京都大学 / 新制・課程博士 / 博士(理学) / 甲第24428号 / 理博第4927号 / 新制||理||1704(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 佐藤 活志, 准教授 堤 昭人, 教授 田上 高広 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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The well-known clinoformal geometry of a basin-fill, with an alluvial to shelf segment, deep-water slope segment, and a basin floor segment, arises from the development of a wedge-shaped body of sediment at the basin-margin that has been termed a basin-margin wedge or a shelf-slope sedimentary prism. The basin-margin wedge characteristically has atopset-foreset clinoformal geometry, with its topset dominated by alluvial, coastal and shelfal processes, while its foreset is dominated by turbidite sedimentation. Tectonic configuration of the basin, sediment supply, and relative sea level variation are some of the major factors that control the development and growth of the basin-margin wedge. This dissertation documents two distinct stages of development of the basin-margin wedge at an Eocene active margin, and relates the observed variability in the nature of the shelf-margin, deep-water slope, and basin-floor deposits with these stages. The Tyee Basin in western Oregon was a forearc basin that was filled during late early Eocene and Middle Eocene under greenhouse climatic condition. The sedimentary succession of the Tyee Basin include continental, shallow-marine and deep-water sandstones that are well exposed in Coast Range area of Oregon. The variability observed within the thick and laterally extensive turbidite sandstones of the Tyee Basin led to contrasting depositional models for the Tyee basin in the past. Notably, the submarine ramp model, which provides an alternative model for deepwater coarse clastic deposition, was proposed based on the sedimentary succession of the Tyee Basin. Reconstruction of the clinoformal geometry of the Tyee Basin succession from detailed field data (more than 1000 outcrop locations) and subsurface data reveals two distinct stages of development of this active basin-margin. Each stage has a distinct style of clinoform development and a distinct character of associated sandy deepwater deposits. At the initial stage the basin-margin clinoforms appear to be small (< 250m clinoform height) and strongly progradational, with clinoform topset dominated by the feeder fluvial deposits. At this stage, sandy unconfined (not channelized) turbidite deposits accumulated on the Tyee deepwater slope and extended to the Tyee basin-floor. Large scale sediment conduits on the deepwater slope, in the form of slope channels or canyons, are notably absent in this stage. The second stage is characterized by larger clinoform height (> 500m), higher degree of topset aggradation with repeated fluvio-deltaic cycles on the shelf, and spectacular, sand-rich, well-organized turbidite channels and canyons on the slope. The slope channels active at this stage supplied coarse sediments to the basin-floor to form unusually thick basin-floor fans. The first infill stage represents the embryonic development of a basin-margin wedge on the Tyee continental margin, and could have some similarity with the previously mentioned submarine ramp model. But this was followed by a much longer period of basin-filling when repeated fluvial and shallow-marine cycles formed on the shelf and well-organized turbidite channels were active on the slope supplying sands to the Tyee Basin floor fans. It was concluded that the two stages of development of the basin-margin wedge in the Tyee Basin is controlled largely by the configuration of the basin, that is a result of the prominent topographic/bathymetric features in oceanic basement underlying the sedimentary succession of the Tyee Basin. Tectonically active hinterland and greenhouse climate may have contributed to a relatively high sediment supply to the basin. The relatively small-amplitude sea level variations expected under greenhouse climatic condition of the Early to Middle Eocene are likely to have relatively minor effect on the architecture of the basin-fill. The present work on Tyee Basin builds on earlier research on this basin, but now establishes a ground trothed clinoformal growth model, revises the existing interpretation of sediment transport direction during a major part of the basin-filling history, and demonstrates a two-stage evolution of margin accretion. The observations from the active Tyee Basin was compared and contrasted with a latest Pleistocene sediment wedge on the New Jersey outer shelf. This sediment wedge, developed under icehouse climatic condition, and on a passive margin, was studied using high resolution seismic data (CHIRP). In contrast to the sedimentary succession of the Tyee Basin, the depositional architecture of the sediment wedge on outer New Jersey shelf, which was interpreted as a set of falling stage deltaic clinothems, appears to be strongly controlled by eustatic sea level variation of latest Pleistocene. / text

La marge d’Équateur est caractérisée par un bassin avant-arc formé par un socle crétacé et une couverture de sédiments marins d’âge Crétacé à Quaternaire. Le relief de cette zone comprend d’une part la cordillère Côtière proprement dite et la plaine Côtière, située entre la cordillère Côtière et les Andes. Ce travail porte sur l’évolution et le soulèvement de la cordillère Côtière durant le Néogène. Dans un premier temps, nous avons réalisé une étude géologique régionale de la cordillère côtière. À partir de l’analyse stratigraphique et structurale des formations géologiques, nous avons réalisé une carte géologique de la cordillère côtière au 1/500 000 qui nous a permis d’effectuer pour la première fois des corrélations stratigraphique et un schéma structural à l’échelle complète de la cordillère. Dans un deuxième temps nous avons réalisé une étude géomorphologique de la zone. À partir de l’analyse de MNT, d’images satellites et aériennes nous avons défini les principales caractéristiques morphologiques de la zone d’étude. En particulier, le travail a porté principalement sur l’analyse de la géométrie du réseau hydrographique, la mesure de la géométrie des vallées et du profil en long des rivières à l’échelle de la cordillère Côtière. En complément nous avons mesuré le profil longitudinal des terrasses alluviales le long du rio Jama et analysé la morphologie des cônes alluviaux qui se déposent au pied des Andes sur la plaine Côtière. Les résultats ont permis de proposer une évolution du soulèvement de la cordillère Côtière. Les mesures des incisions relatives des rivières suggèrent que le soulèvement de la cordillère Côtière n’est pas homogène et que la cordillère est segmentée en plusieurs blocs dont les taux de soulèvement relatif sont variables: les blocs du Nord présentant les incisions les plus importantes. L’analyse des profils longitudinaux des terrasses alluviales du rio Jama montre une activité néotectonique le long des failles du système de Jama. Le taux de soulèvement estimé à partir de cette analyse est de 0.9 à 1.2 mm/ an pour le segment central de la cordillère Côtière. L’analyse du cône de Santo Domingo, situé aux pieds des Andes, révèle une importante interaction entre le soulèvement de la cordillère Côtière et le remplissage sédimentaire de la plaine côtière dont le résultat est la réorganisation du réseau hydrographique en deux grands bassins hydrographiques: Guayas au Sud et Esmeraldas au Nord. A plus long termes, la géologie et la stratigraphie montrent que la partie du Sud a subis une plus forte érosion (soulèvement ?) qu'au Nord. La mise en évidence de plusieurs discordances à l’échelle régionale montre que la cordillère Côtière s’est soulevée de façon hétérogène depuis le Plio-Pléistocène formant un grand antiforme segmenté et contrôlé par des failles régionales qui présentent une direction proche de la direction du mouvement vers le NE-NNE du bloc Nord-Andin. / The Ecuadorian margin is characterized by a forearc basin composed of a Cretaceous basement covered by marine sediments of the Cretaceous to Quaternary age. The topography of this area displays two main morphological domains: the Coastal cordillera in the west and the Coastal plain in the east at the foothills of Andes cordillera. This work focuses on the genesis of the Coastal cordillera during the Neogene. Firstly, we carried out a geological fieldwork throughout the Coastal cordillera. From stratigraphy and structural studies, we produced a regional geological map of the Coastal cordillera at 1:500000 scale, which have allowed for the first time to realize a regional stratigraphy correlation and determine the structural pattern across the Coastal cordillera. In a second step, we carried out a geomorphologic study of the area. From DEM analysis and satellite and aerial imagery processing, we characterized the main landforms features of the study area. In particular, we focused on the geometry of the drainage network and on the river profiles crossing the Coastal cordillera. In addition, we measured the longitudinal profile of the alluvial terrace treads along the Jama River and analyzed the morphology of alluvial fans that are deposited on the Coastal Plain at the foothills of the Andes cordillera. From the different results obtained, we proposed an evolution scheme of the uplift of the coastal cordillera. The measurements of incisions along river valleys suggest that the uplift of the Costal cordillera is heterogeneous: the cordillera is segmented into several blocks with own uplift rates. The incisions of the northern blocks are the highest. The analysis of the longitudinal profiles along the alluvial terrace treads of the Jama River indicates a recent activity along the faults of the system Jama. The uplift rates estimated from this analysis ranges from 0.9 to 1.2 mm/yr for the central segment of the Coastal cordillera. The analysis of Santo Domingo alluvial fan situated at the foothills of the Andes cordillera reveals a large interaction between the sedimentary filling of the Coastal plain and the contemporaneous uplift of the Andes and the Coastal cordilleras. This interaction results into the reorganization of two major drainage basins: Guayas in the south and Esmeraldas in the north. At long timescales, the geology and stratigraphy of the Neogene formations shows that domains in the southern Coastal cordillera were subject to intense erosion (rising?) with respect to the northern domains. The analysis of the several unconformities evidences that the Coastal cordillera was raised in a heterogeneous way since the Plio-Pleistocene as a large and elongated antiform segmented and controlled by regional faults which have a trend between NE and NNE, which is close to that of the movement to the North Andean block.

Thesis (doctoral)--Freie Universität Berlin, 2002. / "January 2003"--P. [2] of cover. Lebenslauf. Includes bibliographical references (p. 137-142). Also available via the World Wide Web.

This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 10^19 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models. / Graduate

This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 10^19 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models. / Graduate

Southern Vancouver Island lies on the forearc of the Cascadia subduction zone, north of a concave bend in the plate boundary centred around the Olympic Mountains. The bend in the margin coincides with a significant decrease in northward-directed trench-parallel forearc migration, and a network of active crustal faults in the Puget Lowland east of the Olympic Mountains accommodates permanent north-south shortening and transpression. The nature of forearc deformation on southern Vancouver Island is less well constrained, due in part to the unknown extent and kinematics of active crustal faulting. Recent work has shown that a brittle fault zone associated with the Eocene terrane-bounding Leech River fault has produced at least two surface-rupturing earthquakes in the late Quaternary. I use LiDAR-derived topographic data, slip-sense indicator analysis of slickenlines on fault planes, electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) to investigate the geometry, kinematics, and Quaternary activity along the eastern half of the active, brittle Leech River fault zone. My mapping reveals a complex, near-vertical zone up to 1 km wide and 25 km long that exhibits many characteristics of a strike-slip fault. Displaced Quaternary deposits are observed directly at two sites in the western 8 km of the study area, and inferred through geophysical imagery, topographic data, and liquefaction features to extend to the eastern terrestrial extent of the fault zone towards previously mapped active faults in the Juan de Fuca Strait and the Darrington-Devil’s Mountain fault zone in western Washington. I use fault kinematics and geometry to show that the eastern Leech River fault zone has been reactivated as a right-lateral strike-slip fault that accommodates forearc deformation within the modern stress field north of the Olympic Mountains. / Graduate / 2018-12-01

The state of stress in forearc regions depends on the balance of two competing factors: the plate coupling force that generates margin-normal compression, and the gravitational force, that generates margin-normal tension. Widespread reversal of the focal mechanisms of small earthquakes after the 2011 Tohoku-oki earthquake indicate a reversal in the dominant state of stress of the forearc, from compressive before the earthquake to tensional afterwards. This implies that the plate coupling force dominated before the earthquake, and that the coseismic weakening of the fault lowered the amount of stress exerted on the forearc, such that the gravitational force became dominant in the post-seismic period. This change requires that the average stress drop along the fault represents a significant portion of the fault strength. Two cases are possible: (1) The fault was strong and the stress drop was large or nearly-complete (e.g. from 50 MPa to 10 MPa), or (2) that the fault was weak and the stress drop was small (e.g. from 15 MPa to 10 MPa). The first option appears to be consistent with the dramatic weakening associated with high-rate rock friction experiments, while the second option is consistent with seismological observations that large earthquakes are characterized by low average stress drops. In this work, we demonstrate that the second option is correct. A very weak fault, represented by an apparent coefficient of friction of 0.032, is sufficient to put the Japan Trench forearc into margin-normal compression. Lowering this value by ~0.01 causes the reversal of the state of stress as observed after the earthquake. A slightly stronger fault, with a strength of 0.045, does not agree well with the observed spatial extent of normal faulting for the same coseismic reduction in strength. We also calculate distributions of stress change on the fault and average stress drop values for the Tohoku-oki earthquake, as predicted from 20 published rupture models which were constrained by seismic, tsunami, and geodetic data. Our results reconcile seismic observations that average stress drops for large megathrust events are low with laboratory work on high-rate weakening that predicts very high or complete stress drop. We find that, in all rupture models, regions of high stress drop (20 – 55 MPa) are probably indicative of dynamic weakening during seismic slip, but that the heterogeneous nature of fault slip does not allow these regions to become widespread. Also, coseismic stress increase on the fault occurs in many parts of the fault, including parts of the area that experienced high slip (> 30 m). These two factors ensure that the average stress drop remains low (< 5 MPa). The low average stress drop during the Tohoku earthquake, consistent with values reported for other large earthquakes, makes it unambiguous that the Japan Trench megathrust is very weak. / Graduate

