the bristol mendip foreland thrust belt the thrusts and associated fold structures of the bristol mendip area are interpreted in terms of a thin skinned piggy back foreland thrust belt of variscan age with dominant northward transport strike normal balanced sections and compatible strike parallel sections have been constructed to illustrate the 3 d geometry of the linked thrust system north south shortening of c 20 km 40 is calculated from deformed and restored strike normal sections a thick and extensive thrust wedge of pre old red sandstone basement in the west of the area is responsible for culminations expressed at the surface as the mendip periclines uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area strike parallel sections and a hanging wall cut off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps or flats the en echelon pattern of the mendip periclines may be related to the oblique ne climb of two of these thrusts the angle of frontal ramp climb through the dinantian stratigraphy is generally low and this explains the presence of numerous klippen of lower on upper carboniferous rocks whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate early pre thrusting movement on basement lineaments in the variscan foreland created uplifts angular unconformities and a predominance of n s structures in the north of the area

the mid paleozoic deformation in the hazen fold belt ellesmere island arctic canada the mid paleozoic deformation of lower paleozoic subgreenschist facies sediments of the hazen fold belt in northern ellesmere island is represented predominantly by chevron style folding folded multilayers display cleavage fans suggesting synchronous fold and cleavage formation bedding parallel slip indicates a flexural slip mechanism of folding the geometry of several large scale anticlinoria has been interpreted as being due to formation of these structures over detachments and thrust ramps the constant fold geometry the parallel orientation of faults and large and small scale folds and the axial plane foliation are related to a single phase of folding with a migrating deformation front in the hazen fold belt during the mid paleozoic orogeny the minimum amount of shortening in the hazen and central ellesmere fold belts has been estimated from surface geology to increase from 40 50 of the original bed length in the external southeastern part to 50 60 in the more internal northwestern part of the

late cretaceous paleomagnetism of the tombstone district and vicinity evidence for a rotational domain boundary in southeastern arizona paleomagnetic data are presented for late cretaceous volcanic and intrusive rocks in the tombstone district and dos cabezas mountains of southeastern arizona the resultant paleopole 73 n 224 e a95 8 is similar to a late cretaceous reference pole for north america 82 n 210 e a95 7 implying that these rocks have not been significantly rotated about vertical or near vertical axes relative to the continental interior generally concordant results have also been published for pennsylvanian terra rosa paleosols within the black prince limestone at localities in the nearby gunnison hills whetstone mountains and naco hills together however these carboniferous data indicate a 9 7 clockwise rotation with respect to north america similar to that determined for the colorado plateau 5 14 since the mid cretaceous published paleomagnetic data for rocks south and west of the tombstone vicinity indicate much larger clockwise rotations ranging from 25 to 60 implying that a rotational domain boundary runs between the mustang mountains and canelo hills to the southeast and between the silver bell and tucson mountains to the northwest this boundary roughly coincides in part with the sawmill canyon fault zone and is parallel to other northwest trending strike slip faults in the region that were active during the laramide orogeny the clockwise rotations in southern arizona are likely part of a larger pattern of dextral shear along the western margin of north america during the late cretaceous and early tertiary caused by northward oblique subduction of oceanic plates relative to the stable craton

limits on the age of the deccan traps of india from paleomagnetic and plate reconstruction data and their uncertainties revised limits on the age of the deccan traps of india are provided by recent improvements to reconstructions of india relative to surrounding plates since late cretaceous time realistic estimates of the uncertainties in the plate reconstructions and recent paleomagnetic data from the deccans traps and from north america we estimate the age of the deccan traps by rotating its paleomagnetic pole into the north american reference frame different assumed ages for the deccan traps result in very different positions for its paleomagnetic pole relative to north america comparing these positions with the north american apparent polar wander path and accounting for uncertainties in both the paleomagnetic poles and in the reconstructions we place limits on the age of the deccan traps of 62 72 ma significantly younger than found in several prior studies the 62 72 ma limits are consistent with ages obtained independently from radiometric dating and paleontology and are consistent with the hypothesis that the deccan traps were extruded rapidly during reversed polarity chron 29r which contains the cretaceous tertiary boundary

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

the elkhorn mountains revisited new data for the late cretaceous paleomagnetic field of north america the late cretaceous portion of north america s apparent polar wander apw path is poorly known due to a dearth of suitable post 85 ma reference poles a paleomagnetic re investigation of elkhorn mountains volcanics 80 ma was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of apw during this time interval correlates with known late cretaceous tectonic events a total of 249 paleomagnetic samples 30 sites were collected from three separate areas within the elkhorn mountains using both alternating field and thermal demagnetization a characteristic direction of magnetization was isolate at 15 sites a95 15 which passes a fold test at the 95 confidence level the mean pole calculated from site virtual geomagnetic poles is located at 80 3 n 189 5 e n 15 a95 9 6 k 16 8 interestingly the elkhorn mountains pole is not concordant with the cretaceous stillstand poles but is nearly coincident with the paleocene reference pole and the recently published pole for the adel mountains dated at 76 ma these new data suggest the onset of apw which brought the pole from its cretaceous position to near its early tertiary position occurred prior to 80 ma and the transition was exceedingly rapid the onset of this rapid apw now appears to coincide more closely in time with many of the significant late cretaceous tectonic events e g the laramide orogeny however since the elkhorn mountains volcanics are floored by a major east directed thrust and have undergone crustal scale deformation the data should be viewed with caution until more results are available for this time interval

the calculation of bulk strain in oblique and inclined balanced sections abstract the calculation of bulk strain from balanced sections is a technique commonly used in the study of orogenic belts the validity of a balanced section is often enhanced by controls such as seismic and or borehole data the construction of a conventional vertical section perpendicular to orogenic strike i e in the transport direction may not allow control data to be directly incorporated into the section the control data can be projected into the plane of the section or alternatively the section may be constructed in an orientation such that it contains the available control data in the case of a vertical section constructed at an oblique angle to the transport direction the bulk strain value obtained is a minimum and can be corrected to the true value of bulk strain in the perpendicular vertical section by a simple trigonometric calculation if the section is perpendicular to strike but inclined at an angle to the vertical the correction of the bulk strain values is mathematically unique to the section this problem can be overcome by using a digitizing table in conjunction with a simple computer program the general case of a section both oblique to the strike direction and inclined to the vertical can be resolved by removing the effects of the inclination to produce a vertical oblique section from which the bulk strain can be obtained and corrected in realistic geological problems the oblique section will be a more common problem than the inclined section in both cases deviations of 10 and less introduce such small errors in the estimates of bulk strain that their effects can be ignored

a balanced section across the pyrenean orogenic belt evolutionary models that have been proposed for the pyrenean orogenic belt involve gravitational gliding of major thrust sheets to both the north and south of an uplifted central axial zone it has been suggested by previous workers that uplift is a result of alpine compression or long lived strike slip movement on the north pyrenean fault we critically assess these models and propose a new model based on the generation of a thin skinned linked thrust system due to alpine collision of iberia and europe pyrenean alpine tectonics is dominated by southward thrusting on a major sole fault dipping north at approximately 6 within hercynian basement northerly directed thrusts of the north pyrenean zone and northern folded foreland are backthrusts from this sole fault a total orogenic shortening of 106 km in the central pyrenees was achieved between the palaeocene and oligocene at a time averaged rate of 0 62 cm yr 1 a thin skinned model requires that the north pyrenean fault is truncated by both northward and southward moving thrusts and occurs in the footwall of the sole fault some 60km north of its present surface trace

paleomagnetism of jurassic volcanic rocks in the patagonia mountains southeastern arizona implications for the north american 170 ma reference pole paleomagnetic study of middle jurassic volcanic and volcaniclastic rocks in southeastern arizona yields a paleomagnetic pole located at 61 8 n 116 0 e k 49 6 95 6 2 these rocks from corral canyon represent an autochthonous segment of the jurassic cordilleran magmatic arc and were deposited directly on upper paleozoic carbonates of the cratonic miogeocline rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism rb sr isotope geochemistry on whole rock samples from eight welded ash flow tuffs yields an isochron age of 172 2 5 8 2 ma the corral canyon pole falls between poles from newark trend group ii 179 3 ma and glance conglomerate 151 2 ma

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

proglacial lakes character behaviour and geological importance proglacial lakes are ubiquitous within the quaternary record and can provide exceptional breadth and depth of palaeoenvironmental information present deglaciation is increasing the number and size of proglacial lakes around the world this study provides a synthesis of knowledge on proglacial lake character and behaviour and critically evaluates the importance of proglacial lakes from a geological perspective we show how ice marginal or ice contact lakes and other distal proglacial lakes can be distinguished from each other by geomorphological sedimentological chemical and biological characteristics the key controls on proglacial lake geomorphology and sedimentology are outlined and discussed proglacial lakes can exacerbate mountain glacier and ice sheet margin ablation via mechanical and thermal stresses but very large lakes can moderate summer air temperatures and relatively retard summer ice ablation proglacial lakes interrupt meltwater flux and are very efficient sediment traps hydrological routing and consequent geomorphological activity can be radically modified by sudden drainage of proglacial lakes and resultant glacial lake outburst floods exceptionally large proglacial lake drainages affected global ocean circulation and global climate during the quaternary overall analyses of proglacial lakes can provide a valuable insight into i patterns character and behaviour of mountain glaciers ice sheets and glaciations and ii the impacts of past present and future deglaciation

deciphering late devonian early carboniferous p t t path of mylonitized garnet mica schists from prins karls forland svalbard this is the peer reviewed version of the following article ko minska k spear f s majka j faehnrich k manecki m piepjohn k dallmann w k 2020 deciphering late devonian early carboniferous p t t path of mylonitized garnet mica schists from prins karls forland svalbard journal of metamorphic geology which has been published in final form at https doi org 10 1111 jmg 12529 this article may be used for non commercial purposes in accordance with wiley terms and conditions for use of self archived versions

sequence analysis of upper pleistocene wisconsinan glaciolacustrine deposits of the north shore bluffs of lake ontario canada abstract up to 90 m of glaciolacustrine upper pleistocene wisconsinan sediments are well exposed along the north shore of lake ontario scarborough and bowmanville bluffs and contain critical information on the effects of glaciers on sedimentation in large proglacial lakes and on the deglaciation history of the great lakes region of north america the sediments vary greatly but the overall vertical trend is of a diamict unit at the base changing upward into distal clay rich rhythmites proximal sand rich rhythmites and well washed deltaic and or coastal sands in some areas gravelly units are interbedded between the basal diamict and the distal rhythmites they are interpreted as subaqueous outwash fan deposits the glaciolacustrine sediments of these bluffs include two different ass mblages one related to overland glaciofluvially derived sediments lacustrine assemblage and another directly related to a glacier with subaqueous terminus glacial assemblage the basic concepts of sequence stratigraphy help in unraveling these complex deposits if conceptually adapted to accommodate the glacial assemblage where a stand is not necessarily controlled by water level but rather by the relative position lowstand of the sediment injection point in the basin thus lowstand equivalent systems tract subaqueous outwash fan deposits of the glacial assemblage may form in ice dammed lakes while at the same time highstand highstand systems tract facies of the lacustrine assemblage are deposited in other parts the basin furthermore as the glacier retrea s subaqueous outwash deposits may be overlain abruptly by deep basin clay rich lacustrine rhythmites locally forming a forced transgressive facies sequence without any change in water level opening and closing of outlets by the glacier also causes rapid changes in lake level leading to development of shelf margin systems tracts and forced regression systems tracts for the lacustrine assemblage

comparison of tectonic styles in the mexican and canadian rocky mountain fold thrust belt despite the fact that most fold thrust belts around the world share many features successfully explained by the critical wedge model the details of their geometric evolution and tectonic style development are poorly understood in the classic section of the southern canadian rocky mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices in the mexican fold thrust belt central mexico the age of deformation the overall structural pattern and the total amount of shortening are similar but the individual thrust sheets exhibit much more internal deformation as manifest by metre scale buckle folds one of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly bedded basinal limestone in the central mexico section strain is concentrated toward the margins between platforms and basins in canada thick platform carbonates form continuous resistant units across the front range possible reasons for the differences in tectonic style between the two sections include the dominant lithology distribution of lithologies taper angle of the tectonic wedges amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks fold thrust belts are common features that developed on the edges of orogenic belts worldwide many have been well studied and their characteristic features established dahlstrom 1970 chapple 1978 boyer elliott 1982 price fermor 1985 mcclay 1992 these features include the classic notions of forward propagation of thrusts piggyback style a basal decollement and decreasing intensity of deformation towards the foreland most fold thrust belts are wedge shaped and since the mid 1980s the critical taper model of their development davis et al 1983 dahlen et al 1984 dahlen 1990 has become generally accepted analogue and numerical models have been developed that are able to capture many of the basic features of fold thrust belts and that are broadly consistent with the critical taper theory e g huiqi et al 1992 stockmal et al 2007 from early studies it has been clear that most of the large folds in fold thrust belts are intimately related genetically to the thrusts and extensive studies of thrust related folds in the field and in analogue and numerical models have established the nature of this relationship e g dahlstrom 1969 1970 elliott 1976 suppe 1983 mcclay 1992 wilkerson et al 2002 spratt et al 2004 despite the many similarities among fold thrust belts in general there are significant differences among belts and along strike within individual belts these reflect among other things differences in the strength of the basal detachment and the composition and distribution of the rocks in the fold thrust wedge in this contribution we compare basic elements of the structures developed roughly synchronously in two widely separated parts of a single orogenic belt the north american cordillera the sections we compare are a well known section across the southern canadian rocky mountains and a new section across the sierra madre oriental in central mexico the dimensions of the fold thrust belts in these two locations are similar and both sections are dominated by carbonates the structural style however is quite different we compare structures in the two locations and speculate on what may be responsible for the differences

