TY - JOUR A1 - Ryberg, Trond A1 - Haberland, Christian A1 - Haberlau, Thomas A1 - Weber, Michael H. A1 - Bauer, Klaus A1 - Behrmann, Jan H. A1 - Jokat, Wilfried T1 - Crustal structure of northwest Namibia: Evidence for plume-rift-continent interaction JF - Geology N2 - The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Parana and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (<100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures). Y1 - 2015 U6 - https://doi.org/10.1130/G36768.1 SN - 0091-7613 SN - 1943-2682 VL - 43 IS - 8 SP - 739 EP - 742 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Scharf, Andreas A1 - Sudo, Masafumi A1 - Pracejus, Bernhard A1 - Mattern, Frank A1 - Callegari, Ivan A1 - Bauer, Wilfried A1 - Scharf, Katharina T1 - Late Lutetian (Eocene) mafic intrusion into shallow marine platform deposits north of the Oman Mountains (Rusayl Embayment) and its tectonic significance JF - Journal of African earth sciences N2 - A silica undersaturated alkali-olivine basanitic magma intruded the late Paleocene/early Eocene Jafnayn Formation near Muscat. Geochemical analyses indicate that a significant amount of host rock (limestone) was assimilated into the magma. We dated the basanite as 42.7 +/- 1.0 Ma (2 sigma error; late Lutetian), using the whole rock Ar-40/Ar-39 step-wise heating technique. Intrusion occurred in the hanging wall of a major regional extensional shear zone (Frontal Range Fault, FRF) bounding the northern margin of two domes within the Oman Mountains (Jabal Akhdar and Saih Hatat domes). Two shear intervals along the FRF have been documented. The first interval lasted immediately after emplacement of the Semail Ophiolite (latest Cretaceous-early Eocene) while the second and poorly constrained interval was assumed to have occurred during the Oligocene. The proximity of the basanite to the FRF suggests that magma used extensional faults for the upper part of its ascent path. Reactivated Permian rift faults of the Pangaea rift or other preexisting faults may have been used for the lower ascent part. We conclude that the basanite intrusion coincided with the onset of the second deformation interval along the FRF, because (1) the position of the basanite is near a dextral releasing bend, associated with the second shear interval, (2) the overlap of our Ar-40/Ar-39 age with the cooling curves for rocks from the nearby Jabal Akhdar Dome, and (3) the basanite postdates the first FRF deformation episode by > 10 Ma. Thus, the second interval along the FRF had started already during the late Lutetian and probably lasted into the Miocene. KW - Ar-40/Ar-39 age KW - Jafnayn formation KW - gravitational collapse KW - Basanite KW - extension KW - Limestone assimilation in basanite Y1 - 2020 U6 - https://doi.org/10.1016/j.jafrearsci.2020.103941 SN - 1464-343X SN - 1879-1956 VL - 170 PB - Elsevier CY - Oxford ER -