In 2007 and 2014, seismic refraction data were acquired in the Mozambique Channel during three scientific expeditions (AISTEK II, MOCOM and PAGE-FOUR cruises). The five seismic refraction lines reviewed here all cross the DFZ. Four of them are east-west oriented and the fifth runs NNE-SSW. Together, they reveal crustal information along more than 1400 km of the East African coast between 11°S and 20°S (Figs. 1 and 2; Leinweber et al. 2013; Müller and Jokat 2017; Vormann et al. 2020; Vormann and Jokat in review). The profiles are labelled A to E from north to south.
In all seismic refraction profiles (Fig. 2), we observe a sedimentary cover up to 6 km thick, with sedimentary velocities reaching values of up to 4.7 km/s just above the acoustic basement, which are typical for this region (Lort et al. 1979; Müller et al. 2016; Müller and Jokat 2017). Crustal velocities along our five lines start at around 5.5 km/s in the southern part, are around 5.1 km/s in the north and increase up to 7.0 km/s in the lower crust (Leinweber et al. 2013; Müller and Jokat 2017; Vormann et al. 2020; Vormann and Jokat in review), as is typical for the Central Mozambican margin (Leinweber et al. 2013; Müller et al. 2016; Müller and Jokat 2017).
Along the 182-km-long northern profile A (AWI-20140150, Vormann and Jokat in review), the DR forms the eastern margin of the Kerimbas Basin (KB, Fig. 2). Some basement elevations are observed beneath the Kerimbas Basin, but there is no distinct basement topography beneath the DR. No steep deepening of the Mohorovičić discontinuity (Moho) is visible in the west of the DR (Fig. 2). The velocity gradient matches to an interpretation of highly stretched continental crust (Christensen and Mooney 1995) and intruded continental crust (Sibuet et al. 2016), both of which are typical for a continent-ocean transition zone (COT) (Vormann and Jokat in review). Here, the DFZ is located below the Kerimbas Basin. East of the DR, the basement and Moho are both flat-lying. The crustal thickness (7 km) and the velocity-depth distribution match those of the old oceanic crust as observed south of this line (White et al. 1992; Leinweber et al. 2013; Müller et al. 2016; Vormann et al. 2020).
Along the second profile, B, at 13°S (AWI-20140130, Vormann and Jokat in review), the seafloor and basement have no elevation in the area of the expected DR (Fig. 2). The western end of the profile represents the COT and shows a steep decrease in crustal thickness seawards, as constrained by gravity modelling. The velocity gradient within the COT matches that of stretched continental crust (Christensen and Mooney 1995). The onset of up to 6.5-km-thick oceanic crust lies to the west of a crustal bulge (CB, Fig. 2), a rather unusual structure that is probably connected to underplated material feeding neo-volcanism at Paisley Seamount (Vormann and Jokat in review). The true extent of the observed underplating cannot be determined from a single profile. Thus, its regional relevance remains speculative. Recent volcanism is also observed further south at the Sakalaves seamount (Courgeon et al. 2018). The DFZ is interpreted to be located over the area of underplated material and to be situated in oceanic crust.
Further south, profile C at 14.5°S (AWI-20140100, Vormann et al. 2020) reveals the DR as a basement elevation covered by thin (2 km) sediments. Close to the Mozambican coast in the west, the data show a crustal thickness of 15 km, which continuously decreases towards the DR. The DFZ is suggested to lie east of DR, in thinned (5.5 km thick) oceanic crust. The presence of oceanic crust (7.1 km) is well characterized by its typical thickness and velocity gradient (White et al. 1992; Vormann et al. 2020).
The three northern profiles (A-C) do not reveal crustal characteristics that would be typical of a purely sheared margin, such as a steep decrease in crustal thickness and/or a narrow COT (Bird 2001). The margin, in contrast, shows a variable and complex crustal structure, with different COT geometries and a fracture zone that is partially located in oceanic crust. This may be explained by an initially oblique phase of separation of the northern Mozambican, Tanzanian, and Kenyan margins from their conjugates during the Jurassic. NNW-SSE-oriented extension of continental crust and formation of early oceanic crust preceded a change in the direction of divergence between East and West Gondwana to N-S between 157 and 144 Ma, leading to the formation of a major shear zone, the Davie Fracture Zone (DFZ). This evolution is responsible for the present-day crustal variability along the East African margins.
The fourth profile, D, at 16.5°S shows strong crustal variations. At its western part off Mozambique (AWI-20140050, Vormann et al. 2020), a thin body of high velocity lower crust (HVLC) is observed beneath the COT (Fig. 2). Eastwards, only a small corridor of oceanic crust (70 km) is found before the western margin of the DR for which a continental origin is encountered. In the centre of the line, the DFZ is observed to coincide with the DR and two additional buried ridges. The velocity distribution is similar to that of stretched continental crust, as observed in continental fragments such as the Beira High (Müller et al. 2016). The crustal thickness and velocity distribution further east do not support the presence of oceanic crust towards the northwestern Madagascan margin. Instead, the crust is interpreted as stretched continental crust that may be underlain by a thin HVLC (120 km wide, 2.5 km thick), consistent with the presence of magmatic intrusions in the sediments off Madagascar (Klimke et al. 2016).
The southernmost profile, E (AWI-20070201, Leinweber et al. 2013, Müller and Jokat 2017), between 16° and 20°S, is oriented NNW. It describes the transition from Jurassic oceanic crust off Central Mozambique to the onshore continental crust of the Mozambique Belt. This profile shows a 220-km-wide HVLC body beneath the COT (Müller and Jokat 2017) with a mean thickness of about 5 km. Oceanic crust appears only 100 km off the coast, where it is constrained by linear magnetic reversal anomalies interpreted as isochrons in the sequence starting at M38n.2n (Müller and Jokat 2017, 2019).
The two southern deep seismic sounding lines (D + E) image a typical volcanic rifted margin with a HVLC body and seaward dipping reflectors (SDRs) within the COT (Leinweber et al. 2013; Müller and Jokat 2017; Vormann et al. 2020). Onshore up to 15.5°S, the landward limit of Jurassic rifted crust is marked by intrusions of middle Jurassic age in the coastal areas (Müller and Jokat 2017). The northern terminations of the intrusions and the HVLC body coincide.