A song of volumes, surfaces and fluxes: The case study of the Central Mallorca Depression (Balearic Promontory) during the Messinian Salinity Crisis

The Central Mallorca Depression (CMD) located in the Balearic Promontory (Western Mediterranean) contains a well-preserved evaporitic sequence belonging to the Messinian Salinity Crisis (MSC) salt giant, densely covered by high- and low-resolution seismic reflection data. It has been proposed recently that the MSC evaporitic sequence in the CMD could be a non-deformed analogue of the key MSC area represented by the Caltanissetta Basin in Sicily. This presumed similarity makes the CMD an interesting system to better understand the MSC events. Physics-based box models of the water mixing between sub-basins, built on conservation of mass of water and salt, help constrain the hydrological conditions under which evaporites formed during the MSC. Those models have been widely used in the literature of the MSC in the past two decades. They have been mostly applied to the Mediterranean Sea as a whole focusing on the Mediterranean–Atlantic connection, or focusing on the influence of the Sicily Sill connecting the Western and Eastern Mediterranean Sea. In this study, we apply a downscaled version of such modelling technique to the CMD. First, we quantify the present-day volumes of the MSC units. We then use a reconstructed pre-MSC paleo-bathymetry to model salinity changes as a function of flux exchanges between the CMD and the Mediterranean. We show that a persistent connection between the CMD and the Mediterranean brine near gypsum saturation can explain volume of Primary Lower Gypsum under a sea level similar to the present. For the halite, on the contrary, we show that the observed halite volume cannot be deposited from a connected CMD-Mediterranean scenario, suggesting a drawdown of at least 850 m (sill depth) is necessary. Comparison between the deep basin halite volume and that of the CMD shows that it is possible to obtain the observed halite volume in both basins from a disconnected Mediterranean basin undergoing drawdown, although determining the average salinity of the Western Mediterranean basin at the onset of drawdown requires further investigation

A Tsunami Generated by a Strike-Slip Event: Constraints From GPS and SAR Data on the 2018 Palu Earthquake

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 Mw = 7.5 Palu earthquake (Sulawesi, Indonesia). With a predominantly strike-slip mechanism, the question remains whether this unexpected tsunami was generated by the earthquake itself, or rather by earthquake-induced landslides. In this study we examine the tsunami potential of the co-seismic deformation. To this end, we present a novel geodetic data set of Global Positioning System and multiple Synthetic Aperture Radar-derived displacement fields to estimate a 3D co-seismic surface deformation field. The data reveal a number of fault bends, conforming to our interpretation of the tectonic setting as a transtensional basin. Using a Bayesian framework, we provide robust finite fault solutions of the co-seismic slip distribution, incorporating several scenarios of tectonically feasible fault orientations below the bay. These finite fault scenarios involve large co-seismic uplift (>2 m) below the bay due to thrusting on a restraining fault bend that connects the offshore continuation of two parallel onshore fault segments. With the co-seismic displacement estimates as input we simulate a number of tsunami cases. For most locations for which video-derived tsunami waveforms are available our models provide a qualitative fit to leading wave arrival times and polarity. The modeled tsunamis explain most of the observed runup. We conclude that co-seismic deformation was the main driver behind the tsunami that followed the Palu earthquake. Our unique geodetic data set constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike-slip faults in transtensional basins

Interpreting GPS observations of the megathrust earthquake cycle: insights from numerical models

