| 1. |
- Bauer, F. U., et al.
(författare)
-
Long-term cooling history of the Albertine Rift : new evidence from the western rift shoulder, DR Congo
- 2016
-
Ingår i: International journal of earth sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 105:6, s. 1707-1728
-
Tidskriftsartikel (refereegranskat)abstract
- To determine the long-term landscape evolution of the Albertine Rift in East Africa, low-temperature thermochronology was applied and the cooling history constrained using thermal history modelling. Acquired results reveal (1) old cooling ages, with predominantly Devonian to Carboniferous apatite fission-track ages, Ordovician to Silurian zircon (U-Th)/He ages and Jurassic to Cretaceous apatite (U-Th-Sm)/He ages; (2) protracted cooling histories of the western rift shoulder with major phases of exhumation in mid-Palaeozoic and Palaeogene to Neogene times; (3) low Palaeozoic and Neogene erosion rates. This indicates a long residence time of the analysed samples in the uppermost crust, with the current landscape surface at a near-surface position for hundreds of million years. Apatite He cooling ages and thermal history models indicate moderate reheating in Jurassic to Cretaceous times. Together with the cooling age distribution, a possible Albertine high with a distinct relief can be inferred that might have been a source area for the Congo Basin.
|
|
| 2. |
- Bauer, Friederike U., et al.
(författare)
-
Tracing the exhumation history of the Rwenzori Mountains, Albertine Rift, Uganda, using low-temperature thermochronology
- 2013
-
Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 599, s. 8-28
-
Tidskriftsartikel (refereegranskat)abstract
- The Rwenzori Mtns form a striking feature within the Albertine Rift of the East African Rift System. They are made up of a dissected Precambrian metamorphic basement block reaching heights of more than 5 km. Applying low-temperature therrnochronology a complex exhumation history becomes evident, where rock and surface uplift can be traced from Palaeozoic to Neogene times. Fission-track and (U-Th-Sm)/He cooling ages and derived cooling histories allow distinguishing different blocks in the Rwenzori Mtns. In the central part a northern and a southern block are separated by a putative NW-SE trending fault; with the northern block showing distinctly younger apatite fission-track ages (similar to 130 Ma) than the southern block (similar to 300 Ma). Cooling ages in both blocks do not vary significantly with elevation, despite considerable differences in elevation. Thermal history modelling reflects protracted cooling histories. Modelled t-T paths show decoupled blocks that were relocated separately along distinct fault planes, which reactivated pre-existing structures, inherited from Palaeozoic folding and thrusting. Initial cooling affected the Rwenzori area in Silurian to Devonian times, followed by Mesozoic and Cainozoic cooling events. Pre-Neogene evolution seems to be triggered by tectonic processes like the opening of the Indian Ocean and the south Atlantic. From thermochronological data the formation of a Mesozoic Albertine high is conceivable. In Cainozoic times the area was affected by rifting, resulting in differentiated surface uplift. Along the western flank of the Rwenzori Mtns, surface uplift was more pronounced. This is also reflected in their recent topography, formed by accelerated rock uplift in the near past (Pliocene to Pleistocene). Erosion could not compensate for this most recent uplift, resulting in apatite He ages of Oligocene to Miocene age or even older.
|
|
| 3. |
|
|
| 4. |
- Bender, Hagen, et al.
(författare)
-
Absolute timing of Caledonian orogenic wedge assembly, Central Sweden, constrained by Rb-Sr multi-mineral isochron data
- 2019
-
Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 344, s. 339-359
-
Tidskriftsartikel (refereegranskat)abstract
- Multi-mineral Rb-Sr isochron ages for mylonites of the Caledonian nappe pile (Jamtland, Sweden) constrain the orogenic evolution of the central Scandinavian Caledonides. An age of 437.8 +/- 3.9 Ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss. Thirteen isotopic ages derived from mylonites across all structural levels are between 434.6 +/- 6.9 Ma and 426.3 +/- 3.1 Ma. These results, complemented by a dense network of kinematic field data, are interpreted to reflect the timing of protracted top-to-the-ESE general shear under decreasing amphibolite- to upper-greenschist-facies conditions across all nappes. Three samples, derived from the top, middle and bottom of the nappe pile, yield additional deformation ages between 416.0 +/- 3.6 Ma and 410.1 +/- 3.0 Ma. These ages are interpreted to represent post-assembly imbrication of the nappe stack under decreasing metamorphic conditions. The new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in Jamtland by three stages of ESE-directed in- and out-of-sequence thrusting. In the first stage, postdating earlier subduction-exhumation of the high-grade metamorphic Seve Nappe Complex, the latter was emplaced in-sequence in the lower/mid-crust above nappes in its footwall (foreland) at c. 430 Ma. This event is related to the collision of Baltica with an island arc. Concurrently at c. 430 Ma, determined by paleogeography and structurally unrelated to the first stage, second-stage out-of-sequence thrusting emplaced the Kali Nappe Complex atop the developing nappe stack during final Baltica-Laurentia collision. Eventually, imbrication and exhumation of the composite Jamtland nappe stack in the uppermost middle crust during the third stage at c. 414 Ma was caused by continuous underthrusting of Baltica beneath Laurentia. This three-stage thrust model reflects the complex interactions of Baltica-arc-Laurentia collisions that were accommodated by two merging subduction zones.
|
|
| 5. |
- Bender, Hagen, 1987-
(författare)
-
Assembly of the Caledonian Orogenic Wedge, Jämtland, Sweden
- 2019
-
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.
|
|
| 6. |
- Bender, Hagen, et al.
(författare)
-
Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones : Sequential Accretion of the Caledonian Internides, Central Sweden
- 2018
-
Ingår i: Tectonics. - : John Wiley & Sons. - 0278-7407 .- 1944-9194. ; 37:10, s. 3545-3576
-
Tidskriftsartikel (refereegranskat)abstract
- Exhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.
|
|
| 7. |
|
|
| 8. |
- Bender, Hagen, 1987-
(författare)
-
Testing Tectonic Concepts in the Seve Nappe Complex, Jämtland, Sweden
- 2016
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When continental plates collide, one of the involved continents is subducted beneath the other one. As a consequence, the lithosphere thickens along the convergent plate boundary and causes the formation of a mountain belt. High-grade metamorphic rocks, formerly parts of the subducted continent, can be exhumed back to the surface and are commonly found in the central domain of mountain belts. The leucogranite- bearing Seve Nappe Complex in the central part of the Scandinavian Caledonides is a good example of such commonly migmatic rock units, which hold the key to understanding the tectonic evolution of a mountain range. This study aims to develop a tectonic model for the Swedish Caledonides, which integrates new structural data, collected during extensive fieldwork, and new geochronological constraints. Here we present the results of the first part of this study. We show how top-to-the-foreland directed shearing affected the migmatic part of the Seve Nappe Complex from bottom to top at amphibolite-facies conditions. Subsequently, the entire Caledonian nappe stack underwent a greenschist-facies overprint, associated with pervasive, again, top- to-the-foreland directed shearing. This last tectonic event resulted in the assembly of the presently observed nappe architecture, which is characterized by the excision of large sections of the lithosphere.
|
|
| 9. |
- Bolhar, R., et al.
(författare)
-
Zircon in amphibolites from Naxos, Aegean Sea, Greece : origin, significance and tectonic setting
- 2017
-
Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 35:4, s. 413-434
-
Tidskriftsartikel (refereegranskat)abstract
- We report U-Pb zircon ages of c. 700-550Ma, 262-220Ma, 47-38Ma and 15-14Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700-550Ma and 262-220Ma are assigned to inherited zircon. Available geochemical data suggest that the 15-14Ma metamorphic rims grew insitu in the amphibolites, corresponding to a high-grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40Ma zircon rims also grew insitu, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late-Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40Ma zircon rims would have grown insitu, and (ii) emplacement in the Miocene as a result mafic underplating during large-scale extension. In this case, only the 15-14Ma metamorphic outer rims would have formed insitu in the amphibolitic host rocks.
|
|
| 10. |
- Craddock, John P., et al.
(författare)
-
Deformation of the European Plate (58-0 Ma) : Evidence from Calcite Twinning Strains
- 2022
-
Ingår i: Geosciences. - : MDPI AG. - 2076-3263. ; 12:6
-
Tidskriftsartikel (refereegranskat)abstract
- We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.
|
|
