Effective elastic plate thickness beneath the East African and Afar plateaus and dynamic compensation of the uplifts

Ebinger, C. J.; Bechtel, Timothy; Forsyth, Donald W.; Bowin, Carl

doi:10.1029/jb094ib03p02883

Cited by 294 publications

(165 citation statements)

References 67 publications

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“…In northeastern Tanzania, extension is accommodated by a diffuse zone ($300 km wide) of block faulting [Dawson, 1992;Ebinger et al, 1997;Foster et al, 1997]. The Western Rift is characterized by several $100-km-long en echelon fault-bounded basins [Ebinger et al, 1989]. Most of the Cenozoic rift system to the south and southwest of the Tanzania craton developed within or adjacent to the Karoo rift system.…”

Section: Geology and Tectonic Historymentioning

confidence: 99%

“…A plume has been proposed to exist beneath the East African Plateau to explain the high elevations, magmatism, and low seismic velocity at the base of the Tanzania craton [Ebinger et al, 1989;Simiyu and Keller, 1997;Nyblade et al, 2000]. Using an estimate of Eocene East Africa lithospheric variations, Ebinger and Sleep [1998] calculated the lateral asthenospheric flow expected for a single plume located beneath Ethiopia.…”

Section: Plume Models and Rift-parallel Flowmentioning

confidence: 99%

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On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

et al. 2004

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[1] East Africa is a tectonically complex region owing to the presence of a rigid craton, paleothrust belts and shear zones, active magmatism and rifting, and possibly even a mantle plume. We present new splitting results of teleseismic shear phases recorded by 21 broadband seismic stations in Tanzania, seven broadband stations in Kenya, and three permanent broadband Global Seismic Network stations in Kenya and Uganda. Inconsistent apparent splitting is observed beneath the craton and along its southern and southeastern flank in Tanzania. Splitting at stations elsewhere in the rifts and orogenic belts is more consistent. We test between different models of anisotropy for stations with inconsistent splitting (single layer with horizontal fast axis, single layer with dipping fast axis, and two layers with horizontal fast axes). However, we show that these more complicated models do not reasonably explain the data. The data are explained better by a laterally (and/or in some places vertically) varying single-layer model with a horizontal fast axis. We arrive at a conceptual model of anisotropy in Tanzania and Kenya that is controlled by (1) active shear along the base of the plate associated with asthenospheric flow beneath and around the moving craton keel, (2) asthenospheric flow from a plume north of central Kenya, (3) fossilized anisotropy in the lithosphere due to past orogenic events, and possibly (4) aligned magma-filled lenses beneath the rifts. Our most robust conclusion is that we can rule out an extension-induced lattice preferred orientation of olivine as a dominant factor, which is surprising given the long history of extension in the region. This indicates that mantle-lithospheric extension in East Africa occurs via dikeintrusion and/or ductile thinning within narrow rift zones and is possibly facilitated by a mechanical lithospheric anisotropy imparted by fossilized north/south structural or mineralogical fabrics.

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Section: Geology and Tectonic Historymentioning

confidence: 99%

Section: Plume Models and Rift-parallel Flowmentioning

confidence: 99%

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

et al. 2004

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“…The former produce and laterally redistribute normal loads of 50 to 500 MPa; that is of the same order as the tectonic forces (5-22 km of sediments over 50-200 km horizontal distances, e.g., Pannonian basin, Albert rift, Baikal rift, Dnieper-Donetz basin [e.g., ure lb and Burov and Cloetingh [1997]). These effects are actually observed [e.g., Ebinger et al, 1989Ebinger et al, , 1999 and, naturally, should influence subsidence phases inferred from tectonic geomorphology and fission track age/length patterns [Rohrman et al, 1995].…”

Section: Introductionmentioning

confidence: 96%

Erosion and rheology controls on synrift and postrift evolution: Verifying old and new ideas using a fully coupled numerical model

Burov¹,

Poliakov²

2001

J. Geophys. Res.

209

163

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Abstract. We use a fully coupled asymmetric dynamic finite element model to study interactions between thermomechanical behavior and surface processes during continental rift evolution. The model accounts for (1) nonlinear brittle-elastic-ductile rheology, layered lithological structure, and faulting; (2) heat transport and thermal buoyancy forces; and (3) "true" erosion and sedimentation (grid elements are eliminated and recreated). Faults are not predefined but are self-localized; their distributions and geometry are model outputs, which provides new geologically sensible constraints on its validity. We test previous ideas on rift evolution based on numerical and analytical component theories (or individual parts) of our model. After demonstrating that our coupled model reproduces classic rift features, we then demonstrate that synrift surface processes result in enhanced lithospheric thinning and widening of the basin, so that the apparent stretching factors increase by a factor of 1.5-2. Sedimentation results not only in thermal but also in localized flexural weakening of the lithosphere, which locally compensates strengthening due to cooling. Erosion on the uplifted flanks produces local strengthening and rebound. Surface processes produce pressure gradients, which drive a ductile crustal flow that (1) provides a fast feedback with tectonic processes and controls subsidence rates and flank stability and (2) drives a secondary extension or compression and uplift on the late synrift/early postrift phase. Our results indicate that kinematic and dynamic rift models that ignore erosion may produce misleading results in many rifts. We reproduced and explained a number of enigmatic synrift phenomena, such as (1) polyphase subsidence provoked by switching of the level of necking between different competent lithological layers and (2) synrift and postrift stagnation and vertical accelerations unassociated with tectonic stress inversion or phase changes.

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“…If coherence exists at wavelengths of 200 km or more, greater than the length of the isochron profiles, the predicted elastic plate thickness would be 227 km. Such a large elastic thickness would only be expected in areas where the age of the lithosphere at the time of loading is >80 Ma [e.g., McNutt, 1984] or in continental zones [e.g., Zuber et al, 1989;Ebinger et al, 1989], but not in young 2.5 Ma oceanic lithosphere. Furthermore, in the study area there is no evidence for off-axis loading.…”

Section: Isochron Coherence and The Strength Of The Oceanic Lithospherementioning

confidence: 99%

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Escartı́n¹

1996

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Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments.In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments.Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis.Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable.Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting. Thesis supervisor:Jian Lin Title:Associate...

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Effective elastic plate thickness beneath the East African and Afar plateaus and dynamic compensation of the uplifts

Cited by 294 publications

References 67 publications

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

Erosion and rheology controls on synrift and postrift evolution: Verifying old and new ideas using a fully coupled numerical model

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

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