rare earth element signatures of economic and sub economic porphyry copper systems in urumieh dokhtar magmatic arc udma iran abstract a wide variety of porphyry copper deposits have been reported along the cenozoic urumieh dokhtar magmatic arc in iran the formation of these deposits is attributed to closure of neo tethys and the subsequent collisional tectonic regime during the tertiary this study presents whole rock rare earth element ree data for the giant to small sarcheshmeh meiduk sungun darreh zerreshk dalli iju parkam and ali abad with sub economic daraloo and reagan porphyry copper systems the data indicate a temporal and geochemical evolution from sub economic porphyry systems to economic deposits all the economic and sub economic porphyry copper systems exhibit lree enrichment relative to hree whereas the economic deposits exhibit a relatively steep downward lree to hree profile reflecting higher lree abundance the eu anomalies vary from distinct negative in sub economic deposits eu eu 0 28 0 70 to either markedly less negative or positive anomalies eu eu 0 45 1 67 in economic deposit the economic porphyry deposits are characterized by relatively high la sm and sm yb values representing high crustal assimilation in a relatively thickened crust and provide insight into fractionation of hornblende with minor garnet in deep crustal parts mash zone compared with sub economic deposits the steep downward lree to mree and flatter to slightly upward mree to hree in economic deposits indicate hornblende involvement magma evolution toward more volatile content it seems that in an ongoing process of closure of neo tethys during compression and crustal shortening the early eocene oligocene sub economic intrusions are followed by adakite like hydrous miocene and younger economic cu bearing intrusions

copper mineralization prevented by arc root delamination during alpine himalayan collision in central iran contrasting geochemical signatures of copper ore hosting eocene and some barren undeformed miocene diorites to granites in central iran temporally overlap with the alpine himalayan collision and provide key implications on the existence and lack of cu mineralization during collisional magmatism high sr and low y and yb contents of eocene arc rocks in the natanz arc segment reflect thickened andean type orogenic arc crust 45 km whereas barren miocene natanz arc rocks 21 19 ma indicate thin arc crust similar to collisional volcanism in anatolia geochemical modeling indicates a change in the mineralogy of the melt residual from precollisional eocene basaltic garnet bearing 5 30 amphibolite to syn or postcollisional miocene metasomatized mantle peridotite which can be explained by collision induced delamination of the arc lithospheric root subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet bearing arc crustal keel explain the low sr and high y and yb contents the relatively enriched initial sr isotope ratios of postcollisional miocene natanz rocks and the lack of copper mineralization in postcollisional miocene natanz arc rocks arc root delamination removes the copper and sulfur enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits lack of the dense melt residues also provides an alternative explanation for the elevated thin crustal iranian back arc plateau 38 km as a result of uplift by isostatic rebound rather than uplift by anomalous shortening miocene arc root delamination implies a minimum age of 21 ma for the alpine himalayan collision in central iran

implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

balancing crustal thickening in arcs by tectonic vs magmatic means igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the late cretaceous to late eocene magmatic arc in north chile 21 45 22 30 s temporally changing major and trace elements of magmatic rocks from this paleo arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening balanced structural cross sections indicate a minimum of 9 km of arc normal shortening that occurred simultaneously with dextral arc parallel movements accounting for orogen parallel lengthening of 10 km this shortening produced 5 4 km of tectonic crustal thickening and resulted in a minimum of 42 km late eocene andean crustal thickness temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder 2 6 km was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression the ratio of tectonic to magmatic crustal thickening is 2 1 whole crustal magmatic addition rates during the 12 m y duration of arc transpression are 35 km 3 per kilometer of model arc length per million years mafic underplating may have thickened the andean crust considerably but most pre neogene crustal thickening was due to discrete episodes of tectonic shortening

cenozoic tectonic evolution in the central andes in northern chile and west central bolivia implications for paleogeographic magmatic and mountain building evolution a review of available stratigraphic structural and magmatic evolution in northernmost chile and adjacent peru and bolivia shows that in this region 1 compression on the paleogene intra arc during the middle eocene incaic phase formed the nne ssw oriented incaic range along the present day precordillera and western cordillera and 2 post incaic tectonic conditions remained compressive until present contrasting with other regions of the andes where extensional episodes occurred during part of this time lapse a late oligocene early miocene peak of deformation caused further uplift the incaic range formed a pop up structure bounded by two thrusts systems of diverging vergencies it represented a major paleogeographic feature that separated two domains with different tectonic and paleogeographic evolutions and probably formed the andean water divide this range has been affected by intense erosion and was symmetrically flanked by two major basins the pampa del tamarugal and the altiplano magmatic activity remained located along the previous late cretaceous early eocene arc with slight eastward shift further compression caused westvergent thrusting and uplift along the western eastern cordillera bounding the altiplano basin to the east by another pop up shaped ridge eastward progression of deformation caused eastvergent thrusting of the eastern cordillera and subandean zone

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

cosmogenic 3he production rates from 39 n to 46 n latitude western usa and france the in situ production rate of cosmogenic 3he was determined in seven different olivine or pyroxene bearing rocks between the latitudes 39 n and 46 n the integrated production rate over the last 17 500 years is about 115 4 atoms gram yr for high latitude at sea level the variation in the integrated production rate between 2000 and 18 000 years is less than 30 and is probably on the order of 20 or less

miocene quaternary structural evolution of the uyuni atacama region andes of chile and bolivia abstract we describe the miocene quaternary geological structural evolution of the region between the salar de uyuni and de atacama andes of chile and bolivia we recognized four main tectonic events based on fold geometry fault kinematics and stratigraphic data the oldest event of miocene age is characterized by folding and reverse faulting of the sedimentary successions with an e w direction of shortening in the northern part of the studied area and a wnw ese shortening in the southern part the following two events of pliocene age are characterized by lower shortening amounts they occurred first by reverse faulting with a nw se trending greatest principal stress 1 computed with striated fault planes and a vertical least principal stress 3 followed by pervasive strike slip faulting with the same nw se trending 1 and a horizontal ne sw 3 the fourth event dating to the late pliocene quaternary is characterized by normal faulting the 3 still trends ne sw whereas the intermediate principal stress 2 exchanged with 1 volcanism accompanied both the contractional transcurrent and extensional tectonic phases the mio pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates the e w to wnw ese direction of shortening of the miocene structures and the nw se 1 of the pliocene structures seem to be more linked to an intra andean re orientation of structures following the wnw directed absolute motion of the south american plate the extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical sw ward collapse of the belt

influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides abstract scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field these scaling factors are analogous to those determined in a similar work dunai t j earth planet sci lett 2000 176 157 169 that were accurate for field strengths equivalent to present day field strength but cannot accommodate field strength variations strategies to derive the relevant geomagnetic field parameters are discussed and are tested with existing production rate calibrations for 3 he a production rate of 103 4 2 atoms g olivine yr sea level and for zero cutoff rigidity was derived from calibration sites from hawaii western usa and canary islands covering the time range of 2 281 kyr

spatial and temporal variations in the geochemistry of cretaceous high sr y rocks in central tibet recent studies based on low temperature chronology and sedimentology have proposed the existence of a proto tibetan plateau p tp however the timing and mechanisms of its formation and evolution remain ambiguous high sr y rocks are an important petrological indicator of thickening here we compile geochemical data of cretaceous rocks to interpret their petrogenesis and to constrain deep geodynamic processes geochemical characteristics in combination with zircon hf isotopic compositions indicate that the high sr y rocks were derived from the partial melting of thickened juvenile lower crust with or without contamination by mantle peridotite comparing geochronological and geochemical data we observe a correlation between magma migration and the composition of high sr y rocks based on these observations we propose a revised tectonomagmatic evolution model for central tibet involving crustal thickening retreating delamination and breakoff our research suggests that the rapid uplift of the p tp was a consequence of the removal of isostatic load during the mesozoic

along strike segmentation of the abanico basin central chile new chronological geochemical and structural constraints the andes of central chile are composed mostly of cenozoic volcanic rocks erupted during the opening and subsequent inversion of the intra arc volcano tectonic abanico basin until recently the internal segmentation of this inverted basin was poorly understood based on a combination of u pb geochronology u th he thermochronology whole rock geochemistry and structural data we propose that the abanico basin can be divided into two main segments separated by the nw striking piuquencillo fault and conjugate ne striking faults stratigraphic units defined in the northern segment abanico and farellones formations cannot be correlated with the rocks of the southern segment coya machali formation and teniente volcanic complex in terms of lithofacies depositional ages and geochemistry the northern and southern segments also show temporal differences in their tectonic evolution an early deformation event beginning at 22 ma affected only the northern segment and is associated with the formation of progressive unconformities between the abanico and farellones formations and also with crustal thickening as reflected in the geochemistry of the farellones formation a second stage of crustal thickening and exhumation began at 12 ma as suggested by a sharp increase of the la yb ratios in the northern segment in the southern segment this event is reflected by only a moderate increase of la yb ratios and by the transition between the coya machali formation and the teniente volcanic complex finally a third stage of exhumation and crustal thickening beginning at 7 ma affected both the northern and southern segments this last stage was the main exhumation event affecting the rocks of the andes of central chile and is recorded in the geochemistry of igneous rocks by a sharp increase in the la yb ratios in the southern segment

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

the effects of secular disequilibrium on u th he systematics and dating of quaternary volcanic zircon and apatite the u th he dating method applied to u rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions however in phases with crystallization ages less than 1 ma chemical fractionation within the 238 u decay series may modify the he ingrowth rate causing he ages computed from the secular equilibrium age equation to be incorrect the resulting systematic error depends on the 230 th 238 u activity ratio of the dated phase when it is erupted and on the eruption age zircons which exclude th relative to u will likely have secular equilibrium he ages that underestimate the eruption age by up to a few tens of decreasing with increasing eruption age apatites tend to accommodate u and th with little fractionation so apatite secular equilibrium he ages will be nearly concordant with eruption age if minerals are erupted immediately after crystallization the disequilibrium effect can be reasonably accounted for based on th u systematics however crystals are likely to reside for unknown but potentially long periods in a magma chamber such that the degree of secular disequilibrium will be reduced prior to the onset of he accumulation u th he analyses of co genetic phases that fractionate the u th ratio differently like apatite and zircon can be used to better constrain eruption age as well as to provide insights into magma chamber residence time we illustrate this approach with u th he analyses of zircons and apatites of the pleistocene age rangitawa tephra new zealand

mid crustal emplacement and deformation of plutons in an andean style continental arc along the northern margin of the north china block and tectonic implications abstract outcrop mapping and thermobarometrical thermochronological and microstructural analyses in five late carboniferous early permian arc plutons in the northern north china block ncb indicate a transition of tectonic regime from arc perpendicular contraction to transpression and successive uplift and exhumation of the crustal rocks during magma emplacement and construction of the continental arc system the early emplaced daguangding and boluonuo plutons 324 302 ma display well developed penetrative moderately to steeply dipping magmatic foliations and high temperature sub solidus foliations that are parallel to host rock foliation near the contact widespread high temperature sub solidus foliations in these plutons indicate that they are syn tectonic with n s arc perpendicular shortening and record arc construction at a convergent continental margin however the late emplaced tianqiao and xianghuangqi wudaoyingzi plutons 288 274 ma display relatively weak magmatic fabrics at their margins and lack sub solidus foliations and pluton related ductile deformation of the contact host rocks these structural differences likely reflect a kinematic transition from early arc perpendicular contraction to late sinistral transpression during continental arc formation aluminum in hornblende barometry shows that their estimated emplacement depths decrease gradually from the late carboniferous 18 7 0 8 km to early permian 13 8 0 9 km indicating successive exhumation of the crustal rocks at an average rate of ca 123 mm ka over 40 million years of arc formation 40 ar 39 ar thermochronological results show that cooling of the continental crust was very slow 4 9 c ma during arc construction in the late carboniferous late permian but increased significantly during the late permian middle triassic after arc termination the mid crust of the northern ncb remained very hot 530 c during arc construction and emplacement of the arc batholiths combined with emplacement depth estimated by aluminum in hornblende barometry paleogeothermal gradients of 42 8 46 5 c km were estimated in mid to upper crustal levels 0 19 km hot mid crust and high mid upper crust geothermal gradients resulted from magmatic advection are likely the most important reasons for crust thickening and uplift during construction of andean style continental arcs and largely influenced the rheological behavior of continental crust

growth and fragmentation of the andean foreland basin during eastward advance of fold thrust deformation puna plateau and eastern cordillera northern argentina 1 the fault bounded cianzo basin represents a cenozoic depocenter between the puna plateau and eastern cordillera of northern argentina analysis of fold thrust relationships nonmarine sedimentation and detrital provenance at 23 24 s helps constrain the origin interconnectedness and subsequent uplift and exhumation of the cianzo basin a potential analog for intermontane hinterland basins in the andes structural mapping reveals a plunging syncline within the 6000 m thick upward coarsening cenozoic clastic succession in the shared footwall of the n striking e directed cianzo thrust fault and transverse ne striking hornocal fault growth strata within upper miocene levels indicate syncontractional accumulation adjacent to the hornocal fault measured stratigraphic sections show upsection changes from 1 paleosol rich distal fluvial sandstones 400 m paleocene eocene santa barbara subgroup to 2 braided fluvial sandstones and mudstones 1400 m upper eocene oligocene casa grande formation to 3 distributary fluvial megafan sandstones and conglomerates 3300 m upper oligocene miocene rio grande formation to 4 alluvial fan conglomerates 1600 m upper miocene pisungo formation the 40ar 39ar geochronological results for interbedded tuffs indicate continuous rio grande deposition from at least 16 34 0 71 to 9 69 0 05 ma sandstone petrographic results define distinct upsection trends in lithic and feldspar content potentially distinguishing the western cordillera magmatic arc from puna eastern cordillera thrust belt sources in addition to growth stratal relationships and 40ar 39ar ages conglomerate clast compositions reflect distinct lithologic differences constraining activation of the cianzo thrust and coeval reverse slip on the reactivated inverted hornocal fault finally detrital zircon u pb ages paleocurrents and facies patterns distinguish local from distal sources revealing a systematic forelandward eastward advance of eocene through upper miocene fold thrust deformation

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

quantifying the diffusion kinetics and spatial distributions of radiogenic 4he in minerals containing proton induced 3he apatite titanite and olivine samples were bombarded with a 150 mev proton beam to produce 10 8 atoms mg of spallation 3he high precision stepped heating experiments were then performed in which the artificial 3he and for apatite and titanite the natural radiogenic 4he were measured to characterize the diffusive behavior of each isotope helium 3 diffusion coefficients are in excellent agreement with concurrently and or previously determined he diffusion coefficients for each mineral our results indicate that proton induced 3he is uniformly distributed and that radiation damage associated with a proton fluence of 5 x 10 14 protons cm 2 does not cause noticeable changes in 4he diffusion behavior in at least apatite and titanite proton induced 3he can therefore be used to establish he diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural 4he distribution is inhomogeneous in addition step heating 4he 3he analysis of a mineral with a uniform synthetic 3he concentration provides a means by which a natural 4he distribution can be determined

geomorphic evidence for post 10 ma uplift of the western flank of the central andes 18 30 22 s 1 the western andean mountain front forms the western edge of the central andean plateau between 18 5 and 22 s latitude the mountain front has 3000 m of relief over 50 km horizontal distance that has developed in the absence of major local neogene deformation models of the evolution of the plateau as well as paleoaltimetry estimates all call for continued large magnitude uplift of the plateau surface into the late miocene i e younger than 10 ma longitudinal river profiles from 20 catchments that drain the western andean mountain front contain several streams with knickpoint bounded segments that we use to reconstruct the history of post 10 ma surface uplift of the western flank of the central andean plateau the generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to pacific ocean capture eustatic change or climate change as causes for creating the knickpoint bounded stream segments observed minor valley filling alluvial gravels intercalated with the 5 4 ma carcote ignimbrite suggest uplift related river incision was well under way by 5 4 ma the maximum age of river incision is provided by the regionally extensive approximately 10 ma el diablo altos de pica paleosurface the river profiles reveal that relative surface uplift of at least1 km occurred after 10 ma

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

long term cosmogenic 3he production rates 152 ka 1 35 ma from 40ar 39ar dated basalt flows at 29 n latitude a set of time integrated cosmogenic 3he production rates in olivines for the last 1 35 ma are presented we investigated three basaltic lava flow tops from lanzarote canary islands spain the 40ar 39ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152 26 ka 281 19 ka and 1 35 0 01 ma 2 three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3he abundances the resulting 3he production rates in olivine at sea level at 29 latitude are 82 14 and 82 8 atoms g 1 a 1 as obtained from the 152 ka and 281 ka old flows respectively considering effects of erosion on the 1 35 ma old flow we find that the production rate of 82 8 atoms g 1 a 1 is consistent with the cosmogenic 3he production during the last 1 35 ma there appears to be a 14 discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of lal are used to compare results this discrepancy is greatly reduced however if the revised scaling factors of dunai this issue are applied using the new scaling factors we derive a production rate for cosmogenic 3he in olivine at sea level and high latitudes 60 of 118 11 atoms g 1 a 1 2 the correspondingly revised value of cerling and craig and ackert et al is 123 6 atoms g 1 a 1 2 the mean value of these two calibrations is 121 6 atoms g 1 a 1 2 we suggest that the production rate of 121 6 atoms g 1 a 1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long term averaged intensity variation in the earth s magnetic field i e over the last 10 ma

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

using pyroxene microphenocrysts to determine cosmogenic 3 he concentrations in old volcanic rocks an example of landscape development in central gran canaria abstract determinations of cosmogenic 3he exposure ages and erosion rates in volcanic rocks older than a few hundred thousand years are complicated by the presence of radiogenic he in addition to the magmatic and cosmogenic he in phenocryst minerals however by analysing microphenocrysts that crystallised on or immediately prior to eruption that have not trapped magmatic he the three component problem can be eliminated and accurate determinations of cosmogenic 3he made in this study we perform three experiments using pyroxene microphenocrysts in basaltic clasts in the pliocene ayacata formation breccias gran canaria that demonstrate they are free of magmatic he exposure ages and erosion rates calculated from the cosmogenic 3he concentrations are combined with a geomorphological study to produce a tentative interpretation of landscape evolution in the mountainous interior of gran canaria long term steady state erosion rates of 14 24 mm ka 1 are recorded from bedrock erosional surfaces on a high plateau headwall retreat rates for a major drainage system of 1 6 m ka 1 have been constrained from the ca 225 ka exposure age of a boulder emplaced on slopes beneath the headwall strath terraces and boulders in a small canyon system yield much younger exposure ages of 47 43 ka

tectonic evolution of the central andean plateau and implications for the growth of plateaus current end member models for the geodynamic evolution of orogenic plateaus predict a slow and steady rise during crustal shortening and ablative subduction i e continuous removal of the lower lithosphere or b rapid surface uplift following shortening which is associated with punctuated removal of dense lower lithosphere and or lower crustal flow this review integrates results from recent studies of the modern lithospheric structure geologic evolution and surface uplift history of the central andean plateau to evaluate the geodynamic processes involved in forming it comparison of the timing magnitude and distribution of shortening and surface uplift in combination with other geologic evidence highlights the pulsed nature of plateau growth we discuss specific regions and time periods that show evidence for end member geodynamic processes including middle late miocene surface uplift of the southern eastern cordillera and altiplano associated with shortening and ablative subduction latest

u pb zircon dating geochemical and sr nd hf isotopic compositions of mafic intrusive rocks in the motuo se tibet constrain on their petrogenesis and tectonic implication abstract mafic intrusive rocks are widely exposed in the motuo tectono magmatic belt southeast lhasa terrane la icp ms u pb zircon dating shows that they have magma crystallization ages of 69 and ca 50 ma these mafic intrusive rocks are characterized by variable sio 2 44 60 57 60 wt high al 2 o 3 17 19 20 86 wt and low mgo 1 85 5 38 wt with mg of 31 55 their chemical composition is comparable with low mgo high al basalts to basaltic andesites they are enriched in liles rb ba k and lree and depleted in hfses nb ta zr hf ti with weakly evolved sr nd hf compositions whole rock 87 sr 86 sr 0 0 7064 to 0 7086 e nd t 3 41 to 1 22 and zircon e hf t 3 8 to 6 4 the mafic rocks were derived from partial melting of metasomatized lithospheric mantle geochemical and sr nd hf isotopic data show that they were insignificantly modified during magma emplacement we provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern lhasa terrane sr nd isotopic composition of the motuo late cretaceous eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle we propose that the late cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern lhasa terrane the foundered ancient materials may subsequently re fertilize the lithospheric mantle not only in the eastern lhasa terrane but also in the surrounding areas

