Jurassic Tectonic Maps of Africa

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Rhaetian/Hettangian Boundary 200+-5Ma

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 (Longacre et al, 2007: Tari et al, 2012). At some point, extension is transferred to the Herodotus (HE) Basin, which hyperextended, leaving the Eratosthenes High as a stranded block (Gao et al, 2020). 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 (as evidenced by paleomagnetics) 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, Escoca et al, 2021).  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: the only reports of these on the African side come from presentations by Petrosen over Senegal.

The ‘Karoo’ rifts are now inactive (Macgregor (2018)).  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.

Pleinsbachian 185+-7Ma

 This stage lies within in the middle of the allowable period for the formation of the Eastern Mediterranean. Tugend at al (2019)  conclude on the basis of palaeomagnetic and other geological data (e.g. Maltese flood basalts) that Ionian Sea spreading was likely initiated in late Early to Middle Jurassic times. Deep marine environments are established by the Aalenian in Sicily  (Catalano et al, 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. SDR formation commences south of the Blake Spur, with these SDRs now located off Surinam and Guinea (Trude, 2023).

A new phase of rifting commences over East Africa, preceding opening of the Somali Oceanic Basin, perhaps as early as Toarcian (Reeves, 2018). 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 et al (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 (2018), 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 (INP, 2023) . 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 event. The lack of any subsequent displacement on Botswanan and Namibian dykes of this age indicates that there have no transform displacements crossing this area at any time since. At 177Ma, volcanism began on the Mozambique margin (Mueller 2019) with the emplacement of seaward dipping reflectors.

Bajocian 170+-5Ma

 The Eastern Mediterranean/Ionian seas may be in a relatively short   drift phase by this time, as extension is now commencing in the Ligurian Sea (Handy et al, 2010),.which will inhibit further Ionian spreading. The portrayal of the Eastern Med. at this time is after Tugend at al (2019). The presence of Middle Jurassic  flood basalts on the western margins of the Ionian Sea (Reeh and Aifa, 2008) may suggest continued activity and it is noteable that these are younger than the Asher Volcanics of Israel, suggesting a E to W propagation of spreading activit.. Many authors believe that the Western Desert (WD) rifts of Egypt were initiated at this time (e.g. Moustafa, 2020) , though Bosworth (2022) places this event in the Late Jurassic. These rifts are reported to be in a transtensional phase, which is speculated to tie to period of spreading of the Eastern Mediterranean (Bosworth (2022)) and/or to transform movements along the Egyptian coast. A SE Asia style model involving initial transtension (and later reversal and transpression), perhaps related to the Trans-African Lineament (TAL), can also be speculated upon.  The rate of drift is accelerating in the Central Atlantic, but with oceanic crust only reaching the Blake Spur (BS) (Teasdale in Casson, 2020) . Sheridan (1981) interprets breakup on the Blake Plateau as  basal Callovian while Trude (2023) still has areas south of here (Guinea/Suriname) still in an SDR phase.

Indian Ocean unzipping and spreading is far more rapid. The Somali Basin is now opening, with oblique spreading extending to Tanzania and perhaps northern Mozambique (Reeves, 2018). The Davie Transform is yet to form (Phethean, 2016). A major transgression is observed in many of these basins around Bajocian times, which may mark break up (Macgregor (2018)). A volcanic margin is forming over the Mozambique margin. Figuerido et al (PESGB presentation, 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 (2023) 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, where a very thick Jurassic section seems to open up over major bounding faults in a possible pull-apart setting.

The Middle Jurassic is a relatively inactive period with respect to intra-plate rifting, the WD excepted. Minor rifting continues in the northernmost Atlas Basins (Escoca et al, 2021), probably associated with sinistral movement on the Agulhas transform.

Kimmeridgian 155+-5Ma

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, supporting the reconstruction of Lovecchio et al, 2020. 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). A suggested 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 (Salman and Abdula, 1995) and could have a common origin. The geodynamic model for the East Falklands Basin remains uncertain. An oceanic crust base 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). The Central Atlantic now extends to Guyana (Trude, 2023) . The North Africa margin is interpreted to be in a transform phase, with rifting in the Western Desert (WD) propagating further west.

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