Crustal Rheology Research Articles

A continuum mechanics model for normal faulting using a strain-rate softening rheology: implications for thermal and rheological controls on continental and oceanic rifting

We use a strain-rate-dependent visco-plastic rheology in a finite element model to investigate the effects of crustal rheology and thermal structure on the development of normal faults in 2-D extending lithosphere. Strain-rate softening in the brittle regime is used to simulate the rate-dependence of frictional strength observed in laboratory studies. Results of numerical experiments show that efficient strain-rate softening can result in localized zones of high strain rate, which develop in response to the rheology and boundary conditions and are not imposed a priori. We argue that these zones of localized shearing are analogous to faults and use the calculated deformation field to predict the preferred location of fault formation for a range of thermal and rheologic conditions. When no regional temperature gradient is imposed, deformation is predicted to be distributed between several sets of conjugate normal faults. However, in the presence of a horizontally varying temperature field, faulting is calculated to focus where the lithosphere is thinnest, forming rift-like topography. Model results predict that when crustal thickness is sufficiently small that no ductile layer is present in the lower crust, deformation is mantle-dominated and rift half-width is controlled primarily by the vertical geothermal gradient. In contrast, when crustal thickness is large enough that the stress accumulation in the upper crust becomes much greater than the stress accumulation in the upper mantle, deformation becomes crust-dominated and the calculated rift half-width is a function of both crustal rheology and the vertical geothermal gradient. This implies that deformation in the brittle upper crust does not behave independently of a strong upper mantle, even in situations where ductile flow occurs in the lower crust.

Crustal properties of Mercury by morphometric analysis of multi‐ring basins on the Moon and Mars

Abstract— We use satellite altitude free‐air and terrain gravity correlations to differentiate regional variations in crustal viscosity and transient cavity diameters of impact basins on the Moon and Mars that we combine with surface roughness for a rheological assessment of the crust of Mercury. For the Moon and Mars, we separate the free‐air anomalies into terrain‐correlated and terrain‐decorrelated components using the spectral properties of the free‐air and computed terrain gravity effects. Adjusting the terrain effects for the terrain‐correlated anomalies yields compensated terrain effects that we evaluate for crustal thickness variations of the impact basins. We interpret the terrain‐correlated anomalies for uncompensated elements of the crustal thickness variations that we find are strongly correlated with the distribution of basin rings from photogeologic analyses. Hence, we estimate the transient cavity diameter from the innermost diameter of the gravity‐inferred rings. Comparing these diameters with the related crustal thickness estimates clearly differentiates regional variations in the crustal rheologies. For the Moon, the analysis points to a farside crust that was significantly more rigid than the nearside crust during bombardment time. For Mars, the growth in transient cavity diameters with apparent crustal age also reflects increased viscosity due to crustal cooling. These results are also consistent with local estimates of surface roughness developed from the root‐mean‐squared topography over 64 times 64° patches centered on the basins. Hence for Mercury where gravity observations are lacking, rheological inferences on its crust may result from comparing photometric estimates of transient cavity diameter and local surface roughness with the lunar and martian estimates. These results for the Beethoven Basin, a typical multi‐ring impact feature of Mercury, suggest that the viscosity of the mercurian crust was relatively great during bombardment time. This enhanced rigidity, despite crustal temperatures that were probably much hotter than those of the Moon and Mars, may reflect an extremely dry crust for Mercury in its early development.

Study on Crustal Movement and Deep Process in Eastern Qinghai‐Xizang Plateau

AbstractSince the material eastward movement of Qinghai1‐Xizang Plateau during the tectonic history of plateau over 50Ma is recorded in the eastern part of the plateau, it draws extensive attention from the world. Modern geodetic study, combined with geological survey, exhibits the crustal movement pattern of the region and provides important boundary constraint conditions for the numerical simulation on geodynamics. The obvious discrepancy between the mantle convection generated stress field in the plateau and its adjacent areas derived from gravity anomalies and the surface crustal movement pattern implies that the crust and the mantle are decoupled. By assuming a depth‐dependant crustal rheology, the numerical simulation exhibits a great difference of crustal thickening in the central and eastern part of the plateau. The crustal thickening in the eastern plateau is mainly contributed by the lower crustal one. The discrepancy between the deformation of mantle and crust also shows the decoupling of them. The study suggests that the low strength of the lower crust may be the important factor which results in the decoupling between the crust and mantle. And, the knowledge concerning the physics and mechanical properties of crust and upper mantle is crucial for the study on the material eastward flow of the plateau.

Archean granite‐greenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion

The structural study of the Kolar greenstone belt and surrounding granite‐gneiss terrains combined with U‐Pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern Dharwar craton developed during a major magmatic accretion event between 2550 and 2530 Ma. The granite‐greenstone pattern resulted from the interference of gravity‐driven sagging of the greenstones (i.e., diapirism), E‐W bulk inhomogeneous shortening combined with horizontal N‐S stretching, and syntectonic juvenile pluton emplacement. Bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive, N‐S trending vertical foliation, shallow stretching lineation, and conjugate strike‐slip shear zone pattern within and outside the greenstone belt, resulting in regional horizontal pure shear deformation. The plutons around the greenstone belt record regional contraction by developing one set of strike‐slip C‐S fabrics of the shear zone pattern. The development of the granite‐greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion, melting, and granulite metamorphism in the lower crust. The Kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain, due to mechanical homogeneity and low viscosity provided by large‐scale melting during the accretion event. This example further suggests specific boundary conditions to the craton that allowed E‐W inhomogeneous shortening to be accommodated by N‐S stretching and spreading of the crust without significant tectonic thickening. Such tectonoplutonic pattern is specific to the Archean and may develop as a consequence of mantle plume activity in intracontinental settings.

Deformation and recrystallization of plagioclase along a temperature gradient: an example from the Bergell tonalite

Syn- to post-mylonitic tilting of the Bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies. At the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains. In contrast, at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration. The temperature increase is inferred to induce this transition in two ways. First, at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower. Hence, the chemical driving potential is small compared with recrystallization at lower temperatures. Second, higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility. The temperature increase is also associated with a transition in the dominant deformation mechanism. Intracrystalline plasticity dominates deformation at lower temperatures, whereas at higher temperatures deformation occurs primarily by diffusion-accommodated grain boundary sliding, as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase–biotite matrix with increasing deformation. Therefore, the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase. Lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding.

About this title

The motion and deformation of rocks are processes of fundamental importance in shaping the Earth, from outer crustal layers to the deep mantle. Reconstructions of the evolution of the Earth therefore require detailed knowledge of the geometry of deformation structures and their relative timing, of the motions leading to deformation structures and of the mechanisms governing these motions. This volume contains a collection of 22 papers on field, experimental and theoretical studies that add to our knowledge of these processes. They are a mixture of review papers oh selected topics in the field of structural geology and tectonics and papers on current issues and new techniques and are grouped into four themes: • The effect of fluids on deformation • The interpretation of microstructures and textures • Deformation mechanisms and rheology of crust and upper mantle minerals • Crust and lithosphere tectonics The volume will appeal to researchers in the fields of structural geology and tectonophysics, both in academia and industry.

Current issues and new developments in deformation mechanisms, rheology and tectonics

Abstract We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on deformation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary misorientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the rheology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localization of deformation.

Seismic activity and neotectonic evolution of theWestern Carpathians (Slovakia)

Seismic activity in the Western Carpathians is closely related to the crustal rheology and combined struc- tural pattern within the brittle upper crust. The structural pattern is a combination of three structural levels. The deep- est level is represented by the Paleo-Alpine suture zones dissected by neo-Alpine fault structures, above all strike- slip fault zones (second level). The recent tectonic regimes are controlled by the contemporary stress field and vertical movements (third level). Following the analyses of structures and various geophysical data, the principal seismo-tectonic zones of the Western Carpathians are defined. The most remarkable and important first-order tectonic structure in the Western Carpathians is the zone of the Pieniny Klippen Belt, which represents the contact of the Western Carpathian internides and the stable European Plat- form. The Mur-Mfault zone in the area of the Vienna Basin represents the contact of the Eastern Alps with the block of Western Carpathian internides. Both these tectonic lines represent subvertical boundaries with Tertiary tectonic activity. The ˇ Certovica fault zone is a surface projection of the thrust plane of the Veporic thick-skinned sheet over the Tatric unit. Based on geological and geophysical data it is as- sumed that the ˇ Certovica zone is recently active due to its extensional reactivation. Earthquake events are released here mostly along the Hron fault system of the ENE-WSW direc- tion.

Volume change during partial melting reactions: implications for melt extraction, melt geochemistry and crustal rheology

The volume change associated with dehydration melting has been investigated experimentally in muscovite and biotite-bearing assemblages because it is a possible driving force for melt segregation during orogenesis. Experiments have been performed on cores of a muscovite+biotite-bearing pelite and on a biotite+plagioclase+quartz gneiss. The muscovite+biotite-bearing pelite produced a similar set of melt-filled cracks to that observed in muscovite-bearing quartzite under partial melting conditions of 700 MPa, and 850 and 900 °C. However, no cracking was observed in the biotite gneiss under a range of temperature conditions (700 MPa, 800–900 °C). The textures of the partially melted rock samples suggest the volume change and associated dilational strain accompanying melting in assemblages with only biotite is insignificant or negative. This is confirmed by calculations of the dilational strain in the biotite gneiss experiments and other experiments in the literature. For example, the dilational strain associated with partial melting of biotite-bearing metagreywacke assemblages ranges from +1.90% to −12.24% (given 30% modal biotite, calculated on a 1-oxygen basis), becoming negative when garnet is produced at higher pressure. In contrast, the dilational strain associated with melt-induced cracking in a muscovite-bearing metapelite is higher, +6.76%, for the same modal abundance. These results suggest that volume change alone is not an important driving force for melt segregation in biotite-only-bearing assemblages, and external deformation at higher melt fractions may be required to segregate melt from the lower crust during partial melting. Reaction-controlled segregation is possible in muscovite-bearing rocks and melt will be more easily expelled in the upper to mid levels of the crust because of rapid pore pressure development during early stages of melting. Major element chemistry of melt in the two-mica assemblage is dominated by muscovite melting, even when assemblage contains reacting biotite. Some implications of these results are that: (1) the melt that escapes at low melt volumes from the mid-crust is likely to have a muscovite-melting chemical signature; and (2) in the lower portions of the crust where melting is controlled by biotite stability, melt may become trapped within and along grains and remain distributed, pervasively, at the grain scale until greater melt fractions are generated. Recent modeling of orogenic belts shows that the evolution of collisional belts likely involves the prolonged presence of a weak crustal layer. Melt trapped along grain boundaries from low dilational strain melting reactions may be a mechanism for keeping melt in the crust and weakening it during active orogenesis.

Grain-size-sensitive flow and shear-stress enhancement at the brittle–ductile transition of the continental crust

Localized shear zones along low-angle normal faults have been identified in regions of extension at the brittle–ductile transition of the continental crust. The possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust. This enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization. The lower crust rheology used to examine this stress increase is strain-rate, temperature and grain-size dependent, due to the coupling of dislocation and diffusion creep. The model structure proposed consists of a top layer, the upper crust, gliding rigidly above a bottom layer, the lower crust, which deforms in simple shear. During a short time interval (1400 years), the equivalent shear stress is found to increase by a factor of up to 3 (67 MPa for anorthite and 17 MPa for quartz). For anorthite, this stress could explain the activation of a Mohr-Coulomb failure with a friction coefficient of 0.2, which is reasonable at the depth of 10 km. Dislocation creep is activated during a rapid change in the prescribed velocity, whereas diffusion creep dominates if the velocity is held constant, highlighting the importance of grain-size sensitivity for lower crustal rheology.

Rheological evolution during extension at nonvolcanic rifted margins: Onset of serpentinization and development of detachments leading to continental breakup

Within the continent‐ocean transition several nonvolcanic rifted margins exhibit a zone of partially serpentinized peridotites which continue under the thinned and faulted continental crust and are thought to represent subcontinental lithosphere serpentinized by contact with water. As water in sufficient volumes can only come from the surface, we suggest that a major condition for the onset of serpentinization is the embrittlement of the entire crust during progressive extension and hence the development of active crustal penetrating faults acting as fluid conduits. We investigate this possibility by modeling the rheological evolution of the lithosphere during extension and hence determining at what stretching factors the lower crust enters the brittle regime for a variety of different strain rates and lower crustal rheologies. Using an initial thermal structure appropriate for nonvolcanic margins, we find that the entire crust becomes brittle at stretching factors of between ∼3 and 5, depending on the strain rate. This compares well with the thickness of the crust observed just landward of the onset of partially serpentinized peridotites west of Iberia. The predicted thickness of the serpentinized peridotites (depth of the thermal limit of serpentinite stability beneath the crust‐mantle boundary) also compares reasonably well with their observed thickness. As serpentinites are characterized by low friction coefficients, we suggest that the onset of mantle serpentinization controls the development of decollements at the crust‐mantle boundary such as the S and H reflectors west of Iberia (leading to crustal separation). The development of thick serpentinites probably contributes to the weakening of the upper lithosphere and hence the localization of final breakup.

Strain localization as a key to reconciling experimentally derived flow-law data with dynamic models of continental collision

Published strength profiles predict strength discontinuities within and/or at the base of continental crust during compression. We use finite element models to investigate the effect of strength discontinuities on continental collision dynamics. The style of deformation in model crust during continued subduction of underlying mantle lithosphere is controlled by: (1) experimental flow-law data; (2) the crustal geotherm; (3) strain localization by erosion; (4) strain-softening and other localization effects. In the absence of erosion and other factors causing strain localization, numerical models with typical geothermal gradients and frictional/ductile rheologies predict diffuse crustal deformation with whole-scale detachment of crust from mantle lithosphere. This prediction is at odds with earlier model studies that only considered frictional crustal rheologies and showed asymmetric, focused crustal deformation. Without localization, model deformation is not consistent with that observed in small collisional orogens such as the Swiss Alps. This suggests that strain localization by a combination of erosion and rheological effects such as strain softening must play a major role in focusing deformation, and that strength profiles derived under constant strain rates and uniform material properties cannot be used to infer crustal strength during collision dynamics.

Rheological structure and dynamical response of the DSS profile BALTIC in the SE Fennoscandian Shield

Numerical modelling was applied to study the present-day state of stress and deformation under different tectonic loading conditions at the seismic BALTIC–SKJ profile in south-eastern Finland and in Estonia. The finite element method was used to solve the numerical problem. The two-dimensional model was constructed using the results from both seismic and thermal studies along the profile. The model is 700 km long and 200 km deep, and is roughly divided into an inhomogeneous, laterally layered crust and a homogeneous mantle lithosphere. Both the linear elastic and non-linear elasto-plastic rheologies were used. Elasto-plasticity was achieved by calculating a rheological strength as a function of depth along the profile. Different tectonic load cases were analysed with displacement, force and pressure type boundary conditions. Also, the effect of different strain rates was investigated. The results suggest that even with relatively low compressive stress levels the lower crust deforms in a plastic manner for a wet crustal rheology. When applying a dry crustal rheology, plastic yielding is attained only with much higher stress fields.

Crustal rheology and faulting at strike‐slip plate boundaries: 2. Effects of lower crustal flow

We present a numerical model of deformation at a strike‐slip plate boundary within a linear viscoelastic crust, which is driven by far‐field plate motions and basal mantle velocities. The crust is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth. Brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding. The locations and depth extents of faults in this model are not prespecified but, instead, are governed by stress evolution within the crust. We find that when a primarily elastic upper crust is underlain by a low‐viscosity lower crustal layer, the deformation zone broadens in time to encompass many parallel strike‐slip faults in an interacting network. In contrast, when the entire crust behaves elastically, the deformation zone remains narrow and focused on a single plate‐bounding fault, reflecting imposed mantle motions. Surface strain rate patterns within the interacting fault network are complex and reflect significant faulting‐related strain rate perturbations that decay over timescales of postseismic relaxation in the lower crust (10–100 years). The fault network has a characteristic spacing, with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology. The maximum depth of faults is limited by stress relaxation and large‐scale viscous flow in the lower crust, which confines brittle failure to shallow and midcrustal levels.

Crustal rheology and faulting at strike‐slip plate boundaries: 1. An analytic model

This paper investigates how rheologic stratification within the crust affects the formation and long‐term evolution of fault systems at a strike‐slip plate boundary. We present an analytic model of deformation at a strike‐slip plate margin within a two‐layer viscoelastic crust, with fixed shear modulus but varying viscosity in each layer. Faulting is represented by static elastic dislocations of fixed, shallow depth extent, imposed at a high critical stress threshold for fracture of a new fault and a lower one for sliding on a preexisting fault. To drive crustal deformation, we impose basal velocity boundary conditions at the Moho representing a narrow zone of high shear in the mantle. In this study we restrict attention to deformation at the surface, where simple analytic solutions exist for velocities and stresses. Our results suggest that when a primarily elastic/brittle upper crust is underlain by a low‐viscosity lower crust, the deformation zone in the upper crust widens in time. At steady state the surface width of the deformation zone may be significantly greater than the prescribed, narrow mantle shear zone. The long‐term width of the deformation zone increases with the viscosity contrast within the crust and with the thickness of the low‐viscosity lower crust. Widening of the deformation zone is accompanied by the fracture of new faults in the upper crust, leading to the formation of a system of parallel strike‐slip faults surrounding the plate boundary. For fixed plate velocity and fracture criteria, we find that the width of the brittle fault network within the total (distributed) deformation zone is primarily governed by strength properties of the upper crust and by the viscosity contrast within the crust.

Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling

Abstract We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from c . 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was c . 120–125 km, with calculated β mean between 1.38 and 1.40. Permo-Triassic β mean estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic β mean estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic β mean varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic β mean , which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic β mean estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.

Ground deformation in a viscoelastic medium composed of a layer overlying a half-space: a comparison between point and extended sources

SUMMARY We obtain and compare analytical and numerical solutions for ground displacement caused by an overpressurized magma chamber placed in a linear viscoelastic medium composed of a layer over a half-space. DiVerent parameters such as size, depth and shape of the chamber, crustal rheology and topography are considered and discussed. Numerical solutions for an axisymmetric extended source are computed using a finite element method (FEM). Analytical solutions for a point source are obtained using the dislocation theory and the propagator matrix technique. In both cases, the elastic solutions are used together with the correspondence principle of linear viscoelasticity to obtain the solution in the Laplace transform domain. Viscoelastic solutions in the time domain are derived inverting the Laplace transform using the Prony series method. The diVerences between the results allow us to constrain the applicability of the point source and the flat surface hypothesis, which are usually implicitly assumed when analytical solutions are derived. The eVect of the topography is also considered. The results obtained show that neglecting the topographic eVects may, in some cases, introduce an error greater than that implicit in the point-source hypothesis. Therefore, for an adequate modelling and interpretation of the time-dependent displacements, topography must be considered.

Heat flow in the Trans‐Hudson Orogen of the Canadian Shield: Implications for Proterozoic continental growth

Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo‐Proterozoic Trans‐Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 42±11 mW m−2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc‐related volcanic rocks. In the Flin Flon‐Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 km) surface layer overlying the mid and lower crust depleted in radioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of ≈40 km. The coincidence between this “cold spot” and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans‐Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming “normal” continental crust with average thickness, which require crustal flow and soft crustal rheology.

The onset of extension during lithospheric shortening: a two-dimensional thermomechanical model for lithospheric unrooting

SUMMARY We model the evolution of the lithosphere during its shortening and consequent gravitational collapse with special emphasis on the induced variations in the surface stress regime and dynamic topography. In particular, we analyse the conditions leading, immediately after lithospheric failure, to local extension, eventually coeval with compression. DiVerent crustal rheologies and kinematic conditions as well as thermally imposed mechanical rupture are considered. Numerical calculations have been performed by using a 2-D finite element code that couples the thermal and mechanical equations for a Newtonian rheology with a temperature-dependent viscosity. The results show that, after the failure of a gravitationally unstable lithospheric root, the replacement of lithospheric mantle by warmer asthenospheric material induces a considerable variation in the dynamic topography and in the surface stress regime. The occurrence of local extension, its intensity and its spatial distribution depend mainly on whether convergence continues throughout the process or ceases after or before the lithospheric failure. Similarly, uplift/subsidence and topographic inversion are controlled by kinematic conditions and crustal rheology. Mechanical rupture produces drastic changes in the surface stress regime and dynamic topography but only for a short time period, after which the system tends to evolve like a continuous model.

Coseismic well-level changes due to the 1992 Roermond earthquake compared to static deformation of half-space solutions

SUMMARY The M w 5.4 Roermond earthquake of 1992 April 13 was one of the strongest events during the last 500 years in Central Europe. For the period March‐May 1992, we collected records of 194 continuously operating well-level sensors, mostly located within 120 km of the epicentre. Nearly all wells penetrate unconfined or poorly confined Quaternary deposits with high hydraulic conductivities. 81 out of 194 raw data sets show a significant dynamic or step-like response of centimetre amplitude to the passage of seismic waves. Precursory anomalies are not obvious in these records. Coseismic well-level fluctuations could reflect a redistribution of stress and pore pressure in the brittle crust. Systematic analyses of such fluctuations may improve our knowledge of the role of pore fluids in crustal rheology and earthquake mechanics. The rather high number of individual observational records for a single event allows a regional correlation of the signs and amplitudes of the coseismic steps to changes in volume strain caused by the earthquake. The coseismic strain field at the surface was calculated for a homogeneous and a layered half-space. The results show reasonable agreement in the sign of the well-level steps but the amplitudes predicted from the wells’ volumetric strain responses are much smaller than those that were recorded. Clearly, the coseismic well-level steps cannot be explained by volume strain changes, as derived from linear elastic models.