quaternary glaciations in the southern hemisphere an overview abstract large glacier systems in antarctica have existed at least since the early miocene glaciers in the patagonian andes advanced in the late miocene and fluctuated several times during the pliocene as did ice masses elsewhere in the southern hemisphere sedimentological evidence of glaciation during the early and middle quarternary is best preserved in volcanic regions patagonia new guinea africa but is poorly constrained by dating there is evidence of glaciation during oxygen isotope stages 10 8 and 6 in several parts of the southern hemisphere but the only well dated advance of the middle quarternary is that of stage 6 in the transantarctic mountains during the last glaciation montane glaciers in parts of the southern hemisphere were as large during isotope stage 4 as during isotope stage 2 the last glaciation maximum stage 2 is well dated in tasmania new zealand and patagonia culminating as 21 18 ka bp following slight recession from the lgm limits late glacial advances peaked at 15 14 ka bp and at 12 10 ka bp major holocene advances occurred on at least three occasions during the last 5 ka in areas presently glacier covered

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

palaeoclimatic implications of mio pliocene sedimentation in the high altitude intra arc lauca basin of northern chile abstract this paper examines the late miocene to pliocene sedimentary succession in the high altitude intra arc lauca basin of northern chile and discusses the palaeoclimatic evidence recorded within the basin fill sedimentological and geochronological data allow the reconstruction of four stages of the fluvial to lacustrine basin fill from the late miocene 6 4 ma to the late pleistocene we interpret the sedimentary evolution to reflect changes in precipitation during a time interval of general aridity climatic changes exerted the dominant control on sediment accumulation volcanism tectonism changes in morphology and sediment supply were factors of subordinate importance stage 1 comprises a clastic red bed sequence developed during the late miocene under relatively humid conditions an abrupt change in depositional style occurred around 6 4 ma when ephemeral saline lake sedimentation prevailed stage 2 this change in depositional style is considered to document the begin of a phase of desiccation that lasted until the early pliocene 3 7 ma and which was succeeded by less arid climatic conditions during the late pliocene and pleistocene stages 3 and 4 the transition between stages 1 and 2 and the subsequent dry period is assumed to coincide with a global cooling event during the latest miocene to early pliocene comparable sedimentary responses to this climatic change can be expected in adjacent basins of the central andes

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

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

autochthonous anisotropy of archaeological materials by the action of water experimental and archaeological reassessment of the orientation patterns at the olduvai sites abstract anisotropic patterns documented indirectly through m leakey s drawings of olduvai archaeological sites have led to questions about the integrity of these sites most experiments on bone transport by water have been carried out using complete elements that do not replicate specimen bone breakage and size as documented in archaeological sites in the present work an experimental framework is provided using experimental proxies of archaeological assemblages results show that autochthonous assemblages affected by hydraulic processes can adopt anisotropy in their fabric archaeological comparisons between drawings and excavated sites at olduvai stress the bias in leakey s drawings of flk zinj and flk north in bed i a large scale open excavation recently carried out at tk bed ii exposes an area comparable in size to leakey s excavations comparing the orientation patterns of this site to those reported by leakey shows how biased the drawing of the site is and how easily this can lead to misinterpretations of isotropy anisotropy with fatal consequences for the understanding of site formation processes

topography and subduction geometry in the central andes clues to the mechanics of a noncollisional orogen the central andean orogen between 12 and 32 s latitude exhibits a high degree of spatial order principally an extraordinary bilateral symmetry that is common to the earth s surface the underlying wadati benioff zone and the nazca south america plate kinematics which has been stable since the mid tertiary this spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen the shapes of the topography and subduction zone can be reduced to symmetric and antisymmeric components relative to any vertical symmetry plane the particular plane which minimizes the antisymmetry and maximizes the symmetry is well resolved and is essentially coincident with the stable euler equator of nazca south america relative motion since the mid tertiary that the topography subduction geometry and persistent mid tertiary plate kinematics share common spatial and geometric elements suggests that the distribution of topography in this orogen depends strongly on the dynamics of subduction other factors that might affect the topography and underlying tectonics such as climate and inherited structural fabric which have different spatial characteristics must be of less significance at a continental scale furthermore the small components of asymmetry among the various elements of the orogen appear to be mutually related in a simple way it is possible that this coupled asymmetry is associated with a late tertiary change in plate kinematics these observations suggest that there is a close connection between plate tectonics and the form of the earth s surface in this noncollisional setting it follows that the distribution of topography near convergent plate boundaries may provide a powerful constraint for understanding the dynamics of subduction

evolution of the west andean escarpment at 18 s n chile during the last 25 ma uplift erosion and collapse through time abstract the geological record of the western andean escarpment warp reveals episodes of uplift erosion volcanism and sedimentation the lithological sequence at 18 s comprises a thick pile of azapa conglomerates 25 19 ma an overlying series of widespread rhyodacitic oxaya ignimbrites up to 900 m thick ca 19 ma which are in turn covered by a series of mafic andesite shield volcanoes between 19 and 12 ma the surface of the oxaya ignimbrites evolved into a large monocline on the western slope of the andes a giant antithetically rotated block oxaya block 80 km 20 km formed on this slope at about 10 12 ma and resulted in an easterly dip and a reversed drainage on the block s surface morphology topography and stratigraphic observations argue for a gravitational cause of this rotation a secondary gravitational collapse 50 km3 extending 25 km to the west occurred on the steep western front of the oxaya block alluvial and fluvial sediments 11 2 7 ma accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall indicating further shortening between 8 and 6 ma flatlying upper miocene sediments

miocene to recent history of the western altiplano in northern chile revealed by lacustrine sediments of the lauca basin 18 15 18 40 s 69 30 69 05 w the intramontane lauca basin at the western margin of the northern chilean altiplano lies to the west of and is topographically isolated from the well known plio pleistocene lake system of fluvio lacustrine origin that covers the bolivian altiplano from lake titicaca to the north for more than 800 km to the salar de uyuni in the south the lauca basin is filled by a sequence of some 120 m of mainly upper miocene to pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin volcanic rocks partly pyroelastic provide useful marker horizons in the first period 6 4 ma of its evolution the lago lauca was a shallow ephemeral lake evaporites indicate temporarily closed conditions after 4 ma the lake changed to a perennial water body surrounded by alluvial plains in the late pleistocene and holocene 2 0 ma there was only marginal deposition of alluvial and glacial sediments the basin formed as a half graben or by pull apart between 10 and 15 ma tectonic displacement of the basal ignimbrite sequence during the quechua phase and 6 2 ma maximum k ar ages of biotites of tuff horizons in the deepest part of the basin apart from this early basin formation there has been surprisingly little displacement during the past 6 ma close to the western cordillera of the altiplano also climate indicators pollen evaporites sedimentary facies suggest that an arid climate has existed for the past 6 ma on the altiplano together these pieces of evidence indicate the absence of large scale block faulting tilt and major uplift during the past 5 6 ma in this area

opening of drake passage gateway and late miocene to pleistocene cooling reflected in southern ocean molluscan dispersal evidence from new zealand and argentina thirty two benthic marine molluscan genus group taxa are newly recognised as common to new zealand and argentina previously suggested examples including panis athlopecten austrovenus large cirsotrema lucinoma aulacomya and zygochlamys are confirmed from new collections the total of 46 cenozoic dispersed taxa now recognised falls into four main groups a oligocene early miocene dispersals from south america to new zealand 8 taxa b oligocene early miocene dispersals from new zealand to south america 22 c latest miocene pliocene dispersals from south america to new zealand 9 and d pleistocene holocene dispersals from south america to new zealand at least 7 taxa dispersal seems to have occurred in bursts both to and from south america when the antarctic circumpolar current acc initiated i e when drake passage opened late in the oligocene while dispersal from south america to new zealand only seems to have occurred again at two later times 1 during the latest miocene pliocene enhancement of acc circulation caused by the initiation of west antarctic glaciation and 2 during pleistocene glaciations however the incompleteness of the argentinian record exaggerates the size of the initial burst and allows no assessment of post middle miocene dispersal to south america formation of the acc through the opening of a passage south of australia at ca 43 40 ma and of drake passage at ca 23 ma not only led to the glaciation of antarctica but also greatly altered southern ocean biogeography

concept of continuum as opposed to periodic tectonism in the andes abstract results of a study of various andean sedimentary basins that are distinct in terms of age and geodynamic position allow us to postulate the idea of a compressional tectonic continuum as opposed to the short tectonic events which have so far been considered as one of the main features of the dynamic evolution of the andean belt in these basins the development of kilometre scale synsedimentary folded structures implies the existence of compressional stress over a period of several million years the existence of tectonic events of long duration allows us to modify the interpretation of the classic andean tectonic model the timing of the continuum tectonism is perhaps dependent on the variation in the convergence rate between the farallon nazca and south american plates

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

tectonic signatures in arid continental basins an example from the upper miocene pleistocene calama basin andean forearc northern chile abstract the calama basin a transtensional basin situated within the andean forearc and its extension to the pampa del tamarugal central depression is filled with 700 m of sediments detailed sedimentary logging and 40ar 39ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick upper miocene pleistocene succession five facies associations fluvio lacustrine diatomite palustrine carbonate massive anhydrite fluvial and alluvial fan have been identified within this part of the basin fill the following events are recognised in the basin evolution 1 deposition of diatomaceous lacustrine sediments in two separate depocentres in the pampa del tamarugal and the central calama basin with alluvial fan accumulation along the basin margins at 8 ma 2 localised deformation within the central calama basin between 7 and 6 ma but continuous sedimentation elsewhere 3 palustrine carbonate deposition across the entire area from 6 to 3 ma except in the pampa del tamarugal where diatomaceous sedimentation continued until 5 ma before being replaced by palustrine carbonate deposition 4 widespread folding after 3 ma and entrenchment as the drainage system cut down to reach a new base level caused by the rio loa breaching the coastal cordillera and reaching the sea localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period comparison with similar age facies elsewhere in northern chile has shown changes such as these to be localised and diachronous indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting the lacustrine deposits do not appear to be a reliable recorder of low frequency climatic change this is exemplified by the change from lacustrine to palustrine deposition at 6 5 ma which would appear to record a drying of the climate however a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run off

lower bound on the amount of crustal shortening in the central bolivian andes balanced cross sections across the cordillera oriental and subandean zone of the central bolivian andes indicate that crustal shortening probably has played the dominant role in orogeny in this convergent margin setting a minimum amount of shortening 210 km is documented which can account for two thirds of the present day crustal cross sectional area along a transect spanning the entire mountain range substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere a large minimum amount of crustal shortening in the bolivian andes shows contrary to common assumptions about orogeny that 1 magmatic addition may be volumetrically less important in orogeny in andean type margins and 2 crustal shortening is not uniquely associated with continental or island arc collision

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

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

the harmattan over west africa nocturnal structure and frontogenesis this article presents observations and model simulations of the low level nocturnal structure of the atmosphere over west africa the measurements are taken from the dry season special observing period sop 0 of the african monsoon multidisciplinary analysis amma at niamey niger on 23 and 24 january 2006 during this time mesoscale structures in the atmospheric aerosol loadings were observed the available observations indicate that these mesoscale features at niamey are consistent with the passage of gravity currents or bores in the northerly harmattan winds model simulations at resolutions down to 1 km indicate that the mesoscale structures are caused by nocturnal frontogenesis in the baroclinic zone to the south of the sahara in the winter months this frontogenesis is a continental scale phenomenon which has significant implications for the uplift and transport of dust and biomass burning aerosols in the region an accompanying frontogenetic feature appears further south in the model simulations associated with the winter intertropical front the frontogenesis is possibly linked with the development of structures showing characteristics of canonical mesoscale phenomena including internal bores and gravity currents representation of these features in the uk met office unified model at different resolutions from 12 km to 1 km is discussed

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

k ar geochronology of the late cenozoic volcanic rocks of the cordillera occidental southernmost peru abstract twenty four k ar radiometric ages are presented for late cenozoic continental volcanic rocks of the cordillera occidental of southernmost peru lat 16 57 17 36 s rhyodacitic ignimbrite eruptions began in this transect during the late oligocene and continued episodically through the miocene the development of andesitic dacitic strato volcanoes was initiated in the pliocene and continues to the present the earliest ignimbrite flows 25 3 22 7 ma are intercalated in the upper coarsely elastic member of the moquegua formation and demonstrate that this sedimentary unit accumulated in a trough parallel to andean tectonic trends largely in the oligocene more voluminous ash flow eruptions prevailed in the early miocene 22 8 17 6 ma and formed the extensively preserved huaylillas formation this episode was coeval with a major phase of andean uplift and the pyroclastics overlie an erosional surface of regional extent incised into a paleogene volcano plutonic arc terrain an age span of 14 2 8 9 ma mid late miocene is indicated for the younger chuntacala formation which again comprises felsic ignimbrite flows largely restricted to valleys incised into the pre huaylillas formation lithologies and at lower altitudes an extensive aggradational elastic facies the youngest areally extensive ignimbrites constituting the sencca formation were extruded during the late miocene in the earliest pliocene the ignimbrites were succeeded by more voluminous calcalkaline intermediate flows which generated numerous large and small stratovolcanoes these range in age from 5 3 to 1 6 ma present day or holocene volcanism is restricted to several large stratovolcanoes which had begun their development during the pleistocene by 0 7 ma the late oligocene early miocene ca 22 23 ma reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the cordilleras occidental and oriental of the central andes

a balanced model of the southern central andes to explain the actual crustal thickness of the southern central andes a balanced cross section at 21 s latitude was modeled a forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations in the model 320 km of shortening since late cretaceous based on balanced geological cross sections are assumed for the whole andes from the trench to the foreland and the initial crustal thickness varies between 30 and 40 km it results that about 20 of the actual crustal volume must be explained by other sources than crustal shortening structures of high velocity material in the upper crust of the eastern cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted nazca plate may contribute to the lower crust of the forearc and arc regions crustal thickening of the eastern cordillera and parts of the altiplano was undoubtedly produced by tectonic thickening whereas the sources of thickening in the western cordillera and forearc region are uncertain

interfering paths of deformation and development of arcs in the fold and thrust belt of the central apennines italy in the foreland fold and thrust belt of the central apennines the major thrust sheets design systems of arcs interfering with noncoaxial trends localization of shortening by thrusting and shearing appears to be controlled by the squeezing of low competence basinal units marche gran sasso monte picca and monte genzana in between the rigid blocks of the lazio abruzzi and adriatic carbonate platforms the geometrical interlocking has been resolved by differential rotations of the superposed thrust sheets as also substantiated by paleomagnetic data the structural analysis and the age of the syntectonic siliciclastic deposits suggest an irregular sequencing of superposed contractional and extensional events during late miocene middle pliocene times interfering kinematic paths characterize the arcuate belts where slip partitioning has adjusted the coeval deformation of the noncoaxially arranged tectonic units in the arcuate gran sasso monte picca thrust belt out of sequence thrusting displacement transfer on high angle strike slip faults and growth of high angle normal faults behind the most rotated blocks have resulted in the back and forth superposition of contractional and extensional structures in adjacent domains our kinematic reconstruction of the central apennines indicates that it is impossible to perform the retrodeformation of tectonic events by assuming a regular outward progression of deformation pulses in such a strongly arcuate belt the irregular trajectories of the displacement field and the complex overprinting relations between different sets of structures reflect the kinematic accommodation of blocks with contrasting rigidity and are not fitted easily within a regionally smoothed stress field

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

upper cenozoic volcanism and tectonism in the andes of northern chile abstract recent geological and radiometric information of the upper cenozoic volcanic sequences of northern chile in conjunction with previous evidences point out the complex evolution of the central andes volcanism volcanic activity developed along the andean range during the last 23 m y appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region since the middle miocene 12 m y there has been a constant overlap both in time and space between lavas and ignimbrites however ignimbrite eruptions seem to predominate up to the upper miocene while lava eruptions are dominant from the pliocene to the quaternary small scale chronological and volumetric variations of the volcanic activity in the studied areas make difficult the correlations with volcanic episodes through long distances units older than pliocene have been folded and thrusted as a result of compressive movements that occurred ca 7 4 m y ago this deformation is mostly remarkable at the borders of major faulted blocks these blocks were structured along pre existing fractures caused by tectonic movements during the lower miocene pliocene and quaternary block movements developed mainly through n s fault systems these faults together with nw se and ne sw systems control the spatial distribution of most volcanic vents in this region locally the n s faults may correspond to reactivations of previous fault zones this allowed successive effusions of volcanic material at least during the last 17 m y

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

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

transition from basement involved to thin skinned thrusting in the cordillera oriental of southern bolivia the eastern flank of the central andes in southern bolivia and northern argentina is underlain by a fold and thrust belt of neogene age it comprises an external thin skinned segment with basal detachment in silurian shales subandean ranges and an internal segment where units as deep as precambrian metasedimentary and igneous rocks are involved in thrusting eastern cordillera oriental between the cordillera oriental and the subandean ranges an intermediate segment interandean zone intervenes north of the argentinian bolivian border 22 s the structure of the interandean zone is characterized by folded and thrust faulted cambrian to triassic strata balanced cross sections across the western subandean ranges the interandean zone and the eastern cordillera oriental are based on previously available data and recent field work they suggest that the geological structure can be interpreted in terms of two major basement involved thrusts here referred to as interandean and subandean thrusts the more internal interandean thrust caused the development of broad basement anticlines in the eastern cordillera oriental however at shallower levels its displacement was accommodated by thin skinned folding and thrusting in the interandean zone in sequence activation of the subandean thrust uplifted the interandean zone on top of an imbricate basement sheet and caused the development of the thinskinned subandean ranges farther east published radiometric data and subsidence history of the subandean foreland basin suggest that the interandean thrust was active between about 10 ma and 5 ma the subandean thrust is still active southward displacement transfer from the subandean thrust onto the interandean thrust and a southward dipping lateral ramp of the interandean thrust result in the gradual transition from the interandean zone north of 22 s to the cordillera oriental of northern argentina a similar but larger scale transfer may be involved in the southern termination of the thin skinned subandean ranges of argentina at 23 s shortening in the western part of the thrust belt is of the order of 80 90 km total shortening in the eastern andean thrust belt from the undeformed foreland to the eastern margin of the cordillera oriental is 140 150 km

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

cenozoic polyphase landscape and tectonic evolution of the cordillera occidental southernmost peru landforms on the pacific slope of the cordillera occidental western cordillera in southernmost peru reflect the episodic epeirogenic uplift of the central andes since the earliest miocene in the geomorphic history three major stages are distinguished initial uplift followed a mid tertiary period of tectonic and magmatic quiescence in which a low lying subdued landscape formed through dissection of an eocene subaerial volcano plutonic terrain uplift and resulting erosional beveling of this landscape coinciding in their later stages with the explosive reactivation of the magmatic arc gave rise to the regionally extensive lower miocene altos de camilaca surface i which was mantled soon after its formation by ignimbrites of the huaylillas formation major relief was achieved in the middle and early late miocene when a succession of uplift episodes generated a sequence of four discrete pediments constituting the multiple pediment stage ii uplift was accompanied by widespread eruption of ignimbrites of the chuntacala formation subsequently in the pliocene pedimentation was succeeded as the major erosional process by canyon incision across the lower slopes of the cordillera during this valley and terrace stage iii the immediate littoral margin experienced the development of two major terraces contemporaneous with a transition from ignimbrite eruption to predominantly andesitic volcanism in the high cordillera in the early pleistocene during this time it is probable that epeirogenic uplift rather than glacio eustatic sea level changes exerted the dominant control on landform development in the absence of evidence for large scale crustal compression in this immediate region since the cretaceous it is inferred that neogene uplift resulted from episodic thickening of the continental crust through underplating by basic and or intermediate magmas the neogene chronology of landform development herein defined is remarkably similar to that outlined in other transects of the central andes implying that the continental margin has on a broad scale responded uniformly to plate subduction since the early miocene however the low relief evident in the mid tertiary in the study area and not for instance in northern chile 27 s suggests that crustal thickening through the neogene was more rapid and more extensive in the vicinity of the arica deflection than in transects to the north and south the estimated neogene uplift rate for the transect was in the range 0 06 to 0 19 mm yr

gigantic paleolandslide associated with active faulting along the bogd fault gobi altay mongolia on the basis of analyses of satellite imagery aerial photographs and field observations we describe the occurrence of one of the largest paleolandslides 50 km3 ever recognized in an intracontinental domain the slide occurred along the active bogd fault in the gobi altay mountain range in mongolia morphological and structural analyses of the relationships between the landslide and the area affected by active tectonics suggest that this gigantic mass movement was associated with surface faulting during a strong earthquake