Ground fissures within the Main Ethiopian Rift: Tectonic,lithological and piping controls

Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping. The fissures start by piping in linear sub-horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation.     

Provenance and Tectonic Setting of the Metasedimentary Rocks of the Neoproterozoic Adola Belt of Southern Ethiopia

The Metasedimentary rocks from the Adola metamorphic belt has been analysed for major, minor, and trace elements, including REEs, in order to investigate the provenance and tectonic setting of these rocks.On the basis of filed work, petrographic data and major element geochemistry the Adola sedimentary rocks are essentially greywackes with subordinate lithic arenite. Among the 27 samples analysed, only two samples are quartz arenite with SiO₂ values above 89%. CaO-Na₂O-K₂O diagrams showed that most sample cluster around the average plots of granites and granodiorites. In order to determine the tectonic setting and provenance of the rocks, the samples are plotted on various binary and ternary diagrams. The plots on Fe₂O₃T+MgO versus TiO₂, K₂O/Na₂O, Al₂O₃/SiO₂ and Al₂O₃/(CaO+Na₂O) plots show that the Adola sediments have Oceanic Island Arc(OIC), Continental Island Arc(CIA), Active Continental Margin(ACM) and Passive Margin(PM) characteristics. Most samples, however, show island arc affinity. Only two samples (the quartz arenites) fall in the Passive Margin (PM) field.The trace element characteristics of these rocks discriminate the rocks only into oceanic and continental arc fields. The relatively high abundance of the transition metals, mainly Co, Ni, Cr and the low concentration of TiO₂ correlates well with the previously determined geochemical affinity of the basic rocks of Adola suggesting the dominance of the low- Ti oceanic tholeiites and even boninites in the source region.The REE patterns show three distinct groupings; a)With strong LREE enrichment, flat HREE and with out Eu anomaly, shows similar patterns with that of the oceanic island arc rocks; b) samples with strong Light REE enrichment, flat HREE pattern and strong negative Eu anomaly showing similar patterns to the Andean type andesites, probably derived from granitic gneisses and are affiliated to Active Continental Margin settings; c) this group is represented by a single plot having an enriched LREE pattern, flat HREE pattern and strong positive Eu anomaly. It is most likely that this pattern is related to a high normative plagioclase content due to local accumulation of feldspar during sedimentation rather than representing excessive Eu content of the precursor rock. The REE pattern represented by this sample is roughly similar to that of the Devonian greywackes of Australia.In conclusion, the use of geochemical characteristics of the sediments coupled with the geological information from the area strengthens the suprasubduction zone (SSZ) ophiolitic tectonic setting interpretation suggested (Yibas 1993) for the Adola belt. The trace element plots and their absolute abundance, and the REE patterns strongly constrain the tectonic setting and the provenance of the metasediments to an arc related setting.