paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

tectonic rotations about vertical axes during the last 4 ma in part of the new zealand plate boundary zone the declinations of the primary magnetization of sedimentary rocks in the northern part of the new zealand plate boundary zone after thermal or alternating field cleaning have been used to determine tectonic rotations about vertical axes of rigid crustal blocks the pattern of rotations during the last 4 ma combined with structural style and continuity defines seven structural domains each ca 100 100 200 km across which contain crustal blocks up to 100 km across and 20 km thick large crustal blocks tens of km across in two of these domains have rotated clockwise more than 20 relative to one of the margins of the plate boundary zone in the last 4 ma their behaviour appears to be controlled by the nature of the plate boundaries such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge small crustal blocks 10 km across may have rotated clockwise through angles greater than 20 during the last 4 ma floating on an underlying zone of more distributed deformation the tectonic rotations of the large crustal blocks and the nature of the deformation at their boundaries combined with an interpolation of the relative plate positions can be used to reconstruct the plate boundary zone at ca 4 ma

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

sea breezes shallow and deep on the california coast abstract analyses of doppler lidar data reveal sea breezes occurring on two different depth and time scales at monterey bay california on a day with offshore gradient flow indicated before sunrise and after sunset the lidar data used in this study consist of vertical cross sections and profiles of the westerly onshore wind component u in the morning after 0900 pst a shallow sea breeze formed which reached a depth of 300 m by noon starting in early afternoon a deeper sea breeze layer formed in the lowest kilometer and by late afternoon the shallow sea breeze blended into the deeper sea breeze and was no longer evident maximum speeds of 6 m s 1 in the shallow sea breeze occurred at the surface whereas those in the deep sea breeze also 6 m s 1 were about 300 m above the surface it is hypothesized that the shallow sea breeze is a local phenomenon responding to a more local temperature contrast between the sea and the region between the ocean and the mountain ranges the deeper sea breeze on the

paleomagnetism in the precordillera of northern chile 22 30 s implications for the history of tectonic rotations in the central andes abstract widespread clockwise rotations in mesozoic and lower tertiary rocks of northern chile have been interpreted as the sum of two rotational events separated in time an early rotation related to local deformation plus a late rotation related to wholesale rotation of northern chile linked to late cenozoic oroclinal bending in the central andes in this paper we report new paleomagnetic data from cretaceous upper oligocene and miocene sedimentary rocks in the precordillera of northern chile the results suggest that all these rocks acquired their remanence at or close to the time of deposition the lack of rotation in undeformed lower miocene strata clearly indicates that clockwise rotations found in underlying faulted and folded cretaceous rocks were completed before the late cenozoic results from nearby localities in deformed upper oligocene strata would argue for little 5 rotation since the late oligocene data from widely separated miocene localities covering an area of about 5000 km2 in the calama basin strongly suggest that northern chile did not undergo significant wholesale rotation during the late cenozoic this together with previous paleomagnetic evidence against neogene rigid body like rotation of the southern peruvian forearc suggests that the curved shape of the central andean forearc was not significantly enhanced during the late cenozoic by inference all of the rotation in most mesozoic and lower tertiary rocks of northern chile was accomplished in the cretaceous and or early cenozoic when the locus of deformation in the central andes was localized in the present forearc region

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

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

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

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

geometry and kinematics of adhesive wear in brittle strike slip fault zones abstract detailed outcrop surface mapping in late paleozoic cataclastic strike slip faults of coastal maine shows that asymmetric sidewall ripouts 0 1 200 m in length are a significant component of many mapped faults and an important wall rock deformation mechanism during faulting the geometry of these structures ranges from simple lenses to elongate slabs cut out of the sidewalls of strike slip faults by a lateral jump of the active zone of slip during adhesion along a section of the main fault the new irregular trace of the active fault after this jump creates an indenting asperity that is forced to plow through the adjoining wall rock during continued adhesion or be cut off by renewed motion along the main section of the fault ripout translation during adhesion sets up the structural asymmetry with trailing extensional and leading contractional ends to the ripout block the inactive section of the main fault trace at the trailing end can develop a sag or half graben type geometry due to block movement along the scallop shaped connecting ramp to the flanking ripout fault leading contractional ramps can develop thrust type imbrication and forces the humpback geometry to the ripout slab due to distortion of the inactive main fault surface by ripout translation similar asymmetric ripout geometries are recognized in many other major crustal scale strike slip fault zones worldwide ripout structures in the 5 500 km length range can be found on the atacama fault system of northern chile the qujiang and xiaojiang fault zones in western china the yalakom hozameen fault zone in british columbia and the san andreas fault system in southern california for active crustal scale faults the surface expression of ripout translation includes a coupled system of extensional trailing ramps as normal oblique slip faults with pull apart basin sedimentation and contractional leading ramps as oblique thrust or high angle reverse faults with associated uplift and erosion the sidewall ripout model as a mechanism for adhesive wear during fault zone deformation can be useful in studies of fault zone geometry kinematics and evolution from outcrop to crustal scales

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

paleomagnetic data from the precordillera of northern chile a multiphase rotation history related to a multiphase deformational history abstract one of the most conspicuous features of the andean chain is the change in its trajectory from nw se to n s at 18 s known as the bolivian orocline although the central andes rotation pattern carp agrees roughly with the geometry of the orocline large variations and greater than expected rotation magnitudes cannot be explained by a progressive oroclinal bending history alone in this work 621 oriented samples from mesozoic and cenozoic rocks in three different areas in the precordillera of northern chile north of calama were analyzed the samples were subjected to different demagnetization processes to isolate the characteristic magnetic remanence and calculate the tectonic rotations by sampling pretectonic syntectonic and post tectonic units the rotation history of different structural blocks was determined and related to specific deformational events the results obtained from lower cretaceous units intruded or overlain by upper cretaceous units not rotated or recording smaller rotations indicate the existence of 20 clockwise and 20 counterclockwise rotated sites related to the late cretaceous peruvian orogenic event data obtained from upper cretaceous lowermost paleocene rocks show areas slightly clockwise rotated 10 15 and not rotated during the early paleocene k t deformation event remanence data from upper cretaceous eocene units across the area indicate as much as 20 clockwise rotation in the hanging wall of the eocene chintoraste quetena rabonas jaspe reverse fault system whereas the footwall was unaffected this eocene rotation occurred during the early stages of the eocene oligocene incaic orogenic phase prior to emplacement of the 40 37 ma fortuna granodiorite complex current models propose that rotations in carp were entirely cenozoic and largely accomplished in a single paleogene event our data indicate that each of the peruvian k t and incaic deformation events contributed to carp with each event generating a non uniform pattern of rotations which was superimposed on prior rotations in a non uniform manner

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

a climatological study of the sea and land breezes in the arabian gulf region 1 this study focuses on observations of the sea and land breeze circulations in the southern arabian gulf during the summer months the indian monsoon creates light northwesterly winds over the arabian gulf allowing for the formation of thermally driven circulations observations from a network of stations are used to develop a wind climatology for the arabian gulf region characteristics of the sea and land breeze circulations such as onset time duration and horizontal and vertical extent are described the dense network of surface stations in the united arab emirates uae allows for a fine scale observational study in this region it is found that the sea breeze occurs during all seasons of the year in this region it occurs in the late afternoon and continues through the evening a land breeze sets in during the night surface offshore winds in the land breeze are strong probably due to drainage flow down the inland hills

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

clockwise block rotations along the eastern border of the cordillera de domeyko northern chile 22 45 23 30 s abstract we report new paleomagnetic results from a study of 38 sites in cretaceous to early tertiary red beds and volcanic rocks belonging to the purilactis group which outcrop along the western border of the salar de atacama basin after detailed thermal demagnetization characteristic directions were determined for 32 units in most cases red bed sediments from the lower members of the purilactis group have a well defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement a large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre tectonic magnetization we interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the cretaceous normal polarity superchron large deviations of the paleomagnetic declinations from the expected ones for stable south america provides new evidence for clockwise tectonic rotations associated with tertiary deformation in the cordillera de domeyko these data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north chilean andes this study however indicates spatial variation in the magnitude of the rotation with rotations 60 in the cerro totola area these rotations have occurred in conjunction with transpressional deformation that affected large tracts of the cordillera de domeyko during eocene deformation the systematic observation of clockwise rotations contemporaneous with sinistral displacements in the cordillera de domeyko can be explained by shear traction at the base of the brittle crust

observations of the sea breeze front during cape part i single doppler satellite and cloud photogrammetry analysis abstract an analysis of two sea breeze events on 6 august an onshore flow event and 12 august an offshore flow event 1991 is presented using single doppler observations satellite images and cloud pictures collected during the convection and precipitation electrification cape experiment documentation of the alongfrontal variability at the leading edge of the sea breeze circulation is presented for the first time the horizontal structure of the front was strongly modulated by the near perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 august these intersection points also appeared to be preferred locations for cloud development along the front horizontal convective rolls were also documented on 6 august however their orientation was nearly parallel to the sea breeze front as a result extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

late mesozoic to paleogene stratigraphy of the salar de atacama basin antofagasta northern chile implications for the tectonic evolution of the central andes abstract the salar de atacama basin the largest pre andean basin in northern chile was formed in the early late cretaceous as a consequence of the tectonic closure and inversion of the jurassic early cretaceous tarapaca back arc basin inversion led to uplift of the cordillera de domeyko cd a thick skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike the almost 6000 m thick upper cretaceous to lower paleocene sequences purilactis group infilling the salar de atacama basin reflects rapid local subsidence to the east of the cd its oldest outcropping unit tonel formation comprises more than 1000 m of continental red sandstones and evaporites which began to accumulate as syntectonic growth strata during the initial stages of cd uplift tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance representing the sedimentary response to renewed pulses of tectonic shortening which were deposited in alluvial fan fluvial and eolian settings together with minor lacustrine mudstone purilactis formation these are covered by 500 m of coarse proximal alluvial fan conglomerates barros arana formation the top of the purilactis group consists of maastrichtian danian alkaline lava and minor welded tuffs and red beds cerro totola formation 70 64 ma k ar deposited during an interval of tectonic quiescence when the el molino yacoraite late cretaceous sea covered large tracts of the nearby altiplano puna domain limestones interbedded with the totola volcanics indicate that this marine incursion advanced westwards to reach the eastern cd slope cd shortening in the late cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north chilean magmatic arc to the west of the cd domain indicating that oblique plate convergence prevailed during the late cretaceous oblique convergence seems to have been resolved into a highly partitioned strain system where margin parallel displacements along the thermally weakened arc coexisted with margin orthogonal shortening associated with syntectonic sedimentation in the salar de atacama basin a regionally important early paleocene compressional event is echoed in the salar de atacama basin by a distinctive angular unconformity which separates paleocene continental sediments from purilactis group strata the basin also records the eocene early oligocene incaic transpressional episode which produced renewed uplift in the cordillera de domeyko and triggered the accumulation of a thick blanket of syntectonic gravels loma amarilla formation

implication of dating of early tertiary volcanic rocks from the north chilean precordillera abstract since the early jurassic subduction induced volcanic activity in the north chilean andes 21 23 5 s has migrated 200 300 km eastward from the present coast jurassic magmatic arc to the western cordillera miocene recent volcanic rocks due to the paucity of chronological data however the exact age and extent of various magmatic sub belts within this area are not well constrained in the precordillera a sequence of 1500 5000 m thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc alkaline tuffs and lavas presently the age of this sequence is considered to be upper jurassic and or cretaceous from this sequence eleven biotites and seven hornblendes were analyzed and yielded 40 ar 39 ar plateau ages of between 48 and 38 ma indicating upper eocene volcanic activity in the eastern part of the precordillera in contrast volcanic activity in the western part of the precordillera has been dated at 92 2 5 ma indicating an earlier onset of volcanism and therefore sedimentation of the red beds in the late eocene the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the nazca plate and the south american continent this final phase of eocene volcanism coincided with a period of important deformation the incaic phase biotites from two ash fall tuffs separated by a distinct angular unconformity yield identical ages of 38 50 0 90 ma and 38 45 0 60 ma these dates provide the best age estimate of the incaic phase in the north chilean precordillera

