Lithospheric structure beneath continental rifts

Isotropic shear wavespeed variations in the upper mantle from the SEMum model [Lekic and Romanowicz, 2008]. The model is developed using the spectral element method to perform full waveform modeling. Note the structural complexity in the uppermost 100-200 km which must be corrected for in order to ensure robust scattered wave imaging. The slow velocity anomalies beneath the East African Rift to great depths (>350 km), and the Afar plume is seen into the transition zone.

Continental rifts are zones of localized extension that play a paramount role in the breakup of continents and the evolution of plate tectonics on Earth. Key debates have focused on how the style and evolution of continental rift systems reflect the relative roles of magmatism, the geometry of mechanical extension, crust and mantle temperature and composition, sediment loading and thermal insulation, and active versus passive asthenospheric flow [e.g. Ruppel, 1995; Wilson et al., 2005; Buck, 2006; Lizarralde et al, 2007; Keranan et al., 2009]. Disentangling the causes and mechanics of continental rifting requires both the accurate mapping of broad asthenospheric and lithospheric features, as well as the detection and characterization of sharp interfaces such as the Moho, the lithosphere-asthenosphere boundary (LAB), and other reflectors that may be related to melting or faulting. Therefore, it is unsurprising that the massive literature concerning continental rifting comprises a plethora of seismic techniques, including body-wave [e.g. Gao et al., 2004; Park and Nyblade, 2006; Bastow et al., 2005; Green et al., 1991] and surface wave tomography [e.g. West et al., 2004; Sebai et al., 2006; Wang et al., 2009], crustal refraction profiles [e.g. Fuis et al., 1984; Thybo and Nielsen, 2009; Mackenzie et al., 2005; Fuis et al., 1984; Lizaralde et al., 2007], and scattered wave imaging [e.g. Dugda et al., 2007, 2005; Wilson et al., 2005; Persaud et al., 2007; Keranan et al., 2009].

 

I am attempting to characterize the geometry, gradients and retention of sharp lithospheric and crustal interfaces. By combining tomographic, waveform-modeling, and scattered-wave imaging methods, I am trying to determine the depth to the LAB and its topography, including how rapidly changes in lithospheric thickness occur, as well as the strength of its associated velocity gradients.