During a megathrust earthquake cycle, the plates accumulate strain in the interseismic stage due to locking of a portion of the megathrust that separates them. This strain is released during the earthquake and following rapid postseismic relaxation. As summarised in Chapter 1, this understanding was built through decades of seismological and geodetic observations and of advances in physics-based models. Models link properties and structures to observable quantities and are crucial to interpreting observed surface deformation in terms of the processes and materials in the inaccessible subsurface. However, different models can approximate the same observations roughly equally well. Furthermore, more sophisticated models do not necessarily reflect better the processes and properties of the subsurface; at the same time, simpler models that can explain some observations do not necessarily reflect the key processes occurring throughout one or multiple earthquake cycles. This thesis uses relatively simple three-dimensional models that still capture the key processes occurring during repeated earthquake cycles, without attempting to reproduce the structure or properties of specific subduction zones, to examine possible explanations of geodetic observations from global navigation satellite systems (GNSSs) at different stages of the cycle. Chapter 2 focuses on the interseismic, horizontal deformation of the overriding plate, which is in apparent contrast with observations of far-reaching coseismic displacement. We estimate the spatial patterns, with uncertainties, of GNSS velocities in South America, Southeast Asia, and northern Japan. Interseismic velocities with respect to the overriding plate generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are small. The hurdle is located 500-1000 km away from the trench for the trench-perpendicular velocity component, and either at the same distance or closer for the trench-parallel component. The trench-perpendicular hurdle generally follows major tectonic or geological boundaries and seismological contrasts. We formulate and test the hypothesis that both the interseismic hurdle and the coseismic response result from a mechanical contrast in the overriding plate. Our models show that overriding plates with a sufficient contrast respond to locked interseismic convergence similarly to observations. The compliance contrast is probably mainly responsible for the observed hurdle and in turn results from thermal, compositional and thickness contrasts. Chapter 3 is concerned with the increased landward velocities that were recorded onshore after 6 megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We find that viscous relaxation, but not afterslip, produces ELM when a depth limit is imposed on afterslip. This ELM results primarily from in-plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer. This conclusion does not significantly change when varying key model parameters. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone. In Chapter 4, we study postseismic landward motion observed on the overriding plate in the vicinity of a major megathrust rupture. We argue that relocking of the megathrust, particularly at shallow depths, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We discuss how that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than two months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity

Gateway exchange, climatic forcing and circulation of the Mediterranean Sea during the late Miocene: A model perspective

Oceanen transporteren - en fungeren als opslag van - grote hoeveelheden warmte, zout, en andere chemische verbindingen, waaronder CO2. Ze zijn daarom een belangrijk onderdeel van het aardse klimaatsysteem. Gezien de huidige bezorgdheid over het veranderende klimaat, is het cruciaal om een goed begrip te hebben van het functioneren van oceaancirculatie. Veranderingen in de circulatie en chemische samenstelling van de oceaan zijn veelal terug te zien in de sedimenten die afgezet worden op de oceaanbodem. Niet alleen het type sediment wordt bepaald door zulke veranderingen; de chemische samenstelling en fossielinhoud van het sediment worden er ook door beïnvloed. Op deze wijze vormen de afgezette sedimenten “het geologische archief” van genoemde veranderingen. Het bestuderen van dit archief is een manier om inzicht te verkrijgen in de processen die de oceaancirculatie aandrijven. De circulatie en sedimentatie in door land ingesloten bekkens zijn bijzonder gevoelig voor veranderingen in het klimaat; dit maakt deze bekkens uitermate geschikt om klimaatverandering te bestuderen. De gevoeligheid voor veranderingen in het klimaat is een gevolg van de beperkte grootte van de bekkens en de gelimiteerde interactie met de open oceaan. Het bekendste voorbeeld van een door land ingesloten bekken is de Middellandse Zee, met zijn ligging tussen het Europese en het Afrikaanse continent. Circulatie in de Middellandse Zee wordt aangedreven door de uitwisseling van water met de Atlantische Oceaan en door de atmosferische condities in het Mediterrane gebied en zijn omgeving, zoals winden, verdamping, neerslag en resulterende zoetwater-instroom van rivieren. Op dit moment is de Straat van Gibraltar de enige zeestraat die de Middellandse Zee en de Atlantische Oceaan met elkaar verbindt; in het verleden is deze verbinding gecompliceerder geweest. De wateruitwisseling door een zeestraat wordt sterk beïnvloed door de zeestraat-bathymetrie; hierdoor was het mogelijk dat tektonische veranderingen in het gebied rond Gibraltar gevolgen hadden voor de grootte van de wateruitwisseling. Genoemde atmosferische condities worden sterk beïnvloed door de intensiteit van de inkomende zonnestraling, die op haar beurt afhangt van de baan van de Aarde en positie van de rotatieas van de Aarde ten opzichte van de zon. Het is het samenspel van klimaat en tektoniek dat leidt tot veranderingen in de circulatie in de Middellandse Zee en dat zijn weerslag vindt in het marien-geologische archief. Het herkennen en onderscheiden van de individuele invloed van de twee drijfveren is lastig maar belangrijk voor het verbeteren van ons begrip van oceaancirculatie. De sedimenten die zijn afgezet in de Middellandse Zee in het laatste (d.w.z. jongste) deel van het Mioceen (het Messinien, van 7,25 tot 5,33 miljoen jaar geleden) vertonen een grote variabiliteit in hun samenstelling. Sedimenten kenmerkend voor afzetting in een open oceaan worden afgewisseld door lagen met een hoge organische inhoud. 5.97 miljoen jaar geleden werd deze afwisseling onderbroken door de afzetting van evaporieten (vooral gips en steenzout). Rondom de gehele Middellandse Zee zijn gipsafzettingen uit deze tijd gevonden en kilometers dikke lagen steenzout zijn verborgen onder de huidige zeebodem. Het uitzonderlijke geologische gebeuren dat geleid heeft tot de afzetting van deze evaporieten staat bekend als de Messinien Zoutcrisis en vond plaats tussen 5,97 en 5,33 miljoen jaar geleden. Deze relatief korte periode van evaporiet-afzetting is, ondanks het onderzoek van generaties van wetenschappers, nog steeds één van de grootste mysteries in de geologische geschiedenis. Voortbouwend op een groot aantal waarnemingen en een uitgebreid kwalitatief inzicht in het laat-Mioceen, heeft mijn promotieonderzoek als doel het verbeteren van het kwantitatieve begrip van de invloed van de grootte van de verbindende zeestraat tussen de Middellandse Zee en de Atlantische oceaan en van het klimaat op circulatie en zoutgehalte in de Middellandse Zee. Dit wordt bereikt door bestaande geologische/geochemische waarnemingen te combineren met een theoretische en model-gedreven aanpak van fysische aard. In dit proefschrift ontwikkel ik (1) een theoretische benadering van de relatie tussen zeestraatgrootte en bekkensaliniteit, (2) een vernieuwende aanpak, gebruik makend van meerdere modellen, om een schatting te maken van het zoetwaterbudget (relatie tussen neerslag, rivier-input en verdamping) van de Middellandse Zee in het Mioceen, (3) een gesimplificeerd model van de oceaancirculatie van de Middellandse Zee, dat de ruimtelijke verdeling van saliniteit in het bekken in eerste orde verklaart. De verkregen resultaten kunnen als volgt kort worden samengevat: Om een zoutgehalte in de Middellandse Zee in stand te kunnen houden dat groter is dan het huidige, moet de verbinding via de zeestraat beperkter (ondieper of smaller, of langer, of een combinatie hiervan) geweest zijn dan de huidige Straat van Gibraltar. Hoewel tijdens het laat-Mioceen de zoetwateraanvoer door de Afrikaanse rivieren mogelijk veel groter was (waarschijnlijk door afwatering in de Middellandse Zee van het destijds zeer grote Chad Meer), was het netto zoetwaterbudget van de Middellandse Zee vergelijkbaar met het huidige. Deze resultaten leiden tot de conclusie dat de Messinien Zoutcrisis voornamelijk op gang gebracht is doordat tektonische processen de verbinding van de Middellandse Zee met de Atlantische Oceaan drastisch verkleind hebben. Mijn onderzoek toont aan dat onder deze omstandigheden betreffende wateruitwisseling en zoetwaterbudget de Mediterrane waterkolom sterk gelaagd geweest kan zijn. Dit resultaat werpt nieuw licht op algemeen aanvaarde percepties van de Messinien Zoutcrisis en levert een nieuwe kwantitatieve basis voor toekomstige studies. De behaalde proces-gerelateerde resultaten zijn niet alleen zinvol voor de Middellandse Zee als case study gebied; tezamen met de geïntegreerde onderzoeksstrategie ontwikkeld in dit proefschrift zullen zij naar verwachting bijdragen tot verdieping van inzicht in oceaancirculatie en sedimentatie in andere vergelijkbare gebieden (tegenwoordige of in het geologische verleden) en in oceaancirculatie, in het algemeen

(Paleo)oceanography of semi-enclosed seas with a focus on the Mediterranean region; Insights from basic theory

To provide context for the interpretation of their sedimentary and paleoceanographic record, semi-enclosed seas are here investigated through the application of basic theory. The principles of conservation of water, salt and heat, in combination with a representation of flow through the seaway to the ocean, are used to chart how basin geometry, connectivity and atmospheric forcing together control basin-averaged salinity and temperature and the exchange flux. Data on present-day semi-enclosed seas of the wider Mediterranean region are used for illustration. First, ignoring the heat balance and with forcing constant in time, a dimensionless form of the governing equations is derived which clarifies the role of the various controlling parameters. This is applied to aspects of the Messinian Salinity Crisis. In the second part of the analysis the forcing is made a generalised periodic function of time. This informs us how basin salinity, its amplitude of variation and lag relative to the forcing, depend on basin and strait properties and varies with the period of forcing. Insights are applied to the precessional variation observed in the record of the Mediterranean Sea. In the third and fourth part of the analysis we include the balance of heat and basin-averaged temperature. Examination of the budget equations allows us to derive a relationship between the average air-sea heat flux and basin restriction. Finally, re-introducing strait flow, we study the interplay of basin temperature and salinity and establish under which conditions heat flux and temperature play a role, in addition to net evaporation and salinity

STEP faults and lithosphere dynamics in the Mediterranean

The locus of continual active tearing at the lateral termination of a subduction zone is dubbed a STEP, and the surface trace left in the wake of a propagating STEP is called the STEP fault. STEP faults have played a major role in the (geologically) fast reorganization of plate boundaries in the Mediterranean, where present-day surface deformation is distributed. This thesis focuses on the effect of passive margins on the propagation of STEPs and lithosphere dynamics in the Mediterranean, studied through physical computer models. Many STEP settings observed on Earth today display a geometry in which the trench is oriented approximately perpendicularly to the STEP fault. Model geometries include a passive margin that is oriented at some angle with respect to the trench. Passive margins steer STEPs in case of favorable orientation with respect to the strike of the trench. In other cases, STEPs will continue to propagate in their original direction, that is, straight into an oceanic basin or a passive margin. Models show that the orthogonal setup of STEP fault and subduction trench is one that STEP systems will evolve towards. Subsequently, Nijholt tailors model setups to resemble the geometry of the African continental margin in the south-central Mediterranean: the domain of the Calabrian subduction zone. Model predictions indicate that the SW STEP of the Calabrian trench propagated eastward along the African (Sicilian) passive margin and then propagated into the Ionian basin. The geodynamic context of deformation in the Ionian basin, offshore Calabria and Sicily, constitutes two factors: STEP fault activity and a wide, lithospheric, dextral shear corridor. Nijholt then focuses on present-day kinematic observations in this same area through an interplay of known tectonic forces. The geodetically measured velocity field, seismicity and sense of slip on regional faults in the south-central Mediterranean can be reproduced by force-based models. These show that the regional imprint of tectonic forces is driven by Africa-Eurasia plate convergence, lateral variations in gravitational potential energy and slab pull. The magnitude of the resistance to fault slip on regional fault zones turns out to be variable. Whereas high resistance on faults that hosted major historical earthquakes confirms the high probability of future natural disasters, the Calabrian subduction interface is unlikely to host a future, major earthquake. In the westernmost Mediterranean, the formation of the Gibraltar arc was also dominated by subduction and STEP activity. However, subduction activity has faded and present-day deformation is distributed. Lastly, Nijholt employs a grid search methodology and finds that it is possible to constrain the rheology, resistance to slip on major fault systems, and slab pull and trench suction forces for the Gibraltar arc domain in light of the available kinematic observations. The observed surface velocity field, seismicity and sense of slip on regional faults in the Gibraltar arc appear to result mainly from Africa-Europe plate convergence and lateral variations in gravitational potential energy. Slab pull from the Gibraltar slab is very likely transmitted poorly into the surface plate

