Agulhas-Falkland Fracture Zone Research Articles

The upper-mantle density variation beneath the South Atlantic Ocean and its implications for hotspots and large igneous provinces

The South Mid-Atlantic Ridge (SMAR), the most typical slow-spreading ridge on the earth, is a significant area to study the oceanic crust-mantle evolution. We used gravity data and the S-wave velocity model to derive a 3D density model of the upper-mantle beneath the South Atlantic Ocean to better understand the relationship between ridges, hotspots, and large igneous provinces. The density variations in the upper-mantle are related to thermal and compositional changes. The segment near the Agulhas-Falkland Fracture Zone (AFFZ) is characterized by low-density but inconspicuous low-velocity anomalies reflecting the depleted composition. Its composition is different from the northern part of the SMAR. A low-density layer at 100 km that extends 200 km from the ridge to the Discovery hotspot and that disappears near the AFFZ, is also present. The higher degree of mantle melting beneath the segment near the AFFZ is most probably influenced by the residual plume materials from the Discovery hotspot. Deep low densities beneath the Rio Grande Rise (RGR) and the Walvis Ridge (WR) indicate a higher degree of melting and their formation is probably influenced by the Tristan da Cunha plume. When the RGR is driven away from the plume, its original hot materials cool down and present as cold and dense uppermost mantle deposits. As the plume is located in the African plate, the WR was continuously formed as a seamount chain by intraplate volcanism of mantle plume.

Coeval development of extensional and contractional features along transform margins: insights from the Diaz Marginal Ridge

Abstract The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian–Hauterivian-aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery. Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east–west-orientated maximum stress and north–south-orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localized magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

Mapping the complexity of transform margins

Abstract Transform margins are a function of the pre-existing crustal architecture (pre-transform) and the interplay of syn- and post-transform geodynamic processes. We use a suite of geospatial databases to investigate four transform margins: East Africa (Davie Deformational Zone, DDZ), Equatorial Africa, and the South African and Falkland (Malvinas) margins (Agulhas–Falkland Fracture Zone, AFFZ). The East African margin is the most complex of the four. This is a consequence of Late Jurassic–Early Cretaceous transform motion affecting highly heterogeneous crust, and post-transform deformation that varies along the margin. Equatorial Africa most closely adheres to traditional definitions of ‘transform margins’, but actually comprises two principal transform systems – the Romanche and St Pauls, dictated by the pre-transform distribution of mobile belts and West African craton. All four margins are spatially associated with volcanism, and each exhibits narrow uplifts associated with transpression or transtension. But the causal relationship of these features with transform processes differ. Volcanism along the East African margin is pre- and post-transform. Syn-transform volcanism on the AFFZ is spatially limited, with the AFFZ possibly acting as a conduit for magmatism rather than as a causal driver. Transform margins are varied and complex and require an understanding of pre-, syn- and post-transform geodynamics.

A revised position for the rotated Falkland Islands microplate

The early stages of transform margin formation are associated with crustal fragmentation and block rotation. The restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo. An example considered here is the Falkland Plateau. This is located adjacent to the Agulhas–Falkland Fracture Zone and its westernmost province is the Falkland Islands microcontinent. The position of the plateau and the islands prior to Gondwana break-up remains contentious. This study integrates seismic reflection and gravity data to propose a revised position of the Falkland Islands microcontinent constrained by (1) the presence of a mega-decollement, controlling the Gondwanide Orogen, described north of the Falkland Islands and underneath South Africa and the Outeniqua Basin, and (2) the similar architecture of fault networks mapped north of the islands and in the northernmost Outeniqua Basin. This revised position requires a re-evaluation of the timing and rate of rotation of the Falkland Islands microcontinent and affects the expected crustal architecture adjacent to the islands. Our model yields rotation rates between 5.5° and 8° Ma−1 and two potential times for rotation, and predicts more unstretched crust beneath the basin east of the Falkland Islands than previous models.

Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau

The south and southeast coast of southern Africa (from 28°S to 33°S) forms a high-elevated transform passive margin bounded to the east by the Agulhas-Falkland Fracture Zone (AFFZ). We analysed the stratigraphic record of the Outeniqua and Durban (Thekwini) Basins, located on the African side of the AFFZ, to determine the evolution of these margins from the rifting stage to present-day. The goal was to reconstruct the strike-slip evolution of the Agulhas Margin and the uplift of the inland high-elevation South African Plateau. The Agulhas transform passive margin results from four successive stages:(1)Rifting stage, from Late Triassic to Early Cretaceous (~200?–134 Ma), punctuated by three successive rifting episodes related to the Gondwana breakup;(2)Wrench stage (134–131 Ma), evidenced by strike- and dip-slip deformations increasing toward the AFFZ;(3)Active transform margin stage (131–92 Ma), during which the Falkland/Malvinas Plateau drifts away along the AFFZ, with an uplift of the northeastern part of the Outeniqua Basin progressively migrating toward the west;(4)Thermal subsidence stage (92–0 Ma), marked by a major change in the configuration of the margin (onset of the shelf-break passive margin morphology).Two main periods of uplift were documented during the thermal subsidence stage of the Agulhas Margin: (1) a 92 Ma short-lived margin-scale uplift, followed by a second one at 76 Ma located along the Outeniqua Basin and; (2) a long-lasting uplift from 40 to 15 Ma limited to the Durban (Thekwini) Basin. This suggests that the South African Plateau is an old Upper Cretaceous relief (90–70 Ma) reactivated during Late Eocene to Early Miocene times (40–15 Ma).

New constraints on the age and style of continental breakup in the South Atlantic from magnetic anomaly data

We present new constraints on the opening of the South Atlantic Ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles. We use 45,000 km of recently collected commercial ship track data combined with 561,000 km of publically available data. The new data cover the critical ocean–continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations. Within the final grids the mean cross-over error is 14 nT computed from 8,227 ship track intersections. The forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep-seismic reflection data. We find the oldest magnetic anomalies to date from M10r (134.2 Ma, late Valanginian) north of the Falkland-Agulhas Fracture Zone and M3 (129.3 Ma, Barremian) south of the Rio Grande Fracture Zone. Hence, assuming the GPTS used is correct, continental breakup was contemporaneous with the Parana and Etendeka continental flood basalts. Many of the landward linear anomalies overlap seismically mapped Seaward Dipping Reflectors (SDRs). We interpret this to mean that a significant portion of the SDRs overlay crust formed by subaerial seafloor spreading. Here crustal accretion is envisaged to be similar to that at mid-ocean ridges, but sheet lava flows (that later form the SDRs) rather than pillow basalts form the extrusive component. Segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows. Our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional (submarine) oceanic crustal generation.

Crustal structure of the Agulhas Ridge (South Atlantic Ocean): Formation above a hotspot?

The southern South Atlantic Ocean contains several features believed to document the traces of hotspot volcanism during the early formation of the ocean basin, namely the Agulhas Ridge and the Cape Rise seamounts located in the southeast Atlantic between 36°S and 50°S. The Agulhas Ridge parallels the Agulhas-Falkland Fracture Zone, one of the major transform zones of the world. The morphology of the ridge changes dramatically from two parallel segments in the southwest, to the broad plateau-like Agulhas Ridge in the northeast. Because the crustal fabric of the ridge is unknown relating its evolution to hotspots in the southeast Atlantic is an open question. During the RV Polarstern cruise ANT-XXIII-5 seismic reflection and refraction data were collected along a 370km long profile with 8 Ocean Bottom Stations to investigate its crustal fabric. The profile extends in NNE direction from the Agulhas Basin, 60km south of the Agulhas Ridge, and continues into the Cape Basin crossing the southernmost of the Cape Rise seamounts. In the Cape Basin we found a crustal thickness of 5.5–7.5km, and a velocity distribution typical for oceanic crust. The Cape Rise seamounts, however, show a higher velocity in comparison to the surrounding oceanic crust and the Agulhas Ridge. Underplated material is evident below the southernmost of the Cape Rise seamounts. It also has a 5–8% higher density compared to the Agulhas Plateau. The seismic velocities of the Agulhas Ridge are lower, the crustal thickness is approximately 14km, and age dating of dredge samples from its top provides clear evidence of rejuvenated volcanism at around 26Ma. Seismic data indicate that although the Cape Rise seamounts formed above a mantle thermal anomaly it had a limited areal extent, whereas the hotspot material that formed the Agulhas Ridge likely erupted along a fracture zone.

Tectonic dissection and displacement of parts of Shona hotspot volcano 3500 km along the Agulhas-Falkland Fracture Zone

In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83–72 Ma and are 10–30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°–55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin.

Morphology, Micro-Structure and Chemistry of A Deformed Garnet Megacryst Suite From Monteleo Kimberlite, Free State Province, South Africa.

