Emplacement mechanisms of a dyke swarm across the Brittle-Ductile transition

2020 ◽  
Author(s):  
Hans Jørgen Kjøll ◽  
Olivier Galland ◽  
Loic Labrousse ◽  
Torgeir B. Andersen
Keyword(s):  
Host Rock ◽  
Mafic Magma ◽  
Large Igneous Province ◽  
Dyke Swarm ◽  
Field Observations ◽  
Transport Pathways ◽  
Influx Rate ◽  
Brittle Ductile Transition ◽  
Iapetus Ocean ◽  
Rifted Margin

<p>Dykes are the main magma transport pathways through the Earth’s crust and, in volcanic rifts, they are considered the main mechanism to accommodate tectonic extension. Most models consider dykes as hydro-fractures propagating as brittle tensile, mode I cracks opening perpendicular to the least principal stress. This implies that dykes emplaced in rifts are expected to be sub-vertical and accommodate crustal extension. Here we present detailed field observations of a well-exposed dyke swarm that formed near the brittle-ductile transition at a magma-rich rifted margin during opening of the Iapetus Ocean. It was related to a ca 600 million year-old large igneous province. Our observations show that dykes were not systematically emplaced by purely brittle deformation and that dyke orientation may differ from the typical mode 1 pattern. Distinct dyke morphologies related to different emplacement mechanisms have been recognized including: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, and en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes with undulating contacts, rounded tips, folding of the host rock and contemporaneous brittle and ductile features; 3) Ductile “dykes” with rounded shapes and mingling between partially molten host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by ~30° that are mutually cross-cutting, demonstrating that the dyke swam did not consist of only vertical sheets oriented perpendicular to regional extension, as expected in rifts. By using the host-rock layers as markers, a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex was performed. This strain estimate shows that the dyke swarm accommodated >100% horizontal extension, but also 27% vertical thickening. This suggests that the magma influx rate was higher than the tectonic stretching rate, which imply that magma was emplaced forcefully, as supported by field observations of the host-rock deformation. Finally, observations of typical “brittle” dykes that were subsequently deformed by ductile mechanisms as well as dykes that were emplaced by purely ductile mechanisms suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition. The abrupt dyke emplacement and associated heating resulted in weakening of the crust that probably facilitated the continental break-up, which culminated with opening of the Iapetus Ocean.</p>

Age of the Grenville dyke swarm, Ontario–Quebec: implications for the timing of lapetan rifting

1995 ◽  
Vol 32 (3) ◽  
pp. 273-280 ◽  
Author(s):  
S. L. Kamo ◽  
T. E. Krogh ◽  
P. S. Kumarapeli
Keyword(s):  
Long Range ◽  
Volcanic Rocks ◽  
Canadian Shield ◽  
Dyke Swarm ◽  
Alkaline Complex ◽  
Southeastern Part ◽  
Time Marker ◽  
Continental Breakup ◽  
Iapetus Ocean

U–Pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the Grenville dyke swarm indicate a single age of emplacement at [Formula: see text] Ma. The 700 km long Grenville dyke swarm, located in the southeastern part of the Canadian Shield, was emplaced syntectonically with the development of the Ottawa graben. This graben may represent a plume-generated lapetan failed arm that developed at the onset of the breakup of Laurentia. Other precisely dated lapetan rift-related units, such as the Callander Alkaline Complex and the Tibbit Hill Formation volcanic rocks, indicate a protracted 36 Ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin. The age of the Grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 Ma Franklin dykes and sills to the 615 Ma Long Range dykes, along the northern and northeastern margins of Laurentia, respectively. Thus, the age for these dykes represents a key time marker for continental breakup that preceded the formation of the Iapetus ocean.

scholarly journals Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

2013 ◽  
Vol 29 ◽  
pp. 1-24 ◽  
Author(s):  
Brian G.J. Upton
Keyword(s):  
Mafic Magma ◽  
Dyke Swarm ◽  
Small Scale ◽  
Alkaline Magmatism ◽  
East African ◽  
The North ◽  
Intrusive Rocks ◽  
Alkali Granites ◽  
African Rift ◽  
Very High

The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c . 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation. A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c . 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations. Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting. There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift.

Paleomagnetism and U–Pb geochronology of the Clarence Head dykes, Arctic Canada: orthogonal emplacement of mafic dykes in a large igneous province

2009 ◽  
Vol 46 (3) ◽  
pp. 155-167 ◽  
Author(s):  
Steven W. Denyszyn ◽  
Don W. Davis ◽  
Henry C. Halls
Keyword(s):  
Remanent Magnetization ◽  
High Arctic ◽  
Large Igneous Province ◽  
Dyke Swarm ◽  
Canadian High Arctic ◽  
The North ◽  
Igneous Province ◽  
The Difference ◽  
Age Range ◽  
Chemical Remanent Magnetization

The north–south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U–Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U–Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ∼800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route.

scholarly journals Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins

2019 ◽  
Vol 518 ◽  
pp. 223-235 ◽  
Author(s):  
Hans Jørgen Kjøll ◽  
Olivier Galland ◽  
Loic Labrousse ◽  
Torgeir B. Andersen
Keyword(s):  
Dyke Swarm ◽  
Brittle Ductile Transition

Spatial variations of sills and implications for magma dispersal across the Karoo basin

2020 ◽  
Vol 123 (4) ◽  
pp. 511-530
Author(s):  
A. Coetzee ◽  
A.F.M. Kisters
Keyword(s):  
Host Rock ◽  
Sedimentary Facies ◽  
Spreading Rate ◽  
Lateral Spreading ◽  
Large Igneous Province ◽  
Propagation Path ◽  
Magma Ascent ◽  
Subsequent Formation ◽  
Karoo Basin ◽  
Lateral Propagation

Abstract Dolerite sill complexes of the Karoo Large Igneous Province (ca. 183 Ma) show systematic variations in emplacement style and size throughout the Karoo basin. These variations are explained in terms of three main, interrelated factors, namely the overburden thickness or emplacement depth, variations in host rock rigidities as a result of sedimentary facies changes in the Karoo basin, and proximity to magma feeders. In the northern parts of the thinner (<500 m) and more coarse-clastic Karoo stratigraphy, sills intrude preferentially below more rigid sandstone horizons that acted as stress barriers causing the arrest of magma ascent and lateral spreading below sandstone beds. The low overburden promotes roof uplift above sills and associated brittle faulting can initiate the formation of inclined sheets that limits the lateral propagation path of inner sills. Roof uplift is further promoted by the proximity to magma feeders in the basement and resulting variations in magma pressure that control the spreading rate and inflation of sills. Localised dyke networks spaced at regular intervals and rooted in underlying sills reflect the stretching of roof rocks above inflating sills. The combination of these effects results in relatively small (<10 km) diameters of sills in the northern parts of the basin. Sills emplaced at intermediate depths (ca. 700 m) in the central Karoo basin are marked by larger diameters (>30 km) and thicknesses of up to 100 m. This reflects the higher overburden pressures and the delay of roof failure and subsequent formation of inclined sheets. Dyke networks in the roof of these sills become more irregular and non-systematic at these greater depths. At even greater depths of up to 2 km in the southern parts of the Karoo basin, mega-sills reach diameters of 50 to 80 km, but thicknesses of only up to 35 m. Thick shale-rich sequences in the southern Karoo basin facilitate sill emplacement through internal host-rock deformation and ductile flow. The thicker overburden and different host rock rigidity delay or suppress roof failure and formation of inclined sheet, thus allowing for the lateral propagation of sills. The deeper-seated sills are typically not associated with local dyke networks.

A 1.78 Ga large igneous province in the North China craton: The Xiong'er Volcanic Province and the North China dyke swarm

Lithos ◽  
2008 ◽  
Vol 101 (3-4) ◽  
pp. 260-280 ◽  
Author(s):  
Peng Peng ◽  
Mingguo Zhai ◽  
Richard E. Ernst ◽  
Jinghui Guo ◽  
Fu Liu ◽  
...  
Keyword(s):  
North China Craton ◽  
North China ◽  
Large Igneous Province ◽  
Dyke Swarm ◽  
Volcanic Province ◽  
The North ◽  
Igneous Province

scholarly journals Enlargement of the area of the Timpton Large Igneous Province (ca. 1.75 ga) of the Siberian craton

2019 ◽  
Vol 10 (4) ◽  
pp. 829-839
Author(s):  
D. P. Gladkochub ◽  
T. V. Donskaya ◽  
R. E. Ernst ◽  
U. Söderlund ◽  
A. M. Mazukabzov ◽  
...  
Keyword(s):  
Siberian Craton ◽  
River Flow ◽  
Large Igneous Province ◽  
Dyke Swarm ◽  
Middle Section ◽  
Geochronological Data ◽  
Igneous Province

We present new geochronological data on dolerites from the Chaya dyke swarm of the Baikal inlier of the Siberian craton. The U‐Pb dating of baddeleyite from one dyke located at the SW end of the Chaya dyke swarm yielded an age of 1752±6 Ma, similar to the previously obtained age of a dyke in the NE end of this swarm. These ages estab‐ lish an age of 1752 Ma for a unified Chaya dyke swarm that extends for more than 200 km in the Baikal inlier of the Siberian craton. These new data confirm that the entire Chaya dyke swarm (as well as the Timpton‐Algamay and Eastern Anabar swarms) is a part of an overall radiating dyke swarm belonging to the Late Paleoproterozoic Timpton Large Igneous Province (LIP), the center of which is located in the middle section of the Vilyuy river flow. Thus, the LIP is enlarged to include the area further west in the Siberian craton.

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