Available online at www.sciencedirect.com Tectonophysics 445 (2007) 245–272 www.elsevier.com/locate/tecto Gondwana breakup via double-saloon-door rifting and seafloor spreading in a backarc basin during subduction rollback A.K. Martin Repsol YPF Exploración, Al Fattan Plaza, PO Box 35700, Dubai, United Arab Emirates Received 2 February 2007; received in revised form 4 July 2007; accepted 21 August 2007 Available online 28 August 2007 Abstract A model has been developed where two arc-parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback. The resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double-saloon-doors about their hinges. As movement proceeds and the two terranes rotate, a gap begins to extend between them, where a third rift initiates and propagates in the opposite direction to subduction rollback. Observations from the Oligocene to Recent Western Mediterranean, the Miocene to Recent Carpathians, the Miocene to Recent Aegean and the Oligocene to Recent Caribbean point to a two-stage process. Initially, pairs of terranes comprising a pre-existing retro-arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents. Terrane docking reduces the width of the subduction zone, leading to a second phase during which subduction to strike-slip transitions initiate. The clockwise rotated terrane is caught up in a dextral strike-slip zone, whereas the counterclockwise rotated terrane is entrained in a sinistral strike-slip fault system. The likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline. By analogy with the above model, a revised five-stage Early Jurassic to Early Cretaceous Gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the Weddell Sea area. Seven features are considered diagnostic of double-saloon-door rifting and seafloor spreading: i) earliest movement involves clockwise and counterclockwise rotations of the Falkland Islands Block and the Ellsworth Whitmore Terrane respectively; ii) terranes comprise areas of a pre-existing retro-arc fold thrust belt (the Permo-Triassic Gondwanide Orogeny) attached to an accretionary wedge/magmatic arc; the Falklands Islands Block is initially attached to Southern Patagonia/West Antarctic Peninsula, while the Ellsworth Whitmore Terrane is combined with the Thurston Island Block; iii) paleogeographies demonstrate rifting and extension in a backarc environment relative to a Pacific margin subduction zone/ accretionary wedge where simultaneous crustal shortening occurs; iv) a ridge jump towards the subduction zone from east of the Falkland Islands to the Rocas Verdes Basin evinces subduction rollback; v) this ridge jump combined with backarc extension isolated an area of thicker continental crust — The Falkland Islands Block; vi) well-documented EW oriented seafloor spreading anomalies in the Weddell Sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback; E-mail address: [email protected]. 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.08.011 246 A.K. Martin / Tectonophysics 445 (2007) 245–272 vii) the dextral strike-slip Gastre and sub-parallel faults form one boundary of the Gondwana subduction rollback, whereas the other boundary may be formed by inferred sinistral strike-slip motion between a combined Thurston Island/Ellsworth Whitmore Terrane and Marie Byrd Land/East Antarctica. © 2007 Elsevier B.V. All rights reserved. Keywords: Gondwana breakup; Double-saloon-door seafloor spreading; Plate tectonics; Backarc basin; Subduction rollback; Opposite rotations of terranes 1. Introduction Fourthly, it is shown that these symptomatic features match the characteristics of Gondwana breakup. Although it is known that Gondwana breakup Fifthly, previously proposed driving mechanisms are initiated in a backarc inboard of a subduction zone on compared to the double-saloon-door model. The its Pacific margin (Storey et al., 1996; 1999), and was geodynamics of double-saloon-door rifting and seafloor contemporaneous with eruption of the Karoo, Ferrar and spreading constitute a new driving mechanism for Chon Aike Large Igneous Provinces (Encarnacion et al., continental breakup. 1996; Duncan et al., 1997; Pankhurst et al., 2000), Finally, some issues are discussed and possible future proposed driving mechanisms (Dalziel and Grunow, tests of the hypothesis are outlined. 1992; Rapela and Pankhurst, 1992; Storey, 1995) explain neither the breakup geometry nor the details of 2. Double-saloon-door rifting and seafloor spreading plate dispersal. Similarly, Weddell Sea magnetic sea- in the Chattian – Langhian Valencia Trough, floor spreading anomalies (e.g. Livermore and Hunter, Liguro-Provencal and Algerian Basins, Western 1996; Ghidella et al., 2002) which are perpendicular to Mediterranean the reconstructed Pacific margin subduction zone do not match models which envisage arc-parallel seafloor The double-saloon-door rifting and seafloor spread- spreading. Furthermore, no coherent geodynamic ing model was developed in the Chattian to Langhian cause has emerged for the opposite rotations of the Western Mediterranean where two arc-parallel rifts Ellsworth Whitmore Terrane and the Falkland Islands propagate in opposite directions from an initial central (Watts and Bramall, 1981; Mitchell et al., 1986). location during backarc seafloor spreading and subduc- Here it is proposed that initial Gondwana breakup tion rollback (Martin, 2006). The resultant geometry (Fig. 1) was driven by double-saloon-door rifting and causes pairs of terranes to simultaneously rotate seafloor spreading and that this process successfully clockwise and counterclockwise like double-saloon- explains breakup geodynamics thereby resolving the doors revolving about their hinges. In Fig. 2, plate West above issues. The purpose of this contribution is six-fold. 2 rotates clockwise about pole P1, whereas plate East 2 First, the double-saloon-door rifting and seafloor rotates counterclockwise about pole P2. Movement is spreading scenario, which was developed for the driven by a centrally located force pulling in the Chattian to Langhian development of the Western direction of subduction rollback (b towards q in Fig. 2b). Mediterranean (Martin, 2006) is summarised. Between a and c as well as d and f, movement is accom- Secondly, it is proposed that double-saloon-door modated by extension of thinned continental crust (dark rifting and seafloor spreading also occurred in the shading) whereas from c to b and from f to e, oceanic Serravalian Gibraltar Arc, the Tortonian — Recent accretion occurs (light shading). With further rotation, Tyrrhenian Basin, the Burdigalian — Recent Pannonian (Fig. 2b and c) rifts propagate towards the rotation poles, as Basin, the Serravalian — Recent Aegean, and the demonstrated by seafloor spreading isochrons (dark lines Oligocene — Recent Caribbean. within light shaded oceanic crust) which successively abut Thirdly, a review of these areas (Lonergan and thinned continental crust. A third rift, orthogonal to the White, 1997; Pindell et al., 1999; Duermeijer et al., subduction zone, opens between the rotating terranes and 2000; Mann et al., 2002; Rosenbaum et al., 2002; propagates in the opposite direction to subduction Wenzel et al., 2002; Van Hinsbergen et al., 2006) rollback. suggests a generalised two-stage model, incorporating a In its nascent stage when it is floored by thinned number of symptomatic features. These characteristics continental crust before oceanic crustal breakthrough, constrain a proposed driving mechanism. the spherical triangular area b i j is akin to a rift formed A.K. Martin / Tectonophysics 445 (2007) 245–272 247 Fig. 1. Reconstruction of pre-breakup (pre-190 Ma) Gondwana which aligns Permo-Triassic fold thrust belts (thick dark dashed lines) of the Sierra de la Ventana in South America (SV), Cape Fold Belt in South Africa (CFB), via the Falkland Islands (FI) in their rotated position (Mitchell et al., 1986), the Ellsworth Whitmore Terrane (EWT) in its rotated position (Watts and Bramall, 1981), and the TransAntarctic Mountains (TAM). West Antarctic Peninsula (WAP) and Thurston Island (TI) positions after Grunow et al (1991) and Grunow (1993a,b). Eastern displacement of southern Patagonia (PAT) along the Gastre Fault (GF) to its pre-breakup position (after Rapela and Pankhurst, 1992; Ben-Avraham et al., 1993; Marshall, 1994). East Antarctica close to the Lebombo volcanic lineament (LE) and the southeastern face of the Maurice Ewing Bank (MEB) (after Martin and Hartnady, 1986; Livermore and Hunter, 1996; Lawver et al., 1999, 160 Ma fit; Jokat et al., 2003). Paleopositions of the Agulhas Plateau (AP) and southern Mozambique Ridge (MR) are not constrained. Accretionary wedges and subduction zones of WAP and PAT overlap (cf Lawver et al., 1999). MBL=Marie Byrd Land. NZ=New Zealand. Dot dash line outlines the Karoo Large Igneous Province in Africa, extending to Dronning Maud Land (DML) in East Antarctica, and the Ferrar Province in East Antarctica (dates after Encarnacion et al., 1996; Duncan et al., 1997). Dot dash lines dated 175 Ma and 160 Ma in southwest Gondwana separate the three divisions of the Chon Aike Large Igneous Province (188–178 Ma, 172–162 Ma, and 157–153