| 1. |
- Barnes, C. J., et al.
(författare)
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Exhumation of the High-Pressure Tsakkok Lens, Swedish Caledonides : Insights From the Structural and White Mica(40)Ar/Ar-39 Geochronological Record
- 2020
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 39:7
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Tidskriftsartikel (refereegranskat)abstract
- Integrated structural, geochemical, and geochronological investigations were conducted on metasedimentary rocks in the eclogite-bearing Tsakkok Lens of the Seve Nappe Complex (Scandinavian Caledonides) to resolve its exhumation history. Three deformation events are defined. D1 is likely related to the prograde to peak-metamorphic stages, represented by a locally preserved S1. D2 resulted in vertical shortening and is defined by a pervasive S2 and cm-/m-scale F2 closed folds. D2 terminated with Scandian thrusting, which emplaced the overlying Koli Nappe Complex. D3 records NE-SW shortening and constitutes m-/km-scale F3 open folds that deformed the Tsakkok Lens and Koli Nappe Complex together. In situ white mica(40)Ar/Ar-39 geochronology was conducted on select metasedimentary samples possessing S1 or S2 to resolve the timing of exhumation. Postdecompression cooling of the Tsakkok Lens is best recorded by samples containing S1 or S2 that yield homogeneous white mica chemistry and(40)Ar/Ar-39 dates. The timing of cooling is resolved to 477.2 +/- 4.1 Ma (S1) and 475.3 +/- 3.5 Ma (S2). Vertical shortening of the lens during exhumation may have proceeded until 458.1 +/- 9.0 Ma. Later-stage deformation during Scandian thrusting penetrated the Tsakkok Lens at 429.9 +/- 9.0 Ma, or younger. This resulted in noncoaxial deformation of the metasedimentary rocks, producing heterogeneous white mica chemistry and partially reset the older(40)Ar/Ar-39 cooling record. Temperatures for deformation are resolved to the upper greenschist-lower amphibolite facies. Altogether, the Tsakkok Lens records rapid exhumation from eclogite-facies conditions to midcrustal depths or shallower, followed by emplacement of the overlying Koli Nappe Complex.
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| 2. |
- Bender, Hagen, et al.
(författare)
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Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones : Sequential Accretion of the Caledonian Internides, Central Sweden
- 2018
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Ingår i: Tectonics. - : John Wiley & Sons. - 0278-7407 .- 1944-9194. ; 37:10, s. 3545-3576
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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.
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| 3. |
- Bergman, S., et al.
(författare)
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Transpressive shear related to arc magmatism: The Paleoproterozoic Storsjön-Edsbyn Deformation Zone, central Sweden
- 2006
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Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 25
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Tidskriftsartikel (refereegranskat)abstract
- The polyphase, 15–30 km wide and >200 km long Storsjön-Edsbyn Deformation Zone (SEDZ) apparently separates two major Paleoproterozoic provinces in central Sweden. Four main phases of deformation have been recognized along the zone; associated high-strain rocks are characterized by their mineralogy, microstructures, kinematic patterns, magnetic signatures, and relative ages. The mineral and stretching lineations are consistently steeply to moderately plunging. Analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with north-south to northeast-southwest bulk regional shortening. The structural analysis suggests that the main ductile deformation along the SEDZ was due to dextral transpression with a steep coaxial component. Field evidence combined with published age data constrain the main deformation to the time interval circa 1.7–1.3 Ga. Synkinematic titanite from a ductile protomylonite in the SEDZ yields an U-Pb age of 1674 ± 6 Ma. The combined results imply transpressive deformation broadly synchronous with, and spatially related to, volcanism and plutonism in a continental margin magmatic arc setting.
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| 4. |
- Brown, D, et al.
(författare)
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Crustal-scale structure and evolution of an arc-continent collision zone in the southern Urals, Russia
- 1998
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 17:2, s. 158-171
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The outcropping geology of the southern Urals contains a well-preserved accretionary complex related to the Paleozoic collision that took place between the Magnitogorsk arc and the former East European Craton. The crustal-scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles. The reflection profiles show the accretionary complex to be highly reflective, allowing direct comparison of many reflections with surface geological features. We interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material, syncollisional sediments derived from the arc, deeply subducted high-pressure gneisses that are intercalated with eclogites and blueschist, and, at the highest structural level, ophiolite complexes. It is bound at the base by a thrust and at the rear by a highly deformed zone (the Main Uralian fault) adjacent to the backstop (the Magnitogorsk arc). Deposition of the Late Devonian volcaniclastic sediments of the Zilair Formation appears to be related to collision, uplift, and erosion of the arc, possibly following the arrival of the full thickness of the East European Craton continental crust at the subduction zone. With the arrival of the continental crust at the subduction zone, offscraping and underplating of Paleozoic slope and platform material took place at the base of the accretionary complex. Uplift of the arc was followed by its collapse and the unconformable deposition of Lower Carboniferous shallow water carbonates on top of it. A time lag of 10 - 15 Myr occurred between the high-pressure metamorphism and the subsequent arrival of the East European Craton at the subduction zone.
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| 5. |
- Chopin, Francis, et al.
(författare)
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The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP-HT Dome During Nuna Assembly
- 2020
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 39:3, s. 1-25
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Tidskriftsartikel (refereegranskat)abstract
- Tectonic evolution of the Paleoproterozoic Vaasa migmatitic complex (VMC) in the central part of the Svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps, deep seismic and magnetotelluric profiles, and structural and metamorphic data. The VMC is a semicircular structure with migmatitic rim and granitic core composed of several subdomes. It evolved in three main tectonic events (D1–D3). The D1 event (ca. 1.89–1.88 Ga) corresponds to the stacking of supracrustal rocks and the formation of an inverted metamorphic gradient. Anatexis at LP‐HT metamorphic conditions enabled the material to flow. The D2 event (ca. 1.88–1.87 Ga) corresponds to large‐scale folding of the partially molten crust within an orocline. It is marked by folds with an E‐W vertical axial planar foliation. The late D3 event resulted from mass redistribution owing to mechanical instabilities within the hinge of the orocline. It is marked by vertical shearing (ca. 1.87–1.85 Ga) in the marginal parts of the complex and along the granitoid subdomes. The seismic reflection profile (FIRE 3a) and magnetotelluric profiles (MT‐PE and MT‐B2) image large‐scale D1 stacking structures within an accretionary prism. Near vertical breaks in crustal‐scale reflectivity and conductivity models are interpreted as D3 shear zones. The VMC is an example of early mass and heat transfer within a collage of hot supracrustal rocks in an accretionary belt. Partial melting enhanced the flow of material, the production, and rise of magma as well as exhumation, marked by magmatic domes in the hinge of the orocline.
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| 6. |
- Cutts, Jamie, et al.
(författare)
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Two‐Stage Cooling and Exhumation of Deeply Subducted Continents
- 2019
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Ingår i: Tectonics. - 0278-7407 .- 1944-9194.
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Tidskriftsartikel (refereegranskat)abstract
- The burial and exhumation of continental crust during collisional orogeny exert a strong controlon the dynamics of mountain belts and plateaus. Constraining the rates and style of exhumation of deeply buried crust has proven difficult due to complexities in the local geology and thermochronometric methods typically used. To advance this field, we applied trace‐element and U‐Pb laser ablation inductively coupled plasma mass spectrometry analyses to rutile from eclogite and amphibolite samples from the Western Gneiss Complex of Norway—an archetypal continental (ultra)high‐pressure (UHP) terrane. Peak temperature and timing of midcrustal cooling were constrained for samples collected along a subduction‐ and exhumation‐parallel transect, using Zr‐in‐rutile thermometry and U‐Pb rutile geochronology, respectively. Peak temperatures decrease from 830 °C in the UHP domain to 730 °C at the UHP‐HP transition, remain constant at 730 °C across most of the terrane, and decrease to 620 °C at the eclogite‐out boundary. U‐Pb results show that most of the terrane cooled through 500 °C at 380–375 Ma except for the lowest grade region, where cooling occurred approximately 20 million years earlier. The results indicate that exhumation was a two stage process, involving (1) flexural rebound and slab flattening at depth combined with foreland‐directed extrusion, followed by (2) synchronous cooling below 500 °C across the, by then, largely flat‐lying Western Gneiss Complex. The latter implies and requires relatively homogeneous mass removal across a large area, consistent with erosion of an overlying orogenic plateau. The Caledonides were at near‐equatorial latitudes at the time. A Caledonian paleo‐plateau thus may represent a so far unrecognized factor in Devonian and Carboniferous atmospheric circulation and climate forcing.
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| 7. |
- Ducharme, T. A., et al.
(författare)
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Stretched Thin : Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece
- 2022
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 41:12
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Tidskriftsartikel (refereegranskat)abstract
- We present field-based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle-ductile conditions, indicative of a component of oblate coaxial strain during overall top-to-E general shear. New white mica Ar-40/Ar-39 geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 +/- 15 degrees C and 7 +/- 1 kbar. Zircon (U-Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low-temperature thermochronology for the HP-LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn-orogenic processes persisting until at least the early Miocene, facilitated in part by normal-sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.
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| 8. |
- Duffy, Brendan, et al.
(författare)
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Arc-parallel extrusion of the Timor sector of the Banda arc-continent collision
- 2013
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 32:3, s. 641-660
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Tidskriftsartikel (refereegranskat)abstract
- Structural studies of synorogenic basins in Timor using field and remote sensing techniques provide new structural and geomorphic evidence for syn-collisional extension in the converging plate boundary zone between the Australian Plate and Banda Arc. Fault mapping and kinematic analysis at scales ranging from outcrop (<1m(2)) to the dimensions of the active orogen in East Timor (similar to 100km(2)) identify a predominance of NW-SE oriented dextral-normal faults and NE-SW oriented sinistral-normal faults that collectively bound large (5-20km(2)) bedrock massifs throughout the island. These fault systems intersect at non-Andersonian conjugate angles of approximately 120 degrees and accommodate an estimated 20km of NE-directed extension across the Timor orogen based on reconstructions of fault-dismembered massifs. Major orogen-parallel ENE-oriented faults on the northern and southern sides of Timor exhibit normal-sinistral and normal-dextral kinematics, respectively. The overall pattern of deformation is one of lateral crustal extrusion sub-parallel to the Banda Arc. Stratigraphic relationships suggest that extrusion began prior to 5.5Ma, before pronounced rapid uplift of the orogen. We link this to progressive coupling of the fore-arc to an underthrust plateau on the Australian Plate and subduction of its ocean crust. Our results enable us to track the structural evolution of the upper crust during dramatic plate-boundary reorganizations accompanying the transition from subduction to collision. The deformation structures that we document suggest that both upper and lower plate deformation during incipient island arc-continent collision was largely controlled by the geometry and topography of the lower plate.
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| 9. |
- Edey, A., et al.
(författare)
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Kinematic variation within the Fars Arc, eastern Zagros, and the development of fold‐and‐thrust belt curvature
- 2020
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Ingår i: Tectonics. - : AGU. - 0278-7407 .- 1944-9194. ; 39:8
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Tidskriftsartikel (refereegranskat)abstract
- We analyze deformation of the Fars Arc in the eastern Zagros, Iran, including earthquake slip vectors, GPS velocities, paleomagnetism data, and fold orientations, to understand how this fold‐and‐thrust belt works, and so better understand the generic issue of fold‐and‐thrust belt curvature. The Fars Arc is curved, convex southwards. GPS‐derived rotation rates are ≤0.5° Myr‐1: rotation is clockwise west of 53° E, and counter‐clockwise to the east. These rotation senses are opposite to previous predictions of passive “bookshelf” models for strike‐slip faults during north‐south convergence. West of 53° E, average GPS vectors, thrust earthquake slip vectors, strain axes derived from GPS data and orthogonal directions to fold trends are all aligned, towards ~218°. East of this meridian, the average GPS vector is towards 208°, but the averages of the other datasets are distinctly different, all towards ~190°. We propose that fault blocks in eastern Fars, each ~20‐40 km long, rotate predominantly counter‐clockwise, whereas in western Fars the regional clockwise rotation takes place mainly on the array of active right‐lateral faults in this area. Thus localized block faulting and rotations accumulate to produce the overall strain and regional curvature. Active folds of different orientations in eastern Fars intersect to produce domal interference patterns, without involving separate deformation phases, indicating that fold interference patterns should not be interpreted in terms of changing stress orientations unless there is clear evidence. Fars Arc curvature is best explained by deformation being restricted at tectonic boundaries at its eastern and western margins, without significant gravitational spreading.
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| 10. |
- Feng, Qianqian, et al.
(författare)
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Thermo-Kinematic Constraints on Restoration of the Eastern Sichuan Fold-And-Thrust Belt, South China
- 2023
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 42:9
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Tidskriftsartikel (refereegranskat)abstract
- Kinematic restoration of the eastern Sichuan fold-and-thrust belt (ESFTB) in South China is calibrated with a new thermo-kinematic model combining analog models, discrete element method, and thermochronology data. Thermo-kinematic analysis provides constraints on the onset, rate, and lateral variation of deformation and exhumation. Results show that the ESFTB experienced a northwestward thrusting and periodic exhumation. Its evolution was characterized by five major stages based on the deformation characteristics; (a) Shortening above a basal decollement and formation of large imbricates separated by narrow synclines during 170-130 Ma; (b) Transition stage during 130-100 Ma; (c) Stepping up of deformation to shallow decollement levels and shaping of the thick-skinned domain during 100-70 Ma; (d) Shaping of the thin-skinned domain during 70-20 Ma; and (e) Continuous exhumation and structural modification from 20 Ma to present. The changes in the exhumation rate have been tectonic responses to the subduction retreat of the Paleo-Pacific Plate and the eastward growth of the Tibetan Plateau. The westward subduction of the Paleo-Pacific Plate resulted in northwestward shortening across South China, progressive deformation of the ESFTB, and rapid exhumation from the Late Jurassic to the Late Cretaceous. The crustal extension associated with the rollback of the Paleo-Pacific slab accounted for the Mesozoic exhumation rate decrease until the Miocene. The accelerated cooling of the thin-skinned domain of the ESFTB since the Miocene was a response to the eastward growth of the Tibetan Plateau, while the continuous slow exhumation in the thick-skinned domain was related to the continuous crustal extension in South China. New thermo-kinematic method combining analog modeling, discrete element method, balanced reconstruction, and thermochronology dataEastern Sichuan fold-and-thrust belt (ESFTB) experienced northwestward thrusting and periodic exhumation and its evolution can be subdivided into five stagesExhumation rate changes are responses of ESFTB to subduction retreat of the Paleo-Pacific Plate and eastward growth of the Tibetan Plateau
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