Breakup Magmatism Research Articles

Jan Mayen Microcontinent Composite Tectono-Sedimentary Element and Jan Mayen Prograded Margin Tectono-Sedimentary Element, Greenland Sea

The Jan Mayen Micro-Continent (JMMC) is the result of two Cainozoic phases of continental breakup, and subsequent seafloor spreading. The first established a spreading ridge system consisting of the Reykjanes, Ægir, and Mohn spreading ridges between Norway and Greenland in early Eocene. The second phase established the Kolbeinsey Ridge in Oligocene/Miocene separating the JMMC form Greenland. Two major stratigraphic elements are established accordingly, the Jan Mayen Micro-Continent Composite Tectono-Sedimentary Element (JMMC CTSE) for the Middle Devonian to Miocene succession, and the Jan Mayen Prograded Margin Tectono-Sedimentary Element (JMPM TSE) for the Miocene to Holocene succession. OBS refraction data show that the JMMC is underlain by a generally thickened crust. Plate reconstructions show that this crust most likely is a remnant of continental crust rifted off East Greenland, and thereby, consist of rocks that correlates with the Late Palaeozoic and Mesozoic strata stratigraphy of the conjugate margins of East Greenland and Norway known to be hydrocarbon. The arrival of the Iceland hot spot and the breakup magmatism greatly influenced the historic and current heat flow. Published maturation models and play analysis, show that there is a potential for oil and gas in the Jan Mayen Ridge within the JMMC.

Vestiges of the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt

The Pre-Caledonian margin of Baltica has been outlined as a tapering wedge with increasing magmatism towards the ocean–continent transition. It is, however, well known that margins are complex, with different and diachronous evolution along and across strike. Baltica’s vestiges in the Scandes have complexities akin to modern margins. It included a microcontinent and magma-poor hyperextended and magma-rich segments. It was probably up to 1500 km wide before distal parts were affected by plate convergence. Characteristic features are exhumed mantle peridotites and their detrital equivalents, some exposed to the seafloor by the pre-orogenic hyperextension. A major change in the architecture of the mountain belt occurred across the NW–SE trending Sveconorwegian front in the Baltican basement. This coincided with the NE termination of the Jotun-Lindås-Dalsfjord basement nappes, the remains of the Jotun Microcontinent (JMC) formed by hyperextension prior to the orogeny. Mantle with ophicalcite breccias exhumed by hyperextension are covered by deep-marine sediments and local conglomerates. Baltican basement slivers are common in the transitional crust basins. Outboard the JMC, the margin was magma-rich. The main break-up magmatism at 605 ± 10 Ma was part of the vast Central Iapetus Magmatic Province. The along-strike heterogeneity of the margin controlled diachronous and contrasting tectonic evolution during the later Caledonian plate convergence and collision.

New Insights Into the Rift to Drift Transition Across the Northeastern Nova Scotian Margin From Wide‐Angle Seismic Waveform Inversion and Reflection Imaging

AbstractSparse wide‐angle seismic profiling supported by coincident reflection imaging has been instrumental for advancing our knowledge about rifted margins. Nevertheless, features of critical importance for understanding rifting processes have been poorly resolved. We derive a high‐resolution velocity model by applying full waveform inversion to the dense OETR‐2009 wide‐angle seismic profile crossing the northeastern Nova Scotian margin. We then create a coincident reflection image by prestack depth migrating the multichannel seismic data. This allows for the first detailed interpretation of the structures related to the final stages of continental breakup and incipient oceanic accretion at the Eastern North America Margin. Our interpretation includes a hyperextended continental domain overlying partially serpentinized mantle, followed by a 10‐km‐wide domain consisting of a continental block surrounded by layered and bright reflectors indicative of magmatic extrusions. A major fault, representing the continent‐ocean boundary, marks a sharp seaward transition to a 16‐km‐wide domain characterized by smoother basement with chaotic reflectors, where no continental materials are present and a 3‐km‐thick embryonic oceanic crust overlying partially serpentinized mantle is created by the breakup magmatism. Further seaward, thin oceanic crust overlies the serpentinized mantle suggesting magma‐poor oceanic spreading with variable magma supply as determined from variable basement topography, 2–4 km thick volcanic layer, and magnetic anomalies. Our results demonstrate that magmatism played an important role in the lithospheric breakup of the area crossed by the OETR‐2009 profile. Considering that the northeastern Nova Scotian margin has been classified as amagmatic, large margin‐parallel variations in magma supply likely characterize a single rift segment.

Along‐Margin Variations in Breakup Volcanism at the Eastern North American Margin

AbstractWe model the magnetic signature of rift‐related volcanism to understand the distribution and volume of magmatic activity that occurred during the breakup of Pangaea and early Atlantic opening at the Eastern North American Margin (ENAM). Along‐strike variations in the amplitude and character of the prominent East Coast Magnetic Anomaly (ECMA) suggest that the emplacement of the volcanic layers producing this anomaly similarly varied along the margin. We use three‐dimensional magnetic forward modeling constrained by seismic interpretations to identify along‐margin variations in volcanic thickness and width that can explain the observed amplitude and character of the ECMA. Our model results suggest that the ECMA is produced by a combination of both first‐order (~600–1,000 km) and second‐order (~50–100 km) magmatic segmentation. The first‐order magmatic segmentation could have resulted from preexisting variations in crustal thickness and rheology developed during the tectonic amalgamation of Pangaea. The second‐order magmatic segmentation developed during continental breakup and likely influenced the segmentation and transform fault spacing of the initial, and modern, Mid‐Atlantic Ridge. These variations in magmatism show how extension and thermal weakening was distributed at the ENAM during continental breakup and how this breakup magmatism was related to both previous and subsequent Wilson cycle stages.

Integrated geophysical characterization of crustal domains in the eastern Black Sea

Abstract Rifting may lead ultimately to continental breakup, but the identification and characterization of the resulting crustal distribution remains challenging. Also, spatial and temporal changes in breakup magmatism may affect the geophysical character of the newly formed oceanic crust, resulting in contrasting interpretations of crustal composition and distribution. In the Eastern Black Sea Basin (EBSB), the evolution from rifting to breakup has been long debated, with several interpretations for the distribution of stretched continental and oceanic crust. We interpret basement morphological variations from long-offset seismic reflection profiles, highlighting a northwest-to-southeast transition from faulted and tilted continental blocks to a rough and then smoother basement. We model magnetic anomalies to further constrain the various basement domains, and infer the presence of a weakly magnetized, stretched continental crust in the northwest, and a 0.4–3.8 A/m layer coinciding with the smooth basement in the central and southeastern area. We conclude that the EBSB oceanic crust extends farther to the northwest than was suggested previously from an abrupt change in crustal thickness and lower-crustal velocity. The apparent discrepancy between these different types of geophysical evidence may result from changes in magma supply during breakup, affecting the thickness and velocity structure of the resulting oceanic crust.

A Mantle Plume Origin for the Scandinavian Dyke Complex: A “Piercing Point” for 615 Ma Plate Reconstruction of Baltica?

AbstractThe origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than ca. 320 million years (pre‐Pangea) are contentious research problems. Here we study the petrology of a 615–590 Ma dolerite dyke complex that intruded rift basins of the magma‐rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1,000‐km‐long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75–250 °C above ambient mantle. Although the trace element modelling solutions are nonunique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present‐day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear‐wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi‐stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

Regional volcanism of northern Zealandia: post-Gondwana break-up magmatism on an extended, submerged continent

Abstract Volcanism of Late Cretaceous–Miocene age is more widespread across the Zealandia continent than previously recognized. New age and geochemical information from widely spaced northern Zealandia seafloor samples can be related to three volcanotectonic regimes: (1) age-progressive, hotspot-style, low-K, alkali-basalt-dominated volcanism in the Lord Howe Seamount Chain. The northern end of the chain ( c. 28 Ma) is spatially and temporally linked to the 40–28 Ma South Rennell Trough spreading centre. (2) Subalkaline, intermediate to silicic, medium-K to shoshonitic lavas of >78–42 Ma age within and near to the New Caledonia Basin. These lavas indicate that the basin and the adjacent Fairway Ridge are underlain by continental rather than oceanic crust, and are a record of Late Cretaceous–Eocene intracontinental rifting or, in some cases, speculatively subduction. (3) Spatially scattered, non-hotspot, alkali basalts of 30–18 Ma age from Loyalty Ridge, Lord Howe Rise, Aotea Basin and Reinga Basin. These lavas are part of a more extensive suite of Zealandia-wide, 97–0 Ma intraplate volcanics. Ages of northern Zealandia alkali basalts confirm that a late Cenozoic pulse of intraplate volcanism erupted across both northern and southern Zealandia. Collectively, the three groups of volcanic rocks emphasize the important role of magmatism in the geology of northern Zealandia, both during and after Gondwana break-up. There is no compelling evidence in our dataset for Late Cretaceous–Paleocene subduction beneath northern Zealandia.

Conjugate volcanic rifted margins, seafloor spreading, and microcontinent: Insights from new high‐resolution aeromagnetic surveys in the Norway Basin

AbstractWe have acquired and processed new aeromagnetic data that cover the entire oceanic Norway Basin located between the Møre volcanic rifted margin and the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit the structure of the conjugate volcanic (rifted) margins and the spreading evolution of the Norway Basin from the Early Eocene breakup time to the Late Oligocene when the Aegir Ridge became extinct. The volcanic margins (in a strict sense) that formed before the opening of the Norway Basin have been disconnected with the previous Jurassic‐Mid‐Cretaceous episode of crustal thinning. We also show evidence of relationships between the margin architecture, the breakup magmatism distribution along the continent‐oceanic transition, and the subsequent oceanic segmentation. The Norway Basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps. The new aeromagnetic compilation also confirms that a fan‐shaped spreading evolution of the Norway Basin was clearly active before the cessation of seafloor spreading and extinction of the Aegir Ridge. An important Mid‐Eocene kinematic event at around magnetic chron C21r can be recognized in the Norway Basin. This event coincides with the onset of diking and increasing rifting activity (and possible oceanic accretion?) between the proto‐JMMC and the East Greenland margin. It led to a second phase of breakup and microcontinent formation in the Norwegian‐Greenland Sea ~26 Myrs later in the Oligocene.

Magmatic development of the outer Vøring margin from seismic data

AbstractThe Vøring Plateau off mid‐Norway is a volcanic passive margin, located north of the East Jan Mayen Fracture Zone (EJMFZ). Large volumes of magmatic rocks were emplaced during Early Eocene margin formation. In 2003, an ocean bottom seismometer survey was acquired over the margin. One profile crosses from the Vøring Plateau to the Vøring Spur, a bathymetric high north of the EJMFZ. The P wave data were ray traced into a 2‐D crustal velocity model. The velocity structure of the Vøring Spur indicates up to 15 km igneous crustal thickness. Magmatic processes can be estimated by comparing seismic velocity (VP) with igneous thickness (H). This and two other profiles show a positive H‐VP correlation at the Vøring Plateau, consistent with elevated mantle temperature at breakup. However, during the first 2 Ma magma production was augmented by a secondary process, possibly small‐scale convection. From ∼51.5 Ma excess melting may be caused by elevated mantle temperature alone. Seismic stratigraphy around the Vøring Spur shows that it was created by at least two uplift events, with the main episode close to the Miocene/Pliocene boundary. Low H‐VP correlation of the spur is consistent with renewed igneous growth by constant, moderate‐degree mantle melting, not related to the breakup magmatism. The admittance function between bathymetry and free‐air gravity shows that the high is near local isostatic equilibrium, precluding that compressional flexure at the EJMFZ uplifted the high. We find a proposed Eocene triple junction model for the margin to be inconsistent with observations.

The eastern Jan Mayen microcontinent volcanic margin

The Jan Mayen microcontinent (JMMC) in the NE Atlantic was created through two Cenozoic rift episodes. Originally part of East Greenland, the JMMC rifted from NW Europe during the Early Eocene under extensive magmatism. The eastern margin is conjugate to the Møre-Faeroes volcanic margin. The western JMMC margin underwent prolonged extension before it finally separated from East Greenland during the Late Oligocene. Here we present the modelling by forward/inverse ray tracing of two wide-angle seismic profiles acquired using Ocean Bottom Seismometers, across the northern and the southern JMMC. Early Eocene breakup magmatism at the eastern JMMC produced an igneous thickness of 7-9 km in the north, and 12-14 km in the south. While the continent is clear in the north, the southern JMMC appears to be affected by later Icelandic magmatism. Reduced seismic velocity and increased crustal thickness are compatible with continental crust adjacent to the volcanic margin in the south, but the continental presence towards the Iceland shelf is less clear. Our magnetic track off the southern JMMC gives seafloor spreading rates comparable to that of the conjugate Møre Margin. Transition to ultraslow seafloor spreading occurs at ~43 Ma, indicating onset of major deformation of the JMMC. Calculating the igneous thickness-mean V P relationship at the eastern volcanic margin gives the typical positive correlation seen elsewhere on the NE Atlantic margins. The results indicate temperature driven breakup magmatism under passive mantle upwelling, with a maximum mantle temperature anomaly of ~50 °C in the north and 90-150 °C in the south.

Thin oceanic crust and flood basalts: India‐Seychelles breakup

Recent seismic experiments showed that separation of India from the Seychelles occurred in two phases of rifting. The first brief phase of rifting between India and the Laxmi Ridge formed the Gop Rift, which is characterized by thick oceanic crust and underplating of the adjacent continental margins. The age of the Gop Rift is uncertain, initiation of seafloor spreading being some time between 71 and 66 Ma. This was then followed by rifting and seafloor spreading between the Laxmi Ridge and the Seychelles, the onset of which is well dated by magnetic anomalies at 63.4 Ma and characterized by thin oceanic crust. Both of these rift events occurred within 1000 km of the center of the Deccan flood basalts, which formed at 65 ± 1 Ma. To constrain the age of the Gop Rift and to explore the reasons for the change in crustal structure between the Gop Rift and Seychelles‐Laxmi Ridge margins, we employ a geodynamic model of rift evolution in which melt volumes, seismic velocity, and rare earth element (REE) chemistry of the melt are estimated. We explore the consequences of different thermal structures, hydration, and depletion on the melt production during the India‐Seychelles breakup to understand the reasons behind the thin oceanic crust observed. Magmatism at the Gop Rift is consistent with a model in which the seafloor spreading began at 71 Ma, ca. 6 Myr prior to the Deccan. The opening occurred above a hot mantle layer (temperature of 200°C, thickness of 50 km) that we interpret as incubated Deccan material, which had spread laterally beneath the lithosphere. This scenario is consistent with observed lower crustal seismic velocities of 7.4 km s−1 and 12 km igneous crustal thickness. The model indicates that when the seafloor spreading migrated to the Seychelles‐Laxmi Ridge at 63 Ma, the thermal anomaly was reduced significantly but not sufficient to explain the observed reduction in breakup magmatism. From observations here of 5.2 km oceanic crust, lower crustal seismic velocities of 6.9 km s−1 and a flat REE profile, we infer that breakup occurred in a region of mantle that became depleted by prior extension related to the Gop Rift.

The importance of rift history for volcanic margin formation

Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Largely on the basis of detailed observations from the North Atlantic Ocean, mantle temperature was identified as the primary factor controlling magmatic production, with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break-up. However, as more detailed observations have been made at other rifted margins worldwide, the validity of this interpretation and the importance of other factors in controlling break-up style have been much debated. One such observation is from the northwest Indian Ocean, where, despite an unequivocal link between an onshore flood basalt province, continental break-up and a hot-spot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension. Our approach allows us to directly compare break-up magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the northwest Indian Ocean and at the better-known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. The inherited extensional history can either suppress or enhance melt generation, which can explain previously enigmatic observations.

Along-strike variations in rifted margin crustal architecture and lithosphere thinning between northern Vøring and Lofoten margin segments off mid-Norway

An extensive geophysical and geological database is used to integrate observations and structural and stratigraphic modelling along the northern Vøring and Lofoten margins, off mid-Norway. Subsidence and fault analysis demonstrates depth-dependent lithosphere stretching along both margin segments, and the modelling concurs with an emergent or relatively shallow-water emplacement environment for the breakup lavas along the northern Vøring margin. However off Lofoten, subsidence modelling predicts ~ 1500 m of bathymetric relief during breakup in contradiction to the view of subaerial volcanic emplacement environment off mid-Norway. A distinct Early Eocene bathymetric gradient from Vøring to Lofoten margin is suggested exhibiting depths in the order of 500–1000 m on the latter margin, and thus an environment for submarine lava emplacement. The water depth may even have reached ~ 1250 m following breakup due to an initial, rapid margin subsidence of the Lofoten margin along a deep-rooted low-angle detachment fault. The differences between northern Vøring and Lofoten margin segments in pre- and post-breakup evolution are attributed to the Bivrost Lineament, a transfer zone separating the two margin segments, which appears as a major across-margin boundary in terms of margin physiography and paleogeography, differential sediment accommodation space, crustal- and lithosphere-scale structure, and breakup magmatism. Furthermore, gravity inversion results appear to independently predict a significantly greater volcanic addition on the Vøring than on the Lofoten margin.

Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.

Rates of continental breakup magmatism and seafloor spreading in the Norway Basin–Iceland plume interaction

In year 2000, an ocean bottom seismometer (OBS) profile was acquired across the Møre margin to the Aegir Ridge, an extinct seafloor spreading axis. The margin is an early Eocene volcanic passive margin, located between the Faeroe‐Iceland Ridge (FIR) and the East Jan Mayen Fracture Zone (EJMFZ). The P wave data were modeled by ray tracing to give a crustal transect showing a 10–11 km thick igneous crust created by breakup magmatism, tapering off to magma‐starved seafloor spreading by C23 time (51.4 Ma). The location of the EJMFZ was reinterpreted from a satellite derived gravity map, and spreading direction in the Norway Basin reevaluated. No other fracture zones were confirmed, and both thin oceanic crust (4–5 km) and lack of fracture zones resemble ultraslow spreading on the Arctic Gakkel Ridge. Magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the OBS profile, and half spreading rates were derived. Early seafloor spreading was slow (15–32 mm yr−1), approaching ultraslow (6–8 mm yr−1) by C20 time (42.7 Ma). A V‐shaped pattern seen in the gravity field located only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the seismic model, recording northeast transport (3–6 mm yr−1) of more melt‐fertile asthenosphere zones. The magma‐starved character of the Norwegian Basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the Iceland plume constructed the FIR to the south, as the asthenosphere is subsequently transported into the Norway Basin.

Crustal structure of the Lofoten–Vesterålen continental margin, off Norway

By compiling wide-angle seismic velocity profiles along the ∼400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, ∼25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of ∼26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels.

Mantle mixing and continental breakup magmatism

The frequent formation of large igneous provinces during the opening of the Atlantic Ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent Pangea. Recent geochemical and geophysical findings from the North Atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis. On the basis of a whole-mantle convection model with chemical tracers, I demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly. When small-scale convection in the upper mantle breaks down into the lower mantle, strong counter upwelling takes place, entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone. This multi-scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non-volcanic rifted margins, with a spatially and temporally varying distribution of fertile mantle.

Early stages of Gondwana breakup: The 40Ar/39Ar geochronology of Jurassic basaltic rocks from western Dronning Maud Land, Antarctica, and implications for the timing of magmatic and hydrothermal events

The timing of magmatic events forming Jurassic basaltic rocks in Dronning Maud Land (DML), Antarctica, and hydrothermal activity that affected them is addressed with detailed 40Ar/39Ar incremental heating dating of feldspars. Plagioclase from an Utpostane gabbro and from a Kirwanveggen dolerite dike yield indistinguishable plateau ages at 177 ± 1.8 Ma. Because of geologic controls, this establishes the age of tholeiites in Vestfjella. These plateau ages demonstrate synchroneity of tholeiitic magmatism of both DML and the well‐documented Ferrar Province of Antarctica, which together with the closely temporal Karoo constitute the Gondwana breakup magmatism. Vug‐filling microclines from Vestfjella yield plateau ages of 150 and 139 Ma which are interpreted to give the ages of secondary mineralization events. These results suggest a lengthy period of extension, possibly accompanied by pervasive low‐temperature hydrothermal activity, between flood basalt magmatism and inception of seafloor spreading (circa 160–165 Ma), with younger moderate temperature events recorded by K‐feldspars and related to early stages of spreading and/or changes in plate motions. The majority of plagioclase samples yield discordant age spectra that reflect primarily overprinting by younger events and incorporation of excess Ar, and they illustrate the complexities that can be produced.

Volcanic margin formation and Mesozoic rift propagators in the Cuvier Abyssal Plain off Western Australia

The western margin of Australia is characterized by synrift and postrift magmatism which is not well understood. A joint interpretation of magnetic anomaly, satellite gravity anomaly and seismic data from the Cuvier Abyssal Plain and margin shows that the breakup between India and Australia started circa 136 Ma (M14) and was followed by two rift propagation events which transferred portions of the Indian Plate to the Australian Plate. Post breakup magmatism continued with the emplacement of the Wallaby and Zenith plateaus (∼17–18 km thick at their centers) along a transform margin. Two narrow magmatic edifices adjacent to the Wallaby Plateau (Sonne and Sonja ridges) represent an extinct ridge and a pseudofault, respectively. They formed by excess volcanism, probably by lateral migration of buoyant melt along upside‐down crustal drainage channels from the melt source underneath the Wallaby Plateau. In a mantle plume scenario a small plume (∼400 km diameter) located underneath the rift could have locally uplifted the Bernier Platform and Exmouth Sub‐basin in the Early Cretaceous and left a track consistent with the azimuth of the Wallaby and Zenith plateaus. In this case, ridge‐plume interaction would have caused two consecutive ridge propagation events towards the plume while the ridge moved away from the hotspot. The abrupt end of the hotspot track west of the Zenith Plateau would be a consequence of the accelerating south‐eastward motion of the spreading ridge relative to the mantle after 120 Ma, leaving the mantle plume underneath the Indian Plate. An alternative nonmantle‐plume scenario is based on the observation that between breakup and chron M0 (∼120 Ma) the ocean crust in the southern Cuvier Abyssal Plain was formed while the spreading ridge abutted Indian continental crust. Small‐scale convection may have been initiated during rifting in the Early Cretaceous and maintained until the Wallaby‐Zenith ridge‐transform intersection passed by the eastern edge of Indian continental crust at chron M0.