Imaging the ramp–décollement geometry of the Chelungpu fault using coseismic GPS displacements from the 1999 Chi-Chi, Taiwan earthquake

We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.     

A thin-skinned thrust model for Variscan Pembrokeshire, Wales

The east and west coasts of Pembrokeshire (SW Wales) provide two sections through the Variscan fold and thrust belt. The evolution of these structures is interpreted in terms of a thin-skinned tectonic model. Balanced cross-sections are constructed for the high-level imbricate sequences, and these allow reasonably accurate estimates of shortening to be made. Basement control on structures developed in the Upper Carboniferous cover rocks is minimal, though some thrust ramp positions may be determined by the location of earlier normal faults.The thrust belt may be divided into two parts, according to the depth to the décollement horizon. In the north, imbricate fans developed from a shallow-level detachment (<1 km) which dips gently south. In the southern part, a deeper level of décollement and thicker sedimentary pile gave rise to large-amplitude folds.Shortening is heterogeneous, and both thrust periodicity and fold style are partly determined by rheology. Cumulative tectonic displacement increases to the west across Pembrokeshire, resulting in a net clockwise rotation of about 40°.     

Evidence for fluid circulation, overpressure and tectonic style along the Southern Chilean margin

The southern Chilean convergent margin, between 50° and 57° S, is shaped by the interaction of the three main plates: Antarctic, South America and Scotia. North of 53° S, the convergence between Antarctic and South America plates is close to orthogonal to the continental margin strike. Here, the deformational style of the accretionary prism is mainly characterized by seaward-verging thrusts and locally by normal faults and fractures, a very limited lateral extension of prism, a very shallow dip ( 6°) décollement, and subduction of a thick and relatively undeformed trench sedimentary sequence. South of 53° S, convergence is oblique to the margin, locally, the trench sediments are proto-deformed by double vergence thrusting and the front of the prism grows through landward-verging thrusting. The décollement is sub-horizontal and deep, involving most of the sediment over the oceanic crust in the accretionary process, building a comparatively wide and thicker prism. A Bottom Simulating Reflector is present across the whole prism to the abyssal plan, suggesting the presence of gas in the sediments.The analysis of P- and S-wave velocity reflectivity sections, derived by amplitude versus offset technique (AVO), detailed velocity information and the velocity-derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre-stack depth migrated seismic lines, aiming to explain the relation between fluid circulation and tectonics.Accretion along double vergence thrust faults may be associated with the presence of overpressured fluid, which decreases the effective shear stress coefficient along the main décollement and within the sediments, and modify the rheolgical properties of rocks. The presence of an adequate drainage network, represented by interconnected faults and fractures affecting the entire sedimentary sequence, can favour the escape of pore fluid toward the sea bottom, while, less permeable and not faulted sediments can favour fluid accumulations. Gravitational and tectonic dewatering, and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench. The results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation.     

Borehole image analysis of the Nankai Accretionary Wedge, ODP Leg 196: Structural and stress studies

Electrical images recorded with Resistivity-At-Bit (RAB) from two sites drilled during Ocean Drilling Program (ODP) Leg 196 were analyzed to study the effects of subduction at the Nankai margin. For the first time in the history of scientific deep-sea drilling in ODP, in situ complete borehole images of the décollement zone were obtained. Analyses of all drilling-induced fracture data indicated that the maximum horizontal compressive stress (S_Hmax) axes have an azimuth of 303°, and analyses of breakout data from RAB images indicated an azimuth of 310°. These azimuths approximate the convergence direction of the Philippine Sea plate towards the Eurasian plate. The frontal thrust at Site 808 was encountered at about 389 mbsf. Density, porosity, resistivity, and gamma ray data change across the frontal thrust. The décollement zone at the deformation front was identified between 937 and 965 mbsf. The base of the décollement is sharply defined as the maximum extent of conductive fracturing and is marked by abrupt changes in physical properties [Mikada, H., Becker, K., Moore, J.C., Klaus, A., Austin, G.L., Bangs, N.L., Bourlange, S., Broilliard, J., Brückmann, W., Corn, E.R., Davis, E.E., Flemings, P.B., Goldberg, D.B., Gulick, S.S., Hansen, M.B., Hayward, N., Hills, D.J., Hunze, S., Ienaga, M., Ishiguro, H., Kinoshita, M., Macdonald, R.D., McNeill, L., Obana, S., Hong, O.S., Peacock, S., Pettigrew, T.L., Saito, S., Sawa, T., Thaiprasert, N., Tobin, H.J., Tsurumi, H., 2002. Proc. ODP, Initial Rep., 196, College Station, TX, (Ocean Drilling Program)]. The upper boundary of the décollement is marked by several sets of conductive fractures and by high variability in physical properties. The décollement zone is characterized by intense brittle fracturing. These fractures are considered to be the consequence of cyclic stresses and high fluid pressures in this zone. We analyzed fracture dips and their orientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites.     

The calculation of horizontal thrust transport using excess area in cross-sections

Existing balancing methods utilizing excess area in cross sections rely heavily on the presence of a perfectly horizontal décollement surface. This is rarely the case in thrust belts, and the commonly observed hindward dip of floor thrusts imparts uplift and internal strain to the thrust wedge during transport. A modified excess area balancing technique is presented to account for hinterland dipping floor thrusts.     

The Siwaliks of western Nepal: I. Geometry and kinematics

The Siwalik Group which forms the southern zone of the Himalayan orogen, constitutes the deformed part of the Neogene foreland basin situated above the downflexed Indian lithosphere. It forms the outer part of the thin-skinned thrust belt of the Himalaya, a belt where the faults branch off a major décollement (MD) that is the external part of the basal detachment of Himalayan thrust belt. This décollement is located beneath 13 Ma sediments in far-western Nepal, and beneath 14.6 Ma sediments in mid-western Nepal, i.e., above the base of the Siwalik Group. Unconformities have been observed in the upper Siwalik member of western Nepal both on satellite images and in the field, and suggest that tectonics has affected the frontal part of the outer belt since more than 1.8 Ma. Several north dipping thrusts delineate tectonic boundaries in the Siwalik Group of western Nepal. The Main Dun Thrust (MDT) is formed by a succession of 4 laterally relayed thrusts, and the Main Frontal Thrust (MFT) is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones. One of the major transfer zones is the West Dang Transfer Zone (WDTZ), which has a north-northeast strike and is formed by strike-slip faults, sigmoid folds and sigmoid reverse faults. The width of the outer belt of the Himalaya varies from 25 km west of the WDTZ to 40 km east of the WDTZ. The WDTZ is probably related to an underlying fault that induces: (a) a change of the stratigraphic thickness of the Siwalik members involved in the thin-skinned thrust belt, and particularly of the middle Siwalik member; (b) an increase, from west to east, of the depth of the décollement level; and (c) a lateral ramp that transfers displacement from one thrust to another. Large wedge-top basins (Duns) of western Nepal have developed east of the WDTZ. The superposition of two décollement levels in the lower Siwalik member is clear in a large portion of the Siwalik group of western Nepal where it induces duplexes development. The duplexes are formed either by far-travelled horses that crop out at the hangingwall of the Internal Décollement Thrust (ID) to the south of the Main Boundary Thrust, or by horses that remain hidden below the middle Siwaliks or Lesser Himalayan rocks. Most of the thrusts sheets of the outer belt of western Nepal have moved toward the S–SW and balanced cross-sections show at least 40 km shortening through the outer belt. This value probably under-estimates the shortening because erosion has removed the hangingwall cut-off of the Siwalik series. The mean shortening rate has been 17 mm/yr in the outer belt for the last 2.3 Ma.     

The mechanisms of overlapping of the Bresse graben by the Jura formations in the Vignoble area (France)

This paper aims to underline how powerful and important the balanced cross-section constructions are for understanding structural mechanisms, especially when several interpretations are possible. In the Vignoble area, between Lons le Saulnier and Arbois — French foothills of Jura, the frontal overlapping is proved by several drillholes. The classical interpretation is a thrusting through listric reverse faults and continuation of the décollement level underlying the Jura at the cover—basement interface. The balanced cross-section construction demonstrates that this assumption is not valid due to the necessary thinning and stretching of the overlapping formations in this region. This paper proposes a new solution where the overlapping, through listric normal faults, is like a huge gravity landslide, without any relation to the shortening and décollement of the internal Jura. The new interpretation is supported by the very consistent balanced crosssection of the displaced cover and by the extension structures observed in the field.     

Roof sequence response to emplacement of the Wills Mountain duplex: the roles of forethrusting and scales of deformation

This paper focuses on the behavior of a roof sequence in the Appalachian Plateau of West Virginia, U.S.A., and emplacement of the Wills Mountain duplex with 17.5 km of displacement. Unlike the Plateau along strike in Pennsylvania and New York where forethrusting was previously documented, this roof sequence lacks an underlying salt-dominated roof décollement. Kinematic analyses reveal that the roof sequence in the West Virginian Plateau accommodated about two-thirds of the 17.5 km of shortening by the adjacent Wills Mountain duplex, as a forethrusting kinematic response. The remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation. This kinematic response matches that along strike in the central Appalachians despite the loss of the salt décollement. We interpret that an Ordovician shale-dominated formation was sufficiently weak to substitute for the salt horizon. Thus, a weak mechanical unit rather than specifically a salt décollement is a necessary prerequisite for forethrusting. A contributing factor to forethrusting may be the subvertical front of the Wills Mountain duplex, which inhibited other responses by the roof sequence. Mesoscale and smaller processes, including grain-to-grain pressure solution, twinning and cleavage formation account for over 75% of the shortening in the roof sequence, and, if ignored, would result in an erroneous interpretation of backthrusting or local compensation. This result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems.     

Diagenesis and fracturing of Paleocene–Eocene carbonate turbidite systems in the Ionian Basin: The example of the Kelçyra area (Albania)

The Kelçyra area is emplaced in a foreland fold-and-thrust belt (FFTB), characterized by a westward thrusting with the Triassic evaporites as the major décollement level. Several secondary features related with this evolution, like backthrusting, folding, duplex structures, evaporite diapirism are present. During the FFTB evolution, the study area has been subjected to several fracturing events with associated stages of fluid migration. During the pre-deformational stage, complex textures such as crack-and-seal features most likely reflect expulsion of overpressured fluids. These fluids were dominantly host-rock buffered. Within the post-deformational stage, a meteoric fluid caused cementation and development of a karst network during a period of emergence after the thrust emplacement. Subsequently, Mg calcite reprecipitated in the more stable carbonate phase calcite and dolomite, which filled part of the karts network. The latter is finally dedolomitized and locally partially dissolved by a second meteoric fluid flow, which greatly increased the secondary porosity.     

Viscous components in Jura folding

Viscous components in Jura folding due to pressure solution include discrete stylolites in limestones and diffuse flowage, particularly viscous folding, in Triassic anhydrites. Though increase of equilibrium solubility on the boundary of the stressed solid is well understood, the kinetic aspects of transport of the solute by diffusion and the movement of pore water are not known quantitatively and can be assessed only qualitatively on the basis of observations in nature and laboratory experiments. Apparently the creation of secondary tectonic pore space — macroscopic shear and tension joints in limestones and microscopic dilatancy cracks in anhydrite — is essential; it is possibly the rate-determining process. Stylolites in limestones, though ubiquitous, are quantitatively negligible, the overwhelming mass transport having occurred along discrete surfaces of failure. Flow of anhydrite, on the other hand, has been essential for basal décollement and cushioning under the folds.     

Fluid flow in the Sicilian accretionary wedge: Primary geochemical signatures or recrystallization mask

Veins in the Sicilian accretionary wedge were studied petrographically and geochemically with the aim to investigate the relation between fluid flow in a décollement horizon and in overlying Mesozoic basinal sediments. Fluids expelled along the décollement horizon precipitated calcite cements that show a broad spread in stable isotope signatures and that generally have rather high Fe and Mn content. The fluids most likely originated from mixing of hot deep metamorphic fluids and dewatering of the clay unit along which the principal overthrusting occurred.Synkinematic veins in the overlying basinal units are cemented with calcite. The trace element content and δ¹³C signatures of these veins are host-rock dependent, pointing to a host-rock buffering effect. Petrographic observations indicate that calcite cements have been recrystallized. Thus the cements could have inherited their geochemical signatures from the host-rock during recrystallization. This is also supported by their δ¹⁸O signature, which is controlled by temperature fractionation.     

Deformation of the Caledonide lithosphere of northwest Scotland

An interpretation of the MOIST seismic reflection profile along the north coast of Scotland has shown the pattern of deep crustal structure across the margin of the Caledonian orogen with the Hebridean cratonic foreland to the northwest. It is proposed that the lower crust of the orogen is characterised by a suite of easterly dipping thrusts that divide it into flakes that were imbricated during lateral compression of the lithosphere. The effects of concomitant and subsequent uplift and erosion led to the removal of the major part of the upper crust within the orogen so that the present crust and Moho are largely the relics of the Caledonian lower crust. As time progressed, the thrust front encroached further into the foreland and cut down to a basal décollement at successively deeper levels. This behaviour is explained as a consequence of the response of the brittle-ductile transition to changing the temperature regime, rock composition and strain rate as the orogeny proceeded. Palinspastic reconstructions of the whole lithosphere illustrate the process. Following the compressional phase of the orogeny, uplift led to initiation of extension and the reactivation of the thrusts as normal listric faults. Rotation of basement blocks gave rise to wedge-shaped sedimentary basins.     

High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil

The Neoproterozoic Ribeira belt is subdivided in two domains with contrasting tectonic characteristics. The northern domain is dominated by shallowly dipping foliations and orogen-normal thrust tectonics. The southern domain is characterized by a 1000-km-long network of anastomosing transcurrent shear zones parallel to the belt. This contrast is interpreted as reflecting continent–continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain. The central, transitional, domain of the Ribeira belt displays the northern termination of the transcurrent shear zone network: the Além Paraíba–Pádua shear zone system (APPSS). The 250-km-long Além Paraíba–Pádua system involves granulites facies mylonites deformed through transpression.A detailed study of the microstructure and lattice preferred orientation (LPO) of the rock-forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust. Plagioclase crystals are plastically deformed; they display curved twins and cleavages, mechanical twins, and evidence of dynamic recrystallization. LPO of plagioclase is consistent with activation of the (010) [100] and (010) [001] slip systems. LPO of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the (100) [001] slip system. Quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration. The LPO of quartz is therefore the result of a static transformation of an initial, syn-kinematic LPO, and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization.     

Paleodeformations d'un massif orthogneissique: Massif des Cammazes, Montagne Noire occidentale, France

The Cammazes orthogneissic massif in the western Montagne Noire (France) has been affected by numerous tectonic phases during the Hercynian orogeny. The principal deformation, synchronous with a mesozonal metamorphism, is represented by different structural features such as foliation, ‘augen’ structures and deformation of megacrystals. These structural features are essentially varied with the concentration of feldspar megacrystals in the gneisses. The structural studies of these gneisses have allowed the recognition of the nature and the orientation of the principal deformation. To determine the intensity of this deformation, large undeformed feldspar crystals have been used as markers. These crystals have been assimilated to rigid ellipsoidal elements disposed in a random fashion in the initial stage of the mesostase which is considered as a viscous Newtonian fluid. The statistical studies of the orientation of these crystals with reference to the plane of flattening (foliation) has caused the appearance of an anisotropy in the final deformed state. This anisotropy, when compared to that obtained theoretically from a population of rigid elements included in a Newtonian fluid flowing at a slow speed, could evaluate the rate of the finite strain.

Résumé

Dans la Montagne Noire occidentale (France) le massif orthogneissique des Cammazes a été affecté lors de l'orogénèse hercynienne par une tectonique polyphasée. La déformation majeure, synchrone d'un métamorphisme mésozonal, se traduit par différents caractères structuraux (foliation, structure oeillée, déformations des mégacristaux) variant essentiellement avec la concentration des mégacristaux feldspathiques dans les gneiss. L'étude structurale a permis de définir la nature et l'orientation de cette déformation majeure. Pour préciser son intensité les grands cristaux indéformés de feldspaths, utilisés comme marqueurs, ont été assimilés à des éléments rigides ellipsoi'daux disposés dans l'état initial de façon aléatoire dans la mésostase considérée comme un fluide visqueux newtonien. L'étude statistique de leur orientation par rapport au plan d'aplatissement (foliation) fait apparaitre dans l'état final (déformé) une anisotropie. Cette anisotropie comparée à celle obtenue par voie théorique, d'une population d'éléments rigides inclus dans un fluide newtonien s'écoulant à faible vitesse, permet d'évaluer le taux de déformation finie.     

Un nouveau critere de sens de mise en place dans une caisse filonienne: Le “pincement” des mineraux aux epontes : Orientation des minéraux dans un magma en écoulement

The flow of a magma into a dike, sill, laccolite, pipe or batholith leads to a preferential orientation of crystals in the magma. A study by computer simulation has brought to light certain types of fabric for oblate and prolate minerals in the case of a plane deformation, on the one hand by pure shear, and on the other hand by simple shear. Theoretical and practical studies suggest a new method for determining the direction of injection (“injection axis”) in a dike, based on the angular relations of minerals near the walls.New developments are proposed on the relations between plane and linear flow-lines, on the distinction between “apparent flow-lines” and the transport-plane of the magma (“real flow-lines”) and on the amount of magma deformation.

Résumé

La mise en place d'un magma dans une caisse filonienne (ou dans des sills, dykes, laccolites, batholites, etc.) s'accompagne d'une orientation préférentielle des minéraux déjà formés. Une étude par simulation sur ordinateur a mis en évidence certains types de fabrique pour les minéraux phylliteux et les minéraux aciculaires dans le cas d'une déformation plane, d'une part par aplatissement pur et d'autre part, par cisaillement simple.En application théorique puis pratique, un nouveau critère de sens de mise en place (axe d'injection) dans une caisse filonienne est défini: le pincement des minéraux aux épontes; en effet, à chacune d'elles, les fluidalités apparentes, observées sur les minéraux, font un angle avec la paroi du filon.Des développements nouveaux sont proposés concernant les relations entre fluidalités planaire et linéaire, la distinction entre fluidalité apparente et plan de transport du magma (fluidalité réelle), le taux de déformation subi par le magma,....     

Slump-initiated and controlled syndepositional sandstone remobilization: an example from the Namurian of County Clare, Ireland

Remobilization of sandstones can dramatically reconfigure original depositional geometries and results in very unusually shaped sandstones, which resemble little, if any, of the original geometry. A number of deformational sandstone bodies, dykes and volcanoes from the upper part of the Carboniferous Ross Formation are described, which offer the opportunity to examine a suite of field-scale reconfigured sandstones. These structures are located in close proximity to the Ross Slide, which outcrops along a 2-km section on the northern coast of the Loop Head Peninsula, County Clare, Ireland. Dome- and ridge-shaped deformational sandstone bodies, dykes and volcanoes are interpreted to be the product of remobilization of a turbiditic sandstone. Liquification and remobilization were triggered by translation, cessation and loading of the underlying turbiditic sandstone by the Ross Slide. Deformational sand body, dyke and volcano development occurred in an asynchronous fashion with deformational sand bodies formed during slump translation. Sand dykes and volcanoes developed after the cessation of slump movement. During slump translation, the minimum principal stress (σ₃) was orientated vertically and the slump behaved in a `ductile' manner. After slump arrest, the minimum principal stress was oriented horizontally, and the unit regained shear strength to behave in a `brittle' manner. The relative change in rheological states with changing applied shear stress is indicative of thixotropic-like behaviour within the slump mass. Ridge-shaped deformational sand bodies are aligned parallel to slump folds, and their morphology is inferred to be controlled by compressional slump deformation associated with heterogeneous cessation of slump movement that was initiated by frontal arrest of the translating mass.     

Strain patterns and shortening in a folded thrust sheet: An example from the southern appalachians

Total strain patterns estimated across the Pulaski thrust sheet of the southwest Virginia Appalachians show an approximately homogeneous, plane strain deformation associated with folding and distortion above a subsurface décollement. Estimated strains are low (1.2 < < 2.0) with a subvertical extension. Chlorite fibers in pressure fringes on framboidal pyrite indicate that non-rotational deformation produced weak cleavage and pencil structure in mudrock. Variations in shape of pencils and fiber lengths in pressure fringes define highest strains in fold hinges and adjacent to contraction faults. Fabric transitions, delineated by distribution and intensity of cleavage, pencil structure and bedding fissility across the thrust sheet are strain dependent. Balanced cross-sections suggest 35% horizontal shortening due to regional folding and faulting within the Pulaski sheet. Strain integration techniques give 17–35% horizontal shortening associated with cleavage formation. Removal of this strain indicates that cleavage was superposed on open to tight, class-3 folds. Pre-existing thickness variations and anomalous low strains in tight folds require early folding accomodated by intergranular deformation (perhaps controlled grainboundary sliding). Suppression of cleavage formation and penetrative strain was possibly due to higher pore fluid pressure in the early stages of thrust sheet deformation. Observed variations in bedding-cleavage angle and low cleavage fans are compatible with this deformation sequence.     

Les rudistes du Crétacé moyen de la province Méditerranéenne occidentale. Evolution, paléoécologie, paléobiogéographie

Dans un premier temps on délimite les plates-formes à rudistes de la province méditerranéenne occidentale, au sein de laquelle on distingue, en fonction des particularités paléogéographiques locales, quatre sous-provinces. On souligne ensuite l'importance et la nature des événements paléogéographiques du Crétacé moyen qui ont joué un rôle dans la distribution et le développement des formations à rudistes; on distingue fondamentalement des événements eustatiques et tectoniques. Les événements biologiques sont caractérisés par un déclin ou une disparition des rudistes éocrétacés, un développement des Caprinidae et des Radiolitidae à l'Albo-Cénomanien, l'apparition des Hippuritidae et le déclin des Caprinidae au Turonien. On analyse les causes, les circonstances et les faits majeurs de l'évolution et de la paléoécologie des rudistes au cours de cette période.The rudistid platforms of the western Mediterranean embrace four palaeogeographical sub-provinces. The palaeogeographical events that decided the distribution and development of the rudistids are discussed. Of significance are eustatic and tectonic events.There was a decline or a disappearance of Early Cretaceous rudistids, a development of the Caprinidae and Radiolitidae around the Albian-Cenomanian, the appearance of the Hippuritidae and a decline of the Caprinidae in the Turonian. Factors of evolutionary and palaeoecological significance are reviewed.     

Gaspé belt subsurface geometry in the northern Québec Appalachians as revealed by an integrated geophysical and geological study: 1 — Potential field mapping

Together, recent gravity and high-resolution aeromagnetic datasets are used to qualitatively investigate the upper- and middle-crustal geometry of the Middle Paleozoic Gaspé belt in the northern Appalachians. Long-wavelength potential field anomalies define two sub-basins that are divided by northeast trending gravity highs. For each sub-basins, gravity lows correlate with the youngest rock units.Maps that highlight anomalies associated with near surface features, at the expense of those related to deeper sources, provide an important supplement to the spatially discrete observations derived from bedrock mapping. Analysis of such maps indicates that the sub-basins are characterized by different structural patterns and that faults trending obliquely compared to the main structural grain have been previously underestimated.The geometry of the Gaspé belt as revealed by this integrated geophysical and geological study bears similarities with orogens exhibiting lateral extrusion. This geometry is interpreted as the result of a heterogeneous strain regime in front of an indenter corresponding to the Early Paleozoic Gander/Dunnage crustal block. The indentation tectonic model is supported by: 1) the various strike and kinematic of faults that suggest a strongly heterogeneous strain regime; 2) the greater geological complexity and the occurrence of faults with a significant thrust component in front of the indenter; 3) the predominance of dextral strike-slip faults in the eastern Gaspé Peninsula that result in lateral material transport away from the indenter; 4) the location of abundant Devonian magmatic dykes, sills and stocks in a fault-bounded zone that experienced local extension; 5) the occurrence of block rotation.     

Thick- and thin-skinned tectonics in the Pyrenees

A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies.The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement.Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere.