Western Rift Research Articles

Melting of a Two-component Source beneath Iceland

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland’s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.

The evolution of the western rift area of the Fimbul Ice Shelf, Antarctica

Abstract. This paper studies the evolution of a zone in the Fimbul Ice Shelf that is characterised by large crevasses and rifts west of Jutulstraumen, an outlet glacier flowing into Fimbulisen. High-resolution radar imagery and radio echo sounding data were used to study the surface and internal structure of this rift area and to define zones of similar characteristics. The western rift area is dominated by two factors: a small ice rumple that leads to basal crevasses and disturbs the homogeneity of the ice, and a zone with fibre-like blocks. Downstream of the rumple we found down-welling of internal layers and local thinning, which we explain as a result of basal crevasses due to the basal drag at the ice rumple. North of Ahlmannryggen the ice loses its lateral constraint and forms individual blocks, which are deformed like fibres under shear, where the ice stream merges with slower moving ice masses of the western side. There, the ice loses its integrity, which initiates the western rift system. The velocity difference between the slow moving western part and the fast moving extension of Jutulstraumen produces shear stress that causes the rifts to form tails and expand them to the major rifts of up to 30 km length.

Comparing extension on multiple time and depth scales in the Corinth Rift, Central Greece

SUMMARY The young ( 10–15 mm yr –1 in the west. We quantify total upper-crust and whole-crust extension on three profiles across the Corinth Rift. Whole-crust extension is greater across the central rift (∼11–21 km) than across the western part of the rift (∼5–13 km). This correlates with the overall rift morphology, which shows maximum basement subsidence, sediment accumulation, rift width and greatest summed Late Quaternary fault displacements in the central basin, but contrasts with the pattern of geodetic extension rates which are greater to the west of the central basin. The E–W increase in strain rates interpreted from geodetic data cannot have persisted over rift history to produce the observed rift morphology. We suggest the discrepancy between shortterm and long-term extension patterns is related to shifts in the loci of maximum extension due to fault growth and linkage during Corinth Rift history, and is likely a characteristic of rift development in general. Total upper-crust and whole-crust extension estimates in the western rift, where extension estimates are best constrained, are within error. We propose that uniform pure-shear extension is a viable extension mechanism in the western rift and crustal extension estimates do not require the existence of a major active N–S dipping detachment fault.

Impacts of Tectonic Earthquakes in the Western Rift Valley of Africa on the Volcanic Activity of Nyiragongo, Virunga Region

Impacts of Tectonic Earthquakes in the Western Rift Valley of Africa on the Volcanic Activity of Nyiragongo, Virunga Region

The Cenozoic western Svalbard margin: sediment geometry and sedimentary processes in an area of ultraslow oceanic spreading

The northeastern high-latitude North Atlantic is characterised by the Bellsund and Isfjorden fans on the continental slope off west Svalbard, the asymmetrical ultraslow Knipovich spreading ridge and a 1,000 m deep rift valley. Recently collected multichannel seismic profiles and bathymetric records now provide a more complete picture of sedimentary processes and depositional environments within this region. Both downslope and alongslope sedimentary processes are identified in the study area. Turbidity currents and deposition of glacigenic debris flows are the dominating downslope processes, whereas mass failures, which are a common process on glaciated margins, appear to have been less significant. The slide debrite observed on the Bellsund Fan is most likely related to a 2.5–1.7 Ma old failure on the northwestern Barents Sea margin. The seismic records further reveal that alongslope current processes played a major role in shaping the sediment packages in the study area. Within the Knipovich rift valley and at the western rift flank accumulations as thick as 950–1,000 m are deposited. We note that oceanic basement is locally exposed within the rift valley, and that seismostratigraphic relationships indicate that fault activity along the eastern rift flank lasted until at least as recently as 1.5 Ma. A purely hemipelagic origin of the sediments in the rift valley and on the western rift flank is unlikely. We suggest that these sediments, partly, have been sourced from the western Svalbard—northwestern Barents Sea margin and into the Knipovich Ridge rift valley before continuous spreading and tectonic activity caused the sediments to be transported out of the valley and westward.

