geophysical mapping of Mesozoic dyke swarms in southern Africa and their origin in the disruption of Gondwana." Journal of African earth sciences 30, no. 3 (2000): 499–513. As two adjacent continents Africa and Antarctica moved in strike-slip motion,

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s 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),Le Gall, B., G. Tshoso, F. Jourdan, G. Féraud, Hélène Bertrand, J. J. Tiercelin, A. B. Kampunzu, M. P. Modisi, J. Dyment, and M. Maia. "40 Ar/39 Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana." Earth and Planetary Science Letters 202, no. 3 (2002): 595–606. which indicates the orientation of the dyke swarm was determined before the occurrence of Karoo magmatism. It is suggested that the dyke emplacement in the Karoo triple junction was controlled by some rock structures of the pre-Karoo basement rocks. The rock fabrics at

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging an ...

boundaries and basement rock control the orientation of Okavango dyke swarmJourdan, Fred, G. Féraud, Hervé Bertrand, M. K. Watkeys, A. B. Kampunzu, and B. Le Gall. "Basement control on dyke distribution in Large Igneous Provinces: case study of the Karoo triple junction." Earth and Planetary Science Letters 241, no. 1 (2006): 307–322. The Okavango dyke swarm inherited the Proterozoic fractures in association with the previous Proterozoic dyke emplacement event. The well-developed

chilled margin A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine-grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid crystalliz ...

and few

xenoliths A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ign ...

found on individual dykes indicate that the intrusion of a dyke was not a vigorous event at 180 Ma. The dyke swarm could extend for such a large area because of the pre-existing basement brittle fracture networks, instead of stress formed by the plate movement.

Dyke emplacement process

The geometry of the Karoo Triple Junction, which is the bulging upward of land surface, might be formed due to extensional tectonics. Cracking and jointing are induced, hence lines of weaknesses are developed in the region. In the

Thune Thunes Mekaniske Værksted A/S, Thune for short, was a Norwegian manufacturing company that among other things built locomotives. The production facilities were last located at Skøyen. History Thune traced its roots to a workshop founded by Ande ...

section of the Okavango dyke swarm, the texture of the rocks show evidence of deformation caused by tensional strain. Planar foliations are found in rock grains which are perpendicular to the dyke plane. It is suggested that magma can flow laterally from east to west. So magma flowing to Okavango Dyke Swarm is sourced from lateral side from the Karoo Triple junction, rather than an uprising active

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

head. Also, Proterozoic dykes and fractures present in the crust may control the orientation of dyke intrusion. When igneous intrusions occurred during the Karoo magmatism in the Jurassic period, they follow the directions of previous dykes. The number of Jurassic dykes accumulates, hence a giant dyke swarm can be developed in the region.

Model 2: Mantle plume hypothesis

Scientists usually relate development of large igneous province with active upwelling magma plumes, which are short-lived, rapid and high volume magmatism. The development of the Karoo triple junction geometry was believed to be formed by uplift of a mantle plume right underneath the triple junction itself.White, R. S. "Mantle plume origin for the Karoo and Ventersdorp flood basalts, South Africa." South African Journal of Geology 100, no. 4 (1997): 271–282. * Stretching of lithosphere is found from 150 km to 50 km depth from the Karoo Large Igneous Province, which favours an upwelling plume head. * A mantle plume is likely to form below extensive continental crust. * Picrites and nephelinites are found in Karoo magmatism. They are known as igneous rocks formed from plume melts. * The mantle plume in Karoo can be induced by distal plate subduction along Pacific margin of southern Gondwana.

However, the Karoo triple junction where the Okavango Dyke Swarm is located may not be compatible with the hypothesis, due to the following reasons:Hastie, Warwick W., Michael K. Watkeys, and Charles Aubourg. "Magma flow in dyke swarms of the Karoo LIP: Implications for the mantle plume hypothesis." Gondwana Research 25, no. 2 (2014): 736–755. *Karoo magmatism lasted from 183–174 Ma which is almost twice the duration of a normal active magma plume. *Lateral intruding melts from the Karoo triple junction are observed at the Okavango Dyke Swarm, which shows that it is not fed by a deep magma plume but instead from a shallow magma source. *The sub-parallel Proterozoic dykes in the Okavango Dyke Swarm are the age constraint for the magma plume hypothesis. The Proterozoic dykes have an age of over 800Ma. But Karoo magmatism occurred at around 180Ma.

Model 3: Passive melting of Gondwana supercontinent

Another hypothesis of the Karoo

(which includes the Okavango Dyke Swarm) is the lack of cooling within the mantle. The lack of movement of the Gondwana supercontinent creates insulating effect to the sub-continental mantle. It may create significant impact to

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

and temperature within the mantle. As the mantle is insulated by the thick subcontinental crust, heat accumulates and mantle temperature underneath the crust increases. Hence the temperature gradient between the mantle near core and lithosphere decreases, and convection cannot be sustained. The high temperature in the lithosphere causes crustal thinning and intrusions. In addition, subduction zones present due to the movement of Gondwana continental fragments further influence the convection of mantle. Therefore, inhomogeneous mantle is formed and causes crustal thinning. Various igneous structures formed by magma intrusion from the mantle to crust and extrusion to the land surface. Passive melting

Significance

The Okavango dyke swarm acts as a

kinematic Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fiel ...

indicator for the Jurassic evolution of Karoo Large Igneous Province related to the fragmentation of Gondwana supercontinent, as it records the paleo-stress and strain. The significant geometry of the Karoo triple junction formed by the giant dyke swarms can be used to identify the site of a mantle plume or anomalies in an easier way. The flow of magma and structure of rocks at the time of Karoo magmatism can be observed by the orientation of dyke swarm. Also, the geology of dyke-swarm and other igneous structures can be used to determine the origin of melts, as well as the tectonic movement during the supercontinent fragmentation.

References

{{Reflist, 30em Geology of Botswana Jurassic Africa