Cunha Hotspot Research Articles

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.

Uppermost mantle and crustal structure at Tristan da Cunha derived from ambient seismic noise

According to classical plume theory, the Tristan da Cunha hotspot, located ∼400 km east off the Mid-Atlantic Ridge, is thought to have played a major role in the rifting while creating an aseismic Walvis Ridge during and after the breakup of the South Atlantic margins. Volcanic activity on the Tristan da Cunha Island shows that the hotspot might still be there influencing the upper mantle and crustal structure. In this study we present ambient noise data from 24 broadband OBS around Tristan da Cunha and a seismic station on Nightingale Island, which provide first constraints on the crustal and uppermost mantle structure around the island. By combining ambient noise techniques, dispersion curve analysis of Rayleigh waves, 2D tomographic inversion of travel times and 3D depth inversion of dispersion data we derived a 3D VS velocity model around the archipelago of Tristan da Cunha. The model shows an isolated, vertically sharp bounded thickened and modified crust beneath the islands surrounded by thin oceanic crust (<5 km). The velocity anomaly shows a typical volcanic structure with a shallow high velocity body and a low velocity root reaching into the upper mantle, which we interpret as shallow solidified magmatic material and volcanic feeding system/magmatic underplating, respectively. The observed simple and localized volcanic structure, embedded in a rather homogeneous crust and upper mantle indicates only minor and very localized magmatic overprinting of the existing lithosphere by the Tristan da Cunha hotspot. The uppermost mantle S wave velocity beneath nearby seamounts and to the SW of the islands is also slow and could indicate a thermal influence from a deeper source, whereas the Tristan da Cunha Fracture Zone shows no signs of modification.

Thickness of the oceanic crust, the lithosphere, and the mantle transition zone in the vicinity of the Tristan da Cunha hot spot estimated from ocean-bottom and ocean-island seismometer receiver functions

The most prominent hotspot in the South Atlantic is Tristan da Cunha, which is widely considered to be underlain by a mantle plume. But the existence, location and size of this mantle plume have not been established due to the lack of regional geophysical observations. A passive seismic experiment using ocean bottom seismometers aims to investigate the lithosphere and upper mantle structure beneath the hotspot. Using the Ps receiver function method we calculate a thickness of 5 to 8km for the oceanic crust at 17 ocean-bottom stations deployed around the islands. Within the errors of the method the thickness of the oceanic crust is very close to the global mean. The Tristan hotspot seems to have contributed little additional magmatic material or heat to the melting zone at the mid-oceanic ridge, which could be detected as thickened oceanic crust. Magmatic activity on the archipelago and surrounding seamounts seems to have only affected the crustal thickness locally. Furthermore, we imaged the mantle transition zone discontinuities by analysing receiver functions at the permanent seismological station TRIS and surrounding OBS stations. Our observations provide evidence for a thickened (cold) mantle transition zone west and northwest of the islands, which excludes the presence of a deep-reaching mantle plume. We have some indications of a thinned, hot mantle transition zone south of Tristan da Cunha inferred from sparse and noisy observations, which might indicate the location of a Tristan mantle plume at mid-mantle depths. Sp receiver functions image the base of lithosphere at about 60 to 75km beneath the islands, which argues for a compositionally controlled seismological lithosphere-asthenosphere boundary beneath the study area.

Marine magnetotellurics imaged no distinct plume beneath the Tristan da Cunha hotspot in the southern Atlantic Ocean

The Tristan da Cunha (TDC) is a volcanic island located above a prominent hotspot in the Atlantic Ocean. Many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot. However, the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area. Marine magnetotelluric and seismological observations were conducted in 2012–2013 to examine the upper mantle structure adjacent to TDC. The electrical conductivity structure of the upper mantle beneath the area was investigated in this study. Three-dimensional inversion analysis depicted a high conductive layer at ~120km depth but no distinct plume-like vertical structure. The conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the TDC islands are located compared to younger segment south of the TDC Fracture Zone, while the bathymetry is rather deeper than prediction for the northern segment. The apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and/or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area. Other observations suggest that 1) the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge, 2) the potential temperature of the segments north of the TDC Fracture Zone is lower than that of the southern segment at least during the past ~30Myr.

Upper mantle structure at Walvis Ridge from Pn tomography

Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break-up. Within the LISPWAL (LIthospheric Structure of the Namibian continental Passive margin at the intersection with the Walvis Ridge from amphibious seismic investigations) project, combined on- and offshore seismic experiments were designed to characterize the Southern African passive margin at the Walvis Ridge in northern Namibia. In addition to extensive analysis of the crustal structures, we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the Walvis Ridge with the Namibian coast. Upper mantle Pn travel time tomography from controlled source, amphibious seismic data was used to investigate the sub-Moho upper mantle seismic velocity. We succeeded in imaging upper mantle structures potentially associated with continental break-up and/or the Tristan da Cunha hotspot track. We found mostly coast-parallel sub-Moho velocity anomalies, interpreted as structures which were created during Gondwana break-up.

The morphostructure of the South Atlantic Ridge in the Tristan da Cunha hot spot area

Investigation of the rift zone and flanks of the South Atlantic midoceanic ridge in the Tristan da Cunha hot spot area revealed that their morphostructure is atypical of slow spreading ridges. Feeding the spreading center with magmatic material, the hot spot changes the morphology of the axial zone and transform faults. It itself forms a relief ensemble that differs from the rift one as well. The continuous migration of the spreading center in the westerly direction stimulates the formation of a spacious volcanic highland, which is responsible for the regional asymmetry of ridge flanks in its extended segments (hundreds kilometers long).

3-D magnetotelluric image of offshore magmatism at the Walvis Ridge and rift basin

The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500Ωm) has three arms of approximately 100km width and 300km to 400km length at 120° angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

Segmentation and volcano-tectonic characteristics along the SW African continental margin, South Atlantic, as derived from multichannel seismic and potential field data

Regional seismic reflection and potential field data document the South Atlantic's break-up history, between 39°S and 19°S, from the Early Cretaceous onwards. Previous maps of distribution of volcanics along the margin showed volcanics along the whole African margin based on extrapolation of data. Based on previously unpublished marine geophysical data, we found the southernmost 460 km long margin segment to be lacking huge volumes of break-up related volcanic effusives. Northwards, break-up was accompanied by the emplacement of huge volumes of volcanic material, prominently featured in seismic sections as huge wedge-shaped seaward dipping reflectors (SDRs). Detailed mapping of offsets (left- and right-stepping) and variations in structural character of the volcanics reveal the segmentation along and the break-up history of the margin. Several superimposed SDR sequences, suggesting episodicity of volcanic emplacement (divided by periods of erosion and sedimentation), are distinct along southerly lines, losing prominence northwards.A main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non-volcanic to a volcanic margin occurs abruptly.We define four distinct First-order Segments along the 2400 km section of the southwestern African margin covered by our seismic data. From south to north these First-order Segments are: Magma-poor Segment I; Segment II with enormous SDRs volumes; decreasing SDRs volumes in Segment III; Segment IV again with enormous volcanic output, likely influenced by Walvis Ridge volcanism.Most important is that there is no systematic increase in the volumes of the effusives towards the Tristan da Cunha hot-spot. Rather there is an alternating pattern in the SDRs' volumes and widths.The boundary between the volcanic and magma-poor margin segments in the southernmost study area is sharp (10s of km), which we propose is reflected in magnetic anomaly data as well. We suggest that this variability along the margin is mainly due to a change in stretching/rifting character from oblique during the early stages of breakup to conventional seafloor spreading from Chron M4 (∼130 Ma) onwards.

Re–Os isotope and highly siderophile element systematics of the Paraná continental flood basalts (Brazil)

Basalts of the Paraná continental flood basalt (PCFB) province erupted through dominantly Proterozoic continental crust during the Cretaceous. In order to examine the mantle source(s) of this major flood basalt province, we studied Os, Sr, Nd, and Pb isotope systematics, and highly siderophile element (HSE) abundances in tholeiitic basalts that were carefully chosen to show the minimal effects of crustal contamination. These basalts define a precise Re–Os isochron with an age of 131.6±2.3Ma and an initial 187Os/188Os of 0.1295±0.0018 (γ187Os=+2.7±1.4). This initial Os isotopic composition is considerably more radiogenic than estimates of the contemporary Depleted Mantle (DM). The fact that the Re–Os data define a well constrained isochron with an age similar to 40Ar/39Ar age determinations, despite generally low Os concentrations, is consistent with closed-system behavior for the HSE. Neodymium, Sr, and Pb isotopic data suggest that the mantle source of the basalts had been variably hybridized by melts derived from enriched mantle components. To account for the combined Os, Nd, Sr, and Pb isotopic characteristics of these rocks, we propose that the primary melts formed from metasomatized asthenospheric mantle (represented by arc-mantle peridotite) that underwent mixing with two enriched components, EM-I and EM-II. The different enriched components are reflected in minor isotopic differences between basalts from southern and northern portions of the province.The Tristan da Cunha hotspot has been previously suggested to be the cause of the Parana continental flood basalt magmatism. However, present-day Tristan da Cunha lavas have much higher 187Os/188Os isotopic compositions than the source of the PCFB. These data, together with other isotopic and elemental data, preclude making a definitive linkage between the Tristan plume and the PCFB.

The crustal structure of the southern Argentine margin

SUMMARY Multichannel reflection seismic profiles, combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost Argentine margin, at the transition from a rifted to a transform margin and outline the extent of the North Falkland Graben. Based on these data, we establish a regional stratigraphic model for the post-rift sediments, comprising six marker horizons with a new formation in the Barremian/Lower Cretaceous. Our observations support that a N–S trending subsidiary branch of the North Falkland Graben continues along the continental shelf and slope to the Argentine basin. During the rift phase, a wide shelf area was affected by the E–W extension, subsequently forming the North Falkland Graben and the subsidiary branch along which finally breakup occurred. We propose the division of the margin in two segments: a N–S trending rifted margin and an E–W trending transform margin. This is further underpinned by crustal scale gravity modelling. Three different tectono-dynamic processes shaped the study area. (1) The Triassic/Early Jurassic extensional phase resulting in the formation of the North Falkland Graben and additional narrower rift grabens ended synchronously with the breakup of the South Atlantic in the early Valanginian. (2) Extensional phase related to the opening of the South Atlantic. (3) The transform margin was active in the study area from about Hauterivian times and activity lasted until late Cretaceous/early Cenozoic. Both, the rifted margin and the transform margin are magma-poor. Very limited structures may have a volcanic origin but are suggested to be post-rift. The oceanic crust was found to be unusually thin, indicating a deficit in magma supply during formation. These findings in combination with the proposed breakup age in the early Valanginian that considerably predates the formation of the Parana–Etendeka continental flood basalt provinces in Brazil and Namibia question the influence of the Tristan da Cunha hotspot during the initial formation of the South Atlantic.

Geochemical characteristics and origin of the Jacupiranga carbonatites, Brazil

New major-, trace-element and Sr, Nd and Pb isotope data are presented for carbonatites and pyroxenites from the 130-Ma-old Jacupiranga complex in southern Brazil. Measured 87Sr 86Sr ranges from 0.7047 to 0.7055, 143Nd 144Nd from 0.51251 to 0.51264 and 206Pb 204Pb from 17.0 to 18.2. Most samples have high measured Th U (5–17), and high 208Pb 204Pb ratios (37.9–40.2) relative to 206Pb 204Pb , with time-integrated Th U > 4 . The pyroxenites have higher initial 206Pb 204Pb (17.63–17.67) and lower initial 87Sr 86Sr (∼ 07047) than the carbonatites (17.05–17.47 and 0.7049–0.7054, respectively). There is a broad negative PbSr isotope array between the carbonatites and the pyroxenites which precludes simple binary mixing, because of their very different Sr/Pb ratios. The isotope differences also appear to preclude models in which the carbonatites segregated by liquid immiscibility from magmas similar to those from which the pyroxenites crystallised, and models in which the isotope arrays are the product of crustal contamination processes. Rather, the initial Sr, Nd and Pb isotope ratios in the Jacupiranga complex, and the negative correlation of Pb and Sr isotope are similar to those in the Paraná high-Ti basalts, and the oceanic basalts of the Walvis Ridge and Tristan da Cunha. Thus, the initial isotope ratios of the Jacupiranga rocks are considered to have been inherited from the mantle source regions associated with incipient magmatism of the Tristan da Cunha hotspot and the opening of the South Atlantic. Finally, the observed variations are used to evaluate suggestions that certain element ratios in upper-mantle rocks, such as high Ca Al and La Yb and low Ti Eu , are features of carbonatite metasomatism. Mantle xenoliths altered by carbonatite metasomatism exhibit elevated Sr contents and low Rb Sr ratios, and most carbonatites have high U Pb , low Rb Sr and Sm Nd . Thus, the intial isotope ratios of the Jacupiranga infiltration and/or metasomatism by carbonatitic melts is one process which may have been responsible for the negative array of U Pb and Rb Sr inferred for the source of certain oceanic basalts.

Paleomagnetism of 122 Ma plutons in New England and the Mid‐Cretaceous Paleomagnetic Field in North America: True Polar wander or large‐scale differential mantle motion?

A paleomagnetic study of Cretaceous White Mountains plutonic complexes in New Hampshire and Vermont yields high unblocking temperature, dual polarity magnetizations in different types of igneous rocks. The resulting pole position for three plutons (71.9°N, 187.4°E, A95 = 6.9°, age = 122.5 Ma) agrees with previously published mid‐Cretaceous poles for North America, which together give a mid‐Cretaceous standstill reference pole slightly revised from Globerman and Irving [1988] at 71.2°N, 194.1°E (A95 = 3.7°, N = 5 studies). We argue on the basis of the wide geographic distribution of these studies, the variety in tectonic settings and rock types, positive reversal tests, and an overall reversal pattern consistent with geomagnetic polarity time scales, that this mean pole represents the North American mid‐Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). The standstill pole limits to within ±4°, the motion of the North American plate relative to the Earth's spin axis. During the same mid‐Cretaceous interval, the New England hotspot track (124 Ma Monteregian Hills, 122.5‐Ma Cretaceous White Mountains, and 103‐ to 84‐Ma New England seamounts) requires 11°±4° of north‐poleward motion of North America, in direct conflict with the paleomagnetic standstill. A similar (∼13°) discrepancy is independently demonstrated between the spin axis and the Tristan da Cunha hotspot track on the African plate during the mid‐Cretaceous interval. The hotspot/spin axis discrepancies ended by ∼90 Ma when it is shown that both Atlantic hotspots agree with North American and African dipole paleolatitudes and present‐day locations. Nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis, leaving as possible interpretations true polar wander and large‐scale (but differential) mantle motion. The southerly motion of the mid‐Cretaceous Louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics. The motions of the three widely separated mid‐Cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid‐Cretaceous “superplume.”

Plate reorganisations around Southern Africa, hot-spots and extinctions

Notwithstanding some inconsistencies between the dates of hot-spot tracks, volcanic lineaments, and seafloor spreading anomalies, relative and absolute motions of the African Plate suggest that: 1. (1) The Tristan da Cunha hot-spot and its track, the Walvis Ridge, are associated both with northward and southward propagating rifts. 2. (2) The Bouvet hot-spot 2.1. (a) tracks the Lebombo Mountain volcanic lineament forming the Mesozoic plate boundary between southeast Africa and East Antarctica, 2.2. (b) is associated with a propagating rift in the Natal Valley during the Cretaceous break-up of the Falkland Plateau and South Africa and 2.3. (c) provides the nucleation point for a plate rearrangement centred on the Agulhas Plateau during the Cretaceous Normal Polarity Epoch. 3. (3) Similarly, the Shona hot-spot is associated with a ridge-jump in the South Atlantic occurring between anomalies 31 and 25, this ridge-jump episode occurred during or just after a Cretaceous-Tertiary boundary plate reorganisation in the Indian Ocean which has been linked to a potential site of a bolide impact. At least two and possibly five of the hot-spot initiated seafloor spreading rearrangements occur when important extinction events occur in the terrestrial biological record. Internal and extraterrestrial triggers are discussed.

Low- and high-TiO 2 flood basalts of southern Brazil: origin from picritic parentage and a common mantle source

The Serra Geral (Paraná) continental flood-basalt province of southern Brazil has two main basalt types: low-TiO 2 (∼ 1 wt.%) basalts occupy the southern portion, and high-TiO 2 (> 3 wt.%) basalts are largely in the northern part. Low-Ti basalts are less evolved (Mg# 60) and more radiogenic (e.g., 87Sr/ 86Sr∼ 0.708) than high-Ti basalts (Mg# 35; 87Sr/ 86Sr∼ 0.705). This is consistent with a model that invokes variable melting of a single mantle source to produce picritic magmas that have relatively lower and higher incompatible element contents. Varying percentages of melting can be related to varying proximity to the early Tristan da Cunha hotspot. The Mg-rich magmas fractionated 60–75% olivine, clinopyroxene, and plagioclase to yield low- or high-Ti flood basalts, assimilating more or less crust in the process. The extent of fractionation and assimilation depended on crustal “warmth” (also tied to location relative to hotspot): (1) above zones of ∼ 25% melting, warm crust relatively easily contaminated crystallizing picritic magma that originated by a high degree of melting (i.e., magma with lower incompatible element contents); additionally, high degrees of melting sustained replenishment of magma with low-Ti magma characteristics; (2) above ∼ 10% melting zones, cooler crust comparatively restricted assimilation during crystallization (of magma with higher incompatible element contents) and permitted magma evolution to high-Ti derivatives; lesser degrees of melting also limited replenishment magma and thereby allowed greater evolution of existing magma. This model refers all diagnostic geochemical and isotopic features of Serra Geral basalts to percentages of partial melting of an essentially homogeneous mantle material.

A geochemical study of rocks from the Walvis Ridge, South Atlantic

Basalts from the Walvis Ridge, an aseismic ridge in the eastern South Atlantic Ocean, have been analyzed to investigate the origin and evolution of this feature, and its relation to Tristan da Cunha. The principal minerals in the basalts are plagioclase, clinopyroxene and Fe-oxides. The alkalic nature of the samples is indicated by the high content of the orthoclase end-member in the feldspars, and the aluminous titanaugites, which contain up to 3.2 wt.% TiO 2. Chemically, the basalts can be divided into two related major groups. Group I is slightly more evolved and is characterized by high TiO 2, FeO∗, V and Zr concentrations, while group II contains relatively low TiO 2 and FeO∗ contents, but has high concentrations of CaO, Na 2O and Al 2O 3. The differences between them are explained by the relative importance of ilmenite precipitation during crystallization in group I and plagioclase precipitation in group II. All of the basalts from the Walvis Ridge are geochemically distinct from Mid-Ocean Ridge basalts originating at spreading centers including those at equivalent latitudes in the South Atlantic. A change in the Zr/Nb ratio of the basalts is observed along the Walvis Ridge — from a value of 10 at the eastern end, 6 in the central section, to ∼4 for Tristan da Cunha. This suggests that the composition of the mantle source was different for the various provinces along the ridge and may have changed progressively with time if the Walvis Ridge represents the trace of the Tristan da Cunha hot spot.