La marge sud péruvienne est située au niveau d’une zone majeure de subduction océan continent depuis au moins le Paleozoique inférieur. C’est dans ce cadre que s’est formé l’un des plus importants orogènes du monde : les Andes Centrales. En effet, l’épaisseur crustale y est >60 km et ce sur une importante surface. Cependant, on considère actuellement que ce surrépaississement a été acquis incrémentalement seulement depuis ~30 Ma. Dans le but de comprendre comment et quand ce surrépaississement est apparu, la majeure partie des études précédentes s’est focalisée sur l’évolution de l’arc magmatique et sur l’histoire de la déformation, du soulèvement et de l’érosion de la zone d’arrière arc. Cependant, l’évolution tectonique et thermique de l’avant arc reste mal connue bien que cette zone soit susceptible de bien enregistrer les changements liés à la dynamique de subduction.Cette thèse à pour objectif de mieux contraindre l’évolution thermique et les couplages entre les processus magmatiques, tectoniques et sédimentaires depuis 200 Ma dans l’avant-arcactuel du sud du Pérou. De nouvelles données géo-thermochronologiques couplées à une nouvelle carte tecto-stratigraphique éclaircissent l’évolution de la marge péruvienne depuis le Jurassique. Trois périodes clefs sont analysées dans ce mémoire : le début de l’épaississement crustal, les déformations de l’avant-arc associées à la formation de l’Orocline bolivien et l’épaississement crustal de l’orogène des Andes Centrales pendant le Néogène.Nous montrons que l’épaississement crustal a probablement commencé entre 90 et 50Ma après plus de 200 Ma d’amincissement, et ce a la faveur d’une évolution en trois étapes :croissance initiale (90-74 Ma), « flare-up » (74-62 Ma) et effondrement extensionnel (62-50Ma). L’extension a ensuite prédominé dans l’avant-arc tout en diminuant progressivement jusqu’à ~30 Ma. Par ailleurs, nous mettons en évidence d’importantes zones de faillesnormales orientées perpendiculairement à la marge sud-péruvienne et qui délimitent de grands blocs basculés vers le nord-ouest. Ces déformations révèlent une extension parallèle à l’orogène dans l’avant arc pendant le Paléogène, probablement due à la formation de l’Orocline bolivien par rotation antihoraire de blocs rigides. Enfin, les traits géomorphiques visibles dans la zone cotiere du sud du Pérou permettent de définir deux périodes de soulèvement de la surface (entre 23 et 10 Ma et depuis ~4.5 Ma), séparées par une période de subsidence (entre ~10 et ~4.5 Ma). La même chronologie ayant été décrite sur le versant Amazonien de l’orogène, nous proposons que cette évolution soit due à des variations à grande échelle de l’épaisseur crustale ; le soulèvement de la surface étant provoqué par addition à la croûte de magma d’origine mantellique et la subsidence par un flux de matériel crustal ductile depuis les zones précédemment sur-épaissies. / The southern Peruvian margin has been located above a major ocean-continentsubduction zone since at least the Early Paleozoic, resulting in the formation of one of thelargest orogens in the world: the Central Andes, where crustal thickness is >60 km over a largearea. This overthickening is currently thought to have occurred incrementally only during thelast 30 Ma. To understand how and when crustal overthickening was acquired, most of theprevious studies have focused on the magmatic arc evolution and on deformation, uplift anderosion history of the backarc. The tectono-thermal Cenozoic evolution of the forearc remainspoorly known, whereas it is a zone prone to recording changes in subduction dynamics.The objective of this dissertation is to address the thermal evolution and the couplingbetween magmatic, tectonic and sedimentary processes over the past 200 Ma in the presentdayforearc of southern Peru where the crust thickens from ~30 km along the coastline tomore than 60 km under the present-day volcanic arc. New geo- and thermochronological datacoupled to a novel geological map illuminate the evolution of the south Peruvian margin sincethe Jurassic. Three key periods of the margin evolution are addressed in this dissertation: theonset of crustal thickening, the deformations associated in the forearc with the formation ofthe Bolivian Orocline and the Neogene crustal thickening of the Central Andean orogen.We show that crustal thickening likely began between 90 and ~50 Ma after more than200 My of lithospheric thinning during a three step evolution of the magmatic arc as follows:growth (90-74 Ma), flare-up (74-62 Ma), extensional collapse (62-50 Ma). Extension prevailedin the forearc since then and waned until ~30 Ma. In addition, we evidence important normalfault zones striking perpendicular to the southern Peruvian margin that delineate largenorthwestward tilted blocks. This deformation reveals orogen parallel extension in the forearcduring the Paleogene likely due to the formation of the Bolivian Orocline by counterclockwiserotation of rigid blocks. Finally, geomorphic features in the coastal area of southern Perureveal two periods of surface uplift (~23 to 10 Ma and since ~4.5 Ma), separated by a period ofsurface subsidence (from ~10 to ~4.5 Ma). The same chronology has been described on theAmazonian side of the Central Andean orogen. We thus propose that this evolution is due tolarge-scale crustal thickness variations; surface uplift being triggered by addition of mantlederivedmagmas to the crust and subsidence by ductile flow away from the previouslyoverthickened crust.