Deformation, hydration, and anisotropy of the lithospheric mantle in an active rift: Constraints from mantle xenoliths from the North Tanzanian Divergence of the East African Rift

Virginie Baptiste,Andréa Tommasi,Alain Vauchez,Sylvie Demouchy,Roberta L Rudnick

doi:10.1016/j.tecto.2014.11.011

Abstract

We have analyzed the microstructures and crystal preferred orientations (CPO), and calculated the seismic properties of 53 mantle xenoliths from four localities within the North Tanzanian Divergence of the East African rift: two within the rift axis and two in the transverse volcanic belt. Olivine OH concentrations were measured in 15 xenoliths.Most samples have harzburgitic to dunitic compositions and high olivine Mg#. Microstructures and olivine CPO patterns vary strongly depending on the location. In-axis peridotites display mylonitic to porphyroclastic microstructures, which record recent deformation by dislocation creep. Highly stretched orthopyroxenes in mylonites indicate that the deformation was initiated under high stress and probably low temperature. Orthopyroxene replacement by olivine in mylonitic and porphyroclastic peridotites suggest syn-kinematic melt–rock reactions and further deformation under near-solidus conditions. Exsolutions in orthopyroxene imply significant cooling between melt-assisted deformation and xenolith extraction. Late metasomatism is evidenced by the occurrence of veins crosscutting the microstructure and interstitial clinopyroxene and phlogopite. Axial-[100] olivine CPOs predominate, suggesting activation of the high temperature, low pressure [100] {0kl} slip systems and, probably, transtensional deformation. In the volcanic belt, Lashaine peridotites display very coarse-granular textures, indicating deformation by dislocation creep under low deviatoric stress conditions followed by annealing. Axial-[010] olivine CPOs are consistent with transpressional deformation or simultaneous activation of the [100](010) and [001](010) slip systems. Intermediate microstructures and CPOs in Olmani suggests heterogeneous deformation within the volcanic belt. Olivine OH concentrations range between 2 and 12ppmwt. H2O. No systematic variations are observed between in- and off-axis samples. Maximum P wave azimuthal anisotropy (AVp) ranges between 3.3 and 18.4%, and the maximum S wave polarization anisotropy (AVs) between 2.3 and 13.2%. Comparison between seismic properties of in-axis peridotites and SKS splitting data suggests transtensional deformation in the lithospheric mantle beneath the rift.

Highlights

Continental rifting is a complex process that results in localized thinning and, in some cases, in disruption of a continental plate
The later model has been further developed by Vauchez et al (1997) and Tommasi and Vauchez (2001), who, based on the analysis of the influence of inherited structures on the localization of continental breakup, suggested that most major rifts start forming through a transtensional deformation regime produced by the reactivation of the olivine crystallographic fabric frozen in the lithospheric mantle
We explore the relations between deformation, melt or fluids percolation, and hydration in a series of mantle xenoliths from four localities in the North Tanzanian Divergence region (East African Rift)

Summary

Introduction

Continental rifting is a complex process that results in localized thinning and, in some cases, in disruption of a continental plate. To account for the observations in the Basin and Range, Wernicke (1981, 1985) proposed an asymmetrical extension model, in which the deformation is localized on a lithospheric-scale detachment fault Such models cannot account for the narrow rift valley and the strong mantle lithosphere thinning observed in East Africa (Dugda et al, 2007, 2009). Numerical models in which the upper mantle has an anisotropic viscosity controlled by the evolution of olivine crystallographic orientations corroborate this assumption (Tommasi et al, 2009) Rheological heterogeneities, both in the crust and in the mantle, have a major effect on the localization of rifting (e.g., Dunbar and Sawyer, 1989; Nyblade and Brazier, 2002; Vauchez et al, 1997)

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Results

Discussion

Conclusion