Rift Border Fault Research Articles

Imaging and modelling the subsurface structure of the Rungwe Volcanic Province in SW Tanzania with aeromagnetic data: An improved structural map to support geothermal exploration

The Rungwe Volcanic Province (RVP) in the East African Rift System, SW Tanzania, provides a unique opportunity to investigate geothermal resources in the context of particularly complex continental rifting processes. To support geothermal resource targeting in the RVP, we present a revised neotectonic structural map based on an interpretation of aeromagnetic data constrained by 2D-forward modelling of magnetic anomalies integrated with the distribution of previously reported faults, seismic epicentre locations, 3D magnetotelluric resistivity models and surface geothermal manifestations. Magnetic anomalies in the RVP, including the nationally prominent Mbeya anomaly, are related to the high magnetic susceptibility or remanent magnetism of Precambrian rocks and Cretaceous carbonatite intrusions buried in the rift under a varying thickness of non-magnetic sediments and volcanic rocks. Magnetic lineaments are related to structures controlling the geometry of the Precambrian rocks and concealed dikes and the thickness of the sediments and volcanics. The recent Ngozi and Rungwe trachyte volcanics have relatively low magnetic susceptibility comparable to the low susceptibility of the sediments in the rift basins. The revised neotectonic structural map shows prominent NW, NE and NS-trending magnetic lineaments that correlate with previously reported faults and alignments of seismic epicentres in the study area and with the regional trend of the rift segments. The NE-trending magnetic lineaments are consistent with interpretations of the current stress field in the RVP. The main volcanoes in the RVP, Ngozi, Rungwe and Kiejo (also known as Kyejo and Kieyo), are aligned with the NW-trending linear magnetic feature joining the Lupa and Livingstone rift border faults. This lineament is intersected and frequently displaced by the NE and NS-trending lineaments, suggesting that the NE to NS-striking structures are younger. The Rungwe and Ngozi volcanoes as well as numerous ‘’monogenetic’’ eruption centres and the Mwakaleli caldera, which originated ca. 2 Ma ago (Ebinger et al., 1989) following a large explosive eruption emplacing widespread ignimbrite deposits, are located within a zone of low-intermediate magnitude magnetic features forming a basin-like structure surrounded by magnetic high anomalies of the Precambrian basement structures. We interpret the intersections between the NW-trending intra-rift faults and the NS and NE-trending faults as favourable locations for wells to target high permeability within the geothermal resource conceptual models previously constructed using 3D MT resistivity imaging integrated with supporting geoscientific data. The intersections provide a focus area for follow-up ground mapping of subtle features that may be associated with very recent fault movement.

Coevolution of craton margins and interiors during continental break-up

Many cratonic continental fragments dispersed during the rifting and break-up of Gondwana are bound by steep topographic landforms known as ‘great escarpments’1–4, which rim elevated plateaus in the craton interior5,6. In terms of formation, escarpments and plateaus are traditionally considered distinct owing to their spatial separation, occasionally spanning more than a thousand kilometres. Here we integrate geological observations, statistical analysis, geodynamic simulations and landscape-evolution models to develop a physical model that mechanistically links both phenomena to continental rifting. Escarpments primarily initiate at rift-border faults and slowly retreat at about 1 km Myr−1 through headward erosion. Simultaneously, rifting generates convective instabilities in the mantle7–10 that migrate cratonward at a faster rate of about 15–20 km Myr−1 along the lithospheric root, progressively removing cratonic keels11, driving isostatic uplift of craton interiors and forming a stable, elevated plateau. This process forces a synchronized wave of denudation, documented in thermochronology studies, which persists for tens of millions of years and migrates across the craton at a comparable or slower pace. We interpret the observed sequence of rifting, escarpment formation and exhumation of craton interiors as an evolving record of geodynamic mantle processes tied to continental break-up, upending the prevailing notion of cratons as geologically stable terrains.

Hydrothermal silicification along rift border faults in the Rio do Peixe basin, Brazil

This study investigates hydrothermal silicification along rift border faults in the Cretaceous Rio do Peixe Basin, Brazil, through field geology, petrography, mineralogy, and fluid inclusion microthermometry. We distinguish three main structural-diagenetic domains: (1) a non-silicified fault damage zone, (2) a silicified damage zone composed of deformation bands and centimeter-scale quartz veins in the hanging wall and footwall, respectively, and (3) a silicified fault core with meter-scale wide quartz veins and mosaic breccia textures. We recognize three main events of silicification in the fault zone. The first silicification event occurs after the widespread development of deformation bands in the hanging wall damage zone that may reach 270 m in width. During subsequent deformation events, multiple fracturing episodes were accompanied with precipitation of the second quartz generation in hydraulic mosaic breccias and the third quartz cement in vein stockworks. Shallow burial conditions of faulting (<2 km), combined with fluid inclusion homogenization temperatures of the second quartz generation, indicate that silicification occurred in a hydrothermal system characterized by fluid temperatures in the range of 71–391 °C (mean value of 141 °C). We conclude that hydrothermal silicification was imprinted during the late synrift stage, when the rift border faults were already established. This contribution has implications for understanding how structural processes may control the geometrical, petrographic and petrophysical attributes of hydrothermal products.

Present-day Fault Kinematics and their Reactivation Likelihood within and South of the North Tanzania Divergence (NTD), East African Rift System: Implication for Geo-hazards Assessment

Abstract This study investigates the relationship between the North Tanzania Divergence (NTD), the southern part of the NTD and the rift structures in the eastern part of the East African Rift System (EARS). It also assesses the susceptibility of these structures to reactivation under the current stress field. Fault slip data and focal mechanisms from various sources have been used to determine the minimum horizontal stress axes (Shmin) of different faults, including the Eyasi rift border fault south of the NTD. The analysis reveals that faults trending NW-SE to NNW-SSE have a sinistral sense of movement, while those trending NE-SW generally have a dextral sense of movement. The N60°E trending faults that dip easterly are not optimally oriented with the current stress field and have a low to moderate likelihood of reactivation. This includes the NE-SW trending Eyasi rift border fault (part of the NTD) and other structures oriented N60°E south of the NTD, with a computed slip tendency (Ts) of less than 0.5. Faults trending N-S, NNW-SSE, and NNE-SSW, on the other hand, have a high to the highest likelihood of reactivation (Ts &amp;gt; 0.85). The other faults in the study area, which trend NE-SW or NW-SE, have Ts values expressed as 0.5 ≤ Ts &amp;lt; 0.85. These findings have important implications for earthquake risk assessment in the NTD and south of the NTD, as geological constraints need to be taken into account for better management and mitigation. The results could be applied elsewhere in the EARS provided that they are under E-W extensional stress regime.

The influence of lithospheric rheology and surface processes on the preservation of escarpments at rifted margins

The formation of rift flank topography along rifted margins is related to the flexural uplift of the footwall on the rift border fault during lithospheric stretching. The preservation of rift flank topography in mature margins can be explained as a feedback mechanism between differential denudation of the margin and regional isostatic response of the lithosphere. However, the contribution of the lithospheric rifting on the escarpment amplitude in both young and mature margins is not easily quantified. In the present work, we used a 2D thermo-mechanical numerical model to simulate lithospheric extension to evaluate the sensitivity of escarpment amplitude and its preservation under different rheological conditions over timescales larger than 100 Myr, comparable with the age of mature rifted margins. We found that the evolution of escarpment uplift and its preservation for tens of millions of years are sensitive to the degree of coupling between the crust and mantle and the regional isostatic response of the lithosphere to surface processes. The flexural uplift as a response to lithospheric stretching are limited to the first 100‐150 km from the rift flank. Without the influence of surface processes, the amplitude of the escarpment monotonically decreases through time due to the lateral flow of the lower crust and thermal cooling of the margin. In the scenarios with surface processes, topographic signature inherited from the rift phase in the post-rift escarpment elevation is possible only in margins where the erosive escarpment retreat is smaller than ∼100 km. In scenarios where the escarpment retreat exceeds this distance, the escarpment elevation is mainly preserved by the combination of the inherited topography (existent before the onset of lithospheric rifting) and flexural rebound of the lithosphere due to the erosion along the margin.

