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The interplay between fracture and flow in the localization of crustal deformation
No full textThe Earth’s crust is generally considered to consist of distinct brittle and viscous (or “ductile”) rheological layers, corresponding to Navier-Coulomb failure or viscous flow, with a “brittle-ductile transition” occurring over a specific and relatively limited depth interval. Depending on the assumed geothermal gradient, a compositionally layered crust could have several such brittle-ductile transitions, but the model still implies that large regions of the crust deform exclusively by either brittle fracture or viscous crystal-plastic flow. However, it is becoming increasingly clear from field observation that, in reality, there is an intimate interplay in space and time between precursor heterogeneities (either structural or compositional), brittle fracture, fluid-rock interaction and more distributed “ductile flow”. In particular, there are now several well-documented examples of brittle precursors localizing subsequent ductile deformation under high grade metamorphic conditions ranging from upper amphibolite (Mancktelow and Pennacchioni 2005, 2007) to even eclogite facies (Austrheim and Boundy 1994). This interplay between fracture and crystal-plastic creep and/or diffusion occurs over a wide range of scales, from 100’s of kilometres down to individual grains.
Localization of strain in the crust can lead to the development of zones of very large relative displacement (such as low-angle thrusts and detachments, and steep strike-slip faults). The mechanics of this localization on a narrow zone and its repeated reactivation can only be considered in terms of a cyclical interaction between fracture, flow, and variation in local pore-fluid pressure. These relatively planar and discrete faults and shear zones are commonly observed to cross-cut layering and foliation at a small angle. Small-scale examples from the field, as well as numerical models, show that viscous localization is strongly controlled by existing compositional and rheological heterogeneity (such as bedding, dykes, veins etc), whereas fractures may crosscut such compositional layering at small angles. This suggests that major crosscutting faults, which may now be dominated by mylonitic fabrics characteristic of crystal-plastic flow (e.g., the Periadriatic Fault in the European Alps), could also have had a large-scale, brittle precursor that controlled subsequent ductile localization.
On a smaller scale, flanking structures (Passchier 2001; Grasemann and Stüwe 2001) developed around brittle fractures of limited length are particularly clear examples of interacting brittle-ductile deformation, because their geometry can only be explained if discrete slip occurred synchronously with more distributed surrounding ductile flow (Exner et al. 2004). Flanking structures that formed in calcite marbles under amphibolite facies conditions (e.g., on the island of Naxos, Greece) demonstrate that brittle fracturing can play an important role even in weak rocks at high temperatures – conditions generally taken to imply exclusively ductile or viscous behaviour. Such flanking structures are common in mylonitic shear zones (e.g., in mylonites in the footwall of the major Simplon low-angle normal fault in the central Alps) and demonstrate the delicate balance between fracture and flow in such high strain zones, with switches back and forth varying locally in space and through time.
This behaviour is not totally unexpected. The reduction of bulk porosity and permeability in rocks with depth raises the local pore fluid pressure from hydrostatic to near lithostatic (as usually assumed in metamorphic petrology), with the result that rocks are generally critically stressed and close to failure. Only minor local changes in the controlling parameters (strain rate, pore fluid pressure, dynamic or “tectonic” pressure) can cause a switch between fracture and flow. In natural examples, the interplay between fracture and flow is observed in middle to lower crustal rocks irrespective of whether they are weak (“wet”) or strong (“dry”). Excellent examples of interacting fracture and flow from glacier-polished outcrops of granodiorite in the Neves area of the eastern Alps developed under wet conditions, with very common quartz vein development and marked fluid-rock interaction along fractures. The deviatoric stress during both flow and fracture was low (<10 MPa), as demonstrated by little deformed calcite porphyroclasts in quartz mylonites, which did not even significantly twin during crystal plastic flow of the matrix quartz under upper amphibolite facies conditions (Mancktelow and Pennacchioni 2010). In contrast, in dry lower crust, such as from the Mont Mary area of the western Alps, stresses were high (as indicated by very small recrystallized quartz grain sizes; Fitz Gerald et al. 2006) and seismic fracture was associated with pseudotachlyte development. Pseudotachylytes subsequently act as rheologically weak layers that strongly localize ductile shearing under dry upper amphibolite facies conditions (Pennacchioni and Cesare 1997).
References:
Austrheim, H., Boundy, T.M., 1994. Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust. Science 265, 82-83.
Exner, U., Mancktelow, N.S., Grasemann, B., 2004. Progressive development of s-type flanking folds in simple shear. Journal of Structural Geology 26, 2191-2201.
Grasemann, B., Stüwe, K., 2001. The development of flanking folds during simple shear and their use as kinematic indicators. Journal of Structural Geology 23, 715-724.
Fitz Gerald, J.D., Mancktelow, N.S., Pennacchioni, G., Kunze, K., 2006. Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust. Geology 34, 369-372.
Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology 27, 645-661.
Mancktelow, N.S., Pennacchioni, G., 2010. Why calcite can be stronger than quartz. Journal of Geophysical Research 115.
Passchier, C.W., 2001. Flanking structures. Journal of Structural Geology 23, 951-962.
Pennacchioni, G., Cesare, B., 1997. Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary Nappe, Italian Western Alps). Journal of Metamorphic Geology 15, 777-791.
Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780
Brittle precursors, fluid‐rock interaction and the localization or spreading of shear zones
No full textHeterogeneous ductile shear zones are generally considered to develop by progressive strain
localization, implying that shear zones become narrower during their development and that individual
zones should affect an ever decreasing volume of rock. We support a diametrically different
model: in granitoid plutons and other non-layered rock bodies, shear zones are strongly localized at
their very initiation, on pre-existing planar rheological discontinuities, and tend to spread into the
adjacent rock with increasing strain. Precursor discontinuities can be either compositional layers
(e.g. dykes or veins) or fractures (Fig. 1), with enhanced fluid flow and fluid-rock interaction along
these fractures leading to localized compositional and rheological change of the original host rock.
Spreading of strain reflects the interplay between two factors: (1) diffusion of fluid away from
the central fracture, which broadens the zone of alteration, and (2) development of new fractures
both in previously intact rock and in already sheared domains. Cycles of fracturing are driven by
local stress concentrations in rocks that remain close to the critical stress state for fracture. Stress
concentration can be due to local mechanical instability (e.g. dyke boudinage) or more generally
due to inherent problems of strain accommodation during deformation of more “rigid” blocks surrounded
by a network of relatively discrete shear zones. As is clear from analogue and numerical
models, such blocks between bounding shear zones must also deform internally to maintain strain
compatibility. A distributed, more homogeneous background strain may develop in the intervening
blocks under higher grade metamorphic conditions, but our field observations demonstrate that
more localized shearing of intact granitic protolith in general develops from a brittle precursor.
The conference location is particularly appropriate for this topic, as one of the first studies that
proposed a brittle precursor to heterogeneous ductile shear zone development was that of Segall and
Simpson (1986), who used the Roses shear zones as relevant examples. There are many published
studies of shear zone development in previously undeformed granitoid plutons, especially with regard
to gradients in microstructure and chemical and/or isotopic changes during “shear localization”.
This interest was in part based on the assumption that plutons are relatively homogeneous,
allowing a direct comparison between the heterogeneous shear zone and the homogeneous background.
However, such intrusive bodies are certainly not homogeneous in detail: they show common compositional boundaries due to enclaves, intrusive contacts and dykes and veins. Cooling
plutons also invariably develop a pervasive set of joints due to thermal contraction, typically involving
a volume decrease on the order of 15% or more. These joints form conduits for late magmatic or
subsequent metamorphic fluids, with the development of veins (especially quartz veins) and localized
new mineral growth (commonly biotite in higher temperature cooling joints). These precursor
discontinuities act as the controlling loci for localizing shear zones either during pluton cooling
(Pennacchioni 2005; Pennacchioni et al. 2010) or later deformation (Mancktelow and Pennacchioni
2005; Pennacchioni and Mancktelow 2007).
However, the process is not necessarily limited to precursor magmatic structures (dykes, veins or
cooling joints). Deformation under higher grade metamorphic conditions (e.g. upper amphibolite
facies, as in Fig. 2) can produce repeated cycles of fracture, fluid rock interaction and ductile shear
localized on these brittle precursors. Already developed broader shear zones can themselves be cut
by discrete fractures, with fluid-rock interaction and new mineral growth once more changing the
local rheology and localizing further shearing to produce new heterogeneous shear zones oblique to
the earlier zone (Fig. 2). Brittle precursors localizing ductile shearing have also been proposed for
greenschist facies shear zones developed in schists of the Cap de Creus (Fusseis et al. 2006).
References
Fusseis, F., Handy, M.R., Schrank, C., 2006. Networking of shear zones at the brittle-to-viscous
transition (Cap de Creus, NE Spain). Journal of Structural Geology 28, 1228-1243.
Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid-rock
interaction on the development of single and paired ductile shear zones. Journal of Structural
Geology 27, 645-661.
Pennacchioni, G., 2005. Control of the geometry of precursor brittle structures on the type of ductile
shear zone in the Adamello tonalites, Southern Alps (Italy). Journal of Structural Geology 27,
627-644.
Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network
within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions.
Journal of Structural Geology 29, 1757-1780.
Pennacchioni, G., Menegon, L., Leiss, B., Nestola, F., Bromiley, G., 2010. Development of crystallographic
preferred orientation and microstructure during plastic deformation of natural coarsegrained
quartz veins. Journal of Geophysical Research-Solid Earth 115.