The enigmatic Chad lineament revisited with global gravity and gravity-gradient fields

Abstract The crustal structure of northern Africa is puzzling, large areas being of difficult access and concealed by the Sahara. The new global gravity models are of unprecedented precision and spatial resolution and offer a new possibility to reveal the structure of the lithosphere beneath the Sahara. The gravity gradients correlate better than gravity with geological features such as rifts, fold belts and magmatic deposits and intrusions. They are an ideal tool to follow geological units (e.g. basement units) below a stratigraphic layer of varying density (e.g. sediments). We focus on the Chad lineament, a 1300 km arcuate feature located between the west and central African rift system. The gravity fields show differences between the lineament and the west and central African rift system. Along the centre of the lineament high-density rocks must be present, which relate to either magmatic or metamorphic rocks. This is very different to the lineaments of the western and central-west African rift system which are filled with sediments. Considering present models of rifting and the absence of topography, the lineament cannot be coeval to the west and central African rift system and is most likely older. We suggest that the lineament is a structural element of the Saharan Metacraton.

A hydrogeological study of the Nhandugue River, Mozambique – A major groundwater recharge zone

The Nhandugue River flows over the western margin of the Urema Rift, the southernmost extension of the East African Rift System, and marks the north-western border of Gorongosa National Park, Mozambique. It constitutes one of the major indispensable water resources for the ecosystem that the park protects. Our study focused on the hydrogeological conditions at the western rift margin by resistivity measurements, soil sampling and discharge measurements. The resistivity results suggest that the area is heavily faulted and constitutes a major groundwater recharge zone. East of the rift margin the resistivity indicate that solid gneiss is fractured and weathered, and is overlain by sandstone and alluvial sediments. The top 10–15m of the alluvial sequence is interpreted as sand. The sand layer extends back to the rift margin thus also covering the gneiss. The sandstone outcrops a few kilometers from the rift margin and dips towards east/south-east. Further into the rift valley, the sand is underlain by lenses of silt and clay on top of sand mixed with finer matter. In the lower end of the investigated area the lenses of silt and clay appears as a more or less continuous layer between the two sand units. The topmost alluvial sand constitutes an unconfined aquifer under which the solid gneiss forms a hydraulic boundary and the fractured gneiss an unconfined aquifer. The sandstone is an unconfined aquifer in the west, becoming semi-confined down dip. The lenses of silt and clay forms an aquitard and the underlying sand mixed with finer matter a semi-confined aquifer. The surface runoff decreases downstream and it is therefore concluded that surface water infiltrates as recharge to the aquifers and moves as groundwater in an east/south-eastward direction.

Landscape genetics of the key African acacia species Senegalia mellifera (Vahl)– the importance of the Kenyan Rift Valley

Acacias across Africa have enormous ecological and economic importance, yet their population genetics are poorly studied. We used seven microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in the widespread acacia, Senegalia (Acacia) mellifera. We quantified variation among 791 individuals from 28 sampling locations, examining patterns at two spatial scales: (i) across Kenya including the Rift Valley, and (ii) for a local subset of 11 neighbouring locations on Mpala Ranch in the Laikipia plateau. Our analyses recognize that siblings can often be included in samples used to measure population genetic structure, violating fundamental assumptions made by these analyses. To address this potential problem, we maximized genetic independence of samples by creating a sibship-controlled data set that included only one member of each sibship and compared the results obtained with the full data set. Patterns of genetic structure and barriers to gene flow were essentially similar when the two data sets were analysed. Five well-defined geographic regions were identified across Kenya within which gene flow was localized, with the two strongest barriers to dispersal splitting the Laikipia Plateau of central Kenya from the Western and Eastern Rift Valley. At a smaller scale, in the absence of geographic features, regional habitat gradients appear to restrict gene flow significantly. We discuss the implications of our results for the management of this highly exploited species.