regional unconformities and the sedimentary architecture of the oak ridges moraine area southern ontario seismic stratigraphy geometry and sediment facies within the oak ridges moraine orm area of ontario record major structural elements and surfaces of the quaternary sedimentary sequence the derived stratigraphic architecture can be used to identify the key elements of a regional erosional surface represented by an unconformity in the subsurface and associated overlying channel sediments the erosional surface unconformity forms a distinct time datum in the quaternary sequence which provides an important aid to lithostratigraphic correlation the architecture also gives improved understanding of the effects of erosion on the late glacial landscape the surfaces of erosional drumlins and intervening troughs and the beds and banks of meltwater channels in the orm area define the regional unconformity highlighted by seismic profiles linked to continuously cored boreholes these features are attributed to regional scale subglacial meltwater flow events the sculpted surfaces which are analogous to water eroded forms the presence of boulder lags and coarse grained deposits on the regional erosional surface and the channels with undulating profiles provide the vital supporting evidence for a meltwater interpretation the inter regional extent of the unconformity is inferred from the coherence of regional paleoflows and the extent of drumlinized uplands tunnel channels and scoured bedrock terrain across 75 of the landscape from the orm area east and south to the finger lakes new york the implied magnitude of erosion suggests a pressing need for directed sedimentological study in those ocean basins that were probable depositional sites for flood deposits

detrital remanent magnetization in hematite heavy mineral placer layers within channel sandstones of the morrison formation western united states have been shown to contain a very stable accurate record of the jurassic geomagnetic field these laminae are composed almost entirely of hematite grains in particular they are largely the hematite pseudomorph of magnetite martite the laminae record the correct geomagnetic field direction during a period of rapid apparent polar wander which suggests an early acquisition of the remanence several other remanence characteristics imply that the remanance is probably a detrital remanent magnetization drm 1 there is large directional dispersion in one lamina which had been heavily burrowed by organisms measurement of individual burrows shows that the burrowing disrupted and randomized the magnetization held in the martite grains 2 directions of laminae that were deposited on sloping surfaces are significantly deflected from the directions of the flat lying laminae 3 more than 50 of the martite grains have high temperature oxidation features suggesting that they may have been martite at the time of deposition these characteristics in particular the burrowing disturbance strongly suggest that the martite was deposited as martite and that the remanence is a drm as opposed to deposition of magnetite grains which were altered to hematite after deposition examination of recent weathering debris on igneous outcrops shows a substantial amount of martite on these outcrops this observation provides further evidence in support of the idea that hematite and in particular martite can be a deposited mineral in sedimentary rocks and thus can carry a drm

tectonics of the bude formation north cornwall the recognition of northerly directed decollement the upper carboniferous bude formation of north cornwall and devon underwent deformation beginning shortly after its deposition which resulted in a complex geometry of folding and thrusting the deformation initially exploited slumped beds as decollement horizons and resulted in a significant northerly translation of thrust sheets the development of chevron folds for which the area is famous commenced at this stage but continued causing folding of thrust planes and cleavages during the deformation the dominant sense of thrust movement switched from a northerly to a southerly direction and this is interpreted as the effect of backthrusting associated with the northwards migration of the deformation front

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

the sedimentary record of drifting ice early wisconsin sunnybrook deposit in an ancestral ice dammed lake ontario canada abstract outcrops of pebbly mud diamict at scarborough in southern ontario canada the so called sunnybrook till are associated with the earliest incursion of the laurentide ice sheet lis into mid continent north america some 45 000 years ago the sunnybrook is a blanket like deposit containing deepwater ostracodes and occurs conformably within a thick 100 m succession of deltaic and glaciolacustrine facies that record water depth changes in a large proglacial lake contextual evidence associated facies sedimentary structures deposit geometry and landforms indicates a low energy depositional setting in an ice dammed ancestral lake ontario in which scouring by floating ice masses was an important process u shaped iceberg cut scours with lateral berms up to 7 m deep occur on the upper surface of the sunnybrook and are underlain by sub scour structures that extend several meters below the scour base ice rafted concentrations of clasts clast layers grooved surfaces formed by floating ice glissading over a muddy lake floor soft sediment striations and melanges of sand and mud mixed by grounding ice keels ice keel turbates are present and are all well known from modern cold environments the wider significance of this depositional model is that the lis margin lay east of scarborough and did not overrun southern ontario this finding is in agreement with recent data from the erie basin of canada ohio and indiana where deposits formerly correlated with the sunnybrook and thus implying an extensive early wisconsin ice sheet are now regarded as illinoian a speculative hypothesis is proposed that relates deposition of the sunnybrook and two younger deposits of similar sedimentology to surge like instabilities of the southern lis margin

paleomagnetism of late jurassic rocks in the northern canelo hills southeastern arizona the canelo hills volcanics are exposed in the canelo hills a northwest trending range in santa cruz county southeast arizona the formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147 6 ma k ar biotite and 149 11 ma whole rock rb sr and a rb sr isochron age reported here which indicates an age of 151 2 ma paleomagnetic samples were collected from 17 cooling units in the northern canelo hills samples from most of these units responded to alternating field af demagnetization and secondary components were generally erased by peak af between 10 and 50 mt samples from five sites showed no response to af demagnetization thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization nrm although within site clustering of nrm directions was improved data from two sites were rejected because of failure to isolate a well determined characteristic nrm of the remaining 15 sites 10 sites were of normal polarity while five sites showed reversed polarity intensities of the characteristic nrm ranged from 4 10 3 to 3 10 1 a m the data from these 15 cooling units yield a formation mean direction of i 29 9 d 334 9 with k 33 4 and 95 6 7 the resulting paleomagnetic pole is at 62 2 n 130 3 dp 4 1 dm 7 4 this pole is between poles obtained from the summerville and lower morrison formations the canelo hills pole is thus consistent both in position and age with the late jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the summerville and morrison formations

