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Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle
Full text linkIn the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred longterm shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ 1 and σ 3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes
From XY tracking to buckling: Axial plane cleavage fanning and folding during progressive deformation
No full textFolding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow. Two field examples of upright (Lachlan Fold Belt, SE Australia) and recumbent (Naukluft Nappe Complex, central Namibia) folds are presented, in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification. Finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded. Cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment. The differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric. For folding in simple shear flow, as specifically modelled here, refraction is only strong on the fold limb that rotates against the imposed sense of shear. In known shear environments, this provides a potential kinematic indicator in folded units at relatively low strain (e.g. in simple shear, γ of around one), where other higher-strain indicators, typical of mylonites, are not yet sufficiently developed or are equivocal. © 2005 Elsevier Ltd. All rights reserved
Time‐Lapse Record of an Earthquake in the Dry Felsic Lower Continental Crust Preserved in a Pseudotachylyte‐Bearing Fault
Full text linkISSN:2169-9313ISSN:0148-0227ISSN:2169-9356ISSN:0148-0227ISSN:2169-935
Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)
Full text linkGarnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lowercrustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼ 1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼ 550 Ma overprint under sub-eclogitic conditions (600–700 °C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lowercrustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte
Interplay between seismic fracture and aseismic creep in the Woodroffe Thrust, central Australia – Inferences for the rheology of relatively dry continental mid-crustal levels
No full textThe over 600 km long Woodroffe Thrust developed at lower to mid-crustal levels during the intracontinental Petermann Orogeny at ca. 560–520 Ma. Ductile deformation with a top-to-north shear sense was accommodated along a shallowly (≤30°) south-dipping surface. Metamorphic conditions during deformation are established along a 60 km N-S transect, providing an ideal framework for studying variation in microstructure and crystallographic preferred orientations with changing temperature (ca. 520–620 °C) and pressure/depth in dominantly dry felsic crust. In the Woodroffe Thrust mylonites, dynamic recrystallization of quartz was dominated by subgrain rotation, whereas feldspar underwent grain size reduction by neocrystallization. Differential stress, estimated from quartz grain size piezometry, decreases with increasing metamorphic grade (i.e., deeper structural levels), and indicates a long-term average strain rate of around 10−11–10−12 s−1. We propose a qualitative rheological model to explain the observed cyclic interplay between ductile shearing (mylonitization) and brittle fracturing (pseudotachylyte formation) in the relatively dry middle crust. The model involves the downward migration of earthquake ruptures from the overlying seismogenic zone, which transiently triggers seismic slip at mid-crustal levels
Oxygen, carbon and strontium isotope constraints on the mechanisms of nappe emplacement and fluid-rock interaction along the subhorizontal Naukluft thrust, central Namibia
No full textThe Naukluft Thrust forms the floor thrust to the Naukluft Nappe Complex, a far-travelled, nappe stack of the Pan-African Damara belt in Namibia. The thrust tectonostratigraphy comprises three dolomitic components, a calc-mylonite horizon, and a discrete brittle fault. Stable isotope data indicate that the leading edge is characterized by positive δ13C values, whereas the trailing edge is characterized by negative δ13C values. There is a significant range in the δ18O values, over 15‰ in different sections, with the leading edge showing a larger range than the trailing edge. δ18O values are characteristic of burial dolomites and secondary dolomitization is indicated by the presence of networks above and below the Naukluft Thrust zone. The large range in δ18O values and variations in δ13C vs. δ18O patterns are interpreted to be the result of interaction between the precursor to the Naukluft Thrust zone dolomites and fluids derived from different footwall lithologies. 87Sr/86Sr isotope ratios indicate that some fluids were derived from the basement. The data presented in this study suggest that an original carbonate-dominated horizon existed prior to thrusting and that the basal thrust of the nappe complex exploited this horizon
Pseudotachylyte formation vs. mylonitization – repeated cycles of seismic fracture and aseismic creep in the middle crust (Woodroffe Thrust, Central Australia)
No full textThe Musgrave Ranges in Central Australia provide excellent exposure of the shallowly south-dipping Woodroffe
Thrust, which placed
∼
1200 Ma granulites onto amphibolite facies gneisses. This
∼
400 km long E-W structure
developed under mid-crustal conditions during the intracratonic Petermann Orogeny around 550 Ma. From
field observations and measurements, the shortening direction is constrained to be N-S and the movement
sense top-to-north. Ductile deformation during this process almost entirely localized in the footwall rocks,
developing a zone of mylonites, ultramylonites and sheared pseudotachylytes, several hundred metres wide, with
pseudotachylyte abundance rapidly decreasing further into the footwall. In contrast, the hanging wall behaved in
a predominantly brittle manner, producing significant volumes of pseudotachylyte breccia and isolated veins, but
was otherwise mostly unaffected and only weakly foliated. The difference in rheological behaviour is reflected
in the pseudotachylyte fabric, which is dominantly sheared in the footwall and largely unsheared in the hanging
wall. Low-strain domains in the footwall show that localized shearing initiated along pseudotachylyte veins and
that shear zones and mylonitic foliations were in turn exploited by subsequent pseudotachylyte veins. Neither
phyllonitization nor synkinematic growth of new muscovite is observed. In contrast to models with a simple
brittle-to-viscous transition, these observations show that a continuous cycle of brittle fracturing and shearing is
active in dry mid-crustal environments. The products of multiple earthquakes and ductile overprint, repeatedly
exploiting the same structural discontinuity, are composite layers of sheared pseudotachylyte. In the Woodroffe
Thrust, these layers are numerous and frequently observed parallel to the foliation in the footwall mylonites. The
thickest of these sheared pseudotachylyte horizons (
∼
15 m thick) mark the immediate contact to the hanging
wall and almost entirely consist of pseudotachylyte matrix. Particularly in the footwall, but locally also in the
hanging wall, shear strain can additionally be concentrated along the margins of dolerite dykes, whose mineral
assemblages will be studied to determine the metamorphic conditions that were active during development of the
Woodroffe Thrust
Strain localization on different scales and the importance of brittle precursors during deformation in the lower crust (Davenport Shear Zone, Central Australia)
No full textHigh strain rocks in the Musgrave Ranges (Central Australia) provide a rather unique insight into the development
of lower crustal shear zones during the 550 Ma Petermann Orogeny, allowing common models for lower crustal
deformation to be critically evaluated. The observed structures in the study area are, from south to north: (1)
The Mann Fault, which is poorly exposed but evident on airborne geomagnetic maps. This regional scale fault
with a component of dextral shear shows a step-over resulting in the formation of a pull-apart basin. (2) The
Davenport Shear Zone, accommodating the horizontal extension in a 7 km wide WNW-ESE-trending mylonitic
zone developed under subeclogitic, lower crustal conditions. This high strain zone is bounded to the north by a
more than 50 km long, continuous, sheared dolerite dyke. North of this dyke, the ∼ 1200 Ma Musgravian fabric is
still preserved, only slightly rotated and typically N-S trending. (3) The Woodroffe Thrust, marking the northern
boundary of the Musgrave Ranges, brings these lower crustal rocks on top of amphibolite facies units, with a
top-to-north sense of movement.
Strain in the Davenport Shear Zone is very heterogeneously distributed, with localization and partitioning
from the kilometre down to the millimetre scale. Pseudotachylyte is commonly associated with dykes, especially
on the boundaries, and is often sheared. The orientation of sheared dykes and localized shear zones is typically at a
high angle to either side of the shortening direction, resulting in a variable sense of shear and a major component of
flattening, with a nearly horizontal extension direction. Detailed outcrop-scale mapping shows that compositional
inhomogeneities, such as quartz veins, are generally not exploited, even when favourably oriented for shear
reactivation. Ultramylonitic shear zones are sometimes only a few millimetres wide but extend for several metres
and are generally oblique to the background foliation. Pseudotachylyte often predates or is coeval with localized
shearing and fracturing clearly played a major role in the nucleation of mesoscale discrete shear zones. In order
to constrain the conditions of pseudotachylyte formation, and to establish whether they developed under lower
crustal subeclogitic conditions, garnet-bearing sheared pseudotachylytes were sampled for geothermobarometric
analysis
Cyclic frictional-viscous slip oscillations along the base of an advancing nappe complex: Insights into brittle-ductile nappe emplacement mechanisms from the Naukluft Nappe Complex, central Namibia
No full textThe Naukluft Nappe Complex (NNC) forms a far-traveled fold and thrust belt klippe of the Panafrican Damara Belt in central Namibia. Estimates of the SE directed displacement range between 50 and 80 km. The entire nappe stack was thrust along an out-of-sequence, nearly planar, horizontal structure, the "Naukluft Thrust." The thrust zone consists of several distinct lithological components whose typical distribution, when all present, from bottom to top is (1) a massive, ochre-yellow weathering dolomite; (2) a polymict "gritty dolomite" (called in the past "Sole Dolomite"), (3) strongly foliated and isoclinally folded calcmylonites, and (4) an upper massive dolomite. A very discrete (<50 mm thick, often <10 mm thick) planar brittle fault (component 5) can occur at any level within this sequence. Our investigations show that the gritty dolomite forms by progressive cataclasis of the massive dolomite (component 1). Moreover, clasts of gritty dolomite are observed randomly oriented within a similar gritty dolomite matrix, suggesting multiple pulses of brecciation and self-brecciation. The gritty dolomite locally forms injection veins into the calcmylonites, and these veins are themselves boudinaged, indicating broadly coeval cataclastic and ductile deformation. The evolution of structures within the thrust zone is linked to the presence and flow of overpressured pore fluids. Field observations suggest that several pulses of fluid-induced brittle deformation overprinted, in a cyclic fashion, ductile structures formed during the emplacement of the nappe edifice. A "fault valve" behavior is suggested for the basal detachment of the NNC, with bulk shortening being accommodated by incremental slip during a history of combined viscous and frictional flow. Copyright 2006 by the American Geophysical Union
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 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼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 ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT
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