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

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

tertiary provenance history of the northern and central altiplano central andes bolivia a detrital record of plateau margin tectonics abstract the internally drained altiplano plateau in the high elevation hinterland of the central andes contains an extremely thick up to 12 km succession of tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins in this study temporal and spatial variations in detrital composition based on 113 sandstone thin sections and 31 conglomerate clast counts combined with sediment dispersal patterns and regional stratigraphic correlations help delineate the tertiary history of altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins stratigraphic and provenance data suggest limited subsidence and west directed sediment dispersal during the early tertiary history of the altiplano region by late eocene time development of a rapidly subsiding basin probable foredeep was underway as recorded by extensive east directed fluvial systems of the 3 6 5 km thick potoco formation potoco compositional data are compatible with sediment sources in the incipient western cordillera and regions farther west including 1 a probable thrust belt source terrane composed of paleozoic mesozoic strata and precambrian granitic gneissic basement and 2 a subordinate magmatic arc source terrane composed of tertiary volcanic rocks increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence due to either enhanced volcanism or eastward arc migration by late oligocene time west directed fluvial and alluvial fan systems of the upper potoco formation and the 3 km thick coniri formation reached the altiplano basin providing a minimum uplift age for the backthrust belt along the western flank of the eastern cordillera paleozoic clast conglomerate and relatively quartz rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly paleozoic strata in the backthrust belt continued late oligocene to quaternary accumulation in the altiplano reveals that the basin persisted as a depositional region during a well documented period of neogene shortening and uplift in the andean thrust belt farther east this intra orogenic hinterland deposition between eastern and western highs during ongoing shortening suggests that the altiplano basin existed as a crustal scale piggyback basin possibly in an internally drained plateau like geomorphic setting since 25 ma provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the altiplano although flexural subsidence due to thrust loading along both plateau margins initially in the west and subsequently in the east may explain most tertiary subsidence possible closed basin conditions due to structural damming could have promoted additional sediment accumulation from late oligocene to quaternary time

sidewall ripouts in strike slip faults abstract distinctive splay fault configurations in wallrocks associated with pseudotachylyte bearing strike slip fault structures at three different localities are recognized as sidewall ripouts sidewall ripout structures occur as asymmetric doubly tapered fault bounded lenses or slabs adjacent to a dominant planar fault surface scallop shaped lens ripouts develop in isotropic or weakly anisotropic host rocks and elongate slab ripouts develop in strongly anisotropic host rocks the tapered ends of these structures consist of leading contractional p type ramp structures at a low angle to the dominant planar fault surface and trailing extensional r or x type ramp structures at higher angles to the dominant fault surface the formation of ripout structures is attributed to transient variations in the coefficient of friction that cause adhesion between fault surfaces leading to rupture of the sidewalls ripouts are interpreted as large outcrop scale examples of adhesive wear during rupture of the sidewalls developed along planar fault surfaces during slip the asymmetry of the ripout structures in these exposures allows their use as a kinematic indicator where the narrow tapered leading end points in the direction of movement of the ripout lens or slab during adhesion to the opposing fault block wall

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

sea breeze circulation over jakarta indonesia a climatology based on boundary layer radar observations abstract characteristics of sea breeze circulation over the tropical site of jakarta indonesia have been documented based on analyses of satellite images and data from long term l band boundary layer radar measurements carried out at serpong 6 4 s 106 7 e inspection of satellite imagery reveals that a sea breeze front develops well along the northern coastal plain of west java and propagates inland until its structure is deformed over complex topography it is found that the sea breeze signal detected by the boundary layer radar is most well defined during the dry season months of july october in all of these months radar observations indicate a late afternoon intensification of sea breeze flow in the 0 5 0 8 km height range between 1700 and 1800 lt which is not elucidated upon by surface measurements the effect of weather conditions on the sea breeze pattern is investigated by using a cloudiness index derived from data of incoming solar radiation the results show that sea breeze intrusion over

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

revisiting the north chile seismic gap segmentation using gps derived interseismic coupling no major earthquake occurred in north chile since the 1877 m w 8 6 subduction earthquake that produced a huge tsunami however geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming which reveals some degree of locking on the subduction interface this accumulation of elastic deformation is likely to be released in a future earthquake because of the long elapsed time since 1877 and the rapid accumulation of deformation thought to be 6 7 cm yr many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high we present a new global positioning system gps velocity field acquired between 2008 and 2012 that describes in some detail the interseismic deformation between 18 s and 24 s we invert for coupling distribution on the nazca south america subduction interface using elastic modelling our measurements require that at these latitudes 10 to 12 mm yr 1 i e 15 per cent of the whole convergence rate are accommodated by the clockwise rotation of an andean block bounded to the east by the subandean fold and thrust belt this reduces the accumulation rate on the subduction interface to 56 mm yr 1 in this area coupling variations on the subduction interface both along strike and along dip are described we find that the north chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling this coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region intersegment zones iquique mejillones correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates the rupture of either the paranal or the loa segment alone could easily produce a mw 8 0 8 3 rupture and we propose that the loa segment from 22 5 s to 20 8 s may be the one that ruptured in 1877

jurassic and cretaceous apparent polar wander relative to south america some tectonic implications late mesozoic apparent polar wander apw with respect to stable south america has been difficult to define it appears that this observation has a simple explanation very little apw occurred and none at all during a 50 m y period 165 to 115 ma it is possible to compute a stillstand pole for this interval 88 8 s 72 4 e k 244 7 a95 4 9 from five reliable studies comprising a total of 523 sites a second stillstand of much shorter duration 70 80 ma is defined by four poles 52 sites this is located at 80 8 s 346 7 e k 457 7 a95 4 3 apw of 9 2 occurred during the interval 115 80 ma a comparable stillstand is found in the north american record e g van fossen and kent 1992 this has a similar degree of scatter k 421 and spans at least 36 m y 124 to 88 ma thus north america and south america both were rotating due westward during a large part of the cretaceous this has some important implications for global tectonics

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

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

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

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

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

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

paleomagnetic tracking of mountain building in the peruvian andes since 10 ma 1 we report paleomagnetic data from 73 sites 628 samples of upper oligocene to pliocene rocks from central and northern peru the data indicate that the subandean zone has not experienced vertical axis rotation since the upper oligocene whereas the coast and the western cordillera record a coherent pattern of counterclockwise rotations emplaced in the last 10 ma this pattern can best be explained by two competing hypotheses 1 a propagation in rotations from the bolivian orocline toward the north or 2 a more punctual and widespread event linked to subduction of the nazca ridge that caused the rotations on the basis of the time space relationship of paleomagnetic rotations deformation magmatism and nazca south america plate convergence the latter appears more likely moreover because the rotations north of the abancay deflection 15 5 s are synchronous with deformation and exhumation they indicate that major mountain building in the peruvian andes has occurred since 10 ma finally we suggest that a succession of aseismic ridge subduction events played an important role in shaping the modern topography of the andean chain

andean oblique folds in the cordillera oriental northwestern argentina insights from analogue models abstract the dominant cenozoic structural grain in northwestern argentina trends north south however several oblique folds have been mapped mainly in the cordillera oriental and the sistema de santa barbara two oblique ne sw trending anticlines are well exposed in the luracatao valley salta along the western border of the cordillera oriental these oblique anticlines are defined by the palaeogene santa barbara subgroup and are cut by the reverse faults bounding the luracatao valley we constructed analogue models to simulate the generation of oblique ne sw anticlines inside the narrow trough like luracatao valley orthogonal compression and basement oblique heterogeneities oblique compression and basement oblique heterogeneities and oblique compression configurations were tested in the models the first two configurations simulate the anticline formation well in addition we analyse dextral rotation and the influence of neoproterozoic basement cretaceous rift structures as the cause of the oblique orientation the analogue models suggest pre andean oblique structures controlling the oblique orientation of these andean folds despite being a second order planes of weakness

tectonic rotations and internal structure of eocene plutons in chuquicamata northern chile abstract a paleomagnetic and ams study on eocene plutonic complexes in the calama area northern chile reveals high temperature high coercivity magnetizations of dominantly thermoremanent origin and magnetic fabrics controlled by magnetite the paleomagnetic results indicate that 43 ma plutons underwent clockwise tectonic rotation whereas adjacent 39 ma plutons did not undergo discernible rotation this points to a middle eocene age for the younger tectonic rotations associated with the central andean rotation pattern in the chuquicamata calama area the petrofabric in these rocks formed under conditions ranging from purely magmatic i e before full crystallization to low temperature solid state deformation ams and paleomagnetism suggest that the plutonic bodies were formed by progressive amalgamation of subvertical magma sheets spanning multiple magnetic polarity chrons the parallelism between magmatic and tectonic foliations suggests that regional tectonic stress controlled ascent emplacement and rock deformation during cooling in this context we suggest that magma ascent and emplacement in the upper crust likely exploited mesozoic structures which were locally reactivated in the eocene

reinterpretation of the ordovician rotations in nw argentina and northern chile a consequence of the precordillera collision early paleozoic paleomagnetic data from nw argentina and northern chile have shown large systematic rotations within two domains one composed of the western puna that yields very large up to 80 counter clockwise rotations and the other formed by the famatina ranges and the eastern puna that shows 40 clockwise rotations around vertical axes in several locations lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the paleozoic previous tectonic models have explained these rotations as indicative of rigid body rotations of large para autochthonous crustal blocks or terranes a different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of precordillera in the late ordovician this collision should have generated dextral shear zones in the back arc region of the convergent sw gondwana margin where systematic domino like clockwise rotations of small crustal blocks accommodate crustal shortening the western puna block bordering the precordillera terrane to the north might have rotated counterclockwise as an independent microplate due to tectonic escape processes in a fashion similar to the present day relationship between the anatolia block and the arabian microplate

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

fold and thrust belt structures and strike slip faulting at the se margin of the salar de atacama basin chilean andes 1 a tectonic reinterpretation is reported for the southeastern margin of the salar de atacama basin of northern chile detailed structural mapping revealed the presence of an east vergent thin skinned fold and thrust belt affecting oligocene miocene paciencia group rocks and the overlying plio pleistocene volcanic rocks along strike segmentation of the main fold implies local foreland influence on footwall ramp geometry leading to local thrust sheet rotation to the east the adjacent western slope of the western cordillera displays two different structural domains probably controlled by preexisting basement structures the southern domain comprises two n s oriented sigmoidal belts of linear arranged pressure ridges indicating left lateral transpression in contrast the northern domain is characterized by east vergent fold and thrust belt structures and reactivated nw se striking sinistral strike slip faults governing clockwise block rotations an indenter driven deformation model is proposed to explain sinistral transpression and clockwise block rotations around vertical axes this variant of a small block rotation mechanism is discussed in the context of oroclinal bending of the central andes emphasizing the significance of ancient structures in controlling rotations

on the nature of buttressing in margin parallel strike slip fault systems margin parallel strike slip faults commonly develop along convergent plate margins where an oceanic plate subducts obliquely beneath a continental plate such faults detach a sliver of continental crust from the leading edge of the overriding plate fault motion then causes the sliver to move parallel to the continental margin in the direction of the transverse component of relative plate motion however few slivers move as rapidly as predicted by simple analyses of the forces acting on them this discrepancy must be due principally to the buttress effect resistance to displacement of the sliver arising from a space problem at its leading edge more simply a sliver can move only if it has somewhere to go buttresses can be geometrical as when the continental margin changes trend abruptly or they can arise from changes in the physical properties of the overriding plate or in other factors such as subduction angle and angle of obliquity that help determine whether sliver motion will occur a strongly buttressed sliver will move only insofar as the buttress can be overcome obvious ways in which a buttress can be overcome are by thickening the crust or widening the sliver examples of both are known crustal thickening entails elevation increase sliver motion is opposed by both gravity and friction widening the sliver is a more complex process that results in areas of extension and compression forming within the sliver as well as a characteristic pattern of overlapping curved fault segments this latter response may be the cause of curvature in many strike slip fault systems

geomorphological markers of faulting and neotectonic activity along the western andean margin northern chile in the atacama desert northern chile some ephemeral channels are developed in the plio quaternary alluvial sequence that caps the neogene atacama gravels formation geomorpho logical studies and high resolution digital elevation data gps along a structural transect in the cen tral depression are used to document modern growth history of subtle folding and faulting in the fore arc region outcrop data of the most recent deposits are combined with observations of warped and faulted late quaternary pediments alluvial fans and terrace surfaces to propose unsuspected neotec tonic processes on the western flank of the domeyko cordillera neotectonic process recognition is here based largely upon the interpretation of alluvial landforms drainage organisation and evolution as the intermittent river network shows systematic patterns of course deflections successive incisions or deposition processes as it encounters the fault scarps or folds in the superficial deposits this area presents both n s trending active vertical faults in the topographically higher pampas and n s trending active folding in the lower pampas these faults seem to accommodate e w extension and compression that could be related to uplift of the western andean margin within a compressive context uplift may have taken place unevenly over the past few million years after the deposition of the superficial alluvial surfaces that cap the neogene atacama gravels copyright 2003 john wiley sons ltd

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

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

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

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

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

structural evidence of orogen parallel strike slip displacements in the precordillera of northern chile the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile is made up of several elongate basement ridges following the trend of the andes these ridges which morphologically rise above the mesozoic tertiary cover rocks are developed as anticlinal cores or as pressure ridges bounded by reverse faults and thus show considerable orogennormal shortening several nearly vertical faults cut through the precordillera parallel or at a very low angle to the mountain ranges from the following structures it can be inferred that orogen parallel transcurrent movements took place along the faults a asymmetric en echelon fault arrays b stratigraphic and structural discontinuities across major faults c fabrics in fault rocks and d vertical folds a dextral displacement of the order of tens of km is probable the orogen parallel strikeslip movements as well as the orogen normal shortening are considered as phenomena of magmatic arc tectonics due to the focussing of central andean igneous activity on the precordillera from the late cretaceous until the paroxysm of deformation 45 30 ma deformation along the precordilleran fault system is related with the development of the large porphyry copper ore deposits of that area the structural evolution of the precordillera can be explained by oblique subduction resulting in dextral transpression

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

cenozoic evolution of the northwestern salar de atacama basin northern chile 1 since 90 ma the nonmarine salar de atacama basin has been the largest deepest and most persistent sedimentary basin of northern chile integration of 200 km of two dimensional seismic reflection data with surface geological data clarifies oligocene and neogene evolution of the northern part of the basin a normal fault with 6 1 km of vertical separation controlled the western boundary of the basin during the accumulation of the oligocene lower miocene paciencia group the combination of this structure a similar one in the calama basin and regional structural data suggests that localized extension played an important role within a tectonic environment dominated by margin perpendicular compression and margin parallel strike slip deformation seismic data substantiate the surface interpretation that much of the cordillera de la sal ridge resulted from diapiric flow of the paciencia group diapiric flow initiated during the late early miocene or middle miocene associated with a deep reverse fault