late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

subtropical anticyclones and summer monsoons the summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans in winter the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography here the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations by prescribing in the model the heatings associated with several of the world s monsoons it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the kelvin wave response to the monsoon heating over the continent to the west a poleward flowing low level jet into a monsoon such as the great plains jet is required for sverdrup vorticity balance this jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region the low level jet into north america induced by its monsoon heating is augmented by a remote response to the asian monsoon heating the rossby wave response to the west of subtropical monsoon heating interacting with the midlatitude westerlies produces a region of adiabatic descent it is demonstrated here that a local diabatic enhancement can lead to a strengthening of the descent longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region below the descent sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through ekman transport feedbacks involving for example sea surface temperatures may further enhance the descent in these regions the conclusion is that the mediterranean type climates of regions such as california and chile may be induced remotely by the monsoon to the east hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems each involving monsoon rains a low level poleward jet a subtropical anticyclone to the east and descent and equatorward flow to the west in winter it is demonstrated how the nonlinear interaction between the strong zonal mean circulation associated with the winter hadley cell and the mountains can define many of the large scale features of the subtropical circulation the blocking effect of the longitudinal mountain chains is shown to be very important subsequent diabatic effects such as a local diabatic enhancement would appear to be essential for producing the observed amplitude of these features

weakening of northeast trade winds during the heinrich stadial 1 event recorded by dune field stabilization in tropical brazil the identification characterization and mapping of large areas of stabilized eolian features along the tropical northeastern brazilian coast enabled recognition of the existence of one of the largest quaternary dune fields 16 000 km 2 in south america this paleodune system is observed inland of the lencois maranhenses transgressive dune field 2 5 s 43 w and comprises deflation plains stabilized parabolic dunes and barchanoid chains developed under the action of northeast ne trade winds optically stimulated luminescence ages coupled with geomorphological analysis were used to constrain the time of dune field stabilization ages of stabilization of parabolic dunes and barchanoid chains throughout this paleodune system range between 19 to 14 ka showing heterogeneous dune stabilization by vegetation growth during a 5 ka time interval dune field stabilization is related to a decrease in ne trade wind strength and increase in precipitation as a consequence of the southward shift of the intertropical convergence zone during the heinrich stadial 1 event 18 15 ka which resulted in a lower eolian drift potential less sand input by alongshore transport and low sediment availability to eolian transport due to an increase in moisture to support vegetation growth and rising relative sea level

brittle failure mode plots for compressional and extensional tectonic regimes abstract equations governing different macroscopic modes of brittle rock failure extensional fracturing extensional shear fracturing compressional shear failure and reshear of existing faults can be represented on plots of differential stress vs effective vertical stress for a set of material properties such plots can be constructed for different tectonic regimes and correlated to depth for particular fluid pressure conditions allowing easy evaluation of the physical controls on brittle rock failure and ready comparison of the fields occupied by the three failure modes in different tectonic settings they emphasize the relative ease in terms of differential stress and fluid pressure levels of deforming a rock mass by brittle fracturing and faulting in extensional regimes compared with compressional aside from their relevance to general structural mechanics these generic failure plots have wide ranging application to understanding the initial development and progressive evolution of fault fracture systems both in sedimentary basins and as hosting structures for hydrothermal mineralization in different tectonic settings

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

vegetation and fire variability in the central cerrados brazil during the pleistocene holocene transition was influenced by oscillations in the sasm boundary belt abstract this study investigates historical fire regimes and arboreal cover variability in the brazilian cerrados a large neotropical savanna ecosystem during the pleistocene holocene transition and early holocene and then tests whether the observed variation is linked to south american summer monsoon sasm variability we present high resolution pollen xrf and charcoal records from lagoa feia located in central cerrados and assess how they compare with other pollen records from the surrounding region to investigate regional trends of vegetation and fire regime variability our results show that the pleistocene holocene transition was marked by a wet episode which included moist forest expansion and rising lake levels that correlates well with increased monsoon activity in a large area of central south america during the same period this wet episode chronologically coincides with the younger dryas cold episode in the northern hemisphere our data revealed moisture declines in central cerrados after 11 2 kyr bp along with centennial scale fluctuations from dry to wet conditions throughout the early holocene period until around 6 kyr bp these dry wet oscillations are associated with weakened sasm activity and repeated shifts of its belt position during this time range fire activity increased in central cerrados just after the onset of drier conditions during the early holocene and likely contributed to decreased arboreal cover at that time a trend of increasing moisture in the region was observed after 6 kyr bp our study reveals how centennial and millennial scale changes in monsoon activity influenced arboreal cover diversity and fire regimes in the central cerrados

kilometre scale structural setting of ore shoots in the frasers gold deposit macraes mine new zealand abstract the hyde macraes shear zone in southern new zealand contains the circa 10 million ounce macraes gold deposit one of the larger phanerozoic orogenic gold deposits discovered to date globally approximately 50 of this 10 million ounce resource is hosted by 5 major ore shoots up to 400 m wide and 1500 m long in the frasers area at the southern end of the mine higher grade 1 5 g t au ore shoots are located along and immediately below the hangingwall shear the principal strand of the hyde macraes shear zone at the frasers deposit they typically trend parallel to the intersection of the shear and foliation in the underlying schist commonly where the foliation dips more steeply that the overlying hangingwall shear especially thick zones of higher grade mineralised rock are located between the hangingwall shear and underlying second order splay shears whose position correlates with minor right hand bends in the strike of the overlying hangingwall shear lower grade

correlated millennial scale changes in surface hydrography and terrigenous sediment yield inferred from last glacial marine deposits off northeastern brazil abstract the stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern brazil stable oxygen isotope records of the planktonic foraminiferal speciesglobigerinoides sacculiferandglobigerinoides ruber pink reveal sub milankovitch changes in sea surface hydrography during the last 85 000 yr warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high xrf intensities of ti and fe and which suggest humid conditions in northeast brazil these tropical signals correlate with climatic oscillations recorded in greenland ice cores dansgaard oeschger cycles and in sediment cores from the north atlantic heinrich events trade winds may have caused changes in the north brazil current that altered heat and salt flux into the north atlantic thus affecting the growth and decay of the large glacial ice sheets

influence of surface processes over africa on the atlantic marine itcz and south american precipitation abstract previous studies show that the climatological precipitation over south america particularly the nordeste region is influenced by the presence of the african continent here the influence of african topography and surface wetness on the atlantic marine itcz ami and south american precipitation are investigated cross equatorial flow over the atlantic ocean introduced by north south asymmetry in surface conditions over africa shifts the ami in the direction of the flow african topography for example introduces an anomalous high over the southern atlantic ocean and a low to the north this results in a northward migration of the ami and dry conditions over the nordeste region the implications of this process on variability are then studied by analyzing the response of the ami to soil moisture anomalies over tropical africa northerly flow induced by equatorially asymmetric perturbations in soil moisture over northern tropical africa shifts the ami southward increasing the climatological pre

disentangling abrupt deglacial hydrological changes in northern south america insolation versus oceanic forcing paleoenvironmental studies and climate models demonstrate that fl uvial runoff and moisture availability in the caribbean hinterland react very sensitively to climatic variations late pleistocene and holocene climate records document pronounced dry and wet periods over tropical south america mainly caused by shifts of the intertropical convergence zone itcz however forcing mechanisms for changes in the itcz position remain controversial here we present high resolution foraminiferal ba ca and 18oseawater records from a core located within the orinoco river outfl ow documenting abrupt hydrological changes in the orinoco catchment area during the deglacial and holocene our data obtained from the surface dwelling foraminifera globigerinoides ruber pink show an abrupt increase in ba ca ratios in the early holocene starting 600 yr after the end of the younger dryas yd cold interval at ca 10 8 ka and suggesting a massive reorganization of moisture sources in northern south america in contrast the salinity dependent 18oseawater from the same samples shows a gradual decrease starting at the end of the yd the offset of our ba ca peak excludes meltwater release in conjunction with the northern andean glacier retreat well before the end of the yd as a forcing mechanism we suggest that the ba ca record documents an abrupt increase in ba rich waters of a northern andean source caused by the insolation driven shift of the itcz and or enhanced monsoon activity

coupling of millennial scale changes in sea surface temperature and precipitation off northeastern brazil with high latitude climate shifts during the last glacial period 1 high resolution records of alkenone derived sea surface temperatures and elemental ti ca ratios from a sediment core retrieved off northeastern brazil 4 s reveal short term climate variability throughout the past 63 000 a large pulses of terrigenous sediment discharge caused by increased precipitation in the brazilian hinterland coincide with heinrich events and the younger dryas period terrigenous input maxima related to heinrich events h6 h2 are characterized by rapid cooling of surface water ranging between 0 5 and 2 c this signature is consistent with a climate model experiment where a reduction of the atlantic meridional overturning circulation amoc and related north atlantic cooling causes intensification of ne trade winds and a southward movement of the intertropical convergence zone resulting in enhanced precipitation off northeastern brazil during deglaciation the surface temperature evolution at the core site predominantly followed the antarctic warming trend including a cooling prior to the younger dryas period an abrupt temperature rise preceding the onset of the bolling allerod transition agrees with model experiments suggesting a southern hemisphere origin for the abrupt resumption of the amoc during deglaciation caused by southern ocean warming and associated with northward flow anomalies of the south atlantic western boundary current

precipitation changes over the eastern bolivian andes inferred from speleothem δ18o records for the last 1400 years abstract here we present high resolution 18 o records obtained from speleothems collected in the eastern bolivian andes the stable isotope records are related to the regional to large scale atmospheric circulation over south america and allow interpreting changes in 18 o during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical andes two distinct phases with more negative 18 o values interpreted as periods of increased convective activity over the eastern andean cordillera in bolivia are observed concomitantly with periods of global climate anomalies during the last millennium such as the medieval climate anomaly mca and the little ice age lia respectively changes in the bolivian 18 o record during the lia are apparently related to a southward displacement of the intertropical convergence zone itcz which acts as a main moisture driver to intensify convection over the tropical continent during the mca however the increased convective activity observed in the bolivian record is likely the result of a different mechanism which implies moisture sourced mainly from the southern tropical atlantic this interpretation is consistent with paleoclimate records further to the north in the tropical andes that show progressively drier conditions during this time period indicating a more northerly position of the itcz the transition period between the mca and the lia shows a slight tendency toward increased 18 o values indicating weakened convective activity our results also reveal a non stationary anti phased behavior between the 18 o reconstructions from bolivia and northeastern brazil that confirms a continental scale east west teleconnection across south america during the lia

structural controls on tertiary orogenic gold mineralization during initiation of a mountain belt new zealand two types of structurally controlled hydrothermal mineralization have occurred during folding of fissile schist in southern new zealand fold related mineralization and normal fault related mineralization both types have the same mineralogy and textures and are dominated by quartz ankerite veins and silicified breccias with ankeritic alteration most mineralized zones are thin centimetre scale although host schist is commonly impregnated with ankerite up to 20 m away thick up to 5 m wide mineralized zones are generally gold bearing and contain pyrite and arsenopyrite with stibnite pods locally some of these auriferous zones have been extensively mined historically despite rugged topography and difficult access mineralization occurred during regional tectonic compression in the initial stages of development of the southern alps mountain belt at the pacific australian plate boundary in the miocene most of the gold bearing deposits occur in east to south east striking normal faults that cut across mesoscopic folds in a belt that coincides with the southern termination of a regional scale north trending antiform mineralized zones have similar structural control and relative timing to a nearby swarm of miocene lamprophyre dykes and carbonatites limited stable isotopic data c and o and trace element geochemistry suggest that there was probably no genetic link between the igneous activity and gold mineralization however these two types of fluid flow have been controlled by the same tectonically created crustal plumbing system this miocene hydrothermal activity and gold deposition demonstrates that orogenic mesothermal mineralization can occur during the inception of an orogenic belt not just in the latter stages as is commonly believed these miocene structures have been preserved in the orogen because the locus of uplift has moved northwards so the early formed gold deposits have not yet been structurally overprinted or eroded

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

structural control of low sulfidation epithermal gold mineralization in the rosario bunawan district east mindanao ridge philippines the rosario bunawan district is situated about 200 km north of davao city the capital of the mindanao island southern philippines gold is produced from the co o mine containing about 2 034 000 t of ore at 10 9 g t au and in numerous small scale operations by local miners epithermal gold mineralization in the rosario bunawan district and the co o mine is confined to narrow 0 2 4 m low sulfidation quartz chalcedony calcite veins in volcanic and volcaniclastic wall rocks three major vein orientations are distinguished 1 the nnw sse trending set with a sinistral strike slip sense of deformation philippine fault trend 2 the ene wsw trending dextral strike slip set palawan trend and associated veins in the riedel geometry and 3 the wnw ese trending conjugate set co o trend three structural stages are defined 1 extensional shear or shear veins formed in the co o the philippine fault and palawan trends during regional nw se compression and near vertical vein opening d1 2 reactivation of veins in the philippine fault veins associated with the palawan and to a lesser extent the co o trends during e w compression and near horizontal n s oriented vein opening d2 new d2 extensional shear or shear veins formed in the philippine fault and structures associated with the palawan and associated riedel trends 3 the d3 stage block faulting subsequently displaced all of the auriferous veins the auriferous rosario bunawan district is situated between two splays of the philippine fault which acted as a lateral ramp system during the oblique convergence of the philippine sea plate and the eurasian plate the oblique convergence resulted in a change from a compressional d1 to a transpressional d2 regime which was a prerequisite for the two stage vein opening and hydrothermal mineralization leading to an economic gold enrichment d1 compressional tectonics may have caused an elevated geothermal gradient in shallow crustal levels forming the heat source for the fluid plumbing system which is at variance to typical epithermal deposits formed in extensional zones d2 thrusting of a limestone nappe together with syn tectonic diorite intrusions may have further increased the geothermal gradient maintaining the fluid plumbing system the limestone nappe may at the same time have represented an aquitard forcing the hydrothermal fluids into the volcanic and volcaniclastic wall rocks which is regarded as critical for the two stage gold mineralization in the rosario bunawan district

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

a 21 000 cal years paleoclimatic record from caco lake northern brazil evidence from sedimentary and pollen analyses abstract sedimentological studies including seismic profiles mineralogy and organic geochemistry on two cores from the center and margin of caco lake maranhao state northern brazil revealed variable climatic and environmental conditions during the last 21 cal kyr bp between 21 and 17 cal kyr bp during the late glacial maximum regional climate was predominantly dry interrupted by short humid phases as reflected by a succession of very thin layers of sand and organic matter the late pleistocene climate was relatively wet as is suggested by rapid lake level rise and forest expansion the late pleistocene humid climate differed significantly from present conditions we suggest that late glacial humid conditions were the consequence of intensification of the inter tropical convergence zone or shifts of its position resulting in antarctic cold front occurrences the abrupt climatic changes during this period were marked by siderite deposition into caco lake which appears to be related to regional hydrologic changes linked to global northern hemisphere events the holocene was characterized by lower moisture availability and a distinct dry period until 7 cal kyr bp in response to south american insolation conditions

millennial scale ice rafting events and hudson strait heinrich like events during the late pliocene and pleistocene a review abstract various types of abrupt millennial scale climate variability such as dansgaard oeschger and heinrich events characterized the last glacial period over the last decade a number of studies demonstrated that such millennial scale climate variability was not limited to the last glacial but inherent to quaternary climate here we review the occurrence and origin of millennial ice rafting events in the north atlantic during the late pliocene and pleistocene last 3 4 ma with a special focus on north atlantic hudson strait hs heinrich like events we show that heinrich layers 5 4 2 and 1 in marine sediment cores from across the north atlantic all bear the organic geochemical fingerprint of the hudson area using this framework and combining previously published results detailed investigations into the organic and inorganic chemistry of ice rafted debris ird found across the north atlantic demonstrate that prior to mis 16 650 ka ird in the north atlantic did not originate from the hudson area of northern canada the signature of this early ird is distinctly different compared to that of hs heinrich layers rather ice rafting events during the late pliocene and early pleistocene predominantly emanated from the calving of the greenland and fennoscandian ice sheets and possibly minor contributions from local ice streams from the north american and british ice sheets compared to north atlantic hs heinrich events these early pleistocene ird events had a limited impact on surface water characteristics in the north atlantic north atlantic hs heinrich like events first occurred during mis 16 at the same time the dominant frequency in silicate rich ird accumulation shifted from the obliquity 41 ka to a 100 ka frequency across the north atlantic iceberg survivability or a change in iceberg trajectory likely did not control this change in ird regime these results lend further support for the existing hypothesis that an increase in size thickness of the laurentide ice sheet controls the occurrence of north atlantic hs heinrich events favoring an internal dynamic mechanism for their occurrence

geochemistry of late metamorphic hydrothermal alteration and graphitisation of host rock macraes gold mine otago schist new zealand abstract the macraes gold deposit in the otago schist new zealand formed during late metamorphic fluid flow through a lower greenschist facies shear zone mineralisation occurred near to the brittle ductile transition at about 300 c large volumes of host rock in a shear zone up to 120 m thick have been hydrothermally altered by this fluid activity most alteration is not structurally controlled apart from proximity to the shear zone ductile and brittle microshears traverse the most mineralised rocks and some structural control of fluid flow occurred as well fluid flow was slow similar to that in metamorphic rocks mm year and diffusion through interconnected fluid was a significant chemical process localised extensional hydrofractures m scale are filled with mineralised quartz most alteration of the host rocks was isochemical with respect to the lithophile elements and mineralised rocks have been variably enriched in as au sb w mo and bi but not co or cd addition of sulphur has occurred to both host rocks and mineralised rocks up to 1 wt above a background of 0 1 wt host rock sulphur is mainly pyritic and is not structurally controlled mineralised rocks have pyrite and arsenopyrite along microshears pyrite chalcopyrite sphalerite and galena have formed from sulphidation of silicates with no addition of metals graphite has been added to mineralised rocks along microshears up to 3 wt locally above a background of 0 1 wt noncarbonate carbon graphite deposition may have occurred as a result of mixing of two fluids water methane and water carbon dioxide graphitisation and sulphidation reactions released low d water which accumulated in the slow moving mineralising fluid distinction between this low d reaction water and meteoric water incursion is difficult

global monsoons in the mid holocene and oceanic feedback the response of the six major summer monsoon systems the north american monsoon the northern africa monsoon the asia monsoon the northern australasian monsoon the south america monsoon and the southern africa monsoon to mid holocene orbital forcing has been investigated using a coupled ocean atmosphere general circulation model foam with the focus on the distinct roles of the direct insolation forcing and oceanic feedback the simulation result is also found to compare well with the ncar csm the direct effects of the change in insolation produce an enhancement of the northern hemisphere monsoons and a reduction of the southern hemisphere monsoons ocean feedbacks produce a further enhancement of the northern africa monsoon and the north american monsoon however ocean feedbacks appear to weaken the asia monsoon although the overall effect direct insolation forcing plus ocean feedback remains a strengthened monsoon the impact of ocean feedbacks on the south american and southern african monsoons is relatively small and therefore these regions especially the south america experienced a reduced monsoon regime compared to present however there is a strong ocean feedback on the northern australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region a new synthesis is made for mid holocene paleoenvironmental records and is compared with the model simulations overall model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations

origin of increased terrigenous supply to the ne south american continental margin during heinrich stadial 1 and the younger dryas we investigate the redistribution of terrigenous materials in the northeastern ne south american continental margin during slowdown events of the atlantic meridional overturning circulation amoc the compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical atlantic shows an extreme rise in sedimentation rates off the parnaiba river mouth about 2 s during heinrich stadial 1 hs1 18 15 ka sediment core geob16206 1 raised offshore the parnaiba river mouth documents relatively constant 143nd 144nd values expressed as end 0 throughout the last 30 ka whereas the homogeneous end 0 data support the input of fluvial sediments by the parnaiba river from the same source area directly onshore the increases in fe ca al si and rb sr during hs1 indicate a marked intensification of fluvial erosion in the parnaiba river drainage basin in contrast the end 0 values from sediment core geob16224 1 collected off french guiana about 7 n suggest amazon sourced materials within the last 30 ka we attribute the extremely high volume of terrigenous sediments deposited offshore the parnaiba river mouth during hs1 to i an enhanced precipitation in the catchment region and ii a reduced north brazil current which are both associated with a weakened amoc

megafauna from the late pleistocene holocene deposits of the upper ribeira karst area southeast brazil abstract the carbonate caves of the upper ribeira valley southeastern sao paulo state brazil preserve an abundant fossil record of pleistocene holocene south american megafauna nevertheless in comparison with other localities of the country its paleontological knowledge still can be considered sparse and in need of further research this work presents an update taxonomic survey of the megafauna material collected since the beginning of the explorations in the area based on this it discusses paleobiogeographic and paleoecological questions the fossil megafauna of the upper ribeira karst region include eremotherium laurillardi nothrotherium maquinense lestodon armatus catonyx cuvieri ocnopus gracilis ahytherium aff aureum glyptodon clavipes g reticulatus hoplophorus cf toxodon spp stegomastodon waringi equidae hippidion cf tapirus terrestris panthera onca and smilodon populator only p onca and t terrestris still remain in the region despite the occurrence of some typical taxa of the pampean paleobiogeographical province the fossil assemblage recorded shows greater affinity to the mega mammal community of the intertropical region the ecology of species found suggests the previous occurrence of fairly different environmental settings than the existing one in the area there were more temperate climates both dry and wet until the current mesophytic forest was finally established the studied material does not show any kind of chrono correlation given the type of genesis of the cave accumulations these fossils depict different faunistic moments related to climate changes during the quaternary to better understand the information provided by these deposits efforts on dating and detailed taphonomic investigations should be conducted the studies of the quaternary fauna in the state of sao paulo fill an important gap in the intricate biogeographical history of the south american megafauna

variations of sea surface temperature wind stress and rainfall over the tropical atlantic and south america abstract empirical orthogonal functions e0fs and composite analyses are used to investigate the development of sea surface temperature sst anomaly patterns over the tropical atlantic the evolution of large scale rainfall anomaly patterns over the equatorial atlantic and south america are also investigated the eof analyses revealed that a pattern of anomalous sst and wind stress asymmetric relative to the equator is the dominant mode of interannual and longer variability over the tropical atlantic the most important findings of this study are as follows atmospheric circulation anomalies precede the development of basinwide anomalous sst patterns over the tropical atlantic anomalous sst originate off the african coast simultaneously with atmospheric circulation anomalies and expand westward afterward the time lag between wind stress relaxation strengthening and maximum sst warming cooling is about two months anomalous atmospheric circulation patterns over northern tropical atlantic are phase

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

maintenance of austral summertime upper tropospheric circulation over tropical south america the bolivian high nordeste low system using the nasa geos reanalysis data for 1980 95 the austral summer stationary eddies in the tropical subtropical southern hemisphere are examined in two wave regimes long and short wave wave 1 and waves 2 6 respectively the basic structure of the bolivian high nordeste low bh nl system is formed by a shortwave train across south america but modulated by the long wave regime the short wave train exhibits a monsoonlike vertical phase reversal in the midtroposphere and a quarter wave phase shift relative to the divergent circulation as inferred from a the spatial relationship between the streamfunction and velocity potential and b the structure of the divergent circulation the short wave train forming the bh nl system is maintained by south american local heating and remote african heating while the long wave regime is maintained by western tropical pacific heating the maintenance of the stationary waves in the two wave regimes is further illustrated by a simple diagnostic scheme that includes the velocity potential maintenance equation which links velocity potential and diabatic heating and the streamfunction budget which is the inverse laplacian transform of the vorticity equation some simple relationships between streamfunction and velocity potential for both wave regimes are established to substantiate the links between diabatic heating and streamfunction of particular interest is a sverdrup balance in the short wave regime this simplified vorticity equation explains the vertical structure of the short wave train associated with the bh nl system and its spatial relationship with the divergent circulation based upon the diagnostic analysis of its maintenance a simple forced barotropic model is adopted to simulate the bh nl system with idealized forcings which imitates the real 200 mb divergence centers over south america africa and the tropical pacific numerical simulations demonstrate that the formation of the bh nl system is affected not only by the african remote forcing but also by the tropical pacific forcing

fossil dripwater in stalagmites reveals holocene temperature and rainfall variation in amazonia abstract most proxy records used for reconstruction of holocene climate of amazonia are unable to quantitatively distinguish between the effect of temperature and rainfall amounts we present a new isotope technique applied to a 13 500 yr stalagmite archive from peruvian amazonia by analysing the coupled isotope composition of fossil dripwater trapped in stalagmite fluid inclusions and that of the calcite hosting the fluid inclusions we were able to calculate independent paleotemperatures and rainfall amounts this stalagmite record shows that holocene climate variation was controlled by orbitally forced southward migration of the inter tropical convergence zone while temperature remained constant isotope variation of rainwater reflected in fluid inclusion water 18o composition suggests a 15 30 increase in convective rainfall through the holocene a comparison of the low land peruvian fluid inclusion record with the high andean huascaran ice core record shows a constant 12 offset of 18o curves for the holocene suggesting that andean vertical temperature gradients lapse rates did not vary much over the last 9000 years during the younger dryas interval however the offset of 18o values was much higher than in the holocene this may be attributed to a relative drop in air temperatures in the highlands higher lapse rate caused by long distance teleconnections to climate perturbations in the north atlantic in a wider perspective fluid inclusion isotope analysis drastically improves paleotemperature reconstructions based on speleothem calcite 18o data because it provides the 18o value of drip water through time which is usually the most important unknown in paleotemperature equations

gold mineralization without quartz veins in a ductile brittle shear zone macraes mine otago schist new zealand greenschist facies schist which hosts the macraes mine in east otago new zealand has been pervasively altered by post metamorphic lower greenschist facies fluids over a 120 m thick section perpendicular to foliation metamorphic titanite has been replaced by rutile and epidote has been replaced by a variety of metamorphic minerals including siderite chlorite muscovite and calcite the early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding the cleavage was widely overprinted by a subparallel set of spaced mm scale microshears which are locally enriched in rutile and hydrothermal graphite strain was then concentrated into narrow m scale zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced 100 m scale microshears the highly strained rocks show a combination of mylonitic and cataclastic microstructures including crystal plastic grain size reduction and recrystallization of micas to form a new foliation muscovite has grown at the expense of albite in the mylonitic cataclasites hydrothermal alteration was accompanied by addition of pyrite arsenopyrite and gold without development of quartz veins gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite the whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold scheelite rutile pyrite and arsenopyrite this deposit displays a continuum of post metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt

late holocene intensification of colds fronts in southern brazil as indicated by dune development and provenance changes in the sao francisco do sul coastal barrier abstract holocene coastal barriers from southern brazil present great geomorphological changes during their late stages of development in this study we investigate the holocene evolution of the sao francisco do sul sfs barrier through geomorphological heavy minerals and grain size analyses constrained by optically stimulated luminescence osl dating the sfs barrier stands out among the southern brazilian barriers due to its well preserved morphology with a parabolic dune belt in the seaward portion the progradation of the sfs barrier started at least 4914 478 years ago and had a pronounced morphodynamic shift around 1891 155 years ago this shift is characterized by episodic development of parabolic dunes migrating to nnw associated with sand coarsening and a marked variation in sediment provenance represented by the input of sands derived from local coastal watersheds southward from the sfs barrier this morphodynamic provenance shift resulted from the strengthening of sse winds and associated wave systems responsible for the northward alongshore drift implying intensification of cold fronts coupled with higher precipitation since 1891 155 years ago osl dating combined with grain size heavy minerals and geomorphological analyses allowed assessing the response of coastal barriers to the impacts of centennial to millenial climate events occurred during the late holocene reconstruction of the late holocene climate variability and associated impacts on coastal sedimentation is a key issue to evaluate the sensitivity of coastal barriers to future climate changes

round hill shear zone hosted gold deposit macraes flat otago new zealand evidence of a magmatic ore fluid the 0 8 moz round hill shear zone hosted gold deposit is one of several located on or near the hyde macraes shear zone macraes flat new zealand field relationships show stockwork veins are the oldest part of the hydrothermal system followed by flat and ramp veins hanging wall shear veins formed early but have been modified by late deformation fluid inclusion microthermometric results for dominant type i inclusions show that flat and ramp veins have the same average wt percent nacl equiv values 2 2 0 6 and were precipitated from the same fluid salinity data for these veins overlap with those from hanging wall veins average 1 3 0 3 and stockwork veins 1 6 0 6 for vein margins 0 9 0 3 for vein centers although both of the latter trend toward lower overall wt percent nacl equiv values suggesting mixing between relatively dilute and more saline end members all of the above veins have similar temperatures of homogenization t h ranging between 110 and 188 c fluid inclusion trapping temperatures are considered to be 300 c equating to a 10 km depth of formation carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities stockwork and hanging wall veins thus have true salinities of 1 wt percent nacl equiv 0 17 molal and flat and ramp veins have true salinities of fluid inclusion volatiles from round hill and other macraes deposits are dominated by h 2 o 99 02 99 69 mol with lesser co 2 0 14 0 76 mol n 2 0 03 0 32 mol and ch 4 0 06 0 17 mol noticeable concentrations of hydrocarbons occur in many of the samples fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid consistent with co 2 corrected microthermometric measurements a range in fluid inclusion d h 2 o values obtained for the round hill deposit is coincident with the different vein types and also indicates mixing between two fluids one fluid is considered to be o isotope shifted meteoric water as trapped by the early stockwork veins and has the lowest d h 2 o values of 80 per mil the other fluid is represented by flat and or ramp and hanging wall veins that have d h 2 o values up to 40 per mil the macraes d h 2 o data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the macraes hydrothermal system

paleolimnological record of the pampean plains argentina as a natural archive of south american hydroclimatic variability since the lgm to the current warm period abstract laguna mar chiquita lmc 30 54 s 62 51 w is a highly variable and shallow saline lake located in the pampean plains of argentina the paleolimnological record of lmc contains information on the environmental variability that occurred in a large area of southern south america since the last glacial maximum lgm until the present as inferred from the diatom assemblages sedimentary features and geochemical proxies prolonged intervals of high and lowstands have caused variations in water salinity trophic state and sedimentary processes this is the first paleolimnological reconstruction covering the hydroclimatic variability that occurred over the last 25 000 cal yr bp in the argentinean pampean region results are in accordance with well known global climatic phases the late pleistocene record is characterized by a scenario dominated by lowstands hypersaline and oligotrophic lake conditions radiocarbon ages 25 000 19 000 cal yr bp indicate that the onset of the record is coeval with the lgm later a progressive lake water level increase was registered at 17 000 cal yr bp which can be assigned to the heinrich stadial 1 a shift toward comparatively drier conditions identified in the record between 14 700 and 13 000 cal yr bp can be chronologically related to the antarctic cold reversal the transition from the late pleistocene to the holocene 13 500 10 500 cal yr bp is recorded by highstand phases while the early holocene record is characterized by high to intermediate water levels the hydrological reconstruction corresponding to the mid holocene is characterized by alternating phases of high lowstands the onset of the late holocene record is marked by the development of dry conditions and thus lowstand phases while around 1300 cal yr bp a distinct water lake level increase is recognized this humid phase ascribed to the medieval climatic anomaly is followed by the record of lowstands between 500 and 1000 cal yr bp being coeval with the little ice age the uppermost part of the record of lmc encompasses the current warm period showing a marked shift towards high lake water level conditions the hydrological variability registered in the paleolimnological record highlights the importance of laguna mar chiquita as an outstanding paleoclimate sensor of hydroclimatic variations for a large area of south eastern south america

tropical ice core records evidence for asynchronous glaciation on milankovitch timescales over the last 28 years ice core records have been systematically recovered from ten high elevation ice fields nine of which are located in the low latitudes each core has provided new information about the regional climate and environmental change and together their records challenge existing paradigms about the earth s climate system when viewed collectively these ice core histories provide compelling evidence that the growth glaciation and decay deglaciation of large ice fields in the lower latitudes are often asynchronous both between the hemispheres and with high latitude glaciation that occurs on milankovitch timescales although stable isotopic records suggest that global scale cooling occurred during the last glacial stage lgs we contend that precipitation is the primary driver of glaciation in the low latitudes this is consistent with the time transgressive nature of precession driven changes in insolation and hence precipitation such that glaciers advance retreat in the tropics north of the equator while glaciers retreat advance in the tropics south of the equator thus the coeval inter hemispheric retreat of glaciers in the 20th century and the beginning of the 21st century is atypical copyright 2005 john wiley sons ltd

structural setting style and timing of vein hosted gold mineralization at the pogo deposit east central alaska gold bearing veins within the liese zone of the pogo deposit display a two stage evolutionary history that records temporal variation in kinematics fluid chemistry and temperature several stacked shallow northwest dipping shear veins are developed at pogo and collectively comprise the liese zone veins consist of 1 early narrow biotite bearing shear veins 2 white quartz veins with pyrite arsenopyrite bands referred to as main stage quartz veins that have sericite fe mg carbonate alteration envelopes and which exploit the early shear veins and 3 extension veins that form as steeper offshoots from the main stage veins the presence and orientation of oblique fabrics developed in the older biotite bearing shear veins are indicative of top to the south displacement under ductile to semi brittle conditions at higher temperatures in contrast the orientation of the extension veins and local sigmoidal shapes indicate a component of top to the northwest normal displacement on the main stage veins in their present orientation and brittle to semi brittle conditions of formation dolomite sericite alteration surrounding main stage veins may represent late to post mineral hydrothermal fluid exploitation of vein margins during ongoing normal displacement along vein systems all types of veining overprint 107 106 ma post metamorphic granitic dykes molybdenite in main stage quartz assemblages has returned re os ages of 104 2 1 1 ma significantly older than 96 to 91 ma 40ar 39ar ages obtained from vein alteration assemblages that may reflect thermal resetting during post mineral fault related hydrothermal activity magmatism and or retrograde cooling of the lithologic sequence unlike typical mesothermal shear vein hosted gold systems pogo is temporally and tectonically separated from metamorphic deformation events and has a comparable kinematic and geometric architecture to cretaceous plutonic gold deposits in the region we interpret the deposit to have formed during a regional cretaceous extensional event during multi stage exploitation of extensional fault surfaces by hydrothermal fluid from a cooling magmatic source

pressure fluctuations phase separation and gold precipitation during seismic fracture propagation gold mineralization at croagh patrick western ireland occurs in quartz veins associated with three synchronous oblique tensile shear systems the veins evolved by progressive thickening of isolated en echelon lensoid tensile fractures that show a power law relation between their length and thickness a change in power law exponent marks the linking of the en echelon arrays by large oblique tensile veins fluid inclusion data show that gold was deposited due to unmixing of an h 2 o co 2 nacl fluid at 320 240 c and 160 30 mpa primarily due to decreases in fluid pressure evidence for phase separation is only observed in the large linking veins and is abundant close to intersections of the vein arrays these sites are where high gold grades occur the distribution of gold is believed to reflect both spatial and temporal variations in fluid pressure fluctuation during seismic fracture propagation controlled by the vein growth mechanism and zones of maximum dilation

geological controls on refractory ore in an orogenic gold deposit macraes mine new zealand the macraes mine is hosted in an orogenic mesothermal gold deposit in metasedimentary rocks of the otago schist belt much gold occurs within altered schist with minimal silica addition and this study focuses on altered schist ore types the unmineralized host schists are chemically and mineralogically uniform in composition but include two end member rock types feldspathic schist and micaceous schist both rock types have undergone hydrothermal alteration along a shallow dipping foliation parallel shear zone but their different rheological properties have affected the style of mineralisation micaceous schist has been extensively recrystallized and hydrothermally altered during ductile deformation to form ores characterized by abundant disseminated millimetre scale pyrite cubes typically 1 2 wt s and minor silicification the earliest pyrite contained ni and or as in solid solution and no gold was imaged in these pyrites or later arsenopyrite grains the ore type is refractory and gold recovery by cyanide leaching is less than 50 with lowest recovery in rocks that have been less affected by later brittle deformation in contrast hydrothermally altered feldspathic schist is characterized by mineralised black microshears and veinlets formed during shear zone related brittle deformation microsheared ore has relatively low sulphur content 0 7 wt and muscovite has been illitised during hydrothermal alteration pyrite and arsenopyrite in microshears are fractured and deformed and contain 1 10 m blebs of gold later pyrite veinlets also contain micron to submicron scale inclusions of sphalerite chalcopyrite galena and gold 10 microns gold in microsheared ore is more readily recoverable than in the refractory ore although encapsulation of the fine gold grains inhibits cyanidation both microsheared ore and disseminated pyritic ore pass laterally into mineralised black shears which contain hydrothermal graphite and late stage cataclastic sulphides this black sheared ore releases gold readily but the gold is then adsorbed on to gangue minerals preg robbed and net cyanidation recovery can be less than 50 hence low gold recovery during cyanidation results from 1 poor liberation of gold encapsulated in microcrystalline quartz and unfractured sulphide grains and 2 preg robbing of liberated gold during cyanidation introduction of pressure oxidation of ore prior to cynidation has mitigated these issues

three dimensional critical wedges tectonics and topography in oblique collisional orogens in orogens deforming in oblique compression both the long wavelength mean topography of averaged slopes and the higher frequency topography first harmonic of valleys and ridges are related to the mechanics of deformation in three dimensions and cannot be adequately modeled in two dimensions a method of analysis of the principal stresses failure planes and topography is presented for three dimensional critical wedges in a critical coulomb orogen which is at failure throughout the components of the stress tensor are interrelated through the failure condition defined in terms of the principal stresses variation of any of the cartesian components must be accommodated by adjustment of one or more of the other components so that the orogen remains at failure this coupling of cartesian components to maintain the wedge at failure permits exploration of the behavior of a three dimensional critical orogen in which shear stresses arise outside of the plane of convergence internal adjustment of shear stresses to externally imposed stress conditions results in the formation of characteristic orogen shapes for an oblique collision zone such as the southern alps of new zealand the resultant mountain belt is one of an obliquely deforming outboard fold thrust belt with subdued relief rising abruptly into a steep backslope to the main divide which falls steeply to the indentor forming the inboard slope the outboard topographic slope is reduced in relation to that of the two dimensional orogen the rigid indentor has a fixed strike and therefore imposes a failure orientation adjacent to the indentor that differs from that of the ideal three dimensional critical wedge solution this produces several different slip orientations and minor partitioning of strain near the indentor first harmonic topography of valleys and ridges arises due to nonlinear heterogeneous behavior of both erosional and mechanical processes and is characterized by uplift controlled ridges in the outboard and incision controlled valleys in the inboard

climate variability and sea level change during the holocene insights from an inorganic multi proxy approach in the se brazilian continental shelf abstract two sediment cores collected in the shelf off rio de janeiro rj13 01b 10 8 cal kyr bp and rj13 02b 4 7 cal kyr bp were investigated in high resolution to evaluate changes in sedimentary processes and paleoclimatic and paleoceanographic variability during the holocene in the southeastern brazil information from inorganic proxies of continental input al ca and fe ca redox sensitive elements fe al v al and mn al paleoproductivity sr al cd al and ba al as well as data for grain size geophysical properties p wave velocity gamma ray density acoustic impedance magnetic susceptibility and porosity si total organic carbon and calcium carbonate were considered the data revealed three main periods of sediment accumulation i from 10 8 cal kyr bp to 7 6 cal kyr bp the sediment was coarse si content was high 27 the magnetic susceptibility was low and the presence of shell fragments all suggest a period of low continental input and the deposition site was shallow i e lower sea level ii from 7 5 cal kyr bp to 4 6 cal kyr bp the elevated ratios of al ca 0 69 0 08 fe ca 0 27 0 04 sr al 31 57 4 47 10 4 cd al 0 09 0 03 10 5 and ba al 0 70 0 16 10 4 are consistent with a period of maximum sea transgression and elevated influence of the nutrient rich south atlantic central water sacw iii from 4 5 cal kyr bp to the present events of higher fluxes of toc ni cu and zn were observed from ca 3 2 cal kyr bp to 3 4 cal kyr bp in both cores related to humid climate overall the multi proxy approach shed light on the effects of climate and oceanographic variability on sediment input and accumulation in a less studied portion of the brazilian shelf which are consistent with other shelf areas and with changes in regional climate systems like the intertropical convergence zone itcz south american monsoon systems sams and south atlantic convergence zone sacz

structural controls and evolution of gold silver and ree bearing copper cobalt ore deposits blackbird district east central idaho epigenetic origins the cu co au ag ni ree ore deposits of the blackbird district east central idaho have previously been classified as besshi type vms sedex and iocg deposits within an intact stratigraphic section new studies indicate that across the district mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems quartz rich and biotite rich veins and alteration zones and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district both early and late quartz veins contain chalcopyrite and pyrite whereas intermediate stage tourmaline biotite veins host the cobaltite barren early and late albite and late carbonate generally siderite form veins or are included in the quartz veins ree minerals principally monazite allanite and xenotime are associated with both tourmaline biotite and late quartz veins the veins are in mineralized intervals along axial planar cleavage intrafolial foliation and shears mineralized intervals are hosted by a variety of metasedimentary rocks including three phyllitic units of mesoproterozoic age and two schistose units all of these units are s tectonites in the footwall of a regional thrust fault specifically the district lies within an oblique thrust ramp containing a series of structural horses three domains in a duplex system the deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower middle greenschist facies in the lower domain an inverted metamorphic sequence early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late tectonic retrograde recrystallization and metamorphic mineral growth to the same extent as the country rocks in each domain where little subsequent deformation occurred early veins are discordant to bedding but where folding was polyphase and fabrics are penetrative mineralized zones are concordant with metamorphic compositional layering late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed 40 ar 39 ar dating of unoriented muscovite from the selvage of a late quartz vein yields a late cretaceous age of about 83 ma the time of retrograde metamorphism associated with introduction of late quartz veins textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to late cretaceous regional thrust faulting heat fluids and conduits for generation and circulation of fluids were part of the regional crustal thickening the faulting also juxtaposed metaevaporite layers in the mesoproterozoic yellowjacket formation over blackbird district host rocks we conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements cl k b na from metaevaporites to produce brines scavenging of metals co cu etc from rocks in the region and finally concentrating metals in the lower plate ramp structures although the ultimate source of the metals remains undetermined the present cu co au ag ni ree blackbird ore deposits formed during late cretaceous compressional deformation

modeling speleothem δ13c variability in a central sierra nevada cave using 14c and 87sr 86sr abstract carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate progressing downward from the soil zone overlying the cave these processes include soil respiration fluid rock interaction in the host limestone degassing of co2 and precipitation of calcite upflow from the speleothem drip site and calcite precipitation at the drip site here we develop a new approach to independently constrain the roles of water rock interaction and soil processes in controlling stalagmite 13c this approach uses the dead carbon proportion dcp estimated from coupled 14c and 230th u measurements in conjunction with sr isotope analyses on stalagmite calcite from a central sierra nevada foothills cave in california a region characterized by a highly seasonal mediterranean type climate to determine the roles of water rock interaction and soil processes in determining stalagmite 13c increases in stalagmite dcp between 16 5 and 8 8 ka are coincident with decreased 13c indicating a varying yet substantial contribution from the soil organic matter som reservoir likely due to significantly increased average age of som in the soil veneer above the cave during wet climatic intervals we use geochemical and isotope mixing models to estimate the host carbonate contribution throughout the 13c time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite the degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals modeled 13c values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite 13c values even during relatively wet intervals these results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem 13c records particularly those from caves that formed in seasonal semi arid to arid environments

evidence of rainfall variations in southern brazil from trace element ratios mg ca and sr ca in a late pleistocene stalagmite trace element ratios in the bt2 stalagmite from botuveracave southern brazil are explored as a proxy for changes in the local rainfall recharge during the last 116 ky bp mg ca and sr ca ratios measured with an electron microprobe are signif icantly positively correlated with one another throughout the entire record and vary in a way that is very consistent with variations of d 18 o in the same speleothem during the last glacial period we suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of mg and sr into calcite of the stalagmite this interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting therefore we conclude that higher lower mg ca and sr ca values are associated with lower higher levels of recharge into the karstic aquifer as such conditions lead to an increase decrease in the volume of calcite precipitated in the unsaturated zone above the cave during dry wet climate periods trace element variations point to generally dryer wetter conditions during lower high phases of summer insolation in the southern hemisphere these periods coincide with decreased increased activity of the south american summer monsoon as revealed by d 18 o stalagmite records in addition trace element variations show that rather wet conditions persisted through out most of the last glacial period from approximately 70 to 17 ky bp we suggest that during this period the glacial boundary conditions especially ice volume buildup in the northern hemisphere played an important role for monsoon rainfall inten sification in the region 2007 elsevier ltd all rights reserved

extension and gold mineralisation in the hanging walls of active convergent continental shear zones abstract orogenic gold bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries the vein systems hosted in extensional structures within compressively deformed rocks are a widespread feature of these orogens in many cases the extensional structures that host gold bearing veins have been superimposed on and locally controlled by compressional structures formed within the convergent orogen exploring these observations within the context of a three dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens our models explore the effect of convergence angle and mid crustal strength on stress states and compare them to the southern alps and taiwan the dilatation zones coincide with the highest mountains in the hanging walls of major plate boundary faults and can extend as deep as the brittle ductile transition extensional deformation is favoured in the topographic divide region of oblique orogens with mid lower crustal rheology that promotes localisation rather than diffuse deformation in the near surface topography influences the stress state to a depth approximately equal to the topographic relief bringing the rock closer to failure and rotating 1 to near vertical the distribution of gold bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens

the 4d microgravity method for waterflood surveillance part ii gravity measurements for the prudhoe bay reservoir alaska between 1994 and 2002 a series of experiments was conducted at prudhoe bay alaska aimed at the development of an effective 4d or time lapse gravity technique theoretical investigations had pointed out the potential for monitoring water injection in the 2500 m deep reservoir but it was not clear that gravity measurements of sufficient accuracy could be made in the arctic environment during the course of these experiments new techniques and instrumentation were introduced and perfected for both gravity and position measurements gravity stations are located using high precision global positioning system gps techniques without permanent monuments robust methods for meter drift control have improved noise resistance in relative gravimeter surveys absolute gravity measurements with a field portable instrument maintain absolute gravity levels among surveys a 4d gravity difference noise of 12 gal standard deviation has been established at prudhoe bay for gps controlled relative gravimeter surveys

reconstruction of regional atmospheric circulation features during the late pleistocene in subtropical brazil from oxygen isotope composition of speleothems two high resolution oxygen isotope records of speleothems from caves located in subtropical brazil provide a broad view of regional climate variations and related forcing during the late pleistocene and holocene here we present a new record precisely dated by u series extending back to 131 kyr bp from a speleothem collected in santana cave comparison with the 116 kyr bp record from botuvera cave confirms that the seasonal change in regional rainfall distribution is primarily controlled by variations in summer insolation on milankovitch timescales however significant negative anomalies of 18 o from 60 kyr bp to 10 kyr bp in the records suggest that the northern hemisphere glacial boundary conditions in particular heinrich events impacted the mean location and or convective activity of the south american summer monsoon sasm resulting in enhanced transport of depleted moisture from the amazon basin into the region in addition comparison of the two records indicates a steep north south gradient in 18 o of rainfall throughout which is interpreted as reflecting different relative contribution of extratropical and monsoonal precipitation to the two locations for some time periods more pronounced negative 18 o anomalies occur in the northern part and more positive anomalies in the southern part of the region which appear to be associated with more abundant monsoonal and extratropical rainfall respectively the last interglacial period is characterized by the highest 18 o values in the entire st8 record indicating a dominance of extratropical winter rainfall and thus the most northward position of the summer sasm for the last 131 kyr bp 2006 elsevier b v all rights reserved

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

interannual and longer term variability of upper air circulation in the northeast brazil tropical atlantic sector the ncep ncar 1958 1997 upper air data set along with surface ship observations have been analyzed for the march april extremum of the annual cycle in the tropical atlantic sector and core of the northeast brazil nordeste rainy season 1 in the long term mean the intertropical convergence zone itcz overlies a band of warmest surface waters and is accompanied by ascending motion and upper tropospheric divergent outflow directed mainly into a band of upper tropospheric convergence and subsidence over the tropical north atlantic 2 during years of extreme drought in the nordeste the band of warmest surface waters is displaced northward and concomitant with this is a northward shift of the strongest lower tropospheric convergence ascending motion and upper tropospheric divergent outflow 3 during the low warm as compared to the high cold so phase the bands of warmest surface waters most intense convection and largest lower tropospheric convergence ascending motion and upper tropospheric outflow are all shifted northward similar to the pattern characteristic of the nordeste drought years 4 long term trends are documented by a southward displacement of the bands of most intense convection largest lower tropospheric convergence ascending motion and upper tropospheric divergence and increasing cloudiness over the equatorial atlantic the study thus substantiates the upper air processes instrumental for the interannual and decadal scale variability in the surface climate of the region

isotopic evidence for late quaternary climatic change in tropical south america the tropical hydrologic cycle affects atmospheric trace gases and global climate change and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology isotopic analysis of calcite from lake junin peru provides a record of hydrologic variability that spans the last glacial interglacial transition in the southern tropics the record reveals a 6 enrichment in 18 o calcite during the late glacial followed by a gradual depletion during the holocene which can be interpreted as a decrease followed by a long term increase in effective moisture close agreement between 18 o calcite and rainy season insolation indicates that long term changes in tropical hydrology were linked to orbital variations furthermore hydrologic change was out of phase in the northern and southern tropics over this time period

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

different precipitation patterns across tropical south america during heinrich and dansgaard oeschger stadials abstract detailed knowledge about tropical south american precipitation patterns during heinrich h and dansgaard oeschger do stadials provides relevant insights into the possible evolution of amazonian hydroclimate under future climate change sediment core geob16224 1 ca 7 n raised from a site in the continental slope off french guiana in western equatorial atlantic under the influence of the amazon river discharge documents the impacts of h and do stadials on both inorganic i e fe ca record and organic i e alkenone c37 concentration and c37 c38 ratio geochemistry between 41 and 13 ka bp our results show millennial scale covariations of increased fe ca values with decreased c37 concentration and c37 c38 ratios during h and do stadials comparing our high temporal resolution data with previously published records from ca 17 n to 4 s we are able to differentiate the influence of h and do stadials upon tropical south american precipitation we find that records under the influence of the south american summer monsoon e g western amazon and the northern edge of the intertropical convergence zone itcz e g northernmost south america exhibit strong climate shifts during both h and do stadials but regions under the influence of the southern edge of the itcz e g northeastern brazil experience a weaker reaction during do stadials than during h stadials

numerical modelling of asymmetric boudinage abstract asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood this paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code flac boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow deformation of boudin trains includes heterogeneous stretching rotation of boudins and offset along the neck regions the sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging layer while kinematic vorticity number of the flow and the orientation of the boudin train with respect to the flow extensional eigenvector usually the shear zone boundary also play a role viscosity ratio and aspect ratio influence the magnitude of offset along the neck regions and the amount of rotation of boudins but not the sense of slip and rotation on the interboudin plane knowledge of the orientation of the interboudin plane itself is insufficient to use asymmetric boudins as independent shear sense indicators details of the boudin geometry such as the sense of deflection of marker horizons along the interboudin plane must be used for this purpose

on the origin of the bolivian high and related circulation features of the south american climate abstract the climatological structure in the upper tropospheric summertime circulation over south america is diagnosed using a gcm with and without south american topography a linear model and observational data emphasis is placed on understanding the origin of observed features such as the bolivian high and the accompanying nordeste low to the east results from the linear model indicate that these two features are generated in response to precipitation over the amazon basin central andes and south atlantic convergence zone with african precipitation also playing a crucial role in the formation of the nordeste low the direct mechanical and sensible heating effects of the andes are minimal acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central andes in the gcm the effects of the andes include a strengthening of the bolivian high and northward shift of the nordeste low primarily through changes in the precipitation field the position of

orbitally driven east west antiphasing of south american precipitation the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales a comparison between a speleothem record of precipitation in northeast brazil and rainfall reconstructions from the rest of tropical south america shows that a similar antiphasing operated in the same hemisphere during the holocene the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales this antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the hadley cells1 2 3 4 here we compare an oxygen isotopic record recovered from a speleothem from northeast brazil for the past 26 000 years with existing reconstructions of precipitation in tropical south america5 6 7 8 during the holocene we identify a similar but zonally oriented antiphasing of precipitation within the same hemisphere northeast brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high whereas the rest of southern tropical south america shows opposite characteristics simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation driven monsoon activity our results suggest that convective heating over tropical south america and associated adjustments in large scale subsidence over northeast brazil lead to a remote forcing of the south american monsoon which determines most of the precipitation changes in the region on orbital timescales

structural controls on gold bearing quartz mineralization in a duplex thrust system hyde macraes shear zone otago schist new zealand the hyde macraes shear zone in east otago new zealand is a low angle north to northeast dipping thrust system in which gold bearing quartz veins have been deposited mineralization accompanied deformation in the shear zone which is preferentially developed in graphitic pelitic schist the shear zone is a duplex system dominated by steep hinterland dipping thrust faults between well defined shallow dipping upper and lower bounding thrusts imbrication and stacking have occurred in the shear zone especially where intrashear schist is psammitic and relatively brittle this stacking contributes to lateral thickness changes from 2 up to 125 m mineralized veins formed mainly in dilational jogs or localized extensional zones on the relatively shallow dipping parts of the intrashear thrust faults late stage shallow south dipping extensional veins crosscut the duplex structures but are related to the same overall deformation from analysis of the shear zone structure it is possible to make predictions about the geometry of mineralized schist within unexposed parts of the shear zone this is a potentially useful tool for further prospecting and mining in hydrothermally mineralized duplex systems

simulated global scale response of the climate system to dansgaard oeschger and heinrich events by using an earth system model of intermediate complexity we have studied the global scale response of the glacial climate system during marine isotope stage mis 3 to perturbations at high northern latitudes and the tropics these perturbations include changes in inland ice volume over north america in freshwater flux into the northern north atlantic and in surface temperatures of the tropical pacific the global pattern of temperature series resulting from an experiment in which perturbations of inland ice and freshwater budget are imposed at high northern latitudes only agree with paleoclimatic reconstructions in particular a positive correlation of temperature variations near greenland and variations in all regions of the northern hemisphere and some parts of the southern tropics is found over the southern oceans a weak negative correlation appears which is strongest at a time lag of approximately 500 years further experimentation with prescribed temperature anomalies applied to the tropical pacific suggests that perturbation of tropical sea surface temperatures and hence the tropical water cycle is unlikely to have triggered dansgaard oeschger d o events however together with random freshwater anomalies prescribed at high northern latitudes tropical perturbations would be able to synchronize the occurrence of d o events via the mechanism of stochastic resonance

critical wedges in three dimensions analytical expressions from mohr coulomb constrained perturbation analysis we present simple analytical expressions for evaluating three dimensional stress fields in critical wedges coulomb failure throughout that are deforming within an oblique convergent zone we assume that the load and geometry variations in the lateral direction and the topographic slope are relatively small the stress state of a two dimensional critical wedge is perturbed by admitting a small lateral shear which is accommodated to maintain criticality by a reduction in the dominant normal compressive stress cohesionless cases are further simplified with taylor series expansions in the topographic slope the analytical expressions distinguish the behavior of strong base wedges associated with steep slopes from weak base wedges with more modest slopes and are reliable for lateral shear values as large as half the lithostatic load in the weak base case where the vertical shear is small the orientations of the principal stresses are very sensitive to the perturbation as the lateral shear increases through relatively small values the roles of the lateral and vertical coordinate axes are interchanged with a resultant transition from predominantly thrust failure to strike slip failure transition to strike slip in the strong base case is more gradual the particular value of horizontal shear for which transition occurs also depends on the difference of the minor normal stresses we produce an analytical expression predicting displacement patterns compatible with those observed in natural and analog orogens displacement partitioning is predicted for weak base wedges due to minor changes in lateral boundary conditions associated with variable orogen geometry

late holocene paleoenvironmental changes in northeast brazil recorded by organic matter in lacustrine sediments of lake boqueirao northeast brazil represents a key area in terms of holocene environmental changes in south america due to its distinct semi arid climate at near equatorial latitudes as well as to a wetter early and middle holocene paleoclimate in comparison with other south america regions which also experienced a reduced monsoon at that time we studied the lacustrine sediments of lake boqueirao which is located at 5 s latitude on the brazilian atlantic coast it is directly influenced by displacement of the intertropical convergence zone itcz a short 1 m core boqc0701 was collected at a 7 5 m water depth during a seismic survey organic matter om geochemical rock eval total organic carbon toc hydrogen index hi oxygen index oi and petrographical studies allowed us to distinguish five sedimentary intervals during the last 3 kyrs phase e 3000 to 2050 cal yr bp presents the largest toc hi and oi fluctuations in the entire core the most probable cause of these variations is a great instability in the lake level during this period phase d 2050 to 1830 cal yr bp is marked by an increase in sedimentation rate and a higher contribution from a well preserved om with an algal origin this was interpreted as a higher and more stable lake level during phase c 1830 to 1550 cal yrs bp low hi and high oi indicate the input of more oxidized degraded and detrital om reflecting a lower lake level in phase b 1550 to 1470 cal yr bp the om quality and quantity hi and toc indicate an algal contribution during phase a 1470 to 570 cal yr bp the high stability of the proxies may be due to a deep environment where om fluctuations are strongly buffered these changes in lake level can be compared with other tropical south america high resolution records a correspondence seems to exist between phases of low enso cool north atlantic ocean and boqueirao wet phases we did not encounter this same pattern during the last 1050 years during which the 1050 570 cal yrs bp interval was generally wet although probable fluctuations are buffered in our proxies while the north atlantic was warm mann et al 2009 a prolonged dryness occurred in lake boqueirao from 570 cal yrs bp until recent decades this phase corresponds to the little ice age characterized by a cooler north atlantic ocean mann et al 2009 these data indicate that the teleconnection pattern between the tropical atlantic enso and northeast brazil rainfalls has changed in the past

south american monsoon response to iceberg discharge in the north atlantic heinrich stadials significantly affected tropical precipitation through changes in the interhemispheric temperature gradient as a result of abrupt cooling in the north atlantic here we focus on changes in south american monsoon precipitation during heinrich stadials using a suite of speleothem records covering the last 85 ky b p from eastern south america we document the response of south american monsoon precipitation to episodes of extensive iceberg discharge which is distinct from the response to the cooling episodes that precede the main phase of ice rafted detritus deposition our results demonstrate that iceberg discharge in the western subtropical north atlantic led to an abrupt increase in monsoon precipitation over eastern south america our findings of an enhanced southern hemisphere monsoon coeval with the iceberg discharge into the north atlantic are consistent with the observed abrupt increase in atmospheric methane concentrations during heinrich stadials

numerical modelling of mechanical controls on coeval steep and shallow dipping auriferous quartz vein formation in a thrust zone macraes mine new zealand the hyde macraes shear zone hmsz is a regionally continuous low angle ne dipping 15 late metamorphic thrust zone in the mesozoic otago schist the shear zone which is host to large volumes of mineralised schist consists of foliated fissile schist with some massive schist pods two sets of quartz veins are found within the hmsz thrust related shallowly dipping veins that were emplaced parallel or sub parallel to the shears and swarms of steeply dipping extensional veins which cut across the metamorphic foliation the latter are restricted to the massive schist pods mutual cross cutting relationships occur between steep extensional veins and shallow dipping veins suggesting that they formed contemporaneously the co existence of these two vein types locally implies local rotation of the principal stress axes to produce extensional veins within a regional thrust setting the steep extensional veins are spatially related to lateral and oblique ramps within the hmsz three dimensional mechanical models show that these lateral or oblique ramps can produce favourable conditions for extensional vein formation when combined with a high fluid pressure and oblique convergence mechanical requirements include a reduced differential stress a positive volumetric strain and an increase in the horizontal shear stress our models show that under certain conditions it is possible for extension related structures to form during shortening because of local changes in the stress state without the need for a regional scale switch in the imposed stress field the convergence direction across the hmsz during formation of the steep extensional veins was wnw