Tectonic history of the western Tethys since the Late Triassic

The tectonic history of the western Tethys since the Late Triassic is illustrated through a set of computer-generated plate reconstructions, which are based on a rigorous plate motions model of this region. The model is constrained by the Atlantic plate kinematics and on-land geologic evidence and defines 13 tectonic phases, spanning the time interval from the late Ladinian (230 Ma) to the present. The kinematics associated with the Late Triassic western Tethyan rifts produced the detachment of a large composite fragment from the northern margin of Gondwana. It can be considered as the eastern propagation of the central Pangea breakup. During the Early Jurassic these rift zones became inactive, while new zones of extension formed along the southern margin of Eurasia, the eastern margin of Iberia, and within the rifted northern Gondwana fragment itself. Plate motions associated with the first two extensional centers can still be considered as an eastern branch of the central Atlantic plate kinematics. Conversely, the kinematic parameters of the latter rift result from the composition of the Euler rotation describing the central Pangea breakup and the Euler pole of closure of the paleo–Tethys ocean. The Late Triassic–Early Jurassic rifting phases determined the formation of a number of independent microplates at the interface between Africa and Eurasia. Starting from the Early Cretaceous, convergence between Africa and Eurasia triggered further deformation within the dispersed continental fragments and the formation of backarc basins at the active margins, ultimately leading to an increase in the number of tectonic elements that were moving independently in the western Tethyan region during the Late Cretaceous and the Cenozoic. The proposed tectonic evolution of the western Tethys area is compatible with both global-scale plate kinematics and geological constraints from on-land data observed across the present-day mosaic of displaced terranes surrounding the Mediterranean region.

Monogenetic vent self-similar clustering in extending continental crust: Examples from the East African Rift System

The spatial clustering of fracture networks and vents in basaltic volcanic fields has been analyzed in three sectors of the East African Rift System, the classical example of an active continental rift. Fracture trace maps and monogenetic basaltic vents have been thus collected in the Afar Depression, in the Main Ethiopian Rift, and in the Virunga Belt (Western Rift). The mapped vents are generally younger than 2 Ma, and most are of Holocene age. All the analyzed fracture networks have self-similar clustering with fractal exponents ( D f ) varying in the 1.54−1.85 range. Also, vents show a self-similar clustering with fractal exponents ( D v ) in the 1.17−1.50 range. For all the studied sectors, the relationship D f > D v has been observed. The fractal exponents for vents ( D v ) of power-law distributions are computed in a range of lengths with a lower and an upper cutoff. The upper cutoff ( Uco ) for the fractal clustering of vents in the studied sectors of the East African Rift System are compared with the respective crust thickness derived by independent geophysical data. The computed Uco s for the studied sectors well match the crust thickness in the volcanic fields. A preliminary conceptual model to explain the relationships between the upper cutoffs of the fractal distribution of vents and the thickness of the crust in the volcanic fields is thus proposed in the light of the percolation theory.

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector, Tanzania

A 500 m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2 Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the Waterfall Sequence at the base are followed by a basanite series, and by a range of evolved nephelinites forming the upper part of the escarpment. Numerous dykes and Strombolian scoria deposits indicate local fissure eruptions as opposed to or in addition to more distant sources. Primitive compositions within each of the series indicate variable candidates for primary magmas. The composition of the basanite suite ranges from primitive mantle melts (high Mg#, Cr, Ni) to more evolved rocks, in particular hawaiites, generated by fractionation of olivine, pyroxene and magnetite. Inter-bedded within the basanite suite, one single olivine melilitite flow with high Mg# and abundant olivine and pyroxene megacrysts is the only primitive candidate for the nephelinite suite. However, in view of the large compositional gap and marked differences in incompatible element ratios, a relation between this flow and the nephelinites remains hypothetical. The variation within the evolved nephelinite series can be partly explained by fractionation of pyroxene, apatite, perovskite (and some nepheline), while magma mixing is indicated by zonation patterns of pyroxene. The most evolved nephelinite, however, differs significantly from all other nephelinites in major and trace elements. Thus the entire sequence is petrologically not a coherent evolution, rather the result of different mantle melts fractionating under variable conditions. Carved into the rift scarp of the study area west of Engare Sero is a young explosion crater, the Sekenge Crater. Sekenge Tuffs are olivine melilitites, similar to other craters and maars of the “Younger Extrusives” on the rift valley floor surrounding Oldoinyo Lengai. Further, still younger alkaline tuffs are found on the top of the rift shoulder.

