new paleomagnetic data from the northern argentine puna central andes rotation pattern reanalyzed 1 along the central andes a pattern of vertical axis rotations carp has been paleomagnetically identified these rotations which are counterclockwise north of arica and clockwise to the south have been detected in mesozoic and cenozoic rocks cropping out in the andean forearc and the altiplano puna different models have been proposed to explain the geodynamic evolution of the central andes but the origin of these rotations remains controversial such models involve oroclinal bending and or local block rotations this lack of agreement is partly due to the scarce paleomagnetic database particularly for the argentine puna here we report new paleomagnetic data obtained in the northern argentine puna from 57 sites drilled from miocene volcaniclastic rocks and 12 sites from the cretaceous eocene salta group it was determined that 1 the morro blanco area did not undergo significant vertical axis rotation since at least the middle miocene 2 the loma blanca mine area underwent a clockwise vertical axis rotation of 10 later than the late miocene and 3 the coranzuli area underwent a clockwise vertical axis rotation of 40 later than the cretaceous eocene the central andes database was analyzed and some features of the carp were determined these features were explained considering domino style models for the forearc and a local block rotation model in response to penetrative shear for the altiplano puna penetrative shear would occur along a high electrical conductivity zone however the possibility of the existence of an oroclinal bending of 5 cannot be discarded

palaeomagnetism of the cretaceous el way and coloso formations from the northern chilean andes summary a palaeomagnetic study mainly of limestones of the el way formation and fine sandstone of the coloso formation near antofagasta northern chile 23 83 s 70 38 w gives an early cretaceous palaeomagnetic pole 76 4 s 150 7 e a 6 4 for the chilean andes comparison of this pole with the saki pole of valencio ef al 1983 for cratonic south america suggests a relative clockwise rotation of 12 f 9 considered in isolation little confidence can be placed in this possible rotation because of its large error and non small flattening of 10 f 10 however if the pole is combined with all available late jurassic and cretaceous palaeomagnetic poles from the andes it is very clear that the poles group according to the geographical location of their sampling areas peruvian andes hinge part of the andes and chilean andes the rotation angle between the two blocks calculated from the peruvian andes mean pole and the chilean andes mean pole amounts to a significant 41 f 18 assuming the rotation centre is located at the hinge of the coast a jurassic sandstone of the arica group from cuya in northernmost chile seems to have been completely thermally remagnetized by cretaceous dike swarms which intrude the sandstone the site mean direction of the sandstone is indistinguishable from that of the dike swarms whose palaeomagnetism has been studied by nomura et al 1983

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

structural constraints on palaeomagnetic rotations south of the arica bend northern chile implications for the bolivian orocline abstract palaeomagnetic data from the central andean fore arc appears to support the bolivian orocline model with clockwise rotations to the south and counter clockwise rotations to the north of the arica bend respectively however counter clockwise rotations in northernmost chile extend south of the arica deflection consistent with mesozoic rotations in southern peru this suggests that the bolivian orocline model needs to be redefined we present a palaeomagnetic study of 140 specimens from 15 sites sampled in middle jurassic sediments and volcanics of the camaraca formation and in the upper jurassic lower cretaceous cuya dykes of northernmost chile 19 2dgs 70 3 w the calculated pole 81 4 s 354 7 e 95 6 7 when compared with the upper jurassic pole for cratonic south america 89 0 s 217 1 e 95 4 6 suggests that the sampled area has undergone a small counter clockwise rotation of approximately 10 the change from counter clockwise to clockwise rotations occurs between 19 s and 23 s and may correspond to a known bend in the peru chile trench these rotations were probably the result of in situ block rotation in response to shear this shear is the result of stresses in the subducted lithosphere with sinistral shear to the north counter clockwise rotations and relatively larger dextral shear to the south clockwise rotations of the bend in the trench respectively the amount of shear is reduced in the area of the bend due to contortion of the subducting slab these stresses resulted due to the shape of the andean margin rather than as a result of oroclinal bending the arica bend is interpreted as an ancient feature which may have evolved from a hot spot generated triple junction rift system the velasco alkaline province of eastern bolivia possibly formed the failed arm of this triple junction