Research Article| December 01, 2015 Morphology, Micro-Structure and Chemistry of A Deformed Garnet Megacryst Suite From Monteleo Kimberlite, Free State Province, South Africa. S. Perritt; S. Perritt Previous address: Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa, Current address: De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: samantha.perritt@debeersgroup.com Search for other works by this author on: GSW Google Scholar R. Preston; R. Preston De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: robin.preston@debeersgroup.com Search for other works by this author on: GSW Google Scholar F. Viljoen; F. Viljoen Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa, e-mail: fanusv@uj.ac.za Search for other works by this author on: GSW Google Scholar G. Van Der Linde G. Van Der Linde De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: gert.vanderlinde@debeersgroup.com Search for other works by this author on: GSW Google Scholar Author and Article Information S. Perritt Previous address: Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa, Current address: De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: samantha.perritt@debeersgroup.com R. Preston De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: robin.preston@debeersgroup.com F. Viljoen Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa, e-mail: fanusv@uj.ac.za G. Van Der Linde De Beers Group Services, Private Bag X01, Southdale, 2135, South Africa, e-mail: gert.vanderlinde@debeersgroup.com Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online Issn: 1996-8590 Print Issn: 1012-0750 © 2015 Geological Society of South Africa South African Journal of Geology (2015) 118 (4): 439–454. https://doi.org/10.2113/gssajg.118.4.439 Article history First Online: 09 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 S. Perritt, R. Preston, F. Viljoen, G. Van Der Linde; Morphology, Micro-Structure and Chemistry of A Deformed Garnet Megacryst Suite From Monteleo Kimberlite, Free State Province, South Africa.. South African Journal of Geology 2015;; 118 (4): 439–454. doi: https://doi.org/10.2113/gssajg.118.4.439 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 SocietySouth African Journal of Geology Search Advanced Search Abstract Garnet megacrysts found in the Cretaceous Group II Monteleo kimberlite (Free State province, South Africa) exhibit an unusual flattened and elongated morphology. Internally, the garnets are characterized by abundant kelyphite veins and microcracks, arranged as preferentially orientated microstructural sets. Numerous rod-like sulphide inclusions are present within the garnets, also characterized by a preferential orientation, with their long axes generally normal to the short axis of the host megacryst. The observed garnet megacryst morphologies and internal micro-structures are consistent with a shear-related deformation event, although the deformation mechanism remains unresolved. The deformation of the garnet megacrysts is inferred to have occurred prior to the entrainment of the garnets in their host kimberlite and is attributed to localized shearing in the lithospheric mantle, possibly associated with overlying crustal shearing that developed parallel to the Agulhas-Falkland Fracture Zone during Gondwana breakup. Variations in major and trace element concentrations indicate that the garnet megacrysts formed as a result of a simple fractional crystallization process, and pressure-temperature modelling indicates that they crystallized from a >1330°C melt at a depth of ~180 km, within a zone of metasomatism within the lithospheric mantle. A similarity in chemical composition between the megacrysts and garnets from associated lherzolitic lithologies suggests that percolation of the megacryst parental melt through the lithospheric mantle may have contributed to the metasomatism. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Post break-up tectonic inversion across the southwestern cape of South Africa: New insights from apatite and zircon fission track thermochronometry

The south-west African margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning. Recent attention focused on this margin has included theoretical modelling studies of rift processes, plate kinematic studies of the opening geometry and timing, and empirical studies focused on documenting the crustal structure and offshore sedimentary record. Here, we examine the onshore geomorphic and tectonic response to rifting and breakup, with a specific focus on the SW Cape of South Africa. We present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of South Africa. The data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the Early Cretaceous (150–130Ma) and Late Cretaceous (100–80Ma), respectively. Both periods of enhanced erosion are regional in extent, involved km-scale erosion, and extend well inland of the current escarpment zone, albeit with spatially variable intensity and style. The Late Cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km-scale differential displacement and erosion. The new AFT data do not exclude the possibility of modest surface uplift occurring during the Cenozoic, but they restrict the depth of regional Cenozoic erosion on the western margin to less than c. 1km. The inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore Orange and Bredasdorp basins. It is suggested that the Late Cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the SW Cape by local tectonic forces arising from dextral displacement of the Falkland Plateau along the Falkland–Agulhas Fracture Zone.

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Abstract. High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity – depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone. Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible.

The Agulhas Ridge, South Atlantic: The Peculiar Structure of a Fracture Zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material.

