Earth Planets Space, Vol. 56 (No. 2), pp. 151-162, 2004
Masaki Kanao1 and Masahiro Ishikawa2
1National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan
2Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan
(Received October 7, 2003; Revised March 2, 2004; Accepted March 2, 2004)
The combination of rock velocities with a seismic signature has been used as a clue to understanding the structure and evolution of the continental lithosphere. The lower crustal reflectivity beneath part of the Pan-African orogeny, the Lützow-Holm Complex (LHC), Western Enderby Land, East Antarctica, has been imaged on single-fold seismic reflection profiles using active seismic studies on the continental ice sheet. The set of velocity layers at middle to lower crustal depths were obtained by modeling the later phases of teleseismic receiver functions observed at Syowa Station (39°E, 69°S), in the LHC. The later phases around 10-16 s from P onset in radial components are explained by assuming a layered lower crustal model with velocity changes of 0.3 km/s in shear waves for 0.5-1.0 km thick layers. The origin of lower crustal reflectivity is discussed in terms of high-pressure laboratory measurements on metamorphic rocks from Western Enderby Land. Lower crustal velocities of 6.9 km/s derived by seismic refraction surveys can be explained by a major composition of mafic pyroxene granulite, as occurs in the Archean Napier Complex. A tectonic model involving collision between the paleo East and West Gondwana blocks during the last stage of Pan-African orogeny is presented to explain the high velocities and reflectivity in the lower crust underlying the LHC. The Napier Complex is considered to have descended eastward under part of the Pan-African belt (LHC), generating a higher-pressure mafic granulite composition. The reflectivity of the lower crust of the LHC may have been enhanced subsequently by extensional stress during the breakup process.
Key words: crustal reflectivity, receiver functions, metamorphic rock velocities, Lützow-Holm Complex, Pan-African orogeny.