Paleomagnetic constraints on stratigraphy and rift-related tectonics of Pliocene and Early Pleistocene volcano-sedimentary strata: the Mt. Galili hominid research area, Southern Afar Depression, Ethiopia

Abstract

Our paleomagnetic investigations in the Northern Main Ethiopian Rift concentrated on ascertaining characteristic remanence directions of volcanic and sedimentary layers embodied within the Mt.Galili Formation (MGF). Magnetic stratigraphy was applied mainly to support anthropological studies on early hominid evolution. The new paleomagnetic results provided implications for stratigraphic age determinations of the MGF, and they also support arguments for rift-related tectonics as block rotation and tilting relative to the stable African crust since the Pliocene. Based on 497 paleomagnetic samples, we applied magnetic stratigraphy on magmatic and sedimentary rocks to determine the 40 39 age of the MGF and correlated the results with published Ar/ Ar age data on tuff and ignimbrite layers which range from ca. 5.4 Ma to ca. 2.3 Ma, consistent with biostratigraphic age constraints. Nine magnetic polarity intervals of the Gilbert zone were identified within the lower four members of the MGF covering a time span from 5.37 to 3.58 Ma. The upper two members of the MGF comprised three polarity intervals assigned to the upper Gauss and lower Matuyama polarity zones. In combination with an 40 39 Ar/ Ar age of 2.35 Ma, located in the middle of the section, we infer that ca. 0.5 Ma and four polarity intervals are missing between the MGF’s lower and upper members. We attribute this significant depositional gap to uplift of the region due to the intrusion of basalt magma, associated with deposition of the Upper Shabeley Laag Member. Extensional rift tectonic processes were intimately affiliated with magmatic activity and generation of rapidly changing sedimentary environments of the MGF`s deposits, but the bulk of the observed tectonic features resulted from post-depositional tectonic overprint, when the re-orientated Wonji Fault System affected the Mt.Galili area in the Quaternary period, thereby creating new internal rift (in older rift) structures. Tectonic movements of MGF`s strata were deduced from our analysed characteristic remanent magnetisation (ChRM) directions compared with Pliocene reference data. The investigation revealed almost unchanged orientation of the ChRM-directions for the marginal internal rift zone in the East of the Mt.Galili area, but moderate counterclockwise rotation for the central internal rift zone in the West. The North of the Mt.Galili area suffered moderate clockwise rotation, probably due to the Mt.Galili area`s regional position, adjoining a transfer zone between the Quaternary Angele and Addo-Do magmatic segments. ____________________________________________________________ ___________________________________ Austrian Journal of Earth Sciences Vienna xxx xxx DOI: 10.17738/ajes.2017.0018 2017 Volume 110/2