Browsing by Author "Gabriel D. Mulibo"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    THE 1994–1995 MANYARA AND KWAMTORO EARTHQUAKE SWARMS: VARIATION IN THE DEPTH EXTENT OF SEISMICITY IN NORTHERN TANZANIA
    (SOUTH AFRICAN JOURNAL OF GEOLOGY,, 2009) Gabriel D. Mulibo
    Seismicity within two earthquake swarms (mb 1.0 to 4.0) in northern Tanzania recorded by the 1994-1995 Tanzania Broadband Seismic Experiment has been investigated through event relocation, modelling of regional depth phases and examination of focal mechanisms. In the Manyara swarm, seismicity is distributed over a region ~20 km wide and extends to >30 km depth. Hypocenters correlate well with the Manyara Rift, and focal mechanisms of many events show normal faulting with nodal planes having the same north-northeast orientation as the Manyara Rift border fault. This result indicates that the events are consistent with slip along the border fault and related faults beneath the Manyara Rift, and shows that the faults extend into the lower crust. Seismicity in the KwaMtoro swarm is distributed over a region ~10 km wide but extends only to ~12 km depth. There are no mapped faults above the swarm, but a strong correlation between the north to south orientation of the swarm, the north to south orientation of nodal planes in focal mechanisms, and north to south striking extensional structures nearby, suggest that events in this swarm could be caused by slip on a system of rift faults. However, a magma-driven origin for either swarm cannot be ruled out either.
  • Thumbnail Image
    Item
    Petrophysical Analysis of the Mpapai well Logs in the East Pande Exploration Block, Southern Coast of Tanzania: Geological Implication on the Hydrocarbon Potential
    (Open Journal of Geology, 2018) Aneth L. Lyaka; Gabriel D. Mulibo
    This study presents results of log analysis from Mpapai well, which is located in the East Pande Block, southern coast of Tanzania. The study aimed at assessing the hydrocarbon potential of lithological units encountered during drilling of Mpapai well. To achieve the general objective, suites of wire-line logs from Mpapai well were used for the analysis. Based on wire-line logs, three types of lithology were identified which include sandstone, shale and limestone. Seven sandstone bodies marked as MpapaiA, B, C, D, E, F and G were identified with their tops and bases at the depth interval from 3004 m to 4008 m. Four zones among seven sandstones bodies marked as MpapaiB, E, F and MpapaiG were identified as reservoir zones. Computed petrophysical parameters for the four reservoir zones gave an average total porosity ranging from 14% to 21% with low permeability in the range of 3.92 mD to 13.67 mD. The low permeability indicates that the reservoir sand bodies are impermeable, that might have been affected by the geology of the area where high content of clay minerals reduces permeability due to filling in open spaces. The fluid type defined in the reservoir zones is basically water with high saturation greater than 75%, which indicates that the proportion of void space occupied by water is high, consequently low hydrocarbon saturation and production. Despite of fair to good porosity, the low permeability and high-water saturation indicate that the quality of Mpapai prospect is poor.
  • Thumbnail Image
    Item
    Seismic Evidence for Plume- and Craton-Influenced Upper Mantle Structure Beneath the Northern Malawi Rift and the Rungwe Volcanic Province, East Africa
    (Geochemistry, Geophysics, Geosystems, 2018-09-28) Ashley Grijalva; ; ; ; Gabrielle Tepp; Marsella Kachingwe; Gabriel D. Mulibo; Richard Wambura Ferdinand; Gabriel Mbogoni; Patrick R. N. Chindandali; Donna J. Shillington; Cynthia J. Ebinger; James B. Gaherty; Natalie J. Accardo; Kyle Homman; Andrew A. Nyblade
    P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low-velocity zone (LVZ) with δVp = ~1.5–2.0% and δVs = ~2–3% centered beneath the RVP, a lower-amplitude LVZ (δVp = ~1.0–1.3% and δVs = ~0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower-amplitude anomaly at ~_10° to 11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting.
  • Thumbnail Image
    Item
    The seismotectonics of Southeastern Tanzania: Implications for the propagation of the eastern branch of the East African Rift
    (Tectonophysics, 2016) Gabriel D. Mulibo
    Seismicity patterns and focal mechanisms in southeastern Tanzania, determined from data recorded on temporary and permanent AfricaArray seismic stations, have been used to investigate the propagation direction of the Eastern branch of the East African Rift System southward from the Northern Tanzania Divergence Zone (NTDZ). Within the NTDZ, the rift zone is defined by three segments, the Eyasi segment to the west, theManyara segment in the middle, and the Pangani segment to the east. Results show that most of the seismicity (~75%) extends to the south of theManyara segment along the easternmargin of the Tanzania Craton, and at ~6–7° S latitude trends to the SE along the northern boundary of the Ruvuma microplate, connecting with a N–S zone of seismicity offshore southern Tanzania and Mozambique. A lesser amount of seismicity (~25%) is found extending from the SE corner of the Tanzania Craton at ~6–7° S latitude southwards towards Lake Nyasa. This finding supports a model of rift propagation via the Manyara segment to the southeast of the Tanzania Craton along the northern boundary of the Ruvuma microplate. However, given the limited duration of the seismic recordings used in this study, the possibility of another zone of extension developing to the south towards Lake Nyasa (Malawi) cannot be ruled out. Focal mechanisms along the boundary between the Victoria and the Ruvuma microplates and offshore southeastern Tanzania show a combination of normal and strike slip faulting indicating mainly extension with some sinistral motion, consistent with the mapped geologic faults and a clockwise rotation of the Ruvuma microplate.