Displays

For very long time, transform margins have been treated and described  based on oceanic transform fault concepts. Their was no change in kinematics nor structures with time and thermally speaking, it was hypothesed that the margin was reheated as the mid-oceanic ridge translated passively along the margin.  In the last 10 years, 3D numerical modelling has been made available and numbers of studies have challenged this view. It is time to review the concepts that have emerge. Interrestingly, many modelling contributions have tackled the obliquity at very different scales, with initial conditions varying from simple flat layered homogeneous lithosphere to subduction of opposite vergence. Moreover some contributions have focus on rheological aspect and other on inheritance at different scale and different physical coupling have been used. Some models were targeting at reproducing the oceanic transform concepts, other at exploring how large scale structure can emerge.  I will therefore try to review  the state of art in numerical modelling of transform margin and oblique extensional system based on my own work and literature review. I will try to emphize the important differences and similarities used in the different modelling. Using different models with different boundary conditions and scale I will try to introduce a new conceptual model of transform margin which captures important characteristics like the delay in continental break-up highlighted by the tracing of sediments and water-depth as well as the obliquity between syn-rift and post-rift subsidence.  Some models of oblique extension have also been producing new type of strike slip ocean continent transition which somehow could be interpreted as steep transform margins but appears to be mainly strike slip and have no conjugate margins. To conclude, all these 3D numerical modelling  allow us today to present a very different view of transform margins than 10 years ago. Some of the new concepts that have emerged  mendate to re assess our interpretation of exisiting datasets.

How to cite: Le Pourhiet, L. and Jourdon, A.: Transform/oblique rift system : what have we learned from numerical modelling and what's next ? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19719, https://doi.org/10.5194/egusphere-egu2020-19719, 2020.

The Dead Sea Transform is an active left lateral, strike-slip plate boundary. The Gulf of Aqaba corresponds to its southern segment, where the largest amount of opening is observed. The gulf itself is deformed by a set of en echelon faults which are bounded by normal faults. These en echelon faults show structural styles of Riedel shears which are typically observed in strike-slip tectonics. However, their orientation is the opposite to the one observed in well described models or natural cases. In this study, we compare a compiled dataset to analogue models which simulate the displacement in various strike-slip systems. This comparison to a sandbox model highlights the importance of the tectonic load in a strike-slip fault system. The model is composed of two base plates with only one straight velocity discontinuity. X-Ray Computed Tomography is used as a technique to carry out a 4D analysis of internal fault structures of the model. The 10°-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. The 30°-transtensional tectonic load shows two major steep bounding faults with a dip-slip component and a set of en echelon faults - opposite Riedel shears in between them. A higher amount of transtension rotates the classic Riedel shear faults to the opposite position. This fault pattern is very similar to the one observed in the Gulf of Aqaba, where the internal fault system is composed of opposite Riedel shears bounded by normal faults. These observations can increase the understanding of the structural styles seen in the Gulf of Aqaba. Moreover, our study describes a new strike-slip fault system.

How to cite: Fedorik, J. and Afifi, A.: New structural model of strike-slip tectonics of the Gulf of Aqaba, southern Dead Sea Transform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13345, https://doi.org/10.5194/egusphere-egu2020-13345, 2020.

The North Anatolian Fault Zone (NAFZ) is a 1200-km-long, dextral intracontinental transform fault zone, and initiated ca. 13–11 Ma ago.  The NAFZ formed in response to the N-S convergence of the Eurasian and Arabian plates, accommodated by the westward motion of the Anatolia plate relative to Eurasia plate.  Mantle xenoliths were sampled in late Miocene (11.68±0.25 to 6.47±0.47 Ma) alkali basalts and basanites, immediately N of the trace of the North Anatolian fault, and were previously interpreted to sample the mantle portion of the North Anatolian fault/shear zone at depth.  The studied xenoliths are mainly spinel lherzolites and harzburgites.  Equilibration temperatures estimated from two-pyroxene geothermometers range from 775 to 975 °C, while pressures estimated from the Cr in clinopyroxene geobarometer and pseudosection modelling range from 12 to 22 kbar, which correspond to depths of 40–80 km.  We used high‐resolution X-ray computed tomography to quantify the xenolith fabric defined by the 3D shape preferred orientation of spinel grains.  Spinel displays dominantly oblate fabric ellispoids, consistent with flattening strain.  Olivine has two main crystallographic preferred orientation patterns, the axial-[010] and the A-type, determined with electron backscatter diffraction.  The axial-[010] pattern is consistent with the spinel fabric and other microstructures that show flattening strains.  To further constrain the strain path, we analyze the crystallographic vorticity axes in olivine, which show a complex pattern.  Our results are consistent with an interpretation of transpressional deformation in the upper mantle below the NAFZ, during the early stages of the development of the transform system.  Transpressional deformation is consistent with collision-induced, strike-slip extrusion of Anatolia.

How to cite: Tikoff, B., Chatzaras, V., Chapman, T., Barshi, N., Aldanmaz, E., and Kiesow, M.: Transpressional deformation in the mantle below the North Anatolian fault system, Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12551, https://doi.org/10.5194/egusphere-egu2020-12551, 2020.

Keywords: inherited structures, fault reactivation, U-Pb geochronology

The E-W striking Inner Moray Firth Basin (IMFB) lies in the western part of the North Sea trilete rift system formed mainly in the Upper Jurassic. The IMFB has experienced a long history of superimposed rifting with plenty of uplift and fault reactivation during Cenozoic. The basin is overlying the Caledonian basement, the pre-existing Devonian-Carboniferous (Orcadian Basin) and a regionally developed Permo-Triassic basin. The potential influence of older structures related to the Orcadian Basin on the kinematics of later basin opening has received little attention, partly due to the poor resolution of seismic reflection data at depth or sparse well data.

By integrating onshore fieldwork with the interpretation of 2D and 3D seismic data and U-Pb geochronology of syndeformationally grown calcite we provide new insights into the kinematic opening of the basin as well as the role of pre-existing Devonian-Carboniferous (Orcadian) basin structures.

The Jurassic opening of the rift basin is known to be associated with major NE-SW trending faults. New detailed mapping of offshore 3D seismic data revealed that at a smaller scale en-echelon E-W to NE-SW trending faults, en-echelon N-S to NNE-SSW and NW-SE fault arrays coexist. This suggests an oblique-sinistral component associated with the major NE-SW rift basin trends. This correlates with onshore findings, which suggest that the inherited Orcadian fault systems (mainly N-S to NE-SW) have been dextrally reactivated. Sinistral WNW-SSE to NW-SE striking faults and associated transtensional folds are also present in the Devonian rocks. This later deformation is consistently associated with calcite mineralization (e.g. slickenfibers, calcite tensile veins or Riedel shear fractures). New U-Pb dating of the calcite mineralization, related to the reactivated faults, shows that the age of fault reactivation is 153 ± 0.68 Ma (Upper Jurassic).

The integration of fieldwork with subsurface interpretations and absolute dating techniques has provided better constraints on superimposed basin development, as well as explaining complexities that have hitherto been ignored. This can reduce subsurface uncertainties regarding the structural evolution of the basin and unlock the full potential of the area and significantly enhance future exploration programs.

How to cite: Tamas, A., Holdsworth, R., Underhill, J., McCaffrey, K., Dempsey, E., Selby, D., and McCarthy, D.: New insights into the kinematics and timing of superimposed rifting events through integration of offshore data, onshore fieldwork and U-Pb geochronology: Inner Moray Firth Basin, Scotland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-528, https://doi.org/10.5194/egusphere-egu2020-528, 2020.

The Beibuwan basin is located along the western margin of the Ailao Shan Red River Shear Zone (ASRRSZ), and also in the north margin of the South China Sea (SCS). This study utilizes 2-D seismic data to investigate the evolution of this basin and discuss its broad tectonic settings. Several stages of rifting and inversion occurred in the Beibuwan basin during Cenozoic: (1) During Paleocene initial rifting (66-56 Ma), the ocean-ward gradual retreat of the Paleo-pacific subduction zone created an extensional tectonic setting in the SCS region. The overall extensional tectonic setting of the northern passive margin of the SCS generated a series of Paleogene NE-striking rift basins, including the Beibuwan basin, the Qingdongnan basin and the Pear River Mouth Basin. (2) During Eocene rifting stage (56-37.8 Ma), the Pacific plate still subducted under the Eurasian plate, and soft collision started to occur between the greater India plate and the Eurasian plate. Subsequently, the NW-SE-direction extension gradually changed to N-S-direction extension, therefore, the NE-striking faults active intensively during this stage, and a small group of EW-striking faults formed in the study area. (3) During the Oligocene rifting stage (37.8-23 Ma), the India-Eurasian collision went into hard collision stage, induced the large-scale left-lateral strike-slip of the ASRRSZ. Furthermore, the subduction of the Pacific plate strengthens the left-lateral shearing of the ASRRSZ. The left-lateral strike-slip of ASRRSZ resulted in the formation of large amount of EW-striking faults in the Beibuwan and Yinggehai basins, and the opening of the South China Sea. (4) After Paleogene, several stage of inversions occurred in the study area, including the end-Oilgocene, end-Miocene and end-Plioence inversions. The regional end-Oligocene inversion is supposed related to the change from major left-lateral transtensional rifting to left-lateral transpression of ASRRSZ. The end-Miocene and end-Pliocene inversions are localized inversions, which also related to the left-lateral transpression of ASRRSZ.

How to cite: Cheng, Y. and Wu, Z.: Cenozoic rifting and inversion in Beibuwan Basin and its relationship with strike-slip motion on the Ailao Shan-Red River Shear Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4522, https://doi.org/10.5194/egusphere-egu2020-4522, 2020.

Plate tectonic history of northeast Indian subcontinent can be viewed as a window to the
evolution of Southeast Asia. One such important tectonic feature is the northern most part of
Indo-Burmese Ranges where this research work has been carried out. Here we propose an
evolutionary model that shows northward moving ‘horse-tail’ feature of the Hukawng Block
from the Burma basin, pushed this region towards the rigid Mishmi Block and Upper Assam
shelf, that caused the hyperoblique pattern of the ranges. It is the juxtaposition of the three
continental blocks: India-Asia-Burma, where there are tectonic and geomorphic influences in
the Block from both the Himalayan and Indo-Burmese orogeny. Stress distribution among
north-easterly moving Indian plate and comparatively stiff Eurasian and Burma plates, within
the India specific reference frame, is resulting in further changes. The study area mainly falls
under Changlang district of Arunachal Pradesh, India; and the regional study has been done in
the quadrangle from 26° to 28°N in latitudes and 95° to 97°E in longitudes. Morphotectonic
study, lineament analysis, fault system characterisation, focal plane mechanism along with
dynamic topography, seismic tomography and gravity anomaly have been incorporated in the
field evidences. Morphotectonic study for Noa-Dihing River basin has resulted in a value of
56.59 for Asymmetric Factor, which shows similar asymmetry result like in the Chi (χ)
analysis. This SW-ward tilted basin is moderately asymmetrical with Transverse Topographic
Symmetric Factor value of 0.42. This indicates that the major river basin along with other subbasins
are under the influence of active oblique rotational component. The regional lineaments
are showing mean orientations of N11°E-S11°W, N70°W-S70°E and EW whereas some local
trends of minor lineaments, in some places have mean orientations of N40°W-S40°E, N82°WS82°
E and N42°E-S42°W. In Mishmi block the major regional trends are N35°W-S35°E and
N40°E-S40°W comprising of probable cross-faults. In Hukawng Block, the lineament
orientation changes from N50°W-S50°E in the west to N30°W-S30°E, N-S and N15°E-S15°W
in the central valley region (north of Jade mines) and then to N50°E-S50°W in the eastern side.
Major fault systems are mostly thrust, with some showing very low angle slip component,
along with some oblique slip faults (e.g. Noa-Dihing River). The dynamic topography and
seismic tomographic studies indicate presence of a high seismic velocity zone beneath Mishmi
block indicating the crystalline rock materials. The block is still actively exhuming. Moreover,
Changlang and Hukawng blocks have undergone uplift and then phases of subsidence during
the last 20Ma. This indicates that the Low Velocity materials that are present underneath were
subjected to some crustal deformations. This tectonic process has also resulted in gravity
anomalies. The role of massive and rigid Mishmi block, comprising older crystalline rocks and,
later forming Burma basins formed the oblique rotation of the Changlang block which is
observed from all stated methods. Hukawng Block, which is controlled by the motion of
Sagaing Fault, have influenced the Changlang Block by its varied strike-slip stress components.
Moreover, Indo-Burmese Ranges also has an influence on this block and vice-versa.