The Kefalonia Transform Fault: A STEP fault in the making

Vertical edges along subducted slabs have been recognized in the majority of subduction zones. Surprisingly, slab edges evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only a few regions; the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone. We investigate the structure and seismicity of the shallow western Hellenic Subduction Zone using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10 to 200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic Subduction Zone is fragmented near the Kefalonia Transform Fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. We also identify a new and major contrast within the lithospheric mantle of the Ionian ocean basin, which aligns with a gradient in free-air anomalies. In the overriding plate, the Kefalonia Transform Fault zone accommodates right lateral strike-slip deformation. We interpret this fault zone as a proto-STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment. Comparing the recent evolution of the NW Hellenic slab edge with currently active STEPs indicates that along-trench variations in convergence velocity are a prerequisite for STEP fault initiation. Such velocity variations may result from subduction of continental crust along part of the trench. Resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur, and thus STEP fault initiation. The amount of time over which a velocity contrast persists will also be relevant for STEP fault initiation. Mechanical coupling between upper- and lower-plate, and the deformability of the upper-plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented

Stress evolution during the megathrust earthquake cycle and its role in triggering extensional deformation in subduction zones

Great (moment magnitude Mw ∼8.0 and larger) subduction megathrust earthquakes are commonly followed by increased rates of normal faulting seismicity. Extensional activity within the subducting slab is amplified when megathrust slip propagates close to the trench, and forearc extension is triggered by the largest magnitude (Mw 8.5 and larger) events. To better understand these observations, we develop an earthquake cycle model with a realistic slab geometry and stresses that are in balance with plate interface slip and bulk viscous relaxation. The modeled stresses represent perturbations to the long-term background tectonic stress field. The steady-state inter-seismic earthquake cycle stresses are compressive and their magnitudes depend on the interface locking configuration, from 25 MPa for a fully locked seismogenic zone to 5 MPa for discrete asperities on an otherwise unlocked plate interface. The co-seismic slip and corresponding co-seismic stress changes are similar in these models, independent of the locking configuration. The co-seismic stress change magnitudes are up to 5 MPa. This implies that the earthquake cycle stresses after the mainshock would still be compressive in the case of a continuous seismogenic zone, but would be widely reset to zero in the case of discrete asperities. Models with co-seismic slip confined to shallow depths produce tensional stress changes in the slab and the forearc near the trench, whereas events rupturing the base of the seismogenic zone produce tensional stress changes limited to region immediately surrounding the rupture. The locations of normal faulting aftershocks generally correlate well with the tensional co-seismic stress changes greater than 1 MPa. Following the mainshock, rapid afterslip occurring down-dip of the megathrust rupture expands the region of relative extension, but reduces its magnitude and does not promote additional normal faulting events. Bulk viscous relaxation does little to the state of stress in the elastic parts of the model. Continued plate convergence across a re-locked interface returns the system to the pre-earthquake state of compression

The mechanism of sapropel formation in the Mediterranean Sea: Insight from long-duration box model experiments

Periodic bottom-water oxygen deficiency in the Mediterranean Sea led to the deposition of organic-rich sediments during geological history, so-called sapropels. Although a mechanism linking the formation of these deposits to orbital variability has been derived from the geological record, physics-based proof is limited to snapshot and short-time-slice experiments with (oceanic) general circulation models. Specifically, previous modelling studies have investigated atmospheric and oceanographic equilibrium states during orbital extremes (minimum and maximum precession). In contrast, we use a conceptual box model that allows us to focus on the transient response of the Mediterranean Sea to orbital forcing and investigate the physical processes causing sapropel formation. The model is constrained by present-day measurement data, while proxy data offer constraints on the timing of sapropels. The results demonstrate that it is possible to describe the first-order aspects of sapropel formation in a conceptual box model. A systematic model analysis provides new insights on features observed in the geological record, such as the timing of sapropels as well as intra-sapropel intensity variations and interruptions. Moreover, given a scenario constrained by geological data, the model allows us to study the transient response of variables and processes that cannot be observed in the geological record. The results suggest that atmospheric temperature variability plays a key role in sapropel formation and that the timing of the midpoint of a sapropel can shift significantly with a minor change in forcing due to nonlinearities in the system