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

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>

scholarly journals Origin and evolution of the Kangâmiut mafic dyke swarm, West Greenland

2006 ◽  
Vol 11 ◽  
pp. 61-86 ◽  
Author(s):  
Kyle R. Mayborn ◽  
Charles E. Lesher
Keyword(s):  
Potential Temperature ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Dyke Swarm ◽  
Granulite Facies Metamorphism ◽  
West Greenland ◽  
Origin And Evolution ◽  
Brittle Ductile Transition ◽  
Depleted Mantle ◽  
Northern Portion

The Kangâmiut dyke swarm in West Greenland intruded Archaean terrains at 2.04 Ga, and its northern portion was subsequently metamorphosed to granulite facies during the Nagssugtoqidian orogeny (c. 1.8 Ga). Mineral and whole-rock major and trace element compositions show that the parental magmas for the dyke swarm differentiated by the fractionation of olivine, clinopyroxene, plagioclase and late stage Fe-Ti oxides. Petrographical observations and the enrichment of K2O during differentiation argue that hornblende was not an important fractionating phase. Field observations suggest emplacement at crustal levels above the brittle–ductile transition, and clinopyroxene geothermobarometry constrains dyke emplacement depths to less than 10 km. Granulite facies metamorphism of the Kangâmiut dykes and their host rocks in the northern portion of the swarm requires subsequent burial to c. 30 km, related to roughly 20 km of crustal thickening between the time of dyke emplacement and peak metamorphism during the Nagssugtoqidian orogeny. Kangâmiut dykes are characterised by low Ba/La ratios (12 ± 5), and high Nb/La ratios (0.8 ± 0.2), compared to subduction related basalts (Ba/La c. 25; Nb/La c. 0.35). These geochemical characteristics argue that the Kangâmiut dykes are not related to subduction processes. Forward modelling of rare-earth element data requires that primitive magmas for the Kangâmiut dykes originated from a moderately depleted mantle source with a mantle potential temperature of c. 1420°C. The inferred potential temperature is consistent with potential temperature estimates for ambient mantle at 2.0 Ga derived from secular cooling models and continental freeboard constraints. The geochemistry and petrology of the Kangâmiut dykes support a model that relates the dyke activity to passive rifting of the proposed Kenorland supercontinent rather than to mantle plume activity or subduction.

40Ar–39Ar dating of alkali basaltic dykes along the southwest coast of Greenland: Cretaceous and Tertiary igneous activity along the eastern margin of the Labrador Sea

1999 ◽  
pp. 19-29 ◽  
Author(s):  
Lotte Melchior Larsen ◽  
David C. Rex ◽  
W. Stuart Watt ◽  
Philip G. Guise
Keyword(s):  
Alkali Basalt ◽  
Dyke Swarm ◽  
Labrador Sea ◽  
Precambrian Basement ◽  
Stress Regime ◽  
Southwest Coast ◽  
Eastern Margin ◽  
Step Heating ◽  
Igneous Activity ◽  
Break Up

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Melchior Larsen, L., Rex, D. C., Watt, W. S., & Guise, P. G. (1999). 40Ar–39Ar dating of alkali basaltic dykes along the southwest coast of Greenland: Cretaceous and Tertiary igneous activity along the eastern margin of the Labrador Sea. Geology of Greenland Survey Bulletin, 184, 19-29. https://doi.org/10.34194/ggub.v184.5227 _______________ A 380 km long coast-parallel alkali basalt dyke swarm cutting the Precambrian basement in south-western Greenland has generally been regarded as one of the earliest manifestations of rifting during continental stretching prior to break-up in the Labrador Sea. Therefore, the age of this swarm has been used in models for the evolution of the Labrador Sea, although it has been uncertain due to earlier discrepant K–Ar dates. Two dykes from this swarm situated 200 km apart have now been dated by the 40Ar–39Ar step-heating method. Separated biotites yield plateau ages of 133.3 ± 0.7 Ma and 138.6 ± 0.7 Ma, respectively. One of the dykes has excess argon. Plagioclase separates confirm the biotite ages but yield less precise results. The age 133– 138 Ma is earliest Cretaceous, Berriasian to Valanginian, and the dyke swarm is near-coeval with the oldest igneous rocks (the Alexis Formation) on the Labrador shelf. A small swarm of alkali basalt dykes in the Sukkertoppen (Maniitsoq) region of southern West Greenland was also dated. Two separated kaersutites from one sample yield an average plateau age of 55.2 ± 1.2 Ma. This is the Paleocene–Eocene boundary. The swarm represents the only known rocks of that age within several hundred kilometres and may be related to changes in the stress regime during reorganisation of plate movements at 55 Ma when break-up between Greenland and Europe took place.

scholarly journals Fault reactivation and strain partitioning across the brittle-ductile transition

Geology ◽  
2019 ◽  
Vol 47 (12) ◽  
pp. 1127-1130 ◽  
Author(s):  
Gabriel G. Meyer ◽  
Nicolas Brantut ◽  
Thomas M. Mitchell ◽  
Philip G. Meredith
Keyword(s):  
Fault Slip ◽  
Fault Reactivation ◽  
Ductile Deformation ◽  
Tectonic Deformation ◽  
Gradual Change ◽  
Bulk Rock ◽  
Total Deformation ◽  
Bulk Strain ◽  
Brittle Ductile Transition ◽  
Ductile Flow

Abstract The so-called “brittle-ductile transition” is thought to be the strongest part of the lithosphere, and defines the lower limit of the seismogenic zone. It is characterized not only by a transition from localized to distributed (ductile) deformation, but also by a gradual change in microscale deformation mechanism, from microcracking to crystal plasticity. These two transitions can occur separately under different conditions. The threshold conditions bounding the transitions are expected to control how deformation is partitioned between localized fault slip and bulk ductile deformation. Here, we report results from triaxial deformation experiments on pre-faulted cores of Carrara marble over a range of confining pressures, and determine the relative partitioning of the total deformation between bulk strain and on-fault slip. We find that the transition initiates when fault strength (σf) exceeds the yield stress (σy) of the bulk rock, and terminates when it exceeds its ductile flow stress (σflow). In this domain, yield in the bulk rock occurs first, and fault slip is reactivated as a result of bulk strain hardening. The contribution of fault slip to the total deformation is proportional to the ratio (σf − σy)/(σflow − σy). We propose an updated crustal strength profile extending the localized-ductile transition toward shallower regions where the strength of the crust would be limited by fault friction, but significant proportions of tectonic deformation could be accommodated simultaneously by distributed ductile flow.

Brittle-ductile transition: semi-brittle behavior

1989 ◽  
Vol 167 (1) ◽  
pp. 75-79 ◽  
Author(s):  
John V. Ross ◽  
Peter D. Lewis
Keyword(s):  
Brittle Behavior ◽  
Brittle Ductile Transition

Geochemistry of syntectonic, crustal fluid regimes along the Lithoprobe Southern Canadian Cordillera Transect

1995 ◽  
Vol 32 (10) ◽  
pp. 1699-1719 ◽  
Author(s):  
Bruce E. Nesbitt ◽  
Karlis Muehlenbachs
Keyword(s):  
Deep Convection ◽  
Canadian Cordillera ◽  
Depth Of Penetration ◽  
Crustal Fluid ◽  
Quartz Veins ◽  
Economic Significance ◽  
Rock Types ◽  
Brittle Ductile Transition ◽  
Fluid Convection ◽  
Convection Model

In conjunction with the Lithoprobe southern Canadian Cordillera program, an extensive examination of geochemical indicators of origins, movement, chemical evolution, and economic significance of paleocrustal fluids was conducted. The study area covers approximately 360 000 km2from the Canadian Rockies to Vancouver Island. Research incorporated petrological, mineralogical, fluid-inclusion, δ18O, δD, δ13C, and Rb/Sr studies of samples of quartz ± carbonate veins and other rock types. The results of the study document a variety of pre-, syn-, and postorogenic, crustal fluid events. In the Rockies, a major pre-Laramide hydrothermal event was identified, which was comprised of a west to east migration of warm, saline brines. This was followed by a major circulation of meteoric water in the Rockies during Laramide uplift. In the southern Omineca extensional zone, convecting surface fluids penetrated to the brittle–ductile transition at 350–450 °C and locally into the underlying more ductile rocks. A principal conclusion of the study is that most quartz ± carbonate veins in metamorphic rocks in the southern Canadian Cordillera precipitated from deeply converted surface fluids. This conclusion supports a surface fluid convection model for the genesis of mesothermal Au–quartz veins, common in greenschist-facies rocks worldwide. The combination of our geochemical results with the results of other Lithoprobe studies indicates that widespread and deep convection of surface fluids in rocks undergoing active metamorphism is a commonplace phenomena in extensional settings, while in compressional-thrust settings the depth of penetration of surface fluids is more limited.

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