Jurassic Paleotectonic and Paleogeographic Maps of Africa
Draft 2 Feb 22
an ongoing project using the plate model of Colin Reeves (www.reeves.nl)
Comments welcomed
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Rhaetic/Hettangian ~ 200Ma
Most of the Eastern Mediterranean likely now opens. Major alkaline flood basalts of latest Triassic age are erupted in Israel and a large unconformity is seen there of Norian to Hettangian age (Gardosh et al, 2011). The available window for the age of the Eastern Mediterranean oceanic crust seems to be from late Triassic to early Middle Jurassic : a younger date is precluded by the opening of the Ligurian Ocean in the Middle Jurassic. Horsts and grabens in the Levantine Basin (LE) trend NNE-SSW , implying transform movement along the sharp Egyptian margin (Tari et al, 2012). At some point, extension is transferred to the Herodotus (HE) Basin, which hyperextend, leaving the Eratothenes High as a stranded block. This can be connected with evidence of transform tectonics in the Gulf of Sirt (GOS, Jagger et al, 2018) and as far as the incipient Azores-Gibraltar transform. However the Menderes-Taurides blocks cannot be moved too far west while holding Apulia in place and the phase of NNE-SSW spreading may thus have been short lived.
The intensity of rifting has decreased in the Newark (NE), Nova Scotia (NS), Atlassic (AT) and Sicilian rifts (SI), though a second milder pulse is proposed in Morocco (Frizon et al, 2008). A major magmatic event occurs at the Triassic-Jurassic boundary, which is defined at the mass extinction associated with the Central Atlantic Magmatic Plume (CAMP). The distribution of these volcanics is largely after McHone (2000). Within tectonically controlled inlets such as the Triassic Basin (TR) and incipient Central Atlantic, evaporites are well developed. On the American side, there is magnetic evidence for the emplacement of seaward dipping reflectors around this time: as yet no convincing evidence of these has been seen on the African side. Instead, Roberts et al (2017) propose that the southern Moroccan margin is a hyperextended margin, with continental crust extending as far as some of the Canaries Islands.
The ‘Karoo’ rifts are now inactive (Macgregor, 1997). Over the Zambian and Tanzanian rifts, an unconformity is seen between Triassic and Late Cretaceous, capping inversion structures that increase in intensity to the SW. The lack of any displacement on Pleinsbachian dyke swarms suggest this inversion could be placed in this time period. This inversion could indicate a reversal of the STASS wrench faults.
Africa is still moving north over the climatic belts, including an expanded arid zone. Despite the widespread evaporites in north Africa, there are still indications of periodically humid conditions. Increasingly hot and arid conditions are evidenced in the Great Karoo (GK) Basin (Catuneanu et al, 2005).
Pleinsbachian - Early Toarcian 185Ma
The Early Jurassic is marked by an expansion of oceanic crust around northern Africa. This stage lies within the allowable period for the formation of the Eastern Mediterranean and an initiation at this time would be consistent with the first phase of rifting of the Western Desert (WD) and a Pleinsbachian transgression accompanying this. Rosenbaum and Lister (2004) conclude on the basis of palaeomagnetic data that the Ionian Sea is most likely of Early Jurassic age. Deep marine environments are established by the Aalenian in Sicily (Catalano, 2013). The model adopted for the opening of the Central Atlantic is that of Labails et al (2010), with first oceanic crust around 190Ma emplaced north of the Blake Spur only . There are indications from AFTA analysis (Charton et al 2020) that the Reguibat massif of NW Africa may be starting a long slow topographic rise, though this is not reflected in any significant supply of coarse clastics to the adjoining margin.
A new phase of rifting commences over East Africa, preceding opening of the Somali Oceanic Basin (Macgregor, 1996). Macgregor (2018) interprets early syn-rift condition in the Pleinsbachian over rifts such as the Mandawa (MA) Basin of Tanzania, preceding the main syn-rift conditions in the Toarcian. Early Jurassic rifts are also interpreted on seismic offshore Somalia (Davidson, 2018), which will go into hyperextension. Marine conditions transgressed in the Pleinsbachian as far south as southern Somalia, prior to of a northern derived marine transgression and salt deposition in the Toarcian (Macgregor 1998) which likely reached Mozambique. Toarcian salt is known from the Mandawa (MA) Basin of Tanzania and the Majunga Basin (MJ) of Madagascar, while salt diapirs are interpreted on seismic in the offshore Angoche (AN) Basin of Mozambique
A major ‘Karoo’ volcanic episode affects southern Africa (Pevye 2015) , Antarctica and Tasmania in the earliest Toarcian (circa 179Ma) and is tied to a significant extinction. . . The lack of any subsequent displacement on Botswanan and Namibian dykes of this age indicates that there have no transforms crossing this area at any time since. At 177Ma, volcanism began on the Mozambique margin (Mueller 2019) as seaward dipping reflectors..
There are relatively few climate datapoints for this interval, These indicate a continuation of the widespread arid conditions of the Late Triassic. Increasingly hyperarid conditions are indicated for the Great Karoo Basin (Bordy and Catuneanu, 2002).
Bajocian 170Ma
The Eastern Mediterranean/Ionian seas may be approaching the end of their drift phase by this time, as extension is now commencing in the Ligurian Sea (Handy et al, 2010). However, the presence of Middle Jurassic flood basalts on the western margins of the Ionian Sea (Reeh and Aifa, 2010) may suggest continued activity . The rate of drift is accelerating in the Central Atlantic, with oceanic crust now reaching south of the Blake Spur (BS) (Teasdale in Casson, 2019) . Sheridan (1988) interprets breakup on the Blake Plateau as basal Callovian.
Indian Ocean unzipping and spreading is far more rapid, with oblique spreading (but no Davie transform) extending to Tanzania and perhaps northern Mozambique (Reeves, 2018). A major transgression is observed in many of these basins around Bajocian times, which may mark break up (Macgregor 2017). A volcanic margin is forming over the Mozambique margin. Figuerido et al (2021) interprets first oceanic crust at 170Ma in the Angoche (AN) Basin and offshore Zambezi (ZA) , though this was abandoned in favour of more outboard spreading centre 10My later, leaving the Beira High (BH) as a stranded block. Roche et al (2021) transfers the extension in this region to the south on a large transform , where the proto-Weddell Sea could be opening. Rifting of the Agulhas Basins in South Africa may be commencing in the deepest half graben of the Gamtoos (GA) Basin (Mcmillan et al, 1997). This is likely to be an extension of the East Falklands (EF) Basin.
The Middle Jurassic is a relatively inactive period with respect to intra-plate rifting . A notable exception is a second phase of rifting in the Western Desert (WD) (Dolson et al, 2014). This and later phases of Western Desert rifting are more difficult to explain in a geodynamic framework : a transtensional model related to the Trans-African Lineament (TAL) can be speculated upon, but more work is needed.
On a plate scale, carbonate deposition has become more widespread. Opposing carbonate platforms are now developed on the two Central Atlantic and East African margins .Limited climatic information suggests a continuation of the dominantly arid conditions of the Early Jurassic. An exception is Increasingly humid conditions present in northern Africa, close to the Tethys ocean.
Kimmeridgian 155Ma
Most Tethyan oceans from the Proto-Caribbean to the Middle East are now rapidly spreading, as are various segments of the Indian Ocean. A switch to N-S spreading in the Somali Basin initiates the Davie Transform (DT), which is entirely developed within oceanic crust (e.g. Pheathan, 2016). A correlation of organic shales at this level between DSDPs on the Maurice Ewing Bank (MEB) to those in the Algoa (AL) and Gamtoos (GA) Basins fixes the relative positions of the African and South American plates at this time. There is no marine strata seen in any of the basins north and west of the Falklands so the marine link in the Agulhas Basins has to come from areas east of the islands. The first marine strata are encountered in the Gamtoos Basin in the Kimmeridgian while marine strata are not seen in the Bredasdorp (BR) Basin to the west till the Tithonian (Mcmillan et al, 1997). Proposed early movement on the Agulhas transform later in the Jurassic probably therefore brought East Falklands Basin waters into contact with the Bredasdorp Basin. A series of rifts are also thought to become active in southern Mozambique at this time and could have a common origin. The geodynamic model for the East Falklands Basin remains uncertain. Oceanic spreading is suggested by Eagles and Eisermann (2020), though a reported significant crustal thickness seems incompatible with such a model. The alternative model is that of a pull apart basin.
NW-SE trending rifts become active in Yemen at this time but exploration for these in Somalia has failed to find thick Jurassic sections (Purcell, 2018).
There is little evidence , either direct or indirect, for significant topography development in Africa during the Late Jurassic. The widespread nature of transgressions, related to high global sea level at this time, is indicative of low topography. A wide carbonate-prone transgression of central eastern Africa occurs, encompassing the Blue Nile gorge and in the Mekele inlier(ME) . Papers in de Wit (2015) tentatively interpret Kimmeridgian shallow marine environments as far inland as the Cuvette Centrale (CC) of the DR Congo. Carbonate deposition dominates all northern regions of the African plate, while the south-western margins are clastic dominated, though without evidence of any major deltas. Paleoclimatic data indicates three clear zones namely, humid tropical conditions on the East Mediterranean margin, widespread arid conditions in the interior and humid and warm temperate conditions in the south and south-west.
