patterns of imbricate thrusting through the use of seismic reflection profiles satellite images and balanced kinematic models we describe patterns that help to identify imbricate structures and define the sequence of thrusting in fold and thrust belts measures of folding shear strains are employed to define break forward systems in which younger deeper faults refold overlying thrust sheets in contrast break backward sequences form a wide range of viable geometries that are not expected to conserve these folding strains thus measures of folding shear strains provide a means to define thrusting sequence we also explore map view patterns of imbricate thrusting by using three dimensional models and stereoscopic satellite images we present an example from the peruvian andes in which these map patterns are combined with seismic reflection profiles to define a complex imbricate system with components of both break forward and break backward thrusting these integrated methods provide new tools for understanding the deformational histories of fold and thrust belts including the definition of complex structural closures with opportunities for hydrocarbon reservoir duplication

lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

tectonic implications of 40ar 39ar muscovite dates from the mt haley stock and lussier river stock near fort steele british columbia the mt haley and lussier river stocks are located northeast of cranbrook b c near the south end of the western main ranges of the southern canadian rocky mountains both are multiphase potassium feldspar porphyritic monzonite plutons that intrude lower paleozoic miogeoclinal strata they crosscut and thermally overprint the lussier river fault and the thrust and fold structures in the east flank of the purcell anticlinorium and the west limb of the porcupine creek anticlinorial fan structure muscovite from the mt haley stock yielded a 40ar 39ar plateau age of 108 2 0 7 ma 2 and a single crystal step heating analysis of muscovite from a skarn in the metamorphic aureole adjacent to the lussier river stock gave a plateau date of 108 7 0 6 ma 2 these dates constrain the timing of thrusting and folding in this portion of the western rocky mountains and of the displacement along the lussier river st mary fault to pre middle albian

transverse folding and superposed deformation mount fisher area southern canadian rocky mountain thrust and fold belt a northeast facing panel of middle proterozoic purcell supergroup strata occurs beneath cambrian and devonian strata along the east side of the rocky mountain trench in the mount fisher area anomalous northeast trending folds faults and cleavage that formed during cretaceous paleocene deformation occur in this panel in the northern part of the area the dominant structure in the southern part is the northwest trending lizard segment of the mesozoic hosmer nappe which folds an older north trending cleavage that probably formed during the east kootenay orogeny 1300 1350 ma thickness and facies variations in purcell strata and changes in the level of erosion beneath the sub devonian unconformity imply that many of the important structural boundaries in the mount fisher area and also the normal faults along the southern rocky mountain trench follow the locus of older structures that were active in the middle proterozoic and the early paleozoic the anomalous northeast trending structures in the moun

cordilleran front range structural features in northwest montana interpreted from vintage seismic reflection data abstract industry seismic reflection data spanning the rocky mountain cordillera front ranges of northwest montana were reprocessed and interpreted in this study five seismic profiles represent 160 km of deep reflection data collected in 1983 that span the eastern purcell anticlinorium rocky mountain trench rmt rocky mountain basal decollement rmbd and lewis thrust the data were reprocessed using modern techniques including refraction statics pre stack time migration pstm and pre and post stack depth migration results indicate the rmbd is 8 13 km below the earth s surface and dip 3 10 west evidence for the autochthonous mesoproterozoic belt and basal cambrian rocks beneath the rmbd is present in all of the profiles and appears to extend east of the rmt the lewis thrust was identified in the seismic profiles and appears to sole into the rmbd east of the rmt the rmt fault system has a dip displacement of 3 4 km and forms a half graben filled with 1 km of unconsolidated tertiary sedimentary deposits the rmt and adjacent flathead fault systems are interpreted to be structurally linked and may represent a synthetic en echelon fault system

structure and evolution of the northern potwar deformed zone pakistan the northern potwar deformed zone npdz is part of the active foreland fold and thrust belt of the salt range and potwar plateau in northern pakistan about 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the deformation style and structure of the npdz with particular emphasis on history of deformation of the dhurnal oil field the seismic lines suggest that the overall structure of the eastern npdz is a duplex structure developed beneath a passive roof thrust the roof thrust is generated from a tipline in the miocene murree formation and the sole thrust is initiated from the same eocambrian evaporite zone that extends 80 km southward beneath the soan syncline and salt range the dhurnal oil field structure is a pop up at the southern margin of the npdz and developed beneath the passive roof thrust the passive roof thrust crops out just north of dhurnal on the steep northern limb of the soan syncline an overstep passive roof thrust sakhwal fault is interpreted west of dhurnal this fault developed due to

fracture abundance and strain in folded cardium formation red deer river anticline alberta foothills canada abstract the folded and thrusted mesozoic clastic sequence of the canadian rocky mountain foothills forms important hydrocarbon reservoirs however the generation of fractures and their timing relative to folding and thrusting have remained unclear understanding the distribution and evolution of natural fractures especially their timing of formation relative to the evolution of the fold and thrust system could potentially improve exploration and development outcomes in these otherwise tight low permeability reservoirs we investigated the relation between folding and fracture formation in the upper cretaceous cardium formation by combining field structural observations and kinematic modeling of the fold and thrust belt evolution we evaluated the effects of structural position and strain on the formation of predominantly opening mode fractures by analyzing fracture intensity and aperture in the backlimb the shallow forelimb close to the crest and the steep forelimb away from the crest of a reservoir scale anticline the red deer river anticline fracture strain determined from both outcrop and microscanlines varies little across the three structural domains of the fold although somewhat lower fracture strain was measured in the forelimb close to the crest these fracture strain measurements are qualitatively consistent with calculated horizontal strain in the tectonic transport direction obtained from kinematic numerical models that simulate fold development associated with slip along the underlying burnt timber thrust the kinematic models predict similar magnitudes of horizontal extension in both the back and forelimbs and somewhat lower extension in the upper forelimb during early development of the red deer river anticline predicted early fracture formation during fold development is consistent with field structural observations of shear reactivation during later stages of folding this combined kinematic modeling and field structural study demonstrates that deforming fold and thrust belts can undergo a complex evolution of bed parallel extension in both space and time resulting in spatially variable fracture formation in structurally complex subsurface reservoirs

ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

the tectonic evolution of the zoumi sandstone western moroccan rif the zoumi sandstone is an oligo miocene syn tectonic flysch sequence deposited on the back of the mesorif thrust sheet whose sediment source was from the foreland to the south variations within the zoumi sandstone from thick to thin bedded turbidite lobes are explained as the result of deposition in small confined basins or larger more extensive basins respectively formation of small basins is related to contractional deformation in the underlying mesorif thrust sheet propagation of the zoumi thrust through the developed basin with areas of relative uplift and subsidence produced complex hanging wall cutoff geometry a complicated footwall geometry exists because deformation in the more external mesorif thrust sheet preceded the final displacement of the more internal intrarif thrust sheet the latter incorporated the zoumi sandstone into its leading edge and thrust it over the mesorif during the miocene this deformation sequence of two or more simultaneously active thrust sheets is described as synchronous thrusting

