Stress Patterns Across South Africa: Something Amiss?
M. A. G. AndreoliZvi Ben‐AvrahamDamien DelvauxRaymond DurrheimÅke FagerengOliver HeidbachNico Van der MerweKerstin SaalmannI. SaudersMichael HodgeA. LogueH. MalephaneJoseph Muaka
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To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuclear, mining (including hydrocarbon extraction) and civil engineering activities in South Africa, the authors are working to improve the data coverage concerning the present day stress field. In principle, this implies constraining the principal compressive stresses (σ1>σ2>σ3) or at least the maximum horizontal compressive stress (σH) because knowledge of these parameters may determine the reactivation potential of known faults, or the behaviour of large excavations and wells. By contrast, much of the subcontinent is under-represented in the World Stress Map database. For this reason we have taken a number of steps, firstly by installing a compact Trillium seismic sensor at Stofkloof (Namaqualand; adjacent to the Vaalputs low and intermediate level radioactive waste disposal facility) and 1-sec sensors at Aggeneys and Koffiemeul (Bushmanland). All stations are equipped with Reftek data loggers and powered by solar panels. The data from these stations will be integrated with data from the national network to obtain focal mechanism solutions for seismic events in the Northern Cape southernmost Namibia region (also known as the Grootvloer cluster). These neotectonic stress tensors are then combined with σH parameters obtained from calliper logs of off-shore wells and from the geometry of joints, faults and sheared fractures in palaeosols (Bushmanland), soils and calcrete (NW Free State) and aeolianites (southern Cape). We also include underground rock engineering phenomenological observations and measurements, and data in the public domain. Our data consistently indicate a NNW-SSE oriented σH (Wegener Stress Anomaly or WSA) that prevails across most of central, southern and western South Africa, Namibia up to the Ruacana hydroelectric power plant at the Angola border. However, in the Congo basin, a few earthquake focal mechanisms suggest rotation of the regional σH to an E-W direction. Geological units affected by the WSA include the Cretaceous oceanic lithosphere (Walvis Ridge), the southern Angola-Kasai craton, the offshore Outeniqua and Orange Basins, the Cape Fold Belt, the Namaqualand metamorphic complex, and the Archaean Craton up to the Witwatersrand basin and the Witbank coal field. In contrast, σH azimuths in the NE-SW quadrants seem prevalent in E Mpumalanga, N Natal, and northern Limpopo. Whereas the origin of these latter stress azimuths are probably linked to the propagation of the E African Rift System, the strike-slip to transpressional character of much of the WSA remains unexplained. Similarly puzzling are a 3-fold increase in seismic events (proxy for strain rate) over the past 20 years in the Grootvloer cluster, and evidence that the WSA is the last of at least 7 successive tectonic regimes to leave their brittle imprints along the SE Atlantic seaboard since the break-up of W Gondwana.Keywords:
Stress field
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Lineament
Stress field
Neotectonics
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East African Rift
Rift zone
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Pull apart basin
Trough (economics)
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The South China Sea has yet to receive a DSDP investigation; consequently, numerous untested models have been proposed for its post-Cretaceous evolution. From a compilation of regional oceanographic heat-flow measurements with offshore and onshore bore-hole temperatures, we thermally model and constrain possible interpretations of its tectonic evolutionary path. The heat-flow data, together with magnetic profiles, depth to basement determinations, and regional sediment isopachs, characterize two principal subbasin extensional elements--one trending east-west (northern area between Hainan and Luzon) and the other trending northeast-southwest (central area between Palawan and Vietnam). The thermal models of simple lithosphere cooling suggest the central area began spreading 55-58 m.y.B.P. and the northern region 34-35 m.y.B.P. These dates of incipient extension correspond to two principal unconformities in Paleocene and Oligocene strata of both offshore China and Palawan, and together they indicate regional uplift of the South China Sea owing to thermal expansion prior to the spreading events. The thermal models also suggest that in the northern region, spreading ceased approximately 19 m.y.B.P., which compares favorably with published magnetic estimates of 17.7 m.y.B.P. A late Cenozoic heating event is evidenced by a thermal anomaly in the southern portion of the central region (southern Vietnam margin) that may be related to incipient spreading along a zone of crustal weakness inherited from the Jurassic-Cretaceous Sunda-Tethys suture. Overall, these data tend to support the hypothesis of spreading occurring first in the central region and then in the northern region. From the geophysical data and observations of Cretaceous ophiolites cropping out to the south in Sabah and Brunei, we kinematically border our model to the south and propose the Palawan Ulugan fault to be a right-lateral suture between continental and intermediate crust. Structurally, within limits of the data presently available, our model further predicts the southern China shelf to have experienced two principal episodes of extension with a net result of younger (<34 m.y.B.P.) east-west trending graben normal faults superimposed upon older (<55 m.y.B.P.) northeast-striking pregraben normal faults. In terms of thermal maturity, geochemical kinetic modeling of Late Cretaceous source rocks suggests depths to the oil ceiling to range from 1.3 to 1.8 km in the northern region and rom 0.98 to 1.6 km in the central region. Similarly, depths to the oil floor are estimated to range from 2.4 to 3.4 km and from 1.8 to 3.2 km in the two respective regions. End_of_Article - Last_Page 303------------
Hydrocarbon exploration
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The earthquake activity of Norway and nearby offshore areas is low to intermediate, with few events above magnitude 5. Recent significant improvements in instrumental coverage in parallel with a better utilization of older (including historical) data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the Viking Graben, while from the northern North Sea to Svalbard the earthquakes in a broad sense follow the continental margin. Fifty‐one focal mechanism solutions from these areas, about half of them new, reveal stress directions that clearly indicate a connection to the plate tectonic “ridge push” force, at least for the areas at a minimum distance from the continental margin. Along the margin, stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects. A dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation. On a regional level, the seismicity in these areas correlate quite well with geologic features such as grabens, fault zones, fault complexes, fracture zones, and the margin itself, indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field. In the northern North Sea, however, an area with quite anomalous stress orientations, with strike‐slip faulting, is found in a region transitional between normal and reverse faulting. Most of the earthquake foci are confined to the presumably brittle parts of the crust, but many events are also located quite close to, and on both sides of, the Moho discontinuity.
Continental Margin
Stress field
Seismotectonics
Passive margin
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The Natal Valley is a ~1000 km long, elongated depression in the seafloor, located within the south-west Indian Ocean, between the coastline of south-east Africa and the offshore aseismic Mozambique Ridge. Knowledge of its present-day crustal structure is important to reconstruct the Gondwana supercontinent break-up history and the concurrent opening of the Indian Ocean, which started during the Jurassic, some ~180 – 150 million years ago. Major geological events relating to this supercontinent break-up including continental rifting, magmatism, the emplacement of extensive oceanic flood basalt as submarine plateaus, vertical processes associated with tectonic uplift, flexural subsidence of the margin and relative global sea-level change are recorded within the crust underlying the south-west Indian Ocean. Although these events have been well modelled, fundamental questions remain regarding the nature and tectonic structure of the crust. Today, the Natal Valley region represents a frontier for geophysical exploration. Early attempts at imaging the crust beneath the seafloor within the south-west Indian Ocean made use of now primitive seismic reflection data acquisition and processing techniques available to them at the time (1960’s - 1970’s). Considering the advancements made in marine seismic reflection imaging, these vintage datasets lacked the horizontal and vertical resolution necessary to most accurately account for and describe features of the upper crust underlying the northern Natal Valley. This study is based on the acquisition, processing and preliminary interpretation of a newly acquired, regional seismic reflection profile, Profile Mz5-007, acquired across the northernmost region of the Natal Valley during the PAMELA-Moz5 cruise in 2016. The methods of marine seismic reflection data acquisition, processing and interpretation are described. The seismic reflection data presented in this dissertation provides a regional-scaled appraisal of the upper crust underlying the northern Natal Valley for the first time. The imagery allows a comprehensive description of crustal structures and features from the very northern Natal Valley, close to the Mozambique coastline, southwards into the southern Natal Valley offshore of Durban. The findings presented here support the notion that the crust underlying the northern Natal Valley experienced roughly N-S orientated trans-tensional stresses in relation to the breakup of West Gondwana. At least two major phases of volcanism are imaged within the seismic stratigraphy. The lower-most event underlies the Limpopo Fan in the northernmost regions of the Natal Valley at a depth of 2 seconds TWT and is interpreted as a seismic expression of the upper surface of the Lower – Middle Jurassic Karoo LIP. The progressive disappearance of this horizon towards the south coincides with the emergence of a second, overlying, as yet unreported volcanic horizon, which extends southwards throughout the Natal Valley. On the basis of its regional distribution, spatial relationship with the…
Seafloor Spreading
Supercontinent
Continental Margin
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A consolidated knowledge of the formation and dispersal of the former supercontinents
reveals important evidence for the earth’s climate and biosphere in the past and
contribute to the prediction of their future evolution. Nowadays, a main objective is
the investigation of the initial break-up of the continental assembly of Gondwana that
serves as a constraint for its subsequent dispersal and the evolution of all oceans and
seas in the southern hemisphere. Evidence of the early rifting stages are expected at
the margins of Southeast Africa and East Antarctica, whereas the latter one is difficult
to access, due to its remote position and ice coverage. To understand the driving forces
and the chronology of the break-up and of the massive volcanism additional detailed
knowledge of the crustal setting along the margins of Southeast Africa is required.
Therefor, a new geophysical dataset was acquired with the RV Sonne in the northern
Mozambique Basin at the beginning of year 2014. This comprises a deep seismic
sounding profile across a so-far unknown structural high, the Beira High. Additional
gravity and magnetic data were systematically recorded across the entire northern
Mozambique Basin. Based on velocity, amplitude, density and magnetic modelling, a
geological model of the continental margin of Central Mozambique was prepared. A
new compilation of all available magnetic data in the Mozambique Basin reveals information
about the age of the sea floor, which serves as constraint for the reconstruction
of the initial Gondwana break-up.
The study depicts a continental origin of up to 23 km thick and partly highly intruded
crust at Beira High. In the adjacent coastal areas of the south-western part
of Central Mozambique, 7 km thin crust is observed, which is covered by more than
11 km thick sediments and implies the continuation of the continent-ocean transition
towards onshore Mozambique. This is in clear contrast to the narrow transition observed
in the north-eastern part of the margin and reveals a clear asymmetric crustal
setting, as supposed for the conjugate margin in the Riiser-Larsen Sea in Antarctica
and consequently suggests a complex break-up scenario.
The presence of a pronounced high-velocity lower crustal body is interpreted as magmatic
material, which underplates the crust and extends about 200 km from the Central
Mozambican margin towards the Mozambique Basin and testifies for the massive
volcanism during the break-up. The distribution of further volcanics along the entire
margin clearly depicts the continuation of the north-eastern branch of the Karoo
large igneous province and are mainly emplaced between 177-157 Ma. The magmatism
in Southeast Africa seemed to be continuous throughout the initial break-up, which
points to the presence of either a mantle plume or a thermal anomaly as source of
the giant magmatism. An additional late stage of rift-volcanism mainly affected the
margin of Dronning Maud Land and causes the difference in the magnetic signature of
the conjugate margins.
The tracing of continuous fractures throughout the Africa-Antarctica Corridor leads
to the reconstruction of a tight Gondwana fit prior rifting, which reveals several geological
links between the plates. A main structure of the East African-Antarctic Orogen
extends from the Namama Shear Zone in Central Mozambique across the Orvin Shear
Zone towards the Forster magnetic anomaly in Dronning Maud Land. During the initial
Gondwana break-up at 182 Ma, Beira High started to separate from West Gondwana
along this suture until it demerged as well from East Gondwana by a rift jump.
The investigation of further partly unknown tectonic structures along the western
and southern coast of Mozambique revealed a possible oceanic origin of the southern
part of the Mozambique Coastal Plains, due to similarities of the magnetic signature to
the oceanic crust south of Beira High as well as the tentative identification of magnetic
spreading anomalies. The subsequent emplaced Mozambique Ridge moved southwards
as part of a micro plate during an additional active spreading centre in the Northern
Natal Valley. The resulting reconstruction of the initial Gondwana break-up in the
Africa-Antarctica Corridor accounts for all present-day available geological, geophysical
and geodynamic constraints and might serve as a basis for the investigation of the
subsequent dispersal of Gondwana.
Continental Margin
Pangaea
Laurasia
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Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion, continental breakup and magmatic processes over a period of more than 3.5 billion years can be studied. The Agulhas-Karoo Geoscience Transect is part of the South African - German cooperative research project Inkaba yeAfrica, which aims to investigate this part of the continent and ocean in a cone-shaped sector from core to space. Geophysical and geological data and samples were collected along this transect which spans from the Agulhas Plateau across the Agulhas-Falkland fracture zone, the Outeniqua Basin, the Cape Fold Belt, the Namaqua-Natal Belt onto the Karoo Craton. A combined onshore-offshore deep crustal seismic reflection and refraction survey as well as several magnetotelluric surveys provide information about tectonic and magmatic structures and constraints for physical parameters from sedimentary sequences to the upper mantle. The main objectives include an understanding of the crustal nature of the Agulhas Plateau, the processes accompanying and succeeding the crustal shearing process along the Agulhas-Falkland Transform/Fracture Zone, the offshore basin formation in relation to breakup, the deep-seated tectonics of the Cape Fold Belt, and the geometries and sources of the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt. The geophysical data are integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model of the evolution of this region and its sedimentary, tectonic and magmatic units.
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The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted Mmax is ~5.8, and that deformation is governed by a NNW-SSE oriented horizontal σ1, typical of an Andersonian strike-slip regime (σ1 > σv > σ3). The history and dynamics of this large scale (≥ 2x 106 Km2) stress field, known as the Wegener stress anomaly, appears to be complex. The palaeostress record suggests that a stress field comparable to the current one became established after the opening of the Atlantic, perhaps at ~102 Ma and waned at about ~72 Ma, when it was replaced by a markedly different Andersonian thrust regime (σ1 > σ2 > σ v) oriented NNE-SSW. It is uncertain when the current Wegener stress field was re-established, but some evidence points to a pre-Quaternary event. Our finds at Vaalputs are consistent with published accounts of mid-Cretaceous, NW-SE oriented crustal shortening through reverse faulting, thrusting, and folding in Namibia and also in the offshore Bredasdorp Basin. This tectonic activity locally resulted in mountain building such as the Groot and Klein Karas Mts. of southern Namibia. As such, this tectonic style is difficult to reconcile with the extensional regime of a classic (passive) “Atlantic-type” continental margin, and calls for a new approach to the way the Kalahari epeirogeny of southern Africa is perceived.
Orogeny
Stress field
Continental Margin
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