the yanque prospect peru from polymetallic zn pb mineralization to a nonsulfide deposit the yanque nonsulfide pb zn deposit inferred resources 12 5 mt 3 7 pb and 3 5 zn is located in the andahuaylas yauri ore province cuzco southern peru the deposit occurs within a base metal mineralized district centered on the medium sized dolores porphyry copper a thorough geological mineralogical and geochemical study has carried out in order to define the relationships between the dolores cu porphyry ore and the yanque zn pb polymetallic mineralization and the characteristics of the economic nonsulfide concentrations both sedimentary and igneous rocks constitute the backbone of the yanque dolores area the sedimentary lithologies belong to the soraya mara and ferrobamba fms upper jurassic middle cretaceous the yanque orebody is hosted by the mara fm which prevailingly consists of a siliciclastic sedimentary breccia the original sulfide mineralization consisted of galena pyrite and sphalerite the host rock has been affected by a strong hydrothermal alteration characterized by prevailing sericite illite as in the typical porphyry related phyllic argillic alteration stage and by minor kaolinite dolomite and quartz minor element geochemistry characterized by sb as mn ag and locally also by cu points to magmatic hydrothermal related mineralizing fluids the pb isotopic compositions from dolores and yanque sulfides are similar and are typical of the tertiary magmatically derived ores in this part of peru the hydrothermally altered rocks at yanque have the same pb isotopic compositions as the sulfides thus confirming the hypothesis that the yanque primary zn pb mineralization may have been produced by hydrothermal circulation related to the emplacement of the dolores cu porphyry as it is the case of other porphyry cu systems associated with polymetallic mineralization elsewhere however no simple genetic model for the mineralization involving just one fluid circulation episode is able to explain the data the yanque economic nonsulfide ore association consists of sauconite hemimorphite smithsonite and cerussite which result from the weathering and alteration of the original sulfide mineralization zinc is allocated mainly in sauconite zn smectite rather than in carbonates a factor strictly related to the prevailing siliciclastic character of the host rock distinctive features of the yanque orebody are the comparable ore grades for both pb and zn 3 5 zn and 3 7 pb and the inverse supergene chemical zoning in fact contrary to other supergene ores of this type zinc prevails in the top zone of the yanque deposit whereas lead content increases with depth considering the different mobility of the two metals in solution it may be assumed that most of the primary zinc that was the source for the yanque nonsulfides was originally located far from the position occupied by the galena mineralization whose remnants have been observed on site zinc sulfides may have been originally contained in the eroded rock volumes that surrounded the actual deposit the zinc rich solutions have possibly migrated through the siliciclastic mara fm and precipitated the nonsulfide minerals by porosity filling and replacement processes in this sense the yanque secondary zn pb deposit could be considered as a special type of exotic mineralization

formation of willemite in hydrothermal environments willemite zinc silicate is the main zinc mineral in some carbonate hosted ore deposits e g franklin new jersey vazante brazil beltana south australia kabwe zambia recent interest in these unconventional zinc deposits has increased because of high zinc grades that exceed 40 wt percent relatively low environmental impact of ore processing owing to the lack of acid generating sulfides in the waste and advances in ore processing technologies in the past most metallogenic studies proposed formation of willemite deposits by supergene or hypogene alteration of preexisting sulfide deposits however recent data on the vazante beltana and kabwe deposits indicate willemite crystallization at temperatures in excess of 150 c raising the possibility of primary precipitation from hydrothermal fluids we use numerical geochemical modeling to examine the formation of willemite under hydrothermal conditions activity activity diagrams reveal that in the presence of dissolved sulfur and quartz willemite instead of sphalerite will precipitate under oxidizing e g hematite stable sulfate predominant and alkaline ph higher than k feldspar muscovite quartz conditions willemite also becomes more stable relative to sphalerite at high temperature and willemite can coexist with magnetite at 300 c the stabilities and solubilities of sphalerite willemite smithsonite hydrozincite and zincite were calculated for wide ranges of temperature 25 300 c chloride concentration dissolved sulfur and carbon concentrations ph quartz saturation and oxidation potential plots of the solubility of the different minerals as a function of two variables e g temperature and redox state ph and redox state allow us to predict the effects of changing chemical conditions which in turn permits an estimate of the efficiency of particular precipitation processes cooling is an effective process for precipitating sphalerite but not willemite whereas ph increase e g by acidic fluids reacting with carbonates is effective for precipitating willemite but not sphalerite dynamic geochemical models that simulate physicochemical processes are used to understand the formation of the beltana willemite deposit in the adelaide geosyncline of south australia this small high grade deposit 850 000 t at 36 zn is hosted in dolomite of the cambrian ajax limestone next to a tectonic contact with the diapiric halite bearing clastic sediments of the callanna group the orebody is associated with hematite alteration and is characterized by the total absence of sulfides willemite is the only zinc ore mineral and the arsenate hedyphane ca 2 pb 3 aso 4 3 cl is the main lead mineral the model results show that willemite will precipitate in response to water rock interaction and fluid mixing processes at temperatures above 120 c the presence of arsenate in the hydrothermal fluid is likely to have been important at beltana in arsenate absent models sulfate is reduced to sulfide by the precipitation of ferrous iron as hematite resulting in the precipitation of sphalerite and galena in contrast in models including arsenate the reduction of sulfate to sulfide is inhibited and willemite is predicted to precipitate

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

a seismically controlled seal breach in a major hydrocarbon province a study from the mauddud formation in the bahrah field kuwait abstract the mauddud formation a late albian aged carbonate oil reservoir is one of the main oil producers in kuwait however it exhibits unreliable oil production from the bahrah field the mauddud formation in the bahrah field is divided into two sub structures by a series of e w faults and shows a slight degradation in oil quality 5api within the up dip structure intersections of the e w faults with nw se faults are associated with deep seated seismic pipes which culminate in pull up features creating the overlying sag like features additionally the mauddud formation shows evidence of baroque saddle dolomite in fracture cements and replacing clasts the baroque dolomite have 3he 4he ratios 0 3 times higher than atmospheric values taken together this seismic and petrographic evidence suggests that the mauddud formation experienced an ingress of radiogenic crustal fluids potentially sourced from the basement seismic pipes therefore are interpreted to be the product of the upward movement of hydrothermal fluids that breach seal rocks causing oil to escape and subsequent calcite cementation an integrated map of a basement magnetic anomaly and current exhalative features on the present day surface shows evidence of a relationship with fault locations on the level of the mauddud formation that were rejuvenated and associated with the remigration of oil this study provides an exceptional example of a seal breach in one of the world s most prolific hydrocarbon provinces it also indicates the importance of a multidisciplinary multi scale approach extending from the basement to the surface in order to understand seal breach in carbonate petroleum systems the overall data is used to assess the main factors that effected the integrity of the seal when this latter is breached by hydrothermal fluids

supergene nonsulfide zn pb mineralization in the mehdiabad world class sub seafloor replacement sedex type deposit iran the world class mehdiabad nonsulfide deposit consists of several accumulations of zn pb fe nonsulfide minerals derived from supergene of a vent proximal sub seafloor replacement sedex type deposit hosted in lower cretaceous clastic carbonate rocks of the taft and abkuh formations the main sulfides are galena sphalerite pyrite and minor amounts of chalcopyrite which have been partially or completely transformed into nonsulfide minerals by supergene processes currently the zn mineralization in the abkuh formation ore horizon ii is almost fully oxidized the mehdiabad deposit which can be assigned to both direct replacement and wall rock replacement types the time of the formation of the nonsulfide deposit has not been determined but it is probable that the main supergene metallogenesis happened during tertiary possibly between post cretaceous to present the mineralogy of the nonsulfide mineralization is generally complex and consists of smithsonite hydrozincite and hemimorphite as the main economic minerals accompanied by cerussite anglesite iron manganese oxy hydroxides sauconite and zn rich clays commonly nonsulfide minerals in this deposit consist of two types of ore red zinc ore rzo rich in zn fe pb as and white zinc ore wzo typically with very high zinc grades but low concentrations of iron and lead the proposed three step scenario for the deposition of nonsulfide mineralization in the mehdiabad deposit comprises 1 the deposition of primary sulfides contemporaneous with lower cretaceous clastic carbonate host rocks to form sedex type mineralization 2 during the late cretaceous to present regional uplift leads to deformation folding and thrusting of sulfide bearing lower cretaceous sequence and 3 as a result zn pb fe sulfides hosted in carbonates experienced oxidation under an arid warm climate to form the supergene nonsulfide ore mineralizations

hydrocarbon plays and global basin classification hydrocarbon plays and the global basin classification system have important connections after a basin has been classified its similarities or differences with other basins may be compared prolific hydrocarbon basins are categorized and the plays are compared to specific tectonic and depositional events some plays are controlled by basin initiating tectonic events such as interior fracture or interior sag and the type of sedimentation inherent in each basin type in more complex polyhistory basins oil and gas plays are commonly associated with combinations of various cycles or by basin modifying tectonics such as episodic wrenching subsidence foldbelts and basin tilt

oxygen and hydrogen isotopes in the hydrologic cycle changes of the isotopic composition of water within the water cycle provide a recognizable signature relating such water to the different phases of the cycle the isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes as a result meteoric waters are depleted in the heavy isotopic species of h and o relative to ocean waters whereas waters in evaporative systems such as lakes plants and soilwaters are relatively enriched during the passage through the aquifers the isotope composition of water is essentially a conservative property at ambient temperatures but at elevated temperatures interaction with the rock matrix may perturb the isotope composition these changes of the isotope composition in atmospheric waters surface water soil and groundwaters as well as in the biosphere are applied in the characterization of hydrological system as well as indicators of paleo climatological conditions in proxy materials in climatic archives such as ice lake sediments or organic materials

miocene fish faunas from the northwestern amazonia basin colombia peru brazil with evidence of marine incursions new evidence indicates marine influences during the miocene in the northwestern amazonia basin this is the first major survey of the ichthyofauna from this area in the miocene fossil fish remains from taxa such as the dasyatoidea myliobatoidea characiformes siluriformes and sciaenidae are found conspicuous finds were two pharyngeal teeth of stephanodus minimus a species that so far had only ever been found in late cretaceous sediments the fish remains of freshwater brackish and marine taxa are deposited together throughout the area this is thought to be related to marine influences during the miocene these finds agree with earlier studies that suggest episodic marine influences which most likely lasted up to the late miocene and disagree with studies that suggest a connection between the atlantic and the caribbean sea over the south american continent the ichthyofaunas also indicate a warm climate as well as shallow and possibly torrential waters c 1998 elsevier science b v all rights reserved

empirical relationships between elevation and the stable isotope composition of precipitation and surface waters considerations for studies of paleoelevation change a compilation of 68 studies from throughout many of the world s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects the isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the himalayas and at elevations 5000 m there are no significant differences in isotopic lapse rates from most regions of the world 0 28 permil 100 m except at the extreme latitudes where isotopic lapse rates are higher given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals this global isotopic lapse rate can be used to place numerical constraints on the topo graphic development of some ancient mountain belts or plateaus there are many complicating factors that can confound interpretation of paleoel evation change based on stable isotopes and many of these are unique to specific mountain belts or time periods relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedo genic or authigenic mineral and the water from which it forms in cases where isotopic proxy minerals are sampled from localities where temperature will change simulta neously with elevation change the apparent change in the isotopic composition of precipitation may be dampened by several permil this suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau

winter monsoon convection in the vicinity of north borneo part i structure and time variation of the clouds and precipitation abstract radar and satellite observations in the vicinity of northern borneo obtained during the international winter monsoon experiment wmonex showed that the convection in that region underwent an extremely regular diurnal cycle over the sea to the north of borneo the general level of convective activity was increased during monsoon surges and during the passages of westward propagating near equatorial disturbances convective activity was decreased during monsoon lulls the diurnal cycle was well defined regardless of whether the general level of convective activity was enhanced or suppressed by synoptic scale events the cycle of convection over the sea was especially well documented it was typically initiated at about midnight when an offshore low level wind began where this wind met the monsoonal northeasterly flow usually just off the coast convective cells formed after midnight the convection continued to develop and by 0800 lst it had evolved into an organized mesoscale system with a

global geologic history and distribution of hydrocarbon reserves a review of global geologic history during seven major periods between the cambrian and the holocene reveals the principal factors influencing the distribution of world hydrocarbon reserves the paleozoic reserves located mainly in north america and europe are mostly related to stable platform environments the mesozoic and tertiary reserves are found in more diverse tectonic environments represented mainly by the mesogean tethyan realm passive margins and the western american mobile belts the mesogea tethys formed a latitudinal feature separating gondwana and laurasia the mesogean realm contains 69 of the world oil and gas reserves divided almost equally between the gondwana and laurasia areas the voluminous reserves in the mesogean realm reflect the history of repeated opening and closing of the mesogea ocean that resulted in several phases of basin development the comparative isolation of the basins from the global ocean circulation favored preservation f organic matter because of diagenesis the thermal history and migration the ultimate hydrocarbon reserves generally decrease as the ages of the reservoir and source rocks increase migration is usually directed toward the higher parts of the stratigraphic section concentrations of reserves within particular age ranges apparently reflect development of two major categories of basins 1 marine platform basins which were dominant in the devonian jurassic and cretaceous periods and 2 rapidly subsiding basins which were dominant in the late tertiary and probably during the late carboniferous pennsylvanian the middle east has an exceptional volume of sediments prolific source rocks and well preserved hydrocarbons and it is not likely that another hydrocarbon province of comparable importance will be found moreover these middle east reserves do not give any anomalous weight to the total reserves of the gondwanian areas when compared to those in the laurasian areas

structural styles of albian rafts in the espirito santo basin se brazil evidence for late raft compartmentalisation on a passive continental margin in recent years hydrocarbon exploration offshore se brazil has been focusing on lower cretaceous strata deformed by gravity gliding above aptian salt a three dimensional 3d seismic volume from the espirito santo basin se brazil is here used to a test the parameters considered to control raft tectonics on a margin tectonically reactivated in the cenozoic and b investigate the impact of prolonged halokinesis on raft deformation offshore espirito santo the combined effects of halokinesis and multiple andean tectonic phases are expressed by local collapse fault reactivation and late segmentation of albian rafts as a result of this deformation we observe four main raft geometries a rolled over rafts b tabular rafts c collapsed rafts and d folded and tilted rafts on the flanks of salt rollers this work shows that salt rollers formed buttresses to moving albian cenomanian rafts with withdrawal of salt from underneath some of the rafts leading to their collapse and welding onto pre salt strata this process occurred in the studied part of the espirito santo basin with minimum control of post raft overburden thickness on raft compartmentalisation salt withdrawal from underneath the rafts is an important phenomenon as it enhanced connectivity between pre salt and post salt units potentially promoting the migration of hydrocarbons from syn rift source units into post salt reservoirs

the cristal zn prospect amazonas region northern peru part ii an example of supergene enrichments in tropical areas abstract the cristal zn prospect is located in the northernmost part of a wide mining district corresponding to the charlotte bongara zinc project which covers an area of approximately 110 km2 in the amazonas region in northern peru the mineralized area consists of many zn occurrences that contain mixed sulfide and nonsulfide mineralizations the nonsulfide ores are interpreted to be the product of weathering of primary mvt sulfide bodies the zn concentrations of the cristal prospect are hosted by platform carbonates of the condorsinga formation early jurassic which belongs to the pucara group the prospect extends over an area of approximately 2 1 km with nearly continuous zones of zn enrichment that has been detected in soil and rock samples the nonsulfide mineralization consists mainly of semi amorphous orange to brown zinc oxides that include hemimorphite smithsonite and fe hydr oxides the most important mineralized areas are the esperanza and yolanda occurrences which were also most intensively explored in both occurrences the supergene zn carbonates and silicates infill solution cavities or replace the carbonate host rocks and or the primary sulfides forming smithsonite and hemimorphite rich mineralizations the analyzed drill core samples have on average 20 wt zn and maximum ge concentrations of 200 ppm the bongara area experienced a prolonged phase of weathering from miocene to recent under tropical climatic conditions in these conditions the weathering processes affected many pre existing sulfide deposits e g cristal florida canyon mina grande where supergene profiles were developed under locally different settings that are defined primarily on the basis of mineralogical and geochemical data contrary to the mina grande deposit at cristal the development of a karst network was minor due to limited uplift and supergene alteration did not completely obliterate the roots of the original sulfide orebody the mineralogy and geochemistry of bongara nonsulfides is dependent on two main factors at the local scale 1 uplift rates and 2 host rock composition the latter may have favored the development of more e g mina grande or less e g cristal alkaline supergene environments uplift was controlled by the activity of local faults which allowed the exposure of sulfide protores at variable elevations in different periods of time and hydrological settings such different settings resulted in the precipitation of isotopically different supergene carbonates e g smithsonites and calcites at mina grande and cristal

the south american retroarc foreland system the development of the bauru basin in the back bulge province abstract the aim of this research is to understand the tectonic setting of the bauru basin this basin in central eastern south america has been classified as intracratonic but the basin fill geometry the involved subsidence mechanisms and the age of the deposits are poorly understood in this work the ranges of the fossil taxa are analyzed and ages are proposed for the lithostratigraphic units isopach maps were used to reconstruct the stratigraphic intervals of the basin fill the stratigraphy of the bauru basin is compared with that of the adjacent basins and the data are integrated with the available information on south american geodynamics the fossil record indicates that sediment accumulated from the cenomanian to early paleocene beginning after the mochica phase of the andean orogeny the basin fill geometry demonstrates migration of the depocenter through time which occurred simultaneously with migration of the andean basin and immediately after the orogenic events of the peruvian phase we propose that the bauru basin is a component of a retroarc foreland system developed during the early stages of the andean evolution and that it was developed in the back bulge province of this system the andean basin constitutes the foredeep depozone of this foreland system including the potosi oriente acre and maranon basins in addition the upper cretaceous of the parecis and solimoes basins were likely also developed in the back bulge province the thickness of the bauru accumulation indicates that other mechanisms might have overlapped the flexural subsidence in this back bulge province

stable isotopes of nonsulphide zn pb ores in britain and ireland fluid characteristics and palaeoclimatic variability stable isotope c o analysis was applied to smithsonites and cerussites from zn pb nonsulphide ores from britain and ireland to determine the nature of the fluids responsible for the precipitation of secondary carbonates and any relationship with palaeoclimatic conditions the carbon isotope compositions of the irish smithsonites show a major contribution of organic carbon whereas those of both zn and pb carbonates in the uk deposits indicate contribution of organic carbon and host rock carbonates 18ovsmow values for cerussite from both areas are about 13 lower than those of smithsonites uk smithsonite 18ovsmow values are more variable than those for cerussites and the irish smithsonites have unusually high values isoscape maps indicate that 18o values for both carbonates are higher in the west and decrease toward the east this trend correlates with a general decrease in the oxygen isotope compositions of rainwater moving eastward and suggests that oxidation of sulphides occurred at temperatures of about 15 20 c under climatic conditions warmer than today supplementary material a comprehensive overview of the investigated sites detailed data for the samples and sem images are available at https doi org 10 6084 m9 figshare c 4579535

beyond the petroleum system geohorizons the first joint aapg amgp association of mexican petroleum geologists hedberg conference on geologic aspects of petroleum systems was held october 2 6 1994 in mexico city mexico this research conference attracted nearly 150 geoscientists from 15 countries 41 papers and 27 posters were presented the opposite response occurred when the petroleum system concept was presented more than 20 yr ago it was largely overlooked in the past decade however interest in this concept as an exploration tool has been growing rapidly the research conference addressed the concept itself its elements and processes the tools and methods used to identify a petroleum system and many case studies we summarize responses by the participants to the following three questions first what is gained from using the petroleum system concept second in what new directions will the petroleum system concept take us in the future third has anything new come out of this research conference

numerical simulation and a geochemical model of supergene carbonate hosted non sulphide zinc deposits abstract the proposed metallogenetic model presented in this paper is based on observations from the iranian non sulphide zinc deposits mehdi abad iran kuh and kuh e surmeh the emplacement of non sulphide ore can be generally subdivided into an oxidation stage followed by a post oxidation stage the formation of carbonate hosted non sulphide zinc deposits is primarily controlled by three factors climate protore composition and geology lithology and structure of the wall rocks the climate controls the oxidation and metal transport predominantly through the amount of meteoric water and the availability of oxygen which indirectly is controlled through the thickness of the soil cover and biogenic oxygen consumption favourable conditions for oxidation are an arid climate with minimal biogenic activity within the soil as a consequence oxygen can reach the sulphides easily resulting in higher metal concentrations of the supergene aqueous solutions dry climates are associated with a low water table preventing descending zn bearing fluids from contact with the aquifer and thus from dilution dispersion and removal of metals commonly non sulphide zinc deposits consist of two types of ore red zinc ore rich in zn 20 fe 7 pb as and white zinc ore typically with very high zinc grades up to 40 but low concentrations of iron 4 2 ions during the oxidation stage leads to the precipitation of highly insoluble anglesite which results in low pb 2 concentration within the fluid most of the zinc up to 80 at ph 6 or 97 at ph 5 leaves the oxidation zone and precipitates as zinc carbonates in adjacent parts of the carbonate host rock the neutralisation processes of the sulphuric acid with the carbonates of the host rock during the oxidation stage lead to the formation of co 2 g consequently the co 2 partial pressure p co2 g increases drastically and forms a co 2 g halo around the active oxidation zone thus most of the zinc precipitates as smithsonite which is stable and occurs exclusively in a high p co2 g environment the p co2 g decreases during the post oxidation stage and reaches the level of atmospheric p co2 g under these conditions hydrozincite becomes stable and starts to replace smithsonite most of the stage i smithsonite becomes corroded and is altered to stage ii hydrozincite the post oxidation stage is also associated with the successive formation of local zinc hydro silicates depending on the availability of sio 2 within the solution generally arid climates provide the best conditions for the preservation of non sulphide deposits the limited availability of meteoric water and deep to very deep water tables protect the non sulphide ore from subsequent dissolution

the karst hosted mina grande nonsulfide zinc deposit bongara district amazonas region peru the mina grande zinc nonsulfide deposit amazonas region peru consists of several accumulations in karst cavities of zn oxide minerals derived from weathering of a mississippi valley type deposit hosted in mesozoic carbonate rocks of the condorsinga formation pucara group the karst cavities hosting the nonsulfide ores were developed in the miocene along northwest southeast faults avg 230 dip 70 dip angle associated with regional structures and locally along stratification joints avg 8 strike 278 dip and 26 dip angle thus far the mina grande zn accumulations have been partially mined in two pits named fase a and fase b and explored in a third area named fase c in 2008 the resources were as follows fase a 160 000 metric tons t 21 2 zn fase b 36 400 t 28 9 zn and fase c 116 700 t 22 5 zn the nonsulfide mineral assemblage consisting mostly of hydrozincite smithsonite and hemimorphite is associated with few remnants of the hypogene ore pyrite and sphalerite mineralogical and petrographic studies revealed several texturally distinct smithsonite and hydrozincite generations which are characterized by extremely negative 13 c compositions pointing to the prevailing contribution of an isotopically light carbon component from the oxidation of organic matter derived carbon and different 18 o isotope compositions 26 9 to 27 2 and 26 0 to 26 3 18 o vsmow for smithsonite and averages 24 6 and 23 7 18 o vsmow for hydrozincite these distinct 18 o compositions indicate that smithsonite and hydrozincite precipitated at least after two depositional stages these stages were probably related to several periods of uplift that occurred in the bongara district since at least 10 ma when the transition from the pebas to acre systems affected the amazonas foreland basin in the miocene the karst activity to which the supergene mineralization is related was restricted to the late miocene and early pliocene periods climatic conditions reconstructed from the ecosystems persisting in the region from late tertiary to recent time suggest that the mina grande supergene mineralization was associated with several weathering episodes that occurred under a climate resembling the present day climatic conditions

pangea extravagant sedimentary resource formation during supercontinent configuration an overview abstract the pangea supercontinent stage the interval from the late carboniferous to earliest jurassic is known for its rich and diverse economic resources from sedimentary systems their uniqueness is not the abundance but the resource association on a global scale and even more the time space distributions the evolution of the supercontinent configuration in context of icehouse to coldhouse stages generated maximum coal evaporite and phosphate deposition furthermore important hydrocarbon uranium and sedimentary metal deposits sedimentary fe and al deposits are very rare also typical is the absence of resources from organic deposition during the early triassic the extravagant spatial distribution of these resource systems but also the timing of their occurrences is linked to the tectonic evolution of pangea during accretion zenith amalgamation and breakup as well as to the resulting paleogeographic climatic oceanographic and ecological reorganization of the biosphere the published data on pangean resources are reviewed and reveal a conspicuous pangean pattern these time space relationships are explained in the context of paleotectonic and ecological changes during the supercontinent stage

winter monsoon convection in the vicinity of north borneo part ii effects on large scale fields abstract an observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large scale environment over the southern south china sea during the field phase december 1978 of the winter monsoon experiment wmonex has been carried out the primary observations used are from three soviet research vessels positioned in a triangular array off the north coast of borneo during the period 6 28 december computations of thermodynamic fields divergence and vertical motion have been made for the duration of the field phase based on six hourly rawinsonde releases at the ship sites analysis of the data indicates that the degree of convective activity over the southern south china sea is modulated by long period synoptic forcing monsoon surges easterly waves and also significantly by diurnal forcing land sea breeze circulations a diurnal cycle of convection persists in the region whether the synoptic scale forcing is weak or strong convection over wate

the cristal zinc prospect amazonas region northern peru part i new insights on the sulfide mineralization in the bongara province the cristal zn prospect consists of a mixed sulfide and nonsulfide mineralization located in the bongara province amazonas region northern peru the mineralization is hosted by carbonate rocks of the pucara group deposited in a mesozoic extensional basin on the western margin of the brazilian guyana shield zinc sulfides at cristal occur in the roots of the nonsulfide concentrations and are locally present also nearer to surface the sulfide mineralization postdates two hydrothermal dolomitization phases dol1 and dol2 and the sulfides occur mainly in veins cavity fillings or as disseminated mineralization within sparry to saddle dolomite they mostly consist of dark brown sphalerite intergrown with smaller amounts of pyrite sphalerite shows a distinct fe zonation with average ca 7 wt fe max ca 12 wt fe and is ge rich mean concentration of 142 ppm galena is rare the cristal sphalerite has sulfur isotopic compositions of 34s 14 to 15 vcdt oxygen isotopic compositions of dolomites are 18o 24 4 to 24 7 vsmow for dol1 and 18 4 to 22 for dol2 34s and 18o values of cristal sulfides and dolomites are similar to those observed in two mississippi valley type mvt deposits located ca 20 30 km south of the cristal prospect namely the florida canyon and florcita deposits this could be consistent with one or more events within a same mvt mineralizing system acting at the district scale the pb isotope compositions of sphalerite from two different areas of the prospect named cristal s s and charlita north define two distinct data point clusters centered around averages of 206pb 204pb 18 850 0 002 207pb 204pb 15 685 0 002 208pb 204pb 38 752 0 004 and 206pb 204pb 19 042 0 002 207pb 204pb 15 712 0 002 208pb 204pb 39 080 0 004 respectively this difference requires distinct metal bearing hydrothermal pulses in the mineralized area and or distinct pb sources the pb isotopic compositions of the cristal s s and charlita north sulfides are intermediate between the compositions of galena from the san vicente and shalipayco mvt deposits and record a contribution from an old crustal component the paleozoic basement which has pb isotopic ratios roughly matching those of dolomites and sulfides from cristal and charlita north areas represents the most reliable candidate to be an end member source of the metals of the cristal sulfide mineralization the second end member could be an igneous source isotopically identical to the late paleozoic to early mesozoic intrusives of the peruvian eastern cordillera or the triassic volcanic rocks occurring within the mitu group

mean state of the troposphere over south east asia and the east indies december 1978 the mean state of the troposphere over south east asia from 8 30 december 1978 is described in the lower troposphere a jet was observed over the south china sea extending south westward from the philippines evidence suggests that this north easterly jet was supported by downstream blocking induced by the borneo land mass at the equator the jet had the appearance of being channelled between the highlands of borneo and sumatra on its left flank near the west coast of borneo was a semi permanent and quasi stationary vortex being warm core this vortex was probably maintained in part by latent heat release in cumulus convection by partitioning the data into means at 00 and 12 gmt the intensity of the rising branch of the hadley circulation over borneo was shown to increase by a factor of two from 100 10 5 to 200 10 5mb s 1 from 00 to 12 gmt

implications for timing of andean uplift from thermal resetting of radiation damaged zircon in the cordillera huayhuash northern peru abstract the cordillera huayhuash is a north south oriented range along the drainage divide of the northern peruvian andes the range has high topography with peaks in excess of 5500 m and the second highest peak in peru nevados yerupaja 6617 m bedrock is dominated by folded mesozoic miogeoclinal rocks unconformably overlain by mid tertiary volcanics intruded by late tertiary granitic rocks and silicic dikes zircon fission track zft and u th he zhe dating of zircons along a west east transect elucidates the thermal evolution of exhumed and uplifted rocks the stability of fission tracks in zircons is a function of single grain radiation damage in samples with grain to grain variability in radiation damage resetting results in variable resetting and multiple age populations low retentive zircons lrzs which have a partly disordered crystalline structure have significant radiation damage and a low temperature of annealing ca 180 200 c high retentive zircons hrzs which are nearly cry

germanium enrichment in supergene settings evidence from the cristal nonsulfide zn prospect bongara district northern peru supergene nonsulfide ores form from the weathering of sulfide mineralization given the geochemical affinity of ge to si4 and fe3 weathering of ge bearing sulfides could potentially lead to ge enrichments in silicate and fe oxy hydroxide minerals although bulk rock ge concentrations in supergene nonsulfide deposits are rarely reported here we present the results of an investigation into ge concentrations and deportment in the cristal supergene zn nonsulfide prospect bongara northern peru which formed from the weathering of a preexisting mississippi valley type mvt sulfide deposit material examined in this study originates from drillcore recovered from oxidized zn rich bodies 15 20 m thick containing 5 45 wt zn and ge concentrations 100 ppm microanalysis and laser ablation icp ms show that precursor sphalerite is rich in both fe mean fe 8 19 wt and ge mean ge 142 ppm using the mineral geothermometer ggimfis geothermometer for ga ge in mn and fe in sphalerite proposed by frenzel et al ore geol rev 76 52 78 2016 sphalerite trace element data from the cristal prospect suggest a possible formation temperature tggimfis of 225 50 c anomalously high for a mvt deposit germanium concentrations measured in both goethite mean values 100 to 229 ppm max 511 ppm and hemimorphite mean values 39 to 137 ppm max 258 ppm are similar to concentrations measured in hypogene sphalerite additionally the ge concentrations recorded in bulk rock analyses of sphalerite bearing and oxidized samples are also similar a persistent warm humid climate is interpreted for the region resulting in the development of an oxidation zone favoring the formation of abundant zn hydrosilicates and fe hydroxides both able to incorporate ge in their crystal structure in this scenario ge has been prevented from dispersion during the weathering of the ge bearing sulfide bodies and remains in the resultant nonsulfide ore

undulatory silver rich polymetallic veins of the castrovirreyna district central peru fault growth and mineralization in a perturbed local stress field strongly undulatory silver veins of the castrovirreyna district central peru formed in a dilatant anastamosing low displacement left lateral fault system during late miocene east west directed compression the steeply dipping veins have multiple orders of undulations as seen in map view and underground mines expose strong dip reversals kinematic indicators from the east northeast to west northwest striking veins record mainly left lateral slip and lesser oblique reverse slip these striation data considered along with vein intersection angles and map patterns indicate that the veins did not form as a simple conjugate fracture set the preferred interpretation is that the curved fracture growth and complex slip distributions developed because of local stress field modifications by the propagating faults and dilating vein segments during regional east west directed compression variably oriented fault striations compound displacement profiles and multiple scales of vein curvature are all characteristics of stress interaction between subparallel veins veins possessing these features may be expected to host oreshoots with a complex geometry that depends not only on local accommodation space produced at dilant jogs but also on the proximity to other veins and changing slip directions

tracing tectonic deformation using the sedimentary record an overview tectonic activity on a range of scales is a fundamental control on sedimentary activity the range of structural deformation within a region extends from the plate tectonic scale governing for example rift initiation to the basin scale with the formation of basin bounding faults internal basin configuration is also strongly influenced by tectonic activity however the relationship between tectonic activity and sedimentation is a complex one given the many additional factors which can also influence sedimentary activity including erosion sediment transport source area lithology groundwater chemistry range of depositional environments climate eustasy and the relative location of an area and its distality to marine influences in this paper we provide a selective overview of the issues associated with the interlinked themes of tectonics and sedimentation examining the main basin types forming in both extensional and compressional plate settings we then review the various models of sedimentation in the selected basins both on a local and a basinal scale finally we look to the future providing a series of possible research areas almost exclusively multidisciplinary which would help to improve existing models of interlinked sediment tectonics systems

fission track dating of granitic rocks from the eastern cordillera of peru evidence for late jurassic and cenozoic cooling apatite and zircon fission track ages of three samples of the huachon granite and one sample of the quiparacra granite from the eastern cordillera of the andes of central peru have been determined zircons from the huachon granite yielded two groups of ages an older group made up of zircons with the lowest uranium content gave an age of c 270 ma the zircons with high uranium content from this granite gave ages of c 160 ma the zircon ages from the huachon granite show no correlation with elevation the age of 270 ma is interpreted as the age of probable emplacement the 160 ma age is evidence of a mild cooling 200 c event that ended in late mid jurassic time this cooling correlates in central peru with thick conglomerates in the sub andes and strong exhumation in the eastern cordillera during bathonian to malm times zircons from the quiparaca granite which is a component of a composite batholith of possible precambrian to tertiary age show ages of c 30 ma which are interpreted as the emplacement age these zircons were totally annealed prior to the cooling in the oligocene this required temperatures 200 c apatite fission track dating of the huachon granite yielded only neogene ages which reflect the total annealing of the tracks present in the apatite before exhumation began c 22 ma apatite ages show a correlation with elevation and indicate two periods of cooling during the neogene at c 21 ma and between 12 ma and the present these young ages suggest approximately 4 to 6 km of erosion in the eastern cordillera of central peru in the past 30 ma these amounts are consistent with known geological data and fission track ages obtained from other parts of the eastern andes

detecting earliest shortening and deformation advance in thrust belt hinterlands example from the colombian andes low temperature thermochronometry and crosscutting relationships identified in newly released reflection seismic data reveal a previously unrecognized zone of early andean shortening in colombia apatite fission track data and thermal modeling help define a 60 50 ma onset of rapid exhumation along the present boundary between the magdalena valley hinterland basin and eastern cordillera thrust belt subsurface angular unconformities localized above fold thrust structures indicate paleogene deposition in a wedge top depozone containing doubly vergent reverse faults retrodeformation of a cross section based on interpreted seismic profiles and thermochronometric data indicates paleocene to early eocene shortening and exhumation occurred through simultaneous activation of east and west directed reverse faults across a broad orogenic front subsequent deformation focused along west directed inversion structures these relationships reveal that deformation operated in a disparate manner rather than following a systematic progression from hinterland to foreland the northern andes also exemplify the potential effects of hinterland sediment loading and fault strength on deformation advance in contractional orogens

life span and fate of basins the life span of each main class of sedimentary basin is estimated from published data life spans vary over at least three orders of magnitude from 100 m y for passive margin and intracratonic basins the life span estimates are used to calibrate a chart of basin groups that focuses on the likely basin fates i e depositional deformational or thermal consequent fates predetermined by the tectonic setting of a particular basin class are distinguished from contingent fates which are independent of basin type accretion of trench basin fill is for example a consequent fate whereas inversion far field shortening of a rift basin is a contingent fate life span data are also used to calibrate the wilson cycle and indicate that it has an average duration of 260 m y this is certainly an underestimate because basin life spans are an imperfect proxy for the duration of subduction and collision zones

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

fracture characterization in upper permian carbonates in spitsbergen a workflow from digital outcrop to geo model abstract carbonates represent major hydrocarbon reservoirs but often exhibit highly heterogeneous reservoir properties outcrop analogues provide important insights into how parameters such as porosity permeability and natural fractures vary as such outcrops can bridge the scale gap between spatially extensive but poor resolution seismic data and 1d high resolution well data however traditional geological fieldwork typically gathers insufficient data to construct robust geological models in this study we have specifically set out to gather key data sets that enable the construction of a geology driven model we illustrate this workflow using the exceptionally well exposed carbonate dominated outcrops of the kapp starostin formation in central spitsbergen arctic norway we fully utilize emerging technologies notably geo referenced digital outcrop models doms to be able to gather quantitative sedimentological structural data from otherwise inaccessible cliffs doms generated from digital photos are used directly for automatic and manual mapping of fractures the digital data are complemented with traditional fieldwork sedimentological logging scanlines structural characterization in order to strengthen the dataset the geo modelling involves traditional facies and petrophysical modelling of the 12 identified facies along with outcrop based discrete fracture modelling finally the static geo model is upscaled and its applications are discussed the presented workflow uses carbonate outcrops of the kapp starostin formation as input but is highly applicable for other studies where outcrops can be utilized as direct input to constrain a geological model

mechanism of formation of hydrocarbon basins the west siberia volga urals timan pechora basins and the permian basin of texas abstract many authors suggest that a large role in the formation of hydrocarbon basins is played by stretching or thrust loading our previous analysis artyushkov and baer 1983 1984a b 1986 has revealed a large number of deep basins on continental crust in fold belts that were formed by rapid subsidence without significant stretching or thrust loading this paper shows that the hydrocarbon basins of west siberia volga urals timan pechora and the permian basin in texas pertain to the same basin type water loaded basins 0 3 0 5 km deep were produced in these regions by rapid subsidence in 1 ma shallow water and deeper water strata cover the same area in the basins which precludes significant stretching during the subsidence there was no folding in the adjacent regions ai the epochs of the subsidence which precludes thrust loading a short duration of the subsidence precludes a significant role of thermal relaxation rapid subsidence was commonly preceded by slight crustal uplift in the volga urals and timan pechora it was also preceded by slight volcanism we suggest destruction of the lower crust under asthenospheric upwelling to the base of the crust as a cause of rapid subsidence the asthenospheric upwelling strongly increases the heat flow which ensures hydrocarbon maturation at shallow depth activization of local tectonic movements from crustal heating occurs after rapid subsidence which produces numerous traps for hydrocarbons deposition of highly organic rocks good source rocks and the formation of reefs and or clinoforms good reservoirs commonly occur after rapid subsidence deposition of shales and evaporites good seals takes place in deeper water basins formed by rapid subsidence diapirism of the latter rocks produces additional hydrocarbon traps the combination of all the above effects associated with rapid subsidence produces very favourable conditions for the generation and preservation of hydrocarbons almost all large hydrocarbon fields in the basins considered are located in regions of rapid subsidence there are no large fields in the adjacent sedimentary basins formed by slow subsidence although the sediment thickness is commonly high

genesis of the mississippi valley type zn pb deposit of san vicente central peru geologic and isotopic sr o c s pb evidence the san vicente zn pb ore deposit is situated 300 km east of lima in central peru within the upper triassic lower jurassic carbonate platform pucara group at the western margin of the brazilian shield production during the last 20 years and present reserves exceed 12 million tons of ore assaying about 12 percent zn and 1 percent pb sphalerite and galena the only ore minerals occur as lens shaped bodies generally parallel to the bedding within the 1 400 m thick pucara sequence three ore bearing dolomite units occur the ore lenses are bound to dolomitized tidal flat and lagoon facies with cryptalgal lamination and evaporite molds and to adjacent oolithic grainstones of barrier facies strontium carbon oxygen and sulfur isotope geochemistry was carried out on consecutive crystallization generations the results obtained display systematic trends the 87 sr 86 sr ratios range between 0 7077 and 0 7084 the first generations display values very similar to those of upper triassic lower jurassic ocean water later generations are slightly enriched in radiogenic strontium the degree of enrichment in radiogenic strontium is low compared to other mississippi valley type deposits the delta 18 o values range between 6 and 10 per mil pdb and the delta 13 c values between 2 and 1 per mil pdb the late crystallization generations are enriched in the light isotopes of oxygen and carbon compared to the first generations the sulfur isotope ratios of sphalerite are relatively homogeneous ranging between 9 9 and 13 0 per mil a trend to lighter sulfur isotope ratios with advancing diagenetic stage can also be recognized the san vicente lead zinc deposit formed during burial diagenesis the temperatures indicated by sulfur isotope geothermometry 75 degrees 92 degrees c would be consistent with temperatures reached at a burial depth of about 2 to 3 km this might have been reached by the end of the jurassic a model based on abiogenic reduction of sulfates at or near the ore site and the introduction of a zinc and lead bearing basinal brine characterized by strontium isotope ratios slightly higher than contemporaneous seawater and highly radiogenic lead is favored the association of the orebodies with peritidal facies with abundant sulfate molds is important evidence favoring the existence of two separate reservoirs for sulfur and metals leaching of detrital material derived from precambrian upper crust from the brazilian shield would explain both strontium and lead isotope ratios

stable isotope geochemistry of carbonate minerals in supergene oxidation zones of zn pb deposits abstract the 18 o 16 o and 13 c 12 c ratios of carbonate minerals formed by supergene oxidation of zn pb deposits and submarine alteration of ancient slags were studied in order to constrain isotope fractionation factors for smithsonite cerussite and phosgenite and to characterize conditions of nonsulfide ore formation we present new isotope data of carbonate hosted ores from the iglesiente district sardinia italy and vila ruiva portugal as well as from the silicate hosted ores of the broken hill district new south wales australia and freihung germany and review previously published isotope data the temperature dependence of oxygen isotope fractionation between pb and zn carbonate minerals and water below 240 c for cerussite and below 100 c for smithsonite and phosgenite can be expressed as 1000ln cerussite water 2 29 10 6 t 2 3 56 1000ln smithsonite water 3 10 10 6 t 2 3 50 1000ln phosgenite water 2 55 10 6 t 2 3 50 with t in kelvin the carbon isotope fractionation between smithsonite phosgenite hydrozincite and calcite is less than about 2 while cerussites are strongly depleted in 13 c by about 10 as compared to the former minerals oxygen isotope variations of individual carbonate minerals within a deposit are relatively small indicating constant formation temperatures and a single meteoric fluid source average formation temperatures of the studied deposits are calculated at 20 5 c using the estimated isotope compositions of local paleometeoric waters we observe a linear relationship between the isotope values of estimated paleometeoric waters and supergene carbonates thus base metal carbonate minerals from supergene deposits may provide paleoclimatic information the carbon isotope values are in most carbonate hosted deposits highly variable more than 10 suggesting at least two isotopically distinct carbon sources the isotopically light component can be related to oxidation of c3 plants soil derived carbon and or microbes that take active part in sulfide oxidation while the isotopically heavy component originates from carbonate carbon from wall rocks and or atmospheric carbon the 13 c enriched component is less dominant in silicate hosted deposits

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

normalized measures of exploration performance in terms of basin sediment volumes the normalized measures of exploration performance defined in this paper are the exploratory drilling density in feet per cubic mile of sediments and the hydrocarbon discovery density in boe per cubic mile these density measures provide a convenient means for summarizing the status of exploration in the sedimentary provinces of the lower 48 states of the united states revealing that the smaller hydrocarbon rich provinces have been explored much more intensely than the larger ones the most important application of the drilling and hydrocarbon discovery densities is in defining normalized discovery functions that is the relationships between the normalized measures of cumulative exploratory drilling and discoveries normalized rather than absolute discovery functions are necessary for proper comparison of the results of exploration in regions or depth zones of differing sizes the normalized drilling and discovery measures are also useful for developing meaningful statistics of exploration performance

the calamines and the others the great family of supergene nonsulfide zinc ores abstract nonsulfides is a term which comprises a series of oxidized zn pb ore minerals it has also been used to define a special deposit type mainly considered as derived from the weathering of zn pb sulfide concentrations however nonsulfide zinc deposits have been distinguished between supergene and hypogene according to their mineralogy geological characteristics and genetic setting the supergene deposits formed by weathering and oxidation at ambient temperatures whereas the hypogene ones are considered hydrothermal or associated with metamorphic processes on primary sulfide ores in this review paper a comparison between a number of several nonsulfide deposits has been carried out typical calamines peculiar calamines and others the whole group comprises deposits of typical supergene origin mixed supergene hypogene mineralizations and oxidized concentrations characterized by different metals only locally associated with zinc the zn pb nonsulfide concentrations hosted in carbonate rocks which are mainly attributed to wall rock replacement and direct replacement supergene processes are the typical calamines liege district belgium iglesias district italy silesia cracow district poland peculiar calamine deposits are those mineralizations that have been generally considered as supergene but which are instead genetically related at least partly to hypogene processes e g angouran iran jabali yemen though mineralogically and texturally similar to supergene nonsulfide deposits the others are prevailingly supergene nonsulfide zinc deposits not hosted in carbonate rocks skorpion namibia yanque peru or characterized by other metals as main commodities like lead magellan australia silver sierra mojada mexico wonawinta australia or vanadium otavi mountainland namibia minerals of current economic importance in most calamine deposits are smithsonite hydrozincite and cerussite this mineralogical association is generally simple but when the calamines are dolomite hosted one of the consequences of the wall rock replacement process is the generation of a series of economically useless zn and mg bearing mixed carbonate phases secondary deposits hosted in silicatic sedimentary or volcanic rocks mainly contain hemimorphite and or sauconite lead ag and v rich nonsulfide ores are characterized by a more complex mineralogical association mixed pb carbonates pb sulfates pb phosphates pb arsenates various ag sulfosalts and zn pb cu vanadates carbon and oxygen stable isotope studies allow distinguishing between supergene and hypogene nonsulfide deposits evaluating the effects of oxidative heating and even gaining indirect paleoclimatic information the oxygen isotope variation of the individual carbonate minerals within a deposit is relatively small indicating constant formation temperatures and a single meteoric fluid source carbon isotope values are highly variable thus suggesting several isotopically distinct carbon sources periods of paleoclimatic switch overs from seasonally humid arid to hyperarid have been considered as the most favorable conditions for the formation and preservation of supergene nonsulfide deposits however while several recent nonsulfide deposits throughout the world are positioned between 15 and 45 n latitude thus pointing to a warm and humid weathering climate others have been deposited in sub arctic regions the economic value of the nonsulfide zn pb ag v ores is highly variable because more than in the case of metallic sulfide deposits it resides not only on the geological setting but also on their mineralogy that can directly influence processing and metallurgy

withdrawn 3d crustal architecture of a magma poor passive margin santos campos and espirito santo basins comparisons with a volcanic passive margin pelotas basin offshore brazil

late permian middle jurassic lithospheric thinning in peru and bolivia and its bearing on andean age tectonics abstract integrated studies and revisions of sedimentary basins and associated magmatism in peru and bolivia 8 22 s show that this part of western gondwana underwent rifting during the late permian middle jurassic interval rifting started in central peru in the late permian and propagated southwards into bolivia until the liassic dogger along an axis that coincides with the present eastern cordillera southwest of this region lithospheric thinning developed in the early jurassic and culminated in the middle jurassic producing considerable subsidence in the arequipa basin of southern peru this 110 ma long interval of lithospheric thinning ended 160 ma with the onset of malm earliest cretaceous partial rift inversion in the eastern cordillera area the lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and or transpressional deformations in particular the altiplano plateau corresponds to a paleotectonic domain of normal lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere the high eastern cordillera of peru and bolivia results from late oligocene neogene intense inversion of the easternmost thinned area

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

from alpine type sulfides to nonsulfides in the gorno zn project bergamo italy recent exploration of the gorno zn pb ag deposit in northern italy identified 3 3 mt of sulfides at 4 9 zn 1 3 pb and 27 2 g t ag indicated inferred resources and a further mineralized nucleus of mixed sulfides nonsulfides in the val vedra area currently under evaluation the ores are hosted in triassic limestone and shale sulfides sphalerite ag bearing galena minor pyrite and chalcopyrite paragenetically follow mn fe bearing saddle dolomite and sparry calcite the mineral association and the carbon and oxygen isotope ratios of the sparry calcite avg 13c 1 0 0 6 v pdb avg 18o 19 63 1 25 v smow are in agreement with precipitation from hydrothermal fluids in a deep burial setting sulfide emplacement occurred before the alpine orogeny likely during the early middle jurassic in analogy to other alpine type deposits the nonsulfide ore formed at the expense of sulfides and mainly consists of smithsonite hydrozincite hemimorphite and cerussite the c o isotope values of the early generations of zn carbonates are characterized by 18o between 24 1 and 26 8 v smow and 13c ratios between 3 1 and 1 7 v pdb the later generations have lower 18o 21 9 to 23 9 and lower 13c 6 2 to 3 9 these compositions as those measured on cerussite 13c 6 3 and 7 7 18o 14 0 and 15 3 agree with the formation of the nonsulfides in a supergene environment under climatic conditions warmer than today the 18o decrease from early to late generations suggests progressive involvement of meteoric water sourced from higher altitudes these characteristics indicate that the nonsulfides formed during the exhumation of the gorno area from miocene to pliocene

erosion rates and mechanisms of knickzone retreat inferred from 10be measured across strong climate gradients on the northern and central andes western escarpment escarpment edge deep gorges and distinct knickzones in river profiles characterize the landscape on the western escarpment of the andes between 5 s and 18 s northern peru to northern chile th

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

major controlling factors on hydrocarbon generation and leakage in south atlantic conjugate margins a comparative study of colorado orange campos and lower congo basins abstract we present a supra regional comparative study of the major internal and external factors controlling source rock sr maturation and hydrocarbon hc generation and leakage in two pairs of conjugate margins across the south atlantic the brazil campos basin angola lower congo basin margins located in the central segment and the argentina colorado basin south africa orange basin in the southern segment our approach is based on the analysis and integration of borehole data 1d numerical modeling 2d seismic reflection data and published reports coupling of modeling results sedimentation rate calculation and seal bypass system analysis reveal that 1 oil window is reached by syn rift srs in the southern segment during the early to late cretaceous when thermal subsidence is still active while in the central segment they reach it in late cretaceous neogene during a salt remobilization phase and 2 early hc generation from post rift srs in the southern segment and from all srs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates the latter seems to be associated with the andes uplift history for the western south atlantic basins campos and colorado and to a possibly climate driven response for the eastern south atlantic basins orange and lower congo additionally we observe that the effect of volcanism on sr maturation in the southern segment is very local the comparison of cretaceous mass transport deposit mtd episodes with hc peak of generation and paleo leakage indicators in the southern segment revealed the possible causal effect that hc generation and leakage have over mtd development interestingly paleogene leakage indicators which were identified in the argentina south africa conjugate margins occur contemporaneously to low sedimentation rate periods nonetheless present day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea level drops

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

structural styles and basin evolution in eromanga region eastern australia the eromanga region of eastern australia has undergone at least four major tectonic cycles the results of which are recognized in the structures seen on seismic sections the late proterozoic early phanerozoic cycle ended with the formation of the thomson foldbelt which forms basement in the region the subsequent reactivations of the fault systems within this basement and deeper lithosphere have produced the structural styles now important as exploration targets most reactivations are interpreted as resulting from crustal shortening events affecting sedimentary strata deposited in an epicontinental shallow water environment with small strike slip adjustments affecting some structures the dominant style of deformation evident from the seismic sections is that seen across high angle thrust faults in other parts of the world with the amount of overthrusting being comparatively small generally the maximum throw on faults in the devonian strata can be measured in thousands of meters whereas those in the permian triassic and jurassic cretaceous strata are a few hundred meters at most the devonian sequences of the adavale basin and associated troughs were deformed during two carboniferous events the first with crustal shortening in a north south direction and the second in an east west direction associated small strike slip movements were accommodated along a number of faults including the warrego grenfield warbreccan and canaway faults southwest of our study area in the permian triassic cooper basin other workers have interpreted a northwest southeast crustal shortening event during the late permian and another wrench induced northeast southwest crustal shortening during the triassic during the middle tertiary a crustal shortening event regionally affected the jurassic cretaceous eromanga basin sequence and older sedimentary rocks this event is seen pro inently in a deformation zone between the canaway and cunnavalla faults with the direction of crustal shortening being northeast southwest small strike slip movements are interpreted as partially decoupling events from neighboring provinces the adavale basin and cheepie shelf to the east and southeast and the cooper basin and warbreccan dome to the southwest and northwest respectively in this partial decoupling the canaway cunnavalla warrego and warbreccan faults are interpreted as playing an important role the basin modifying tectonic episodes within the lithosphere control structural style these relatively small scale events have resulted in structures that provide attractive petroleum prospects

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

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

carbon and oxygen isotope study of hydrothermal carbonates in the zinc lead deposits of the san vicente district central peru a quantitative modeling on mixing processes and co2 degassing abstract carbon and oxygen isotope studies of the host and gangue carbonates of mississippi valley type zinc lead deposits in the san vicente district hosted in the upper triassic to lower jurassic dolostones of the pucara basin central peru were used to constrain maiels of the ore formation a mixing model between an incoming hot saline slightly acidic radiogenic pb sr fluid and the native formation water explains the overall isotopic variation 13 c 11 5 to 2 5 relative to pdb and 18 o 18 0 to 24 3 relative to smow of the carbonate generations the dolomites formed during the main ore stage show a narrower range 13 c 0 1 to 1 7 and 18 o 18 7 to 23 4 which is explained by exchange between the mineralizing fluids and the host carbonates combined with changes in temperature and pressure this model of fluid rock interaction explains the pervasive alteration of the host dolomite i and precipitation of sphalerite i the open space filling hydrothermal white sparry dolomite and the coexisting sphalerite ii formed by prolonged fluid host dolomite interaction and limited co 2 degassing late void filling dolomite iii or calcite and the associated sphalerite iii formed as the consequence of co 2 degassing and concomitant ph increase of a slightly acidic ore fluid widespread brecciation is associated to co 2 outgassing consequently pressure variability plays a major role in the ore precipitation during the late hydrothermal events in san vicente the presence of native sulfur associated with extremely carbon light calcites replacing evaporitic sulfates e g 13 c 11 5 altered native organic matter and heavier hydrothermal bitumen from 27 0 to 23 0 13 c points to thermochemical reduction of sulfate and or thiosulfate the 13 c and 18 o values of the altered host dolostone and hydrothermal carbonates and the carbon isotope composition of the associated organic matter show a strong regional homogeneity these results coupled with the strong mineralogical and petrographic similarities of the different mvt occurrences perhaps reflects the fact that the mineralizing processes were similar in the whole san vicente belt suggesting the existence of a common regional mineralizing hydrothermal system with interconnected plumbing

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

climate and tectonic influences on continental erosion of tropical south america 0 13 ma is tectonic uplift or climate change the primary driving force behind continent scale erosion here we examine these two hypotheses for tropical south america by comparing a high resolution record of terrigenous sediment accumulation rates from the western tropical atlantic ocean with continental tectonic reconstructions and both low latitude and high latitude climate indices that span the past 13 m y over the full record terrigenous sedimentation rates most closely follow lowland aridity as recorded by abundance ratios of terrigenous oxide minerals an indication that basin integrated climate is an important control of erosion step like transitions in the flux and composition of terrigenous material near 8 and 4 5 ma however follow dated tectonic events which suggests progressive expansion of the drainage basin into less humid or less vegetated regions that were more prone to erosion on long time scales high latitude climate variability and sea level as recorded by 1 8 o do not closely track terrigenous sedimentation we conclude that regional climate change provides a mechanism for long term erosion of tropical south america tectonics plays a significant but secondary role in this area perhaps by shaping the drainage basin to include regions in which the climate is more favorable to erosion

global basin classification system a proposed system classifies sedimentary basins worldwide into specific as well as general categories the geologic history of each basin may be subdivided into cycles using three parameters basin forming tectonics depositional sequences and basin modifying tectonics sedimentary basins may be simple with one or two tectonic sedimentary cycles or they may be complex polyhistory basins with many different cycles and events there are eight simple cycle types in this classification which cover continental continental margin and oceanic areas the eight basic cycle types their depositional fills and tectonic modifiers have been given letter and number symbols so that the specific geologic history of each basin may be written as a formula these formulas may then e compared and similarities or differences between basins noted

testing global synchronism in peri pacific arc volcanism abstract episodicity of arc volcanism along the pacific rim is quantitatively analysed based on the record preserved by ash layers in deep sea sediments drilled by the deep sea drilling project and the ocean drilling program two major pulses of arc volcanism have been found one during middle miocene times 18 13 ma the other during pliocene quaternary times 5 0 ma these pulses are shown to relate to the tectonic stress regime of the arc lithosphere rather than to subduction rate these episodes of arc volcanism activity coincide with major tectonic events documented on circum pacific subduction zones as a consequence a global control of plate kinematics could influence both volcanic and tectonic evolution of pacific convergent margins

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

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

major neogene events of the southeastern pacific the chilean and peruvian record abstract according to the biochronostratigraphic model proposed the neogene marine deposits in chile and peru represent two independent transgressive sequences alloformations separated by a major regional hiatus the earlier sequence nseps i would stretch from the top of the early miocene up to the base of the late miocene ca 19 10 m y b p the upper sequence nseps ii would correspond in turn to the late pliocene ca 3 5 1 6 m y b p the regional hiatus separating both sequences would encompass the late miocene and the early pliocene ca 10 3 5 m y b p the planktonic foraminiferal events described suggest an age of terminal early miocene middle miocene and late pliocene respectively the paleoclimatic events recorded are three a warming event lasting from the terminal early miocene to the end of the middle miocene mmwe 18 11 5 m y b p a cooling event of the middle late pliocene lpce 2 5 0 2 m y b p and warming event spanning most of the late pliocene lpwe 3 5 2 2 m y b p restricted to the surface and intermediate sea waters of the middle and low latitudes the last paleoclimatic event not recorded as such so far is both synchronous and mutually exclusive in latitudinal terms with the second one three tectonic regional events are analyzed the quechuan 1 phase 19 1 5 m y b p the quechuan 2 phase 10 1 m y b p and the geographic rhodanian phase 3 5 0 5 m y b p in the studied area there is no late miocene marine sedimentary record to confirm the terminal tortonian or quechuan 3 tectonic phase ca 7 m y b p admitted to occur in the volcanoclastic continental sequences of northern chile and southern peru