1,721,016 research outputs found
Brittle-viscous deformation cycles in the dry lower continental crust
No full textMany rheological models of the lithosphere (based on “strength envelopes”) predict a weak aseismic lower
crust below the strong brittle upper crust. An alternative view, based on the distribution of crustal seismicity,
is that the lower crust could also be strong and seismic. It has been suggested that a strong, seismogenic lower
crust results from the dry conditions of granulite facies rocks, which inhibit crystal plastic flow. This study
investigates exhumed networks of shear zones from Nusfjord (Lofoten, northern Norway) to understand initiation
and localization of viscous shearing in the dry lower crust. In the study area, different sets of ultramylonitic shear zones are hosted in the massive coarse-grained anorthosite. Metamorphic conditions of 720 ◦C, 0.9 GPa have been estimated for ductile deformation using amphibole-
plagioclase geothermobarometry. Field evidence indicates that ductile shearing exploited pseudotachylyte veins
and the associated damage zone of extensive fracturing. Undeformed pseudotachylyte veins locally overprint
mylonitic pseudotachylytes suggesting that frictional melting occurred at the same metamorphic conditions of
mylonitization. The deep crustal origin of the pseudotachylytes is also indicated by (1) the presence of microlites
of labradoritic plagioclase and clinopyroxene, and of dendritic garnet, and (2) the recrystallization of clinopyrox-
ene in the damage zone flanking the pseudotachylyte veins. Therefore the association of pseudotachylytes and
mylonites records brittle-viscous deformation cycles under lower crustal conditions.
The ultramylonites show phase mixing, fine grain size (5-20 μm) and equant shape of all minerals. Nucleation
of amphibole in triple junctions and dilatant sites is common. EBSD analysis indicates that the minerals in the
matrix are internally strain free and do not show a crystallographic preferred orientation. Taken together, these
observations suggest that diffusion creep and grain boundary sliding were the main deformation mechanisms in
the ultramylonites. Nucleation of hornblende indicates synkinematic fluid infiltration. Ongoing measurements of
intracrystalline water content along gradients from the pristine anorthosite to the ultramylonite will shed light on
the effect of water infiltration on the deformation mechanisms of plagioclase and clinopyroxene.
In summary, this study indicates that brittle (coseismic) fracturing was essential to induce grain size reduction
and fluid infiltration in the dry and strong lower crust. These processes promoted weakening by activating grain
size sensitive creep in the fine-grained hydrated material and resulted in the ductile shear zones localized to
the brittle precursors. In the absence of intense fracturing dry granulites would not undergo deformation and
metamorphism, and would survive metastably in the course of Wilson cycles. This has obvious implications for
long-term continental dynamics and for strain localization at plate boundaries, and will need to be included in
future geodynamic models
Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)
No full textThe Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern
Nucleation and growth of myrmekite during ductile shear deformation in metagranites
No full textMyrmekite is extensively developed along strain gradients of continuous, lower amphibolite facies shear
zones in metagranites of the Gran Paradiso unit (Western Alps). To evaluate the role of stress, strain
energy and fluid phase in the formation of myrmekite, we studied a sample suite consisting of weakly
deformed porphyric granites (WDGs), foliated granites (FGs) representative of intermediate strains, and
mylonitic granites (MGs). In the protolith, most K-feldspar is microcline with different sets of perthite
lamellae and fractures. In the WDGs, abundant quartz-oligoclase myrmekite developed inside Kfeldspar
only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental
shortening direction. In the WDGs, stress played a direct role in the nucleation of myrmekites along
interfaces already characterized by high stored elastic strain because of lattice mismatch between Kfeldspar
and albite. In the FGs and MGs, K-feldspar was progressively dismembered along the growing
network of microshear zones exploiting the fine-grained recrystallized myrmekite and perthite
aggregates. This was accompanied by a more pervasive fluid influx into the reaction surfaces, and
myrmekite occurs more or less pervasively along all the differently oriented internal perthites and
fractures independently of the kinematic framework of the shear zone. In the MGs, myrmekite forms
complete rims along the outer boundary of the small K-feldspar porphyroclasts, which are almost
completely free of internal reaction interfaces. Therefore, we infer that the role of fluid in the nucleation
of myrmekite became increasingly important as deformation progressed and outweighed that of stress.
Mass balance calculations indicate that, in Al–Si-conservative conditions, myrmekite growth was
associated with a volume loss of 8.5%. This resulted in microporosity within myrmekite that enhanced
the diffusion of chemical components to the reaction sites and hence the further development of
myrmekite
Dissolution-precipitation creep of K-feldspar in mid-crustal granite mylonites
No full textThe deformation of K-feldspar within lower amphibolite facies granite mylonites from the Gran Paradiso nappe (North-Western Alps, Italy) is primarily accomplished by dissolution, replacement and precipitation processes, with little or no evidence for dislocation glide or creep. New elongate grains of K-feldspar precipitate in dilational domains within and around K-feldspar porphyroclasts as a part of the process of myrmekite formation that progressively consumes the porphyroclasts. The new grains grow in epitaxial continuity with the parent porphyroclasts, which
hinders the development of a bulk crystallographic preferred orientation (CPO) in the aggregates of new grains when large amounts of relict K-feldspar are still present. In mylonites, the magmatic K-feldspar is eventually transformed into 100-300 μm thick, nearly monomineralic, fine-grained (20-50 μm in size) aggregates of new grains showing an oblique shape fabric and a CPO. The observed CPO is not consistent with the activity of any slip system in K-feldspar. Instead, it is interpreted to result from dissolution-precipitation creep, with a slow reaction rate parallel to [010] and [001] crystallographic axes and a fast reaction rate parallel the [100] axis. Consistent with such a dissolution-precipitation mechanism, boundaries of new K-feldspar grains are highly corroded when oriented approximately parallel to the extensional instantaneous stretching axis, whereas boundaries approximately orthogonal to the same stretching axis show well-developed crystal facets. The CPO developed is weak, which suggests that the anisotropy in the dissolution/growth rate of K-feldspar is also weak
Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla unit, Western Alps)
No full textWe have studied quartz microstructures and the c-axis crystallographic preferred orientations (CPOs) in four granitoid samples representative of increasing ductile shear deformation, from a weakly deformed granitoid (stage 1) to a mylonitic granitoid (stage 4). The quartz c-axis CPO measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area. All the samples belong to the Austroalpine Arolla unit (Western Alps) and were deformed at greenschist facies conditions. The quartz c-axis CPO was analyzed using a U-stage and the optical orientation imaging technique. The magmatic plagioclase, forming more than 50% of the volume of the granitoid, is extensively replaced by a mica-rich aggregate even in weakly deformed samples of stage 1. These aggregates flow to form an interconnected weak matrix with increasing deformation, wrapping relatively less strained quartz grains that undergo dominantly coaxial strain. Recrystallization of quartz ranges from less than 1% in the weakly deformed granitoid to up to 85% in the mylonitic granitoid, with average grain strain of 41% and 64%, respectively. With increasing strain and recrystallization, quartz grains in the granitoids show a sequence of transient microstructures and CPOs. Crystal plastic deformation is initially accomplished by dislocation glide with limited recovery, and at 50% grain strain it results in a CPO consistent with dominantly basal 〈a〉 slip. At 60% grain strain, recrystallization is preferentially localized along shear bands, which appear to develop along former intragranular cracks, and the recrystallized grains develop a strong c-axis CPO with maxima orthogonal to the shear band boundaries and independent of the host grain orientation. Within the granitoid mylonite, at an average quartz grain strain of 64%, recrystallization is extensive and the c-axis CPO of new grains displays maxima overlapping the host c-axis orientation and, therefore, unrelated to the bulk sense of shear. The host-controlled CPO is inferred to reflect pervasive recrystallization by progressive subgrain rotation. The switch from ‘shear band-control’ to ‘host-control’ on c-axis CPO occurred between 40% and 70% of recrystallization. In the quartz mylonite, the quartz c-axis CPO develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions. This study indicates that the CPO evolution of quartz may significantly differ in cases of polymineralic vs. monomineralic rocks under the same deformation conditions, if quartz in the polymineralic rock behaves as a ‘strong’ phase
High temperature pseudotachylytes and ductile shear zones in dry rocks from the continental lower crust (Lofoten, Norway)
No full textUnderstanding the mechanisms of initiation and growth of shear zones under lower crustal conditions is of funda-
mental importance when assessing lithosphere rheology and strength. In this study we investigate brittle-ductile
shear zones developed under lower crustal conditions in anorthosites from Nusfjord, Lofoten (northern Norway).
Steep ductile shear zones trend E-W to ESE-WSW and have a stretching lineation plunging steeply to the SSW or
SSE. The shear sense is normal (south block down to the south) as indicated by SC and SC’ fabrics and sigmoidal
foliations. The shear zone show a mylonitic to ultramylonitic fabric, sharp boundaries to the host anorthosites,
and abundant anastomosing dark fine-grained layers along the main foliation. The fine-grained layers localized
much of the strain. Relatively lower strain domains within or adjacent to shear zones indicate that the fine dark
bands of mylonites represent transposed pseudotachylyte which still locally preserve the pristine structures such
as chilled margins, breccia textures with angular clasts of the host rock and injection veins; intersecting veins of
pseudotachylyte record multiple stages of seismic slip. The orientation of injection veins and marker offset along
the most preserved pseudotachylyte fault veins indicate approximately a sinistral strike slip kinematic during
faulting event responsible for the friction-induced melting.
These observations indicate that ductile shear zones exploited pre-existing brittle fault zones including a network
of pseudotachylytes, and that the fine-grained “ultramylonites” derive from former fine-grained pseudotachylytes.
The pseudotachylyte microstructure is dominated by plagioclase microlites dispersed in a groundmass of fine-
grained clinopyroxene. Clinopyroxene recrystallizes in the damage zone flanking the pseudotachylytes, indicating
high metamorphic grade during pseudotachylyte formation. Small idioblastic or cauliflower garnet are scattered
through the matrix and overgrow the plagioclase porphyroclasts; in some cases small garnets nucleated along
thin microfractures discordant to the pseudotachylyte vein or along the pseudotachylyte boundary. In the host
rock garnet form thin continuous coronitic rims surrounding biotite and opaque and discontinuous one around
pyroxene. The mineral assemblage of ultramlylonites is also consistent with high grade metamorphic conditions
(recrystallized plagioclase and clinopyroxene, biotite and amphibole).
Nucleation of ductile shear zones is dictated by the availability of pseudotachylyte veins; remarkably, lithological
boundaries have not been exploited by ductile shear zones. Brittle deformation and extreme grain size reduction
are likely to be necessary conditions in order to promote ductile strain localization in dry rocks in the lower crust
Three-dimensional characterization of a crustal-scale fault zone: The Pusteria and Sprechenstein fault system (Eastern Alps)
No full textThe characterization and representation of fault zones is of paramount importance for studies of fault and earthquake mechanics, since their rheological and geometric complexity controls seismic/aseismic behaviour and fluid circulation at depth. We present a 3D geological model of a fault system, created by integrating borehole and surface structural data, which allows us to bridge the gap between outcrop-scale
descriptions and large-scale geophysical models. The model integrates (i) fault geometry and topology, (ii) fault-rock distribution, and (iii) characterization of fracturing in damage zones at the km-scale. The dextral-reverse Pusteria and Sprechenstein-Mules Faults (Italian Eastern Alps) provide an
opportunity to study fault rocks and damage distribution as a function of host-rock lithology and fabric, and of fault geometry. A first-order control is exerted by the composition of protoliths (quartzo-feldspathic vs. phyllosilicate-rich) and/or by the presence of an inherited anisotropic fabric (massive vs. foliated), resulting in a marked asymmetry of damage zones. Interestingly, the pervasive foliation typical
of some protoliths may explain both this asymmetry and the relative weakness of one of the faults. The importance of geometrical factors is highlighted when the damage zone thickness increases five times in proximity to a km-scale contractional jog. On the other hand, the type of fault rock present within the fault core does not show a direct relationship with damage intensity. In addition, the thickness of damage zones along planar fault segments does not appear to grow indefinitely with displacement, as might be
envisaged from some scaling laws. We interpret both of these observations as reflecting the maturity of
these large-displacement faults
The effect of Dauphin, twinning on plastic strain in quartz
Full text linkWe present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal (regions without twinning) to {π} and {π′} (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks
Brittle-ductile-brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)
No full textLate- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature (∼ 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at ∼ 250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures (∼ 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar–40Ar ages of ∼ 30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition
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