space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

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

marine incursions and the influence of andean tectonics on the miocene depositional history of northwestern amazonia results of a palynostratigraphic study abstract new palynological and sedimentological data permit the age of neogene sediments in northwestern amazonia to be established more precisely and indicate that major environmental changes occurred in the area during this time based on a study of borehole samples the age of the solimoes formation solimoes basin northwestern brazil is determined as miocene and five pollen zones are distinguished which are correlated with existing zonations for northern south america in addition 30 new sporomorphs are described which belong to the following 12 genera psilamonocolpites retimonocolpites retitricolpites retibrevitricolpites psilatriporites nov gen bombacacidites psilatricolporites retitricolporites rugutricolporites psilastephanoporites nov gen psilastephanocolporoties and heterocolpites a correlation is made between the brazilian wells and some of the studied outcrops in colombian and peruvian amazonia based on palynological marker species the presence of coastal elements such as mangroves zonocostites indicates that during the miocene this area was influenced by marginal marine conditions caused by several marine incursions these incursions may be related to global sea level fluctuations miocene marine phases also are known from sites elsewhere in northern south america which at present like northwestern amazonia are entirely ruled by continental conditions sediment composition shows that during the early miocene the guyana shield was the major source area of sediment input in the basins of northwestern amazonia in the interval from early to middle miocene the andes became the major sediment source area the change in provenance is related to the uplift of the eastern cordillera this event caused a major change in palaeoenvironment and palaeogeography in northwestern amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial lacustrine system with an estuarine character

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

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

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

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

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

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

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

miocene volcanism and deformation in the western cordillera and high plateaus of south central peru new radiometric ages on tuffs from south central peru support the postulated flare up of volcanic activity during early miocene time in the region of huancavelica julcani and lircay lower miocene rocks lie on folded strata of pre cenozoic age the absence of units of eocene and early oligocene age indicates that this area remained positive after incaic deformation in eocene time conglomerate beds reflecting erosion attendant on the first pulse of late cenozoic quechuan compressive deformation do not appear in the stratigraphic record until after 21 5 m y b p in one section beds of coarse conglomerate are underlain by tuff dated at 18 3 0 6 m y and overlain by tuff dated at 17 3 0 2 m y at another locality beds of conglomerate conformably overlie tuff dated at 19 6 0 8 m y if we incorporate published data from other areas in central and southern peru it appears that quechuan deformation began in early miocene time between 19 5 and 17 m y ago a firm age for the end of the first pulse of quechuan deformation in this region is provided by an ash flow sheet dated at 12 to 12 5 m y that unconformably overlies strata of eocene to early miocene age

late oligocene early miocene compressional tectosedimentary episode and associated land mammal faunas in the andes of central chile and adjacent argentina 32 37 s abstract a reassessment of the geologic and land mammal fossil evidence used in attribution of a tectosedimentary episode in the andes between 32 and 37 s to the middle eocene incaic tectonic phase of peru indicates that the episode occurred during late oligocene early miocene times 27 20 ma from west to east three structural domains are recognized for this time span in the study area a volcanic arc chile a thin skinned e verging fold thrust belt cordillera principal chile argentina border strip and a foreland basin argentina initiation of thrusting in the cordillera principal fold thrust belt produced the coeval initiation of sedimentation in the foreland basin of adjacent argentina this onset of foreland deposition postdates strata bearing a divisaderan land mammal age fauna i e 35 30 ma and is marked at 36 30 s by the base of the rodados lustrosos conglomerates which are conformably overlain by sedimentary rocks containing a deseadan land mammal age fauna i e 29 21 ma geologic relationships between the thick volcanic abanico coya machali and farellones formations also demonstrate that this tectosedimentary episode practically ended at 20 ma at least in the volcanic arc and was therefore roughly coeval with the major tectonic crisis 27 19 ma known in northwestern andean bolivia some 1500 km to the north this strongly suggests that a long outstanding tectonic upheaval affected at least an extended 12 37 s segment of the andean margin of south america during late oligocene and early miocene times

new temporal constraints on the rotation of the peruvian central andes obtained from paleomagnetism new paleomagnetic data from middle eocene to middle miocene volcanic and plutonic formations of the western cordillera of central peru document a counterclockwise rotation of 11 7 4 this rotation similar to previous results reported for the northern peruvian margin is post middle miocene in age it is thus coeval with the acceleration of the andean uplift and supports isacks model

palaeodunes intercalated in loess strata from the western chinese loess plateau timing and palaeoclimatic implications abstract dune systems represent important palaeoenvironmental archives and therefore provide valuable information on past climate conditions of dryland regions this study reports on several palaeodunes thickness ca 10 20 m intercalated in thick loess accumulations located in the lanzhou area of the western chinese loess plateau the intercalated dune sands are well sorted and have a typical aeolian dune structure with distinct cross bedded sand layers the chronology for these palaeodunes and the related loess sediments was established by quartz optically stimulated luminescence osl dating of 19 samples the dating results indicate that sand dune accumulation occurred at ca 35 25 ka which implies a period of increased aridity during dune formation this interpretation is in contrast to previously retrieved data from lake archives of the nearby deserts e g tengger and badain jaran desert indicating a humid episode during marine isotope stage 3 mis 3 the previously proposed humid mis 3a may not be universal over all of northwest china

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

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

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

paleomagnetic study of an arcuate fold belt developed on a marginal orogen the cajamarca deflection northern peru abstract a paleomagnetic study of over 500 cores from 48 sites sampled in albian to turonian sedimentary formations along the cajamarca deflection in northern peru indicates that these lithologies carry a stable remanent magnetization in the two major segments of the deflection positive fold tests establish that the magnetization predates the folding except for five sites in the western part which appear to be remagnetized by the intrusion of paleocene granites the eastern part of the deflection does not appear to have undergone any significant rotation whilst a post late eocene counterclockwise rotation of about 25 is recorded in the central and western parts this angular difference only accounts for about half of the difference in the strikes of structures in the two sections of the deflection when considered with the available tectonic sedimentological and geodynamic data these results indicate that the arcuate shape of the cajamarca fold belt results from complex processes early multi phased compressional events of paleocene eocene age created the folds and built an arcuate structure with non rotational mechanisms then in post late eocene times the structure reached its present form after rotation of its central and western parts the bearing of these results on the geodynamic evolution of the andean cordillera is discussed

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the 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

terrigenous sediment on ceara rise a cenozoic record of south american orogeny and erosion abstract ceara rise located east the amazon river mouth is covered with a thick blanket of pelagic carbonate and hemipelagic terrigenous sediment the terrigenous component has been extracted from 57 bulk sediment samples at ocean drilling program odp sites 925 and 929 on ceara rise to obtain a cenozoic record of riverine discharge from northern south america from the early eocene to early miocene 55 20 ma terrigenous accumulation was dominated by moderate amounts of generally large grained gray to green sediment especially depleted in elements that are enriched in post archaean shale e g cs th yb however pulsed inputs of relatively small grained gray to green terrigenous sediment less depleted in the above elements occurred in the late eocene and oligocene the accumulation of terrigenous sediment decreased significantly until 16 5 ma in the middle miocene 16 5 13 ma terrigenous accumulation was dominated by small amounts of small grained tan sediment notably depleted in na and heavy rare earth elements the accumulation rate of terrigenous sediment increased markedly from the latest miocene 10 ma to the present day a change characterized by deposition of gray green sediment enriched in elements that are enriched in post archaean shale observed changes in terrigenous sediment at ceara rise record tectonism and erosion in northern south america the brazil and guyana shields supplied sediment to the eastern south american margin until the middle miocene 20 16 5 ma when a period of thrusting shortening and uplift changed the source region probably first to highly weathered and proximal phanerozoic sediments by the late miocene 9 ma there was a transcontinental connection between the andes and eastern south america weathering products derived from the andes have increasingly dominated terrigenous deposition at ceara rise since the late miocene and especially since the late pliocene

origin of the high plateau in the central andes bolivia south america the bolivian altiplano in the central andes of south america is part of the second largest high plateau on earth it is an elongate region of subdued relief 1 2 105 km2 and 4 km above sea level bounded by the eastern cordillera and volcanic arc western cordillera here the crust is up to 75 km thick we describe the cenozoic geological evolution of this region using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure crustal shortening and magmatic addition and locally sedimentation are the main mechanisms of cenozoic crustal thickening leading to nearly 4 km of surface uplift since the paleocene addition of mafic melts appears to be a first order mechanism of cenozoic crustal growth contributing 40 of the crustal thickening beneath the volcanic arc removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind arc mafic volcanism at 23 ma and 0 5 ma contributing to the surface uplift the altiplano originated as a sedimentary basin several hundred kilometers wide between the proto western cordillera and a narrow zone of uplift proto eastern cordillera farther east the latter zone formed by inversion of the center of a wide lacustrine or marine cretaceous paleocene basin close to sea level at 45 ma a thickness of 2 4 km of paleogene continental elastics accumulated in the proto altiplano basin subsequently in the oligocene we estimate that this region and the western margin of the eastern cordillera were technically shortened 22 65 km resulting in 9 km of average crustal thickening the altiplano basin was rejuvenated at 25 ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting eastern and western cordilleras between 25 and 5 ma folding and thrusting in the western margin of the eastern cordillera migrated westward into the center of the altiplano basin essentially terminating deposition except in local subbasins and accommodating 13 30 km of shortening and an estimated 7 km of average crustal thickening subsequently there has been strike slip deformation and limited local thrusting 5 km of shortening geomorphological and geochronological evidence for 1 5 2 km of surface uplift of this region since the late miocene suggests 14 km of lower crustal thickening beneath an essentially rigid lid and can be explained by 100 150 km of underthrusting of the brazilian shield and adjacent regions beneath the eastern margin of the central andes the present subdued relief in the altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin

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

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

paleomagnetic evidence for rapid vertical axis rotation in the peruvian cordillera ca 8 ma paleomagnetic results from 31 neogene sites in the peruvian andes yield primary magnetizations as demonstrated by positive fold and reversal tests strata dated as 18 9 ma record a significant counterclockwise rotation 11 5 whereas unconformably overlying younger strata 7 6 ma are not rotated the age of rotation thus is between 9 and 7 ma a period that coincides with the widespread quechua 2 deformation phase moreover eight independent studies on 107 9 ma rocks from peru between 9 s and 15 s reveal similar and significant rotations 15 6 this suggests that the region rotated during a 2 m y period of deformation ca 8 ma when the andes underwent rapid uplift and important deformation commenced in the subandean zone

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate

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

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

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

the i xe record of alteration in the allende cv chondrite abstract complex i xe and mineralogical studies have been performed on four heavily altered allende fine grained spinel rich ca al rich inclusions cais and four allende dark inclusions dis showing various degrees of iron alkali metasomatic alteration the cais are largely composed of fe rich spinel al diopside and secondary nepheline and sodalite the dis consist of chondrules and allende like matrix composed of lath shaped fayalitic olivine nepheline sodalite and ca fe rich pyroxene andradite feni sulfide nodules chondrule phenocrysts are extensively or completely replaced by fayalitic olivine nepheline and sodalite metal nodules are replaced by feni sulfides andradite and ca fe rich pyroxenes the chondrules and matrices are crosscut by ca fe rich pyroxene feni sulfide fayalitic olivine veins dis are surrounded by continuous ca rich rims composed of andradite wollastonite kirschsteinite and ca fe rich pyroxenes whereas the outer portions of the inclusions are depleted in ca three cais yield well defined i xe isochrons with ages 3 1 0 2 3 0 0 2 and 3 7 0 2 ma younger than the shallowater internal standard 4566 2 ma similar release profiles suggest the same iodine carrier most probably sodalite for all four cais the allende dis yield i xe ages from 0 8 0 3 to 1 9 0 2 ma older than shallowater based on the petrographic observations we infer that the dis experienced at least two stage alteration during an early stage of the alteration which took place in an asteroidal setting but not in the current location of the dis chondrule silicates were replaced by secondary fayalitic olivine nepheline and sodalite calcium lost from the chondrules was redeposited as ca fe rich pyroxene veins and ca fe rich pyroxene andradite nodules in the matrix the second stage of alteration resulted in mobilization of ca from the dis and its re deposition as ca rich rims composed of ca fe rich pyroxenes andradite and wollastonite around the dis we interpret i xe ages of the dis as time of their alteration prior incorporation into allende the younger i xe ages of the fine grained spinel rich cais may reflect hydrothermal alteration of the allende host which could have occurred contemporaneously with the second stage of alteration of the allende dis the lack of evidence for the disturbance of i xe system in the allende dis may suggest that fluid responsible for the alteration of the allende cais was in equilibrium with the i and xe bearing phases of the dis