3d interpretation by integrating seismic and potential field data in the vicinity of the proposed cosc 1 drill site central swedish caledonides the scientific drilling project cosc collisional orogeny in the scandinavian caledonides designed to study key questions concerning orogenic processes aims to drill two fully cored boreholes to depths of c 2 5 km each at carefully selected locations in west central sweden the first of these cosc 1 is scheduled for start late spring 2014 and will target the seve nappe complex characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion in this study available seismic sections have been combined with surface geology to produce a 3d interpretation of the tectonic structures in the vicinity of the cosc 1 borehole constrained 3d inverse gravity modelling over the same area supports the interpretation and the high density seve nappe complex stands out clearly in the model interpretation and models show that the maximum depth extent of the seve nappe complex is less than 2 5 km consistent with reflection seismic data the gravity modelling also requires underlying units to comprise low density material consistent with the lower allochthon but the modelling is unable to discern the decollement separating the allochthons from the crystalline precambrian basement

the application of reflection seismology to the investigation of the geometry of near surface units and faults in the blake river group abitibi belt quebec reprocessing of part of a lithoprobe high resolution seismic reflection line across the southern part of the abitibi belt has improved the imaging of shallow reflections and allowed correlation of the data with surface geology enhancement of early reflections was accomplished by focusing on the high frequency content of the data this improved resolution of reflections at two way traveltime as early as 0 3 s and attenuated noise such as shear waves the shallow reflections are interpreted as impedance contrasts at the contact between a metadiabase diorite body and metavolcanics rocks offsets of the reflectors correlate with faults mapped at the surface and indicate a downdropped block which may be of interest for mineral exploration

reflection seismic investigations in the western part of the paleoproterozoic vhms bearing skellefte district northern sweden the skellefte district forms part of the svecofennian ca 1 90 to 1 80 ga supracrustal sequence and associated intrusive rocks in the northern part of sweden the western part of the skellefte district which is the most important metallogenic province in northern sweden today hosts major volcanic hosted massive sulfide vhms deposits e g the 23 million metric tons mt kristineberg cu zn pb ag au deposit in order to obtain a better understanding of the vhms ore potential at depth new seismic reflection data were acquired along two parallel and 25 km long profiles in the kristineberg area in 2003 the data were collected with the purpose of obtaining high resolution images of the top 10 km of the crust and are presented here for the first time although the structural setting is very complex the stacked sections reveal numerous reflections that can be correlated with surface geology visible on both profiles is a pronounced north dipping band of reflections marking a boundary between relatively transparent crust above and significantly more reflective crust beneath it we interpret this reflective crust to represent a structural basement to the ore bearing skellefte group possibly constituting bothnian basin metasedimentary rocks bordering the skellefte district to the south this new interpretation is important for the understanding of the tectonic evolution of the skellefte district and for defining exploration strategies in the area the seismic results suggest that the kristineberg and ravliden deposits occur on the northern limb of a kilometer scale local second order syncline within the hinge zone of a major antiform results from a profile located approximately 8 km to the west of the kristineberg mine indicate that the revsund granitoid has a thickness of about 3 to 3 5 km ultramafic rocks are also imaged clearly diffraction patterns and bright spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions or a mineralization zone at 3 to 5 km depth these results help to identify new prospective areas and mineral potential both downplunge from known ores and on the same stratigraphic horizon on the southern limb of the ore bearing syncline the seismic reflection profiling has been effective in imaging the major structures around the kristineberg orebody demonstrating that this technique can be used for delineating complex structures significant for mineral exploration

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

vertical seismic profiling using distributed acoustic sensing with scatter enhanced fibre optic cable at the cu au new afton porphyry deposit british columbia canada abstract wireline logs and vertical seismic profile data were acquired in two boreholes intersecting the main mineralized zone at the cu au new afton porphyry deposit canada with the objectives of imaging lithological contacts fault zones that may have acted as conduits that channelled the mineralization and alteration zones log data provide physical rock properties for the main lithologies and alteration zones calliper logs reveal many faults and caved in zones generally indicating rocks with low integrity at the borehole wall the preponderance of these zones as indicated by the logs suggests that their response may dominate the seismic reflection wavefield outside fault zones compressional and shear wave velocities exhibit significant variability due to porosity the heterogeneity of volcanic fragmental rocks and alteration distributed acoustic sensing was used to acquire vertical seismic profiling data in the two boreholes surveyed with wireline logs straight and helically wound fibre optic cables housed standard fibres and a fibre engineered to increase the intensity of backscattering at the distributed acoustic sensing interrogator standard and engineered optical fibres placed in the two boreholes were daisy chained together to form two 5 km long continuous fibres that were interrogated at once with two interrogators a new generation of interrogator connected to the engineered fibres provided field data with lower noise level and higher signal to noise ratio these data with higher signal to noise ratio from straight fibre optic cable were processed and used for depth imaging depth images benefitted from new migration weights that account for the directional sensitivity of the straight fibre optic cable and limit the extent of migration artefacts migration results show several reflectors with shallow dips to the northwest some explained by faults intersecting the surveyed boreholes the main sub vertical lithological and alteration contacts at new afton generated downgoing reflections that were not considered in the migration

rock uplift and exhumation of continental margins by the collision accretion and subduction of buoyant and topographically prominent oceanic crust understanding the causes of rock and surface uplift is important because they control the location of mountain building depocenters and drainage characteristics and can influence climate here we combine previous thermochronological data with field observations to determine the amount of exhumation rock and surface uplift that occurs in the upper plate of central and south american subduction zones during the collision accretion and subduction of oceanic plateaus and aseismic ridges the collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 ma uplift appears to be an immediate response to collision and is generally independent of the slab dip the amount of rock uplift is controlled mainly by excess topography associated with the ridge ultimately linked to buoyancy and erosion while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel the quantity of exhumation is strongly dependant on climate induced erosion and the lifespan over which the topography is uplifted and supported sediment draining into the trench may leave the elevated ridge axis sediment starved increasing the shear stresses at the ridge subduction interface leading to positive feedback between ridge subduction rock uplift and exhumation trench parallel variations in exhumation have a direct impact on exploration paradigms for porphyry related metalliferous deposits and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

the paleoproterozoic kristineberg mining area northern sweden results from integrated 3d geophysical and geologic modeling and implications for targeting ore deposits the kristineberg mining area in the western part of the paleoproterozoic skellefte ore district northern sweden is well known for its base metal and recent gold discoveries a pilot 3d geologic model has been constructed on a crustal scale covering an area of 30 30 km to depths of 10 km constrained 3d inverse and forward gravity modeling have been performed to confirm and refine previous modeling along seismic profiles using mainly 2 5d techniques the 3d inverse gravity modeling was geared to generating isodensity surfaces that enclose regions within the model of anomalous density contrast the 3d forward gravity modeling was conducted to include faulting and folding systems that are difficult to include in the inversion the 3d geologic model supports many previous interpretations but also reveals new features of the regional geology that are important for future targeting of base metal and gold deposits the margins of a thick granite in the south dip steeply inward suggesting the possibility of

demonstration of two pulses of paleogene deformation in the andes of peru abstract new radiometric ages of about 25 m y on volcanic materials in a marine intercalation within clastic continental strata of the upper moquegua formation near caraveli southern peru together with an age of 25 3 0 4 m y obtained by tosdal et al from a locality about 300 km to the ese show that the formation contains strata of late oligocene as well as miocene age and demonstrate that the coastal region was at a low elevation during latest oligocene time because the unconformities between the upper moquegua formation and the underlying lower moquegua formation and between the lower moquegua formation and underlying paleocene rocks cannot both represent the same tectonic event two discrete paleogene events must be present in the andes of peru although the exact timing of these events is uncertain the unconformities are likely to be of paleocene and middle eocene age or possibly of middle eocene and oligocene age

regional structural profiles in the western part of the palaeoproterozoic skellefte ore district northern sweden abstract the kristineberg mining area is situated in the western part of the palaeoproterozoic skellefte ore district northern sweden and is well known for its vhms base metal and gold deposits this paper presents five upper crustal geological cross sections that have been constructed and mainly constrained by seismic reflection data potential field modeling as well as geological field observations these profiles are visualized in 3d to highlight the three dimensionality and internal consistency of structures across the region the resulting structural model for the skellefte volcanics and overlying metasediments comprises two thrust sheets that expose the skellefte volcanics in the cores of hanging wall anticlinal structures a shear zone is imaged as a band of seismic reflectivity terminated by the southern revsund granite unit another shear zone possibly a continuation of the skellefte shear zone ssz runs through the centre of the region and accounts for some of the structural complexity and shearing observed between the two anticlinal exposures of the volcanics additional smaller scale shear zones have been identified from geological and geophysical mapping within the main structural blocks of the skellefte volcanics the mala volcanic rocks in the north are separated from the skellefte volcanics by a fault that cuts discordant to the strike of the mala volcanics a structural basement has been proposed to the skellefte volcanics constrained by seismic reflection data exposures of bothnian basin rocks south of the revsund granite outcrops suggest that the domain beneath the north dipping reflectivity is associated with bothnian basin stratigraphy the preferred interpretation for the contact between the skellefte volcanics and the bothnian basin rocks is a thrust fault that brings the felsic volcanic rocks over the metasedimentary domain the revsund granites are divided into two major groups based on their present day thickness and shapes although parts of the viterliden intrusion are almost undeformed it is cut by a series of shear zones causing the magnetic lineations seen within these rocks the structural profiles presented demonstrate that the kristineberg ore is situated in the northern limb of a local synformal structure the new crustal scale structural model demonstrates the potential of integrating geophysical and geological data when modelling structures hosting mineralization in a complex region like the skellefte district the structural profiles presented in this paper have greatly improved our understanding of the 3d tectonostratigraphy and architecture of the poly deformed ca 1 9 ga the skellefte ore district

building a 3d model of lithological contacts and near mine structures in the kevitsa mining and exploration site northern finland constraints from 2d and 3d reflection seismic data the kevitsa mafic ultramafic intrusion in northern finland hosts a large disseminated nickel copper sulphide ore body the kevitsa intrusion is an active mining and exploration site for which we

the henkel petrophysical plot mineralogy and lithology from physical properties the henkel plot logarithm of magnetic susceptibility versus density of rock samples reveals that most rocks fall on either a magnetite trend or a paramagnetic trend i

metalliferous mining geophysics state of the art after a decade in the new millennium mining exploration was very active during the first decade of the twenty first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration development came from different sources instrumentation improvements new numerical algorithms and cross fertilization with the seismic industry in gravity gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration in potential field methods in general numerous techniques have been developed for automatic interpretation and 3d inversion schemes came into frequent use these inversions will have even greater use when geologic constraints can be applied easily in airborne electromagnetic em methods the development of time domain helicopter em systems changed the industry in parallel improvements in em modeling and interpretation occurred in particular the strengths and weaknesses of the various algorithms became better understood simpler imaging schemes came into standard use whereas layered inversion seldom is used in the mining industry today improvements in ground em methods were associated with the development of squid technology and distributed acquisition systems the latter also impacted ground induced polarization ip methods developments in borehole geophysics for mining and exploration were numerous borehole logging to measure physical properties received significant interest perhaps one reason for that interest was the desire to develop links between geophysical and geologic results which also is a topic of great importance to mining geologists and geophysicists

shallow high resolution seismic imaging at the ansil mining camp in the abitibi greenstone belt volcanic rocks in and around the ansil mining camp host a large number of massive sulfide mineral deposits a high resolution seismic reflection profile was shot across the camp with the objective of mapping the contacts between the different volcanic units at which most of the ore bodies have been found numerous exploration boreholes define the geology to a depth of 1600 m and allow a precise comparison with the recorded reflections geophysical logs obtained in one deep borehole suggest that reflection coefficients between the andesite rhyolite units of the volcanic stratigraphy are around 0 05 but few corresponding reflections can be identified in the seismic data those reflections in the surface seismic profile that can be correlated with the subsurface geology originate from diorite sills at which reflection coefficients are between 0 05 and 0 11 we suggest that reflections are observed from the diorite sills because the sills were intruded as sheets some along fault planes resulting in inte

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

delineating the tuwu porphyry copper deposit at xinjiang china with seismic reflection profiling the tuwu deposit is one of a series of recently discovered porphyry copper deposits in the eastern tian shan range of xinjiang china since its discovery in 1997 more than ten boreholes have been drilled and a suite of geophysical surveys has been acquired to delineate the deposit as part of the geophysical program a set of eight seismic reflection profiles was acquired in 2000 followed by a physical rock property study in 2001 the ores are characterized by slightly higher density 0 1 g cm3 and significantly higher p wave velocity vp 1 0 1 5 km s than the dioritic host rocks the seismic surveys used 0 6 to 0 9 kg shallow dynamite sources with a 24 channel end on spread and offsets up to 350 m the orebody and associated igneous layers dip steeply 45 toward the south so careful processing of the seismic data was required weak reflections from stratigraphic contacts are visible on most of the profiles including the top of the intrusion and the base of the orebody since the obser

thermotectonic history of the maranon fold thrust belt peru insights into mineralisation in an evolving orogen fold thrust belts along convergent margins around the world host major ore deposits the origin of such ore deposits is commonly episodic and so are the temporal and spatial variations in deformation styles in these fold thrust belts here we focus on the maranon fold thrust belt mftb of the peruvian andes and demonstrate a link between the spatio temporal distribution of ore deposits and thick skinned tectonics we present low temperature thermochronology results from the mftb that document the uplift exhumation history and timing of thick skinned tectonics and vergence reversal our results suggest that the thermal history of the mftb involved two discrete intervals of opposed vergence large scale crustal deformation since the late cretaceous an early interval at 80 20 ma was associated with folding and east vergent thin skinned tectonics and was followed by west vergent thick skinned deformation at 20 0 ma furthermore our findings suggest that thick skinned contraction was coincident with increased focusing of mineralising fluids and ore deposition in the mftb during the early miocene and in the mid miocene both mineralisation and deformation were enhanced by subduction of the nazca ridge underneath the south american plate

geological interpretation of bouguer gravity and aeromagnetic data from the gobi desert covered area eastern tianshan china implications for porphyry cu mo polymetallic deposits exploration abstract in this study bouguer gravity and aeromagnetic data have been used to better understand the geology and mineral resources near the late carboniferous late permian porphyry cu mo polymetallic mineralization in the chinese eastern tianshan belt which is extensively covered by gobi desert the reduced to pole rtp transformation of regional scale aeromagnetic data shows that the porphyry cu mo deposit is within a cluster of magnetic anomaly highs that overprint on a northeast trending magnetic gradient belt generally along the crustal scale kanguertag huangshan fault the 10 km upward continuation transformation of both bouguer gravity and aeromagnetic data indicates that the known porphyry cu mo polymetallic deposits are located on the flanks of prominent gravity and magnetic anomaly highs these anomalies are spatially correlated with the late carboniferous late permian igneous rocks and in the tuwu yandong mineralization district are centered over the granodiorite rocks genetically related to porphyry copper systems in order to minimize interpretational ambiguities a useful approach that is correlation analysis ca based on correlation coefficient cc given by gravity and magnetic data was employed to separate positively and negatively correlated anomalies features the ca procedure is applied to 10 km upward continuation transformation of both bouguer gravity and rtp transformed aeromagnetic data for mapping correlative magnetization and density contrast anomalies from deep sources which may be associated with the porphyry cu mo polymetallic mineralization five prominent cc positive anomalies have been found in the southern margin of dananhu tousuquan arc those anomalies zones could be interpreted to reflect a late carboniferous late permian magmatic belt that is favorable for additional discoveries of late carboniferous to late permian porphyry copper systems in north region of eastern tianshan

three dimensional distribution of igneous rocks near the pebble porphyry cu au mo deposit in southwestern alaska constraints from regional scale aeromagnetic data abstractaeromagnetic data helped us to understand the 3d distribution of plutonic rocks near the pebble porphyry copper deposit in southwestern alaska usa magnetic susceptibility measurements showed that rocks in the pebble district are more magnetic than rocks of comparable compositions in the pike creek stuyahok hills volcano plutonic complex the reduced to pole transformation of the aeromagnetic data demonstrated that the older rocks in the pebble district produce strong magnetic anomaly highs the tilt derivative transformation highlighted northeast trending lineaments attributed to tertiary volcanic rocks multiscale edge detection delineated near surface magnetic sources that are mostly outward dipping and coalesce at depth in the pebble district the total horizontal gradient of the 10 km upward continued magnetic data showed an oval deep magnetic contact along which porphyry deposits occur forward and inverse magnetic modeling showed that the magnetic rocks in the pebble district extend to de

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

controls on timing of exhumation and deformation in the northern peruvian eastern andean wedge as inferred from low temperature thermochronology and balanced cross section in northern peru a 500 km long regional balanced section has been constructed across the eastern andean wedge using fieldwork industrial seismic sections and wells the structure is characterized by a thin skinned thrust system involving the eastern cordillera ec the sub andean zone saz and the maranon foredeep in the saz and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events permian thrust faults had been reactivated to form a basement duplex underlying the saz and the foredeep at the same time a triassic jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault giving rise to the ec the impingement of the ec was responsible for the deformation of the saz and the propagation of the thrust wedge the minimum shortening calculated is 142 km representing a shortening strain of 28 a sequential restoration calibrated by u th he and fission track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7 1 mm yr 1 between 17 and 8 ma to 3 6 mm yr 1 between 8 ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 ma when compared with other andean thrust wedges we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the bolivian orocline and the propagation is not controlled by the precipitation regime we rather suggest that reactivated basement faults favored thrust wedge propagation

tectonic significance of gravity and magnetic variations along the lithoprobe southern canadian cordillera transect correlation of potential field data to regional geological features within the lithoprobe southern canadian cordillera transect corridor allows characterization of anomaly patterns according to their likely sources long wavelength bouguer gravity anomalies are attributed to isostatic effects of topography which in most areas is compensated two notable exceptions occur in the foreland belt a large positive isostatic anomaly is likely due to mechanical support of topography formed as cordilleran thrust sheets were emplaced over the thick craton and on the west coast isostatic anomalies are related to active subduction long wavelength magnetic anomalies in the foreland belt are associated with cratonal basement beneath the thrust sheets and these can be followed westward to near the omineca belt a prominent positive magnetic anomaly along the western coast belt is probably associated with mafic rocks generated during subduction elsewhere relatively short wavelength gravity and magnetic anomalies c

identifying mineral prospectivity using seismic and potential field data in the hongniangyu district inner mongolia china abstract since the discovery of several large porphyry mo deposits 0 1 mt mo in the jining region of inner mongolia china during the past decades the area has been receiving wide interest for mining activities however mineral exploration in this area is challenging because most of it is thickly covered by cenozoic sediments geophysical surveys especially the seismic method are becoming increasingly important for investigating deep seated ore controlling structures e g porphyry granites faults and even orebodies supporting the discovery of buried ore deposits recently potential field data and seismic reflection data have been acquired for the hongniangyu area south of jining region in this study we mainly use seismic reflection and potential field data to obtain subsurface geological model in support of mineral exploration targeting gravity and magnetic data were firstly used for spatial analysis of mineral system by mapping the distribution of the faults concealed granites and the sedimentary basin through wavelet analysis tilt angle and euler deconvolution to address the common problem of low signal to noise ratio of seismic reflection data obtained in hard rock terranes we applied a sophisticated workflow including refraction static correction noise filtering surface consistent deconvolution and velocity analysis the obtained post stack migrated seismic profile clearly imaged the geometry of the concealed graben located at the north end of the datong basin and the south east dipping kouquan normal fault controls the depression of the graben and offsets cenozoic sediments by several hectometers up to 500 m joint modeling of seismic gravity and magnetic data further provided evidence of the three buried belts of granites intriguingly one of the concealed granitic belts located in the north of hongniangyu area was found to have both significant chargeability and mo geochemical anomalies indicating a high potential of hosting porphyry type mo ore deposits as expected mo and w mineralized cores were discovered in this target area but limited to only one deep drilling hole the presented results provide the geological framework of hongniangyu district and guidelines for delineating new potential mo mineralization in the jining region

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

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

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

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

lithoprobe crustal reflection structure of the southern canadian cordillera 2 coast mountains transect the lithoprobe seismic reflection transect across the southern coast mountains of the canadian cordillera images fundamental crustal structures presumably related to collision of the intermontane and insular composite terranes and deep levels in the upper plate of the offshore cascadia subduction belt the eastern part of the coast mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors they are truncated by subhorizontal to west dipping middle and upper crustal reflectors these geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust and a later phase of east directed shortening caused by wedging of the intermontane belt into the lower and middle crust of the tectonic stack subsequently the coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the intermontane belt and surface in the omineca and foreland belts extensional faults bounding the east flank of the coast mountains and west flank of the central nicola horst in the intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage within the western coast mountains east dipping reflections spanning the middle crust to upper mantle are traced updip to vancouver island and the underlying cascadia subduction zone the c reflector on vancouver island is believed to separate wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the moho if the moho is not a young feature then some accreted material appears to have wedged into the continental framework above the crust mantle boundary possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data the structurally lower e reflections interpreted as shear zones originate at the subduction contact offshore and project landward into sub moho reflections within the upper plate on the mainland the region between the e reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle

lithoprobe crustal reflection cross section of the southern canadian cordillera 1 foreland thrust and fold belt to fraser river fault seismic reflection data from the south central canadian cordillera covering the interval from the easternmost metamorphic core complexes near arrow lakes to the fraser river fault system along the fraser river reveal a highly reflective and complex crust the base of the crustal reflectivity interpreted as the reflection moho is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12 0 s in the east to about 10 5 s in the west this virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust even though part of this area was subjected to large amounts of eocene extension the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the u s cordillera three structural culminations the monashee complex the vernon antiform and the central nicola horst are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during jurassic to eocene compression and then to have been modified and exposed during early and middle eocene extension an example of a compressional structure observed on the profiles is the monashee decollement which can be traced from the surface westward into the lower crust extension is manifested along a variety of normal faults including the regionally extensive low angle okanagan valley eagle river fault system moderately dipping faults such as the columbia river and slocan lake faults and high angle faults such as the quilchena creek and coldwater faults both jurassic to eocene compressional shear zones and early to middle eocene extensional shear zones are listric into the lower crust or moho under the intermontane belt

the pierina epithermal au ag deposit ancash peru paragenetic relationships alunite textures and stable isotope geochemistry the pierina high sulfidation epithermal au ag deposit 9 26 5 s 77 35 w was emplaced in the middle miocene into a hypabyssal to extrusive pumice tuff and an underlying older dacitic flow dome complex both of which are cut by hydrothermal breccias and small dacitic domes stage i advanced argillic alteration generated a core of vuggy silica focused in the tuff and surrounded successively by zones of quartz alunite dickite kaolinite pyrophyllite and illite montmorillonite kaolinite laser ablation icp ms analysis of the sulfide minerals of the succeeding stage ii cu pb bi sb zn as barite mineralization largely confined to the vuggy silica zone reveals that both au and ag were introduced at that time this assemblage was almost entirely obliterated during stage iii when oxidation by low temperature meteoric waters generated botryoidal hematite goethite assemblages which are now the main precious metal hosts stage iv barite acanthite mineralization shows limited temporal overlap with stage iii but dominantly overprints the hematite goethite assemblage the deposit incorporates alunite exhibiting a wide range of modes of occurrence grain size and morphology disseminated alunite dominates stage i alteration which replaced phenocrysts and fragments and locally hosts corroded 40 m alumino phosphate sulfate aps inclusions the alunite shows limited na substitution molar na na k 0 2 but alunite natroalunite assemblages occur sporadically throughout the quartz alunite alteration zone isotopic analysis yields 34s values of 16 6 31 0 consistent with a magmatic hydrothermal origin rare occurrences of disseminated alunite and pyrite in textural equilibrium occur in unoxidized areas of the deposit and yield 34salu py precipitation temperatures of 179 to 250 c with the majority below 200 c disseminated alunite in shallower oxidized portions of the deposit where pyrite has been destroyed yield 18oso4 values of 6 3 to 14 4 the lighter compositions occur along the axis of the quartz alunite alteration zone and imply precipitation at higher temperatures albeit with some meteoric water involvement porcelaneous alunite 10 100 m forms the matrix of breccias that cut stage i alteration sodium contents are low na na k molar 0 1 and aps inclusions have not been identified the 34s values of 21 8 27 1 are consistent with a magmatic hydrothermal origin but the 18oso4 values of 11 4 to 14 6 indicate deposition under cooler temperatures and the involvement of meteoric water that interacted substantially with igneous country rocks coarse open space filling alunite which occurs as veins or coatings on breccia fragments exhibits limited na substitution is not associated with natroalunite and does not host aps inclusions its 34s and 18oso4 values of 12 9 to 26 2 and 6 5 to 8 6 respectively imply a magmatic hydrothermal origin and deposition at higher temperatures dike like bodies and mantos of friable alunite have distinctive 34s 16 2 19 3 and 18oso4 7 6 9 2 values and may record a steam heated environment however these compositions may also indicate loss of h2s gas during oxidation or overprinting of steam heated alunite by magmatic hydrothermal alunite or vice versa all forms of alunite exhibit higher 18oso4 values in the central part of the deposit and the coherent isotopic chemistry casts doubt on the reliability of the widely accepted textural criteria for the origin of this mineral in hydrothermal systems

applications of geophysical inversions in mineral exploration increases in computing power and advances in mathematical optimization theory have combined to produce a new generation of algorithms that can invert geophysical data to recover 1 d 2 d or 3 d images of the earth s physical properties these images may indicate mineralization directly or delineate the associated structures in both cases they are valuable aids to mineral exploration and they provide information that previously had to be distilled from data maps alone in this short article we illustrate both the practicability of inverting geophysical data and the important even decisive information that it provides applications are in time domain electromagnetics dc resistivity and induced polarization gravity and magnetics

solving the production diffusion equation for finite diffusion domains of various shapes part i implications for low temperature u th he thermochronology abstract we propose an accurate fast and easy to use method to derive numerical solutions for production diffusion equations for finite diffusion domains of various shapes and arbitrary cooling histories previous studies provide solutions for spheres infinite cylinders and infinite sheets we extend this range and provide solutions for finite bodies i e finite cylinders and rectangular blocks of any aspect ratio this approach is important as recently it has become clear that for example the physical grain is the diffusion domain for he diffusion in apatite and titanite j geophys res 105 2000 2903 geochim cosmochim acta 63 1999 3845 we discuss the use of the new approach for forward modelling u th he production diffusion in apatite taking results with finite cylinders as a good approximation for apatite crystals it is found that approximating instead with spheres or infinite cylinders having the same radius yields differences in calculated ages that can easily be as large as 20 35 the relative differences are most pronounced in thermal histories that spend significant time at or near the closure temperature on the other hand reasonable agreement is found with spheres having the same surface to volume ratio

a vibroseis reflection seismic survey at the buchans mine in central newfoundland we present results from the first major vibroseis seismic reflection survey at a mine site in north america it is demonstrated that conventional high stack fold reflection seismic techniques can image fault structures associated with volcanogenic massive sulfide bodies despite the fact that these structures are locally steeply dipping and produce records with low signal to noise ratios the new lines were recorded at the locations of two earlier experimental explosive surveys that failed to image many strong reflectors the principal reasons for the success of the vibroseis experiment were the proper choice of sweep frequencies for maximum signal to noise ratio the use of high stack fold and the careful analysis of velocities and statics during processing a comparison of the new seismic sections with borehole and other geophysical data indicates that the origin of reflections at buchans can be attributed to various mechanisms including contrasts in lithologies and rock competence and intrusion of diabase sills into pre existing fault zones the best reflections emanate from shallow dipping brittle ductile thrust fault zones characterized by fault gouge within broader fractured zones the two seismic lines presented have proven to be a useful and cost effective supplement to existing borehole and geophysical data and have provided enough new information on the nature of thrust faulting at the mine to significantly influence current exploration plans

timing of porphyry cu mo and base metal zn pb ag cu mineralisation in a magmatic hydrothermal system morococha district peru the morococha district in central peru is characterised by economically important cordilleran polymetallic zn pb ag cu vein and replacement bodies and the large toromocho porphyry cu mo deposit in its centre u pb re os and 40ar 39ar geochronology data for various porphyry related hydrothermal mineralisation styles record a 3 5 ma multi stage history of magmatic hydrothermal activity in the district in the late miocene three individual magmatic hydrothermal centres were active the codiciada toromocho and ticlio centres each separated in time and space the codiciada centre is the oldest magmatic hydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz molybdenite veins the hydrothermal events are recorded by a titanite u pb age at 9 3 0 2 ma and a molybdenite re os age at 9 26 0 03 ma these ages are indistinguishable from zircon u pb ages for porphyry intrusions of the composite stock and indicate a time span of 0 2 ma for magmatic hydrothermal activity the small ticlio magmatic hydrothermal centre in the west of the district has a maximum duration of 0 3 ma ranging from porphyry emplacement to porphyry mineralisation at 8 04 0 14 ma 40ar 39ar muscovite cooling age the toromocho magmatic hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1 3 ma and is responsible for the formation of the giant toromocho cu mo deposit at least two hydrothermal pulses are identified post dating a first pulse of molybdenite mineralisation wide spread hydrous skarn covers an area of over 6 km2 and is recorded by five 40ar 39ar cooling ages at 7 2 6 8 ma these ages mark the end of the slowly cooling and long lived toromocho magmatic hydrothermal centre soon after last magmatic activity at 7 26 0 02 ma district wide 50 km2 cordilleran base metal vein and replacement bodies post date the youngest recorded porphyry mineralisation event at toromocho by more than 0 5 ma polymetallic veins 5 78 0 10 and 5 72 0 18 ma 40ar 39ar ages and the manto italia polymetallic replacement bodies 6 23 0 12 and 6 0 0 2 ma 40ar 39ar ages are interpreted to have been formed by a single hydrothermal pulse hydrothermal activity ceased after the formation of the base metal vein and replacement bodies overlapping monazite u pb 8 26 0 18 ma and muscovite 40ar 39ar ages 8 1 0 5 ma from the early base metal stage of one cordilleran vein sample in the sulfurosa area provide evidence that a discrete hydrothermal pulse was responsible for polymetallic vein formation 2 6 ma prior to the district wide polymetallic veins these ages pre date those of toromocho porphyry cu mo formation and show that zn pb ag cu mineralisation formed during several discrete magmatic hydrothermal pulses in the same district

constraints on the geometry of the suasselka post glacial fault northern finland based on reflection seismic imaging abstract unloading of the ice during the last glacial period in northern fennoscandia is believed to have generated major faulting these faults often referred to as post glacial faults typically have clear surface exposures but their geometry at depth is poorly known in order to better understand the geometry at depth of the suasselka post glacial fault in finland three high resolution 2d reflection seismic profiles over the fault were reprocessed their total profile length is about 60 km and they were acquired as part of a major effort in finland to map the uppermost crust in mining areas the reprocessing led to significantly improved images that could be used to map the fault at depth two approximately n s striking profiles and one e w striking profile were reprocessed the different azimuths and the crooked nature of the profiles allowed the fault geometry to be relatively well constrained clear reflections from the fault dipping towards the se can be traced from the shallow subsurface down to about 3 km the strike and dip of two sets of dipping reflections in the stacked data along with geometrical constraints and cross dip analysis give a consistent dip of about 35 45 towards the se for the fault the strike and dip vary from n55e with a dip of 35 in the east to a strike of n48e with a dip of 45 in the west existence of the two sets of reflections indicates that the fault surface is non planar aside from allowing the geometry of the fault to be determined the seismic data show a complex reflectivity pattern in the area and indications of both reverse and normal movement along fault planes with similar orientation to the suasselka post glacial fault these images can be used as a basis for better characterizing the 3d geology of the area

seismic and potential field imaging of the guichon creek batholith british columbia canada to delineate structures hosting porphyry copper deposits the guichon creek batholith gcb located in south central british columbia contains several large low grade copper deposits of considerable economic importance the surface geology of the guichon batholith and its surrounding region have been well mapped however little information about subsurface features is available the batholith consists of four major phases emplaced radially outward which can be separated on the basis of their texture and composition previous interpretation of gravity data suggests a mushroom shaped structure for the batholith data from lithoprobe seismic reflection line 88 11 acquired across the batholith in 1988 reveal weakly coherent east dipping reflections on the west side and west dipping reflections on the east in the upper 10 km to determine if these are related to structures associated with the batholith we reprocessed the upper 6 s with particular emphasis on applications of signal enhancement techniques e g pattern recognition methods refraction statics

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

seismic imaging and potential field modelling to delineate structures hosting vhms deposits in the skellefte ore district northern sweden the skellefte district in northern sweden is a roughly 150 by 50 km2 large early proterozoic massive sulphide belt based on high resolution reflection seismic data along two parallel seismic profi

physiographic and tectonic settings of high sulfidation epithermal gold silver deposits of the andes and their controls on mineralizing processes abstract gold and silver ores in the vast majority of andean high sulfidation epithermal au ag deposits occur at high present day elevations and typically 200 500 m below low relief landforms situated at 3500 to 5200 m a s l most deposits are middle miocene and younger and include el indio tambo pascua lama veladero el indio belt chile argentina cerro de pasco central peru pierina lagunas norte yanacocha northern peru quimsacocha ecuador and the california vetas mining district santander colombia jointly accounting for 130 moz au resources slightly older examples are only preserved in the atacama desert and include the middle eocene el guanaco and el hueso and the late oligocene early miocene la coipa deposits the absence of paleocene and older high sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift these conditions prevailed predominantly in segments of shallow angle subduction of the nazca or caribbean plate below the south american continent a tectonic setting also common for porphyry style cu au mo deposits stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent recording the terminal stages of local arc magmatism however dacitic domes are important at e g yanacocha and la coipa at lagunas norte a small stratovolcano largely pre dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high elevation low relief surfaces in the california vetas mining district and el indio belt hydrothermal alunite ages become generally younger upstream along the incising valleys indicating that hydrothermal activity and by inference ore deposition were facilitated by erosion the lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids ultimately enhancing ore deposition the host rock composition permeability and location of the water table control the distribution of alteration zones and ore intermediate volcanic rocks are the most common ore hosts but they typically pre date mineralization by several ma however high sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks california vetas mining district significantly older plutonic rocks pascua lama or quartzites lagunas norte large vuggy quartz alteration zones and commonly oxidized low grade large tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies whereas coherent volcanic plutonic or metamorphic rocks may host fault and breccia controlled ores the near surface steam heated zone can attain a thickness of several hundred meters in dry climates e g veladero pascua lama tambo but is typically poorly developed and less than 20 m thick in humid climatic zones the physiographic and tectonic settings of high sulfidation epithermal deposits are distinct from low sulfidation epithermal districts such as those of patagonia el penon chile or fruta del norte ecuador the latter range to significantly older ages jurassic to early eocene occur at mainly lower elevations and were emplaced in extensional settings a temporal coincidence between uplift erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low sulfidation districts

a scenario for late neogene andean shortening transfer in the camisea subandean zone peru 12 s implications for growth of the northern andean plateau precise knowledge of the timing of deformation in the subandean zone of the andean plateau is a prerequisite for deciphering the late neogene growth of the andean plateau in this paper we report new apatite fission track aft and vitrinite reflectance ro data for a regional balanced cross section of the camisea basin in the central peruvian subandean zone adjacent to the northern andean plateau the balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust related anticlines the balanced cross section restoration shows 53 km 39 of total horizontal shortening we sampled paleozoic to cenozoic sedimentary strata for aft and ro analyses along the 4 km thick vertical profile of the mainique back thrust passive roof thrust the innermost preserved subandean structure young components of aft ages are spread between ca 6 ma and ca 24 ma a break in the slope in the aft ages determines the geometry of the miocene partial annealing zone and the exhumation of the mainique back thrust at ca 6 ma sequential restoration calibrated by aft and ro data indicates that the last 23 km horizontal shortening were accommodated by the camisea thrust system over the past 6 m y giving a mean shortening rate of 3 8 mm yr using this shortening rate for the first 30 km horizontal shortening we calculate that the andean shortening transfer into the peruvian subandean zone initially started at ca 14 ma this result suggests that the transfer of shortening from the northern andean plateau to the subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca 10 ma and ca 7 ma we rather propose that the late neogene growth of the northern andean plateau mostly resulted from a continuous crustal shortening combined with lower crustal flow

natural age dispersion arising from the analysis of broken crystals part i theoretical basis and implications for the apatite u th he thermochronometer abstract over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite u th he thermochronometry and it is now standard practice and generally seen as best practice to analyse single grain aliquots these individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite this is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains and evidence of freshly broken ends when grains are viewed using a scanning electron microscope this matters because if the 4 he distribution within the whole grain is not homogeneous because of partial loss due to thermal diffusion for example then the fragments will all yield ages different from each other and from the whole grain age here we use a numerical model with a finite cylinder geometry to approximate 4 he ingrowth and thermal diffusion within hexagonal prismatic apatite crystals this is used to quantify the amount and patterns of inherent natural age dispersion that arises from analysing broken crystals a series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios in addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets the results indicate that natural dispersion of a set of single fragment ages defined as the range divided by the mean arising from fragmentation alone varies from c 7 even for rapid c 10 c ma monotonic cooling to over 50 for protracted complex histories that cause significant diffusional loss of 4 he the magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius and is of a similar magnitude to dispersion expected from differences in absolute grain size alone spherical equivalent radii of 40 150 m this source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses c 6 and standard deviations on multiple grain analyses from a single sample c 10 20 where there is a significant difference in the u and th concentration of individual grains eu the effect of radiation damage accumulation on 4 he diffusivity assessed using the rdaam model of flowers et al 2009 is the primary cause of dispersion for samples that have experienced a protracted thermal history and can cause dispersion in excess of 100 for realistic ranges of eu concentration i e 5 100 ppm expected natural dispersion arising from the combined effects of reasonable variations in grain size radii 40 125 m eu concentration 5 150 ppm and fragmentation would typically exceed 100 for complex thermal histories in addition to adding a significant component of natural dispersion to analyses the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration eu considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic an important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 he distribution within the whole grains thus providing an important source of information about the thermal history of the sample we suggest that if the primary focus of a study is to extract the thermal history information from u th he analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages not minimise it and should aim to analyse circa 20 30 grains from each sample the key observations and conclusions drawn here are directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

signal enhancement using pattern recognition techniques with application to near vertical crustal seismic reflection experiments most near vertical crustal seismic reflection experiments are conducted over regions where in the past the crust has undergone some form of tectonic deformation the deep structures differ from the near surface sedimentary structures in that they are generally not layered seismic energy after travelling through long ray paths in such media arrives at slightly different times at the receivers and hence tends to be poorly aligned along its respective theoretical travel time curves conventional seismic processing routines which attempt to increase the signal to noise ratio s n by some stacking technique can in many cases actually destroy the poorly aligned signals such that the resultant subsurface images are not very satisfactory our study focuses on a signal enhancement technique based on a pattern recognition approach which is applied in the prestack processing stream in an effort to increase the s n ratio the discrimination between signal and noise is based on attributes such as the frequency amplitude waveshape and lateral continuity of the signal the method was first tested on synthetic data and then applied to real data acquired over the canadian shield signal enhancement using a pattern recognition approach as outlined in this paper dramatically improved the subsurface images over those obtained by more conventional stacking methods

geological analysis of aeromagnetic data from southwestern alaska implications for exploration in the area of the pebble porphyry cu au mo deposit aeromagnetic data are used to better understand the geology and mineral resources near the late cretaceous pebble porphyry cu au mo deposit in southwestern alaska the reduced to pole rtp transformation of regional scale aeromagnetic data shows that the pebble deposit is within a cluster of magnetic anomaly highs similar to pebble the iliamna kijik and neacola porphyry copper occurrences are in magnetic highs that trend northeast along the crustal scale lake clark fault a high amplitude short to moderate wavelength anomaly is centered over the kemuk occurrence an alaska type ultramafic complex similar anomalies are found west and north of kemuk a moderate amplitude moderate wavelength magnetic low surrounded by a moderate amplitude short wavelength magnetic high is associated with the gold bearing shotgun intrusive complex the rtp transformation of the district scale aeromagnetic data acquired over pebble permits differentiation of a variety of jurassic to tertiary magmatic rock suites jurassic cretaceous basalt and gabbro units and late cretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs tertiary basalt units also produce magnetic highs but appear to be volumetrically minor eocene monzonite units have associated magnetic lows the rtp data do not suggest a magnetite rich hydrothermal system at the pebble deposit the 10 km upward continuation transformation of the regional scale data shows a linear northeast trend of magnetic anomaly highs these anomalies are spatially correlated with late cretaceous igneous rocks and in the pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems the spacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northern chile these anomalies are interpreted to reflect a late cretaceous magmatic arc that is favorable for additional discoveries of late cretaceous porphyry copper systems in southwestern alaska

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

isotopic evidence for microbial activity during supergene oxidation of a high sulfidation epithermal au ag deposit the light stable isotope chemistry of barite and acanthite in the pierina au ag deposit peru provides strong support for microbial involvement at the economically critical supergene oxidation stage early hypogene barite yields 34s values of 23 6 to 28 5 and 18o values of 5 8 to 10 9 and was precipitated with the original precious metal enriched sulfide assemblage in contrast late barite deposited during formation of supergene goethite and hematite the main ore hosts yields 34s and 18o values of 1 4 to 14 2 and 2 8 to 4 7 correlated mutually and with the 34s values of coprecipitated acanthite 34s 0 4 to 3 9 low 34s values of barite and acanthite record sulfide mineral oxidation by meteoric waters increasing 34s and 18o values of barite and 34s values of acanthite indicate enrichment of the sulfate reservoir in 34s and 18o reflecting the preferential utilization of light isotopes by microbes during aqueous sulfate reduction continued sulfate reduction locally reduced supergene fluids remobilizing iron and locally destroying the goethite hematite assemblage precious metals were released but reacted with sulfide in the reduced waters to form au rich acanthite

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

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

timing of tectonic and magmatic events in the central andes of peru the study of fault displacements in a region of well dated sequential granitic intrusions suggests a close time relationship between two peaks of deformation and intrusive activity mantle derived basic magmas were associated with both episodes which are dated as mid cretaceous and early palaeocene it is concluded that in each case a period of tensile tectonics with mafic dyke injection preceded the intense compressive deformation which accompanied the generation and rise of granitoid magmas these events may be related to alternations of slow and rapid subduction

seismic imaging of massive sulfide deposits part ii reflection seismic profiling seismic reflection profiling the dominant geophysical method for hydrocarbon exploration has the potential to provide images of regional structure for mineral exploration in the crystalline crust and direct detection of large massive sulfide orebodies previous tests of traditional seismic methods in hard rock environments have had mixed success based on these earlier results we have tailored acquisition and processing strategies to conditions in the canadian shield we illustrate these methods using an example from the sudbury basin a rich mineral producing region because of structural complexity that complicates interpretation of the seismic images the seismic data are integrated with detailed three dimensional forward modeling based on physical properties studies mine geology and well logging through this integrated approach we show that massive sulfides can produce a characteristic seismic reflection response and that surface seismic reflection profiling may be used to detect and delineate deep large massive sulfide deposits accurately in a complex geologic setting characterized by moderate dips

interpreting regional 3d fault networks from integrated geological and geophysical data sets an example from the guichon creek batholith british columbia abstract the effects of fault processes on rocks commonly include a lowered magnetic susceptibility and an increased predisposition to erosion we present a workflow that uses these characteristics of faults and geological observations to interpret a 3d fault network the location of faults on the topographic surface is first interpreted by a multi layer lineament mapping method that includes geological and geophysical data sets we then demonstrate how magnetic intensity data can be used to estimate the dip of fault related magnetic anomalies by performing 2d inverse modeling along profiles extracted from a magnetic intensity grid the accuracy of the method is assessed by modeling the dip of low magnetic anomalies of known geometry created in a synthetic 3d magnetic susceptibility model modeled dips are consistently accurate to better than 5 for input dips 60 a requirement of the method however is that the magnetic susceptibility contrast between the background field and the magnetic anomaly must be 75 for the anomaly to be accurately modeled comparison between fault orientations modeled from magnetic data and measured in the field in the guichon creek batholith confirms that the methodology can be successfully applied to brittle faults in real albeit relatively simple geological environments

seismic reflection imaging over the south portuguese zone fold and thrust belt sw iberia 1 we reprocessed an 35 km long part of the ibesreis seismic reflection profile which runs over the iberian pyrite belt section of the south portuguese zone sw iberia with the goal to image the upper crust 15 km depth the applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5 s travel time relative to high amplitude source generated noise a complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked line prestack migration scheme for coherent imaging to interpret sources of diffracted energy we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections the final seismic images show south vergent imbricate fold and thrust tectonics documenting the contractive deformation that the south portuguese zone experienced during the variscan orogeny based on surface geological information we correlate a low reflective unit with the shallow upper carboniferous flysch group a highly reflective unit ranging in depth from 2 to 4 km with the middle carboniferous volcano sedimentary complex group which hosts massive sulfide deposits and a moderately reflective unit with the upper devonian phyllite quartzite group below these units another low reflective facies is present which may represent older paleozoic metasediments in addition the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions these proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in early carbonifereous times

carbon isotope studies of auriferous quartz carbonate veins from two orogenic gold deposits from the neoarchean chitradurga schist belt dharwar craton india evidence for mantle magmatic source of auriferous fluid abstract carbon and oxygen isotopic compositions of carbonates from auriferous quartz carbonate veins qcvs of two orogenic gold deposits ajjanahalli and guddadarangavvana halli g r halli from the neoarchean chitradurga schist belt of the dharwar craton southern india are examined to understand the origin of the mineralizing fluids the average carbonate carbon 13cpdb and oxygen 18osmow isotope compositions of qcvs of ajjanahalli are 5 5 1 3 and 14 1 2 7 respectively the same ratios for the qcvs of g r halli are 6 2 1 9 and 14 1 0 5 the corresponding average fluid 13c and 18o compositions are 5 81 1 14 13 78 5 1 for ajjanahalli and 4 64 0 7 6 50 0 6 for g r halli the 13cpdb of syn sedimentary carbonates of bif of ajjanahalli 1 8 0 1 carbonated metabasalts of ajjanahalli 1 4 and g r halli 1 3 fall in the compositional range of marine carbonates 0 2 as dissolution decarbonation reactions during metamorphism of pre existing carbonate carbonated rocks produce co2 with 13c values similar to or more enriched than parent rock the carbonate or fluid 13c ratios of the qcvs which fall in the compositional range of mantle magmatic derived co2 or carbonates obtained in this work cannot be the result of metamorphism it is proposed that gold mineralizing fluids were derived from juvenile magmatic melts and were channeled through crustal scale shear zones to give rise to the gold deposits

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

geophysical inversion contributions to mineral exploration lessons from the footprints project magnetic and gravity inversions are used to create 2d or 3d models of the magnetic susceptibility and density respectively using potential field data unconstrained inversions generate an output

the structure and kinematics of the southeastern zagros fold thrust belt iran from thin skinned to thick skinned tectonics 1 we present the first complete balanced cross section across the southeastern zagros fold thrust belt zftb the following main structural features emerge from this section 1 in the south of the zftb the proterozoic to recent sedimentary sequence has been decoupled from its panafrican basement along the ductile basal evaporites and folded into a series of large detachment anticlines ongoing shortening of these structures has resulted in migration of the basal salt layers into the cores of the anticlines and propagation of forelimb thrusts 2 in the north of the zftb deep seated ramps have folded the hanging wall rocks and produced imbrications and duplex structures within the higher levels of the sedimentary sequence 3 out of sequence thrusts linked to major seismogenic basement faults have cut through the structures in the cover of the zftb a three step incremental restoration of the section shows that two main phases of deformation can be separated in the tectonic evolution of the zftb a mio pliocene thin skinned phase in the course of which most of the structures in the cover were generated followed by a pliocene to recent thick skinned phase expressed as out of sequence faulting in the cover which is currently underlined by the seismicity within the basement in plan view the initial structures of the southeastern zftb developed with a curved shape essentially controlled by the shape and thickness of the underlying proterozoic salt basin i e the jura style in the following basement involved phase out of sequence thrusts cut at oblique angles through the preexisting structures of the cover the total shortening absorbed in the cover amounts to at least 45 km corresponding to a ratio of 22

apatite u th sm he thermochronology of rapidly cooled samples the effect of he implantation apatite u th sm he ahe thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface results of ahe thermochronology however are often in conflict with apatite fission track aft thermochronology yielding an inverted age relationship with ahe dates older than aft dates of the same samples this effect is mainly explained by radiation damage of apatite either impeding he diffusion or causing non thermal annealing of fission tracks so far systematic age inversions have only been described for old and slowly cooled terranes whereas for young and rapidly cooled samples too old ahe dates are usually explained by the presence of undetected u and or th rich micro inclusions we report apatite u th sm he results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post depositional thermal history robust age constraints are provided independently through sample biostratigraphy all studied apatites have low u contents 5 ppm on average while aft dates are largely in agreement with deposition ages most ahe dates are too old for leg 43 where deposition age of sampled sediment is 26 5 29 5 ma alpha corrected average ahe dates are up to 45 ma indicating overestimations of ahe dates up to 50 this is explained by he implantation from surrounding host u th rich sedimentary components and it is shown that ahe dates can be corrected by mechanically abrading the outer part of grains we recommend that particularly for low u th apatites the possibility of he implantation should be carefully checked before considering the degree to which the alpha ejection correction should be applied

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

new 40ar 39ar alunite ages from the colquijirca district peru evidence of a long period of magmatic so2 degassing during formation of epithermal au ag and cordilleran polymetallic ores we present 40ar 39ar data acquired by infra red co2 laser step heating of alunite crystals from the large miocene colquijirca district in central peru combined with previously published data our results show that a long at least 1 3 my and complex period of magmatic hydrothermal activity associated with epithermal au ag mineralization and base metal cordilleran ores took place at colquijirca the new data indicate that incursion of magmatic so2 bearing vapor into the colquijirca epithermal system began at least as early as 11 9 ma and lasted until 10 6 ma four alunite samples associated with high sulfidation epithermal au ag ore gave 40ar 39ar plateau ages between 11 9 and 11 1 ma compared to the previously documented 11 6 to 11 3 ma by combining individually these new ages with crosscutting relationships the duration of the au ag deposition period can be estimated to at least 0 4 my three new 40ar 39ar plateau ages on alunite associated with the base metal cordilleran ores are consistent with previously obtained ages all of them between 10 83 0 06 and 10 56 0 06 ma suggesting that most of the sulfide rich polymetallic deposits of smelter and colquijirca formed during this short period the recognition of consecutive alunite bearing and alunite free mineral assemblages within both the au ag and the base metal cordilleran ores may suggest that so2 bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses a number of which was not necessarily associated in time with ore fluids it is likely that a period of so2 bearing vapor degassing longer than 11 9 to 10 6 ma may be recognized with further more detailed work

zircon fission track thermochronology and applications to fault studies zircon is one of the most useful minerals used to unravel the earth s history recorded in rocks after the success of etching fission tracks in zircon fleischer et al 1964 naeser 1969 krishnaswami et al 1974 early studies focused primarily on dating young volcanic horizons for stratigraphic purposes e g naeser et al 1973 hurford et al 1976 seward 1979 gleadow 1980 kowallis et al 1986 kohn et al 1992 subsequently fission track analysis of zircon has been extensively employed along with other radiometric dating methods such as u pb and u th he techniques e g davis et al 2003 reiners 2005 to understand the thermochronology of rocks in a variety of geological settings i e thermal history analysis of basement rocks for orogenic studies e g zeilter et al 1982 hurford 1986 kamp et al 1989 sorkhabi 1993 seward and mancktelow 1994 hasebe and tagami 2001 spikings et al 2001 thermochronology using detrital zircon grains in sedimentary rocks for provenance analysis and thermal history analysis in or near faults which will be highlighted below in this present chapter the thermal sensitivity of the zircon fission track thermochronometry is reviewed in the context of laboratory and field studies then laboratory procedures of the zircon fission track analysis using the external detector method are outlined as practiced in the fission track laboratory of kyoto university for basic fundamentals of these fission track analyses including data analysis and graphical displays see the chapter by tagami and o sullivan 2005 finally as an example of recent geological applications of the technique the results of zircon fission track analysis of the nojima fault zone japan are presented these rocks were the target of systematic drilling of an active fault system using five boreholes and a trench applications to other geological settings and the