Thermal and exhumation history of the central Rwenzori Mountains, Western Rift of the East African Rift System, Uganda

The Rwenzori Mountains (Mtns) in west Uganda are the highest rift mountains on Earth and rise to more than 5,000 m. We apply low-temperature thermochronology (apatite fission-track (AFT) and apatite (U–Th–Sm)/He (AHe) analysis) for tracking the cooling history of the Rwenzori Mtns. Samples from the central and northern Rwenzoris reveal AFT ages between 195.0 (±8.4) Ma and 85.3 (±5.3) Ma, and AHe ages between 210.0 (±6.0) Ma to 24.9 (±0.5) Ma. Modelled time–temperature paths reflect a protracted cooling history with accelerated cooling in Permo-Triassic and Jurassic times, followed by a long period of constant and slow cooling, than succeeded by a renewed accelerated cooling in the Neogene. During the last 10 Ma, differentiated erosion and surface uplift affected the Rwenzori Mtns, with more pronounced uplift along the western flank. The final rock uplift of the Rwenzori Mtns that partly led to the formation of the recent topography must have been fast and in the near past (Pliocene to Pleistocene). Erosion could not compensate for the latest rock uplift, resulting in Oligocene to Miocene AHe ages.

Cladogenesis and endemism in Tanzanian mole-rats, genus Fukomys: (Rodentia Bathyergidae): a role for tectonics?

African mole-rats of the family Bathyergidae are subterranean hystricomorph rodents found throughout sub-Saharan Africa, where the distributional ranges of the most speciose taxa are divided by the African Rift Valley. In particular, mole-rats of the genera Heliophobius and Fukomys are distributed widely, and their adaptive radiation appears to have been strongly influenced by the geological process of rifting. As a result, virtually all members of the genus Fukomys occur in locations west of the Rift Valley. However, a small number of isolated populations occur east of the Rift Valley in Tanzania, where Heliophobius is widespread and is the predominant bathyergid rodent. Phylogenetic analysis of mitochondrial cytochrome b sequences of previously unstudied Tanzanian mole-rats (genus Fukomys) and geographically adjacent populations strongly suggests that vicariance in the Western Rift Valley has subdivided populations of mole-rats and, together with climatic changes, played a role in the isolation of extralimital populations of Fukomys in Tanzania. Together with molecular clock-based estimates of divergence times, these results offer strong support for the hypothesis that the observed patterns of cladogenesis are consistent with tectonic activity in the ‘Mbeya triple junction’ and Rungwe volcanic province between Lakes Rukwa and Nyasa. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 337–352.

The 1375 Ma “Kibaran event” in Central Africa: Prominent emplacement of bimodal magmatism under extensional regime

