Western Rift Research Articles

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

AbstractUnusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system. The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. Four hundred seventy‐four earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes, and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (≥5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half‐graben morphology over at least 12 Myr of basin evolution. Fault scaling based on our results suggests that M > 7 earthquakes along Tanganyika border faults are possible.

Extension direction re-orientation in the oceanic rift of Iceland, and comparison with continental rifts

In this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of Iceland, namely (1) the northern part of the Eastern Rift segment (Northern Volcanic Zone, NVZ) and (2) the Western Rift segment, that include the Western Volcanic Zone (Thingvellir area) and the Reykjanes Peninsula. In the N-S-trending NVZ (mainly Krafla volcanic area), the obtained extension directions trend around ESE. At Thingvellir, the extension directions trend ca. NW-SE and are perpendicular to the main NE-SW-trending faults. In the ENE-WSW-trending tectono-magmatic segments of the Reykjanes Peninsula, our data show a marked difference with the GPS data and spreading directions. The extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors. Extension shows an overall southward increase in divergence from the regional directions indicated by GPS vectors. This behavior has been compared with the continental Main Ethiopian Rift of East Africa, where structural studies show a re-orientation of the extension direction between the rift margins and the internal sectors, resulting in a divergence between local and regional extension directions. Aimed at investigating the causes of these differences, we performed a new series of analogue models by varying different parameters such as rift obliquity and width. Our findings suggest that rift width exerts a primary control on the boundary-axis variation of the extension direction. In particular, significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys (e.g., continental rift of Ethiopia); conversely, fault orientation and local extension direction are much more homogeneous in narrower rift valleys (e.g., rifts of Iceland), where a variations in the direction of extension from the margins to the rift axis are not observed.

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (∼24 km long and up to 10 km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705 Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement.This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition.The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility.The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.

REGREENING OF THE NORTHERN ETHIOPIAN MOUNTAINS: EFFECTS ON FLOODING AND ON WATER BALANCE


 
 
 The hydro-geomorphology of mountain catchments is mainly determined by vegetation cover. This study was carried out to analyse the impact of vegetation cover dynamics on flooding and water balance in 11 steep (0.27-0.65 m m-1) catchments of the western Rift Valley escarpment of Northern Ethiopia, an area that experienced severe deforestation and degradation until the first half of the 1980s and considerable reforestation thereafter. Land cover change analysis was carried out using aerial photos (1936,1965 and 1986) and Google Earth imaging (2005 and 2014). Peak discharge heights of 332 events and the median diameter of the 10 coarsest bedload particles (Max10) moved in each event in three rainy seasons (2012-2014) were monitored. The result indicates a strong reduction in flooding (R2 = 0.85, P<0.01) and bedload sediment supply (R2 = 0.58, P<0.05) with increasing vegetation cover. Overall, this study demonstrates that in reforesting steep tropical mountain catchments, magnitude of flooding, water balance and bedload movement is strongly determined by vegetation cover dynamics.
 KEY WORDS: HYDRO-GEOMORPHOLOGY, REFORESTATION, CREST STAGE, PEAK DISCHARGE, BEDLOAD
 
 

Geochemistry and possible origins of biodegraded oils in the Cretaceous reservoir of the Muglad Basin and their application in hydrocarbon exploration

The Muglad Basin is one of the largest petroliferous basins within the Central and Western African Rift System (WCARS). A total of 42 oil samples including light, normal and heavy crude oils from the Cretaceous reservoirs of the Muglad Basin were systematically analyzed to fully understand their geochemical characteristics and possible origins. The results of oil–oil correlation show that these Cretaceous crude oils are genetically identical and derive from the same source kitchen. Differences in physical properties and geochemical compositions may be attributed to secondary alteration, such as biodegradation. Lightly, moderately and severely biodegraded oils can be divided on the basis of the relative abundance of n–alkanes and acyclic isoprenoids in the oils and the presence of “humps” on gas chromatogram baselines, as well as of the relative abundance of 25–norhopanes. The biodegradation degree of Cretaceous oils in the Muglad Basin is closely related to the burial depth of oil reservoirs and the distance between the locations of oil reservoirs and adjacent faults. This study may have practical application for further petroleum exploration and development in the Muglad Basin.

Evolution of the northern Turkana Depression (East African Rift System, Kenya) during the Cenozoic rifting: New insights from the Ekitale Basin (28‐25.5 Ma)

Two sedimentary basins are identified in the Turkana Depression (East African Rift System, Kenya). One of them, the Ekitale Basin, is presented in detail. Located on the western rift shoulder of the northern Turkana Depression, the preserved portion of the Ekitale Basin is 3–5‐km wide and bordered by N40°–50° normal faults. These normal faults were inherited from the reactivation of pre‐existing basement structures. The Ekitale Basin is filled by the ~75‐m‐thick Topernawi Formation. The lower portion of the formation provides evidence that the basin was occupied by a lake, bordered by alluvial fans, and into which river‐derived turbiditic complexes were deposited. The upper portion is comprised of pyroclastic deposits, originating from volcanic activity, and interbedded with fluvial deposits emplaced during periods of volcanic quiescence. The Ekitale Basin opened just after 28 Ma and was abandoned prior to 25 Ma, placing it after the deposition of the Oligocene Traps (ca. 33.9–27 Ma) and prior to the rift climax (after 14 Ma). This syn‐rift basin largely impacts our understanding of the Cenozoic rift evolution in the Turkana Depression. It helps to identify, for the first time, the first pulse of extension of the Cenozoic rifting, revealing a two‐step rifting scenario for the northern Turkana Depression. The Ekitale Basin, and analogous syn‐rift basins, is believed to have developed from the reactivation of pre‐existing structures during a period of extension marked by low differential stress and a low amount of extension.

Relationships Between Lithospheric Structures and Rifting in the East African Rift System: A Rayleigh Wave Tomography Study

AbstractThe East African Rift System (EARS) provides a unique location for exploring factors influencing the development and maturation of continental rifting. In particular, the geographical relationships between Cenozoic rifts and Pre‐Cambrian lithospheric structures suggest that such preexisting structures exert an influence on early‐stage rift geometry and behavior. This study uses Rayleigh wave phase velocity at periods of 20 to 100 s to study lateral variability in the lithospheric structures of rift segments and preexisting structures in the central and southern EARS. The model is constructed using records of 789 earthquakes, recorded by a composite station array of 235 stations from nonconcurrent seismic networks between 1994 and 2015. In the central EARS, we observe fast velocities beneath the Tanzania Craton, isolated low‐velocity regions along the Western Rift Branch, and low velocities in all resolved portions of the Eastern Rift Branch, consistent with previous regional surface wave studies. South of the Tanzania Craton, we observe linear low‐velocity zones trending both southeast and southwest from the Tanzania Divergence Zone, suggesting a southern bifurcation of the Eastern Rift Branch. In the southern portions of the Western Rift Branch, the Malawi Rift borders fast velocities associated with the Bangweulu Block and Irumide Belt. Anomalously fast velocities in these regions persist to long periods, confirming the existence of cratonic lithosphere inferred from previous studies. Fast velocities observed beneath the Irumide Belt extend across the southernmost portion of the Malawi Rift, suggesting that strong lithosphere in this region may hinder the southern propagation of the rift.

Magmatic-tectonic control on the generation of silicic magmas in Iceland: Constraints from Hafnarfjall-Skarðsheiði volcano

Hafnarfjall-Skarðsheiði (H-S) central volcano, located at the edge of the Western rift zone in Iceland, provides a snapshot into silicic magma generation that occurred soon after establishment of a rift. We present in situ zircon U-Pb ages and oxygen and hafnium isotope compositions, complemented by whole-rock major and trace element and Pb, Nd, and Hf whole rock isotope data, from the dominant silicic units erupted throughout H-S's eruptive history. Zircon U-Pb ages (ca. 5.4 to 3.9 Ma) and field relationships indicate silicic magmatism was episodic. However, relatively low (for Iceland) whole rock εHf (+11.9 to 13.3) and εNd (+7.2 to 7.6), in addition to Pb isotope data from basalt and rhyolite units indicate that the same mantle-derived source is dominantly responsible for the geochemical characteristics observed in both magma types, which are more similar to those from magmas from a propagating rift than an established one. This observation is consistent with a role for fractional crystallization of mantle melts in addition to contributions of partially melted altered crust to explain the low δ18Ozrc values (1.5 to 4.6‰) observed. This study highlights the importance of the evolutionary state of the rift, crustal history, and mantle heterogeneity all as contributing factors to the isotopic composition of silicic Icelandic magmas. We invoke a petrogenetic model where the timescales of rift drift explain the long-lived, episodic silicic magmatism produced during rift propagation at Hafnarfjall-Skarðsheiði volcano.

Relocation of Light and Moderate‐Magnitude (M4–6) Seismicity Along the Central Mid‐Atlantic

AbstractWe estimate precise relative epicentroid locations of earthquakes along the Mid‐Atlantic Ridge (MAR) between 12°N and 19°N using intermediate‐period Rayleigh and Love wave cross‐correlation measurements. We relocate 152 earthquakes with magnitudes ranging between 4.0 and 6.0 and when possible use P waveform modeling to estimate centroid depths. We compare surface‐wave locations to locations in the U.S. Geological Survey, Global Centroid Moment Tensor, and Pacific Marine Environmental Laboratory hydroacoustic catalogs (Smith et al., , https://doi.org/10.1029/2001GL013912) and demonstrate that surface‐wave based relative location techniques provide significant improvement on precision. Our analyses shift locations to more tectonically consistent alignments that correlate strongly with bathymetric features. Relocations along the Fifteen‐Twenty and Marathon Fracture Zones (FZs) move to within a few kilometers of the transform bathymetry. Events along ridge segments scatter over a distance of about 10–20 km perpendicular to the local plate boundary and most ridge‐segment seismicity clusters spatially. North of the Fifteen‐Twenty FZ, ridge events along several segments locate preferentially in the North American Plate, suggesting recent asymmetric seismic strain release and perhaps asymmetric spreading. Along the ridge between 13°N and 14°N, moderate‐magnitude seismic strain release concentrates in an area with two active oceanic core complexes (OCC) along the western rift boundary, but the largest events along this segment appear to occur along the eastern boundary rift valley faults and represent deformation opposite the OCC detachment surface processes. Precise moderate‐magnitude earthquake epicentroid locations for events that occurred over the last few decades complement the results of temporary seismometer deployments that have imaged microseismic activity along the MAR.

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

AbstractAlthough the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.

The Electrical Structure of the Central Main Ethiopian Rift as Imaged by Magnetotellurics: Implications for Magma Storage and Pathways

AbstractThe Main Ethiopian Rift is part of the East African Rift with its unique geological setting as an active continental breakup zone. The Main Ethiopian Rift includes a number of understudied active volcanoes with potentially high risks for this densely populated part of Ethiopia. Using newly recorded (2016) magnetotelluric data along a 110 km long transect crossing the whole rift, we present a regional 2‐D model of electrical resistivity of the crust. The derived model endorses a previous study that drew the surprising conclusion that there was no highly conductive region associated with a magma chamber directly under the central rift volcano Aluto. This has implications for the estimation of the amount of magma present, its water content, and the storage conditions, as the volcano is actively deforming and results from seismicity and CO2 degassing studies all indicate magma storage at about 5 km depth. Additionally, the existence of a strong conductor under the Silti Debre Zeyt Fault Zone approximately 40 km to the northwest of the rift center is confirmed. It is located with a slight offset to the Butajira volcanic field, which hosts a number of scoria cones at the boundary between the NW plateau and the rift. The magnetotelluric model reveals different electrical structures below the eastern and western rift shoulders. The western border is characterized by a sharp lateral contrast between the resistive plateau and the more conductive rift bottom, whereas the eastern flank shows a subhorizontal layered sequence of volcanic deposits and a smooth transition toward the shoulder.

Trace element systematics and Nd, Sr and Pb isotopes of Pliocene flood basalt magmas (Ethiopian rift): A case for Afar plume-lithosphere interaction

We present high-precision trace element and Sr-Nd-Pb isotope data for Pliocene flood basalts located at Sire (eastern rift wall) and Chefe Donsa (western rift wall) sections from the main Ethiopian rift (MER). These basalts are tholeiitic to mildly alkaline and were derived by moderate degrees of melting (~10–20%) of a spinel lherzolite containing minor amounts of amphibole within the lithospheric mantle. Strontium, Nd and Pb isotopic compositions (87Sr/86Sr: 0.7040–0.7057, 143Nd/144Nd: 0.52164–0.51287, 206Pb/204Pb: 17.75–19.20, 207Pb/204Pb: 15.53–15.64 and 208Pb/204Pb: 38.12–38.86) are between the inferred composition of the Afar mantle plume and an unspecified enriched lithospheric component, suggesting interaction of melts from asthenospheric and lithospheric mantle domains rather than melting of homogeneous mixed sources or magma mixing. It is proposed that in the Oligocene the impinging Afar plume creates zones of metasomatically enriched mantle. Later, lithospheric thinning as a result of incipient rifting leads to decompression and melting of this metasomatically enriched mantle due to its lower solidus temperature. The chemistry of the Pliocene lavas therefore reflect two associated scenarios namely (i) upwelling plume material from the advancing Afar plume may have induced metasomatism in the lithospheric mantle (ii) mobilization and melting of this enriched lithospheric mantle material resulted in melts characterized by variable mixtures of the two mantle reservoirs.

Regreening of The Northern Ethiopian Mountains: Effects on Flooding and on Water Balance

The hydro-geomorphology of mountain catchments is mainly determined by vegetation cover. This study was carried out to analyse the impact of vegetation cover dynamics on flooding and water balance in 11 steep (0.27-0.65 mm-1) catchments of the western Rift Valley escarpment of Northern Ethiopia, an area that experienced severe deforestation and degradation until the first half of the 1980s and considerable reforestation thereafter. Land cover change analysis was carried out using aerial photos (1936,1965 and 1986) and Google Earth imaging (2005 and 2014). Peak discharge heights of 332 events and the median diameter of the 10 coarsest bedload particles (Max10) moved in each event in three rainy seasons (2012-2014) were monitored. The result indicates a strong reduction in flooding (R2 = 0.85, P<0.01) and bedload sediment supply (R2 = 0.58, P<0.05) with increasing vegetation cover. Overall, this study demonstrates that in reforesting steep tropical mountain catchments, magnitude of flooding, water balance and bedload movement is strongly determined by vegetation cover dynamics.

Crustal Structure at a Young Continental Rift: A Receiver Function Study From the Tanganyika Rift

AbstractThe southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use 11 broadband seismometers to implement receiver function analysis using H‐κ stacking, a method sensitive to crustal thickness and VP/VS ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance, and phase‐weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by VP/VS ratios of 1.75–1.77, implying a felsic bulk composition. Crust beneath the fault‐bounded basins has high VP/VS (>1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high VP/VS. However, elevated VP/VS values within fault‐bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively unextended Tanzanian craton and Bangweulu Block are 41.6–42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault‐bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early stage continental rifting in the Western rift, Africa.

Brittle Shear Tectonics in a Narrow Continental Rift: Asymmetric Nonvolcanic Barmer Basin (Rajasthan, India)

Our field studies emphasizing brittle shear P- and Y-planes along the margins of the Barmer basin (Rajasthan, India) support its two-phase (NW-SE, followed by NE-SW) extension during Early Cretaceous and Late Cretaceous–Paleocene periods. We also document nearly NE-trending megascale transfer zones along the northern margin of the Barmer basin. Preexisting brittle planes in the Malani basement rocks guided the relay structures here. Structures at the western basin shoulder margin indicate NE-SW extension, and the crosscut relation connotes the relative timing of the two extension phases. The crosscutting conjugate fault sets are non-Andersonian. The NW-trending faults produced by the second-phase extension and the inherited NNW-trending brittle features are dominantly dip-slip. Prior fractures of the Malani rocks at ≥45° to the NE-SW principal extension direction extended the Barmer basin obliquely during the Late Cretaceous–Paleocene period. The asymmetric nature of the rift, too, connotes its oblique rifting. The extension direction of the first phase probably rotated clockwise. This is derived mainly from WSW-trending faults cutting NE-trending faults. Brittle planes of shear and fracture significantly promoted fluid flow, as understood from secondary hydrothermal mineral deposits (Barmer hill area) and pre-Deccan basalts (Sarnoo area). Reverse slip detected along subvertical faults on the western and eastern rift shoulders are probably due to isostatic flexure–related contraction or might be related to the far-field effect of ridge-push forces. The Mesozoic subsurface stratigraphy there and elsewhere within the Barmer basin requires more study to substantiate the potential for structural entrapment of hydrocarbon.

Genetic structuring, dispersal and taxonomy of the high-alpine populations of the Geranium arabicum/kilimandscharicum complex in tropical eastern Africa.

The scattered eastern African high mountains harbor a renowned and highly endemic flora, but the taxonomy and phylogeographic history of many plant groups are still insufficiently known. The high-alpine populations of the Geranium arabicum/kilimandscharicum complex present intricate morphological variation and have recently been suggested to comprise two new endemic taxa. Here we aim to contribute to a clarification of the taxonomy of these populations by analyzing genetic (AFLP) variation in range-wide high-alpine samples, and we address whether hybridization has contributed to taxonomic problems. We identified only two genetic groups. One corresponded to G. kilimandscharicum, which has been reported as exclusively high-alpine and confined to the eastern Rift mountains in East Africa. The other corresponded to G. arabicum, reported from lower altitudes on the same mountains as well as from a wide altitudinal span in Ethiopia and on the western Rift mountains in East Africa. The four populations analyzed of a recently described species from the Bale Mts in Ethiopia were admixed, indicating that they result from recent long-distance dispersal of G. kilimandscharicum from East Africa followed by hybridization with local G. arabicum in naturally disturbed habitats. Some admixture between the two genetic groups was also inferred on other mountains, supporting earlier suggestions of introgression based on morphology. We did not find support for recognition of the recently suggested new subspecies of G. arabicum in Ethiopia. Interestingly, the high-alpine G. kilimandscharicum lacked clear geographic structuring, suggesting a recent history of colonization of the different mountains or extensive intermountain gene flow.

Seismic inversion as a predictive tool for porosity and facies delineation in Paleocene fluvial/lacustrine reservoirs, Melut Basin, Sudan

The Melut Basin is a rift basin in the interior Sudan linked to the Mesozoic-Cenozoic Central and Western African Rift System. The Paleocene Yabus Formation is the main reservoir deposited in heterogeneous fluvial/lacustrine environment. Delineation of channel sandstone from shale is a challenge in reservoir exploration and development. We demonstrate a detailed 3D quantitative seismic interpretation approach that integrates petrophysical properties derived from well logs analysis. A porosity transform of acoustic impedance inversion provided a link between elastic and rock properties. Thus, we used seismic porosity to discriminate between different facies with appropriate validation by well logs. At the basin scale, the results revealed lateral and vertical facies heterogeneity in the Melut Basin. Good reservoir quality is observed in the Paleocene Yabus Formation. The sand facies indicated high porosity (20%) corresponding to low acoustic impedance (20000–24000 g ft/(cm3.s)). However, lower quality reservoir is observed in the Cretaceous Melut Formation. The porosity of sand/shale facies is low (5%), corresponding to high acoustic impedance (29000–34000 g ft/(cm3.s)). This suggests that the Yabus Sandstone is potentially forming a better reservoir quality than Melut Formation. At the reservoir scale, we evaluated the facies quality of Yabus Formation subsequences using petrophysical analysis. The subsequences YB1 to YB3, YB4 to YB7 and YB8 to YB10 showed relatively similar linear regressions, respectively. The subsequence of YB4 to YB7 is considered the best reservoir with higher porosity (25%). However, subsequence YB1 to YB3 showed lower reservoir quality with higher shale volume (30%). This attributed to floodplain shale deposits in this subsequence. Similarly, the high porosity (20%) recognized in deeper subsequences YB6 to YB9 is due to clean sand facies. We learnt a lesson that appropriate seismic preconditioning, exhaustive petrophysical analysis and well log validation are important keys for improved reservoir quality prediction results in fluvial/lacustrine basins.

Onshore and offshore apatite fission-track dating from the southern Gulf of California: Insights into the time-space evolution of the rifting

We present the results of a apatite fission-track (AFT) study on intrusive rocks in the southern Gulf of California, sampled along the eastern margin of Baja California Sur (western rift margin), as well as from islands and submerged rifted blocks within the Gulf of California, and from the conjugate Mexican margin (Nayarit state). For most of the samples U-Pb zircon and 40Ar-39Ar mineral ages were already available (Duque-Trujillo et al., 2015). Coupled with the new AFT data these ages provide a more complete information on cooling after emplacement. Our samples span a wide range of ages between 5.5±1.1 and 73.7±5.8Ma, and show a general spatial distribution, with late Miocene AFT ages (about 6Ma) aligned roughly NW-SE along a narrow offshore belt, parallel to Baja California Peninsula, separating older ages on both sides. This pattern suggests that in Late Miocene, deformation due to plate transtension focused at the eastern rheological boundary of the Baja California block. Some Early Miocene AFT ages onshore Baja California could be related to plutons emplaced at shallow depths and thermal resetting associated with the onset of volcanism at ~19Ma in this part of the Peninsula. On the other hand, an early extensional event similar to that documented in the eastern Gulf cannot be ruled out in the westernmost Baja California.

Spatial distribution of helium isotopes in Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and evolution of the Icelandic mantle plume

Abstract The distribution of helium isotope ratios (3He/4He) in Icelandic geothermal fluids, volcanic glasses and phyric lavas is investigated. Along with presenting a new helium isotope dataset using phyric lavas largely from off-rift regions, we compiled published data and constructed a database of all available helium isotope data from Iceland. The new dataset reveals an exceptionally high 3He/4He ratio from a phyric lava in NW-Iceland (47.5 RA, where RA is the 3He/4He ratio of air), which is among the highest values measured in any mantle-derived magma to date. Modifications of primary (i.e., mantle-derived) helium isotope ratios, due to additions of air-derived helium and He from radiogenic ingrowth, were evaluated and the database was filtered accordingly. The geographical information system ArcGIS (ESRI) was used to perform spatial analysis on the filtered database and the interpolation method, Natural Neighbor, was used to calculate representative helium isotope ratios for all parts of Iceland, including off-rift regions. The results show that helium isotope ratios for the whole of Iceland vary from 5.1 to 47.5 RA. However, this study allows for a fine-scale distinction to be made between individual rift segments and off-rift regions. The results clearly reveal that each rift zone has its own distinctive mean isotope signature: 12–17 RA in the Western Rift Zone, 8–11 RA in the Northern Rift Zone and 18–21 RA in the Eastern Rift Zone. Our isoscape map places new constraints on a previously inferred high-helium plateau region in central Iceland (Breddam et al., 2000). The plateau continues southward along the propagating Eastern Rift Zone and through to the South Iceland Seismic Zone and the Mid-Iceland belt. Its location coincides with many geological features, e.g., eruption rates, location of abandoned rift segments, seismic velocity and gravity anomalies. Such high helium isotope ratios have been associated with undegassed and primordial mantle sources that have been isolated in the lower mantle over Earth's history. Thus, high-helium domains throughout Iceland are interpreted to mark the loci of present and past plume conduits which help explain the considerable spatial variation in the sampling of a primordial mantle He component beneath the Iceland hotspot.

Relic populations of Fukomys mole-rats in Tanzania: description of two new species F. livingstoni sp. nov. and F.hanangensis sp. nov.

Previous studies of African mole-rats of the genera Heliophobius and Fukomys (Bathyergidae) in the regions of East and south central Africa have revealed a diversity of species and vicariant populations, with patterns of distribution having been influenced by the geological process of rifting and changing patterns of drainage of major river systems. This has resulted in most of the extant members of the genus Fukomys being distributed west of the main Rift Valley. However, a small number of isolated populations are known to occur east of the African Rift Valley in Tanzania, where Heliophobius is the most common bathyergid rodent. We conducted morphological, craniometric and phylogenetic analysis of mitochondrial cytochrome b (cyt b) sequences of two allopatric populations of Tanzanian mole-rats (genus Fukomys) at Ujiji and around Mount Hanang, in comparison with both geographically adjacent and more distant populations of Fukomys. Our results reveal two distinct evolutionary lineages, forming clades that constitute previously unnamed species. Here, we formally describe and designate these new species F. livingstoni and F. hanangensis respectively. Molecular clock-based estimates of divergence times, together with maximum likelihood inference of biogeographic range evolution, offers strong support for the hypothesis that vicariance in the Western Rift Valley and the drainage patterns of major river systems has subdivided populations of mole-rats. More recent climatic changes and tectonic activity in the “Mbeya triple junction” and Rungwe volcanic province between Lakes Rukwa and Nyasa have played a role in further isolation of these extra-limital populations of Fukomys in Tanzania.