Ductile Rheology Research Articles

Can subduction initiation at a transform fault be spontaneous?

Abstract. We present an extensive parametric exploration of the feasibility of “spontaneous” subduction initiation, i.e., lithospheric gravitational collapse without any external forcing, at a transform fault (TF). We first seek candidates from recent subduction initiation events at an oceanic TF that could fulfill the criteria of spontaneous subduction and retain three natural cases: Izu–Bonin–Mariana, Yap, and Matthew and Hunter. We next perform an extensive exploration of conditions allowing for the spontaneous gravitational sinking of the older oceanic plate at a TF using 2-D thermomechanical simulations. Our parametric study aims at better delimiting the ranges of mechanical properties necessary to achieve the old plate sinking (OPS). The explored parameter set includes the following: crust and TF densities, brittle and ductile rheologies, and the width of the weakened region around the TF. We focus on characterizing the OPS conditions in terms of (1) the reasonable vs. unrealistic values of the mechanical parameters and (2) a comparison to modern cases of subduction initiation in a TF setting. When modeled, OPS initiates following one of two distinct modes, depending mainly on the thickness of the overlying younger plate. The asthenosphere may rise up to the surface above the sinking old plate, provided that the younger plate remains motionless (verified for ages ≥5 Myr, mode 1). For lower younger plate ages (typically ≤2 Myr), the younger plate is dragged toward the older plate, resulting in a double-sided subduction (mode 2). When triggered, spontaneous OPS is extremely fast. The parameters that exert the strongest control over whether OPS can occur or not are the brittle properties of the shallow part of the lithosphere, which affect the plate resistance to bending, the distance away from the TF over which weakening is expected, and the crust density. We find that at least one mechanical parameter has to be assigned an unrealistic value and at least two other ones must be set to extreme ranges to achieve OPS, which we do not consider realistic. Furthermore, we point out inconsistencies between the processes and consequences of lithospheric instability, as modeled in our experiments and geological observations of subduction infancy, for the three natural candidates of subduction initiation by spontaneous OPS. We conclude that spontaneous instability of the thick older plate at a TF evolving into mature subduction is an unlikely process of subduction initiation in modern Earth conditions.

Characterization of Oligo-Miocene evaporite-rich minibasins in the Sivas Basin, Turkey

Abstract The Sivas Basin in Turkey displays in its central part an Oligo-Miocene halokinetic province which acts as a major outcrop analogue to study salt-sediment interactions. Based on field geology observations, the present paper focuses on the geometry and sedimentology of several minibasins having the particularity of being mainly filled by gypsiferous deposits. Such type of evaporite-rich minibasins remain difficult to identify and are poorly studied in other halokinetic provinces. In the Sivas Basin, the evaporites were recycled from diapiric salts and precipitated in saline ponds emplaced above deflating diapiric stems. Diapir deflation resulted either from local transtensive strain, cessation of diapir feeding and/or subsurface dissolution of the diapiric salt. Minibasin subsidence was likely enhanced by the fast emplacement rate of the capping evaporites, together with the high density of the depositional sulfates compared to the diapiric halite. The evaporite-rich minibasins stand out from their surrounding siliciclastic counter-parts by their small dimension (lower than 1 km-wide), their encased teardrop shape, and their high internal deformations. The later include well-developed halokinetic sedimentary wedges, aerial mega-slumps or inverted flaps. Such structural features probably resulted from the ductile rheology of the evaporite infill and the complex pattern of downbuilding. Although secondary evaporitic minibasins have never been identified in other ancient halokinetic settings, our study highlights that they could developed in any evaporitic environments, coastal or continental, such as in the Precaspian Basin. The secondary minibasins described here can also act as field analogues of other primary evaporite-rich minibasins already suspected in salt giant basins (e.g. in the Santos Basin, Brazil).

Spectrum Research on Rheology of High Rank Coal

Aiming to discuss the change characteristic of macromolecular structures of high rank coal in different rheological conditions, the high rank undeformed coal from southern Qinshui basin and the coal after variable temperature and variable pressure rheology experiments were investigated and analyzed in detail through Fourier transform infrared spectroscopy (FTIR) and laser Raman spectra analysis. The result shows that the texture and composition of different types of rheological coals under different temperature and pressure exhibit significant differences. Experiments of variable temperature and pressure of high rank coal (temperature: 300-400 °C, confining pressure: 50-100 MPa, strain: less than 10% and strain rate: 10(-4)-10(-7) · s(-1) will distort their macromolecular structures and recombine the chemistry structures. When the temperature is 300 °C or 350 °C, the high rank coal generates brittle or brittle-ductile rheology easily, mechanical energy transforms to heat energy, some branches and functional groups with weaker bond energy break and fall off, which split as dissociative micromolecule, with stress degradation as principal role, and stress polycondensation occurs with aromatic, texture increasing. When the temperature is up to 400 °C, ductile rheology of the high rank coal occurs with the secondary defects increasing, mechanical energy transforms to strain energy which helps the early shedding small molecules be embedded or adsorbed in the defect or on the surface of macromolecular preferentially and change the aliphatic and aromatic structures. It is affected by stress degradation and polycondensation progress, and the latter is dominated. The confining pressure and water injection of coal do not have much effect on the macromolecular structure obviously.

Alpine lithosphere slab rollback causing lower crustal seismicity in northern foreland

Beneath the northern foreland of the Central Alps deep crustal earthquakes up to magnitude 4 regularly occur in the continental lithosphere. At 20 km to 30 km depths, where most of these earthquakes are located, temperatures above 450 °C are expected. This leads to a more ductile rheology of the lower continental crust. To better understand occurrence and underlying processes of these unusual earthquakes, we homogenize and improve hypocenter locations of events in the period 1984 to 2012 using a high-precision multi-phase earthquake location method in combination with a reliable three-dimensional P-wave velocity model of the crust and uppermost mantle. With the new approach, the average uncertainty in focal depth of well-locatable earthquakes is less than ±1 km. The homogeneously relocated hypocenters suggest a relatively uniform depth distribution throughout the lower crust. In agreement with previous studies, seismicity is entirely restricted to the crust and no evidence for seismicity in the mantle lithosphere beneath the northern Central Alpine foreland was found. The geographical distribution of lower crustal earthquakes in the foreland correlates remarkably well with the lateral extent of the European slab beneath the Central Alps where it is still attached to the European lithosphere. In addition, the directions of extensional axes derived from focal mechanisms of the deep crustal earthquakes are predominantly parallel to the Alpine front. This consistency of extensional axes can be interpreted as the result of the transferred buoyancy force of the lower crust in the subduction, transformed to a compressional force in the foreland perpendicular to the Alpine front. Existing 2-D thermo-mechanical models predict such viscous bending and stress transfer to the foreland. In our proposed model, the anomalously deep crustal seismicity is driven by stresses transferred into the foreland interrelated with the exhumation of the crust form the orogenic root caused by the rollback of the European lithosphere.

Fabric development as the key for forming ductile shear zones and enabling plate tectonics

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.

Evaluating Seal Quality for Potential Storage Sites in the Norwegian North Sea

Caprock quality is crucial when wanting to evaluate the storage potential of a reservoir. The objective of this study is to evaluate the quality of the primary and secondary seal in case of CO2 storage in sandstones form the Mid to Late Jurassic in the Norwegian North Sea. These seals are composed respectively of Lower Cretaceous and Upper Jurassic shales for the primary, and from the three Cenozoic groups (Nordland, Hordaland and Rogaland) for the secondary.The primary seal is in general of sufficient thickness according to the recommendations from the Norwegian Petroleum Directorate (over 50 meters), except in a thin North-South corridor to the West of the platform, in the inner part of the grabens. This seal is brittle in the entire study area due to a deep burial depth. The only area where it could be ductile is to the North of the Utsira High, which is due to a warmer geothermal gradient but also possibly the movement of a basement fault beneath. Since its rheology is brittle, the primary seal could be breached during fault movements. To ensure its efficiency, it is thus important to evaluate the properties of the faults from this area that have a direction less than 30° away from the regional maximum horizontal stress (ENE-WSW)The secondary seal is much shallower and therefore has a ductile rheology which makes it less affected by fault movements. This is due to the nature of ductile deformation which creates less conductive faults that have self- healing properties. The thickness is also at least 500 meters close to the shore, but most commonly between 1000 and 2000 meters. These properties make it a high quality secondary seal, as has been demonstrated by the successful Sleipner storage site in the Utsira sands that are sealed by Nordland shales.

Sand intraclasts as evidence of subglacial deformation of Middle Pleistocene permafrost, North Norfolk, UK

Sand intraclasts are common within the Bacton Green Till Member, a glacitectonic mélange subjected to polyphase deformation during the Middle Pleistocene in North Norfolk, UK. The intraclasts range from a few tens of centimetres to >10 m in length and have sharp contacts with the surrounding till. Sand within the intraclasts is unconsolidated and contains well-preserved primary stratification. The wrapping of glacitectonic foliation around the intraclasts and the development of folds relating to mechanical instabilities indicate that the intraclasts acted as competent masses within a more easily deformable fine-grained till that accommodated the majority of the strain. Sharp contacts and distinctive heavy-mineral assemblages indicate little intermixing between the sand and till. Five hypotheses about the entrainment and evolution of the intraclasts are tested against sedimentological, structural and mineralogical observations. The most reasonable hypothesis attributes the intraclasts to glacitectonic deformation of “warm” permafrost. Initial ice advance caused large-scale thrusting of proglacial permafrost that led to the stacking of pre-glacial and ice-marginal sediments that were subsequently deformed sub-marginally to generate the intraclasts. Preservation of primary stratification within the intraclasts is attributed to deformation at temperatures slightly below the pressure-melting point, when pore ice cemented the intraclasts as rigid bodies. At the same time deformation was concentrated into the surrounding finer-grained till because of its significant liquid water content and ductile rheology. It is concluded that the intraclasts provide a criterion to identify past glacier–permafrost interactions and a potential means of differentiating between subglacial deformation under unfrozen and partially-frozen conditions.

Influence of tectonic overpressure on P–T paths of HP–UHP rocks in continental collision zones: thermomechanical modelling

AbstractThe principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P–T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P–T paths of HP–UHP rocks is located in the bottom corner of the wedge‐like confined channel with the characteristic magnitude of pressure deviation on the order of ∼0.3 GPa and 10–20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP‐rocks subjected to significant (≥0.3 GPa) overpressure at intermediate subduction depth (50–70 km, P = 1.5–2.5 GPa) then underpressured at depth ≥100 km (P ≥ 3 GPa); (ii) HP‐rocks subjected to ≥0.3 GPa overpressure at peak P–T conditions reached at 50–70 km depth in the bottom corner of the wedge‐like confined subduction channel (P = 1.5–2.5 GPa); (iii) lower‐pressure rocks formed at shallower depths (≤40 km depth, P ≤ 1 GPa), which are not subjected to significant overpressure and/or underpressure.

Mechanics of the transition from localized to distributed fracturing in layered brittle–ductile systems

The mechanical coupling between brittle and ductile layers in the continental lithosphere produces rheological contrasts, which are supposed to trigger localized or distributed mode of faulting. A plane-strain 2D finite-element model is used to highlight the mechanical role of the brittle–ductile coupling in defining the patterns of fracturing. The coupling is performed through the shortening of a Von Mises elasto-viscoplastic layer rimmed by two ductile layers behaving as Newtonian incompressible fluids. By varying the viscosity of the ductile layers or the amount of softening in the brittle layer, the fracturing mode evolves from localized to distributed. The mechanics of brittle–ductile coupling is explained by the limitation of the fault displacement rate imposed by both brittle and ductile rheologies. On these bases, an analytical approach is presented in order to estimate the maximum velocity along each fault permitted by both brittle and ductile media. This velocity is then compared to the velocity required by the boundary shortening rate. If the velocity in the fault is not large enough, the development of new faults is necessary. From this analysis, we define four fracturing modes in a brittle–ductile media: the localized mode with the onset of a few large faults, the distributed mode with very dense fault patterns, and finally, the ductile-control mode and the brittle-control mode, where the number of faults increases with an increase in the ductile viscosity and a decrease in the brittle softening respectively.

Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia

Piercement structures such as hydrothermal vent complexes, pockmarks, and mud volcanoes, are found in various geological settings but are often associated with faults or other fluid-focussing features. This article aims to investigate and understand the mechanisms responsible for the formation of piercement structures in sedimentary basins and the role of strike-slip faulting as a triggering mechanism for fluidization. For this purpose four different approaches were combined: fieldwork, analogue experiments, and mathematical modeling for brittle and ductile rheologies. The results of this study may be applied to several geological settings, including the newly formed Lusi mud volcano in Indonesia (Mazzini et al., 2007). Lusi became active the 29th of May 2006 on the Java Island. Debates on the trigger of the eruption rose immediately. Was Lusi triggered by the reactivation of a fault after a strong earthquake that occurred two days earlier? Or did a neighbouring exploration borehole induce a massive blow-out? Field observations reveal that the Watukosek fault crossing the Lusi mud volcano was reactivated after the 27th of May 2006 earthquake. Ongoing monitoring shows that the frequent seismicity periodically reactivates this fault with synchronous peaks of flow rates from the crater. Our integrated study demonstrates that the critical fluid pressure required to induce sediment deformation and fluidization is dramatically reduced when strike-slip faulting is active. The proposed shear-induced fluidization mechanism explains why piercement structures such as mud volcanoes are often located along fault zones. Our results support a scenario where the strike-slip movement of the Watukosek fault triggered the Lusi eruption and synchronous seep activity witnessed at other mud volcanoes along the same fault. The possibility that the drilling contributed to trigger the eruption cannot be excluded. However, so far, no univocal data support the drilling hypothesis, and a blow-out scenario can neither explain the dramatic changes that affected the plumbing system of numerous seep systems on Java after the 27-05-2006 earthquake. To date (i.e. April 2008) Lusi is still active.

Gravity influenced brittle‐ductile deformation and growth faulting in the lithosphere during collision: Results from laboratory experiments

Because of the competition between brittle and ductile rheologies and their interplay with tectonic and buoyancy forces, lithospheric deformation results in very contrasting styles. In continental collision, especially with unconfined boundaries, deformation can be either homogeneously distributed or localized on complex fault patterns, and different deformation modes such as contraction, extension, and strike‐slip interfere. Using scaled lithospheric analog experiments made of dry sand, silicone putties, and dense honey, we investigate the mechanical parameters that control the deformation style in colliding systems, with a particular focus on the roles of buoyancy and brittle‐ductile coupling. The analysis of tens of experiments shows that the principal deformation features depend on two main parameters: a brittle‐to‐ductile strength ratio Γ, which controls deformation localization at the largest scale, and a buoyancy‐to‐strength ratio Ar, which fixes the relative amount of contractional, extensional, and strike‐slip structures. Strain localization occurs only for Γ larger than a critical value (∼0.5), and the range of Γ values, over which the transition from nonlocalized to localized deformation occurs, is small. The three main deformation regimes (contraction, strike‐slip, and extension), which coexist in most of the collision experiments, occur in relative proportions that depend mainly on Ar and on the nature of the boundary conditions.

Continental plate collision, P– T– t– z conditions and unstable vs. stable plate dynamics: Insights from thermo-mechanical modelling

There was much debate recently on the mechanisms of continental convergence and related pressure–temperature ( P– T) conditions, both in modeling and petrologic community. Depending on the mechanisms of convergence (subduction, collision, folding or RT instability) one can argue about the possibility of large-scale deviations of pressure and temperature in the accretion prism and below it, from “reference” (i.e. lithostatic) conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks. These deviations can be caused, for example, by tectonic overpressure that, as suggested in some studies, may be responsible for formation of UHP (Ultra High Pressure) rocks. However, overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling (pure shear, folding). In this study, we analyze conditions that define various mechanisms of convergence, and consequently, of exhumation. These mechanisms can be represented as a super-position of (1) simple shear (subduction), (2) pure shear (collision), (3) folding and (4) Rayleigh–Taylor instability. We study these scenarios using a thermo-mechanical model that accounts for brittle–elastic–ductile rheology, surface processes, and metamorphic changes. It appears that stable, “oceanic-type” subduction may occur in the case of cold lithospheres ( T Moho < 550 °C) and relatively high convergence rates (> 3–5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the whole (upper and lower) crust or only the lower crust can be involved in subduction. In case of weak metamorphic rheologies, phase changes improve chances for stable subduction. Pure shear becomes a dominant mechanism when T Moho > 550 °C or convergence rates are lower than 3 cm/yr. Large-scale folding is favored in case of T Moho = 500–650 °C and is more effective in the case of mechanical coupling between crust and mantle (e.g., strong lower crust). Gravitational (Rayleigh–Taylor) instabilities overcome other mechanisms for very high values of T Moho (> 800 °C) and may lead to development of subvertical “cold spots.” However, it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate, which is rarely slower than 5 cm/yr. This rate is sufficient to drive continental subduction during the first several Myr of collision. In this case, the subduction channel is characterized by nearly lithostatic pressure conditions. Large-scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks. Overpressure may be built inside the channel in the short moment of its closure. We suggest that most continental orogenic belts could have started their formation from continental subduction. This evokes small tectonic overpressures and thus deep origin of the UHP rocks that may be brought to the surface via the suggested multi-level mechanism of exhumation.

PN Wave Velocity Structure and Anisotropy in the Central Tien Shan Region

AbstractPn wave velocity structure and anisotropy in the central Tien Shan are inversed by using seismic arrival data from the broadband temporary network (GHENGIS) and the Kyrgyzstan seismic network (KNET). Results reveal lower average Pn velocities and a strong heterogeneity at the uppermost mantle beneath the central Tien Shan, which are often found in tectonically active zones. Particularly, very low Pn velocities are found in the south‐central Tien Shan, which is believed to be underlain by an unusual mantle with higher heat flows. There is a clear correlation between seismic activity and Pn velocity variation. Most earthquakes are concentrated in the north‐central Tien Shan with hight Pn velocities, while almost no earthquakes located in the south‐central Tien Shan with very low Pn velocities. Our result suggests that the very low Pn velocity beneath the southcentral Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has been conducted well up into the crust, inducing a ductile rheology that prevents significant seismic activity. Moreover, strong anisotropy is observed in the north‐ and south‐central Tien Shan. Fast Pn velocity anisotropy in the south‐central Tien Shan is nearly in N‐S directions, which is consistent with the SKS wave anisotropy concerning mantle flow. However, fast Pn velocity directions in the north‐central Tien Shan show a southward rotation variation. We estimate that it is probably related to the inserting of the crust of the Chu basin from the southern Kazakstan platform to the central Tien Shan, which caused a change of stress fields and lithosphere deformation.

Uppermost mantle structure and its relation with seismic activity in the central Tien Shan

Using arrival data from GHENGIS and Kyrgyz Network (KNET) stations, we determined the uppermost mantle structure of the central Tien Shan from Pn tomography. Pn velocities in the north‐central Tien Shan are slower than normal for stable lithosphere. They are very slow in the south‐central Tien Shan, where a hot mantle previously has been postulated. The Pn velocity variation correlates well with seismicity. Most earthquakes are concentrated in the north‐central Tien Shan, but almost no earthquakes in the south‐central Tien Shan, even though both geologic and GPS observations show active convergence there. This correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle. Specifically, we propose that the very slow Pn velocity beneath the south‐central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has conducted well up into the crust, inducing a ductile rheology that prevents significant seismicity.

A genetic classification of sinkholes illustrated from evaporite paleokarst exposures in Spain

This contribution analyses the processes involved in the generation of sinkholes from the study of paleokarst features exposed in four Spanish Tertiary basins. Bedrock strata are subhorizontal evaporites, and in three of the basins they include halite and glauberite in the subsurface. Our studies suggest that formation of dolines in these areas results from a wider range of subsidence processes than those included in the most recently published sinkhole classifications; a new genetic classification of sinkholes applicable to both carbonate and evaporite karst areas is thus proposed. With the exception of solution dolines, it defines the main sinkhole types by use of two terms that refer to the material affected by downward gravitational movements (cover, bedrock or caprock) and the main type of process involved (collapse, suffosion or sagging). Sinkholes that result from the combination of several subsidence processes and affect more than one type of material are described by combinations of the different terms with the dominant material or process followed by the secondary one (e.g. bedrock sagging and collapse sinkhole). The mechanism of collapse includes any brittle gravitational deformation of cover and bedrock material, such as upward stoping of cavities by roof failure, development of well-defined failure planes and rock brecciation. Suffosion is the downward migration of cover deposits through dissolutional conduits accompanied with ductile settling. Sagging is the ductile flexure of sediments caused by differential corrosional lowering of the rockhead or interstratal karstification of the soluble bedrock. The paleokarsts we analysed suggest that the sagging mechanism (not included in previous genetic classifications) plays an important role in the generation of sinkholes in evaporites. Moreover, collapse processes are more significant in extent and rate in areas underlain by evaporites than in carbonate karst, primarily due to the greater solubility of the evaporites and the lower mechanical strength and ductile rheology of gypsum and salt rocks.

Structural softening of the lithosphere

AbstractStructural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust‐mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10‐fold.

Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark of trench‐parallel terrane migration?

We studied the back azimuth dependence of Ps converted phases at GSN station COR (Corvallis, Oregon) using broadband P receiver functions computed from 602 teleseismic earthquakes. The amplitudes and polarities of the transverse Ps phases are largely two‐lobed, which indicates anisotropy with a tilted symmetry axis. A double‐peaked Ps conversion at 4.5–6.5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the Ps converted phase by the dipping interface. A polarity flip on the transverse receiver function near north‐south back azimuth indicates an anisotropic symmetry axis aligned north‐south, far from the convergence direction (N68°E). The Ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed (VP ∼ 6.0–6.5 km/s) near 40 km depth, at the slab interface with the overriding North American plate. One‐dimensional modeling suggests 10% anisotropy in the supraslab layer, with a slow symmetry axis oriented N5°W at a 60° tilt from the vertical. Adjustments for the effect of slab interface dip on Ps amplitude suggest a somewhat lower 7% anisotropy. We infer a thin (∼7 km) anisotropic detachment zone for the northward slippage of the Siletz forearc terrane along the top of the descending slab, a motion consistent with GPS measurements and models of regional lithospheric dynamics. Serpentinite is a likely constituent for the deep anisotropic layer, owing to its reduced VP and high Poisson ratio. The ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone.

Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan‐Tibetan orogen

Plane strain, thermal‐mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self‐heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large‐scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high‐relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 1019 Pa s. This reduction occurs from 700°C to 750°C in the models and is attributed to a small percentage of in situ partial melt (“melt weakening”). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan‐Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.

Microcosmic analysis of ductile shearing zones of coal seams of brittle deformation domain in superficial lithosphere

The ductile shearing zones of coal seams in a brittle deformation domain in superficial lithosphere are put forward based on the study on bedding shearing and ductile rheology of coal seams. The macrocosmic and microcosmic characteristics include wrinkle fold, mymonitized zones and ductile planar structure of coal seams, etc., while the microcosmic characteristics may also include different optic-axis fabrics and the anisotropy of vitrinite reflectance as well as the change of chemical structure and organic geochemistry components. The forming mechanism is analyzed and the strain environment of ductile shearing zones of coal seams discussed. The result indicates that, in the superficial brittle deformation domain, the coal seams are easy to be deformed, resulting in not only brittle deformation but also ductile shearing deformation under the action of force. Because of simple shearing stress, the interlayer gliding or ductile rheology may take place between coal seams and wall rocks. Therefore, many ductile shearing zones come into being in superficial lithosphere (<5 km). The research on ductile shearing zone of brittle deformation domain in superficial lithosphere is significant not only theoretically for the study of ductile shearing and ductile rheology of the lithosphere but also practically for the structural movement of coal seams, the formation and accumulation of coal-bed methane, and the prevention and harness of gas burst in coal mine.

How soft is the crust?

In a series of thermomechanical models of the lithosphere, applied to the determination of apparent elastic thickness, extension, and collision and exhumation, Burov and coworkers (cf. e.g. Burov and Diament, 1995; Cloetingh and Burov, 1996; Burov and Poliakov, 2001; Burov et al., 2001) model the ductile behaviour of the upper crust (sometimes of the whole crust) assuming a quartz-controlled rheology. An extensive comparison of rheological input parameters used in geodynamic modelling (Fernandez and Ranalli, 1997) has shown that the dry quartz rheology employed by Burov and coworkers results in a crust (or upper crust) which is about three orders of magnitude softer than in any other model claiming to use the same dry quartz rheology (in one case— Cloetingh and Burov, 1996—a different set of parameters was included for the purpose of comparison, but not used in the calculations). The same conclusion is reached by comparing Burov and coworkers’ rheology with the review of lithosphere strength estimates by Kohlstedt et al. (1995). Although experimentally determined rheological parameters show wide scatter, this usually results in uncertainties of the order of one order of magnitude or less in estimates of the ductile rheology of the crust of a given composition (see Ranalli, 1995, 1997). Any larger difference may conceal some problems with the input parameters. This would be particularly important in this case, as the models by Burov and coworkers are at the forefront in the present attempt to understand geodynamic processes, and their results are widely used to interpret tectonic features. The equation for the strength of a material deforming in power-law creep is