Rift Model Research Articles

A multidisciplinary study of the final episode of the Manda Hararo dyke sequence, Ethiopia, and implications for trends in volcanism during the rifting cycle

Abstract The sequence of dyke intrusions between 2005 and 2010 in the Manda Hararo rift segment, Ethiopia, provided an opportunity to test conceptual models of continental rifting. Based on trends up to dyke 13 in the sequence, it was anticipated that, should magma supply continue, dykes would shorten in length and eruptions would increase in size and decrease in distance from the segment centre as extensional stress was progressively released. In this paper we revisit these predictions by presenting a comprehensive overview of the May 2010 dyke and fissure eruption, the 14th and last in the sequence, from InSAR, seismicity, satellite thermal data, ultraviolet SO 2 retrievals and multiple LiDAR surveys. We find the dyke is longer than other eruptive dykes in the sequence, propagating in two directions from the segment centre, but otherwise fairly typical in terms of opening, propagation speed and geodetic and seismic moment. However, though the eruption is located closer to the segment centre, it is much smaller than previous events. We interpret this as indicating that either the Manda Hararo rifting event was magma limited, or that extensional stress varies north and south of the segment centre.

A scalable, matrix-free multigrid preconditioner for finite element discretizations of heterogeneous Stokes flow

Abstract In this paper we describe a computational methodology that is specifically designed for studying three-dimensional geodynamic processes governed by heterogeneous visco-plastic Stokes flow. The method employs a hybrid spatial discretization consisting of a Q 2 - P 1 disc mixed finite element formulation for the Stokes problem, coupled to a material-point formulation which is used for representing material state and history-dependent variables. The applicability and practicality of this methodology is realized through the development of an efficient, scalable and robust variable viscosity Stokes preconditioner. In this work, these objectives are achieved through exploiting matrix-free operators and a geometric multigrid preconditioner employing hybrid coarse level operators, Chebyshev smoothers and hybrid Krylov coarse level solvers. The robustness and parallel efficiency of this strategy is demonstrated using an idealized geodynamic model. Lastly, we apply the new methodology to study geodynamic models of continental rifting and break-up in order to understand the diverse range of passive continental margins we observe on Earth today.

Tectono-stratigraphic signature of a rapid multistage subsiding rift basin in the Tyrrhenian-Apennine hinge zone (Italy): A possible interaction of upper plate with subducting slab

The Campania Plain is a rapidly subsiding Quaternary basin that formed on the eastern margin of the Tyrrhenian Sea in association with the younger phase of Tyrrhenian rifting. It is located in the hinge area between the Apennines fold-thrust belt and the Tyrrhenian extensional backarc basin. By combining original stratigraphic analyses of well logs and seismic profiles we built a basin subsidence curve, mapped the fault pattern of the Campania Plain and analyzed the impact of the block faulting on the sedimentology and stratigraphic architecture of the basin fill. Well data indicate that the Quaternary succession consists of offshore, shoreface and coal-bearing coastal plain deposits arranged to form thick aggradational and retrogradational units. The sequence stratigraphy interpretation of well logs permitted us to recognize thirteen depositional sequences and the stratigraphic signatures of the rift stages. The study area corresponds to a sediment overfilled/balanced infill basin type that resulted from superposition of several rifting events characterized by high rates of basin subsidence.Taking into account the geological data of the adjacent areas, we propose a Pliocene-Quaternary rifting evolution of the upper Tyrrhenian plate consisting of four episodes. Two peculiar features of the Tyrrhenian rifting are a skip of the extensional axial zone eastwards leaving the previous zone of high strain localization (Vavilov basin), followed by a dramatic change (90°) of the direction of extension. Because these Tyrrhenian features cannot be accounted for by the current rifting models we hypothesized a link between the evolution of upper plate and subducting slab. The proposed geodynamic scenario is characterized by a progressive rupture of the subducting plate and formation of extensional basins in the upper plate.

Tectonic evolution and setting of the Sa’al Complex, southern Sinai, Egypt: A Proterozoic continental back-arc rift model

The Sa’al Complex is a mainly low grade metamorphosed polydeformed volcanosedimentary sequence exposed in the northern central Sinai basement, Egypt. Details of the stratigraphy, sedimentology and petrography of the three formations: Agramiya, Ra’ayan and Zaghara Formations are described. The earliest deformation (D1) is related to extensional tectonism and HT-LP regional metamorphism. The main D1 structure is a bedding-parallel S1 foliation with at least 50% vertical shortening in the well-foliated Ra’ayan phyllites. Earlier models that explained S1 by bedding-parallel shearing are rejected. The Sa’al volcanism, D1 extension and HT metamorphism were probably associated with back-arc rifting in a continental arc setting, similar to the modern Taupo Zone of New Zealand. Later deformations, D2 and D3, involved folding about NE-trending and NW-trending axial planes, respectively. D2 was probably a result of compressional stresses typical of continental back-arc regions, and resulted in development of steep NW-vergent imbricate thrusts and NE-trending F2 meso- and macrofolds. The Firinga gabbro and the Wadi Murad foliated diorite intruded along D2 backthrusts, while the main diorite intrusion dominating the centre of the complex intruded along D2 steepened imbricate thrusts. F3 deformation may be related to the latest convergence of the east and west Gondwana, and has correlatives in the Kid and Feiran Complexes. A final deformation D4 that generated the main strike-slip faults in the area correlates with NE–SW trending σ1, inconsistent with a Najd origin. Recent geochronological results from U/Pb zircon studies are difficult to reconcile with stratigraphic and intrusion field evidence, and apparently require very tight time constraints on the main metamorphism, D1 and D2 deformations of the complex.

The Baikal rift: Pliocene (Miocene) – Quaternary episode or product of extended development since the Late Cretaceous under various tectonic factors. A review

The Baikal rift: Pliocene (Miocene) – Quaternary episode or product of extended development since the Late Cretaceous under various tectonic factors. A review

Passive rifting and continental splitting in the Jurassic Ligurian Tethys: the mantle perspective

Abstract Based on present knowledge of mantle peridotites from the Ligurian Tethys ophiolites, this paper presents new ideas and a new model for passive rifting to ocean spreading in the slow–ultraslow rifting Europe–Adria realm. Relevant points include: (i) the positive feedback between deformation and melt percolation during passive magmatic rifting; (ii) the positive feedback between natural evidence and experimental data on the behaviour of the mantle lithosphere during passive rifting; (iii) the significance of hidden magmatism and the associated melt thermal advection; (iv) the role of the wedge-shaped weakened and softened axial zone; and (v) the evidence of a transition from passive to active rifting in the Ligurian Tethys. Passive rifting induced passive asthenospheric upwelling and the onset of partial melting. Fractional melts migrated through the mantle lithosphere and stagnated at shallow levels (the hidden magmatism). Melt thermal advection heated the mantle lithosphere to temperatures ( T ) of ≥1200°C and formed a wedge-shaped axial zone of rheological softened/weakened mantle peridotites that served as the future locus of continental break-up. The hotter/deeper asthenosphere ascended within this axial zone, underwent partial melting and formed aggregated mid-ocean ridge basalts (MORBs) that migrated within dunite channels to form olivine gabbro intrusions and basaltic lava flows. Rifting evolved from passive to active, and the actively upwelling asthenosphere established a ridge-type system and thermal regime.

Numerical modeling of mantle diapirism as a cause of intracontinental rifting

The numerical model of mantle diapirism and active rifting is developed. The model describes the possibility of extension and thinning of the Earth’s crust under the action of a local 100-km long heat source in the sublithospheric mantle, which causes melting and rising of the magmatic diapir through the cratonic lithosphere. The model combines the mechanisms of the uplifting of the anomalously hot material due to its gravitational instability, underplating of magma beneath the continental crust, and its extension by the forces of the convective flows at the base of the plate. The obtained results shed light on some geological features of the joint formation of the large Vilyui igneous province and Vilyui sedimentary basin.

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10−8 yr−1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.

The effect of crustal melt on rift dynamics: case study of the Taupo Volcanic Zone

The effect of crustal melt on extension in the central Taupo Volcanic Zone, New Zealand is investigated using a 3D numerical model of rifting. Partial melt bodies are directly inferred from magnetotelluric imaging, and are used to determine heat flow and crustal viscosity in the rift. We show how the pattern and narrow width of active faulting in the modern Taupo Rift and the change in fault strike south of the Okataina Volcanic Centre can be partly explained by the thermal weakening effects of partial melt at depth.

Kinematics of the Neogene Terror Rift: Constraints from calcite twinning strains in the ANDRILL McMurdo Ice Shelf (AND-1B) core, Victoria Land Basin, Antarctica

We report new strain analyses of mechanically twinned calcite in veins hosted by Neogene (13.6–4.3 Ma) sedimentary and volcanic rocks recovered from the Terror Rift system in the southern Ross Sea, Antarctica, by the ANDRILL (ANtarctic geological DRILLing) McMurdo Ice Shelf (MIS) Project. Strain analyses of the ANDRILL MIS AND-1B drill core samples yield prolate and oblate ellipsoids with principal shortening and extension strains ranging from −7% to 9%, respectively. The majority of samples show ≤25% negative expected values, indicating homogeneous coaxial strain characterized predominantly by subvertical shortening. We attribute the subvertical shortening strains to mechanical twinning at relatively shallow depths in an Andersonian normal faulting stress regime induced by sedimentary and ice sheet loading of the stratigraphic sequence and characterized by low stress magnitudes. Oriented samples yield a northwest-southeast average extension direction that is subparallel to other indicators of Neogene extension. This northwest-southeast extension is consistent with strain predicted by Neogene orthogonal rifting in a north-northeast–trending rift segment, as well as models of right-lateral transtensional rifting. The overall paucity of a noncoaxial layer-parallel shortening signal in the AND-1B twin populations favors orthogonal extension in the Neogene Terror Rift system, but could also be due to spatial partitioning of strain in a transtensional rift regime.

Evolution of stress and fault patterns in oblique rift systems: 3‐D numerical lithospheric‐scale experiments from rift to breakup

AbstractRifting involves complex normal faulting that is controlled by extension direction, reactivation of prerift structures, sedimentation, and dyke dynamics. The relative impact of these factors on the observed fault pattern, however, is difficult to deduce from field‐based studies alone. This study provides insight in crustal stress patterns and fault orientations by employing a laterally homogeneous, 3‐D rift setup with constant extension velocity. The presented numerical forward experiments cover the whole spectrum of oblique extension. They are conducted using an elastoviscoplastic finite element model and involve crustal and mantle layers accounting for self‐consistent necking of the lithosphere. Despite recent advances, 3‐D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. This issue is addressed by applying a post processing method that identifies the stress regime and preferred fault azimuth at each surface element. The simple model setup results in a surprising variety of fault orientations that are solely caused by the three‐dimensionality of oblique rift systems. Depending on rift obliquity, these orientations can be grouped in terms of rift‐parallel, extension‐orthogonal, and intermediate normal fault directions as well as strike‐slip faults. While results compare well with analog rift models of low to moderate obliquity, new insight is gained in advanced rift stages and highly oblique settings. Individual fault populations are activated in a characteristic multiphase evolution driven by lateral density variations of the evolving rift system. In natural rift systems, this pattern might be modified by additional heterogeneities, surface processes, and dyke dynamics.

Did a proto-ocean basin form along the southeastern Rae cratonic margin? Evidence from U-Pb geochronology, geochemistry (Sm-Nd and whole-rock), and stratigraphy of the Paleoproterozoic Piling Group, northern Canada

The Paleoproterozoic Piling Group along the southeastern Rae margin, northern Canada, is characterized by thick, deep marine turbidite deposits not observed in time-equivalent, intracratonic basin units further southwest. Models invoked to explain this feature include development of a full-ocean, back-arc, or proto-ocean basin followed by turbidite sedimentation. We present new and existing U-Pb geochronological, Nd isotope, geochemical, and stratigraphic evidence that support a proto-ocean basin model, and we explore the events leading to the formation and closure of such a rift basin during the middle Paleoproterozoic. Sedimentation initiated largely after ca. 2160 Ma with deposition of craton-derived, shallow marine siliciclastic strata (Dewar Lakes formation). Continued extension resulted in accumulation of south-facing carbonate beds (Flint Lake formation) and likely concomitant, arc-like tholeiitic to picritic volcanism and voluminous volcaniclastic sedimentation (lower Bravo Lake formation) farther outboard at ca. 1980 Ma. Accumulation of intrabasinal siliciclastic strata above lower Bravo Lake formation rocks may mark a hiatus in mafic-ultramafic magmatism. By ca. 1923 Ma, upper Bravo Lake formation, within-plate–type alkaline sill emplacement and volcanism occurred within highly extended crust. The oceanic island basalt–like signatures of the Bravo Lake formation rocks (but lack of depleted, mid-ocean-ridge basalt–type compositions) suggest that by this time the thinned Rae continental lithosphere had fragmented into small crustal block(s) and narrow zone(s) of incipient oceanic crust farther outboard of the Piling Group basin. Rapid subsidence of the southeastern Rae margin ensued, leading to deposition of euxinic (Astarte River formation) and overlying turbiditic strata (Longstaff Bluff formation). The post–ca. 1915 Ma northern turbiditic sedimentary units were likely derived from a thoroughly mixed, two-component source with possible input from the Snowbird tectonic zone and Bravo Lake formation, whereas the post–ca. 1930 Ma southern turbidite unit may have been sourced from the Meta Incognita microcontinent, presently exposed further south. We favor a rift margin over a foreland basin setting for the deposition of the northern turbidite deposits. Subsequent mantle upwelling associated with incipient ocean formation may have triggered melting of highly thinned continental crust resulting in emplacement of late-stage, ca. 1897 Ma, contaminated rapakivi granite and highly differentiated mafic sills. Our results are most consistent, albeit not exclusively, with the much debated model of asthenospheric upwelling and incipient rifting along the Rae-Hearne boundary farther southwest at ca. 1.9 Ga. Later accretion of the Meta Incognita microcontinent led to basin closure and development of a north-verging fold-and-thrust belt after ca. 1883 Ma.

Wide rift model in Bohai Bay Basin: insight into the destruction of the North China Craton

The Bohai Bay Basin is a Cenozoic extensional basin along the eastern aspect of Asia. Whether the Bohai Bay Basin is a pull-apart or rift basin is controversial. The Bohai Bay Basin exhibits a high density of extensional faults and records destruction of the North China Craton. Many structural analyses have been performed on the Bohai Bay Basin, especially the Tan-L and Taihang Mountain fault systems which control its boundary. The initial deposition of Kongdian Formation was mainly distributed along the boundary of Bohai Bay Basin during the Palaeocene–early Eocene. Subsequently, tectonic activity migrated toward the interior of the basin during deposition of Shahejie Formation in the middle Eocene–early Oligocene. Bohai Bay Basin crust was thickened in early Mesozoic time and has thinned since late Mesozoic time. The crustal strength profile of Bohai Bay Basin is characterized by very weak lower crust, which differs from that of adjacent crust. In regard to the crustal structure, lithospheric thickness, and extensional style, an alternative rift model is proposed. Initial Bohai Bay Basin rifts were characterized by metamorphic core complexes affecting the North China Craton, which reflects collapse of parts of the early Mesozoic intra-plate orogen. Furthermore, westward subduction of the Palaeo-Pacific Plate led to upwelling of asthenosphere mantle. Persistent upwelling of mantle decreased the strength of lower crust and led to the warm heat-flow regime and generation of a lower crustal fluid layer and wide rifting. Outward flow of ductile lower crust following late Cretaceous extension thinned the lower crust and generated the overall sag appearance of the basin in early Cenozoic time. The model supports a model whereby a wide rift narrows with time. For the Bohai Bay Basin, extension and strike-slip faulting were two independent deformation systems superimposed on each other.

Geodetic measurements and numerical models of rifting in Northern Iceland for 1993–2008

Geodetic measurements and numerical models of rifting in Northern Iceland for 1993–2008

The Faroe–Shetland Basin: a regional perspective from the Paleocene to the present day and its relationship to the opening of the North Atlantic Ocean

Abstract The Faroe–Shetland Basin is located offshore NW Scotland on the SE margin of the Atlantic Ocean and comprises numerous sub-basins and intra-basin highs that are host to a number of significant hydrocarbon discoveries. The principal hydrocarbon discoveries are in Paleocene–Eocene strata, although earlier strata are known, and their existence is therefore intimately linked to the opening and evolution of the North Atlantic from 54 Ma. The final rifting and separation of Greenland from Eurasia is commonly attributed to the arrival of a mantle plume which impacted beneath Greenland during early Tertiary time. Moreover, the ensuing plate separation is commonly described in terms of instantaneous unzipping of the North Atlantic, whereas in reality proto-plate boundaries were more diffuse during their inception and the linked rift system which we see today, including connections with the Arctic, was not established until Late Palaeogene–Early Neogene time. From a regional analysis of ocean basin development, including the stratigraphic record on the adjacent continental margins, the significance of the Greenland–Iceland–Faroe Ridge and the age and role of Iceland, we propose a dual rift model whereby North Atlantic break-up was only partial until the Oligo-Miocene, with true final break-up only being achieved when the Reykjanes and Kolbeinsey ridges became linked. As final break-up coincides with the appearance of Iceland, this model negates the need for a plume to develop the North Atlantic with rifting reliant on purely plate tectonic mechanisms, lithospheric thinning and variable decompressive upper mantle melt along the rifts.

Tectono-stratigraphic evolution of the Upper Jurassic–Neocomian rift succession, Araripe Basin, Northeast Brazil

The rift succession of the Araripe Basin can be subdivided into four depositional sequences, bounded by regional unconformities, which record different palaeogeographic and palaeoenvironmental contexts. Sequence I, equivalent to the Brejo Santo Formation, is composed of fluvial sheetflood and floodplain facies association, while Sequence II, correspondent to the lower portion of the Missão Velha Formation, is characterised by braided fluvial channel belt deposits. The fluvial deposits of Sequences I and II show palaeocurrents toward SE. The Sequence III, correspondent to the upper portion of Missão Velha Formation, is composed of fluvial sheetflood deposits, which are overlain by braided fluvial channel deposits displaying a palaeocurrent pattern predominantly toward SW to NW. Sequence IV, equivalent to the Abaiara Formation, is composed of fluvio–deltaic–lacustrine strata with polimodal paleocurrent pattern. The type of depositional systems, the palaeocurrent pattern and the comparison with general tectono-stratigraphic rift models led to the identification of different evolutionary stages of the Araripe Basin. Sequences I, II and III represent the record of a larger basin associated to an early rift stage. However, the difference of the fluvial palaeocurrent between sequences II and III marks a regional rearrangement of the drainage system related to tectonic activity that compartmentalised the large endorheic basin, defining more localised drainage basins separated by internal highs. Sequence IV is associated with the renewal of the landscape and implantation of half-graben systems. The high dispersion of palaeocurrents trends indicate that sedimentary influx occurs from different sectors of the half-grabens.

Insights into the mode of the South Georgia rift extension in eastern Georgia, USA

The South Georgia rift (SGR) lies oblique to the east coast margin of North America and across the Alleghenian suture between Laurentia and Africa in southern Georgia. Regionally, the SGR can be divided into a southwest compartment and a northeast compartment across the Jacksonville structure that is located in the vicinity of that suture. Analytical and numerical models are used to characterize the mode of rifting in the northeast compartment. Borehole, COCORP seismic, and regional geophysical information from the compartment, that were used previously to infer the geometry of the basin, are reassessed with the use of those models to analyze the lithospheric conditions influencing Triassic extension. This approach led to the interpretation of core complex mode extension and to the proposal of a model of progressive rifting. The model shows how the Riddleville and Main SGR basins are associated and how changes in structural style of those two basins resulted from changing lithospheric conditions during extension. The core complex model also indicated that extension was influenced by distributed deformation of a younger, warmer, and less stable lithosphere adjacent to the Permian suture; whereas extension in other east coast rifts that lie subparallel to structural fabric was probably localized by preexisting zones of weakness.

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along western Laurentia. The formation is divided into seven informal members, from bottom to top: (1) arkosic grit deposited as proximal turbidites during onset of rifting; (2) quartzite and grit deposited as medial turbidites; (3) pebbly slate with dropstones deposited during an older glacial episode; (4) slate with interbedded quartzite deposited mostly as distal turbidites; (5) mafic volcanic and intrusive rocks; (6) diamictite with basement and volcanic clasts deposited during younger glaciation; and (7) graywacke with rare carbonate layers. The formation has a basal nonconformity on Archean to Paleoproterozoic basement rocks of the Facer Formation and is overlain by arkosic quartzite of the Maple Canyon Formation and argillite of the Kelley Canyon Formation. Changes in member thicknesses reflect syndepositional faulting. Vertical and lateral changes in provenance and maximum depositional ages were evaluated from detrital zircon (DZ) U-Pb geochronology of five well-exposed stratigraphic sections across the sedimentary basin. DZ patterns for the arkosic grit record local sources during onset of rifting, patterns for the pebbly slate and slate members record distal Laurentian sources and possible sediment recycling, and patterns for diamictite and graywacke record basement sources along the rift margin and reworking of mafic to felsic volcanic rocks. Young zircon grains derived from volcanic rocks give maximum depositional ages of 703 ± 6 Ma and 667 ± 5 Ma for the lower part of the diamictite and graywacke members, respectively, suggesting a late Sturtian age for the younger glaciation. Associated cap carbonates, however, have Marinoan-style features, indicating that global correlations based only on lithology may not be valid. DZ data for the Kelley Canyon Formation record a change to distal Laurentian sources during final stages of early rifting. Mafic volcanic and intrusive rocks are subalkaline to alkaline and have trace-element ratios indicative of a rift setting. Volcanic clasts in diamictite include basalt, trachyte, and rhyolite. Volcanic zircon grains have e Hf = –1 to –21, which record crustal contamination during igneous activity. Rare mafic to ultramafic dikes in the upper part of the section were intruded during waning stages of igneous activity. Early rifting spanned ca. 720 Ma to 660 Ma but was incomplete and followed by deposition of mature quartzite of the overlying lower Brigham Group across a broad basin, indicating that Rodinia rift models showing pre–720 Ma separation of western Laurentia may need revision. Final rifting and transition to drift occurred ca. 550 Ma during deposition of the upper Brigham Group.

The rift origin of the Vilyui basin (East Siberia), from reconstructions of sedimentation and mechanical mathematical modeling

Results of modeling of the formation of the Vilyui sedimentary basin are presented. We combine backstripping reconstructions of sedimentation and thermal regime during the subsidence with a numerical simulation based on the deformable solid mechanics. Lithological data and stratigraphic sections were used to “strip” the sedimentary beds successively and calculate the depth of the stratigraphic units during the sedimentation. It is the first time that the evolution of sedimentation which is nonuniform over the basin area has been analyzed for the Vilyui basin. The rift origin of the basin is proven. We estimate the spatial distribution of the parameters of crustal and mantle-lithosphere extension as well as expansion due to dike intrusion. According to the reconstructions, the type of subsidence curves for the sedimentary rocks of the basin depends on the tectonic regime of sedimentation in individual basins. The backstripping analysis revealed two stages of extension (sediments 4–5 km thick) and a foreland stage (sediments >2 km thick). With the two-layered lithosphere model, we conclude that the subcrustal layer underwent predominant extension (by a factor of 1.2–2.0 vs. 1.1–1.4 in the crust). The goal of numerical experiments is to demonstrate that deep troughs can form in the continental crust under its finite extension. Unlike the oceanic rifting models, this modeling shows no complete destruction or rupture of the continental crust during the extension. The 2D numerical simulation shows the possibility of considerable basement subsidence near the central axis and explains why mafic dikes are concentrated on the basin periphery.

Seismicity from February 2006 to September 2007 at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms

Abstract. We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from –0.5 to 5.1. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the east and west of the Rwenzoris with the highest seismic activity observed in the northeastern area, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders, seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley, seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly normal faulting. T-axis trends are highly uniform and oriented WNW–ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN–SWS, which may be indicative of a local perturbation of the regional stress field.