HR: 0830h
AN: S11C-0314    [PDF]
TI: Anisotropic Layering in the Proterozoic Crust of the Arabian-Nubian Shield
AU: * Park, J
EM: jeffrey.park@yale.edu
AF: Yale Univ., PO Box 208109 Dept of Geology, New Haven, CT 06520-8109 United States
AU: Levin, V
EM: vadim@ldeo.columbia.edu
AF: Rutgers Univ., Dept of Geological Sciences 610 Taylor Road, Piscataway, NJ 08854-8066 United States
AB: The Arabian-Nubian shield (ANS) assembled in the late Proterozoic at the climax of the Pan-African orogeny, primarily from "juvenile" crustal terranes that have been identified as oceanic plateaus or island arcs. Geochemical data suggests that the crust of the shield and the underlying mantle lithosphere have not separated since that time. The shield remained tectonically stable and intact until the Oligocene opening of the Red Sea separated its Nubian and Arabian portions. We contrast two seismic stations on opposite sides of this nascent ocean basin, Ar-Rayn in Saudi Arabia (RAYN) and Kottamiya in Egypt (KEG), with a receiver function (RF) technique that incorporates both regional and teleseismic earthquake data. A broad range of source distances provides opportunity for migration of converted phases, making their interpretation more robust. High frequencies in the P coda allow us to construct RF gathers with useful energy up to 2.5--3 Hz. Crustal thickness is very similar at both sites. Anisotropic layering is found beneath the crust-mantle transition. In their early, "crustal" parts, the RFs display numerous features with characteristic attributes of anisotropic layering, i.e., strong transversely-polarized pulses in broad backazimuth sectors. These pulses change polarity with backazimuth. Because the temporal moveout that would be expected from inclined interfaces is not evident, we conclude that anisotropy is the cause. At KEG the first 4 s of the transverse RF contain multiple phases with clear polarity reversals. The pulse shapes depend on the RF passband, suggesting their origin in fine-scale lamination of the crust. RAYN shows evidence for localized anisotropic layering at the Moho and in the middle crust. Interpretation of the middle crust is challenging, because some pulses exhibit polarity reversals with back azimuth in both radial and transverse receiver functions. RAYN lies more than 700 km from the zone affected by the continental breakup, and thus lithospheric structure here likely reflects Proterozoic tectonics of shield assembly, or earlier processes that shaped the individual terranes that comprise the ANS. KEG lies within the zone affected by the rifting in the Gulf of Suez, and thus may bear marks of both Proterozoic and relatively recent (Miocene) activity.
DE: 3902 Creep and deformation
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting