| 1. |
- Hansman, Reuben
(författare)
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Constraining the Uplift History of the Al Hajar Mountains, Oman
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mountain building is the result of large compressional forces in the Earth’s crust where two tectonic plates collide. This is why mountains only form at plate boundaries, of which the Al Hajar Mountains in Oman and the United Arab Emirates is thought to be an example of. These mountains have formed near the Arabian–Eurasian convergent plate boundary where continental collision began by 30 Ma at the earliest. However, the time at which the Al Hajar Mountains developed is less well constrained. Therefore, the timing of both the growth of the mountains, and the Arabian–Eurasian collision, needs to be understood first to be able to identify a correlation. Following this a causal link can be determined. Here we show, using apatite fission track and apatite and zircon (U-Th)/He dating, as well as stratigraphic constraints, that the Al Hajar Mountains were uplifted from 45 Ma to 15 Ma. We found that the mountains developed 33 Myr to 10 Myr earlier than the Arabian–Eurasian plate collision. Furthermore, the plate collision is ongoing, but the Al Hajar Mountains are tectonically quiescent. Our results indicate that the uplift of the Al Hajar Mountains cannot be correlated in time to the Arabian–Eurasian collision. Therefore the Al Hajar Mountains are not the result of this converging plate boundary.
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| 2. |
- Peillod, Alexandre, 1988-
(författare)
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The metamorphic history of Naxos (central Cyclades, Greece) : Deciphering the Oligocene and Miocene exhumation events
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence ofpast subduction zones. Understanding the tectonics processes that control the exhumation of HP-LT metamorphic rocks in these subduction zones requires full comprehension of the pressure-temperature-time (P–T–t) cycle that the rocks experienced. In the Cyclades, Greece, the Cycladic Blueschist Unit (CBU) hosts eclogite and blueschist facies rocks. However, the processes that exhumed them are debated. The overall aim of this thesis is to understand how the Eocene HP-LT rocks were exhumed in the central Cyclades based on a study of the metamorphic history of Naxos Island and nearby Syros Island. In this thesis, I carried out a systematic geothermobarometric and geochronological investigation on Naxos to better constrain the P–T–t paths that are recorded by the rocks. The data indicate that high-P metamorphism on Naxos occurred in the Eocene at c. 40 Ma and the HP-LT rocks were exhumed by two tectonic events. The first exhumation event occurred in the Oligocene. The HP-LT rocks were exhumed in a convergent setting by an extrusion wedge. The top of the sequence reached greenschist facies conditions at c.32 Ma, whereas the bottom of the sequence remained at greater depth (equating to pressures of 8–12 kbar). Additionally rocks from southeastern Syros recorded a similar Eocene/Oligocene P–T–t history to that recorded by the top of the sequence on Naxos, suggesting a common Eocene/Oligocene metamorphic history for the central Cyclades. The second exhumation event occurred in the Miocene. The rocks were further exhumed in an extensional setting from c. 20 to 8 Ma. The top of the sequence on Naxos was already in the brittle crust at that time and therefore did not record this Miocene metamorphism. The bottom of the sequence was first isothermally exhumed at high-T conditions and thereafter cooled rapidly.
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| 3. |
- Bender, Hagen, 1987-
(författare)
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Assembly of the Caledonian Orogenic Wedge, Jämtland, Sweden
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Collisional orogeny creates the largest mountain belts on Earth. The Caledonides of Scandinavia are a deeply eroded, ancient mountain belt, which today exposes a deep section through the former orogenic interior. The orogenic internides hold important geological information necessary to understand the geodynamic processes shaping collisional plate boundaries. This thesis explores the kinematics and timing of orogenic wedge formation in Jämtland, central Sweden. An integrated approach of structural field mapping, microstructural analysis, Rb–Sr radiogenic dating and rock magnetism yielded new and comprehensive tectonochronologic data. A regionally extensive network of kinematic field data demonstrated pervasive ductile top-to-the-ESE shearing across the entire tectonostratigraphy. Rb–Sr multi-mineral isochron ages constrained the absolute timing of ductile deformation to c. 430 Ma and c. 415 Ma. Local structural and magnetic data showed that final nappe emplacement and exhumation had occurred before extensional deformation initiated. The new data presented in this thesis contradicted a tectonic model previously proposed for Caledonian nappe stacking. These findings were used to develop an alternative tectonic model consistent with both the new and other available structural, petrological and chronological data. The new model for orogenic wedge assembly comprises three stages of foreland-directed, top-to-the-ESE thrusting. It reflects the complex interactions caused by the merging of two subduction zones accommodating Baltica–arc–Laurentia collisions during Ordovician to Devonian time.
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| 4. |
- Hansman, Reuben Johannes, 1986-
(författare)
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Cryptic Orogeny : uplift of the Al Hajar Mountains at an alleged passive margin
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mountains evolve and grow because of the large forces that occur from the collision of tectonic plates. Plate boundaries change and move through time, and regions that were once stable, shallow-marine environments can be dragged into subduction zones and get transformed into vast mountain ranges. The Al Hajar Mountains in Oman consist of carbonate rocks which show that during most of the Mesozoic (c. 268 Ma – 95 Ma) they had not yet formed but were flat and below sea level. Following this, in the Late Cretaceous (c. 95 Ma), a major tectonic event caused oceanic crust to be obducted onto this Mesozoic carbonate platform. Then after obduction a shallow marine environment resumed, and Paleogene sedimentary rocks were deposited. Currently, the central mountains are located on the Arabian Plate and are 200 km away from the convergent plate boundary with Eurasia. Here, Arabia is being subducted. Further towards the northwest Arabia and Eurasia are colliding, forming the Zagros Mountains which initiated no earlier than the Oligocene (c. 30 Ma). At this time the mountains were even further away from the plate boundary. The problem with the Al Hajar Mountains is that they record a collision, but are not in a collisional zone. To better understand the formation of the Al Hajar Mountains, a multidiscipline approach was used to investigate the timing at which they developed. This included applying low-temperature thermochronology, U-Pb dating of brittle structures, and balanced cross-sections. Results indicate that the orogeny began in the late Eocene and had concluded by the early Miocene (40 Ma – 15 Ma). Therefore, the uplift of the Al Hajar Mountains is not related to either the older Late Cretaceous ophiolite obduction or the younger Zagros collision, and a new tectonic model is proposed. This research shows that the Cenozoic tectonic history of northern Oman is more cryptic than what has been formerly presented.
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| 5. |
- Lewerentz, Alexander, 1987-
(författare)
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Vein controlled crystal size distributions of Barrovian index minerals
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912, and Barrovian metamorphism is still applied by metamorphic petrologists. Today the importance of metamorphic fluids for driving metamorphic reactions is widely recognised. Even so, the general view is that Barrovian metamorphism is solely controlled by pressure, temperature, and protolith composition. This thesis aims to establish if and how fluids control index mineral formation and distribution during Barrovian metamorphism. To do so, samples from Barrow’s own type locality in Glen Esk, Southeast Scottish Highlands, are used to investigate possible relationships between veining and index mineral distribution. This was done using a combination of petrographic and textural observations and analyses, whole rock chemistry, mineral chemistry, as well as oxygen isotope analyses. These data show a grade dependant association of index mineral distribution and veining: in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. For the garnet and staurolite zones, index mineral abundance is shown to decrease away from veins. The kyanite zone shows a fairly homogenous kyanite distribution, but also indications of extensive fluid-rock interaction. Based on these observations and the analysis datasets, it is concluded that fluid played a major role in the stabilisation and distribution of the Barrovian index minerals in Glen Esk, and that the fluid control was larger at metamorphic grade.
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| 6. |
- Peillod, Alexandre, 1998-
(författare)
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How were high-pressure rocks exhumed in Naxos, Greece?
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Combining petrological and geochronological data we were able to show that rocks in south Naxos completed a full blueschist-/greenschist-facies metamorphic loop in about 10 Myr, distinctly faster than hitherto believed. We show that the high-pressure rocks reached peak pressure (16.3±0.9 kbar - 619 ±32°C) between 40.5±1.0 and 38.3±0.5 Ma and were re-equilibrated in the middle crust (3.8±1.1 kbar - 384±30 °C) under greenschist-facies metamorphism during in the Oligocene ~32 Ma. Our data indicate that this exhumation occurred at rates of 7.4±4.6 km Ma-1. Therefore, the Oligocene greenschist-facies overprint of the blueschist-facies rocks from south Naxos is unrelated to the Miocene amphibolite-/greenschist-facies metamorphism observed in Naxos.
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