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1.
Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a ∼10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, north–northeast-striking, shallowly dipping mylonites of the SLSZ form three 1–10-m-thick splays. Oblique stretching lineations and shear sense in both shear zones record components of dip-slip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from ∼280–500 °C in the HSZ to ∼280–600 °C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga. 相似文献
2.
Thomas Leydier Philippe Goncalves Pierre Lanari Emilien Oliot 《Journal of Metamorphic Geology》2019,37(8):1129-1149
The inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings. 相似文献
3.
Lithospheric deformation on Earth is localized under both brittle and ductile deformation conditions. As high-temperature ductile rheologies are fundamentally strain-rate hardening, the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature. In this contribution, I develop a localization potential that quantifies the weakening associated with these changes. The localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions. I provide analytical expressions for the localization potential associated with a temperature increase, grain size reduction, an increase in water fugacity, melt content, or the abundance of a weak mineral phase. I show that these processes cannot localize deformation from a mantle convection scale (103 km) to a ductile shear zone scale (1 km). To achieve this, is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers. This process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non-linear rheology. Micas, melt, and fine-grained aggregates – unless dry rheologies are used – have the necessary characteristics. As none of these phases is expected to be present in the dry lithosphere of Venus, this concept can explain why Venus, unlike the Earth, does not display a global network of plate boundaries. The diffuse plate boundary in the Central Indian Ocean may be as yet non-localized because serpentinization has not reached the ductile levels of the lithosphere. 相似文献
4.
Using analogue model experiments this study investigates the effects of lithostatic pressure and temperature in controlling the pattern of shear localization around rigid inclusions. Compression experiments were conducted on polymethylmethaacrylate (PMMA) by varying confining pressure (P = 30–70 MPa), homologous temperature (To = 0.67–0.80 corresponding to room temperature – 80 °C) and axial strain (yield – 15%) in a triaxial setup. Mechanical data showed temperature has a greater influence on the bulk yield strength and steady-state flow of the analogue material than confining pressure. Increasing confining pressures multiply the shear bands in number, and their overall pattern becomes progressively more complex, leading to composite band structures. On the other hand, under ambient pressure increasing temperature results in a transition from incipient high-strain zones to shear bands with sharp boundaries. Further increase in temperature switches the mode of shear localization, sharp to diffuse type. We finally show the inclusion-induced shear localization as a two-stage process, and provide a micro-mechanical explanation for the P-T dependent shear band patterns, attributed to three mechanically distinct domains of the inclusion-matrix interface: compression, extensional and shear localization. 相似文献
5.
《Journal of Structural Geology》2001,23(6-7):1007-1013
The phenomenon of shear-heating is generally difficult to recognise from petrologic evidence alone. Establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia, there is a clear spatial association between shear zones and interpreted elevated temperatures. Eclogite facies shear zones that formed at ∼550 Ma record temperatures of ∼650–700°C. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (∼450°C in the 40Ar–39Ar and Rb–Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater than about 450°C at ∼550 Ma for any extended period. Thus, the shear zones record temperatures that are ∼200°C higher than the surrounding country rocks. Simple calculations show that the combination of relatively high shear stresses (∼100 MPa) and high strain rates (∼10−11 s−1) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones, while the country rock remained at relatively lower temperatures. 相似文献
6.
Schistose mylonitic rocks in the central part of the Alpine Fault (AF) at Tatare Stream, New Zealand are cut by pervasive extensional (C′) shear bands in a well-understood and young, natural ductile shear zone. The C′ shears cross-cut the pre-existing (Mesozoic—aged) foliation, displacing it ductilely synthetic to late Cenozoic motion on the AF. Using a transect approach, we evaluated changes in geometrical properties of the mm–cm-spaced C′ shear bands across a conspicuous finite strain gradient that intensifies towards the AF. Precise C′ attitudes, C′-foliation dihedral angles, and C′–S intersections were calculated from multiple sectional observations at both outcrop and thin-section scales. Based on these data the direction of ductile shearing in the Alpine mylonite zone during shear band activity is inferred to have trended >20° clockwise (down-dip) of the coeval Pacific-Australia plate motion, indicating some partitioning of oblique-slip motion to yield an excess of “dip-slip” relative to plate motion azimuth, or some up-dip ductile extrusion of the shear zone as a result of transpression, or both. Constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary (SZB). Across all examined parts of the shear zone, the mean dihedral angle between the C′ shears and the mylonitic foliation (S) remains a constant 30 ± 1° (1σ). The aggregated slip accommodated on the C′ shear bands contributed only a small bulk shear strain across the shear zone (γ = 0.6–0.8). Uniformity of per-shear slip on C′ shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain-hardening rheology, such that they locked up at a finite shear strain (inside C′ bands) of 12–15. Shear band boudins and foliation boudins both record extension parallel to the SZB, as do the occurrence of extensional shear band sets that have conjugate senses of slip. We infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with Wk < 0.8, and perhaps even as low as <0.5. The C′ shear bands near the AF formed in a thinning/stretching shear zone, which had monoclinic symmetry, where the direction of shear-zone stretching was parallel to the shearing direction. 相似文献
7.