Segall, P., Simpson, C., 1986. Nucleation of ductile shear zones on dilatant fractures. Geology 14,
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The influence of grain boundary fluids on the microstructure of quartz-feldspar mylonites
No full textQuartz-rich domains in pre-Alpine, water-deficient, amphibolite facies (510–580 °C, 250–450 MPa), pegmatite mylonites from Mont Mary (MM), western Alps, preserve a fine dynamically recrystallized grain size, without significant annealing, despite the high synkinematic temperatures and subsequent static greenschist facies Alpine overprint. The microstructure is dramatically different from more typical water-rich amphibolite facies mylonites, such as from the Simplon Fault Zone in the central Alps, where the recrystallized grain size is on the millimetre-scale. The difference reflects the dominant strain-induced recrystallization mechanism: (1) progressive subgrain rotation and grain boundary bulging for the dry MM examples; and (2) fast grain boundary migration for the wet Simplon examples. The grain boundary microstructure imaged with SEM is also very different, with most grain boundaries in the dry MM samples lacking porosity, whereas grain boundaries in the wet samples are decorated by a multitude of pores. Quartz grain boundaries from both wet and dry samples are locally coated by thin (100’s of nanometres), possibly amorphous, silica films. Despite the differences in microstructure, the crystallographic preferred orientations (CPOs) of quartz-rich domains from both areas are very similar. Water-deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism(s) but apparently have little influence on the intracrystalline slip systems, as reflected in the CPOs (strong c-axis Y maxima). In MM mylonites, both K-feldspar and plagioclase (An33-38) dynamically recrystallize, consistent with the inferred metamorphic conditions. Under water-deficient conditions, mid- to lower-crustal rocks can deform heterogeneously under transitional ductile-brittle conditions at high differential stress (for MM ca. 300–500 MPa, as estimated for dry Mohr–Coulomb failure) and preserve this high-stress microstructure, because of the low mobility of dry grain boundaries
Analogue modelling of the influence of shape and particle/matrix interface lubrication on the rotational behaviour of rigid particles in simple shear
No full textThe rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear (Couette) flow was studied in 2D rock-analogue experiments. The influence of particle shape (elliptical vs. monoclinic), aspect ratio and the nature of the matrix/particle interface (lubricated vs. unlubricated) was investigated. Both matrix (PDMS) and lubricant (liquid soap) were linear viscous, with a viscosity ratio of ca. 10^4. Without lubricant, the rotational behaviour of all particles closely approximates the Jeffery theory. Lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position. Lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize. Weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation. In general for elliptical particles, rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle. For thick mantles (initially >10% of the volume of the particle), rotation rates are faster than Jeffery theory. For very thin mantles they are markedly slower compared with thick mantles, particularly when the long axis is nearly parallel to the shear direction. Rotation rates are never strictly zero, so true stabilization does not occur. However, for more elongate particles (axial ratio=6) rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population. In experiments, the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain. In natural examples of porphyroclast systems, the weak mantle continually develops by recrystallization and/or cataclasis of the rigid clast core and a steady state between production and thinning could be attained, potentially leading to true stabilization for particles with a high axial ratio
Analogue modelling of reverse fault reactivation in strike-slip and transpressive regimes: Application to the Giudicarie fault system, Italian Eastern Alps
No full textSandbox analogue models were used to study the reactivation of a reverse fault in strike-slip and transpressive regimes, for comparison with the evolution of the Giudicarie fault system in the Central Eastern Alps. The Giudicarie system is interpreted as resulting from Late Miocene sinistral transpressive reactivation of an older, Late Oligocene reverse fault. The 'old' reverse fault was reproduced as a pre-cut dilatant surface obtained by pulling a stiff metal wire through the model sand layer. The position of the pre-existing fault with respect to the base plate fault accommodating the strike-slip and transpressive faulting phase controlled the extent and geometry of reactivation. The clearest reactivation in a pure strike-slip regime was achieved in experiments where the basal strike-slip fault was immediately below the pre-existing fault plane. This strong reactivation involved lateral extrusion and lateral stepping of secondary faults from the basal fault to the pre-existing reverse fault. In the case of transpression, the most spectacular reactivation was achieved for a convergence angle of 10°. Strongly asymmetric structures developed on either side of the pre-cut dilatant zone. The analogue experiments reproduced very closely the structural features of the Giudicarie fault system, supporting a model involving a twofold tectonic evolution for the Giudicarie fault system, with later reactivation in sinistral transpression of an older reverse fault. © 2003 Elsevier Ltd. All rights reserved
Small-scale ductile shear zones: Neither extending, nor thickening, nor narrowing
No full textThe length, thickness and strain gradients of small-scale (10 −3 –10 −1 m thick) ductile shear zones are pre-determined by the presence of surface precursors (e.g. fractures or compositional layers) and associated fluid-rock interaction. Fractures, with their surrounding host-rock damage zone and concurrent tectonic underpressure during dilation, provide efficient fluid pathways and networks with diffusive fluid-rock interaction at their margins. The fluid-induced, gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types, including single homogeneous-to-heterogeneous shear zones and paired shear zones. As a consequence, geochemical differences between shear zone and host rock may reflect fluid-rock interaction during the precursor brittle history rather than during slip, especially considering that shear zones with even a small component of stretch may be over-pressured and therefore unable to drain fluids from the surrounding rocks. Support from field observation for this interpretation is given by (1) the lack of correlation between shear zone thickness and accumulated displacement; and (2) the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors. Most small-scale shear zones in massive rocks (granitoids) are initially neither thickening nor narrowing with increasing strain. This concept may also apply to more foliated rocks, but does not necessarily hold for larger-scale shear zones
Depth of ancient seismicity along the Woodroffe Thrust (Central Australia): Constraints from pseudotachylytes in peraluminous gneisses
Full text linkThe Woodroffe Thrust (WT) is a regional‐scale mylonitic shear zone that developed during the Petermann Orogeny (630–520 Ma) in lower to mid‐crustal rocks of the Musgrave Ranges, central Australia. In the upper part the WT hosts the largest volume worldwide of tectonic pseudotachylytes (coseismic quenched frictional melts). The pseudotachylytes were only marginally reworked along the mylonitic belt marking the WT, which mainly derived from the footwall amphibolite‐facies rocks. Mid‐crustal conditions of deformation along a shallowly dipping (<6°) WT were previously inferred from estimates of the P‐T conditions of mylonitization along a regional transect in the N‐S direction of thrusting. However, the pressure estimates were subject to large uncertainties. To better constrain the ambient conditions of the ancient seismic faulting along the WT, we investigate pseudotachylytes within peraluminous gneisses, a rock type more sensitive to P‐T changes in the range of interest. Microstructural analysis allows the sequence of minerals (corundum, sillimanite, cordierite, andalusite, kyanite and garnet) developed during melt quenching and subsequent solid‐state growth to be established. Critical observations are the growth of andalusite during pseudotachylyte cooling, constraining faulting at <0.45 GPa, and of kyanite during the immediately following ductile reactivation of pseudotachylytes. Seismic faulting is inferred to have occurred at ambient conditions of ∼0.4 GPa and 450°C, that is at much shallower conditions than previously assumed. These new P‐T estimates imply an inclination of 20–25° of the WT, if the main stage of seismic faulting and mylonitization along the WT were coeval
Determining the timing of formation of the Rawil Depression in the Helvetic Alps with the use of paleomagnetic and structural methods
No full textAnisotropy of magnetic susceptibility, palaeomagnetism and structural methods are used in order to test the relative timing of antiform updoming and formation of the Rawil Depression in the Helvetic Alps. Samples were collected from all nappes currently exposed in the study region. The magnetic fabric is consistent with extension oblique and parallel to the regional fold trend and with palaeostress reconstructions from fault planes and veins. Palaeomagnetic analyses show a stable characteristic remanence (ChRM), with samples recording both normal and reverse polarity. A successful fold test performed across the antiformal dome structure suggests that the palaeomagnetic signal was acquired prior to doming. By comparison with thermochronometric data, the ChRM was acquired between 25 and 10 Ma and is pre- to synfolding. A secondary post-doming palaeomagnetic component (A), whose magnetization is likely to have occurred between 10 and 3.5 Ma, appears to be too steep with regards to the inclination of the Earth's field, suggesting recent large-scale tilting has occurred in the region. These combined analyses indicate that widespread orogen-parallel extension occurred prior to the formation of the Rawil Depression, which is finally interpreted as the result of a stepover structure at the curvature between Central and Western Alps
The earthquake cycle in the dry lower continental crust: Insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)
Full text linkThis paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25-40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'
Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust
No full textThe Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating Ar-40-Ar-39 dating. Three deformation-related crystallization ages can be distinguished: (1) D-1, characterized in the PBT hanging wall by an S-1 foliation defined by white mica + chloritoid, began at or before similar to 38.0 Ma; (2) D-2 formed a pervasive S-2 cleavage and synchronous white-mica rich veins dated at similar to 27 Ma; (3) D(3)produced an S-3 crenulation cleavage and chlorite + white-mica veins dated at similar to 23 Ma. Older ages of similar to 96 Ma (footwall) and similar to 115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K-Ar age of similar to 19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D-1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D-2 reflects Oligocene top-to-NW shearing, with both in- and out-of-sequence thrusting. D-3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT
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