error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

an accreted continental terrane in northwestern peru abstract a paleomagnetic study of over 250 cores from 26 sites sampled in early to late cretaceous and paleogene volcanic plutonic and sedimentary formations of the lancones basin in the piura province of northern peru indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable south america a clockwise rotation ranging from 90 for the lowermost units to 35 for the uppermost ones has been documented although the lack of precise chronology has not allowed a detailed temporal description four sites from late carboniferous pennsylvanian formations in the amotape tahuin range also show a 110 clockwise rotation and yield evidence for a northward displacement when considered together with previous geological studies these data are consistent with the hypothesis of the accretion of an amotape tahuin continental terrane to the peruvian margin in neocomian times the accretion was followed by in situ rotation suggesting a dextral shear regime these results indicate that the geodynamical evolution of northern peru is more closely related to the processes observed in ecuador than to those classically assumed for the central andes of peru

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

plate tectonics of east qinling mountains china abstract this paper presents the first detail study of the tectonic evolution and the petrologic and geochemical characteristics of the eastern qinling mountains of central china and their relation to plate tectonics theory the field investigations and petrological and geochemical studies have verified that the plate tectonic regime of the east qinling mountains resulted from the middle late proterozoic evolution of the continental margins of the north china and yangtze plates and the late paleozoic triassic collision of the two plates

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

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

palaeomagnetism of neogene ocros dyke swarm the peruvian andes implication for the bolivian orocline summary remanent magnetization directions of 32 dykes and lava flows sampled near ayacucho the peruvian andes revealed 14 2 5 5 counterclockwise rotation after the neogene intrusion of this dyke swarm palaeo magnetic results of these rocks and other palaeomagnetic evidences from the central andean mesozoic rocks suggest relatively recent occurrence of the andean oroclinal bending around the axis at the peru chile border

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

paleomagnetism and the orocline hypothesis eldredge s bachtadse v and van der voo r 1985 paleomagnetism and the orocline hypothesis in n l carter and s uyeda editors collision tectonics deformation of continental lithosphere tectonophysics 119 153 179 oroclines were originally defined by carey as curved mountain belts which initially were straight or at feast straighter than they are today in the last few years the definition has been broadened to include any curved mountain belt regardless of its original shape since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt the arcuate belts investigated are the sicilian calabr n arc and the umbrian arc of italy the swiss portion of the jura mountains the central portion of the appalachian mountains from pennsylvania to virginia u s a the wyoming idaho overthrust belt of western north america and the hercynides of western and central europe the jura mountains and the pennsylvania virginia portion of the appalachians fail to show significant oroclinal bending the wyoming idaho belt shows a combination of rotated possibly oroclinai and unrotated thrust sheets in the sicilian calabrian arc significant oroclinat bending caused by the impingement of the calabria peloritani nappes in the late tertiary can be demonstrated while the umbrian arc of similar age in the northern apennines also shows oroclinal bending on a smaller scale hercynian europe the only belt included in which deformation of basement rocks can be demonstrated shows oroclinal bending at least so as well as a marked original curvature 70 in its western part common to all the orociines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny causing subsequent deformation and bending of the fold belt

rotation of the peruvian block from palaeomagnetic studies of the central andes palaeomagnetic studies were performed on the rocks of the central andes to test the hypothesis of oroclinal bending of the andes around the peru chile border natural remanent magnetization of mesozoic volcanic and sedimentary rocks of peru shows a counterclockwise shift of declination by several tens of degrees relative to the field directions of the same ages in the stable south american craton two mesozoic dyke swarms from arica region northernmost chile also indicate about 20 of declination shift in a counterclockwise sense these results suggest a considerable amount of counterclockwise rotation of large area north of arica that is oroclinal bending of the andes around some hinge near the peru chile border one neogene dyke swarm sampled in central peru also gave declination which deviates counterclockwisely by 15 if this declination shift is a result of the wide area tectonic rotation as suggested by the mesozoic rocks oroclinal bending appears to have continued until relatively recent times

late cenozoic paleomagnetism and chronology of andean basins of bolivia evidence for possible oroclinal bending new paleomagnetic and radioisotopic data are reported from two late tertiary high elevation non marine basins in the eastern cordillera of bolivia 1 quebrada honda located at 22 s lat consists of a 300 m thick section containing abundant santacrucian or friasian middle miocene fossil mammals this locality is constrained by mean 40 k 40 ar ages of 12 83 0 11 ma and 11 96 0 11 ma and the local magnetostratigraphy is correlated to chrons c5aa through c5a on the magnetic polarity time scale mpts the fossil mammals from quebrada honda have an extrapolated age of about 13 0 to 12 7 ma 2 micana located at 17 s lat consists of a 205 m thick section containing late miocene fossil mammals including a megatheriid sloth and the tiny mesothere microtypotherium cf m choquecotense this locality is constrained by a fission track age determination of 6 9 1 1 ma and the local magnetostratigraphy is correlated to chron 7 on the mpts the fossil mammals from this section have an extrapo

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

bending the bolivian orocline in real time global positioning system gps data from the central andes record vertical axis rotations that are consistently counterclockwise in peru and bolivia north of the bend in the mountain belt and clockwise to the south in southern bolivia argentina and chile these geologically instantaneous rotations have the same sense as rotations that have accrued over millions of years and are recorded by paleomagnetic and geologic indicators the change in sign of the rotation at both decadal and million year time scales occurs across the axis of topographic symmetry that defines the bolivian orocline when extrapolated to a common time interval the magnitudes of rotation from geologic features and from gps are surprisingly similar given that a significant part of the instantaneous deformation field is probably elastic and due to interseismic locking of the plate boundary some of the interseismic deformation field must reflect permanent deformation and or some of the current elastic deformation will be converted to upper plate permanent deformation over time rather than be recovered by elastic rebound during interplate earthquakes we suggest that the spatial patterns of the elastic and the permanent modes of bending are similar because they are driven by the same stress field

spatial and temporal variations in the andean subduction zone a comprehensive global study of the process of subduction of oceanic lithosphere has made it evident that a major role is played by the age of the subducted lithosphere the age dependence of subduction is of special interest in an understanding of the late cenozoic history of the andean subduction zone many of the complexities of this zone result from the peculiar circumstance that the zone is in transition from an early stage of consumption of old lithosphere 70 ma producing deep earthquakes towards the latest stage of consumption of young lithosphere without deep earthquakes this transition has induced an evolutionary pattern with significant temporal variations notably changes in dip and length of the subducted slab moreover the various segments of the south american subduction zone are in different stages of this evolution in comparison with the other segments the northern chile segment is in the earliest stage and therefore shows a much closer resemblance to the classic deep subduction zones in the western pacific than do other parts of the south american zone a roughly n s symmetry in development is found around northern chile with the ecuadorian and southern chilean near 40 45 s segments representing the latest stage to a large extent the temporal and spatial variations in lithospheric age at the trench originate from the reorganization in plate configuration which started with the break up of the farallon plate into the cocos plate and the nazca plate at about 25 ma

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

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

paleomagnetic study of cretaceous rocks of peru south america evidence for rotation of the andes abstract paleomagnetic data for the cretaceous volcanic and sedimentary rocks in the andean region of peru are given reliable paleomagnetic field directions were obtained for three cretaceous albian to cenomanian formations from calcareous sediments in northern peru stable remanent magnetization directions were also derived from twelve cretaceous lava flows and dikes in coastal peru paleomagnetic data of the same age from the stable areas of south america such as brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole but peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts this suggests counterclockwise tectonic rotation of an extensive block which includes the whole of andean peru

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

paleomagnetism of the puente piedra formation central peru abstract paleomagnetic samples were collected from 15 sites in the early cretaceous puente piedra formation near lima peru this formation consists of interbedded volcanic flows and marine sediments and represents the oldest known rocks of the andean coastal province in this region the puente piedra formation is interpreted as a submarine volcanic arc assemblage which along with an overlying sequence of early cretaceous clastic and carbonate rocks represents a terrane whose paleogeographic relationship with respect to the peruvian miogeocline in pre albian time is unknown moderate to high coercivities blocking temperatures below 320 c and diagnostic strong field thermomagnetic behavior indicate that pyrrhotite is the dominant magnetic phase in the puente piedra formation this pyrrhotite carries a stable crm acquired during an event of copper mineralization associated with the intrusion of the santa rosa super unit of the coastal batholith at about 90 5 m y b p the tectonically uncorrected formation mean direction of d 343 2 i 28 6 95 3 4 is statistically concordant in inclination but discordant in declination with respect to the expected direction calculated from the 90 m y reference pole for cratonic south america the observed declination indicates approximately 20 of counterclockwise rotation of the puente piedra rocks since about 90 m y this is consistent with other paleomagnetic data from a larger crustal block which may indicate modest counterclockwise rotation during the cenozoic associated with crustal shortening and thickening in the region of the peru chile deflection

snow peak oregon latest miocene low k tholeiite volcanism in the cascadia forearc abstract snow peak oregon is a moderate size basaltic shield volcano 50 52 wt sio2 7 4 km3 located within the forearc of the cascadia subduction zone 50 km west of the current arc front herein we present new whole rock geochemistry mineral chemistry and 11 new 40ar 39ar ages together with petrologic modeling that allow us to constrain the timing and origin of volcanism in contrast to previous k ar ages that suggested volcanism occurred at 3 ma our new 40ar 39ar ages show that snow peak formed between 5 3 and 6 million years ago the volcano lies unconformably on 30 ma volcanic rocks of the western cascades volcanism occurred over a total duration of 14 wt mgo snow peak lavas have trace element characteristics transitional between the calc alkaline basalt cab and low k tholeiite lkt primary magma types recognized throughout the cascade range but are closer to lkt and are classified as such estimates based on phase equilibria models and plagioclase hygrometers suggest that the primary magmas contained moderate amounts of water 1 5 2 wt consistent with lile hfse ratios that are greater than morb values snow peak is part of a widespread suite of lkt magmas that erupted between 5 8 ma throughout the central oregon cascade range in response to intra arc rifting and snow peak shows that lkt magmatism at this time extended well into the forearc of the central oregon cascade range overall lkt magmas of this age occur over a distance of 120 km normal to the arc front we speculate that the presence of snow peak may relate to the western extension of the brothers fault zone into the cascade forearc

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

the geology and tectonic setting of the velasco alkaline province eastern bolivia the velasco alkaline province of mesozoic age is situated near to the bolivian brazilian border and is enclosed in precambrian gneisses of the brazilian shield it consists of an early volcanic suite and a series of 14 interfering circular ring and elliptical plutons and is associated with a silicified carbonatite complex the combined length of the province is nearly 80 km and it outcrops over an area of 440 km 2 the plutonic rock types range from foyaite to aegirine granite but each individual pluton may be classified as either over or under saturated the province was built up by a series of plutonic events which for the most part were aligned along a ne trending lineament normal faulting was active during the development of the province although the velasco alkaline province lies over 1500 km from the eastern continental margin it is deduced that it defines a plume generated triple point initiated at the same time as the break up of the south american africa plate