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

zircon u pb age and sr nd hf o isotope geochemistry of the paleocene eocene igneous rocks in western gangdese evidence for the timing of neo tethyan slab breakoff northward neo tethyan oceanic lithosphere subduction beneath southern tibet in the mesozoic early cenozoic produced continental arc magmas in the 1600 km long gangdese belt the most voluminous magmatism occurred in the paleocene eocene and is characterized by extensive i type calc alkaline to high k calc alkaline linzizong volcanic rocks and coeval plutons these rocks have been extensively studied in the eastern gangdese belt east of 89 e but few data exist from the western gangdese belt new data for eleven samples of these rocks combined with existing data from the literature show that they are similar to the eastern gangdese belt rocks with relative depletions in nb ta p and ti and enrichments in rb ba th u k pb zr and hf on a primitive mantle normalized trace element diagrams typical of continental arc related igneous rocks however compared to the east western gangdese igneous rocks range to higher k2o contents up to 6 1 wt higher 87sr 86sr i ratios up to 0 7151 and lower endi values down to 8 1 suggesting that an evolved crustal source was involved in arc magmatism the gangdese arc magmatism lasted to 80 ma and has a gap or quiescent period afterwards starting at 69 ma the arc magmatism initiated and shifted southward from 30 5 n to 29 5 n in southern tibet with an abrupt change of india asia convergence rate the magmas through the whole gangdese belt at 69 53 ma are characterized by intermediate endi values 0 6 to 4 0 positive ehfi values 3 8 to 7 1 intermediate 18o values 5 0 to 6 5 and low th y and la yb ratios 20 these magmas were likely derived from the mantle with crustal contamination by mash process in response to neo tethyan slab rollback as proposed by previous studies the sr nd hf o isotopic compositions of linzizong volcanic rocks and coeval intrusions are relatively homogeneous during 69 53 ma but show a wide range at 53 50 ma zircon temperature values from these rocks are distinctively high during the latter period and whole rock th y and la yb ratios increased significantly after 50 ma we suggest that the high temperature and heterogeneous geochemistry of these magmas at 53 50 ma reflect neo tethyan slab breakoff during india asia collision which triggered asthenospheric mantle upwelling mantle derived magmas resulted in extensive heating induced crustal melting and generated magmas with variable isotopic compositions tibetan crust was significantly thickened after 50 ma

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

pluton emplacement during transpression in brittle crust new views from analogue experiments analogue experiments were used to investigate pluton emplacement during transpression in brittle crust models consisted of silicone putty placed within slots in a rigid basement and an overburden of dry sand the models were transpressed causing emplacement of silicone intrusions into the overlying sand space was made for intrusions by upward displacements of overburden along oblique reverse shear zones in some cases accompanied by extensional shearing above the intrusions silicone putty was progressively accreted to the bases of uplifted blocks and growing intrusions rode up the shear zones leaving space for accretion of more silicone the resulting intrusion shapes depended largely on the geometries of shear zone systems which in turn depended on overburden thickness the intrusion profiles and the structural relationships between intrusions and host in the experiments are comparable to some natural examples a magma accretion model may thus account for pluton emplacement and batholith assembly within some transpressive orogens

high magmatic flux during alpine himalayan collision constraints from the kal e kafi complex central iran voluminous eocene gabbros to granites of the kal e kafi backarc composite intrusion were emplaced prior to the alpine himalayan collision in the central iranian backarc but the reasons for precollisional high arc and backarc magmatic productivity 60 53 ma are unclear diagnostic geochemical signatures are high k2o sr contents and successively depleted middle rare earth element ree patterns reflecting a highly metasomatized source and an increasing role for amphibole and garnet 0 10 in the relatively younger granites release of concealed k sr rich fluids from oceanic fractures and faults during buckling and bulging of a precollisional choking oceanic slab and melting of phlogopite bearing lithosphere with subsequent interaction of the melt with lower crustal garnet amphibolite of a 40 km thick crust can explain the kal e kafi geochemical and isotopic signatures gravimetric data indicating a 39 km present day backarc crustal thickness are consistent with geochemical results but also imply little if any collisional crustal thickening of the central iranian plateau high eocene arc backarc melt flux prior to collision in this region reflects vigorous thermal convection which may in fact be diagnostic of collisional magmatism explaining the presence of postcollisional shoshonitic melts in this and other collisional orogenic settings

long term cosmogenic 3 he production rates from 40 ar 39 ar and k ar dated patagonian lava flows at 47 s exceptionally well preserved basaltic lava flows on the cool arid eastern side of the southern andes near lago buenos aires lba at lat 47 s provide ideal sites for calibration of in situ produced cosmogenic 3he we report new 3he measurements and independent age determinations using unspiked k ar and 40ar 39ar incremental heating measurements from two basalt flows both age techniques give concordant ages with analytical uncertainties of 5 demonstrating their utility for dating surfaces older than 35 ka the limit of 14c dating the weighted mean age of the cerro volcan flow is 109 3 ka and of the rio pinturas flow is 68 3 ka scaled to sea level and high latitude d lal earth planet sci lett 104 1991 424 439 and standard atmospheric pressure j o stone j geophys res 105 2000 23 753 23 759 3he production rates on the cerro volcan flow are 130 4 atoms g 1 a 1 in clinopyroxene and 139 4 atoms g 1 a 1 in olivine 2 consistent with a small compositional dependence of the production rate the 3he production rate in olivine from the younger rio pinturas flow is 129 6 atoms g 1 a 1 which is indistinguishable from that obtained on cerro volcan the 3he production rate is at least 11 higher than previous determinations from low mid latitudes in the northern hemisphere corrections for magnetic field strength variability lower the scaled lba production rates by only 1 and do not account for the discrepancy we infer that the enhanced production rates are caused by lower atmospheric pressure in patagonia during glacial periods the southern westerlies which are associated with the steep circumpolar atmospheric pressure gradient are presently centered on lat 50 s but based on independent marine and terrestrial evidence are thought to have migrated north at least 5 during glacial periods the inferred paleobarometry provides independent evidence for northward shifts in the patterns of southern hemisphere atmospheric circulation

the geology of vertical movements of the lithosphere an overview defining and quantifying horizontal movements of the earth s crust has been a common activity for geoscientists during the last decades due to the success of structural geology in measuring magnitudes of shortening or extension via balanced cross sections and strain analysis coupled with the popularization of continental drift and plate tectonics theory on the larger scale on the other hand vertical movements of rocks or of the surface of the earth although perceived for over two centuries have been more elusive because of the smaller magnitude of translations the difficulties in finding unambiguous markers and because more often their characterization requires the concurrence of integrated approaches some of them recently developed downward movements i e subsidence associated with the formation of sedimentary basins are better quantified and understood but often not coupled with the areas experiencing upward vertical movements over the past decades considerable progress has beenmade in the understanding and quantification of vertical motions of the lithosphere and in particular of the earth s surface thanks to the implementation of new tectonic geomorphologic geochemical and geophysical techniques largely motivated by the strong implications of uplift and subsidence for landscape and environmental issues in addition there is an increased perception that areas experiencing upward and downwardmovements mountains and basins should be looked at as parts of a single tectonic geomorphic and sedimentologic system a significant number of challenges remain to be addressed an inspection of the recent literature clearly indicates that the geology of vertical movements is a field of very active multidisciplinary research the esf sponsored topoeurope initiative with its national counterparts topoiberia toposcandia etc is an obvious demonstration of the increased attention of the geoscience community towards processes controlling the development of continental topography with this in mind on 28 31 september 2007 we convened a workshop entitled the geology of vertical movements uplift and subsidence mountains and basins in the framework of the international lithosphere program s task force on sedimentary basins the workshop was hosted by the 1 convention of the moroccan association of petroleum geologists mapg and the venue was the town of marrakech in the frame of the superb moroccan mountains and basins e g fig 1 which have been the recent objective of many studies thus the workshop intended a particular emphasis on african

role of the plate margin curvature in the plateau buildup consequences for the central andes 1 the influence of convergent plate boundary curvature on the stress distribution in an overriding plate is explored using analytical and numerical modeling techniques trench parallel compression can be produced near the symmetry axis of a seaward concave plate boundary if the interplate friction is high and or if the subducting lithosphere has a low flexural rigidity which produces little nonhydrostatic normal stress on the plate boundary this situation favors the formation of trench parallel thrusts with minor trench parallel component of slip trench parallel compression is reduced along the most oblique parts of the plate boundary which favors the formation of strike slip faults with major trench parallel slip both the stress conditions on the interplate zone and the 3 d geometry of this zone control whether the trench parallel stress in the center of a seaward concave curvature is a tension or compression low dip angle and high convergence obliquity angle favor trench parallel compression in the central andes n s minor shortening in the center of the arica bend and strike slip systems north and south of the symmetry axis suggest that the effect of shear traction dominated during cenozoic time when the curvature of the plate boundary was forming this result suggests that the processes responsible for the formation of the plate boundary curvature were assisted by enhanced interplate friction and or reduced compressive nonhydrostatic normal stress for a geometry resembling the present day south american plate margin estimations of normal and shear stresses on the plate boundary suggest that the trench parallel stress in the center of the curvature is compressive

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

combining 3 he cosmogenic dating with u th he eruption ages using olivine in basalt this paper presents new u th he crystallization and 3 he exposure ages on olivine for a suite of basalt samples from the snake river plain idaho the results provide the first demonstration that u th he geochronology can be successfully applied to late quaternary basalts using measurements of olivine phenocrysts as opposed to u rich minerals the approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with 4 he implantation from the groundmass and a specially designed extraction system that allows u th and he concentrations and isotopic compositions to be measured on the same olivine grains we demonstrate the viability of the u th he technique on basalts with ages of 373 ka and 3400 ka previously dated by ar ar and show that concurrent measurements of u th 4 he ages and 3 he cosmogenic exposure ages improve the accuracy of both methods the combinedu th 4 he 3 hetechniqueisappliedtoaseriesofbouldersandflowsfromboxcanyonnearhagerman idaho toevaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode offormation of the canyon the u th he ages of the box canyon basalts are between 86 12 ka and 130 12 ka 1 sigma errors and suggest at least two periods of eruption the ages are similar to a ca 95 ka ar ar age previously assigned and distinguish the box canyon basalts from nearby ca 395 ka basalt exposure ages of boulders within the canyon at the mouth head and middle of box canyon are 21 1 19 3 and 48 3 ka scoured bedrock at the canyon head has an exposure age of 45 5 ka these data indicate that the canyon was carved before the

cosmogenic 3he production rates revisited from evidences of grain size dependent release of matrix sited helium measurements of the cosmogenic 3 he 3 hec content of various size aliquots of exposed olivines show that the fine fraction 140 m has 3 hecconcentrations between14and100 lowerthanthatofthecoarsefractions 0 14 1mm suchdifferencesattest to a grain size dependent partial release of 3 hec from the phenocrysts matrix during the preliminary in vacuo crushing this result mighthave importantimplications since most 3 hecmeasurements haveusedfor 20yrastandardroutine basedonthe fusion ofbulk powdered phenocrysts whatever their grain size a suite of new data obtained from coarse olivine grains yielded a mean sea level high latitude 3 hec production rate slhl p3 of 128 5 and 136 6 at g 1 yr 1 depending on the scaling factors used this new value which is 15 higher than previously published rates is obtained from 5 ropy flow surfaces of mt etna 38 n and hawaiian 19 n volcanoes at elevations between sea level and 870 m and ranging in age from 1 47 0 05 to 149 23 ka according to independent 14 c or k ar dating 3 he loss during the crushing step might account for the discrepancy between the standard reference valueof110 115at g 1 yr 1 andthehigherslhlp3proposedhere moregenerally removal ofthe powderedfraction beforefusion is an important point to consider in further studies in order to avoid any 3 hec systematic underestimates

fossil cosmogenic 3 he record from k ar dated basaltic flows of mount etna volcano sicily 38 n evaluation of a new paleoaltimeter this study investigates the reliability of a paleoaltimeter based on the elevation dependency of the cosmogenic nuclides production rate the proposed method relies on fossil cosmogenic record indeed measuring the cosmogenic nuclide concentration in an exposed and then buried lava flow combined with an independent estimate by k ar or 40ar 39ar dating of the past exposure duration theoretically allows recording fossil cosmogenic production rates which can thus be converted in paleoelevations this approach was tested by measuring cosmogenic 3he 3hec in olivines and clinopyroxenes of quaternary k ar dated basaltic flows of mount etna volcano sicily 38 n the agreement within uncertainties 1 500 m between current and cosmogenic derived elevations demonstrates the paleoaltimeter reliability however dating imprecision earth s paleomagnetic fluctuations and erosion contemporary to the flow exposure are parameters that may affect the accuracy and the precision of the method on older geological scales their respective influences were thus carefully evaluated and discussed to conclude that the paleoaltimeter can potentially reach resolutions better than 1000 m even for cenozoic records

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

u pb zircon chronology geochemical and sr nd isotopic composition of mesozoic cenozoic granitoids in the se lhasa terrane petrogenesis and tectonic implications abstract whole rock geochemistry sr nd isotope and zircon u pb isotope data are reported for seven granitoid intrusions from the eastern lhasa terrane our zircon u pb data exhibit three periods of magmatism 164 125 ma 83 ma and 71 45 ma granitoids from the middle jurassic to early cretaceous 164 125 ma display evolved nd isotope composition with end t 8 6 to 15 5 and t2dm 1 6 to 2 2 ga in contrast the late cretaceous 83 ma granitoids with adakitic characteristics high sr y ratios display less evolved nd isotopic composition with end t 0 3 to 3 0 and t2dm 0 9 to 1 1 ga geochemical and sr nd isotopic data indicate that the middle jurassic to early cretaceous granitoids were derived from partial melting of proterozoic crustal basement and the late cretaceous adakitic granitoids were derived from partial melting of over thickened lower crust geochemical and sr nd isotopic data of granitoids from the latest cretaceous to eocene period 71 45 ma reveal that they result from diverse magma sources including both juvenile and reworked mature crustal materials our compilation of new and published data from the eastern lhasa terrane show a marked variation in sr y ratios and a step change in sr nd isotope compositions during the late cretaceous we suggest that the middle jurassic to early cretaceous granitoids resulted from the northward neo tethyan ocean slab subduction the increasing sr y ratios from the middle jurassic to late cretaceous granitoids and northeastward migration of arc magmatism in the eastern lhasa terrane during the interval ca 125 95 ma are attributed to the shallowing angle of subduction of the neo tethyan ocean slab the late cretaceous magmatism in the eastern lhasa terrane probably resulted from the neo tethyan mid ocean ridge subduction and subsequent delamination of the arc root the latest cretaceous to eocene granitoids could be interpreted as a magmatic response to roll back and or break off of the subducted neo tethyan slab

tectonic shortening and crustal thickness in the central andes how good is the correlation exceptional thickening of continental crust beneath the central andes is believed to be mainly a result of tectonic shortening of the south american plate in neogene time this shortening has been estimated to have contributed as much as 70 80 of the present crustal volume a compilation of published shortening values and our own estimates based on balanced cross sections from the central andes between lat 3 s and 40 s suggest that 70 80 is a maximum rather than an average value for this part of the andes tectonic shortening and the crustal cross section area are only loosely correlated variations in tectonic shortening are more abrupt than those of crustal areas particularly near the northern and southern ends of the altiplano puna high plateau where thick crust is associated with relatively small amounts of shortening shortening there may account for no more than about 30 of the present crustal cross section area the processes that created the remaining crustal area are not clear but are likely to involve poorly constrained pre neogene tectonic shortening moderate magmatic additions to the crust tectonic underplating of material derived from the forearc and possible flow of ductile lower crust along strike

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

contrasting ore styles and their role in understanding the evolution of the altaids abstract the altaids is a paleozoic orogenic collage in central asia it occupies 10 of the eurasian continent and it is one of the richest concentrations of mineral deposits in the world comparable to those of the circum pacific and tethyan orogenic belts in particular it contains many types of hydrothermal ore deposits that are sensitive to the nature and origin of their host terranes including porphyry copper gold molybdenum volcanogenic massive sulfide copper lead zinc and orogenic gold deposits which are all diagnostic of convergent continental margins extensional arc related submarine basins and transpressional terrane boundaries within the altaids porphyry copper deposits are mostly concentrated within the kazakhstan and tuva mongolia micro continents where they formed almost continuously from the early ordovician to permian only a few are cambrian and triassic and most are late carboniferous most volcanogenic massive sulfide ore deposits are distributed along the south urals and rudny altai in the western altaids where they formed in a short time window from the early to middle devonian important orogenic gold deposits formed along regional strike slip faults between craton island arc or micro continent boundaries from the neoproterozoic to permian mostly in the early permian in limited key areas volcanogenic massive sulfide deposits formed upon much older 100 ma basement rocks than those of coeval porphyry deposits indicating that the altaids could not have developed from a single arc furthermore the porphyry deposits outline the distribution of several continental fragments the combined area of which is far less than that of the altaids which suggests that extensive new juvenile material has been added into island arc systems which may have been accreted to surrounding continental margins these metallogenic distributions in localized areas were a result of the mechanisms of crustal growth within the overall tectonic evolution of the altaids and they are consistent with the interpretation that the altaids is a multiple subduction accretion collage

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

cretaceous to middle cenozoic exhumation history of the cordillera de domeyko and salar de atacama basin northern chile

adakites from collision modified lithosphere 1 adakitic melts from papua new guinea png show adakitic geochemical characteristics yet their geodynamic context is unclear modern adakites are associated with hot slab melting and or remelting of orogenic mafic underplate at convergent margins rift propagation over collision modified lithosphere may explain the png adakite enigma as png was influenced by rapid creation and subduction of oceanic microplates since mesozoic times in a new rift tectonic regime decompressional rift melts encountered and melted remnant mafic eclogite and or garnet amphibolite slab fragments in arc collisional modified mantle and partially equilibrated with metasomatized mantle alternatively hot slab melting in a proposed newborn subduction zone along the trobriand trough could generate adakitic melts but recent seismic p wave tomographic models lack evidence for subducting oceanic lithosphere in the adakite melt region however they do show deep subduction zone remnants as a number of high p wave anomalies at lithospheric depths which supports our proposed scenario

precambrian tectonic discontinuities in western laurentia broadband seismological perspectives on the snowbird and great falls tectonic zones

scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces abstract the decrease in rates of production of cosmogenic nuclides occurs because of shielding of cosmic rays by mountains sloped surfaces and local rock formations that block them when a large part of the sky is blocked this correction is large and requires detailed model calculations this paper considers three geometries a rectangular obstruction a triangular obstruction and a sloped surface other geometries can be considered as a combination of these the results are presented in terms of formulas and graphs so that the reader can easily apply them to common field situations any use of cosmogenic nuclides in the study of geomorphic processes or forms must consider factors that introduce variations in the production of nuclides

geochemical differences between subduction and collision related copper bearing porphyries and implications for metallogenesis abstract porphyry cu mo au deposits occur not only in continental margin arc settings subduction related porphyry cu deposits such as those along the eastern pacific rim eprim but also in continent continent collisional orogenic belts collision related porphyry cu deposits such as those in southern tibet these cu mineralized porphyries which develop in contrasting tectonic settings are characterized by some different trace element e g th and y concentrations and their ratios e g sr y and la yb suggesting that their source magmas probably developed by different processes subduction related porphyry cu mineralization on the eprim is associated with intermediate to felsic calc alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting assimilation storage and homogenization mash whereas k enriched collision related porphyry cu mineralization was associated with underplating of subduction modified basaltic materials beneath the lower crust with subsequent transformation into amphibolites and eclogite amphibolites and resulted from partial melting of the newly formed thickened lower crust these different processes led to the collision related porphyry cu deposits associated with adakitic magmas enriched by the addition of melts and the subduction related porphyry cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks all of which were associated with fluid dominated enrichment process in subduction related cu porphyry magmas the oxidation state fo2 the concentrations of chalcophile metals and other volatiles e g s and cl and the abundance of water were directly controlled by the composition of the primary arc basaltic magma in contrast the high cu concentrations and fo2 values of collision related cu porphyry magmas were indirectly derived from subduction modified magmas and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust and in part by the contribution from the rising potassic and ultrapotassic magmas both subduction and collision related porphyries are enriched in potassium and were associated with crustal thickening their high k2o contents were primarily as a result of the inheritance of enriched mantle components and or mixing with contemporaneous ultrapotassic magmas

strain accumulation controls failure of a plate boundary zone linking deformation of the central andes and lithosphere mechanics 1 we make use of observations on orogenic strain accumulation and deformation partitioning in the central andes to explore the backarc strength evolution at the lithospheric scale in plan view the altiplano puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the cenozoic beyond the maximum lateral extent reached around 10 15 ma 40 50 of entire proto andes undergoing deformation at 10 20 total strain rapid localization initiated at the eastern flank of the altiplano inter and subandean thrust belt but not at the puna latitude localization was associated with a significant increase in bulk shortening rate average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the middle miocene strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle we find that lithosphere scale failure resulting from strain weakening beyond a critical strain threshold c 20 and fault coalescence with formation of a weak detachment in shales effective coefficient of friction 0 1 plays a key role in the evolution of the andes strain related lithosphere weakening appears to dominate over the impact of external forcing mechanisms such as variations of plate convergence mantle assisted processes or erosion comparison of these orogen scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

cosmogenic 3he and 21ne age of the big lost river flood snake river plain idaho the big lost river flood in southeastern idaho occurred 20 500 calibrated yr b p on the basis of dates derived from cosmogenic 3 he and 21 ne measurements of samples from flood deposited boulders and from scour features this date corresponds to a date of 16 900 14 c yr b p and is close in age to several other cataclysmic flood events in western north america it may mark evidence for widespread warming at the end of the pleistocene in western north america the big lost river flood was smaller than some other late pleistocene floods such as the bonnevi1le flood and the missoula floods thus some samples exposed after the flood had significant amounts of cosmogenic 3 he and 21 ne that was acquired before the flood occurred

timing and significance of gabbro emplacement within two distinct plutonic domains of the peninsular ranges batholith southern and baja california evaluating the crucial role postulated for mantle derived mafic magmas in the formation of silicic batholiths requires well determined igneous crystallization ages from mafic rocks as well as petrologic data reflecting possible mantle source heterogeneities and or variations in subcrustal processes the abundant mafic plutons that crop out within the mid cretaceous peninsular ranges batholith prb southern and baja california provide an important opportunity to investigate the relationship of mafic and silicic magmatism in cordilleran batholiths zircon u pb laser ablation inductively coupled plasma mass spectrometry ages are reported for 24 samples of prb gabbro collected throughout its 1400 km long extent the samples span the entire compositional spectrum of prb gabbro from cumulate olivine gabbro with 10 ppm whole rock zr content to gabbronorite to more fractionated hornblende gabbro eighteen of the samples are from the western zone gabbro belt of the batholith including the type locality for san marcos gabbro while six are from the eastern zone where gabbro is rare composing 1 of the outcrop area the morphology of zircon in prb gabbro ranges from anhedral fragments to euhedral grains and this zircon has distinctly lower uranium concentrations and higher th u ratios than zircon from prb granitoids most samples exhibit simple u pb systematics but several contain inherited or entrained zircons and two eastern gabbro samples contain distinctly bimodal age populations weighted mean 206pb 238u zircon ages are interpreted as crystallization ages and range from 130 5 1 5 ma to 100 2 3 0 ma for western zone samples the ages demonstrate that gabbros were emplaced throughout the 30 m y interval during which the bulk of the western zone was constructed and conflict with the earlier interpretation of western gabbros as mafic forerunners that preceded emplacement of granitoid plutons eastern zone gabbros in contrast yield u pb ages that range mostly from 109 4 2 9 ma to 97 8 2 3 ma narrowly predating the ca 92 98 ma eastern zone tonalite trondhjemite granodiorite batholith flareup and thus may have played a role in crustal thickening and triggering of the flareup despite their partially cumulate character whole rock major and trace element compositions of prb gabbro closely match high alumina olivine tholeiite and arc basalt and basaltic andesite from modern arcs gabbros from the west and east sides of the batholith transverse to regional strike have similar nearly flat rare earth element patterns that reflect derivation from a common partially contaminated depleted mantle source the apparent across strike petrologic uniformity of the gabbros contrasts strongly with prb granitoids which are well known for across strike asymmetries in trace elements initial 87sr 86sr and 18o reflecting progressively deeper partial melting of mafic source rocks from west to east with additional input from a high 18o high 87sr end member on the east side of the batholith the western prb gabbros represent a plausible deep crustal source composition for derivation of western granitoids by partial melting or fractional crystallization for the eastern batholith the mafic source region requires modification by input of accretionary complex supracrustal rocks delivered via forearc subduction erosion underthrusting

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

rise and emplacement of magma during horizontal shortening of the brittle crust insights from experimental modeling 1 magmatic activity tends to concentrate at plate margins at divergent margins extensional tectonics provide steep conduits for magma to reach the surface at rapidly convergent margins such as the andes one might imagine that horizontal compression prevents the rise of magma nevertheless volcanoes are also common in order to study the mechanisms by which magma rises in a compressional context we resorted to laboratory experiments in which a brittle crust was shortened while magma was intruding our model materials were 1 cohesive fine grained silica powder representing brittle crust and 2 molten low viscosity vegetable oil representing magma in general horizontal shortening and injection were coeval but independent processes thrust faults accommodated the shortening while overpressured oil formed hydraulic fractures in those experiments where there was no shortening injection resulted in a saucer shaped intrusive body in the other experiments where there was shortening oil formed a basal sill before rising along thrust faults once in place the sill lubricated the base of the model so that arcuate thrusts formed at the leading edge of a plateau uplift of the plateau promoted further intrusion of oil at depth in general the pattern of deformation and intrusion depended on the kinematic ratio r between rates of shortening and injection the lengths of the basal sill and plateau increased with decreasing r on the basis of these results we have reexamined two natural examples of magmatic complexes which were emplaced in compressional tectonic settings tromen volcano in argentina and the boulder batholith of montana

hot paleocene eocene gangdese arc growth of continental crust in southern tibet abstract the 1600 km long gangdese magmatic belt features extensive paleocene eocene i type intrusive rocks and coeval volcanic successions which can be divided into group i 69 53 ma group ii 53 49 ma and group iii 49 43 ma corresponding to neo tethyan slab rollback neo tethyan slab breakoff and ongoing indian asian collision respectively the magmas from these three groups show significant variations in geochemical and isotopic compositions which provide the information of the growth of continental crust in southern tibet the most voluminous magmatism in the gangdese belt occurred during 53 49 ma high zircon saturation temperature up to 800 c and ti in zircon temperature up to 980 c estimations suggest there is a period of thermal anomaly during 53 49 ma starting from 53 ma magmas have increased k2o contents and their zircons have decreased th u ratios and y and yb contents zircons from group ii have the most heterogeneous hf isotopic compositions ehf t 5 3 to 15 1 these are evident of ingress of asthenosphere mantle in the arc extensive crustal melting and magma mixing magma underplating during this time is the main mechanism for the growth of continental crust with the indian asian collision going on the magmas in group iii show high th y and la yb ratios and k2o contents but significantly low tzr and t ti zr values mostly below 750 c these features suggest the water fluxed melting of early arc residues occurred in the late stage of growth of continental crust the crust has been thickened and nearly mature at this stage this study has great implication on understanding of growth of continental crust in orogenic belts

helium isotopic variability within single diamonds from the orapa kimberlite pipe abstract the distribution and isotopic composition of helium has been measured in a suite of well characterized one carat diamonds from the orapa kimberlite botswana crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix generally greater than 90 rather than by the inclusions step heating experiments performed on inclusion free fragments remaining after crushing indicate that the3he 4he ratio is variablewithin individual diamonds the fragments as small as 10 mg were heated in two timed steps both at 2000 c in every case lower3he 4he ratios are observed in the first graphitization step 0 05 3 atmospheric while the last heating step releases helium with systematically higher3he 4he ratio 30 80 atmospheric we suggest that this internal isotopic variability is the result of stepwise graphitization the first heating step initiates graphitization which nucleates around defects and the second heating step graphitizes the relatively defect free regions of the diamond the3he 4he ratio measured using the partial graphitization technique differs by up to a factor of 100 within a single specimen the inclusion free fragments release small quantities of helium below 2000 c which suggests that helium release is obtained only by graphitization the3he contents of the monocrystalline diamonds are relatively constant at 3 10 13 cm3 stp gram and indicate that most of the isotopic variability is due to radiogenic4he the variations in4he content are either related to zoning of th and u in the diamonds i e in situ decay to zoning of inherited4he or to implantation of particles from a th and u rich environment i e kimberlite because the orapa diamonds were mined from roughly 40 m depth in the kimberlite spallation reactions from cosmic ray interactions are not a significant source of3he however calculations based on the age of the kimberlite 90 m y and reasonable th and u abundances suggest that most of the3he in the orapa diamonds could be produced by6li n t in the diamond although this may not be true of all diamonds nuclear reactions in the crust and mantle including spallation reactions at the surface can explain many of the high3he 4he ratios previously reported for diamonds

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

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

composition and thickness of the southern altiplano crust bolivia slant stacking of broadband seismograms recorded in the western united states for two 1993 intermediate depth earthquakes that occurred near the bolvia argentina chile borders reveals small but clear precursors to both the compressional p wave and shear s wave depth phases we interpret and model these precursors as underside reflections from the thickened andean altiplano crust we model the crustal structure of the altiplano with a grid search technique to match the timing and amplitudes of the depth phases and precursors in the waveforms of both the p wave and s wave our best fit model has an average crust velocity of 5 9 6 0 km s a crustal v sub p v sub s of 1 6 a crustal thickness of 75 80 km and a high velocity v sub p 8 4 km s high v sub p v sub s 1 9 mantle wedge assuming isotropy the low v sub p v sub s ratio of 1 6 for the crust corresponds to an anomalously low poisson s ratio of 0 18 such a low value in conjunction with the low average v sub p estimate is consistent only with a felsic composition and high upper crustal temperatures the finding of a thick felsic crust overlying a high velocity mantle supports models of altiplano uplift due predominantly to crustal shortening as opposed to mafic magmatic additionmore and is inconsistent with recent mantle delamination 19 refs 3 figs 1 tab less

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

recycling of orogenic arc crust triggers porphyry cu mineralization in kerman cenozoic arc rocks southeastern iran pre collisional eocene oligocene arc diorites quartzdiorites granodiorites and volcanic equivalents in the kerman arc segment in central iran lack porphyry cu mineralization and ore deposits whereas collisional middle late miocene adakite like porphyritic granodiorites without volcanic equivalents host some of the world s largest cu ore deposits petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment ore hosting kuh panj porphyry intrusions exhibit high sr 400 ppm low y 20 no negative eu anomalies eu eu 1 and relatively non radiogenic sr isotope signatures 87sr 86sr 0 7042 0 7047 relative to eocene oligocene granitoids mainly sr 12 la yb 15 eu eu 1 87sr 86sr 0 7053 0 7068 trace element modeling indicates peridotite melting for the barren eocene oligocene intrusions and a hydrous garnet bearing amphibolite source for middle late miocene ore hosting intrusions the presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30 35 to 40 45 km or 12 kbar the changes in residual mineralogy in the source of eocene to miocene rocks in the kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel underplating of cu and sulfur rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust mantle boundary and dehydration melting under oxidizing conditions produced syn and post collisional ore hosting intrusions while the lack of post collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur rich and oxidized adakitic magmas

experimental high pressure granulites some applications to natural mafic xenolith suites and archean granulite terranes high pressure granulite assemblages have been produced as residues in partial melting experiments on a natural alkalic basalt amphibolite between pressures of 12 and 18 kbar in particular at 18 kbar partial melting of the hornblende plagioclase quartz assemblage under fluid absent conditions produces garnet clinopyroxene new albitic plagioclase melt seismic velocities v p are estimated from the modal data for the experimental assemblages and range from 6 90 km s for hornblende bearing residues to 7 62 for the dominantly garnet clinopyroxene residues these values are typical for rock types in the lowermost crust transitional to mantle the experimental results help place additional pressure temperature a h 2 o constraints on the source region for the natural high pressure granulites the experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the delegate pipes in australia where the pipes intrude upper ordovician and lower devonian continental crust and in the bearpaw mountains montana where the xenolith bearing magmas intrude older crust of archean age the combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust mantle boundary in these areas the delegate pipes xenoliths suggest that the crust mantle boundary may be the site for partial melting and assimilation whereas the bearpaw mountains samples indicate that magmatic underplating may have been a major process in generating thick continental archean crust the experimental data and bearpaw mountains xenoliths suggest that the large range of rock types found in archean granulite terranes may not be representative of the lowermost continental crust

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

he diffusion in olivine helium diffusion in olivine dunite xenolith has been measured in the temperature range 1180 1460 c a linear arrhenius function was obtained with an activation energy of 120 27 32 kcal mole and a pre exponential factor d0 2 2 108 cm2 s diffusion mechanisms are not a viable means of degassing he from the mantle olivine phenocrysts can be expected to retain previously trapped he during cooling in extrusive basalts provided the flow units are thinner than 50 m xenoliths will retain mantle he signatures only if magma transport times are less than 50 years or if the he fugacity in the magma is high enough to prevent xenolith degassing the lower oceanic crust is probably substantially degassed of he trapped he will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 1014 c m y glass at several centimeters depth in a basalt flow near the spherulite zone will have cooling rates lower than this so he loss may be significant in many basalt glass samples

cosmogenic and nucleogenic 3he in apatite titanite and zircon cosmogenic 3he was measured in apatite titanite and zircon and cosmogenic 21 ne in quartz at 13 depth intervals in a 2 7 m long drill core in a miocene ignimbrite from the altiplano of bolivia all three 3he depth profiles as well as the 21 ne profile attenuate exponentially with depth indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources the attenuation lengthscale for 3he production of 180 11 g cm 2 is consistent with expectations for neutron spallation and is identical to that found for the cosmogenic 21 ne in quartz by normalizing the measured 3he concentrations to 21 ne and using the independently known cosmogenic 21 ne production rate the apparent cosmogenic 3he production rates in apatite titanite and zircon were respectively found to be 112 97 and 87 atoms g yr at sea level and high latitude the formal uncertainty on these estimates is 20 2 and arises in equal parts from uncertainties in the measured 3he 21 ne ratios and the uncertainty in the 21 ne production rate however an additional factor affecting the apparent 3he production rate in these phases arises from the long stopping range of spalled 3he and tritium which decays to 3he because all three accessory phases have higher mean atomic number than major rock forming minerals they will have lower 3he production rates than their surroundings as a consequence the long stopping ranges will cause a net implantation of 3he and therefore higher apparent production rates than would apply for purely in situ production thus these apparent production rates apply only to the specific grain sizes analyzed analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size from 50 to 100 m cross section yields a 10 decrease in apparent production rate while this effect warrants further study the grain sizes analyzed here are typical of the accessory phases commonly encountered so the apparent rates provide an appropriate starting place for surface exposure dating using 3he in these minerals

structural evidence of orogen parallel strike slip displacements in the precordillera of northern chile the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile is made up of several elongate basement ridges following the trend of the andes these ridges which morphologically rise above the mesozoic tertiary cover rocks are developed as anticlinal cores or as pressure ridges bounded by reverse faults and thus show considerable orogennormal shortening several nearly vertical faults cut through the precordillera parallel or at a very low angle to the mountain ranges from the following structures it can be inferred that orogen parallel transcurrent movements took place along the faults a asymmetric en echelon fault arrays b stratigraphic and structural discontinuities across major faults c fabrics in fault rocks and d vertical folds a dextral displacement of the order of tens of km is probable the orogen parallel strikeslip movements as well as the orogen normal shortening are considered as phenomena of magmatic arc tectonics due to the focussing of central andean igneous activity on the precordillera from the late cretaceous until the paroxysm of deformation 45 30 ma deformation along the precordilleran fault system is related with the development of the large porphyry copper ore deposits of that area the structural evolution of the precordillera can be explained by oblique subduction resulting in dextral transpression

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to