The Cape Fold Belt and Syntaxis and the rotated Falkland Islands: dextral transpressional tectonics along the southwest margin of Gondwana

Two enigmas concerning the Cape Fold Belt (CFB), part of the Gondwanide Orogen that formerly stretched across southern Gondwana, are (1) its apparent development far-removed (≥1 500 km) from the convergent margin of Gondwana; and (2) the origin of the Cape Syntaxis, a 40–80° bend in the strike of the belt that occurs near Cape Town. An additional puzzle is how the Falkland Islands, which are believed to have originally been situated off the southeastern coast of Africa and may have formed the eastern continuation of the CFB, came to be rotated 180° during, or prior to, the break-up of Gondwana. In an attempt to address these enigmas, I review recent developments in the study of the CFB and of the South American, Falkland Islands and Antarctic portions of the Gondwanide Orogen, provide reinterpretations of some data and suggest a new tectonic model. Structural data (orientations relative to an east-west-trending CFB) are consistent with interpretation of the South America and Antarctic portions of the Gondwanide Orogen as northwest-trending dextral transpressional belts. The east-west-trending CFB, including the Falkland Islands, forms an inboard- or left-step within this intracontinental dextral shear zone. Dextral margin-parallel translation of the crustal block outboard of the orogen (extending from the Gondwanide Belt south to the margin of Gondwana) was accommodated by strike-slip deformation in South America and Antarctica, and by convergence and the development of a foreland-verging fold and thrust belt, across the CFB. The Cape Syntaxis, and the partially obscured Port Elizabeth Antitaxis, are oroclinal bends of the CFB that developed in response to continued dextral shear along the Gondwanide Belt. Clockwise rotation of the Falkland Islands occurred in two stages: (1) ~ 90° during oroclinal bending (the islands were incorporated in the short east limb of the Port Elizabeth Antitaxis); and (2) > 60° during solid body rotation about the Euler Pole to the Agulhas-Falkland Fracture Zone, a major dextral transform fault that propagated through the hinge region of the Port Elizabeth Antitaxis during break-up of Gondwana.

Role of continental break-up, mantle plume development and fault reactivation in the evolution of the Gamtoos Basin, South Africa

In response to the break-up of Gondwana the Gamtoos Basin rifted in the Oxfordian/Kimmeridgian with reactivation of Cape Fold Belt thrusts as extensional structures. Rifting was rapid with an initial divergent syn-rift sequence passing into a parallel passive infill sequence as rifting waned. Dextral movement on the Agulhas–Falkland Fracture Zone during the Valanginian resulted in reactivation of Oxfordian/Kimmeridgian structures with extensional, compressional and strike-slip senses of movement. Post-rift thermal subsidence phase of basin evolution began during latest Valanginian times with passive infill and onlap of the topography created during the Valanginian. Subsequently the basin began to be uplifted due to the westward migration of the Falkland Plateau and Natal Valley spreading centre past the basin resulting in a non-depositional hiatus and local channelling before erosion during the Cenomanian. Following renewed subsidence the basin experienced uplift during the Palaeocene and Eocene the cause of which remains problematic but is contemporaneous with igneous intrusion in southern Africa.

Tectonostratigraphical development of the Upper Karooforeland basin: Orogenic unloading versus thermally-induced Gondwana rifting

The origins of the Karoo Basin have never been adequately explained, but twomain models have been suggested: (1) a purely foreland basin (retro-arc) model; and (2) a polyphase successor basin, beginning with extension during Upper Table Mountain Group (Silurian) times to a foreland basin in the Early Karoo (Permian) and extension in the Upper Karoo (Upper Triassic-Jurassic). Subsidence and forward modelling on the known stratigraphy of the Karoo Basin do not fit a simple foreland basin model. This is particularly true for the Upper Karoo where the stratigraphy, stacking patterns and age of the succession suggest that the foreland basin model is not viable, and that continental extension played a major role in late basin development. Evidence for extension, and when this occurred, is provided by: (1) sequence stratigraphical identification of a major Middle-Late Carnian low stand sequence boundary unconformity, related to uplift of a newly emergent source area in the southeast; and (2) the presence of volcanic detritus in Upper Karoo sediments, chemically similar to the Karoo basalts, suggesting a possible chemogenetic link between them and, by implication, with early volcanism and incipient Gondwana rifting, now well-preserved in the rift sequences of East Antarctica. This early volcanism, which began not later than the Carnian (∼230 Ma), and provenance uplift, may be related to a thermal anomaly linked to mantle plume-induced updoming, crustal extension and faulting approximately 40 Ma prior to maximum uplift preceeding Karoo Basalt eruption at 183 Ma. Thermal uplift occurred some 200 km off the present southeast coast of South Africa, just south of the Karoo Hotspot, but away from the direct influence of any known hotspot or hotspot track. This uplift, aligned parallel to the southeast coast of South Africa, at a high angle to the Cape Fold Belt, was located close to the site of later rifting and separation along the Agulhas Falkland Fracture Zone.

