depositional sequence of argillite diamictite hyaloclastite and lava flows within the franciscan complex northern california an originally conformable sequence of argillite diamictite hyaloclastite pillow breccia pillow basalt exists in a 3 km long up to 200 m thick fault bounded slab among franciscan rocks 6 km south of the klamath river mouth argillite and diamictite grade laterally into one another over short distances diamictite contains clasts of chert and graywacke that exhibit detrital rounding greenstone clasts are irregularly shaped and compositionally similar to overlying volcanic rocks hyaloclastite consists of globules and granules of altered basalt and basaltic glass with plagioclase phenocrysts and minor plagioclase crystals and altered glass the hyaloclastite is complexly interstratified with diamictite and features suggesting soft sediment deformation exist locally tongues of chloritized basaltic glass have intruded diamictite matrix pillow breccia has hyaloclastite matrix breccia fragments and whole pillows are abundant at the top of the sequence trace element geochemical data suggest that the seq

paleomagnetic evidence of an early paleozoic rotated terrane in northwest argentina a clue for gondwana laurentia interaction paleomagnetic results from three lower ordovician units in northwest argentina are reported when assessed with available geologic evidence these data permit the definition of the puna oriental famatina rotated terrane paleogeographic reconstructions based on these paleomagnetic results place puna oriental famatina in the south iapetus ocean close to the gondwana margin and near some of the lower middle ordovician central mobile belt terranes of the northern appalachians the accretion of puna oriental famatina may have resulted from the closure of this ocean in middle ordovician time

a multi proxy record of holocene climatic change in southwestern spain the laguna de medina cadiz palaeolimnological data diatoms ostracods foraminifera molluscs aquatic pollen and lithology from a radiocarbon dated sediment core from a saline lake the laguna de medina provide the first complete record of holocene lake level change for southwest spain for the last c 9000 cal years the lake has always been relatively shallow but has exhibited marked fluctuations in salinity and water depth especially in the earlier record when oscillations culminate in maximum lake levels from c 6960 6680 cal bp c 6070 5830 bp indicating enhanced early to mid holocene humidity and a mid holocene humidity maximum prolonged shallowing thereafter reflects in part increased aridity in the later holocene lake desiccation followed by a c 800 yr phase zone 2 of major limnological change commencing at c 8000 cal bp c 7200 bp and a number of other abrupt desiccation events are also noteworthy the mid holocene maximum is consistent with widespread evidence for high lake levels around 6000 bp but t

does the mid atlantic ridge affect the distribution of abyssal benthic crustaceans across the atlantic ocean abstract a trans atlantic transect along the vema fracture zone was sampled during the vema transit expedition in 2014 15 the aim of the cruise was to investigate whether the mid atlantic ridge mar isolates the abyssal fauna of the western and eastern abyssal basins based on two genetic datasets of macrostylidae and desmosomatidae nannoniscidae studied by riehl et al and brix et al in this issue we found that most of the therein delimitated species were found at only one side of the mar we analysed those species of macrostylidae and desmosomatidae that were sampled across the mar and complemented these with one species of a third family munnopsidae with these datasets we were further able to consider the effect of different niche adaptations macrostylidae are infaunal burrowing munnopsidae are considered epifaunal with pronounced swimming capabilities and desmosomatidae and nannoniscidae are partly able to swim but are not as well adapted to swimming as munnopsidae we concluded that the mar seems to be a dispersal barrier for the non swimming macrostylidae as well as weakly swimming desmosomatidae and nannoniscidae however four species of macrostylidae and desmosomatidae did cross the mar but evidence for regular unrestricted gene flow is still lacking for the swimming munnopsidae we were able to detect persistent gene flow across the mar

lateglacial and late holocene environmental and vegetational change in salada mediana central ebro basin spain the salada mediana lacustrine sequence central ebro basin spain 41330 10an 0344 w 350 m a s l provides an example of the potential and limitations of saline lake records as palaeoclimate proxies in the semi arid mediterranean region sedimentary facies analyses chemical stratigraphy stable isotopes d18o and d13c of authigenic carbonates d13c values of bulk organic matter and pollen analyses from sediment cores provide paleohydrological and vegetation change reconstructions for the lateglacial and late holocene in the central ebro basin a preliminary chronology is based on 210pb and 14c ams dates the lacustrine sequence is composed of three sedimentary sections the lower section was deposited in a permanent saline to brackish lake this stage represents the most humid period in the record and it was accompanied by the expansion of temperate trees particularly corylus the middle section was deposited in an ephemeral playa lake complex frequent subaerial exposure conditions favour the colonisation of the playa lake oor by chenopodiaceae during a low water table period this interval re ects the most arid conditions in the salada mediana record including the current environment a secondary temperate tree expansion occurred after the maximum aridity period aquatic plants and cyanobacterial mats spread in the lake during periods of raised water tables this paleohydrological and vegetational evolution attests to large changes in e ective moisture during the lateglacial in the semi arid northeastern spain the abundance of corylus during the lateglacial indicates that refugia for temperate trees were located along the ebro valley during the last glacial maximum the holocene sediments in the salada mediana records have been eroded and the upper section represents deposition during the last few centuries 2000 elsevier science ltd and inqua all rights reserved

franciscan complex olistostrome at crescent city northern california an olistostrome and bounding turbidites are exposed within the late mesozoic franciscan complex along the crescent city california coastline facies grade in character from mutti ricci lucchi 1978 mixed facies b c and d to f the olistostrome to mixed a b and e progressing upwards within the franciscan stratigraphic section the facies f unit outcrop is up to 600 m thick and extends 12 km along strike it consists of oblate to tabular blocks up to 200 m in maximum dimension of greenstone tonalite radiolarian chert limestone phyllite and greywacke dispersed in a scaly argillite matrix the olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites by the presence of abundant recycled sedimentary clasts within the unit and by the presence of sedimentary breccias and associated dismembered slump folded turbidites both within the olistostrome and among subjacent turbidites sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected late mesozoic sierran klamath magmatic are franciscan rocks record an early pervasive layer parallel flattening strain in such features as extensional faults necking and pinch and swell structures several scales of extensional faulting account for the juxtaposition of turbidites of different facies and or with varying degrees of stratal disruption the formation of sandstone lozenges and the formation of scaly foliation in the olistostrome matrix the latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay these were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome the presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix and of sandstone dykes that intrude fractures and associated drag folded turbidite beds indicate that franciscan sediments were not lithified during their early deformation these sediments were deposited in either a trench or trench slope basin and were first deformed either by gravity collapse of the trench slope cover or less likely by vertical loading beneath the toe of the accretionary wedge they later were folded during internal shortening of the growing franciscan accretionary prism

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

the distribution of geothermal fields on the juan de fuca ridge near bottom water temperatures were mapped along 400 km of the strike of the juan de fuca ridge as part of a combined sea marc seabeam experiment to image the variability of morphology and structure along a spreading center segment the water temperature data collected by a continuously towed thermistor chain in addition to salinity data indicate that there are four geothermal areas spaced at distances of 100 km from each other south of the cobb propagator and one field just to the north of the propagator on the endeavor ridge segment each thermal region is located above a morphological dome on the spreading center these domes are an average of 100 200 m shallower than the rest of the axis the structure of bottom water suggests that the geothermal regions are on average 20 km long and that the heat from these fields raises the temperature in the water column by a minimum of 0 06 sup 0 c up to 300 m above the bottom two simple models are used to estimate the heat flux associated with these features

the accretionary history of southern south america from the latest proterozoic to the late palaeozoic some palaeomagnetic constraints

melange diapirs into the cambria slab a franciscan trench slope basin near cambria california near cambria california a franciscan trench slope basin of late cretaceous age is intruded by two diapirs of blueschist block bearing mud matrix melange the rise of the diapirs caused major upturning of the bedded strata within a few hundred meters of the contact and complex interfingering of mud matrix melange with basin sediments near the contacts at one locality a fragment of basin sandstone is observed partially detached from the wall rock the mud matrix melange is highly mobile and the intrusive character during diapirism is clearly analogous to that of magma in igneous intrusions diapirism may be responsible for large scale intercalation of slope and slope basin sediments with subduction generated melange at actively convergent plate margins

quantifying crustal thickness variations in evolving orogens correlation between arc basalt composition and moho depth a compilation of geochemical data from 50 active volcanoes where seismically defined moho depth measurements are available representing most volcanic arcs around the world indicates a relation exists between crustal thickness moho depth and the trace element composition of basalts the best correlation exists for maximum light heavy rare earth element ree ratios and we use maximum ce y r2 0 90 to show the exponential function with increasing depth to 50 km application to new zealand part of the eastern gondwanan margin through much of the phanerozoic shows that higher ce y ratios in arc basalts correspond to two periods of recognized crustal thickening plus an additional event at ca 240 ma which links new zealand to the gondwanide orogeny conversely low ce y ratios in basalts correlate with periods of recognized extension in eastern gondwana very high ce y ratios in ca 120 ma rocks imply an unusually thickened 50 km arc crust consistent with independent geological estimates of crustal thickness at that time the correlation shows that crustal thickness variations during orogenesis can be conservatively estimated to within 3 km

the holocene and upper pleistocene pollen record from padul granada spain a new study two borings were made at the site of padul granada pollen analysis of 200 spectra from two successive sequences 14 8 and 8 m deep enables a description of the vegetational and climatic history of this region the most southern one in europe from early wurmian times the chronology is supported by twenty one 14c dates original equivalents are found for the great european prewurm interstadials a markedly arid though thermically not extreme episode exactly equivalent to stage 4 of the marine isotopic stratigraphy closes this relatively temperate period the middle part of the last glacial shows climatic fluctuations that are poorly characterized as is often the case in europe for this period a long section very probably corresponding to the complete last pleniglacial does not show any climatic amelioration towards 15 000 yr b p a change in the diagram is noted that can only be interpreted as reflecting a larger expansion of a regional steppe cover this event also reported on three occasions in southeast france marks the beginning of the oldest dryas the climatic amelioration of ca 13 000 yr b p is far more pronounced here than anywhere else in europe whereas that at 10 000 yr b p is not so clearly marked this unexpected result may be accounted for by the fact that padul is the first continental site so far south and so near pleniglacial refuges to be known on the other hand this result is consistent with what is known from oceanic isotopic stratigraphy a quite new late glacial and holocene vegetational history is revealed characterized by the early appearance and dominance of quercus ilex oak forests of a southern type and the early occurrence of quercus suber and olea it is the first account of a complete history of the postglacial reafforestation in a region with a modern semi arid mediterranean climate from the palaeoclimatic point of view it shows that the hot and humid holocene optimum was attained slightly before 8000 yr b p and that the holocene climatic fluctuations were of but small amplitude

diatom responses to holocene environmental changes in a small lake in northwest spain abstract the holocene history of lagoa grande 1360 m a s l northwest spain has been investigated through the analyses of diatom remains found in a sediment core changes in the lake have been interpreted in terms both of climatic variability and major changes in the catchment area the diatom record begins at about 9800 yr bp and exhibits three marked changes at 8200 6000 4200 and at 2600 yr bp which are interpreted as responses to climatic changes detected by marine and other terrestrial palaeoclimatic records the initial lake was shallow and aulacoseira species characteristic of acidic and oligotrophic waters were dominant the early holocene is characterised by the presence of aulacoseira ambigua which suggests an increase in water depth and nutrient levels throughout the holocene diatom assemblages underwent several changes linked to fluctuating limnological conditions these changes in lake characteristics coincide with the regional succession of vegetation suggesting that climate forcing initiated these limnological changes during the last 2000 years catchment processes appear to be the major factor controlling lake characteristics in lagoa grande site diatom responses reflected the progressive peatland development and growth of littoral vegetation in the lake and not only a change in the regional environment

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

a meridional 14c and 39ar section in northeast atlantic deep water 14c 39ar and complementary hydrographic and nutrient data are presented for deep water below 2500 m depth from stations along a meridional section 8 s to 45 n through the romanche trench and along the deep northeast atlantic basins f s meteor cruise 56 leg 5 the large scale 14c distribution along the section is resolved at the 14c data precision of 2 bottom water 14c decreases by 6 from the equator to 45 n and farther up there is a weak 14c minimum 123 over much of the section the 14c data are interpreted as giving a turnover time of about 30 years for the waters below the depth of the 14c minimum 4250 m it is found that water of 1 50 0 05 c potential temperature enters the east atlantic from the west through the romanche trench sill depth about 4000 m and a preliminary value for the inflow rate of 3 6 106 m3 s is deduced this rate greatly exceeds estimated deep inflow rates through the vema fracture zone or across the northern boundary of the east atlantic 39ar data that cover an entire deep ocean circulation system are presented for the first time the observed 14c and 39ar distributions are mutually consistent transit times from the source regions to the equator for water from northern and southern deepwater sources are estimated to be about 170 and 105 years respectively and the 39ar concentration of young antarctic bottom water is deduced as 60 7 modern the 39ar 14c correlation in the ocean appears to be affected by mixing of waters of different age and by more efficient raising of 39ar in the deepwater formation processes

the geological significance of some triassic microfossils from the south orkney islands scotia ridge late triassic radiolaria have been recovered from a bedded chert collected early in 1977 in the south orkney islands this is the first reliable palaeontological evidence for the age of the so called basement complex that forms most of the exposed rock of a microcontinent on the southern limb of the scotia ridge the fossils place a lower limit on the age of deformation of at least part of the complex as post karnian or norian the basement is overlain by undeformed jurassic to cretaceous conglomerates which suggests this event took place during the early mesozoic gondwanian orogeny the new data are consistent with the interpretation that the basement complex of the south orkney islands represents part of a subduction complex and was accreted to the pacific margin of gondwanaland during the late triassic or early jurassic time

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

proto andean evolution of the eastern cordillera of peru abstract the eastern cordillera of peru represents one of the longest 1200 km paleozoic metamorphic and magmatic belts exposed along the western andean margin of south america in this study we examine the tectonothermal evolution of a key segment of the metasedimentary basement of the eastern cordillera of peru the huaytapallana complex and demonstrate that it has experienced a hitherto undocumented high grade orogenic event at 260 ma latest middle permian based on u pb and th pb monazite age data from paragneisses and u pb dating of zircon rims from leucosomes these ages are interpreted as recording crystallization and are consistent with 255 ma rutile growth in lower grade units u pb apatite data c 260 230 ma in all units are consistent with slow cooling from this 260 ma metamorphic peak u pb zircon geochronology of pre tectonic plutons yield ages ranging from c 302 ma to c 260 ma these geochronological data are augmented by new u pb apatite age data from other segments along the eastern cordillera of peru a regional synthesis of existing geochronological constraints from the eastern cordillera of peru demonstrates that the margin has experienced a polycyclic orogenic history deformation and magmatism occurred at c 480 ma and c 435 ma during the famatinian orogenic cycle was followed by a late silurian to early carboniferous c 420 350 ma magmatic and metamorphic gap and terminated with gondwanide magmatism and metamorphism at c 315 ma and c 260 ma these famatinian and gondwanide orogenic phases can be correlated into the proto andean margin of argentina and chile and are thus of regional extent the evolution of the proto andean margin is thus best explained by changes in tectonic plate reorganization in a long lived paleozoic accretionary orogen which was undergoing phases of advance and retreat resulting in magmatic pulses and orogenic phases which can be correlated along the length of the plate boundary

radiometric evidence for the age of the subduction complex in the south orkney and south shetland islands west antarctica new k ar and rb sr data are presented for the age of part of the supposed late palaeozoic early mesozoic metamorphic complex in west antarctica here renamed the scotia metamorphic complex these results together with a review of other radiometric and palaeontological evidence fail to confirm a pre mesozoic age for the subduction complex in this region although rocks belonging to the albite epidote amphibolite facies could be as old as permian blueschists and greenschists from the south shetland is appear to be significantly younger and were probably formed in part at least during cretaceous subduction

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

provenance analysis and tectonic setting of the ordovician clastic deposits in the southern puna basin nw argentina provenance studies on early to middle ordovician clastic formations of the southern puna basin in north western argentina indicate that the sedimentary detritus is generally composed of reworked crustal material tremadocquartzrich turbidites tolar chico formation mean composition q t 89 f7 l4 are followed by volcaniclastic rocks and greywackes tolillar formation mean q t 33 f42 l25 these are in turn overlain by volcaniclastic deposits mean qt24 f30 l46 of the diablo formation late arenig early llanvirn that are intercalated by lava flows all units were deformed in the ocloyic orogeny during the middle and late ordovician sandstones of the tolar chico formation are characterized by th sc ratios 1 la sc ratios 10 whereas associated fine grained wackes show slightly lower values for both ratios lree light rare earth elements enrichment of the arenites is 50 chondrite eu eu values are between 0 72 and 0 92 and flat hree heavy rare earth elements patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition the e n d t s e d values scatter around 11 to 9 the calculated nd t d m residence ages vary between 1 8 and 2 0 ga indicating contribution by a palaeoproterozoic crustal component the th sc and la sc ratios of the tolillar formation are lower than those of the tolar chico formation normalized ree rare earth elements patterns display a similar shape to paas post archaean average australian shale but with higher abundances of hrees eu eu values range between 0 44 and 1 17 where the higher values reflect the abundance of plagioclase and feldspar bearing volcanic lithoclasts average e n d t s e d values are less negative at 5 1 and nd t d m are lower at 1 6 ga this is consistent with characteristics of regional rocks of upper continental crust composition which most probably represent the sources of the studied detritus the rocks of the diablo formation have the lowest th sc and la sc ratios lower lree abundances than the average continental crust and are slightly enriched in hrees eu eu values are between 0 63 and 1 17 the nd isotopes e n d t s e d 3 to 1 t d m 1 2 ga indicate that one source component was less fractionated than both the underlying early ordovician and the overlying middle ordovician units synsedimentary vulcanites in the diablo formation show the same isotopic composition our data indicate that the sedimentary detritus is generally composed of reworked crustal material but that the diablo formation appears to contain 80 of a less fractionated component derived from a contemporaneous continental volcanic arc there are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the gondwana margin during the ordovician

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

a complete arc trench system recognized in gondwana sequences of the antarctic peninsula region prior to late triassic early jurassic times the geological history of the antarctic peninsula region was dominated almost entirely by gondwana sequences that together comprised a major arc trench system subduction complex trench slope break and fore arc basin sedimentation can all be recognized and deposition was at least partly on early palaeozoic or older continental crust the only evidence for a contemporaneous magmatic arc situated in the antarctic peninsula at this time consists of patchy occurrences of metavolcanic rocks possibly representing the frontal edge of the arc and the major outcrop area of these rocks is believed to lie under the broad shallow continental shelf e of the antarctic peninsula this is contrary to most current hypotheses in which a marginal basin presumably floored by oceanic crust is thought to crop out close to the e coast of the antarctic peninsula however the complete absence of substantial outcrops of pre jurassic volcanic rocks anywhere in eastern antarctica and south africa which are the closest and most likely places in which these should exist supports the new proposal moreover the identification of back arc elements of the arc trench system foreland fold thrust belt and retro arc foreland basin in eastern antarctica and south africa greatly strengthens the likelihood of the arc cropping out in the area suggested towards the end of the triassic and during the early jurassic periods an intense diastrophic event or culmination of events of orogenic magnitude occurred gondwanian orogeny causing substantial redistribution of the pre existing elements of the arc trench system in particular all the fore arc sequences were strongly deformed some possibly for the first time e g in the fore arc basin and became firmly accreted to the continental margin moreover the magmatic foci migrated trenchwards to intrude the deformed rocks in the antarctic peninsula because many of the plutons were emplaced synkinematically they often closely resemble basement gneisses the distinction between these rocks remains a serious problem in the antarctic peninsula and it can be solved only partially by radiometric dating

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

lower paleozoic relative motion of the arequipa block and gondwana paleomagnetic evidence from sierra de almeida of northern chile paleomagnetic results have been obtained from a suite of paleozoic samples from sierra de almeida within the high eastern portions of the atacama desert characteristic directions are discussed for two sequences of pre silurian lavas of probable cambro ordovician age the late cambrian choscas pluton three late ordovician early silurian plutons the devonian carboniferous lila formation and late paleozoic volcanic units of the pular and cas formations the choschas pluton and one lava series yield similar northerly and shallow directions which for the presence of reversals and their concordance are suggested to represent early paleozoic arequipa plate directions directions in the three late ordovician early silurian plutons pass a tilt test using an overlying erosional unconformity and these also include reversals together with directions from a second lava series a roof pendant in an early silurian pluton these define poles compatible with silurian gondwana results from africa and australia in situ directions from the basal red beds of the devonian lila formation are inconsistent with devonian gondwana or stable south american poles but like the devonian strata of the appalachians they are consistent with a tilt corrected overprint of kiaman superchron age these results together with previous results from the late paleozoic cas and pular formations are discordant from the gondwana path only for the latest cambrian earliest ordovician the discordance in paleomagnetic data together with regional geologic constraints can be explained by a model in which the arequipa block representing a paraautothonous finger of gondwana like japan or the iberian peninsula rotated about a nearby pole but was then resutured during the silurian such a scenario resolves much of the discrepancies in the models which have emerged from peru chile and argentina

late neogene and quaternary tectonics associated with northward growth of the san andreas transform fault northern california strain patterns within the forearc at a convergent margin adjacent to a passing fault fault trench triple junction show a systematic evolution we describe late neogene and quaternary deformation in northern coastal california in the last 3 m y associated with the migration of the mendocino triple junction deformation in the north american forearc near humbolt bay consists of a 30 km wide zone of on land contraction with a narrower zone of translation to the east the zone of subaerial contraction is part of the relatively wide southernmost extent of forearc contraction within the cascadia subduction zone net quaternary northeast southwest contraction across forearc thrust faults is at least 7 9 km and minimum fault slip rates on the six major thrust faults are 0 8 2 3 mm yr net right slip within the zone of translation is a minimum of 3 km south of the forearc translation is predominant in the north american plate and is accommodated along two major right lateral fault zones of the san andreas transform boundary the lake mountain and garberville fault zones discontinuous exposures of late neogene sediments along these faults near covelo and garberville respectively indicate that crustal contraction expressed by thrust faults and associated folding predated translational deformation at each site inception of contraction near covelo was related to inception of internal deformation of the gorda plate at 3 ma this contraction due to coupling of the convergent plates migrated northward in the forearc in advance of and at the rate of migration of the triple junction present day deformation near humboldt bay thus reflects contraction associated with subduction of the gorda plate and translation farther east associated with oblique convergence at the plate boundary as the triple junction migrates the strike slip faults of the san andreas transform boundary will extend to the north northwest into the present forearc and forearc strike slip faults will become faults of the san andreas system contractional structures near humboldt bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south

abyssal circulation change in the equatorial atlantic evidence from cenozoic sedimentary drifts off west africa abstract seismic profiling in the equatorial atlantic reveals deep water 4500 m sediment bodies formed by current controlled deposition near the intersection of large offset fracture zones with the african margin a 600 km long drift accumulation the ivory coast rise lies north of the st paul transform near 3 n a smaller drift deposit has been identified along the northern side of the guinea transform at 10 n antarctic bottom water aabw which presently enters the eastern atlantic basins through the romanche and vema fracture zones probably played an important role in the development of these features proto aabw may have reached the equatorial region as early as mid eocene time before the establishment of permanent ice sheets in antarctica the ivory coast rise existed as a distinct sedimentary drift by the mid eocene as a result of deposition from bottom water moving southwards along the african margin and westwards parallel to the st paul fracture zone this early flow pattern is in the opposite sense to the present movement of deep water in the sierra leone basin a reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the cenozoic

bottom water observations in the vema fracture zone the vema fracture zone trough at 11 n between 41 and 45 e is open to the west at the 5000 m level but is silled at the 4650 m level on the east where it intersects the axis of the mid atlantic ridge the trough is filled with antarctic bottom water aabw with a potential temperature of 1 32 c and salinity of 34 82 ppt the bottom water is thermally well mixed in a nearly homogeneous layer about 700 m thick the great thickness of this bottom layer as compared with the bottom water structure of the western atlantic basin may result from enhanced mixing induced by topographic constriction at the west end of the fracture zone trough a benthic thermocline with potential temperature gradients of about 1 2 mdeg m 1 is associated with an abrupt increase in turbidity with depth at about 1200 m above bottom a transitional layer of more moderate temperature gradients about 0 4 mdeg m 1 lies between the benthic thermocline above and the aabw below the aabw layer whose depth averaged suspended paniculate concentrations range from 8 to 19 g l 1 is consistently higher in turbidity than the overlying waters at the eastern end of the trough 140 m below sill depth very low northeastward current velocities with maximums of 3 cm s 1 were recorded for an 11 day period

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

documentation and significance of progressive microfabric changes in middle ordovician trench mudstones middle ordovician mudstones of the greenwich slice of the hamburg klippe an inferred early paleozoic subduction complex in the central appalachians comprise three mesoscopically distinct fabric zones that reflect progressive development of scaly foliation from undeformed zone mudstones through deposits of the transition zone into stratally disrupted deformed zone mudstones scanning electron microscopic analysis of these deposits demonstrates an over all decrease in porosity from undeformed mudstone into the deformed zone undeformed zone mudstones illustrate a porous clay fabric that becomes more closely packed in the transition zone transition zone mudstones also show 1 discrete discontinuous shear zones characterized by compact domains of oriented platy grains and 2 fracture surfaces along which there has been no grain reorientation the bulk of the mudstone however has apparently been compacted by distributed fabric collapse rather than shear related compaction mudstones of the deformed zone display a complex array of continuous well defined in some cases striated or grooved scaly folia bound on one side by very compact domains of strongly oriented platy grains that pass outward into a more open or porous clay fabric deformation of greenwich slice mudstones can be explained by strain hardening of accreted trench fill deposits at the toe of the early paleozoic accretionary prism initial deformation was probably concentrated in weak water rich sediment horizons sandwiched between more consolidated strata strain hardening of the early deformed sediment in response to shear related fabric collapse and dewatering as evidenced by the scaly fabric of deformed zone mudstone probably resulted in lateral propagation of deformation and progressive scaly fabric development the extent to which stratal disruption propagated may have been controlled by the slope of the water content gradient of the deforming sediment that is the deformation front migrated outward from the initially deformed water rich sediment to that position where the strength of the sediment as determined by its relatively low water content precluded further sediment yield the microscopic shear zones and fractures within transition zone mudstones suggest that these deposits were dry and therefore strong enough to arrest further stratal disruption thus the final width of a deformation zone characterized by stratal disruption and scaly clays may be inversely proportional to the slope of the water content gradient that existed between deforming water rich underconsolidated sediment and stronger more consolidated deposits

probing the evolving andean lithosphere mid late tertiary magmatism in chile 29 30 30 s over the modern zone of subhorizontal subduction in the mid late tertiary 27 6 m y ago volcanism diminished then ceased in the main andean cordillera in chile between 29 and 30 30 s over the same time span the subduction angle shallowed under this region which today lies in the modern shallowly dipping nonvolcanic segment of the south central andes 28 33 s the trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition volcanic changes include in addition to a decreased rate of volcanism a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks and an increase in crustal assimilation these volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space time pattern of deformation and volcanism in the foreland region although the model stands alone support for the tectonic setting and magmatic source region of these tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern andean segments the older dona ana formation 27 18 m y is similar to volcanic rocks in the southern volcanic zone while the younger cerro de las tortolas 14 10 m y and vallecito 6 m y formations are similar to volcanic rocks over regions of thicker crust in the central volcanic zone and the northern part of the southern volcanic zone

origin and age of ultramafic rocks and gabbros in the southern puna of argentina an alleged ordovician suture revisited ultramafic rocks and gabbros are exposed in the southern puna nw argentina in tectonic association with continental arc related ordovician volcano sedimentary successions and granitoids the origin of this mafic rock suite has been debated for three decades as either representing an ordovician terrane suture primitive ordovician arc related rocks or relics of the pre ordovician basement in tectonic contact with the ordovician retro arc basin successions we present the first u pb ages of primary and inherited zircon from gabbros of this mafic ultramafic assemblage la icp ms analyses on cores and rims of these zircon grains yielded a concordia age of 543 4 7 2 ma for the gabbroic rocks other analysed zircons have mesoproterozoic and early ediacaran core and rim ages indicating that the magmas also assimilated meso and neoproterozoic crustal material prior to final crystallization the mafic rocks witnessed higher metamorphic grade than associated ordovician rocks which are unmetamorphosed or only affected by anchimetamorphism the gabbros are mostly tholeiitic and enriched in zr th as well as other incompatible elements and have end t 540ma ranging from 1 3 to 7 4 with most of the values between 5 and 7 147sm 144nd ratios show evidence of weak crustal contamination the mafic rocks do not reveal any affinity to mid ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment chromites from ultramafic rocks show typical ti al cr fe3 abundances found in magmatic arc rocks the formation of the gabbros and the associated ultramafic rocks in the southern argentine puna is related to the evolution of the margin of the pampia terrane including the puncoviscana basin during the late neoproterozoic and earliest cambrian in contrast to previous interpretations the rocks predate the ordovician evolution of the central proto andean active margin consequently interpretations assuming these rocks to represent an oceanic terrane suture of ordovician age have to be dismissed as much as all palaeotectonic models that define ordovician terranes in the central andes based on assumption that the ultramafic rocks and gabbros exposed in the southern puna mark plate boundaries

the structure of the precambrian and lower paleozoic basement of the central andes between 22 and 32 s lat three major diastrophic cycles defined by their structural style and their spatial distribution are recognized in the andean basement of this region the oldest structures are related to the panamerican orogeny 500 to 700 m a which produced in the central craton multiply deformed complexes of schists gneisses and granites that are covered discordantly by unmetamorphosed cambrian and ordovician beds west of the central craton ordovician sedimentary beds are folded with a simple structural style and intruded by granites both the sedimentry beds and the granites are covered discordantly by undeformed devonian sequences the folding of the ordovician is attributed to the ocloyic phase of the caledonian movements west of the ocloyic belt is another foldbelt consisting of strongly folded devonian beds attributed to the chanic phase hercynian the chanic belt is intruded by carboniferous and permian granites and covered discordantly by carboniferous and permian sequences

chemical and isotopic composition of lower to upper ordovician sedimentary rocks central andes south bolivia implications for their source abstract fifty million years of erosional history of the early paleozoic orogen in the central andes are documented in the 12 km thick ordovician siliciclastic sedimentary section from south bolivia transport directions in the north south trending basins allow a distinction between sediment sources from the proterozoic brazilian shield to the east and the early paleozoic mobile belt to the west major and trace element and isotopic nd sr pb compositions of stratigraphically well defined samples of tremadocian to arenigian ages have been studied the element patterns and isotopic composition of the ordovician sedimentary rocks end450 ma 7 5 to 11 87sr 86sr450 ma 0 71 0 72 206pb 204pb450 ma 17 2 18 8 207pb 204pb450 ma 15 60 15 73 208pb 204pb450 ma 36 9 39 1 nd depleted mantle model ages tdm 1 7 2 2 ga are typical of upper crustal felsic rocks which underwent one or more cycles of erosion and deposition the compositional range of the ordovician sedimentary rocks is similar to that of

late palaeozoic early mesozoic fore arc basin sedimentary rocks at the pacific margin in western antarctica we infer that the trinity peninsula group of the antarctic peninsula was deposited in a fore arc basin s of composite type located between the gondwanian arc massif consisting of plutonic and older gneissose basement rocks and a subduction complex now represented by greenschists blueschists and albite epidote hornblende bearing rocks of the pre jurassic metamorphic complex

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

deep tow studies of the vema fracture zone 2 evidence for tectonism and bottom currents in the sediments of the transform valley floor the deep tow geophysical instrument package has been used to survey a 65 km segment along the principal transform displacement zone ptdz of the vema fracture zone away from either ridge transform intersection the surface trace of the ptdz is a small groove 5 80 m deep and 20 200 m wide incised in the otherwise flat lying turbidites of the transform valley floor the fault trace cuts across the transform valley diagonally from ridge crest to ridge crest hugging the southern flank of the median ridge the groove is straight or gently sinuous relatively free of the splays and bifurcations which characterize the transform fault zone near the ridge transform intersection macdonald et al this issue a pull apart basin was observed at the location of a slight clockwise bend in the fault trace vertical offsets of turbidites suggest that the west end of the median ridge and the east end of the south wall are rising relative to the transform valley floor at rates of the order of millimeters per year sediment waves were observed buried under 10 20 m of sediment suggesting that antarctic bottom water may have flowed through the transform valley more vigorously several tens of thousands years ago than at present

