Rift Dynamics Research Articles

The Gulf of Suez rifting: implications from low-temperature thermochronology

ABSTRACT The Gulf of Suez is a continental rift basin that has been developed during the Oligocene-Miocene as a northern extension of the Red Sea rift system. The Oligo-Miocene rift-activated flanks are represented by exposures of the Neoproterozoic basement rocks of the Arabian-Nubian Shield. An additional thermal overprint (i.e. mantle plume) and the preceding tectono-thermal events that also affected the rifting dynamics and the uplift of its flanks remain uncertain. We here present thermochronological data for 20 basement samples collected from the Gabal Somr ElQaa area at the northwestern flank of the Gulf of Suez. The resulted zircon fission-track ages could be divided temporospatially into two groups: the Cambrian and the Silurian-Devonian with average ages of 503 ± 14 Ma and 407 ± 11 Ma, respectively. While apatite fission-track data reveal three different age groups of the Devonian-Carboniferous, the Permian-Triassic, and the Late Cretaceous, with average ages of 349 ± 10 Ma, 246 ± 6 Ma, and 81 ± 5 Ma, respectively. The modelled t-T histories indicate four cooling pulses during the Neoproterozoic, the Devonian-Carboniferous, the Cretaceous, and the Oligocene-Miocene. Integrating our findings with the regional tectonism and depositional records indicates that these cooling pulses could be attributed to synchronous activities of the post-accretion erosional event, the Variscan tectonic event, the Mid-Atlantic Ridge spreading, and the Gulf of Suez rifting. The Gulf of Suez is a passive rift, formed by a single and rapid rifting pulse within the framework of the Red Sea rift system. An additional east-southward far-field thermal overprint from the Arabian marginal plume partially affected the Suez rifting process. This thermal print affected only the rift’s southern portion of its eastern flank, causing syn-rift cooling ages, highly elevated rift flanks, and an increased heat flow. The western rift flank and the northern segment of the eastern flank are characterized by pre-rift cooling ages, modest rift flanks, and reduction in heat flow.

Ocean Coupling Limits Rupture Velocity of Fastest Observed Ice Shelf Rift Propagation Event

AbstractThe Antarctic ice sheet is buttressed by floating ice shelves that calve icebergs along large fractures called rifts. Despite the significant influence exerted by rifting on ice shelf geometry and buttressing, the scarcity of in situ observations of rift propagation contributes considerable uncertainty to understanding rift dynamics. Here, we report the first‐ever seismic recording of a multiple‐kilometer rift propagation event. Remote sensing and seismic recordings reveal that a rift in the Pine Island Glacier Ice Shelf extended 10.53 km at a speed of 35.1 m/s, the fastest known ice fracture at this scale. We simulate ocean‐coupled rift propagation and find that the dynamics of water flow within the rift limit the propagation rate, resulting in rupture two orders of magnitude slower than typically predicted for brittle fracture. Using seismic recordings of the elastic waves generated during rift propagation, we estimate that ocean water flows into the rift at a rate of at least 2,300 m3/s during rift propagation and causes mixing in the subshelf cavity. Our observations support the hypotheses that large ice shelf rift propagation events are brittle, hydrodynamically limited, and exhibit sensitive coupling with the surrounding ocean.

Contrasting transform and passive margin subsidence history and heat flow evolution: insights from 3D thermo-mechanical modelling

Abstract Transform and passive margins developed during the continental rifting and opening of oceanic basins are fundamental elements of plate tectonics. It has been suggested that inherited structures, plate divergence velocities and surface processes exert a first-order control on the topographic and bathymetric evolution and thermal history of these margins and related sedimentary basins. Their complex spatial-temporal dynamics have remained controversial. Here, we conducted 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to simulate the dynamics of continental rifting, continental transform fault zone formation as well as persistent oceanic transform faulting and zero-offset oceanic fracture zones development. Numerical modelling results allow to explain the first-order observations from passive and transform margins, such as diachronous rifting, heat flow rise and cooling in individual depocentres as well as contrasting basin tectonics of extensional and transtensional origin. In addition, the models reproduce the rise of both marginal ridges and transform marginal plateaus, and their interaction with erosion and sedimentation. Comparison of model results with observations from natural examples yields new insights into the tectono-sedimentary and thermal evolution of several key passive and transform continental margins worldwide.

Controls on Asymmetric Rift Dynamics: Numerical Modeling of Strain Localization and Fault Evolution in the Kenya Rift

AbstractComplex, time‐dependent, and asymmetric rift geometries are observed throughout the East African Rift System (EARS) and are well documented, for instance, in the Kenya Rift. To unravel asymmetric rifting processes in this region, we conduct 2D geodynamic models. We use the finite element software ASPECT employing visco‐plastic rheologies, mesh‐refinement, distributed random noise seeding, and a free surface. In contrast to many previous numerical modeling studies that aimed at understanding final rifted margin symmetry, we explicitly focus on initial rifting stages to assess geodynamic controls on strain localization and fault evolution. We thereby link to geological and geophysical observations from the Southern and Central Kenya Rift. Our models suggest a three‐stage early rift evolution that dynamically bridges previously inferred fault‐configuration phases of the eastern EARS branch: (1) accommodation of initial strain localization by a single border fault and flexure of the hanging‐wall crust, (2) faulting in the hanging‐wall and increasing upper‐crustal faulting in the rift‐basin center, and (3) loss of pronounced early stage asymmetry prior to basinward localization of deformation. This evolution may provide a template for understanding early extensional faulting in other branches of the East African Rift and in asymmetric rifts worldwide. By modifying the initial random noise distribution that approximates small‐scale tectonic inheritance, we show that a spectrum of first‐order fault configurations with variable symmetry can be produced in models with an otherwise identical setup. This approach sheds new light on along‐strike rift variability controls in active asymmetric rifts and proximal rifted margins.

Lithospheric Structure of a Transitional Magmatic to Amagmatic Continental Rift System—Insights from Magnetotelluric and Local Tomography Studies in the North Tanzanian Divergence, East African Rift

Continental break-up is controlled by several parameters and processes (rheology, inherited structures, magmatism, etc). Their impact, chronology and interactions are still poorly known and debated, particularly when rifting interacts with cratons. In order to better understand the rifting initiation in a cratonic lithosphere, we analysed 22 magnetotelluric (MT) soundings collected along two East-West profiles in two different rift segments of the North Tanzanian Divergence. The North Tanzanian Divergence, where the East African Rift is at its earliest stage, is a remarkable example of the transition between magmatic to amagmatic rifting with two clearly identified segments. Only separated by a hundred kilometers, these segments, Natron (North) and Manyara (South), display contrasted morphological (wide versus narrow), volcanic (many versus a few edifices) and seismic (shallow versus deep activity) signatures. Magnetotelluric profiles across the two segments were inverted with a three-dimensional approach and supplied the resistive structure of the upper lithosphere (down to about 70 km). The Natron segment has a rather conductive lithosphere containing several resistive features (Proterozoic Belt), whereas the Manyara segment displays highly resistive blocks probably of cratonic nature encompassing a conductive structure under the axial valley. The joint interpretation of these models with recent local and regional seismological studies highlights totally different structures and processes involved in the two segments of the North Tanzanian Divergence. We identified contrasted CO2 content, magma upwelling or trapping, in depth regarding the Manyara or the Natron branch and the influence of inherited cratonic structures in the rifting dynamics.

Numerical modelling of Cretaceous Pyrenean Rifting: The interaction between mantle exhumation and syn‐rift salt tectonics

AbstractThe preshortening Cretaceous Pyrenean Rift is an outstanding geological laboratory to investigate the effects of a pre‐rift salt layer at the sedimentary base on lithospheric rifting. The occurrence of a pre‐rift km‐scale layer of evaporites and shales promoted the activation of syn‐rift salt tectonics from the onset of rifting. The pre‐ and syn‐rift sediments are locally affected by high‐temperature metamorphism related to mantle ascent up to shallow depths during rifting. The thermo‐mechanical interaction between décollement along the pre‐existing salt layer and mantle ascent makes the Cretaceous Pyrenean Rifting drastically different from the type of rifting that shaped most Atlantic‐type passive margins where salt deposition is syn‐rift and gravity‐driven salt tectonics has been postrift. To unravel the dynamic evolution of the Cretaceous Pyrenean Rift, we carried out a set of numerical models of lithosphere‐scale extension, calibrated using the available geological constraints. Models are used to investigate the effects of a km‐scale pre‐rift salt layer, located at the sedimentary cover base, on the dynamics of rifting. Our results highlight the key role of the décollement layer at cover base that can alone explain both salt tectonics deformation style and high‐temperature metamorphism of the pre‐rift and syn‐rift sedimentary cover. On the other hand, in the absence of décollement, our model predicts symmetric necking of the lithosphere devoid of any structure and related thermal regime geologically relevant to the Pyrenean case.

Melt-modified lithosphere beneath Ross Island and its role in the tectono-magmatic evolution of the West Antarctic Rift System

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1–88.2), lherzolite (Fo90.6–87.4), and pyroxenite xenoliths (Fo89.3–87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo89.7–88.0), with more ferroan matrix olivine grains (Fo83.7–81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ± 0.3RA; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with an initial 187Os/188Os ratio of 0.1286 ± 0.0001. The initial 187Os/188Os ratio is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279–0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land.

Breakup Without Borders: How Continents Speed Up and Slow Down During Rifting

AbstractRelative plate motions during continental rifting result from the interplay of local with far‐field forces. Here we study the dynamics of rifting and breakup using large‐scale numerical simulations of mantle convection with self‐consistent evolution of plate boundaries. We show that continental separation follows a characteristic evolution with four distinctive phases: (1) an initial slow rifting phase with low divergence velocities and maximum tensional stresses, (2) a synrift speed‐up phase featuring an abrupt increase of extension rate with a simultaneous drop of tensional stress, (3) the breakup phase with inception of fast sea‐floor spreading, and (4) a deceleration phase occurring in most but not all models where extensional velocities decrease. We find that the speed‐up during rifting is compensated by subduction acceleration or subduction initiation even in distant localities. Our study illustrates new links between local rift dynamics, plate motions, and subduction kinematics during times of continental separation.

2. 45 Ga break-up of the Archaean continent in Northern Fennoscandia: Rifting dynamics and the role of inherited structures within the Archaean basement

Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5–2.4 Ga and later pulses of mafic magmatism (2.32–1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits.

Impact of rift dynamics on palaeoenvironmental conditions and hydrocarbon system development (northern Upper Rhine Graben, SW Germany)

The Upper Rhine Graben (URG), a classical hydrocarbon (HC) province, is part of the European Cenozoic Rift System. The rift graben development has led to a complex basin fill of terrestrial and marine Cenozoic deposits, providing several HC source and reservoir units. The aim of this study is the multidisciplinary analysis of the palaeoenvironmental conditions and source-rock development within the transgressive marine intervals, and the palaeothermal history of the graben system to improve the understanding of the HC system development. Palaeoenvironmental conditions are strongly influenced by rift-related tectonic activity. Transgressive marine intervals in times of major subsidence show high terrestrial input from the graben shoulders, leading to mainly terrigenous gas-prone kerogen, while transgressive marine intervals during weak tectonic activity are dominated by marine–brackish palaeoenvironments and oil-prone kerogen. This differs clearly from the previously suggested HC potential of these intervals. Thermal maturation analysis shows nearly constant maturation with depth, which is atypical for burial-controlled maturation. It indicates significant secondary thermal overprinting related to long-lasting very hot hydrothermal fluid systems, concentrated along fault zones. Therefore the development of the depositional setting, kerogen composition, thermal maturation and the HC potential is directly linked to the dynamics of the rift system development. Supplementary material: detailed results of all three wells are available at https://doi.org/10.6084/m9.figshare.c.4183106

Late Paleozoic Woniusi basaltic province from Sibumasu terrane: Implications for the breakup of eastern Gondwana’s northern margin

Research Article| September 01, 2015 Late Paleozoic Woniusi basaltic province from Sibumasu terrane: Implications for the breakup of eastern Gondwana’s northern margin Shi-Yong Liao; Shi-Yong Liao † 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China2Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China †E-mail: liaosy@hotmail.com or syliao@pmo.ac.cn. Search for other works by this author on: GSW Google Scholar Dong-Bing Wang; Dong-Bing Wang 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar Yuan Tang; Yuan Tang 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar Fu-Guang Yin; Fu-Guang Yin 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar Shu-Nan Cao; Shu-Nan Cao 3Department of Earth Sciences, University of Hong Kong, Hong Kong, China Search for other works by this author on: GSW Google Scholar Li-Quan Wang; Li-Quan Wang 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar Bao-Di Wang; Bao-Di Wang 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar Zhi-Ming Sun Zhi-Ming Sun 1Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China Search for other works by this author on: GSW Google Scholar GSA Bulletin (2015) 127 (9-10): 1313–1330. https://doi.org/10.1130/B31210.1 Article history received: 20 Sep 2014 rev-recd: 30 Jan 2015 accepted: 02 Mar 2015 first online: 08 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 Shi-Yong Liao, Dong-Bing Wang, Yuan Tang, Fu-Guang Yin, Shu-Nan Cao, Li-Quan Wang, Bao-Di Wang, Zhi-Ming Sun; Late Paleozoic Woniusi basaltic province from Sibumasu terrane: Implications for the breakup of eastern Gondwana’s northern margin. GSA Bulletin 2015;; 127 (9-10): 1313–1330. doi: https://doi.org/10.1130/B31210.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The tectonic mechanism responsible for detachment of the Sibumasu terrane in the eastern Cimmerian continent from the Australian Gondwana margin remains poorly understood and debated. Our study on the late Paleozoic Woniusi basaltic province in the northern Sibumasu terrane may provide a new perspective on this problem, and further, on the rifting dynamics of eastern Gondwana’s northern margin. In this study, we revealed that the Woniusi basaltic province is spread over an area of ∼12,000 km2 with a thickness of ∼300–500 m for the eruptive phase. Direct zircon U-Pb dating of mafic dikes indicates that the Woniusi basaltic province was emplaced between the latest Carboniferous and late Early Permian (301 Ma to 282 Ma), synchronous with basaltic rocks from the Panjal Traps, Tethyan Himalaya (Bhote Kosi, Selong, Abor), Lhasa, and southern Qiangtang. Geochemically, the Woniusi basaltic rocks show subalkaline tholeiitic affinity and relatively homogeneous features of enriched light rare earth elements (LREEs) and large ion lithophile elements (LILEs) but depleted Nb and Ta patterns. They were most likely derived from an enriched subcontinental lithospheric mantle source and triggered by the mantle plume–induced rifting process. The Woniusi basaltic province, together with other coeval basaltic provinces from the Panjal Traps, Tethyan Himalaya, Lhasa, and southern Qiangtang, belongs to a fragmented large igneous province that may have had an original area of over 2 × 106 km2, which is comparable to that of the Ontong-Java, Deccan, or Siberian Traps. These Tethyan basaltic provinces share a common mantle plume centered in northern Greater India. It was this mantle plume that may have triggered the initial rifting of the eastern Cimmerian terranes (e.g., Sibumasu, southern Qiangtang) from both the northern Indian and Australian Gondwana margins, as well as the opening of Meso-Tethys Ocean. This study, moreover, sheds light on the paleopositions of the Lhasa and Tengchong terranes before their breakups. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin

Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong post-rift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multi-channel seismic profile and other published geophysical data. We show that an 80 km-wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex”, is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north-dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios, a first one involving a simple coupling between mantle extension due to slab retreat and crustal extension, and a second one involving extraction of the lower crust and mantle from below the margin by the south-eastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow south-eastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

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.

Insights into the development of major rift‐related unconformities from geologically constrained subsidence modelling: Halten Terrace, offshore mid Norway

AbstractDue to the effects of sediment compaction, thermal subsidence and ‘post‐rift’ fault reactivation, the present‐day geometry of buried, ancient rift basins may not accurately reflect the geometry of the basin at any stage of its syn‐rift evolution. An understanding of the geometry of a rift basin through time is crucial for resolving the dynamics of continental rifting and in assessing the hydrocarbon prospectivity of such basins. In this study, we have restored the Late Jurassic–Early Cretaceous geometry of the southern Halten Terrace, offshore mid Norway, using a combination of well log‐ and core‐derived, sedimentological and stratigraphic data, seismic‐stratigraphic observations and reverse subsidence modelling. This integrated geological and geophysical approach has allowed the large number of input parameters involved in flexural backstripping and post‐rift thermal subsidence modelling to be constrained. We have thus been able to determine the regional structure of the basin at the end of the Late Jurassic–Early Cretaceous rift phase and the associated amount of crustal stretching. Our basin geometry reconstructions reveal that, during the latest syn‐rift period in the Late Jurassic–Early Cretaceous, the Halten Terrace was characterized by a series of isolated depocentres, located between footwall islands, which were not connected into a single depocentre until the Late Cretaceous (Coniacian). We show that two major unconformities, which are now vertically offset by ca. 2 km and located ca. 60 km apart, formed at similar subaerial elevations in the Late Jurassic–Early Cretaceous and were subsequently vertically offset by thermally induced tilting of the basin margin. Cretaceous sediments were deposited in a single, relatively unconfined basin in water depths of 1–1.5 km. The β profile that best restores palaeobathymetry to match our geological constraints is the same as that derived from summing visible post‐Late Triassic heave on faults plus 25–60% additional extension to account for sub‐seismic deformation. This indicates that, at least in the southern part of the Halten Terrace, the amount of upper‐crustal stretching during the Late Jurassic–Early Cretaceous rift phase is comparable to the total amount of lithospheric stretching, supporting a uniform pure‐shear stretching model.

Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean

Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ∼20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.

The Rwenzori Mountains of western Uganda – Aspects on the evolution of their remarkable morphology within the Albertine Rift

The Rwenzori Mountains form a narrow mountain range within the western branch of the East African Rift System. They rise to heights of more than 5000m a.s.l. and are composed of Precambrian metamorphic rocks. Lacking a volcanic origin the Rwenzoris are regarded as an extreme example of rift flank uplift and a key area to study rift-related uplift processes within an extensional setting.To determine these processes and underlying rift dynamics, the presented study provides constraints on the temporal and spatial exhumation history of the Rwenzori Mts.Recent and former geomorphology is investigated, addressing geomorphologic proxies as well as the cooling history. Associated erosion rates are derived from thermochronological data, allowing constraining relief changes over time.It could be demonstrated, that inherited tectonic structures play a major role in routing erosion processes. The cooling history of the Rwenzori Mts derived from low-temperature thermochronology reaches back to Jurassic times. Final exhumation was recorded for Neogene times, with differentiated erosion and uplift movements during the last 10Ma and a fast final uplift of the Rwenzoris in the near past, where erosion could not compensate for (Bauer et al., 2010a).This paper integrates geomorphologic and thermochronological constraints, in order to provide a basis for understanding the development of the evolution of the Albertine Rift area, with focus placed on the Mesozoic and Cenozoic history of the central Rwenzori Mts.

Piecing together rifts

Earth's crust is formed where tectonic plates rift apart and upwelling magma solidifies. Disparate observations from rifts beneath the oceans and on land provide insights into the dynamics of rifting and opportunities for synthesis.

Crustal accretion at high temperature spreading centres: Rheological control of crustal thickness

New determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as Iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides ( Bjarnason and Schmeling, 2009). To understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass, momentum and energy with melting, melt extraction, and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume. The dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust. Four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone, the accretional heating rate, which can be associated with the thickness of the surface layer in which magmatic emplacement takes place, and the spreading rate. Mode 1: zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone. Mode 2: strongly or moderately crustal thickening away from the axis develops in case of warm (deep reaching) accretion and wide emplacement zones. Mode 3: nearly constant crustal thickness develops in case of warm (deep reaching) accretion but narrow emplacement zones. Dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis. Modes 2 or 3 may be identified with the situation in Iceland. Mode 4: a stagnating central crustal block evolves for cool accretion and narrow emplacement. This mode disappears for increasing spreading rates. No accretional mode with maximum crustal thickness above the plume at the rift axis has been found. The absence of mode 1 accretion (with zero crust at ridge axis) on earth may be an indication that in general crustal accretion is not cold (and shallow). The model is also applied to other hotspot-ridge settings (Azores, Galapagos) and suggests modes 2–3 accretion.

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Spatial and temporal variations in magma-assisted rifting, Taupo Volcanic Zone, New Zealand

Taupo Volcanic Zone (TVZ), New Zealand, is a NNE-trending rifting arc, active for ~ 2 Myr, with a 125-km-long central segment characterized by exceptionally voluminous rhyolite volcanism. The volcanic segmentation reflects along-axis variations in magmatism with implications for the thermal state of the crust and consequent rifting dynamics. Along the zone to the north and south of Central TVZ, the limbs of broad monoclines, disrupted to various degrees by normal faults, dip SE against major NW-facing fault zones. In these northern and southern segments, the loci of magmatism (shown by the position of volcanoes) and rifting (manifested by the distribution of seismicity and modern (< 61 ka) faulting in the Taupo Fault Belt (TFB)) coincide. Mantle-derived magmas are localized within the crust in a plexus of small bodies, dikes and sills, and dike-assisted rifting operates at times (but not always) as shown by the historic record. In contrast, throughout most of Central TVZ the loci of magmatism and tectonism (shown by the distribution of high-temperature geothermal systems and inferred from geophysical models and surface fault studies) are offset laterally and extensional strain appears to be partitioned accordingly. Geological, geophysical and geodetic studies indicate the following magma-assisted mechanisms of extension in Central TVZ: 1) mafic dike intrusion of length scale > 20 km and width > 1 m oriented perpendicular to the extension direction; 2) fault slips of < 2 m on structures along-strike from and coeval with silicic eruptions, some of which were triggered by mafic dike intrusion; 3) rifting episodes associated with regional-scale uplift, multi-fault rupture (slips < 2 m) and transient subsidence, arguably driven by changes in state at shallow depths. Volcanic studies of < 340 ka deposits demonstrate that an additional, but less frequent, mechanism involves temporally higher rates of fault slip with regional-scale collapse of rift basins in association with large-scale silicic eruptions. TVZ rifting mechanisms thus vary in space and time according to magmatic style and result in unpredictable fault behaviour over millennial time scales.