distribution and mechanism of neogene to present day vertical axis rotations pacific australian plate boundary zone south island new zealand remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust steeply dipping basement rocks in south island new zealand provide an opportunity to map the distribution of rotations across the pacific australian plate boundary zone and to delineate boundaries of rotated blocks in unusual detail we synthesize new structural data with new and existing paleomagnetic data with geodetic data and with patterns of neogene quaternary faulting in the strike slip marlborough fault system for the past 20 m y vertical axis rotations have been hinged about two crustal scale boundaries near the east coast the ne hinge accommodated 50 of early middle miocene clockwise rotation which caused deformation of the eastern ends of the alpine wairau and clarence strike slip faults the sw hinge has accommodated a further 30 50 of finite clockwise rotation since 4 ma and deflects active fault traces the locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction rotating rocks are pinned to the south against a locked collision zone where the continental chatham rise impinges against the margin slip on inland strike slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the pacific plate the rotation mechanism is a migrating hinge which resembles a flexed telephone book strike slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock

a new middle late jurassic flora and hot spring chert deposit from the deseado massif santa cruz province argentina we present an initial report of a well preserved and relatively diverse gondwanan plant assemblage from bahia laura group chon aike formation strata of the estancia flecha negra area central western region of the deseado massif santa cruz province patagonia argentina the locality contains the first richly fossiliferous chert with a diverse and well preserved plant assemblage reported from the mesozoic which is demonstrably associated with hot spring activity a compression flora and petrified forest contained in associated clastic and volcaniclastic environments provide an indication of regional plant diversity during this as yet poorly represented stratigraphic interval

paleostress adjacent to the alpine fault of new zealand fault vein and styolite data from the doctors dome area doctors dome 75 km north of christchurch new zealand is an early pleistocene to recent structure being deformed along the southeast edge of the pacific australian plate boundary paleostress in the area has been determined in basement rocks of the mesozoic meta graywacke torlesse supergroup which lies unconformably beneath cretaceous and younger cover rocks pre late cretaceous multiple folding and bedding plane faulting produced a complex low grade metasedimentary basement with predominantly steeply plunging fold hinges and largely isotropic overall behavior with respect to later deformation probable late cretaceous wnw ese shortening and nne wsw extension of the basement produced widespread laumontite calcite and silica veins widespread but sparse slickensided minor fault surfaces are probably related to the same stress system during the late cenozoic dome stage basement was uplifted rotated and severely thrust along some of the range fronts to produce local high concentrations of small slickensided surfaces in the frontal areas inversion of basement fault data for the area indicates a general northwest compression with two peaks one wnw ese parallel to the shortening suggested by the older vein system and the other parallel to southeast northwest stylolite columns in the cover rocks this direction is approximately parallel to regional indicators of contemporary deformation in and adjacent to the alpine fault zone and suggests that the stress field affecting these rocks has not changed significantly since the late pliocene early pleistocene like the san andreas system this compression is at a high angle to the strike of the zone as a whole but is compatible with the direction of plate convergence and motion of the major faults between the overlapping ends of the alpine fault and the hikurangi subduction zone the alpine fault may become subhorizontal at middle lower crustal levels partially decoupling the crust from underlying structures and thus allowing oblique motion to be transferred directly onto the fault from the subduction complex while aiding the change from subduction to continental collision

the oroclinal bend in the south island new zealand abstract most of the south island of new zealand lies within an eocene recent continental shear zone related to pacific australia plate motion macroscopic finite strain in this shear zone has in the past been tracked through the deformation of the dun mountain ophiolite belt this paper identifies additional sub vertical basement strain markers including buller takaka terrane boundary darran suite and jurassic volcanic belt within the median batholith taieri wakatipu goulter synform axial trace esk head melange and bedding form surfaces within the buller takaka and torlesse terranes an analysis of the oroclinal bend over the entire zealandia continent shows that it is a composite feature involving pre as well as post eocene bending of basement structures satisfactory paleogeographic reconstructions of zealandia cannot be made without the use of substantial regional scale non rigid intracontinental deformation

rotation of ductile fabrics across the alpine fault and cenozoic bending of the new zealand orocline this study examines the role and kinematic mechanism of vertical axis rotations and folding during early development phases of a transform plate boundary in continental crust near the alpine fault in new zealand this involved an initial period of widely distributed shear including wrench folding together with vertical axis rotations and orocline development followed by a focusing of slip onto a single fault in the south island the haast schist has been displaced horizontally in the neogene by c 480 km of dextral slip on the alpine fault ls tectonites on either side of the fault lie on different limbs of a profound z shaped orocline rotation of these limbs took place about a vertical axis resulting in 25 30 of clockwise rotation of fabrics to the nw of the alpine fault relative to those to the south foliation anisotropy caused bending to take place locally by folding about an inclined hinge in the limbs of a pre existing synform which tightened during the deformation on both sides of the fault involvement of post oligocene age structures in wrench folding and bending suggests that the bending took place entirely in the cenozoic before or during miocene development of the alpine fault as a through going structure during this early phase of plate boundary deformation shear was distributed across a zone 100 km wide vertical axis rotation affected fault bounded basement slivers that shortened perpendicular to their strike a style of deformation like that in the transverse ranges in california today

large volume silicic volcanism along the proto pacific margin of gondwana lithological and stratigraphical investigations from the antarctic peninsula jurassic magmatism in western gondwana produced the most voluminous episode of continental volcanism in the phanerozoic era during the early to middle jurassic some 2 5 3 million km3 of dominantly basalt and to a lesser extent rhyolite were erupted onto a supercontinent in the early stages of break up the major silicic portion of the gondwana magmatic province is exposed in patagonian south america the volcanic rocks of patagonia have been collectively termed the chon aike province and constitute one of the world s most voluminous silicic provinces the volcanic rocks are predominantly pyroclastic dominated by ignimbrite units of rhyolite composition volcanic rocks crop out sporadically across much of the once contiguous antarctic peninsula and are considered to form an extension of the chon aike province a continuation of the province to include the antarctic peninsula would extend its strike length along the active pacific margin by c 2000 km volcanic rocks exposed along the east coast of the antarctic peninsula defined here as the mapple formation are also dominated by rhyolitic ignimbrite flows with individual units up to 80 m in thickness and a total thickness of c 1 km the ignimbrites vary in degree of welding from high grade rheomorphic ignimbrites with parataxitic textures to unwelded lithic rich ignimbrites rhyolite lava flows air fall horizons debris flow deposits and epiclastic deposits are volumetrically minor occurring as interbedded units within the ignimbrite succession the lithology and stratigraphy of the jurassic volcanic rocks of the mapple formation are presented and comparisons are made to the chon aike province a consistent stratigraphy of permo triassic metasedimentary rocks unconformably overlain by terrestrial mudstone siltstone sequences which are in turn conformably overlain by largely silicic subaerial volcanic rocks is present at several localities along the antarctic peninsula and at localities in the chon aike province precise zircon u pb middle jurassic ages exist for two volcanic formations from the antarctic peninsula and a middle lower jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis the antarctic peninsula chronostratigraphy coupled with lithological similarities indicate a close relationship to those sequences of the chon aike province

validation of tectonic reconstructions by crustal volume balance new zealand through the cenozoic we describe a procedure that allows us to quantitatively test hypotheses about the tectonic evolution of a region including areas for which there is little or no geologic record and apply it to the pacific australian plate boundary in the new zealand region since 65 ma the derived models are a first attempt to quantify the effects of the development of the modern plate boundary on four major components of the tectonic evolution of new zealand i e sedimentation paleogeography growth of crustal root and nonrigid deformation the technique balances total rock volume through time using estimates of these factors although the solutions are non unique the uncertainties in each factor can be estimated and used to define the models the plate reconstructions we present are minor modifications of earlier work developed to test whether those proposed reconstructions are self consistent and consistent with present day crustal structure and sediment distribution they are digital and geographically referenced and they have the advantage that new data can be readily incorporated and new concepts tested against several criteria the model of the evolution of new zealand9s crustal root is new and incorporates a previously unpublished compilation the models show that rock redistribution is accounted for by erosion and sedimentation during the period 65 25 ma the pacific australian plate boundary began propagating through new zealand ca 45 ma in these models development of the boundary has no effect on nonrigid deformation until ca 25 ma that time marked the start of a significant increase in land area and of the nonrigid deformation of a large region in the center of the new zealand continent since ca 20 ma 575 000 km 3 of sediments were deposited in basins around new zealand and a crustal root with a volume of 2 200 000 km 3 is interpreted to have developed beneath the landmass a large area in the center of late cretaceous and tertiary reconstructions of the new zealand region inferred from outcrop well and seismic reflection data deformed eroded and was eventually amalgamated within the continental crust of new zealand a redistribution of 3 080 000 km 3 of rock at 0 porosity

behavior of geogrid sand interface in direct shear mode the contribution of transverse ribs to the soil geogrids interaction under pullout mode has been well documented however the contribution of transverse ribs to the soil geogrid interaction under direct shear mode is at best unclear consequently this paper presents the results of a comprehensive direct shear testing program aimed at evaluating the contribution of transverse ribs to the interface shear the direct shear tests involved ottawa sand and several polyester geogrids with a variety of material tensile strength percent open area and aperture pattern the test results show that the shear strength of sand geogrid interfaces under direct shear mode is significantly higher than that of sand geotextile interfaces analysis of shear displacement strength response of the interfaces indicates that in addition to interface shear components due to sand rib friction and sand sand shear at the location of the openings the transverse ribs provide additional contribution to the overall sand geogrid interface resistance specifically analysis of the results reveals that the transverse ribs of the geogrid used in this study provide approximately 10 of interface shear resistance this contribution is positively correlated with the tensile strength and the stiffness of geogrid ribs but is negatively correlated with the percent open area of the geogrid a simple model is proposed to quantify the contribution of transverse ribs to the interface shear strength under direct shear mode

biota and palaeoenvironment of a high middle latitude late triassic peat forming ecosystem from hopen svalbard archipelago a siliceous permineralized peat block recovered from hopen in the svalbard archipelago hosts a low diversity late triassic flora dominated by autochthonous roots and stems of bennettitaleans and lycophytes and parautochthonous leaves sporangia spores and pollen from a small range of pteridophytes and gymnosperms some parenchymatous bennettitalean root cells show interactions with chytrid fungi and bacteria the remains of other fungi and fungi like organisms are dispersed within the peat s detrital matrix cavities excavated through some roots and compacted detritus contain abundant coprolites probably derived from sapro xylophagous oribatid mites although no body fossils have yet been identified sparse larger coprolites containing leaf fragments attest to the presence of invertebrate folivores in the ancient ecosystem the low diversity flora relatively few trophic levels and simple nutritional web together with sedimentological aspects of the host formation and the peat structure collectively favour accumulation of the organic mass as a fibric root dominated peat within a temperate high middle latitude well aerated mire

patterns of late cenozoic exhumation deduced from apatite and zircon u he ages from fiordland new zealand new apatite and zircon u th he ages from the fiordland region of new zealand s south island expand on earlier results and provide new constraints on patterns of late cenozoic exhumation and cooling across this region zircon u th he cooling ages in combination with increased density of apatite ages show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study there is also a trend toward younger cooling ages to the east distinct breaks in cooling age patterns on southwestern fiordland appear to be correlated to the location of previously mapped faults the northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation or it may reflect synchronous regional exhumation that exposed different structural levels across fiordland or some combination of these effects in either case differential exhumation accommodated by major and minor faults that dissect fiordland basement rocks apparently played an important role in producing the resulting age patterns

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

the lithospheric geodynamics of plate boundary transpression in new zealand initiating and emplacing subduction along the hikurangi margin and the tectonic evolution of the alpine fault system abstract in contrast to the normal wilson cycle sequence of subduction leading to continental collision and associated mountain building the evolution of the new zealand plate boundary in the neogene reflects the converse initially a period of continental convergence that is followed by the emplacement of subduction plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes relative plate motions and reconstructions since the early to mid miocene require significant continental convergence in advance of the emplacement of the southward migrating hikurangi subduction a sequence of tectonism seen in the present plate boundary geography of hikurangi subduction beneath north island and convergence in the southern alps along the alpine fault in contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable the transition from a continental convergent regime with its associated crustal and lithospheric thickening to subduction of oceanic lithosphere requires substantial thinning removal of upper plate continental lithosphere to make room for the slab the simple structure of the wadati benioff zone seen in the present day geometry of the subducting pacific plate beneath north island indicates that this lithospheric adjustment occurs quickly associated with this rapid lithospheric thinning is the development of a series of ephemeral basins younging to the south that straddle the migrating slab edge based on this association between localized vertical tectonics and slab emplacement the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting pacific slab although the new zealand plate boundary is often described as simply two subduction zones linked by the transpressive alpine fault in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction

la josefina au ag deposit patagonia argentina a jurassic epithermal deposit formed in a hot spring environment abstract the la josefina jurassic epithermal au ag deposit located in patagonia argentina developed in an extensional setting of a back arc environment associated with a widespread middle late jurassic calc alkaline volcanism block faulting has juxtaposed shallow level features evidenced by hot spring manifestations hydrothermal eruption breccias and au rich veins which suggest that mineralization in these veins could extend far below the depths already tested by core drilling veins are filled by quartz chalcedony opal and minor adularia and barite with massive comb cockade colloform crustiform bandings and lattice bladed textures ore minerals include electrum ag rich sulfosalts freibergite pyrite galena sphalerite chalcopyrite and specular hematite with minor arsenopyrite marcasite tetrahedrite and bornite four mineralizing stages have been identified the first two s1 and s2 are au and ag rich with temperatures ranging from 225 to 290 and salinities from up to 15 wt in s1 decreasing to 1 wt nacl in s2 the third stage s3 displays higher base metal contents at lower temperatures 200 c finally the last stage s4 is barren with temperatures lower than 100 c veins are surrounded by a proximal alteration halo of quartz pyrite adularia illite followed outwards by illite smectite interstratified clays and smectites with less chlorite to a propylitic zone stable isotope values calculated for the fluids show a mostly meteoric origin for mineralization fluids such distinct features place the la josefina deposits in a hot spring environment with evidences of being formed at a proximal position of the jurassic paleosurface and paleowater level

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

organic composition and diagenetic mineralization of microfossils in the ediacaran doushantuo chert nodule by raman and petrographic analyses abstract we studied microfossils and their embedding matrix minerals from chert nodules in the ediacaran doushantuo formation of the yangtze gorges area in south china using raman spectroscopy as well as transmitted and polarized light microscopy the microfossils are composed of organic carbonaceous material with weak structural organization ordering which indicates a low degree of thermal maturation and alteration raman spectral disparity of carbonaceous material among different portions vesicle inclusion of microfossils reveals some differences in organic composition and structure of their precursor biological matters the mineral phases of silica matrix that entombs the microfossils are recognized as opal ct cryptocrystalline quartz and their transitional phases overall raman spectral analyses and petrographic observations document a complete and continuous sequence of silica phase transformation from opal ct to quartz when reconstructing the diagenetic history of sediments and mineralization process of microfossils in a localized microenvironment the silica in pore waters precipitated preferentially at the periphery of microorganisms in the form of opal gradually decreasing in abundance as it transformed into more stable cryptocrystalline quartz the recrystallization took place in cryptocrystalline quartz to form grains authigenic fluorapatite readily precipitated in the vicinity of some microfossils and within their degraded organic fragments while apatite was crystallizing around organisms and their biostructures silica precipitated in the surrounding voids inhibiting the space of potential apatite crystallization the silicification and phosphatization of microfossils occurred in the microbial sulfate reduction msr zone of the sediment column and both were driven by the localized ph change resulting from msr and pyrite precipitation in the microenvironment thus facilitating the exceptional fossil preservation in the doushantuo chert nodules

paleomagnetic constraints on cenozoic deformation along the northwest margin of the pacific australian plate boundary zone through new zealand 1 new zealand straddles the boundary between the australian and pacific plates a zone of oblique continental convergence and transform motion the actively deforming region offers a unique opportunity to study the dynamics of deformation including vertical axis rotation of rigid blocks within a transcurrent plate boundary zone we present and interpret paleomagnetic data from three new and three previously published sites from the nw part of the south island nw nelson region where sedimentary strata dated between 36 and 10 ma overlie the crystalline paleozoic basement assemblages of the gondwana margin compared with reference directions from the australian apparent polar wander path none of the results provide evidence of post eocene vertical axis rotation this suggests that for the past 36 myr nw nelson has remained a strong coherent block that has moved as a contiguous part of the australian plate this is in marked contrast to the strongly rotated nature of more outboard accreted terranes to the east for example the hikurangi margin in the north island nw of the alpine fault and the marlborough region in the ne of the south island se of the alpine fault have both undergone diverse clockwise rotations of up to 140 since the early paleogene the nw tip of the south island seems to have acted as a rigid backstop relative to these more complex oroclinal deformations we infer that because of its relatively stiff bulk rheology it has not been drawn into the distributed plate boundary rotational deformation associated with the new zealand orocline

stiffness of soil geosynthetic composite under small displacements i model development abstractwhile significant emphasis has been placed on the quantification of soil geosynthetic properties under failure conditions studies of properties that are suitable for characterizing this in

improved understanding of geogrid response to pullout loading insights from three dimensional finite element analysis soil reinforcement has rapidly become one of the most common soil improvement techniques used in geotechnical engineering understanding the behavior of a geogrid under pullout loading is essential

age and origin of the new zealand orocline in relation to alpine fault movement the alpine fault and the recurved arc structures orocline in the late palaeozoic mesozoic rangitata orogen are two of the first order structures of the new zealand subcontinent a commonly held view is that the orocline originated as a gigantic drag fold with alpine fault movement in view of the post oligocene age of the alpine fault however this concept involving a single recurved arc cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the rangitata orogen and its cover rocks these data indicate at least 62 of dextral rotation during each of the mesozoic rangitata orogeny and the late cenozoic kaikoura orogeny a new model presented here postulates that there are actually two dextral recurved arcs a western arc which lies mainly outside the australia pacific plate boundary zone has a multiple origin having formed chiefly during the late triassic phase of the rangitata orogeny by the subduction accretion of the exotic rakaia terrane an eastern arc coincides with the area of the present plate boundary zone it formed as a mega brittle ductile shear zone with the late cenozoic development of the modern australia pacific plate boundary which includes the alpine fault this solves the problem of the nature and location north of the alpine fault of the relative plate movement evident as 480 km of displacement on the alpine fault a major implication of this model for plate tectonic reconstruc tions of the sw pacific is that only approximately 500 km of dextral displacement has occurred through new zealand during the late cretaceous cenozoic compared with the commonly adopted figure of 1000 km

cenozoic tectonic evolution of the new zealand plate boundary zone a paleomagnetic perspective abstract new zealand straddles the obliquely convergent boundary between the pacific and australian plates with subduction of pacific plate along the hikurangi margin finite plate reconstructions predict 800 km of relative plate motion since 43 ma with 80 90 clockwise rotation of the hikurangi margin since 20 ma at an average rate of 4 4 5 myr and the short term deformation

limestone and chert in tectonic blocks from the esk head subterrane south island new zealand the esk head subterrane is a continuous belt generally 10 20 km wide of tectonic melange and broken formation on the south island of new zealand this subterrane separates older and younger parts of the torlesse terrane which is an extensive accretionary prism composed mostly of quartzo feldspathic submarine fan deposits ranging from permian to early cretaceous in age the torlesse is the most pacific ward of several permian and mesozoic accreted terranes in new zealand that record tectonic amalgamation and ultimate accretion against the pacific facing gondwana margin the esk head subterrane of the torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt associated seamount and sea floor limestones and cherty rocks thought to be representative of the subducted plate limestones in tectonic blocks are of late triassic and probably jurassic ages and include 1 submarine cemented pelagic bivalve geopetal packstone grainstone 2 brachiopod bryozoan encrinite and 3 radiolarian pelagic lime mudstone most of the triassic blocks have been dated using conodonts which have remarkably low color alteration index cai values paleogeographic inferences drawn from megafossils bioclasts and radiolarians as well as from carbonate cements indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the new zealand part of the gondwana margin but higher than paleoequatorial latitudes these oceanic sediments and their basaltic substrates were evidently emplaced in the torlesse accretionary prism following off scraping from an extensive subducting oceanic plate probably the phoenix plate which was obliquely convergent with the northwest trending gondwana margin during late jurassic and or early cretaceous time

jurassic hot spring deposits of the deseado massif patagonia argentina characteristics and controls on regional distribution abstract the deseado massif santa cruz province argentinean patagonia hosts numerous middle to late jurassic age geothermal and epithermal features represented by siliceous and calcareous chemical precipitates from hot springs sinters and travertines respectively hydrothermal breccias quartz veins and widespread hydrothermal silicification they indicate pauses in explosive volcanic activity marking the final stages in the evolution of an extensive jurassic ca 178 151 ma volcanic complex set in a diffuse extensional back arc setting heralding the opening of the atlantic ocean published paleo hot spring sites for the deseado massif plus additional sites identified during our recent field studies reveal a total of 23 locations five of which were studied in detail to determine their geologic and facies associations they show structural lithologic textural and biotic similarities with miocene to recent hot spring systems from the taupo and coromandel volcanic zones new zealand as well as with modern examples from yellowstone national park u s a these comparisons aid in the definition of facies assemblages for deseado massif deposits proximal middle apron and distal siliceous sinter and travertine terraces and mounds with preservation of many types of stromatolitic fabrics that likely were controlled by formation temperature ph hydrodynamics and fluid compositions locally the mapped hot spring deposits largely occur in association with reworked volcaniclastic lacustrine and or fluvial sediments silicic to intermediate lava domes and hydrothermal mineralization all of which are related to local and regional structural lineaments moreover the numerous geothermal and significant epithermal those with published minable resources deposits of the deseado massif geological province mostly occur in four regional nnw and wnw hydrothermal structural belts northwestern northern central and southern defined here by alignment of five or more hot spring deposits and confirmed as structurally controlled by aeromagnetic data the northern and northwestern belts in particular concentrate most of the geothermal and epithermal occurrences hence jurassic hydrothermal fluid flow was strongly influenced by the most dominant and long active geological boundaries in the region the outer limits of the deseado massif horst itself

jurassic epithermal au ag deposits of patagonia argentina abstract important precious metal deposits have been discovered during the last 10 years in the deseado massif region of patagonia argentina this region is a plateau consisting of middle to upper jurassic volcanic rocks that host fracture controlled epithermal au ag mineralization these mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism probably during the late jurassic period the presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved with erosion of younger sedimentary and volcanic rocks now exposing these systems these deposits represent diverse erosional levels ranging from sinter formed at the paleosurface to intermediate au ag rich quartz veins to base metal bearing au ag veins that represent deeper levels of the epithermal systems the best known deposits are in the cerro vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km geological mapping of the district has resulted in the definition of over 140 km of veins current reserves are over 3 2 million ounces au equivalent with grades of 10 g t au and 120 g t ag the geologic characteristics of the region and the presence of major au ag deposits provide targets for continuing exploration in the deseado massif

the marianas san marcos vein system characteristics of a shallow low sulfidation epithermal au ag deposit in the cerro negro district deseado massif patagonia argentina the cerro negro district within the argentinian deseado massif province has become one of the most significant recent epithermal discoveries with estimated reserves plus resources of 6 7 moz au equivalent the marianas san marcos vein system contains about 70 of the au ag resources in the district mineralization consists of upper jurassic 155 ma epithermal au and ag rich veins of low to intermediate sulfidation style hosted in and genetically related to jurassic intermediate composition volcanic rocks 159 156 ma veins have a complex infill history represented by ten stages with clear crosscutting relationships that can be summarized in four main episodes a low volume metal rich initial episode e1 an extended banded quartz episode with minor mineralization e2 a barren waning stage episode e3 and a silver rich late tectonic hydrothermal episode e4 the first three episodes are interpreted to have formed at the same time and probably from fluids of similar composition a 290 230 c fluid dominated by meteoric and volcanic waters 3 to 0 18owater with 3 nacl equivalent salinity and with a magmatic source of sulfur 1 to 2 34swater metal was mainly precipitated at the beginning of vein formation episode 1 due to a combination of boiling at 600 to 800 m below the paleowater table and associated mixing cooling processes as evidenced by sulfide rich bands showing crustiform colloform quartz adularia and chlorite smectite banding during episodes 2 and 3 metal contents progressively decrease during continuing boiling conditions and veins were filled by quartz and calcite during waning stages of the hydrothermal system and the influx of bicarbonate waters 6 to 8 5 18owater hydrothermal alteration is characterized by proximal illite adularia and silica zone with chlorite and minor epidote intermediate interlayered illite smectite and a distal chlorite halo this assemblage is in agreement with measured fluid inclusion temperatures a striking aspect of the marianas san marcos vein system is that the high grade high temperature veins are partially covered by breccia and volcaniclastic deposits of acidic composition and are spatially associated with hot spring related deposits and an advanced argillic alteration blanket a telescoped model is therefore proposed for the marianas san marcos area where deeper veins were uplifted and eroded and then partially covered by non explosive post mineral rhyolitic domes and reworked volcaniclastic deposits together with shallow geothermal features the last tectonic hydrothermal mineralization episode e4 interpreted to have formed at lower temperatures could be related to this late tectonic and hydrothermal activity

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

jurassic geothermal landscapes and fossil ecosystems at san agustin patagonia argentina abstract an extensive well preserved late jurassic c 150 ma geothermal system at san agustin farm in the deseado massif patagonia argentina is described this deposit along with others previously known from the same region partially fills a considerable gap between cenozoic and scattered palaeozoic hot spring localities reported worldwide the san agustin deposit is novel because it represents a large 1 4 km 2 and nearly complete geothermal landscape siliceous hot spring facies both subaerial and subaqueous are exposed side by side in their original spatial and geological context set amongst intrusive rhyolite domes and fluviolacustrine sediments the jurassic hot springs have preserved an entire local ecosystem containing microbes arthropods gastropods and plants exhibiting lagerstatten style preservation plant preservation in particular ranges from decayed litter to seedling sprouts and to dense stands in life orientation with intact anatomy the san agustin deposit shares some ecological taphonomic and sedimentological characteristics with modern hot springs as it formed in a pre angiosperm world it is akin to the famous hot spring related devonian rhynie cherts of scotland it differs in having excellent exposure and thus will probably contribute to a better understanding of biosignal preservation in extreme environments in the geological record

multitrophic interactions in a geothermal setting arthropod borings actinomycetes fungi and fungal like microorganisms in a decomposing conifer wood from the jurassic of patagonia abstract a permineralized araucarioid conifer stem preserved in state of partial decay is described from the middle to late jurassic chon aike formation bahia laura group at laguna flecha negra in the deseado massif santa cruz province argentina the tree stem which comprises periderm to the secondary xylem shows evidence of interactions with xylophagous borers fungi actinomycetes and other fungal like microorganisms in the secondary phloem it displays rows of traumatic resin ducts which host abundant mycelium comprising very thin hyphae differentiated into spore producing structures comparable to actinomycete bacteria and spherical fungal like resting structures in the cortical and phloem regions there are cell groups with structural changes which appear variably degraded by fungi the secondary xylem displays decay of cellulose and lignified components at various spatial scales comparable with fungal decay patterns in modern wood further septate fungal filaments sometimes bearing clamp connections are associated with decayed regions the degraded secondary xylem shows circular to oval borings filled with frass containing degraded tracheids comparable to those bored by different wood boring beetles development of traumatic resin ducts in the phloem could have been triggered by penetration of the living stem by these xylophagous arthropods the association of the different components present suggests pathogenic and saprotrophic interactions promoting decomposition of the stem and represents a rarely documented example of multitrophic biological interactions involving several different biological groups in jurassic terrestrial paleoecosystems