The mode of deformation in the orogenic mid‐crust revealed by seismic attribute analysis

Torvela, Taija; Moreau, Julien; Butler, Robert W.; Korja, T.; Heikkinen, Pekka

doi:10.1002/ggge.20050

Cited by 23 publications

(27 citation statements)

References 63 publications

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“…Hammond [2014] shows how back azimuth variations from receiver functions can be used to determine the symmetry axis and thus orientation of melt layers in the crust via anisotropic H-j stacking. Exposed sections of migmatitic middle and lower crust clearly indicate that nonhorizontal migmatitic layering (e.g., due to shear zone formation or metamorphic dome formation) is a common phenomenon [e.g., Torvela et al, 2013;Platt et al, 2015].…”

Section: Discussionmentioning

confidence: 99%

Modeling the impact of melt on seismic properties during mountain building

Lee

Walker

Lloyd

et al. 2017

Geochem Geophys Geosyst

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Initiation of partial melting in the mid/lower crust causes a decrease in P wave and S wave velocities; recent studies imply that the relationship between these velocities and melt is not simple. We have developed a modeling approach to assess the combined impact of various melt and solid phase properties on seismic velocities and anisotropy. The modeling is based on crystallographic preferred orientation (CPO) data measured from migmatite samples, allowing quantification of the variation of seismic velocities with varying melt volumes, shapes, orientations, and matrix anisotropy. The results show nonlinear behavior of seismic properties as a result of the interaction of all of these physical properties, which in turn depend on lithology, stress regime, strain rate, preexisting rock fabrics, and pressure-temperature conditions. This nonlinear behavior is evident when applied to a suite of samples from a traverse across a migmatitic shear zone in the Seiland Igneous Province, Northern Norway. Critically, changes in solid phase composition and CPO, and melt shape and orientation with respect to the wave propagation direction can result in huge variations in the same seismic property even if the melt fraction remains the same. A comparison with surface wave interpretations from tectonically active regions highlights the issues in current models used to predict melt percentages or partially molten regions. Interpretation of seismic data to infer melt percentages or extent of melting should, therefore, always be underpinned by robust modeling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation.

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Section: Discussionmentioning

confidence: 99%

Modeling the impact of melt on seismic properties during mountain building

Lee

Walker

Lloyd

et al. 2017

Geochem Geophys Geosyst

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“…Torvela et al () proposed another alternative tectonic model for the Svecofennian orogeny. They interpret the deep crustal seismic reflections as a proxy for crustal‐scale S‐C structure (Berthé et al, ) of extensional shear bands (Kidan & Cosgrove, ).…”

Section: Introductionmentioning

confidence: 99%

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

et al. 2020

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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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“…Betics (Platt and Vissers, 1980) and Aegean (Jolivet et al, 2010), strike-slip: e.g. Red River Fault (Leloup et al, 1995) and Cap de Creus (Fusseis et al, 2006)) and are known to exist at larger scale from seismic reflection data (Torvela et al, 2013). Focus has been put here on detachment zones because their footwalls exhibit exhumed rocks deformed at the brittle-ductile transition and also because we can use active analogues to calibrate strain rates in the models.…”

Section: Introductionmentioning

confidence: 99%

Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling

et al. 2013

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Abstract.We have designed a series of fully dynamic numerical simulations aimed at assessing how the orientation of mechanical layering in rocks controls the orientation of shear bands and the depth of penetration of strain in the footwall of detachment zones. Two parametric studies are presented.In the first one, the influence of stratification orientation on the occurrence and mode of strain localisation is tested by varying initial dip of inherited layering in the footwall with regard to the orientation of simple shear applied at the rigid boundary simulating a rigid hanging wall, all scaling and rheological parameter kept constant. It appears that when MohrCoulomb plasticity is being used, shear bands are found to localise only when the layering is being stretched. This corresponds to early deformational stages for inital layering dipping in the same direction as the shear is applied, and to later stages for intial layering dipping towards the opposite direction of shear. In all the cases, localisation of the strain after only γ = 1 requires plastic yielding to be activated in the strong layer.The second parametric study shows that results are lengthscale independent and that orientation of shear bands is not sensitive to the viscosity contrast or the strain rate. However, decreasing or increasing strain rate is shown to reduce the capacity of the shear zone to localise strain. In the later case, the strain pattern resembles a mylonitic band but the rheology is shown to be effectively linear.Based on the results, a conceptual model for strain localisation under detachment faults is presented. In the early stages, strain localisation occurs at slow rates by viscous shear instabilities but as the layered media is exhumed, the temperature drops and the strong layers start yielding plastically, forming shear bands and localising strain at the top of the shear zone. Once strain localisation has occured, the deformation in the shear band becomes extremely penetrative but the strength cannot drop since the shear zone has a finite thickness.

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The mode of deformation in the orogenic mid‐crust revealed by seismic attribute analysis

Cited by 23 publications

References 63 publications

Modeling the impact of melt on seismic properties during mountain building

Modeling the impact of melt on seismic properties during mountain building

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling

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