干旱区生态环境知识资源中心

Africa was a key tectonic unit in the geologic evolution of the Tethyan realm in the Middle-Late Cretaceous during the rifting of the South-Equatorial Atlantic Ocean. This research sheds light on the tectonic evolution of Africa during the Albian-Campanian time. The two end members of the Wadi Natash alkaline volcanic province [WNAVP] (104-78 Ma) in the South Eastern Desert of Egypt were studied paleomagnetically. The WNAVP (24.5 degrees N-34.25 degrees E) is made up of a thick succession of differentiated lava flows grading from alkali olivine basalt [AOB] followed by hawaiite-mugearite and benmoreite flows with abundant sandstone interbeds, all intruded by a large number of trachyte/phonolite [Tr/Ph] circular lava domes and ring dykes forming conspicuous landmarks in the province. The oldest flows of the AOB (104 7 Ma) and the youngest Tr/Ph plugs and ring dykes (86-78 Ma) as well as the interbedded Turonian "Taref" sandstone formation [previously known as "Nubian" sandstone] were sampled all over the province which covers more than 500 km(2).

The isothermal remanent magnetization [IRM] study pointed to magnetite family as the major remanence in Wadi Natash volcanics, with little contribution from goethite/hematite minerals. On the other hand, goethite and/or hematite are the dominant remanence carriers in the interbedded "Taref" sandstone. After the progressive stepwise thermal demagnetization of all samples, the characteristic remanence [ChRM] direction for each rock type was visually isolated and its best-fit line was calculated, followed by the site-means, rock-unit means and the paleomagnetic poles. This paleomagnetic study revealed that:

1- In the tilt-corrected coordinates, the mean ChRM of the oldest AOB flows [N = 12 sites] was Dec./ Inc. = 340 degrees/-10 degrees [K = 54, alpha 95 = 6 degrees] yielding a north pole at 55 degrees N/250 degrees E. The ChRM of the youngest Tr/Ph ring dykes and plugs [N = 10 sites] was Dec./Inc. = 354 degrees/3.5 degrees [K = 60, alpha 95 = 6.3 degrees] yielding a north pole at 66.5 degrees N/229 degrees E.

2- On the other hand, the site-mean ChRMs of the "Taref" sandstone interbeds [N = 8 sites], in the present-day in-situ coordinates, are clustered around the present-day geomagnetic field with a mean Dec./ Inc. = 3 degrees/33 degrees [K = 37, alpha 95 = 9.3 degrees] yielding a north pole at 83 degrees N/191 degrees E.

The two paleomagnetic poles of the AOB and the Tr/Ph are in accordance with Albian-Campanian poles from Africa [including Madagascar] and those rotated from the main Cratons such as the North American Craton, Europe [including Greenland and Iberia], South America and India using the published Plate Kinematics and Paleomagnetic Euler Pole [PEP] rotation parameters.

Paleogeographically, the two obtained poles of Wadi Natash volcanics place the African Plate at two different azimuths and paleolatitudes during Albian-Cenomaniam and Santonian-Middle Campanian. Tectonically, these two paleomagnetic poles point to an active synchronous northwards translation associated with a counterclockwise rotational convergence of Africa towards Eurasia during the Albian-Campanian time. Between the time of the AOB eruption [104+/-7 Ma] and the intrusion of the TR/Ph [86-78 Ma] Africa concomitantly rotated counterclockwise about 14 degrees and translated northwards towards Eurasia about 6 degrees of latitude. (C) 2011 Elsevier B.V. All rights reserved.