Bruce R. Rosendahl

Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data

Abstract Northwest-southeast extension has opened the East African Rift System along two main branches, the Western and Eastern Branches. Rift zones along the Western Branch are marked by narrow lakes floored by thick piles of fluvial clastic and ‘pelagic’ sediment. Magmatism is restricted to a few small areas in the ‘arches’ between the lakes. In contrast, rift zones along the Eastern Branch are largely filled with volcanic and volcaniclastic materials and magmatism is generally perceived to be an integral part of the rifting process. In an attempt to sort out the significance and meaning of these and other differences, we have compared multifold seismic data from three Western Branch rift zones (Tanganyika, Rukwa and Malawi) and one Eastern Branch zone (Turkana). The Tanganyika and Malawi Rift Zones are composed of half-graben basins linked in complex ways by accommodation zones which generally trend oblique to the rift axes, and sometimes oblique to the extension direction. Half-grabens alternate basinal polarities where the rift crosses Proterozoic dislocation zones. Complex fault geometries are associated with some accommodation zones; elsewhere faults are almost exclusively planar. Sedimentary fill reaches at least 4–5 km and the section is mostly Cenozoic in age. Patches of Permo-Triassic sedimentary rocks are believed to occur within both rift zones. The Rukwa Rift is a pull-apart zone that connects the northern (Livingstone) basin of Lake Malawi to the Kalemie Basin in central Lake Tanganyika. The entire pull-apart system may be a series of down-to-the-east half-grabens. An accommodation zone develops along a short stretch of the Rukwa Rift, but no full polarity reversal occurs. The break-away faults of the Livingstone, Rukwa and Kalemie basins are essentially coincident with the Proterozoic Rukwa dislocation zone, which sub-parallels the inferred extension direction. Fault geometries in the Rukwa Rift are markedly listric, especially in the pre-Cenozoic section. Sedimentary fill ranges in age from pre-Karroo through Cenozoic and locally exceeds 10 km in thickness. The Turkana Rift is composed of short, linear, NNE-trending normal fault segments that are offset in a left-lateral sense by numerous, NW-SE trending transfer faults, linking facing border fault segments together. The overall trend of the rift zone is oblique to the opening direction, like the Tanganyika and Malawi cases, but the border fault segments are sub-perpendicular. Fault geometries are highly variable, but flower structures associated with transfer faults predominate. Igneous activity is ubiquitous and appears to be localized along the transfer faults. Basin fill reaches 4–5 km in thickness and is dominated by fluvial clastic, volcaniclastic and volcanic materials. The structural differences within the Tanganyika-Rukwa-Malawi system stem mainly from the modifying effects of pre-rift anistropies on strain expressions. Fundamentally, this system is a NW-SE trending series of single-polarity pull-apart basins. At the two ends of the pull-apart zone, the rift is deflected into more N-S trending basins which have a high tendency to alternate polarities along strike. This explanation does not account for the differences in fault forms between the Tanganyika-Malawi (planar) and Rukwa (listric) Rifts. For the time being, we presume these differences arise from systematic differences between Tanganyika-Malawi and Rukwa in the age ranges of the fill and/or the maximum depths of seismic imaging. Rifting in Turkana is profoundly different than in the Tanganyika-Rukwa-Malawi sub-branch and seems to involve a softer, more ductile, more organized style of extension which may be closer to the ideal case. In a thermal sense, rifting has progressed further in Turkana than along the Western Branch zones. This does not preclude original, fundamental difference in the thermal states of two branches.

Deep-imaging seismic and gravity results from the offshore Cameroon Volcanic Line, and speculation of African hotlines☆

Deep-imaging multi-channel seismic reflection data show that volcanic centers along the offshore part of the Cameroon Volcanic Line (CVL) are composed of uplifted, Aptian to Late Cretaceous oceanic crust, >4 km of sedimentary overburden, and Neogene igneous rocks, with volcanic material forming a cap < 1.5 km thick over pre-uplift sedimentary deposits. At Principe Island, the underlying oceanic basement has been uplifted by as much as 3 km to form a crustal arch less than 200 km wide perpendicular to the CVL trend. Vertical faults having small offsets and dikes are common across this arch. Reflection Moho shallows parallel to the uplifted crust along margins of the arch, but is not observed directly below the arch axis where volcanism and faulting are pervasive. The episode of crustal uplift is marked by a prominent reflection unconformity. This unconformity occurs at other CVL islands and seamounts and represents a synchronous period of crustal uplift and volcanism. Reflectors from this unconformity have been correlated to offshore boreholes indicating a Miocene age. Gravity modelling indicates that an elongate wedge of relatively less dense lithospheric mantle (Δρ = −0.1 g/cm3) underlies Principe Island to a depth of 40 km. This interpreted zone of lighter mantle material may form by a combination of intruded mafic partial melt and reheating of the lithosphere. Dynamic support from asthenospheric upwelling may also have contributed to uplift. Other NE-trending volcanic chains and rises off West Africa (Canary Islands, Cape Verde Rise, Sierra Leone Rise and Walvis Ridge) display similar features to CVL islands. These volcanic chains exhibit crustal uplift unconformities and intraplate volcanism occurring during the Miocene and later; Miocene and older marine sediments crop out on most of the islands; there are no flexural depressions surrounding volcanic centers; their ocean island basalts (OIB) have similar geochemical characteristics; the OIB does not appear to be the main construction material of each chain; anomalously high modern heatflow occurs along their lengths; and hotspot-like age progression of volcanism is not clearly defined along their lengths. It is apparent that the CVL is not the product of a single mantle plume or hotspot, and we speculate that the CVL and possibly other NE-trending volcanic chains off West Africa (and perhaps linear belts of Neogene volcanism on the African continent) are the result of linear, mantle upwelling zones or ‘hotlines’. These hotlines are suggested to form above upwelling flow currents in between cylindrical Rayleigh-Bernard convection rolls in the upper mantle. Such convection may be driven by heat transfer across and/or shear along the 670 km discontinuity as a result of convection in the lower mantle.

Development of coarse-grained facies in lacustrine rift basins: Examples from East Africa

Detailed studies of high-resolution and multifold seismic reflection data from the two largest East African rift lakes, Malawi and Tanganyika, reveal a complex suite of coarse-grained depositional facies. These facies occur within specific regions of the controlling half grabens that compose the rift lakes. Sand-prone environments include subaqueous channels and small drowned fluvial complexes. Channel systems range from large erosional canyons to deep-water turbidite channel-levee systems. Lowstand and highstand deltas of axial and shoaling-side rivers are volumetrically important coarse-grained facies. Fan deltas develop along the base of major border faults during lake lowstands; subaqueous talus fan deposits occur along the base of the border faults during lake highstands. Lowstand deltas are the best-preserved progradational facies in these rift lakes. In addition to simple tectonic control, drastic tectonically or climatically induced lake-level change significantly regulates the production of coarse-grained lacustrine synrift deposits.

Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa

We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC. In the Douala Basin, sedimentary units deposited in an Early Cretaceous continental rift now form marginal basin highs overlain by < 1 km younger of sediment beneath the shelf. RCC here has undergone little attenuation, as more highly attenuated RCC is believed to occur below the conjugate Sergipe-Alagoas Basin in eastern Brazil. In the North Gabon Basin, RCC has been markedly thinned and resulting rift structures are deeply buried by post-rift deposits. Reflective lower crust within RCC forms discontinuous bands, where normal dip-slip faults sole into it and transform related faults crossthrough it. Interpreted Moho reflections atthe base of RCC form a discontinuous horizon that is sometimes offset at crustal deformation zones; elsewhere it transects the reflective grain of crustal deformation zones. Interpreted WAPS MCS data show that the margin is composed of rift-units separated at ca 40 km intervals by NE trending fracture zones and transfer fault zones of variable length, causing the margin to step progressively southwestward by alternating between normal and transformfaulted segments. Correlation of rift structures between these West African basins and the conjugate Sergipe-Alagoas Basin show that the two margins formed a connected rift-branch that underwent NE-SW oriented extension, ca 35° oblique to the rift axis. This orientation favoured occupation of NE-SW trending preexisting shear fabrics situated in the Late Proterozoic Pan-African/Braziliano mobile belt. At the RCC-POC boundary, wedges of seaward-dipping reflectors abut against RCC and overlie non-reflective lower crust and reflection Moho. They correlate with pronounced negative magnetic anomalies (−200 to −600 nT), probably caused by the juxtaposition of weakly magnetised RCC and more magnetic, mafic volcanic rocks of the POC. Steeply-dipping faults are interpreted to occur at transform-faulted segments, where they separate blocks of RCC from POC, or in some places OC. These NE trending transfer fault zones propagate into oceanic fracture zones (FZs) that compartmentalise and isolate both continental and oceanic rift-units. For example, the Ascension FZ is shown to intersect the African continent south of Bata, Equatorial Guinea, where it forms a 200 km long transform-faulted boundary between RCC and POC. The thickness of POC from the top of the seaward-dipping reflectors to reflection Moho ranges between 2–5 s two-way travel time (twtt). This POC probably formed by volcanic outpouring of partial melt at the time of continental rupture, and then during initial seafloor spreading. Phases of magmatic and amagmatic extension formed within rift-unit spreading cells, owing to variable widths of POC (20–200 km wide) trending parallel to flow lines before evolving seaward into normal OC. Aptial salt extends over RCC and POC, and in some places, salt has been mobilised farther seaward (<10 km) as horizontal sills emanating from salt diapirs. The northernmost known occurrence of this Aptial salt lies 100 km south of Douala, Cameroon. There is no evidence in the MCS data that salt was deposited behind structural highs rimming the margin of the salt basin. Instead, salt overlies sub-horizontal reflectors (0.2–1 s twtt thick) believed to represent sedimentary rocks that drape POC and OC of presumably Aptial age. This interpretation implies that salt in the part of West Africa was deposited in nearshore to deeper basin margin environments without restriction from central basin waters.

Structure and stratigraphy of the Rukwa rift

Abstract Combining recently acquired multifold seismic data with well and gravity information and field mapping, a comprehensive picture of the structure and stratigraphy of the Rukwa rift has emerged. The Rukwa rift lies between the Tanganyika and Nyasa (Malawi) rifts in the western branch of the East African rift system in southwest Tanzania. The Rukwa rift is a NW-trending half-graben basin that is 350 km long and 40 km wide. Unlike the neighboring Tanganyika and Nyasa rifts, there is no evidence of half-graben polarity reversals in the Rukwa rift. The NW-trending boundary fault system lies on the northeastern side of the basin and comprises a series of listric faults. Most internal faults also show listric forms and trend N–S, oblique to the boundary fault. The basal sedimentary section is the Permo-Triassic (Karroo) Sequence. This is overlain by the Red Bed Sandstone Sequence, in which both Mesozoic and Tertiary fauna have been found. The Cenozoic Lake Bed Sequence is the highest unit and covers nearly all of the present basin. Sediment thicknesses commonly reach 7 km and attain a maximum of 12 km at the southeastern end of the basin. The Lake Bed Sequence is the thickest unit in the main depocentre, but the Karroo Sequence is often the thickest unit on the shoaling side of the half-graben. The Rukwa rift is here interpreted to have evolved as a strike- to oblique-slip pull-apart basin, based on numerous indications of NW-trending strike-slip faulting.

Oblique‐slip deformation in extensional terrains: A case study of the lakes Tanganyika and Malawi Rift Zones

The East African Rift system (EAR) is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins. The three-dimensional structure of parts of the EAR has been recently elucidated by a multifold seismic (MFS) survey of Lakes Tanganyika and Malawi (Project PROBE). Analysis of fault geometries displayed on the PROBE MFS data coupled with the more extensive 28-kHz echosounder data has improved understanding of the geometry and kinematics of the linked fault systems that underlie the lakes. In particular, it has been recognized that profiles across (i.e., at high angles to) the rift elongation commonly display fault geometries that are not readily retrodeformable. There must therefore be significant displacement out of the plane of what would conventionally be regarded as “dip” lines, that is, perpendicular to rift elongation and parallel to the tectonic transport or extension direction. Careful determination of fault plane dip and the dip of synrift reflectors in the hanging wall demonstrates that the dip directions of the shallowest faults and the steepest hanging wall sediment dips are oriented either NW or SE. This defines the direction of maximum fault block rotation and therefore the direction of extension or tectonic transport as NW/SE, oblique to the trend of the regional rift axis and to the apparent strike of many of the major border faults in the survey area. The border faults and many other faults imaged on the PROBE MFS data must therefore have significant components of strike-slip motion and the Tanganyika and Malawi rift zones have undergone extension oblique to and not perpendicular to their axes. Near dip-slip normal faults, steeply dipping oblique-slip faults and subvertical strike-slip or transfer faults have all been recognized in a complex, linked fault system that accomplishes the extension. The overall orientation of the rift lakes and their internal segmentation are influenced by major, preexisting, subvertical fault zones within the basement. Complex accommodation zones act principally as transfer zones that allow switches in gross polarity of the border fault system. The detailed geometries of the accommodation zones result from the specific relations between juxtaposed half-graben and the strike and internal geometry of the influencing basement structure. The variations in fault geometry, subsidence, water depth and basin or rift morphology can be better explained by an oblique-slip extensional model influenced by basement structures than by a simple orthogonal extensional model.

North Viking Graben: An East African Perspective

Rifting models derived from Project PROBEs (Proto-Rifts and Ocean Basin Evolution) East African rift system investigations are applied to various facets of the geology of the North Viking graben between lats. 59° and 62°N. Simple cross-sectional comparisons show striking similarities, particularly in regard to the occurrences of asymmetrical half graben and the creation of intrarift highs (accommodation zones) related to linking modes of adjacent half grabens. The positions of accommodation zones and structurally elevated platforms also are consistent and predictable by this application. A plausible evolutionary scenario and ready explanation for some midrift structures are derived by using a scheme of half-graben overprinting in space and time. The analysis al o may provide a qualitative understanding of switching depositional patterns and facies in both space and time.

Deep seismic reflection study of a passive margin, southeastern Gulf of Guinea

A large grid of deep-imaging, marine seismic reflection data has been acquired in the Gulf of Guinea. The data show that the architecture of old Atlantic igneous oceanic crust and upper mantle is highly variable, particularly if reflection Moho is taken to be the base of the crust. Most abrupt changes in oceanic basement thickness and depth to Moho can be correlated with fracture- zone crossings, but significant variations can occur between fracture zones and along flow lines, especially near the ocean-continent transition. Reflection Moho is usually continuous from ocean to continent and does not display any systematic changes in character, continuity, or reflection time even beneath the innermost shelf areas. There are several varieties of intra-crustal reflectors, including those that mark different levels within the oceanic gabbroic complex and events that diagonally link the top of oceanic seismic layer 2 and Moho. Different types of sub-Moho dipping reflections also are observed. Some are associated with fracture zones, some originate within continental crust and dip toward the ocean, dissecting Moho without offsetting it, and still others originate at the oceanic Moho and dip toward the continent. The transition from oceanic to continental crust is generally quite sharp north of lat 1°S, but the exact nature of the transition ranges from rift-block geology to abrupt juxtapositions of oceanic and continental crustal rocks. South of about lat 1°S, the transition to continental crust is more gradual, involving a progressive thickening of oceanic crust toward land. This difference may relate to the occurrence of much more oblique initial rifting north of 1°S.

Structure and evolution of an obliquely sheared continental margin: Rio Muni, West Africa

Abstract Oblique-shear margins are divergent continental terrains whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni margin, equatorial West Africa, where the ca. 70-km-wide Ascension Fracture Zone (AFZ) exhibits oblique–slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole lithosphere failure and continental breakup (ca. 117 Ma). Oblique extension is recorded also by strike–slip and oblique–slip fault geometry within the AFZ, and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of continental margin kinematics. At transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and oblique-shear margins may experience several tens of millennia between breakup and drift. Noncoeval breakup and drift have important consequences for the fit of the equatorial South American and African margins because, in reconstructing the configuration of conjugate continental margins at the time of their breakup, it cannot be assumed that highly segmented margins like the South Atlantic will match each other at their ocean–continent boundaries (OCBs). Well known ‘misfits’ in reconstructions of South Atlantic continental margins may be accounted for by differential timing of breakup and drifting between oblique-shear margins and their adjacent rifted segments.

Seismic reflection character of the Cameroon volcanic line: Evidence for uplifted oceanic crust

Deep-imaging multifold seismic fines across submarine parts of the Cameroon volcanic line (west Africa-Gulf of Guinea) show asymmetric uplift of oceanic crust associated with extensive magmatism. The main pulse of uplift occurred after creation of a regional sequence boundary believed to be Miocene in age. The apparent synchroneity of uplift argues against the Cameroon line being a simple hotspot trace, as previously inferred. One plausible theory of origin for the seaward part of the Cameroon volcanic line and its asymmetric uplift geometry combines regional asthenospheric upwelling with restriction of magmatic egress to regularly spaced weak spots, corresponding to fracture-zone crossings. Horizontal motion and buckling also may have occurred along the Cameroon volcanic line.