interaction of basement uplift and thin skinned thrusting moxa arch and the western overthrust belt wyoming a hypothesis immediately north of big piney in western wyoming the moxa arch is modeled as emplaced during the late cretaceous along an east dipping low angle thrust moxa thrust that has precambrian basement and paleozoic and younger cover rocks in the hanging wall the west verging moxa thrust cut up section from the basement cover contact and flattened to the west along a detachment in lower triassic rocks thaynes detachment during motion on the moxa thrust the leading edge of the hanging wall wedged westward along the thaynes detachment peeling back triassic and younger rocks and thrusting them relatively eastward along the ancestral prospect thrust the 5 3 km of westward movement along the moxa thrust was matched by eastward movement along the west dipping ancestral prospect thrust the ancestral prospect thrust moved an additional 5 1 km during the late paleocene when thrust belt deformation progressed eastward to the moxa arch it appears that in the snider basin area the prospect thrust does not share a ramp with the darby thrust and that the emplacement of the moxa arch and prospect thrust determined the position of the later and more westerly darby thrust

regional variations in deformation mechanisms and structural styles in the central appalachian orogenic belt orogenic belts such as the central appalachian fold and thrust belt consist of distinct tectonic provinces characterized by different structural styles as a result of deformation under different physical conditions a quantitative study of deformation mechanisms was undertaken for quartzites and sandstones of the central appalachian orogenic belt in order to understand the controls of different physical parameters on the relative importance of these mechanisms and on the geometry and mechanics of formation of structures in the different provinces in the blue ridge province the dominant deformation mechanisms are pressure solution dislocation creep and extension fracturing with some accompanying grain boundary sliding and recrystallization the proportion of dislocation creep to pressure solution strain increases both with temperature and deviatoric stress an axial plane slaty cleavage which contains the maximum and intermediate principal directions of the strain ellipsoid maintains a uniform orientation in both the blue ridge and the great valley provinces folds in the blue ridge province have a similar geometry formed by progressive flattening of parallel folds due to internal deformation ductile deformation zones on the overturned limbs of folds acted as precursors for the propagation of thrust faults in the late stages of folding in the valley and ridge and massanutten synclinorium important deformation mechanisms in the sandstone units include pressure solution plastic deformation cataclasis brecciation extension fracturing and slip along minor faults and bedding planes major folds are strongly disharmonic and show variations between concentric and angular geometries they are associated with thrust ramps that propagated through progressively developing folds in the late stages of folding brittle deformation zones characterized by high intensities of extension fractures and cataclastic bands formed on the steep forelimbs these features are interpreted to be the result of layer parallel extension and the propagation of splay faults through these limbs cleavage developed during layer parallel shortening prior to the onset of folding and was rotated to its present fanned geometry during folding on the basis of the results of this study the central appalachian orogenic belt can be subdivided into two deformational provinces dominated by different deformation mechanisms and characterized by different geometries and mechanics of formation of the structures for orthoquartzites the tectonite front which separates the tectonites of the hinterland from the non tectonites of the foreland is located along the blue ridge front for carbonates and argillaceous sandstones the tectonite front is located along the north mountain fault

distribution and mechanisms of strain within rocks on the northwest ramp of pine mountain block southern appalachian foreland a field test of theory fractures faults stylolites and strain determined from an analysis of twinning in calcite in the mississippian newman limestone at the base of the pine mountain block show that deformation was accomplished primarily by a combination of bedding plane slip and layer parallel shortening poles to joints are either parallel to strike of the thrust sheet or are in the transport plane small slip surfaces with syntaxial calcite fibers lie either parallel to bedding or parallel to the strike of the pine mountain block and dip steeply movement along the former is in the direction of transport of the block whereas movement on the latter is approximately along strike tectonic stylolites are rare but appear to be controlled by subtle changes in lithology wackestones contain more stylolites than do packstones strains in calcite from closely spaced samples show the converse relationship they are highest in units without stylolites maximum shortening strain e 1 10 4 and lowest in units with stylolites or small faults e 1 2 0 the maximum principal shortening strain directions however are identical within the error and are parallel to both bedding and the transport direction a sequence of events in this portion of the pine mountain block is 1 layer parallel shortening producing tectonic stylolites and a strong preferred orientation of compression axes 2 development of fractures parallel to the strike of the block on the lower hinge of the ramp to accommodate stretching parallel to the transport direction these strike parallel fractures can also be interpreted as having occurred quite late 3 during 2 above and continuing the start of bedding plane slip marked by syntaxial calcite fibers which trend in the direction of transport and 4 complications such as shear parallel to the strike of the pine mountain fault as a result of motion of the entire block between bounding tear faults events 2 through 4 most likely overlapped in time these data suggest that the acquisition of dip of the rocks on the ramp was primarily accomplished by bedding plane slip on the slip surfaces which experienced a shear stress of

synfolding and prefolding magnetizations in the upper devonian catskill formation of eastern pennsylvania implications for the tectonic history of acadia the upper devonian catskill formation was sampled for paleomagnetic study in east central pennsylvania 41on 76ow in one area the dominant component of magnetization se is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660oc a conventional fold test is positive at the 99 confidence level however statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3 4 unfolding the magnetization is therefore secondary with a mean direction of d 166 6 o i 1 8 o the

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

jurassic magnetostratigraphy 2 middle late oxfordian of aguilon iberian cordillera northern spain abstract a reproducible magnetic polarity pattern has been obtained from four overlapping stratigraphic sequences of ammonite rich gray pelagic limestones spanning part of the middle and late oxfordian g transversarium toe bimammatum ammonite zones progressive thermal demagnetization removed a present field overprint fold tests both between and within outcrops are positive the mean direction of magnetization is 322 45 95 6 which implies a paleolatitude of 26 n frequent changes of polarity were observed the magnetostratigraphic sequence contains the ammonite zones subzones and horizons of the tethyan faunal realm using the reference horizons provided by this detailed ammonite zonation the polarity zones can be readily correlated among all sections the composite polarity pattern is predominantly of reversed polarity punctuated by a number of relatively shorter duration normal polarity intervals in order to compare this sequence to that of the marine magnetic anomalies we have revised the age estimates for the pre m25 marine magnetic normaly pattern according to the most recent time scale based on those ages the sedimentary record at aguilon suggests a much greater frequency of polarity changes than does either model derived from the two sets of marine magnetic anomalies of this time this sedimentary record probably is a truer approximation of the actual geomagnetic polarity history because of the continuity of sedimentation in these sections and the inherent ambiguities involved in determining a polarity pattern from the very low amplitudes that everywhere characterize this part of the marine record

styles of folding within thrust sheets examples from the appalachian and rocky mountains of the u s a and canada abstract folds in a single thrust sheet can be classified as trailing edge formed over footwall ramps intraplate due to shortening within the body of the sheet and leading edge leading edge folds are usually removed by erosion at the front of a thrust sheet but their unfaulted equivalents can be examined at the lateral termination of the thrusts most folds of these various classes possess a kink band geometry with sharp hinges and long planar limbs the orderly nature of kink band folds breaks down when thin incompetent units separate thicker competent members the resulting fold geometry ranging from simple disharmonic to hinge collapse is controlled by the thickness of interbedded incompetent materials the appalachian and rocky mountain thrust belts provide examples of these various classes and styles of folds

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

relationship of spaced cleavage to folds and thrusts in the idaho utah wyoming thrust belt the idaho utah wyoming thrust belt of the rocky mountain cordillera preserves structures that developed at very shallow levels of the crust t 200 c the geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts spaced cleavage and pencil fracturing are developed within micrites of the jurassic twin creek formation the cleavage developed dominantly by pressure solution early in the deformation history of each thrust sheet aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier overriding sheets cleavage within any sheet is well developed in the area covered by the pre erosional extent of the previous sheet forelandward from this area cleavage intensity decreases drastically thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes cleavage is axial planar to small folds and formed concurrently with small scale buckle folds and contraction faults during the early history of movement on each thrust however cleavage strongly fans about large scale folds formed later during movement suggesting passive rotation of cleavage with bedding during the later stages of thrust history the rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping or during movement on a later lower thrust cross folding may also develop as a result of lateral ramping thus complex patterns of interfering folds multiple cleavages and associated fiber filled fractures may be produced during motion on a single thrust fault and so magnify the problem of correlating cleavages and deciphering multiple deformations

cretaceous magnetizations in northwestern nevada and tectonic implications paleozoic and mesozoic arc related rocks of the black rock desert region of northwestern nevada have acquired a very stable postfolding secondary magnetization probably in late cretaceous time the remagnetization which appears to be of regional extent probably accompanied emplacement of granitic plutons in a northeast trending belt across northwestern nevada near the end of mesozoic time the region has experienced no appreciable tectonic rotation since that time

origin of magnetization in the phosphoria formation at sheep mountain wyoming a possible relationship with hydrocarbons hydrocarbon impregnated fenestral dolomites of the phosphoria formation were sampled on both flanks of sheep mountain anticline wyoming demagnetization results indicate one dominant component that is either synfolding laramide in age or predates folding cretaceous in age thermal demagnetization yields stable decay to temperatures between 500 c and 550 c and rock magnetic experiments suggest that the magnetization is dominated by a low coercivity component magnetic extracts contain spheres 15 diameter as well as other authigenic forms energy dispersive analysis demonstrates that fe is the only detectable element present in the spheres and x ray diffraction analysis indicates that magnetite is present in the extracts the spheres are interpreted to be authigenic magnetite the magnetization is interpreted as a chemical remanent magnetization crm residing in this authigenic magnetite chemical conditions created by the hydrocarbons may have caused precipitation of the authigenic magnetite and acquisition of the associated crm

grain scale deformation and the fold test evaluation of syn folding remagnetization the paleomagnetic fold test examines for pre syn or post folding re magnetization generally passive limb rotation is used to restore folded units to their original position and the position of the fisherian maximum deter mines the relationship between folding and magnetization in a more complete analysis the reorientation of material lines in different fold models is used however the magnetic vec tor should not be considered a material line as it represents the sum of vectors in individual magnetic carriers and does not cross grain boundaries therefore the role of grain scale deformation mechanisms was examined using two end mem bers rigid body rotation spin and homogeneous strain from this study it is concluded that 1 the fisherian maximum will in general occur before complete unfolding 2 the sign of the inclination may change during unfolding and consequently cross over unfolding is not representati ve of syn folding remagnetization 3 statistical analysis will not necessarily produce significant results a number of recommendations are given which include stepwise unfol ding and unplunging should be carried out routinely local rather than regional fold tests should be performed both limbs and the hinge of a single folded layer should be exa mined in order to determine the active deformation mecha

solution cleavage and its relationship to major structures in the idaho utah wyoming thrust belt micrites of the jurassic twin creek formation in the idaho utah wyoming thrust belt show spaced cleavage developed during folding and thrusting cleavage in each thrust sheet forms mainly by pressure solution aided by elevated temperatures and pressures caused by increased overburden as a result of emplacement of earlier overriding thrust sheets within each thrust sheet cleavage develops early and is contemporaneous with microscopic to mesoscopic buckle folds and faults large scale ramp folds formed during major motion on the thrust postdate the cleavage and passively rotate it into major fans

paleomagnetism of the upper devonian catskill formation from the southern limb of the pennsylvania salient possible evidence of oroclinal rotation multiple components of magnetization were isolated in the natural remanent magnetiza tion of samples of the upper devonian catskill formation red beds taken from the southern limb of the pennsylvania salient the dominant ther mally distributed component sf previously thought to predate folding is demonstrably syn folding in origins the mean direction for sf based on data from the current study and a pre vious study is declination inclination 161 6o 7 9 o a95 3 9 o pole position 127 3oe 43 1on a95 3 1 o n 14 sites although the remagnetization is clearly synfolding in most areas the relative ages of folding and remagne tization vary locally a subordinate high unblocking temperature component ht has a mean tilt corrected direction of 160o 36 o a95 16 o pole position 123 5oe 26 ion a95 15 4 o s n m 7 samples comparison of ht with the prefolding magnetization isolated in the northern limb of the salient suggests that the paleolatitude of this part of north america was about 16os and that part of the curvature of the salient was acquired during orogenesis

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

late paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east tennessee evidence for a post ore chemical event thermal demagnetization studies on samples from mineralized and unmineralized rocks in the lower ordovician knox group of east tennessee reveal a mean direction of d i 157 18 95 7 7 k 52 9 after 60 unfolding combining stable endpoint and remagnetization circle data although not significant at the 95 confidence level the fold test is suggestive of a synfolding alleghenian age of the magnetization the magnetization cannot be postfolding i e permian or younger because the in situ directions do not resemble those for post carboniferous times the magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the early permian but unlike poles for pre late carboniferous times characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test the best estimate for the paleopole resulting from this study falls at 40 n 126 e within the late carboniferous segment of the north american paleozoic apparent polar wander path thus the magnetization is interpreted as synfolding and secondary due to widespread kiaman remagnetization blocking temperatures of up to 520 c and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres secondary dolomites have the same magnetic characteristics as the primary limestones and it is inferred that similar magnetite spheres are the carriers of their magnetization because this magnetization and therefore its carrier occurs in sphalerite and secondary dolomite it must be coeval with or younger than the ore which itself is thought to be of middle to late paleozoic age it is argued that the magnetization was acquired during growth of magnetite associated with a late paleozoic chemical event

the origin of the basse normandie duplex boulonnais france abstract two hercynian duplexes are developed in visean limestones in the basse normandie quarry the lower duplex is completely exposed in a subvertical quarry wall the partially exposed upper duplex lies immediately above the lower duplex the duplexes are both located in the footwall of the hydrequent thrust which emplaced devonian clastic sediments above the visean limestones the lower duplex exposes all the internal thrusts a reference bed of chalky limestone the roof and floor thrusts and the duplex tip the duplex has been graphically restored to its pre deformation geometry by line length and area balancing and its resultant geometry is close to the model of boyer elliott the lower duplex shortened by two different mechanisms an initial phase of layer parallel shortening which produced no cleavage followed by thrust imbrication the average contraction of the front portion of the duplex was 49 natural strain of which 27 is layer parallel shortening and 22 is thrust imbrication however locally the bulk shortening increases from zero at the duplex tip to over 120 in a down dip direction the area balancing provides the most accurate estimates of bulk shortening line length balance calculations give minimum estimates only an area balance on the whole of the lower duplex gives a bulk shortening of 84 an area balance of the upper duplex yields an average contraction of 75 and the total contraction produced by both duplexes is 92

jurassic magnetostratigraphy 3 bathonian bajocian of carcabuey sierra harana and campillo de arenas subbetic cordillera southern spain abstract four sections in majocian bathonian middle jurassic pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300 c to in excess of 450 c the polarity patterns indicate that the majority of the bajocian and bathonian is characterized by quite frequent reversals of the magnetic field lengthy periods of constant polarity particularly constant normal polarity were not observed the average frequency of reversals is about 6 per ammonite zone which roughly may be interpreted as a frequency of a reversal every 260 000 years a rate comparable to that of the miocene pliocene paleolatitudes of these sites 25 28 are about 10 south of their present positions variable clockwise block rotations within the subbectic region have rotated these sites relative to stable iberia

cretaceous paleomagnetism of the methow pasayten belt washington detailed demagnetization experiments isolated a characteristic remanent magnetization in ten stable sites from the upper cretaceous winthrop and midnight peak formations in the methow pasayten belt of north central washington this remanence agrees best between opposite limbs of a fold the goat peak syncline when corrected for 46 of tilt this is consistent with magnetization acquired during deformation synfolding magnetization may have been facilitated by a thermo chemical event associated with synkinematic intrusions along the axis of folding the mean direction d 12 0 i 61 1 alpha 95 4 8 is highly discordant with respect to the expected direction for north central washington this discordance points to about 1 400 km of poleward transport and 48 of clockwise rotation between 93 and 45 48 ma

effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes france abstract the relationships among the anisotropy of magnetic susceptibility the finite strain and the characteristic remanent magnetization chrm have been investigated in deformed red sediments from the alpes maritimes in southeast france the magnetic properties of these rocks are due to hematite the finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and x ray texture goniometry studies of chlorite and illite the strain data permit a classification of the magnetic results according to four main stages of progressive deformation in the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading the chrm directions approximate to a fisherian distribution about a mean value close to the known permian direction for the region as deformation proceeds pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes with increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial at these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage the remanence directions in deformed rocks do not correspond to a fisher distribution and eventually deviate strongly from permian directions these results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock simple tilt correction without compensation for the strain would yield incorrect paleopole positions an attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker march 1932 equations for strain removal were applied to the remanence in two ways 1 a using strain data directly obtained in the field for each site and 2 b by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value correcting for the finite strain by both methods improves the within site grouping of paleomagnetic vectors in strongly deformed rocks strain removal assuming the march model results in overcorrection of the remanent magnetization data in low strain regions distortion may produce mechanical rotation of the hematite particles but recrystallization may alter the deformation mechanism in regions of high strain the limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the alpes maritimes

thrust fault geometry of the snake river range idaho and wyoming the snake river range is located at the north end of the absaroka fault system in the idaho wyoming utah thrust belt and it adjoins the teton mountains nine imbricate sheets of the absaroka system form a shingled array of overlapping thrusts most fault nomenclature in the range dates from early discontinuous reconnaissance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are or ever were connected as mechanical blocks fault names are revised here to consider the lateral continuity of sheets as the major criteria for defining the block to which a name should be applied the absaroka thrust and st john thrust are recognized to be equivalent different parts of a major transfer zone using the continuity of sheets as the main criteria the map pattern of the range is dominated by major re entrants to the west in the traces of the st john and absaroka thrusts the re entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the southeast the folding of the st john thrust above the indian creek culmination also folds the thrust sheets above it helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east the absaroka system thrusts formed from 20 to 40 km west of their present position they are folded by underlying faults of the jackson or farther south the darby system indicating that the absaroka system was emplaced into its present position by motion on these underlying faults discussions concerning mechanical interaction between the thrust belt and a foreland buttress ancestral teton or targhee uplift concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress the individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress

a paleomagnetic study of tectonically deformed red beds of the lower glarus nappe complex eastern switzerland a paleomagnetic study at 15 sites 315 samples in permo triassic red beds of the helvetic nappes of eastern switzerland isolated stable characteristic and secondary remanent directions thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction secondary directions are scattered but may have been acquired during the alpine deformation in the oligocene and miocene the characteristic directions also show large between site scatter anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage and that there is a north south lineation this anisotropy pattern implies that the magnetic grains were realigned by the alpine deformation it is not possible to explain the characteristic remanent magnetization deviations on a site by site basis but the site mean directions are streaked along a great circle the down dip direction of which is approximately parallel to the regional fold axes 60 20 this spread could be explained by a penetrative simple shear deformation associated with the nappe internal deformation

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

folding strain and graham s fold test in palaeomagnetic investigations summary graham s fold test is an important test of stability of magnet ization and is widely used in palaeomagnetic studies the change in dispersion of palaeomagnetic directions achieved by returning layers to their original orientation can give an indication of the relationship between the time of deformation and of magnetization acquisition however mcelhinny s criteria for application of the test are regarded by mcfadden jones to be inappro priate additionally simple rotation of directions about the strike of the layer may not be a sufficient procedure the rock type and style and amount of deformation may often appreciably affect the orientation of a vector or magnetic direction during deformation hence detailed observations are necessary to provide the basis of the steps used in applying the fold test apparent failure of the test may not indicate the absence of pre deformation stable magnetization in a rock although graham s fold test is important in palaeomagnetic and related tectonic investigations of deformed rocks application of the test may be very complex

remanence rotation due to rock strain during folding and the stepwise application of the fold test numerical modeling was conducted to determine the effects of simple models of folding strain on prefolding fisherian distributions of magnetization i found that the constant volume simple shear associated with flexural slip flexural flow folding can rotate a prefolding magnetization to have a maximum precision parameter k value at partial unfolding and thus appear to be synfolding in age if the magnetization is treated as a passive line marker this behavior only occurs for steep initial magnetizations the initial homogeneity of the fisherian precision parameters from each limb is degraded by this type of deformation and may be used to detect a strain modified remanence if the magnetization is carried by rigid mechanically active ellipsoidal or spherical particles a maximum k value at partial unfolding will occur for any initial inclination with the special case that rigid spheres will be rotated to cause a maximum k value at 50 unfolding for any initial inclination homogeneity of limb precisions is not affected in this case a discontinuous model of strain during folding in which pressure solution removes material at an axial planar spaced cleavage and causes limb dips steeper than bedding dips could also produce a maximum k value at partial unfolding strain and remanence studies of flexural slip folding in the allentown formation dolomite and the bloomsburg formation red sandstones and mudstones suggest that 1 the rigid grain model may be a more realistic model of magnetic particle behavior during shear strain and 2 about one quarter to one third of the theoretically predicted simple shear strain occurs in these rocks this amount of strain cannot explain the synfolding behavior observed in the allentown formation but could explain the maximum k values at 80 90 unfolding seen in the bloomsburg formation and perhaps other appalachian clastic rocks

time of initial thrusting in the sevier orogenic belt idaho wyoming and utah reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the sevier thrust belt suggests that initial thrust movement may be no older than aptian age this interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the idaho wyoming and utah thrust belts a major episode of middle cretaceous aptian cenomanian subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent sevier orogenic belt an earlier subsidence event took place during middle jurassic time more than 30 m y prior to the cretaceous event and may be the result of tectonic events to the west that are unrelated to thrust deformation in the idaho wyoming and utah thrust belts we find no verifiable evidence to support previous interpretations that sevier belt deformation and uplift began in late jurassic time

further paleomagnetic evidence for oroclinal rotation in the central folded appalachians from the bloomsburg and the mauch chunk formations renewed paleomagnetic investigations of red beds of the upper silurian bloomsburg and the lower carboniferous mauch chunk formations were undertaken with the objective of obtaining evidence regarding the possibility of oroclinal bending as contributing to the arcuate structural trend of the pennsylvania salient these formations crop out on both limbs of the salient and earlier but less definitive paleomagnetic studies on these units indicate that early acquired magnetizations can be recovered oriented samples were obtained from nine sites on the southern limb of the salient and eight sites from the northern limb in the bloomsburg the natural remanent magnetizations are multivectorial dominated by a component b with a distributed spectrum of unblocking temperatures ranging up to 670 c and a component c with a higher and very discrete distribution of unblocking temperatures the b component is uniformly of reverse polarity shows a statistically significant synfolding character and represents a late paleozoic remagnetization the c component passes fold tests with normal and reverse polarity site means the c component directions from the southern limb 345 1 31 6 and the northern limb 359 3 29 7 are significantly different in declination 14 2 10 4 but not in inclination 1 9 9 samples were also analyzed from seven additional sites in the mauch chunk on the southern limb of the salient inclusion of these new data gives a revised estimate of the difference between southern and northern limb mean directions of prefolding magnetizations in the mauch chunk of 23 3 12 5 in declination and 4 8 11 in inclination paleomagnetic data from the bloomsburg mauch chunk and revised results recently reported for the upper devonian catskill formation together indicate 22 8 11 9 of relative rotation accounting for approximately half the present change in structural trend around the pennsylvania salient the oroclinal rotation can be regarded as a tightening of a less arcuate depositional package that developed across a basement reentrant to achieve a curvature closer to that of the earlier zigzag continental margin outline

integrating laboratory creep compaction data with numerical fault models a bayesian framework 1 we developed a robust bayesian inversion scheme to plan and analyze laboratory creep compaction experiments we chose a simple creep law that features the main parameters of interest when trying to identify rate controlling mechanisms from experimental data by integrating the chosen creep law or an approximation thereof one can use all the data either simultaneously or in overlapping subsets thus making more complete use of the experiment data and propagating statistical variations in the data through to the final rate constants despite the nonlinearity of the problem with this technique one can retrieve accurate estimates of both the stress exponent and the activation energy even when the porosity time series data are noisy whereas adding observation points and or experiments reduces the uncertainty on all parameters enlarging the range of temperature or effective stress significantly reduces the covariance between stress exponent and activation energy we apply this methodology to hydrothermal creep compaction data on quartz to obtain a quantitative semiempirical law for fault zone compaction in the interseismic period incorporating this law into a simple direct rupture model we find marginal distributions of the time to failure that are robust with respect to errors in the initial fault zone porosity

ordovician docking of the carolina slate belt paleomagnetic data paleomagnetic results from eight sites in argillaceous metasediments from union county north carolina in the central carolina slate belt yield a consistent high blocking temperature magnetization component the precision of the mean declination and inclination of all site means is maximized at 50 tilt correction and the improvement in grouping relative to 0 and 100 tilt correction is significant at the 95 confidence level this tends to indicate that the high blocking temperature component was acquired during folding the metamorphism and deformation in this area are rather well constrained by recent 40ar 39ar data to the late ordovician 450 ma we therefore interpret the computed pole position 29 6 n 122 1 e 95 5 11 as late ordovician in age and note that it lies close to some of the late ordovician poles for cratonic north america the paleolatitude 22 2 s of the sampling site is certainly unlike those derived from armorica and gondwana for this time interval and is rather similar to that proposed for the iapetus coast of laurentia during the late ordovician indeed the magnetization and deformation may be contemporaneous with the docking of the carolina slate belt onto the laurentian continent

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

analysis of microstructures in the meade plate of the idaho wyoming foreland thrust belt u s a abstract kinematic histories of several imbricate slices of the meade plate in the idaho wyoming foreland thrust belt were determined using analyses of e lamellae in calcite f lamellae in dolomite and sub basal i deformation lamellae in quartz supporting these data were analyses of stylolites strained fossils and other markers and vein filled extension fractures compression was always parallel or sub parallel to bedding on upright fold limbs and on overturned fold limbs was either sub parallel or more commonly at a high angle to bedding extension in both limbs was generally either perpendicular to bedding or parallel to the fold axes observations in the hinge zones of folds were inconsistent while near thrust faults the microstructures record compression at a high angle to the nearby thrust surface measurement of different minerals in the same sample as well as cross cutting stylolite vein relations indicate that early compression was parallel to bedding while later compression was at a high angle to bedding these data suggest that regional shortening and probably compressive stress was horizontal or sub horizontal throughout the deformation and that most of the strain within thrust plates is directly related to folding and not thrusting the following structural history is proposed 1 1 initial layer parallel compression 2 2 buckle folding soon yielding to more complex folding in a three dimensional strain field 3 3 locking of folds and 4 4 locking of thrusts the fold locking angle as well as fold geometry indicates that these structures might be characterized as kink folds but the asymmetry is not consistent with theoretical analyses of kink folds strain due to basal traction along the fault surface is only important within 50 200 m of that surface depending on lithology

source of anomalous magnetization in area of hydrocarbon potential petrologic evidence from jurassic preuss sandstone wyoming idaho thrust belt the jurassic preuss sandstone which crops out in the central part of the wyoming idaho thrust belt on trend with a hydrocarbon producing region to the south has been previously identified as the source of anomalous magnetization in the area elsewhere anomalous magnetization in sedimentary rocks near hydrocarbon accumulations has been attributed to hydrocarbon engendered magnetic minerals but magnetization of the preuss is controlled by detrital magnetite evidence of a detrital origin for magnetite includes 1 concentration of magnetite grains along laminations containing other heavy minerals 2 the presence of exsolved ilmenite hematite and spinel in the magnetite grains and 3 titanium contents typical of igneous derived magnetite that detrital magnetite is responsible for the anomalous magnetization in the preuss is further indicated by the systematic eastward decrease in magnetite abundance corresponding to a similar eastward decrease in magnetic susceptibility and remanent magnetization of the unit petrologic and vitrinite reflectance studies indicate a complex low temperature 150 c or 302 f diagenetic history for the preuss nevertheless preservation of detrital magnetite the presence of diagenetically early ferric oxide minerals and the absence of sulfide minerals all indicate that the preuss has not experienced sulfidic reducing conditions common in areas of hydrocarbon seepage the marine carbon isotopic composition of calcite that cements most preuss sandstones dgr13c values ranging from 2 47 to 1 48 pmil is evidence that carbonate diagenesis also was not influenced by hydrocarbons the results of this multidisciplinary study of the preuss underscore the importance of similar studies when evaluating the sources of aeromagnetic anomalies in areas of hydrocarbon potential

strain reorientation of hematite and its bearing upon remanent magnetization using a computer based nume rical model we suggest that passive reorientation of hematite planar particles induced by strain perturb significantly the remanent magnetization carried by the hematite the evolution of its direction and dispersion in a progressively incre asing strain can be correlated with the evolution of passive 1ines in real rocks the amount of reorientation can be appro ched by measuring one of the thr ee follo wing tensors strain tensor orientation tensor of some hematite cristallographic axes or magnetic susceptibility tensor from examples taken in the maritime alps and in brittany we emphasize the possible correlations between these tensors orien tation of principal axes and axial ratios fina1ly we use our best estimate of strain to correct the paleomagnetic results by the strain removal technique we have already used in the maritime aips cogne and perroud 1985 when applied to the preliminary paleom gnetic results ob tained from ordovician deformed redbeds of brittany it allows to recover a conclu sive primary magnetization direction although the characteristic magnetizations

a comparison between airguns and explosives as wide angle seismic sources the relative merits of a 48 gun 9324 cu in 153 litre airgun array and a 200 kg explosive source are considered for the purposes of long range 0 400 km refraction seismic work with particular reference to traveltime modelling theoretical source calculations indicate that in the frequency range 2 5 12 0 hz the airgun source will produce an rms pressure 8 of that produced by the explosive source and an initial burst pressure 17 of that produced by the explosive source observed data support these calculations at short ranges and illustrate the greater attenuation of the airgun signal with range due to its lack of very low frequency 5 hz content at short offsets the airgun array provides a preferable seismic source to the explosives due to densely spaced shots and a consistent waveform resulting in excellent trace to trace coherence with increasing offsets it may be necessary to stack the airgun data to enhance its signal to noise ratio here we use a 4 fold stack large explosive shots although more powerful produce a less consistent waveform and are more widely spaced due to operational constraints the offset at which airguns provide a preferable source is dependent on the ambient noise this practical comparison of real sources demonstrates that even without advanced processing a well tuned airgun array may provide a preferable source to explosives at offsets up to 160 km under favourable experimental conditions

jurassic magnetostratigraphy 1 kimmeridgian tithonian of sierra gorda and carcabuey southern spain abstract two coeval sections of red to white ammonite rich pelagic limestones spanning the complete kimmeridgian and most of the tithonian were sampled in detail all samples were treated by progressive thermal demagnetization to remove a present field overprint characteristic magnetization is carried primarily by magnetite polarity intervals are easily identified and correlate well between the two sections the tithonian polarity sequence can also be correlated to sections in northern italy the similarity between the polarity sequence and the m sequence of marine magnetic anomalies coupled with the precise biostratigraphic control allows assignment of the following ages to the m sequence the late early tithonian boundary is correlated to the end of m 20 the tithonian kimmeridgian boundary to the end of m 23 the late early kimmeridgian boundary to the latter part of m 24 and the kimmeridgian oxfordian boundary within or slightly after m 25 the mean directions of characteristic magnetization have 95 s less than 3 and demonstrate extensive differential block rotation within the subbetic province paleolatitudes during the kimmeridgian tithonian are in the range of 16 24 n