TS5.3

Fascinating feedback relationships between surface processes and tectonic deformation have long been highlighted for convergent settings. Mountains influence local climate, with precipitation increasing with mountain height and focusing at windward-facing slopes. The resulting erosion reduces the elevation and width of mountain belts, in turn leading to a focussing of tectonic deformation and exhumation at eroding regions. Thus, in convergent settings, erosion and tectonic deformation show positive feedback by enhancing each other. In comparison, the role of surface processes in extensional settings has received less attention, which does not mean that erosion or sedimentation might not equally affect tectonics deformation during extension. In this presentation, i will review theoretical expectations, discuss numerical experiments, and pose questions on how, when, and where surface processes interplay with tectonic deformation during extension.

How: The removal of material by erosion is expected to decrease vertical crustal stress and reduce brittle strength (which is the main process leading to focussing of deformation in shortening). Sedimentation conversely increases brittle strength. However, sediments of low thermal conductivity in extensional basins can trap heat, increasing crustal temperatures, and reducing viscous crustal strength. Will brittle strengthening or viscous weakening dominate during sedimentation? And during rifting, is erosion the controlling surface process, or sedimentation, or both?

When: Usually, subsidence needs to create accommodation space before sedimentation occurs and rocks should uplift before they can be eroded. This would imply that surface processes need time to start up and cannot play a decisive role in initial stages of deformation. This then begs the question: once an extensional system starts to deform in a certain style, can surface processes still change the style? For rift basins, we find from numerical experiments that sedimentation favours symmetric basins over asymmetric half-graben and single basins over distributed deformation. For rifted margins, i have found that sedimentation promotes hyperextension by forming wide areas of thinned continental crust, thus supressing early break-up. These experiments point out that surface processes seem to be able to exert a control on the style of rifting. But at which stage in rift evolution do surface processes start to play a role? And is there a crucial timing, after which erosion and sedimentation no longer influence the extensional style?

Where: Analogous to convergent tectonic settings, erosion of rift footwalls can enhance tectonic deformation and, on a large-scale, turn a ‘passive’ margin ‘active’ in a tectonic sense. Footwall uplift provides a sediment source region, linking erosion to offshore sedimentation. For rifted margins, where does deposition of sediments (whether they are brittle strengthening or viscous weakening) play the most influential role in the rifting process? Can strong near-footwall sedimentation suppress footwall uplift, thus providing a negative feedback in the system?

How to cite: Buiter, S.: A discussion on how, when and where surface processes interplay with extensional tectonic deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8665, https://doi.org/10.5194/egusphere-egu21-8665, 2021.

Normal faulting acts as a forcing on the morphodynamics of alluvial rivers by changing the topographic gradient of the river valley and channel around the fault zone. Normal faulting affects river morphodynamics either instantaneously by surface rupturing earthquakes, or gradually by continuous vertical displacement. The morphodynamic responses to normal faulting range from longitudinal bed profile adjustments to channel pattern changes. However, the effect of faulting on river morphodynamics and morphology is complex, as they also depend on numerous local, non-tectonic characteristics of flow, river bed/bank composition and vegetation cover. Moreover, river response to faulting is often transient. Such time-dependent river response is important to consider when deriving relationships between faulting and river dynamics from a morphological and sedimentological record. To enhance our understanding of river response to tectonic faulting, we used the physics-based, two-dimensional morphodynamic model Nays2D to simulate the responses of a laboratory-scale alluvial river to various faulting and offset scenarios. Our model focusses on the morphodynamic responses at the scale of multiple meander bends around a normal fault zone. Channel sinuosity increases as the downstream meander bend expands as a result of the faulting-enhanced valley gradient, after which a chute cutoff reduces channel sinuosity to a new dynamic equilibrium that is generally higher than the pre-faulting sinuosity. Relative uplift of the downstream part of the river due to a fault leads to reduced fluvial activity upstream, caused by a backwater effect. The position along a meander bend at which faulting occurs has a profound influence on channel sinuosity; fault locations that enhance flow velocities over the point bar result in a faster sinuosity increase and subsequent chute cutoff than locations that cause increased flow velocity directed towards the outer floodplain. Our study shows that inclusion of process-based reasoning in the interpretation of geomorphological and sedimentological observations of fluvial response to faulting improves our understanding of the natural processes involved and, therefore, contributes to better prediction of faulting effects on river morphodynamics.

How to cite: Woolderink, H., Weisscher, S., Kleinhans, M., Kasse, C., and Van Balen, R.: Modelling the effects of normal faulting on alluvial river morphodynamics., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1108, https://doi.org/10.5194/egusphere-egu21-1108, 2021.

Dilatant fissures are a common feature at the Earth’s surface in active rift systems where faults cut mechanically-strong rocks, such as igneous rocks, metamorphic basement or carbonates. Much attention has focused on modern examples of large-aperture fissures in basaltic rocks, where in most cases, only the near-surface-expression is accessible to depths of ~100 m. Numerous mechanisms have been proposed for the formation of such dilatant fractures, including near-surface tensile fracturing along active normal faults at depth, geometric mismatch along faults, and fault-block rotation. However, fissure system architecture and connectivity in the subsurface, and the depth to which dilatant sections can grow are less well understood, as are the ways in which such structures may interact with surface processes.

In this presentation, we focus on dilatant faults and fractures from the ancient rock record, including examples hosted in rocks below regional erosional unconformities, commonly on the upfaulted flanks of nearby sedimentary basins. Such fissures are typically sub-vertical Mode I fractures that can be kilometres long, tens of metres wide and can extend to depths of 1 km or more below the palaeosurface. They are filled with a remarkably diverse range of high porosity, high permeability fills which act as natural proppants holding fractures open for tens to hundreds of million years. Fills include: wall rock collapse breccias; clastic or carbonate sediment; fossiliferous materials, and a variety of epithermal mineral deposits with characteristically vuggy forms and cockade-like textures. Alteration related to weathering and/or near-surface epithermal mineralization may extend down fissure systems to depths of many hundreds of metres. The subterranean clastic fills are commonly water-lain and preserve a unique record of the stratigraphic or fossil record that may be missing due to erosion at the overlying unconformity. Fissures can form along active normal faults at depth, as later-stage reactivations of pre-existing exhumed fault zones and along regional joint sets associated with folds. Some fissures form along the margins or interior of pre-existing mafic dykes or may act as sites of subsequent dyke emplacement – or both. Sub-unconformity fissure systems and their associated fills are likely to be a major influence on both the fluid storage capacity and flow behaviour in subsurface reservoirs including those hosting hydrocarbons, geothermal resources, and in aquifers worldwide.

How to cite: Holdsworth, B., Hardman, K., Walker, R., Bubeck, A., Greenfield, C., Lee, J., McCaffrey, K., and Dempsey, E.: Geological fissures: linking sub-surface structures to surface processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-96, https://doi.org/10.5194/egusphere-egu21-96, 2021.

The Danakil depression in the northern part of the Afar is the only modern example of a rift undergoing the active transition from continental to marine settings, a crucial stage in rift and passive margin development. Thick evaporite deposits in its central part, and fringing Pleistocene coralgal reef terraces along its margins evidence at least four Red Sea incursions in to the basin and subsequent desiccation. The two youngest coralgal reef terraces were dated as respectively MIS 5e and MIS 7. Recent field expeditions measuring the paleo-shorelines' elevation provide a precious record of neotectonic activity in the basin. The margins show varied uplift while outcrops situated closer to the rift axis subsided below sea level. MIS 7 sediments at the northern, western margin, were uplifted up to 170 masl. Neotectonic movements are smaller on the eastern margin of the Danakil depression but moderate uplift was sufficient to avoid flooding of the depression during the Holocene. Syn-rift sedimentary patterns in the Danakil basin illustrate that the transition from continental to marine conditions is not gradual but marked by alternating marine and continental episodes. This alternation is controlled by the interaction between eustatic, tectonic and volcanic processes. Significant increase in accommodation space and sediment deposition can happen at very short time intervals.

How to cite: Rime, V., Foubert, A., Perrochet, L., Jaramillo-Vogel, D., Negga, H., Atnafu, B., and Kidane, T.: Tectonic controls on the sedimentation patterns in the Danakil Depression, Afar, Ethiopia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6135, https://doi.org/10.5194/egusphere-egu21-6135, 2021.

The East African Rift Systems (EARS) is a modern example of a divergent plate boundary at early stages of development. In Tanzania, the rift has evolved in two branches since the Early Miocene. In addition, recent studies have proposed the existence of a marine branch of the rift in the western Indian Ocean, corresponding to the Kerimbas Graben – Davie Ridge (DR) system offshore northern Mozambique and southern Tanzania. North of this region, putative passive margin structures are present: the islands of Zanzibar and Pemba, and the troughs that separate them from the mainland. Although different theories for their formation have been proposed, a clear understanding of how the islands relate to the regional tectonic regime and the effect on the deep-water sediment routing system is lacking. 