In previous literature, the “Kibara belt” has often been portrayed as a Mesoproterozoic belt trending NE over 1300 km across the Central African Congo craton, from the Angola-Zambia-D.R.Congo border triple-junction in the SW, through Katanga and Kivu-Maniema (DRC), Rwanda and Burundi, up to SW Uganda and NW Tanzania in the NE. However, north of Katanga in the DRC, there is a clear break in continuity of the thus-defined “Kibara belt”, cross-cut by Palaeoproterozoic (Rusizian) terranes, in structural continuity with the NW-SE trending Ubende shear belt further south in Tanzania. In this paper, we redefine the use of the term “Kibara belt” (“KIB”), restricting it henceforward to the belt occurring SW of the Ubende-Rusizian terranes, i.e. in the Kibara Mountains type area of Katanga (DRC). The other belt situated NE of the Ubende-Rusizian terranes and east of the Western Rift, previously referred to as the “Northeastern Kibaran Belt” (NKB), is henceforward and for clarity reasons re-named “Karagwe-Ankole belt” (“KAB”). In our re-definitions, we do not take into account the Kivu-Maniema (DRC), because of its geological complexity apparent from satellite imagery and the lack of recent field data, although “some continuity” of the KAB in Kivu-Maniema is obvious. For the KAB, we document 10 new SHRIMP U–Pb zircon ages, in addition to new 40Ar/ 39Ar and laser-ablation zircon Hf data, all of them obtained from previously already isotopically “dated” rock specimens. Contrary to previous belief, magmatism in the KAB (and the KIB) is punctuated by the profuse emplacement of bimodal intrusions between 1380 and 1370 Ma. Moreover, the occurrence of Palaeoproterozoic basement within the KAB is confirmed. The prominent c. 1375 Ma bimodal magmatism in the KAB consists of (1) the 350 km long Kabanga-Musongati (KM) alignment of mafic and ultramafic, Bushveld-type, layered complexes, originating from an enriched lithospheric mantle source and (2) voluminous S-type granitoid rocks with accompanying subordinate mafic intrusive rocks. Both coeval magmatic suites are interpreted to have been emplaced under extensional regime in a regional-scale intra-cratonic setting. During ascent the mantle-derived magmas have taken advantage of the regionally occurring crustal-scale zone of weakness in the KAB, i.e. the rheological boundary between the Archaean craton of Tanzania, to the east, and the adjacent Palaeoproterozoic basement (2.1 Ga mobile belt), to the west, both overlain by Mesoproterozoic (meta)sedimentary rocks. The mantle-derived magmas initiated concomittantly and under extension, large-scale crustal melting preferentially of the Palaeoproterozoic basement, and characterised by the absence of a thick lithospheric profile in contrast to the nearby Archaean craton. Such petrogenetic processes have intra-plate characteristics and are thus not associated with normal plate boundary processes nor with their typical magmatism. On the contrary, they may include rift-related packages, characteristically associated with successful or attempted, though unsuccessful, continental break-up as was the case here. In the KAB, later magmatic events occurred respectively at c. 1205 (A-type granitoids) and c. 986 Ma (“tin-granites”). They represent minor additions to the crust. For decades the term “Kibaran” has been used to name the orogenic cycle and/or orogeny occurring in (Central) Africa in “late” Mesoproterozoic times (1.4–1.0 Ga), which was considered to have a protracted character. Here, we propose to restrict henceforward the term “Kibaran” only to the prominent tectono-magmatic “event”, giving rise to the coeval c. 1375 Ma bimodal magmatism emplaced under extensional regime. This “Kibaran event” pre-dates compressional deformation, reflecting far-field effects of global orogenic events, external to the craton and possibly related to Rodinia amalgamation.

Some aspect of seismicity prior to the 27 November 2006 eruption of Nyamuragira volcano and its implication for volcano monitoring and risk mitigation in the Virunga area, Western Rift Valley of Africa

The temporal variation in the seismicity in the Nyamuragira area was investigated for the period 1 July 2004–27 November 2006, prior to the 27 November 2006 eruptions of Nyamuragira. It is found that this eruption was preceded by 11 months by progressive increase in number of long-period earthquakes. This pattern of seismicity, integrated with other geophysical, geological and geochemistry data, is useful for volcano monitoring and risk mitigation.

Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift

Lithospheric thinning is a fundamental process associated with the transition from continental to oceanic regimes during continental rifting. Precisely how and when this lithospheric thinning proceeds are first order controls on rift basin evolution. The Main Ethiopian Rift, part of the ∼ 2000 km long East African Rift System, is the archetypical modern example of continental rifting, and a key location in which to study the evolution of the lithosphere during extension. This study explores lithospheric modification in the interface region between the Main Ethiopian rift and northward propagating rifting from Kenya through a major and trace element study of rift initiation and maturation using the 19 myr magmatic record preserved in this region. Initial rifting in southern Ethiopia is coincident with the eruption of basalts along the rift shoulders that are characterized by deep fractionation trends (0.5 GPa) and poorly developed magmatic pathways. The earliest of these basalts are derived from melting columns where the aluminum phase is garnet-dominated (Tb N/Yb N ∼ 1.8–2) and has geochemical characteristics interpreted as melting of the lithospheric mantle. The transition from initial rift shoulder magmatism to Quaternary magmatic–tectonic fault belts on the modern rift floor at Arba Minch (6°N) is coincident with a shallowing of the melting column (Tb N/Yb N ∼ 1.3–1.7), less significant contributions from the lithospheric mantle, and the establishment of a shallow fractionation regime (0.1 GPa). At Chencha (∼ 6.3°N) newly dated (12.32 ± 0.17 Ma) magmatism on the rift shoulder has similar fractionation paths to contemporaneous magmatism to the south (0.5 GPa), but is derived from a different, shallower mantle source (Tb N/Yb N ∼ 1.3–1.5) that we interpret results from lithospheric thinning associated with the now-inactive Chow Bahir rift. Between 6.5 and 8°N, significant surface faulting and shallow magmatic fractionation paths (0.1 GPa) in the dominant Quaternary structure of the Main Ethiopian Rift (the Wonji Fault Belt and Silti-Debre Zeyit Fault Zone), highlights the strong connection between magmatism and extensional tectonics in these structures. Along the eastern rift margin, Wonji Fault Belt magmas are derived from a dominantly shallow melting column (Tb N/Yb N ∼ 1.4–1.7) that is similar in composition to the older rift shoulder lavas at Chencha. Adjacent to the western rift margin, magmas erupted in the Silti-Debre Zeyit Fault Zone are interpreted to have erupted through a thicker lithosphere as these magmas are derived from a deeper melting column (Tb N/Yb N ∼ 1.7–2.1) that contains some minor apatite. The inferred variations in lithospheric thickness in southern Ethiopia outlined in this study illustrate the interaction between northward and southward rift propagation in addition to lateral variations across the rift floor as extension migrates into zones of focused magmatic intrusion. The results of this investigation show that geochemical techniques can be applied to probe the history of lithospheric modification during rifting and provide new constraints for models of rift development.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

Rhyolites at Kerlingarfjöll, Iceland: the evolution and lifespan of silicic central volcanoes

Kerlingarfjoll central volcano is Iceland’s second largest outcrop of Quaternary rhyolite and is part of the Icelandic Western Rift Zone. Geochemical and Ar/Ar age data show that at least 21 different rhyolite eruptions have taken place at Kerlingarfjoll over the last 350 ka. Ar/Ar dating was carried out on samples of obsidian which showed variable reproducibility, illustrating the difficulty in dating young Icelandic volcanics. Nevertheless, reasonable estimates of eruption age have been derived for a number of eruptive units that are consistent with observed stratigraphy, enabling an understanding of the temporal evolution of Kerlingarfjoll. Two rhyolite magma types are present. The first is an older, low-Nb rhyolite that was erupted episodically along a cryptic curved fracture system, to form a discontinuous ring of rhyolite mountains, between 350 and 250 ka. This discontinuous ring is similar to structures observed at other volcanoes in Iceland, suggesting that the development of a curved fracture that acts as a pathway for episodic silicic eruptions is a feature of central volcano development. The second magma is a younger, high-Nb rhyolite that was erupted episodically between 250 and 68 ka in the northern part of Kerlingarfjoll, forming two clusters, both of which have areas of intense hydrothermal activity. Repose periods for rhyolite volcanism are thought to be on the order of tens of thousands of years, and it is possible that Kerlingarfjoll will erupt rhyolite again in the future.

PROBABILISTIC SEISMIC HAZARD ASSESSMENT FOR THE DEMOCRATIC REPUBLIC OF CONGO AND SURROUNDING AREAS

Research Article| December 01, 2009 PROBABILISTIC SEISMIC HAZARD ASSESSMENT FOR THE DEMOCRATIC REPUBLIC OF CONGO AND SURROUNDING AREAS T. MAVONGA; T. MAVONGA University of the Witwatersrand, Private Bag 3, WITS, 2050 South Africa. Goma Volcanic Observatory, 142 Rond Point Avenue, Goma, North Kivu, Democratic Republic of the Congo, e-mail: mavotulu@gmail.com Search for other works by this author on: GSW Google Scholar R.J. DURRHEIM R.J. DURRHEIM University of the Witwatersrand, Private Bag 3, WITS, 2050 South Africa, Council for Scientific and Industrial Research, PO Box 91230, Auckland Park, 2006, South Africa, e-mail: raymond.durrheim@wits.ac.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (3-4): 329–342. https://doi.org/10.2113/gssajg.112.3-4.329 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. MAVONGA, R.J. DURRHEIM; PROBABILISTIC SEISMIC HAZARD ASSESSMENT FOR THE DEMOCRATIC REPUBLIC OF CONGO AND SURROUNDING AREAS. South African Journal of Geology 2009;; 112 (3-4): 329–342. doi: https://doi.org/10.2113/gssajg.112.3-4.329 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract A probabilistic approach was used to map the seismic hazard in Democratic Republic of Congo and surrounding areas, and assess the seismic hazard level for 14 cities in the region. Seismic hazard maps for 2%, 5% and 10% chance of exceeding the indicated ground accelerations in 50 years were prepared using a homogenised 90-year catalogue compiled for moment magnitudes; the attenuation relations of Mavonga (for the Western Rift Valley of Africa), Jonathan (for eastern and southern Africa) and Atkinson and Boore (for eastern North America); and the EZ-Frisk software package. The highest levels of seismic hazard were found in the Lake Tanganyika Rift seismic zone, where peak ground accelerations in excess of 0.32 g, 0.22 g and 0.16 g are expected to occur with 2%, 5% and 10% chance of exceedence in 50 years, respectively. The seismic hazard in the Congo Basin diminishes with distance away from the Western Rift Valley until, at a distance of about 450 km, the chance of exceeding 0.05 g (the threshold value of engineering interest) is less than 10% in 50 years. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal structure beneath two seismic broadband stations revealed from teleseismic P-wave receiver function analysis in the Virunga volcanic area, Western Rift Valley of Africa

The shear velocity structure beneath the Virunga volcanic area was estimated by using an average solution in the time domain inversion of stacked teleseismic receiver functions provided by two seismic broadband stations KUNENE (KNN) and KIBUMBA (KBB). These two stations are 29 km apart and located at the eastern and western escarpment of the Western Rift Valley of Africa in the Virunga area, respectively. The velocity model was presented as P-wave velocity models. From these models, the crust mantle transition zone beneath the area sampled by KNN and KBB in the Virunga area was determined at depth from about 36 to 39 km and 30 to 41 km, respectively. A low velocity zone was observed below stations KNN and KBB at depths between 20–30 km and 18–28 km, respectively, and with average velocity 5.9 km/s and 6.0 km/s. This low velocity zone may probably related to a magma chamber or a melt-rich sill. The models show also high velocity material (6.8–7.4 km/s) lying beneath stations KNN and KBB at depths 3–20 km and 3–10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. The result obtained in this study was applied to the determination of epicentres during the period prior to the 27 November 2006 Nyamuragira eruption. This eruption was preceded by a swarm of hybrid volcanic earthquakes with clear P-waves onset. Using the receiver function model was found to improve the location of events. The located events correlate well with the location of the eruptive site and data provided by the INSAR observations of surface deformation associated with eruption.