| 1. |
- Buntin, Sebastian, et al.
(författare)
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Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield
- 2019
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Saucer-shaped intrusions of tens of meters to tens of kilometres across have been observed both from surface geological mapping and geophysical observations. However, there is only one location where they have been reported to extend c. 100 km laterally, and emplaced both in a sedimentary basin and the crystalline basement down to 12 km depth. The legacy BABEL offshore seismic data, acquired over the central Fennoscandian Shield in 1989, have been recovered and reprocessed with the main goal of focusing on this series of globally unique crustal-scale saucer-shaped intrusions present onshore and offshore below the Bothnian Sea. The intrusions (c. 1.25 Ga), emplaced in an extensional setting, are observed within both sedimentary rocks (<1.5 Ga) and in the crystalline basement (>1.5 Ga). They have oval shapes with diameters ranging 30-100 km. The reprocessed seismic data provide evidence of up-doming of the lower crust (representing the melt reservoir) below the intrusions that, in turn, are observed at different depths in addition to a steep seismically transparent zone interpreted to be a discordant feeder dyke system. Relative age constraints and correlation with onshore saucer-shaped intrusions of different size suggest that they are internally connected and fed by each other from deeper to shallower levels. We argue for a nested emplacement mechanism and against a controlling role by the overlying sedimentary basin as the saucer-shaped intrusions are emplaced in both the sedimentary rocks as well as in the underlying crystalline basement. The interplay between magma pressure and overburden pressure, as well as the, at the time, ambient stress regime, are responsible for their extensive extent and rather constant thicknesses (c. 100-300 m). Saucer-shaped intrusions may therefore be present elsewhere in the crystalline basement to the same extent as observed in this study some of which are a significant source of raw materials.
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| 2. |
- Farzipour-Saein, A., et al.
(författare)
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Basin evolution in the Lurestan region of the Zagros fold-and-thrust belt, Iran
- 2009
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Ingår i: Journal of Petroleum Geology. - : Wiley. - 0141-6421 .- 1747-5457. ; 32:1, s. 5-19
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of the central part of the Lurestan region in the Zagros fold-and-thrust belt has been studied using newly generated isopach maps for different time intervals between the Late Cretaceous and the Miocene. The study was based on existing geological maps, gravity data, measured stratigraphic surface sections, original field work and well data. Understanding the processes which have influenced facies and thickness variations in the study area will have a significant impact on future hydrocarbon exploration. Cenomanian carbonates assigned to the Sarvak Formation, the main reservoir unit in the study area, are composed of both pelagic and neritic facies. These facies occur along the roughly north-south trending "Anaran lineament", interpreted to represent a palaeohigh, which influenced patterns of sedimentation in the Cretaceous-Tertiary. The palaeohigh formed as a result of the reactivation of a basement lineament in the Late Cretaceous. The continuing influence of this lineament on patterns of sedimentation during Oligocene - early Miocene time is indicated by a range of evidence including the presence of clinoform geometries. Analysis of sedimentary thicknesses in the Zagros foreland basin between the Late Cretaceous and the early Miocene indicates progressive SWward migration of the depocentre. Late Cretaceous ophiolite obduction and plate margin convergence exerted a major influence on stratigraphic architecture, and controlled depocentre migration and foreland basin evolution.
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| 3. |
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| 4. |
- Aswad, Khalid J. A., et al.
(författare)
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Cr-spinel compositions in serpentinites, and their implications for the petrotectonic history of the Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 802-818
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Tidskriftsartikel (refereegranskat)abstract
- Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.
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| 5. |
- Aziz, Nabaz R. H., et al.
(författare)
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Contrasting Settings of Serpentinite Bodies in Northwestern Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 819-837
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Tidskriftsartikel (refereegranskat)abstract
- Protrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.
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| 6. |
- Bahroudi, A, et al.
(författare)
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Effect of ductile and frictional decollements on style of extension
- 2003
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Ingår i: Journal of Structural Geology. - 0191-8141 .- 1873-1201. ; 25:9, s. 1401-1423
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Tidskriftsartikel (refereegranskat)abstract
- Scaled analogue models were used to study the effect of frictional and ductile detachments on thin-skinned extension. Models consisted of two halves; one half is the ductile and the other has a frictional detachment. Extension occurred above two different basal configurations: a stretchable rubber sheet and a folded, banded sheet intended to produce homogeneous and heterogeneous extension, respectively. Model parameters varied systematically and included the brittle/ductile thickness ratio, rheologies, and bulk strain. Structures in the two halves are compared in profiles and plan views. A series of graben developed above both halves of models extended above a banded sheet, although there were differences in style, propagation rate and width of the deformation zone between the two halves. Different rates of propagation of structures in the two halves led to the formation of an accommodation or transfer zone parallel to the extension direction. Most relay ramps and inflection of normal faults in this zone indicate differential extension between the two halves.In contrast, in models extended above a stretchable rubber sheet, extensional structures such as horst and graben developed only above the ductile detachment. Model results indicate that heterogeneous mechanical stratigraphy and displacement rate have no effect on extensional structure above a rubber sheet. However, above 20% bulk extension, deformation becomes heterogeneous along multiple sets of conjugate faults oblique to the extension direction.
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| 7. |
- Burliga, Stanislaw, et al.
(författare)
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Modelling cover deformation and decoupling during inversion, using the Mid-Polish Trough as a case study
- 2012
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Ingår i: Journal of Structural Geology. - : Elsevier BV. - 0191-8141 .- 1873-1201. ; 42, s. 62-73
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Tidskriftsartikel (refereegranskat)abstract
- Seismic sections across the NW part of the Polish Basin show that thrust faults developed in the sedimentary units above the Zechstein evaporite layer during basin inversion. These cover thrust faults have formed above the basement footwall. Based on the evolution of the basin, a series of scaled analogue models was carried out to study interaction between a basement fault and cover sediments during basin extension and inversion. During model extension, a set of normal faults originated in the sand cover above the basement fault area. The distribution and geometry of these faults were dependent on the thickness of a ductile layer and pre-extension sand layer, synkinematic deposition, the amount of model extension, as well as on the presence of a ductile layer between the cover and basement. Footwall cover was faulted away from the basement only in cases where a large amount of model extension and hanging-wall subsidence were not balanced by synkinematic deposition. Model inversion reactivated major cover faults located above the basement fault tip as reverse faults, whereas other extensional faults were either rotated or activated only in their upper segments, evolving into sub-horizontal thrusts. New normal or reverse faults originated in the footwall cover in models which contained a very thin pre-extension sand layer above the ductile layer. This was also the case in the highly extended and shortened model in which synkinematic hanging-wall subsidence was not balanced by sand deposition during model extension. Model results show that inversion along the basement fault results in shortening of the cover units and formation of thrust faults. This scenario happens only when the cover units are decoupled from the basement by a ductile layer. Given this, we argue that the thrusts in the sedimentary infill of the Polish Basin, which are decoupled from the basement tectonics by Zechstein evaporites, developed due to the inversion of the basement faults during the Late Cretaceous-Early Tertiary. (C) 2012 Elsevier Ltd. All rights reserved.
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| 8. |
- Carboni, F., et al.
(författare)
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Modelling the 4D kinematics of extensional structures developed above discontinuous inclined ductile basal detachments
- 2022
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Ingår i: Journal of Structural Geology. - : Elsevier. - 0191-8141 .- 1873-1201. ; 157
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Tidskriftsartikel (refereegranskat)abstract
- A series of scaled analogue models were used to a preliminary study on the influence of the basal detachment configuration on the evolution of extensional systems, developed above an inclined basal ductile detachment. Three different basal configurations were aimed to test extension above: i) a homogeneously distributed ductile detachment; ii) two sectors with a different distribution of the ductile detachment, separated by an abrupt, transversal boundary; ii) a laterally variable width of the ductile detachment, driven by an abrupt oblique boundary. Fault activity was monitored through a series of digital elevation models obtained from the laser scanning of the models' surface. Results of the models analysis show how the geometry, the distribution, the kinematic, and the timing of faulting is strongly controlled by the geometry and orientation of the ductile-brittle interface in respect to the extension direction. Faults close to the brittle-ductile interface develop following the interface geometry, whose effect is reduced moving away from it. In addition, we observe a major number of faults which accommodates lower amount of extension and are less closely spaced above longer ductile detachments. Our results can be compared with natural case-histories along continental passive margins, where extension occurs above a dipping salt detachment, with inhomogeneous distribution (e.g., the northern Gulf of Mexico, central Brazilian margin, Angola margin).
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| 9. |
- Carrillo, Emilio, et al.
(författare)
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Structural significance of an evaporite formation with lateral stratigraphic heterogeneities (Southeastern Pyrenean Basin, NE Spain)
- 2017
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Ingår i: Marine and Petroleum Geology. - : Elsevier. - 0264-8172 .- 1873-4073. ; 86, s. 1310-1326
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Tidskriftsartikel (refereegranskat)abstract
- We run a series of analogue models to study the effect of stratigraphic heterogeneities of an evaporite formation on thin-skinned deformation of the Southeastern Pyrenean Basin (SPB; NE Spain). This basin is characterized by the existence of evaporites, deposited during the Early-Middle Eocene with lateral variations in thickness and lithological composition. These evaporites are distributed in three lithostratigraphic units, known as Serrat Evaporites, Vallfogona and Beuda Gypsum formations and acted as decollement levels, during compressional deformation in the Lutetian. In addition to analogue modeling, we have used field data, detailed geological mapping and key cross-sections supported by seismic and well data to build a new structural interpretation for the SPB. In this interpretation, it is recognized that the basal and upper parts of the Serrat Evaporites acted as the main decollement levels of the so-called Cadi thrust sheet and Serrat unit. A balanced restoration of the basin indicates that thrust faults nucleated at the stratigraphic transition of the Serrat Evaporites (zone with lateral variations of thickness and lithological composition), characterized by a wedge of anhydrite and shale. The analogue models were setup based on information extracted from cross-sections, built in two sectors with different lithology and stratigraphy of the evaporites, and the restored section of the SPB. In these models, deformation preferentially concentrated in areas where thickness change, defined by wedges of the ductile materials, was inbuilt. Based on the structural interpretation and model results, a kinematic evolution of the SPB is proposed. The kinematic model is characterized by the generation of out-of-sequence structures developed due to lateral stratigraphic variations of the Serrat Evaporites. The present work shows a good example of the role of stratigraphic heterogeneities of an evaporite formation which acts as decollement level on structural deformation in a fold-thrust belt. The results of this work have implications for hydrocarbon exploration and are relevant for studying structural geometry and mechanics in shortened evaporite basins. (C) 2017 Elsevier Ltd. All rights reserved.
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| 10. |
- Cruden, A.R., et al.
(författare)
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Diapiric basal entrainment of mafic into felsic magma.
- 1995
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Ingår i: Earth and Planetary Science Letters. - 0012-821X. ; 131:3-4, s. 321-340
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Tidskriftsartikel (refereegranskat)abstract
- One consequence of partial melting of the lower crust by heat transfer from a mantle-derived underplate is that the resultant buoyant, felsic magma layer (density ϱ2, viscosity μ2, thickness h2) will overllie a denser mafic layer (density ϱ3, viscosity μ3, thickness h3, which can be fully liquid to completely solid, depending on its thermal history. Laboratory experiments and finite-difference numerical models have been used to determine the conditions that favour the entrainment of the mafic layer into the overlying felsic magma as it ascends diapirically. Large amounts of entrainment occur when (where ϱ1 is the density of the crust), , and . When these conditions occur, the buoyancy and viscous effects acting to maintain the stability of the felsic-mafic layer interface are minimized. The role of m is much more important in the diapiric entrainment phenomenon than in the comparable problem of axial withdrawal from a density- and viscosity-stratified magma chamber with rigid walls.Favourable conditions for entrainment are likely to occur during the evolution of many lower crustal felsic magma source regions with a mafic underplate. Low amounts of entrainment result in minimal interaction (i.e., mixing) between the felsic host and entrained mafic material. If a large amount of entrainment occurs, our models combined with other studies show that mafic magma can remain in the centre of the conduit (low to high Reynolds number (Re), m ≈ 1), become fully mixed with the felsic host (high Re, low m), or become encapsulated by the felsic magma (low Re, m < 0.6). Such mechanical processes may account for the textural and compositional complexity shown by some plutons.
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