Geolunch seminar @ UiB

Leonardo Muniz Pichel
2020-11-18 12:15 - 13:00 (Europe/Oslo)
Salt Tectonics in the Santos Basin, Brazil: the effects of translation, base-salt relief and the enigma of the Albian Gap
Salt-driven translation over variable base‐salt relief produces three‐dimensionally complex and multiphase deformation. Understanding how pre- and base-salt structures influence salt‐related deformation is important to constrain regional salt tectonic kinematics (i.e. direction, timing and magnitude of salt flow) and the distribution of salt tectonic domains. The São Paulo Plateau, Santos Basin, Brazil is characterized by a >2 km thick, Aptian salt deposited above prominent base‐salt relief. We use 3D seismic data, numerical and kinematic models to investigate how gravity‐driven translation above thick salt, underlain by variable base‐salt relief, generated a complex framework of salt structures and minibasins. We show that ramp‐syncline basins (RSBs) developed above and downdip of the main pre‐salt highs and record c. 30 km of post-Albian basinward translation. As salt and overburden moved downdip, salt flux variations over base‐salt relief resulted in the coeval development of extensional, contractional and load-driven salt structures in the São Paulo Plateau. This style of salt deformation and the magnitude of translation afforded an improved kinematic model for the enigmatic Albian Gap, located immediately updip of the São Paulo Plateau.
The Albian Gap is a uniquely large (up to 65 km wide and >450 km long) structure characterized by the near-complete absence of Albian strata above depleted Aptian salt, and a 8-9 km thick basinward-dipping post-Albian rollover. Owing to its unique geometry, size and counter-regional aspect, the origin and evolution of the Albian Gap has been the centre of debate for >25 years. This debate revolves around two competing models; that is, did it form as a result of thin-skinned (i.e. supra-salt) extension, or progradational loading and salt expulsion? We combine regional 2D PSDM seismic data and novel structural restoration workflows that incorporate flexural isostasy, in addition to a detailed, sequential reconstruction of the intra-gap rollover sequences. We show that the geometry and kinematics of the Albian Gap vary along-strike, and that both post-Albian extension and expulsion play a significant role in its evolution. Where the Albian Gap is relatively wide (>50 km), these processes alternate and operate at approximately equal proportions. Our results seemingly reconcile one of the longest-running debates in salt tectonics, having also more general implications for understanding the regional kinematics and dynamics of salt tectonics in other basins, in particular the controls on the development of large, counter-regional faults and the effects of translation and pre-salt relief.

Thomas Theunissen
2020-11-04 12:15 - 13:00
Relative continent/mid-ocean ridge elevation: a reference case for isostasy in geodynamics
The choice on crustal and mantle densities in numerical geodynamics model is usually based on common habits. However, lithospheric deformation involving regional or fault-related flexure or involving strong change in elevation as distal rifted margin formation or long-term erosion of mountain belt require good constrain on densities to provide a reasonable estimation of isostatic response. Accommodation space for sediments, absolute elevation (gravity potential and conditions for erosion and deposition) and long term survival of relief depends on isostasy.
We use 2-D incompressible plane strain finite element method for viscous-plastic creeping flows coupled with a melt prediction routine to solve for the dynamic evolution during extension from continental rifting to mid-ocean spreading. Two independent melting prediction procedure to compute density structure below the mid-ocean ridge are compared in order to evaluate sensitivity to phase changes on solution. The first approach considers the use of P-T density maps and melt prediction from thermodynamic solution. A simple conversion from batch to incremental melting is proposed in order to use only one thermodynamic solution. The second approach considers a simpler approach that uses approximate mantle solidus to compute melt fraction and degree of depletion.
Both approaches provide a solution to fit mid-ocean ridge elevation. Crustal and mantle densities and mid-ocean ridge elevation are calibrated based on observations. We use thermodynamic solution using characteristic bulk mantle compositions to estimate the average degree of depletion of the lithospheric mantle repectively -15 kg/m3, -28.5 kg/m3 and -35 kg/m3 respectively for 125 km, 180 km and 240 km thick lithospheres. The average crustal density is 2850 kg/m3 in this reference model in agreement with global and regional estimations. Finally the reference model fits the characteristic mid-ocean ridge elevation of -3000 m +/- 100 m and continental elevation of 300 m +/- 100 m obtained from a global analysis. This study provides a reference case for isostasy in geodynamics and we discuss the importance of such reference model to use geodynamic numerical models to calculate subsidence and melting histories which could be compared with observation.

Hasbi Ash Shiddiqi
2020-10-28 12:15 - 13:00
More details soon !

Christian Haug Eide
2020-10-21 00:15 - 13:00
Underneath the volcano: Trying to understand how igneous sills are emplaced using outcrops and seismic from East Greenland

Here I will tell you about our ongoing work to understand sill emplacement, using data from Jameson Land in East Greenland. It is a unique onshore basin, in many ways analogous to the Mid-Norwegian margin, where host-rocks and sills occur in both outcrops AND crustal-scale seismic data. This makes it possible to investigate how igneous intrusions are imaged in seismic data, how lithology influences sill emplacement, and possibly how large-scale tectonic stresses influence geometries of the igneous system.

Christopher Jackson
2020-10-14 12:15 - 13:00
Hot rocks under our feet; seismic reflection imaging of the processes and products of crustal magmatism
Volcanoes are hot, loud and scary, thus we know little of their internal structure or underlying ‘plumbing systems’. Magma intrusion in the Earth’s crust, and the extrusion of this material on its surface, represent a natural (geo)hazard; however, these processes can also result in the formation of economic quantities of hydrocarbons. In this talk I will show how 3D seismic reflection data can illuminate the structure and emplacement mechanics of these ‘hot rocks’. I will also demonstrate the key role igneous processes and products play in the development of petroleum systems on continental margins.

———————————————————–
Professor Christopher Aiden-Lee Jackson
Equinor Professor of Basin Analysis
Basins Research Group (BRG)
Department of Earth Science & Engineering
Imperial College
Prince Consort Road
LONDON
SW7 2BP
UK

Email: c.jackson@imperial.ac.uk
Web: www.imperial.ac.uk/c.jackson
Twitter: @seis_matters
———————————————————–

Vilde Dimmen
2020-05-20 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Seismic Modelling of Normal Faults

Kui Xiang
2020-05-13 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Seismic Waveform Modelling

EGU practice talks
2020-04-29 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway

Gang Lu
2020-04-22 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Rifting models with magmatism

Lorenz Michel
2020-04-01 00:15 - 13:00
Investigating feedbacks between tectonics and earth surface processes during rifting and continental breakup
Disentangling the interactions and possible feedbacks between tectonic and earth surface processes has been a focus of geoscientific research. Many previous studies were conducted in convergent settings and fewer studies focused on extensional settings. Here, we model the formation of continental extensional rift systems, using a geodynamic model coupled with a landscape evolution model.
We compare, how the structure of the subsequent rifts, topography and sedimentary yield evolve over time depending on the combination of five key parameters. This includes varying the crustal rheology, duration of extension, distribution of inherited strain, efficiency of erosion and the base level for erosion. The modeling results show that structure and topography of the intra-continental rifts strongly depend on crustal strength, on the distribution of inherited strain and on the duration of extension. High erosion rates are observed for models with significant topography and elevated rift shoulders. Hence, models that produce high topography (i.e. models with higher crustal strength) display a significant sediment flux during the syn-rift phase. In the post-rift phase, the sediment flux is often impacted by river capture events, which are more likely to occur for models with lower rift shoulders. Taken together, these observations suggest that there is a strong dependency of the sedimentary flux on the respective tectonic setting but also highlight the role played by earth surface processes, in particular in the post rift phase.

Thomas Theunissen
2020-03-25 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Distal margin formation

Björn Nyberg
2020-03-11 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Assessing First-order BQART Estimates for Ancient Source-to-Sink Mass Budget Calculations: The Sognefjord Source-to-Sink system, Northern North Sea Rift.
Constraining the timing and volume of sediment dispersal in an ancient sedimentary system is vital to understand a basins infill history. In recent years, first-order approximation of ancient sediment load to a basin has relied on empirical observations of modern riverine systems. The empirical relationships apply simplified understanding of hinterland controls such as basin morphology, topography, climate, runoff and bedrock characteristics to observed river sediment loads. Despite the popularity of those methods, a comprehensive assessment on the validity of modern observations to measure sediment load on geological timescales is lacking. Here we investigate the uncertainties, sensitivities and practicalities surrounding the use of modern empirical observations to evaluate ancient sediment river loads using a case study of the Middle to Late Jurassic Sognefjord source to sink (S2S) system, offshore Norway. Combined with new regional seismic data, the full extent of the shallow-marine siliciclastic successions across the Horda Platform provides a well-constrained sedimentary system to measure unaccounted volume of sediment.

Hasbi Ash Shiddiqi
2020-03-04 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Crustal Structure of Nordland: a preliminary result from seismic tomography
Nordland is the most seismically active region in mainland Norway. Historically, the 1819 Lurøy / Rana earthquake (magnitude estimate 5.8 – 6.0) was the largest earthquake in northern Europe for the last 200 years. In recent years, several series of earthquake swarms occurred along the coastal area of Nordland. In this study, we aim to investigate the causative mechanism of intraplate seismicity and swarm in Nordland by using seismic tomography. Additionally, we also intend to investigate the crustal structure variation in this region, especially beneath Lofoten and the Northern Scandinavian mountain. We used 602 earthquakes with a total of ~8000 P- and ~6500 S-wave arrival times recorded by 89 seismic stations. In the inversion, hypocenters, Vp, Vp/Vs and station delays are jointly inverted. Our preliminary Vp model shows a good agreement with the crustal thickness estimate from receiver function studies. We also found a relatively high Vp/Vs ratio in the lower crust below the mainland Nordland. However, more synthetic tests and other quality assessment are needed to confirm the reliability of this anomaly.

Atle Rotevatn
2020-02-19 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
High in the sky: geology and flying
Atle Rotevatn, who in addition to being a professor in our department is also well into his training to become a pilot (PPL-A for those familiar with flying). In this talk he will show pictures and video from geology spotting flying trips in New Zealand, as well as from spectacular flights in Greenland (although these latter ones were piloted by actual proper pilots). He will introduce us to the combined joys of flying and geology, and we will spend time in the New Zealand Alps, the southern coast of New Zealand, and see some beautiful braided river systems from low altitude in a single-engine aircraft. We will also spend some time in Greenland, looking down at the amazing deltas and coastal geomorphology of the NE Greenland national park. Perhaps also other members of staff or students will want to get into flying after this?

Casey Nixon
2020-02-12 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Increasing fault-slip rates within the Corinth Rift: A localising active rift fault network
The young (<5 Ma) and active Corinth Rift, Greece, is one of the fastest extending rift zones in the world with present day extension rates of up to ~15 mm/yr. High extension rates combined with high sedimentation rates make the rift a perfect laboratory for investigating early rift development and processes of fault evolution within rifts. The offshore syn-rift sediments of the rift have recently been drilled during the International Ocean Discovery Program (IODP) Expedition 381, which importantly provides details and constraints on the nature and age framework of the basin infill for the first time. Combing results from IODP Expedition 381 with a refined rift fault network and stratigraphic framework for the entire rift, we examine fault activity throughout the Corinth Rift determining the time-scales and slip-rate variation during strain localisation and the establishment of a border fault system.
Results from IODP Expedition 381 confirm 10s-100s of kyr cyclic variations in basin paleoenvironment, alternating between marine and isolated/semi-isolated intervals as eustatic sea level fluctuated with respect to basin sills. Correlation of paleomagnetic data and palaeontological assemblages with relative sea-level curves provides key age constraints and timing of marine intervals over the last ca. 800 kyr. A dense network of marine seismic reflection profiles allows these age constraints to be extrapolated and applied to horizons throughout the entire rift, for which detailed displacement data have been collected for the rift fault network.
Along strike cumulative displacement profiles of faulting within the Gulf of Corinth indicate that overall basement deformation has been equally distributed between major S-dipping and N-dipping faults over the last ca. 2-2.5 Myr. This shows two peaks in cumulative displacement that coincide with the early development of two depocentres before ca. 800 ka. However, since this time the distribution of displacement has been proportionally increasing on N-dipping faults within the rift. Fault maps show that this is related to a decrease in activity on major S-dipping faults between ca. 800-340 ka and is accommodated by numerous small faults that form within the syn-rift deposits at this time. This process is reflected in isochore maps of the syn-rift sediments, which show that the central Gulf of Corinth experienced a switch in rift polarity from a dominant N-thickening depocentre to a dominant S-thickening depocentre.
Since ca. 800 ka the rift has experienced an increase in activity on numerous N-dipping faults along the southern margin, which form the present basin bounding border fault system. Fault slip-rate patterns for these faults show an evolution towards a coherent bell-curve distribution in activity by ca. 130 ka, indicating a linked fault system at depth. As these faults have become kinematically and geometrically linked, slip-rates across the fault system have continued to significantly increase up to ca. 5 mm/yr by ca. 130 ka. Thus the majority of the basins deformation has rapidly localised onto the border fault system during its establishment.

Sebastian Wolf
2020-02-05 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
How does topography evolve during growth and decay of a mountain belt – inferences from coupled tectonic-surface process models
It has been a long-standing problem how mountain belts gain and loose topography during their tectonically active growth and inactive decay phase. It is widely recognized that mountain belt topography is generated by crustal shortening, and lowered by river bedrock erosion, linking climate to tectonics. However, it remains enigmatic how to reconcile high erosion rates in active orogens as observed in Taiwan or New Zealand, with long term survival of topography for 100s of Myrs as observed for example
in the Uralides and Appalachians. Here we use for the first time a tight coupling between a landscape evolution model (FastScape) with an upper mantle scale tectonic (thermo-mechanical) model to investigate the different stages of mountain belt growth and decay. Using two end-member models, we demonstrate that growing orogens with high erosive power remain small (<200 km), reach steady state between tectonic in- and erosional material eff-flux, and are characterized by transverse valleys. Contrarily, mountain belts with medium to low erosive power will not reach growth steady state, grow wide, and are characterized by longitudinal rivers deflected by active thrusting. However, during growth both types of orogens reach the same height, controlled by rheology and independent of surface process efficiency. Erosional efficiency controls orogenic decay, which is counteracted by regional isostatic rebound. Rheological control of mountain height implies that there is a natural upper limit for the steepness index of rivers on Earth. To compare model results to various natural examples, we quantify the degree of longitudinal flow of modeled rivers with river ³longitudinality² in several active or recently active orogens on Earth. Application of the river ³longitudinality index² gives information whether (parts of) an orogen is or was at steady state during orogenic growth.
Model snapshot

Hallgeir Sirevaag
2020-01-29 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Detrital zircon inventory of the Triassic Greater Barents Sea Basin: sediment transport and geodynamics
Hallgeir Sirevaag, Christian Haug Eide, Albina Gilmullina, Leif-Erik Rydland Pedersen
The intra-cratonic Greater Barents Sea Basin (GBSB) represents one of the last underexplored petroleum provinces in the world. It is believed to contain most of the yet-to-find hydrocarbon resources on the Norwegian continental shelf, and significant resources are also present within the Russian sector of the Barents Sea (e.g. Shtokman discovery). Several of the hydrocarbon reservoirs within the Barents Sea are of Triassic age (e.g. Goliat field). The reservoir properties are influenced by the mineralogical composition, and the reservoir quality is therefore also strongly linked to the source area for the sediment. Within the GBSB, c. 2200 previously published U-Pb detrital zircon ages from 26 Triassic samples have provided information on the provenance for the Triassic reservoirs. These analyses indicate two main sources for the Triassic succession in GBSB: a south-southwestern source related to the Fennoscandian Shield and the Caledonides contributes with mature sediments with favorable reservoir qualities, while an eastern source from the Urals and the Siberian Traps provided immature sediments with poor reservoir properties. Using U-Pb analyses on detrital zircons for pin-pointing the source area and understanding their geodynamic evolution is therefore useful for predicting the reservoir quality.
Decades of hydrocarbon exploration in the GBSB have made it possible to obtain detrital zircon data from time-equivalent formations over large areas. The GBSB is therefore an ideal basin for a large-scale source-to-sink study of an intra-cratonic basin. Within the ISBAR-project at UiB, we aim to combine previously published U-Pb data with new U-Pb analyses on stratigraphic intervals and geographic areas with low sampling density. This will be done in order to obtain a holistic view of the Greater Barents Sea Basin throughout the Triassic, with respect to temporal and spatial variations in sediment influx. These data can provide information on the relative contribution from the different sediment sources in the basin, and furthermore, and can be used for gaining insight into the geodynamic evolution of the source areas.

From Eide et al. (2018)

Mahmoud Eltayieb
2019-12-10 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Electromagnetic modelling and inversion using integral equation methods

AGU practice talks
2019-12-04 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
4 AGU practice talks:

Helene Meling Stemland | Applications of surface seismic on thawing saline permafrost: examples from the Norwegian Arctic

Anneleen H. Geurts | Drainage integration in active continental rifts: tipping the balance between basin filling and subsidence

Hasbi Ash Shiddiqi | Implications of 3D teleseismic ray-tracing on focal mechanism determination

Felix Julian Halpaap | Earthquakes track subduction fluids from their source in the slab to their sink in the mantle wedge

Applications of surface seismic on thawing saline permafrost: examples from the Norwegian Arctic
Helene Meling Stemland1,3, Tor Arne Johansen1,2,3 , Bent Ole Ruud1,3
1 University of Bergen, Department of Earth Science, PO Box 7803, 5020 Bergen, Norway.
2 The University Center in Svalbard (UNIS), PO Box 156, 9171 Longyearbyen, Norway.
3 ARCEx, UiT The Arctic University of Norway, PO Box 6050 Langnes, 9037 Tromsø, Norway.
Arctic surface temperatures are increasing more rapidly than the world average due to feedback mechanisms, and an implication of this is thawing of currently frozen ground. Saline permafrost is particularly prone to following geomorphic consequences because saline pore-water gradually changes phase at subzero temperatures, hence even minor subzero temperature variations can lead to significant changes in the degree of freezing in such areas. To detect and monitor permafrost degradation, non-intrusive methods are desirable due to efficiency and environmental constraints. The effective elastic properties of the subsurface depend partly on the degree of freezing of pore-water. Since seismic surface waves are sensitive to changes in shear-wave velocity in particular, thawing of even very thin layers is reflected by changes in surface wave characteristics.
We have acquired seismic data in Adventdalen on Svalbard in the Norwegian Arctic during several field seasons using a variety of seismic sources, and we here investigate how the data vary with time and source type. Vertical geophones record surface waves of Rayleigh type. Data acquired with explosives show the strongest surface waves, but are dominated by an air wave. Thawing in the uppermost sediments usually affects high frequencies, but we here observe variation with time even at frequencies <50 Hz, likely related to higher modes of Rayleigh waves. We observe an inversely dispersive trend in frequency-velocity spectra of seismic records from winter, indicating dominant higher modes. This is typical for low-velocity sediments embedded in higher velocity sediments, and Rayleigh wave dispersion curves extracted from synthetic seismograms made for such a geometry provide a good match with observations. Previous studies have suggested that Adventdalen consists of eolian low-salinity sediments overlying raised marine high-salinity sediments. Thus, the upper sediments are more frozen (i.e. higher velocity) than those below during winter. Such an inverse velocity gradient makes traditional refraction seismic methods fail, and our results indicate that seismic surface wave methods are a viable option for monitoring thawing of frozen sediments. However, inversion of surface wave data with higher modes is not straightforward and requires further attention.

Drainage integration in active continental rifts: tipping the balance between basin filling and subsidence
Anneleen H. Geurts, Alexander C. Whittaker, Rob L. Gawthorpe, and Patience A. Cowie
Drainage integration is an important phenomenon in the evolution of continental rifts. While fault-bounded basins are often internally drained in the early stages of extension, they tend to become progressively integrated with one another over time which has implications for both basin stratigraphy and landscape evolution. However, the drivers of drainage integration remain poorly understood. Here we use the actively extending central Italian Apennines to investigate the contribution of the river network itself to long-term (~3 Myr) drainage integration. We integrate detailed stratigraphic and geomorphic datasets for one of the largest river systems in the area (the Aterno river system) with results from regional-scale numerical modelling work. Our data compilation shows a disordered spatio-temporal pattern of drainage integration that we interpret to mainly result from the infilling and overspilling of fault-bounded basins and not from headward erosion from externally-draining catchments. Rates of sedimentation and basin subsidence in the central Apennines were almost matched during the initial phases of faulting, and only a small climate- or tectonic-induced increase in sediment supply was sufficient to tip the balance towards overfilled conditions. Slip acceleration associated with fault interaction in this region can explain why we observe in some stratigraphic records the re-establishment of underfilled conditions. Drainage integration abruptly modifies sediment dispersal patterns, as well as the magnitude and characteristics of sediment fluxes. Moreover, it induces successive waves of fluvial erosion associated with individual integration events. The rates at which these erosional waves propagate upstream suggest that the effects of drainage network evolution can persist in landscapes and sediment routing systems for millions of years following complete drainage integration. We conclude that drainage integration is just as important as tectonics and climate in controlling transient landscape evolution and basin stratigraphy in continental rifts and we recommend it to be considered explicitly as a driver of landscape evolution and sediment routing in areas of active extension, rather than simply as a product of external forcing.

Implications of 3D teleseismic ray-tracing on focal mechanism determination
Katharina Newrkla1, Hasbi Ash Shiddiqi1, Annie Elisabeth Jerkins2, Henk Keers1 and Lars Ottemöller1, (1)University of Bergen, Bergen, Norway, (2)NORSAR, Kjeller, Norway
This study investigates the apparent mismatch of first motion polarities at teleseismic distances in focal mechanism determination. We hypothesize that especially deviations from the 1D model in the larger source region are responsible for this mismatch. In this study, four seismic ray tracing algorithms are implemented in a spherical coordinate system to compute ray paths and travel times. We compare the results of the Euler, symplectic Euler, midpoint, and 4th-order Runge-Kutta numerical methods in order to determine their accuracy. The Runge-Kutta method, which produces the most accurate result, is used to compute the take-off angles from the hypocenter of the 24 August 2016 Chauk Mw 6.8 earthquake (depth 90 km) in Central Myanmar to four seismic stations in Europe using a 3D regional velocity model of the upper mantle below Asia. In a previous study, the observed waveform polarities from these stations did not match the focal mechanism solution of the 2016 Chauk earthquake derived using a 1D velocity model. The differences in the azimuthal angles calculated in the 1D and 3D velocity models are considerable, with a maximum of 19.6 degree. We can resolve an apparent polarity mismatch by using the take-off angles for the 3D velocity model. If this is done then these stations move from the dilatational to the compressional quadrant. The polarities of synthetic waveforms change accordingly when we use the take-off angles corresponding to the 3D model. This method has the potential to improve the focal mechanism solutions, especially for historical earthquakes for which limited waveform data are available.

Earthquakes track subduction fluids from their source in the slab to their sink in the mantle wedge
Felix Julian Halpaap1, Stephane Rondenay1, Alexander Perrin2, Saskia D B Goes3, Lars Ottemöller1, Florian Millet4, Håkon Austrheim5, Robert Shaw6 and Thomas Eeken7, (1)University of Bergen, Bergen, Norway, (2)Imperial College London, London, SW7, United Kingdom, (3)Imperial College London, Department of Earth Science and Engineering, London, SW7, United Kingdom, (4)LGLTPE Laboratoire de Géologie de Lyon : Terre, Planètes et Environnement, Villeurbanne Cedex, France, (5)University of Oslo, Oslo, Norway, (6)Imperial College London, London, United Kingdom, (7)Madison, WI, United States
Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. The relationship between these fluids and earthquakes has been debated. Here we combine new seismic observations and new thermal-petrologic models from the Western Hellenic subduction zone in Greece to investigate the relationship between fluids and earthquakes. From seismic images and relocated hypocenters of deep earthquakes (>35 km), we observe that earthquakes occur in three domains of the subduction zone: in the slab, on the plate interface, and in the mantle wedge. These observations are supported by a systematic difference in focal mechanisms and secondary arrival patterns (caused by the interactions of the local seismic waves with subduction discontinuities) between the domains. By comparing across other subduction zones where mantle wedge earthquakes occur (Greece, Lesser Antilles, E Honshu, New Zealand), we find that earthquakes in these cold slabs effectively track the flow of fluids from their source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface. In some locations, this seal breaks and fluids escape from the slab, through a vent on the interface, into a dry mantle wedge. Along their migration path, the fluids cause earthquakes in the slab, on the plate interface, and in the overlying mantle wedge, while the forearc slab updip of the vents consistently lacks earthquakes due to being fluid-starved. In eleven years (2006-2017) of detailed observations for Greece, the interface vents persistently hosted only small (Mw ≤ 3.7) earthquakes. Their combined release of seismic moment points to a small (55 km), suggesting that the vents are relevant for the nucleation of damaging earthquakes.

Jan Michalek
2019-11-20 00:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
The European Plate Observing System portal

Pr Renato de Paes Almeida
2019-11-13 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway

Jhon Meyer Munoz
2019-11-06 00:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway
Supra-detachment basin – Norwegian passive margin

Raisya Noor Pertiwi
2019-10-30 12:15 - 13:00 | Kontinentalsokkelen (2G16e), Realfagbygget, Allégaten 41, Bergen, 5007, Norway

Felix Halpaap | Seismology
2019-10-23 12:15 - 13:00
Earthquakes track subduction fluids from slab source to mantle wedge sink

Felix Halpaap1, Stéphane Rondenay1, Alexander Perrin2, Saskia Goes2, Lars Ottemöller1, Florian Millet1,3, Håkon Austrheim4, Robert Shaw2 and Thomas Eeken2

1) Department of Earth Science, University of Bergen, Bergen, Norway
2) Department of Earth Science and Engineering, Imperial College London, London, UK
3) Laboratoire de Géologie, Claude Bernard University Lyon, Lyon, France
4) Physics of Geological Processes (PGP), The Njord Centre, Department
of Geosciences, University of Oslo, Oslo, Norway

Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. The relationship between these fluids and earthquakes has been debated. Here we combine new seismic observations and new thermal-petrologic models from the Western Hellenic subduction zone in Greece to investigate the relationship between fluids and earthquakes. From seismic images and relocated hypocentres of deep earthquakes (>35 km), we observe that earthquakes occur in three domains of the subduction zone: in the slab, on the plate interface, and in the mantle wedge. These observations are supported by a systematic difference in focal mechanisms and secondary arrival patterns (caused by the interactions of the local seismic waves with subduction discontinuities) between the domains. By comparing across other subduction zones where mantle wedge earthquakes occur (Greece, Lesser Antilles, E Honshu, New Zealand), we find that earthquakes in these cold slabs effectively track the flow of fluids from their source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface. In some locations, this seal breaks and fluids escape from the slab, through a vent on the interface, into a dry mantle wedge. Along their migration path, the fluids cause earthquakes in the slab, on the plate interface, and in the overlying mantle wedge, while the forearc slab updip of the vents consistently lacks earthquakes due to being fluid-starved. In eleven years (2006-2017) of detailed observations for Greece, the interface vents persistently hosted only small (Mw ≤ 3.7) earthquakes. Their combined release of seismic moment points to a small (<0.4 %) fraction of coseismic compared to aseismic slip between the plates. However, the vents coincide with the locations of three major earthquakes (Mw 5.7, 6.8, ~7.0) that occurred at unusually large depths on the plate interface (>55 km), suggesting that the vents are relevant for the nucleation of damaging earthquakes.

Carolina Cavalcante
2019-09-18 00:15 - 13:00
When hot orogeny goes samba: the case of the Araçuaí belt, SE Brazil

Kyrre Heldal Kartveit
2019-05-29 12:15 - 13:00
soon…

anonymous
2019-05-22 12:15 - 13:00
soon… This slot is free

Edoseghe Osagiede
2019-05-15 12:15 - 13:00
Selective influence of pre-existing intra-basement shear zones on subsequent rift-related normal faults
Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift-related normal faults. However, the extent to which physical and kinematic interaction between pre-existing shear zones and younger rift faults control the growth of normal faults is less-well understood. Using 3D reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-SW- and N-S-striking rift faults that define a coeval Middle Jurassic – Early Cretaceous, non-colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear zone. The faults either tip-out above or physically merge with the underlying shear zone. For faults that merges with the basement shear zone, a change from tabular to wedge-shaped geometry of the hangingwall synrift strata records a transition from non-rotational to rotational extension faulting, which we attribute to the time of rift fault’s linkage with the shear zone, following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset (rather than link with) intra-basement shear zones. These observations highlight the selective influence pre-existing intra-basement shear zones have on evolving rift-related normal faults.

