a window into the lower crust trace element systematics and the occurrence of inclusions intergrowths in granulite facies rutile abstract rutile occurs as an accessory mineral in many high temperature metamorphic assemblages and has the potential to identify and investigate uht metamorphic terranes whilst the use of zr in rutile geothermometry to identify ultrahigh temperature terranes is appealing its application to granulites can be difficult owing to diffusional resetting of zr concentrations during cooling and decompression in order to provide constraints on p t conditions and trace element systematics during ultrahigh temperature metamorphism metamorphic rutile in granulites from the archean napier complex and the palaeozoic rauer group antarctica have been investigated for trace element composition and mineral inclusions intergrowths textural observations and zr in rutile temperatures show that unlike the large proportion of rutiles grains in the matrix rutile grains that are shielded by phases with low zr diffusivities e g orthopyroxene and or are in chemical isolation from zircon have the potential to retain zr concentrations that correspond to ultrahigh temperatures principal component analysis reveals that v cr nb mo ta and w are significantly enriched in rutile from archean and proterozoic ultrahigh temperature terranes high concentrations of elements such as cr and v indicate a spinel rich source demonstrating the effect of protolith bulk chemical composition on rutile trace element signatures rutile is shown to contain inclusions and intergrowths of aluminium silicate quartz corundum and feldspar including the first reported occurrence of prograde kyanite which provides direct evidence that the napier complex experienced a typical clockwise p t evolution

multivariate characterization of meandering abstract a suite of 40 morphometric variables has been measured on 57 sections of freely meandering channels from 33 rivers these objective variables are computed from digitized traces of the channel centerline and include measures of sinuosity meander wavelength curvature moments meander asymmetry and pattern irregularity several variables are measured on the half meanders defined by successive inflection points principal components analysis reveals that morphological variations between freely meandering streams can be resolved into four factors related to meander wavelength meander irregularity sinuosity and large scale wandering of the meander axis discriminant analysis using the morphometric variables results in almost perfect separation between natural streams and two theoretical models of meander planform ferguson s disturbed periodic model and howard and knutson s simulation model despite visual similarities the simulated meanders are too regular in size and shape in relation to natural ones perhaps because the interaction of effects of curvature and alternate bars upon topography and flow structure in natural channel bends produces multiple scales of bank erosion not accounted for in present simulation models

remagnetization dating of roman and mediaeval masonry abstract the earth s magnetic field may partially remagnetize rocks often by the process of viscous remanent magnetization vrm thus when masonry is turned and placed in a monument it magnetizes parallel to the earth s field partly overprinting any older geological magnetizations the intensity of this new overprint will increase with time if the masonry is undisturbed older magnetizations require higher demagnetization temperatures thus the unblocking temperature tub relates directly to the age of the remanence and therefore the age of stabilization of the masonry for suitable rocks we can calibrate the remagnetization time scale for the rock using historically dated structures the scale then permits us to estimate the age of more enigmatic structures by interpolation or modest extrapolation some samples reveal multiple episodes of remagnetization showing that masonry was recycled acquiring a differently oriented remanence each time that it stabilized in a new architectural configuration masonry from the bishop s palace lincoln u k includes roman c ad 300 mediaeval 1160 1450 post civil war 1650 1720 and victorian phases that have characteristic tub consistent with their ages some mediaeval masonry recycled roman building materials

effects of experimental deformation on the remanent magnetization of sediments abstract changes in direction and in intensity of the remanent magnetization of sediments were studied during experimental deformation leading to an axial shortening of 20 30 the remanence was produced before shortening by anhysteretic magnetization in an alternating field of 90 mt superimposed on a parallel steady field of 1 mt at various inclinations with respect to the sample the samples were sandstones marls and volcanic sediments the results show that the direction of magnetization is stable during the deformation within the experimental precision the intensity of magnetization decreases during shortening by an amount which depends on the type of rock about 30 for sandstones and about 5 for marls and volcanic sediments the march model does not account for the data and a discontinuous deformation must be invoked the evolution of the young modulus of sandstones during cyclic loading and unloading shows first a decrease in relation to crack density growth then an increase showing a localization of the fracturation and finally a deformation by displacement of microblocks an interpretation is given after a discussion of the results obtained by other authors with plasticine and artificial clays the amplitude of the decrease and of the rotation of the remanent magnetization could be a function of the size of the magnetic grains compared with the scale of the fracturation during non homogeneous processes of deformation

low temperature demagnetization and ice pressure demagnetization in magnetite and haematite summary low temperature demagnetization ltd is confirmed as an effective method of removing soft low coercivity components of remanence in magnetite bearing specimens by passing the magnetite through its low temperature transition 130 k haematite has its low temperature transition at approximately 258 k when some specimens bearing fine grained haematite pigment pass through this transition the directional component which is hardest highest coercivity may be more reduced than the soft components in other samples of the same material the opposite may occur depending on the range of coercivities represented in each directional component thus ltd does not consistently isolate hard directional components of remanence in haematite it is therefore an unsuitable method for routine palaeomagnetic cleaning of natural remanent magnetization carried by haematite specimens that contain moisture for example from recent drilling with watercooled equipment show greater reductions in remanence intensity after ltd this is attributed to ice pressure demagnetization due to the effects of stress as pore ice thaws when the specimens warm to room temperature it is recommended that porous samples are thoroughly dried out before ltd is performed if consistent results are required

dynamic equilibrium between an active uplift and the sevier river utah in a zone of active tectonic uplift one expects a river to change morphology simply because valley slope deforms but by cutting through a transverse uplift the sevier river in southern utah also adapts to the tectonics by eroding and depositing these localized changes of transport are caused by varying channel geometry pattern sedimentology and slope in the uplift the river is narrow efficiently shaped coarser grained and expends a greater proportion of energy on bed particles it deposits sediments downstream in exaggerated meandering reaches with wider less efficient channels localized erosion and deposition create a dynamic equilibrium with the structural growth but this equilibrium includes nonlinear responses like knickpoints and depositional lobes in response to the required erosion and deposition the river so dramatically alters channel grain size and channel geometry that the increase of sinuosity expected in the tectonically steepened valley does not occur

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

paleomagnetic direction obtained by strain removal in the pyrenean permian redbeds at the col du somport france abstract in order to investigate the possibility and limitations of paleomagnetic works within strained regions a paleomagnetic study related with strain analysis has been conducted in the deformed pyrenean permian redbeds in the col du somport area paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold the results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that 1 the shale beds have undergone a penetrative strain 2 the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding showing that they recorded the compaction it is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric the paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn or post tectonic magnetic component as both components appear to be carried by hematite pigment their separation using classical demagnetization procedures has been difficult a characteristic remanent magnetization chrm has however been determined when possible as the hardest component in demagnetization curves then the chrm direction distributions are represented in stereographic density plots although these chrm directions exhibit a clear tendency towards se declinations and shallow inclinations characteristic of permian paleomagnetic field direction for the iberian plate the tilt correction does not induce a clustering of these directions strain is inferred to be responsible for this situation assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of march 1 an attempt has been made to remove the effect of strain upon the remanent magnetization it is shown that when using a reconstructed tectonic strain tensor i e the total strain tensor as measured in the field corrected for an estimated compaction we obtain a significant clustering of chrm directions the computation of the relevant vgp gives a pole position 210 5 e 42 0 n compatible with the reference apwp for the iberian plate it is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks

significance of active growth faulting on marsh accretion processes in the lower pearl river louisiana abstract neotectonic processes influence marsh accretion in the lower pearl river valley active growth faults are suggested by groupings of ponded river channel sections transverse and linear river channel sections and down and across valley contrasts in channel sinuosity seismic profiles identified several likely fault induced structural anomalies two of which parallel the axes of surface distributary networks lithostratigraphy and biostratigraphy of six cores from across a suspected fault in the west middle river combined with 14 c based age control yielded evidence of vertical offsets indicating that this river section is on the plane of a growth fault these data were used to estimate fault slip rates over two time intervals 1 2 mm y over the last 1300 yr and 0 2 mm yr 1 over the last 3700 yr and delineated a sinusoidal pattern of deformation moving distally from the fault which we interpret as resulting from fault propagation folding higher rates of sediment accumulation of the order of cm yr 1 from 210 pb xs and 137 cs activity data on the down thrown side are consistent with sedimentary response to increased accommodation space and mass based sediment accumulation rates g cm 2 yr 1 exhibit a pattern inverse of that shown by fault driven sinusoidal deformation we contend that near surface growth faults are critically important to driving accretion rates and marsh response to sea level rise

tectonic strain and paleomagnetism experimental investigation a porous clay rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation at 100 mpa room temperature and constant natural strain rates of 1 10 5 s 1 the paleomagnetic vector does not simply rotate away from the axis of compression when the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding as expected but the remanence also rotates toward bedding when the rock is shortened parallel to bedding moreover in some cases the remanence vector changes azimuth within the specimen during experimental deformation thus bedding anisotropy and the accompanying mineral fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study this anisotropy also influences the post deformational behaviour of remanence 4 months after deformation some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre deformational attitude whereas some other cores continued to show progressive changes intragranular recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

evaluating influence of active tectonics on spatial distribution pattern of floods along eastern tamil nadu india flooding is a naturally recurrent phenomenon that causes severe damage to lives and property predictions on flood prone zones are made based on intensity duration of rainfall carrying capacity of drainage and natural or man made obstructions particularly the lower part of the drainage system and its adjacent geomorphic landforms like floodplains and deltaic plains are considered for analysis but stagnation in parts of basins that are far away from major riverine systems is less unveiled similarly uncharacteristic flooding in the upper and middle parts of drainage especially in zones of an anomalous drainage pattern is also least understood even though topographic differences are attributed for such anomalous spatial occurrence of floods its genetic cause has to be identified for effective management practice added to structural and lithological variations tectonic movements too impart micro scale terrain undulations because active tectonic movements are slow occurring long term geological processes its resultant topographical variations and drainage anomalies are least correlated with floods the recent floods of tamil nadu also exhibit a unique distribution pattern emphasizing the role of tectonics over it hence a detailed geoinformatics based analysis was carried out to envisage the relationship between spatial distribution of flood and active tectonic elements such as regional arches and deeps block faults and graben and drainage anomalies such as deflected drainage compressed meander and eyed drainages the analysis reveals that micro scale topographic highs and lows imparted by active tectonic movements and its further induced drainage anomalies have substantially controlled the distribution pattern of flood

magnetic characterization using a three dimensional hysteresis projection illustrated with a study of limestones summary limestones provide an important source of palaeomagnetic information despite their low content of submicroscopic remanence bearing minerals the chief sources of these minerals are thought to be clastic volcanic magnetite and titanomagnetite and organic magnetite the latter mostly from bacterial sources chemically remagnetized limestones carry magnetite or pyrrhotite three hysteresis properties prove useful in identifying and characterizing these mineralogical influences on limestones the ratio of zero field maximum remanence to saturation remanence m r m s in an applied field coercivity of remanence b cr and coercivity b c to a lesser extent k f m s may be useful where k f is the ferrimagnetic susceptibility traditionally these have been plotted on a combination of 2 d graphs that of necessity only preserve two variables day et al 1977 wasilewski 1973 however we found that magnetic discrimination and characterization of the limestones was much easier on a three axis hysteresis projection that preserves the values of b cr b c and m r m s as independent variables using logarithmic scales the regression surfaces through the data become almost planar and distinguish pelagic shallow marine shelf and remagnetized limestones on the basis of the slope and intercept of the associated regression surface clearly there are sensitive sedimentological geochemical or organic influences that dictate the magnetic mineralogy through sedimentary environment moreover the 3 d plot of hysteresis criteria avords easy recognition of remagnetized limestones and may permit the rejection of material unsuitable for palaeomagnetic study the 3 d hysteresis projection may be useful for the characterization of other rocks and magnetic materials

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

apparent polar wander paths and the close of late archean crustal transpression northern ontario 1 lamprophyre dikes of the southern superior province of the canadian shield crosscut archean structures including a late archean unconformity and its overlying younger archean timiskaming sedimentary and volcanic rocks the dikes also cut the single schistosity that formed in these rocks nevertheless the lamprophyre dikes show a weak internal tectonic fabric that is approximately parallel to the schistosity in the bedrock and oblique to dike walls the consistent orientation of this internal tectonic fabric has been recognized in every lamprophyre dike using anisotropy of magnetic susceptibility ams thus the lamprophyre dikes provide a useful chronological marker trapping the last pulse of archean tectonism unfortunately our previous attempts to compare the lamprophyre paleopoles with those of the age calibrated apparent polar wander path apwp failed because it was not possible to isolate separate young and old components of magnetization that failure occurred due to the overlap of blocking temperatures or coercivities for the different characteristic vector components borradaile et al 1994a using new cores from the same specimens here we distinguish successfully between consistently oriented vector components by using low temperature demagnetization ltd before thermal demagnetization although the paleopoles appear well defined comparisons with published apwps for the superior province from late archean irving 1979 dunlop 1983 symons et al 1994 to early proterozoic buchan et al 2000 are difficult moreover the earlier parts of the paths are progressively less precise and complete nevertheless a component remanences for the dikes give a paleopole near the path at 2650 ma for a timiskaming volcanic breccia with a u pb age of 2696 ma the a component paleopole may lie between 2600 and 2650 ma b component remanences yield paleopoles in the nw pacific region incompatible with any proposed archean path but perhaps representing some much younger proterozoic remagnetization event e g keweenawan 1100 ma

influences of relative sea level rise and mississippi river delta plain evolution on the holocene middle amite river southeastern louisiana abstract the holocene geomorphic history of southeastern louisiana s middle amite river is recorded in the stratigraphy of three alloformations identified in decreasing age as the watson wat denham springs ds and magnolia bridge mag the wat meander belt formed by at least 9000 yr b p when sea level was lower and the amite river was tributary to a larger ancestral drainage basin the ds became an active meander belt by at least 3000 yr b p in response to relative sea level rise and eastward progradation of the mississippi river delta plain the mag developed its meander belt in part during the european settlement of the drainage basin and is now attempting to adjust to modern anthropogenic influences geomorphic influences on the middle amite river floodplain have temporal and spatial components that induce regional and local scale effects regional extrinsic influences caused meander belt avulsion that produced alloformations however local influences produced intrinsic geomorphic thresholds that modified channel morphology within a meander belt but did not induce alloformation development base level influences of the relative sea level rise and the mississippi river delta plain were so dominant that the effects of possible climate change were not recognized in the holocene amite river system

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

use of alloformations for definition of holocene meander belts in the middle amite river southeastern louisiana geomorphological studies in drainage basins of the northern gulf of mexico coastal plain have indicated similarities in flood plain character and comparable influences on basin evolution alluvial morphology and stratigraphy can be linked to flood plain evolution through the use of alloformations unconformity bounded stratigraphic units this procedure extends classical terrace mapping concepts to conform with formally defined stratigraphic procedures a meander belt segment of the middle amite river valley in southeastern louisiana was selected to test the applicability of alloformation mapping in part of a regional scale 5 000 km 2 drainage basin three alloformations identified as magnolia bridge mag denham springs ds and watson wat can be differentiated by unconformable boundaries landscape morphology and relative pedogenic development the valley fill consists of a lower sandy and upper silty facies preserved deposits consist of channel lag point bar and scroll bar channel fill and overbank sequences lithofacies and sedimentary environments are dependent on fluvial process and a suite of meander belt environments can be identified within each alloformation surface horizons of alluvial soils are all similar ochric epipedons but subsoil horizons are chiefly stratified parent material on the mag entisols cambic horizons on the ds inceptisols and minimally developed argillic horizons on the wat alfisols allostratigraphy provides an objective method for definition of geologic units that contain genetically related but heterogeneous sedimentary deposits alluvial sediments are differentiated in a manner that successfully integrates geomorphic sedimentologic and pedologic data into significant stratigraphic models this approach is applicable to a wide variety of quaternary settings where prior stratigraphic techniques have resulted in ambiguity

remagnetisation of a rock analogue during experimental triaxial deformation abstract synthetic specimens containing pseudo single domain magnetite were given a univectorial anhysteretic remanent magnetisation they were deformed in a ductile manner at room temperature under a confining pressure of 150 mpa 1 5 kbar the strain rate was 10 5 s 1 and the maximum strain achieved was 23 shortening a weak 0 035 mt magnetic field was present in the pressure vessel during deformation hydrostatic compaction alone produced pressure demagnetisation of low coercivity components of remanence but did not change remanence directions macroscopic differential stress was needed to deform the specimens and this also permitted syndeformational remagnetisation for values of more than 20 mpa 200 bars this added new components of piezo remanent magnetisation prm approximately parallel to the direction of the magnetic field in the pressure vessel they were acquired in all coercivity intervals but the largest components were added below 35 mt the final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence prm contributions almost completely masked the changes related to strain induced grain rotation at these low strains the directions of the externally imposed stress strain axes may not significantly influence the direction of new remanence components prm acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress history of the magnetite grains in part this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation or thermal or chemical processes

the role of deep processes in late 20th century subsidence of new orleans and coastal areas of southern louisiana and mississippi 1 geodetic leveling observations from biloxi ms to new orleans la and water level gauge measurements in the new orleans lake pontchartrain area were analyzed to infer late 20th century vertical motions these data were used to test the validity of previous subsidence rate measurements and the models that predict the location and causes of subsidence water gauges attached to bridge foundations and benchmarks affixed to deep rods that penetrate holocene strata subsided as much as 0 8 m locally between 1955 and 1995 the observed deep seated subsidence far exceeds model predictions and demonstrates that shallow processes such as compaction and consolidation of holocene sediments are inadequate by themselves to explain late 20th century subsidence deep seated subsidence occurring east and north of the normal faults marking the gulf of mexico basin margin can be explained by local groundwater withdrawal and regional tectonic loading of the lithosphere by the modern mississippi river delta mrd sharp changes in subsidence coincide with strands of the basin margin normal faults displacements are consistent with activity and show motions consonant with fault creep deep subsidence of the region to the south including new orleans can be explained by a combination of groundwater withdrawal from shallow upper pleistocene aquifers the aforementioned lithospheric loading and perhaps nongroundwater related faulting subsidence due to groundwater extraction from aquifers 160 to 200 m deep dominated urbanized areas and is likely responsible for helping to lower local flood protection structures and bridges by as much as 0 8 m

active near surface growth faulting and late holocene history of motion matagorda peninsula texas abstract the structural framework of the northern gulf of mexico coastal zone includes numerous growth fault systems neotectonic processes in coastal environments here have been shown to be important contributors to subsidence and relative sea level rise as well as having significant influences on sedimentary accretion processes one active growth fault at east matagorda peninsula texas was subjected to detailed study to address the hypotheses that 1 this fault was present and active before the peak of regional hydrocarbon recovery in the mid 20th century and that 2 slip along this fault has been episodic over the resolved period of record to characterize the late holocene behavior of this fault three core transects were established normal to the surface fault trace and stratigraphic analysis was conducted with the use of radiocarbon 14c for chronological control correlation of time equivalent stratigraphic boundaries reveals a maximum total late holocene offset of 0 7 m the matagorda fault was in place long before the peak of regional oil and gas extraction but experienced a late holocene rejuvenation initiated between 2500 and 1500 years bp the kinematic behavior of the fault expressed as temporally and spatially variable measures of expanded stratigraphic thicknesses on the downthrown extent suggest that sediment loading from the transgressive matagorda barrier complex may have contributed to this rejuvenation and that slip along the fault persists to the present radiocarbon chronology also revealed clear differences in the magnitude of fault throw during the late holocene with maximum displacement 0 7 m occurring at the southwestern most transect and minimum throw at the northeastern most transect 0 2 m

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

low temperature demagnetization isolates stable magnetic vector components in magnetite bearing diabase summary it may be difficult to isolate stable palaeomagnetic vectors of different ages if they lie in grain assemblages with overlapping ranges of coercivity or of unblocking temperature this is because some moments associated with either vector may demagnetize at the same stage of experimental demagnetization the sharp transition between vector components may be obscured and stable components may appear less linear on the demagnetization plot thermal and alternating field af demagnetization techniques remove vector components on a quantitative basis according to a discrete limiting unblocking temperature or coercivity low temperature demagnetization ltd differs in that it removes vector components discretely wherever there are mobile domain walls experiments tested the ability of ltd to improve the effectiveness of af demagnetization on isothermal and anhysteretic remanent magnetizations irm arm in diabase multicomponent nrms were simulated irms or arms in different non overlapping coercivity ranges along three orthogonal axes or along two non orthogonal directions the known directions of the experimentally applied vector components were always more successfully verified by af demagnetization if ltd was first applied for the same specimens ltd reduced the same artificial remanences by 50 per cent for the coercivity range 0 15 mt by 25 per cent for the range 15 30 mt and negligibly for higher coercivity fractions ltd demagnetizes polydomain magnetite as domain walls rearrange on passing through a low temperature transition near 120 k

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

influence of growth faults on coastal fluvial systems examples from the late miocene to recent mississippi river delta abstract the details of how fluvial systems respond to spatial changes in land surface subsidence produced by active faulting remain incompletely understood here we examine the degree to which the positioning of individual channels and channel belts is affected by local maxima in subsidence associated with the hanging walls of growth faults the channel forms and faults are imaged using a seismic volume covering 1400 km2 of breton sound and barataria bay in southern louisiana usa we look at the consequences of interactions between channels channel belts and faults in late miocene to recent strata more than fifty individual channels that crossed the traces of active growth faults were examined of these channels only three appear to have been redirected by the faults there also appeared to be no systematic change in the cross sectional geometries of channels or channel belts associated with crossing a fault though the orientation of the channel belts appears to be more influenced by faulting than the orientation of individual channels seven out of ten mapped channel belts appear to have been steered by growth faults we propose that channel belts are more likely to be influenced by faults than individual channels because channel belts are longer lived features unlikely to shift their overall position before experiencing a discrete faulting event in addition the style of influence in the few cases where an individual channel is affected by a fault is different from that of larger systems while downstream of a fault channel belts generally become oriented perpendicular to fault strike the individual channels are directed along the hanging wall of the fault running parallel to the fault trace we relate this to the ratio of the length scale of fault rollover relative to the channel or channel belt width fluvial fault interactions with higher values for this ratio are more likely to be carried parallel to the fault trace than systems with lower ratio values

experimental deformation of two component irm in magnetite bearing limestone a model for the behaviour of nrm during natural deformation abstract experimental room temperature triaxial deformation at 200 mpa 2 kbar and at a controlled strain rate of 10 5 s 1 is performed on three limestones with magnetic behaviour dominated by single domain magnetite permanent deformation is achieved mainly by twinning of calcite isothermal remanent magnetisation irm in the samples is reduced in magnitude especially when parallel to compressive stress the decrease in magnitude correlates slightly better with peak stress than with strain in accord with previous work in uniaxial tests indicating the role of stress in deformational demagnetisation remanences have their vector plots condensed after deformation particularly in the range 0 30 mt a two component irm is used as a simple model of a multi component natural remanent magnetisation nrm the resultant vector is deflected by the action of stress towards whichever components are least reduced by deformation in the case of magnetite bearing rocks this appears to be the most coercive components in the case of a natural deformation it is predicted that the effects of stress at any stage should be to remove progressively any low coercivity components added to an nrm by remagnetisation during tectonism especially those parallel to compressive stress

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

evidence of possible tectonic upwarping along the south carolina coastal plain from an examination of river morphology and elevation data although the fault s responsible for the 1886 charleston south carolina earthquake have not yet been identified primarily because of the lack of surface rupture evaluation of landsat imagery aerial photography and topographic maps have revealed an 200 km long 15 km wide north northeast trending zone composed of subtle topographic highs and morphologic changes in rivers that may be associated with tectonic activity river anomalies observed within this zone include river bends that are convex toward the north northeast incised channels changes in river patterns and convex upward longitudinal profiles analyses of geologic and geophysical data further indicate that these surface features may be the result of ongoing tectonic uplift along a north northeast trending fault zone possibly associated with recent seismicity near charleston

experimental and numerical modeling of irm rotation in deformed synthetic samples abstract the effect of strain upon isothermal remanent magnetization irm carried by hematite particles embedded in a plasticine matrix has been investigated magnetized artificial hand samples were deformed by simple coaxial shortening after deformation the hand samples were cut into 8 15 specimens and each specimen s irm measured hence a mean irm direction was obtained for each shortening step the main results are 1 when the initial direction of irm is parallel to the shortening axis 3 the hand sample mean irm stays parallel to 3 during deformation however individual magnetization directions are deflected towards the flattening plane and thus the specimen directions are scattered by strain this is accompanied by a significant decrease of magnetization intensity 2 in contrast when the initial direction of irm is perpendicular to 3 the shortening results in a clustering of specimen magnetization directions the mean direction being unchanged this is accompanied by a slight increase of magnetization intensity 3 in all other cases where initial irm direction makes an intermediate angle with shortening direction deformation induces a deviation of each specimen irm toward the flattening plane this results in an overall deflection of hand sample mean irm the amount of rotation increases with strain intensity changes of grouping and intensity depend on the initial angle between irm and 3 defining a continuum in behavior between irm directions being either parallel to 3 or perpendicular to 3 finally it is shown that all the experimental changes in irm due to deformation direction grouping intensity can be modeled by a passive marker rotation of hematite planar particles

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

tectonic control of subsidence and southward displacement of southeast louisiana with respect to stable north america 1 gps data collected between 1995 and 2006 suggest that southeast louisiana including new orleans and the larger mississippi delta are both subsiding vertically and moving southward with respect to stable north america both motions are likely related due to their common tectonic setting subsidence in the new orleans area occurs in part because it is located in the hanging wall of a large listric normal fault system that forms the northern boundary of a 7 10 km thick allochthon that is detached from stable north america southward motion of this allochthon relative to stable north america occurs at 2 2 0 6 mm yr the average subsidence rate for gps sites located on the allochthon is 5 2 0 9 mm yr relative to earth s center of mass or 7 mm yr relative to mean sea level motion of the allochthon is likely due to the gravity instability created by rapid holocene sediment deposition in the delta following continental glacial retreat and is facilitated at depth by weak salt horizons because new orleans and other communities of southeastern louisiana lie atop this active allochthon future motion of this body should be considered during rebuilding of the region following hurricanes katrina and rita

remanent magnetism and ductile deformation in an experimentally deformed magnetite bearing limestone abstract a limestone with pseudo single domain psd pure magnetite is given an isothermal remanent magnetization irm samples are deformed triaxially at 200 mpa 2 kbar confining pressure at room temperature and at a strain rate of 10 5 s 1 the limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20 shortening the uniform saturation irm rotates away from the shortening axis approximately by an amount expected for the homogeneous strain of a non material line marker hydrostatic compaction alone shows that this is time dependent and compatible with progressive damage of the ferrimagnetic grains strain produces an increase in coercivity of the magnetite an increase in its saturation remanence and an increase in its anisotropy of magnetization with respect to irm these observations are compatible with reduction in effective grain size and change of shape of magnetite experimental deformation reduces the intensity of magnetisation chiefly by the removal of low coercivity components of remanence when there is a pure irm vector

response of alluvial rivers to slow active tectonic movement alluvial rivers respond to valley slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river on the basis of experimental results and field examples hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers an experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift with subsidence aggradation in the central reach was the main response transverse bars developed downstream of the subsidence axis an experimental meandering channel responded to slope steepening with a sinuosity increase bank erosion and point bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis upstream of the uplift axis and downstream of the subsidence axis where the slope was flattened water flooded over bars local convexity in longitudinal profiles of the middle rio grande new mexico is considered to be formed by a domal uplift local aggradation and degradation could be explained by the effect of uplift the san joaquin river california which is now highly controlled does not show clear adjustment to the rapid subsidence due to ground water withdrawal it shows however a channel pattern adjustment to active tectonic subsidence that has been occurring for a long time the san antonio and guadalupe rivers in texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes

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

strain magnetic fabric and paleomagnetism of the deformed red beds of the pont rean formation brittany france a joint study of strain anisotropy of magnetic susceptibility ams and paleomagnetism has been conducted in the deformed red beds of the ordovician pont rean formation brittany france analysis of results from 13 sampling sites shows that homogeneous strain estimated from the shape of elliptical reduction spots in each site controls the development of a magnetic fabric during the hercynian folding of this formation the good correlation between strain and ams tensors is interpreted as mainly arising from a strain induced progressive rotation of planar hematite that tends to align within cleavage plane this rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage because of the high angle between the initially subvertical ordovician remanent magnetization and the regionally subhorizontal shortening direction this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within site initial dispersions for the same reason the overall distribution of mean site pretectonic magnetization vectors is clustered at the in situ deformed state this results in an anomalous behavior of the classical fold test that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors it allows recovery of the initial direction of these pretectonic ordovician magnetizations d 235 i 75 95 6 5

channel sinuosity of the koros river system hungary romania as possible indicator of the neotectonic activity abstract the koros river drainage basin is located in the eastern part of the great hungarian plain west of the apuseni mts in the middle of the pannonian basin the channels of the river and its tributaries are mostly meandering characterized by large loops and multiple bends in their pre regulation form the channel sinuosity of this river system is analyzed in order to draw conclusions on the neotectonic activity of the eastern region of the great hungarian plain the original pre regulation status of the river thalwegs was digitized from rectified historical maps sinuosities at different sample section lengths were computed in a gis software environment providing so called sinuosity spectra for each point of the analyzed channels high sinuosity sections were correlated to linear features identified in seismic survey sections indicating their neotectonic activity some low sinuosity sections former marshlands and a special river section having anastomosed planform suggest zones of rapid subsidence all of the above features cannot be verified by the topography as the relief in this area is below 10 m

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

experimental strain of isothermal remanent magnetization in ductile sandstone abstract a macroscopically ductile sandstone to which a homogeneous isothermal remanent magnetization irm was applied has been deformed at 150 mpa confining pressure and constant strain rate of 10 5 s 1 hydrostatic compaction does not produce a deflection of the irm vector although it is reduced in intensity pure shear producing shortening in the range 2 to 35 steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening the amount of rotation is slightly less than that expected for a non material line undergoing homogeneous strain deformation selectively removes weakly coercive components of remanence as revealed by alternating field af demagnetization during deformation a weak deformational viscous remanent magnetization dvrm is acquired from the pressure vessel this is different from a conventional viscous remanent magnetization vrm in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone even for periods three times longer than the longest deformation test

the influence of valley aggradation and listric normal faulting on styles of river avulsion a case study of the brazos river texas usa abstract the holocene avulsion history of the lower brazos alluvial valley of east texas usa was studied using 10 drill cores 26 radiocarbon dates aerial photos and a digital elevation model this study shows that two long term processes aggradation and localized valley tilting along a normal listric fault are responsible for generating two styles of avulsion the first process precedes avulsion by progradation while the second process precedes avulsion by annexation as valley aggradation migrated updip over the last 7 5 ka three regional backstepping avulsions occurred along the lower 140 km of the valley and each generated sizable deposits a pattern emerges of landward stepping progradational avulsions tracking the locus of valley aggradation and of valley aggradation migrating inland even after the rate of sea level rise diminishes at the same time several local nodal avulsions occurred between 50 and 55 km updip of the current highstand shoreline but generated no observable deposits geomorphic evidence indicates that since the late pleistocene active movement along a previously undocumented normal listric fault has occurred at the location of the nodal avulsion these two long term processes do not operate mutually exclusive of each other to promote avulsions rather they operate concurrently only aggradation promotes avulsions that affect floodplain alluviation although the total volume of these deposits comprises a small portion of the valley fill

paleoremanence dispersal across a transpressed archean terrain deflection by anisotropy or by late compression the quetico belt is an archean metasedimentary terrain sandwiched between greenstone subprovinces a single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aarm aligned micas are the primary cause of ams aligned pyrrhotite is the sole cause of aarm each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history their slight obliquity may be due to crystallization during noncoaxial strain magnetic fabric ellipsoids suggest varying severity of flattening very strong flattening on the northern boundary less severe on the southern boundary and least severe in the interior the interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually from ns in the north to nnw in the south the paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations late stress effects rotated paleoremanences into this pattern we discount deflection of the paleofield by anisotropy the pattern of dispersal indicates uniaxial oblate flattening of the region with nnw sse shortening late in the metamorphic cooling history 300 c this postdates schistosity ams and aarm fabrics but it is controlled by the same kinematic pattern

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

rapid and widespread response of the lower mississippi river to eustatic forcing during the last glacial interglacial cycle the lower mississippi valley provides an exceptional fi eld example for studying the response of a continental scale alluvial system to upstream and downstream forcing associated with the large orbitally controlled glacialinter glacial cycles of the late quaternary however the lack of a numerical chronology for the widespread pleistocene strata assemblage known as the prairie complex which borders the holocene fl oodplain of the lower mississippi river has so far precluded such an analysis here we apply optically stimulated luminescence osl dating mainly on silt sized quartz from prairie complex strata in total 27 osl ages indicate that the prairie complex consists of multiple allostratigraphic units that formed mainly during marine isotope stages 7 5e and 5a thus the aggradation of the prairie complex is strongly correlated with the sea level highstands of the last two glacialinterglacial cycles fluvial incision during the sea level fall associated with the mis 5a mis 4 transition extended as far inland as 600 km from the present day shoreline testifying to the dominant downstream control of fl stratigraphic architecture in the lower mississippi valley in addition the short reaction time of the lower mississippi river suggests that large fl uvial systems can respond much more rapidly to allogenic forcing than is commonly believed

reorientation of remanent magnetism during tectonic fabric development an example from the waynesboro formation pennsylvania u s a abstract a paleomagnetic study of the late early early middle cambrian waynesboro formation in south central pennsylvania reveals a pre folding magnetization which is close to the expected cambrian field at three sites while other site mean directions are rotated by as much as 82 from the expected cambrian field these results are based on detailed thermal demagnetization analysis of 84 red siltstone and sandstone samples from nine sites collected over a 163 km 2 area local faulting fault block rotation and remanence acquisition during rapid apparent polar wander do not appear to be responsible for the observed remanence deviations total strain at the sites as indicated semiquantitatively by the anisotropy of magnetic susceptibility ams of 28 samples has been studied to test the hypothesis that deformation has rotated the remanence directions ams data vary considerably both between sites and within sites and show a path of progressive deformation on a flinn diagram there is a strong positive correlation between the deviation of a sample s remanence from the expected cambrian field and its position along the deformation path suggesting that finite strain fabric development has rotated the magnetizations the study area is structurally contained in the south mountain anticlinorium whose southern margin forms the displacement transfer zone to the southern appalachian alleghenian thrust sheets elliott 1976 if thrusting had continued and incorporated the waynesboro rocks into these thrust sheets the rotated magnetizations could have been incorrectly interpreted to result from rigid thrust sheet rotation

geomorphic and sedimentary response of rivers to tectonic deformation a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings abstract rivers are extremely sensitive to subtle changes in their grade caused by tectonic tilting as such recognition of tectonic tilting effects on rivers and their resultant sediments can be a useful tool for identifying the often cryptic warping associated with incipient and smaller scale epeirogenic deformation in both modern and ancient settings tectonic warping may result in either longitudinal parallel to floodplain orientation or lateral normal to floodplain orientation tilting of alluvial river profiles alluvial rivers may respond to deformation of longitudinal profile by 1 deflection around zones of uplift and into zones of subsidence 2 aggradation in backtilted and degradation in foretilted reaches 3 compensation of slope alteration by shifts in channel pattern 4 increase in frequency of overbank flooding for foretilted and decrease for backtilted reaches and 5 increased bedload grain size in foretilted reaches and decreased bedload grain size in backtilted reaches lateral tilting causes down tilt avulsion of streams where tilt rates are high and steady down tilt migration combing where tilt rates are lower each of the above effects may have profound impacts on lithofacies geometry and distribution that may potentially be preserved in the rock record fluvial sedimentary evidence for past tilting is traditionally based on the assumption that depositional features reminiscent of modern fluvial tectonic effects are evidence for past tectonic effects where it is closely associated with historically active structures or where non tectonic causes cannot be invoked however caution must be exercised when using these effects as criteria for past or current tectonic warping as these effects may be caused by non tectonic factors these non tectonic causes must be eliminated before tectonic interpretations are made

response of active tectonics on the alluvial baghmati river himalayan foreland basin eastern india active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope the baghmati an anabranching foothills fed river system draining the plains of north bihar in eastern india has responded to ongoing tectonic deformation in the basin the relatively flat alluvial plains are traversed by several active subsurface faults which divide the area in four tectonic blocks each tectonic block is characterized by association of fluvial anomalies viz compressed meanders knick point in longitudinal profiles channel incision anomalous sinuosity variations sudden change in river flow direction river flow against the local gradient and distribution of overbank flooding lakes and waterlogged area such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through dem and field observations to understand the nature of vertical movements in the area the sub surface faults in the baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes

factors controlling the holocene avulsion history of the rhine meuse delta the netherlands abstract the avulsion history of the holocene rhine meuse delta the netherlands was reconstructed on a timescale of millenia using detailed paleogeographic maps based on approximately 200 000 lithological borehole descriptions over 1150 14c ages and 36 000 archaeological artifacts gradient lines were constructed for all channel belts these allowed determination of gradients paleo flow direction relative age of channel belts and time correlation of undated channel belt fragments avulsion sites were inferred from the paleogeographic reconstruction at least 91 avulsions occurred over the past 10 000 years of which 82 could be dated with an accuracy of 200 14c years the location and shifting of holocene avulsion sites in the rhine meuse delta in space and time are related to 1 relative sealevel rise in the early holocene avulsions could not take place because rivers were still incised around 7500 yr bp avulsions occurred in the western part of the present delta as a result of backfilling of the late weichselian valley between 7500 and 3700 yr bp the zone where avulsions occurred shifted inland as a result of relative sealevel rise 2 neotectonics between 4900 and 1700 yr bp the location of avulsion sites seems to have been influenced by neotectonic movements of the upthrown peel horst four out of six avulsion nodes in the rhine meuse delta were located in the peel horst fault zones 3 increased discharge and or within channel sedimentation from 2800 until about 1500 yr bp avulsion sites were located all over the delta during this time the number of channels was high and avulsion frequency reached a maximum at a time when aggradation rate decreased with a reduction in the rate of sealevel rise after 2000 yr bp meander wavelength of alluvial channels increased considerably the increased meander wavelength and the high avulsion frequency are attributed to increased bankfull discharge or within channel sedimentation leading to channel widening or both 4 human influence between 1000 and 650 yr bp all the rivers were embanked and avulsions could no longer take place the few that occurred were induced by humans

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

strain and magnetic remanence abstract experimental data may be compatible with the hypothesis that a single direction of magnetic remanence rotates as a rigid marker with strains up to 40 shortening in coaxial perfect flattening x y z detailed agreement with the passive line model is relatively poor for the specimens in which remanance is carried by magnetite however for this range of strains the differences with the passive line model wettstein s equation are so slight that the latter model may be more easily employed to de strain or restore deformed remanance to its original attitude in the case of hematite bearing remanences the differences between the passive line and rigid marker model are even smaller because of the higher aspect ratios of grains of hematite therefore it is suggested that wettstein s equation may be safely used to restore remanence after even higher strains where the remanence is carried by hematite

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed