geochemical constraints on the origin of calcite veins and associated limestone alteration ordovician viola group arbuckle mountains oklahoma u s a abstract isotopic and elemental analyses constrain the origins of two types of calcite veins and associated limestone alteration in the ordovician viola group arbuckle mountains oklahoma u s a calcite from small millimeter wide veins has similar 87 sr 86 sr ratios 0 70781 0 70792 and 13c values 1 4 to 1 6 pdb to host viola limestone 87 sr 86 sr 0 70779 0 70794 13c 2 2 to 1 7 pdb indicating that the fluids from which the vein calcite precipitated were chemically buffered by the host limestone the low 18o values 8 2 to 5 5 pdb of the vein calcite relative to the host limestone 5 0 to 3 1 pdb suggest that the vein calcite probably precipitated from 18o depleted meteoric water calcite from large veins 5 50 cm wide is characterized by radiogenic 87 sr 86 sr ratios 0 70887 0 70915 and low 13c values 8 0 to 2 5 pdb relative to host viola limestone such isotopic compositions demonstrate that the fluids responsible for the formation of the large veins were enriched in 87sr and depleted in 13c relative to the host limestone the 18o values 4 8 to 3 7 pdb of calcite from the large veins coupled with the presence of hot 50 90 c and saline fluid inclusions constrain the formation of the vein calcite to 18o enriched 18 o 2 smow fluids on this basis calcite from the large veins is believed to have precipitated from basin derived fluids adjacent to the large veins alteration zones up to 0 5 m wide have developed in host limestone relative to other limestones the altered limestones display radiogenic 87 sr 86 sr ratios 0 70792 0 70880 low 13c values 8 2 to 2 0 pdb and low sr concentrations the similar isotopic compositions of the altered limestone to those of the large veins suggest that alteration of limestones was also caused by basinal fluids

associations between burial diagenesis of smectite chemical remagnetization and magnetite authigenesis in the vocontian trough se france results of a paleomagnetic rock magnetic geochemical and petrographic study on jurassic and cretaceous carbonates in the vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization crm carried by magnetite where smectite has altered to other clay minerals limestones are characterized by a prefolding secondary normal polarity magnetization throughout the basin the magnetization is interpreted to be a crm based on low burial depths which cannot cause thermoviscous resetting where significant smectite is still present the crm is absent weakly developed and where the clays show no evidence for burial alteration the units are characterized by a primary magnetization crm intensity also varies with the amount of smectite and burial isothermal anhysteretic and natural remanent magnetization intensities increase where smectite has altered both stratigraphically and geographically this is interpreted to indicate magnetite authigenesis associated with clay diagenesis superparamagnetic magnetite is more dominant in highly altered units based on the results of low temperature experiments all sections away from the alps have 87sr 86sr values that are similar to coeval seawater and stable isotopes of carbon and oxygen show no sign of alteration orogenic type fluids therefore are not a likely agent of remagnetization near the alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the crm through a reversal a postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic type fluids we conclude that burial diagenesis of smectite is the likely cause for the development of the widespread crm in the vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere

thrust fault growth and segment linkage in the active ostler fault zone new zealand abstract thrust faulting is a fundamental mode of crustal deformation yet many of the key geometrical attributes of thrust faults and the controls on fault rupture growth and linkage remain poorly resolved numerous surface rupturing thrust faults cut through upper quaternary glacial outwash terraces within the ostler fault zone an active thrust system in the southern alps new zealand we use these deformed marker surfaces to define the three dimensional deformation field associated with their surface expression and to map displacement and length on 40 fault segments displacement transfer across two fault segment arrays occurs in distinctly different styles in one displacement is transferred between en echelon fault segments to produce a smooth linear displacement gradient in the other large scale folding and a population of small faults transfer displacement between two non overlapping fault segments with a residual displacement minimum within the transfer zone size distribution of fault segment length and maximum displacement follow a power law scaling relationship maximum displacement d max scales linearly with and represents 1 of segment trace length l d max l data from the segments of the ostler fault zone display similar scaling to a global fault dataset this similarity is surprising given that the observed displacements along the ostler fault represent only 20 ky of slip on fault segments that are likely to be many times older

kinematic models of fluvial terraces over active detachment folds constraints on the growth mechanism of the kashi atushi fold system chinese tian shan during detachment folding the relationship between differential uplift and shortening depends on the mechanism of fold growth such as limb rotation or hinge migration and may vary over the lifetime of a fold thus neither long term shortening rates nor the present fold geometry unambiguously constrain the kinematics of fold growth where rivers cut through growing anticlines flights of abandoned fluvial terraces act as passive kinematic markers as shortening progresses the terraces become deformed and thereby preserve critical information about the kinematics and evolution of active fold growth to constrain recent fold growth across three detachment folds in the kashi atushi fold system in the sw tian shan china we surveyed flights of deformed terraces and compared them with geometric models of successively emplaced horizontal unconformities terraces across pregrowth strata deformed by hinge migration limb rotation and a combination of the two migration of angular hinges and curved hinge zones were also compared each kinematic model predicts both a distinct geometry for the deformed terraces and contrasting angular relationships between the terraces and the pregrowth strata notably limb rotation and migration of curved hinge zones result in progressively rotated terraces that cut across pregrowth strata whereas all limb lengthening models result in parallelism between pregrowth strata and terrace straths across much of the fold the kashi atushi terraces show clear evidence of abandoned axial surfaces concentrated deformation near the core of the folds and progressive tilting with age when compared to the model predictions the folds are likely growing by a combination of limb rotation in the tight cores of the folds and hinge zone migration of pregrowth strata across the flanks of the folds

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

age and paleoenvironmental reconstruction of partially remagnetized lacustrine sedimentary rocks oligocene aktoprak basin central anatolia turkey the age and paleoenvironmental record of lacustrine deposits in the aktoprak basin of south central turkey provides information about the evolution of topography including the timing of development of an orographic rain shadow caused by uplift of the mountain ranges fringing the central anatolian plateau new magnetostratigraphy based age estimates in combination with existing biostratigraphic ages suggest that the partially remagnetized kurtulmus tepe section of the basin is chattian upper oligocene the mean carbon and oxygen stable isotope ratios 18o 24 6 2 0 13c 4 9 1 1 are largely constant through the section and indicative of a subtropical open freshwater lake these isotopic values are also similar to those of the chattian mut basin to the south on the mediterranean side of the modern orographic barrier tauride mountains and indicate absence of an orographic barrier during late oligocene basin deposition post depositional partial remagnetization occurred after tilting of the basin sequence and was mineralogically controlled affecting grey carbonate rich rocks average caco3 82 whereas interlayered pink carbonate poor rocks average caco3 38 carry a primary pretilt magnetization the pink rocks are rich in clay minerals that may have reduced the permeability of these rocks that carry a primary magnetization concentrating basinal fluid flow in the carbonate rich grey layers and leading to the removal and reprecipitation of magnetic minerals the normal and reverse polarities recorded by the remagnetized rocks suggest that remagnetization occurred over a protracted period of time

geomorphic constraints on listric thrust faulting implications for active deformation in the mackenzie basin south island new zealand 1 deformed fluvial terraces preserved over active thrust related folds record the kinematics of folding as fault slip accumulates on the underlying thrust in the mackenzie basin of southern new zealand the kinematics revealed by folded fluvial terraces along the active ostler and irishman creek fault zones are inconsistent with traditional models for thrust related folding in which spatially uniform rock uplift typically occurs over planar fault ramps instead warped and tilted terraces in the mackenzie are characterized by broad continuous backlimbs and abrupt forelimbs and suggest folding through progressive limb rotation by relating this pattern of surface deformation to the underlying thrust with a newly developed simple geometric and kinematic model we interpret both faults as listric thrusts rooted at depth into gently dipping planar fault ramps constraints on the model from detailed topographic surveying of deformed terraces ground penetrating radar over active fault scarps and luminescence dating of terrace surfaces suggest slip rates for the ostler and irishman creek faults of 1 1 1 7 mm yr and 0 5 0 7 mm yr respectively the predicted depth of listric faulting for the ostler fault 0 7 0 2 0 1 km and the irishman creek fault 1 3 0 5 0 1 km generally agrees with geophysical estimates of basin depth in the mackenzie and suggests control of preexisting basin architecture on the geometry of active thrusting despite the potential effects of changes in fault curvature and hanging wall internal deformation the methodology presented here provides a simple tool for approximating the kinematics of surface deformation associated with slip along listric or curviplanar thrust faults

structural analysis and interpretation of the surface deformations of the el asnam earthquake of october 10 1980 the el asnam earthquake of october 10 1980 represents the major seismotectonic event of these last decades in the west mediterranean area it has induced a large amount of surface breaks this earthquake reveals the importance of compressional phenomena that characterize nowadays the tectonics of north africa the tectonic analysis of surface rupture shows the complexity of the deformation mechanism the principal mechanism consists of the activation of a nw se trending thrust fault accompanied with a left lateral motion and with an intense deformation of the northwestern overthrusting block an accurate description of these deformations shows that for a large part the breaks geometry depends closely on the substratum structure finally the analysis of the dislocations on irrigation ducts just above the fault traces allowed a quantitative study of the deformation

lake sediment geochemistry reveals 1400 years of evolving extractive metallurgy at cerro de pasco peruvian andes the geochemical record preserved in lake sediments is a potentially powerful tool in archaeometallurgy here sediments from llamacocha a small lake in the central peruvian andes are used to reconstruct a 1400 year legacy of metal extraction from cerro de pasco once the largest silver mine in the world the earliest evidence for anthropogenic lead pb enrichment occurs ca a d 600 and is confi rmed by pb stable isotope ratios that match those of cerro de pasco ores early pb pollution is attributed to precolonial smelting for silver production which relied on galena based fl uxes following colonial control of the mine ca a d 1600 the switch to mercury hg amalgamation for winning silver resulted in atmospheric hg emissions as registered in llamacocha sediments both pb and hg deposition increased through the twentieth century attaining peak values in a d 1968 and 1942 respectively principal components analysis pca identifi es a gradient that differentiates anthropogenic from natural metals within the record confi rming that early smelting led to the volatilization of trace metals associated with local ore mineralogy these results represent the fi rst evidence for a major precolonial mining industry at cerro de pasco provide a chronological framework for evolving extractive technologies and are the fi rst to document widespread hg pollution associated with colonial hg amalgamation 80 w 70 w

delle phosphatic member an anomalous phosphatic interval in the mississippian osagean meramecian shelf sequence of central utah phosphatic rocks are a distinctive part of the lower and lower upper mississippian delle phosphatic member of the woodman formation at the type locality of the member in the lakeside mountains northwestern utah there as elsewhere in central and western utah the delle is composed of a relatively thin anomalous depositional interval characterized by phosphorite siliceous microfossils secondary chert distinctive dense lime mudstone and argillaceous rocks stratigraphically sandwiched within a thick sequence of subtidal and peritidal mississippian shelf carbonate and craton derived siliciclastic rocks strongly reducing in sediment conditions evidently prevailed at the onset of delle deposition yet sedimentologic features in the delle such as endolithic algal borings in pisolitic phosphorite and features in the rocks above and below the delle such as tidally laminated fecal pellet limestones and exclusively tractional sedimentary structures indicate shallow shelf depositional environments these rocks do not support the concept of a deep sediment starved depositional basin the deseret deep starved basin of previous authors rather the nature of the delle suggest incursion far onto the broad early mississippian shelf of nutrient rich organically productive waters possibly produced by upwelling in the coeval antler foreland trough in central nevada this interpretation places the early mississippian shelf margin to the west ofmore the lakeside mountains probably west of utah rather than in central utah as required by the deseret deep starved basin model less

late quaternary slip rates across the central tien shan kyrgyzstan central asia 1 slip rates across active faults and folds show that late quaternary faulting is distributed across the central tien shan not concentrated at its margins nearly every intermontane basin contains neogene and quaternary syntectonic strata deformed by holocene north south shortening on thrust or reverse faults in a region that spans two thirds of the north south width of the central tien shan slip rates on eight faults in five basins range from 0 1 to 3 mm yr fault slip rates are derived from faulted and folded river terraces and from trenches radiocarbon optically stimulated luminescence and thermoluminescence ages limit ages of terraces and aid in their regional correlation monte carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95 confidence limits faults in basins appear to merge at relatively shallow depths with crustal scale ramps that underlie mountain ranges composed of pre cenozoic rocks the sum and overall pattern of late quaternary rates of shortening are similar to current rates of north south shortening measured using global positioning system geodesy this similarity suggests that deformation is concentrated along major fault zones near range basin margins such faults separated by rigid blocks accommodate most of the shortening in the upper crust index terms 8107 tectonophysics continental neotectonics 9320 information related to geographic region asia 8010 structural geology fractures and faults

remagnetization and orogenic fluids testing the hypothesis in the central appalachians summary palaeomagnetic and geochemical studies on devonian sedimentary rocks in the valley and ridge province in west virginia indicate that the hypothesized relationship between orogenic fluids and remagnetization is not straightforward the helderberg group an aquitard and the oriskany formation a palaeoaquifer both contain similar synfolding late palaeozoic chemical remanent magnetizations crms residing in magnetite the results from the oriskany formation are consistent with remagnetization by orogenic fluids because the unit contains geochemical evidence for the migration of such fluids in contrast there is no geochemical evidence for the widespread migration of orogenic fluids into the helderberg group and thus the crm cannot be related to such fluids another remagnetization mechanism which remains elusive must be responsible for the crm in the helderberg group

the magnetization of clay rich rocks in sedimentary basins low temperature experimental formation of magnetic carriers in natural samples abstract rocks are often magnetically altered during burial thus providing a useful method to study their thermo chemical history in order to improve the understanding of burial diagenetic magnetization we experimentally synthesized magnetite in natural samples lower jurassic argillites from the paris basin were heated to 150 c under a confined atmosphere of argon when a magnetic field was applied during the heating a chemical remanent magnetization was acquired rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite using anhysteretic remanent magnetization as a measure of the quantity of magnetite an increase of up to 150 after heating was observed ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3 calcite and new magnetite is sometimes completely absent in samples containing less than 0 5 calcite hysteresis measurements suggest that the new magnetite is in the pseudo single domain state scanning electron microscope studies indicate that the new magnetic carriers are associated with pre existing pyrite the magnetic transformations during the experiments involved a fluid which reacted with iron bearing and sulfur bearing minerals in ways depending on local redox conditions this study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins without any external supply from long range fluid flow

separating multidomain and single domain like remanences in pseudo single domain magnetites 215 540 nm by low temperature demagnetization it has long been hypothesized that the pseudo single domain properties of submicron magnetites are a result of coexisting multidomain and single domain like remanences we have separated these two types of remanence by low temperature demagnetization ltd in which samples are cooled through the low temperature magnetite isotropic point and rewarmed to room temperature all in zero field at the isotropic point the first crystalline anisotropy constant vanishes and multidomain remanence due to displaced domain walls will be unpinned we measured stepwise alternating field af and thermal demagnetization curves of weak field thermoremanent magnetization trm and anhysteretic remanent magnetization and of saturation isothermal remanent magnetization irms both with and without prior ltd for magnetites with mean particle sizes of 215 390 and 540 nm the memory fraction of remanence recovered after ltd which represents the single domain like component decreased from 44 for 215 nm grains to 23 for 540 nm grains in the case of irms memory of trm was approximately twice as large af decay curves were soft and multidomain like before ltd but became harder and single domain like in shape after ltd ltd removed most low unblocking temperature trm and irms distributed low unblocking temperatures are therefore a multidomain phenomenon probably resulting from successive adjustments of domain walls to changing internal fields and pinning strengths during heating

chronology and rates of faulting of ventura river terraces california new evidence concerning the chronology of four late pleistocene terraces of the ventura river near oak view california that are vertically offset and tilted by reverse and reverse flexural slip faults provides a means of estimating rates of fault movement and downcutting by the ventura river radiocarbon ages of charcoal contained within terrace deposits date two of the terraces correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the arroyo parida santa ana fault not a flexural slip fault date the others resulting age estimates for the four main late pleistocene terraces are qt5b 30 000 qt6a 38 000 qt6b 54 000 and qt6c 92 000 yr b p vertical slip rates on flexural slip faults range from 0 3 to 1 1 mm yr and apparently are related to the rate form and mechanics of folding of the north limb of the ayers creek canada larga syncline average rates of downcutting of the ventura river for several intervals during the late pleistocene estimated from the chronology and relative elevation of river terraces north of the arroyo panda santa ana fault upstream from the zone of faulting vary from 0 5 to 1 3 mm yr the range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level climatic conditions and or regional deformation of the western transverse ranges the average rate of downcutting of the ventura river north of the zone of flexural slip faulting is 0 8 mm yr compared with 1 2 to 2 2 mm yr in the deformation zone immediately to the south this apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system

illite and hydrocarbon exploration illite is a general term for the dioctahedral mica like clay mineral common in sedimentary rocks especially shales illite is of interest to the petroleum industry because it can provide a k ar isotope date that constrains the timing of basin heating events it is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap anticline etc that is to hold the oil illite also may precipitate in the pores of sandstone reservoirs impeding fluid flow illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial k ar ages are apparent ages of mixtures of detrital and diagenetic end members and what we need are the ages of the end members themselves this paper describes a methodology based on mineralogy and crystallography for interpreting the k ar ages from illites in sedimentary rocks and for estimating the ages of the end members

remagnetization of lower silurian black shale and insights into shale gas in the sichuan basin south china the organic rich lower silurian shale of the longmaxi formation in the sichuan basin is the most important target for shale gas exploration in china most paleozoic rocks of the sichuan basin have experienced extraordinarily pervasive remagnetizations to test a hypothesized connection between hydrocarbon generation and remagnetization and contribute to shale gas exploration in the region we undertook an integrated magnetic geochemical and petrographic study of 160 specimens from the shale the results suggest that the shale contains a reliable remanent magnetization dec 41 4 inc 40 8 and 95 6 8 the magnetization predates tilting and the paleopole plots close to the late triassic segment of the south china apparent polar wander path the rock magnetic data and scanning electron microscope sem observations confirm that framboidal magnetites carry the bulk of the magnetization which suggest a late triassic chemical remanent magnetization in the shale 87sr 86sr and magnetic analyses indicate that the amount of magnetite was unaffected by fluid alterations around the veins but is strongly covariant with the amount of total organic matter moreover sem observations reveal possible evidence of the replacement of pyrite framboids by magnetite probably in the presence of organic acids these analyses therefore suggest that the remagnetization was caused by organic maturation rather than orogenic fluids and that the maturation occurred in the late triassic this timing of organic maturation has been validated by independent modeling studies and provides important constraints on the complex thermal history of the longmaxi shale as well as contributing to shale gas exploration efforts

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

an integrated paleomagnetic rock magnetic and geochemical study of the marcellus shale in the valley and ridge province in pennsylvania and west virginia an integrated paleomagnetic rock magnetic and diagenetic study of the devonian marcellus subgroup from threefolds in the valley and ridge province in pennsylvania and west virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization crm with south southeast declinations and shallow negative inclinations residing in pyrrhotite and a crm with more southerly declinations and shallow positive inclinations residing in magnetite rock magnetic results confirm the presence of pyrrhotite and magnetite tilt tests indicate that pyrrhotite and magnetite crms are syntilting to posttilting and paleopoles for both crms are similar and plot on the permian part of the apparent polar wander path for north america the marcellus subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation the magnetite and pyrrhotite crms formed in the permian probably due to burial diagenetic processes and not orogenic fluids the base of the unit contains the highest total organic content toc values and highest magnetic intensities both of which decrease up section this connection between toc and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis the pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir the similar permian crm ages may be explained by thrusting which resulted in rapid burial of the marcellus subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite respectively

direct dating of carbonate remagnetization by 40ar 39ar analysis of the smectite illite transformation abstract the occurrence of remagnetizations in carbonate rocks is a longstanding issue the result of a cryptic process that affects rocks in both fold thrust belts and undisturbed continental interiors this paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the cantabrian asturian belt northern spain detailed x ray diffraction patterns indicate that mixed layer clays from these rocks are mostly illitic with an increasing proportion of authigenic illite polytype 1m d in finer grain size fractions the 40 ar 39 ar age data from the different grain size fractions coupled with quantitative polytype modeling indicates an authigenic age that is coeval with the established late paleozoic remagnetization age of these rocks we propose that the transformation of fe rich smectite to fe poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks the irreversible kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization and may explain such vexing phenomena as synfolding magnetizations this technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences

application of trishear fault propagation folding to active reverse faults examples from the dalong fault gansu province nw china abstract determining accurate fault slip rates at 1 ka to 1 ma timescales requires well constrained palinspastic reconstructions of dateable geomorphic and or geologic markers although general kinematic models have been developed to simultaneously reconstruct both bedrock e g bedding and fault attitudes and neotectonic markers e g strath terraces along active strike slip and thrust faults it is not clear if these models can also account for deformation along steeply dipping 45 reverse faults to address this problem we have investigated the active 50 dipping dalong reverse fault system this 40 km long fault system forms part of the aksai restraining stepover along the active left slip altyn tagh fault in northwestern china our geometric and kinematic analyses show that conventional fault bend fold models cannot satisfy the steeply dipping fault geometry we observe in the bedrock record likewise standard fault propagation fold models fail to match our measurements of a set of fluvial terraces however by expanding the trishear model of fault propagation folding to track both bedrock and neotectonic markers we are able to match both sets of records in particular we have developed trishear kinematic models for two sites liuchenzi and qingyazi using the numerical modeling program fault fold v 5 0 this work indicates that an important implication of active trishear fault propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace thus to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall terrace profiles across active reverse faults must extend 1 2 km on either side of this zone of deformation

backlimb trishear a kinematic model for curved folds developed over angular fault bends abstract numerous analog models and some natural examples display smoothly curving backlimb fold hinges over sharp angular bends in the underlying thrust surface we present a new kinematic model that can reproduce this geometry by defining a triangular zone focused on the fault bend the model presupposes incompressible flow in the triangular zone and makes common assumptions about velocities on either side of the fault bend either symmetric or asymmetric triangular zones can be defined with parallel kink folding and similar folding being special cases of the two respectively where the apical angle of the triangular zone is zero the results of the numerical model compare well to analog experiments the model is conceptually analogous to forelimb trishear at the tip of a propagating fault and hence we refer to it as backlimb trishear

strain distribution across the australian pacific plate boundary in the central south island new zealand from 1992 gps and earlier terrestrial observations as part of a geodetic experiment aimed at understanding the deformation associated with the australian pacific plate boundary in the south island of new zealand in 1992 we reoccupied using global positioning system gps techniques a first order triangulation and trilateration network established in 1978 between christchurch on the east coast and hokitika on the west the network crosses the south island a few tens of kilometers southwest of the region where the plate boundary changes from a single throughgoing oblique slip fault the alpine fault to a series of subparallel strike slip faults the marlborough faults the gps data have been analyzed as daily network solutions with about 12 stations in each solution rms repeatabilities for stations with multiple occupations are 4 mm 8 mm and 15 mm in the north east and vertical components respectively the observed strain across the network is consistent with the entire nuvel ia pacific australia plate velocity being accommodated on land across this part of the south island shear strain rates derived from the gps and terrestrial data show that the highest strain rate 0 4 rad yr occurs in the region of the southern alps and alpine fault this rate is about two thirds of the rate predicted from the nuvel ia plate velocity model assuming that all the plate boundary deformation occurs across this region implying that about two thirds of the plate motion is accommodated in the vicinity of the alpine fault significant shear strain rates greater than 0 2 rad yr are also observed farther east particularly in the region of the porters pass amberley fault zone demonstrating that this zone accommodates a significant part of the plate boundary deformation using dislocation modeling we show that the variation in fault parallel shear strain rates across the alpine fault is reasonably consistent with a nonoptimized dislocation model involving a 50 dipping fault that is presently locked to a depth of 12 km one explanation for this observation is that the upper 12 km of the crust in this area is storing most of the right lateral shear component of the relative plate motion as elastic energy that will be released in a future major earthquake the variation in the other component of shear strain which may be interpreted as fault normal relative contraction is not explained by a simple dislocation model implying that the normal component of plate motion is taken up in other ways

geochemistry of the mississippian delle phosphatic event eastern great basin u s a abstract the middle mississippian delle phosphatic event marks a significant change in depositional conditions in the devonian mississippian antler foreland basin of north america stratigraphic and petrographic studies show that deposition of delle event rocks ended a long period of open marine carbonate sedimentation when facies shifted to phosphorites cherts fine grained siliciclastics and lime mudstones from highly productive waters that may have been episodically anoxic this drastic change in lithology is reflected in the chemistry of the two suites of rocks 18o values of the delle event lime mudstones and phosphatized wackestones packstones affected by the delle event are 3 5o oo lighter than underlying and overlying open marine wackestones packstones sr concentrations of delle event lime mudstones and open marine wackestone packstone facies are low suggesting that both of these lithologies have undergone similar postdiagenetic alteration the uniform sr concentrations and variable 18o data suggest that either the younger delle event rocks were exposed to meteoric water during diagenesis or that the relatively light 18o values represent a primary brackish water depositional signal either interpretation is consistent with sedimentological evidence of the delle event occurring in a predominantly shallow water setting 13c values of delle event lime mudstones are also anomalously light indicating a high input of organic carbon during their formation mn concentrations of the delle event lime mudstones and stratigraphic equivalents are highly variable suggesting fluctuating oxygen concentrations during their deposition and diagenesis rare earth element data from phosphorites and cherts of the delle event indicate oxic to suboxic depositional conditions

quaternary climate change and the formation of river terraces across growing anticlines on the north flank of the tien shan china nested stream terraces warped upward over actively growing anticlines along the north flank of the tien shan in western china appear to record alternating phases of valley widening and incision differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines we infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr the crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm a the maximum heights of 25 and 35 10 m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr 10 kyr hence they suggest that flood plains which were abandoned to form the terraces developed adjacent to active stream beds during the last glacial period when climates were relatively

tectonic geomorphology of active folding over buried reverse faults san emigdio mountain front southern san joaquin valley california investigation of the tectonic geomorphology of active folding over buried reverse faults at the san emigdio mountain front southern san joaquin valley california provides insight concerning the tectonic and geomorphic development of mountain fronts produced by active folding and faulting monoclinally flexed gravels with dips as great as 50 and a minimum age of about 65 ka provide evidence of late pleistocene deformation at the active range front studies of the surface folding of alluvial fans and fluvial terraces indicate a holocene vertical deformation rate of 1 9 3 0 m k y at the active range front and 0 8 1 3 m k y 2 km basinward geomorphic evidence also indicates that the locus of active folding and vertical deformation along the northern flank of the san emigdio mountains has migrated and continues to migrate basinward this evidence includes a relict mountain front now within the uplifted block 5 km from the present active mountain front and the existence of recently initiated folds in the active alluvial fan 2 km basinward from the mountain front northward migration of tectonic activity results in the progressive widening of the uplifted block as the location of active folding moves basinward this migration of tectonic activity appears to occur through the onset and subsequent increase of vertical deformation along more northerly folds and faults accompanied by reduction and eventual cessation of activity along the older more southerly structures

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

challenges in isolating primary remanent magnetization from tethyan carbonate rocks on the tibetan plateau insight from remagnetized upper triassic limestones in the eastern qiangtang block abstract carbonate rocks widely used for paleomagnetically quantifying the drift history of the gondwana derived continental blocks of the tibetan plateau and evolution of the paleo meso neo tethys oceans are prone to pervasive remagnetization identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests here we apply comprehensive paleomagnetic rock magnetic and petrographic studies to upper triassic limestones in the eastern qiangtang block our results reveal that detrital biogenic magnetite which may carry the primary natural remanent magnetization nrm is rarely preserved in these rocks in contrast authigenic magnetite and hematite pseudomorphs after pyrite and monoclinic pyrrhotite record three episodes of remagnetization the earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite probably related to the collision between the northeastern tibetan plateau and the qiangtang block after closure of the paleo tethys ocean in the late triassic the second remagnetization residing in hematite and minor goethite which is the further subsurface oxidation product of pyrite magnetite is possibly related to the development of the localized cenozoic basins soon after india asia collision in the paleocene the youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite respectively our analyses suggest that a high supply of organic carbon during carbonate deposition prevailing sulfate reducing conditions during early diagenesis and widespread orogenic fluid migration related to crustal shortening during later diagenesis have altered the primary remanence of the shallow water tethyan carbonate rocks of the tibetan plateau we emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the nrm

remagnetization in bituminous limestones of the neoproterozoic araras group amazon craton hydrocarbon maturation burial diagenesis or both 1 neoproterozoic carbonates of the araras group exhibit two distinct magnetic components across the same carbonate succession in a cross section between the amazon craton and the paraguay fold belt pink dolostones of the mirassol d oeste formation carry a dual polarity primary component whereas black bituminous limestones of the guia formation yield a secondary postfolding component magnetic signatures of the guia limestones such as high anhysteretic remanence magnetization saturation isothermal remanence magnetization ratios high frequency dependent magnetic susceptibility and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates unblocking temperatures suggest that the stable high temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations the different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence crm the presence of pyrrhotite as one of the main carriers of crm in these rocks and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage however hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

palaeomagnetic palaeolatitudes of the ontong java plateau from 120 to 55 ma implications for the apparent polar wander path of the pacific plate we present palaeomagnetic inclinations from late cretaceous early tertiary deep water carbonates obtained during odp leg 192 drilling at the ontong java plateau ojp for the following periods magnetochrons c33n 73 6 79 1 ma c32 71 1 73 6 ma c27r c31n 61 3 68 7 ma and c25 c26 55 9 60 9 ma compaction induced inclination shallowing is considered to be negligible for the ojp sediments examined here because i their palaeomagnetic inclinations are in excellent accord with those recently obtained from underlying ojp basement rocks ii the studied sediments have anisotropy of anhysteretic remanent magnetization fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations and iii in the few cases where we observe a significant difference between our new ojp sedimentary palaeomagnetic data and data from other parts of the pacific plate obtained mainly from seamount magnetization studies and skewness analyses the sedimentary inclinations are not systematically lower and therefore cannot be explained in terms of inclination shallowing combining our new data with existing ojp palaeomagnetic data we obtain an internally consistent data set that we interpret to indicate northward motion of the plateau from its formation at similar to 120 ma until 55 ma at 120 ma the central plateau was located at 24 degrees 4 degrees s approximately 45 myr later similar to 76 ma the plateau is located near 21 degrees s our data therefore indicate that the ojp was either stationary or exhibited a slow northward latitudinal drift during this interval in contrast from similar to 76 ma until 68 ma the plateau moved rapidly through approximately 10 degrees of latitude to similar to 10 degrees s these intervals of slow and rapid motion track similar motions previously proposed for the entire pacific plate during the cretaceous but extends the interval of slow motion into the late cretaceous from 68 ma until 56 ma the plateau was again almost latitudinally stationary before moving slowly northward more direct palaeomagnetic data are necessary to better define pacific plate motions during this time interval

quaternary rate of folding of the ventura avenue anticline western transverse ranges southern california upper quaternary terraces of the ventura river california are uplifted tilted and folded over the ventura avenue anticline rates of uplift and tilting have decreased since inception of the structure over the past 200 ka assuming that the chronology based on amino acid racemization 14c dates and soils correlation is approximately correct then the minimum possible average rate of uplift in the axial region of the fold has decreased from 14 mm yr to 2 mm yr during the past 200 ka interval rates of uplift for the periods 200 ka to 80 or 105 ka 80 or 105 ka to 30 ka and 30 ka to present are respectively about 20 mm yr 9 mm yr and 5 mm yr the rate of tilting shows a similar trend decreasing from 5 8 urad yr 2 5 urad yr and 1 2 urad yr for the same time intervals respectively based on the mechanics of flexural slip folds in stratified sedimentary rocks these data suggest that the rootless ventura avenue anticline is a fold that has been shortening at a relatively constant rate of about 9 mm yr since its inception

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

near synchronous interhemispheric termination of the last glacial maximum in mid latitudes isotopic records from polar ice cores imply globally asynchronous warming at the end of the last glaciation however 10be exposure dates show that large scale retreat of mid latitude last glacial maximum glaciers commenced at about the same time in both hemispheres the timing of retreat is consistent with the onset of temperature and atmospheric co2 increases in antarctic ice cores we suggest that a global trend of rising summer temperatures at the end of the last glacial maximum was obscured in north atlantic regions by hypercold winters associated with unusually extensive winter sea ice

diachronous pervasive remagnetization in northern iberian basins during cretaceous rotation and extension article i nfo northern iberia exposes a series of mesozoic sedimentary basins whose final formation history is tied to a rotation episode of iberia during the aptian related to the opening of the bay of biscay many of these basins experienced widespread remagnetization previously loosely tied to a mid cretaceous thermal event here we make use of the improved apparent polar wander path apwp of iberia to narrowly constrain the age of their remagnetization to show that several basins experienced a regional diachronous remagnetization we apply the small circle intersection sci method to reconstruct the paleomagnetic field direction during remagnetization in the organya basin in the southern pyrenees spain where the age of remagnetization is stratigraphically well dated to around the barremian aptian boundary and where the paleomagnetic direction during remagnetization has been well established the resulting direction of d i 316 8 5 7 54 8 3 3 where the uncertainty in declination is approximated by 95 cos i and the error in inclination by 95 correspondscloselytoarecently improvedapwpofiberia providingsupportfortheapplicabilityof thismethod applicationofthescimethodallowsustoshowthattheolderportionoftheremagnetizedbedswasalreadytilted

insights into the mechanism for orogen related carbonate remagnetization from growth of authigenic fe oxide a scanning electron microscopy and rock magnetic study of devonian carbonates from northern spain 1 a rock magnetic and sem study of devonian carbonates from the cantabria asturias region northern spain was undertaken to further our understanding of the pervasive remagnetization of carbonate rocks during the late paleozoic and the mechanism by which these remagnetizations occur these rocks contain three ancient late paleozoic magnetizations the rock magnetic properties of mineral extracts were compared with those of whole rock chips and nonmagnetic residue to deduce magnetic carrier s and grain sizes hysteresis measurements for rock chips show typical wasp waisted loops whereas extract shows typical pseudosingle domain like psd unrestricted loops within all sites there is a noticeable contribution of superparamagnetic sp grains seen in hysteresis properties and low temperature magnetization measurements of whole rock chips whereas a trend away from a strong sp contribution is seen when hysteresis properties of whole rock are compared with those of residue and extract consequently our extraction process predictably removes sp grains while preserving the characteristic fraction of remanence carrying material which behaves like a typical mixture of single domain sd and psd magnetite paradoxically the typical fingerprint of remagnetized carbonates as seen in the whole rock data seems to be a response to abundant sp grains associated with the acquisition of chemical remanent magnetizations crm and not the actual remanence carrying population itself scanning electron microscopy sem observations of magnetic extract reveal abundant authigenic fe oxides characterized as either 10 100 m ni free spherules or individual 0 1 10 m euhedral grains sem observations of thin sections reveal abundant evidence of fluid flow driven chemical reactions that resulted in formation of new fe oxide such reactions occurred along cracks and grain boundaries and within void space and are associated with fe rich clay and calcite dolomite reactions or as oxidation of fe sulfide framboids together the sem observations and rock magnetic experiments reveal that the three late paleozoic remagnetizations experienced by cantabria asturias paleozoic carbonates are crms facilitated by the presence of fluids activated during late paleozoic variscan deformation

late cenozoic evolution and earthquake potential of an active listric thrust complex above the hikurangi subduction zone new zealand in the center of the frontal wedge of the hikurangi subduction zone new zealand mahia peninsula and its submarine continuation lachlan ridge are being uplifted and folded above an active landward dipping thrust fault complex that is 80 km long high quality marine seismic reflection profiles reveal complex deformation of a cretaceous to holocene sedimentary section and enable a detailed analysis of the stratigraphy structural evolution deformation rates and future earthquake potential the structural analysis is facilitated by uplifted marine terraces on mahia peninsula and by 14 submarine unconformities in the hanging wall sequence five of which are correlated across the eroded crest of lachlan ridge and into the footwall basin the ages of the unconformities are determined by seismic ties to an offshore exploration well onshore outcrops on the peninsula and seabed samples dated by pollen coccolith nannoflora and foraminifera biostratigraphy nine regional quaternary unconformities which developed in response to eustatic fluctuations in sea level and are not older than ca 1 ma are correlated with oxygen isotope stages in equatorial pacific ocean drilling project core 677 the ages of fault growth strata and progressive restorations of deformed stratigraphy indicate that lachlan ridge developed during three phases of deformation since subduction of the pacific plate commenced beneath the australian plate in the early miocene these include an initial phase of thrust faulting a subsequent phase dominated by extensional faulting and the current mainly pleistocene to holocene phase of structural inversion reactivated listric thrust faulting and folding early to middle miocene thrusts in the deeper core of the complex developed out of sequence by sequentially stepping up into the hanging wall section creating an imbricate fan with emerging thrust tips now buried within the forelimb basin in the middle miocene early pliocene listric extensional faults developed in the active thrust wedge possibly as a result of substantial relief on the subducted pacific plate and controlled the development of lachlan basin to the west of the ridge the principal active thrust is the lachlan fault a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right stepping segments depth converted seismic profiles indicate that the fault dips westward at 15 20 6 8 km beneath the western flank of lachlan ridge and steepens to 55 70 or even steeper in the upper 1 2 km of section beneath the eastern flank syninversion pleistocene fault growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold limb rotation resulting from the listric fault geometry a geometric analysis of the fault growth strata and deformed terraces was used to derive a maximum dip slip displacement rate of as fast as 3 0 6 5 mm yr the implied shortening rate of 2 6 6 3 mm yr represents 8 20 of the total 31 mm yr of orthogonal plate convergence across this part of the upper plate of the hikurangi subduction zone the top of the pre fault growth paleogene section reveals as much as 5 8 1 5 km of vertical separation in the north decreasing to 30 50 of this value in the south temporal 10 3 10 6 yr and spatial 10 3 10 4 m of strike length variations in vertical deformation rate have occurred during the past 1 m y maximum rates occurred in the holocene and middle quaternary a long term increase in vertical separation rate on all segments during the pleistocene largely reflects a change in thrust kinematics associated with structural inversion the relatively greater increase in uplift rate on the northern part of the fault during the past 1 m y could be related to the possibility that a subducted seamount lies 10 km beneath the peninsula estimates of earthquake source parameters incorporating paleoseismic uplift data from mahia peninsula indicate a potential moment magnitude of up to m w 7 6 8 0 for an earthquake that ruptures all three segments of the lachlan fault the average recurrence interval for such events is estimated to be 615 2333 yr which is consistent with a mean recurrence interval of 1062 yr for four late holocene earthquakes thus the uplift and folding of mahia peninsula and lachlan ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large magnitude earthquakes

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

differences in magnetic properties of fragments and matrix of breccias from the rupture of the 2008 wenchuan earthquake china relationship to faulting abstract rock magnetic and mineralogical analyses were performed on fragments and matrix of six fault breccias from the zhaojiagou outcrop of yingxiu beichuan fault which was the main fault ruptured during the 2008 wenchuan mw 7 9 earthquake at leigu town beichuan county sichuan province china the matrix which is generally enriched in dolomite feldspar and clay minerals but depleted in calcite has much higher low field magnetic susceptibility and magnetization and lower coercivity in comparison with fragments and bulk samples magnetic behavior of the bulk samples and fragments is dominated by dia paramagnetic components in contrast small amounts of partially oxidized magnetite and lepidocrocite are present in the matrix a simple conceptual model associating faulting related effects was proposed to explain these significant differences in magnetic properties of fragments and matrix during coseismic slip intense shear may crush the pre existing magnetic grains in fault rocks into finer ones while stress and frictional heating to a little extent induce magnetic changes and thermochemical alterations of magnetic mineralogy in fault breccias respectively during interseismic periods meteoric fluids would infiltrate and percolate into fault zone and cause dissolution precipitation and recrystallization of fe bearing minerals as the diverse permeability structure and grain size of fragments and matrix these effects would modify their magnetic mineralogy at various levels consequently faulting related effects especially the fluid movements taking place over many previous earthquake cycles would be the most likely reasons for the observed different magnetic properties in fragments and matrix it further proposes that magnetic studies of fault breccias would provide clues to help understand seismic faulting and history of fault activity

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

construction of the mississippian pennsylvanian and permian seawater 87sr86sr curve abstract the estimated path of seawater 87 sr 86 sr during the mississippian pennsylvanian and permian is based on 228 analyses the samples are largely from the southern interior of the u s a where rocks of this age are widely exposed multiple analyses from single rock units are used to define the seawater ratio for different time periods the seawater 87 sr 86 sr declines from 0 70812 sw 95 mississippian to an estimated low in the early meramecian of near 0 70755 sw 152 the value rises to near 0 70812 sw 95 in latest mississippian the ratio rises to 0 70830 sw 77 in mid pennsylvanian and decreases to near 0 70809 sw 98 during virgilian time the earliest permian value is near 0 70793 sw 114 after rising to 0 70804 sw 103 in the early leonardian the seawater 87 sr 86 sr falls sharply to reach a low of 0 70672 sw 235 in ochoan time the end of the paleozoic is estimated to be near 0 70707 sw 200 based on samples from china analysis of the paleozoic curve shape shows an alternating dominance of oceanic and continental sr contributions the extended and precipitous decline in the 87 sr 86 sr during mid permian is unmatched during the phanerozoic the contrast of the paleozoic seawater sr curve shape to that in the mesozoic and cenozoic indicates a potentially fundamental change in the balance of oceanic and continental sr contributions at the end of the permian

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

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

crossover plots a useful method for plotting sirm data in paleomagnetism the effective grain size of magnetic minerals and therefore domain type controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create crossover plots for paleomagnetic specimens where sirm is plotted as a function of the field applied using a logarithmic scale this paper presents templates of such plots for magnetite pyrrhotite hematite and goethite onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain size

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

remagnetization of the folded belden formation northwest colorado paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in belden formation pennsylvanian limestones on an asymmetric fold in colorado the natural remanent magnetizations contain three components a modern viscous magnetization removed at low temperatures a reversed tertiary magnetization declination d 186 7 inclination i 54 6 of thermoviscous origin removed at intermediate temperatures and an apparently synfolding late cretaceous early tertiary characteristic magnetization d 338 7 e i 62 5 precision parameter k 54 9 radius of 95 probability cone 95 5 9 65 unfolding removed by 580 c hysteresis properties suggest that the characteristic magnetization resides in single domain magnetite low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization the 87sr 86sr values 0 7088 0 7100 show the limestones to be radiogenic with 87sr enriched halos around calcite cemented tectonic fractures there is no control on the distribution and intensity of the magnetization by the veins or structural position e g highly mineralized versus less mineralized rock lithology rather than fluid alteration controls the magnetization fecal pellets contain pyrite with alteration rims identified as magnetite by scanning transmission electron microscope x ray diffraction patterns selective demagnetization has established that the fecal pellets carry the characteristic magnetization it is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding a synfolding magnetization caused by the syndeformational mineralizing fluids however can be ruled out structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result the replacement of pyrite by magnetite is interpreted as prefolding in origin perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons

pervasive palaeogene remagnetization of the central taurides fold and thrust belt southern turkey and implications for rotations in the isparta angle the turkish anatolide tauride block rifted away from the northern margin of gondwana in the triassic which gave way to the opening of the southern neo tethys by the late palaeocene to eocene it collided with the southern eurasian margin leading to the closure of the northern neo tethys ocean to determine the position of the anatolide tauride block with respect to the african and eurasian margin we carried out a palaeomagnetic study in the central taurides belt which constitutes the eastern limb of the isparta angle the sampled sections comprise carboniferous to palaeocene rocks mainly limestones our data suggest that all sampled rocks are remagnetized during the late palaeocene to eocene phase of folding and thrusting event related to the collision of the anatolide tauride block with eurasia to further test the possibility of remagnetization we use a novel end member modelling approach on 174 acquired isothermal remanent magnetization irm curves we argue that the preferred three end member model confirms the proposed remagnetization of the rocks comparing our data to the post eocene declination pattern in the central tauride belt we conclude that our clockwise rotations are in agreement with data from other studies after combining our results with previously published data from the isparta angle that includes our study area we have reasons to cast doubt on the spatial and temporal extent of an earlier reported early to middle miocene remagnetization event we argue that the earlier reported remagnetized directions from triassic rocks in tilt corrected coordinates from the southwestern antalya nappes western taurides are in good agreement with other studies from the area that show a primary origin of their characteristic remanent magnetization this implies that we document a clockwise rotation for the southwestern antalya nappes since the triassic that is remarkably similar to the post eocene 40 rotation of the central taurides for the previously published results that are clearly remagnetized we argue that their remagnetization has occurred in the palaeocene to eocene

basement balancing of rocky mountain foreland uplifts restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts local zones of shortening and extension occur in hinge zones above transitions in fault curvature high curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover 20 references 4 figures

active folding in the algerian earthquake of 10 october 1980 a fold controlling the form of the local topography near el asnam algeria increased in amplitude at the time of the earthquake of 10 october 1980 the deformation occurred rapidly in an elastic brittle manner and not in the gradual ductile fashion in which folds are normally thought to develop

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

geometric and numerical model of progressive limb rotation in detachment folds a simple area balanced geometric model of progressive limb rotation in detachment folds allows the quantification of displacement uplift and rotation and the assessment of their effect upon growth strata geometries translation of this geometric model into a velocity model of deformation allows the modeling of associated growth strata resulting from background sedimentation and local erosion transport and deposition we show that with constant displacement and sedimentation rate this model predicts a progressive unconformity indicating the onlap and eventual overlap of the fold limb caused by a decreasing uplift rate this decrease is a consequence of the kinematics of progressive limb rotation increases in displacement rate with sedimentation rate kept constant can lead to offlap and angular unconformities although subsequent onlap does not necessarily reflect a return to a lower displacement rate modeled growth strata geometries exhibit many of the features reported from the literature

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

integrated rock magnetic and mineralogical study of the lower triassic sedimentary rocks from spitsbergen abstract a rock magnetic mineralogical and mossbauer spectroscopy study was undertaken on the lower triassic sediments from the west spitsbergen fold and thrust belt wsftb in svalbard as well as the foreland of this orogen for a thorough recognition of magnetic mineralogy necessary for a better constrained interpretation of paleomagnetic results sem and mossbauer spectroscopy indicate the presence of magnetite and pyrrhotite both most probably of secondary origin but altered clasts of volcanic origin and grains of ilmenite and detrital magnetite are also observed thermomagnetic curves of whole rock samples exhibit paramagnetic behavior whereas ferromagnetic separates indicate the presence of magnetite irm component analysis shows three major contributors magnetite hematite and pyrrhotite to the remanence with ferromagnetic iron oxides the most dominant identified ferromagnetic minerals display a wide range of grain size and magnetic behavior with the domination of psd particles rock magnetic characteristics of both areas are similar but sites in the fold belt were most likely remagnetized whereas those in the foreland carry the primary magnetization the age of remagnetization was estimated as pre eurekan related to rift controlled mesozoic extensional phases of the barents sea shelf clark et al 2014 and or cretaceous injections of dolerites in the svalbard archipelago nejbert et al 2011 as well as early stages of the wsftb formation however triassic remanence might persist in volcanic lithoclasts and detrital magnetite indicated along the wsftb the multicomponent nature of the nrm displaying a broad range of grain size might impede the paleomagnetic interpretation of triassic rocks in spitsbergen

environmental dose rates and radioactive disequilibrium from some australian luminescence dating sites abstract determination of geological archaeological ages by luminescence dating requires the measurement of the rate of delivery of radiation dose from the sample and its environment for all age determinations it is necessary to assess whether the dose rate has been constant during the time that the dose has been accumulating and to make allowance for it if it was not we report here results from a number of australian sites to illustrate the information that can be found about element concentrations about their distribution and about dose rates tests for radioactive disequilibrium form part of these measurements such tests take the form of comparison of independent measurements of dose rates and or individual elements in general there is little or no evidence for disequilibrium in aeolian dune systems even when the age of the system exceeds 500 ka however disequilibrium has been found in several sites subject to wet conditions

application of geometric models to inverted listric fault systems in sandbox experiments paper 1 2d hanging wall deformation and section restoration abstract fault geometry is a primary control on hanging wall deformation in order to examine their geometrical relationships a positive inversion analogue experiment was conducted using a rigid fault surface of listric geometry the hanging wall deformation observed on a representative vertical section was examined with conventional 2d geometric models and was restored to its pre inversion phase with two techniques these results suggest that the deformation can be best approximated by inclined simple shearing iss the iss model can determine the inclination of the apparent shear plane and the amount of apparent horizontal shortening which is equivalent to that calculated with the conventional depth to detachment method this estimated apparent shortening was generally smaller than the actual amount of the experiments probably due to tectonic compaction

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

chemical remagnetization and burial diagenesis testing the hypothesis in the pennsylvanian belden formation colorado lower pennsylvanian belden formation carbonate rocks from colorado were subjected to paleomagnetic rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization crm carried by authigenic magnetite and burial diagenesis thermal demagnetization results indicate the presence of two components of natural remanent magnetization nrm after removal of a low unblocking temperature nrm 250 c remanence that is interpreted to be a modern viscous magnetization an intermediate unblocking temperature 250 400 c remanence component with normal and reversed polarity tertiary directions is interpreted to be a thermoviscous remanent magnetization many limestones also contain a high unblocking temperature 400 570 c remanence component which is interpreted to be a crm fold tests from different parts of the basin indicate that the crm was acquired either before or during laramide folding this crm is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the crm is carried by single domain pseudo single domain magnetite although elevated 87sr 86sr values indicate passage of radiogenic fluids through the limestones the results of contact vein tests do not support the hypothesis that these fluids were responsible for the crm the time of crm acquisition which varies from late paleozoic to cretaceous coincides with the modeled time of organic matter maturation in different parts of the basin this suggests that diagenetic reactions that were triggered by low to moderate burial temperatures may have caused the magnetite authigenesis and probably gave rise to the crm

origin of orogenic remagnetizations in mississippian carbonates sawtooth range montana 1 paleomagnetic results are presented from mississippian madison group carbonates in the sawtooth range northwestern montana samples were collected from sites along two east west trending transects perpendicular to the thrust faults in the sun river canyon and in the north and south forks of the teton river and from three folds the madison group contains a widespread characteristic remanent magnetization chrm that resides in magnetite with southerly declinations and moderately steep up inclinations tilt test results suggest that the chrm is pretilting in the thrust sheets and teton anticline but syntilting in the clary coulee and swift dam folds the chrms all have the same characteristics and were probably caused by the same remagnetization event yet the tilt test results are different one explanation involves the difference in fold types between the teton anticline fault bend fold geometry and the clary coulee and swift dam folds fault propagation fold geometries the deformation that produced the two geometries could have caused variations in strain stress which may have altered an original pretilting into a syntilting chrm a mean paleopole for the three pretilting tilt test results 67 2 n 177 9 e a95 13 1 suggests remanence acquisition in the late jurassic early tertiary the chrm is interpreted as a chemical remanent magnetization crm geochemical studies indicate that the mississippian carbonates were altered by evolved fluids with radiogenic 87sr 86sr values and petrographic analysis indicates that hydrocarbons migrated through the carbonates the crm is interpreted to be related to alteration by one of these fluids

strontium isotopic evolution of oil field waters from carbonate reservoir rocks in bindley field central kansas u s a abstract oil field waters produced from mississippian carbonate reservoir rocks in bindley field kansas had an average salinity of about 42 8 mg l they were enriched in ca sr na k rb and li and depleted in mg relative to sea water at the same level of either cl or br concentration the average abundances of different elements are as follows na 13 460 mg l k 325 mg 1 rb 0 9 mg l li 12 mg l ca 1 515 mg l sr 42 mg l mg 430 mg l cl 23 000 mg l so4 2 630 mg l br 32 mg l the 87 sr 86 sr values of the waters ranged between 0 7221 and 0 7230 whereas the values of the host carbonate rocks were between 0 7090 and 0 7093 the very high 87 sr 86 sr values of the waters probably occurred as a result of their reaction with alkali feldspar minerals in buried precambrian crystalline rocks to the east of the bindley field the study demonstrates that despite their residence in carbonate reservoir rocks oil field waters can retain sufficiently distinct isotopic memory that may provide important dues about mineral water interactions in the chemical evolutionary history of the waters the study further indicates that very late diagenetic reactions in many carbonate rocks are of minor extent difference in the sr isotopic data among the oil field waters suggests that the sr isotopic data can be used as a reliable guide in recognising existence of separate pools in an oil field

imprinting chemical remanent magnetization in claystones at 95 c abstract thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization crm we present an experimental study of crm production by heating claystones at 95 c a vertical magnetic field of 2 mt was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months solid fragments 9 to 26 g of claystones are included in a teflon holder that is placed in the oven under a controlled atmosphere newly formed grains acquire a crm and a thermoviscous magnetization tvrm both being parallel to the applied magnetic field crm is related to the amount of newly formed grains that pass the critical volume during the reaction to measure the acquired remanence the claystones are first cooled in a zero magnetic field and then measured using a 2g squid magnetometer in the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation we investigate the magnetic transformation of mont terri lower dogger claystones switzerland due to thermal imprinting at 95 c we simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones during dehydration the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0 23 0 07 ma m 1 day between 3 and 14 days the magnetic susceptibility increases by a few percent the increase of the remanence and of the magnetic susceptibility stops after 15 days mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes during rehydration the remanence increases again whilst magnetic susceptibility drops by a few percent after 20 days the remanence during rehydration follows a rate of 0 42 0 15 ma m 1 day by contrast when rehydration takes place later after 66 days the rate is much lower 0 09 0 04 ma m 1 day low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite newly formed magnetite and hematite carry the remanence we propose that magnetite is formed at the expense of pyrite hematite results from the progressive oxidation of newly formed magnetite our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature

rock magnetism of remagnetized carbonate rocks another look authigenic formation of fine grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks authigenic magnetite grains dominantly in the super paramagnetic and stable single domain size range also give rise to distinctiverock magnetic prop erties now commonly used as a fingerprint of remagnetization we re examine the basis of this association in terms of magnetic mineralogy and particle size distribution in remagnetized carbon ates having these characteristic rock magnetic properties including wasp waisted hysteresis loops high ratios of anhysteretic remanence to saturation remanence and frequency dependent susceptibility new measurements on samples from the helderberg group allow us to quantify the proportions of superparamagnetic stable single domain and larger grains and to evaluate the mineralogical composition of the remanence carriers the dominant magnetic phase is magnetite like with sufficient impurity to completely suppress the verwey transition particle sizes are extremely fine approximately 75 of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single domain although it has been pro posed that the single domain magnetite in these remagnetized carbonates lacks shape anisotropy and is therefore controlled by cubic magnetocrystalline anisotropy we have found strong exper imental evidence that cubic anisotropy is not an important underlying factor in the rock magnetic signature of chemical remagnetization the usefulness of a palaeomagnetic remanence is directly related to the accuracy with which its age is known e g van der voo 1990 primary remanence acquiredduringorverysoonafterrockformation pro videsdirectinformationonpalaeofieldorientationand strengthatthattime thepossibilityofpartialorcom plete remagnetization at a later time complicates palaeomagneticinterpretation ashaslongbeenrecog nized e g graham 1949 every robust palaeomag netic study must therefore include some effort to constrain the ages of identified components of the natural remanent magnetization nrm relative dating with respect to sedimentary processes struc tural tilting or cross cutting relationships is the basis of the classical geometric palaeomagnetic tests fold test and conglomerate test graham 1949 baked contact test everitt clegg 1962 unconformity test kirschvink 1978 the special circumstances required for application of these tests are often not available however or positive test results may providerelativelylooseconstraintsallowingmagneti zation ages that significantly post date the age of the rock formation as a result other evidence although generally more indirect becomes essential in evaluating the age origin and significance of nrm components such evidence commonly includes petrographic observations isotopic andgeo chemical data and rock magnetic characterization by their very existence magnetic overprints provide evidence of some process or event of