chronology of internal drainage development and uplift southern puna plateau argentine central andes fundamental to understanding the late cenozoic evolution of the central andes is knowledge of the age that the puna altiplano plateau became a unique tectonomorphic entity within the orogen the two attributes that define the puna altiplano are 1 its average elevation of 3 km and 2 regions of high elevation that correspond spatially to regions with internal drainage the chronology of plateau uplift to elevations similar to modern ones is equivocal owing to an inability to define the absolute elevation of the plateau surface through time however because regions of the puna altiplano have nonmarine evaporites nad because such evaporites must accumulate within regions of internal drainage the chronology of these strata elucidates when the plateau became internally drained we show that for the southern puna between lat 24 and 26 s internal drainage was established between 14 1 and 24 2 ma and probably by 15 ma these results corroborate and refine other workers inferences that the puna altiplano plateau became uplifted and paleogeographically isolated from the adjacent fore arc and foreland during middle to late miocene time

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

a model for tectonic rotations about a vertical axis abstract the rotation of a rigid ellipsoidal inclusion within a highly viscous fluid orientated so that two of the principal axes remain horizontal is used as a model for the rotation of crustal inclusions in wide zones of continental deformation this model is also applicable to other geological problems involving the rotation of inclusions in a matrix the pattern of behaviour in such a model is shown to be complex in general the rotation rate of the inclusion is a function of all components of the velocity field of the deforming medium and the horizontal aspect ratio of the inclusion however for a given velocity field this aspect ratio must exceed a critical value before the inclusion can rotate continuously inclusions with lower aspect ratios will rotate for a certain range of orientations in an opposite direction to the sense of shear in the deforming zone the possibility of the inclusion changing shape during rotation adds to the complexity of behaviour

lithospheric scale structure across the bolivian andes from tomographic images of velocity and attenuation for p and s waves we have developed a three dimensional lithospheric scale model across the bolivian andes at 20 s based on tomographic images of velocity and attenuation for both p and s waves observations of travel time and attenuation for this study are from regional mantle earthquakes in the subducted nazca plate recorded on a portable broadband seismic array broadband andean joint experiment and seismic exploration of the deep andes in bolivia and chile the shallow mantle under the altiplano from 18 s to 21 s is high velocity and moderately high q vp 8 3 vs 4 7 qp 500 and qs 200 suggesting lithospheric mantle high velocity material in the altiplano extends to a depth of 125 150 km the shallow mantle of the western cordillera is characterized by high vp vs 1 83 suggesting a correlation between vp vs and arc volcanism seismic velocity in the western cordillera mantle is on average only slightly reduced from global averages however velocity and attenuation anomalies are locally strong vp 7 8 vs 4 3 qp 200 and qs 100 consistent with partial melt conditions under the los frailes volcanic field in the eastern cordillera shallow mantle velocity and q decrease drastically from the neighboring altiplano vp 7 8 vs 4 3 qp 300 qs 100 however high vp vs is not as pervasive as it is in the western cordillera we believe that slab derived water and perhaps other volatiles strongly influence the western cordillera while the eastern cordillera low velocity region is more affected by partial melt and or compositional changes average velocity and q in the shallow mantle across the bolivian andes where the tomographic images are best resolved are significantly higher than in most mantle wedge environments where corresponding images are available this is likely the result of a compressional back arc setting in the andes this implies that lithospheric shortening and thickening associated with the formation of the andes has profoundly influenced the shallow mantle structure across the range shallow mantle structure is locally influenced by the subduction processes particularly under the western cordillera however the differing volcanism and seismic character under the two cordilleras suggest that the volcanic process in the eastern cordillera may be distinct from arc volcanism tertiary volcanism in the eastern cordillera is located in the region where mantle shortening is suspected to be greatest both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere

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

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

magnetic polarity stratigraphy of inchasi a pliocene mammal bearing locality from the bolivian andes deposited just before the great american interchange abstract the newly discovered section at inchasi located about 50 km southeast of potosi bolivia in the eastern cordillera consists of about 120 m of undeformed terrestrial sediments containing fossil mammals paleomagnetic analysis of 54 sites indicates a polarity pattern with an estimated duration of about 0 64 ma the rich inchasi local fauna indicates a montehermosan and or chapadmalalan land mammal age pliocene given these constraints inchasi correlates from the interval between the late gilbert within the cochiti subchron to the early gauss within the mammoth subchron chrons that is between about 4 0 and 3 3 ma the distinct lack of north american mammals in the inchasi local fauna provides some of the first well calibrated evidence that the great american interchange occurred after about 3 0 ma as has been previously stated based on other calibrations of the earliest immigrant uquian faunas

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

clockwise block rotations in northern chile indications for a large scale domino mechanism during the middle late eocene the andean deformational pattern of the northern chilean precordillera between 26 30 and 27 00 s suggests significant influence of an orogen oblique northwest trending pre andean basement structure the left lateral reactivation of this structure and the associated deformation can be explained by clockwise vertical axis rotations of forearc wide crustal blocks during the middle late eocene ca 39 3 ma induced by the coupling stress due to oblique northeastward subduction of the oceanic farallon plate beneath the south american continent the compressive stress at the leading edges of the blocks may have been accommodated 200 km away from the peru chile trench in what is now the precordillera where the magmatic arc was then located the postulated large scale domino mechanism explains the paleomagnetically observed in situ block rotations as well as the orientation segmentation and shear sense of major structural elements within large parts of the present forearc in northern chile

paleomagnetism of upper miocene ignimbrites at the puna an analysis of vertical axis rotations in the central andes the origin of the rotations detected paleomagnetically in the central andes is controversial tectonic models proposed to explain it involve late cenozoic oroclinal bending the bolivian orocline and or small block rotations driven by oblique subduction in this paper we report paleomagnetic data from upper miocene ignimbrites from the puna in the presumed southern limb of the orocline these rocks showed high unblocking temperature and high coercivity magnetization the resulting pole position latitude 85 7 s longitude 80 5 e a95 7 9 k 22 n 17 indicates that the study area has not been significantly affected by vertical axis rotation since late miocene onward this implies that the whole puna has not undergone significant regional rigid body rotation during the same time span a farther analysis based on neogene paleomagnetic data from 31 andean localities distributed from 11 s to 31 s shows the presence of rotated and unrotated areas systematic counterclockwise rotations in peru and northern bolivia are observed whereas clockwise rotations are present in southern bolivia northern chile and perhaps northwestern argentina no systematic rotations are found farther south the overall time spatial distributions of these neogene paleomagnetic data suggest that if orogenic bending occurred it must have taken place before the middle miocene thus the oroclinal hypothesis cannot explain the rotations observed in middle miocene rocks or younger ones which also show orocline like declination anomalies we suggest that small block rotations driven by distributed shear may be a single process that can account for the whole rotations in the central andes it is hypothesized that horizontal shear in the andean crust could be controlled by ancient structures

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

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

resistivity cross section through the southern central andes as inferred from magnetotelluric and geomagnetic deep soundings to investigate the upper lithospheric structure and subduction induced processes across an active continental margin magnetotelluric mt and geomagnetic deep sounding gds experiments were done in the southern central andes 21 25 s of northern chile southern bolivia and northwestern argentina using two dimensional modeling we have constructed an e w resistivity cross section at latitude 21 45 s that involves all the andean units from the pacific coast to the lowland plains the crust of the coastal cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted nazca plate the electrical resistivity of the normal andean crust seems to be low having values of only 200 ohm m a wide zone of extremely high electrical conductivity with an e w extent of more than 330 km was detected this high conductivity zone hcz is found at a depth of about 25 km under the western cordillera and has a total conductance of more than 23 000 s here it may be caused by partially melted crust an interpretation which would correlate with those from gravity and seismic data the hcz can be followed in the lower crust eastward below the altiplano where it has a total conductance of about 15 000 s in the western part of the eastern cordillera total conductance first increases and then drops abruptly at about longitude 65 10 w the latter hczs can be explained in terms of thrust tectonics where detachment zones may involve fluids as well as increased mineralizations the lowlands of the sub andean and the chaco are characterized by a cover of anisotropic low resistivity the resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m while the deeper mantle gets unexpectedly much more conductive

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

geochemical investigations to explain iodine overabundances in antarctic meteorites abstract iodine bromine and chlorine concentrations were determined in different antarctic meteorite specimens eucrites high iron and low iron chondrites with isotope dilution mass spectrometry in all antarctic meteorites i overabundances have been analysed compared with the concentrations for non antarctic meteorites of the same class half of the antarctic specimens show especially high concentrations of more than 1 g a similarly high enrichment effect in antarctic meteorites was not found for br and cl but smaller cl excesses could be observed in some eucrites and high iron chondrites by analysing different types of antarctic rocks a significant decrease of the i concentration but not of the br and cl concentration was determined from the surfaces to the centers of the rocks this shows that atmospheric i compounds interact with the surfaces of antarctic rocks and therefore with those of antarctic meteorites as well analyses of atmospheric halogens show that the gaseous compound ch 3 i is probably responsible for the i overabundance in antarctic meteorites and rocks chloride and bromide are found in particles derived from sea spray in antarctic aerosols deposition of halogens in antarctic snow near the coast shows i cl ratios 10 190 higher than the average value for seawater the corresponding br cl ratios are in the same range as seawater on the basis of our results and of other reported data we construct a preliminary hypothesis for a geochemical i cycle in antarctica taking into consideration long distance and short distance transportation of different i compounds from the coast to inland antarctica

cenozoic dextral transpression and basin development at the southern edge of the puna plateau northwestern argentina abstract in northwestern argentina the southern edge of the altiplano puna is the most remarkable positive topographic anomaly average altitude 3700 m of the andean chain to the south are the sierras pampeanas a less elevated domain made of alternating compressive basins and ranges developed during the cenozoic at about 27 s the transition between the puna and the sierras pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the pacific subduction zone nazca plate from 30 e below the plateau to subhorizontal below the sierras pampeanas we argue that this structural transition which can be identified at the scale of the andean topography is a major dextral transpressional transfer zone the tucuman transfer zone in this region ene wsw to e w directed subhorizontal shortening related to bulk convergence between the nazca and south american plates interferes with nw se directed subhorizontal shortening strains due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics structural observations digital mapping spot satellite images and a microtectonic analysis of fault populations provide general insights into the neogene kinematic evolution of transpressional basins and ranges at the edge of the puna

active tectonics of the andes nearly 90 mm a 1 of relative plate convergence is absorbed in the andean plate boundary zone the pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved along any traverse across the plate boundary zone the sum of relative velocities between points must equal the relative plate motion we have developed a kinematic synthesis of displacement and strain partitioning in the andes from 47 s to 5 n relevant for the last 5 ma based upon 1 relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080 2 moment tensor solutions for over 120 crustal earthquakes since 1960 3 structural studies of deformed plio pleistocene rocks 4 topographic geomorphic studies 5 palaeomagnetic data and 6 geodetic data we recognize four neotectonic zones with subzones and boundary transfer zones that are partitioned in different ways these zones are not coincident with the classic zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form the long term segmentation and finite evolution of the andes may not occur in constantly defined segments in space and time in segment 1 47 39 s the slip vector is partitioned into roughly orthogonal benioff zone slip with large magnitude large slip surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50 and dextral slip in the curved liquine ofqui fault system giving 5 10 of anticlockwise fore arc rotation in segment 2 39 20 s the slip vector is partitioned into benioff zone slip roughly parallel with the slip vector andean crustal shortening and a very small component of dextral slip including that on the atacama fault system between 39 and 34 s a cross strike dextral transfer which deflects the chile trench and the volcanic arc absorbs the shortening contrast between segments 1 and 2 in segment 3 20 6 s the slip vector is partitioned into roughly orthogonal benioff zone slip crustal shortening sinistral trench parallel faulting and northeast southwest extension compatibility between segments 2 and 3 is maintained by the sinistral east southeast trending cochabamba shear zone and north trending dextral faults in segment 4 6 s to 5 n the slip vector is partitioned into roughly orthogonal benioff zone slip and dextral strike slip faulting in the fore arc and volcanic chain

paleomagnetic determinations of vertical axis tectonic rotations from late cretaceous and paleocene strata of bolivia structural development of the altiplano during the past 10 m y involved crustal thickening and enhanced curvature of the central andes oroclinal bending of 30 explains reasonably the pattern of vertical axis rotations observed from units with ages 10 to 30 ma new paleomagnetic data from late cretaceous through paleocene strata of southeastern peru and the eastern cordillera of bolivia document rotations that are also counterclockwise on the north limb of the orocline and clockwise on the south limb however the rotations of units with ages 30 to 70 ma are more variable nearby areas have undergone differential vertical axis rotations of up to 60 these larger rotations that have affected 30 70 ma units in the central andes may represent 1 local rotations during late oligocene to present orogeny 2 local rotations during eocene orogeny and or 3 oroclinal bending during eocene orogency

magnetostratigraphy of the miocene corque basin bolivia implications for the geodynamic evolution of the altiplano during the late tertiary a magnetostratigraphic study of a thick miocene continental red bed sequence totora formation located in the north central altiplano the corque syncline yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the altiplano paleomagnetic results obtained in 653 samples from a composite 4 5 km thick red bed sequence across the corque syncline indicate that most of the totora sequence was deposited in the middle miocene from 14 to 9 ma with high sedimentation rates up to 970 m myr from 12 to 9 ma the high rate of infill of the corque basin demonstrates active subsidence of the northern altiplano during the middle miocene deformation ceased in the northern altiplano domain only at the end of the miocene two major tuff beds within the sequence have been previously dated by 39 ar 40 ar of sanidine minerals marshall et al 1992 the excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by cande and kent 1995 the numerous paleomagnetic results obtained in middle miocene sediments within the corque syncline a structure which can be traced for more than 100 km along its axis demonstrate that the corque syncline rotated counterclockwise by 10 8 2 9 since 9 ma as a consequence of the internal deformation of the altiplano inclination flattening of 17 is recorded in these red bed sediments this result provides additional evidence that detrital magnetizations especially in red beds should be taken with caution when estimating paleolatitudes or long term variations of the geomagnetic field

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

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

vertical axis rotations across the puna plateau northwestern argentina from paleomagnetic analysis of cretaceous and cenozoic rocks between 10 s and 30 s the central andes are marked by both a major topographic anomaly the altiplano puna plateau and a westward concave geometry whose origin remains controversial the arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes indeed in the central andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc throughout peru northernmost chile and northern bolivia and clockwise rotations on the southern limb throughout southern bolivia northwestern argentina and northern chile to fill a gap in data from northern argentina and to contribute to the ongoing debate on the origin of rotations in the central andes we have undertaken a paleomagnetic study of 373 cores taken at 29 sites grouped into seven localities the samples are from sediments and lava flows of cretaceous to tertiary age located in intermontane basins of the puna plateau in northwestern argentina vertical axis rotations calculated from paleomagnetic declinations are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central andes however significant variations in the amount of rotation occur from one locality to another suggesting that they are at least in part influenced by local tectonics as most faults in the puna plateau have reverse dip slip components we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults whether or not this mechanism has operated across the entire area of thickened crust in the central andes remains to be demontrated even if such faulting has locally influenced rotations cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central andes

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

biot slow wave effects in stratified rock the transmission of p waves through the stratified layers of a sedimentary basin is modeled numerically using biot theory the effects on the transmissivity of frequency angle of incidence layer thickness permeability and elastic compliance of the rocks are all considered consistent with previous analytical work it is found that the equilibration of fluid pressure between the fine layers of a sedimentary sequence can produce significant p wave attenuation at low frequencies for this attenuation mechanism to act within the surface seismic band say 3 300 hz we find that there must be layering present at the scale of centimeters to tens of centimeters if the layering is restricted to layers of roughly 1 m thickness or greater then for typical sandstone formations the attenuation caused by the interlayer flow occurs below the seismic band of interest such low frequency interlayer flow is called biot slow wave diffusion in the context of biot theory and is likely to be the dominant source of low

paleomagnetic study of upper miocene rocks from northern chile implications for the origin of late miocene recent tectonic rotations in the southern central andes paleomagnetic studies in the southern central andes have shown the widespread presence of clockwise vertical axis rotations rock units sampled in these studies however are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern chile being from mesozoic and lower tertiary rocks whereas most data in the southern altiplano puna and cordillera oriental are from upper tertiary rocks in this paper we present the results of a paleomagnetic study on upper miocene sedimentary rocks and ignimbrites from the precordillera of northern chile at 22 s these rocks are coeval with the initiation of crustal shortening in the eastern foreland fold thrust belt which some tectonic models relate to oroclinal rotation of northern chile primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites suggesting the study area has acted as a single coherent block with respect to vertical axis rotational deformation although minor inadequate sampling of paleosecular variation can affect our data set its time averaged paleomagnetic direction is indistinguishable from the expected late miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 ma this suggests that late miocene recent oroclinal rotation of the northern chilean forearc if present must be either very low or nonuniform a direct implication of this result is that much of the unquestionable tectonic rotations detected from upper miocene rocks in the southern central andes is of local origin available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the andes

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

3he evidence for a wide zone of active mantle melting beneath the central andes we report results of a regional survey of helium isotopes measured in water and gas samples in volcanic sulfataras and geothermal springs from the central andes of northern chile and bolivia between the latitudes 15 s and 23 s the highest 3he4he ratios reported as rra ratios r sample 3he4he ra air 3he4he are associated with the active volcanic arc of the western cordillera 0 92 rra 5 52 and approach ratios found at other convergent margins in the circum pacific region a significant 3he component is also present in fluid and gas samples from the high altiplano plateau 0 48 rra 3 56 and the eastern cordillera 0 03 rra 1 2 up to 300 km east of the active arc and more than 300 km above the subducting slab this wide zone of 3he anomalies is delineated both to the east and the west by regions with low 3he4he ratios 0 2ra typical of radiogenic helium production in the crust studies of the regional groundwater regime suggest that the wide zone of elevated 3he4he values away from the active volcanic arc is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of 3he the high 3he4he ratios are interpreted as reflecting degassing of volatiles from mantle derived magmas emplaced over an area 400 km wide beneath and into crust up to 75 km thick the subducting slab is at depths of 100 350 km in this region in the west underneath the active volcanic arc mantle melting is probably largely controlled by mantle hydration and dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at depth in contrast mantle melting behind the arc beneath the altiplano and eastern cordillera may be a result of convective removal of the base of the lithosphere the sharp cut off in the mantle helium signal in the east is interpreted as marking the western edge of thick and relatively cold lithosphere devoid of mantle melts which could transport mantle volatiles towards the surface this may coincide with the limit of underthrusting of the brazilian shield beneath the eastern margin of the central andes

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

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

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

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

new evidence of clockwise vertical axis rotations south of the arica elbow argentine puna abstract the central andes show a pronounced bend at 18 s this bend is called the arica elbow the existence of counterclockwise vertical axis rotations north of the arica elbow and clockwise rotations to the south has been determined from palaeomagnetic data various authors have proposed different mechanisms to explain this but whether rotations are local or regional and whether the arica elbow is a primary or secondary feature of the central andes remains controversial in order to contribute to the palaeomagnetic data available for the argentine puna 102 oriented cores were obtained from cenozoic rocks cropping out in the bajo de las siete curvas area of the southern argentine puna different demagnetization techniques were applied pretectonic remanent magnetizations were isolated and indicate the existence of a clockwise vertical axis rotation of approximately 7 18 taking into account the presence of a major thrust fault to the west of the studied area this rotation could be considered to be of local character induced by a miocene pliocene compression

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

paleomagnetic evidence for rotation in the precordillera of northern chile structural constraints and implications for the evolution of the andean forearc abstract palaeomagnetic results are reported for two late mesozoic to early cenozoic continental sedimentary sequences from the precordillera cordillera de domeyko of northern chile comparison of isolated components of magnetisation from the late cretaceous palaeocene purilactis formation and the unconformably overlying oligo miocene paciencia group with reference poles for south america indicate that significant post palaeocene and post lower miocene clockwise rotation has taken place a structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the cenozoic stratigraphie structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the purilactis formation lower eocene incaic orogeny and the paciencia group lowest miocene peheunche orogeny and resulted in the folding uplift and clockwise rotation of the two sequences differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets 12 of rotation took place prior to deposition of the paciencia group followed by 17 of post lower miocene clockwise rotation this study demonstrates that palaeomagnetically detected rotation in the precordillera of northern chile 250 km inland of the peru chile trench is related to the periodic movement of an easterly propagating thin skinned thrust front between 22 30 and 23 30 s the north chilean forearc contains areas of non rotation thin skinned compressional related rotations and large block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the central andes should be modified in particular evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event

paleomagnetism of middle tertiary volcanic rocks from the western cascade series northern california the western cascade series wcs is a 3 5 km thick crudely homoclinal east dipping calcalkaline volcanic sequence of mid oligocene to early miocene age that crops out near the southern tip of the cascade range in northern california the mean direction of remanent magnetization in the wcs is d 4 9 i 57 6 n 53 k 14 4 95 5 3 when compared to a reference direction for the north american craton the wcs direction indicates that the southern cascade range has rotated 14 0 9 0 since the wcs accumulated a difference in mean direction between the lower and upper halves of the wcs suggests that much of this rotation occurred during the late oligocene six other paleomagnetic studies of rock units of an age roughly comparable to the wcs also are available from western oregon northwestern california and southwestern washington all show statistically significant clockwise rotation inclinations of all but one are concordant comparison of directions indicates that the pacific northwest did not rotate as a rigid body but neither did it behave as a collection of small independently rotating domains the amount of rotation found throughout the area increases to the west or northwest suggesting a driving force for rotation operating at the continental margin