Storfjorden Trough Mouth Fan (Western Barents Sea): slope failures in polar continental margins; significance of stress changes and fluid migration induced by glacial cycles

The development of two Arctic Trough Mouth Fans (TMFs), the Storfjorden and Kveithola TMFs, is investigated by means of sub-bottom and seismic reflection profiles, multibeam bathymetry and sediment samples allowing their detailed stratigraphic architecture to be defined. We find that the TMFs mainly consist of an alternation of rapidly deposited glacigenic debris flows during glacial maxima and a sequence of well-layered plumites and hemipelagic sediments, which were mainly deposited during the deglaciation phase of the adjacent glacial trough. We have identified eight units above regional reflector R1, which indicate that the ice sheet reached the shelf edge within the Storfjorden Trough on at least three occasions during the last ~200 ka. A shallow subsurface unit of glacigenic debris flows suggests that the ice sheet had a short re-advance over the northern and central part of Storfjorden after the Last Glacial Maximum. From stratigraphy, core and literature data, we estimate that ice sheets reached the shelf edge between 19.5 to 22.5 ka, 61 to 65 ka and 135 to 167 ka. Detailed seismic imaging allows us to refine the sedimentary model of Arctic TMFs. The main differences to previous models involve gully formation during not only the deglaciation phase, but also during interglacials by dense shelf water cascading, and a specific timing for the occurrence of slope failures (i.e., shortly after the deglaciation phase). High mean sedimentation rates during glacial maxima of up to 18 kg m-2 yr-1 likely allow excess pore pressure to develop in the water rich plumites and hemipelagic sediments deposited in the previous deglacial period, particularly where such plumites attain a significant thickness. Climate variations control sediment supply to the continental slope as well as glacial advances and retreats. This hypothesis has been investigated by using a combination of geophysical and geotechnical data. The results of compressibility and permeability testing are used together with margin stratigraphic models obtained from seismic reflection data, as input for numerical finite elements models to understand focusing of interstitial fluids in glaciated continental margins. BASIN software has been used to model the evolution of the Storfjorden TMF from 2.7 to 0.2 Ma. The model results show that onset of glacial sedimentation (~1.5 Ma) has a significant role in developing permeability barriers (tills) on the shelf decreasing the vertical fluid flow towards the sea floor and switching it towards the slope. Prior to 220 ka, the higher values (up to 0.6) are located below the shelf edge and in the distal area. In turn, Plaxis software has been used to perform a detailed high resolution hydrogeological model of the four glacial/inter-glacial cycles. Ice loading during Glacial Maxima caused a rapid consolidation of the shelf sediments with high fluid expulsion rates towards the shallower plumite sediments on the slope. This caused an overpressure increase inside these layers up to 0.6. The higher overpressures occurred during the Last Glacial Maximum and few thousand years after. Present day overpressures reach maximum values of 0.7 on the shelf. A detailed slope stability analysis through the Last Glacial Maximum, last deglaciation and Holocene has been carried out. Safety factor (SF) of the margin was reduced to one half due to overpressures development during the Last Glacial Maximum with minimum values of ~1.2 which were preserved through the last deglaciation. These low values occurrence match with the time when most of the landslides identified in the study are occurred. Although the safety factor values are close to the instability, mechanisms as glacio- eustatic rebound induced earthquakes can be considered as a final trigger.L’evolució de dos ventalls glacials, Storfjorden i Kveithola, s’ha investigat fent servir dades geofísiques i mostres de sediment. D’aquesta manera s’ha pogut establir de forma detallada la seva estratigrafia. A partir de l’anàlisi estratigràfic i dades de literatura s’ha pogut establir que el gel va ocupar la plataforma continental com a mínim en tres ocasions durant els últims ~200 ka: 19.5 a 22.5 ka, 61 a 65 ka i 135 to 167 ka. A més la bona cobertura i resolució de les dades geofísiques ha permès de refinar el model d’evolució dels ventalls glacials. Les diferències més important respecte a treballs previs corresponen a que la formació de xaragalls no dóna només en els deglacials, sinó també en els interglacial degut a les aigües denses de plataforma, i també l’ocurrència de esllavissades durant la deglaciació del marge. En aquest estudi queda palès el control climàtic en la tassa de sedimentació, els sediments dipositats i els avanços i retrocessos de gel. Aquestes variacions influeixen en el patrons de fluid del subsòl i el desenvolupament de sobrepressions en els sediments. Aquesta hipòtesi ha estat investigada utilitzant models d’elements finits (BASIN i Plaxis) a partir de les dades geofísiques i assajos geotècnics. La modelització dels darrers 2.7 Ma mostra que l’inici dels períodes glacials, fa aproximadament 1.5 Ma, va promoure la formació bandes de baixa permeabilitat (tills) a la plataforma. La càrrega de gel a la plataforma durant els màxims glacials, va provocar una ràpida consolidació dels sediments subjacents i l’expulsió de l’aigua intersticial, incrementant així el flux de fluids cap al talús superior. Alhora, aquest flux focalitzat cap als sediments més permeables del talús (plumites) va incrementar dràsticament les sobrepressions. Les sobrepressions màximes van tenir lloc durant l’últim màxim glacial i els següents mil·lennis. Degut a aquestes sobrepressions, el factor de seguretat del talús es va reduir a la meitat, arribant a valors de 1.2 al final de l’últim màxim glacial. Tot i els que aquest valors són baixos, els terratrèmols provocats pel rebot glacio-eustàtic han de ser considerats com a desencadenants finals de les esllavissades identificades a la zona