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

air clathrate crystals from the grip deep ice core greenland a number size and shape distribution study we performed microscopic observations and a statistical study of the number size and shape distribution of clathrates in the grip greenland ice core project deep ice core using 185 samples from a depth range of 1016 3014m spanning a period of 6 to 110 ka sp and encompassing the holocene wisconsin and eemian periods the number concentration of the clathrates varied considerably with climatic changes it was possible to detect the rapid climatic oscillations in the last glacial between 13 and 110 ka bp the dansgaard oeschger cycles in the number concentration profile of clathrates the mean volume of clathrates is less clearly influenced by climatic factors with a tendency towards greater volumes in warmer periods but also a growth of clathrates with depth could be detected this growth rate was calculated to be 3 1 x 10 12 cm 3 a 1 the amount of gases captured in the clathrates is estimated to be significantly smaller than the total amount of air determined by gas concentration measurements this points to diffusion processes of atmospheric gases within the ice matrix

paleogene clockwise tectonic rotations in the forearc of central andes antofagasta region northern chile 1 for the central valley of northern chile antofagasta region a paleomagnetic analysis of data from 108 sites mainly in mesozoic and paleogene volcanic rocks has yielded stable remanent magnetization directions for 86 sites from these data we infer clockwise tectonic rotations of up to 65 within the forearc domain of the central andes the apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the incaic orogenic event of eocene early oligocene age a few paleomagnetic results obtained in neogene rocks do not show evidence of clockwise rotations hence the development of the bolivian orocline during late neogene time cannot be explained by simple bending of the whole margin these results demonstrate that tectonic rotations within the forearc and pre cordillera are key elements of early andean deformation which should be taken into account by kinematic models of mountain building in the central andes

upper neoproterozoic series in the central iberian cantabrian and west asturian leonese zones spain geochemical data and statistical results as evidence for a shared homogenised source area abstract analyses 100 of six groups of upper neoproterozoic shales from different areas in central and northern spain covering an extension of over 60 000 km2 reveal relatively uniform major and trace element abundances and values of relevant ratios such as al2o3 tio2 k2o al2o3 mol ti zr ti nb rb zr rb th sm nd eu eu la yb n la nb zr y zr nb th nb th yb th u and cr zr the six groups of shales define indistinguishable chondrite normalised and upper continental crust normalised patterns for rare earth and trace elements respectively and show relatively low fractionated geochemical features compared with those of the continental upper crust statistical calculations demonstrate a normal distribution of the abundances of major and trace elements and element ratios they suggest a remarkable geochemical homogeneity for the upper neoproterozoic series studied and they separate this series from the lower cambrian one in the central iberian zone on the basis of 15 element ratios the extension of the upper neoproterozoic detrital series and the similar geochemical features of shales in all the studied areas suggest an extensive and homogenised source region thus the mean of the 100 analyses neoproterozoic iberian average shale nibas is proposed as a standard of comparison to identify the specific recycled and homogenised source for detrital upper neoproterozoic sediments in central and northern iberia and probably also in other european peri gondwanan areas the ranges of statistically distinctive geochemical parameters separating the upper neoproterozoic from the lower cambrian series are given as possible criteria to help in the studies of areas where no other data provide conclusive information about the series ages

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

the significance of the pdaeomagnetism of jurassic cretaceous rocks from south america predrift movements hairpins and magnetostratigraphy summary in this paper we show that 1 the positions of the cretaceous palaeomagnetic poles pp for south america and africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole 2 the positions of the middle late jurassic virtual geomagnetic poles for south america exhibit an elongated distribution along the meridians 20 200 e it is suggested that this is due to a rapid shift of south america in middle late jurassic time 3 the late early early late cretaceous sections of the apparent polar wandering paths for south america and africa are consistent with south atlantic seafloor spreading data on the basis of the comparison of the reliable late palaeozoic late cretaceous pps for south america and africa taking into account the restrictions established by geological palaeontological and seafloor spreading data it is suggested that minor movements could have occurred within western gondwana in middle late jurassic time along a narrow zone which later became the south atlantic divergent boundary four hairpins are defined in the late palaeozoic late cretaceous section of the apparent polar wandering path for south america the two youngest of these can be correlated with the origin of the south atlantic ocean basin and the onset of the andean orogeny respectively the magnetostratigraphy for the serra geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption

palaeomagnetic studies of a cretaceous molasse sequence in the central andes coloso formation northern chile the coloso formation is an early cretaceous conglomerate dominated molasse sequence in northern chile the sequence was deposited rapidly and records the unroofing of granodioritic and andesitic source rocks thin sand units throughout the sequence have been studied palaeomagnetically the results are complex but can be related to the diagenesis of arid zone alluvium natural remanent magnetization nrm may be normal or reversed but is usually a combination of the two isolation of magnetic components by vector analysis of thermal and af demagnetization data yields both normal and reversed components which are approximately antiparallel and yield an average pole position at 70 s 192 e this is consistent with late cretaceous data from northern chile but not with the established apparent polar wander apw path for south america it may indicate late cretaceous remagnetization but is considered to be more likely an indication that the present apw path assumes no tectonic rotation and that important relative movements have taken place between northern chile and argentina

physical analysis of an antarctic ice core towards an integration of micro and macrodynamics of polar ice microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail ice core parameters crystal lattice preferred orientation lpo grain size grain shape mesostructures visual stratigraphy as well as borehole deformation were measured in a deep ice core drilled at kohnen station dronning maud land dml antarctica these observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the edml ice core drilling site european project for ice coring in antarctica in dml the results suggest a division of the core into five distinct sections interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock parallel shear region 1 uppermost approx 450 m depth with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization region 2 approx 450 textendash 1700 m depth shows a girdle type lpo with the girdle plane being perpendicular to grain elongations which indicates triaxial deformation with dominating horizontal extension in this region approx 1000 m depth the first subtle traces of shear deformation are observed in the shape preferred orientation spo by inclination of the grain elongation region 3 approx 1700 textendash 2030 m depth represents a transitional regime between triaxial deformation and dominance of shear which becomes apparent in the progression of the girdle to a single maximum lpo and increasing obliqueness of grain elongations the fully developed single maximum lpo in region 4 approx 2030 textendash 2385 m depth is an indicator of shear dominance region 5 below approx 2385 m depth is marked by signs of strong shear such as strong spo values of grain elongation and strong kink folding of visual layers the details of structural observations are compared with results from a numerical ice sheet model pism isotropic for comparison of strain rate trends predicted from the large scale geometry of the ice sheet and borehole logging data this comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet this article is part of the themed issue textquoteleft microdynamics of ice textquoteright

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

a tale of two continents some tectonic contrasts between the central andes and the north american cordillera as illustrated by their paleomagnetic signatures comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large scale tectonic processes comparison of the paleomagnetic signatures of the western cordillera of north america and the central andes shows that the western edges of these belts have had profoundly different mesozoic and cenozoic histories specifically the north american cordilleran pattern shows strong evidence of post middle cretaceous relative northward displacement of outboard crustal blocks but there is almost no comparable evidence of margin parallel displacement in the andes we speculate that this may largely be a consequence of a simple difference in shape the convex westward western margin of north america facilitates margin parallel displacement as a response to oblique subduction whereas the concave westward margin of south america inhibits it the patterns of block rotations found along the western edge of the two orogens also are quite different nearly everywhere within the western north american cordillera crustal blocks have rotated clockwise since mid cretaceous time reflecting a pervading state of dextral shear within the western andes of peru and northern chile however mesozoic and cenozoic rocks in peru and northernmost chile are rotated strongly counterclockwise whereas rocks of the same age in the remainder of chile to about latitude 48 s are rotated clockwise a model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations

the lower cretaceous way group of northern chile an alluvial fan fan delta complex abstract alluvial fan sediments of the lower cretaceous coloso basin in northern chile were deposited in a half graben and derived from andesitic volcanics of a former island arc transport directions were towards the east away from the present day peru chile trench grain flow density modified grain flow and sheetflow processes were responsible for most of the sediment deposition with cohesive debris flows playing only a minor part an early phase of conglomerate deposition coloso formation into a restricted basin records the transition from proximal fan facies with abundant grain flows and remobilized screes to mid fan facies dominated by sheetflows stratiform copper mineralization near the top of the lower conglomerates is related to the unroofing of the jurassic island arc this mineralization comprises copper sulphide cemented sands and gravels and formed by the reaction of mineralized detritus with diagenetic and hydrothermal solutions a later phase of deposition lombriz formation includes sandstones siltstones and conglomerates with a source area different from the coloso formation this change in source may be related to strike slip tectonics as the basin extended the lombriz conglomerates pass distally eastwards into red sandstones and purple siltstones with thin limestones deposited under marine conditions this sequence is interpreted as a major fan delta complex it passes conformably into marine carbonates of the tableado formation signifying the complete drowning of the basin in lower cretaceous times

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

self reversed magnetization held by martite in basalt flows from the 1 1 billion year old keweenawan rift canada in some basalt flows of the 1 1 billion year old keweenawan rift exposed at mamainse point ontario there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession the paleomagnetic and geological context of this component demonstrates that it is not a chemicaloverprint whoseremanence is attributableto a subsequently reversed geomagnetic field but that the component is a self reversal of the primary magnetization here we use rock magnetic experiments and mossbauer spectroscopy to show that this phase occurs in the most oxidized flowsandisheldbya fine grainedpopulationofhematitethatacquiredits self reversedremanencethrough interactions with a phase of lower blocking temperature we propose that this self reversed hematite is a manifestation of the self reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and thathas been attributed to negative exchange coupling across the crystal lattices we suggest that hematite formed in association with iron silicates in the basalt flows carries a remanence reflective of the field in which it formed but that martite hematite pseudomorphed after magnetite which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self reversed remanence this study marks the second time that a naturally occurring self reversed magnetization has been attributed to this mechanism and marks the oldestreportedinstanceof anytypeof self reversedremanence themartitethatformsthrough thisprocess has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years

concordant late paleozoic paleomagnetizations from the atacama desert implications for tectonic models of the chilean andes abstract paleomagnetic results are reported from three formations of late paleozoic age from the northern chilean andes of the atacama desert for the first time primary nrm components are resolved for paleozoic units along the western flank of the central andes pole positions are calculated for the formations and compared with apw data for cratonic south america these comparisons reveal that the collecting sites in the northern domeyko and almeida ranges of the central andes have undergone no paleomagnetically defined rotations or translation with respect to cratonic south america since the time of nrm acquisition which is likely to have been in the lower parts of the kiaman reverse interval if growth of the south american lithosphere has involved accretion of exotic microplates they are either likely to be substantially older than units sampled here or be restricted to more coastal terranes the results taken together with other paleomagnetic data from northern chile and southern peru which have showed a wide range of discordance in their declinations when compared to each other or apw data lead to the conclusion that this region of the andes during the mesozoic or cenozoic has not been affected by simple processes of clockwise oroclinal bending from peru to chile nor regionally consistent patterns of block rotations

conglomerate hosted copper mineralization in cretaceous andean molasse the coloso formation of northern chile alluvial fan sediments of the lower cretaceous coloso formation of northern chile were deposited during the evolution of a half graben arid intra arc basin the sediments were derived from an andesitic volcanic arc and show compositional variation which can be interpreted as an unroofing sequence copper mineralization is stratigraphically restricted to the upper 150 m of the sequence in conglomerates of predominantly granodioritic composition the presence of mineralized clasts at this level indicates the erosion of bedrock mineralization lead isotopic signatures of ore sulphide support the sedimentological evidence for derivation of the sediments and metals from an island arc volcano plutonic complex to the west of the basin a genetic model is proposed in which uneconomic metal concentrations were transported into the basin as detrital phases and released by diagenetic reactions the precipitation of copper minerals followed authigenic formation of quartz feldspar analcime and calcite and also some secondary parosity generation copper minerals include the oxychloride atacamite and the suphides bornite digenite djurleite and covellite sulphur isotope values for the sulphides are relatively heavy isotope enriched 34 s 3 6 to 12 8 although these results could be interpreted in terms of closed system bacterial sulphide reduction there is no known occurrence of organic material within the coloso basin an alternative explanation involving the mixing of diagenetically derived copper rich brines and sulphur rich hydrothermal fluids is proposed to account for the coloso mineralization

self reversal of chemical remanent magnetization on the transformation of maghemite to haematite summary self reversed chemical remanent magnetization crm has been observed in haematite formed on heating maghemite which has been produced by the dehydration of acicular crystals of synthetic lepidocrocite 1 to 2 pm in length our experimental evidence suggests that self reversal of haematite remanence only occurs when the parent maghemite is still blocked at the temperature of its transformation to haematite when the transformation temperature is above the blocking temperature of the parent maghemite and it is unblocked the resulting haematite remanence is normally magnetized it is suggested that the strong dependence on remanent state supports exchange control of the self reversal process we propose that the self reversal is probably a general feature of the maghemite to haematite transition and the significance of the source lepidocrocite in our experiments is that it produces maghemite of a suitable grain size so that much of it remains blocked at the elevated temperatures required to make the transformation to haematite occur over the short time scale of the laboratory experiments in nature transformation of maghemite to haematite will occur at much lower temperatures due to the much longer time scales involved and self reversal of the resulting crm may occur over a much less restricted grain size range of parent maghemite