tunnel valleys current knowledge and future perspectives tunnel valleys are elongated incisions that are commonly interpreted as being the result of erosional processes by subglacial meltwater occurring under continental ice sheets the abundance size and the primarily coarse grained infill of these features have made tunnel valleys important hydrocarbon and groundwater reservoirs although numerous tunnel valleys have been described over the last century their formation and infill remain poorly understood this review summarizes and discusses the current knowledge of tunnel valleys providing an overview of the observations around the world morphological aspects that separate tunnel valleys from other landforms are discussed as well as the wide variety of sedimentary environments found to contribute to the infilling of these features the depth of the incision and the character of ice retreat significantly determine the final infill architecture the formational hypotheses proposed in the literature are assessed to test their wider applicability to all other tunnel valleys in order to find a generic model that helps in the prediction of the morphology and infilling of both pleistocene and pre pleistocene age a quasi steady state model with small meltwater outbursts that erode tunnel valleys near the ice margin seems compatible with most of the known valleys other proposed models require specific geographical or climatic conditions

paleomagnetism of the roskruge and gringo gulch volcanics southeast arizona paleomagnetic data were obtained from nine cooling units of the late cretaceous 72 ma roskruge volcanics and from 25 flows in the lower paleocene gringo gulch volcanics both from southeastern arizona alternating field demagnetization successfully erased the infrequent secondary components of magnetization the paleomagnetic pole position obtained from the roskruge volcanics is at 73 6 n 176 0 e with dp 6 2 and dm 8 8 the gringo gulch volcanics pole position is at 77 0 n 201 0 e with dp 1 2 and dm 1 7 in conjunction with other recently published paleocene and eocene paleomagnetic poles these data provide details of north american apparent polar wander during the laramide orogeny 80 to 40 ma an episode of rapid apparent polar wander from the cretaceous pole position in the bering strait to the eocene pole position near the present rotation axis occurred during that interval of time the initiation of this episode of apparent polar wander appears to be coincident with the major plate reorganizations which occurred at the onset of the laramide orogeny

structural interpretation of the eastern sulaiman foldbelt and foredeep pakistan the sulaiman foldbelt is an active tectonic feature on the northwestern margin of the indian plate seismic reflection data have been combined with surface geological mapping and drill hole data to interpret the structural style and tectonic shortening of the eastern sulaiman foldbelt and its adjacent foredeep the data show that the basement is more than 8 km deep near the deformation front and that it deepens towards the west the foredeep adjacent to the eastern sulaiman foldbelt is a broad synclinal structure with a steep western limb the more gentle eastern limb has monoclinal dips from near zero to 2 5 several salt cored anticlines are observed on the eastern part of the monocline but the salt structures are lacking in the western foredeep and in the frontal part of the sulaiman foldbelt the basal decollement under the sulaiman lobe lies either in ductile eocambrian salt or within a deep zone of other material that undergoes ductile behavior near the deformation front ramps from the basal detachment become flats in lower triassic and lower cretaceous shales a steep and highly elevated zone immediately to the west of the frontal folds is interpreted to be underlain by a passive roof duplex the culmination wall of this oblique duplex is separated from the overlying roof sequence by a passive roof thrust in lower cretaceous shales the roof thrust has a backthrust sense of movement relative to the forelandward propagating duplex and may extend obliquely 100 km into the interior of the foldbelt fault propagation and fault bend folds are present in the frontal portion of the eastern sulaiman foldbelt palinspastic restoration of a balanced cross section shows 108 km of shortening in the eastern sulaiman foldbelt east of the kingri strike slip fault the presence of more than 11 km of pre neogene strata on top of basement suggests that the mesozoic rifted margin of the indian subcontinent may lie beneath the sulaiman foldbelt this interpretation is also favored by the presence of ophiolites and flysch deposits overthrusted on late cretaceous shelf strata

conceptual hydrogeological model of the yonge street aquifer south central ontario a glaciofluvial channel fan setting the yonge street aquifer ysa in the greater toronto area of south central ontario is a prolific municipal supply aquifer it has been considered to be channelized sand and gravel linked to a bedrock valley despite considerable work the fundamental conceptual model for the ysa is not well developed and documented based on high quality data a revised conceptual model of the aquifer is presented seismic profiles define the geometry of the regional stratigraphy with four distinct units bedrock lower sediments newmarket till and oak ridges moraine orm sediment seismic data reveal two generations of roughly north south channels older sub newmarket till channels within lower sediments termed thorncliffe channel and orm related channels termed orm channel that incise both newmarket till and lower sediments the ysa is interpreted to occur within a thorncliffe channel with possible vertical connection to younger orm channels and lateral connection to inter channel lower sediments thorncliffe c

from proterozoic strata to a synthesized seismic reflection trace implications for regional seismic reflection patterns in northwestern canada calculation of a synthetic seismic reflection trace from detailed descriptions of exposed proterozoic strata in northwestern canada permits correlation of reflections on regional seismic profiles to surface outcrop approximately 5 4 km composite thickness of paleo and mesoproterozoic strata are exposed in the muskwa anticlinorium that is located within the foreland of the cordillera in northeastern british columbia the tuchodi anticline is the easternmost structure of the muskwa anticlinorium and has the deepest levels of proterozoic strata exposed at this location prominent seismic reflection layering rises toward the surface and is easily correlated to the deeper formations of the muskwa assemblage stratigraphy these layers are followed westward into the middle crust where they are overlain by dramatically thickened by about five times strata primarily of the tuchodi formation along the same line of section the muskwa assemblage reflections overlie additional subparallel layered reflection

crustal structure and lithology of the northern canadian cordillera alternative interpretations of snorcle seismic reflection lines 2a and 2b seismic reflectors at shallow crustal levels recorded in snorcle slave northern cordillera lithospheric evolution lines 2a and 2b in the northern canadian cordillera can be reconciled with many stratigraphic and structural elements known from geological mapping clearly evident examples are the crustal penetrating northern rocky mountain trench nrmt fault the several kilometres thin slices of slide mountain sylvester allochthon and cache creek terranes and the northward dipping hotailuh fault interpretations of lithological successions and structures at deeper crustal levels are more speculative of fundamental significance is a southwesterly tapering wedge of continental crust overlain in its distal part by the allochthonous terrane stikinia and northeast of thibert fault by a thick probably structurally thickened sequence of sedimentary rocks and the extensive granitic cassiar batholith the top of the wedge is interpreted to be a tectonic accretion surface resulting from collision of sti

the neogene structural evolution of the western margin of the pelagian platform central tunisia abstract the deformation front of the atlas mountains in central tunisia is a structure termed the north south axis to the west of kairouan the north south axis consists of ne sw to nne ssw trending folds and thrusts a main decollement thrust in triassic evaporites has been mapped with secondary detachment of out of sequence thrusts in tertiary rocks and backthrusts at the base of the tertiary and in upper cretaceous sediments the upper cretaceous and tertiary sediments are backfolded in the hangingwalls of the backthrusts which are in turn folded in the se verging anticlines the backthrusts are roof thrusts to passive roof duplexes and imply the extensive development of thin skinned blind se verging thrusts a similar structural style is seen further west in the tunisian atlas but major nw verging backthrusts are not exposed adjacent to the pelagian platform the thrust structures are affected by later strike slip faults the thin skinned thrusting at the deformation front is middle miocene in age and was synchronous with africa europe collision the north south axis is therefore interpreted as the frontal structure of a collision related thrust belt the thrust belt is underlain by a nw se to e w trending middle jurassic to early cretaceous rift basin with probable mid cretaceous to oligocene post rift fill the ne sw trending thrust belt is detached from nw se to e w trending syn rift basement faults but changes in thrust belt geometry across the syn rift faults indicate that basement structures have compartmentalized the thrust belt the later strike slip faulting resulted from the reactivation of the syn rift basement faults in the post collisional framework of the thrust belt

hierarchical dynamic stratigraphy in various quaternary gravel deposits rhine glacier area sw germany implications for hydrostratigraphy the approach of dynamic stratigraphy aims to understand genetic processes that form stratigraphic units in a hierarchy of spatial and temporal scales this approach was used to investigate quaternary gravel deposits in terms of their sedimentology and in order to characterize the various sedimentary units in terms of their hydrogeological properties facies analysis within 62 gravel pits laboratory permeability measurements of field samples and geophysical surveys 3 d georadar 2 d seismic reflection led to the detection and classification of sedimentary heterogeneity according to the following six scales whereby each scale can be translated into defined hydrostratigraphic units 1 particles and pores micro scale that reflect depositional and diagenetic fluid dynamics as well as source material behaviour e g grain size roundness lithological composition this was found to be important for the hydrogeochemistry of groundwater in gravel aquifers e g higher sorption capacity of carbon rich limestone particles for organic pollutants 2 strata meso scale contain the recognition of sorting fabric texture and stratinomic features which can give an indication of transport and depositional dynamics five major lithofacies groups for example were distinguished within fluvial gravel bed deposits their variable hydraulic properties led to their subdivision into 12 hydrofacies types they form the smallest mappable hydrostratigraphic units which may result in either preferred pathways for fluid flow or flow barriers 3 depositional elements macro scale enable reconstruction of sedimentary geomorphic elements and their dynamics within a depositional system e g gravel bed braided river systems are dominated by gravel sheet gravel dunes and scour pool depositional elements hydrostratigraphically the architecture of depositional elements influences the hydraulic connectivity and local permeability structure distribution within an aquifer body five types of depositional elements in fluvial gravel bed deposits were distinguished and their geometries dimensions quantified 4 facies bodies mega scale composed of a stack of depositional elements and strata recording distinct environmental systems and their dynamics e g a coarse grained prograding delta system hydrostratigraphically facies bodies represent major compartments of an aquifer six major types of meltwater controlled facies bodies were identified in the study area 5 genetic sequences mega scale reflect the shifts of depositional environments caused by allocyclic changes e g glacial advance recorded by a coarsening upward sequence or autocyclic changes of landscape shaping events these sequences may form separate hydrostratigraphic units or aquifer storeys 6 basin fill giga scale comprising the lateral and vertical stacking of facies bodies and genetic sequences controlled by either long term glacier dynamics or short term flood events the regional distribution of permeable gravel units and for example less permeable diamicts builds the larger scale hydrostratigraphy

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

description and numerical model of pleistocene iceberg scours and ice keel turbated facies at toronto canada near toronto in southern ontario canada glaciolacustrine muds and shoreface sands of late pleistocene age early wisconsin glaciation 45 ka were deformed by icebergs ploughing the floor of an ancestral ice dammed lake ontario outcrop mapping and a ground penetrating radar survey identify ice scour structures up to 4 m deep up to 10 m wide and at least several hundred metres long with high standing lateral margins berms ploughing of lake floor sediment is also recorded by ice keel turbated facies in which storm deposited shoreface sands have been mechanically mixed with underlying muds palaeoenvironmental information for one prominent scour such as water depths estimates of the mass and draft of the ice mass and scouring forces can be determined using a simple finite element numerical model a mass of 0 04 million tonnes a keel draft of between 10 and 30 m and a scouring force of 5 103 kn is consistent with scouring by a small iceberg bergy bit or pressure ridged lake ice the former is preferred because of the considerable depth of the scour and the overall glacial depositional setting estimates of ice mass dimensions agree with independent reconstructions of palaeobathymetry no greater than 20 m derived from storm deposited hummocky and swaley cross stratified facies that fill the scour data presented here aid the identification of ice scour structures and turbated sediment in the ancient record despite being a common process on modern high latitude shelves descriptions of ancient structures and facies are few

north america during the lower cretaceous new palaeomagnetic constraints from intrusions in new england summary six late aptian and albian intrusions in maine new hampshire and vermont dated between 113 and 100 ma were sampled for palaeomagnetic analysis the majority of samples unblock between 550 c and 580 c and curie temperatures fall in a discrete interval between 556 c and 580 c this indicates that the dominant remanence carrier is a trm and is carried by titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete cl c3 grains as exsolution lamellae in the silicates and as fine reaction rims on the silicates the palaeomagnetic results are combined with additional data from four barremian and early aptian intrusions in southern maine to produce nine reliable virtual geomagnetic poles these possess a higher latitude than most published poles for the lower cretaceous ar4o ar3 ages on biotite and hornblende from these intrusions demonstrate three intrusive pulses at approximately 120 ma 111 ma and 100 ma constraining the ages of the calculated vgps the corresponding palaeopoles are 72 3 on 198 2 e a 3 3 74 5 on 195 2 e a 3 8 and 76 6 on 167 5 e a 5 3 based on new palaeomagnetic data reported here significant changes to the north american mesozoic apw path are necessary these changes constrain the palaeoposition of the north american plate in the lower cretaceous

the eurekan deformation in the arctic an outline the evolution of the eurekan deformation zones in the arctic is closely related to the development of the circum greenland plate boundaries in early cenozoic times 53 34 ma mostly the eurekan orogeny or deformation has been interpreted as a predominantly compressive tectonic event but the eurekan deformational history in the arctic was not the result of a single tectonic episode it rather represents a complex sequence of successive tectonic stages which produced a number of intra continental deformation zones with changing sometimes opposing lateral oblique and convergent kinematics in the canadian arctic archipelago north and ne greenland and svalbard the interaction between the continental plates especially in combination with the development of transform faults resulted onshore in the formation of several complex deformation zones and areas of eurekan deformation the eurekan deformation can be divided into two major tectonic stages the first phase in the early eocene was dominated by orthogonal compression in the west spitsbergen fold and thrust belt along the west margin of the barents shelf and contemporaneous sinistral strike slip tectonics along the wegener fault and on ellesmere island whereas the second phase in the late eocene was characterized by dextral strike slip and compression on ellesmere island and contemporaneous dextral transpression and transtension along the de geer fracture zone or hornsund fault complex between ne greenland and spitsbergen

a new late cretaceous paleomagnetic pole from the adel mountains west central montana north america s apparent polar wander path has been poorly defined between the mid cretaceous and paleocene reference pole positions existing data allowed 13 of apparent polar motion over about 22 m y 87 65 ma roughly coinciding with the beginning of laramide deformation 80 ma we report on a paleomagnetic study of the adel mountain volcanic rocks to refine the north american apparent polar wander path for this interval the shonkinite rocks of the adel mountain volcanic field are on the eastern edge of the cretaceous paleocene fold and thrust belt some of these structures disturb the western edge of the volcanic pile we obtained two new k ar dates from the adel rocks one date on biotite from a shonkinite dike that crosscuts most of the volcanic rocks is 71 2 2 7 ma the other a whole rock date from a flow deep in the volcanic pile is 81 1 3 5 ma we collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows dikes and laccoliths positive fold and conglomerate tests along with alternating field and thermal demagnetization indicate that our characteristic remanent directions are primary magnetizations acquired before late cretaceous to paleocene thrust belt deformation averaging the virtual geomagnetic poles from 26 reliable sites all of normal polarity yields a paleopole at 82 2 n 209 9 e 95 6 80 k 18 38 this pole is concordant with the paleocene reference pole 82 0 n 170 2 e 95 3 5 k 18 6 diehl et al 1983 and is 11 6 from the globerman and irving 1988 mid cretaceous pole at 71 n 196 e the youngest information in the cretaceous stillstand pole is from the niobrara formation shive and frerichs 1974 at about 85 89 ma if we take the average age of the adel mountain volcanics to be 76 ma then 12 of apparent polar motion occurred between 87 ma and 76 ma thus rapid apparent polar motion correlates well with the onset of laramide deformation

a paleomagnetic test of the proposed mojave sonora megashear in northwestern mexico abstract three upper triassic lower jurassic formations in southwestern united states and northwestern mexico have been investigated paleomagnetically as a means of testing for displacement along the proposed mojave sonora megashear luning formation of west central nevada and sil nakya formation of southern arizona lie north of the megashear whereas in northwestern sonora antimonio formation lies south of the megashear previously published results from the nazas formation of presumed lower jurassic age from north central mexico were incorporated into the interpretations these four units compose two sets of correlative formations on opposite sides of the proposed megashear restoration of 800 km of left lateral displacement along the megashear about the pole of rotation determined by anderson and schmidt 1983 improves the grouping of the two sets of paleomagnetic poles noticeably with the improvement having statistical significance at the 95 confidence level we feel that the data support but do not prove the postulated displacement along the megashear all four poles are displaced from irving s 1979 smoothed apparent polar wander path for cratonic north america in the intervals 190 and 200 ma the displacement needed to bring all of these poles into coincidence is a pure rotation of about 8 counterclockwise about the sampling area two probable cretaceous poles were obtained from the sand wells formation of southern arizona and from unnamed volcanics in northwest sonora these poles are essentially identical and suggest that displacement along the megashear had ceased by the cretaceous data from the early jurassic mulberry wash and pitoikam formations in southern arizona appear to have been remagnetized in a later cretaceous event and were not used in this study

paleomagnetism stratigraphy and petrology of the upper cretaceous roskruge volcanics at bell mountain southeast arizona reconnaissance geologic mapping and petrologic study of the upper cretaceous roskruge volcanics at bell mountain in the roskruge mountains southeastern arizona have identified five compositionally distinct ash flow tuffs including the basal red viopuli tuff which comprise six or seven separate cooling units previous paleomagnetic investigators inferred eleven cooling units at bell mountain but insufficient information was given to determine which tuffs had been sampled this study was undertaken therefore to establish characteristic remanence directions for each tuff sheet the available paleomagnetic data corroborate the identification of five map units in the bell mountain area but the number of independent observations is too few for the calculation of a reliable paleopole paleosecular variation of the geomagnetic field would probably not be averaged out no correctable tuffs are present at bell mountain so relative vertical axis rotations cannot be determined although they are probably not large the absence of regionally correctable tuffs suggests the presence of additional potentially mineralized volcanic centers perhaps farther north in the roskruge mountains or buried nearby beneath cenozoic basin fill

remagnetization of jurassic volcanic rocks in the santa rita and patagonia mountains arizona implications for north american apparent polar wander paleomagnetic poles for the jurassic corral canyon sequence and glance conglomerate in southern arizona have been used to construct apparent polar wander apw paths for the north american plate but they are controversial and conflict with higher latitude poles from new england lower jurassic dacites and ash flow tuffs of the mount wrightson formation in the santa rita mountains were initially sampled to provide an additional paleopole for southern arizona these rocks however have a predominantly reversed polarity characteristic magnetization in situ i 47 d 154 95 9 which is statistically indistinguishable from that for the nearby latest cretaceous elephant head pluton i 48 d 165 95 8 although magnetizations of both polarities are observed in the ash flow tuffs they are mostly carried by hematite and dual polarity components are observed within some specimens moreover widespread mineralization and a k ar age of 67 ma for altered rocks of the mount wrightson formation imply that these rocks were subjected to a prolonged episode greater than one polarity interval of low temperature alteration and remagnetization middle jurassic ash flow tuffs of the corral canyon sequence in the patagonia mountains to the south have a similar geologic setting they are exposed along strike amid mineralized rocks on the eastern flank of a mountain range cored by paleocene intrusions hematite is also the dominant remanence carrier in most of the corral canyon sequence and its predominantly normal polarity direction in situ i 51 d 326 95 9 is indistinguishable from that for the nearby patagonia granodiorite i 49 d 342 95 8 rocks of the corral canyon sequence therefore are likely remagnetized as well problems also exist with the glance conglomerate pole these rocks are situated within a caldera structure and have been potassium metasomatized this potassic alteration could have occurred shortly after emplacement or at a later time postdeformation the low latitude jurassic apw path for north america and j 2 cusp therefore are not well supported and may need revision

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

late cretaceous paleomagnetism of the tucson mountains implications for vertical axis rotations in south central arizona the tucson mountains of southern arizona are the site of an upper cretaceous caldera from which the rhyolitic cat mountain tuff was erupted at about 72 ma two magnetic units within the cat mountain tuff are distinguished by paleomagnetic data in both the northern and southern tucson mountains the resurgent amole pluton 72 ma in the northern tucson mountains was emplaced soon after eruption of the cat mountain tuff but cooled and was magnetized after northeastward tilting 50 85 of the adjacent caldera fill sequence petrologic and paleomagnetic data indicate that the lower magnetic unit of the cat mountain tuff caps the silver bell mountains to the northwest a previous paleomagnetic investigation n 34 indicates that the silver bell mountains have been rotated clockwise 30 16 95 confidence level about a vertical axis relative to cratonic north america a similar paleomagnetic study of upper cretaceous volcanic volcaniclastic and intrusive units in the tucson mountains n 26 indicates that these rocks have been rotated 7 14 clockwise relative to stable north america a direct comparison of paleomagnetic directions for the lower unit of the cat mountain tuff shows a 17 10 clockwise rotation between the silver bell mountains and the tucson mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges preliminary paleomagnetic directions for middle tertiary units from the silver bell and tucson mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks clockwise rotation of crustal blocks in southern arizona likely was associated with strike slip movement on major northwest trending faults in the region and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in late cretaceous and early tertiary time the available paleomagnetic data indicate that rocks in southern arizona have not remained unrotated with respect to north america since late cretaceous time and that vertical axis rotations may have played an important role in the region during laramide deformation

late cretaceous paleomagnetism and clockwise rotation of the silver bell mountains south central arizona late cretaceous ash flow volcanism in the silver bell mountains of southern arizona 32 3 n 248 5 e was associated with caldera formation and porphyry copper mineralization oriented samples from 34 sites in volcanic volcaniclastic and intrusive units related to this episode of igneous activity 73 69 ma yield a mean paleomagnetic direction of i 61 2 d 24 0 95 7 6 primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping 95 confidence level of site mean directions for a widespread tuff unit after structural correction a comparison of this direction with a direction at silver bell calculated from contemporaneous units for stable north america i 58 8 d 342 3 95 7 7 indicates a late cretaceous paleolatitude anomaly of 2 4 7 6 not significant at the 95 confidence level and a declination anomaly of 41 7 14 3 for the silver bell mountains previously determined paleomagnetic data for southeastern arizona suggest that this apparent clockwise rotation in the silver bell mountains is a local phenomenon although preliminary the average paleomagnetic direction for oligocene and lower miocene rocks in the silver bell area i 43 8 d 357 3 95 16 5 is similar to that calculated for stable north america i 50 2 d 352 2 95 3 9 implying that the observed rotation in the silver bell mountains occurred before 26 ma and was most likely associated with dextral strike slip movement along the ragged top and related wnw trending faults bounding the silver bell mountain block these data in conjunction with plate reconstructions and other paleomagnetic data from southwestern north america imply that wnw trending strike slip faults may have played an important role during late cretaceous to early tertiary deformation in southern arizona

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

facies characteristics of middle pleistocene saalian ice margin subaqueous fan and delta deposits glacial lake leine nw germany abstract the blocking of major river valleys in the leinebergland area by the early saalian scandinavian ice sheet led to the formation of a large glacial lake referred to as glacial lake leine where most of the sediment was deposited by meltwater at the initial stage the level of glacial lake leine was approx 110 m a s l the lake level then rose by as much as 100 m to a highstand of approx 200 m a s l two genetically distinct ice margin depositional systems are described that formed on the northern margin of glacial lake leine in front of the retreating scandinavian ice sheet the bornhausen delta is up to 15 m thick and characterized by a large scale tangential geometry with dip angles from 10 28 reflecting high angle foreset deposition on a steep delta slope foreset beds consist of massive clast supported gravel and pebbly sand alternating with planar parallel stratified pebbly sand deposited from cohesionless debris flows sandy debris flows and high density turbidity flows the finer grained sandy material moved further downslope where it was deposited from low density turbidity currents to form massive or ripple cross laminated sand in the toeset area the freden ice margin depositional system shows a more complex architecture characterized by two laterally stacked sediment bodies the lower part of the section records deposition on a subaqueous ice contact fan the upper part of the freden section is interpreted to represent delta slope deposits beds display low to high angle bedding 3 30 and consist of planar and trough cross stratified pebbly sand and climbing ripple cross laminated sand the supply of meltwater transported sediment to the delta slope was from steady seasonal flows during higher energy conditions 2 d and 3 d dunes formed migrating downslope and passing into ripples during lower energy flow conditions thick climbing ripple cross laminated sand beds accumulated also on higher parts of the delta slope

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

glacially streamlined hard and soft beds of the paleo ontario ice stream in southern ontario and new york state abstract an extensive tract of glacially streamlined terrain across a large part of southern ontario canada is recognized as the footprint of the paleo ontario ice stream ois within the easternmost great lakes sector of the last laurentide ice sheet the upstream part is a drumlinized and megagrooved hard bed underlain by cambro ordovician carbonates and sandstones subglacial plucking of jointed limestone on the lateral margins of drumlinized escarpment interfluves and rock drumlins generated a large flux of coarse debris within the ice base recorded by sporadic spreads of hummocky rubble moraine downstream the hard bed passes underneath a streamlined soft bed of till cored drift drumlins and megaridges of the classic peterborough and new york state drumlin fields the boundary between the two bed types is a 10 km wide mixed bed of isolated drift drumlins resting on drumlinized rock suggesting a common erosional origin spatial variation in the geomorphology of 2500 drift drumlins indicates that megaridges are clones resulting from the erosion and dissection of larger parent drumlins a large moraine system may mark the final collapse and melt of the ice stream accompanying abrupt flow switching of its margin

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

magnetic mineralogy of continental deposits san juan basin new mexico and clark s fork basin wyoming magnetic concentrates were obtained from nine bulk samples from the late cretaceous through middle paleocene continental sedimentary section in the san juan basin new mexico and from two bulk samples from the late paleocene and early eocene section in the clark s fork basin wyoming strong field thermomagnetic js t curves of almost all the san juan basin concentrates show only a single curie temperature in the 175 195 c range indicating that the dominant ferrimagnetic mineral is detrital titanomagnetite fe3 xtixo4 of composition 0 51 x 0 54 the clark s fork basin concentrates exhibited curie temperatures of 200 c and 580 c indicating a mixture of x 0 5 titanomagnetite and magnetite respectively no evidence of hematite or other ferric oxides or oxyhydroxides was observed in the js t data isothermal remanent magnetization irm acquisition curves were determined for 56 samples from the clark s fork basin and 114 samples from the san juan basin although dominated by irm acquired in magnetizing fields of 300 mt additional irm was acquired in magnetizing fields from 300 to 700 mt the irm acquired above 300 mt is attributed to hematite or other ferric oxides or oxyhydroxides and is thought to be the result of minor oxidation probably during recent weathering these results indicate that irm acquisition behavior may be used to monitor the hematite content in continental sedimentary rocks and may indicate stratigraphic intervals within which the natural remanent magnetization nrm could contain significant chemical remanence overprints although minor hematite content is indicated the detailed sampling of irm acquisition behavior for the san juan basin and clark s fork basin sedimentary sequences did not reveal any significant correlation of irm behavior with polarity of the characteristic nrm these data thus support the previous conclusion based on paleomagnetic data that the characteristic nrm is a depositional remanence which provides a valid recording of the geomagnetic polarity sequence during deposition of these continental sedimentary sequences

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

roof sequence response to emplacement of the wills mountain duplex the roles of forethrusting and scales of deformation abstract this paper focuses on the behavior of a roof sequence in the appalachian plateau of west virginia u s a and emplacement of the wills mountain duplex with 17 5 km of displacement unlike the plateau along strike in pennsylvania and new york where forethrusting was previously documented this roof sequence lacks an underlying salt dominated roof decollement kinematic analyses reveal that the roof sequence in the west virginian plateau accommodated about two thirds of the 17 5 km of shortening by the adjacent wills mountain duplex as a forethrusting kinematic response the remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation this kinematic response matches that along strike in the central appalachians despite the loss of the salt decollement we interpret that an ordovician shale dominated formation was sufficiently weak to substitute for the salt horizon thus a weak mechanical unit rather than specifically a salt decollement is a necessary prerequisite for forethrusting a contributing factor to forethrusting may be the subvertical front of the wills mountain duplex which inhibited other responses by the roof sequence mesoscale and smaller processes including grain to grain pressure solution twinning and cleavage formation account for over 75 of the shortening in the roof sequence and if ignored would result in an erroneous interpretation of backthrusting or local compensation this result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems

sequence analysis of early saalian glacial lake deposits nw germany evidence of local ice margin retreat and associated calving processes abstract this paper presents a detailed analysis of high resolution facies architecture of glaciolacustrine ice margin deposits which formed at the southern margin of the scandinavian ice shield the ice margin depositional systems are characterised by coarse grained deltas and subaqueous fans which are formed by stacked transgressive regressive sequences recording an overall lake transgression interrupted by minor short term lake level falls the delta complexes at the northern margin of glacial lake rinteln are thought to reflect a relatively stable position of the ice margin in front of the weser chains mountain ridges the onset of delta progradation probably represents a halt in ice advance and a related high sediment supply period the sedimentary facies and depositional architecture resemble those of nonglacial gilbert type deltas except for the deposition of glacial debris the main delta progradation is recorded from the highstand systems tract when high meltwater and sediment discharge occurred during the melt season a forced regression during opening of outlets led to the formation of subaerially exposed sequence boundaries and the erosion of the highstand systems tract deeply incised channels incised valleys were filled during the subsequent transgression at the eastern lake margin subaqueous fan deposits reflect an unstable ice front that was rapidly retreating and subject to periodic calving which resulted in the generation of floating icebergs dumping ice rafted debris and ploughing into subaqueous fan deposits the rapid retreat of this eastern ice margin is interpreted to result from the overall lake level rise the fan stacking pattern and bulk geometry were determined by the ice front fluctuations the shifting of meltwater outlets and the short term lake level fluctuations depositional processes reflect the character of sediment supply and distance from the ice margin ice proximal upper fan deposits are characterised by coarse grained gravel deposited by debris flows that document continuous discharge slump and slide deposits are related to steep depositional slopes the mid fan contains basinward fining and thinning deposits of quasi steady and surge type high and low density turbidity currents indicating more ice distal and periodic deposition the outer fan deposits mainly consist of surge type low density turbidites and glacial debris dumped by icebergs the forcing parameters governing the development of depositional sequences in both delta and fan settings were lake level fluctuations sediment yield rates and physiography the depositional sequences were deposited on a time scale of seventh and eighth order high frequency cycles 101 102 years in this short time span lake level fluctuations were on the order of 120 m therefore accommodation space was largely controlled by lake level and subsidence can be ignored

a barremian aptian early cretaceous north american paleomagnetic reference pole four intrusive complexes from southern maine containing rocks ranging in composition from olivine gabbro to granodiorite were sampled to determine an early cretaceous reference pole for the north american plate sampling included the lebanon diorite the tatnic complex alfred complex and the cape neddick complex dated radiometrically by the 40ar 39ar or k ar method on biotite between 125 and 120 ma the majority of samples have unblocking temperatures between 550 c and 580 c and curie temperatures between 550 c and 570 c indicating that the dominant remanence carrier is titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete c1 c3 grains as exsolution or oxidation exsolution lamellae in the silicates as a product of magmatic reactions forming magnetite ilmenite symplectites with pyroxene or olivine or as ultrafine dust hemo ilmenite is present as a minor component in the lebanon diorite and in this case the remanence direction carried by the hemo ilmenite is indistinguishable from the titanomagnetite detailed petrographic observations and rockmagnetic experiments demonstrate that despite the coarse average grain size of the oxides in these rocks which might lead one to expect unstable magnetic remanence the magnetic remanence in most samples is extremely stable and caused by single domain or pseudo single domain behavior in magnetite containing fine oxidation exsolution lamellae of ilmenite the tatnic and alfred intrusions are of normal polarity d 333 i 62 pole 70 5 n 210 3 e a95 3 7 n 24 sites the cape neddick and lebanon intrusions are of reversed polarity d 161 i 57 pole 73 7 n 185 7 e a95 5 0 n 17 sites together these intrusions yield a mean paleomagnetic pole at 72 2 n 198 9 e a95 3 3 n 41 sites the well characterized reversed polarity recorded in the precisely dated cape neddick complex may represent the best age for the magnetic polarity chron m0r the last reversal before the onset of the cretaceous normal superchron at the barremian aptian stage boundary

late stage evolution of fault propagation folds principles and example abstract fault propagation folding commonly occurs near the fronts of mountain belts this type of folding may even represent in some thrust systems the major mode of deformation however fault propagation folds are frequently altered by two kinds of late stage evolution breakthrough thrusting or transport on the flat we model the geometric features of these kinds of fold in particular the late stage modification and present the algorithms and equations used in an original program an application based on an example from the atlas mountains of algeria illustrates the main features of the model

paleomagnetic results from granitic basement rocks in the cajon pass scientific drillhole natural remanent magnetizations in granitic rocks intersected by the cajon pass scientific drillhole are a composite of two superimposed magnetizations one magnetization is a primary trm acquired in late cretaceous times upon initial cooling while the other is an irm induced by the coring device coring induced irm has obscured any pre coring vrm and thus core orientation using recent vrm was not possible primary inclinations are internally consistent and agree with expected values derived from the adjacent sierra nevada and southern california batholiths

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex