three dimensional model and late stage warping of the plattengneis shear zone in the eastern alps abstract the plattengneis shear zone is a 250 600 m thick flat lying cretaceous eclogite facies mylonitic shear zone with north over south transport direction that is exposed over almost 1000 km2 in the koralpe region along the eastern margin of the alps although the shear zone is one of the largest in the alps its role in the eoalpine metamorphic evolution and the subsequent exhumation of the region remain enigmatic and its large scale geometry is not well understood the outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn and antiforms with wavelengths of about 10 km eclogite bodies occur above within and below the shear zone and there is no metamorphic grade change across the shear zone in the south the fold axes strike east west and plunge shallowly to the east in the north the fold axes are oriented in north south direction and form a dome shaped structure of the shear zone total shortening during this late stage warping event was of the order of 5 indirect evidence constrains this folding event to have occurred between 80 and 50 ma and the fold geometry implies that the final exhumation in the koralpe occurred somewhat later than further north interestingly the shear zone appears to strike out of the topography in the south and dip into the topography in the north so that north of the shear zone only hanging wall rocks are exposed and south of it only foot wall rocks possibilities for the geometric relationship of the plattengneis shear zone with the surrounding south dipping detachments are discussed

tectonic significance of paleobotanically estimated climate and altitude of the late eocene erosion surface colorado erosion beveled the laramide front range uplift in colorado to a surface of low relief by the end of the eocene this study uses j a wolfe s new multivariate climate analysis techniques to determine the paleoelevation of this regional surface by examining the overlying 35 ma florissant flora a multiple regression model explaining 93 3 of the variance in mean annual temperature was developed using wolfe s dataset of 31 leaf physiognomic character states for 86 modern vegetation sites these character states were scored on 29 species collected from one facies of the florissant lake beds the paleotemperature estimate of mean annual temperature 10 7 plus minus 1 5c derived from these data when combined with sea level temperature and terrestrial lapse rate implies a late eocene paleoelevation of 2 4 2 7 km pliocene uplift is thus not required to explain the present elevation of 2 5 km it is unclear when and why the southern rocky mountains achieved this elevation magmatic crustal thickening can explain the late eocene high elevation of the southern rockies but neither this mechanism nor compressive thickening explains why the great plains which are tied to the florissant elevation by the wall mountain tuff were also high this paleoelevation estimate indicates that regional surfacesmore of planation could be formed at high elevation in the eocene probably because of peculiarities of the eocene climate less

geomorphically driven late cenozoic rock uplift in the sierra nevada california geologists have long accepted that the sierra nevada california experienced significant late cenozoic tectonically induced uplift a flexural isostatic model presented here shows however that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the sierra nevada and deposition in the adjacent central valley and therefore requires less tectonic forcing than previously believed the sum of range wide erosion and the resultant isostatic rock uplift would have lowered sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production

the candelaria punta del cobre iron oxide cu au zn ag deposits chile several iron oxide rich cu au zn ag deposits define an approximately 5 km wide and at least 20 km long belt along the eastern margin of the coastal batholith near copiapo chile this belt includes the large candelaria mine and a group of middle and small sized mines in the punta del cobre district which is located about 3 km northeast of the candelaria deposit estimated geologic resources of the belt are on the order of 700 to 800 million metric tons mt at 1 0 percent cu the ore occurs in veins breccia and stringer bodies and in replacement bodies that are roughly concordant to bedding the orebodies are hosted mainly by volcanic and volcaniclastic rocks of the punta del cobre formation and in places also occur in volcaniclastic intercalations in the lower part of the overlying early cretaceous chanarcillo group most of the larger orebodies in the belt are located where northwest trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks these northwest faults and a major northeast trending ductile shear zone control portions of the ore of the candelaria deposit chalcopyrite is the only hypogene cu mineral the cu au ore is characterized by abundant magnetite and or hematite and by locally elevated concentrations of ag zn mo and light rare earth elements the ore is hosted mainly in zones with biotite potassium feldspar calcic amphibole epidote alteration at candelaria in the punta del cobre district ore in the deeper parts of the deposits is similarly associated whereas at shallow levels it occurs in zones of biotite potassium feldspar or albite chlorite calcite alteration mineralization at candelaria punta del cobre took place under relatively oxidized conditions manifested by the formation of specular hematite in parts of the district the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures the bulk of the magnetite probably formed at temperatures of about 500 to 600 c the main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of 470 to 328 c subsequent martitization of the magnetite points to a temperature decrease cooling of the hydrothermal system is also indicated by the homogenization temperatures of 236 c of saline fluid inclusions in late stage calcite oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid e g basinal brines or meteoric waters during the late stages of hydrothermal activity sulfur isotope ratios of chalcopyrite pyrite pyrrhotite and sphalerite from the bronce candelaria las pintadas santos and socavon rampa deposits range from 34scdt values of 0 7 to 3 1 per mil this narrow range of sulfur isotope ratios near 0 per mil is consistent with sulfur of magmatic origin anhydrite from the candelaria mine paragenetically overlaps with chalcopyrite fluid inclusions in this anhydrite homogenize between 340 and 470 c and it has 34scdt values between 14 5 and 17 5 per mil a sulfate sulfide value of 13 4 per mil for a sample with coexisting anhydrite and chalcopyrite is consistent with sulfide sulfate fractionation at temperatures on the order of 400 c ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearby intrusive rocks radiometric ages including new 40ar 39ar ages for hydrothermal alteration at candelaria point to a main cu au mineralization event at candelaria punta del cobre at around 115 ma the ages indicate that ore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift they further imply that the cu au zn ag deposits formed at shallow crustal levels 3 km the stable isotope data the presence of previously reported hypersaline co2 rich fluid inclusions in the main cu ore stage and saline fluid inclusions in late stage calcite the oxidized character of the first ore fluid pulses and the mineralization age coeval with nearby intrusive activity are consistent with but not unequivocally evidence of magmatic fluid contribution into the hydrothermal system

synchronous opening of the rio grande rift along its entire length at 25 10 ma supported by apatite u th he and fission track thermochronology and evaluation of possible driving mechanisms one hundred and forty seven new apatite u th he ahe ages are presented from 32 sample locations along the flanks of the rio grande rift in new mexico and colorado these data are combined with apatite fission track aft analyses of the same rocks and modeled together to create well constrained cooling histories for rio grande rift flank shoulders the data indicate rapid cooling due to extension from ca 28 to 5 ma in the sawatch range ca 28 ma to quaternary in the sangre de cristo mountains ca 25 to 5 ma in the albuquerque basin and ca 25 to 10 ma in the southern rio grande rift in southern new mexico rapid cooling of rift flanks followed the oligocene ignimbrite flare up and the northern section of the rio grande rift in colorado exhibits semicontinuous cooling since the oligocene overall however rift flank cooling along the length of the rift was out of phase with high volume magmatism and hence is inferred to have been driven mainly by exhumation due to faulting although each location preserves a unique cooling history when combined with existing ahe data from the gore range in northern colorado and the sandia mountains in new mexico together these data indicate that extension and exhumation of rift shoulders were synchronous along 850 km of the length of the rio grande rift from 25 to 10 ma these time space constraints provide an important new data set with which to develop geodynamic models for initiation and evolution of continental rifting models involving northward propagation of rifting and colorado plateau rotation are not favored as primary mechanisms driving extension instead a geodynamic model is proposed that involves upper mantle dynamics during multistage foundering and rollback of a segment of the farallon plate near the laramide hinge region that extended between the wyoming and se new mexico high velocity mantle domains the first stage of flat slab removal accompanied ca 40 20 ma volcanism in the san juan and mogollon datil ignimbrite centers which initiated asthenospheric upwelling and circulation a second stage involved a ca 30 25 ma detachment of remaining fragments of the farallon slab intensifying asthenospheric upwelling and focusing it along a n s trend beneath colorado and new mexico by 25 ma the north american lithosphere had become weakened critically along this narrow zone so that extension was accelerated resulting in the observed 25 10 ma cooling indicated by the thermochronologic data this developed a central graben with increased fault related high strain rates and resulted in maximum sediment accumulation in the rio grande rift our geodynamic model thus involves oligocene removal of parts of the farallon slab beneath the ignimbrite centers followed by a major oligocene miocene slab break that instigated the discrete n s rio grande rift through focused upper mantle convection beneath the southern rocky mountain rio grande rift region

structural analysis of dyke emplacement directions as an aid to palaeomagnetic studies an example from northern chile summary a major problem in palaeomagnetic studies of intrusive rocks lies in determining whether or not such rocks have been subjected to post emplacement tilting structural analysis of dyke emplacement directions can be used to show the current attitude of the extension direction for the dyke swarm if the original extension direction at the time of emplacement can be deduced from geological evidence this then provides a field test for post emplacement tilting of the dyke swarm and its host rocks in the example given from northern chile we were able to make a palaeomagnetic study of and structurally analyse three successively younger dyke swarms that intrude similarly younging plutons all three dyke swarms yielded extension directions close to horizontal but with markedly diverent azimuths it is argued that the similarity in the plunge of the extension directions cannot be coincidental and that the dykes and their host plutons have not suvered significant post emplacement tilting this simple technique should be widely applicable in the assessment of post emplacement tilting of dykes in palaeomagnetic studies

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

sodic calcic alteration in fe oxide cu au districts an origin via unmixing of magmatic h 2 o co 2 nacl cacl 2 kcl fluids iron oxide cu au deposits particularly those formed in deeper level plutonic environments are commonly characterized by regional scale sodic calcic alteration which typically formed pre or syn cu au mineralization the sodic calcic assemblages include albite scapolite pyroxene actinolite apatite titanite epidote and calcite the consistent presence of coexisting hypersaline aqueous and co2 rich fluids in minerals from sodic calcic alteration and associated fe oxide cu au deposits is the result of unmixing of h2o co2 nacl cacl2 kcl magmatic fluids experimental evidence indicates that the na na k ratio of fluids in equilibrium with two alkali feldspars in co3 2 bearing parent fluids would be significantly higher than in unmixed chloride bearing aqueous fluids therefore fluid unmixing caused by decreases in temperature and or pressure will result in albitization of wall rocks as is observed in most deeper level fe oxide cu au deposits this alteration style may be succeeded by k feldspathization with decreasing temperature because of the increase in equilibrium na na k in chloride bearing fluids buffered by alkali feldspars

plutonic and metasedimentary rocks from the coastal range of northern chile rbsr and upb isotopic systematics more than 50 of the surface in the coastal range of northern chile between the towns of taltal and chan aral consists of granitoid rocks age determinations by means of upb isotopic analyses on sized zircons and by means of rbsr isotopic measurements in conjuction with other geochemical studies reveal a complex pattern for the development of the plutonites though closely associated two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data in a west east transect southeast of chan aral several calc alkaline plutonites with basic to acid members intruded mainly into paleozoic metasedimentary rocks the upb data of the zircons from the plutonites yielded concordant ages ranging from lower jurassic to lower cretaceous upb ages are identical with rbsr biotite ages within the error limits and almost coincide with the kar mineral ages both the ages and the initial87sr86sr ratios decrease continuously from west to east the low initial sr isotopic ratios 0 703 0 705 and other geochemical parameters indicate i type characteristics in the west east segment northeast of chan aral mainly alkaline plutonic rocks intruded in an interval from upper permian to upper triassic the upb data of zircons are slightly discordant and indicate a small portion of inherited crustal material 1 ga old rbsr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks the initial 87sr86sr ratios of 0 710 point to an origin of these rocks by anatexis of crustal rocks s type the oldest rocks in the area of chan aral consist of thick sequences of metasediments the material for these rocks was derived from a source 2 5 ga old as indicated by the upb data of the zircons the lower intercept age of the zircons points to metamorphism of the source area at about 600 ma due to uplift in the early paleozoic the source area supplied the material for the sediments which were deposited in the devonian contact metamorphism in the triassic transformed the sediments into metasedimentary rocks as demonstrated by rbsr whole rock data this event was caused by the emplacement of the s type granitoid rocks which probably originated however from a deeper level of the crust

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

resolving time space histories of late cenozoic bedrock incision along the upper colorado river usa abstract the upper colorado river basin drains the western slope of the rocky mountains province of north america and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the late miocene here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods luminescence cosmogenic burial 40ar 39ar basalt and detrital sanidine and u pb detrital zircon to reconstruct a detailed record of bedrock incision in the upper colorado river over timescales ranging from miocene to late pleistocene late miocene ca 11 8 ma basalt flows perched 800 1700 m above the colorado river and the oldest known ancestral colorado river deposits preserved beneath basalt flows on grand mesa provide the basis for average long term 107 yr incision rates rates range from 110 160 m ma between grand junction downstream and glenwood canyon upstream with rates decreasing upvalley intermediate scale 105 106 yr incision rates are 200 320 m m y over the past 1 3 ma and these rates are faster than the average over the past 8 11 ma short term

basins and bedrock spatial variation in 10be erosion rates and increasing relief in the southern rocky mountains usa we used measurements of cosmogenic 10 be in alluvium to estimate erosion rates on a 10 3 10 4 yr time scale for small 0 01 47 km 2 unglaciated basins in northern colorado southern wyoming and adjacent western nebraska western united states basins formed in proterozoic cores of laramide ranges are eroding more slowly 23 7 mm k y 1 n 19 than adjacent basins draining weakly lithified cenozoic sedimentary rocks 75 36 mm k y 1 n 20 erosion rates show a relationship to rock resistance and for granitic rocks to basin slope but not to mean annual precipitation we estimated longer term 10 5 yr time scale erosion rates for the granitic core of the front range by measuring the concentration of 10 be and 26 al produced mainly by muon interactions at depths 1 7 10 m below the surface concentrations imply erosion rates of 9 31 mm k y 1 similar to shorter term erosion rates inferred from alluvial sediment the spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern rocky mountains increased in the late cenozoic modern relief probably dates from post middle miocene time

dynamic interactions between erosion deposition and three dimensional deformation in compressional fold belt settings 1 this study investigates whether the three dimensionality of erosion and deposition can influence three dimensional folding even when compression is purely convergent this problem is studied by employing a three dimensional mathematical model incorporating dynamic thin plate deformation coupled with fluvial and hillslope sediment transport the approach taken is to impose several fundamentally different sediment routing patterns by changing the initial regional topographic slope and the ratio of the characteristic timescales for deformation to fluvial surface processes r and to investigate how deformation responds numerical results indicate that when and or r are small i e initial topography is regionally flat and or rates of surface processes are slow compared to rate of imposed deformation folding is two dimensional and unaffected by surface processes since loads created by surface processes are imposed after deformation this situation favors development of small scale drainage networks transporting sediment from anticlines to adjacent basins when and or r are relatively large the development of a large scale transverse drainage network which can be maintained during folding is able to strongly amplify and localize folding this leads to interesting features where the largest transverse rivers coincide with the greatest structural and topographic amplitude of doubly plunging folds which is caused by erosion enhanced deformation for intermediate parameter values folding and surface processes interact in a complex manner giving rise to three dimensional en echelon structures that migrate in a transient manner this study demonstrates that under some circumstances one can anticipate major two way interaction between surface processes and three dimensional deformation

implication of dating of early tertiary volcanic rocks from the north chilean precordillera abstract since the early jurassic subduction induced volcanic activity in the north chilean andes 21 23 5 s has migrated 200 300 km eastward from the present coast jurassic magmatic arc to the western cordillera miocene recent volcanic rocks due to the paucity of chronological data however the exact age and extent of various magmatic sub belts within this area are not well constrained in the precordillera a sequence of 1500 5000 m thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc alkaline tuffs and lavas presently the age of this sequence is considered to be upper jurassic and or cretaceous from this sequence eleven biotites and seven hornblendes were analyzed and yielded 40 ar 39 ar plateau ages of between 48 and 38 ma indicating upper eocene volcanic activity in the eastern part of the precordillera in contrast volcanic activity in the western part of the precordillera has been dated at 92 2 5 ma indicating an earlier onset of volcanism and therefore sedimentation of the red beds in the late eocene the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the nazca plate and the south american continent this final phase of eocene volcanism coincided with a period of important deformation the incaic phase biotites from two ash fall tuffs separated by a distinct angular unconformity yield identical ages of 38 50 0 90 ma and 38 45 0 60 ma these dates provide the best age estimate of the incaic phase in the north chilean precordillera

postdepositional tilt of the miocene pliocene ogallala group on the western great plains evidence of late cenozoic uplift of the rocky mountains the rocky mountains and adjacent western great plains share a common history of late cenozoic stream incision both epeirogenic uplift and climate change with no associated uplift have been proposed as the cause of thissubcontinental scale erosional episode however the lack of a well defined cenozoic paleoelevation history for the region has hampered our ability to distinguish between the two a tilt analysis of the cheyenne tablelands in the western great plains of wyoming and nebraska provides us with a datum from which postdepositional changes in slope can be determined miocene to earliest pliocene gravels of the ogallala group 17 5 5 ma cap the tablelands which currently tilt down to the east at slopes as great as 10 2 however paleohydraulic analysis of the ogallala gravels indicates depositional slopes of 10 3 to 10 4 implying a postdepositional increase in tilt if a hinge point at the eastern edge of the cheyenne tablelands is assumed this tilt translates into differential uplift of 680 m 815 410 m at the western edge of the great plains flexural isostatic rebound of the tablelands due to known young erosion in surrounding basins only produces a few hundred meters of uplift therefore even if all of the recent erosion in the region can be attributed to climate change the resulting rebound is insufficient to account for the observed uplift of the tablelands thus the tilting of the cheyenne tablelands is most consistent with broad wavelength tectonic uplift centered under the rocky mountains initiated during ogallala deposition and continuing since deposition ceased

time scales of tectonic landscapes and their sediment routing systems abstract in regions undergoing active tectonics the coupling between the tectonic displacement field the overlying landscape and the redistribution of mass at the earth9s surface in the form of sediment routing systems is particularly marked and variable coupling between deformation and surface processes takes place at a range of scales from the whole orogen to individual extensional fault blocks or contractional anticlines at the large scale the attainment of a steady state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system with a time scale dependent on the vigour of the erosional system generally in the range 10 6 10 7 years the catchment fan systems associated with extensional fault blocks and basins of the western usa are valuable natural examples to study the coupling between tectonic deformation landscape and sediment routing systems even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation these response times originate from the development of uniform steady state relief during the accumulation of displacement on a normal fault c 10 6 years the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate c 10 6 years or the relaxation times of the integrated catchment fan system in response to changes in climatic and tectonic boundary conditions 10 5 10 6 years the presence of extensive bedrock or alluvial piedmonts increases response times significantly the sediment efflux of a mountain catchment is a boundary condition for far field fluvial transport but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre fluvial systems appear to act as buffers to incoming sediment supply signals with a diffusive time scale c 10 5 10 6 years dependent on the length of the system and the extent of its floodplains stream channels and proximal gravel fans the vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems landscapes are best differentiated as buffered or reactive depending on the ratio of the response time to the repeat time of the perturbation furthermore landscapes may be regarded as steady or transient depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions the response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture

local relief and the height of mount olympus a three dimensional assessment of the net volume of rock differentially eroded from below mountain tops to form valleys yields a range wide constraint on feedback between valley development and the height of mountain peaks the superelevation of mountain peaks potentially attributable to differential removal of material from below peaks in the olympic mountains washington was constrained by fitting a smoothed surface to the highest elevation points on a 30 m grid digital elevation model of the range high elevation areas separate into two primary areas one centred on mount olympus in the core of the range and the other at the eastern end of the range the largest valleys and hence areas with the greatest volume of differentially eroded material surround mount olympus in contrast the highest mean elevations concentrate in the eastern end of the range calculation of the isostatic rebound at mount olympus attributable to valley development ranges from 500 to 750 m 21 to 32 per cent of its height for a 5 to 10 km effective elastic thickness of the crust comparison of cross range trends in mean and maximum elevation reveals that this calculated rebound for mount olympus corresponds well with its superelevation above the general cross range trend in mean elevation it therefore appears that the location of the highest peak in the olympics is controlled by the deep valleys excavated in the centre of the range copyright 2000 john wiley sons ltd

valley incision and the uplift of mountain peaks simple end member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks these models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes analyses of topographic profiles across major drainages of the central sierra nevada and the tibetan plateau indicate that isostatically compensated valley incision could account for at most 5 10 of the present elevation of mountain peaks in contrast analyses of topographic profiles across the himalaya show that as much as 20 30 of the present elevation of himalayan peaks could be explained by isostatically compensated valley incision the degree to which valley incision is compensated locally however depends on the flexural rigidity of the lithosphere the size of the range and its tectonic boundary conditions these results show that while a tectonic control is required for late cenozoic surface uplift of the sierra nevada a significant portion of the elevation of himalayan peaks may reflect late cenozoic valley incision on the margin of the tibetan plateau

comment on origin of the highly elevated pyrenean peneplain by julien babault jean van den driessche and stephane bonnet sebastien castelltort and alain crave

displacement history of the atacama fault system 25 00 s 27 00 s northern chile in the cordillera de la costa of the central andes in northern chile mesozoic arc complexes are cut by a trench parallel strike slip fault system the atacama fault system brittle faulting in the atacama fault system is superposed on steeply dipping foliations in ductile shear belts between 25 s and 27 s the western part of the fault system was active in early cretaceous time as an upper amphibolite facies down to the east dip slip ductile shear zone in the eastern part of the fault system ductile deformation is of similar early cretaceous age but occurred under lower grade metamorphic conditions at the greenschist lower amphibolite facies transition the mylonites in the eastern part of the fault system were formed by sinistral strike slip displacement the dip slip and sinistral strike slip displacements are contemporary with the development of a magmatic arc and they imply that the tectonic environment in this part of the arc was transtensional the ductile deformation was partitioned spatially into a dip slip component associated with the emplacement of magmas and a sinistral strike slip component brittle fault zones in the el salado segment of the atacama fault system define large scale sidewall ripout structures subhorizontai slickenlines ripout asymmetry and s c type fabrics in fault gouge indicate that brittle deformation involved sinistral strike slip displacements the transition from ductile to brittle sinistral strike slip displacements may have occurred due to cooling in mid cretaceous time when the magmatic arc was abandoned

some metallogenic features of gold and copper deposits related to alkaline rocks and consequences for exploration porphyry copper gold skarn copper gold sediment hosted carlin style gold breccia pipe low sulphidation epithermal gold pluton related mesothermal or orogenic gold vein and volcanogenic massive sulphide deposits associated with alkaline rocks are commonly broadly similar to those hosted by their calc alkaline counterparts in contrast porphyry molybdenum gold deposits are confined to alkaline igneous centres alkaline suites are notably deficient in high sulphidation epithermal gold deposits and even in the advanced argillic lithocaps which host them this is surprising given that the required oxidised sulphur species are seemingly more abundant than in calc alkaline igneous centres highly efficient buffering of acidic fluid by metasomatised alkaline rocks may offer a viable explanation all types of intrusion related zinc deposits also appear to be poorly developed in alkaline provinces the characteristics of several gold and copper deposits associated with alkaline rocks including the giant porgera cripple creek ladolam olympic dam and phalaborwa examples are judged to diverge appreciably from their most closely related deposit types rendering them arguably unique most of these aberrant mineralisation styles may be due to variations in magmatic fluid compositional and liberation characteristics consequent upon the extreme diversity of ore related alkaline magmas most types of gold and copper deposits developed in calc alkaline provinces also constitute exploration targets in and around alkaline igneous centres although porphyry copper gold and low sulphidation epithermal gold deposits are considered to possess the greatest potential perhaps of even greater interest however is the possibility of encountering unconventional giant gold and copper deposits which lack closely analogous examples if already defined aberrant deposits are truly unique then exploration designed specifically to detect additional examples is pointless furthermore exploration for new unique deposits is difficult because their defining geological parameters are unknown alkaline rocks within or behind calc alkaline arcs at convergent plate boundaries probably offer the greatest exploration potential although anorogenic intracontinental extensional settings should not be ignored

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

paleozoic age of the walden creek group ocoee supergroup in the western blue ridge southern appalachians implications for evolution of the appalachian margin of laurentia in the foothills of the great smoky mountains in tennessee low grade metamorphic rocks of the walden creek group ocoee supergroup contain a fossil assemblage composed of trilobites bryozoans ostracodes pelmatozoans algae and agglutinated foraminifers indicating a silurian or younger age the new paleontological data contradict older ideas on the neoproterozoic age of the walden creek group the walden creek group was not deposited in a continental rift basin preceding the opening of the iapetus ocean but possibly in a pull apart basin formed in a transtensional event during the acadian orogeny there is no field evidence indicating that the walden creek group is separated from the lower units of the ocoee supergroup by a basin wide unconformity and hiatus a tentative geodynamic model is presented for the ocoee supergroup basin the model is dependent on paleontological data indicating a paleozoic age for the walden creek group and concentrates on lithologic petrographic and basin characteristics the model allows for the possibility that the entire ocoee supergroup was deposited in a paleozoic transtensional basin

geomorphic and tectonic forcing of late cenozoic warping of the colorado piedmont late cenozoic warping of the colorado piedmont involved interplay of tectonic forcing river erosion and isostatic response to erosion modeled erosional isostasy closely replicates the observed pattern of deformation but accounts for only about half its magnitude the remainder reflects tectonic rock uplift that increases southward across the piedmont likely reflecting proximity to the northward propagating rio grande rift this differential uplift triggered differential erosion concentrated on southern piedmont river systems particularly the arkansas river which led in turn to differential isostatic rock uplift focused on the arkansas drainage covariation of tectonic uplift erosion and isostatic compensation across the piedmont reflects a positive feedback between uplift induced erosion and erosion induced isostasy which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing

a tectonic redefinition of the southern rocky mountains abstract the southern rocky mountains of the western united states physiographically defined and described by n m fenneman nearly 60 yrs ago are tectonically redefined and extended they are shown to constitute the crestal range of a mammoth continental arch like feature here named the alvarado ridge its axis trends south from casper wyoming at least as far south as el paso texas beyond which the ridge begins to lose morphological identity maximum elevations along the crest of the ridge exceed 4 2 km the summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km falling to elevations of less than 400 m modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out a major sediment filled axial graben exists over the southern two thirds of the ridge but equivalent parts of it farther north were stripped of their neogene fill by erosion in the headwaters areas of the colorado and north platte rivers the maximum elevation of earlier laramide mountains in this area has been estimated to have been no more than 2 km in colorado half that of the present range related topography was nearly obliterated by erosion prior to late eocene time lateral stream planation produced a southeast sloping major late eocene erosion surface across the region that had what was probably an isostatically adjusted average maximum elevation of less than 900 m present day elevations and relief on the ridge crest are the result of steep crestal normal faulting pronounced block uplift and regional arching with extensional strain limited to an axial corridor less than 200 km wide this episode of mountain building began in middle miocene time 17 to 12 ma culminating in latest miocene and early pliocene time between 7 and 4 ma ago debris from the newly elevated range the southern rocky mountains sensu lato was shed along the full length of the neogene alvarado ridge down parts of both rises as well as into its medial graben the uplifted and tilted east rise the great plains ramp has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle miocene to earliest pliocene time its fluviatile lacustrine and eolian sediments are undergoing dissection today but a sufficient portion remains to define the unmodified profile of the east rise as well as the original length of the ridge it is coincidentally similar in configuration and scale to the west rise of the mid atlantic ridge the topography of the crestal mountain ranges on both structures is also remarkably similar the southern rocky mountains are as high as the alps but are not the product of lithospheric compression and crustal thickening rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge

high elevation of low relief surfaces in mountain belts does it equate to post orogenic surface uplift we present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level sea level after active uplift the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins

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

late cenozoic flexural deformation of the middle u s atlantic passive margin despite the century long recognition of regional epeirogeny along the middle atlantic passive margin relatively few studies have focused on understanding postrift uplift mechanisms here we demonstrate that epeirogenic uplift of the central appalachian piedmont and subsidence of the salisbury embayment represent first order flexural isostatic processes driven by continental denudation and offshore deposition our results show that regional epeirogenic processes present on all atlantic type passive margins are best resolved by specific stratigraphic and geomorphic relationships rather than topography a simple one dimensional geodynamic model constrained by well dated baltimore canyon trough coastal plain and lower susquehanna river piedmont stratigraphy simulates flexural deformation of the u s atlantic margin the model represents the passive margin lithosphere as a uniformly thick elastic plate without horizontal compressive stresses that deforms flexurally under the stress of strike averaged vertically applied line loads model results illustrate a complex interaction among margin stratigraphy and geomorphology the isostatic response to denudational and depositional processes and the modulating influence of exogenic forces such as eustasy the current elevation with respect to modern sea level of fluvial terraces and correlative coastal plain deposits or unconformities is successfully predicted through the synthesis of paleotopography eustatic change and margin flexure results suggest that the middle u s atlantic margin landward of east coast magnetic anomaly is underlain by lithosphere with an average elastic thickness of 40 km flexural rigidity d 4 1023 n m the margin experiences an average long term denudation rate of approximately 10 m m y and the piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m y long term isostatic continental uplift resulting from denudation and basin subsidence resulting from sediment loading are accommodated primarily by a convex up flexural hinge physiographically represented by the fall zone our results elucidate an inherent danger in using topography alone to constrain late stage passive margin deformation mechanisms only through careful synthesis of field stratigraphic and geomorphic elements such as fluvial terraces coastal plain deposits and offshore stratigraphy can age control be extended from the offshore depositional setting to the erosionally dominated continent this study demonstrates that despite a relatively subdued topography the middle u s atlantic margin experiences progressive flexural isostatic deformation similar to that proposed for high relief margins characterized by great escarpments thus margin topographic diversity remains a function of other factors such as lithospheric composition and or structure supracrustal stratigraphy and structure degree of drainage integration drainage divide migration and climate

volcanic dykes in the north chilean coast range dense swarms of predominantly mafic dykes outcrop in the north chilean coast range these rocks are not lamprophyres they mark channelways used by jurassic to lower cretaceous extrusives

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

low grade regional metamorphism in the mesozoic cenozoic volcanic sequences of the central andes all the mesozoic and cenozoic volcanic rocks of the central andes from southern ecuador to central chile except recent ones have been affected by episodes of regional metamorphism without change in texture and structure the metamorphism which ranges from low zeolite to greenschist facies can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities some sequences display metamorphic patterns transitional to ocean floor and to geothermal field types reflecting variations along and across the andes in tectonic setting and thermal gradients volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the central andes the episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series where strata of higher grade may even overlie those of lower grade the existence of permeability controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity rate of reaction and pfluid were acting in addition to temperature and p tot overall gradients during the regional metamorphism the alteration is accompanied by chemical changes and disturbances of the k ar and rb sr isotope systems similarities between mesozoic facies series in the western and eastern flanks of the andes are consistent with a mechanism of ensialic spreading subsidence

middle to late cenozoic cooling and high topography in the central rocky mountains constraints from clumped isotope geochemistry abstract the timing and mechanism of the formation of the high elevations of the central rocky mountains rockies in the western interior of north america remain controversial we examine the middle and late cenozoic paleorelief between the great plains and the central rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near surface conditions and retain primary paleoclimate and paleoelevation signals carbonate clumped isotope temperatures decrease 10 c and calculated water o 18 values decrease 4 from 35 ma to 5 ma in both the central rockies and the western great plains following middle and late cenozoic global cooling trend a persistent longitudinal gradient in temperature and o 18 value existed between the central rockies and the great plains during the oligocene and miocene with temperatures 4 8 c higher and calculated water o 18 values 3 6 higher in the great plains these observations suggest the mean elevation of the central rockies was 1 km higher than the western great plains during the middle and late cenozoic similar to the present day regional relief when placed in the context of other paleoaltimetry studies and geological observations our findings support the hypothesis that the high mean topography of the rockies was developed during the late eocene possibly related to isostatic adjustment or dynamic uplift caused by foundering of lower mantle lithosphere or the farallon slab or both

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

unsteady late pleistocene incision of streams bounding the colorado front range from measurements of meteoric and in situ 10be 1 dating of gravel capped strath terraces in basins adjacent to western u s laramide ranges is one approach to document the history of late cenozoic fluvial exhumation we use in situ 10be measurements to date the broad surfaces adjacent to the eastern edge of the rocky mountains in colorado and compare these calculated ages with results from meteoric 10be measurements we analyze three sites near boulder colorado gunbarrel hill table mountain and pioneer that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the plio pleistocene transition our in situ 10be results reveal abandonment ages of 95 129 ka at table mountain 175 27 ka at pioneer and ages of 251 10 ka and 307 15 ka at gunbarrel hill all are far younger than previously thought inventories of meteoric 10be support this interpretation yielding ages that are comparable to table mountain and 20 lower than pioneer in situ ages we argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate and that vertical incision rates of several mm yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions in contrast to the traditional age chronology in the area our ages suggest that the deep exhumation of the western edge the high plains occurred relatively recently and at an unsteady pace

the kinematic and geodynamic significance of the atacama fault zone northern chile abstract the atacama fault zone afz is the dominant feature in the structure of the north chilean coastal cordillera left lateral displacement took place along its system of longitudinal faults during the jurassic and early cretaceous this development was contemporaneous with arc magmatism and was later reactivated resulting in a steep normal fault strike slip movements along the afz consist of two sets of ductile shear zones of different ages one jurassic formed under amphibolite facies conditions the other early cretaceous with greenschist facies mylonites structural asymmetries point to a sinistral sense of shear in both sets the afz can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate trench linked strike slip fault the sinistral sense of shear is consistent with reconstructions of late jurassic to early cretaceous plate configurations in the se pacific

late cenozoic increase in accumulation rates of terrestrial sediment how might climate change have affected erosion rates abstract accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents although not everywhere apparently erosion has increased in elevated terrain regardless of when last tectonically active or what the present day climate in many regions sediment coarsened abruptly in late pliocene time sparser data suggest increased sedimentation rates at 15 ma approximately when oxygen isotopes in benthic foraminifera imply high latitude cooling if climate change effected accelerated erosion understanding how it did so remains the challenge some obvious candidates such as lowered sea level leading to erosion of continental shelves or increased glaciation account for increased sedimentation in some but not all areas perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until 3 4 ma when large oscillations in climate with periods of 20 000 40 000 years developed and de

tectonic and climatic significance of a late eocene low relief high level geomorphic surface colorado new paleobotanical data suggest that in the late eocene the erosion surface which capped the front range colorado was 2 2 3 3 km in elevation which is similar to the 2 5 km present elevation of surface remnants this estimated elevation casts doubt on the conventional belief that the low relief geomorphic surface was formed by lateral planation of streams to a base level not much higher than sea level and that the present deeply incised canyons must represent neogene uplift of colorado description of the surface calculations of sediment volume and isostatic balance and fluvial landsculpting models demonstrate that while the high elevation of the erosion surface was due to tectonic forces its smoothness was mostly a result of climatic factors a sediment balance calculated for the front range suggests that from 2 to 4 km of material were eroded by the late eocene consistent with fission track ages this amount of erosion would remove a significant portion of the 7 km of laramide upper crustal thickening isostatic modeling implies that the 2 2 3 3 km elevation was most likely created by lower crustal thickening during the laramide a numerical model of fluvial erosion and deposition suggests a way that a late eocene surface could have formed at this high elevation without incision a humid climate with a preponderance of small storm events will diffusively smooth topography and is a possible mechanism for formation of low relief high level surfaces paleoclimate models suggest a lack of large storm events in the late eocene because of cool sea surface temperatures in the equatorial region return to a drier but stormier climate post eocene could have caused the incision of the surface by young canyons by this interpretation regional erosion surfaces may represent regional climatic rather than tectonic conditions

impact of rock uplift on rates of late cenozoic rocky mountain river incision 1 the high relief of the modern rocky mountain landscape formed in the late cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the yellowstone hot spot and to growth of the northern portion of the rio grande rift calculated incision rates are 60 m myr and total depth of erosion of sedimentary basins is 300 m well below the long term incision rates and amounts of erosion interpreted from the geologic record broad scale tilting of the region toward the east accomplished by a gradient in rock uplift of 1 km along the north south axis of the central rockies declining to zero 1000 km to the east can account for the additional erosion needed to match observations in each modeling scenario stream incision is nonsteady with rock uplift outpacing erosion for 1 myr in perimeter basins and 1 5 myr in interior basins three factors dominate the spatial and temporal pattern of regional landscape evolution 1 the time since uplift began 2 the uplift pattern and 3 the distribution of relatively resistant bedrock within the region our results suggest that the spatial variability in late cenozoic exhumation can be explained by a long lived transience in the stream network response to these various late cenozoic geophysical events

a late cenozoic age for long wavelength surface uplift of the atlas mountains of morocco the atlas mountains have been uplifted by two mechanisms cenozoic thickening of the crust and thinning of the mantle lithosphere due to a buoyant thermal anomaly previously inferred by indirect criteria to have started some 15 ma because crustal shortening related uplift and mantle related uplift affect the topography at different spatial scales we use scattered direct surface evidence to clarify the palaeoelevation dynamics uplifted messinian shallow marine sediments in the

age of mineralization of the candelaria fe oxide cu au deposit and the origin of the chilean iron belt based on re os isotopes re os isotopes are used to determine the age of iron oxide cu au mineralization at candelaria chile and to explore possible genetic links with the batholithic intrusions in the area re os ages calculated from molybdenite are 114 2 0 6 ma and 115 2 0 6 ma and they are interpreted to represent the age of mineralization these ages are consistent with previously reported ages for biotite alteration that range from 114 to 116 ma an isochron calculated by re os ratios from hydrothermal magnetite and sulfides constrains an initial 187 os 188 os ratio of 0 36 0 10 the initial 187 os 188 os ratio for sulfide from bronce mine a small satellite of the candelaria orebody is 0 33 0 01 these values are broadly similar to the calculated initial 187 os 188 os ratio for magmatic magnetite in nearby batholithic rocks that range from 0 20 to 0 41 the relatively radiogenic initial 187 os 188 os ratio represents a mixture of mantle and crustal components in the ores and batholitic rocks the similarity in initial 187 os 188 os ratio of the ore and magmatic oxides suggest that the granitic plutons could be the source of metals in the candelaria district these data are consistent with the existence of a significant magmatic fluid component in the hydrothermal system as suggested by previously published work in order to establish a regional perspective we analyzed ore minerals from other chilean deposits of the iron oxide cu u au rare earth elements ree class magnetite from the manto verde iron oxide cu au deposit has os and re concentrations of 11 to 17 parts per trillion ppt and 4 to 6 parts per billion ppb respectively the initial 187 os 188 os ratio is approximately 0 20 and is similar to that of ore minerals from candelaria and of the early cretaceous batholithic intrusions these data indicate a similar metal source for the mineralization at manto verde and candelaria magnetite from three magnetite apatite deposits of the chilean iron belt have re concentrations between 0 8 and 3 ppb and os concentrations between 11 and 76 ppt calculated initial 187 os 188 os ratios of these magnetites range from 1 2 to 8 4 and are distinctly different from those of the iron oxide cu au deposits the cause of the comparatively high radiogenic signatures in the magnetite apatite ores is probably related to fluid interactions with i e leaching the surrounding sedimentary rocks during their genesis in contrast to the iron oxide cu au systems the os in magnetite apatite ores could be derived from sedimentary rocks this evidence suggests that predominantly basin derived nonmagmatic brines formed these magnetite apatite deposits

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

evaporitic source model for igneous related fe oxide ree cu au u mineralization we propose that many igneous related fe oxide rich ree cu au u bearing deposits form by hydrothermal processes involving evaporitic ligand sources either coeval salars or older evaporites these deposits are abundant in both phanerozoic and proterozoic extensional continental and continent margin settings they commonly form in global arid zones but they also occur where magmatism is superimposed upon older evaporites magmatic compositions exert only second order control mainly on alteration mineralogy and on element abundances hot s poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments siderophile lithophile and hydrothermal alteration sodic locally alkaline found in these systems this model contrasts with immiscible oxide melt and magmatic hydrothermal origins commonly proposed for these deposits although all three mechanisms can occur 31 refs 3 figs 1 tab

kuroko type deposits in the middle cretaceous marginal basin of central peru barite massive sulfide and siliceous stockwork deposits of kuroko type in the lima region are associated with the casma group a sequence of submarine volcanic rocks of middle cretaceous age these deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill volcanic hosted deposits occur in the entire region sediment hosted deposits are restricted to the eastern casma volcanic facies which intercalate with limestones and shales deposited on a shelf platform adjacent to the marginal basin in most cases mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz serieite alteration the latter have locally been dated at 116 to 106 m y by the k ar method strata bound deposits of bedded barite pyrite sphalerite and pyrrhotite overlie the feeder zones the most important deposits of this kind are leonila graciela and juanita with 4 million tons of produced barite and 2 5 million tons of production plus reserves of massive sulfide ore they are located in a roof pendant of folded strata intruded by two plutons of the coastal batholith contact metamorphism of hornblende hornfels facies affects both ore deposits and wall rocks k ar ages on hornblende biotite pairs from the granitic rocks indicate that they were emplaced 82 and 65 m y ago whole rock ages on postmetamorphic dikes vary between 31 and 39 m y p t conditions for contact metamorphism of hornblende hornfels facies at leonila graciela are estimated at 2 1 to 2 6 kb and 300 degrees to 500 degrees c on the basis of sphalerite geobarometry stratigraphic reconstruction metamorphic mineralogy and interpretation of discordant k ar age patterns mole percent fes in sphalerites increases in a prograde sense from the actinolite zone at juanita to the biotite muscovite zone at gracicla in massive sulfide specimens it varies correspondingly from 15 4 or 0 2 to 17 6 or 0 7 sphalerites from siliceous stockworks show the same trend with 14 7 or 0 4 mole percent fes and 17 6 or 1 1 mole percent fes metamorphic equilibration was reached only in the biotite muscovite zone at graciela this is demonstrated by the homogeneity of high mole percent fes values detected in sphalerite which coexists in mutual contact with pyrite and hexagonal pyrrhotite

abrupt landscape change post 6 ma on the central great plains usa the principal control on landscape evolution in the central great plains of the united states over the past 10 m y is a contentious subject new sedimentary data collected from late miocene ogallala group and pliocene broadwater formation of the nebraskan great plains demonstrates a twofold increase in the median grain size from 20 mm to 40 mm exported from the rocky mountains across the miocene pliocene boundary paleoslope reconstructions derived from these data support the tilting of the miocene ogallala group after 6 ma but demonstrate that the transport slope of the lower part of the unconformably overlying pliocene succession is identical to the present day slope these data allow us to constrain the timing of differential uplift in the great plains to between 6 and 3 7 m y the wavelength and short duration of this tilting are best explained by the initiation of localized dynamic topography our results also suggest a threefold to fourfold increase in specific stream power at this time meaning that pliocene rivers draining the central rockies were considerably more competent than their miocene predecessors incision during this period was not continuous a significant episode of aggradation from 3 7 to 2 5 ma is best explained by high rates of sediment supply relating to the warm wet mid pliocene climate optimum the modern pattern of incision on the great plains occurred from 2 5 ma and not from the end of the miocene as is sometimes supposed reflecting the onset of major northern hemisphere glaciation

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

magmatic arc fault systems deformation partitioning and emplacement of granitic complexes in the coastal cordillera north chilean andes 25 30 s to 27 00 s three fault systems were responsible for permian to late cretaceous deformation of the overriding plate of the andean convergent margin in the coastal cordillera of northern chile 25 30 to 27 00 s displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs the tigrillo fault system active from triassic to early cretaceous time was characterized by arc normal extension with increasing left oblique extension transtension from early jurassic to early cretaceous time stretching of the crust created space for triassic early jurassic and early cretaceous arc basins where epiclastic volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments tabular plutonic complexes were emplaced by roof uplift floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension the atacama fault system was initiated at c 132 ma as a mainly left strike slip fault during left oblique extension of the margin elongate tabular plutonic complexes were emplaced within the atacama fault system between c 132 and c 106 ma again by roof uplift floor subsidence mechanisms ductile brittle transitions in synplutonic mylonitic rocks of the atacama fault system provided the setting for kiruna type fe apatite and fe oxide with cu and or au ores the chivato fault system was active as an extensional fault system at the eastern side of the coastal cordillera during displacement on the tigrillo fault system and later between c 125 and c 93 ma as a partitioned left oblique extensional fault system in post early cretaceous time the chivato fault system was inverted by left oblique contraction transpression when nw trending transfer faults some probably reactivated lateral ramps in the tigrillo fault system accommodated clockwise vertical axis rotations of 35 45 contraction inverted the atacama fault system and tigrillo fault system and was responsible for west vergent thin skinned fold thrust deformation in stratified rocks throughout the margin

pleistocene relief production in laramide mountain ranges western united states gently sloped summits and ridges collectively referred to as summit flats are abundant in many laramide ranges in the western united states the erosion rate of summit flats is 10 m m y on the basis of the concentrations of cosmogenic radionuclides because erosion rates in valleys between summit flats are an order of magnitude faster relief within these ranges is currently increasing by about 100 m m y if summit flat erosion is slower than rock uplift driven by the isostatic response to valley erosion then this relief production could result in increased summit elevations the mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four laramide ranges based on geographic information system gis analyses of digital elevation models this erosion would result in a maximum of 250 300 m of rock uplift assuming airy isostasy however because the laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere erosionally driven rock uplift is limited to 50 100 m over the past several million years summit erosion would approximately offset this rock uplift therefore we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced on the basis of valley and summit erosion rates and the average depth of erosion we estimate that relief production in laramide ranges began at ca 3 ma we hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges

ant nest ichnofossils in honeycomb calcretes neogene ogallala formation high plains region of western kansas u s a abstract two new ant nest trace fossils are described from calcic sandy paleosols of the neogene ogallala formation in western kansas the ichnofossils are preserved within and below calcrete beds weathering in positive relief as carbonate filled casts or as cavities in negative relief daimoniobarax ichnogenus nov is established for burrow systems composed of vertically tiered horizontally oriented pancake shaped chambers connected by predominantly vertical and cylindrical shafts 0 8 cm in diameter ichnospecies of daimoniobarax are differentiated based on differences in the plan view outline of chambers shaft orientation and junctions between chambers and shafts daimoniobarax nephroides ichnospecies nov is composed of an 24 76 cm long vertical sequence of distinctly lobed chambers 2 20 cm wide and 1 cm high arranged along sinuous to helical shafts chamber shape in plan view ranges from small teardrops to larger kidney and u shaped forms shafts intersect at chamber edges such that chambers appear to bud from the central shafts daimoniobarax nephroides is most similar to the nests of extant seed harvester ants of the new world genus pogonomyrmex such ants are specialized granivores and prefer sandy soils in arid to semi arid grassland and desert regions daimoniobarax tschinkeli ichnospecies nov is 30 80 cm in vertical extent chambers 2 30 cm wide and 1 cm high are circular to elongate or pseudopodial in plan view vertical shafts are straight to slightly sinuous and intersect most often toward the center of the chambers the generalized architecture of d tschinkeli is similar to that of the nests or nest portions of several extant ant genera though it does not closely resemble any known modern nest ant ichnofossils provide valuable information on hidden biodiversity paleohydrologic regimes paleopedogenic processes and paleoclimate during the time of nest occupation depth related changes in chamber size and vertical spacing may also help interpret paleosurfaces and paleodepths and serve as geopetal features

time interval between volcanism and burial metamorphism and rate of basin subsidence in a cretaceous andean extensional setting 40ar 39ar ages were obtained from basaltic flows belonging to a 9 km thick sequence generated in an extensional ensialic setting of an arc back arc basin type during the early cretaceous and presently exposed along the coastal range of central chile the basalts have been affected by very low to low grade burial metamorphism mostly under prehnite pumpellyite facies age values obtained from primary volcanic and secondary metamorphic minerals permit to quantify the time interval between volcanism and burial metamorphism a plateau age of 119 1 2 ma from primary plagioclase represents the best estimation of the age of the volcanism whereas adularia in low variance assemblages contained in amygdules gave a plateau age of 93 1 0 3 ma which is interpreted as the age of the metamorphism considering the p t conditions estimated for this metamorphic event the c 25 ma time interval between volcanic emplacement and prehnite pumpellyite facies metamorphism the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150 180 m ma

geochemistry and origin of proterozoic skarns at the mount elliott cu au co ni deposit cloncurry district nw queensland australia diopside rich skarn hosted copper gold ore derived primarily from carbonaceous metapelites at mount elliott forms a distinctive member of the spectrum of cu au fe oxide deposit styles in the cloncurry district of the paleoproterozoic to mesoproterozoic mount isa block the mine sequence is a package of carbonaceous metapelites and metagreywackes containing amphibolites derived from tholeiitic basic rocks a 40ar 39ar age spectrum with an extensive plateau like segment at 1 510 3 ma from an actinolite associated with sulfides is taken to represent the age of mineralization and is identical within error to the ages of most of the nearby batholithic granitoids the mine sequence is locally intruded by 1 to 10 m thick late to post tectonic trachyandesite dykes which were emplaced during the hydrothermal activity that created the orebodies and have affinities with the regional high potassium eureka supersuite granitoids stable isotope data are consistent with dominantly magmatic fluids during mineralization and the regionally distinctive skarn ca mg and cu au ni co te se low pb zn ag sb chalcophile element associations may reflect a primitive magmatic fluid source and or leaching of these elements from country rocks mount elliott is an unusual skarn deposit characterized by pronounced early albitization k fe mg depletion of the host rocks succeeded by predominantly open space deposition of sodic diopside actinolite scapolite andradite magnetite sulfides apatite allanite tourmaline calcite the ca fe mg na rich manganese poor chemistry was imposed from the fluid phase in the absence of carbonate rich protoliths immobile trace element ti zr nb geochemistry shows that mount elliott skarns formed in both metasedimentary and mafic metavolcanic host rocks but the former are the main hosts of ore in upper and lower ore zones that represent most of the resource banded skarns derived from a distinct calc silicate marble package at the nearby swan prospect have higher nb tio2 and zr tio2 ratios than the mount elliott metasediment derived skarns consistent with different provenance of the detrital components in the two sequences medium to coarse grained massive skarn and skarn breccia in the mount elliott lower ore zone formed in pelites and the trachyandesite dykes are the only intrusive rocks that could be genetically related to the mineralization in the immediate vicinity of the orebodies

detrital cosmogenic 21ne records decoupling of source to sink signals by sediment storage and recycling in miocene to present rivers of the great plains nebraska usa the preservation of conglomerates far from mountainous sources is commonly interpreted in terms of tectonic or climatic forcing to relate a depositional signal to changing conditions in source areas the process and duration of sediment routing from source to sink need to be determined for the first time we quantified sediment transport histories using cosmogenic 21ne in quartzite pebbles from modern river gravels and neogene conglomerates from the modern and ancient north platte river of the great plains of nebraska united states we demonstrate that at 400 km distance from the front ranges of the rocky mountains the majority of pebbles were stored in older channel deposits for up to several millions of years before being recycled this was enabled by very slow to zero basin subsidence rates this implies that upstream tectonic or climatic controls on surface processes were decoupled from the downstream depositional record a result supported by the similarities in cosmogenic 21ne values among miocene pliocene and modern river channel pebbles despite known changes in tectonic and climatic forcing

does climate change create distinctive patterns of landscape incision 1 incised fluvial systems are typically interpreted as recording geologically recent changes in either climate or tectonics however few diagnostic tools exist to evaluate whether particular incised landscapes primarily reflect climatic or tectonic perturbation here we summarize the results of a simple fluvial sediment transport model that allows us to contrast patterns of fluvial incision driven by changes in hydrology and sediment flux climate with those driven by changes in rock uplift patterns relative to sea level tectonics our modeling suggests that there may be diagnostic differences between the spatial and temporal patterns of incision caused by these different processes in particular incision driven by climate change is most commonly accompanied by downstream migrating waves of incision and decreases in channel gradient while under most circumstances incision driven by tectonics will be accompanied by upstream migrating incision and increases in channel gradient we apply our modeling to a case study of the north american high plains where regionally elevated surfaces east of the rockies have been incised up to 500 m by the fluvial systems draining the core of the range although this incision has been interpreted as reflecting a tectonic rejuvenation of the high plains our analysis suggests that climate change is at least as plausible an explanation to explain the incision of this landscape diagnostic differences between the climate and tectonic end member scenarios might be obtained via detailed study of the spatial and temporal patterns of terrace abandonment providing a potentially fruitful target for field investigation

landscape dissection isostatic uplift and the morphologic development of orogens we examine the morphologic factors that determine the isostatic response to landscape dissection in orogenic terrains in order to quantify the relative contributions of active tectonic uplift and passive isostatic uplift in generating mountain topography we demonstrate that although peaks many times the elevation of an assumed initial plateau can theoretically be generated only moderate amounts of peak uplift can be explained by isostatic rebound in real terrains analysis of topographic data from the european alps and other orogens indicates that isostatic uplift in response to dissection can account for about one fourth of the elevation of the highest peaks if local isostatic compensation is assumed and less if the lithosphere has significant flexural rigidity we conclude that although isostatic uplift can be significant in orogens high peaks are predominantly a consequence of tectonic processes in convergent settings

headwater channel dynamics in semiarid rangelands colorado high plains usa incised ephemeral channels provide a window into the fluvial processes that help sculpt rangeland landscapes this paper presents observations of ephemeral channels and valley networks in the high plains of colorado usa with an eye toward painting a picture of the ingredients that must be included in mathematical models of landscape evolution in such environments channel incision in the study area is driven by summer thunderstorms which can with reasonable frequency 3 5 yr generate boundary shear stresses high enough to penetrate the highly resistant vegetation armor but only within erosional hot spots where hydraulic forces are amplified by channel constriction and locally steep gradients focusing of erosion at these hot spots which correspond to knickpoints and channel heads is amplified by the small areal footprint and short erosional reach of most convective storms upstream migration of knickpoints creates a pattern of short active channel reaches separated by unchanneled or weakly channeled fully vegetated stable reaches based on our observations we interpret the necessary and sufficient conditions leading to the observed channel forms and dynamics as 1 a resistant vegetation layer overlying an erodible substrate which sets up a conditional instability through which erosional perturbations can grow by positive feedback 2 high flow variability 3 moderate to high substrate cohesion and 4 a high volume fraction of fine grained erodible material concave upward valley long profiles are interpreted as a trade off between downstream increasing flood frequency and downstream decreasing flood effectiveness the observed process dynamics imply that long term rates of valley incision should be especially sensitive to climatic oscillations between episodes of drought and warm season convective rainfall

buoyant sub surface loading of the lithosphere in the great plains foreland basin a recent development1 in the study of foreland sedimentary basins is the recognition of the important role played by negatively buoyant sub surface loads in the evolution of some of these basins models2 for the subsidence of the apennine and carpathian foreland basins in particular show that sub surface loads can equal or exceed the topographic load of the neighbouring mountain belt here we show that part of the rocky mountain foreland is underlain by a positively buoyant sub surface load which counterbalances almost half of the total topographic load on the western great plains the presence of this load explains the shallowness of the denver basin and the large bouguer gravity anomalies over the great plains we suggest that this load was emplaced during the miocene when according to geological evidence3 5 the region underwent epeirogenic uplift the sub surface load may result from lithospheric thinning which often accompanies or precedes crustal extension6 7

role of river incision in enhancing deformation many active compressional belts contain examples of transverse rivers intersecting anticlines at their highest structural and topographic position this unusual association between rivers and doubly plunging anticlines at relatively small scales is usually explained by some form of antecedence or superposition in this article i suggest an alternative explanation i e that river erosion unloads the crust and that this process causes local deformation to be enhanced in the vicinity of rivers i show the plausibility of this hypothesis with the aid of a coupled three dimensional mechanical surface process mathematical model the conclusion is that river incision can have a major influence on deformation of the surrounding crust if incision occurs at the same time that the crust is deforming plastically in response to regional compression in this case incision amplifies background deformation at a relatively small scale leading to the formation of noncylindrical folds with culminations coinciding with river incision in the absence of regional deformation the response of the crust to river incision is small and of long wavelength because it is governed by flexural isostasy of relatively rigid elastic crust thus whether rivers exert an influence on local deformation depends critically on the timing between river incision and regional deformation this point may explain why in some cases rivers appear to have had no significant influence on local deformation whereas in other cases they have

multiple sources for the coastal batholith of central chile 31 34 s geochemical and sr nd isotopic evidence and tectonic implications abstract the coastal batholith of central chile between 31 and 34 s represents a locus of long lived c 200 ma plutonic activity it is composed of four plutonic complexes santo domingo limari papudo quintero and illapel these complexes which occur as north trending discontinuous belts that decrease in age eastward were emplaced during four major episodes of arc magmatism the santo domingo complex carboniferous comprises hornblende biotite bearing tonalites and granodiorites with abundant mafic enclaves and enclave free granites the association of enclave bearing granitoids and enclave free granites resulted from different degrees of mixing of crustal and mantle derived magmas the limari complex lower jurassic is a bimodal association of crustal leucogranites and mantle derived gabbros the lithology of the papudo quintero complex middle jurassic varies from hornblende pyroxene diorite to biotite granite the main rock types namely tonalites and granodiorites commonly contain mafic enclaves the granitoids and the enclaves have remarkably similar ree patterns and are similar in sr nd isotope composition which suggest that they represent a cogenetic suite of mantle origin the illapel complex cretaceous consists of hornblende bearing tonalites trondhjemites and granodiorites that resemble the rocks of ttg suites the end values and initial 87 sr 86 sr ratios sri for the mafic rocks of the carboniferous complex end 2 0 and 3 5 sri c 0 7057 differ markedly from those of the mesozoic mafic rocks end 1 9 to 5 4 sri 0 7033 to 0 7039 the cretaceous rocks being the most depleted with regard to sr nd a change from carboniferous lithosphere dominated sources to mesozoic asthenosphere dominated sources of the magmas that started in the early jurassic is recognized slab components can be recognized in the source of the cretaceous rocks the crustal source of the lower jurassic leucogranites was more isotopically depleted and refractory than that of the carboniferous granites on the other hand no crustal participation in the origin of the middle jurassic and cretaceous felsic granitoids is recognized they are differentiated from mantle derived magmas progressive removal of old lithospheric mantle during the mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source as well as the melting and modification towards a more refractory nature of the continental crust

early magnetite amphibole plagioclase alteration mineralization in the island copper porphyry copper gold molybdenum deposit british columbia the initial hydrothermal event in the evolution of the ca 377 mt 0 41 percent cu island copper porphyry cu au mo deposit northern vancouver island was the formation of an extensive alteration mineralization facies dominated by magnetite calcic amphibole and intermediate to sodic plagioclase early stage quasipervasive magnetite rich alteration and associated magnetite rich veinlets in this middle jurassic island arc hosted hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata the magnetite rich alteration zone 500 to 700 m wide and originally extending over at least 450 m vertically exhibits an outward zonation from quartz magnetite albite an sub or 6 amphibole trace apatite trace through quartz magnetite amphibole albite an sub 6 10 or oligoclase andesine an sub 15 36 apatite trace or scapolite trace to amphibole magnetite oligoclase or sodic andesinc an sub 15 39 or quartz or apatite trace assemblages definition of early stage mineral assemblages impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration was facilitated by the use of novel petrographic techniques including incident light nomarski differential interference contrast microscopy early stage mineralization comprises several well defined vein veinlet types which were demonstrably emplaced prior to the main stage chalcopyrite pyrite stockwork and associated k silicate alteration minor chalco pyrite and pyrite occur in some early stage veins and magnetite is a constituent of many later main stage sulfide rich veins but the distinction between these two major vein systems is clear mass exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks with an fe enrichment approaching 450 percent 20 g 100 cc in the dacite dike and lesser and more variable na enrichment with both ti and al behaving as mobile constituents paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite rich alteration mineralization and imply that this event was initiated above 650 degrees c and at pressures of at least 1 kbar and was generated by fecl 2 rich brines at first single phase and moderately saline avg 15 wt nacl equiv but latterly boiling at ca 560 degrees to 645 degrees c and or ca 0 55 kbars the main stage potassic alteration zone has the configuration of an annulus ca 100 to 150 m in width which was entirely superimposed on the more extensive early alteration mineralization zone at ca 430 degrees to 575 degrees c and below 450 bars gold although correlated overall with cu and potassic alteration was probably extensively introduced in the early stage it is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids in which so super 2 exceeded both h 2 s and so super 2 4 the deposition of abundant magnetite and the widespread and intense fe metasomatism which define the early stage record the high solubility of fe as fecl 2 in high temperature brines in equilibrium with magnetite bearing quartzofeldspathic rocks as well as the retrograde solubility of magnetite between ca 550 degrees and 750 degrees c whereas the high so 2 h 2 o so 4 ratios of the initial hydrothermal fluids at island copper and in other similar deposits may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating s rich mafic melts the mineral assemblages of the early stage veins and associated quasi pervasive alteration cannot be assigned to the accepted alteration mineralization facies of porphyry systems such as a veins and potassic alteration and record specific and distinct conditions of fluid rock interaction