How to cite: Zaman, F., Goswami, U., and Bezbaruah, D.: Hyperobliquity of Changlang Block of eastern Arunachal Pradesh, India and the role of Mishmi Block, India and Hukawng Block, Myanmar in its development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-185, https://doi.org/10.5194/egusphere-egu2020-185, 2020.

Large-offset transform faults (TFs) in the Atlantic juxtapose hot spreading segments against older, colder oceanic lithosphere, leave permanent traces as fracture zones in ageing oceanic crust and represent a significant proportion of the plate boundary along the Mid-Atlantic Ridge (MAR). The manifestation of the thermal contrast and the structure and composition of TFs however, are not well understood. The Romanche TF, situated in the Equatorial Atlantic, offsets the MAR by ~950 km, has a slip of ~1.7 cm/yr, and divides the northern MAR from its equatorial and southern spreading systems. Close to the eastern ridge-transform intersection (RTI), shallowing of the seafloor from north to south across the TF reflects the change from old, cold African lithosphere to the warmer and younger South American plate close to the MAR axis, however the bathymetry and structures across the fault itself are complex. Over 100 km distance, a large northern transverse ridge reaches depths of <1000 m and contains a fossil transform trace, before steeply descending into a 45‑km wide transform valley containing ~7000 m‑deep basins, which is bounded to the south by a further shallow structure reaching ~2500 m‑depth. Previous studies using seafloor sampling, seismic reflection and bathymetry data have suggested these features comprise a mix of uplifted magmatic crustal blocks and serpentinized mantle peridotites. However, these studies cannot effectively determine the sub‑seafloor structure.

The ILAB-SPARC experiment in 2018 obtained an active-source wide-angle refraction profile across the eastern Romanche TF, consisting of twenty-eight ocean-bottom seismometers spaced at ~14 km. We present a P-wave velocity model produced by the inversion of seismic travel time picks which reveals variations in crustal structure from ~40 My lithosphere to the north to ~7 My lithosphere to the south. Within the TF, a ~15 km-wide low-velocity anomaly extends from the top basement through to >10 km below basement. A lack of Moho reflections suggests no abrupt crust/mantle boundary exists beneath the TF, likely indicating the presence of a deep column of fractured and sheared basalts, breccias and peridotites. Low mantle velocities suggest faulting and water penetration to depths of ~16 km, causing widespread and extensive serpentinization. The crust to the south of Romanche is relatively thin (~5 km‑thick) compared to north of Romanche (~6 km‑thick), and contains areas of high velocity indicative of a predominantly gabbroic crust. This may be attributed to the irregularity of the MAR segment as it approaches the RTI, as it jumps to the west in several non-transform discontinuities and exhibits seafloor fabric indicative of magma-starved, tectonic spreading with exhumation along detachment faults.

These results suggest the shearing and transtensional/transpressional forces present at large-offset transform faults result in mantle exhumation and form deep conduits for fluid circulation. At Romanche, these tectonic forces combined with the thermal contrast and magma-starved ridge axis, stretch and deform magmatic oceanic crust within the TF such that it is thin and patchy. This may suggest that crustal structure within transforms is linked to the fault offset, valley width, and the magma supply at the closest ridge segment.

How to cite: Gregory, E., Marjanović, M., Wang, Z., and Singh, S.: Structure and composition of large-offset Atlantic transform faults: an extreme example at the Romanche transform from wide-angle refraction data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10151, https://doi.org/10.5194/egusphere-egu2020-10151, 2020.

A significant portion of the Maures-Tanneron Massif (SE branch of the European Variscan Belt) is occupied by late orogenic, anatectic crustal granitoids that were emplaced at ca. 325-300 Ma (Upper Carboniferous)^1,2. The Camarat granite³ is one of the smallest representatives of these granitoids (~2.5 km²). It is a composite intrusion exposed in migmatitic gneisses of the Maures Massif, along the southern shore of the Saint-Tropez Peninsula. From west to east, it consists of an E-W strip of Ms-Bt-Crd leucogranite where coarse- and fine-grained facies are found in similar amounts, and two bodies of Bt-Ms leucogranite, dominantly coarse-grained.

Zircon and monazite from two samples of the Camarat granite have been analyzed by LA-ICP-MS for U-Pb dating. Sixteen monazite analyses from the fine-grained facies of the E-W granite strip give a Concordia age of 303.5 ± 1.8 Ma (2 S.E., MSWD = 0.9). Sixteen zircons from the coarse-grained facies of the easternmost intrusion provide a Concordia age of 304.6 ± 2.1 Ma (2 S.E., MSWD = 1.2). The two dates are identical within uncertainty and are considered to constrain crystallization of the Camarat granite at ~304 Ma (Kasimovian–Gzhelian limit).

Twenty-one measurements of the anisotropy of magnetic susceptibility (AMS) and direct textural quantifications through image analysis (IA) of 10 samples give agreeing results that reveal the fabric orientation in the Camarat granite. The foliation has a variable orientation, with a weighted average of N65°E/26°NNW for the AMS data and N77°E/17°NNW for the IA data (D = 10°). The lineation pattern is more homogeneous, displaying a consistent northerly shallow plunge (mean of N12°E/22°NNE vs. N22°E/20°NNE; D = 10°). The Camarat granite lineations are parallel to lineations in the gneissic country rocks. These were produced during the last Variscan tectonic event evidenced in the area, a partitioned transpression phase, localized along ca. N-S sinistral strike-slip shear zones⁴. It is proposed that the ascent of the Camarat granite was favoured by such strike-slip structures and that pull-aparts represent the sites of emplacement, as best exemplified by the E-W granite strip.

In the Corso-Sardinian Block, another portion of the SE Variscides formerly juxtaposed to the Maures-Tanneron Massif⁵, a model of progressive transition from late orogenic, Upper Carboniferous transpression to post orogenic, Permian extension has been recently proposed⁶. A similar model may be extended to other areas of the SE Variscan Belt, in particular to the Maures-Tanneron Massif which is cut and bordered by Permian grabens⁷, the ca. E-W orientation of these grabens implying that a ca. N-S direction of stretching, as recorded by the 304 Ma Camarat granite, was still prevailing in Permian times.

1. Duchesne et al., Lithos 162-163, 195-220 (2013). 2. Schneider et al., Geol. Soc. Spec. Pub. 405, 313-331 (2014). 3. Amenzou & Pupin, C. R. Acad. Sc. Paris (Série II) 303, 697-700 (1986). 4. Corsini & Rolland, C. R. Geoscience 341, 214-223 (2009). 5. Edel et al., Geol. Soc. Spec. Pub. 405, 333-361 (2014). 6. Casini et al., Tectonophysics 646, 65-78 (2015). 7. Toutin-Morin, Ann. Soc. géol. Nord 106, 183-187 (1987).

How to cite: Bolle, O., Corsini, M., Diot, H., Laurent, O., and Melis, R.: The Camarat granite (Maures Massif, SE France): a tectonic marker of the late orogenic evolution of the South European Variscan Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10167, https://doi.org/10.5194/egusphere-egu2020-10167, 2020.

The palaeogeographic reconstruction of the Falkland Plateau transform margin in a Gondwana pre-break-up configuration has been the subject of debate for years. This is mainly due to the uncertainty in the position of the Falkland Islands microplate. The islands were an extension of the south-east coast of South Africa, being either i) part of a rigid Falkland Plateau fixed to the South American plate or ii) undergoing a vertical-axis clockwise rotation of between 80° to 120° along the transform faults generated during the initial stages of fragmentation of south-western Gondwana. The absence of documented evidence of this rotation within the sedimentary infill of the basins surrounding the Falkland Islands represents an ongoing issue. Furthermore, a structural framework of the eastern continental shelf of the islands that takes into account the most recent seismic reflection surveys has not been published yet.

This study presents an updated description of the structural configuration of the Falkland Plateau Basin, focusing on the Volunteer and Fitzroy sub-basins. This structural framework, based on extensive 2D and 3D seismic reflection data and aided by seismic attribute mapping, provides new insights into the evolution of the Falkland Islands microplate and the Falkland Plateau Basin.

Three main structural trends were identified across this section of the Falkland Plateau. WNW-ESE trending half-grabens were mapped north-west of the Volunteer sub-basin; these correlate laterally with linear gravity anomalies following the same trend north of the Falkland Islands. NNE-SSW to N-S normal faults are predominant west of the Volunteer sub-basin and are believed to control the western margin of the Falkland Plateau Basin. Locally, the NNE-SSW trend is subdued by NNW-SSE striking en-échelon normal faults suggestive of left-lateral movement along a NNE-SSW direction. A similar trend is interpreted in the southern part of the Fitzroy sub-basin, supporting sinistral wrenching along the western margin of the Falkland Plateau Basin.

These results suggest intra-plate deformation that is consistent with a clockwise rotation of the Falkland Islands microplate along the transform faults that accommodated the initial fragmentation of Gondwana. The interpreted fault network allows us to understand the temporal variation in the orientation of the minimum horizontal stress across the Falkland Islands microplate. By comparing this variation with the regional stress regime in south-western Gondwana, the timing and mechanism of the rotation of the islands can be better constrained.

How to cite: Stanca, R. M., Paton, D., Mortimer, E., Hodgson, D., and McCarthy, D.: The structural evolution of the Falkland Plateau , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9240, https://doi.org/10.5194/egusphere-egu2020-9240, 2020.

The western slope is the most promising area for hydrocarbon accumulation in Xihu Sag. Since Cenozoic, the western slope has undergone multiple stages of evolutions, which resulted in the complex structure of the slope and complicated the hydrocarbon exploration in the study area. Based on the fine seismic interpretation, fault activation rate calculation and balanced cross section restoration, this paper analyzes the structural characteristics and evolution process of the western slope, in order to provide supports to the hydrocarbon exploration. The results show that the western slope is controlled by NNE-striking and NW-striking faults. Seperated by NW-striking faults, the western slope can be divided into sub-Hangzhou slope, sub-Pinghu north slope, sub-Pinghu south slope and sub-Tiantai slope from north to south. (1) The sub-Hangzhou slope is a faulted-step gentle slope. In faulting episode I (Cretaceous-Paleocene), the slope was controlled by step faults. In faulting episode II (Early Eocene), the slope changed from fault-controlled slope to gentle strata slope. In depressional period (Late Eocene-Middle Miocene), the slope was a gentle strata slope. (2) The sub-Pinghu north slope is a graben-horst slope. In faulting episode I (Cretaceous-Paleocene), the slope was controlled by two sets of step faults with opposite tendencies. In faulting episode II (Eocene), the slope changed from fault-controlled slope to gentle strata slope. In depressional period (Oligocene-Middle Miocene), the slope was a gentle strata slope. (3) The sub-Pinghu south slope is a faulted-step steep slope. In faulted period (Cretaceous-Paleocene), the slope was controlled by Pinghu fault and secondary step faults. In depressional period (Oligocene-Middle Miocene), the activation of Pinghu fault became weak, but this fault still divided the strata. (4) The sub-Tiantai slope is a single-fault steep slope. In faulted period (Cretaceous-Paleocene), the slope was controlled by Baoshi fault. In depressional period (Oligocene-Middle Miocene), the activation of Baoshi fault became weak, but this fault still divided the strata. Differences of structural characteristics and evolution process influence the hydrocarbon accumulation in different sub-slopes.

How to cite: Dai, Y., Wu, Z., Cheng, Y., and Xu, B.: Structural Characteristics and Evolution Process of the Western Slope of Xihu Sag, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8839, https://doi.org/10.5194/egusphere-egu2020-8839, 2020.

The general understanding of the major tectonic structures that are traced on Crete Island is of great importance to decipher the geodynamic regime of the leading edge of the overriding Aegean microplate and consequently Eurasia’s southernmost active margin. The aim of this multi-disciplinary methodology is to provide useful information for more reliable mapping of buried structures, which in turn supplement the dynamic and kinematic model of this key area of high interest.

Several indicators for the existence of oblique fault block displacement were identified with the use of earth observation data, as strike slip faulting expressions on the surface are more efficiently identified by vertical observations. Tectonic structures which are usually created along lateral displacements require different working scales. Hence, earth observation data (satellite images, aerial photographs) with various spatial characteristics need to be included.

Therefore, the methodology presented in this paper involves high spatial resolution digital elevation models and several remote sensing multispectral datasets, in many cases merged with higher spatial resolution panchromatic aerial photographs. The co-registration and ortho-rectification of all datasets proved to be a very significant part of this work in order to produce high resolution coloured 3D scenes at selected sites in central Crete, where the observed N-S trending strike slip fault zones crosscut arc parallel low angle normal faults and higher angle fault scarps.

Additionally, deep seismic reflection datasets along the major geomorphic structure of Messara basin were combined and highlighted the strike slip mechanism, since the continuation of the sub-vertical structures in depth has become clearer after the exact positioning of the sections and further interpretation.

How to cite: Vassilakis, E., Alexopoulos, J., and Farangitakis, G.-P.: Combination of Earth Observation and Seismic Reflection Data Analysis for The Definition of Strike Slip Fault Zones in Central Crete, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2882, https://doi.org/10.5194/egusphere-egu2020-2882, 2020.

Accommodation zone is an important deformation structure in sedimentary basin, which is significant to understanding the basin structure. The formation and evolution of the Xihu Sag is controlled by the NNE-striking faults, whereas the NNE-striking deformation is offset by the NW-striking accommodation zone. However, the structure and evolution of the accommodation zone are poorly known. Based on the dips and activation rates of related NNE-striking faults on two sides of the NW-striking accommodation zone, 8 styles of NW-striking accommodation zones are divided in this sag, including the synthetic approaching style, synthetic broken line style, synthetic overlapping style, reverse approaching style, reverse broken line style, composite approaching style, composite broken line style, composite overlapping style. The relative accommodation ratio of the accommodation zone can be reflected by the difference-value of the faults activation rate of the NNE-striking faults. The results show that: (1) the most of the NW-striking accommodation zones formed at Early Cretaceous with low relative accommodation ratio, and reached its peak at Eocene, and disappeared at Late Oligocene. (2) The temporal and spatial differences of the NW-striking accommodation zones are very common in the Xihu Sag. Spatially, the accommodation zones are mainly developed in the western slope of Xihu Sag, and rarely developed in the middle and eastern of the Xihu Sag. Temporally, the accommodation zones developed in the northern area of the western slope of the Xihu Sag during the early stage, whereas, these zones migrated to the southern area of the western slope of the Xihu Sag during the late stage. This study on the tectonic evolution of the accommodation zone provides significant support to the study on the tectonic evolution of the Xihu sag.

How to cite: Xu, B., Wu, Z., Cheng, Y., and Dai, Y.: The characteristics and evolution of accommodation zone in Xihu Sag, East China Sea Shelf Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5146, https://doi.org/10.5194/egusphere-egu2020-5146, 2020.

Pull-apart basins are structural features closely linked to the interactions between strike-slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, width/length ratio, or changes in the extension direction. In this work, we focus on changes in extension direction during the formation of a pull-apart basin as a basis to further understand the evolution of the northern Gulf of California through a series of physical analogue modelling experiments.

We investigate the effect of a variation in the basin extension direction, using a two-layer ductile-brittle configuration to simulate continental crust rheology. Pull-apart basin development is accomplished by displacing a plastic sheet at the bottom of the experiment, with pre-cut geometry resembling interconnected rift and strike-slip segments, orthogonal to the evolving rift axes. Subsequently, we change the relative motion of the base plate by 7^o in accordance with the reconstructed plate vector from the Gulf of California. Oblique extension continues on this new plate motion vector to the end of the experiment.

To analyse the results, we inserted the model cross-sections in a seismic interpretation software generating 3D interpretations for faulting and sedimentary thickness. Preliminary results show that the shift in the direction of plate motion produces sigmoidal oblique slip faults that become normal when deformation adjusts to the new plate motion vector. Furthermore, it appears that sediment distribution is controlled heavily by the relative plate rotation.

Finally, we compare our observations with seismic reflection images, sedimentary package thicknesses and fault interpretations from the pull-apart structure in the Northern Gulf of California transtensional margin, where we find good agreement between model and nature.

How to cite: Farangitakis, G.-P., McCaffrey, K. J. W., Willingshofer, E., Kalnins, L. M., van Hunen, J., Persaud, P., and Sokoutis, D.: The structural evolution of pull-apart basins in response to relative plate rotations; A physical analogue modelling case study from the Northern Gulf of California., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5378, https://doi.org/10.5194/egusphere-egu2020-5378, 2020.

The Owen transform fault in the northwest Indian Ocean is a >300 km-long active structure that constitutes the active plate boundary between Somalia and India. The first-order fault geometry was reached in the Early Miocene when the Carlsberg Ridge propagated westward into the African plate to open the Gulf of Aden. Presently, it accommodates ~23 mm/yr of left-lateral strike-slip motion between the Sheba and Carlsberg spreading centers.  The fault was recently surveyed in the Spring of 2019 during the VARUNA and CARLMAG cruises on BHO Beautemps-Beaupré, an oceanographic ship operated by the French Navy. Along with geophysical measurements (multibeam bathymetry, gravity and magnetic fields) a set of high-resolution seismic lines (> 5000 km) was acquired across both the active and fossil traces of the fault between 9°N and 15°N. The area is largely buried under the distal Indus turbiditic sediments and therefore offers a fairly unique continuous high-resolution stratigraphic record of past regional tectonic events. Here we present the first multibeam map of the Owen Transform system. A remarkable transpressive ridge borders the active trace of the fault along most of its length. At the intersection with the Carlsberg Ridge, the Owen Transform marks an 11° bend characterized by ~1200 m of seafloor uplift.  Our preliminary interpretation of the seismic lines brings to light the key unconformities related to Global Plate Reorganization Events. Off the main fault, new data reveal the magmatic nature of the Varuna Bank and similar partially buried highs. These have likely grown in the very early stage of formation of the oceanic crust carrying them, although tectonic emplacement cannot be completely ruled out. Some of the highs show internal structure, which can be interpreted either as carbonate caps or layered volcanic formations. This dataset, combined with previous cruises, offers unprecedented coverage of a 1500 km-long transform corridor along the Arabia-India and India-Somalia plate boundaries.

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.: Imaging the past and present Owen transform fault: preliminary results from the VARUNA seismic cruise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7974, https://doi.org/10.5194/egusphere-egu2020-7974, 2020.

A spectrum of rift types from orthogonal, through oblique to pure transform-type systems have been documented. In addition, it is well-established that during rift evolution transition between these kinematic regimes may occur. The effects of obliquity are extensive, often leading to distinctive structural settings dominated by transtensional and transpressional processes. The complexity of these settings, as well as their global prevalence, emphasises the need for better understanding, so that the role of oblique rifts and transforms in larger-scale plate tectonics can be fully appreciated.

The development of oblique rifts and transforms is influenced by a number of interrelated factors including: 1) oblique crustal and mantle inheritance, 2) a reduced force required for plastic yielding, 3) changes in far-field forces, 4) asthenospheric dynamics, and 4) grain size changes in the lower crust and mantle. However, although their development is controlled by this array of processes, it is known that the influence of oblique crustal and mantle inheritance, as well as changes in far-field forces is substantial. Yet, the relative importance, and prevalence, of these two factors amongst rift systems globally is insufficient. As such, the aim of this study is to determine to what extent these two processes prevail.

Structural inheritance refers to heterogeneities produced by previous geological processes that proceed to influence subsequent geological events. This process plays a substantial role in oblique rift and transform development at both the crustal and mantle scale. Specifically, large-scale mantle structures may localise crustal deformation, whilst reactivation of discrete structures in the pre-rift crystalline basement can influence the geometry and kinematics of rift basins and margins. On the other hand, changes in the orientation and magnitude of far-field forces mean that as a rift proceeds from inception to possible breakup, the kinematic regime may evolve such that the orientation of extension with respect to the rift boundary is spatiotemporally variable. Such changes in rift kinematics allow structures established under one kinematic regime to be subsequently reactivated, overprinting multiple rift episodes, whilst variable extension magnitude may introduce further complexities.

To better understand these processes we systematically compare the structural and tectonic evolution of several oblique rifts and transform margins, which were chosen to represent a diversity of rift types. Specifically, we compare: 1) the Davis Strait, 2) the Bay of Biscay 3) the Gulf of California, 4) the Red Sea, and 5) the East African margin. This is achieved by extracting rift velocity and extension directions from published plate tectonic models using GPlates, which are then compared with model results, as well as geological and geophysical observations. Preliminary results indicate that most oblique rifts and transforms express a strong influence of structural inheritance and a substantial change in kinematics during their evolution, emphasising the importance of these factors in oblique rift development.

How to cite: Peace, A. L., Cadenas Martínez, P., Farangitakis, G.-P., Phethean, J. J. J., and Watremez, L.: Structural inheritance and evolving rift kinematics in transform and oblique rift systems: A comparison of global examples , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12637, https://doi.org/10.5194/egusphere-egu2020-12637, 2020.

Slow and ultra-slow spreading systems gives way to complexes seafloor morphologies characteristic of different modes of tectono-magmatic activity at the ridge: crust accretion by episodic magma supply, low-angle brittle/ductile normal faulting and high-angle normal faults leading to the formation of oceanic core complexes (OCC). Previous studies have established that the magma supply exert a first order control on the tectono-magmatic activity at ultra-slow ridges (Howell et al., 2019; Lavier et al., 2000). However, other parameters are likely to play a significant role in the mode of spreading and therefore the seafloor morphology. For instant, transform faults are ubiquitous in slow spreading systems and are therefore likely to impact the mode of spreading by redistributing the stress field in the oceanic lithosphere. This seems to be supported by the observation that OCC are typically occurring in the inside corners of intersections between the ridge axis and major transform faults (Tremblay et al., 2009). Yet, little work has been done to investigate this question, leaving a significant gap in the understanding of slow and ultra-slow spreading systems.

This contribution investigates the interaction between ultra-slow spreading ridge and transform faults within the framework of a case study of the Fram Strait using high-resolution 3D numerical modelling. This study rely on the latest advances in geodynamics, namely the grain-damage rheology (Bercovici and Ricard, 2012) – which allows for internally consistent modelling of long-lived transformed faults. Numerical experiments are compared to the tectonic history of the Fram Strait over the last 10 Ma. A significant amount of geophysical and geological data available in the region allows us to asses how well the models reproduce observable structures in near-surface. Results show that ridge obliquity and ridge-transform interplay strongly affect the ridge spreading mode. Oblique ridge favour the formation of OCC over low-angle detachment fault and are systematically formed in the vicinity of major transform faults. Overall, results are in accordance with the highly complex seafloor morphology of the Fram Strait, in particular in the vicinity of the Molloy ridge. This study opens the way for a better understanding of complex ridge and abyssal hills structures in ultra-slow and slow spreading systems.

Bibliography

Bercovici, D., Ricard, Y., 2012. Mechanisms for the generation of plate tectonics by two-phase grain-damage and pinning. Phys. Earth Planet. Inter. 202–203, 27–55. doi:10.1016/j.pepi.2012.05.003

Howell, S.M., Olive, J.-A., Ito, G., Behn, M.D., Escartín, J., Kaus, B., 2019. Seafloor expression of oceanic detachment faulting reflects gradients in mid-ocean ridge magma supply. Earth Planet. Sci. Lett. 516, 176–189. doi:10.1016/J.EPSL.2019.04.001

Lavier, L.L., Buck, W.R., Poliakov, A.N.B., 2000. Factors controlling normal fault offset in an ideal brittle layer. J. Geophys. Res. Solid Earth 105, 23431–23442. doi:10.1029/2000JB900108

Tremblay, A., Meshi, A., Bédard, J.H., 2009. Oceanic core complexes and ancient oceanic lithosphere: Insights from Iapetan and Tethyan ophiolites (Canada and Albania). Tectonophysics 473, 36–52. doi:10.1016/J.TECTO.2008.08.003

How to cite: Beaussier, S. J., Plaza Faverola, A., Gerya, T., and Buenz, S.: Effect of the interplay between ultra-slow spreading ridge and transform faults on seafloor morphology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9390, https://doi.org/10.5194/egusphere-egu2020-9390, 2020.

The Gulf of Aden is a young and active oceanic basin, which separates the south-eastern margin of the Arabian Plate from the Somali Plate. The rifting leading to the formation of the north-eastern Gulf of Aden passive margin started ca. 34 Ma ago when the oceanic spreading in this area initiated at least 17.6 Ma ago. The opening direction (N26°E) is oblique to the mean orientation of the Gulf (N75°E), leading to a strong structural segmentation.

The Encens cruise (2006) allowed for the acquisition of a large seismic refraction dataset with profiles across (6 lines) and along (3 lines) the margin, between the Alula-Fartak and Socotra-Hadbeen fracture zones, which define a first order segment of the Gulf. P-wave velocity modelling already allowed us to image the crustal thinning and the structures, from continental to oceanic domains, along some of the profiles. A lower crustal intermediate body is observed in the Ashawq-Salalah segment, at the base of the transitional and oceanic crusts. The nature of this intermediate body is most probably mafic, linked to a post-rift thermal anomaly. The thin (1-2 km) sediment layer in the study area allows for a clear conversion of P-waves to S-waves at the top basement. Thus, most seismic refraction records show very clear S-wave arrivals.

In this study, we use both P-wave and S-wave arrivals to delineate the crustal structures and segmentation along and across the margin and add insight into the nature of the rocks below the acoustic basement. P-wave velocity modelling allows for the delineation of the structure variations across and along the margin. The velocity models are used as a base for the S-wave modelling, through the definition of Poisson’s ratios in the different areas of the models. Picking and modelling of S-wave arrivals allow us to identify two families of converted waves: (1) seismic waves converted at the basement interface on the way up, just before arriving to the OBS and (2) seismic waves converted at the basement on the way down, which travelled into the deep structures as S-waves. The first set of arrivals allows for the estimation the S-wave velocities (Poisson’s ratio) in the sediments, showing that the sediments in this area are unconsolidated and water saturated. The second set of arrivals gives us constraints on the S-wave velocities below the acoustic basement. This allows for an improved mapping of the transitional and oceanic domains and the confirmation of the mafic nature of the lower crustal intermediate body.

How to cite: Watremez, L., Leroy, S., d'Acremont, E., and Rouzo, S.: Crustal structures across the young and oblique North-eastern Gulf of Aden margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19421, https://doi.org/10.5194/egusphere-egu2020-19421, 2020.

A strong edge effect is predicted at the intersections between long-offset transforms and mid ocean ridge segments. The Equatorial Atlantic hosts several megatransforms, where the connections of potentially low mantle temperatures due to the large lithospheric age contrast with melt production are poorly understood. The SMARTIES cruise focused on the Romanche transform that offsets the Mid Atlantic Ridge (MAR) laterally by 900 km with an age offset of 55 Ma. The eastern Ridge-Transform Intersection (RTI) markedly shows the effects of the lateral cooling of the ridge segment. To better understand the thermal regime at these complex domains, we acquired surface geophysical data and bathymetry of the area, and geological observations and sampling during 25 HOV Nautile dives. The integrated study of rock characteristics and of geophysical surveys allows tackling the connections between magmatism and tectonics. A network of 19 OBS was also deployed to study the seismic activity during the cruise in collaboration with the ILAB project.

There is a striking change in deformation patterns along the ridge axis moving away from the transform southwards. The bathymetry is extremely complex, with several structural directions, partly resulting from transtension. A low melt supply is focused at the ridge axis resulting in a long oblique axial domain, that forms a relay zone between the roughly north-south ridge axis in the south and the area close to the transform fault, while the transform fault domain is highly complex. Trends oblique to both the main spreading axis direction and the transform fault direction are widespread. A clear Principal Transform Displacement Zone (PTDZ) can be followed as a long, near continuous alignment, on the seafloor of the wide Romanche valley. However, the valley morphology suggests a migration of the PTDZ and intense deformation within the transform domain. The RTI is complex and the position of the spreading axis clearly evolved with time, through at least two and possibly three eastward ridge jumps.

Six Nautile dives explored the northern wall of the Romanche, the damaged zone of the transform fault, and the exceptionally deep nodal basin. The north wall exposes a very thick basalt unit covered with a thick layer of sediments. Eight dives explored the southern flank of the Romanche identifying fragments of old Oceanic Core Complexes (OCCs) formed by highly deformed peridotites, and a large OCC located at the RTI that exposes mylonitized peridotites and is dissected by several normal faults. The magmatic zones of the axial domain (nine dives) are formed by volcanic ridges affected by important tectonic activity. The dives show pillow and tube volcanic flows with intersecting faults. An oblique elongated faulted and sedimented ridge (2 dives) parallel to the oblique relay zone was shown to be of peridotitic nature Recent faults have been observed, as well as traces of high-T hydrothermal activity consistent with black-smoker type venting, recently overprinted by low temperature diffuse venting related to active faulting.

How to cite: Maia, M. and Brunelli, D. and the SMARTIES Cruise Scientific Party: The Eastern Romanche ridge-transform intersection (Equatorial Atlantic): slow spreading under extreme low mantle temperatures. Preliminary results of the SMARTIES cruise., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10314, https://doi.org/10.5194/egusphere-egu2020-10314, 2020.

Transform continental margins known across the Earth represent 31% of passive margins. Resulting from first-order plate tectonic processes, transform margins record a diachronous evolution mainly defined by three successive stages, including intra-continental transform faulting, active and passive transform margin. Due to their high complexity and a lack of large hydrocarbon discoveries (i.e. not a target for oil industry), they have only been sparsely studied, especially when compared with other margin types (i.e. divergent or convergent).

                  We present the structure and evolution of the NS-trending Limpopo Transform Fault Zone (LTFZ), corresponding to the main fracture zone from western part of the Africa-Antarctica Corridor (AAC). Here, we combine published and unpublished dataset (seismic reflection profiles, wells, multibeam bathymetry, gravity, magnetic data) in order to propose an interpretation of the LTFZ structure and adjoining segments and their evolution through time, from rifting to spreading.

The LTFZ is composed of two main segments: the East Limpopo segment and the Astrid conjugate one and the North and South Natal segment including the Dana-Galathea Plateau (Mozambique side) and the Maud rise/east of Grunehogna craton (Antarctica margin). The LTFZ offsets the segments of divergent conjugate margins (Southern Natal-off Grunehogna craton in the west and Beira High Angoche-Riiser Larsen Sea in the east) since 155 Ma (chron M25). We focus on the evolution of the transform fault zone from its initiation at chron M25 up to chron M0 (~126 Ma, Barremian). Oceanic spreading onset at chron M25 in the south of Beira High segment and Dana-Galathea Plateau triggered the uplift and erosion of the proximal parts of the margin and the formation of several seaward dipping reflectors wedges. Plate kinematic implies an NNW-SSE opening of the LTFZ. The oblique component of opening promotes the setting up of several volcanic wedges. These wedges rejuvenate southward trough time, which is consistent with the sliding of Antarctica with respect to Africa and thus confirm the diachronous evolution of the transform fault zone.

How to cite: Leroy, S., Roche, V., Guillocheau, F., Dietrich, P., Revillon, S., Watremez, L., Robin, C., Despinois, F., and Vetel, W.: Magma-rich transform margins: example from the Limpopo transform fault zone between Mozambique and Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10508, https://doi.org/10.5194/egusphere-egu2020-10508, 2020.

The Romanche Transform Fault (TF) in the equatorial Atlantic Ocean is the largest oceanic transform fault on Earth, offsetting the slow-spreading (2 cm/ yr) Mid-Atlantic Ridge (MAR) by 900-km and producing a maximum age contrast at the Ridge-Transform Intersection (RTI) of 45 Myr. This offset could cause a large thermal variation in the lithosphere around the RTI, but it is not known how this thermal variation would manifest itself. Here we present a ~21-day-long micro-earthquake study using a temporary deployment of 19 ocean-bottom seismometers (OBSs) during the 2019 SMARTIES cruise. 1363 earthquakes were detected on at least three OBSs and 622 could be located, of which 351 have high location accuracy (mean semi-major-axis of 3.9 km).

Linear (HYPOSAT) and non-linear (NonLinLoc) location algorithms reveal a similar earthquake distribution. Two event groups cluster at depths of 1) 0 km to ~18 km and 2) ~20 km to 30 km. Along the Romanche TF, micro-earthquakes are located beneath the southern border of the 30 km wide transform valley; no events are observed beneath the central or northern sections of the valley. These events' depths increase rapidly and linearly from a few km at the RTI to 30 km at 40 km along the transform fault, indicating a rapid increase in the thickness of the seismogenic zone (and lithosphere) along the transform fault. The presence of earthquakes on the southern border of the transform fault, which is younger and hence warmer, suggests that these events, and hence the seismogenic zone, follow an isotherm separating the brittle-ductile boundary. The absence of seismicity beneath the centre and northern boundary of the transform fault could be due to a much colder lithosphere and hence deeper ductile-brittle boundary.  

An aseismic gap exists beneath the pull-apart basin observed on bathymetry data. Beneath the RTI, earthquakes mainly occur in the 0-18 km depth range. Eight well-constrained focal mechanisms, derived from P-wave polarities, suggest that strike-slip faulting dominates along the transform fault. Normal faults are also observed, which may be attributed to an active detachment fault or pull-apart basin formation.

From the RTI to the tip of the southern MAR segment, micro-earthquakes show an undulating focal depth distribution from north to south. They can be summarized into three clustering groups: the RTI, the 16.6°W group, and the 16.2°W group. Micro-earthquakes beneath the MAR are mainly located in the axial valley. Events in the 16.6°W group mainly occur in the mantle at depths of 12-20 km, whereas those in the 16.2°W group are located at shallow depths of 2-12 km, which is similar to that observed along other slow-spreading Mid-Ocean Ridges. This evidence indicates that there are significant variations in the along-axis thermal structure of the lithosphere along the rift axis.

ZY acknowledges the China Postdoctoral Science Foundation (2019M652041, BX20180080); DB acknowledges funding PRIN2017KY5ZX8.

How to cite: Yu, Z., C. Singh, S., Gregory, E., Crawford, W., Maia, M., and Brunelli, D.: Microseismicity constrains on the lithospheric structure at the ridge-transform intersection at the Romanche Transform Fault and Mid-Atlantic Ridge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6006, https://doi.org/10.5194/egusphere-egu2020-6006, 2020.

Vast areas of the deep ocean floor are still insufficiently explored with respect to tectonic processes, exchange processes between the lithosphere and the ocean, and potential deep chemosynthetic energy sources for life. Transform faults and fracture zones, which are dominant seafloor morphological features in the abyssal ocean, deserve specific attention in this regard as they provide potential pathways for fluid recycling. One of them is the Gloria Fault, a unique feature in the Central North Atlantic. It has been the source of large magnitude earthquakes (namely the 1941, M8.4, the second largest instrumental earthquake on a fracture zone) and is a special case of a plate boundary, corresponding to the transform reactivation of an old oceanic fracture zone. Seismic refraction has shown an anomalous layer between normal lower crust and uppermost mantle, possibly a 4 km thick layer of hydrated mantle. We present first results of RV Meteor cruise M162 (March-April 2020) dedicated to the groundtruthing of potential fluid emanation sites.

How to cite: Hensen, C., Terrinha, P., Duarte, J., Kaul, N., Schmidt, M., Schmidt, C., Batista, L., Magalhāes, V., Liebetrau, V., Kipfer, R., Hübscher, C., and Lever, M.: Quest for Fluid Flow along the Gloria Fault – First results of R/V Meteor expedition 162, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8034, https://doi.org/10.5194/egusphere-egu2020-8034, 2020.

Along the Ecuadorian margin, the North Andean Sliver is moving in the northeastward direction due to the oblique subduction of the Nazca plate. The opening of the gulf of Guayaquil is a consequence of this motion. Two principal models compete to explain the opening. One proposes an opening achieved essentially with strike-slip motion along a single major fault through the gulf, the other with a combination of strike-slip and normal faulting on both sides of the gulf. The consequences in term of seismic hazard are very different. A single strike-slip fault model could imply a long fault segment capable of generating large magnitude events. In contrast, a multi-segments composite fault system will give conditions for producing small to medium size earthquakes. The southern Ecuador subduction zone is characterized by the absence of large historical earthquake. Data from the historical and instrumental seismicity for magnitude above 4 show the forearc has a high level of moderate seismic activity within and around the gulf that connects to the crustal seismic activity of the volcanic arc. In contrast, the forearc elsewhere shows very little or no seismic activity between the marine forearc zone and the volcanic arc. Regional and global CMTS data show a large number of mechanisms within the gulf that do not line up on a simple straight fault system. We present new earthquake data from the recently upgraded national seismic network of Ecuador. They provide the first image of SW-NE trending crustal faults stretching in the central part of the gulf and running eastward south of the Puna island. The main seismic belt appears to be discontinuous, made of short length segments with variable trends. The variety of focal solutions also indicates complex faulting. As the shape of this seismic belt is in good agreement with the orientation of the GPS velocity vectors, this new fault zone is readily interpreted as the southernmost segment of the actual NAS boundary. Others seismic clusters are observed parallel to the northern coast of the gulf, indicating active structures eventually accommodating the North-South opening of the gulf through normal faulting. b-value analysis of the main seismic belt seismicity shows high b value (>1) indicating either highly fractured or heterogeneous medium, or/and low stress level within the gulf of Guayaquil. This is again in agreement with a multi-segmented faulting system and also with the lack of large magnitude event in the historical seismic data. A cross-section for the entire seismic belt shows a depth extend of the crustal seismic activity down to 30 km which confirms the seismic belt to be a sliver boundary.

How to cite: Regnier, M., Ponce, G., Saillard, M., Audin, L., Vaca, S., Alvarado, A., and Ruiz, M.: Characterization of Active Faults Through the Gulf of Guayaquil, Ecuador: implication for the southern boundary of the North Andean Sliver, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11437, https://doi.org/10.5194/egusphere-egu2020-11437, 2020.