The Luangwa Rift Active Fault Database and fault reactivation along the southwestern branch of the East African Rift

Abstract. Seismic hazard assessment in slow straining regions is challenging because earthquake catalogues only record events from approximately the last 100 years, whereas earthquake recurrence times on individual faults can exceed 1000 years. Systematic mapping of active faults allows fault sources to be used within probabilistic seismic hazard assessment, which overcomes the problems of short-term earthquake records. We use Shuttle Radar Topography Mission (SRTM) data to analyse surface deformation in the Luangwa Rift in Zambia and develop the Luangwa Rift Active Fault Database (LRAFD). The LRAFD is an open-source geospatial database containing active fault traces and their attributes and is freely available at https://doi.org/10.5281/zenodo.6513691. We identified 18 faults that display evidence for Quaternary activity, and empirical relationships suggest that these faults could cause earthquakes up to Mw 8.1, which would exceed the magnitude of historically recorded events in southern Africa. On the four most prominent faults, the median height of Quaternary fault scarps varies between 12.9 ± 0.4 and 19.2 ± 0.9 m, which suggests they were formed by multiple earthquakes. Deformation is focused on the edges of the Luangwa Rift: the most prominent Quaternary fault scarps occur along the 207 km long Chipola and 142 km long Molaza faults, which are the rift border faults and the longest faults in the region. We associate the scarp on the Molaza Fault with possible surface ruptures from two 20th century earthquakes. Thus, the LRAFD reveals new insights into active faulting in southern Africa and presents a framework for evaluating future seismic hazard.

Fault control and timing of manganese deposits at Abu Ramad, South Egyptian margin of the Red Sea

ABSTRACT Over thirty manganese deposits have been discovered and mined along the southern Egyptian Red Sea coast (Abu Ramad area). The manganese deposits are hosted in Miocene-Pliocene syn-rift sediments of the downdip portion of the NW-oriented rift-border fault and are in a tectonically complex area. The NW-oriented rift-related extensional fault system is overprinting the old NE to ENE-oriented Onib-Sol Hammad suture. The latter was reactivated as sinistral faults during the Pliocene. The rift-related fault pattern, including the reactivated, inherited Precambrian faults and shear zones, plays a vital role in the distribution and genesis of manganese deposits. In this work, results from field investigation, geological mapping, and boreholes data were integrated to establish a geological model for the fault-controlled manganese deposits east Abu Ramad city. Manganese minerals occur as fault-hosted lenses and vein-like bodies (Type I) and cementing material in the Pliocene manganiferous sandstone (Type II). These two types are genetically related and mapped together at or near the intersection zones between NW-oriented normal faults and ENE-oriented transfer faults. The damage zone of the cross-trend at their intersections with the NW-oriented faults hosted the high-grade manganese deposits (Type 1), while the low-grade type II deposits were formed as the cement of the Pliocene sandstone. Pliocene reactivation of the ore-rich shear zones causes high permeability damage zones that act as a corridor through low-permeable Pliocene shale and brings the manganese-rich solution into localized fault-bounded basins. Additionally, the sinistral offset for the NW-oriented faults enhances the permeability at faults intersection zones, which act as a structural trap of the manganese deposits. This study recognizes multi-reactivated faults and fault intersection zones as crucial targets for manganese mineral exploration along the Red Sea margin.

Strain localization and migration during the pulsed lateral propagation of the Shire Rift Zone, East Africa

We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: Rift-Phase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: Rift-Phase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2 period. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ with a rotation of border faults across the shear zone, and terminates farther northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) favorably-oriented zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the lateral propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes localized later-phase deformation in the form of border fault hard-linking transverse faults that exploited strength contrasts within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited crustal strength anisotropies may serve as both strain-localizing, refracting, and ‘strain barrier’ tectonic structures.

The depositional model of the Lower Cretaceous Nantun Formation and its implications for oil accumulation in the northern Wuerxun Sag of Hailar Basin, NE China

The northern Wuerxun Sag is a lacustrine rift basin which experienced multiple faulting and subsidence during the Lower Cretaceous. The tectonic evolution with uplifted regions as the provenances caused the complex sedimentary fill of the Lower Cretaceous Nantun Formation. In this paper, we use well log, core and 3D seismic data to characterise the sedimentary facies and depositional environment of the Nantun Formation, and consider how this affects the distribution of hydrocarbons within the basin. Four depositional systems are recognised in this study: fan deltas located along the steep slopes; braided deltas along the gentle slopes; sublacustrine fans and lacustrine in the centre of the basin. Fan deltas are mainly composed of coarser sediments with lower sediment maturity than braided deltas, and are mainly located in steep slopes with rift border faults. Sublacustrine fans form lenticular depositional bodies in the basin centre, with higher sediment maturity as a result of reworking. Three sediment-transport pathways were recognised which governed the sediment deposition in this basin, which are (1) the steep rift border fault in the northwest, (2) the NE–SW Surennuoer strike-slip fault zone (SSFZ), and (3) the major faults in the south-eastern gentle slope. Most oil is found in sublacustrine fan sandstones which were transported along the SSFZ mainly from shallower fan deltas, with underlying mudstones (SQN1) as effective source rocks and overlying thick and laterally continuous lacustrine mudstones (SQD1) as seal rocks. The good source–reservoir–seal assemblages enable prolific near-source oil distribution in this area, suggesting the sedimentary controls on oil accumulation, which may guide future oil exploration and production activities in this area.

Climate-driven stress changes and normal fault behavior in the Lake Malawi (Nyasa) Rift, East Africa

Climate-triggered fluctuations of surface masses, including ice and water, can cause transient stress in the Earth's crust, further affecting the slip behavior of faults over different temporal and length scales. In particular, lakes developed within active continental rifts may modulate the stress states and slip rates of rift border faults and intrarift faults. Here, we utilize a numerical model in a case study of the Malawi (Nyasa) Rift to understand the response of faults to mass fluctuations on the Earth's surface. The water level of Lake Malawi rose 600 m over the last 150 kyr, and significantly influenced the stress state of faults in rift valley. We find that such water load fluctuations can exert 4.6 MPa normal stress on fault planes and produce a negative Coulomb stress (down to −2.0 MPa) on fault planes as well as a pronounced reduction of slip (∼2 m) on fault planes within the rift. Moreover, along-strike differences in fault geometries and their position relative to the center of the water column load resulted in variable along-strike stress and slip changes. These results suggest that lakes that develop within continental rifts play an important role in the evolution of extensional faulting. Our case study provides a basis for evaluating the relationship between climate-driven surface mass variations and the subsurface stress state of fault planes and associated seismic potentials.

Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and “simpler” electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF’s maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580–837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

Evolution of a complex early Permian coarse-grained shoreline along a rift basin margin

ABSTRACT Tectonic activity in extensional basins has a profound control on accommodation and sediment supply through the interplay between footwall uplift and hanging-wall subsidence, and thus largely influences the three-dimensional architecture of syn-rift sequences. This is emphasized in areas close to major rift-border faults, where steep coastal reliefs and fluvial gradients produce compound facies zonation and stratigraphic styles with strong lateral variability. The lower Permian High Cliff Sandstone was deposited in an array of shallow marine environments along the margin of the northern Perth Basin during a protracted late Paleozoic rifting episode in Western Australian basins. The formation is composed of fluvio-deltaic and nearshore strata sharply overlying a thick succession of offshore mudstone that was deposited during a phase of tectonic quiescence. This basal contact likely reflects submarine erosion and is, therefore, interpreted as a regressive surface of marine erosion generated in response to forced regression. The facies arrangement consisting of interbedded sandstone, conglomerate, and heterolithic facies chiefly records the evolution of a low- to high-gradient paleoshoreline punctuated by coastal streams, steep sea cliffs, and back-barrier lagoons. Extraformational outsized clasts were probably emplaced by the erosion of exhumed basement and older sedimentary rocks through fluvial incision, wave sapping, or landsliding. The along-strike variability between low- and high-gradient shoreline deposits indicates a dynamic depositional setting with a complex tectonic influence. The basal regressive surface of marine erosion is attributed to footwall uplift during the early reactivation stage of basin-bounding normal faults and, therefore, records the initiation of a new syn-rift phase in the northern Perth Basin.

Quantitative Constraints on Faulting and Fault Slip Rates in the Northern Main Ethiopian Rift

AbstractThe Boset magmatic segment (BMS) of the northern Main Ethiopian Rift (MER) is an ideal natural laboratory to investigate the kinematics, interaction, and rates of activity within a fault network in a magma‐rich rift. In this paper we take advantage of the availability of (1) high‐resolution remote sensing data (LiDAR, ASTER); (2) absolute age chronology on offset reference surfaces; and (3) well‐exposed active normal fault arrays to place new constraints on rift kinematics and strain distribution, and to quantify the architecture and fault slip rates at different temporal scales within a magmatic segment. We found that the rift border faults strike approximately NE, while the younger faults in the rift segments strike NNE. Analyses of geometric rift parameters show that the axial active part of the rift is transtensional with an increase of the shear component northward. The fault displacement analyses and displacement:length ratios increase toward the segment tips, suggesting a significant contribution of fault growth by linkage. In contrast, magmatism is focused on the segment center and localized to a narrow zone. Estimated fault slip rates vary, with rates of up to ~0.37 mm/year in ~0.3 Ma old rift floor deposits, whereas higher rates of up to ~4.4 mm/year are observed for faults cutting through ~6 Ka lavas. The difference in slip rates indicates short‐term variability or a very active recent episode compared to long‐term low average slip rates.

Tectonics and Landscape of the Central African Plateau and their Implications for a Propagating Southwestern Rift in Africa

AbstractThe Central African Plateau (CAP) covers a million square kilometers of African lithosphere absent of recent volcanism and intense seismicity. Treating the CAP erosion surface as a reference frame for measuring continental deformation reveals an active landscape of normal fault systems and crustal flexures. Free‐air gravity anomalies over the CAP reveal both a short‐wavelength (100–200 km) flexural and a longer‐wavelength (&gt;500 km) mantle convective signature. Apatite fission track thermochronometry records the onset of regional cooling of the erosion surface below 60 °C between 38 and 22 Ma. The erosion surface was formed by the Latest Miocene and elevated to its present altitude (1,200 ± 50 m) in the Latest Miocene/Pliocene. High‐resolution Shuttle Radar Topography Mission‐ and LIDAR‐based digital elevation models of the erosion surface show active fault terraces and alluvial fan deformation associated with pre‐existing rift border faults. Flexural modeling of the footwall uplift of the Luangwa Rift border fault yields an effective elastic thickness of the CAP lithosphere of ~35 km. The rifting initiated in the Pliocene with, or soon after, elevation of the CAP. Subsequent Plio‐Pleistocene deformation of the CAP surface controls the Congo and Zambezi drainage systems and wetland locations. The CAP rifts link southwestward through the Zambezi, Kafue and Muchili Rifts to the Pleistocene aged Okavango and Eiseb Rifts of Botswana and Namibia, defining a propagating Southwestern Rift cutting the Nubian Plate. This active rift system developed along relatively thin (~150 km) lithosphere between the Congo and Kalahari cratons within crust inherited from Neoproterozoic collisional tectonics.

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

AbstractThe distribution of deformation during the early stages of continental rifting is an important constraint on our understanding of continental breakup. Incipient rifting in East Africa has been considered to be dominated by slip along rift border faults, with a subsequent transition to focused extension on axial segments in thinned crust and/or with active magmatism. Here, we study high‐resolution satellite data of the Zomba Graben in southern Malawi, an amagmatic rift whose topography is dominated by the west‐dipping Zomba fault. We document evidence for five subparallel fault scarps between 13 and 51 km long spaced ~10–15 km apart. The scarps consist of up to five segments between 4 and 18 km long, separated by minima in scarp height and river knickpoints. The maximum height of each fault scarp ranges from 9.5 ± 4.2 m to 35.3 ± 14.6 m, with the highest scarp measured on the intrabasin Chingale Step fault. We estimate that the scarps were formed by multiple earthquakes of up to Mw7.1 and represent a previously unrecognized seismic hazard. Our calculations show that 55 ± 24% of extensional strain is accommodated across intrabasin faults within the ~50 km wide rift. This demonstrates that a significant proportion of displacement can occur on intrabasin faults during early‐stage rifting, even in thick continental lithosphere with no evidence for magmatic fluids.

The role of pre-existing Precambrian structures in the development of Rukwa Rift Basin, southwest Tanzania

In this study, Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and aeromagnetic data are used to analyse the trends of pre-existing basement structures within the Rukwa Rift Basin. The NW-trending Rukwa Rift Basin on the western branch of the East African Rift System, southwest Tanzania, is developing on the NW-trending Paleoproterozoic Ubendian orogenic belt, a belt that experienced multiple orogenic collisions associated with subduction in Proterozoic time and comprises several distinct terranes bounded by faults or shear zones. The results obtained using magnetic edge enhancement (derivatives) methods highlight major magnetic domains identified based on their distinctive magnetic patterns in relation to geology and tectonic setting of the studied area. The results also highlight the Precambrian Chisi shear zone which trends in a NW-SE direction in the subsurface, below the Lake Beds Formation sedimentary succession of the Neogene Rukwa Rift Basin. The orientation of rift border faults and other major faults and their relationship with basement fabrics inferred from SRTM DEM and magnetic data trend mainly NW-SE, which is consistent with those of the NW-trending Paleoproterozoic Ubendian orogenic belt. Thus, Paleoproterozoic Ubendian orogenic belt structures have played a significant role on geometry and orientation of the Rukwa Rift Basin. These structures facilitated strain localisation within the border faults of the rift basin by exploiting the existence of inherited lithospheric heterogeneity. It can be concluded that the pre-existing faults and shear zones define a mechanical anisotropy in the basement of the Rukwa Rift Basin and facilitated the strain localisation within the border faults of the rift.

Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion: The Main Ethiopian Rift, East Africa

The Cenozoic-Quaternary Main Ethiopian Rift (MER) is characterized by extended magmatic activity. Although magmatism has been recognized as a key element in the process of continental breakup, the interaction between melts and intruded lithosphere is still poorly understood. We have performed a 2D thermo-rheological modeling study of continental crust incorporating rheological variations due to melt intrusion-related thermal perturbation. The model is calibrated based on the characteristics of lithologies occurring in the MER and its extensional history, and includes the effect of metamorphism and anatexis on crustal strength and rheological features. During Miocene early rift phases strain in the MER was mainly accommodated through rift border faults, whereas Pliocene-to-recent extension history is characterized by magma assisted rifting with most strain accommodated across magmatic segments in the rift axis. Consequently, very little strain is distributed in the old Pan-African to Paleogene crust during Pliocene to Holocene times. The magmatic activity along the rift axis created ≈20km thick magmatic segments, with growth rate estimated to range from ≈3.5mmyr−1 to ≈6mmyr−1. Our model suggests that the strain transfer from Miocene rift border faults to magmatic segments was favored by a moderate increase in crustal strength, due to prograde metamorphism subsequent to the melt-induced thermal perturbation. Under such conditions, crustal stretching may not constitute an effective extension mechanism, thus strain may be preferentially accommodated by melt injection along hot, partially molten magmatic segments. Anatexis has been detected in our simulations, with melt fractions sufficient to break-up the crust solid framework and migrate. This determines local variations of rheological behavior and may induce seismicity. However, resulting melt percentages are not sufficient to induce widespread, crust-derived volcanic activity. Subsequently, volcanism in the MER is mainly due to mantle-derived melts, subjected to a various degree of fractionation, with minimal contribution from crust-derived magmas.

Rapid spatiotemporal variations in rift structure during development of the Corinth Rift, central Greece

AbstractThe Corinth Rift, central Greece, enables analysis of early rift development as it is young (&lt;5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high‐resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620–340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at ~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at ~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

Subsurface structure and stratigraphy of the northwest end of the Turkana Basin, Northern Kenya Rift, as revealed by magnetotellurics and gravity joint inversion

In order to understand the subsurface stratigraphy and structure of the northwest end of the Turkana Basin, Northern Kenya Rift, we used 2-D joint inversion of magnetotelluric (MT) and gravity data acquired along 3 profiles perpendicular to the main Murua Rith-Lapur Rift Border Fault. The regional geology is characterized by a basement of Precambrian age overlain by a ≤500-m thick sandstone formation named the Lapur Sandstone of upper Cretaceous-lower Eocene in age, covered by thick rhyolitic and basaltic lavas of late Eocene-middle Miocene age, known as the “Turkana Volcanics”. Final interpretation of the resistivity and density models, until 5 km depth, obtained by the joint inversion approach confirms the previous general knowledge about the half-graben geometry of the northern part of the Turkana Basin. The main Murua Rith-Lapur Rift Border Fault is well identified by both gravity and MT. At least, two other important secondary faults without surface expression are also identified. A new small half-graben basin, named the Kachoda Basin, parallel to the main Turkana Basin and filled by 1.5 km of sediments, has been also characterized. This study also highlights strong thickness variations of the three main geological units that could be expected in the subsurface of the Turkana Basin. For example, the sedimentary Nachukui and Kibish Formations reach up to >3 km in thickness at the eastern end of the north and central profiles. Lateral variations of the topography of the Precambrian basement are also evidenced. Conceptual geological models, which result from the combination of the obtained density and resistivity models as well as from geological and reflection seismic data, are proposed. In such an area of intensive and promising oil exploration, these models are essential in terms of identification of reservoirs, source rocks and trapping mechanisms.

Diking‐induced moderate‐magnitude earthquakes on a youthful rift border fault: The 2002 Nyiragongo‐Kalehe sequence, D.R. Congo

AbstractOn 24 October 2002, Mw 6.2 earthquake occurred in the central part of the Lake Kivu basin, Western Branch of the East African Rift. This is the largest event recorded in the Lake Kivu area since 1900. An integrated analysis of radar interferometry (InSAR), seismic and geological data, demonstrates that the earthquake occurred due to normal‐slip motion on a major preexisting east‐dipping rift border fault. A Coulomb stress analysis suggests that diking events, such as the January 2002 dike intrusion, could promote faulting on the western border faults of the rift in the central part of Lake Kivu. We thus interpret that dike‐induced stress changes can cause moderate to large‐magnitude earthquakes on major border faults during continental rifting. Continental extension processes appear complex in the Lake Kivu basin, requiring the use of a hybrid model of strain accommodation and partitioning in the East African Rift.