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

exhumation history and timing of supergene copper mineralisation in an arid climate new thermochronological data from the centinela district atacama chile

evolutionary cycles during the andean orogeny repeated slab breakoff and flat subduction tectonomagmatic similarities between the modern chilean flat slab region and pre neogene magmatic episodes suggest that they represent analogues to flat subduction evolutionary patterns in each magmatic suite include i increasing la yb ratios and sr and nd isotopic enrichment through time ii eastward migration of magmatism after periods of transpressional transtensional intra arc deformation and iii subsequent termination and virtual absence of main arc activity for 5 10 myr these patterns may reflect slab shallowing followed by flat subduction and thickening of the overlying crust if repeated they require interchanging episodes of slab steepening increasing convergence rates force slab kinking and eventual failure of the oversteepened slab followed by rebound of the slab tip owing to lack of further slab pull flat subduction and termination of subduction related magmatism rapid subduction leads to shallow overriding of the detached slab fragment eclogitization of the gradually steepening slab tip at depth and subsequent slab pull permits asthenospheric corner flow and subduction related magmatism

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

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

40ar 39ar and rbsr analyses from ductile shear zones from the atacama fault zone northern chile the age of deformation abstract the influence of deformation on the k ar and the rb sr isotope system is investigated it is assumed that due to the diffusion processes involved deformation has a similar effect on isotopic equilibrium as has temperature in order to examine the influence of deformation on the k ar and the rb sr isotope systems two shear zones from the atacama fault zone afz situated in the north chilean coastal cordillera have been investigated the afz which was active as a sinistral strike slip fault during the mesozoic has two sets of shear zones one formed under amphibolite sz1 one under greenschist facies conditions sz2 rb sr and 40ar 39ar age determinations were conducted on samples from cross sections of each set in sz1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153 152 and 150 1 ma respectively biotite from the center of the shear zone of sz1 gave an isochron of 143 9 0 3 ma mswd 0 04 which is interpreted as the age of deformation which produced resetting of the mineral system in sz2 hornblendes yielded 40ar 39ar plateau cooling ages of 138 ma biotites from undeformed samples gave rb sr and 40ar 39ar total degassing ages of 130 1 ma whereas biotite from the mylonitic rocks yielded 126 125 ma which dates the time of deformation sr isotope homogenization occurred in the mylonitic rocks and is most likely a result of deformation the formation of sz1 can be correlated to the araucanian nevadan phase the deformation in sz2 is related to the onset of uplift and cooling of the coastal cordilleran magmatic arc

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

geometry and structural evolution of the central andean backthrust belt bolivia the central andean backthrust belt is a large scale west vergent thrust system along the western side of the eastern cordillera in the generally east vergent andean fold thrust belt of bolivia although west vergent structures in the central andes have been recognized previously we describe the backthrust belt at a regional scale emphasizing its implications for the kinematic development of the andes and the subsequent influence of these kinematics on amounts of tectonic shortening we use techniques such as line length balancing restorability and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and altiplano the cross sections are taken to a regional depth of detachment basement to examine the relationship between mapped surface structures and inferred subsurface structures the relationship of the backthrust belt to the altiplano suggests that the altiplano basin is a crustal scale piggyback basin created as a basement megathrust propagated up and over a half crustal scale ramp located just west of the physiographic boundary of the eastern cordillera this basement megathrust was the means by which a narrow paleocene fold thrust belt located to the west of the altiplano propagated eastward and emerged in the present eastern cordillera the relationship between the basement thrusts and the physiographic boundaries of the central andean plateau as defined by isacks 1988 suggests that extensive megathrust sheets involving crystalline basement or quartzite may play an important role in the formation of orogenic plateaus the kinematic development of the andean fold thrust belt indicates that the backthrust belt developed as a taper building mechanism after the basement megathrust overextended the system eastward the mechanism proposed in this study for the development of the central andean backthrust belt requires a minimum of 200 km of shortening within the altiplano eastern cordillera alone this increases minimum shortening estimates across the fold thrust belt in bolivia to as much as 300 340 km

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

internal structure of the precordilleran fault system chile insights from structural and geophysical observations abstract geological field work and audiomagnetotelluric data from two profiles crossing the west fissure zone in northern chile were used to describe the geometry and structure of the fracture damaged zone surrounding this strike slip fault system the w e profiles show that the width of the west fissure zone based on the fracture density distribution derived from aerial photographs is 4000 m on profile a and 7000 m on profile b the estimated widths correspond roughly to the region where the fault is kinematically uniform the ratio of the fault width to the fault length ranges from 0 024 to 0 041 this ratio compares favourably with the measured ratio of small metre scale natural faults audio frequency magnetotelluric imaging shows low electrical resistivity zones resistivity of 5 30 m coincident with the mapped surface traces of the fault however these zones are very narrow width about 100 m and only 50 m profile a and 200 m profile b deep respectively on profile b the shallow high conductive zone is underlain by a resistive zone 1000 m the conductivity enhancement in these shallow and narrow zones contrasts with the broad process zone revealed by structural analysis we assume that the conductivity enhancement is due to meteoric water entering a zone of ruptured rocks along the fault trace fault core at present we have no indication for seismic slip along the investigated segments of the west fissure zone fault models have shown that during aseismic periods of fault evolution fault healing i e strength recovery due to compaction and cementation is active fluids and fluid transport which are probably responsible for enhanced conductivity are confined to the remaining fractures of the fault core at shallow depth

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

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

eocene exhumation and basin development in the puna of northwestern argentina 1 the puna is part of the larger puna altiplano plateau also known as the central andean plateau characterized by high elevation low relief and aridity located in the central andes of bolivia and argentina tertiary sedimentary rocks preserved within the puna contain a unique archive of information regarding the paleogeography depositional environments and timing of sediment source exhumation during the early stages of andean mountain building the eocene geste formation in the salar de pastos grandes area within the central puna of northwestern argentina consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly braided fluvial system and alluvial fans proximal to the source terrane paleocurrent data document an overall eastward flow direction up section coarsening of the geste formation suggests that topographic relief in the source area increased through time possibly owing to enhanced tectonic activity and source terrane unroofing sandstone petrography and conglomerate clast count data document quartzose and phyllitic compositions typical of ordovician rocks preserved just west of the salar de pastos grandes area paleocene eocene detrital apatite fission track age populations p1 35 52 ma p2 52 65 ma of the geste formation and their consistent trends up section suggest moderate to rapid 0 4 mm a to 1 mm a exhumation of western sediment sources during the early to mid tertiary stages of andean mountain building sedimentation rates increase up section from 0 1 mm a to 1 mm a our data when combined with other structural stratigraphic and seismic evidence from surrounding regions suggest that the geste formation was deposited in response to crustal shortening and resulting erosion and sedimentation which started as early as cretaceous in the chilean cordillera de domeyko and in the salar de pastos grandes area by eocene time the geste formation could be interpreted either as a local wedge top accumulation on the eastward propagating central andean orogenic wedge or as a local intermontane basin the similarities between wedge top deposits preserved in bolivia and eocene deposits in northwestern argentina south of 25 s lead us to favor the wedge top scenario for the geste formation if correct this implies that the deformation front of the andean orogenic wedge incorporated both thin and thick skinned structures as it migrated possibly unsteadily from the cordillera de domeyko during the cretaceous paleocene to areas within the puna and eastern cordillera by mid late eocene time contemporaneously a regional scale foreland basin system developed over an along strike distance of at least 650 km

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

tectonic interaction between mesozoic to cenozoic extensional and contractional structures in the preandean depression 23 25 s geologic implications for the central andes abstract a multidisciplinary study supported by field data and 2 d seismic information was conducted to understand the tectonic interactions between mesozoic to cenozoic extensional and contractional structures along the preandean depression 23 25 s in northern chile which have been largely debated this work gives some new ideas about the control that ancient basement extensional faults exert on the crustal shortening and growth of those orogenic belts located over normal subduction zones similar to the central andes in this work two sedimentary basins were analyzed the salar de atacama and the salar de punta negra both of which correspond to intra montane basins bounded by the domeyko cordillera and the current volcanic arc of the central andes the interpretation of a series of west to east oriented 2 d seismic profiles revealed three types of structural interaction i pure positive reactivation ii decapitation of previous normal faults by reverse faults and iii transport of previous normal faults by thrust ramps inversion anticlines and basement involved contractional structures were the most common observed structures indicating that the geometry and distribution of inherited normal faults of the paleozoic and mesozoic basement played an important role in basin contraction the different modes of tectonic interaction frequently created variations on the geometry and kinematic of the final structural styles present in the region commonly the upper cretaceous to cenozoic synorogenic tectonosequences recorded the ages of contractional deformation that affected the region similar to other regions of northern chile basin inversion occurred during the late cretaceous and subsequent propagation of reverse faults occurred from the upper late cretaceous to the present the basement involved reverse fault structures usually create the most important positive topographic relief in this part of the western central andes using as example our study area we finally conclude that the original position and distribution of ancient basement pre orogenic normal faults in the orogenic belts condition the final vergence and kinematic of the subsequent thrust systems related to shortening episodes

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

response of supergene processes to episodic cenozoic uplift pediment erosion and ignimbrite eruption in the porphyry copper province of southern peru the jurassic to middle eocene porphyry copper deposits and prospects exposed on the pacific slopes of the central andean cordillera occidental of southern peru between latitudes 16 309 and 18 s record a protracted ca 30 m y history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment itself a response to successive regional tectonic events including the late eocene incaic the late oligocene to earliest miocene aymara and the middle to late miocene quechuan events weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern peru at 25 5 ma following a 27 m y amagmatic interval and supergene processes were variously interrupted or terminated by ignimbrite blanketing although in several locations supergene profiles were preserved by such cover the landform chronology for the area surrounding the cuajone quellaveco and toquepala deposits ca 17 s is revised and extended northwestward through field mapping to the cerro verde santa rosa district ca 16 309 s the 40 ar 39 ar incremental heating dates of supergene alunite group minerals from the angostura 38 1 and 38 8 ma and posco 38 8 ma prospects and the cerro verde deposit 36 1 38 8 ma demonstrate that supergene processes were underway in the late eocene beneath a subplanar topography resulting from uplift and erosion during the incaic orogeny now represented by a regional unconformity in the cenozoic volcanic sedimentary rock succession broadly contemporaneous supergene processes were probably active in the cuajone quellaveco toquepala district slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the incaic supergene profiles aymara uplift subsequently led to the incision of the 23 8 to 24 ma altos de camilaca and the 18 8 to 19 1 ma pampa lagunas pediplains and their regional correlatives the ensuing water table lowering was associated with intense leaching and sulfide enrichment from the late oligocene 24 4 28 ma natroalunite at cerro verde 26 27 ma natroalunite at santa rosa and 28 6 ma jarosite at la llave to the early miocene 23 ma alunite and 21 ma natroalunite at cerro verde and 19 2 ma jarosite at la llave and was plausibly responsible for much of the upgrading of the cuajone and toquepala deposits and thr quellaveco prospect which are intersected by both the altos de camilaca pediplain and erosional features representing upslope extensions of the pampa lagunas pediplain the younger supergene profiles were widely superimposed on the remnants of those generated during the incaic orogeny middle miocene 14 2 ma biotite age chuntacala formation flows protected the cuajone supergene profile from destruction by erosion but at 13 0 ma interrupted supergene processes at quellaveco revision of volcano stratigraphic relationships in the latter area reveals that subsequent erosion of the chuntacala formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10 1 ma ash flow tuff of the asana formation elsewhere supergene activity persisted at the cachuyito prospect through 11 4 ma and minor jarosite development occurred at least until 4 9 ma both there and at cerro verde during and following the multiple pediment landform stage ca 7 9 15 0 ma the occurrence of relics of late eocene alunite group minerals within considerably younger late oligocene to late miocene supergene alteration profiles suggests that the overall physiographic configuration of the pacific piedmont of southern peru remained remarkably consistent from the late eocene to the middle miocene moreover the new age data confirm that as in northern chile semiarid climatic conditions prevailed along much of the plate boundary from the mid eocene until the late miocene or early pliocene onset of hyperaridity the local geomorphologic and volcanic conditions in southern peru however conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late eocene to early oligocene porphyry copper belt of northern chile which underwent supergene upgrading over relatively brief periods

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

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

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

on postmidnight low latitude ionospheric irregularities during solar minimum 1 equatorial atmosphere radar and gps tec observations in indonesia 1 using the 47 mhz equatorial atmosphere radar ear in west sumatra indonesia 10 36 s dip latitude it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere mu radar in midlatitudes 29 3 n dip latitude utilizing the rapid beam steering capability of the ear the spatial structure of the postmidnight irregularities is clearly presented for the first time it is found that they usually propagate westward and can be categorized into two types one shows sharp upwelling plumes near local midnight which should not be a mere passage of fossil plasma bubbles the other has successive tilted structures which have the same orientation as medium scale traveling ionospheric disturbances typically observed at midlatitudes we suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector the displacement between geographic and magnetic equators may also be important for seasonal longitudinal variation of the postmidnight irregularities

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

aeromagnetic signature of porphyry copper systems in northern chile and its geologic implications the compilation of a large aeromagnetic high resolution dataset acquired for base metals exploration in northern chile between lat 19 45 s and 27 15 s revealed a pattern of transorogenic regional magnetic anomalies these anomalies occur as generally east west stripes of negative residual magnetic intensity of more than 100 nanotesla nt they have an east west extension of some tens of kilometers and a north south width of some 5 to 10 km some of them can be observed continuously from the coast to the volcanic belt their residual intensity is typically more than 100 nt they also show a loose north south spacing it is observed that all known porphyry copper deposits of the investigated segment are spatially related to these transverse magnetic anomalies we propose that the transverse magnetic anomalies are the magnetic response to the loci of emplacement of intrusive bodies of batholithic size along paths of the advancing magmatic front of the active continental margin the occurrence of several important porphyry copper deposits grouped in clusters within a district is explained as being related to a common parental intrusive complex which is geophysically signaled by a corresponding transverse magnetic anomaly a possible implication of the observed relationship between porphyry copper deposits and transverse magnetic anomalies is the quasideterministic restriction of porphyry copper deposits to certain orogen transverse loci necessary condition and the corresponding consequences for future exploration strategies

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

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

geometry and late mesozoic cenozoic evolution of the salar de atacama basin 22 30 24 30 s in the northern central andes new constraints from geophysical geochronological and field data abstract the late mesozoic cenozoic evolution of the central andes of northern chile consists of a series of events related to changing conditions along the plate margin leading to different episodes of compression and extension a long lasting basin which records these changes corresponds to the salar de atacama basin its tectonic history has been blurred by several interpretations of seismic reflection lines and multiple chronostratigraphic correlations between subsurface surface units as such many aspects of its tectonic evolution are still unclear in this study we performed an e w gravimetric profile across the barros arana syncline and the basin we analyzed interval velocities obtained from seismic reflection studies and reviewed the profiles closest to the gravity survey four u pb samples for detrital zircon analysis were obtained from late cretaceous paleogene units geophysical data show a segmentation of physical properties within the basin with denser units and faster interval velocities around the late mesozoic early cenozoic units and lighter slower units in the actual salt pan the main contrast in physical properties is observed in the evaporitic and fine grained continental deposits of the san pedro formation geophysical and geological constraints show that its thickness varies between 800 and 1200 m and that it was deformed during later events the u pb data show that the rocks within the barros arana syncline reach well into the paleogene and that they were mainly deformed during the late eocene incaic event where an east verging thrust system also involving permo triassic units was developed establishing the actual structural configuration the late oligocene early miocene units were then deposited in a post orogenic setting with minor compression a stronger phase around the late miocene reactivated previous structures and folded these deposits this continuous compressive history of the basin which is mostly a result of the incaic event is in agreement with the regional evolution of the central andes

development of a continental forearc a cenozoic example from the central andes northern chile in order to understand the response of a continental forearc to changes in subduction zone geodynamics we constructed a high resolution chronostratigraphic cross section across the central andean forearc of northern chile 21 24 s the tectono stratigraphic development of the forearc differs from established models no relationship was found between changes in rate of relative plate convergence and amount and style of deformation forearc response to continual compression since the oligocene has been uplift and segmentation into discrete tectono stratigraphic zones from west to east these zones are the extensional coastal cordillera the extensional and or transtensional central depression and the transpressional and or compressional precordillera preandean depression each area has recorded almost continuous sedimentation from oligocene eocene time to the present day accommodation space has been generated by basin margin uplift rather than active subsidence we propose a model in which uplift of the leading edge of the south american plate is driven by subcrustal accretion of material removed at the trench by subduction erosion uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge the tectono stratigraphic complexity exhibited within the central andean forearc is likely to be representative of cordilleran type margins and would be difficult to recognize in an ancient continental forearc

cenozoic stratigraphic development in the north chilean forearc implications for basin development and uplift history of the central andean margin abstract analysis of the cenozoic stratigraphic development of the forearc of northern chile between 18 s and 23 30 s allows constraints to be placed on the timing and nature of basin formation and the uplift history of the central andes chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc sections were taken from the longitudinal valley central depression calama basin salar de atacama precordillera and the western flank of the western cordillera correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections three chronostratigraphic units are defined based upon the presence of regional unconformities deposition of the late eocene to early miocene chronostratigraphic unit 38 19 ma commenced across an irregular unconformity surface between 38 and 30 ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites aggradation after 25 ma resulted in development of a large broad basin over much of northern chile that expanded eastwards through onlap onto basement deposition terminated around 19 ma with the development of an angular unconformity over much but not all of the study area during deposition of the early to late miocene chronostratigraphic unit 18 10 ma emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins fold growth affected sedimentation restricting accommodation space to small intra thrust basins in the precordillera and localised disruption and unconformity development in the longitudinal valley the late miocene to present day chronostratigraphic unit 10 0 ma followed the development of a regional angular unconformity at 10 ma sedimentation was restricted to a series of thrust bounded endorheic basins in both the central depression and the precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets alluvial fan fluvial and lacustrine sedimentation dominated within the endorheic basins from 8 to 3 ma after development of a regional unconformity at 3 ma a change to isolated evaporite sub basins took place in the central depression with small lacustrine basins developed along the flank of the western cordillera the scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the western cordillera by 30 ma and that this has persisted to the present day this area also shed material eastwards into the altiplano the presence of such a topographic feature by 30 ma suggests that a significant proportion of andean uplift had occurred prior to the late miocene this important uplift phase should be incorporated into any model of andean uplift the evidence from the basin fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 ma and that a transition to a thrust bounded foreland style basin took place after the development of the unconformity at 19 ma

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

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

nazca south america interactions and the late eocene late oligocene flat slab episode in the central andes 1 the most prominent features of the andean range are the altiplano and puna plateaus which were constructed by crustal shortening and uplift over the past 45 myr the early construction of these plateaus may have controlled subsequent growth of the orogen proposed models have suggested that an abrupt acceleration in relative motion between the nazca plate and the south american plate at 30 ma may have led to compression of the continent however the major plate motion change occurred at 25 23 ma and paleomagnetic rotations and crustal shortening of the andean forearc require that the arica bend formed prior to about 25 ma inferred history of flat slab subduction along the altiplano section of the andean margin and the structure of the adjacent south american cratonic shield combine to suggest an alternate scenario based partly upon geodynamic models of oceanic continental plate interactions in subduction zones we propose that central andean tectonism may have been controlled by two distinct regimes of subduction 1 oblique subduction along the central andean margin during the late eocene and oligocene accompanied by downdip alignment with the center of the amazonian shield flat slab activity in this phase of orogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction and 2 an abrupt transition to trench normal subduction after 25 ma toward the more distal sao francisco craton was accompanied by a return to normal angle subduction similar interactions are hypothesized to have occurred during the laramide orogeny in western north america

ionospheric irregularities in the low latitude valley region observed with the equatorial atmosphere radar 1 the geomagnetically low latitude valley region between the upper e region and the lower f1 region is studied with the equatorial atmosphere radar ear in indonesia three meter scale field aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread f esf in the period from sunset to midnight the valley region echoes usually appear at above 150 km and propagate downward with time rapid beam scanning of the ear revealed that spatial structure temporal variation and drift velocity of the valley region echoes resemble those of esf which indicates that the dynamoelectric field in the equatorial f region controls the low latitude valley region irregularities perturbed electric fields associated with esf map down to the low latitude valley region and can produce the perturbed plasma density structures as images of esf structures image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the ear it is suggested that intermediate layers should supply a plasma density gradient for excitation of 3 m scale irregularities in the valley region through gradient drift instability

zongo san gaban zone eocene foreland boundary of the central andean orogen northwest bolivia and southeast peru discordant muscovite and biotite k ar dates of samples taken in six transects through the cordillera oriental of southeast peru and northwest bolivia combined with sup 40 ar sup 39 ar age spectra reveal a northwest trending structurally cryptic tectonothermal zone of late eocene age ca 38 ma that overprints triassic and older igneous and metamorphic rocks the zone is at least 450 km long but is less than 25 km wide temperatures along its northeast boundary are estimated to have reached 350 sup 0 c no paleogene magmatism has been recognized that could account for the thermal resetting of the k ar isotopic system uplift erosion and southwest verging thrusting accompanied the development of this domain which defined the foreland boundary of the orogen prior to inception of northeast verging andean thrusting in miocene time it is proposed that this tectonothermal zone the zongo san gaban zone zsgz which involves thrusting and basement shortening is the southerly manifestation of the incaic compressional deformation which produced the thin skinned maranon thrust and fold belt mtfb north of the abancay deflection of central peru

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

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