It is becoming increasingly apparent that continental rifting, breakup, and ocean spreading contain significant complexities not easily explained by standard models. Recent discoveries of the importance of obliquity during rifting and continental material far offshore, such as beneath Iceland, the Comoros, Kerguelen, Jan Mayen and Mauritius, challenges conventional tectonic models. The coincidence of many regions of anomalous intraplate- or on-ridge volcanism with continental material, often detected geochemically, hints at imminent breakthroughs in our geodynamic understanding of the ocean floor and rifting processes. New models for the complex dynamics of continental breakup, including precursory deformation and magmatism, the role of shearing, structural inheritance, the structure and meaning of magnetic anomalies, the structural variability at passive margins, the development of spreading centres and the difficult birth of new oceans are required. These models must account for the complex features that are observed, including hybrid crust, marginal ridges, rift axis migration, isolated blocks of heavily rotated lithosphere in the ocean, anomalous bathymetry, and the geochemistry of lavas.
In this session, we explore the formation, evolution, structure, composition and underlying mechanisms controlling the formation of complex oceanic regions and continental margins. We seek case histories from around the globe addressing different geoscience disciplines, such as marine geophysics, seismology, ocean drilling, geochemistry, plate kinematics, tectonics, structural geology, numerical and analogue modelling, sedimentology and geochronology. We particularly encourage cross-disciplinary presentations, thought-provoking studies that challenge conventions, and submissions from early career researchers.
vPICO presentations: Tue, 27 Apr
In order to study opening mechanisms and their variation in the Atlantic ocean basins, we compiled existing wide-angle and deep seismic data along conjugate margins and performed plate tectonic reconstructions of the original opening geometries to define conjugate margin pairs. A total of 23 published wide-angle seismic profiles from the different margins of the Atlantic basin were digitized, and reconstructions at break-up and during early stages of opening were performed. Main objectives were to understand how magma-rich and magma-poor margins develop and to define more precisely the role of geologic inheritance (i.e., preexisting structures) in the break-up phase. At magma-poor margins, a phase of tectonic opening without accretion of a typical oceanic crust often follows initial rupture, leading to exhumation of serpentinized upper mantle material. Along volcanic margins the first oceanic crust can be overthickened, and both over- and underlain by volcanic products. The first proto-oceanic crust is often accreted at slow to very slow rates, and is thus of varied thickness, mantle content and volcanic overprint. Accretion of oceanic crust at slow to very slow spreading rates can also be highly asymmetric, so the proto oceanic crust at each side of conjugate margin pairs can differ. Another major aim of this study was to understand the mechanisms of formation and origins of transform marginal plateaus. These are bathymetric highs located at the border of two ocean basins of different ages and are mostly characterized by one or several volcanic phase during their formation. They often form conjugate pairs along a transform margin as it evolves and might have been the last land bridges during breakup, thereby influencing mammal migration and proto-oceanic currents in very young basins. At these plateaus, volcanic eruptions can lead to deposits of (at least in part subaerial) lava flows several km thick, better known by their geophysical signature as seaward dipping reflectors. Continental crust, if present, is heavily modified by volcanic intrusions. These marginal plateaus might form when rifting stops at barriers introduced by the transform margin, leading to the accumulation of heat in the mantle and increased volcanism directly before or after the cessation of rifting.
How to cite: Klingelhoefer, F., Biari, Y., Franke, D., Funck, T., Loncke, L., Sibuet, J.-C., Basile, C., Austin, J., Rigoti, C., Sahabi, M., Benabdellouahed, M., and Roest, W.: Deep structure of the Atlantic margins and neighboring oceanic crust from wide-angle seismic data and plate kinematic reconstructions., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4078, https://doi.org/10.5194/egusphere-egu21-4078, 2021.
The kinematics of the Iberian plate during Mesozoic extension and subsequent Alpine compression and their implications on the partitioning of strain experienced across the Iberia-Europe plate boundary continue to be a subject of scientific interest, and debate. To date, the majority of plate tectonic models only consider the motion of rigid tectonic plates. In addition, the lack of consideration for the kinematics of intra-continental domains and intervening continental blocks in-between has led to numerous discrepancies between rigid plate kinematic models of Iberia, based mainly on tight-fit reconstruction of M-series magnetic anomalies, and their ability to reconcile geological and geophysical observations. To address these discrepancies, deformable plate tectonic models constrained by previous plate reconstructions, geological, and geophysical studies are built using the GPlates software to study the evolution of deformation experienced along the Iberia-Eurasia plate boundary from the Triassic to present day. These deformable plate models consider the kinematics of small intra-continental blocks such as the Landes High and Ebro Block situated between large tectonic plates, their interplay with pre-existing structural trends, and the collective impact of these phenomena on the deformation experienced during Mesozoic rifting and Alpine compressional re-activation along the Iberia-European plate boundary. Preliminary results suggest that the independent kinematics of the Landes High played a key role on the distribution of oblique extension between different rift arms and resultant deformation within the Bay of Biscay. Within the Pyrenean realm, deformation experienced prior to and during the Alpine Orogeny was more largely controlled by the interplay between the Ebro Block kinematics and rift segmentation induced by the orientation of inherited trends.
How to cite: King, M., Welford, K., Cadenas, P., and Tugend, J.: Investigating the plate kinematics of continental blocks and their role on the deformation experienced along the Iberia-Eurasia plate boundary using deformable plate tectonic models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1402, https://doi.org/10.5194/egusphere-egu21-1402, 2021.
Models of Cenozoic plate motions between Greenland and North America often use magnetic anomalies in the Labrador Sea and Baffin Bay regions. The crustal origin of some of the older magnetic signatures, (pre C24, Paleocene) is questioned, and these models often portray Paleogene motions inconsistent with geological data from Nares Strait region. We test for a connection between the (mis)interpretation of anomalies and inconsistencies between model predictions and geological evidence by constructing a regional model that is not based on magnetic data in the Labrador Sea region. We do this by closing the North America – Greenland – Eurasian plate circuit from the Paleocene to Eocene – Oligocene Boundary (C25 – C13). Our findings show seafloor spreading in the Labrador Sea initiated during Eocene, and not Paleocene, times. In turn, we argue that C24 and older isochrons in the Labrador Sea are not suitable as isochron markers for modelling plate motions. We further show that the previously noted counterclockwise rotation of Greenland, marking the beginning of plate convergence in the eastern Canadian Arctic, is not a result of changes in seafloor spreading direction, but instead of the initiation of seafloor spreading in the Labrador Sea. Our model shows ~160km of shortening in the Eastern Canadian Arctic.
How to cite: Causer, A., Eagles, G., Pérez-Díaz, L., and Adam, J.: Cenozoic relative movements of Greenland and North America by closure of the North Atlantic – Arctic plate circuit, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2372, https://doi.org/10.5194/egusphere-egu21-2372, 2021.
Oceanic transform faults are seismically and tectonically active major plate boundaries. Their inactive traces are called fracture zones and may cross entire ocean basins. Plate tectonics idealizes transforms to be conservative two-dimensional strike-slip boundaries where lithosphere is neither created nor destroyed, and along which the lithosphere cools and deepens as a function of plate age. Here, we present constraints from a new compilation of high-resolution multibeam bathymetric data from 41 oceanic transforms covering all spreading rates. Statistical data show that all transform faults are considerably deeper than adjacent spreading segments and that the depth of transform valleys increases with decreasing spreading rate. The trend of increasing transform depth seems to be governed by age-offset. Further, accretion at ridge-transform intersections appears strongly asymmetric, with outside corners showing shallower relief and more extensive magmatism while inside corners have deep nodal basins and appear magmatically starved. We use a three-dimensional viscoplastic numerical model to survey the relationship between transform depth and age-offset and use high-resolution bathymetric data to study the interaction between adjacent spreading segments and transform faults at their intersection, the ridge-transform intersection or RTI. Our global compilation of multibeam bathymetry suggest that processes acting at RTIs are independent of spreading rate, contradicting deductions from gravity field observations which seemed to imply a strong spreading rate dependence of processes shaping transform faults and fracture zones.
How to cite: Grevemeyer, I., Rüpke, L., Morgan, J., Iyper, K., and Devey, C.: Transform faults revisited- a global approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2261, https://doi.org/10.5194/egusphere-egu21-2261, 2021.
The Gulf of Mexico is an intraplate oceanic basin where rifting started in the Late Triassic, leading to drifting by Middle Jurassic and ensuing oceanic accretion, which ceased by the Early Cretaceous. Its tectonic evolution encompasses multiple rifting phases dominated by orthogonal extension, major strike-slip structures, transtensional basins, variable magmatism, and salt deposition. This complex tectonic history is captured in the rifted margins of the Gulf of Mexico, especially along the eastern part of the basin; where considerable debate remains regarding the crustal configuration and tectonic evolution.
This study presents new insights into the crustal types and an updated tectonic framework for the Florida margin. An integrated analysis of seismic, gravity, and magnetic data allows us to characterise the continental crust, which shows wide zones of hyperextension that we relate to pull-apart basins, magmatic underplating, seaward dipping reflection (SDR) packages, and a narrow zone of exhumed mantle. In addition, we identified NW-SE trending sinistral strike-slip faults altering the typical crustal configuration expected in a rifted margin.
Our results suggest the need for a new plate model of the Florida margin at the Eastern Gulf of Mexico that invokes the polyphase rifting, accounts for the Yucatan’s block counter-clockwise rotation, explains the increase in magma supply, and captures the influence of strike-slip faults on the crustal boundaries and the magmatic budget.
How to cite: Vasileiou, A., Gouiza, M., Mortimer, E., Paton, D., Nanfito, A., and Lewis, D.: New insights into the crustal architecture and tectonic evolution of the Eastern Gulf of Mexico., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8762, https://doi.org/10.5194/egusphere-egu21-8762, 2021.
During evolution of the South Sandwich subduction zone, which has consumed South American plate oceanic lithosphere, somehow continental crust of both the South American and Antarctic plates have become incorporated into its upper plate. Continental fragments of both plates are currently separated by small oceanic basins in the upper plate above the South Sandwich subduction zone, in the Scotia Sea region, but how fragments of both continents became incorporated in the same upper plate remains enigmatic. Here we present an updated kinematic reconstruction of the Scotia Sea region using the latest published marine magnetic anomaly constraints, and place this in a South America-Africa-Antarctica plate circuit in which we take intracontinental deformation into account. We show that a change in fracture zone orientation in the Weddell Sea requires that previously inferred initiation of subduction of South American oceanic crust of the northern Weddell below the eastern margin of South Orkney Islands continental crust, then still attached to the Antarctic Peninsula, already occurred around 80 Ma. We propose that subsequently, between ~71-50 Ma, the trench propagated northwards into South America by delamination of South American lithosphere: this resulted in the transfer of delaminated South American continental crust to the overriding plate of the South Sandwich subduction zone. We show continental delamination may have been facilitated by absolute southward motion of South America that was resisted by South Sandwich slab dragging. Pre-drift extension preceding the oceanic Scotia Sea basins led around 50 Ma to opening of the Drake Passage, preconditioning the southern ocean for the Antarctic Circumpolar Current. This 50 Ma extension was concurrent with a strong change in absolute plate motion of the South American Plate that changed from S to WNW, leading to upper plate retreat relative to the more or less mantle stationary South Sandwich Trench that did not partake in the absolute plate motion change. While subduction continued, this mantle-stationary trench setting lasted until ~30 Ma, after which rollback started to contribute to back-arc extension. We find that roll-back and upper plate retreat have contributed more or less equally to the total amount of ~2000 km of extension accommodated in the Scotia Sea basins. We highlight that viewing tectonic motions in a context of absolute plate motion is key for identifying slab motion (e.g. rollback, trench-parallel slab dragging) and consequently mantle-forcing of geological processes.
How to cite: van de Lagemaat, S., Swart, M., Vaes, B., Kosters, M., Boschman, L., Burton-Johnson, A., Bijl, P., Spakman, W., and van Hinsbergen, D.: Subduction initiation in the Scotia Sea region and opening of the Drake Passage: when and why?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2816, https://doi.org/10.5194/egusphere-egu21-2816, 2021.
The North West Shelf of Australia has experienced numerous rift events during its prolonged evolution that most likely started in the Lower Palaeozoic and continued through to the formation of the present day passive margin in the Lower Cretaceous. Carboniferous and Permian is associated with rifting of the Lhasa terrane, a phase extension in the Lower and Middle Jurassic associated with the separation of the Argo terrane Upper Jurassic to Lower Cretaceous extension culminated in the separation of Greater India and Australia. Investigations based on interpretation of extensive, public domain seismic data, combined with numerical mechanical modelling, demonstrate that crustal structure, rheology and structural fabrics inherited from older events exert a significant control on the architecture of younger rifts.
Defining the older, more deeply buried rift episodes is challenging, but with seismic data that now images deeper structures more effectively, it is clear that NE-SW oriented Carboniferous to Permian aged rift structures control the overall geometry of the margin. Variations in the timing, distribution and intensity of that rift may account for some of the complexity that governs the Triassic – a failed arm of the rift system might account for the accumulation of thick sequences of fluvio-delatic sediments in an apparent post-rift setting, while active deformation and igneous activity continued elsewhere on the margin.
A renewed phase of extension began in the latest Triassic in the western part of the Northern Carnarvon Basin, but became progressively younger to the NE. High-resolution mechanical numerical experiments show that the dual mode of extension that characterises the Northern Carnarvon Basin, where both distributed and localised deformation occurs at the same time, is best explained by necking and boudinage of strong lower crust, inherited form the Permian rift event, proximal to the continental margin, and a subdued extensional strain rate across the distal extended margin. A very clear and consistent pattern of ENE oriented extension, which interacts obliquely with the older NE-SW oriented Permian aged structures, is apparent across the whole of the Northern Carnarvon Basin and extends north east into the Roebuck and Browse Basins. This is at odds with the NW-SE oriented extension predicted by the separation of the Argo terrane which occurs at this time. This may be explained by the detached style of deformation that characterises the Mesozoic interval. Alternatively, the separation of Greater India may have exerted a stronger influence on the evolution of the margin during the Jurassic than hitherto recognised.
How to cite: Elders, C. and Moron, S.: Multiphase oblique extension on the North West Shelf of Australia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14515, https://doi.org/10.5194/egusphere-egu21-14515, 2021.
Recent advances in knowledge have led to the recognition of continental crust beneath the Comoros islands offshore East Africa and conflicting fracture zone patterns in isolated regions of the Indian ocean. Furthermore, whilst the presence of continental crust within the Davis Straight has been known for some time, its origin remains debated.
Here, using gravity lineament analysis, plate kinematic modelling, seismic reflection interpretations, and 3D crustal thickness inversions (constrained by a new composite sedimentary thickness dataset), we investigate the origin of microcontinents and proto-subduction events in the Western Somali Basin, Indian Ocean, and the Labrador Sea. We find the role of plate motion changes, which induce transpression along active transform faults, play a critical role in the cleaving of the Comoros microcontinent and inducing previously poorly understood plate convergence and missing crustal sections along the Chain and Owen ridges. Furthermore, the temporal and spatial patterns of thrust and normal faulting in the Davis Straight indicates an analogous mechanism emplaced continental crust in this region, suggesting a generic and predictable mechanism may be applicable to the production of this type of microcontinent around the globe. The Davis Straight proto microcontinent (i.e., incompletely rifted microcontinent) began development during the 53 Ma spreading axis reorientation and ceased separation at 33 Ma, when the basin became extinct. We postulate that the extinction of ocean spreading in the Labrador Sea, and possibly also the Western Somali Basin, may have been influenced by increasing transpression across long-offset fracture zones and suggest further study of this phenomenon.
How to cite: Longley, L., Martin, L., Satchwell, B., Tomlinson, L., and Phethean, J.: Microcontinent cleaving and enigmatic proto subduction events (?): The role of transpressional transforms in plate tectonics of the Indian and North Atlantic Oceans, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5049, https://doi.org/10.5194/egusphere-egu21-5049, 2021.
Orthogonal, oblique and transform rifted margins are defined by the comparison of the structural trend of the margin versus the orientation of the oceanic spreading ridge marked by marine magnetic anomalies. However, when neither transform fault nor marine magnetic anomalies can be identified in the oceanic domain, the determination of the obliquity of extension is delicate and deduced from the architecture of the rifted margins. This setting is illustrated by the Eastern Mediterranean Sea, which is a relic of an oceanic domain, now partly subducted northward underneath Anatolian, Aegean and Calabrian domains. Although the Southern and Eastern margins, from Malta to Lebanon, escaped compressional reactivation during Late Cretaceous and Cenozoic, their potential orthogonal, oblique or transform components have been the subject of extensive debates. Multiple geodynamic scenarios implying different ages and directions of oceanic opening have been proposed suggesting that either the southern or the eastern margins had a transform motion (or highly oblique).
In this contribution, we investigate the architecture of the different margin segments using 2D and 3D seismic data combined with available stratigraphic records and potential field maps. Based on these observations, we identified and mapped the different rift domains of the Eastern Mediterranean margins, adapting the terminology developed for hyper-extended rifted margins. The Eastern Mediterranean rifted margins are characterized by Mesozoic thick post-rift carbonate platforms developed over moderately thinned continental crust. Distal domains are dominated by thick sedimentary basins (>10 km) where the top basement is barely visible on reflection seismic data. Between the carbonate platform and the distal basin, the transition is always sharp (<30km in width) and marked by large normal faults. The resulting rift domain map highlights different structural trends, which are not coherent with a simple pair orthogonal-transform margins. Moreover, we reconstructed the extensional evolution of the former Northern and Western conjugate margins, which are now integrated in the Alps, Balkanides, Hellenides and Taurides by compiling boreholes and onshore geological data. These fossil margins recorded evidence for different tectonic extensional phases from Permian to Cretaceous.
Our preliminary conclusion suggests that poly-phased and poly-directional extension led to distinct breakup ages in the Herodotus and northern Levant Basins. It results in the superposition of extensional structures of different orientations and ages, which inhibit the clear determination of orthogonal, oblique or transform margins. We tentatively explain this architectural complexity by the close position of the East Mediterranean Sea to the migrating rotation pole between Africa and Eurasia during the Mesozoic in relation with the Central Atlantic spreading to the West and the multiple subduction systems of the Neo-Tethys to the North.
How to cite: Nirrengarten, M., Mohn, G., Sapin, F., Charlotte, N., and Tugend, J.: Obliquity of the Eastern Mediterranean Sea rifted margins: Reconciling structures and kinematic models. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2885, https://doi.org/10.5194/egusphere-egu21-2885, 2021.
The oceanic crust and lithosphere are commonly treated as geologically simple, their fundamental properties encapsulated by the 1D model of layered oceanic crust and the plate-cooling model of lithospheric thickness we learnt as undergraduates. The question of directionality or anisotropy in the behaviour and deeper structure of oceanic plates is relatively rarely considered, despite formation processes, such as rifting and seafloor spreading, and surface topography, such as abyssal hills, that are clearly highly anisotropic. In this presentation, we bring together evidence from a variety of sources from regional studies of rifting and volcanism to numerical modelling and global analyses of bathymetry and gravity data. We show how anisotropy is imprinted into the oceanic lithosphere at formation, both in the early rifting phases and at mature spreading centres, and how that anisotropic signature persists for many millions of years, potentially strengthened by preferential alignment of mineral phases as the moving plates cool and thicken. We then consider how this directionality impacts later deformation, volcanism, and eventually subduction.
How to cite: Kalnins, L. M., Simons, F. J., Farangitakis, G.-P., and Richards, F. D.: The role of anisotropy in oceanic lithosphere from 'cradle to grave', EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10747, https://doi.org/10.5194/egusphere-egu21-10747, 2021.
The Owen oceanic transform fault is a 300-km long linear structure connecting the Carlsberg and Sheba spreading centers in the northwest Indian Ocean. It presently forms with the Carlsberg ridge the active plate boundary between India and Somalia. The Owen transform fault accommodates the left-lateral strike-slip motion between India and Somalia at a rate of about 23 mm/yr. Firstly identified by Tuzo Wilson in the 60s, this oceanic transform remains poorly described. The fault was recently surveyed in the Spring of 2019 during the VARUNA and CARLMAG cruises (https://doi.org/10.17600/18001108, https://doi.org/10.17600/18000872) along its entire length aboard BHO Beautemps-Beaupré, an oceanographic ship operated by the French Naval Hydrographic and Oceanographic Services (SHOM) and the French Navy.
During these missions a set of high-resolution seismic lines (>5000 km) were acquired together with high resolution multibeam bathymetry. The data cover both the active and fossil traces of the transform fault between 9°N and 15°N, at a place where continuous deposition of the distal Indus turbiditic sediments offers a unique high-resolution stratigraphic record of past regional tectonic events.
The new bathymetric mapping reveals two remarkable transpressive ridges on the active fault trace. A precise stratigraphic work using seismic profiles and drilling data of the ODP leg 117 allows the time calibration of the new seismic lines as far south as the Carlsberg ridge.
We show that a major compressive event occurred on the Owen Oceanic Transform Fault recently between 1.5 Ma and 2.4 Ma. Compression is still active today as evidenced by Sub-bottom profiler data (3.5 kHz) and two compressive focal mechanisms found in the historical seismicity records. At the intersection with the Carlsberg ridge, the southern transpressive ridge bends and stands ~1200 m above the seafloor at its apex, suggesting a maximum surrection rate near 800 m/Ma. These new geophysical dataset combined with previous cruises offers an unprecedented window on the recent evolution of the India-Somalia plate boundary.
How to cite: Janin, A., Rodriguez, M., Chamot-Rooke, N., Rabaute, A., Delescluse, M., Dyment, J., Fournier, M., Huchon, P., Olive, J.-A., and Vigny, C.: Active transpression along the Owen oceanic transform fault, India - Somalia plate boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3285, https://doi.org/10.5194/egusphere-egu21-3285, 2021.
The northern border of the Caribbean plate is characterized by the oblique collision between the Caribbean and North American tectonic plates. Increasing obliquity of the collision between these two plates lead to complex strike-slip fault zones, which successively jump southward to accommodate the eastward escape of the Caribbean plate and the collisional indentation against the Bahama carbonate platform. The present-day Septentrional–Oriente Fault zone (SOFZ) defines the northern limit of the Caribbean plate, accommodating much of the obliquity of the convergence. Since its inception, at the end of the Oligocene, the current active style of the strike-slip boundary evolves over time. We focus our study on the Windward Passage area between the south-east of Cuba and the north-west of Haiti coast. Currently crossed by the SOFZ, the tectono-sedimentary framework of this large strait displays critical evidences to constrain the Neogene evolution of the northern boundary of the Caribbean plate. Based on seismic reflection and swath-bathymetric dataset we shed light on the structure and tectonic pattern of the Windward Passage. Our study provides structural and stratigraphic insights into relative timing of deformation along the Windward Passage and new elements to constrain the southeastward shift of the north Caribbean plate boundary until its present-day position. Contrasts in patterns of deformation on the Windward Passage area reveal a polyphase tectonic history of dominant strike-slip faulting impacted by the rate and obliquity variations of the convergence. Deformation phases recorded by the offshore sedimentary cover in the Windward Passage correlate well with the major paleogeographic reorganization episodes described onland (Late Eocene, Late Oligocene, Middle Miocene and Late Pliocene). A left-lateral shift of at least ~80 km is demonstrated by the restoration of the offset of the seismic units, estimating a Pliocene age for the onset of the SOFZ segments activity in this area.
How to cite: Oliveira de Sa, A., d’Acremont, E., Leroy, S., and Lafuerza, S.: The Septentrional–Oriente strike-slip Fault Zone: polyphase deformation and fault strand switching in the Northern Caribbean plate boundary (Windward Passage), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5512, https://doi.org/10.5194/egusphere-egu21-5512, 2021.
Under the influence of the three global tectonic systems of the Paleo-Asian, Pacific and Tethyan dynamic systems, East Asia underwent diverse assemblies of many minor plates, blocks, micro-blocks or micro-plates after the Triassic. We refer to these assemblies as super-convergence related to the Supercontinent Amersia over the subsequent 300 Ma. Three cratons in China – the North China, South China and Tarim blocks – form the center of this super-convergent region. The peak of the super-convergence event is the Yanshannian Movement, which occurred in the Jurassic. This was related to the breakup of the Supercontinent Pangea and the assembly of the future Supercontinent Amersia (Pangea Ultima).
Opening of the South China Sea Basin in the Cenozoic is thought to have been driven by two tectonic systems, the western Pacific Subduction Zone and the Neo-Tethyan Collision-Subduction System. Its tectonic setting is different from that of the North Atlantic. Since 16 Ma, the Cenozoic South China Sea has been closing in the tectonic setting of the circum-East Asian subduction system. Closing of the South China Sea Basin indicates the initial assembly of the Supercontinent Amersia. Tomographic images show the Pacific slab in the mantle transition zone is broken into many mantle micro-blocks and developed later than 30 Ma although its ages are 90 - 130 Ma. This indicates the super-convergence must be driven by powerful forces that fragment the single large-scale oceanic plate.
The Atlantic Ocean has been opening since 150 Ma, from south to north. It is related to the breakup of the Supercontinent Pangea. Its opening mechanism has been much discussed. The two main models are a) a chain of deep-mantle plumes along which the mid-Atlantic ridge formed, and b) “back-arc” extension behind the Alpine subduction zones.
It is unlikely that Pangea was a young supercontinent that emerged from an earlier Proto-Pangea (Sanzhong Li et al., 2018). Instead, it is likely an intermediate stage of the long process of supercontinent evolution from Proto-Pangea to a future Amersia.
How to cite: Li, S., Suo, Y., Foulger, G., Jiang, Z., and Liu, Y.: Superdivergence of the Atlantic Ocean and Superconvergence around the South China Sea: A Comparison, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8581, https://doi.org/10.5194/egusphere-egu21-8581, 2021.
The Orphan Basin on the eastern edge of the Newfoundland continental margin formed as a Mesozoic rift basin prior to continental breakup associated with the opening of the North Atlantic. Few exploration wells exist in the basin, and until recently regional interpretations have been based on sparse seismic data coverage - because of this the structural evolution of the Orphan Basin has historically not been well understood. Key uncertainties include the timing and amount of rift-related extension, dominant extension directions, and the structural styles that accommodated progressive rift development in the basin.
Interpretation of newly acquired modern broadband seismic data and structural restoration of three regional, WNW-ESE oriented cross-sections across the Orphan Basin and Flemish Cap provide new insights into rift evolution and structural style in the area. Our results show that major extension in the basin occurred between 167 Ma and 135 Ma, with most extension occurring prior to 151 Ma. We show that extension after 135 Ma largely occurred east of Flemish Cap due to a shift in the locus of rifting from the Orphan Basin to east of Flemish Cap. We find no evidence for discrete rifting events in the Orphan Basin, as has been suggested by other authors. Kinematic restoration and associated heave measurements for the Orphan Basin show that extension was both widespread and relatively evenly distributed across the basin from Middle-Late Jurassic to Early Cretaceous.
We provide evidence for more widespread deposition of Jurassic strata throughout the Orphan Basin than previously interpreted, and show that Jurassic deposition was controlled by the occurrence and displacement of crustal-scale extensional detachment faults. Structure in the three regional cross sections is dominated by large-scale, shallowly dipping extensional detachment faults. These faults mainly dip to the northwest and control the geometry and position of extensional basins – grabens and half-grabens – which occur at a range of scales. Stacked detachment surfaces, hyperextension, and attenuation of the crust are observed in central and eastern parts of the Orphan Basin. Zones of extreme crustal attenuation (to ca. 3.7 km) are interpreted to be coincident with large-displacement (up to 60 km) low-angle detachments. Results from crustal area balancing suggest that up to 41% of extension is not recognized through structural seismic interpretation, which we attribute to subseismic-scale ductile and brittle deformation, and uncertainties in the identification of detachment surfaces or complex structural configurations (e.g., overprinting of early extensional deformation).
Rifting style in the central, northern, and eastern parts of the Orphan Basin is dominated by low-angle detachment faulting with maximum extension perpendicular to the incipient rift axis. In contrast, structural geometries in the southwestern part of the basin are suggestive of transtensional deformation, and interplay of normal and strike-slip faulting. Results from map-based interpretation show that strike-slip faults within this transtensional zone are associated with displacement transfer between half-grabens of opposing polarity, rather than regional strike-slip displacement. These structures are interpreted as contemporaneous and kinematically linked to displacement along low-angle detachment surfaces elsewhere, and are not attributed to distinct episodes of oblique extension.
How to cite: Cawood, A. J., Ferrill, D. A., Morris, A. P., Norris, D., McCallum, D., Gillis, E., and Smart, K. J.: Rift evolution and structural style in the Orphan Basin: results from regional structural restorations and crustal area balancing , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6527, https://doi.org/10.5194/egusphere-egu21-6527, 2021.
Magnetite formation of serpentinized ultramafic rocks leads to variations in the magnetic properties of serpentinites; however, magnetite precipitation is still on debate.
In this work, we analyzed 60 cores of ultramafic rocks with a variety of serpentinization degrees. These rocks belong to the ultramafic-mafic San Juan de Otates complex in Guanajuato, Mexico. Geochemical studies have been previously conducted, enabling us to compare changes in the magnetic properties against the chemical variations generated by the serpentinization process. By studying the density and magnetic properties such as anisotropy of magnetic susceptibility, hysteresis curves as well as magnetic and temperature-dependent susceptibility and, we were able to identify the relationship between magnetic content and serpentinization degree, the predominant magnetic carrier, and to what extent the magnetite grain size depends on the serpentinization. Variations in these parameters allowed us to better constrain the temperature at which serpentinization occurred, the generation of other Fe-rich phases such as Fe-brucite and/or Fe-rich serpentine as well as distinctive rock textures formed at different serpentinization degrees.
How to cite: Ramírez-García, S. B. and Alva-Valdivia, L. M.: Variations in magnetic properties of Mexican serpentinites and related micro-textures , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8554, https://doi.org/10.5194/egusphere-egu21-8554, 2021.
Oblique rift systems form when the axis of rifting is not orthogonal to the direction of tectonic extension, normally due to pre-existing zones of weakness that influence the location and orientation of new faults. Irrespective of the regional-scale obliquity, most individual extensional faults will tend to nucleate according to the orientation of the tectonic stress orientations, and therefore normal to the direction of maximum extension. Transfer faults in oblique systems will tend to form parallel to the direction of extension and, in contrast to orthogonal rifting, will play a major role in the architecture and development of the rift and its sedimentary basins.
An intriguing feature in oblique rift systems is the formation of reverse structures evocative of wrench tectonics during the syn-rifting stage. This stems from the orientation of geological structures relative to the direction of tectonic extension. Even slight changes in tectonic transport direction or stress orientations during the development of the rift system can lead to events of transpression or transtension along transfer structures. Because of the relevance of transfer structures in oblique systems, transpression can result in the appearance of discontinuities in the sedimentary record that are often interpreted as, somewhat incongruent, inversion events.
Oblique structures also play a crucial role during the full inversion of the rift system during convergence, particularly so because tectonic shortening will strike at an angle to the orientation of faults. Irrespective of the evolution of oblique rifting and inversion, the initial fault pattern is also normally preserved in fully inverted systems involved in fold-and-thrust systems. In many of cases, when the original rift obliquity is not well understood, the characteristic rhomboidal pattern is interpreted to relate to wrench tectonics. In this presentation we will review evidence from Iberia, Northwestern Africa and the Eastern Alps to discuss the role that obliquity plays in rift development and its inheritance in fold-and-thrust belts with different degrees of inversion.
How to cite: Fernández, O., Ramos, A., García-Senz, J., and Pedrera, A.: Control of obliquity directions on structural development, from rifting to inversion: Examples from the Tethyan domain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16343, https://doi.org/10.5194/egusphere-egu21-16343, 2021.
The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin.
As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge.
Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.
The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.
The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.
How to cite: Jegen, A., Dannowski, A., Kopp, H., Barckhausen, U., Heyde, I., Schnabel, M., Schmid, F., Beniest, A., and Hannington, M.: Dynamics of the extension in the Fonualei Rift in the northern Lau Basin at 16 °S, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7605, https://doi.org/10.5194/egusphere-egu21-7605, 2021.
The active volcanic arcs of the Scotia Plate and Caribbean Plate are two prominent features along the otherwise passive margins of the Atlantic Ocean, where subduction processes of oceanic crust is verifiable. Both arcs have been, and continue to be, important oceanic gateways during their formation. Trapped between the large continental plates of North- and South America, as well as Antarctica, the two significantly smaller oceanic plates show striking similarities in size, shape, plate margins and morphology, although formed at different times and locations during Earth’s history.
Structural analyses of the seafloor are based on bathymetric datasets by multibeam-echosounders (MBES), including data of the Global Multi Resolution Topography (GMRT), Alfred Wegener Institute (AWI), MARUM/Uni-Bremen, Geomar/Uni-Kiel, Uni-Hamburg and the British Antarctic Survey (BAS). Bathymetric data were processed to create maps of ocean floor morphology with resolution of 150-250 meters in accuracy. The Benthic Terrain Modeler 3.0 (BTM), amongst other GIS based tools, was utilized to analyse the geomorphometry of both plates. Furthermore, we used the bathymetric datasets for three-dimensional modelling of the seafloor to examine large-scale-structures in more detail.
The modelling of ship-based bathymetric datasets, in combination with the GEBCO 2014 global 30 arc-second interval grid, included in the GMRT bathymetric database, delivered detailed bathymetric maps of the study area. With the help of the fine- and broad-scale bathymetric position index (BPI), comparable to the topographic position index (Weiss, 2001), we present the first detailed interpretation of combined bathymetric datasets of the Scotia Sea, including the entire Scotia Plate and adjacent areas, such as the East Scotia Plate. We identified typical morphological features of the abyss, based on the determination of steep and broad slopes, ridges, boulders, flat plains or flat ridge tops and depressions in various scales. Additional data analyses of gravimetric and magnetic properties of the crust should help to understand the plate tectonic history of both areas in more detail.
Ryan, W. B. F; Carbotte, S.M.; Coplan, J.; O'Hara, S.; Melkonian, A.; Arko, R.; Weissel, R.A.; Ferrini, V.; Goodwillie, A.; Nitsche, F.; Bonczkowski, J. and Zemsky, R. (2009): Global Multi-Resolution Topography (GMRT) synthesis data set, Geochem. Geophys. Geosyst., 10, Q03014, doi:10.1029/2008GC002332.
Walbridge, S.; Slocum, N.; Pobuda, M.; Wright, D.J. (2018): Unified Geomorphological Analysis Workflows with Benthic Terrain Modeler. Geosciences 2018, 8, 94. doi: 10.3390/geosciences8030094
Weiss, A. D. (2001): Topographic Positions and Landforms Analysis (Conference Poster). Proceedings of the 21st Annual ESRI User Conference. San Diego, CA, July 9-13.
How to cite: Burmeister, C., Wintersteller, P., and Meschede, M.: Similarities of the Scotia and Caribbean Plates: Implications for a common plate tectonic history?!, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16364, https://doi.org/10.5194/egusphere-egu21-16364, 2021.
In the West Somali Basin, the classic plate tectonic reconstructions describe an early Cretaceous intraplate deformation of oceanic crust (Hauterivian to Aptian) followed by the activation of a major transform fault (Davie Fracture Zone) displacing Madagascar southward for more than 1000 km. In this contribution, using vintage and new high-resolution 2D, 3D seismic reflection data and exploration wells, we show the first clear images of a poorly known tectonic structure: the Seagap fault. The Seagap fault is represented by a complex fault zone of several hundred kilometres of extent, oriented parallel to the Davie Fracture Zone and defined by segment faults, relay zones and step overs structures. It appears to have continuously acted as left-lateral strike slip fault during the Paleogene and most of the Neogene. From structural and stratigraphic observations of both existing and newly interpreted 3D seismic data, the Seagap appears nucleating as a strike-slip fault by reactivating failed Jurassic oceanic spreading zones. At regional scale the main fault appears to cut the main Neogene pervasive extensional oblique rift structures and at place to re-work some of the major Cenozoic inherited structure, creating apparent restraining bend structure. The sinistral kinematic nature of the transcurrent history, suggests that the Seagap fault acted as an independent feature respect to the Davie Fracture Zone. During the Quaternary the Seagap, which also parallels the seismically active Kerimbas rift, shows reduced offsets and appears to slip with normal displacement. We discuss the tectonic significance of the Seagap fault with respect to both to the major extensional oblique rift structural trend offshore Tanzania and the Davie Fracture Zone.
How to cite: Iacopini, D., Tavani, S., Pentagallo, S., Ebinger, C., Dottore Stagna, M., Reynolds, D., and Maselli, V.: Architecture and Neogene kinematic of the Seagap fault, offshore Tanzania, West Somali Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8991, https://doi.org/10.5194/egusphere-egu21-8991, 2021.
The NE Atlantic formed by complex, piecemeal breakup of Pangea in an environment of structural complexity. North of the present-day latitude of Iceland the ocean opened by southward propagation of the Aegir ridge. South of the present-day latitude of Iceland breakup occurred along the proto-Reykjanes ridge which formed laterally offset by ~ 100 km from the Aegir ridge to the north. Neither of these new breakup axes were able to propagate across the east-westerly striking Caledonian frontal thrust region which formed a strong barrier ~ 400 km wide. As a result, while sea-floor spreading widened the NE Atlantic, the Caledonian front region could only keep pace by diffuse stretching of the continental crust, which formed the aseismic Greenland-Iceland-Faroe ridge. The magmatic rate there was similar to that of the ridges to the north and south and so the stretched continental crust is now blanketed by thick mafic flows and intrusions. The NE Atlantic also contains a magma-inflated microcontinent – the Jan Mayen Microplate Complex, and an unknown but probably large amount of stretched continental crust blanketed by seaward-dipping reflectors in the passive margins of Norway and Greenland. The NE Atlantic thus contains voluminous continental crust in diverse forms and settings. If even a small portion of the sunken continental material contiguous with the Greenland-Iceland-Faroe ridge is included the area exceeds a million square kilometers, an arbitrary threshold suggested to designate a sunken continent. We have called this region Icelandia. The conditions and processes that funneled large quantities of continental crust into the NE Atlantic ocean are common elsewhere. This includes much of the North and South Atlantic oceans including both the seaboards and the deep oceans. Nor are such processes and outcomes confined to oceans bordered by passive margins. They are also found around the Pacific rims where subduction is in progress. Indeed, these conditions and processes likely are generic to essentially all the world's oceans and are potentially also informed by observations from intracontinental extensional regions and land-locked seas.
How to cite: Foulger, G., Gernigon, L., and Geoffroy, L.: Icelandia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13797, https://doi.org/10.5194/egusphere-egu21-13797, 2021.
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