Stéphane Rondenay
2019-05-08 12:15 - 13:00
A preview of the Bergen Arcs Seismic Experiment (BASE) and how the data collected will be automatically processed
In this talk, I will give an overview of the Bergen Arcs Seismic Experiment (BASE), which is currently ongoing and aims to image the deep structure associated with the Bergen Arcs and Hardangerfjord Shear Zone. Though we don’t have any images to show yet (the stations are still in the field), I will discuss how we plan to analyse the data with an automated processing scheme that was developed as part of project GLImER – a global database of teleseismic Receiver Functions that allows us to image structure of the crust and upper mantle at continental scale.

Timothy Reston
2019-03-13 12:15 - 13:15 | The usual place, Room 2G16c, 2nd floor, Realfabyyget, 41 Allégaten, Bergen, 5007, Norway
Detachment faulting during continental breakup and slow/ultraslow seafloor spreading
Detachment faults, apparently gently-dipping long offset normal faults, are observed in some continental rifts, at some rifted margins, and at both slow and ultraslow spreading ridges. Key questions revolve around their angle and mechanics of slip: many papers have suggested that such detachments only ever slipped at high angles and have passively rotated to low angle. In this talk, I will discuss the mechanics of detachment faults at rifted margins, at slow- and ultra-slow spreading ridges.
At rifted margins, detachments include the S reflectors (Galicia and N Biscay margin), the P detachment (Porcupine Basin) and the H detachment (S Iberia Abyssal Plain). The best constrained of these is the Galicia S reflector, imaged by a 3D seismic volume we collected in 2013. The data have been processed through to prestack time migration, subsequently depth-converted, revealing the true 3D geometry of S and its relationship with the overlying block-bounding faults. S is corrugated, confirming it is a slip surface: moving oceanwards the corrugations swing round from E-W to ESE-WNW, suggesting that rifting may have migrated. The faults above S die out and link laterally, requiring several faults to have ben active simultaneously: heave analysis allows us to group the faults into sets of contemporaneous structures. Where the landward-most fault in each set detaches onto S, S is distorted as if the fault continues oceanwards as S, cutting across the more landward and older portions of S. These observations suggest that S formed by some sort of rolling hinge mechanism, in which an apparently continuous detachment comprises the rotated root zones of successive normal faults, each cutting steeply through the hanging wall of the preceding, more landward fault. However, the synrift geometries show that S must have slipped at ~20°, and as noted above, several faults (rather than a single fault) were active simultaneously. We suggest that the latter is a requirement of the 3D nature of the world, and that the former indicates that S only developed once the entire crust had become brittle, allowing mantle hydration and the formation of weak (serpentine?) fault rocks. IODP Proposal 943-Full sets out to test this interpretation, among other aspects of the rifting to breakup, by dating the synrift sequences across the margin.
To the south, a similar detachment (H) in the South Iberia Abyssal Plain is truncated oceanwards by another detachment that exhumes mantle. The wide expanse of mantle that lies oceanward is itself characterized by landward-dipping reflections, which we interpret as the root zone of successive detachments, each of which moves asymmetrically with its hanging wall to be cut by a new fault cutting through its footwall. The same model explains the unroofing of wide expanses of mantle during amagmatic spreading at the Southwest Indian Ridge; kinematic analysis suggest however that these fault must have initiated as very steep structures in marked contrast to S. Seismicity also suggests that oceanic detachments, controlling the formation of oceanic core complexes (e.g 13°20’N, which we investigated during a cruise in 2016), also dip steeply (75-80°) at depth, and thus might have developed from dikes. Thus there appear to be detachment and detachments active during rifting and seafloor spreading.
