Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean Christian Heine1 and Sascha Brune1,2 1EarthByte Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia 2Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 2.5, Geodynamic Modelling, Telegrafenberg, D-14473 Potsdam, Germany ABSTRACT poles of relative motions between the African Rifting between large continental plates results in either continental breakup and the for- plates, describing the lithospheric extension mation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. in the WARS and CARS, have been generated The nonlinear interaction between key parameters such as plate boundary confi guration, litho- from published extension estimates (e.g., Ge- spheric architecture, and extension geometry determines the dynamics of rift evolution and nik, 1992; McHargue et al., 1992) and fi tting ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the of restored sediment basin widths (Heine et al., contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa 2013; see the GSA Data Repository1 for paleo- and South America that was preceded by ~20–30 m.y. of extensive intracontinental rifting tectonic maps in 1 m.y. time steps). prior to the fi nal separation between the two plates. While the South Atlantic and Equato- Relative motions between the main rigid rial Atlantic conjugate passive margins continued into seafl oor-spreading mode, forming plates are initiated at 140 Ma and progress at the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon slow extensional velocities, compounding to after South America broke away from Africa. We investigate the spatiotemporal dynamics ~4 mm a–1 between South America and south- of rifting in these domains through a joint plate kinematic and three-dimensional forward ern Africa until 126 Ma (southern Africa fi xed numerical modeling approach, addressing (1) the dynamic competition of Atlantic and Afri- reference frame, full spreading rates at 37.5°W, can extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the 5°S). Modeled plate motions between South acceleration of the South America plate prior to fi nal breakup. Oblique rifts are mechanically America and northwest Africa result in ~10– favored because they require both less strain and less force in order to reach the plastic yield 15 km displacement during the initial phase. limit. This implies that rift obliquity can act as selector between successful ocean basin forma- Nondeforming South American and northwest tion and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and African plates surrounding this region (Heine et ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of al., 2013) imply that an incipient, diffuse plate the last continental connection between Africa and South America produced a severe strength- boundary along the future Equatorial Atlantic velocity feedback responsible for the observed increase in South America plate velocity. region may have existed during the Early Cre- taceous, contemporaneous with rifting in the INTRODUCTION the east (Basile et al., 2005). While extension in CARS and WARS. Marine magnetic anomalies Lithospheric extension related to the fi nal the SARS and EqRS ultimately led to the forma- in the southernmost South Atlantic document dispersal of western Gondwana started with tion of the South Atlantic and Equatorial Atlan- breakup and subsequent seafl oor spreading in the formation of large intracontinental rift sys- tic (Nürnberg and Müller, 1991; Torsvik et al., the southern rift segment (Nürnberg and Müller, tems within and between the Africa and South 2009; Moulin et al., 2010; Heine et al., 2013), 1991; Moulin et al., 2010), while the northern America plates in the Early Cretaceous (Burke the CARS and WARS never went beyond rift part still undergoes continental extension (Tors- and Dewey, 1974; Unternehr et al., 1988). Four mode and eventually failed, being preserved as vik et al., 2009; Moulin et al., 2010; Heine et al., extensional domains developed between the subsurface graben structures (Burke and Dewey, 2013). Relative plate velocities based on seafl oor main rigid continental lithospheric blocks dur- 1974; Fairhead, 1986; Genik, 1992). spreading patterns indicate an ~10-fold increase ing that time (Fig. 1A; Heine et al., 2013): (1) Here we investigate the spatiotemporal evo- of spreading and/or extensional velocities from the Central African Rift System (CARS), ex- lution of continental extension leading to the 4 mm a–1 to >39 mm a–1 toward the early Ap- tending from Sudan to the eastern part of the abandonment of these large intracontinental rift tian (120.6 Ma; Heine et al., 2013). From then Benoue Trough (Fairhead, 1986), (2) the West systems and the breakup between Africa and onward, breakup occurs successively in isolated African Rift System (WARS), extending from South America. We analyze the geodynamics segments of the northern SARS and EqRS, with the eastern part of the Benoue Trough north- of rifting by combining plate kinematic and for- complete breakup achieved by 104 Ma. ward toward southern Libya (Burke and Dew- ward numerical models. ey, 1974; Genik, 1992), (3) the South Atlantic NUMERICAL MODEL SETUP Rift System (SARS), comprising the present- PLATE KINEMATIC MODEL We investigate the dynamics of rift competi- day conjugate South Atlantic marginal basins Our study builds upon a new plate kinematic tion and the reason for the observed multiphase with the Benoue Trough–northeast Brazil at model for the evolution of the West Gondwana velocity behavior using the three-dimensional its northernmost extent (Nürnberg and Mül- rift systems (SARS, CARS, WARS, and EqRS) (3-D) thermomechanical code SLIM3D (Popov ler, 1991), and (4) the Equatorial Atlantic Rift that quantitatively integrates crustal deforma- and Sobolev, 2008) with boundary conditions as System (EqRS), covering the conjugate West tion from Cretaceous African and South Ameri- specifi ed in Figures 1C and 1D. The program African and South American margins from the can intraplate deforming zones as well as from solves the thermomechanically coupled con- Guinea Plateau–Demarara Rise in the west to the conjugate passive margins of the equatorial servation equations of momentum, energy, and the Benoue Trough–northeasternmost Brazil in and South Atlantic (Heine et al., 2013). Stage mass. It includes a free surface and rheological 1GSA Data Repository item 2014073, methodological information on numerical model setup, paleo-tectonic reconstruction maps for the Equatorial Atlantic region for the time between 140 and 100 Ma in 1 m.y. time steps, and numerical model animations (Apple Quicktime®), is available online at www.geosociety.org/pubs /ft2014.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, March 2014; v. 42; no. 3; p. 1–4; Data Repository item 2014073 | doi:10.1130/G35082.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | March Society 2014 of| www.gsapubs.orgAmerica. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 Figure 1. Overview map 10°W 0° 10°E 20°E and numerical model A B setup. Southern Africa N WARS km (SAf) is held fi xed in WARS NEA 0 500 IuB present-day position in Termit Basin 20°N NWA all reconstructions. A: Guinea Jos Lambert azimuthal equal Plateau NWA CARS CARS Bida area view of pre-rift con- EqRS 10°N fi guration at 140 Ma, BeT showing major rigid 0° plates and deforming crustal regions (Heine SAm SAf Demerara EqRS Ceará B. et al., 2013). Present-day Rise SLC Piaui B. South America and Africa BPB SAf SARS FdA MB PotB plates are separated into 20°S c) 0° MarB SARS four major rigid plates in N G.G. region of interest during SAm Late Jurassic–Early Cre- taceous: NEA—northeast Present−day extended continental crust Non−deforming crust Africa, NWA—northwest 40°S Actively deforming continental crust Oceanic crust Africa, SAf—southern Af- 20°W Proterozoic rocks Present−day coastline rica, SAm—South Amer- 0° Sedimentary rocks (Jurassic−Cretaceous) Metamorphic basement grain 40°E ica. Rift systems (white 20°E Landward limit oceanic crust Rift bounding normal fault font, red background): Rift zone Plate interior CARS—Central African C D Temperature (°C) rift, EqRS—Equatorial At- WARS lantic rift, SARS—South lar movement, No-slip 0 500 1000 0 500 1000 Atlantic rift, WARS— boundary Upper crust: s condition NWA condition wet quartz West African rift. Red tres 20 CARS Lower crust: and white polygon indi- EqRS No face-perpendicu SARS mafic granulite 40 cates map frame (in B), continuous s SAf white dashed lines are Lithospheric mantle: Depth present-day 5° graticule. constantIf velocity force (15< 39 TN/m) mm/yr: dry olivine constant velocity (39 mm/yr) SAm (km) B: 117 Ma reconstruc- tion (Heine et al., 2013). LAB Depth Rift zone LAB Northernmost segment else: (1350°C isotherm) 120 Asthenospheric mantle: of SARS is already in 120 km Rift Zone wet olivine seafloor-spreading 150 km Plate Interior 150 0 0 0 mode; in EqRS, breakup Top boundary: Free surface 200400 600 800 20040 600 800 is incipient. Numerically Bottom boundary: Winkler support Differential stress (MPa) modeled region is indi- Plate interior LAB cated by white rectangle. Minor rigid plates: BPB—Borborema province block (northeast Brazil), Jos—Jos subplate (northern Nigeria), SLC—São Luis craton. Basins (B): BarB—Barreirinhas Basin, BeT—Benoue Trough, FdA—Foz do Amazon Basin, G.G.—Gurupi graben, IuB—Iullemmeden Basin, MarB— Marajó Basin, MB—Maranhão Basin, PotB—Potiguar Basin. C: Initial geometric setup of three-dimensional numerical model involves pro- spective rift zones as thermal heterogeneities. Model size is 2400 × 1600 km horizontal and 200 km vertical.