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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.

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.

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.

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.