scholarly journals Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse

2020 ◽  
pp. jgs2020-199
Author(s):  
J. D. Wiest ◽  
J. Jacobs ◽  
H. Fossen ◽  
M. Ganerød ◽  
P. T. Osmundsen
Keyword(s):  
Shear Zones ◽  
Melt Crystallization ◽  
Western Gneiss Region ◽  
Content Type ◽  
Metamorphic Core Complexes ◽  
Collision Zone ◽  
Orogenic Wedge ◽  
Transfer Zones ◽  
Supplementary Material ◽  
Metamorphic Core

The (ultra)high-pressure Western Gneiss Region (WGR) of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continent-continent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite U-Pb zircon samples date melt crystallization at ∼405 Ma in the Øygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Øygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398 Ma, recording exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the WGR show abrupt breaks (10 – 30 Myrs) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge vs. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments. Supplementary material:https://doi.org/10.6084/m9.figshare.c.5241710

Geochemical evaluation of the in-situ regolith of Madengi hills of Dodoma, Tanzania. Implication on bedrock mapping and delineating gold mineralization target

2021 ◽  
pp. geochem2021-074
Author(s):  
Godson Godfray
Keyword(s):  
Gold Mineralization ◽  
Early Stage ◽  
Shear Zones ◽  
Soil Geochemistry ◽  
Residual Soils ◽  
Content Type ◽  
Geochemical Evaluation ◽  
Supplementary Material ◽  
Pathfinder Elements

Successful gold exploration projects depend on a piece of clear information on the association between gold, trace elements, and mineralization controlling factors. The use of soil geochemistry has been an important tool in pinpointing exploration targets during the early stage of exploration. This study aimed to establish the gold distribution, the elemental association between gold and its pathfinder elements such as Cu, Zn, Ag, Ni, Co, Mn, Fe, Cd, V, Cr, Ti, Sc, In, and Se and identify lithologies contributing to the overlying residual soils. From cluster analysis, a high similarity level of 53.93% has been shown with Ag, Cd, and Se at a distance level of 0.92. Au and Se have a similarity level of 65.87% and a distance level of 0.68, hence is proposed to be the most promising pathfinder element. PCA, FA, and the Pearson's correlation matrix of transformed data of V, Cu, Ni, Fe, Mn, Cr, and Co and a stronger correlation between Pb and U, Th, Na, K, Sn, Y, Ta and Be shows that source gold mineralization might be associated with both hornblende gneisses interlayered with quartzite, tonalite, and tonalitic orthogneiss. From the contour map and gridded map of Au and its pathfinder elements, it has been noted that their anomalies and target generated are localized in the Northern part of the area. The targets trend ESE to WNW nearly parallel to the shear zones as a controlling factor of Au mineralization emplacement.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5721965

scholarly journals Reactivated normal-sense shear zones in the core of the Greater Himalayan Sequence, Solukhumbu District, Nepal

2017 ◽  
Vol 53 ◽  
pp. 99-105
Author(s):  
Mary Hubbard ◽  
David R. Lageson ◽  
Roshan Raj Bhattarai
Keyword(s):  
Shear Zones ◽  
The North ◽  
Orogenic Wedge ◽  
Kinematic Models ◽  
Thrust System ◽  
Greater Himalayan Sequence ◽  
Displacement Based ◽  
Metamorphic Core ◽  
Normal Sense ◽  
Over Time

We present preliminary observations from the Solukhumbu region of Nepal, coupled with structures described in the literature, to suggest the importance of structural and metamorphic discontinuities within the Himalayan metamorphic core (Greater Himalayan Sequence) and reactivation of at least one of these thrust discontinuities with a normal (down-to-the-north) sense of displacement. Based on preliminary geochronologic data, development of these discontinuities may have evolved over time. In the Dudh Kosi Valley near Ghat, gneissic rocks and pegmatites exhibit tectonized fabrics and yield argon cooling ages of ~4 Ma for K-feldspar and ~9 Ma for biotite. Just north of Khumjung there is a prominent topographic break from which sheared gneissic rocks indicate a top-to-the-north, or normal, sense of shear. Near Pangboche, a repeated section of kyanitebearing rocks interleaved with sillimanite-muscovite schist suggests structural imbrication and/or interleaved retrograde metamorphism. Below the peaks of Nuptse and Lhotse, the Khumbu thrust (Searle 1999) appears to form the floor of a thick succession of leucogranite sills. We suggest that these discontinuities were formed over time, possibly from early MCT and STDS deformation at ~21 Ma to as recent as ~4 Ma, and need to be considered in kinematic models that combine channel flow with critical taper and tectonic denudation. Moreover, orogenic collapse in the Himalayan core may be migrating southward through time as the orogenic wedge continues to uplift in response to underthrusting of India and southward propagation of the Main Frontal Thrust system.

Tectonic significance of synextensional ductile shear zones within the Early Miocene Alaçamdağ granites, northwestern Turkey

2009 ◽  
Vol 147 (4) ◽  
pp. 611-637 ◽  
Author(s):  
FUAT ERKÜL
Keyword(s):  
Hanging Wall ◽  
Shear Zones ◽  
Early Miocene ◽  
Aegean Region ◽  
Menderes Massif ◽  
Metamorphic Core Complexes ◽  
Ductile Shear Zones ◽  
Northwestern Turkey ◽  
Ductile Shear ◽  
Metamorphic Core

AbstractSynextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts. One of the highly extended regions, the Aegean region, includes a number of metamorphic core complexes and synextensional granitoids that developed following the Alpine collisional events. The Alaçamdağ area in northwestern Turkey is one of the key areas where Miocene granites crop out along the boundary of various tectonic units. Structural data from the Early Miocene Alaçamdağ granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes. (1) Steeply dipping ductile shear zones caused emplacement of syn-tectonic granite stocks; they include kinematic indicators of a sinistral top-to-the-SW displacement. This zone has also juxtaposed the İzmir–Ankara Zone and the Menderes Massif in the west and east, respectively. (2) Gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the Menderes Massif on the structurally lower parts. Kinematic data from the foliated granites indicate a top-to-the-NE displacement, which can be correlated with the direction of the hanging-wall movement documented from the Simav and Kazdağ metamorphic core complexes. The gently dipping shear zones indicate the presence of a detachment fault between the Menderes Massif and the structurally overlying İzmir–Ankara Zone. Mesoscopic- to map-scale folds in the shallow-dipping shear zones of the Alaçamdağ area were interpreted to have been caused by coupling between NE–SW stretching and the accompanying NW–SE shortening of ductilely deformed crust during Early Miocene times. One of the NE-trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the Cycladic and the Menderes massifs. This crustal-scale wrench corridor, the İzmir–Balıkesir transfer zone, may have controlled the asymmetrical and symmetrical extensions in the orogenic domains. The combination of the retreat of the Aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique-slip tearing within the Eurasian upper plate appears to be a plausible mechanism for the development of such extensive NE-trending shear zones in the Aegean region.

Fluids sources in basement-hosted unconformity-uranium ore systems: tourmaline chemistry and boron isotopes from the Patterson Lake corridor deposits, Canada

2021 ◽  
pp. geochem2021-037
Author(s):  
E.G. Potter ◽  
C.J. Kelly ◽  
W.J. Davis ◽  
G. Chi ◽  
S-Y. Jiang ◽  
...  
Keyword(s):  
Shear Zones ◽  
Isotopic Signature ◽  
Strong Interactions ◽  
Content Type ◽  
B Values ◽  
Isotopic Signatures ◽  
Link Type ◽  
Basement Rocks ◽  
Supplementary Material ◽  
Over Time

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost one kilometer below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation only observed in shallow samples from the Triple R deposit (Tur 1) contain the heaviest isotopic signature (δ11B ≈ +26 to +19 ‰), whereas subsequent generations (Tur 2, Tur 3) yield lighter and more homogeneous isotopic signatures (δ11B ≈ +17.5 to +19.9 ‰). These results are consistent with precipitation from low temperature, NaCl- and CaCl2-rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ11B values). The lower δ11B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ11B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact.Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathwaysSupplementary material:https://doi.org/10.6084/m9.figshare.c.5727555

Exhumation of the crustal-scale Gaoligong strike-slip shear belt in Southeast Asia

2021 ◽  
pp. jgs2021-038
Author(s):  
Yanlong Dong ◽  
Shuyun Cao ◽  
Franz Neubauer ◽  
Haobo Wang ◽  
Wenyuan Li ◽  
...  
Keyword(s):  
Shear Zone ◽  
Early Cretaceous ◽  
Vertical Motion ◽  
Shear Zones ◽  
Strike Slip ◽  
Western Boundary ◽  
The Tibetan Plateau ◽  
Content Type ◽  
Lateral Extrusion ◽  
Supplementary Material

Lateral extrusion of blocks is a well-known geological process during continent–continent collision, which always expresses by either brittle strike-slip faults or ductile shear zones. However, vertical motion along such fault systems remains poorly constrained. The Gaoligong shear zone (GLG-SZ) formed the western boundary of the Indochina block during the India–Eurasia collision, resulting in the exhumation of deep crustal rocks, including a large volume of syntectonic granites. Combined zircon U-Pb dating and 40Ar/39Ar thermochronology revealed that both the unfoliated and foliated granitic intrusions were emplaced during the Early Cretaceous (112–125 Ma), post-magmatic melting occurred from the Early Oligocene (ca. 35 Ma), and subsequent cooling during the Middle Miocene (ca. 13 Ma). The average emplacement depth of Early Cretaceous samples revealed that at least 15 km of hangingwall of the GLG-SZ must have been removed by vertical motion during shearing. Syn-shearing exhumation underlines the role of the lateral motion of the shear zone initiation by magma-assisted rheological weakening and exhumation at high ambient temperatures within the shear zone. A new model links magmatic channel flow underneath the Tibetan Plateau with magma intrusions and the high geothermal gradients along the shear belts, such as the GLG-SZ.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5598365

In-situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: Campaign-style dating of metamorphism

2021 ◽  
pp. jgs2021-094
Author(s):  
Renée Tamblyn ◽  
Martin Hand ◽  
Alexander Simpson ◽  
Sarah Gilbert ◽  
Ben Wade ◽  
...  
Keyword(s):  
Laser Ablation ◽  
Ultrahigh Pressure ◽  
Western Gneiss Region ◽  
Content Type ◽  
Link Type ◽  
Geological Processes ◽  
History Of ◽  
Supplementary Material ◽  
Wilson Cycle

The development of in-situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in-situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogen. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometers across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronologic ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages.Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycleSupplementary material:https://doi.org/10.6084/m9.figshare.c.5715453

scholarly journals From orogeny to rifting: insights from the Norwegian ‘reactivation phase’

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Gwenn Peron-Pinvidic ◽  
Per Terje Osmundsen
Keyword(s):  
Continental Crust ◽  
Shear Zones ◽  
Detachment Faults ◽  
Metamorphic Core Complexes ◽  
Crustal Thinning ◽  
Collapse Mode ◽  
Collapse Structures ◽  
Normal Thickness ◽  
Further Development ◽  
Metamorphic Core

Abstract Based on observations from the Mid-Norwegian extensional system, we describe how, when and where the post-Caledonian continental crust evolved from a context of orogenic disintegration to one of continental rifting. We highlight the importance of a deformation stage that occurred between the collapse mode and the high-angle faulting mode often associated with early rifting of continental crust. This transitional stage, which we interpret to represent the earliest stage of rifting, includes unexpected large magnitudes of crustal thinning facilitated through the reactivation and further development of inherited collapse structures, including detachment faults, shear zones and metamorphic core complexes. The reduction of the already re-equilibrated post-orogenic crust to only ~ 50% of normal thickness over large areas, and considerably less locally, during this stage shows that the common assumption of very moderate extension in the proximal margin domain may not conform to margins that developed on collapsed orogens.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

2016 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Lower Zone ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada) suggest the presence of two distinct rheological transitions in the middle crust. (1) The brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear. (2) The localized-distributed transition or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. In some examples (e.g. the Whipple Mountains) the lower zone is spatially distinct from the detachment, although high-strain rocks from the lower zone were subsequently exhumed along the detachment. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization.

scholarly journals Interactions of plutons and detachments: a comparison of Aegean and Tyrrhenian granitoids

Solid Earth ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 1357-1388
Author(s):  
Laurent Jolivet ◽  
Laurent Arbaret ◽  
Laetitia Le Pourhiet ◽  
Florent Cheval-Garabédian ◽  
Vincent Roche ◽  
...  
Keyword(s):  
Root Zone ◽  
Numerical Models ◽  
Shear Zones ◽  
Normal Faults ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear Zones ◽  
Mountain Belts ◽  
Back Arc ◽  
Metamorphic Core

Abstract. Back-arc extension superimposed on mountain belts leads to distributed normal faults and shear zones interacting with magma emplacement within the crust. The composition of granitic magmas emplaced at this stage often involves a large component of crustal melting. The Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCCs) below crustal-scale low-angle normal faults and ductile shear zones. Intrusion processes interact with extension and shear along detachments, from the hot magmatic flow within the pluton root zone to the colder ductile and brittle deformation below and along the detachment. A comparison of the Aegean plutons with the island of Elba MCC in the back-arc region of the Apennine subduction shows that these processes are characteristic of pluton–detachment interactions in general. We discuss a conceptual emplacement model, tested by numerical models. Mafic injections within the partially molten lower crust above the hot asthenosphere trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain localization on persistent crustal-scale shear zones at the top that guide the ascent until the brittle ductile transition. Once the system definitely enters the brittle regime, the detachment and the upper crust are intruded, while new detachments migrate upward and in the direction of shearing.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Solid Earth ◽  
2017 ◽  
Vol 8 (1) ◽  
pp. 199-215 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
High Stress ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

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