extensive felsic lavas and rheoignimbrites in the keweenawan midcontinent rift plateau volcanics minnesota petrographic and field recognition the 8 km thick sequence of plateau lavas north shore volcanic group in northeastern minnesota that was erupted during the formation of the 1 1 ga midcontinent rift system includes an unusually large proportion 10 25 of felsic flows rhyolites and icelandites individual flows are now recognized with estimated volumes up to 600 km3 and lateral extents up to 40 km some were probably originally much broader many are anomalously large compared to most silicic lava flows primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows in particular the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas and others as strongly rheomorphic tuffs it may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top where the only evidence poorly to strongly welded shards may exist of the pyroclastic origin of a large otherwise lava like flow interior textures of these crystalline flows provide evidence of high temperature eruption especially in the form of abundant paramorphs of magmatic tridymite in the groundmass commonly arranged in a pilotaxitic flow structure based on structures textures field relations and geochemistry we conclude that a all the icelandites and some rhyolites were emplaced as large moderately to very fluid lava flows that spread widely while crystallizing tridymite and that b some large rhyolites which are only sparsely phyric were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites the mode of emplacement of one of the largest aphyric flows remains enigmatic vent areas or characteristics are unknown the low apparent viscosity of the erupted magmas is probably attributable to compositional factors f content high fe2 as well as to high temperatures and eruption rates estimated temperatures of about 1000 1100 c are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting

widespread lavalike silicic volcanic rocks of trans pecos texas several silicic units of the trans pecos volcanic field have outcrop and thin section scale features of lava flows but areal extents and aspect ratios of ignimbrites these voluminous rocks up to hundreds of cubic kilometres per unit are quartz trachytes to low silica rhyolites 68 to 72 sio 2 lava flow features include flow banding and folding elongated vesicles and autobreccias and vitrophyres at the base and top of units pyroclastic flow features include sheetlike geometry lateral extents up to 70 km aspect ratios as low as 1 700 and areal extents up to 3000 km 2 a few of these units are clearly rheomorphic ignimbrites but others show no unambiguous evidence of a primary pyroclastic origin although no adequate explanation currently exists for the origin of the latter we evaluate two end member hypotheses 1 they are ignimbrites in which extreme rheomorphism has obliterated primary internal features and 2 they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas either origin requires a rock type and eruptive mechanism not commonly recognized

crustal structure across the northern cordillera yukon territory from seismic wide angle studies omineca belt to intermontane belt a seismic refraction wide angle reflection experiment shot in 1997 in the southern yukon territory crosses the omineca belt which includes the strike slip tintina fault and terminates within the intermontane belt of the northern canadian cordillera lithospheric structure is interpreted from two dimensional forward and inverse modelling of traveltimes combined with forward amplitude modelling and from 2 5 dimensional modelling of gravity data beneath the cassiar terrane and the north america miogeocline average velocities in the upper 20 km of crust are 6 1 km s in the west beneath the accreted cache creek slide mountain and yukon tanana terranes average velocities increase to 6 3 km s in the upper crust the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the cassiar terrane the tintina fault is a crustal scale structure across which significant structural differences occur a mid crustal reflector terminates

the middle cambrian mount roosevelt formation new of northeastern british columbia evidence for rifting and development of the kechika graben system the eastern margin of the kechika graben in the vicinity of muncho and moose lakes northeastern british columbia contains middle cambrian rift related strata herein named the mount roosevelt formation the formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late early cambrian gataga group bonnia olenellus zone and is conformably overlain by a thick unnamed carbonate sequence of middle cambrian age plagiura poliella zone the mount roosevelt formation is subdivided into three members the lowermost member is characterized by ooid bearing siltstone and sandstone interbedded with dolostone limestone and hematitic conglomerate conformably above this the middle member is a thick sequence of polymict cobbly pebble conglomerate the upper member includes karstified dolostone calcareous cemented conglomerate and sandstone and limestone collectively the mount roosevelt formation reflects alluvial fan delta progradation into a transgressive mar

recent crustal movements in switzerland and their geophysical interpretation abstract in this paper a review is presented on the most recent results of geodetically determined recent crustal movements in switzerland these data are compared with the new bouguer gravity map of switzerland and an attempt is made to discuss the geodetic and gravity results in connection with the crustal structure of the swiss alps in terms of seismic compressional wave velocities and depths to the crust mantle boundary

studies of the rooiberg group bushveld complex south africa no evidence for an impact origin abstract it has been suggested that the bushveld complex of south africa could be the result of multiple large meteorite or comet impacts according to this hypothesis part of the lower rooiberg group which forms the roof of the complex represents a sheet of impact melt breccia and other impact breccias the present study is an attempt to test the viability of the impact hypothesis for the bushveld complex by interpreting newly acquired field geochemical petrographic and textural data for rooiberg group and associated rocks extensive field work throughout the rooiberg group and particularly at the contact between this unit and the underlying pretoria group metasediments has failed to identify any material that could be interpreted as impact related the rooiberg group is predominantly composed of individual volcanic flows and pyroclastic units representing several geochemically distinct magma types these volcanic units are interbedded with thin laterally extensive sedimentary units a few of which are sedimentary breccias the presence of needles of quartz that may represent paramorphs after tridymite in some rooiberg group units has been used as evidence to support the contention that these rocks represent superheated impact melt however quartz paramorphs after tridymite have been recognized in terrestrial volcanic provinces e g the north shore volcanic group in northeastern minnesota structural data including dips of rooiberg group strata suggest that the lobate shape of the complex which resembles several closely spaced ring features is the result of post rooiberg group deformation microdeformation features in quartz from bushveld related rocks do not satisfy the criteria of shock metamorphic planar deformation features pdfs which would be characteristic of impact induced shock pressures between 10 and 30 gpa these data especially the absence of macroscopic and microscopic evidence of shock deformation in pre bushveld rocks are inconsistent with formation of the bushveld complex by impact

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

cassiar platform north central british columbia a miogeoclinal fragment from idaho the allochthonous cassiar platform in north central british columbia is a cratonal fragment of ancestral north america juxtaposed against autochthonous north american crust along the tintina northern rocky mountain trench fault the cassiar platform records a neoproterozoic to early paleozoic rift to passive margin history that includes lower cambrian archeocyathan bearing limestones of the rosella formation in the cassiar mountains this study indicates that an extensive oolitic shoal developed toward the western edge of this carbonate platform during the deposition of the nevadella zone parallel to the western limit of thick continental crust initial sr 0 706 isopleth paleogeographic studies from other archeocyathan bearing units in the cordillera indicate that a semicontinuous oolitic shoal was along the western margin of the continental shelf from alaska to mexico there is a distinctive gap in the passive margin record from southeastern washington to southern idaho paleogeographic constraints from the rosella formation and published paleomagnetic data from the overlying sylvester allochthon suggest that this miogeoclinal slice was originally deposited near present day idaho and was transported northward along poorly constrained dextral strike slip faults

three dimensional structure across the tintina strike slip fault northern canadian cordillera from seismic refraction and reflection tomography accepted 2006 june 5 received 2006 june 3 in original form 2005 september 4 summary the development of the northern canadian cordillera involved major strike slip displacement of accreted terranes relative to north america along faults such as the tintina which has experienced 425 km of dextral motion since the palaeocene the snore seismic refraction wideangle reflection experiment was carried out in 1997 as one component of lithoprobe s slavenorthern cordillera lithospheric evolution snorcle transect in addition to four 2 d profiles two sets of broadside recordings were acquired to image the 3 d structure across the tintina fault tf in areas centred at about 59 5 n and 62 n simultaneous and independent refraction and reflection traveltime tomography were applied to the combined inline and broadside data set for each region to establish the range of lower crustal velocity moho depth and upper mantle velocity structure consistent with the data our preferred models are the average of the simultaneous and independent models since they represent the robust features required by the data the preferred 3 d models are generally consistent with the 2 d models obtained from the inline data in previous independent studies there are along strike variations across the tf perhaps due to the change in strike direction or the amount of motion along the fault in the north compared to the south in the lower crust the only correlation with the tf that is required by the data is a 0 1 km s 1 drop in velocity to the southwest of the fault in the northern study area the absence of a strong correlation with the tf in the lower crust is consistent with the interpreted continuity of lower crustal units across the fault in the snorcle reflection data the moho is relatively flat throughout the study area 34 35 km depth but there is broad crustal thickening of a few kilometres centred 50 km southwest of the tf in the northern and southern study areas this thickening may be the result of a period when there was a component of compression along the tf there is strong evidence for a 0 3 0 4 km s 1 drop in upper mantle velocity to the west of the tf in the south and weak evidence for 0 1 km s 1 drop to the southwest in the north the upper mantle variations in the south indicate that the tf is the boundary between cooler and or more refractory ancestral north american mantle to the east and warmer and or more fertile mantle beneath accreted north america to the west in the north the mantle appears to be more intermediate in its properties on both sides of the tf

extension in compressional orogenic belts the eastern alps extension in the eastern alps encompasses the following 1 extension accompanying simple shear crustal stacking subhorizontal stretching occurs along flat lying foliations and regionally consistent stretching lineations with associated high strains on a local scale stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust sheet imbrication the overall geometric effect is crustal thickening 2 classical extensional tectonics extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks the overall geometric effect is crustal thinning extension is generally oriented subparallel to the strike of the orogen

subaqueous paleoproterozoic metarhyolite dome flow cone complex flin flon greenstone belt manitoba canada abstract two sparsely plagioclase phyric rhyolite domes 50 100 m thick and 125 250 m long as observed in 2 dimensional vertically dipping exposures are partly mantled by a rhyolite volcaniclastic complex that on the south side of the domes also forms a half cone like mound at least 60 m thick and 500 m long the domes and mantling volcaniclastic complex are overlain by three aphyric to sparsely phyric rhyolite flows that are 70 120 m thick and 550 1200 m long domes and flows have a lower columnar jointed highly fractured locally brecciated subfacies that grades upward into 40 95 crackled and disaggregated breccia crackled breccia is a highly fractured subfacies in which clast like areas are bounded by closely spaced joints and minor matrix but there is only limited rotation of clasts crackled breccia was produced by quench fracturing combined with hydraulic action of water converted to steam or supercritical fluid within the fractures overlying disaggregated breccia comprises rotated particles and 25 50 matrix it is most abundant near flow margins and is a crumble breccia produced by flow advance or expansion the volcaniclastic mound comprises lenticular interlayered facies of 1 resedimented phreatomagmatically generated heterolithic rhyolitic tuff and lapilli tuff 2 isolated to close packed rhyolite lobes that are 0 5 5 m thick and 0 5 50 m long the lobes are at least in part pillows and 3 units comprising small

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

thermal consequences of thrust faulting simultaneous versus successive fault activation and exhumation abstract when converting temperature time curves obtained from geochronology into the denudation history of an area variations in the isotherm geometry should not be neglected the geothermal gradient changes with depth due to heat production and evolves with time due to heat advection if the deformation rate is high furthermore lateral variations arise due to topographic effects ignoring these aspects can result in significant errors when estimating denudation rates we present a numerical model for the thermal response to thrust faulting which takes these features into account this kinematic two dimensional model is fully time dependent and includes the effects of alternating fault activation in the upper crust furthermore any denudation history can be imposed implying that erosion and rock uplift can be studied independently to each other the model is used to investigate the difference in thermal response between scenarios with simultaneous compressional faulting and erosion and scenarios with a time lag between rock uplift and denudation hereby we aim to contribute to the analysis of the mutual interaction between mountain growth and surface processes we show that rock uplift occurring before the onset of erosion might cause 10 to more than 50 of the total amount of cooling we applied the model to study the cenozoic development of the sierra de guadarrama in the spanish central system aiming to find the source of a cooling event in the pliocene in this region as shown by our modeling this temperature drop cannot be caused by erosion of a previously uplifted mountain chain the only scenarios giving results compatible with the observations are those incorporating active compressional deformation during the pliocene which is consistent with the ongoing nw se oriented convergence between africa and iberia

the geology and structural relationships of the southern lebombo volcanic and intrusive rocks south africa a brief account is presented for the lebombo volcanic succession which crops out in natal south africa the volcanic belt is of late karoo age and is composed of a thick sequence of basaltic lavas sabie river formation overlain by an equally voluminous succession of acid flows jozini formation erupted over a period of about 70 m y field relationships indicate that the lebombo basalt pile consists of simple and compound flow units the rhyolite succession consists of thick 80 284 m flows units characterised by features found in both ignimbrites and rhyolitic lavas respectively it is postulated that they were extruded as high temperature low volatile pyroclastic flows the bumbeni volcanic complex which crops out near the southern termination of the lebombo mountains disconformably overlies the jozini formation and is characterised by a suite of rocks that includes rhyolite lavas air fall and ash flow tuffs syenite intrusions and basic intermediate lavas dolerite dykes are ubiquitous throughout the succession and an extremely dense concentration of basic intrusions located along the western margin of the belt gives rise to the rooi rand dyke swarm rare sill forms are found associated with the mafic volcanies acid intrusives are represented by simple and composite quartz porphyry intrusions and rhyolite dykes the structure of the lebombo is that of a faulted monocline tilted to the east developed prior to the fragmentation of eastern gondwanaland the volcanic belt is located at the tectonic contact between two major precambrian elements the 3 000 m y kaapvaal craton to the west and the southerly extension of the 550 m y mozambique belt to the east it is bounded to the south by the 1 000 m y old natal namaqua mobile belt

one and two dimensional thermal modelling of orogenic crustal extension in the tormes gneissic dome nw iberian massif spain situated in the inner zone of the variscan iberian massif the tormes gneissic dome offers a good opportunity for thermal modelling of orogenic crustal extension because the p t t loops are well constrained by an extensive set of thermobarometric structural and geochronological data as an example of feedback between forward and inverse methods the aim of this study was to establish one and two dimensional thermal models that reproduce the contrasting petrological p t paths of two structural units separated by an extensional tectonic contact in the metamorphic complex and to explain the spatial and temporary development of the low pressure metamorphism in the rocks located just above this contact in one dimension the syn extension path of the lower unit resulting from modelling is characterized by an isothermal decompression phase followed by near isobaric cooling which is typical of exhumed rocks the upper unit path records a syn extension near isobaric heating more important in rocks just above the tectonic contact condensed isograds of low pressure high temperature metamorphism in the basal upper unit are thus interpreted as a consequence of advective crustal extension and conductive upward heat transfer in two dimensions the delaminated simple shear geometric model of crustal extension explains the observed temperature rise in excess of 500 c in the basal upper unit and is consistent with the spatial distribution of m2 low pressure high temperature isograds this demonstrates the important role of extensional structures produced during the collapse of the thickened crust in the thermal evolution the heating phase well explained with intermediate dip angle for extensional fault in the upper crust 45 and finite extension of 75 km is followed by cooling thus reflecting normal erosional process

geodynamics of rapid voluminous felsic magmatism through time abstract two end member geodynamic settings produce the observed examples of rapid voluminous felsic rhyolitic magmatism through time the first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2 45 ga this style produces high temperature 1100 c low aspect ratio rheoignimbrites and lavas that exhibit high sio 2 al 2 o 3 ratios high k 2 o na 2 o ratios and where available data exists high ga al 2 o 3 ratios 1 5 with high f in thousands of parts per million and low water content f concentration is significant as it depolymerizes the silicate melt influencing the magmas physical behavior during development and emplacement these rhyolites are erupted as part of rapidly emplaced 10 15 myr mafic lips and are formed primarily by efficient assimilation fractional crystallization processes from a mafic mantle parent the second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the phanerozoic slips silicic large igneous provinces develop over periods 2 al 2 o 3 ratios and lower k 2 o na 2 o ratios rapid voluminous felsic magmatism represents both extreme examples of continental growth since the archean and also dramatic periods of crustal recycling and maturation during the phanerozoic

evolution and architecture of a large felsic igneous province in western laurentia the 1 6 ga gawler range volcanics south australia the gawler range volcanics grv of south australia are an intracontinental subaerial large igneous province lip formed during supercontinent assembly in the mesoproterozoic like many lips the grv are intraplate were erupted over a geologically short time 2 million years and are dominated by voluminous lavas however the grv are overwhelmingly dominated by felsic units the grv are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved furthermore they are well exposed in very gently northward and eastward dipping sections up to 4 km thick allowing assessment of the facies architecture and evolution of this felsic volcanic lip the evolution of the grv can be clearly separated into two main stages initial eruptions at numerous volcanic centres produced small to moderate volume geochemically distinct felsic lavas and lava domes together with ignimbrites and minor mafic and intermediate lavas forming a sequence 0 5 to 3 km thick volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from phanerozoic felsic volcanic provinces however the second stage produced at least three voluminous felsic units each of which represents about 1000 3000 km3 of magma moreover the dominance of evenly porphyritic textures and lack of pyroclastic textures pumice shards broken crystal lithic clasts in these units suggests that they were erupted effusively and flowed as lavas each of these felsic lavas are generally dominated by a single uniform composition and commonly mingled with a subordinate and compositionally distinct lava

textural evidence for high grade ignimbrites formed by low explosivity eruptions parana magmatic province southern brazil abstract the parana etendeka province is a lower cretaceous huge bimodal tholeiitic volcanic province 1 million km 3 that predated the gondwana breakup its silicic portion makes up a total volume of at least 20 000 km 3 and in southern brazil it comprises the chapeco porphyritic high ti trachydacites dacites and the palmas microporphyritic aphyric low ti dacites rhyolites the widespread silicic sheets are debated in the literature because they bear similarities between lavas and high grade ignimbrites here we provide new observations and interpretations for flow units with large dark and vesicle poor lens shaped blobs surrounded by a light colored matrix the textural features macro to micro scale of these blobs are different from typical pumice and or fiamme and support a low explosivity pyroclastic origin possibly low column fountain eruptions with discharge rates high enough to produce laterally extensive high grade ignimbrites such an interpretation combined with a conspicuous absence of lithic fragments in the deposits is aligned with a lack of identified calderas in the parana etendeka province maximum timescales of crystallization associated with the juvenile blobs and estimated from csd slopes are on the order of millennia for phenocryst populations and on the order of decades for microphenocryst populations

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

the eucarro rhyolite gawler range volcanics south australia a 675 km3 compositionally zoned lava of mesoproterozoic age the eucarro rhyolite is remarkable in being far more voluminous 675 km3 and extensive than most felsic lavas 1 km3 its dimensions are comparable to the largest known volcanic units felsic ignimbrites and flood basalts it is one of several voluminous felsic units within the mesoprot erozoic ca 1592 ma gawler range volcanics an intracontinental intraplate volcanic province in south australia and is typical of the voluminous felsic magmatism associated with laurentia cf belt supergroup midcontinent united states the lava is 300 m thick with groundmass textures and lithofacies characteristics consistent with cooling and degassing as a single unit there are gradational compositional and textural variations throughout its highly elongate distribution 225 km east west 74 wt sio2 pyroxene is the dominant ferromagnesian phase but at the base in the west primary amphibole and olivine are present granitic and mafic clasts are ubiquitous but most abundant 5 vol in western and eastern parts of the unit the main eucarro magma was compositionally zoned and several smaller batches of compositionally distinct magma were tapped during the eruption concentrations of granitic clasts coincide with changes in bulk composition and may record the rupture of wall rock between nearby but separate magmas the distribution subtle lateral variations in composition and texture and local presence of compositional flow banding along the northern margins suggest that the eucarro rhyolite was derived from several source vents located along an east trending fissure system although very widespread outflow of the eucarro lava was probably no more than a few tens of kilometers from source vents

in situ formation of welded tuff like textures in the carapace of a voluminous silicic lava flow owyhee county sw idaho the badlands rhyolite on the owyhee plateau of southwestern idaho can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes its well exposed near vent features and its response to pre existing topography however samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures including material which closely resembles the compressed welded glass shards which are characteristic of ignimbrites formation of these tuff like textures involved processes probably common to emplacement of most silicic lava flow units decompression upon extrusion causes inflation of pumice at the surface of the lava flow some of this pumice is subsequently comminuted producing loose bubble wall shards bits of pumice chips of dense glass and fragments of phenocrysts this debris sifts down around loose blocks and into open fractures deeper in the flow where it can be reheated compressed and annealed to varying degrees the end result is a dense vitrophyre layer beneath the true upper non welded carapace breccia which can be extremely texturally heterogeneous with areas of flow foliated lava occurring very near lava which in many aspects looks like welded ignimbrite complete with flattened pumices identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow foliated rock which otherwise appears to be lava the textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites at least at scales ranging from thin sections to outcrops voluminous silicic units with scattered fragmental textures but with otherwise lava like features are probably true effusive lava flows

partially melted lithic megablocks in the yardea dacite gawler range volcanics australia implications for eruption and emplacement mechanisms lithic megablocks ranging from 200 m felsic volcanic unit in the mesoproterozoic gawler range volcanic province grv of south australia throughout its vast extent the yardea dacite shows typical lava like features in that it is massive columnar jointed and evenly porphyritic with 30 40 crystals in a spherulitic and granophyric groundmass in addition flow banding is present at many locations the megablocks are abundant at two sites 50 km apart but isolated megablocks and smaller 6 cm lithic clasts are also scattered throughout the unit at both sites the megablocks are matrix supported non graded randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite the most abundant and largest megablocks are granitoids derived from older basement and from early crystallised plutons of the hiltaba suite which is broadly coeval and comagmatic with the grv the granitoid megablocks have been partially melted most likely prior to eruption when resident in the thermal aureole of the yardea dacite magma chamber the lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas consistent with the lava like character of the host dacite using reasonable estimates of megablock density magma density and magma viscosity we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks implying that they could have been entrained and erupted effusively all but the largest and least melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow the rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber dislodging megablocks that were later entrained and effusively erupted

distinguishing strongly rheomorphic tuffs from extensive silicic lavas high temperature silicic volcanic rocks including strongly rheomorphic tuffs and extensive silicic lavas have recently been recognized to be abundant in the geologic record however their mechanisms of eruption and emplacement are still controversial and traditional criteria used to distinguish conventional ash flow tuffs from silicic lavas largely fail to distinguish the high temperature versions we suggest the following criteria ordered in decreasing ease of identification to distinguish strongly rheomorphic tuffs from extensive silicic lavas 1 the character of basal deposits 2 the nature of distal parts of flows 3 the relationship of units to pre existing topography and 4 the type of source as a result of quenching against the ground basal deposits best preserve primary features can be observed in single outcrops and do not require knowing the full extent of a unit lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix such deposits are unique to lavas because the chilled base of an ashflow tuff generally does not participate in secondary flow primary pyroclastic features are best preserved there massive flow banded bases are more consistent with a lava than a pyroclastic origin lavas are thick to their margins and have steep abrupt flow fronts ashflow tuffs thin to no more than a few meters at their distal ends where they generally do not show any secondary flow features lavas are stopped by topographic barriers unless the flow is much thicker than the barrier ash flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits these criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava like ignimbrite the latter have characteristics of lavas except for great areal extents up to many tens of kilometers these rocks have been interpreted as ash flow tuffs that formed from low boiling over eruption columns based almost entirely on their great extents and the belief that silicic lavas could not flow such distances however we interpret the best known examples of lava like ignimbrites to be lavas this interpretation should be tested through additional documentation of their characteristics and research on the boiling over eruption mechanism and the kinds of deposits it can produce flow bands flow folds ramps elongate vesicles and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic pumice and shards also occur in both tuffs and lavas although they occur throughout ash flow tuffs and generally only in marginal breccias of lavas dense welding secondary flow and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick rheomorphic tuffs and thick lavas high temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts extensive silicic lavas could be appropriately termed flood rhyolites

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

effusive silicic volcanism in the parana magmatic province south brazil evidence for locally fed lava flows and domes from detailed field work abstract lava flows and dome complexes of silicic composition were identified in the lower cretaceous parana magmatic province pmp at rio grande do sul state southern brazil detailed mapping and image analysis reveals significant volumes of effusive deposits aligned according to main lineaments likely representing the fissural systems that fed the three palmas type silicic units different structures indicative of effusive emplacement lava domes lobated flows sheet flows and autobreccias are very common in the study area and are probably also more abundant than previously thought in whole pmp silicic magmatism in fact effusive deposits seem predominant in the three distinct silicic units identified in the area since no remnants of pyroclastic components have been identified the vitreous dacites that make up the upper flows of the basaltic andesite to dacite barros cassal sequence are clearly effusive as indicated by their occurrence as thin sheet flows the much thicker early caxias do sul dacites occur mostly as lava flow lobes and pancake like of low to moderate viscosity and lava domes the younger high sio2 santa maria rhyolite unit shows unequivocal examples of effusive deposits at its lower portion as lobated flows formed by vesicle rich obsidian in spite of higher viscosities relative to the previous units 106 pa s it is probable that the very low h2o contents 1 wt of these rhyolite melts associated with high discharge rates resulted in an effusive nature in most to this unit

two dimensional thermal modelling of normal faulting the simplon fault zone central alps switzerland abstract two dimensional finite difference thermal modelling of the major extensional simplon fault zone in the central alps establishes that the shape of cooling curves for samples from the footwall and hanging wall at various distances from the detachment are very sensitive to the actual displacement history if the displacement had occurred within a very limited time span 18 15 ma samples from the hanging wall close to the detachment should have significantly increased in temperature with a peak around 14 ma this is not discernible in the cooling curves interpreted from the mineral ages if the rate of normal fault displacement had been lower but constant from 18 ma to present the cooling curves for the footwall should show a correspondingly steady decrease in temperature the interpreted cooling curves from mineral ages are steep after 18 ma becoming shallower in more recent times this shape can be reproduced by the thermal modelling for a variable rate of fault displacement initially rapid total relative displacement 10 mm a across the fault zone between 18 and 15 ma but reducing to a more steady 0 4 mm a between 15 ma and 3 ma the corresponding exhumation rate of the footwall was 4 6 mm a from 18 to 15 ma and 0 6 mm a from 15 to 3 ma whereas the hanging wall was exhumed at a constant rate of 0 4 mm a the difference in cooling curves for samples with increasing horizontal distance into the footwall reflects a distributed ductile shear with the sample at 20 km distance initially at 25 km depth at 25 ma whereas that at 70 m distance was only at 19 km depth at the same time samples in the hanging wall were at 10 km depth total relative displacement parallel to the fault plane was around 36 km the thermal modelling demonstrates that during rapid fault displacement the apparent exhumation rate calculated assuming a constant geothermal gradient is too low whereas the apparent rate calculated for the subsequent period of slower displacement is too high

volcanism of the palaeoproterozoic bushveld large igneous province the rooiberg group kaapvaal craton south africa abstract the volcanic rocks of the rooiberg group represent the uppermost unit in the palaeoproterozoic transvaal supergroup and form one of the largest provinces of silicic volcanic rocks in the world although stratigraphically associated with the transvaal basin fill the rooiberg group is petrogenetically linked with the larger bushveld magmatic event for which emplacement was preceded by the extrusion of the vast rooiberg lava flows in the northern part of the kaapvaal craton like many silicic dominated large igneous provinces lips which are increasingly recognised in the rock record the rooiberg group volcanics are intracontinental subaerial and are dominated by voluminous lava flows originally the rooiberg group is inferred to have covered an area of more than 200 000 km2 of which after erosion an area of 50 000 67 000 km2 remains the lava flows form a stratigraphic succession up to 6 km thick and are divided into four formations in ascending order dullstroom damwal kwaggasnek and schrikkloof due to a scarcity of reliable geochronological data the temporal span of the rooiberg group is poorly understood the rooiberg group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 c minor explosive eruptions are represented by pyroclastic rocks and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes the rocks are essentially undeformed and have not been buried so that their original textures are well preserved the bushveld complex and the associated rooiberg group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume this thorough review of the geochronology physical volcanology and geochemistry of the rooiberg group enables construction of a geodynamic model

lateral displacement during neogene convergence in the western and central alps although major deformation in the western alps is clearly the result of north northwest directed thrust tectonics there is evidence suggesting that orogen parallel deformation may have been important in the late tectonic history of the western alps the authors propose that this northeast trending deformation includes 1 southwest directed normal fault movement along the simplon line 2 a diffuse zone of northeast striking dextral strike slip deformation along the rhone valley in switzerland between the mont blanc and aiguilles rouges massifs and through the belledonne massif and 3 southwest directed thrusting in the embrunais ubaye and digne nappe systems of southeastern france the correlation of these three broad regions of deformation is based on similar amounts of minimum displacement consistent kinematics and timing of deformation

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting

k ar dating of synkinematic clay gouges from neoalpine faults of the central western and eastern alps abstract with ongoing alpine convergence and deformation during exhumation there is a general progression from ductile to brittle behaviour on the periadriatic fault and kinematically related faults farther north e g the simplon rhone fault and a diffuse discontinuous zone of generally dextral strike slip movement on the southern boundary of the aar massif k ar dating of fine grained illite from clay fault gouges provides a reliable method for establishing the maximum age of this transition to brittle faulting and for short lived faults the approximate time of faulting itself the new results establish that brittle faulting on the northeastern segment of the canavese fault i e the part of the periadriatic fault southwest of lago maggiore occurred around 20 ma with south side up kinematics a k ar age of ca 17 ma for the crosscutting giudicarie fault in the eastern alps is effectively identical with an already published pseudotachylyte age and places a lower limit on major and continuous strike slip movements of the periadriatic fault the age of brittle faulting farther north on the southern border of the aar massif is from 13 6 0 3 to 8 3 1 1 ma consistent with the younger cooling and exhumation in this area the dominantly dextral brittle faulting becoming younger to the north reflects the continued indentation and anticlockwise rotation of adria as alpine units become exhumed and progressively welded to the southern block in contrast to the eastern alps there is no field evidence for westward lateral extrusion of alpine units relative to adria and the european foreland

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

evolution and emplacement of high fluorine rhyolites in the mesoproterozoic gawler silicic large igneous province south australia the gawler range volcanics grv and the hiltaba suite hs of south australia form a silicic dominated large igneous province the gawler slip emplaced in an intracontinental setting during the mesoproterozoic emplacement of the grv lasted for a short period of time 2 ma and can be separated into two main phases the first phase lower grv is composed of thick 3 km sequences erupted from distinct centres and includes small to moderate volume up to 150 km3 felsic lavas ignimbrites and minor mafic and intermediate lavas the upper grv include extensive felsic lavas that are up to 1000 of km3 in volume and 200 km across using well preserved quartz hosted melt inclusions we investigated the composition of the lower grv including major trace and volatile elements the results indicate high concentrations of k2o 7 8 wt rare earth and high field strength elements and low concentrations of ca mg ni cr sr and ba in comparison with felsic continental crust overall melt inclusion compositions match whole rock geochemical characteristics we demonstrate that the grv magma was f rich 1 3 wt and had high temperature for a silicic magma high f concentrations and high temperature would have resulted in lower than usual polymerisation of the melt and relatively low viscosity these characteristics help explain how very voluminous felsic magma was erupted effusively and emplaced as lavas other intracontinental slip contain extensive felsic lavas and ignimbrites which appear to share similar geochemical characteristics we also show that selective alteration caused depletion of whole rock compositions in some trace elements namely pb u and sn

welded tuffs deformed into megarheomorphic folds during collapse of the mcdermitt caldera nevada oregon the montana mountains contain part of a resurgent dome complex within the southwestern sector of the mcdermitt caldera exposed intracaldera units consist of welded tuffs lavas domes and epiclastic deposits the majority of volcanic units have a comenditic composition but a few units are nonperalkaline and biotite bearing a deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick the geometry of vitrophyres flow bands foliations and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes however the contacts between units are conformable the vitrophyres are continuous from one fold to the next and the units display pumice shards and other features common in welded tuffs the deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted but not folded and beneath relatively undeformed tuffs and lake deposits this relationship suggests that their large scale folds in the deformed sequence developed shortly after caldera collapse

exhumation of the central wasatch mountains utah 2 thermokinematic model of exhumation erosion and thermochronometer interpretation 1 the wasatch fault is a 370 km long normal fault in utah that marks the boundary between the stable colorado plateau to the east and the extending basin and range to the west understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones e g east africa rift transantarctic mountains we explore the thermokinematics of footwall exhumation and erosion in the cottonwood intrusive belt of the central wasatch mountains emphasis is placed on using low temperature thermochronometers to quantify 1 the spatial and temporal variability of exhumation and erosion rates 2 the geometry of footwall tilt 3 the fault dip angle and 4 the magnitude and duration of exhumation these processes are investigated using two dimensional 2 d thermal kinematic models coupled with cooling rate dependent kinetic models which predict exhumed apatite fission track aft and u th he ages the range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0 2 and 2 0 mm yr 1 footwall tilt hinge positions between 15 and 40 km distance from the fault a single planar normal fault with dip angles of 45 and 60 and exhumation magnitudes of up to 15 km at the fault simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat producing layers erosion maintains a constant topographic profile the kinematic and exhumation history of the wasatch mountains is investigated by comparing model predicted thermochronometer ages to observed aft zft and u th he ages predicted and observed ages are compared using a reduced chi square analysis to determine a best fit kinematic model for the wasatch mountains the preferred model includes exhumation occurring on either a 45 or 60 dipping fault a footwall hinge located a minimum of 20 25 km from the fault and a step decrease deceleration in the footwall exhumation rate at the fault from 1 2 to 0 8 mm yr 1 at around 5 ma the model also suggests an exhumation duration of 12 myr 2 myr index terms 1035 geochemistry geochronology 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8109 tectonophysics continental tectonics extensional 0905 8130 tectonophysics heat generation and transport keywords exhumation erosion normal faults numerical modeling heat flow thermochronometers

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

geochemistry and geochronology of gneiss pebbles from foreland molasse conglomerates geodynamic and paleogeographic implications for the oligo miocene evolution of the eastern alps abstract gneiss pebbles deposited in foreland fans during the miocene epoch were studied with geochemical and thermochronological methods to reconstruct the paleodrainage evolution of the eastern alps and the geodynamic behavior of the source areas clear discrimination between upper plate austroalpine and lower plate penninic gneiss pebbles was achieved by single pebble paleocooling paths austroalpine gneisses showed highly variable and regionally distinctive paleocooling paths which record variscan to cretaceous rb sr and k ar cooling ages penninic gneisses showed paleogene to lower miocene high temperature rb sr k ar and miocene low temperature zircon and apatite fission track cooling ages cooling rates of penninic gneisses indicate maximum exhumation rates at 17 ma during the climax of lateral tectonic extrusion recording dominance of tectonic unroofing the paleodrainage evolution was characterized by a relatively stable catchment configuration in the western part of the eastern alps wh

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

distribution and significance of crystalline perlitic and vesicular textures in the ordovician garth tuff wales diverse spherulitic and granular crystalline fabrics perlitic textures and fabrics related to the growth and migration of vesicles occur in the garth tuff a largely welded ordovician ignimbrite defining the distribution of such textures helps to constrain the ignimbrite s cooling and degassing history suites of spherulitic and perlitic textures closely reflect variation in cooling rates seven facies are defined based on the style and intensity of crystallisation 1 a medium to coarsely crystalline equigranular facies 2 an intensely spherulitic facies 3 a sparsely spherulitic facies 4 a pectinate facies 5 a microcrystalline to cryptocrystalline equigranular facies 6 a lithophysal facies and 7 a transitional perlite pectinate facies textural changes from facies 1 to 5 reflect progressively higher cooling rates facies 1 occurs in proximal settings in the ignimbrite s core facies 2 to 5 successively envelop facies 1 with facies 2 becoming the dominant fabric in the ignimbrite s core in medial settings facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite s welded margins crystallisation under hydrous conditions is reflected by second boiling textures in the sporadically developed lithophysal facies the seventh facies reflects a subtle interplay between cooling hydration and crystallisation which locally prevented perlitic fracturing the distribution of amygdales reflects patterns of volatile migration and entrapment in the lower levels of the ignimbrite amygdales occur in irregular concentrations or rare subvertical pipe like structures pipe like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state amygdales are widespread and evenly distributed in the upper levels of the ignimbrite however the top of the welding profile is characterised by a thin poorly vesiculated originally vitrophyric horizon that abruptly caps an intense concentration of amygdales ductile and brittle fabrics developed during the upward migration of gas microscopic drag folds occur around some amygdales jigsaw fit to clast rotated breccias originated through both late stage pneumatic fracturing and autobrecciation vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation secondary alteration has enhanced or modified some fabrics perlitic zones were susceptible to patchy chlorite sericite carbonate diagenetic alteration diagenesis and metamorphism have contributed to the infilling of vesicles

on the growth of colloform textures a case study of sphalerite from the galmoy ore body ireland abstract colloform textures have been described from many of the world9s sulphide ore deposits and involve formation of distinct microcrystalline layers these textures provide information related to sequential stages of ore formation yet their mechanism of growth is poorly understood this study has analysed a series of colloform samples from the galmoy zn pb mine ireland results indicate that the growth sequence is not always intuitive and layers that appear stratigraphic upon cursory observation may not be the crystal preferred orientation cpo of discrete colloform layers abruptly switches between different orientation axes examination of the same layers using in situ laser sulphur isotope analysis reveals equally striking changes in 34 s signature between end member bacteriogenic 25 and hydrothermal 10 sources although these results combined with petrographic observation allow the determination of a probable growth history there is no correlation between shifting cpo and 34 s signature trace element analysis reveals changes in trace element sequestration between colloform layers cadmium and chlorine rich layers correspond to a hydrothermal sulphur source whereas iron rich layers correspond to a bacteriogenic sulphur source in the absence of alternative mechanisms we suggest that the most likely factors influencing cpo changes are temperature and degree of supersaturation during two fluid mixing

voluminous acid volcanism in the busheveld complex a review of the rooiberg felsite the 2 1 b y old rooiberg felsite roofs and is intruded by the mafic layered rocks and granites of the bushveld complex the felsite unit which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300 000 km3 is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types volcanic rocks of more mafic composition occur towards the base of the sequence the rooiberg episode essentially terminated the volcanic activity in the transvaal basin and heralded the emplacement of the bushveld complex

on the welding of pyroclasts from very low viscosity magmas examples from kimberlite volcanoes abstract lithofacies in kimberlite pipes in southern africa exhibit features consistent with welded pyroclastic rocks these include flared conduit filling geometries abundant lithic clasts lithic clast layers subhorizontal clast fabrics gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash and lapilli and melt coated particles the welding dynamics of kimberlite pyroclasts differ from those of glassy vesiculated pyroclasts in silicic volcanic systems low melt viscosity 0 1 pa s results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets the glassy state is difficult to form in silica deficient magmas because the crystallization kinetics are fast in low viscosity melts three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition 1 densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure mel

numerical modelling of the thermal history of the nw himalayas kullu valley india abstract this paper demonstrates two dimensional time dependent finite difference modelling of himalayan metamorphism illustrated by the kullu s lahul area himachal pradesh after the collision between india and asia and the over thrusting of the higher himalayas over the lesser himalayas it is shown that a one dimensional model with crustal thickening by a single thrust cannot satisfy the observations present in the section along the kullu valley a two dimensional solution demonstrates that during thrust sheet emplacement at geologically realistic strain rates of several centimetres a year a temperature inversion does not occur in contrast to the inversion assumed by the one dimensional instantaneous thrust models therefore the variability of two other model parameters were explicitly considered i there is a significant conductivity contrast between the different types of rocks as conductivity layers function as a barrier to the transport of heat the conductivity limit should be variable for different regions in the grid ii rocks in the higher himalayas record a phase of metamorphism that preceded the movements on the main central thrust this observation together with the knowledge that rocks with different radiogenic heat production and conductivity are not uniformly distributed in the crust emphasizes that there were considerable horizontal temperature variations across the whole region for that reason the initial temperature profile too has to be adapted for a realistic model

from trash to treasure three dimensional basement imaging with excess data from oil and gas explorations modern oil and gas seismic surveys commonly use areal arrays that record continuously and thus routinely collect excess data that are not needed for the conventional common reflection point imaging that is the primary goal of exploration these excess data have recently been recognized to have utility not only in resource exploration but also for addressing a diverse range of scientific issues here we report processing of such discarded data from recent exploration surveys carried out in southeastern new mexico these have been used to produce new three dimensional 3 d seismic reflection imagery of a layered complex within the crystalline basement as well as elements of the underlying crust this enigmatic basement layering is similar to that found on industry and academic seismic reflection surveys at many sites in the central united states correlation of these reflectors with similar features encountered by drilling in northwestern texas suggest that they may be part of an extensive continental scale network of tabular mafic intrusions linked to keweenawan rifting of the igneous eastcentral unites states during the late proterozoic more importantly this analysis clearly demonstrates that the new generation of continuously recorded 3 d exploration datasets represent a valuable source of fresh information on basement structure that should be examined rather than discarded such basement information is not only important to understanding crustal evolution it is directly relevant to assessing risks associated with fossil fuel extractions such as induced seismicity related to waste water injection

the hypothetical mesoproterozoic supercontinent columbia implications of the siberian west laurentian connection abstract the siberian west laurentian connection is a postulated reconstruction of a proterozoic craton that formed between 1800 ma and 1500 ma correlation of geological trends and of rift facies between the siberian platform and western laurentia indicates that although the two cratons underwent significant intracontinental rifting in mesoproterozoic and neoproterozoic times they did not undergo complete separation until early cambrian ca 530 ma although this interpretation does not support the hypothetical columbia supercontinent model of rogers 2000 and rogers and santosh 2002 some elements of it are compatible with that model

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

trace and minor element variations and sulfur isotopes in crystalline and colloform zns incorporation mechanisms and implications for their genesis abstract various models have been proposed to explain the formation mechanism of colloform sphalerite but the origin is still under debate in order to decipher influences on trace element incorporation and sulfur isotope composition crystalline and colloform sphalerite from the carbonate hosted mississippi valley type mvt deposit near wiesloch sw germany were investigated and compared to sphalerite samples from 52 hydrothermal vein type deposits in the schwarzwald ore district sw germany to study the influence of different host rocks formation mechanisms and fluid origin on trace element incorporation trace and minor element incorporation in sphalerite shows some correlation to their host rock and or origin of fluid gangue paragenetic minerals and precipitation mechanisms e g diagenetic processes fluid cooling or fluid mixing furthermore crystalline sphalerite is generally enriched in elements like cd cu sb and ag compared to colloform sphalerite that mainly incorporates elements like as pb and tl in addition sulfur isotopes are characterized by positive values for crystalline and strongly negative values for colloform sphalerite the combination of trace element contents typical minerals associated with colloform sphalerite from wiesloch sulfur isotopes and thermodynamic considerations helped to evaluate the involvement of sulfate reducing bacteria in water filled karst cavities sulfate reducing bacteria cause a sulfide rich environment that leads in case of a metal rich fluid supply to a sudden oversaturation of the fluid with respect to galena sphalerite and pyrite this however exactly coincides with the observed crystallization sequence of samples involving colloform sphalerite from the wiesloch mvt deposit

walker creek fault zone central rocky mountains british columbia southern continuation of the northern rocky mountain trench fault zone walker creek fault zone wcfz well exposed in the western rocky mountains of central british columbia near 54 comprises a 2 km wide zone of variably deformed neoproterozoic and cambrian strata in fault bounded slivers and lozenges extensional shear bands subhorizontal extension lineations slickensides mesoscopic shear bands and other minor structures developed within and immediately adjacent to the fault zone consistently indicate right lateral displacement offset stratigraphic changes in correlative neoproterozoic strata indicate at least 60 km of right lateral displacement across the zone wcfz is the southern continuation of the northern rocky mountain trench nrmt fault zone it shows a through going moderate displacement strike slip fault system structurally links the nrmt and the north central part of the southern rocky mountain trench strike slip motion on the wcfz occurred in the late cretaceous to early eocene at the same time as northeast directed shortening in the fold and thrust

high temperature ash flow wet sediment interaction in the makwassie formation ventersdorp supergroup south africa abstract the late archaean ventersdorp supergroup in south africa constitutes a supracrustal succession consisting of mafic lava at the base that is overlain by a clastic wedge deposit known as the kameeldoorns formation which formed in grabens that were the result of extensional tectonics overlying these units is an intermediate to felsic ash flow deposit the goedgenoeg and makwassie formations that are now represented by feldspar and quartz feldspar porphyries at t kuip in the northern cape province spherical structures or clasts varying greatly in size occur at the contact of the makwassie formation with the underlying sedimentary kameeldoorns formation the larger clasts have cavities of different shapes and are regarded to have developed as a result of the deposition of a high temperature ash flow onto water saturated sediments

strontium isotopes and magma dynamics insights from high temperature rhyolites volcanic rocks often exhibit internal heterogeneity in radiogenic isotopes isotopic disequilibrium between coexisting phenocrysts and isotopic zoning within single crystals has been demonstrated in basalts andesites dacites and rhyolites high temperature snake river type rhyolites appear to be an exception despite the occurrence of snake river plain rhyolites in a region of isotopically highly variable crust and mantle and significant differences from rhyolite unit to rhyolite unit little to no sr isotopic zoning is found within their feldspar phenocrysts and feldspars within a single unit define tightly grouped unimodal populations high temperature rhyolitic magmas possess a unique combination of temperature and melt viscosity although typically 200 c hotter than common rhyolites the temperature effect on viscosity is offset by lower water contents 5 10 6 pa s however magmatic temperatures are in the same range as basaltic andesites and andesites consequently cation diffusion rates are orders of magnitude greater than common rhyolites we hypothesize that this combination of characteristics promotes crystal isotopic homogeneity viscosities are too high to permit crystal transfer between liquids of contrasting 87 sr 86 sr on time scales shorter than those required for diffusive homogenization of sr within phenocrysts 500 10 000 yr this is not true for most magma types in which crystal transfer is rapid 5 10 6 yr

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

structures and lithofacies of inferred silicic conduits in the parana etendeka lip southernmost brazil abstract extensive silicic units in the parana etendeka lip have been long interpreted as pyroclastic density currents rheomorphic ignimbrites derived from the messum complex in namibia in recent literature however they have been characterized as effusive lava flows and domes in this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at mato perso sao marcos and jaquirana cambara do sul areas in southern brazil inferred conduits are at least 15 25 m in width and the lithofacies include variably vesicular monomictic welded and non welded breccias in the margins to poorly vesicular banded spherulitic and microfractured vitrophyres in the central parts flat lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits large scale regional tectonic structures in southern brazil include the ne sw aligned porto alegre suture leao and acotea faults besides the antas lineament a curved tectonic feature accompanying the bed of antas river south of the antas lineament smaller scale nw se lineaments limit the exposure areas of the inferred conduits ne sw and subordinate nw se structures within this smaller scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub vertical banding in the widespread banded vitrophyre lithofacies upper lava flows display flat lying foliation pipe like and spherical vesicles and have better developed microlites petrographic characteristics of the silicic vitrophyres indicate that crystal poor magmas underwent distinct cooling paths for each inferred conduit area the vitrophyre chemical composition is defined by the evolution of trachydacitic dacitic vitrophyres with 62 65 wt sio 2 to rhyodacite and rhyolite with 66 68 wt sio 2 the more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions based on the documented lithofacies architecture we attribute the source of the silicic lava flows in the studied localities to tectonic controlled local conduits rather than pyroclastic density currents from distant vent areas

proterozoic deep marine volcanic eruptive product obsidian glass in chanda limestone penganga group india its implications for basin evolution and metallogenesis abstract subaqueous effusive to low explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine grained volcaniclastic debris flow conglomerate and turbidite occurring in a deep marine sequence of thin and even bedded grey siliceous micritic limestone of the chanda limestone the siliceous limestone conformably encloses stratiform manganese deposit obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass oriented microlites and microphenocrysts of quartz and k feldspars euhedral crystals resorbed rounded and embayed crystals spherical vesicles microlites and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava the blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion gravitational instability of the resulting slurry of glass fragments and the ambient sea water sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition this study records maiden evidence of volcanism in the chanda limestone of the neoproterozoic penganga group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica the inferred tectono sedimentary setting facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal

microfabric memory of vein quartz for strain localization in detachment faults a case study on the simplon fault zone abstract this manuscript deals with the adaptation of quartz microfabrics to changing physical deformation conditions and discusses their preservation potential during subsequent retrograde deformation using microstructural analysis a sequence of recrystallization processes in quartz ranging from grain boundary migration recrystallization gbm over subgrain rotation recrystallization sgr to bulging nucleation blg is detected for the simplon fault zone sfz from the low strain rim towards the internal high strain part of the large scale shear zone based on i the retrograde cooling path ii estimates of deformation temperatures and iii spatial variation of dynamic recrystallization processes and different microstructural characteristics continuous strain localization with decreasing temperature is inferred in contrast to the recrystallization microstructures crystallographic preferred orientations cpo have a longer memory cpo patterns indicative of prism and rhomb glide systems in mylonitic quartz veins overprinted at low temperatures 400 c suggest inheritance of a high temperature deformation in this way microstructural textural and geochemical analyses provide information for several million years of the deformation history the reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large and small scale temperature changes during the evolution of such a long lived shear zone the spatially resolved quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large scale shear zones

conodonts from the kechika formation and road river group lower to upper ordovician of the cassiar terrane northern british columbia this study examines the lower paleozoic stratigraphy of the cassiar terrane from three key sections comprising an east west transect across the terrane the cassiar terrane west of the northern rocky mountain trench consists of a neoproterozoic to triassic succession and is a fragment of the cordilleran miogeocline that has been displaced northward the amount of displacement from its original position remains controversial conodonts from the cassiar terrane have been previously reported from only a few reconnaissance studies more than 3000 m of strata have been measured and examined in detail and 85 conodont samples collected a total of 926 identifiable conodont elements are assigned to 31 species representing 21 genera the conodonts are mainly representative of the midcontinent faunal realm but some also represent the atlantic realm conodonts from the upper kechika formation and base of the road river group are early ordovician tremadocian in age and those from the upper road river group rang

kinematics of the northern part of the simplon line central alps abstract structural studies were carried out along the northern part of the simplon line in order to constrain the kinematic history of the line during tertiary times the ductile strain accumulated within both basement units and mesozoic cover reveals the upper units sheared towards the west southwest the simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line this shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature lepontine episode finally it was followed by the formation of the berisal syncline back folding episode which generated some brittle movements towards the south localized at the contact between the lower and upper pennine units i e on the simplon line

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

the 1 1 ga midcontinent rift system central north america sedimentology of two deep boreholes lake superior region abstract the midcontinent rift system mrs of central north america is a 1 1 ga 2500 km long structural feature that has been interpreted as a triple junction rift developed over a mantle plume as much as 20 km of subaerial lava flows mainly flood basalts are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds this rock sequence known as the keweenawan supergroup has been penetrated by a few deep boreholes in the search for petroleum in this paper two deep boreholes in the upper peninsula of michigan are described in detail for the first time both the amoco production 1 29r test herein referred to as the st amour well and the nearby hickey creek well drilled by cleveland cliffs mining services were 100 cored the former is 7238 ft 2410 m deep and the latter is 5345 ft 1780 m deep the entirety of the stratigraphic succession of the hickey creek core correlates very well with the upper portion of the st amour core as determined by core description and point counting of 43 thin sections selected out of 100 studied thin sections two lower paleozoic units and two keweenawan red bed units the jacobsville sandstone and the underlying freda sandstone are described the jacobsville is largely a feldspatholithic sandstone and the freda is largely a lithofeldspathic sandstone below the freda the remaining footage of the st amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate sandstone siltstone and shale black shale individual basalt flows and a basal ignimbritic rhyolite this lower portion of the st amour core presents an enigma as it correlates very poorly with other key boreholes located to the west and southwest while a black shale sequence is similar to the petroleum bearing nonesuch formation farther west there is no conglomerate unit to correlate with the copper harbor conglomerate other key boreholes are distributed over a 1300 km distance along the better known southwest arm of the triple junction mrs and can be correlated rather well with the units that are exposed in the lake superior region however a definitive explanation of the anomalous deeper st amour stratigraphy is elusive and any explanation is tenuous a possible explanation for this anomalous stratigraphy may be the geographic proximity of the st amour borehole to the keweenawan hot spot mantle plume the suggested thermal force behind the development of the mrs similarly a drastic change in structural architecture may be explained by this geographic relationship thus within the locale of this rifting center complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

lower paleozoic stratigraphic and biostratigraphic correlations in the canadian cordillera implications for the tectonic evolution of the laurentian margin the ancient laurentian margin rifted in the latest neoproterozoic to early cambrian but appears not to have developed as a simple passive margin through a long post rift drift phase stratigraphic and conodont biostratigraphic information from four platform to basin transects across the margin has advanced our knowledge of the early paleozoic evolution of the margin in northeastern british columbia two northern transects span the macdonald platform to kechika trough and ospika embayment and a third transect spans the parautochthonous cassiar terrane in the southern rocky mountains new conodont biostratigraphic data for the ordovician succession of the bow platform is correlated to coeval basinal facies of the white river trough in total from 26 stratigraphic sections over 25 km of strata were measured and 1200 conodont samples were collected that yielded over 100 000 conodont elements key zonal species were used for regional correlation of uppermost cambrian to middle devonian strata along the cordillera the biostratigraphy temporally constrains at least two periods of renewed extension along the margin in the latest cambrian and late early ordovician alkalic volcanics associated with abrupt facies changes across the ancient shelf break intervals of slope debris breccia deposits and distal turbidite flows suggest the margin was characterized by intervals of volcanism basin foundering and platform flooding siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the peace river arch prominent hiatuses punctuate the succession including unconformities of early late ordovician sub llandovery possibly early to middle silurian and early devonian ages

the effect of fluorine on viscosities in the system na2o al2o3 sio2 implications for phonolites trachytes and rhyolites the effect of fluorine on melt viscosities of five compositions in the system na2o al2o3 sio2h as been investigateda t one atmospherea nd 1000 1600 cb y concentric cylinder viscometry the compositions chosen were albite jadeite and nepheline on the join naalolsio2 and two others of the join at 75 mole percent sio2 one peralkaline and one peraluminous all melt viscosities were independent of shear rate over two orders of magnitude indicating newtonian behavior all viscosity temperature relationships were arrhenian within error fluorine reduces the viscosities and activation energies of all melts investigated the viscosity reducing power of fluorine increases with the sio2 content of melts on the join naalo2 sio2 and is a maximum at na al molar i for melts containing 75 mole percent sio2 fluorine and water have similar effects on aluminosilicate melt viscosities probably due to depolymerization of these melts by replacement of si o si al bridges with si oh and si f bonds respectively evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts the viscous flow of phonolites trachytes and rhyolites will be strongly afected by fluorine it appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts