Active concomitant counterclockwise rotation and northwards translation of Africa during the Albian-Campanian time: A paleomagnetic study on the Wadi Natash alkaline volcanic province (104-78Ma), South Eastern Desert, Egypt

摘要

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] (104-78Ma) in the South Eastern Desert of Egypt were studied paleomagnetically. The WNAVP (24.5° N-34.25° 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±7Ma) and the youngest Tr/Ph plugs and ring dykes (86-78Ma) as well as the interbedded Turonian "Tare" sandstone formation [previously known as "Nubian" sandstone] were sampled all over the province which covers more than 500km~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 "Tare" 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°/-10° [K=54, α95=6] yielding a north pole at 55° N/250° E. The ChRM of the youngest Tr/Ph ring dykes and plugs [N=10 sites] was Dec./Inc.=354°/3.5° [K=60, α95=6.3] yielding a north pole at 66.5° N/229° E. 2- On the other hand, the site-mean ChRMs of the "Tare" 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°/33° [K=37, α95=9.3°] yielding a north pole at 83° N/191° 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° and translated northwards towards Eurasia about 6° of latitude.