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Dynamics of Continental Rift Propagation: the End-Member Modes

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Dynamics of Continental Rift Propagation: the End-Member Modes Earth and Planetary Science Letters 229 (2005) 247–258 www.elsevier.com/locate/epsl Dynamics of continental rift propagation: the end-member modes J.W. Van Wijk*, D.K. Blackman IGPP, Scripps Institution of Oceanography UCSD, La Jolla CA 92093-0225, USA Received 1 June 2004; received in revised form 8 October 2004; accepted 20 October 2004 Available online 15 December 2004 Editor: S. King Abstract An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three- dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin. D 2004 Elsevier B.V. All rights reserved. Keywords: lithosphere extension; rift propagation; transfer zone; finite element model; Laptev margin; Gulf of Aden 1. Introduction axis during rift propagation. Away from the pole of opening, lithosphere thinning increases and may An important aspect of continental rifting is that the eventually result in seafloor spreading. While con- deformation style varies progressively along the rift tinental deformation may be distributed, the rift to drift transition requires a focusing of deformation. In * Corresponding author. Fax: +1 858 534 5332. regions of rift propagation, specifically transition E-mail address: [email protected] (J.W. Van Wijk). zones from continental rifting to seafloor spreading, 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.10.039 248 J.W. Van Wijk, D.K. Blackman / Earth and Planetary Science Letters 229 (2005) 247–258 J.W. Van Wijk, D.K. Blackman / Earth and Planetary Science Letters 229 (2005) 247–258 249 contrasting styles of deformation along the axis of rift rheological provinces. It thus seems that continental propagation are often bounded by shear zones. The rift propagation and rift to drift transition may evolve transitional phase from continental rifting to active in an episodic manner, in which transform or shear seafloor spreading can be found in a few places on zones are formed between the various extensional earth. In the East Gakkel Ridge-Laptev Sea Margin modes along the rift axis. It is a topic of debate whether area in the Arctic Ocean for example, an active large oceanic transform faults develop from such spreading axis approaches a continental margin (Fig. transfer or shear zones in continental rifts [9,15,16]. 1). Here, the Gakkel spreading center terminates The time break between successive propagation against a transform zone (the Khatanga-Lomonosov periods varies considerably both within and between Fracture) that offsets the ridge from a wide zone of different extensional regions, ranging from less than 3 continental rifting (e.g., [1,3–5]). The currently most My (Gulf of Aden) to over 50 My (e.g., Gakkel active continental rift segment on the Laptev Sea Ridge). Margin is the Bel’Kov-Svyatoi Nos Rift, which is It has generally been accepted that continental offset from the Gakkel Ridge over a distance of 150– extension through heterogeneous lithosphere may ease 200 km. This situation has persisted for over 50 or delay the rift propagation process. Rifting prefer- million years (e.g., [1]), and magnetic lineation entially occurs along weak trends in the lithosphere, patterns indicate that the Gakkel Ridge is not like thrust belts. In contrast, propagation of continental propagating further at present; the rift to drift transition extension through a blockedQ zone in the lithosphere has stalled [6]. Different from the stalled rift situation may delay the rift to drift transition [17–19]. A locked of the Arctic in terms of a successful evolution of the zone could be formed, for example, by variations in rift to drift transition, is the Woodlark Basin off Papua rheological properties of the crust, an abnormally New Guinea in the western Pacific. The Woodlark thickened crust or a less favorably oriented weak trend Basin-Papuan Peninsula region forms a continuous (i.e., a weak zone that is not parallel to the axis of system of continental rifting and seafloor spreading maximum tensile stress), inherited from earlier tec- (e.g., [7–12]; Fig. 1). Rapid continental extension tonic events. Rift progression thus seems to be not initiated synchronously about 6 Ma [10] and was only controlled by plate kinematics, but also by the subsequently followed by the rift to drift transition. inherited lithospheric structure. The different styles of rifting on the continental part of The focus of this study is to look at dynamic the rift and the oceanic spreading center are bounded processes near the tip of a weak trend in continental by a currently developing transform fault system [7]. lithosphere. Our approach is probably best viewed as Formation of transform faults during pauses in the rift an attempt to understand the tectonic forces involved propagation process has also been observed in the Gulf in rift propagation that may help to determine the of Aden [13,14]. The longest pause was at the Shukra- interaction between continental and oceanic exten- El Sheik discontinuity (SSD, Fig. 1), where rift sional regimes and processes responsible for the origin propagation stalled for about 13 My [13], an earlier, and evolution of continental breakup. A numerical shorter pause of 4 My occurred at the Alula-Fartak model is used to study dynamic processes near a rift tip Fracture Zone. Shorter pauses and a slowing down and possibilities for rift propagation. More specifi- propagation rate have been observed at other fracture cally, this study explores how modeled rift behavior zones in the Gulf of Aden. Manighetti et al. [13] found depends on the value of rheological parameters. Two evidence for a relation between the occurrence of time end-member modes are distinguished. The stalled rift breaks in propagation, formation of transform zones, phase is characterized by absence of rift propagation and the encounter of the rift tip with different for a certain amount of time. In the alternative rift Fig. 1. (A) Multibeam bathymetry of Woodlark Basin [8]. Spreading ridge shown by thick black lines, major transform zones by thinner black lines. PP=Papuan Peninsula, SI=Solomon Islands. (B) Bathymetry and sketch of major fault structures of the Laptev Sea Margin. Dashed lines indicate strike-slip zones. Structural interpretation after [1]. 1. New Siberian Rift, 2. Anisin Rift, 3. Bel’Kov-Svyatoi Nos Rift, 4. Ust Lena Rift, 5. South Laptev Rift Basin, 6. West Laptev Rift Basin. (C) Topography and bathymetry of the Gulf of Aden and surrounding regions. Thick black line denotes the spreading ridge. SSD: Shukra El-Sheik discontinuity, AFTF: Alula Fartak transform fault, STF: Socotra transform fault, OTF: Owen transform fault. Segmentation pattern from [2,13]. 250 J.W. Van Wijk, D.K. Blackman / Earth and Planetary Science Letters 229 (2005) 247–258 Table 1 where e is strain, l is dynamic viscosity, r is stress, t Thermal and rheological model parameters (same for all simulations) is time and E is Young’s modulus. To represent the Density [kg mÀ3] 2700 (uc), 2800 (lc), rheology of the lithosphere at low temperatures and 3300 (ml) stresses, where it approximates brittle deformation, it Thermal expansion [KÀ1]1Â10À5 Crustal heat production [W mÀ3]1Â10À6 is assumed that rocks are fractured extensively, and Specific heat [J kgÀ1 KÀ1] 1050 strength is controlled by friction along preexisting Conductivity [W mÀ1 KÀ1] 2.6 (uc), 2.6 (lc), faults, and Byerlee’s law is adopted [25–28]. At high 3.1 (ml) stresses, the value of the yield stress is given by the 10 10 Young’s modulus [Pa] 8Â10 (uc), 5Â10 (lc), power law breakdown stress [29]. The state of stress is 1Â1010 (ml) Poisson’s ratio 0.25 constrained by the force balance 6 Power-law breakdown stress [Pa] 600Â10 Br =Bx þ qg ¼ 0 ð2Þ Power-law exponent n 3.0 (olivine) ij j i 1 Activation energy Q [kJ molÀ ] 555 (olivine) q Àn À1 À14 in which g is gravity, is density, and i,j vary 1–3 for Material constant A [Pa s ] 7.0Â10 (olivine) x, y and z directions. In our simulations density Parameters from [29,55,56].
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