the ages and tectonic setting of the faja eruptiva de la puna oriental ordovician nw argentina abstract the ordovician faja eruptiva de la puna oriental is a magmatic predominantly intrusive belt in the puna of northwestern argentina with a n s extension of ca 400 km scarce isotope geochemical ages and biostratigraphic data on some of the folded faja eruptiva country rocks assign the magmatism either to the lower and lower middle ordovician or to the latest ordovician interpretations of origin and tectonic framework of the faja eruptiva are controversial and vary between arc back arc and collisional orogenic settings we present high resolution la icp ms u pb age and hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200 km in the northern argentinian puna the xenocrystic and magmatic zircon age data have a wide spread between 2700 ma and 440 ma concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460 ma and between 453 and 444 ma and constrain the time of the last intrusions at 444 3 ma and at 445 2 ma thus defining this last and main phase of intrusion at 444 ma ehf t values define a main vertical trend centered at 500 ma with ehf t values between 3 and 16 indicating significant mixing of juvenile early paleozoic melts with paleoproterozoic crustal components a second trend is formed by zircons with ages between 1 1 ga and c 500 ma and predominantly positive ehf values between 8 and 3 and originates in juvenile mantle compositions between 1 6 and 1 1 ga the spread of the zircon and hf data document that the faja eruptiva intrusives have experienced large scale contamination by the hosting crustal basement it follows that the basement of the puna is formed either by the upper proterozoic lower cambrian puncoviscana formation as an erosional product of the proterozoic orogenic belts of sw amazonia or that the puna including its puncoviscana basement is underlain by a crust shaped by these orogenies the main intrusive event at 444 ma has been linked to the ocloyic orogeny in the late ordovician the plutons intruded very likely in a sinistral strike slip regime after the main folding phase of the ocloyic orogeny had deformed the ordovician sedimentary country rocks

the influence of deformation partitioning on dissolution and solution transfer in low grade tectonic melange dissolution of quartz and the recryst allization and re equilibration of phyllosilicates produce the dark anastomosing seams that dominate microstructures of tectonic melange which occur in a low grade imbricated and multiply deformed mid palaeozoic intracra tonic fiysch sequence in northeastern australia seams are composed of very closely spaced or coalesced cleavage lamellae which are very thin layers of extremely fine grained phyllosilicates cleavage seams enclose lenses of silt stone or greywacke which formerly occurred in continuous sedimentary layers indicating extremely heterogeneous and disruptive deformation microphacoids enclosed by cleavage seams have subtle shape asymmetries analogous to those of porphyroclasts phyllosilicate preferred orientations within microphacoids commonly lie at a low angle to enclosing seams and asymmetric relationships occur within seams the shape and fabric asymmetries appear to be constant and are regarded as analgous to s and c planes the number and extent of seams the amount of dissolution they indicate and the efficiency of deformation partitioning imply some enhancement of chemical activity and substantial silica loss from the system this in turn suggests the passage of large amount of silica undersaturated fluid and melanges may be zones of high fluid flow however the microstructures and the disruptive nature of the fabrics may also reflect the influence of high bulk shear strains and suggest some relationship between the shearing component of deformation and the development of cleavages and foliations

early paleozoic accretionary orogenies in nw argentina growth of west gondwana abstract two early paleozoic orogenic cycles in the gondwana margin of nw argentina were driven by subduction and interrupted by collision of continental ribbons of laurentian affinity subduction started at the passive margin of gondwana possibly in response to the end of amalgamation of the supercontinent the passive margin was underlain by a hyper extended continent ocean transition and low angle subduction gave rise to a wide and hot fore arc region during the pampean orogenic cycle the arrival of the first continent ribbon ended the pampean cycle steepened the subduction further heating the fore arc and restarting arc magmatism after a magmatic lull 250 300 km trenchwards initiating the famatinian orogenic cycle this cycle started with a 30 ma long period of extension and marine sedimentation followed by the arrival of the second continent ribbon and inversion the back arc initiating a 20 ma long shortening event culminating with the shutting down of the arc this event gave rise to a 300 km wide low topography hot orogeny thus the many peculiarities of the two paleozoic orogens of nw argentina result from a subduction history that efficiently transferred heat to a 300 400 km wide belt of turbidite dominated sediments that was first part of the pampean fore arc and then part of the famatinian retroarc developed at the extended continental margin of gondwana the two orogenies with continued high heat flux lasted 110 ma giving rise to two calc alkaline arcs separated by a 300 km belt of high t low p migmatites and peraluminous granites formed by anatexis of sediments deposited on the passive margin as well as those deposited in the wide pampean fore arc these turbidite dominated sequences were metamorphosed and melted to form a continental crystalline basement and accreted to the cratonic margin adding 500 km of crystalline rocks to the margin of gondwana at the same time that a similar process was happening in eastern australia along the same continental margin interestingly this wide accretionary orogen has now become the region where the andean system developed a wide orogeny above flat slab subduction

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated