TS8.1

Fracture zones were recognized to be an integral part of the seabed long before plate tectonics was established. Later, plate tectonics linked fracture zones to oceanic transform faults, suggesting that they are the inactive and hence fossil trace of transforms. Yet, scientist have spent little time surveying them in much detail over the last three decades. Recent evidence (Grevemeyer, I., Rüpke, L.H., Morgan, J.P., Iyer, K, and Devey, C.W., 2021, Extensional tectonics and two-stage crustal accretion at oceanic transform faults, Nature, 591, 402–407, doi:10.1038/s41586-021-03278-9) suggests that the traditional concept of transform faults as being conservative (non-accretionary) plate boundary faults might be wrong. Instead, transform faults are always deeper than the associated fracture zones and numerical modelling results suggest that transform faults seem to suffer from extensional tectonics below their strike-slip surface fault zone. During the cruise M170 of the German research vessel METEOR early in 2021, we aimed to test this hypothesis by collecting, in a pilot study, micro-seismicity data from the Oceanographer transform fault which offsets the Mid-Atlantic Ridge by 120-km south of the Azores near 35°N. Preliminary analysis of 10-days of seismicity data recorded at 26 ocean-bottom-seismometers and hydrophones showed 10-15 local earthquakes per day. Along the transform fault the distribution of micro-earthquakes and focal mechanisms support strike-slip motion. However, at both ridge-transform intersections seismicity does not mimic a right-angular plate boundary; instead, seismicity occurs below the inside corner and focal mechanism indicate extensional tectonics. Therefore, micro-seismicity supports features found in numerical simulations, revealing that transform faults have an extensional as well as a strike-slip component.

How to cite: Grevemeyer, I., Lange, D., Klaucke, I., Beniest, A., Gómez de la Peña, L., Ren, Y., Hilbert, H.-S., Li, Y., Murray-Bergquist, L., Unger, K., Devey, C. W., and Ruepke, L.: The Oceanographer transform fault revisited – preliminary results from a micro-seismicity survey reveals extensional tectonics at ridge-transform intersections, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2279, https://doi.org/10.5194/egusphere-egu22-2279, 2022.

Oceanic Transform faults are one of the three major tectonic plate boundaries and yet their evolution and deformational mechanism is not well understood. They are broadly considered to be dominated by strike-slip displacement along simple planar vertical faults and to be conservative in nature with no magmatic addition. Observations from Pre-Stack depth-migrated (PSDM) 3D seismic of Cretaceous-aged transforms in the eastern Gulf of Guinea allow complex internal architectures to be described, including crustal scale detachments and rotated packages of volcanics.

These insights demonstrate additional complexity previously only predicted in numerical simulations of spreading ridge-transform interaction, namely intra-transform extension at a high angle to the spreading orientation, and the addition of significant extrusive volcanic material. In the study area of São Tomé and Príncipe, several Oceanic Fracture Zones (OFZ) are identified, consisting of a broad deformational zones that can be described from top to base crust. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho below, necking the adjacent crust along the length of the OFZ in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. This constrains a process of oblique extension at a high angle to the spreading orientation along a low angle shear zone which also acts as a conduit for decompression related melt.

We demonstrate that transforms in São Tomé and Príncipe were both non-conservative and not a simple strike slip fault zone, contradicting the current understanding of modern systems. This style of deformation has similarities with anomalously deep and smooth nodal basins which form at slow spreading inside-corner crust. Our model adds strong observational constraints to complement recent numerical models that predict oblique extension within transform zones.

How to cite: Heine, C., Thomas, M., van Itterbeeck, J., Ostanin, I., Seregin, A., Spaak, M., Morales, T., and Oude Essink, T.: A new model for the evolution of oceanic transform faults based on 3D PSDM Seismic observations from São Tomé and Príncipe, eastern Gulf of Guinea., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4993, https://doi.org/10.5194/egusphere-egu22-4993, 2022.

Gondwana splitting started during the Early Jurassic (ca. 180 Ma) with the separation of Antarctica and Madagascar from Africa, followed by the separation of South America and Africa during the Middle Jurassic. Thanks to recent seismic profiles for petroleum exploration, the architecture of rifted margins and the transform faults zones, which developed as a result of the relative motion between tectonic plates have been recently evidenced and studied along the whole eastern and south-eastern Africa (i.e., in the Western Somali Basin, the Mozambique Basin, the Natal Basin, and the Outeniqua Basin). Yet, the structure and overall kinematic evolution of the three major transform faults zones together – i.e., the Agulhas, the Davie, and the Limpopo Fracture Zones – that control the opening of these major oceanic basins remain poorly studied. The interpretation of an extensive regional multichannel seismic dataset coupled with recent studies allows us to propose an accurate regional mapping of the crustal domains and major structural elements along the rifted margins along the whole eastern and south-eastern Africa. We provide new constraints on the structuration and evolution of these three transform systems. Although our findings indicate common features in transform style (e.g., a right-lateral transform system, a wide sheared corridor), the deformation and the thermal regime along these systems appear quite different. In particular, we show that the Davie and Agulhas Fracture Zones recorded spectacular inversions during the transform stage whereas transtensional deformation is observed along the Limpopo Fracture Zone during its activity. This suggests that faults activity controls vertical displacements along transform margins, minimising other processes such as thermal exchanges between the oceanic and continental lithospheres across the transform fault and flexural behaviour of the lithosphere. This different style of deformation may be explained by two main forcing parameters: (i) the magmatic conditions that may modify the rheology of the crust, and (ii) the far-field forces that may induce a rapid change of regional tectonic stress. Further, in the Davie and Agulhas cases, the major transform faults postdate the development of the rift zone-controlling faults. Thus, there are no pre-existing structures that control the initiation of a transform fault zone. Conversely, the Limpopo margin shows an intracontinental transform faulting stage. In both cases, a minimum of several Ma is required to establish a complete kinematic linkage between the two-active spreading centers. During this period, the rifted segments opening possibly triggered rift-parallel mantle flow, which progressively favors the decoupling in-between the continental domain and the future oceanic domain. In the post-drift history, rapid changes of regional tectonic stress are recorded and show that some transform margins are excellent recorders of large plate kinematic changes.

How to cite: Roche, V., Leroy, S., Ringenbach, J.-C., Sapin, F., Revillon, S., Guillocheau, F., Vetel, W., and Watremez, L.: East African Fracture Zones: a long lifespan since the breakup of Gondwana, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12463, https://doi.org/10.5194/egusphere-egu22-12463, 2022.

The faults that accommodate Pacific - North America plate motion along the San Andreas plate boundary occupy a region that previously served as part of the upper plate of the Cascadia subduction zone plate boundary. After the passage of the Mendocino triple junction (MTJ), several fault systems develop within the newly formed Pacific-North America plate margin, with one fault system eventually evolving to become the primary plate boundary structure (termed the San Andreas Fault in central California). As a result of the northward migration of the MTJ, the Cascadia subduction zone, undergoing NNW-directed shortening at a rate of ~ 50 km/Ma, replaced by the equivalent lengthening of the San Andreas system.  In northern California, three primary fault systems are identified:  on the west (along the western margin of the North America plate) is the San Andreas fault (which does not serve as major component of the lithospheric scale plate boundary structure in northern California; moving inland (eastward) is the Maacama - Rodgers Creek (M-RC) fault system; further east is the Lake Mountain - Bartlett Springs (LM-BS) fault system.  These latter two faults primarily accommodate Pacific -North America motion in the region just to the south of the MTJ. 

New tomography imagery of this region of northern California provides crustal constraints on deformation and fault localization, both within Cascadia, north of the MTJ, and south of the transition from subduction to translation. Using these tomographic images and analyses of GPS data within the region, we have developed a tectonic model that both explains the present fault systems north and south of the MTJ, and helps us understand why one of these fault systems - the M-RC fault system - develops to become the primary plate boundary structure over several million years after MTJ passage. Two fundamental aspects of the North America and Pacific plates control the location of these primary fault systems - the existence of relatively rigid upper-plate backstops  (the Great Valley and Klamath blocks), and a small remnant (the Pioneer fragment) of the subducted Farallon plate accreted to the eastern margin of the Pacific plate and migrating northward with it. As a result of these structures, the LM-BS fault system develops as an upper-crust (brittle) fault system, while the M-RC system initially forms as a shear zone (ductile) along the eastern margin of the Pioneer fragment, with the upper-crustal faults developing in response to the deeper plate boundary shear zone. This lithospheric shear zone localizes the plate boundary development and leads to the M-RC system becoming the main plate boundary fault.

How to cite: Furlong, K. P. and McKenzie, K. A.: Formation and Development of the San Andreas Fault System with Migration of the Mendocino Triple Junction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10112, https://doi.org/10.5194/egusphere-egu22-10112, 2022.

The southern Dead Sea Transform (SDST) is an active left-lateral transform plate boundary that extends from the Sinai triple junction to the Lebanon restraining bend, separating the Arabian and Sinai plates. In this study, we analyze structural variations along the SDTS, and reproduce these variations in a 4D analogue model.  

From south to north, the structural styles along the SDTS indicate (1) rotational transtension within the Gulf of Aqaba, (2) pure strike-slip in Wadi Araba and Jordan River valley, and (3) pull-apart basins in the Dead Sea, Sea of Galilee and Hula basin. These different structural styles were replicated experimentally in an analogue model incorporating transtension with minor rotation along a kinked plate boundary. Our 4D model produced a deep southern depression with en echelon faults corresponding to the Gulf of Aqaba, a simple strike-slip fault system without vertical displacement reflecting the Wadi Araba and Jordan Valley, and a set of pull-apart basins reminiscent of the Dead Sea, Sea of Galilee and Hula basins. The accurate reproduction of the structural styles along this 600km-long plate boundary segment constrains the relative movement between the Arabian and Sinai plates to a simple combination of transtension with minor rotation, thereby negating the earlier hypothesis of Euler pole shift during the tectonic evolution of the SDST. 

How to cite: Fedorik, J., Afifi, A., Zwaan, F., and Schreurs, G.: A kink in the plate boundary, rotation and transtension: new 4d insights into the tectonics of the southern Dead Sea Transform, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2128, https://doi.org/10.5194/egusphere-egu22-2128, 2022.

The region around the Dead Sea Transform represents a unique example of the structures that form around restraining and releasing bends in a strike-slip environment. With our 3D numerical models, we aim to understand the processes that shaped the region including the Dead Sea Basin, the Dead Sea Transform Fault, and the Lebanese Restraining Bend.

In our study, we employ geodynamic modelling using the software ASPECT coupled to the surface processes code FastScape. Our model setup includes a compressive and a tensional stepover along a strike-slip fault with periodic along-strike boundary conditions. Even though we use a simplistic setup with horizontally homogeneous rock layers, we can reproduce many of the present-day features of the Dead Sea Transform region, including the sediment thicknesses in the Dead Sea basin, heat flow patterns, relative topographical height differences, and the general outlines and activity of the main faults along the Dead Sea basin, the Mount Lebanon and Anti Lebanon ranges.

With our models we can investigate the influence of surface processes on the underlying stepover strike-slip tectonics and the resulting crustal-scale flower structures: (1) Along the tensional stepover, the horizontal distance between the bounding faults of the pull-apart basin increases with greater efficiency of surface processes due to an increasing sediment load filling the basin. The sediments hinder the border faults in approaching each other at the surface, thereby enforcing basin-ward fault dip, resulting in wider and deeper basins with greater surface process efficiency. (2) In the uplifted compressive stepover, the erosional efficiency has a direct feedback on the longevity of faults and the rheological state of the crust through its influence on the uplift rate. Elevated erosion-induced uplift rates lead to a connection of the brittle parts of lower and upper crust, because the upper crustal viscous part is moved into a zone of lower temperatures and thus becomes brittle. This drastic change of the underlying rheology manifests in the formation of a new fault, which cuts through the centre of the compressional area. When no erosion is assumed a similar fault is observed in map view, but cross sections reveal that without erosion this fault has a different origin and the flower structure is more complex and more symmetric than for models that include erosion.

How to cite: Heckenbach, E., Brune, S., Glerum, A., and Neuharth, D.: 3D geodynamic evolution of strike-slip restraining and releasing bends modulated by surface processes: application to the Dead Sea Transform, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5427, https://doi.org/10.5194/egusphere-egu22-5427, 2022.

The North Victoria Land structural framework is characterized by the long-lived tectonic activity along major crustal lineaments, including the NNW-SSE and NW-SE trending Rennick Graben Fault (RGF) system and Aviator Fault (AF). This tectonic corridor is characterized by an important strike-slip component that is easily connected to the main strike-slip fracture zone that characterizes the Southern Ocean between Australia and East-Antarctica. Structural analysis of field data along the RGF evidences a poly-phased activity with multiple reactivations related to the Paleozoic juxtaposition of NVL to the East Antarctic craton (as resulting from the Gondwana breakup) and to the Meso-Cenozoic plate tectonics associated to the Australia-East Antarctica separation and characterized by both offshore and onshore crustal strike-slip deformation. Both the northward, offshore propagation of the RGF system and its southward prosecution and link with the AF are inferred but still need to be proved/better framed.

During the XXXVII Italian Antarctic campaign in the framework of the LARK project 92 field measurement sites have been surveyed between latitude 71.5°S and 73.5°S. To better frame the link between the RGF and AF the evidence of brittle deformation (including faults with the associated kinematic indicators and fracture attitude, dimension and sets) have been measured. This deformation involve rocks with ages ranging from Lower Paleozoic to Lower Jurassic. Where time constraints from stratigraphy are lacking and to better frame the age of the tectonics with its associated vertical displacement, ad hoc field samples have been collected for thermochronology dating.

Open, un-mineralized fracture sets are important indicator of recent paleo-stress (tectonic) activity, since their formation is limited to shallow depth and their presence testify a short erosion time, thus representing a good indicator of the last, recent stress regime. The intensity of brittle deformation associated to this last tectonic setting can be quantified by the H/S adimensional parameter, where H represents the size of the fracture and S is the spacing between nearest fractures belonging to the same azimuthal family and having comparable dimensions. This parameter has been proved (Cianfarra & Salvini 2016) to be proportional to the total energy released by the stress during fracture generation though time. The analysis of the recently collected field structural data is still in progress and will allow to prepare both a map of the spatial distribution of H/S values and to infer the (multiple) paleostress responsible for the observed brittle deformations by the application of original methodologies that include the inversion of fault and near orthogonal fracture systems. The latter inversion methodology solves both the identification and grouping of the fractures into the two systematic and non-systematic families, and the orientation of the responsible paleostress by a Monte Carlo approach.

Results from the central RGF system area shows the increase of the H/S values by approaching the RGF central zone, due to the increase of the local stress produced by its kinematics.

Cianfarra P. and Salvini F., (2016). Quantification of fracturing within fault damage zones affecting Late Proterozoic carbonates in Svalbard. Rend. Fis. Acc. Lincei, 27(19), 229-241. DOI 10.1007/s12210-016-0527-5

How to cite: Cianfarra, P., Salvini, F., Crispini, L., Locatelli, M., and Federico, L.: Linking the strike-slip kinematics of the Rennick Graben Fault system and the Aviator Fault from field structural data, North Victoria Land, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10205, https://doi.org/10.5194/egusphere-egu22-10205, 2022.