Continental Breakup Research Articles

Utilization of continental transforms in break-up: observations, models, and a potential link to magmatism

Abstract Reactivation of continental transform faults (hereafter transforms) is identified herein as a significant factor in continental break-up, based on a global review of divergent margins and numerical modelling. Divergent margins that have reactivated transforms are characterized by linear and abrupt terminations of thick continental crust. Transforms represent some of the largest structures on Earth, and these megastructures represent major lithospheric weaknesses and are therefore prone to reactivation upon changes in the stress field, which typically occur during plate break-up. The blunt termination of the margins is consistent with observations of very limited pre-break-up lithospheric thinning of such margins. This mode of break-up appears to occur abruptly, and contrasts notably with highly tapered and slowly extended divergent margins. Magma leakage along transforms is well-known worldwide where divergence occurs across such features. This leakage may evolve to dyke injections, further reducing the plate strength. We observe that many of the blunt margins we attribute to transform reactivation have been prone to above-normal magmatism and are marked by seaward-dipping reflectors underlain by high-velocity lower crustal intrusions. The magmatism may be directly related to the separation of abruptly terminated margins, whereby the large resulting lateral thermal gradients trigger edge-driven convection and melt addition.

Constraints on the formation of basaltic magmas. Comment on “Lithosphere thickness controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth – a review and new perspectives” – by Niu Y. (2001)

Niu (2021) reviews fundamental topics of basaltic magma generation in the oceans and continents, including passive and active margins as well as intraplate regions. In his approach, Niu (2021) focuses on the role exerted by lithospheric (“LID”) thickness in controlling basalt compositions and volumes, proposing only a minor role, if any, for the thermal state of the mantle. A consequence of this is rejection of mantle plumes in controlling the compositions of MORB and OIB, albeit not excluding completely their existence to explain basalt petrogenesis.Although the fundamental role of the lithospheric mantle on continental breakup, deformation and volcanism was recognized over 50 years ago, this region has only rarely been considered a candidate source for basaltic rocks, being thought to be too cold, stiff and sterile to produce significant amounts of partial melt. However, the stability of C-H-bearing mineral phases such as carbonates, phlogopite and pargasitic amphibole at lithospheric mantle depths indicates relatively low solidus temperature, and the possibility to produce partial melts even under relatively cold thermal regimes.We contribute to this debate by proposing that a wide range of variables explain igneous activity on Earth and, likely, the other rocky planets. The “lid effect” is certainly important but there is evidence that other factors and variables also influence the compositions and volumes of melts produced in the upper mantle. Among these, bulk- and trace-element chemical heterogeneities, the distribution and speciation of volatiles, and large-wavelength variations in temperature caused by radiogenic decay, are the most relevant.

Thermal history along the Araçuaí Orogen and São Francisco Craton border, eastern Brazilian continental margin, based on low-temperature thermochronologic data

Passive margins form in response to rifting affecting a continental area. Rifting evolution is dependent on structural and rheological inheritance, generally controlling differential denudation, and deposition. The Brazilian continental margin developed over a complex basement, composed of the orogenic lithosphere of the Brasiliano-Pan-African orogenic belts (e.g., the Araçuaí Orogen), and the cratonic lithosphere of the São Francisco Craton. In this work, we focus on the transitional zone between the warm and cold lithosphere of the northern segment of the Araçuaí Orogen to the São Francisco Craton. This study provides the first suite of two low-temperature thermochronometers, i.e., zircon and apatite (UTh)/He, for the region. Using this approach, we examine the main cooling events, the geodynamic forces acting upon the lithosphere, the denudation rates affecting this region since West Gondwana amalgamation, and the correlation between thermochronology and structural inheritance. Our results indicate that ZHe corrected ages range from 483.1 ± 38.6 to 93.8 ± 7.5 Ma and eU concentrations vary from 10.91 to 632.51 mg/g. AHe corrected ages span from 129.9 ± 7.79 to 32.7 ± 1.96 Ma and eU concentrations range from 5.6 to 62.8 mg/g. Both ZHe and AHe ages display a positive correlation with eU concentration. Based on this dataset, we identify three cooling episodes, constrained from Lower Devonian to Carboniferous, Triassic/Jurassic to the present day, and Late Cretaceous/Paleocene to the present day. Additionally, two reheating episodes are constrained from Middle Jurassic to Early Cretaceous and from Late Cretaceous to Paleocene. Furthermore, we analyzed lineament directions in the orogenic, cratonic, and Cenozoic sedimentary cover domains to understand how structural inheritance affects the thermochronological record. Our observations reveal that reactivation of the Brasiliano NW-SE inherited structures during continental breakup up until the Paleocene possibly kept the AHe system open, resulting in the youngest AHe ages of our data set.

The crustal structure in the Northwest Atlantic region from receiver function inversion – Implications for basin dynamics and magmatism

The Labrador Sea and Baffin Bay form an extinct Palaeogene oceanic spreading system, divided by a major continental transform, the Davis Strait, with the whole region defined as the Northwest Atlantic. The Davis Strait hosts the Ungava Fault Zone and is the central structural element of the Davis Strait Large Igneous Province (DSIP) that formed broadly coeval with continental breakup to its north and south. While constraints on the crustal structure in this region primarily exist in the offshore, crustal models are limited onshore, which makes an interpretation of regional structures as well as the extent, and therefore origin of the DSIP extremely difficult to ascertain. Here, we have collected all available teleseismic data from the Northwest Atlantic margins and applied a receiver function inversion to retrieve station-wise velocity models of the crust and uppermost mantle. We integrate the outcomes with published controlled-source seismic data and regional crustal models to make inferences about the crustal structure and evolution of the Northwest Atlantic. In particular, we focused on constraining the spatial extent and origin of high velocity lower crust (HVLC), and determining whether it is generically related to the Davis Strait Igneous Province, syn-rift exhumed and serpentinised mantle, or pre-existing lower crustal bodies such as metamorphosed lower crust or older serpentinised mantle rocks. The new results allow us to better spatially constrain the DSIP and show the possible spatial extent of igneous-type HVLC across Southwest Greenland, Northwest Greenland and Southeast Baffin Bay. Similarly, we are able to relate some HVLC bodies to possible fossil collision/subduction zones/terrane boundaries, and in some instances to exhumed and serpentinised mantle.

The Role of Magma in the Birth of the Atlantic Ocean

High-resolution seismic models of the Nova Scotia margin reveal a role for magmatism in continental breakup, even at magma-poor sections of the eastern North American margin.

Stratigraphic record of continental breakup, offshore NW Australia

AbstractContinental breakup involves a transition from rapid, fault‐controlled syn‐rift subsidence to relatively slow, post‐breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn‐breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth‐dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift‐associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup‐related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non‐erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra‐Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra‐Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth‐dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition.

Lead isotope geochemistry of plagioclase in the Skaergaard intrusion by LA-ICP-MS: Assessing the effects of crustal contamination and link with East Greenland flood basalts

The Skaergaard intrusion of East Greenland is one of the best studied layered intrusions on Earth and preserves an exceptional rock record produced during low-pressure, closed-system crystallization of Fe-rich tholeiitic magma. The lead isotope compositions of plagioclase feldspar in cumulates and related igneous rocks of the Skaergaard intrusion were determined directly from single spot analyses in thin section by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) (n = 757). Plagioclase primocrysts, typically with <2 ppm Pb, and feldspar in interstitial pockets from the Layered Series and from most of the Marginal Border Series have the same Pb isotope compositions within analytical uncertainty (208Pb/206Pb = 2.20 ± 0.06; 207Pb/206Pb = 0.889 ± 0.028). There are no significant Pb isotope differences between cores and rims of individual plagioclase primocrysts or between different types of interstitial pockets (e.g., interstitial granophyre, symplectites). These Pb isotope compositions are less radiogenic than those from most of the contemporaneous East Greenland flood basalts indicative of crustal contamination at deeper crustal levels prior to emplacement. Plagioclase from three samples of the Marginal Border Series adjacent to host gneisses (<5 m from the contact) have higher 208Pb/206Pb (2.30 ± 0.06, 2.35 ± 0.02, 2.51 ± 0.21) compared to the rest of the intrusion. These results confirm that crystallization of the Skaergaard intrusion occurred under essentially closed-system conditions with localized minor incorporation of “amphibolitic” gneiss. Hydrothermal alteration occurred at near-solidus to subsolidus conditions by circulating meteoric fluid that had equilibrated with the overlying flood basalts. This alteration resulted in albitized interstitial granophyre and rare spot analyses from plagioclase primocrysts characterized by Pb isotope compositions comparable to those of the flood basalts. Granophyres of different origins (e.g., transgressive granophyre, granophyric segregation) have variable Pb isotope compositions (208Pb/206Pb = 2.09–2.51; 207Pb/206Pb = 0.844–1.013) that are distinct from the Skaergaard cumulates. Based on Pb isotope systematics, combined with published SrNd isotopic compositions and 40Ar/39Ar geochronology from East Greenland and Faroe Island basalts, the Skaergaard intrusion is linked to eruption of the earliest of the Plateau Basalts in East Greenland (Milne Land Formation) during continental breakup and opening of the North Atlantic Ocean. This microanalytical study highlights the ability to conduct rapid Pb isotopic measurements in low-Pb concentration materials (e.g., <2 ppm Pb in plagioclase) at high spatial resolution by LA-ICP-MS with precision sufficient to constrain a range of magmatic processes in layered intrusions and other feldspar-bearing rocks preserved in the geological record.

Definition of tectono-sedimentary elements for rifted continental margins of the Norwegian and Greenland seas

Abstract The geology of the conjugate continental margins of the Norwegian and Greenland Seas reflects 400 Ma of post-Caledonian continental rifting, continental breakup between early Eocene and Miocene times, and subsequent passive margin conditions accompanying seafloor spreading. During Devonian-Carboniferous time, rifting and continental deposition prevailed, but from the mid-Carboniferous, rifting decreased and marine deposition commenced in the north culminating in a Late Permian open seaway as rifting resumed. The seaway became partly filled by Triassic and Lower Jurassic sediments causing mixed marine/non-marine deposition. A permanent, open seaway established by the end of the Early Jurassic and was followed by the development of an axial line of deep marine Cretaceous basins. The final, strong rift pulse of continental breakup occurred along a line oblique to the axis of these basins. The Jan Mayen Micro-Continent formed by resumed rifting in a part of the East Greenland margin in Eocene to Miocene times. This complex tectonic development is reflected in the sedimentary record in the two conjugate margins, which clearly shows their common pre-breakup geological development. The strong correlation between the two present margins is the basis for defining seven tectono-sedimentary elements (TSE) and establishing eight composite tectono-sedimentary elements (CTSE) in the region.

Crustal structure and lateral variations in the Gulf of Mexico conjugate margins: From rifting to break-up

The Gulf of Mexico opened as a Late Triassic-Mid Jurassic continental rift that was first largely covered by the Mid-Jurassic Louann Salt and later split apart by a triangular-shaped oceanic crust. Salt in the Gulf of Mexico largely hampers the imaging and interpretation of underlying pre-salt and crustal geometries, which are fundamental for assessing the early kinematic evolution of the margin. To better define these deep geometries and their lateral variations, we built three seismic-based crustal-scale cross-sections across the Florida-Yucatan conjugate margins, in the areas where the Louann Salt is thinner. They were used, together with magnetic and gravity anomalies data, to build a crustal domains map for the study area. Cross-sections show a meaningful along-strike variation: the South Florida-East Yucatan area is characterized by a narrower rifted continental crust that evolves sharply to oceanic crust whereas in the North Florida and central-western Yucatan areas, the rifted continental crust is wider and the transition to the oceanic crust corresponds to a narrow magmatic or exhumed mantle domain. Bulk continental crust extension was determined using the area balancing method. Estimated horizontal extension values vary from a minimum of ∼120 km in the South Florida-East Yucatan conjugate to a minimum of ∼240 km in the North Florida-Central Yucatan conjugate, being systematically larger in the northern margin. Based on our observations and considering previous models, we propose that the study area evolved from an early rift involving magmatism, to a magma-poor margin, with continental break-up (OCT formation) being characterized by mantle exhumation and associated magmatism along the North Florida and central-western Yucatan areas. As in other analogues such as the Gulf of Guinea, the thick evaporite deposited during the late rift, while mantle was being exhumed.

Moho carbonation at an ocean-continent transition

Abstract Carbonation of mantle rocks during mantle exhumation is reported in present-day oceanic settings, both at mid-ocean ridges and ocean-continent transitions (OCTs). However, the hydrothermal conditions of carbonation (i.e., fluid sources, thermal regimes) during mantle exhumation remain poorly constrained. We focus on an exceptionally well-preserved fossil OCT where mantle rocks have been exhumed and carbonated along a detachment fault from underneath the continent to the seafloor along a tectonic Moho. Stable isotope (oxygen and carbon) analyses on calcite indicate that carbonation resulted from the mixing between serpentinization-derived fluids at ~175°C and seawater. Strontium isotope compositions suggest interactions between seawater and the continental crust prior to carbonation. This shows that carbonation along the tectonic Moho occurs below the continental crust and prior to mantle exhumation at the seafloor during continental breakup.

Lithosphere Structure and Seismic Anisotropy Offshore Eastern North America: Implications for Continental Breakup and Ultra‐Slow Spreading Dynamics

AbstractThe breakup of supercontinent Pangea occurred ∼200 Ma forming the Eastern North American Margin (ENAM). Yet, the precise timing and mechanics of breakup and onset of seafloor spreading remain poorly constrained. We investigate the relict lithosphere offshore eastern North America using ambient‐noise Rayleigh‐wave phase velocity (12–32 s) and azimuthal anisotropy (17–32 s) at the ENAM Community Seismic Experiment (CSE). Incorporating previous constraints on crustal structure, we construct a shear velocity model for the crust and upper ∼60 km of the mantle beneath the ENAM‐CSE. A low‐velocity lid (VS of 4.4–4.55 km/s) is revealed in the upper 15–20 km of the mantle that extends ∼200 km from the margin, terminating at the Blake Spur Magnetic Anomaly (BSMA). East of the BSMA, velocities are fast (&gt;4.6 km/s) and characteristic of typical oceanic mantle lithosphere. We interpret the low‐velocity lid as stretched continental mantle lithosphere embedded with up to ∼15% retained gabbro. This implies that the BSMA marks successful breakup and onset of seafloor spreading ∼170 Ma, consistent with ENAM‐CSE active‐source studies that argue for breakup ∼25 Myr later than previously thought. We observe margin‐parallel Rayleigh‐wave azimuthal anisotropy (2%–4% peak‐to‐peak) in the lithosphere that approximately correlates with absolute plate motion (APM) at the time of spreading. We hypothesize that lithosphere formed during ultra‐slow seafloor spreading records APM‐modified olivine fabric rather than spreading‐parallel fabric typical of higher spreading rates. This work highlights the importance of present‐day passive margins for improving understanding of the fundamental rift‐to‐drift transition.

Assessing the Rotation and Segmentation of the Porcupine Bank, Irish Atlantic Margin, During Oblique Rifting Using Deformable Plate Reconstruction

AbstractPublished plate reconstructions have provided insights regarding the formation of the North Atlantic, in which the motion of the Porcupine Bank, on the Irish Atlantic margin, underlain by orogenic pre‐rift crustal basement terranes, is investigated and restored. Previous reconstructions of the Porcupine Bank mainly relied on potential field data rather than seismic constraints and failed to reveal the role of inherited crustal terranes during rifting and subsequent crustal deformation. In this study, five deformable plate tectonic models with distinct structural inheritance trends are established in GPlates by adjusting a previously published restoration model for the North Atlantic. For each model, driving factors such as the inclusion of the Orphan Knoll, the Flemish Cap poles of rotation, and the motion of the eastern border of the Porcupine Basin are also considered. To assess the validity of deformable plate models, crustal thickness estimates obtained from gravity inversion and seismic data modeling are compared with those calculated via deformable plate models. The preferred deformable plate model proposes the subdivision of the Porcupine Bank into four blocks with each block experiencing poly‐phased rotation and shearing prior to the final continental breakup, implying strong inheritance and segmentation of the Porcupine Bank and Porcupine Basin. The reconstructed paleo‐positions of the Flemish Cap and Porcupine Bank within deformable regions reveal evolving conjugate relationships during rifting, which are assessed using regional seismic transects from both margins. Finally, extensional obliquity between both margins is quantitatively restored, showing time‐variant orientations due to the rotation and shearing of associated continental blocks.

Back-arc underplating provided crustal accretion affecting topography and sedimentation in the Adria microplate

Supported by evidence of deep crustal sources for the observed magnetic anomalies in Central Italy and by outcropping gabbros in the Croatian archipelago, we model the observed gravity and magnetic anomalies in the Central Adriatic Sea and surroundings. We suggest that the major magnetic anomalies in the area are related to a wide underplating and propose that this volume represents the first stage of the back-arc Adria continental breakup in Early Permian times. During the Palaeotethys-Adria collision, underplating has controlled topography and palaeogeographic domains resulting in the observed asymmetrical sedimentary evolution since the Triassic across the Adria microplate. Finally, we propose that the Palaeotethys-Adria boundary in the Early Permian was similar to the current Pacific-Okhotsk plate boundary.

Mesozoic dike swarms in Borborema Province (NE Brazil): A structural analysis based on airborne geophysical data and field work

Mapping and analysis of dike swarms support a detailed understanding of the orientation, geometry, spatial distribution, geochronology, emplacement mechanism, structural control, and environments at the time of the magmatic events. Therefore, dike swarms have become a key tool for studying mantle plume activity, large igneous provinces, and continental breakup. Before the Pangea breakup, the Neoproterozoic Borborema Province (NE Brazil) was intensively intruded by sets of dike swarms, whose precise mapping and classification are essential for understanding the tectonic plate movements and evolution of the South American and African continents. In this sense, we promoted a regional mapping of dike swarms and their relationships between orientation, emplacement mechanism and structural control to provide a better understanding of the role of Mesozoic and Cenozoic large magmatic events in their tectonic evolution. We apply anomaly enhancement techniques to high-resolution airborne magnetic data to obtain the geometry and estimate the depth of causative sources. A structural analysis was carried out integrating magnetic patterns, field data and a compilation of previous geological maps to describe the detailed distribution of dike swarms in Borborema Province. Our study demonstrated that dike swarms extend over 5.4 × 105 km2. The 1388 mapped mafic dikes are grouped into three distinct dike swarms (DS): 1135 – Rio Ceará-Mirim DS; 168 – Canindé DS; and 85 – Riacho do Cordeiro DS. The dike swarms show three preferential trends: NE-SW, E-W and NW-SE. The geometry and structural analysis of the dikes allowed us to establish the paleostress field active at the time of intrusion. For the Rio Ceará-Mirim DS, the paleostress trajectories represent an N–S extension, rotating to NW-SE in the western portion of the swarm. For the Canindé DS, the paleostress orientation shows a NW-SE extension rotating to NE-SW. For the Riacho do Cordeiro DS, the trajectories are similar to those traced for the Rio Ceará-Mirim DS, showing a predominant NW-SE trend.

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.

Crustal architecture and evolution of the southwestern South China Sea: Implications to continental breakup

The South China Sea (SCS) is a marginal basin characterized by propagating breakup and subsequent seafloor spreading during the Cenozoic. Our study investigates the crustal architecture and tectonic evolution of the Xisha Islands - Northwest Borneo conjugate margins using a 1000 km long seismic profile and two shorter seismic lines. This system is characterized by wide margins showing a succession of rift basins with thinned continental crust, associated with large extensional detachment faults soling on top of the lower crust. In relation with crustal extension, several syn-rift sequences have been identified. The lower syn-rift unit (syn-rift I) shows well expressed wedge-shape growth strata. The upper one (syn-rift II) presents a more uniform thickness, and is slightly offset by normal faults dipping toward the depocenters. To illustrate the evolution of the southwestern SCS margins during continental breakup, we restore two conjugate sections near the oceanic magnetic anomaly C5En (18.8 Ma). The conjugate COT presents an asymmetrical hyper-extension architecture in cross sections, with the southern margin being the lower plate margin. In map view, the spatial variation of crustal structures illustrates a bayonet shape characterized by an initial N–S extension coeval with the first phase of seafloor spreading in the east sub-basin in the SCS. The extension direction changed shortly later (circa 23Ma) to NW-SE opening in the southwest sub-basin in relation to a ridge jump. We suggest that variable extension direction is linked to changes in the compressional tectonic regime along Borneo and Palawan.

Lateral coexistence of ductile and brittle deformation shapes magma-poor distal margins: An example from the West Iberia-Newfoundland margins

A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain.

Catching the Main Ethiopian Rift evolving towards plate divergence

Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.

Provenance of the Incipient Passive Margin of NW Laurentia (Neoproterozoic): Detrital Zircon from Continental Slope and Basin Floor Deposits of the Windermere Supergroup, Southern Canadian Cordillera

Abstract The origin of the passive margin forming the paleo-Pacific western edge of the ancestral North American continent (Laurentia) constrains the breakup of Rodinia and sets the stage for the Phanerozoic evolution of Laurentia. The Windermere Supergroup in the southern Canadian Cordillera records rift-to-drift sedimentation in the form of a prograding continental margin deposited between ~730 and 570 Ma. New U-Pb detrital zircon analysis from samples of the post-rift deposits shows that the ultimate source area was the shield of NW Laurentia and the near uniformity of age spectra are consistent with a stable continental drainage system. No western sediment source area was detected. Detrital zircon from postrift continental slope deposits are a proxy for ca. 676-656 Ma igneous activity in the Windermere basin, likely related to continental breakup, and set a maximum depositional age for slope deposits on the eastern side of the basin at 652±9 Ma. These results are consistent with previous interpretations. The St. Mary-Moyie fault zone near the Canada-U.S. border was most likely a major transform boundary separating a rifted continental margin to the north from intracratonic rift basins to the south, resolving north-south variations along western Laurentia in the late Neoproterozoic at approximately 650-600 Ma. For Rodinia reconstructions, the conjugate margin to the southern Canadian Cordillera would have a record of rifting between ~730 and 650 Ma followed by passive margin sedimentation.

Geodynamic events leading to formation of passive western continental margin of India

The geodynamic events of continental breakup and origin of northwest Indian Ocean led to the development of passive continental margin, off western India. However, causal mechanisms and relative chronology of these geodynamic events are not clearly known because of complex regional-scale ridges-basin physiography, multi-stage rifting in a short-time span and thick sediment cover. The Laxmi and adjacent Gop basins constitute key tectonic elements and geophysical investigations on them have come up with sharply divergent explanations of continental rifting and ocean spreading. We present geochemical results of the Laxmi Basin (LB) basement, recovered by the International Ocean Discovery Program Expedition-355 and interpreted in light of existing geophysical results. The basement is identified as continental rift basalt, different from the Deccan/Madagascan basalts. We suggest the basement eruption at ~75 Ma causing igneous underplating which triggered the extension/rifting in Laxmi and Gop basins. The rifting translated into ocean spreading only in the Gop Basin and not in the Laxmi Basin. The geodynamic events echoed soon with similar relative chronology in western India with Reunion plume impact and the Deccan eruption followed by second extension/rifting that culminated in India-Seychelles breakup.