Dissertations / Theses on the topic 'Deep subduction'

This thesis seeks to improve our understanding of the processes involved in continental collision zones, with a particular focus on subduction-exhumation. The main objective of this work has been to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex (SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology to constrain the P-T-t paths of the SNC rocks in Sweden. The research has focused on the high grade rocks of the SNC and resulted in the discovery of metamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten. Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) and subduction of continental rocks to mantle depths. The UHPM in these rocks was confirmed by calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnet pyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings provide compelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along the strike of this allochthonous unit). The SNC rocks followed nearly isothermal decompression paths and paragneisses have locally experienced partial melting during exhumation. Formation of the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is related to the latter stage. In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supports previous geochronological data inferring that the peak of metamorphism is probably Middle Ordovician and was followed by Early Silurian partial melting. The exact timing of the UHPM here still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneiss from northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism of the SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlier UHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca. 470 Ma. Based on the P-T-t data obtained in this thesis, particularly on the evidence for Middle Ordovician UHPM and subsequent Silurian exhumation, a new tectonic model for the Scandinavian Caledonides has been proposed. The outcomes of this thesis therefore improve our understanding of the tectonometamorphic history of the Caledonides.

In this study new tomographic models of Colombia were calculated. I used the seismicity recorded by the Colombian seismic network during the period 2006-2009. In this time period, the improvement of the seismic network yields more stable hypocentral results with respect to older data set and allows to compute new 3D Vp and Vp/Vs models. The final dataset consists of 10813 P- and 8614 S-arrival times associated to 1405 earthquakes. Tests with synthetic data and resolution analysis indicate that velocity models are well constrained in central, western and southwestern Colombia to a depth of 160 km; the resolution is poor in the northern Colombia and close to Venezuela due to a lack of seismic stations and seismicity. The tomographic models and the relocated seismicity indicate the existence of E-SE subducting Nazca lithosphere beneath central and southern Colombia. The North-South changes in Wadati-Benioff zone, Vp & Vp/Vs pattern and volcanism, show that the downgoing plate is segmented by slab tears E-W directed, suggesting the presence of three sectors. Earthquakes in the northernmost sector represent most of the Colombian seimicity and concentrated on 100-170 km depth interval, beneath the Eastern Cordillera. Here a massive dehydration is inferred, resulting from a delay in the eclogitization of a thickened oceanic crust in a flat-subduction geometry. In this sector a cluster of intermediate-depth seismicity (Bucaramanga Nest) is present beneath the elbow of the Eastern Cordillera, interpreted as the result of massive and highly localized dehydration phenomenon caused by a hyper-hydrous oceanic crust. The central and southern sectors, although different in Vp pattern show, conversely, a continuous, steep and more homogeneous Wadati-Benioff zone with overlying volcanic areas. Here a "normalthickened" oceanic crust is inferred, allowing for a gradual and continuous metamorphic reactions to take place with depth, enabling the fluid migration towards the mantle wedge.

The Scandinavian Caledonides are defined by long transported thrust sheets emplaced in a nappe stratigraphic succession onto the Paleozoic Baltica platform, as a result of the collision between the paleo-continents Baltica and Laurentia. This Palaeozoic collisional orogen is nowadays exposed at mid-crustal levels, thus provides an excellent ground for in situ studies of mountain building processes. The complex nappe stack is subdivided into the Lower, Middle, Upper and Uppermost allochthons. The tectonostratigraphic highest unit in the Middle Allochthon is the Seve Nappe Complex (SNC), itself segmented into Lower, Middle and Upper Seve nappes, which all experienced different metamorphic evolution. The SNC is known for high pressure (HP) and ultrahigh pressure (UHP) subduction related rocks and the target for the Collisional Orogeny in the Scandinavian Caledonides (COSC-1) scientific drilling programme. The drilling resulted in a continuous c. 2.4 km long drill core through the Lower Seve Nappe, drilled in the eastern slope of Åreskutan Mt in west-central Jämtland. Above the COSC-1 profile lies the high grade Middle Seve Nappe (i.e. Åreskutan Nappe), which experienced UHP verified by the presence of microdiamonds in kyanite bearing gneisses. Recently, microdiamonds have also been discovered in gneisses (described here) further north close to Saxnäs in southern Västerbotten.     The metamorphic history of the Lower Seve Nappe is reconstructed based on material from the COSC-1 drill core, which also enables evaluation of the tectonometamorphic relationship to the overlying high grade Middle Seve Nappe. The Lower Seve Nappe comprise calc-silicates, calcareous gneisses and mylonitic micaschists and two tectonometamorphic events are recognized, prograde metamorphism (M1-D1) and retrograde thrust related metamorphism (M2-D2). Pressure and temperature (PT) conditions of the Lower Seve Nappe is constrained by state-of-the-art Quartz-in-Garnet (QuiG) barometry based on the shift in Raman band position of quartz inclusions in garnet, and Titanium-in-Quartz (TitaniQ) thermometry (satellite masters project). Supplementary conventional barometry based on phengite composition is applied where the use of QuiG is limited. The PT conditions of the M1-D1 is constrained to ~ 8-13 kbar, 525-695 o C and the M2-D2 event ~7-10 kbar, 450-550 o C. Conclusively, the Lower Seve Nappe was metamorphosed in upper greenschist-amphibolite to lower eclogite facies conditions at depths around 40-60 km and later suffered from greenschist overprint during thrusting. Lu-Hf garnet geochronology confirm that the overlying high-grade Åreskutan Nappe experienced UHP conditions around 450 Ma at depths around 120 km. Likewise, Ar-Ar dating implies peak conditions of the Lower Seve around 460-450 Ma. Moreover, their respective lower shear zones were active at the same time, c. 424 Ma. Conclusively, they were juxtaposed in their current tectonostratigraphic positions in a subduction channel in the early Silurian as a result of exhumation. Additionally, the microdiamond bearing kyanite-garnet gneisses from Saxnäs indeed show similarities to the Åreskutan gneisses, which strongly implies that the UHPM in this unit of the Scandinavian Caledonides is of regional character.
De Skandinaviska Kaledoniderna har bildats genom en kollision mellan de två kontinentalplattorna Baltika och Laurentia då Japetushavet stängdes omkring 400 miljoner år sedan. Till följd av de starkt komprimerande krafterna transporterades stora flak (skollor) av havsbottenberggrund och kontinentalskorpa hundratals kilometer upp på Baltikakontinenten. Skollorna är överskjutna på varandra omlott och benämns som undre, mellersta, övre och översta skollberggrunderna och återfinns idag i vår fjällkedja. Innan kollisionen med Laurentia krockade Baltika med en vulkanisk öbåge, vilket resulterade i att delar av Baltika pressades ner så pass djupt att bland annat diamanter bildades till följd av det ultrahöga trycket. Bevis för omvandling under extremt tryck finns i den så kallade Seveskollan som utgör en del av den mellersta skollberggrunden. Seveskollan är ett komplex av tre olika enheter, som utsatts för olika grad av metamorfos till följd av tryck och temperatur. Till följd av väder och vind under miljontals år så är fjällkedjan idag nederoderad och därav väl exponerad. Det gör att de Skandinaviska Kaledoniderna är en av världens bästa platser att studera och förstå bergskedjebildade processer. Av den anledningen borrade djupborrningsprojektet COSC-1 en cirka 2.4 km långt kärnborrhål genom den lägst belägna enheten i Seve komplexet (lägre Seveskollan) strax nedanför Åreskutan i Jämtlandsfjällen. Över COSC-1 profilen ligger den berggrund som tillhör den mellersta Seveskollan, även kallad Åreskutanskollan. Åreskutanskollan är en del av Baltika som utsattes för ultrahöga tryck, och i kyanitförande gnejser har diamanter inneslutna i det motståndskraftiga mineralet granat påträffats. Nyligen, längre norrut i Saxnäs (södra Västerbotten) har ytterligare diamantförande gnejser påträffats i den mellersta Seveskollan, som karaktäriseras i den här studien.      Material från COSC-1 borrkärnan har använts för att bestämma under vilka tryck och temperatur bergarterna i den lägre Seveskollan har metmorfoserats, för att förstå den tektoniska och metamorfa utvecklingen och även relationen till den överliggande högmetamorfa Åreskutanskollan. Trycket har bestämts genom den relativt oprövade metoden QuiG -barometri. Små kristaller av kvarts inneslutna i granat har analyserats med Raman spektroskopi och de fysikaliska parametrarna av kvarts och granat kan direkt översättas till tryck. Temperatur har erhållits genom det temperaturkänsliga ämnet titan i kvartsinneslutningarna. Resultatet visar att den lägre Seveskollan har genomgått minst två metamorfa faser genom tektonisk påverkan. Den första fasen varierar från övre grönskiffer-amfibolit till lägre eklogitfacies under tryck och temperatur av ca 8-13 kbar, 525-695 o C. Den andra fasen är associerad med överskjutning och skjuvning, vilket orsakade retrograd metamorfos i grönskifferfacies under lägre tryck och temperatur (ca 7-10 kbar, 450-550 o C). Datering baserat på radioaktivt sönderfall av lutetium till hafnium i granat fastställer att Åreskutanskollan utsattes för ultrahögt tryck för omkring 450 miljoner år sedan, samtidigt som lägre Seveskollan nådde metamorft klimax. Resultaten visar även att lägre och mellersta Seveskollorna skjuvades samtidigt, omkring 424 miljoner år sedan. Det betyder att de erhöll sina nuvarande tektonostratigrafiska positioner på stort djup innan överskjutningen på Baltika. Detaljerad petrografi påvisar att de diamantförande kyanit-och granatförande gnejserna från Saxnäs visar påtagliga likheter med Åreskutanskollans högtrycksgnejser. Det tyder på att berggrunden i Saxnäs kan kopplas samman med Åreskutanskollan och att ultrahögtrycksmetamorfos av den mellersta Seveskollan omfattar ett större område än vad som tidigare antagits.

La sédimentation marine récente dans les fosses de subduction est caractérisée par l'interstratification de sédiments hémipélagiques et de turbidites localement intercalées avec les coulées de débris, qui peuvent résulter de la destabilisation des pentes continentales par de tremblements de terre. La marge d’Equateur est constituée par une forte érosion tectonique qui contribue à la formation d'une fosse profonde remplie d'une suite complexe de faciès sédimentaires. La sédimentation par écoulements gravitaires est omniprésente le long de la marge et les faciès vont de dépôts de transport de masse d'épaisseur métriques latéralement continus à des turbidites d'épaisseur centimétriques isolées intercalées avec des couches d'hémipélagites, de volcanoclastiques et de téphras. Nous présentons l'interprétation de la bathymétrie, des profils sismiques à haute résolution et des données pétrophysiques des carottes sédimentaires. L'objectif de cette étude est de décrire la complexité morphologique à la frontière équatorienne de la plaque de Nazca où un ensemble d'aspérités marines profondes ont subducté à différentes échelles, et ses conséquences sur la distribution latérale des sédiments dans les différents sous-bassins. La marge équatorienne comprend trois segments géomorphologiques: Le segment nord, situé au nord de la crête Carnegie, est caractérisé par une large (5-10 km) et profonde fosse (3800-4000 m), une pente continentale ravinée et une plate-forme (10-40 km de large) avec subsidence active. Le segment central en face de la crête de Carnégie montre une fosse étroite (0-5 km de large) et peu profonde (3100-3700 m), la pente escarpée et ravinée, sans canyons, et plateau continental étroit de 15 à 40 km de large caractérisé par des zones d'affaissement et de soulèvement actifs. Enfin, le segment sud, situé au sud de la crête Carnegie, présente une large (5-10 km) et profonde fosse (4000-4700 m), une pente continentale pauvre en sédiments avec des systèmes de canyons bien définis et une large plate-forme de subsidence (20-50 km). La dynamique sédimentaire le long de la marge est évaluée par l'analyse de 15 carottes sédimentaires dont la description visuelle, les photographies à haute résolution, l'imagerie par rayons X, les données XRF et les propriétés pétrophysiques conduisent à l'identification de 11 faciès sédimentaires caractérisant 7 processus sédimentaires: dépôts de turbidite, hémipélagites, téphras, dépôts de coulées de débris, homogénites, des slumps et des dépôts de carbonate de ooze. Les âges des dépôts sont définis par la datation au radiocarbone des sédiments hémipélagites. Les âges vont de 500 à 48000 ans BP. Les profils sismiques à haute résolution permettent de définir 3 echo-faciès: transparent, stratifiés et chaotiques. Le facies transparent est principalement associé aux dépôts d'homogénites, le facies stratifié est associé aux dépôts interstratifiés turbiditique-hémipélagique et le facies chaotique est associé à des dépôts gravitaires grossiers. Le remplissage de la fosse représente un enregistrement lacunaire mais important de l'histoire de la marge de subduction. De grandes coulées de débris se déplaçant vers l'est dans les deux séquences inférieures du remplissage de la fosse sont initiées le long de la paroi extérieure de la fosse, le long de grandes failles normales dues à la flexion de la plaque océanique subductante. Les sédiments de la séquence supérieure du remplissage qui nappent la fosse sont plus largement fournis par la paroi interne de la fosse mais avec un fort contrôle de la ride de Carnegie. En conséquence, la profondeur, la fréquence, l'épaisseur, la composition et la disposition latérale des dépôts sédimentaires varient grandement entre le nord et le sud. Les grands méga-lits simples, les slumps, les coulées de débris et les homogénites sont situés dans les segments nord et sud. Ils sont déclenchés par de grands escarpements de failles régionales, dans le Nord
Recent deep marine sedimentation in subduction trenches is characterized by the inter-stratification of hemipelagic and turbidite sediments locally interbedded with debris flow, which can result from continental slope shaking triggered by earthquakes. The active margin of Ecuador comprises tectonic erosion that contributes to the formation of a deep trench filled by a complex suite of sedimentary facies. Gravity flow sedimentation is ubiquitous along the margin and facies range from laterally continuous m-thick mass transport deposits to isolated cm-thick turbidites intercalated with hemipelagite, volcanoclastics and tephra. In this study we show interpretation of swath bathymetry, high-resolution seismic profiles and petrophysical data from cores. The objective is to describe the morphologic complexity on the Ecuadorian border of the Nazca plate where a set of deep marine asperities is subducting at different scales, and their consequences on the distribution of sediments in the different sub-basins. Ecuadorian margin comprises three geomorphological segments: The northern segment, northward of the Carnegie Ridge, is characterized by a wide (5-10 km) and deep trench (3800 – 4000 m), a gentler gullied continental slope and a shelf (10-40 km wide) with active subsidence. The central segment facing the Carnegie Ridge, is strongly influenced by the subduction of the Carnegie ridge which induces a narrow (0–5 km wide) and shallow trench (3100 – 3700 m depth), a steep and gullied slope with no canyons and a 15–40 km wide shelf characterized by areas with active subsidence and uplift. Finally, the southern segment, southward of the Carnegie Ridge, presents a wide (5–10 km) and deep (4000–4700 m) trench, a starved continental slope with well-defined canyon systems and a wide subsiding shelf (20–50 km). The sedimentary dynamics along the margin is evaluated by the analysis of 15 cores. Visual description, high-resolution photographs, X-Ray imagery, XRF data and petrophysical properties led to the identification of 11 sedimentary facies that characterize seven sedimentary processes: turbidites, hemipelagites, tephras, debris flows, homogenites, slumps, and ooze carbonate deposits. Age of the deposits is defined by radiocarbon age dating of hemipelagic sediments. Ages range from 500 to 48,000 years BP. High-resolution seismic profiles allow definition of three echo-facies: transparent, layered and chaotic. Transparent echo-facies is mainly associated to homogenite deposits, layered echo-facies is associated to the turbiditic-hemipelagic interbedded deposits and chaotic echo-facies is associated to reworked gravity flow deposits. The trench fill represents a lacunar but important record of the subduction margin history. Large eastward debris flows in the lower two sequences of the trench fill are provided by the trench outer wall as a results of slope failures along normal faults due to the downward bending of the oceanic plate. The sediment of the upper sequence of the trench fill draping the trench floor, are largely provided by the inner trench wall strongly controlled by the Carnegie Ridge. As a result, depth, frequency, thickness, composition and lateral disposition of the deposits vary greatly from those at north and south. The large, simple mega-beds like slump, debris flows and homogenites are located at the northern and southern segments. They were triggered by large regional faults in the North and enhanced by the activity of sets of splay faults in the South overhanging the seafloor at the slope toe. Small-size, fluid rich events were triggered by subduction of isolated seamounts at the edges of the Carnegie Ridge due to frequent but small destabilizations of an inner trench wall preconditioned by the impacts of successive seamounts. Sets of partly volcanoclastic turbidites in central segment might have been triggered by the complex interaction of slope and continental shelf deformation by seamount subduction

The spatial distribution of deep earthquakes remains elusive, as the earthquakes below 30 km depth cannot be explained using the brittle frictional processes due to the fluid behavior of rocks under high pressure and temperature conditions. Several models that have been developed to identify the source distribution fall largely into categories like negative buoyancy and viscous friction to the flow, anti-crack faulting due to metastable olivine, volume reductions from phase transformations etc. Still none of them were able to satisfactorily explain the spatial distribution of deep earthquakes. We propose a new method using the visco-elastic nature of the earth material to model the deformation, stress, and elastic energy of the subducting lithosphere using “Marker in cell method” in combination with a conservative finite difference scheme. The software is written in Python and NumPy. We have tested this code for the known results of a Rayleigh–Taylor instability of solid-fluid interaction, and for a general subduction benchmark (Schmeling et al., 2008). We show a large set of numerical models in which we investigate the role of volatiles in the transition zone by varying the viscosity of the lithosphere and the presence of a high viscosity zone below the upper-lower mantle transition zone. Finally, we compare the rate of inner energy dissipation and the stored elastic energy in the subducting lithosphere with deep earthquake spatial distribution and discuss which constrains geodynamic models offer to deep earthquake location.