In this study, we use 2D seismic reflection profiles and exploration wells to investigate the Oligocene to recent stratigraphy offshore northern Tanzania to examine the following two questions: When did the Pemba and Zanzibar islands form? And how does the evolution of deep-water depositional systems record rift tectonics? Regional correlation of dated seismic horizons, integrated with 3D reconstruction of canyons/channels network through time, allow understanding of the main depositional events and their timing. A net decrease in the number of slope channels is visible offshore Pemba during the middle-late Miocene, which we interpreted to mark the onset of the uplift of the island. At the same time, deep-water channels were still aggrading offshore Zanzibar, indicating that the uplift of this island occurred later, likely during the late Miocene to early Pliocene. The uplift of the islands promoted the formation of a newly discovered giant canyon, characterized by a modern width of > 30 km and depth of > 485 m at > 2,200 m water depth.

The timing of the islands’ uplift indicates a potential relation with the EARS tectonics. While the structures which form the anticlines of Pemba and Zanzibar Islands may be related to Tertiary (EARS) inversion of Mesozoic-aged rift faults,  numerous high-angle normal faults, both antithetic and synthetic, dissect the post-Oligocene stratigraphy. These create horsts and grabens on a variety of scales, some of which (e.g. Kerimbas Graben and Zanzibar/Pemba trough) show comparative shape and size respect to onshore rift basins. The stratigraphic evolution of deep-water channel systems provides a tape-recorder with which to determine the modification of EARS’ tectonics on sedimentation of the older Tanzania margin.

Supported by these new results, we propose a new alternative conceptual model for the evolution of the central East African margin during the Neogene and Quaternary, highlighting the main tectonic structures and their timing of formation.

How to cite: Dottore Stagna, M., Maselli, V., Grujic, D., Reynolds, P., Reynolds, D., Iacopini, D., Richards, B., Underhill, J., and Kroon, D.: Effects of the tectonics of the East African Rift System on the evolution of the Tanzania margin offshore Zanzibar and Pemba Islands., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2456, https://doi.org/10.5194/egusphere-egu21-2456, 2021.

Little is still known about the structural fabric of a potential continuation of the East African Rift System (EARS) offshore Tanzania in the West Somali Basin. This continuation has been established mostly through sparse GPS measurements, earthquake slip vector data, spatial distribution of teleseismically detected earthquake focal mechanisms, and some recent seismic reflection data. West of the Davie Ridge (which part of a larger structure named the Davie Fracture Zone) and across its northern extension, regional seismic reflection profiles indicate the occurrence of continental - oceanic crust transition, which is characterized by early Cretaceous reverse faulting localized along deformation corridors. After the Aptian, the seafloor spreading ceased and the Tanzania margin evolved into a passive margin dominated by clastic deep-water deposition. In this contribution, we describe some results obtained from structural mapping of a 3D seismic dataset, calibrated by few explorations well, covering an area located between the Davie Ridge and the continent, south of Mafia Island.  The seismic data maps suggest a major structural style change across the Neogene that is still active today. The recent structures are represented by two main interacting fault trends: some NS boundary faults corridors and a NW-SE internal arcuate segmented fault, both depicting a widely and diffused distribution of normal fault (with an overall cumulative amount of horizontal brittle extension ranging between 5 to 10 km). Some of the largest faults appear to reactivate older extensional structures but the general absence of growth faults cutting across the Paleo-Neogene depositional units suggest very recent rift re-activation. The recent rift system appears to show a component of obliquity with respect to the orientation of the Davie Ridge, and to the onshore structure related to the EARS tectonics.

How to cite: Ruocco, S., Iacopini, D., Tavani, S., Ebinger, C., Dottore Stagna, M., Reynolds, D., and Maselli, V.: Neogene rift tectonic activity in the West Somali Basin, offshore Tanzania: example of a segmented oblique rift structure., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9066, https://doi.org/10.5194/egusphere-egu21-9066, 2021.

The Congo basin (CB), considered as a typical intracratonic basin, due its slow and long-lived subsidence history and the largely unknown formation mechanisms, occupies a large part of the Congo craton, derived from the amalgamation of different cratonic pieces. It recorded the history of deposition of up to one billion years of sediments, one of the longest geological records on Earth above a metamorphic basement. The CB initiated very probably as a failed rift in late Mesoproterozoic and evolved during the Neoproterozoic and Phanerozoic under the influence of far-field compressional tectonic events, global climate fluctuation between icehouse and greenhouse conditions and drifting of Central Africa through the South Pole then towards its present-day equatorial position. Since Cretaceous, the CB has been subjected to an intraplate compressional setting due to ridge-push forces related to the spreading of the South Atlantic Ocean, where most of sediments are being eroded and accumulated only in the center of the basin.

In this study, we first reconstructed the stratigraphy, the depths of the main seismic horizons, and the tectonic history of the CB, using geological and exploration geophysical data. In particular, we interpreted about 2600 km of seismic reflection profiles and well log data located inside the central area of the CB (Cuvette Centrale). We used the obtained results to constrain the gravity field data that we analyzed, in order to reconstruct the depth of the basement and investigate the shallow crustal structure of the basin. To this purpose, we used a gravity inversion method with two different density contrasts between the surface sediments and crystalline rocks.

The results evidence NW-SE trending structures, also revealed by magnetic and seismic data, corresponding to the alternation of highs and sediments filled topographic depressions, related to rift structures, characterizing the first stage of evolution of the CB. They also show a general good consistency between the seismic and gravity basement along the seismic profiles and evidence the presence of possible high-density bodies in the shallow to deep crust. The identified structures are prevalently the product of an extensional tectonics, which likely acted in more than one direction.

Therefore, we performed 3D numerical simulations to test the hypothesis of the formation of the CB as multi-extensional rift in a cratonic area, using the thermomechanical I3ELVIS code, based on a combination of a finite difference method applied on a uniformly spaced Eulerian staggered grid with the marker-in-cell technique. To this purpose, the numerical tests have been conducted considering a sub-circular weak zone in the central part of the cratonic lithosphere and applying a velocity of 2.5 cm/yr in two orthogonal directions (N-S and E-W). We repeated these numerical tests by increasing the size of the weak zone and varying its lithospheric thickness. The results show the formation of a circular basin in the central part of the cratonic lithosphere, characterized by a series of highs and depressions, consistent with those obtained from geophysical/geological reconstructions.

How to cite: Maddaloni, F., Delvaux, D., Tesauro, M., Gerya, T., and Braitenberg, C.: The Congo basin: an example of failed rift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5950, https://doi.org/10.5194/egusphere-egu21-5950, 2021.

Before Break-Up, the opening of the South China Sea Passive Margin (SCS) was characterized by a wide rift mode during Cenozoic rifting. Such wide extensional margin (>600 km wide) is controlled by a set of hyper-extended sub-basins separated by basement highs.

These basins infill recorded a polyphased extensional deformation hence resulting in complex 3D sedimentary evolution. Based on a recent industrial 3D seismic reflection survey along the Sabah area (southern margin of the SCS), this contribution aims to investigate the detailed 3D geometries of extensional structures as well as their control on the overlying successive sedimentary sequences and relation to crustal deformation.

We mapped and analyzed several crustal-scale rolling hinge structures controlled by a series of low-angle normal faults. Deeper crustal levels are likely exhumed along the core of these rolling hinge structures, separated by extensional allochthones blocs of upper continental crust. Our structural analysis enables us to identify three main extensional phases corresponding to distinct sedimentary packages: (1) a synrift sequence 1 controlled by small offset normal faults formed during incipient rifting; (2) an intermediate synrift sequence 2 recording the development of extensional detachment faults. (3) a thick syn-rift sequence 3 recording a continuation of extension along the detachment faults resulting in the dismembering of the syn-
rift sequence 2. Intra-basement seismic reflectors dipping towards the north-west are observed, onto which extensional structures often seem to root. Some of these reflectors are interpreted as interleaved thrust sheets from a dismantled accretionary wedge of the former Mesozoic active margin (Yenshanian magmatic Arc).

Our results provide new key observations on the 3D mechanisms of detachment faulting and its control on sedimentary evolution as well as coeval crustal deformation. 3D approach throw some light on the detailed geometries of a metamorphic core-complex in relation with crustal boudinage, shear zones and lower/middle crust exhumation below the syn- rift sediments. These geometries can be compared to those described in the Basin and Range province or the Aegean Sea. Consequently, our results have implications for our understanding of rift and breakup mechanisms of marginal basins as a whole.

How to cite: Legeay, E., Mohn, G., Ringenbach, J.-C., and Vetel, W.: 3D structure of detachment faulting and related tectono-sedimentary processes in the SE South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14568, https://doi.org/10.5194/egusphere-egu21-14568, 2021.