Tectono-stratigraphic evolution of the North Falkland region

Abstract An interpretation of regional seismic data acquired between 1993 and 1995 has been used to establish a regionally consistent tectono-stratigraphic framework for the North Falkland Shelf. This incorporates three rift (extensional) and two post-rift sequences separated by important tectono-stratigraphic breaks representing unconformities at near-Base Upper Jurassic, near-Base Cretaceous, Mid-Cretaceous and Early Tertiary levels. In the structural framework derived from seismic interpretation two major zones are recognized. The northeast Graben Zone constitutes three tectono-stratigraphic provinces, the north-south orientated North Falkland Graben, and the Western Rift Complex and Southern Half-graben which are dominated by northwest-southeast rift trends. The Falkland Islands form part of the southwest Uplift Zone which has a tectono-stratigraphic history, shown by the interpretation of apatite fission track data, to be compatible with that of the North Falkland Shelf and southern South America. A regional northeast-southwest seismic line shows an extensional northern edge to the Falkland Plateau and no apparent westerly extension of the Agulhas-Falkland Fracture Zone. A ‘rigid’ or ‘non-rotational’ reconfiguration of southwest Gondwanaland (i.e. with no independent rotation of a Falkland Islands microplate) shows a regional consistency of Late Triassic-Mid-Jurassic back-arc and Late Jurassic-Early Cretaceous South Atlantic rift trends. This is presented as circumstantial evidence for a ‘non-rotational’ model for the break-up of southwest Gondwanaland.

Falkland Plateau evolution and a mobile southernmost South America

Abstract Assessment of southwest Gondwana break-up and its implications for regional hydrocarbon prospectivity must now take into account the origin of the southeast margin of the Falkland Islands as a volcanic rifted continental margin, and of the floor of the major part of the Falkland Plateau Basin as elevated oceanic crust. A reconstruction of the Falkland Plateau against southern Africa shows a southward extensional widening of the Outeniqua Basin across the line of the Falkland-Agulhas Fracture Zone, changing from stretched continental to oceanic crustal structure. Such a model for Outeniqua Basin opening, and the independent westward and clockwise rotation of the Falkland Islands block, suggests that southernmost South America was also a collection of microplates moving independently within a generally extensional environment in the Late Triassic-Early Jurassic. This is incompatible with assumptions of a rigid southernmost South America over this time, and a dominant role for a continuous dextral strike-slip Gastre Fault.

When did the Falklands rotate?

Similarities in the styles and relative timings of tectonic events in the Outeniqua Basin, South Africa and the North Falkland Basin suggest that basin formation in both regions may have preceded rotation of the Falklands microplate. Contrary to previous models for the break-up of Gondwana, which suggest Jurassic rotation, the data implies Valanginian rotation, contemporaneous with the first recorded motion on the Agulhas Falkland Fracture Zone and South Atlantic rifting. The data also suggests that the formation of the Falkland Plateau Basin may also be a Cretaceous event as opposed to the previously assumed Jurassic age. Such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for Jurassic rotation does not exclude later rotation as a possibility.

Transtension in southeastern Africa associated with Gondwana break-up

Abstract The southeastern margin of Africa is marked by a zone of complex faulting associated with crustal extension related to the Mesozoic break-up of Gondwana. The oldest age of faulting (180–175 Ma) displays differences in style in regions with Archaean as opposed to Proterozoic basement. Faulting in the Archaean region shows extension normal to the proto-plate boundary whereas transtension prevailed in the Proterozoic region. The north-south trending normal faulting in the Archaean craton, which cuts across basement trends, displays an asymmetry indicating simple shear extension of the lithosphere. In contrast, transtension occurred in the region of Proterozoic basement by ENE strike-slip movement of blocks along terrane boundaries. As these blocks moved, normal faults developed behind them as well as within them at high angles to the terrane boundaries, to accommodate extension. Analogue modelling reproduced the observed different fault patterns. It is proposed that sinistral movement along the interconnected Gastre Fault System and Falkland-Agulhas Fracture Zone, together with the presence of a releasing bend, are the reasons for extension in the Lebombo region. This movement may have been caused by plate edge events between the Patagonia block and the proto-Pacific Ocean.