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

lateral and vertical changes of deformation style in the osen roa thrust sheet oslo region abstract changes in deformation style and amounts of shortening in the osen roa thrust sheet of the oslo region occur vertically and laterally approaching the thrust front in the south deformation in the cambromiddle ordovician sequence passes laterally from closely spaced imbricates in the north 50 60 shortening through triangle pop up and imbricate zones toward the south 20 37 shortening to widely spaced zones of deformation up to 20 shortening approaching the thrust front changes in deformation style are attributed to changing boundary conditions across the klekken thrust declining end of orogenic forces and an increase in thickness of competent units in the ordovician rocks to the south vertical changes in deformation style are attributed to the increasing percentage of competent units upward in the cambro silurian sedimentary rocks in the north the accompanying decrease in shortening upwards requires a structurally necessary upper detachment horizon to separate folded late middle ordovician silurian sediments from imbricated early cambro middle ordovician sediments below while southward in the oslo area the upper detachment needs to be placed between silurian and cambro ordovician units finally in eiker with less than 20 shortening the whole cambrosilurian sequence appears to have deformed as a single unit in the northern oslo region the upper detachment probably has a backthrust sense of motion above an imbricate stack passive roof duplex further south the upper detachment is probably directed toward the foreland

evolution of the triangle zone in the rocky mountain foothills near coalspur central alberta a triangle zone which commonly occurs along the external margin of a foreland thrust and fold belt with a buried thrust front is underlain by a subhorizontal blind foreland verging thrust that ends against a foreland dipping hinterland verging thrust these contemporaneous thrusts active towards the end of orogenesis enclose an intercutaneous wedge that moved towards the foreland during orogenesis a triangle zone evolves through periodic replacement of faults bounding the active wedge replacements occur in cycles during each of which a lower fault tends to be replaced by one in a lower stratigraphic horizon an upper fault by one farther away from the foreland each cycle ends with the lower fault moving to a younger horizon where it joins a new more external upper fault near coalspur the triangle zone exposes the remnants of several wedges involving upper cretaceous and paleocene molasse most of these wedges developed during the last cycle but one and have a combined displacement of about 5

physical and chemical properties of the sulphur mountain thermal springs banff national park and implications for endangered snails nine thermal springs in three groups occur along the flank of sulphur mountain in banff national park the principal recharge zone is suggested to be above 2000 m elevation on mount rundle circulation depths are estimated to be 3 2 0 6 km and discharge is focused along the sulphur mountain thrust springs show constant temperature and total dissolved solids load throughout the winter whereas both drop in association with spring snowmelt the degree and timing of temperature drops are a function of elevation with the highest springs showing the earliest and most significant temperature drop the highest elevation springs also show the greatest seasonal variability in water chemistry unusual seasonal flow stoppages of these springs are related to extreme low precipitation years the biogeochemistry of the thermal springs and population fluctuations of the endemic and endangered banff springs snail physella johnsoni are strongly related to seasonal variability in flow rates changes in redox condi

understanding kinematic data from the moine thrust zone in terms of a kinematics based mathematical model of deforming thrust wedges abstract a salient recess pair along the moine thrust zone exhibits significant lateral kinematic variations specifically the more steeply dipping portion of the thrust surface associated with the recess shows evidence for greater strain magnitudes and more out of the plane motion than the adjoining salient additionally a subset of the grain shape data has grain shape approximations with their long axes oriented perpendicular to the regional transport direction to more fully understand these kinematic features we developed a kinematics based mathematical model of thrust wedge deformation our numerical model predicts an increase in strain magnitude as well as non plane strain flattening in the region of a thrust ramp consistent with the strain patterns observed within the recess along the moine thrust zone furthermore in the absence of a lateral confining boundary condition much of the model thrust wedge experiences transport perpendicular maximum extension except for the region closest to the thrust where simple shear deformation is dominant using our numerical model we predict the amount of lateral confining strain needed to induce plane strain deformation ps as well as the amount of lateral confining strain necessary to make the long axis of the strain ellipsoid parallel with the transport direction parallel we have found that the incremental parallel can be as much as 42 of the incremental strain magnitude thus we consider transport perpendicular lineations like those seen along the moine thrust zone to be the result of lateral confining boundary conditions that are weak enough to allow transport perpendicular maximum extension

middle jurassic exhumation along the western flank of the selkirk fan structure thermobarometric and thermochronometric constraints from the illecillewaet synclinorium southeastern british columbia new thermobarometric and thermochronometric data from the illecillewaet synclinorium a broad regional southwest verging structure in the western flank of the selkirk fan structure indicate that accretion of the intermontane superterrane resulted in deep burial 20 25 km of the outer margin of ancestral north america and that subsequent southwest verging folding and thrusting were accompanied by rapid exhumation southwest verging folding of lower paleozoic rocks in the illecillewaet synclinorium was initiated during regional peak metamorphic conditions at 6 7 kbar crystallization of late synkinematic granitoid plutons that intrude the illecillewaet synclinorium also occurred at pressures of 6 7 kbar however equilibration of the contact aureoles of the plutons occurred at 3 3 5 kbar during the latter stages of the southwest verging deformation evidently a decompression on the order of 3 kbar occurred concurrently with southwest verging folding and thrusting in the illecillewaet synclinorium thermochronometric analyses further indicate that plutonic rocks from the illecillewaet synclinorium cooled rapidly between times of crystallization and times of ar closure in biotite regional relationships indicate that the accretion of the intermontane superterrane to the edge of the north american margin in southeastern british columbia occurred between 187 and 173 ma toarcian bajocian subsequent southwest verging deformation occurred approximately between 173 and 168 ma bajocian bathonian thus the removal of at least 10 km of rocks from above the illecillewaet synclinorium inferred from thermobarometric data occurred approximately between 173 and 168 ma rapid denudation of the selkirk fan structure was probably the result of synorogenic extension or extensional collapse contemporaneous with the development of southwest verging structures at deeper levels of the orogen the data presented here indicate that all significant deformation and most of the exhumation along the western flank of the selkirk fan structure were complete by late middle jurassic time after the middle jurassic the selkirk fan structure must have been transported passively northeastward along the basal decollement of the cordilleran foreland thrust and fold belt together with the intermontane superterrane as the fold and thrust belt tectonically overrode the margin of the north american craton

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

thrust tectonics thin skinned or thick skinned and the continuation of thrusts to deep in the crust abstract the paper analyses the geometry of thin skinned thrust zones where the thrusts shallow out at depth and of thicker skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards in the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones the energy involved in the internal deformation of hanging wall rocks may prohibit many faults becoming more shallow upwards such shallowing upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities but displacements on the faults are unlikely to be large another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the himalayas these carry large thicknesses 30 to 100 km of crustal and upper mantle rocks to the south causing flexuring and isostatic depression of the indian plate the steeply dipping thrusts are not footwall ramps these may be some distance behind the steepened zone this thrust induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics

deformation of stratiform znpb barite deposits in the northern canadian cordillera abstract the major stratiform sediment hosted zn pb barite deposits of the northern canadian cordillera occur in the palaeozoic selwyn basin kechika trough they formed in euxinic fine grained clastic dominated extensional basins that developed within the carbonate platform of the passive margin of ancestral north america during the late jurassic to mid cretaceous these basins and the stratiform ore deposits were subjected to low greenschist facies regional metamorphism and fold and thrust deformation as they were incorporated into the western part of the rocky mountains mackenzie mountains foreland fold and thrust belt this paper summarises the results of detailed field and microscopic studies on four of these deposits the tom deposit the howards pass deposit the cirque deposit and the driftpile creek deposit the dominant mineralogy of these deposits is sphalerite galena pyrite and in some cases significant barite is present low greenschist grade metamorphism and deformation is characterised by cataclastic flow and brecciation in coarse grained massive pyritic sulphides and by folding recrystallisation and cleavage development in fine grained sulphides and barite relict primary depositional diagenetic structures are found within the orebodies deformation is characterised by strong pressure solution fabrics in barite in fine grained laminated pyrite layers and in mixed sulphide carbonate beds marked local transposition is found with significant remobilisation of galena barite and pyrite the deformation mechanics of these sulphide barite deposits are illustrated using schematic deformation mechanism maps

challenges of structural diagenesis in foreland fold and thrust belts a case study on paleofluid flow in the canadian rocky mountains west of calgary foreland fold and thrust belts are challenging because their structural complexity highly impacts fluid flow and diagenetic processes paleofluid flow in the rocky mountain fold and thrust belt west of calgary was investigated following a structural diagenetic approach paleofluid flow or mass transfer mechanisms have been inferred based on vein morphology completely filled en echelon slickenfibre bed parallel open or micro breccia veins orientation and geochemistry stable oxygen and carbon isotope and strontium isotopes bed parallel veins probably developed by flexural slip during laramide folding and most completely filled and slickenfibre veins formed or were reactivated during the laramide orogeny both types of veins associated with the laramide orogeny precipitated under low fluid flux conditions dominated by diffusional processes as inferred from stable isotopic signatures similar to the host rock the open veins with meteoric calcite cement marked by cathodoluminescence zonations and strongly depleted oxygen isotopic signature point to advectional fluid flow processes in a post laramide extensional stress field some slickenfibre and completely filled veins that originally formed during the compressional phase of the laramide orogeny were probably reactivated at time of the open vein formation the latter inference is based on the occurrence of the same cement in the open veins as in the centre of the slickenfibre and completely filled veins in conclusion there is no evidence for large scale fluid flow during the laramide orogeny but evidence points to post laramide large scale fluid advection being prevalent challenges in this structural diagenetic study consist of i the lack of abundant vein vein and vein stylolite cross cutting relationships in the outcrops studied ii the difference between structural versus diagenetic concepts iii the uncertainty of chronology and attribution of fracture sets and diagenetic cements to either a single or discrete episodes and iv reactivation of veins

an evaluation of models for the kinematic evolution of thrust and fold belts structural analysis of a transverse fault zone in the front ranges of the canadian rockies north of banff alberta abstract the prevailing piggyback conceptual model for the kinematics of thin skinned thrust and fold belts maintains that the main faults develop sequentially from the hinterland to the foreland and from the top to the bottom of the accretionary wedge moreover it presumes that when younger thrust faults originate overlying older thrust faults become inactive and are carried forward passively this appears to contradict the prevailing mechanical model for the evolution of thin skinned thrust and fold belts the critical coulomb wedge model which requires that lateral growth of the wedge must be accompanied by vertical thickening of the wedge crosscutting relationships along a transverse fault zone in the front ranges of the canadian rockies north of banff alberta and patterns of overprinting of thrust related folding on pre existing thrust sheets demonstrate substantial overlap in the times of displacement on four major thrust faults in this part of the front ranges the presumption that displacement on one major thrust fault ends when displacement on a younger underlying thrust begins is a fallacy there is no contradiction between the piggyback conceptual kinematic model and the critical coulomb wedge mechanical model for the evolution of thin skinned foreland thrust and fold belts the main faults do originate sequentially from the hinterland to the foreland and from the top to the bottom of the evolving wedge but displacement occurs simultaneously on several major faults

the cordilleran ribbon continent of north america the north american cordilleran orogen is the result of a two stage process a triassic jurassic accretion within panthalassa forming saybia a composite ribbon continent and b late cretaceous collision of saybia with north america this model requires that a large portion of the continental foreland of the orogen is exotic the exotic continental component of saybia cassiar platform is distinguished from the autochthon on the basis of its a triassic eurasian fauna b involvement in a major late triassic early jurassic orogenic event and c young in part grenvillian basement and mantle a mid cretaceous magmatic arc records west dipping subduction beneath the east margin of saybia the related accretionary prism consists of imbricated shale chert and deep water limestones the medial basin and overlies an isotopically juvenile mantle domain carbonatite complexes delineate the cryptic suture separating saybia and the autochthon paleomagnetic and paleobotanical data place saybia 2000 km to the south relative to the autochthon at 80 ma late cretaceous thrust belt development records transpression between the north moving ribbon continent and the autochthon pinning against the okhotsk chukotka arc in siberia buckled saybia giving rise to the alaskan promontory

mesozoic deformation in the nevada test site and vicinity implications for the structural framework of the cordilleran fold and thrust belt and tertiary extension north of las vegas valley detailed studies in the cp hills and mine mountain area of the nevada test site nts together with analysis of published maps and cross sections and a reconnaissance of regional structural relations indicate that the cp thrust of barnes and poole 1968 actually comprises two separate oppositely verging mesozoic thrust systems 1 the west vergent cp thrust which is well exposed in the cp hills and at mine mountain and 2 the east vergent belted range thrust located northwest of yucca flat regional structural relations indicate that the cp thrust forms part of a narrow sigmoidal belt of west vergent folding and thrusting traceable for over 180 km along strike the belted range thrust represents earlier mesozoic deformation that was probably related to the last chance thrust system in southeastern california as suggested by earlier workers a reconstruction of the pre tertiary geometry of the cordilleran fold and thrust belt in the region between the nts and the las vegas range bears a close resemblance to other regions of the cordillera and suggests that west vergent deformation developed in the hinterland of a part of the sevier fold and thrust belt characterized by substantial structural relief reconstruction of the fold and thrust belt also suggests that previous estimates of upper crustal tertiary extension north of the las vegas valley shear zone e g 80 guth 1981 are 2 or 3 times too large

mesoscale and regional kinematics of the northwestern yalakom fault system major paleogene dextral faulting in british columbia canada the northwestern yalakom fault system yfs lies on the eastern margin of the coast belt in southwestern british columbia the yalakom fault is the most significant structure in the fault system with at least 115 km of dextral slip mostly of early tertiary age the nw yfs is up to 25 km wide and is bounded on the southwest by the tchaikazan fault zone which had 5 km or more of dextral slip kinematic and paleostress analysis of mesofaults and map scale structures shows that secondary faults within the nw yfs had both dextral and sinistral oblique slip three large synclines had increasing amounts of clockwise vertical axis rotation from southeast to northwest block rotation between two synthetic strike slip faults with large offset yalakom and moderate offset tchaikazan is similar to current kinematics between the san andreas and san jacinto faults in southern california the nw yfs was active from latest cretaceous to mainly eocene time and postdates early late cretaceous thrust faulting to the southwest early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike slip faulting commenced the abrupt northwest termination of the yfs may be related to extension within the adjacent tatla lake metamorphic complex some dextral slip may have been transferred farther northwest through the coast belt but these structures have not been documented in west central british columbia most of the 120 km of dextral slip on the yfs was transferred to the north via a mosaic of normal and strike slip faults in central british columbia thus the yfs forms the southwestern margin of a broad zone of transtensional structures that extends to the tintina rocky mountain trench fault zone these structures accommodated northwestward displacement of the northwestern cordillera in paleogene time

outcrops as guides to subsurface natural fractures example from the nikanassin formation tight gas sandstone grande cache alberta foothills canada abstract outcrop studies are essential for understanding subsurface fractures because some key attributes such as length and height distribution cannot be measured using wellbores discrepancies between outcrop and subsurface fracture attributes arise because loading paths due to burial exhumation and thermal paths differ consequently even where rock type and structural setting are close matches differences may exist in mechanical properties and fracture growth here we investigate petrologic criteria and disseminated arrays of quartz filled microfractures as tools for comparing and contrasting fractures in outcrop with those as sampled by core for tight gas sandstones fracture cements are key diagnostic criteria for proving correspondence between outcrop and core we offer guidelines on selecting suitable outcrop analogs that differ markedly from current practice our illustration uses outcrops and core samples of late jurassic to early cretaceous nikanassin formation a tight gas sandstone but aspects of the outcrop vetting procedure can be applied to other rock types in some core and outcrops nikanassin formation fractures contain quartz and ankerite cement assemblages crack seal and rind textures and aperture size distributions that show that the outcrop and subsurface localities we examined share the same fracture stratigraphy and similar diagenesis these outcrops can thus be used to measure attributes including those that are unlikely to be captured in core such as fracture height and length distributions relative and absolute fracture intensity with structural position and degree of fill by synkinematic cement in our example absolute fracture intensity and distribution of postkinematic carbonate cements are site specific and differ between outcrop and core but in ways that can be quantified

duplex structures in the lewis thrust sheet crowsnest pass rocky mountains alberta canada abstract the lewis thrust sheet of the southern canadian rocky mountains contains many spectacular examples of small scale duplex structures this paper presents the results of a detailed analysis of such structures found in the mississippian carbonates of the banff formation at crowsnest pass southwestern alberta foreland dipping hinterland dipping and antiformal stacked duplexes are found in the hangingwall of the lewis thrust out of sequence thrusts back thrusts and folds that push out of the plane of the cross section termed lateral lobes give rise to complex internal geometries dominant slip vectors are towards 080 090 but the complex fault geometries have generated significant variations in slip away from this direction the duplex structures occur as discrete thrust fault bounded packages with each package having different slip vectors the panels above and below the duplex structures show consistent slip vectors towards 080 090 whereas the duplexes exhibit a wide scatter of slip vectors from 350 160 the stacking of duplexes with many horses can be likened to the stacking of many inverted soup bowls herein termed turtle back structures and will involve a wide scatter of slip directions particularly if the horses are of limited lateral extent such a stacking mechanism involving out of section movement invalidates the assumption of two dimensional plane strain in the plane of the cross section that contains the regional tectonic transport direction correctly balanced cross sections cannot be constructed through such stacked duplex structures as described in this paper

aspects of the three dimensional structure of the alberta foothills and front ranges ever more detailed subsurface images of the alberta foothills and front ranges obtained from improving seismic techniques coupled with an ever expanding base of information from deep wells and improved structural analysis techniques have produced an understanding of fold thrust deformation that is probably applicable to other fold thrust belts comprehensive subsurface data obtained by the petroleum industry are used to outline and analyze the three dimensional configuration of fold thrust deformation of paleozoic carbonate strata these data indicate that displacement changes smoothly along individual thrusts but displacement changes along strike may be large marked by considerable horizontal rotation 20 30 or more of hanging wall stratigraphic cutoffs relative to the footwalls stacking of thrusts with varying displacements has produced summed horizontal rotations of hanging wall cutoffs of as much as 45 50 the three dimensional trajectories of the thrusts through the stratigraphy are smooth for large distances interrupted by abrupt offsets at lateral and oblique ramps these are more common than previously realized the intersections of these ramps with other thrusts produce complex three dimensional links between thrusts with concomitant abrupt displacement changes some simple examples of these linkages are examined with a three dimensional geometric analysis the predictions of the analysis are consistent with structural geometries observed in two case studies of subsurface structures the links at lateral ramps imply simultaneous displacement on parts of the thrusts connected by the ramps since the ramps extend downdip 20 km or more displacement occurred simultaneously on structures occupying relatively interior and exterior positions in the thrust belt the smooth and long radius of curvature folding of many upper detachments indicated by reflection seismic data also is consistent with simultaneous or overlapping periods of motion on groups of thrusts distributed along regional dip most major alberta foothills thrust sheets terminate southward at lateral ramps at the paleozoic level the abrupt southwest northeast offsets at these features are most clearly apparent on a palinspastic reconstruction these offsets cause the paleozoic deformation front to step out east and northeast around the southern and eastern margins of the main arc of the foothills in contrast northward thrust terminations at the paleozoic level commonly occur at both tip lines and lateral ramps subsurface data reveal that there have been some misconceptions regarding large scale structural relationships for example the mcconnell and livingstone sheets are actually portions of the same thrust sheet as are the bighorn and nikanassin sheets when corrected for these misconceptions the data from the alberta foothills suggest that maximum displacement has a nonlinear relationship to thrust length although a linear relationship cannot be ruled out

continental collisions and seismic signature processes active during continental collisions and orogenic collapse create distinct structures which are imaged by reflection seismic profiling examples are shown and discussed

strain analysis and fold shape in a limestone layer and implications for layer rheology abstract folds are developed in thin limestone layers within slates of the mckay formation exposed to the east of the rocky mountain trench british columbia canada they possess geometrical characteristics expected of development by buckling strain in the profile plane of a selected fold is similar to that predicted by tangential longitudinal strain except that magnitudes are too low for the observed curvature this is attributed to inhomogeneity of strain on the scale of measurement largely because of pressure solution material removed by pressure solution from the inner arc of the fold appears to form veins perpendicular to the hinge a direction of tectonic stretching bedding parallel stylolites developed diagenetically prior to tectonism layer parallel shortening during the initiation of buckling was less than 20 and probably less than 10 the mean arclength thickness ratio is 6 5 and 7 1 with a dispersion of 0 48 and 0 37 for local and regional populations of 29 and 212 folds respectively application of buckling theory to this data suggests that folding followed a non linear flow law the viscosity contrast between limestone and slate would be higher and the power law exponent lower if initial irregularities in the layers were in the form of a constant amplitude spectrum rather than one of white roughness the data do not allow a choice of initial amplitude spectrum to be made nor do they closely constrain estimates of n the power law exponent and viscosity contrast deformation in the limestone layers was accommodated by intracrystalline flow twin gliding pressure solution and extensional veining the last two linked by diffusive mass transfer the first two dominated deformation in the profile plane of the fold and the last in association with fracturing allowed for extension parallel to the hinge experimental and theoretical considerations suggest that deformation by a combination of these processes should be non linear the non linear flow law deduced from buckling analysis is consistent with expectations based on observations of active deformation mechanisms

zebra dolomitization as a result of focused fluid flow in the rocky mountains fold and thrust belt canada discordant zebra dolomite bodies occur locally in the middle cambrian cathedral and eldon formations of the main ranges of the canadian rocky mountains fold and thrust belt they are characterized by alternating dark grey a and white b bands forming an abba diagenetic cyclicity these bands developed parallel to both bedding and cleavage dark grey a bands consist of fine 300 m non planar crystalline impure dolomite the white b bands are composed of coarse up to several millimetres milky white pure saddle dolomites b1 which are often covered by pore lining zoned dolomite b2 the b phases often possess a saddle shaped morphology in contrast to the replacement origin of the a dolomite the zoned b2 dolomite rims are interpreted as a cement formed in open cavities the b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non carbonate components all the zebra dolomites studied are nearly stoichiometric and are characterized by enriched na and depleted sr concentrations fe and mn concentrations in these dolomites differ depending on the sample locality fluid inclusion data indicate that the dolomites formed from relatively hot th 130 200 c saline 20 23 wt cacl2 eq fluids a diagenetic high temperature origin is also supported by depleted 18o values 20 to 14 vpdb a contribution of 87sr enriched fluids is reflected in the 87sr 86sr values 0 7091 0 7123 zebra dolomite development is explained by focused fluid flow which exploited areas of structural weaknesses e g basin platform rim areas faults etc expulsion of hot basinal brines in a tectonically active regime generated overpressures which explains the development of secondary porosity during zebra dolomitization as well as the intra zebra fracturing at decimetre to micrometre scale

lithoprobe crustal reflection cross section of the southern canadian cordillera 1 foreland thrust and fold belt to fraser river fault seismic reflection data from the south central canadian cordillera covering the interval from the easternmost metamorphic core complexes near arrow lakes to the fraser river fault system along the fraser river reveal a highly reflective and complex crust the base of the crustal reflectivity interpreted as the reflection moho is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12 0 s in the east to about 10 5 s in the west this virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust even though part of this area was subjected to large amounts of eocene extension the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the u s cordillera three structural culminations the monashee complex the vernon antiform and the central nicola horst are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during jurassic to eocene compression and then to have been modified and exposed during early and middle eocene extension an example of a compressional structure observed on the profiles is the monashee decollement which can be traced from the surface westward into the lower crust extension is manifested along a variety of normal faults including the regionally extensive low angle okanagan valley eagle river fault system moderately dipping faults such as the columbia river and slocan lake faults and high angle faults such as the quilchena creek and coldwater faults both jurassic to eocene compressional shear zones and early to middle eocene extensional shear zones are listric into the lower crust or moho under the intermontane belt

middle palaeozoic thrusting in the eastern lachlan fold belt southeastern australia abstract early to middle palaeozoic clastic and volcanic strata in the goulburn bungonia region 150 km southwest of sydney are divided by the n trending yarralaw fault into two domains in the western domain major e w shortening formed folds axial planar cleavage and w dipping contraction faults in all pre upper devonian units this deformation formed an anticlinorium cored by a w dipping thrust system with an overturned e younging limb in the eastern domain upright flattened chevron folds occur in ordovician strata with stratal repetition along steep contraction faults spaced at intervals of 1 km or less zones of tectonic melange occur in the hangingwalls of these faults and have a well developed scaly fabric with a steeply plunging striation overprinting a bedding parallel cleavage in mudstone in interbedded silurian shale and limestone a classical stair step trajectory is recognized for the frome hill fault which duplicates the succession and has undergone back rotation to its present steep dip overall thrusting progressed from east to west with deformation in the eastern domain pre dating intrusion of the early devonian marulan batholith whereas farther west deformation continued into the middle devonian the folds and thrusts in the goulburn bungonia region are part of a major fold thrust zone that extends throughout the eastern lachlan fold belt and which was formed in the middle silurian to middle devonian during a period of plate convergence between gondwana and the palaeo pacific ocean the fold thrust zone is inferred to have an arc frontal arc setting above a w dipping subduction zone and the deformation relates to underthrusting of an allochthonous terrane with major shortening in the hangingwall block

using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts abstract we investigate changes in shortening displacement and fold geometry to understand the detailed along strike structural variation within fold thrust belts and infer thrust growth and linkage mechanisms field observations from the vercors in se france are used to characterise deformation style in the region parallel cross sections are constructed analysed and used to create shortening and thrust displacement profiles from the northern to southern vercors sections show changes in structural style and shortening accommodation from thrust dominated in the north to fold dominated in the south the total shortening distance in the vercors does not change significantly along strike 3400 4650 m however displacements along individual thrust zones do vary significantly and displacement profiles show a range in displacement gradients 16 107 m km despite relatively simple shortening patterns in the vercors sections show a more complex 3d internal structure of the fold thrust belt thrust displacements and geometries suggest both large scale thrust zones and small scale thrusts are soft linked transferring displacement along strike through transfer zones short soft linked thrust segments indicate an intermediate stage of thrust growth and linkage well documented for normal fault systems which form prior to the formation of thrust branches and hard linked displacement transfer

structure and tectonic development of the southern rocky mountain trench the rocky mountain trench is one of the youngest most prominent and most enigmatic structures of the canadian cordillera approximately 650 km of seismic reflection data providing regional three dimensional coverage over an area of 10 000 km include six crossings of the rocky mountain trench between 49 n and 50 15 n prominent reflections from mid proterozoic moyie sills outline thrust and fold structures of a late jurassic to early cretaceous fault system that was cut by the rocky mountain trench fault in the tertiary the near basement reflections outline a 10 km high west facing basement ramp the hinge line of which spatially coincides with the rocky mountain trench in this area this ramp is part of a mid proterozoic margin upon which the belt purcell supergroup was deposited and is preserved beneath the trench during mesozoic contraction the basal detachment of the foreland belt closely followed the craton cover contact across the basement ramp thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp in the eocene miocene the basement ramp and the culmination above it focused stress reactivating the basal detachment and causing extensional faulting in the southern rocky mountain trench the rocky mountain trench fault may be linked via the basal detachment to the flathead fault on the east and the eocene extensional faults that flank the omineca belt on the west

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

stresses beneath a ramping thrust sheet abstract there is a systematic progression of both the timing of motion of thrust sheets and the spacing of thrust ramps during the development of some thrust dominated foreland fold and thrust belts on the basis of these observations we suggest that a regional mechanical process that controls frontal ramp formation may dominate local effects such as stratigraphic pinchouts and interaction with basement to investigate this process we have developed elastic plane strain models of the state of stress in the footwall of the last formed or most forward thrust ramp these models include the load of an overriding thrust sheet and a synorogenic sedimentary cover the effect of emplacing a thrust sheet or developing a synorogenic sedimentary cover above an incipient thrust sheet is to stabilize the footwall toward the hinterland with respect to failure and thereby move failure more to the foreland failure is localized where there is a favorable trade off between the spatial rates at which mean stress and differential stress decrease toward the foreland the models also predict that frontal ramps nucleate well above the basal decollement and propagate upward and downward eventually linking with the regional decollement fault trajectories predicted by using a coulomb criterion are subhorizontal in the hinterland portion of the model and curve upward to the foreland resulting in a curved flat and ramp geometry these analyses show that each successive thrust sheet would be thinner as well as shorter than the previous thrust sheet based on these results we propose a model for the formation of the first thin skinned thrust sheet in the external portions of orogenic belts the synorogenic sediments that form during uplift of the internal zones of mountain belts will cover the preorogenic strata to the foreland as deformation progresses to the foreland and deformation becomes more shallow and more brittle in nature the preorogenic strata will interact with the synorogenic strata and failure will occur creating the first brittle thrust sheet the location of this first thrust will be controlled by the mechanisms outlined in our models to the foreland of this point each sucessive thrust fault will localize as a result of the combined effects of an overriding thrust sheet and synorogenic deposits

the structure and tectonic evolution of the aguilon fold nappe sierra alhamilla betic cordilleras se spain abstract detailed structural work in the sierra alhamilla se spain shows that the aguilon nappe comprising triassic and older metasedimentary rocks is a fold nappe the most prominent set of small scale folds changes from dominantly n vergent in the upper right way up limb to s vergent in the greatly thinned lower limb the nappe closes to the north and must have been emplaced in this direction nappe formation was accompanied by small scale folding and extensive solution transfer producing a pronounced differentiated crenulation cleavage these structures overprint an earlier set of folds and cleavage and are overprinted in turn by late n vergent structures the lower limb of the nappe was thinned and disrupted during continued nappe transport pre triassic schist in the core of the nappe was affected by medium grade metamorphism of probable post triassic age the contact with low grade permo triassic sediments above and below the schist coincides with a distinct change in metamorphic grade this contact may be a post metamorphic extensional fault that is now folded around the nappe which suggests that nappe formation was preceded by extensional faulting this is consistent with gravity spreading as the driving process for nappe emplacement

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

a dual doubly vergent orogen in the banda arc continent arc collision zone as observed on deep seismic reflection profiles new deep seismic reflection profiles across the banda arc of indonesia reveal reflectors in the uppermost 50 km of the lithosphere combined with existing earthquake hypocenter locations and focal mechanisms the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m y and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge a clearly defined wadati benioff zone and recent deformation of shelf sediments observed on older shallow seismic profiles indicated to previous workers that substantial convergence occurred at the timor trough a few focal mechanisms seafloor escarpments and recent geodetic surveying indicate that convergence at 7 cm yr 1 currently last 200 kyr occurs at the northern margin of the now inactive volcanic arc the wetar thrust zone reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the timor trough and wetar thrust specifically basement reflectors beneath the toe of the accretionary complex have reverse sense offsets that imply blind thrusts the whole crust is horizontally shortened not only the sedimentary cover rocks that previously deformed into duplexes above a decollement reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting thus each arc represents a doubly vergent fold and thrust belt but only the northern one is currently active crustal thicknesses inferred from seismic velocities reflectors and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3 4 km of topographic relief in the region of eastern timor

crustal evolution of the eastern canadian cordillera the cordillera is not like the himalayas the himalayas are supported by the indian continent that was thrust under the asian continent the eastern cordillera is supported not by imbricated continents but by imbricated continental margin of the ancient western north america that ancient margin was compressed and imbricated as it was confined between the north american craton and the minicontinent of stikinia in the eastern canadian cordillera tectonic thickening is in the collision zone and not on either side of it as for example in the tibetan plateau where the underthrust indian continent has duplicated the thickness of the crust in the eastern canadian cordillera jurassic and early cretaceous thrusts and faults are directed away from the collision zone toward the converging blocks the oppositely directed thrust faults outline wedge shaped sheets of rock sedimentary and volcanic rocks cover the crystalline rocks on which they were deposited basement and the underlying upper mantle may have been detached from each other during the compression the basement therefore would have imbricated internally between the cover and the mantle as a duplex structure confinement of the ancient margin s basement between the colliding blocks requires that the volume of basement under the margin be the same as that presently between the colliding blocks the possibility of such an equality has been tested and confirmed in cross section the cross section of the ancient western margin from that test has many features comparable to cross sections recently drawn through the eastern continental margin of canada

some elements of continental subduction along the himalayan front abstract detachment tectonics in which the subducting basement is not internally affected by thrusting and a steady state system in which the pattern of deformation and topography are invariant uplift is balanced by erosion and the material can experience subsequent phases of deformation by migrating through the system are proposed to be the fundamental mechanisms of convergence at the himalayan arc both surface and subsurface data are more consistent with these concepts than with more familiar concepts widely used in current models intracrustal thrusting and evolutionary tectonics where superimposed phases of deformation are interpreted as distinct phases the belt of intermediate magnitude thrust earthquakes the topographic front at the high himalaya and the main central thrust mct are associated with the same fundamental element the basement thrust front btf which traces a small circle in the central portion of the himalayan arc fault plane solutions indicate thrusting in the radial direction of this arc this radial convergence at the btf implies that tibet is extending laterally at a rate similar to the rate of convergence across the btf this extension cannot be unidirectional and must be parallel to the btf if the circular shape of the btf is invariant

magnetic models of the moyie anticline purcell anticlinorium southeastern canadian cordillera two dimensional magnetic models of the moyie anticline contribute to an independent project targeting development of a regional three dimensional 3d model of much of the purcell anticlinorium by compensating for shortcomings in reflection seismic signatures and distributions of drill holes and surface structural data modelling has focused on prominent anomalies related to relatively strong magnetizations within the creston formation and moyie sills models of the western flank of the anticline indicate the creston formation forms the bulk of a westward thickening wedge shaped sedimentary package whose west dipping layers are truncated by a steeply west dipping moyie fault truncating also moyie sills on the opposite side near surface dips of the fault range from 54 to 87 stratigraphic comparisons across the fault indicate minimum reverse dip displacements ranging from 5 6 to 8 5 km the eastern flank of the moyie anticline is modelled consistently as a monoclinal eastward dipping stratigraphic pa

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

structural divergence and transpression in the teslin tectonic zone southern yukon territory the structural and tectonic evolution of the teslin tectonic zone a complex belt of ductilely deformed rocks forming the southern extension of the yukon tanana terrane in the northern canadian cordillera is elucidated with new field data and by the reinterpretation of existing data the zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the nisutlin and anvil assemblages characterized by s and l s tectonite fabrics primary contact relationships and ages show that most rocks in the zone were contiguous by mississippian time mapping identified a number of strain domains which preserve s fabrics dipping northeasterly or southwesterly variable mineral lineation orientations and a variety of shear directions including easterly and westerly vergent thrust shear down to the east and down to the west normal shear and dextral strike slip shear regional constraints and microstructures suggest that latest ductile deformation was late triassic to early jurassic in age structural characteristics are explained most effectively by convergent dominated transpression in combination with tectonic wedging and associated back thrusting above a low angle west dipping detachment oblique eastward and upward movement of the rocks over the detachment which may have coincided with the top of thinned north american continental crust produced easterly vergent shear and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear the transpressive deformation postdates formation of the nisutlin assemblage by mississippian time and subduction of the slide mountain ocean beneath the nisutlin assemblage in permian to early triassic time the western margin of the teslin tectonic zone was truncated by cretaceous strike slip faults and translated northward rocks of the peraluminous orthogneiss assemblage of the yukon tanana terrane in central yukon acted as the tectonic wedge and were overthrust by rocks of the nisutlin and anvil assemblages in early jurassic time before unroofing by local extension in cretaceous time

proterozoic craton to basin crustal transition in western canada and its influence on the evolution of the cordillera deep seismic reflection data have imaged a crustal scale west facing ramp or ramp system in the subsurface of western canada in northwestern canada the ramp is within proterozoic crust east of the cordillera and is unconformably overlain by paleozoic sedimentary rocks indicating that it was formed during the proterozoic in this region similar structures are visible within the cordillera in southern canada and the northwestern united states along a south projection of the ramp observed in the north in the monashee complex of british columbia and in the priest river complex in northern washington reflections are visible that dip westward from the surface to near the base of the crust and are structurally discordant with underlying more horizontal reflections thus outlining footwall ramps we propose that the proterozoic crustal ramp in the north and the footwall ramps in the cordillera probably coincide with a proterozoic crustal transition from thick craton on the east to thin crust on the west thi

including strain data in balanced cross sections abstract almost all published balanced cross sections include cleaved and or strained rocks and may therefore violate the requirement that bed lengths and formation areas are preserved between undeformed and deformed states knowledge of distortions presence and type of cleavage and overprinting relations in poly deformed segments are necessary to define area changes related to volume loss strike elongation and poly deformation work in the southern and central appalachians has shown variation in structural complexity and changing deformation styles both along and across the thrust belt this necessitates separate treatment of individual sheets during balancing regions of little or no interstratal slip pinning points should be chosen for each thrust sheet and each fold strain in the sheet in external sheets where deformation intensity increases towards thrusts pin points should be back limb inflection points in intermediate and internal sheets where cleavage is approximately coeval with folding pin points should be at fold hinges where cleavage and strain are heterogeneously developed throughout a stratigraphic sequence a line length balance using massive relatively uncleaved beds e g knox group southern appalachians can be used as a basis for restoring the section unstraining of deformed layers given strain data and cleavage trajectories for individual thrust sheets provides line lengths and formation areas which can be fitted to the key bed of the undeformed section cleavages which partially or fully postdate folding indicate continued shortening in the thrust wedge and modify earlier pinning points in upper sheets these cleavages and poly deformed segments are difficult to restore as strain data are generally scarce in the metamorphosed internal thrust sheets

structural evolution of the argentine precordillera the rio san juan section abstract the argentine precordillera ap represents a high level fold and thrust belt which was formed during the andean tertiary crustal shortening in a w e transect studied mostly e directed imbrications are combined with folding and involve a pile of cambrian to tertiary sediments the thrust belt is detached above a main decollement within the ordovician to lower devonian strata to the east it is bounded by a back thrust zone directed westwards restored sections allow an estimate of crustal shortening and also provide insight into the structural development during pre tertiary times carbonate platform sequences of cambrian to ordovician age grade into a volcani clastic succession with transitional melanges representing deposits along a western continental slope in the western and easternmost parts of the ap cambrian to silurian deposits are affected by a first folding which to the west is combined with a slight metamorphic overprint during carboniferous to permian times intensified mobility along the eastern rim and in the western parts of the ap leads to crustal extension which partly was triggered by the formation of synsedimentary normal faults in the central ap no angular unconformities at the base of carboniferous permian deposits were found there was no field evidence found to postulate a terrane boundary within the ap area and an onset of thrust tectonics during cretaceous to early tertiary times

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

a reappraisal of the structure of the spanish segment of the iberian pyrite belt the major structural features of the iberian pyrite belt are described in terms of geometry deformation mechanisms scale timing kinematics and the mutual relationships among the various architectural elements the result of such an analysis allows this zone to be considered as a s verging thin skinned fold and thrust belt propagating southwards over a mid crustal basal detachment this was the response in the footwall of the suture to the major phase of hercynian oblique collision between the south portuguese plate and the ossa morena zone of the iberian autochthon this thin skinned event inverted a previous extensional structure acquired during the initial stages of the collisional process and intimately linked to the formation of the ore deposits that make this region a world class metallogenic province

three dimensional horizon reconstruction from outcrop structural data restoration and strain field of the baisahi anticline western nepal abstract within many fold and thrust belts preservation of bed thickness is generally assumed during deformation a three dimensional 3d reconstruction method is proposed from outcrop structural data this technique accounts for along strike thickness variations by a series of 3d spatial projections of dip and strike data the thickness of strata from any point at the ground surface to reference horizons on stratigraphic columns can be calculated across strike and projected along the poles of the associated strata providing sets of dots in space which represent folded horizons the technique is applied to the baisahi antiform located on the frontal subhimalayan thrust an along strike deformation gradient is evidenced by two dimensional cross section balancing which shows a passive roof duplex structure the 3d geometry of the roof is reconstructed allowing other approaches such as unfolding to be done the residual strain field induced by restoration shows distortion features which are interpreted as internal strain due to the propagation of the fault

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

the reflection moho beneath the southern canadian cordillera the transition from the crust to the mantle beneath the canadian portion of the north american cordillera varies in depth geometry and tectonic age across the orogen these variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data the moho in this region is thus interpreted to be a long lived feature perhaps as old as proterozoic in the eastern part of the cordillera that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching recognition of the moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen they likely project to the base of the crust where they flatten and

controls on c s ratios in hemipelagic upwelling sediments the relationship between organic carbon and reduced sulfur concentrations was investigated in hemipelagic sediments underlying highly productive regions of the ocean and oxic to suboxic waters the benguela upwelling area peru and oman margins at all three sites the degree of pyritization averaged close to 0 4 indicating that reactive iron was probably not limiting for pyrite formation complete sulfate reduction occurred in the upper sections of the sediments from the oman and peru margins however dissolved sulfate never decreased below 5 mm in the benguela upwelling sediments the authors attribute this to more advanced organic matter degradation at the benguela upwelling site c s ratios increased with increasing organic carbon concentrations at all sites for sediments with greater than 3 wt toc the c s ratio averaged about 1 3 and 1 8 times the normal marine ratio of 2 8 at the benguela upwelling and peru margin sites respectively but 2 9 times this value in the oman margin sediments they hypothesize that the quality of the organic matter undergoing burial and rates of near interfacial transport processes are the dominant controls on sulfur burial in these sediments

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

travertine mounds of the cave and basin national historic site banff national park the cave and basin national historic site is a fan shaped travertine deposit associated with four thermal spring outlets tentative age dating of the travertine mound indicates growth initiated with onset of the late holocene shift to more humid and cool climate conditions and suggests that the flow of thermal waters was limited during the hypsithermal which in turn places constraints on the evolutionary biology of endemic species in the spring system two large caves and one collapsed cave structure are developed within the deposit cave development is in response to both physical erosion of till underlying the travertine and acid gas attack of calcite that makes up the deposit this process is buffered by formation of reaction crusts of gypsum on the interior cave walls only minor modern travertine growth occurs due to historic flow control measures understanding the flow of water through the historic site is critical for long term preservation

the geometry of folded and thrusted rocks in the rocky mountain foothills near grande cache alberta lower cretaceous clastic rocks make up the inner foothills of the rocky mountains along the smoky river near grande cache alberta shortening of these rocks is accomplished by folding and thrusting and averages 33 over the area studied folds in the area are of the chevron variety they are caused by shortening of a multilayered sequence of alternating competent and incompetent strata where the thickness of the competent layers is fairly constant the folds are generally cylindrical and maintain their shapes over distances of up to 2 km along trend however at their tapering ends they are conical the majority of the faults are southwest dipping thrusts displaying ramps that cut up stratigraphic section and flats that are parallel to bedding the best exposed fault in the area is the mason thrust displacements along this fault increase from 150 m in the northwest to 1500 m in the southeastern part of the area a prominent anticline below the mason thrust diminishes gradually towards the southeast a conical fold with a half apical angle of 7 describes this geometry shortening by this fold decreases from about 30 in the northwest to 10 in the southeastern part of the area this indicates that a decrease in shortening by folding is compensated by an increase in thrust fault displacement it is an example of fault to fold displacement transfer

area length and thickness conservation dogma or reality abstract the basic assumption of quantitative structural geology is the preservation of material during deformation however the hypothesis of volume conservation alone does not help to predict past or future geometries and so this assumption is usually translated into bed length in 2d or area in 3d and thickness conservation when subsurface data are missing geologists may extrapolate surface data to depth using the kink band approach these extrapolations preserving both thicknesses and dips lead to geometries which are restorable but often erroneous due to both disharmonic deformation and internal deformation of layers first the bolivian sub andean zone case is presented to highlight the evolution of the concepts on which balancing is based and the important role played by a decoupling level in enhancing disharmony second analogue models are analyzed to test the validity of the balancing techniques chamberlin s excess area approach is shown to be on average valid however neither the length nor the thicknesses are preserved we propose that in real cases the length preservation hypothesis during shortening could also be a wrong assumption if the data are good enough to image the decollement level the chamberlin excess area method could be used to compute the bed length changes

comment on linking the mesoproterozoic belt purcell and udzha basins across the west laurentia siberia connection abstract paleogeographic reconstruction of the northern siberian craton against southwestern laurentia in accordance with paleomagnetic data basement piercing points and the best fit of the rift margins aligns mesoproterozoic dike and sill swarms in northeast siberia with correlative ones in montana and wyoming it also juxtaposes the mesoproterozoic belt purcell basin of west laurentia against the mesoproterozoic udzha basin of the northern siberian craton we review the veracity of this hypothetical belt purcell udzha basin as a test of our siberia west laurentia reconstruction various elements of the structural framework of the basins are closely aligned in the reconstruction the udzha trough is aligned with the st mary moyie fault zone and vulcan basement structure of southwestern canada the khastakh trough is aligned with the helena embayment of central montana parts of the belt purcell supergroup appear to correlate with parts of the riphean section in northeastern siberia however the siberian section appears to be much thinner than the belt purcell section and precise correlation is not possible with present stratigraphic and geochronological data the reconstruction leads to the prediction that the udzha rift channeled sediment from a cratonic pediment into the delta and alluvial fan complex in the deep belt purcell rift basin distributary channels may have shifted within the udzha basin to feed shifting depocenters in the belt purcell basin details of age and tectonic evolution for the udzha basin are less clear than for the belt purcell basin but we outline specific geological relationships predicted by the reconstruction model and suggest tests for future research

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

structural evolution of basement gneisses and hadrynian cover bulldog creek area rocky mountains british columbia two gneiss bodies are contained in thrust sheets on the west edge of the rocky mountain main ranges near valemount british columbia the bulldog gneiss comprises aphebian or older paragneiss and amphibolitic gneiss intruded by aphebian orthogneiss sheets the yellowjacket gneiss is granodioritic orthogneiss of unknown age both gneiss bodies are basement highs with thin hadrynian metasediment cover sequences the cover sequences are assigned to the lower miette group and are correlated with horsethief creek group internal shortening of gneiss thrust sheets is expressed by recumbent folding and stacking of thin thrust sheets of gneiss and cover the bulldog gneiss and its cover were carried on the postmetamorphic purcell thrust the yellowjacket gneiss and its cover were carried on the pre to synmetamorphic bear foot thrust northeast and northwest displacement is documented on the moderately southwest dipping bear foot thrust and a dextral oblique slip thrust model is proposed to explain the duality

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

investigation of the martensitic like transformation from mg2geo4 olivine to its spinel structure polymorph a coherent mechanism for the transformation of olivine to its spinel structure polymorph has been investigated in mg 2 geo 4 olivine at conditions ranging from 6 gpa to 18 gpa at 300 to 1250 c using a multi anvil apparatus this transformation mechanism which produces plates of spinel lying parallel to the 100 planes in olivine becomes increasingly important above 10 gpa at temperatures between 300 and 400 c the primary effect of temperature above 400 c is to increase the length of the lamellae the primary effect of pressure is to increase the number of lamellae that have nucleated the distribution of transformation is very inhomogeneous lamellae first appear in patches associated with grain edges and corners as the degree of transformation increases patches become more extensive and more heavily populated until the majority if not all grains contain large numbers of lamellae that span large portions of their host grains the mechanism that produces the lamellae requires three factors high pressure overstep of the equilibrium boundary some level of differential stress either microscopic or macroscopic and high enough temperature to facilitate rearrangement of the cations it is most consistent with a martensitic like transformation mechanism the results of this study suggest that olivine may exhibit similar behavior in cold rapidly subducting lithospheric slabs if conditions are too cold for olivine to transform by reconstructive nucleation and growth it could instead progressively transform by the martensitic like mechanism as conditions deviate further and further from the equilibrium phase boundary

development of normal faults during emplacement of a thrust sheet an example from the lewis allochthon glacier national park montana u s a abstract geologic mapping in southern glacier national park montana reveals the presence of widespread e dipping normal faults within the basal portion of the lewis allochthon the displacement along the normal faults increases downward from less than 1 m at the highest exposure 200 300 m above the lewis thrust to a maximum of 200 m near or at the lewis thrust the normal faults are located below discrete bedding parallel shear zones associated with mesoscopic structures characterized by ne or sw trending striations on bedding surfaces and asymmetric e verging folds these shear zones lie directly below the e directed brave dog fault a major bedding subparallel fault within the lewis allochthon the shear zones are interpreted to have formed during the development of the brave dog fault striations on the brave dog fault normal faults and shear surfaces in the shear zones are consistent with the transport direction of the lewis thrust the kinematic compatibility of the normal faults with the lewis thrust the concentration of the normal faults along the basal part of the lewis plate and the increase in displacement along them toward the lewis thrust all suggest that their development was synkinematic with eastward emplacement of the lewis allochthon the normal faults may have formed as riedel shears r that accommodated a bulk simple shear strain within the thrust plate between the simultaneously moving subhorizontal brave dog and lewis faults

detachment levels in the marathon fold and thrust belt west texas surface and subsurface data are integrated to characterize the structural architecture of the marathon fold and thrust belt in west texas multiple detachment levels are present within the thrust belt and result in distinct structural domains in addition to the basal decollement whose stratigraphic position varies along strike we recognize a regionally extensive detachment zone in the late mississippian to early pennsylvanian lower tesnus formation the lower tesnus detachment forms a structural domain boundary that can be observed along strike in the surface data and at depth in the subsurface the stratigraphic intervals above and below this detachment exhibit characteristic patterns of deformation the lower tesnus detachment is folded by imbrication and the formation of duplexes in the early mississippian to ordovician section suggesting that the detachment may have initially formed as a perched decollement in the foreland that was subsequently exploited as a roof thrust in a duplex system as deformation progressed in a break forward sequence and older strata were incorporated into the toe of the allochthonous wedge the structural model presented here for the marathon region may be applicable across much of the ouachita orogenic system

sequence of thrusting and origin of culminations in the northern and central oman mountains abstract detailed mapping and structural analysis of three large scale culminations sumeini and asjudi half windows and haybi hawasina window in the oman mountains shows a considerably more complex history of deformation than a simple foreland or downward sequence of thrust development early thrusting processes tended to create a regular stacking order of imbricate slices and major thrust sheets complying with the rules of thrust propagation assembled progressively downwards and forwards in the direction of translation out of sequence thrusts can also be demonstrated in places by truncation of footwall structures folds imbricate slices etc gross strain differences between thrust sheets downward facing structures in footwall units and elimination of thrust sheets beneath late stage thrusts frequently cut up section through the previously assembled stack putting previously younger lower thrust sheets over previously older higher ones many of the culminations in the northern and central oman mountains were formed by ramping associated with this late stage leap frog rethrusting event

fault dating in the canadian rocky mountains evidence for late cretaceous and early eocene orogenic pulses comment comment the bold assertions by van der pluijm et al 2006 1 do not survive close scrutiny the technologically advanced procedures employed involve great ingenuity but the results of the fault dating are suspect because 1 2 1 beaumont c 1981 foreland basins geophysical journal

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

convection tectonics some possible effects upon the earth s surface of flow from the deep mantle until a little more than a century ago the land surface not only was the only part of the earth accessible to humans but also was the only part for which geophysical and geochemical methods could then provide any details since then scientists have developed ways to study the ocean floors and some details of the interior of the earth to ever greater depths these discoveries have followed one another more and more rapidly and now results have been obtained from all depths of the earth new methods have not contradicted or greatly disturbed either old methods or old results hence it has been easy to overlook the great importance of these recent findings within about the last 5 years the new techniques have mapped the pattern of convection currents in the mantle and shown that these rise from great depths to the surface even though the results are still incomplete and are the subject of debate enough is known to show that the convection currents take two quite different modes one of these breaks the st

speculations on the petroleum geology of the accretionary body an example from the central aleutians abstract in the 300 km wide adak amlia sector of the central aleutian trench 36 000 km 3 of offscraped trench fill makes up the wedge shaped mass of the aleutian accretionary body within this wedge seismic reflection profiles reveal an abundance of potential hydrocarbon trapping structures these structures include antiforms thrust and normal faults and stratigraphic pinchouts maximum closure on these features is 2 km in addition the silt and possibly sand size sediment within the offscraped turbidite deposits and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge may define suitable reservoirs for the entrapment of hydrocarbons potential seals for these reservoirs include diagenetically altered and produced siliceous and carbonate sediment the organic carbon input into the central aleutian trench based on carbon analyses of dsdp legs 18 and 19 core samples suggests that the average organic carbon content within the accretionary body is approximately 0 3 0 6 heat flow across the aleutian terrace indicates that at present the oil generation window lies at a depth of 3 6 5 km at depths of 8 km which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope the probable high 170 180 c temperatures prohibit all but gas generation the dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body these fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage in the aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist the size and ultimate preservation of these deposits however are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes