Spider texture and amphibole preferred orientations

doi:10.1016/0191-8141(94)90120-1

Journal of Structural Geology

Volume 16, Issue 5, May 1994, Pages 709-717

https://doi.org/10.1016/0191-8141(94)90120-1 Get rights and content

Abstract

Foliation in blueschist facies chert from California is defined by layers of oriented alkali-amphibole which consistently curve towards and converge on pyrite (and possibly pyrrhotite) crystals. These foliation nodes, not previously described, are called here ‘spider texture’. The texture is interpreted in terms of perturbations of the stress field in a matrix undergoing strain about rigid pyrite (or pyrrhotite) crystals, and it has important implications for understanding the mechanisms of amphibole preferred orientation development. Geometrical relationships between spider texture, pressure shadows and quartz preferred orientations suggest that amphiboles grew with a strong preferred orientation along planes of maximum shearing stress. The mechanism of foliation and preferred orientation development probably involved competitive anisotropic growth of amphibole prisms within the small gaps that open at steps on shear planes, followed by additional (micro-) porphyroblastic growth. The first stage of the mechanism is similar to slickenfibre development.

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The relation of seismic anisotropy to the deformation and flow of the lower crust relies on the intrinsic properties of the minerals in the crust and on the formation of crystallographic preferred orientation (CPO). Hornblende is an important constituent of the continental and oceanic lower crust and often exhibits a strong CPO. However, the formation of CPO in hornblende is not well understood, and it is difficult to uniquely relate its texture and anisotropy to conditions of metamorphism-deformation. We present a petrofabric analysis, using electron backscatter diffraction, of two hornblende-rich amphibolites from the Mamonia complex (Cyprus) with peak pressure and temperature of 0.6 GPa and 610 ± 40 °C. We distinguish between microstructures with varied strains and show that, while the common hornblende CPO—in which the [001] is aligned with lineation—is independent of strain, its symmetry does depend on strain: it is hexagonal (axial-[001]) under low strains but becomes orthorhombic ([001] and (100) parallel to the lineation and foliation normal of the deformation, respectively) under increasing strains. This transition is correlated with a decrease in the shape-preferred orientation and an increase in the intragrain misorientation, whose rotation is consistent with the easy slip system (100)[001] of hornblende. Thus, we interpret the hexagonal CPO to represent a metamorphic fabric (a texture formed under quasi-static conditions) and the orthorhombic CPO to represent a solid-state deformation fabric (a texture formed by strain accommodated via dislocation-mediated deformation). We consider the seismic anisotropy of the natural hornblende textures and compare two tectonic scenarios of lower crust deformation - crust thickening and channel flow with marked differences in the expected seismic anisotropy between the two scenarios. Taken together, the dataset presented here can be used as a new framework for understanding and interpreting natural textures and seismic anisotropy from the lower crust.
Crustal anisotropy and deformation of the southeastern Tibetan Plateau revealed by seismic anisotropy of mylonitic amphibolites
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Citation Excerpt :
These observations suggest that the dominant deformation mechanism of amphibole grains in these samples is not mainly dominated by dissolution-precipitation and intragranular plasticity deformation process. It is generally accepted that amphibole is the least plastic of the diagenetic silicate minerals under crustal conditions (Shelley, 1994), and only at temperatures above 800 °C does amphibole deformation become dominated by intracrystalline plasticity (Huang et al., 2003). Process involving rotation of non-equant amphibole grains in a deforming magma chamber may result in moderate intragranular deformation, and the amphibole content decreases progressively with increasing solid-state deformation to yield more biotite, epidote and quartz (Berger and Stünitz, 1996).
The composition and deformation patterns in the deep crust of southeastern Tibet are remain controversial. In this study, we present microstructures, fabric and seismic properties of amphibolites from the Ailao Shan-Red River shear zone, western Yunnan, China. The amphibolites display a distinct mylonitic foliation, and the amphibole grains show low intracrystalline deformation, slight compositional zoning but strong crystallographic preferred orientations (CPOs). We explain these CPOs could have formed by rigid body rotation during deformation accommodated by diffusion creep and grain boundary sliding. The amphibolites show strong seismic anisotropies (AV_p = 4.6–9.3% and Max. AV_s = 3.2–9.0%). Rock recipe modelling results indicate the seismic anisotropies of the mylonitic amphibolites are dominated by the content of biotite and amphibole. The delay time (0.28–1.01 s, average of 0.58 s) calculated for a 40 km-thick amphibolite layer is consistent with the observed delay time (0.52 s) of Pms wave splitting in this region, which indicates a thick amphibolitic layer in the crust of the southeastern Tibetan Plateau. The observed Pms wave splitting, extensive fold deformation and low velocity zones (LVZs) in the Diancang Shan (DCS) massif indicate two anisotropic layers in the crust. The upper layer related to frozen fabrics of amphibolites, which deformed during compressional folding and strike-slip shear. The lower layer related to the alignment of amphibole and melt pocket induced by present-day channel flow. The observed Pms wave splitting in the Ailao Shan (ALS) massif is attributed to amphibolites with vertical foliation and horizontal lineation which are generated by compressional folding and strike-slip shearing.
Dissolution precipitation creep as a process for the strain localisation in mafic rocks
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The lower crust is, on average, mafic in composition and composed of minerals that remain mechanically strong up to high temperatures. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro bodies at upper amphibolite facies conditions. The Kågen gabbro, N. Norway, is comprised of undeformed gabbro lenses enclosed by mylonitised margins that deformed at 690 ± 25 °C and 1.0–1.1 GPa. The evolution of the microstructures and fabric of the low strain gabbro to high strain margins were investigated. Original clinopyroxene and plagioclase dissolved during mineral reactions and precipitated as new minerals phases: new plagioclase and clinopyroxene (different compositions relative to the magmatic parents) and additional amphibole and garnet. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism. Amphibole shows a strong CPO that is primarily controlled by orientated growth in the stretching direction. The progression of mineral reactions and weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than required by dislocation creep. Initially strong lithologies can become weak, provided that reactions proceed during deformation.
Development of a synorogenic composite sill at deep structural levels of a magmatic arc (Odenwald, Germany). Part 2: Rheological inversion and mullion formation under bulk constriction
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At T > 600 °C, a CPO in hornblende might result from dislocation glide along the most favorable slip system (100) [001] (Cao et al., 2010; Gómez Barreiro et al., 2010). However, there is also some evidence that amphibole behaves as a rigid phase, which does not undergo significant crystal-plastic strain even at high temperature (Ildefonse et al., 1990; Shelley, 1994; Berger and Stünitz, 1996; Ko and Jung, 2015), and/or dissolving and reprecipitating during deformation (Berger and Stünitz, 1996; Hacker and Christie, 1990; Kruse and Stünitz, 1999; Marti et al., 2017). The hornblende CPO of the Acebuches amphibolites of the Aracena metamorphic belt is interpreted to result from rigid body rotation of amphibole prisms within the weaker plagioclase matrix.
We present structural, microfabric and strain data of a spessartite sill and its quartzmonzodiorite host that were affected by bulk constriction at T = ca. 660 °C. Constrictional deformation led to (1) subvertical prolate grain-shape fabrics, (2) increase of the sill's dip, (3) doubling of the sill's thickness, (4) mullions with cusps pointing into the host, and (5) boudinage of felsic veins cutting through the mafic sill. The sigmoidal shape of the foliation/lineation within the sill was caused by sill rotation, which led to shear stresses along the contacts with the host resulting in antithetic rotation of the grain-shape fabric that is subvertical in its central part and subparallel along the margins. The shape of the mullions, the boudinage of the felsic veins and the sigmoidal foliation/lineation indicate that the mafic sill was incompetent and the felsic material of host and veins was competent. This inversion in rheology can be explained by phase-boundary diffusion that was more effective in the fine grained mafic sill than in the coarse grained felsic host. Evidence for phase-boundary diffusion in form of strongly lobate phase boundaries is widespread and concerns all of the constituent phases present within the sill, host and felsic veins.
Stresses during pseudotachylyte formation - Evidence from deformed amphibole and quartz in fault rocks from the Silvretta basal thrust (Austria)
2021, Tectonophysics
Citation Excerpt :
The higher the distance to the seismic active fault, the more distributed the deformation. During this long-term creep, quartz-rich layers accumulate the higher amount of strain in relation to the amphibole-rich gneisses, consistent with a lower flow strength of quartz related to dislocation creep (e.g., Gleason and Tullis, 1995; Hirth et al., 2001; Luan and Paterson, 1992) as opposed to amphibole (e.g., Berger and Stünitz, 1996; Brodie and Rutter, 1985; Cao et al., 2010; Díaz Aspiroz et al., 2007; Imon et al., 2004; Shelley, 1994). Dislocation glide-controlled deformation of quartz is preferentially occurring in coarse quartz grains as opposed to the fine-grained recrystallized aggregates consistent with the inverse relationship between strength and grain size during dislocation glide, where grain boundaries act as obstacles (e.g., Fredrich et al., 1990; Hall, 1951; Hirth, 1972; Petch, 1953).
Pseudotachylyte-bearing, amphibole-rich gneisses with quartz-rich layers concordant to the foliation from the base of the Silvretta nappe were analyzed by polarized light microscopy and scanning electron microscopy to constrain the deformation history. Amphibole shows microfractures, kink bands and mechanical twins associated with pseudotachylytes. (−101)[101] twins document high differential stresses of ≥ 400 MPa. Quartz grains in contact with twinned amphibole in gneisses show subbasal deformation lamellae, short-wavelength undulatory extinction and new grains along cracks with diameters of < 15 μm and crystallographic preferred orientation (CPO) controlled by the orientation of host grains. This quartz microstructure indicates dislocation glide-controlled deformation coeval with high-stress amphibole deformation and pseudotachylyte formation.
Quartz-rich layers concordant to the gneiss' foliation commonly show evidence of dislocation creep by homogenous recrystallized grain aggregates with CPO indicating <a> dislocation glide. Systematic differences in distribution of the recrystallized grains and grain sizes, indicate deformation at differential stresses from a few tens to hundred MPa. Yet, no evidence of increasing strain towards the contact to amphibole-rich gneisses is observed. In contrast, a decreasing number of recrystallized grains towards the contact occurs locally. In relation to pseudotachylytes, the recrystallized quartz is affected by shear fractures offsetting the recrystallized microstructure. This, and the missing or decreasing strain gradient of dislocation creep towards the contact to amphibole-rich gneisses, indicate that 1) dislocation creep in quartz-rich layers is preceding and 2) not directly related to the high-stress and high strain-rate deformation, ruling out a major influence of thermal runaway for the formation of the pseudotachylytes. The differences in the deformation behavior is interpreted to be controlled by the distance to the propagating fault tip. This study demonstrates the potential of microstructures to constrain the deformation history controlling seismic faulting and creep at hypocentral depth not directly accessible for in-situ stress measurements.
Petrofabric of forsterite marbles and related rocks from a low-pressure metamorphic terrain (Almadén de la Plata massif, Ossa-Morena Zone, SW Spain) and its kinematic interpretation
2018, Journal of Structural Geology
A fabric study of forsterite marble and calc-silicate rock paragenetic minerals is presented for samples collected in the Almadén de la Plata massif of SW Spain, that experienced Variscan low-pressure metamorphism related to the tectonic evolution of the suture between the Ossa-Morena and South-Portuguese zones of the Iberian Massif. Two tectonically stacked metamorphic units contain marble, metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist (structuraly upper unit) and the upper amphibolite/granulite facies (structuraly lower unit). Different reactional paths lead to formation of forsterite marbles in both units. Dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn-metamorphic ductile deformation under temperatures between 500 and 700 °C. Dolomite formed (harder) porphyroclast systems and boudinaged bands, whereas weaker calcite underwent dynamic recrystallization. In both cases deformation was facilitated by activation of various intracrystalline slip systems, complemented by other deformation mechanisms. The fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral/stretching lineation, including a systematic asymmetry suggestive of non-coaxial deformation components. Foliations and lineations are interpreted as solid-state flow planes and directions bearing a kinematic significance. In particular, regional sinistral shear at mid-deep crustal realms coeval with low-pressure metamorphism explains the imbrication of metamorphic units and their exhumation, concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the Ossa-Morena/South-Portuguese suture between the late Devonian (∼360 Ma) and the latest Carboniferous (∼300 Ma).

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Spider texture and amphibole preferred orientations

Abstract

Tectonophysics

Tectonophysics

Tectonophysics

Earth Planet. Sci. Lett.

Tectonophysics

J. Struct. Geol.

J. Struct. Geol.

J. Struct. Geol.

J. Struct. Geol.

Tectonophysics

J. Struct. Geol.

Brittle deformation of hornblende in a mylonite: a direct geometrical analogue of ductile deformation by translation gliding

Can. J. Earth Sci.

Shear sense: a new approach that resolves conflicts between criteria in metamorphic rocks

J. metamorph. Geol.

(100) deformation twins in naturally deformed amphiboles

Nature

On the relationship between deformation and metamorphism, with special reference to the behavior of basic rocks

Dynamic recrystallisation and chemical evolution of clinoamphibole from Senja, Norway

Contr. Miner. Petrol.

Burgers vector determination in clinoamphibole by computer simulation

Am. Miner.