structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

pumpellyites and coexisting minerals in different low grade metamorphic facies of liguria italy pumpellyite from four phase assemblages pumpellyite epidote prehnite chlorite pumpellyite epidote actinolite chlorite pumpellyite epidote na amphibole chlorite together with common excess phases considered to be low variance in a cao mgo feo al2o3 fe2o3 na2o sio2 h2o system have been examined in areas which underwent metamorphism in the prehnite pumpellyite pumpellyite actinolite and low temperature blueschist facies respectively the analysed mineral assemblages are compared for nearly constant basaltic chemical composition at varying metamorphic grade and for varying chemical composition basic intermediate acidic at constant metamorphic conditions low temperature blueschist facies in the studied mineral assemblages coexisting phases approached near chemical equilibrium at constant basaltic bulk rock composition the mgo content of pumpellyite increases and the xfe3 of both pumpellyite and epidote decreases with increasing metamorphic grade the fe3 being preferentially concentrated in epidote both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions pumpellyite becomes progressively enriched in fe and depleted in mg from basic to intermediate and acidic bulk rock compositions the compositional comparison of pumpellyites from high variance 1 3 phases assemblages in various bulk rock compositions basic intermediate acidic rocks greywackes gabbros shows that the compositional fields of both pumpellyite and epidote are wide and variable broadly overlapping the compositional effects observed at varying metamorphic grade in low variance assemblages the intrinsic stability of both fe and al rich pumpellyites extends across the complete range of the considered metamorphic conditions element partitioning between coexisting phases is the main control on the mineral composition at different p t conditions

determining the significance of high grade shear zones by using temperature time paths with examples from the grenville orogen ductile shear zones preserve essential information on processes that are active in orogenic roots but the significance of these zones is often difficult to interpret structural petrologic and geochronologic data from shear zones yield elements of the history that are not necessarily synchronous however by combining these data with temperature time t t paths insights are obtained into the nature of shear zones the relation between bounding blocks and orogenic evolution of the deep crust this procedure is illustrated with two examples from the mid proterozoic grenville orogen t t paths from 1160 to 900 ma are based on u pb dating of metamorphic minerals including garnet closure temperature t c 800 c monazite t c 725 c sphene t c 600 c and rutile t c 400 c and 40 ar 39 ar hornblende ages t c 480 c comparison of t t paths from adjacent blocks allows predictions about the significance kinematics and timing of displacement of shear zones in the grenville orogen t t paths can distinguish between major terrane boundaries e g the carthage colton shear zone and within terrane shear zones e g the bancroft shear zone thus these data can also be used to identify individual tectonic terranes in the deep crust this integrated approach to analysis of shear zones provides constraints needed to determine the nature and rate of deep orogenic processes in areas that are complicated by high metamorphic grades

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

strain analysis of heterogeneous ductile shear zones based on the attitudes of planar markers using the simple measurement of the attitude of planar features such as foliation shear planes and displaced tabular markers such as veins or dykes fundamental information is obtained on deformation types and rheology in heterogeneous ductile shear zones the method outlined in this study is based on the subdivision of the shear zone in n deformed layers each of them being characterized by approximately homogenous deformation for each layer paired measurements of and are made where is the angle that the foliation forms with the shear plane and is the effective shear strain by means of suitable grids constructed according to strain boundary conditions values of stretch k shear strain strain ratio r and kinematic vorticity number wk are calculated the method has been tested on three ductile wrench zones exposed in a deformed granitoid pluton in the eastern alps italy the data indicate that the shear zones are of dominantly transtensional or transpressional type furthermore the finite shear strain and strain ratio peaked profiles suggest that in all instances shear zone evolution was characterized by strain softening analysis of the kinematic vorticity indicates that strain softening essentially affected the simple shear component of the deformation

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

deformation and geochronology of syntectonic granitoids emplaced in the major gercino shear zone southeastern south america abstract the major gercino shear zone is one of the ne sw lineaments that separate the neoproterozoic dom feliciano belt of brazil and uruguay into two different domains a northwestern supracrustal domain from a southeastern granitoid domain the shear zone striking ne is composed of protomylonites to ultramylonites with mainly dextral kinematic indicators in santa catarina state southern brazil the shear zone is composed of two mylonite belts the mylonites have mineral orientations produced under greenschist facies conditions at a high strain rate strong flattening and coaxial deformation indicate the transpressive character while the role of pure shear is emphasized by the orientation of the mylonite belts in relation to the inferred stress field component the quartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity were the dominant mechanisms of deformation during the mylonitization process additionally the fabrics suggest that the glide systems are activated for deformation conditions compatible with the metamorphism in the middle greenschist facies elongated granitoid intrusions belonging to two petrographically geochemically and isotopically distinct rock associations occur between the two mylonite belts the structures observed in the granites result from a deformation range from magmatic to solid state conditions points to a continuum of magma straining during and just after its crystallization conventional u pb analysis of multi crystal zircon fractions yielded essentially identical ages of 609 16 ma and 614 2 ma for the two granitic associations and constrain the transpressive phase of the shear zone k ar ages of biotites between 585 and 560 ma record the slow cooling and uplift of the intrusions some k ar ages of micas in regional mylonites are similar suggesting that thermo tectonic activity was intense up to this time probably related to the agglutination of the granite belt to the supracrustal belt nw of the mgsz

the tandilia system of argentina as a southern extension of the rio de la plata craton an overview the southernmost outcrops of the rio de la plata cratonic region are exposed in the tandilia system in eastern argentina the geological evolution comprises mainly an igneous metamorphic paleoproterozoic basement named buenos aires complex which is covered by neoproterozoic to early paleozoic sedimentary units which display subhorizontal bedding the basement of calc alkaline signature consists mainly of granitic tonalitic gneisses migmatites amphibolites some ultramafic rocks and granitoid plutons subordinate rock types include schists marbles and dykes of acid and mafic composition tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids the tectonic scenario seems related to juvenile accretion event 2 25 2 12 ga along an active continental margin followed by continental collision 2 1 2 08 ga after u pb zircon data the collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks the tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution the basement was preserved from younger orogenies such as those of the brasiliano cycle after a long paleoweathering process the sierras bayas group c 185 m thick represents a record of the first neoproterozoic sedimentary unit siliciclastic dolostones shales limestones superposed by cerro negro formation c 150 400 m thick siliciclastics assigned to upper neoproterozoic age the final sedimentary transgression during early paleozoic was the balcarce formation c 90 450 m thick deposited over all the mentioned precambrian units based on all the geological background a tectonic evolution is offered

phanerozoic low temperature evolution of the uruguayan shield along the south american passive margin the crystalline basement of uruguay was assembled during the brasiliano orogeny in the neoproterozoic era and was later affected by discrete tectonic activity a new multi method low temperature dataset including u th he ages from both zircon and apatite t t modelling and k ar dating of fine sericite fractions and fault gouge reveal a detailed post orogenic geological history spanning the phanerozoic eon the juxtaposition of the terranes that compose the area was achieved in the ediacaran period and post collision was marked by intense exhumation in which the crystalline basement reached near surface conditions by the early to mid palaeozoic regional subsidence promoted sedimentation in the parana basin until the permian covering and reheating much of the basement that is at present exposed afterwards deposition and volcanism were mostly confined to its current limits regional exhumation of the shield during the permo triassic exposed much of the northern portion of the basement and the south was further affected by the opening of the south atlantic ocean during the mesozoic little exhumation affected the uruguayan shield during the cenozoic as reflected in its modest topography the reactivation of inherited neoproterozoic structures influenced the development of mesozoic basins and the present day landscape supplementary material supplementary data sample locations u th he data and k ar data are available at https doi org 10 6084 m9 figshare c 3702043

post glacial environmental change and hunting gathering societies of the southern prairie peninsula abstract evidence from several disciplines supports the proposition that following the wisconsinan and an initial but brief moist phase of the holocene there was a gradual drying trend in the mid continent prairie peninsula that peaked about 7000 years ago a pattern that was not reversed until about 4000 years ago except for minor perturbations conditions during the past four millennia have essentially been similar to those of the present studies in palynology and geomorphology have shown that the climatic regimes of the early mid post glacial had a marked influence on both the biota and landscape erosional patterns especially along the margins of the prairie peninsula measurable changes in the landscape and attendant biota show a time transgressive pattern of drought related phenomena recorded palaeo ecologically from west to east few studies of archaeological sites in the prairie peninsula have examined the evidence in and around the sites themselves for data reflecting local environmental dynamics in fact earlier models proposed for explaining the evolution of food procuring systems in eastern north america virtually ignored potential changes in the natural environment this paper examines data from two archaeological sites along the prairie forest border in missouri that contain long cultural sequences evidence is presented that pronounced changes in the natural environment occurred and that these biophysical variables are indeed critical for understanding the evolution of food procuring societies in the american midwest

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

the laxford shear zone an end archaean terrane boundary the lewisian gneiss complex of northwestern scotland consists of archaean gneisses variably reworked during the proterozoic it can be divided into three districts a central granulite facies district between districts of amphibolite facies gneiss to the north and south recent work has interpreted these districts in terms of separate terranes initiating a controversy that has implications for how precambrian rocks are understood worldwide the northern district of the lewisian gneiss complex the rhiconich terrane is separated from the central district the assynt terrane by a broad ductile shear zone known as the laxford shear zone this paper reviews the geology of the laxford shear zone clarifying field relationships and discussing other evidence to consider whether or not it does indeed represent a terrane boundary a detailed review of field geochemical and geochronological evidence supports the recognition of the separate assynt and rhiconich terranes mafic dykes the scourie dyke swarm and granitoids of palaeoproterozoic age occur on both sides of the laxford shear zone and thus the terranes were most probably juxtaposed during the late archaean to early palaeoproterozoic inverian event subsequently the less competent more hydrous amphibolite facies gneisses of the rhiconich terrane were affected by later palaeoproterozoic laxfordian deformation and partial melting to a greater extent than the more competent granulite facies gneisses of the assynt terrane

tectonics of the southern portion of the ribeira belt apiai domain abstract the name ribeira belt was originally used in a reference to a broad belt of deformed crust which strikes subparallel to the southeastern brazilian coastline it was originally believed to be neoproterozoic brasiliano panafrican but present investigations indicate that this age seems to be mainly related to the extensive arc related plutonism and tectono metamorphism that affected its most representative formations whereas the time of deposition of the supracrustals would be significantly older in this study the main tectono stratigraphic features of the apiai domain of the ribeira belt are described the lithostratigraphy is reviewed as well the paleogeographic structural geochemical and radiometric data and their implications for the tectonic evolution models for the region the marine sediments of the apiai domain were laid down on the borders of the parana and luis alves cratons and on an intervening mesoproterozoic 1 8 1 1 ga ocean crust they were intensively deformed by ocean closure and collision of the cratonic fragments and they constitute the majority of the local supracrustals designated as the acungui supergroup symmetrical continental passive margin deposits are represented to the northwest by a more proximal facies itaiacoca group probably related to the parana paleocontinent and by the capiru formation to the southeast between them lies the central domain formed by the votuverava group characterized by the remains of gradually deepening carbonatic platform sediments of the lajeado subgroup which grade eastward into distal turbiditic sequences of the ribeira subgroup where ocean floor and immature island arc magmatic rocks are present during the neoproterozoic the apiai domain behaved as a single continental mass into which basic rocks were intruded between 1 1 and 0 85 ga between 0 7 and 0 6 ga the apiai domain evolved into a cordilleran type magmatic arc related to the evolution of a subduction zone possibly situated to the southeast of the luis alves cratonic fragment this subduction caused metamorphism and migmatization of the supra and infra crustal rocks of the region and produced the extensive calc alkaline granitoid plutonism between 0 6 and 0 45 ga the majority of the southeastern continental blocks amalgamated during closure of the adamastor ocean followed by development of a system of regional dextral transcurrent shear zones related to oblique collision and or escape tectonics pull apart basins opened up and were filled with molasse sequences the region was intruded by post tectonic granitoids and then underwent regional cooling

oblique collision and evolution of large scale transcurrent shear zones in the kaoko belt nw namibia abstract early structures in the central part of the kaoko orogenic belt of nw namibia suggest that the initial stage of collision was governed by underthrusting of the medium grade central kaoko zone below the high grade western kaoko zone resulting in the development of an inverted metamorphic gradient in the western zone early structures were overprinted by a second phase of deformation which is associated with localization of the transcurrent puros shear zone along the contact between the western and central zones during this second phase extensive partial melting and intrusion of 550 ma granitic bodies occurred in the high grade western zone in the central zone the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the puros shear zone and to the development of kilometre scale folds in the more distal parts strain partitioning into transcurrent deformation along the puros shear zone and ne sw oriented shortening in the central zone is consistent with a sinistral transpressional regime during the second phase of deformation transcurrent deformation continued during cooling of the entire belt giving rise to the localized low temperature village mylonite zone that separates a segment of elevated mesoproterozoic basement from the rest of the western zone in which only pan african ages have so far been observed the data suggest that the boundary between the western and central kaoko zones represents a modified thrust zone controlling the tectonic evolution of the pan african kaoko belt

evolution of the major gercino shear zone in the dom feliciano belt south brazil and implications for the assembly of southwestern gondwana the dom feliciano belt developed during the brasiliano pan african orogenic cycle due to the tectonic interaction between the rio de la plata congo and kalahari cratons along with the amalgamation of smaller continental fragments together with its prolongations to the south the major gercino shear zone constitutes one of the main lineaments of the orogenic system establishing a more than 1000 km long ne trending tectonic boundary between a granitic batholith and a metavolcano sedimentary association based on combined field structural microstructural and textural data together with new and published geochronological data a refined model for the geological evolution of the major gercino shear zone is presented regional nw verging low dipping structures were generated between 650 and 615 ma preceding the formation of the shear zone this was followed by the main deformation phase corresponding to pure shear dominated dextral strike slip interpreted do be controlled by regional transpression during oblique convergence of the continental terranes this stage lasted until ca 585 ma and was coeval with the continuous emplacement of granitic magmatism along the structure strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions late stage ductile deformation along the structure was active during the cooling of the intrusions until ca 550 ma while the deformation front of the orogenic system migrated to counterpart orogenic belts in the african continent after tectonic stabilization the major gercino shear zone recorded episodic brittle reactivation possibly related to the phanerozoic evolution of the active margins and intracratonic basins in gondwana 2018 springer verlag gmbh germany part of springer nature

timing of terrane accretion in the neoproterozoic eopaleozoic ribeira orogen se brazil abstract new id tims u pb geochronological data of the oriental terrane combined with detailed geological data of other terranes support the presented tectonic evolutionary model for the central segment of the neoproterozoic ribeira belt southeastern brazil that can be subdivided in four major tectonic units a the occidental terrane regarded as the reworked margin of the sao francisco craton to the northwest b the paraiba do sul klippe c the oriental terrane comprising arc complexes and d the cabo frio terrane to the southeast the oriental terrane and the cabo frio terrane were progressively accreted to the reworked continental margin of the sao francisco paleoplate the occidental terrane contains pre 1 8 ga basement slivers interlayered with neoproterozoic passive margin successions andrel ndia group the oriental terrane focus of the work was firstly accreted to the sao francisco margin at ca 580 ma it displays three structural domains thrusted to nw toward the reworked sao francisco margin from west to east a the cambuci domain that is interpreted as the fore arc association b the costeiro domain that contains a distal passive margin succession intruded by the ca 790 and 635 620 ma rio negro arc complexes and c the italva domain that comprises marbles psamitic gneisses and amphibolites of ca 840 and is regarded both as the proximal passive margin of the oriental terrane or a back arc basin syn to late collisional granitoids and three metamorphic episodes ca 605 580 and 550 ma indicate a complex history for the collision i they were also described for the kaoko damara and west congo only the youngest one pan african belts located at occidental africa and are supposed to record the main episode of convergence in western gondwana a second collisional stage at ca 530 ma accreted the cabo frio terrane to the oriental terrane and is possibly related with the closure of a back arc basin located between the oriental terrane and kalahari craton that remained open until the ordovician the effects of the collision ii were also detected at the oriental terrane finally late to post tectonic granites marked the tectonic collapse of the orogenic building between 513 480 ma

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions

the transpressional connection between dom feliciano and kaoko belts at 580 550 ma a new u pb shrimp age of 551 4 ma on a mylonitic porphyry that intruded into the sierra ballena shear zone southernmost dom feliciano belt uruguay and a review of relevant published data make possible a more refined correlation and reconstruction of brasiliano pan african transpressional events paleogeographic reconstruction kinematics and timing of events indicate a connection between the shear systems of the dom feliciano and kaoko belts at 580 550 ma sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the congo and rio de la plata cratons the correlation is strengthened by the similarity of magmatic and metamorphic ages in the coastal terrane of the kaoko belt and the punta del este terrane of the dom feliciano belt this post collisional sinistral transpression brought these units near to their final position in gondwana and explains the different evolution at 550 530 ma while in the kaoko belt an extensional episode resulted in exhumation as a consequence of collision in the damara belt in the dom feliciano belt sinistral transpression occurred associated with the closure of the southern adamastor ocean due to kalahari rio de la plata collision

black dimensional stones geology technical properties and deposit characterization of the dolerites from uruguay dimensional stones with a black color occupy a prominent place on the international market uruguayan dolerite dikes of andesitic and andesitic basaltic compo sition are mined for commercial blocks of black dimen sional stones a total of 16 dikes of both compositions were studied and samples collected for geochemical and petro graphical analysis color measurements were performed on different black dimensional stones in order to compare them with the uruguayan dolerites samples of the two commercial varieties absolute black and moderate black were obtained for petrophysical analysis e g density porosity uniaxial compressive strength tensile strength etc detailed structural analyses were performed in sev eral quarries geochemistry and petrography determines the intensity of the black color the uruguayan dolerite absolute black is the darkest black dimensional stone analyzed in this study the petrophysical properties of uruguayan dolerites make them one of the highest quality black dimensional stones structural analyses show that five joint sets have been recognized two sub vertical joints one horizontal and two diagonal these joint sets are one of the most important factors that control the deposits since they control the block size distribution and the amount of waste material

quartz microstructures and crystallographic preferred orientation which shear sense do they indicate abstract crystallographic preferred orientation cpo and shape fabrics of dynamically recrystallized quartz are currently used as shear sense indicators we show that the cpo and shape fabric in a polyphase rock do not necessarily indicate the shear sense at the global scale of the shear zone but rather at the local scale of the deforming and recrystallizing quartz aggregates in lower amphibolite facies shear zones in the gran paradiso metagranodiorite magmatic quartz grains have recrystallized dynamically by subgrain rotation and grain boundary migration and deform inside a very fine grained feldspar mica matrix the quartz cpo has a peripheral c axis maximum inclined synthetically with the local shear sense the orientation of the surface fabric is related to the orientation of the c axis maximum and the microscopic shear sense in the quartz aggregate the geometry of the surface fabric ranges from monoclinic to symmetric depending on the relative contribution of grain boundary migration it is inferred that flow partitioning between the quartz aggregates and the matrix controls the local kinematics cpos are only reliable shear sense indicators in polyphase rocks if the most highly strained parts are analyzed where spin of the aggregates with respect to the shear zone boundary has ceased

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369

zeolite to prehnite pumpellyite facies metamorphism of the late proterozoic zig zag dal basalt formation eastern north greenland abstract the zig zag dal basalt formation 1300 m maximum thickness of late proterozoic age exposed in eastern north greenland has been variably affected by low temperature metamorphism resulting in the development of calc silicate and mafic phyllosilicate bearing assemblages relatively fresh flows sometimes preserving primary olivine calcic plagioclase augite and pigeonite and showing randomly mixedlayer chlorite smectite are sandwiched between flows showing not only more extensive alteration but also higher temperature assemblages accordingly in these flows the primary igneous phases are replaced with the exception of augite and prehnite and pumpellyite along with rare zeolites are sporadically developed while regular mixed layer chlorite smectite and or true chlorite predominate temperatures have been estimated on the presence absence of various secondary minerals and also using the chlorite geothermometer and are indicative of progressive metamorphism in the range from below 125 c to approximately 200 c i e in the zeolite facies to lowermost prehnite pumpellyite facies range no depth dependent temperature related zonation of secondary mineral phases as seen in other thick altered basalt successions has been identified in the zig zag dal basalts however instead alteration appears to have been influenced by the physical character of the flows themselves including flow thickness degree of vesicularity and the presence or absence of jointing all of which appear to have exerted an influence over access of fluid this relation between flow character and intensity of alteration suggests that fluid flow through the zig zag dal basalts must have been predominantly lateral rather than vertical

40ar 39ar and rb sr geochronology of the uruguayan dike swarm rio de la plata craton and implications for proterozoic intraplate activity in western gondwana abstract the uruguayan dike swarm intrudes country rocks of the rio de la plata craton that were deformed and metamorphosed to high grade during the 2200 1900 ma transamazonian orogeny slow cooling of the continental crust following this event is suggested by the 40 ar 39 ar plateau ages of hornblende 2016 11 ma and biotite 1817 10 ma from the pintos post tectonic granodiorite there are seven 40 ar 39 ar age determinations on mafic dikes and felsic veins from the swarm between 1727 and 1700 ma one dike yielded concordant plateau ages for igneous hornblende 1727 10 ma and biotite 1725 10 ma these ages are taken as the best estimate for the emplacement of the uruguayan swarm a rb sr whole rock best fit line calculated for 15 dikes yielded 1766 124 ma in agreement within error with the 40 ar 39 ar plateau ages the origin of the uruguayan swarm is tectonically related to the emplacement of the illescas and minas de corrales rapakivi granites that crop out in the nico perez domain the tectonic setting of these units is also compatible with bimodal magmatism and basin formation associated with extensional events in the uruguayan and brazilian shields on the whole the geologic framework suggests that the uruguayan dike swarm marks a transcontinental intraplate episode within the south america continent which occurred shortly after the transamazonian orogeny the results of initial release portions of 40 ar 39 ar spectra dikes and felsic veins coupled with a rb sr mineral isochron from one felsic vein are between 1370 and 1170 ma eight k ar ages range from 1600 to 1170 ma suggesting variable argon loss in the systems these isotopic disturbances were probably favored by low grade hydrothermal overprints exemplified by differences in the 40 ar 39 ar spectra from one felsic vein muscovite and biotite and suggested also by the secondary mineralogy that may be present in the dikes these post emplacement ages could result from mesoproterozoic crustal shortening recognized in the nico perez domain interpreted to be tectonically associated with the namaqua orogeny of southern africa this inference is important in understanding the late mesoproterozoic evolution of western gondwana and brings potential implications for a paleogeographic reconstruction between the paleocontinent plata and amazonia at the time of the grenville orogeny

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

low grade metamorphism and geotectonic setting of the macuchi formation western cordillera of ecuador the cretaceous eocene basic to intermediate marine volcanic rocks of the mucuchi formation constitute the western cordillera in northern ecuador their chemical features mostly correspond to those of tholeiitic basalts with some calc alkaline affinities and suggest an oceanic island arc setting the macuchi rocks are affected by low grade non deformative metamorphism characterized by zeolite prehnite pumpellyite and lower greenschist facies assemblages depth zonation is suggested by the downward mineral sequence i laumontite pistacitic epidote pumpellyite prehnite ii pumpellyite prehnite pistacitic epidote iii actinolite biotite pistacitic epidote chlorite this broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea water under a moderate to high thermal gradient 75 c km alteration appears to have been dependent primarily on fluid control volume pressure composition temperature and reaction kinetics which together partly overshadow the role of load pressure compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently metastable equilibrium commonly exists however a progressive approximation to equilibrium as a result of p t control is shown by the mineralogy a high fo2 of the fluid phase is evident from the mineral chemistry the metamorphism of the macuchi volcanics is similar to the hydrothermal burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment however some of its characteristics point to a transition toward systems defined by a higher t p ratio such as those found in ocean floor metamorphism a model is proposed in which the macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north south attenuation of the continental crust due to mantle diapirism

great slave lake shear zone canadian shield reconstructed vertical profile of a crustal scale fault zone abstract the great slave lake shear zone gslsz of the northwestern canadian shield a 25 km wide early proterozoic transcurrent dextral mylonite zone is a type example of a crustal scale fault zone profile with time 1 1 the metamorphic grade decreased from granulite to greenschist facies 2 2 the locus of high strain narrowed and jumped laterally abandoning relatively older mylonites progressively younger mylonites represent both cooler temperatures and lower pressures thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the gslsz 3 3 deformation evolved through the brittle ductile transition initially producing non dilational penetrative breccias followed first by discrete non dilational faulting and subsequently by dilational quartz stockworks developing crustal scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate independent of variations in relative plate motion mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks

shear zone evolution and timing of deformation in the neoproterozoic transpressional dom feliciano belt uruguay abstract new structural microstructural and geochronological u pb la icp ms ar ar k ar rb sr data were obtained for the dom feliciano belt in uruguay the main phase of crustal shortening metamorphism and associated exhumation is recorded between 630 and 600 ma this stage is related to the collision of the rio de la plata and congo cratons at ca 630 ma which also involved crustal reworking of minor crustal blocks such as the nico perez terrane and voluminous post collisional magmatism subsequent orogen parallel sinistral shearing gave rise to further deformation up to ca 584 ma and resulted from the onset of the convergence of the kalahari craton and the rio de la plata congo cratons sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation dextral ene striking shear zones were subsequently active at ca 550 ma coeval with further sinistral shearing along n to nne striking shear zones the tectonothermal evolution of the dom feliciano belt thus recorded the collision of the rio de la plata and congo cratons which comprised one of the first amalgamated nuclei of gondwana and the subsequent incorporation of the kalahari craton into western gondwana

titaniferous accessory minerals in very low grade metamorphic rocks keweenaw peninsula michigan usa abstract titanite tio 2 and pseudorutile are associated with primary igneous fe oxide grains in basalt and rhyolite clasts from the keweenaw peninsula michigan usa which were metamorphosed to the prehnite pumpellyite facies pseudorutile occurs with titanite tio 2 in broad lamellae within titaniferous magnetites it also occurs as intergrowths with titanite and fe oxide in embayments within primary oxide grains and in composite fe oxide titanite grains that appear to represent nearly complete replacement of original ti bearing fe oxides thin 111 lamellae in titaniferous magnetite grains contain titanite titanite tio 2 and titanite tio 2 fe oxide tio 2 also occurs by itself in networks of closely spaced small elongated lenses in fe ti oxides concentrations of caalsio 4 f caalsio 4 oh in titanite range from 0 to 30 the large variation in titanite compositions suggests that equilibrium was not achieved except on a very local scale a conclusion also supported by local variations in the assemblages of ti bearing secondary minerals textures and mineral assemblages indicate that ulvospinel lamellae were altered to titanite and or tio 2 polymorphs while ilmenite was altered to pseudorutile and titanite tio 2 the relative proportions of tio 2 and titanite appear to reflect local variations in the composition of the metamorphic fluid phase that may be linked to the degree of interaction with a hydrothermal fluid titanite rich regions may indicate a greater degree of fluid flushing than tio 2 rich regions textures and mineral assemblages cannot distinguish between models in which pseudorutile and a tio 2 polymorph formed during weathering or diagenesis followed by conversion of tio 2 to titanite during metamorphism and models in which pseudorutile formed together with titanite and tio 2 during metamorphism in either case pseudorutile was able to persist through prehnite pumpellyite metamorphism suggesting that it can be a significant ti rich accessory mineral in very low grade metamorphic rocks

lithospheric shear zones as constant stress experiments below the seismogenic zone plate boundaries are defi ned by lithospheric ductile shear zones these localize strain and are the main reason that convective motions in the earth s interior are expressed at the surface as plate tectonics we argue here that lithospheric shear zones operate at approximately constant stress equal to the yield strength of the surrounding rocks this places constraints on the bulk strength of the lithosphere and allows us to calculate the cumulative width of the shear zones as a function of depth the concept applies most clearly to strike slip shear zones but is also applicable to thrust and normal sense shear zones although these evolve with time due to temperature changes associated with burial or exhumation if shear zones operate at constant stress this affects their microstructural evolution and hence their rheology decreases in grain size due to dynamic recrystallization may cause a switch in the dominant deformation mechanism to grain size sensitive creep leading to weakening and strain localization a common argument based on a constant strain rate approach has been that such transitions will be inhibited by grain growth in a constant stress shear zone however dynamic recrystallization continues to maintain the low grain size even after the switch has occurred grain growth is inhibited under these conditions and hence the switch is permanent as long as the boundary conditions remain the same grain size reduction in shear zones is therefore a critical factor in maintaining plate tectonic processes

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the paleogeographic evolution of southwestern gondwana abstract the ediacaran to lower cambrian arroyo del soldado group represents a thick marine sequence cropping out in the eastern border of the rio de la plata craton in central uruguay this work addresses the provenance of different siliciclastic units in the arroyo del soldado group in terms of petrography geochemistry nd isotope and detrital zircon geochronologic data the lower part of the arroyo del soldado group the yerbal formation and the overlying carbonates of the polanco formation are related to an open shelf which formed after rifting along eastern border of the rio de la plata craton u pb zircon dating of reworked detrital material of the yerbal formation shows a provenance with mainly palaeoproterozoic ages 2 0 2 2 ga the yerbal formation shows t dm model ages of 2 1 ga and nd t of 15 the middle part of the arroyo del soldado group is characterized by deposition of coarse sediments barriga negra formation that could be related to a sea level fall during non global glaciation the zircon ages reflect a major contribution from the palaeoproterozoic and secondly from neoproterozoic basement rocks and constrain the maximum age of deposition to 566 ma t dm model ages reflect archaean 2 7 ga and palaeo mesoproterozoic 1 6 1 9 ga mantle extraction ages for the sources the archaean component shows geochemical similarities with a mafic source characterized by high concentration of cr 350 ppm and low th sc 0 7 up section during a sea level rise the deposition of bif shale and chert of the cerro espuelitas formation occurred t dm ages of 2 1 ga suggest a provenance mainly derived from the rio de la plata craton u pb zircon dating of the cerros san francisco formation representing the upper part of the arroyo del soldado group shows a provenance mainly from palaeoproterozoic 2 0 2 1 ga and archaean 2 5 2 9 3 5 ga rocks the scarcity of neoproterozoic zircons along with t dm ages of 2 2 2 9 ga and the extreme recycling that affected the cerros san francisco formation as indicated by petrography and geochemistry do not fit with the hypothesis that the arroyo del soldado group was deposited close to a syntectonic magmatic arc a tectonic scenario in which the arroyo del soldado group was deposited on a passive margin before strike slip collision with the cuchilla dionisio pelotas terrane at ca 530 ma is supported

transition from the zeolite to prehnite pumpellyite facies in the karmutsen metabasites vancouver island british columbia

isotope geochemistry and geochronology of the nico perez terrane rio de la plata craton uruguay u pb shrimp zircon geochronology bulk rock geochemistry and sr nd isotopic data are herein presented in order to better constrain the tectonic evolution of the nico perez terrane npt uruguay the studies are focused on two key geological units located in the southern part of the npt namely carape complex cc and lavalleja metamorphic complex lmc the carape complex previously interpreted as part of a remnant magmatic arc formed at 1 75 ga mainly by partial melting of archean crust t dm model ages between 2 8 and 3 0 ga with strong negative e nd t values the lavalleja metamorphic complex previously interpreted as a back arc basin has volcanic units formed at 0 59 ga during the brasiliano cycle but may also contain older paleoproterozoic units the geochemistry of the lmc meta igneous rocks indicates a complex scenario two distinct signatures have been recognized one similar to morbs and the other similar oibs sm nd t dm model ages of basic rocks present a bimodal paleoproterozoic and archean distribution which is also recognized in several other units of the npt in the light of new results we conclude that the npt represents a cratonic margin setting characterized by the stacking of lithotectonic assemblages with archean paleoproterozoic and neoproterozoic age at the margin of the rio de la plata craton

calc silicate mineralization in active geothermal systems detailed study of the calc silicate mineralogy in the cerro prieto geothermal system was used to demonstrate examples for thermodynamic evaluation of phase relations among minerals of variable composition and to calculate the chemical characteristics of hydrothermal solutions compatible with the observed calc silicate assemblages in general there is a close correlation between calculated and observed fluid compositions calculated fugacities of o 2 at about 320 degrees c in the cerro prieto geothermal system are about five orders of magnitude less than those at the nearby salton sea geothermal system this observation is consistent with the occurrence of fe super 3 rich epidotes and andradite garnets in the latter system and the presence of prehnite at cerro prieto modified journal abstract

zeolite to prehnite pumpellyite facies metamorphism in the toa baja drill hole puerto rico secondary mineral assemblages in eocene volcanic and volcaniclastic rocks of the toa baja drill hole progressively change as a function of depth four metamorphic zones are delineated with increasing depth caclinoptilolite heulandite laumontite prehnite purnpellyite zone the deepest belongs to the prehnite pumpellyite facies whereas the three zones at shallower depths 2350 m belong to the zeolite facies ca clinoptilolite and heulandite exhibit continuous solid solution with varying si al 2 9 5 2 and ca ca na k ratios 0 45 0 89 pumpellyites are characterized by extensive variation in the fe fe al mg ratio 0 29 0 74 and extreme fe enrichment in the laumontite zone temperatures estimated from mineral assemblages and fades transitions suggest that the north coast basin puerto rico has been subject to a metamorphic field gradient of 50 to 70 c km

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

paleozoic crustal blocks of onshore and offshore central argentina new pieces of the southwestern gondwana collage and their role in the accretion of patagonia and the evolution of mesozoic south atlantic sedimentary basins abstract the southwestern gondwana basement block configuration in the central argentinean offshore area was analyzed using gravimetric magnetic and seismic data and existing onshore tectonic models the resultant maps the distribution of the mesozoic and cenozoic basins and paleozoic structural features were used to validate the interpretations and to produce a new regional tectonic model pre carboniferous southwestern gondwana of south america was interpreted as an open margin formed from east to west by the dom feliciano belt the rio de la plata craton the pampean belt and the pampia and cuyania terranes the collision of the patagonia allochthonous terrane during the late paleozoic resulted in the development of the ventania cape fold belt which was mapped for the first time off the argentinean coast out to 600 km from the shore a strong change in the orientation of the fold belt is referred to as the colorado syntaxis a mirror image of the cape syntaxis in south africa this change reflects the buttressing effect of the cratonic blocks that hamper the northward propagation of syncollisional deformation and resulted in a 180 km shift of the orogenic front the mesozoic basins and the basement block distribution were analyzed the pampean belt a deformed area produced by the pampia accretion to the cratonic area is the locus to two episutural basins the general levalle and macachin basins the salado basin was interpreted as an episutural basin controlled by a 2 1 2 0 ga suture within the rio de la plata craton the colorado basin is composed of four segmented depocenters that reflect different emplacement controls the location of the western colorado basin was controlled by the upper paleozoic orogen the distributions of the central and eastern colorado depocenters orthogonal to the continental boundary were also strongly influenced by the upper paleozoic structures and were offset by lineaments that reflect dom feliciano fabric the colorado basin external depocenter that parallels the continental margin was also controlled by these lineaments we interpret a time gap of some 50 ma between the beginning of the evolution of the margin orthogonal depocenters and the atlantic breakup

timing of deformation in the sarandi del yi shear zone uruguay implications for the amalgamation of western gondwana during the neoproterozoic brasiliano pan african orogeny u pb and hf zircon sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms ar ar hornblende and muscovite and rb sr whole rock muscovite isochron data from the mylonites of the sarandi del yi shear zone uruguay were obtained in order to assess the tectonothermal evolution of this crustal scale structure integration of these results with available kinematic structural and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630 625 ma during the collision of the nico perez terrane and the rio de la plata craton the shear zone underwent dextral shearing up to 596 ma under upper to middle amphibolite facies conditions which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 ma after emplacement of the cerro caperuza granite at 570 ma the shear zone underwent only cataclastic deformation between the late ediacaran and the cambrian the sarandi del yi shear zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the rio de la plata craton and the nico perez terrane furthermore the obtained data reveal that strain partitioning and localization with time magmatism emplacement and fluid circulation are key processes affecting the isotopic systems in mylonitic belts revealing the complexity in assessing the age of deformation of long lived shear zones

constraints on deformation path from finite strain gradients we present a methodology for analyzing finite strain gradients in high strain zones in order to place constraints on the zone s possible deformation path to accomplish this the high strain zone is divided into several sub zones within each of which finite strain is described by a single ellipsoid based on knowledge of the spatial and temporal evolution of deformation strain observed in less deformed sub zones is then mathematically removed from more deformed sub zones to calculate incremental finite strains the mean kinematic vorticity number of each of these increments must be constant if deformation was steady state we use the methodology to analyze two natural examples of high strain zones at different scales both natural examples have non steady state likely deformation paths which may be a common characteristic of high strain zones although results of the methodology are not unique identification of likely paths considerably narrows the range of possible deformation paths consequently useful kinematic information can be extracted from finite strain data even in instances where the detailed spatial and temporal evolution of a high strain zone cannot be determined conclusively

multiple accretion at the eastern margin of the rio de la plata craton the prolonged brasiliano orogeny in southernmost brazil the neoproterozoic eoplalaeozoic brasiliano orogeny at the eastern margin of the rio de la plata craton in southernmost brazil and uruguay comprises a complex tectonic history over 300 million years the southern brazilian shield consists of a number of tectono stratigraphic units and terranes the sao gabriel block in the west is characterized by c 760 690 ma supracrustal rocks and calc alkaline orthogneisses including relics of older c 880 ma old igneous rocks both igneous and metasedimentary rocks have positive end t values and neoproterozoic tdm model ages they formed in magmatic arc settings with only minor input of older crustal sources a trondhjemite from the sao gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation d3 yield an u pb zircon age la icp ms of 701 10 ma giving the approximate minimum age of the sao gabriel accretionary event the encantadas block further east containing the supracrustal porongos belt and the pelotas batholith is in contrast characterized by reworking of neoarchean to palaeoproterozoic crust the 789 7 ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the porongos belt provides a time marker for the basin formation zircons of a sample from tonalitic gneisses constituting the palaeoproterozoic basement of the porongos belt form a cluster at 2 234 28 ma interpreted as the tonalite crystallization age zircon rims show ages of 2 100 2 000 ma interpreted as related to a palaeoproterozoic metamorphic event the porongos basin formed on thinned continental crust in an extensional or transtensional regime between c 800 700 ma the absence of input from neoproterozoic juvenile sources into the porongos basin strongly indicates that the encantadas and sao gabriel blocks were separated terranes that became juxtaposed next to each other during the brasiliano accretional events the tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the rio de la plata craton i accretion of an intra oceanic arc at c 880 ma passinho event and ii accretion of the 760 700 ma cambai vila nova magmatic arc sao gabriel event the latter event also includes the collision of the encantadas block with the rio de la plata craton to the west collision and crustal thickening was followed by sinistral shear along sw ne trending orogen parallel crustal scale shear zones that can be traced from southern brazil to uruguay and have been active between 660 and 590 ma voluminous granitic magmatism in the pelotas batholith spatially related to shear zones is interpreted as late to post orogenic magmatism possibly assisted by lithospheric delamination it marks the transition to the post orogenic molasse stage localized deformation by reactivation of preexisting shear zones continued until c 530 ma and can be assigned to final stages of the amalgamation of west gondwana

comment on provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeographic evolution of southwestern gondwana by blanco et al precambrian res 171 2009 57 73 thus it is truly tectonics which governs stratigraphy and thetwobranchesofthegeologicalsciencesareinseparable astruc tural geologists who is not a stratigrapher is only a geometer not a geologist for he reasons about abstract surfaces and vol umes emptied of their history and a stratigrapher who neverconcerned himself with tectonics would produce only a deadstratigraphy gignoux 1950 p 3 the paper provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeo graphicevolutionofsouthwesterngondwana blancoetal 2009 presented new u pb shrimp sm nd and geochemical data ofsome units of arroyo del soldado group asg in order to betterconstraining the evolution of sedimentary protoliths and there fore to justify a rift tectonic setting for deposition of those units unfortunately the data presented are incapable of answering thetectonic evolution of neoproterozoic lower paleozoic brasilianounits located at the east of the uruguayan territory we point outthattheauthors interpretationisnotsubstantiatedonthedata thescarcity of geological and isotopic data obtained for dom felicianobelt dfb determinesawidespanofgeotectonicmodels blancoetal 2009 use force andadapttheavailableisotopicdatatosustaintheir tectonic model other plausible models can also explain the

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

discrimination of annealed and dynamic fabrics consequences for strain localization and deformation episodes of large scale shear zones abstract large scale shear zones are often characterized by long lasting deformation with complex strain localization behavior based on a drill core across polymineralic carbonate mylonites of the doldenhorn nappe helvetic alps switzerland the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones to infer the deformation mechanisms and to reconstruct the strain localization history these goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone particularly the occurrence of two distinct zener trends i e the relationship between grain size and volume fraction of matrix and second phase grains allows for the identification of annealed from dynamic microstructures the data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation induced cooling resulted in progressive strain localization at the base of the shear zone while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

strain localization in homogeneous granite near the brittle ductile transition a case study of the kellyland fault zone maine usa abstract we present a detailed case study of a strike slip shear zone that cut homogeneous granite near the brittle ductile transition this zone contains three distinct strain facies 1 a 2 3 km wide belt of foliated granite 2 a 100 300 m wide belt of small localized shear zones and 3 a 200 400 m wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites the foliated granite formed via dislocation creep of quartz and minor dissolution precipitation creep and dislocation creep of feldspar under amphibolite facies conditions shortly after granite crystallization the localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite and they deformed by granular flow grain boundary sliding coupled with dissolution precipitation creep these deformation features reveal a three phase rheologic evolution in granite near the brittle ductile transition phase 1 is recorded by the foliated granite and the rheology was governed by dislocation creep of quartz phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone phase 3 was a long lived period of ductile deformation and the rheology was governed by the granular flow mechanism strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation

development of sigmoidal en echelon microfractures in beryl and the role of strain localization evidenced by fluid inclusions abstract a combined fluid inclusion and microstructural study was carried out in beryl crystals from the san cayetano nb ta bearing pegmatite san luis argentina primary aqueous carbonic fluids t0 were subsequently re trapped during shearing resulting in en echelon microfractures the more brittle behaviour of beryl compared to quartz makes this mineral more suitable for the preservation of fluid inclusions and microstructures the primary inclusions t0 are preserved in strain free domains whereas the pseudo secondary t1 to t3 type inclusions occur in domains showing intracrystalline deformation co2 was relatively immobile or reacted to form carbonate in early t1 type inclusions whereas h2o preferentially migrated along microfractures and was trapped as t2 and t3 type inclusions short intragranular trails of fluid inclusions representing en echelon healed microfractures resulted from progressive strain localization contrary to previous proposals this new model implies the progressive thickness reduction of intracrystalline micro shear zones besides hydrolytic weakening linked to dislocation glide is the most likely mechanism to explain the evolution of fluid inclusions with intracrystalline deformation enhancing anisotropic diffusion this study highlights the potential of combined fluid inclusion and microstructural studies in order to understand the interaction between fluid activity and deformation in this way valuable insights can be provided into the progressive development of overprinted fabrics and low to medium temperature deformation mechanisms of minerals

strain distribution within a km scale mid crustal shear zone the kuckaus mylonite zone namibia the subvertical kuckaus mylonite zone kmz is a km wide crustal scale proterozoic dextral strike slip shear zone in the aus granulite terrain sw namibia the kmz was active under retrograde amphibolite to greenschist facies conditions and deformed felsic and minor mafic gneisses which had previously experienced granulite facies metamorphism during the namaqua orogeny lenses of pre to syn tectonic leucogranite bodies are also deformed in the shear zone pre kmz deformation d1 is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering shear strain within the kmz is heterogeneous and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges strain is localized along the edge of leucogranites and between gneissic lozenges preserving d1 migmatitic foliations strain localization appears controlled by pre existing foliations grain size and compositional anisotropy between leucogranite and granulite the local presence of retrograde minerals indicate that fluid infiltration occurred in places but most ultramylonite in the kmz is free of retrograde minerals in particular rock composition and d1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space with deformation localization along planes of rheological contrast and along pre existing foliations therefore the spatial distribution of strain in crustal scale ductile shear zones may be highly dependent on lithology and the orientation of pre existing fabric elements in addition foliation development and grain size reduction in high strain zones further localizes strain during progressive shear maintaining the anastomosing shear zone network established by the pre existing heterogeneity

paleoecology of an early holocene faunal and floral assemblage from the dows local biota of north central iowa abstract the faunas and floras from the dows local biota provide an opportunity to compare holocene taxa without a cultural bias the dows local biota is located in a large depression on the back side north of the altamont i moraine complex within the des moines lobe the dows silt fauna flora dsf ca 9380 130 yr b p one horizon of the dows local biota was collected for plant macrofossils mollusks and micromammals dsf terrestrial gastropods are upland mesic forest dwellers although one species strobilops affinis is characteristic of more xeric forests and may represent open woods the aquatic gastropods reflect both permanent and periodic waters dsf micromammals prefer an open mesic deciduous forest the micromammal sympatry is restricted to a small area within the tension zone and deciduous forest belt of west central wisconsin dsf plants are characteristic of upland forests moist meadowlands or disturbed areas and aquatic habitats the dsf plant sympatry is large but restricted to the conifer hardwood and deciduous forests along the great lakes new england regions quantitative climatic data for the combined dsf sympatries suggest that dows ca 9380 yr b p was cooler than at present and is nearly identical to that achieved by pollen analyses at the cherokee sewer lake west okoboji sites ca 9000 yr b p in northwest iowa based on common habitat interpretations and sympatries about 9380 yr b p north central iowa was cooler and moister than at present and was occupied by an open deciduous forest

measurement and interpretation of strain in the syntectonic solis de mataojo granitic complex uruguay the neoproterozoic solis de mataojo granitic complex is an intrusive body elongate north south emplaced in the sarandi del yi arroyo solis grande shear zone in the present work a quantification of the magmatic strain has been made by analysis of fabric enclave geometry and minor structures structural evidence indicates that deformation began in the magmatic state and continued through the sub solidus stage the observed distribution of magmatic foliations and lineations the fabric the geometry of the enclaves and the late to post magmatic structures are related to a non coaxial flattening regime that took place after the emplacement and late in the magmatic history these structures reveal that magmatic strain involved a shortening of the order of 80 with a simple shear component between 3 and 4 5 the solid state deformation records an additional mean of 2 7

widespread remagnetization of late proterozoic sedimentary units of uruguay and the apparent polar wander path for the rio de la plata craton summary a palaeomagnetic study was performed on three neoproterozoic sedimentary successions exposed in central and eastern uruguay in order to better constrain the palaeogeographic evolution of the rio de la plata craton by the latest proterozoic these units comprise the latest ediacaran to early cambrian calcareous cerro victoria fm exposed in the central nico perez terrane the late ediacaran clastic yerbal fm and the calcareous polanco fm exposed in the dom feliciano belt and the ediacaran clastic rocha fm exposed in the easternmost punta del este suspect terrane the study showed that none of the units carry the original remanence and that they were affected by remagnetizations the most widespread remagnetization is shown by the cerro victoria yerbal and rocha formations a dual polarity post folding remanence carried by hematite was isolated in all these units mean directions from them are cv cerro victoria dec 179 1 inc 59 0 95 2 9 n 79 samples y yerbal dec 182 4 inc 61 9 95 4 5 n 38 samples r rocha dec 4 2 inc 64 9 95 2 7 n 42 samples their corresponding pole positions are virtually identical cv 82 6 s 309 3 e a95 3 9 y 77 0 s 298 4 e a95 5 9 r 76 6 s 291 0 e a95 4 2 suggesting the same magnetization age and process comparison with the south american path suggests the late permian early triassic or the latest cretaceous palaeogene as the most likely times for its acquisition these poles are also identical to the pole of the late proterozoic la tinta fm which is interpreted as affected by the same remagnetization and therefore invalid for palaeogeographic reconstructions a different magnetic component also dual polarity and post folding was defined at two sites of the cerro victoria fm and other two of the polanco fm possibly carried by magnetite the mean geomagnetic poles from both units cvc 4 2 n 343 2 e a95 13 8 and p 3 2 n 325 8 e a95 15 2 fall on the middle cambrian to early ordovician segment of the gondwana apwp suggesting that age for the remagnetization this permits to establish the age of the tectonic event affecting the neoproterozoic units of the rio de la plata craton as early cambrian ca 525 ma the available palaeomagnetic poles for this craton permit to constrain its palaeogeographic evolution only since approximately 600 ma

petrography and geochemistry of the carape granitic complex southeastern uruguay the southern sector of uruguay was intruded by numerous granitic plutons during the brasiliano cycle the granites and granodiorites of the carape complex comprise a large part of the neoproterozoic terrain exposed in southern uruguay typological and age relationships show that the characteristic of the granitic rocks changed during the brasiliano cycle four groups of granitoids can be distinguished according to their emplacement the first group corresponds to the campanero unit interpreted as a pre brasiliano basement which includes mainly preorogenic granites the second group pan de azucar and related granitoids includes synorogenic granites and granodiorites the third granitoid group dos hermanos granite and related plutons is classified as late to postorogenic granites finally the fourth group consisting of the aguila granite and related plutons is represented by alkaline amphibole biotite granites and are considered as post collisional alkaline granites which we assign to an extensional event associated with post collisional slab break off marking the end of the late proterozoic brasiliano orogenic cycle most of the granitic plutons in this area 2 300 km2 are relatively well exposed and have well defined intrusive relationships with the metamorphic country rocks these granitic rocks are the result of successive magma pulses from similar magma chambers through the late proterozoic to early paleozoic times

mass transfer and porosity evolution during low temperature water rock interaction in gneisses of the simano nappe arvigo val calanca swiss alps late alpine fissures and fractures in amphibolite facies basement gneisses at arvigo val calanca swiss alps show distinct cm sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high grade gneiss assemblages the process of selvage or reaction vein formation occurred in the brittle deformation regime and at temperatures characteristic of first the prehnite pumpellyite facies and then later of the zeolite facies the vein formation occurred during uplift and cooling at very late stages of the alpine orogeny the reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space the central zone of the arvigo veins contains an early assemblage with epidote prehnite and chlorite and a late succession sequence of various species of zeolite the veins of the arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate ca zeolite this is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of ca zeolite species vein formation during late uplift and cooling of the alps occurred at continuously decreasing t and at hydrostatic pressure 1 coexisting prehnite epidote records temperatures of 330 380 c 2 chlorite formation at temperature of 333 32 c and 3 formation of zeolites 250 c in the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss in addition to albitisation chloritisation of biotite is the second important reaction in the alteration process the reactions release components for the formation of ca al silicates the water rock interaction is associated with a depletion of al si ca fe and k in the altered wall rock the overall reaction is associated with an increase in porosity of up to 14 2 2 2 in the selvage zone altered wall rock caused by the volume decrease during albitisation and the removal of biotite the propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution reprecipitation mechanism zeolite formation is tied to the plagioclase alteration reaction in the rock matrix which releases components for zeolite formation to a co2 poor aqueous liquid

the black lake shear zone a boundary between terranes in the adirondack lowlands grenville province abstract the black lake shear zone has been proposed as a potentially important tectonic boundary between the frontenac and adirondack lowlands terranes based on geologic and isotopic evidence we present new structural geochronologic and anisotropy of magnetic susceptibility ams results from the black lake area that constrain the location kinematics and timing of deformation in the region both structural and ams results document a northeast trending zone of high strain between black lake and the saint lawrence river within this zone strain is heterogeneous and includes a strongly developed sub vertical northeast trending foliation less commonly two orthogonal lineation sets are present one that is sub horizontal and another that is down dip the foliation and granitic dikes are locally folded forming tight to isoclinal northeast trending folds kinematic indicators show thrust deformation in areas of down dip lineation but are mixed in areas of sub horizontal lineation with a slight majority suggesting left lateral shear these results suggest major northwest directed shortening accommodated by flattening and ductile thrust faults possibly with a component of sub horizontal shear the two lineations may have formed during discrete episodes or synchronously during transpression u pb shrimp geochronology on igneous titanite from deformed dikes in combination with previous geochronology show that significant deformation occurred from ca 1170 1100 ma this deformation likely occurred during the late stages of the shawinigan orogeny and may record accretion of the adirondacks to the margin of laurentia during terrane assembly

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

mineralizations of the lavalleja group uruguay a probable neoproterozoic volcano sedimentary sequence abstract the lavalleja group is located in the southern extreme of the dom feliciano belt being tentatively correlated with the porongos and brusque groups of brazil the basement of the lavalleja group is probably represented by granitic gneissic rocks of the campanero unit with ages in the southern portion ranging from 1 75 to 2 1 ga u pb in zircon the lavalleja group is characterized by narrow bands of metasedimentary and metavolcanic rocks and it is separated in three formations namely from base to top zanja del tigre fuente del puma and minas outcrops assigned to the minas formation have been recently correlated with the arroyo del soldado group only the fuente del puma formation hosts base metals au and ag ocurrences the fuente del puma formation is divided into three informal units sedimentary volcanic and hornblenditic gabbros the sedimentary unit is characterized by an important amount of carbonates syn collisional to post tectonic granitic bodies carape complex intrudes the lavalleja group and the campanero unit several mineralizations are located in the fuente del puma formation those associated to arrospide ramallo reus chape valencia la oriental apolonia redondo hill la china and la paloma mines are the most important in addition many ocurrences of cu zn pb were recognized in the region the cu zn pb mineralization includes massive sulfides with pyrite chalcopyrite sphalerite galena pyrrothyte arsenopyrite hematite into small bodies with lenticular shape the host rock shows frequently hydrothermal alteration the geochemistry and the geological features of the mineralizations suggest besshi massive sulphide zn cu pb and sedex zn pb as most probably genetic models for the deposits related to the neoproterozoic orogeny early mineralizations are syngenetic and were formed on the sea floor although the main mineralizations are related to remobilization during syn to late metamorphic events and thrusting

aeromagnetic patterns in southern uruguay precambrian mesozoic dyke swarms and mesozoic rifting structural and tectonic evolution abstract new high resolution airborne magnetic data of uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms main faults and other magnetic bodies which compose the uruguayan shield we combined geophysical analyses vertical derivatives upward continuation euler deconvolution structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the uruguayan shield and contribute to a better understanding of its tectonic evolution the magnetic maps revealed several outstanding features in the uruguayan shield the paleoproterozoic dyke swarm is larger denser more widespread and complex than originally thought suggesting a possible plume origin in addition a new mesozoic dyke swarm as complex as the previous one was identified crosscutting the paleoproterozoic dyke swarm and the neoproterozoic orogenic structures moreover this swarm is connected to volcanic calderas in the merin basin and shows displacements along neoproterozoic shear zones in the magnetic maps revealing its brittle reactivation during mesozoic times the new observations clarify how proterozoic basement structures controlled the development of the mesozoic rift paleoproterozoic dyke swarms were reactivated as normal faults and neoproterozoic structures hindered the rift growth deflecting the deformation in transcurrent movements meanwhile the mesozoic dyke swarm was developed in a perpendicular direction to the neoproterozoic structures moreover these findings contradict the current rift model for uruguay and rise a new model in which the mesozoic rift developed as two rift basins connected by a central transfer zone generated by the reactivation of dom feliciano belt structures between the sierra ballena and sarandi del yi shear zones

structural analysis of mylonitic rocks in the cougar creek complex oregon idaho using the porphyroclast hyperbolic distribution method and potential use of sc type extensional shear bands as quantitative vorticity indicators abstract mylonitic rocks of the cougar creek complex of northeastern oregon and west central idaho provide an opportunity to document the deformational structures produced during general non coaxial shear within quartz feldspar mylonites and to explore the potential role of sc type extensional shear bands in vorticity analysis well developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity w k using the porphyroclast hyperbolic distribution phd method w k values for the cougar creek mylonites range from w k 0 26 to w k 0 37 synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated w k values and compared to the non coaxial flow field geometries and eigenvector orientations in each mylonite zone synthetic sc type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector ab of the eigenvectors similarly antithetic shear band populations show a range of inclination near the obtuse bisector ob of the eigenvectors we infer that sc type extensional shear bands form initially parallel to ab and ob and rotate towards the flow plane with progressive deformation decreasing their inclination relative to the shear zone boundary ab and ob have significance in the strain field in that they represent orientations of maximum angular strain rate thus planes perpendicular to ab and ob are mechanically favorable for small zones of localized simple shear shear bands within the heterogeneous bulk strain of the mylonite orientation analysis of populations of sc type shear bands may provide a direct quantitative means of estimating w k

geochronology of shear zones a review abstract shear zones play a major role in the deformation of the crust at a variety of scales as expressions of strain localization during orogeny and rifting and also as reactivated structures they influence the geometry and evolution of orogenic belts and rifts crustal rheology magma ascent and emplacement and fluid flow consequently assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere the interpretation of thermochronologic data from shear zones is however not straightforward in the first place closure temperatures depend on a number of factors grain size cooling rate mineral composition and pressure among others on the other hand deformation related processes such as dynamic recrystallization neocrystallization and fluid circulation seem to be crucial for isotopic systems and thus the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks for this reason geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo or recrystallization fluid assisted deformation and inheritance of the protolith age s in order to robustly reconstruct p t e t paths of long term crustal scale shear zones structural microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems in addition geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications one of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion particularly the coupling with deformation processes based on natural theoretical and experimental data future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks

structural analysis of the carolina inner piedmont terrane boundary implications for the age and kinematics of the central piedmont suture a terrane boundary that records paleozoic laurentia gondwana interactions new field mapping along the carolina inner piedmont terrane boundary in south carolina and eastern georgia reveals preaccretionary synaccretionary and postaccretionary faults the dextral strike slip lowndesville shear zone is adjacent to a 50 km long segment of the terrane boundary however the lowndesville shear zone is correlated eastward with the dextral strike slip beaver creek shear zone which is within the carolina terrane and predates 415 ma the lowndesville shear zone is overprinted by a dextral strike slip phyllonite zone named the deal creek shear zone the deal creek shear zone is correlated eastward with the gold hill silver hill shear zone which is also within the carolina terrane and records dextral strike slip motion between 400 and 325 ma the cross anchor and mulberry creek faults both truncate the lowndesville and the deal creek shear zones and form the terrane boundary the mulberry creek fault probably of triassic jurassic age juxtaposes the lowndesville shear zone adjacent to the carolina inner piedmont terrane boundary west of waterloo south carolina the cross anchor fault is the terrane boundary east of waterloo south carolina and forms the southeastern boundary of the whitmire reentrant crosscutting relationships indicate that the cross anchor fault is the oldest fault which juxtaposes the carolina and inner piedmont terranes in the study area these structural interpretations and available geochronological data indicate that the cross anchor fault is a 325 ma thrust fault and may be the central piedmont suture an early alleghanian suture resolves the problem of inserting the carolina terrane into the western iapetus ocean

the mutual climatic range technique is usually not the area of sympatry technique when reconstructing paleoenvironments based on faunal remains abstract names applied to particular analytical techniques must be explicit and distinctive the potential that two distinct techniques of paleoenvironmental reconstruction that use ancient faunal remains each with a particular name might be confused in the future has recently become a reality the name mutual climatic range mcr coined and developed in the 1980s and concerning the modern climatic not geographic co occurrence of species represented in a prehistoric collection of remains has been applied multiple times to a different technique first published in 2009 the new mcr technique is very similar to a distinct one developed in the 1950s and 1960s that focuses on the location of the geographic co occurrence of species in an assemblage relative to the location of the deposit producing the remains the older method has been known as the area of sympatry aos technique since 1966 histories of the aos and original mcr techniques suggest they were developed independently the new mcr technique seems to have been developed without knowledge of the two earlier ones to avoid future confusion of one technique with the other it is suggested that the new one be renamed the utm mcr technique because rather than use isopleths of climatic variables as the aos technique does it considers climatic variables within 10 10 km utm squares

compositional variations in mafic phyllosilicates from regional low grade metabasites and application of the chlorite geothermometer mafic phyllosilicates in metabasites affected by low grade regional metamorphism from wales and eastern north greenland show variations in their structure and chemistry these variations are related to four mineral zones in these metabasites which are recognized on the presence absence of various key calc silicate minerals and also actinolite zones 1 and 2 equate with the zeolite facies zone 3 with the prehnite pumpellyite facies or prehnite actinolite facies in rocks with appropriate bulk rock composition and zone 4 with the greenschist facies whilst variations in fe fe mg in chlorite correlate closely with fe fe mg ratios in the whole rock other chemical variations are clearly unrelated to whole rock compositions contents of aliv are seen to increase systematically in samples from zone 1 through to zone 4 which relate to an increase in temperature calibration of alteration temperatures calculated using the chlorite geothermometer based on aliv contents developed for meta andesites in the los azufres geothermal system mexico against x values an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates shows a decrease in the swelling component in passing from zone 1 to zone 4 i e with an increase in temperature calculated temperatures compare favourably with published stability estimates for the various key calc silicates and actinolite these data indicate that the chlorite geothermometer although developed for meta andesites from a hydrothermal system does show a correlation with temperatures estimated from calc silicate assemblages in metabasites affected by low grade metamorphism developed on a regional scale

u pb id tims baddeleyite ages and paleomagnetism of 1 79 and 1 59 ga tholeiitic dyke swarms and position of the rio de la plata craton within the columbia supercontinent abstract the tandilia terrane southernmost fringe of the rio de la plata craton is an igneous and metamorphic complex produced by an accretionary orogeny 2 25 2 02 ga calc alkaline acidic dykes with e w strike and a major shear zone with similar orientation are related with the late orogeny stage as supported by field relations in a previous study the acid dykes gave 40 ar 39 ar ages of 2007 24 ma to 2020 24 ma a n and nw trending tholeiitic dyke swarm tandil swarm is also present in the tandilia terrane one sample from the nw trending subset previously gave a u pb id tims baddeleyite age of 1588 11 ma new precise u pb id tims baddeleyite dating of both n and nw trending tandil dykes yielded crystallization ages of 1589 3 ma 1588 3 ma and 1588 3 ma significantly older tholeiitic dykes known as the florida swarm occur in the northern rio de la plata craton for which a u pb id tims baddeleyite age of 1790 5 ma was previously reported consequently intermittent rifting 1 79 1 59 ga took place after tectonic stabilization of the late paleoproterozoic lithosphere proto rio de la plata craton the available geochemical data for the 1 59 ga tandil dykes define low and high tio 2 trends although only the low tio 2 subgroup is firmly dated both the tandil and florida dykes have geochemical and nd sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects the tandil dykes may be linked with the 1 57 0 02 ga capivarita anorthosite which occurs to the east of the northern part of the craton correlatives on other crustal blocks may include those in baltica such as bimodal rock association including the breven hallefors and aland aboland diabase dykes and in the reconstructed gawler craton nw laurentia dolerites bimodal magmatism and iocg deposits contemporary within plate bimodal associations are also present in the sw amazonian craton paleomagnetic data for the 1790 ma florida dykes allows three possible reconstruction scenarios for the position of rio de la plata craton i in the southern hemisphere linked to the pampia terrane and the amazonia rio apa landmass as close as they are in present day ii in the northern hemisphere as a nearest neighbor to the reconstructed nw laurentia and gawler blocks proto australia or iii in the northern hemisphere near the boundary of amazonia proto amazonian craton and baltica positions 2 and 3 are consistent with all three criteria paleomagnetic poles and proximity to matching ca 1790 ma and 1590 ma mafic magmatism our data are consistent with the idea that the columbia supercontinent started major break up at 1 59 ga

dating recurrent shear zone activity and the transition from ductile to brittle deformation white mica geochronology applied to the neoproterozoic dom feliciano belt in south brazil abstract argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation as temperature ranges for mineral crystallization and isotopic closure of different grain size fractions overlap between ca 275 425 c this study presents over 40 k ar ages of coarse grained muscovite and multiple clay sized white mica fractions extracted from mylonites and fault gouges in southernmost brazil complemented by detailed structural and xrd characterization the widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low grade ductile deformation into brittle faulting the onset of the ibare shear zone is constrained in the tonian followed by reactivation during the ediacaran evolution of the adjacent dom feliciano belt which records a remarkable long term history of recurrent deformation for over 100 myr new data extends the geochronological constraints of the dorsal do cangucu shear zone into low temperature conditions estimating the onset of brittle activity at 525 470 ma by dating early fault gouges formed shortly after final syn kinematic white mica crystallization in the hosting mylonites dating of recurrent faulting events during the paleo mesozoic complement the regional sedimentary and thermochronological record

post collisional transition from calc alkaline to alkaline magmatism during transcurrent deformation in the southernmost dom feliciano belt braziliano pan african uruguay abstract in the southernmost dom feliciano belt of uruguay highly fractionated calc alkaline granites mildly alkaline granites shoshonitic volcanics and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones four representative magmatic unites of this diverse association were petrographic and geochemically investigated the solis de mataojo complex a medium to high k2o calc alkaline granite with signature typical of mature continental arcs and post collisional settings the maldonado granite highly fractionated calc alkaline to alkaline with characteristics that are transitional between both types of series the pan de azucar pluton with characteristics typical of post collisional alkaline granites and the las flores shoshonitic basalts geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high k calc alkaline magmas solis de mataojo and maldonado shortly after collision extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity las flores 15and finally a shift to magmas with alkaline signatures pan de azucar simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination this evolution took place between 615 and 575 ma according to available data contrary to previous proposals which considered this magmatism to represent the root of a continental magmatic arc a post collisional environment transitional from orogenic to anorogenic during transcurrent deformation is proposed

a connection between the neoproterozoic dom feliciano brazil uruguay and gariep namibia south africa orogenic belts evidence from a reconnaissance provenance study abstract a provenance study of neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the pan african gariep belt stinkfontein subgroup and oranjemund group respectively in southwestern africa as well as in the rocha group of the punta del este terrane dom feliciano belt in uruguay revealed that the oranjemund and rocha groups can be correlated and most likely formed in the same basin thus the rocha group is considered to represent the fill of the westernmost part of a re activated ediacaran gariep basin the lower parts of the oranjemund and rocha groups reflect erosion of mafic rocks whereas the upper parts are derived from a predominantly felsic source area first major and trace element geochemical data are consistent with oceanic islands of within plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the oranjemund group whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper oranjemund group age spectra obtained by shrimp u pb analyses of detrital zircon grains from the stinkfontein subgroup port nolloth group the oranjemund group and the rocha group are very similar except for a lack of the youngest age group around 600 ma in the stinkfontein subgroup in all three units zircon grains of 1000 1200 ma dominate with a further peak in the age distribution between 1700 and 2000 ma these ages compare well with the pre gariep basement geology in southwestern africa where the former age range corresponds to magmatic and high grade metamorphic activity in the mesoproterozoic namaqua natal belt and the latter to an extensive palaeoproterozoic andean type volcanic arc richtersveld terrane comparable ages are conspicuously absent in the basement of the rio de la plata craton in south america derivation of the rocha group sediments from a similar source as the contemporaneous oranjemund group sediments is therefore suggested the most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640 590 ma magmatic arc of the dom feliciano belt this interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the gariep belt two stages of basin evolution are distinguished for the gariep basin an early failed continental rift that subsequently became re activated as back arc basin related to the cryogenian dom feliciano arc consequently the gariep basin does not represent the main suture between the rio de la plata and the kalahari cratons this suture is now suspected to be west of the dom feliciano volcanic arc and might be connected with the sierra ballena major gercino lineament

extrusional tectonics in the core of a transpressional orogen the kaoko belt namibia the pan african kaoko belt of nw namibia provides a wellexposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis the kaoko belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time 575 550ma it is divided into a barrovian style escape zone with inclined nappes and thrusts that verge outward towards the foreland a steep high grade orogen core that experienced intense wrench style deformation and in the hinterland an older 650ma exotic coastal terrane that was reworked at moderate t low p conditions during transpressional orogenesis the kinematic lineation array defined by stretching lineations and shear sense indicators shows variation in degree and polarity of lineation obliquity to the grain of the belt lineation plunge and different polarity of vergence along the plunge with respect to the overall sinistral shear sense the gross regional lineation pattern defines an arcuate array from near orogen parallel in the orogen core to higher angle obliquity across the escape zone reflecting lateral escape towards the orogen margin two highgrade lobes within the orogen core are coincident with obliqueinclined upward flow trajectories based on the kinematic lineation array and represent oblique extrusion of lower crustal material into middle crustal levels an intervening low grade metamorphic trough within the orogen core and the coastal terrane coincides with acute oblique downward and outboard directed apparent flow trajectories these kinematic patterns are consistent with barometric constraints for peak metamorphism of 5 2kbar and 4 5kbar in the low grade trough and coastal terrane and 8 0kbar in the two high grade lobes integration of the kinematic and metamorphic datasets shows that oblique shallowly inclined extrusion trajectories occurred within the orogen core of this classic transpressional orogen and that stretching lineations approximately reflect the particle paths experienced across orogen metamorphic gradients indicate a marked contrast between the escape zone and orogen core the escape zone experienced an inverted barrovian style high p moderate t metamorphism and low thermal gradients of 20 c km the orogen core experienced moderate p high t metamorphism and high thermal gradients of 30 40 c km clockwise p t paths were experienced in all parts of the kaoko belt tight and shallow conduction dominated p t paths were experienced in all orogen core domains involving heating dominated prograde paths followed by decompression consistent with extrusional tectonics steeper and more open advection dominated p t paths were experienced in the crustal over thrust escape zone which experienced low thermal gradients