The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite-facies Permian shear zone that developed after the emplacement of
a deep-seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially
distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature
range from ~800 to ~400°C. During the earlier high-grade deformation stage, which was accompanied by decompression from ~7
to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ≥1 km wide. Strain became increasingly localized
as temperature decreased, but rather than reactivating pre-existing shear zones as commonly expected, younger mylonites expanded
into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the
SLSZ suggests that pre-existing compositional and grain size heterogeneities in the starting material played a key role in
governing superposed generations of shear zones. 相似文献
8.
We conducted axial compression and general shear experiments, at T = 900 °C and P = 1.5 GPa, on samples of banded iron formation (BIF) and synthetic aggregates of quartz, hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization, and the role of weak second phases (oxides) on the rheology and microstructural evolution of the aggregates. Experiments showed strain localization into oxide rich layers, and that the oxide content and oxide distribution are key factors for the strength of the aggregate. Only 2–10 wt.% hematite leads to pronounced weakening and increasing hematite content above ∼10% has only a minor additional effect. Where oxide grains are dispersed, the initial strength contrast with quartz induces stress concentrations at their tips, promoting high stress recrystallization-accommodated dislocation creep of quartz. Fine recrystallized quartz reacts with oxide, forming trails of fine reaction product (ferrosilite/fayalite) leading to the interconnection/percolation of a weaker matrix. The strength contrast between the quartz framework and these fine-grained trails promotes strain localization into micro-shear zones, inducing drastic strain weakening. Thus dynamic recrystallization of quartz promotes syn-deformational reactions leading to a microstructurally-controlled evolution of phase strength contrast. It results in a rheologic transition from load-bearing framework to a matrix-controlled rheology, with transition from S–C′ to S–C fabric with increasing strain. 相似文献
9.
Interpretations of deformation processes within ductile shear zones are often based on the characterisation of microstructures preserved in exhumed rocks. However, exhumed microstructures provide only a snapshot of the closing stages of deformation and we need ways of understanding how microstructures change through time and at what rate this occurs. To address this problem, we study optical microstructures and electron backscatter diffraction (EBSD) data from samples of quartz layers deflected around garnet porphyroclasts (which generate local stress and strain rate perturbations) during mylonitic deformation in the Alpine Fault Zone of New Zealand.During shearing around rigid garnet porphyroclasts, quartz undergoes grain size reduction in response to locally increased stresses, while c-axes reveal increasing components of rhomb <a> and prism <a> slip, reflecting a local increase in shear strain and strain rate. TitaniQ thermobarometry and quartz microstructures suggest a rather narrow range of recorded quartz deformation temperatures around 450–500 °C, which we propose reflects the cessation of grain boundary migration driven deformation. Given that temperatures well above the brittle–ductile transition for quartz (∼350 °C) are preserved, we anticipate that rapid cooling and exhumation must have occurred from the 500 °C isotherm. Ultimately, we propose a modified geotherm for the central Alpine Fault Zone hanging wall, which raises the 500 °C isotherm to 11 km depth, near the brittle–ductile transition. Our updated Alpine Fault Zone geotherm implies a hotter and weaker middle to lower crust than previously proposed. 相似文献
10.
Alberto Ceccato Philippe Goncalves Giorgio Pennacchioni 《Journal of Metamorphic Geology》2020,38(8):881-903
Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid-saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid-saturated to fluid-undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load-bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid-undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid-mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid-undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn-kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids. 相似文献
11.
The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D1 is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D2 with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D3, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D3 shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D3 deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10−12 to 10−14 s−1. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D3 deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton. 相似文献
12.
The Ronda peridotites form the largest mass of subcontinental mantle outcropping on land. Unlike other orogenic lherzolite massifs, the two main bodies of Ronda (the Sierra Bermeja and Sierra Alpujata massifs) are unique cases where ductile shear zones linked to the hot thrusting of mantle over continental crustal rocks are well exposed. We present a new insight into the deformation localization in these shear zones based on structural, fabric and petrological data. The Ronda peridotites show increasing deformation towards the continental footwall rocks, from porphyroclastic rocks to ultramylonites. Garnet-pyroxenites from the basal shear zone of the Alpujata massif yield ca. 1100 °C and 1.4 GPa for the mylonitization. Such conditions promoted partial melting and the formation of felsic dynamothermal aureoles from the underlying crustal rocks. Subsequent deformation is mainly localized in the dynamothermal aureoles, since they are weaker than the peridotites. Both aureoles show marked strain gradients towards the contact but record different kinematics. In Sierra Alpujata, kinematic criteria indicate a top-to-the ENE shear sense, whereas in Sierra Bermeja the felsic mylonites provide a top-to-the NNW motion. A transpressional setting is proposed to explain such kinematic shift. 相似文献
13.
In the Bear Creek area of the Sierra Nevada batholith, California, the high temperature postmagmatic deformation structures of the Lake Edison granodiorite include steeply-dipping orthogneiss foliations, joints, and ductile shear zones that nucleated on joints and leucocratic dykes. Exploitation of segmented joints resulted in sharply bounded, thin shear zones and in large slip gradients near the shear zone tips causing the deformation of the host rock at contractional domains. The orthogneiss foliation intensifies towards the contact with the younger Mono Creek granite and locally defines the dextral Rosy Finch Shear Zone (RFSZ), a major kilometre-wide zone crosscutting the pluton contacts. Joints predominantly strike at N70–90°E over most of the Lake Edison pluton and are exploited as sinistral shear zones, both within and outside the RFSZ. In a narrow (∼250 m thick) zone at the contact with the younger Mono Creek granite, within the RFSZ, the Lake Edison granodiorite includes different sets of dextral and sinistral shear zones/joints (the latter corresponding to the set that dominates over the rest of the Lake Edison pluton). These shear zones/joints potentially fit with a composite Y–R–R′ shear fracture pattern associated with the RFSZ, or with a pattern consisting of Y–R-shear fractures and rotated T′ mode I extensional fractures. The mineral assemblage of shear zones, and the microstructure and texture of quartz mylonites indicate that ductile deformation occurred above 500 °C. Joints and ductile shearing alternated and developed coevally. The existing kinematic models do not fully capture the structural complexity of the area or the spatial distribution of the deformation and magmatic structures. Future models should account more completely for the character of ductile and brittle deformation as these plutons were emplaced and cooled. 相似文献
14.
传统构造地质学用摩尔-库伦准则和贝克尔的应变椭球体理念分别解释地壳中的脆性断层和塑性变形,将变形局部化的韧性剪切带形成解释为平行应变椭球体的圆切面,却无法解释变形局部化的共轭剪切带稳定夹角~110°面对应缩短方向。变形局部化是独立于脆性和塑性变形外的变形领域,受最大有效力矩准则控制。20世纪末提出的变位形分解理念,摆脱连续介质力学的束缚,合理地说明广泛存在的走滑断层平行俯冲带或逆冲断层带。非均匀变形和非连续介质力学理念的建立,为地质学与力学的结合开辟了新的前景。文章试用上述两理念概略分析中国和邻区中新生代构造格局,以期引发讨论。 相似文献
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16.
The fabric, mineralogy, geochemistry, and stable isotope systematics of auriferous shear zones in various hydrothermal gold deposits were studied in order to discuss the role of fluids in rock deformation at temperatures between 500 °C and 700 °C. The strong hydrothermal alteration and gold mineralization indicates that effective permeability development goes ahead with high-temperature rock deformation. The economic gold enrichment is often hosted by breccias and quartz veins in the ductile shear zones, which either formed at fast strain rates or by low strain continuous deformation at slow strain rates. Both processes require (1) a close-to lithostatic to supralithostatic fluid pressure and/or (2) a strong rheology contrast of the deformed lithologies that is often developed during progressive hydrothermal alteration. Compartments of high fluid pressure are sealed from the rest of the shear zones by high-temperature deformation mechanisms, e.g. intracrystalline plasticity and diffusion creep, and compaction. In contrast, in mylonites with heterogeneous crystal plastic and brittle deformation mechanisms for the various minerals, an interconnected network of a grain-scale porosity forms an effective fluid conduit, which hampers fluid pressure build-up and the formation of veins.The auriferous shear zones of the various gold mines represent fluid conduits in the deeper crust, 100 m along strike and up to 1000 m down-dip. The hydrothermal fluids infiltrated may be responsible for low magnitude earthquakes in the Earth's lower crust, which otherwise deforms viscously. 相似文献
17.
In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid–rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms. 相似文献
18.
The interaction between reaction and deformation: an experimental study using a biotite + plagioclase + quartz gneiss 总被引：3，自引：0，他引：3
Metamorphic reactions commonly accompany ductile deformation of crustal rocks. We performed an experimental study to determine: (i) the effect of syn-deformation reaction on strain weakening and localization, and (ii) the effect of crystal plastic deformation on reaction extent and distribution. Experiments were conducted on a fine-grained gneiss (58 vol.% quartz, forming the interconnected matrix, 13 vol.% biotite, 28 vol.% plagioclase and 1 vol.% garnet/Fe-Ti oxides). General shear experiments were performed at 745 and 800 °C, 1.5 GPa, two strain rates, and shear strain ( γ ) from 0.6 to 5, yielding three suites with initial phase strength contrast between the matrix quartz and weak biotite of 45×, 25× and 10×; hydrostatic experiments were performed on cores and powders at 750 and 800 °C and 1.5–2 GPa for the same times. At these conditions, biotite reacts with plagioclase and quartz to form garnet, K-feldspar and water (no melt was observed). Greater reaction extent was observed in deformed samples than in equivalent hydrostatic samples, because of the increased surface area and internal strain energy. In all of the deformed samples, reaction contributes to strain weakening, due principally to a switch to grain boundary sliding in the fine-grained reaction products. The degree to which syn-deformational reaction causes strain weakening and localization in this polyphase aggregate depends on the phase strength contrast and how it evolves. In samples with low-phase strength contrast, strain and reaction are homogeneously distributed; however, in samples with high-phase strength contrast, ductile strain and reaction interact positively to produce a narrow ductile shear zone. Similar concentration of reaction is observed in some natural ductile shear zones. 相似文献
19.
The Helvetic nappes in Switzerland consist of sediments, which have been sheared off and thrust over the crystalline basement of the European passive continental margin during Alpine orogeny. Their basal shear zones usually root above the external crystalline massifs. However, the mechanisms that initiated the shear zones and the associated nappe formation are still debated. We perform two-dimensional numerical simulations of the shearing of linear viscous fluids above a linear viscous fluid with considerably higher viscosity (quasi-undeformable). The boundary between the fluid, mimicking the sediments, and the quasi-undeformable fluid, mimicking the basement, exhibits geometrical perturbations, mimicking half-grabens. These geometrical perturbations can trigger significant strain localization and the formation of shear zones within the linear viscous fluid although no rheological softening mechanism is active. This kinematic, ductile strain localization is caused by the half-grabens and the viscosity ratio between basement and sediments. The viscosity ratio has a strong control on the kinematics of strain localization, whereas the depth of the half-grabens has a weak control. For sediment viscosities in the order of 1021 Pas and typical half-graben geometries of 5 km depth and 25 km width the localization generates (a) low-angle shear zones at the basement-sediment interface, but also entirely within the sediments, (b) horizontal transport >10 km associated with the shear zones, (c) shear zones with thickness in the order of 100 m, (d) an ordered stacking of model nappes and (e) shear zones that root above the basement. The results suggest that tectonic inheritance in the form of half-grabens and associated kinematic strain localization could have been the triggering mechanism for Helvetic nappe formation, and not rheological softening mechanisms, which might, however, have subsequently further intensified shear localization significantly. 相似文献
20.
Pressure–temperature–structural distance relationships within Greater Himalayan rocks in eastern Bhutan: implications for emplacement models 
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下载免费PDF全文 K. S. Agustsson S. M. Gordon S. P. Long G. G. E. Seward K. Zeiger M. Penfold 《Journal of Metamorphic Geology》2016,34(7):641-662
In the Himalayan orogen, Greater Himalayan (GH) rocks were buried to mid‐ to lower‐crustal levels and are now exposed across the strike of the orogen. Within the eastern Himalaya, in the Kingdom of Bhutan, the GH is divided into structurally lower (lower‐GH) and upper (upper‐GH) levels by the Kakhtang thrust (KT). Pressure–temperature estimates from lower‐ and upper‐GH rocks collected on two transects across the KT yield similar P–T–structural distance trends across each transect. In the eastern transect, temperatures are similar (from 730 to 650 °C) over a structural thickness of ~11 km, but peak pressures decrease from ~10 to 6 kbar with increasing structural level. In comparison, peak temperatures in the central Bhutan transect are similar (from 730 to 600 °C), but pressures decrease from 10 to 6.5 kbar with increasing structural level over a structural thickness of ~6 km. The structurally highest sample reveals slightly higher pressures of 8.0 kbar in comparison to pressures of ~6.5 kbar for samples collected from within the KT zone, ~4 km below. Within each transect, there are increases in pressure ± temperature within the overall upright P–T gradient that may demarcate intra‐GH shear zone(s). These P–T results combined with evidence that the timing of initial melt crystallization becomes older with increasing structural level suggest that the intra‐GH shear zones emplaced deeper GH rocks via progressive ductile underplating. These shear zones, including the KT, likely aided in the initial emplacement and construction of the GH as a composite tectonic unit during the Late Oligocene to Early Miocene, from c. 27 to 16 Ma. 相似文献