crystallization and welding variations in a widespread ignimbrite sheet the rattlesnake tuff eastern oregon usa the 7 05 ma rattlesnake tuff covers ca 9000 km2 but the reconstructed original coverage was between 30000 and 40000 km2 thicknesses are remarkably uniform ranging between 15 and 30 m for the most complete sections only 13 of the area is covered with tuff thicker than 30 m to a maximum of 70 m the present day estimated tuff volume is 130 km3 and the reconstructed magma volume of the outflow is 280 km3 dre dense rock equivalent the source area of the tuff is inferred to be in the western harney basin near the center of the tuff distribution based mainly on a radial exponential decrease in average pumice size and is consistent with a general radial decrease in welding and degree of post emplacement crystallization rheomorphic tuff is found to a radius of 40 60 km from the inferred source four facies of welding and four of post emplacement crystallization are distinguishable they are non welded incipiently welded partially welded and densely welded zones and vapor phase pervasively devitrified spherulite and lithophysae zones the vapor phase pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones at constant thickness 20 3 m and over a distance of 1 3 km nonrheomorphic sections can cary between two extremes a entirely vitric sections grading from nonwelded to incipiently welded and b highly zoned sections highly zoned sections have a basal non to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae dominated zone to a zone of pervasive devitrification which in turn is overlain by a zone of vapor phase crystallization and is capped by partially welded vitric tuff a three dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections strong local variations are interpreted to be the result of threshold governed welding and crystallization controlled by residence time above a critical temperature which is achieved through differences in thickness and accumulation rate

deep crustal expressions of exhumed strike slip fault systems shear zone initiation on rheological boundaries abstract the formation of major exhumed strike slip faults represents one of the most important dynamic processes affecting the evolution of the earth s lithosphere detailed models of the potential initiation their properties and architecture of orogen scale exhumed strike slip faults which are often subparallel to mountain ranges are rare the initiation of strike slip faults is at depth where temperature controlled rheological weakening mechanisms play the essential role localizing future strike slip faults in this review study we highlight that in pluton and metamorphic core complex mcc controlled tectonic settings as end members the initiation of strike slip faults occurs by rheological weakening along hot to cold contacts deep within the crust and mantle lithosphere respectively these endmember processes are potential mechanisms for the initiation of orogen scale exhumed strike slip faults at depth result in a specific thermal and structural architecture similar processes guide the overall displacement and ultimately the exhumation at such deep levels these types of exhumed strike slip dominated fault zones expose a wide variety of mylonitic cataclastic and non cohesive fault rocks on the surface which were formed at different structural levels of the crust during various stages of faulting and exhumation exhumation of mylonitic rocks is therefore a common feature of such reverse oblique slip strike slip faults implying major transtensive and or transpressive processes accompany pure strike slip motion during exhumation a major aspect of many exhumed strike slip faults is their lateral thermal gradient induced by the lateral juxtaposition of hot and cold levels of the crust controlling relevant properties of such fault zones and thus the overall fault architecture e g fault core damage zone shear lenses fault rocks and its thermal structure these properties of the overall fault architecture include strength of fault rocks permeability and porosity the hydrological regime as well as the nature and origin of circulating hydrothermal fluids

conduction model for the thermal influence of lithic clasts in mixtures of hot gases and ejecta the thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows in order to study this aspect we present a heat conduction model which considers both the temperature distributions of the lithic clasts and the exchange of thermal energy between the lithic clasts and other constituents gases and juvenile pyroclasts the model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast using this energy function and considering some hypothetical arrangements of lithic clasts it is possible to calculate by means of heat balance the temperature variations in mixtures of hot gases and ejecta as a function of time volume fraction of particles to gas and thermal diffusivity initial temperature diameter and volume fraction of lithic clasts theoretical results show that in an eruption a volume fraction of 12 of cool lithic clasts 20 100 c whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300 c we suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows

axtell and allende a mossbauer spectroscopic study abstract mossbauer spectra showed the cv3 oxidized subgroup meteorites allende and axtell to be similar in olivine content and in a surprising lack of pronounced magnetic components but different in fe3 phases one atypical allende sample showed an 8 area under the curve for magnetite which is still less than a reported value for a cv3 reduced subgroup member allende s unusual fe3 spectral region distinguishes it from most other stony meteorites and is difficult to fit to known mineral parameters introduction axtell cv3 carbonaceous chondrite strongly resembles the well characterized 1969 fall allende however axtell s thermo luminescence suggests a terrestrial age of 100 years and its 6oco abundance and details of its refractory inclusions also differ from those of allende simon et al 1995 we present the fe mossbauer spectrum of axtell which resembles that of allende but with distinct differences especially obvious during attempts at curve fitting we report such attempts for allende which draw attention to its unusual ferric phases both meteorites are conventionally placed in the oxidized subgroup of the cv3 chondrites

tectono thermal evolution magma emplacement and shear zone development in the caruaru area borborema province ne brazil abstract the neoproterozoic borborema province bp in northeastern brazil is characterized by a regional flat lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones combined field micropetrographic thermobarometric and magmatic fabric studies carried out on plutons country rocks and shear zones in the eastern domain of the province the caruaru area reveal that 1 an early episode of regional deformation with a top to the ne displacement was followed by the development of conjugate strike slip shear zones 2 similar low pressure 650 c metamorphic conditions occurred during the two events 3 plutons intruded the flat lying foliation 4 magma emplacement slightly predated strain localization in transcurrent shear zones but 5 plutons underwent strike slip deformation before complete crystallization therefore regional deformation pluton emplacement and shear zone development were successive events occurring over a relatively short time span 40ar 39ar laser dating of amphibole 584 ma and biotite 545 553 ma single grains supports slow cooling 5 c ma of country rocks through the argon closure temperatures of these minerals complex sometimes discordant amphibole ages bracketed between 552 and 575 ma for shear zones may be attributed to episodic activity and or dike swarm intrusions into them which disturbed the ar system biotite ages for the shear zones 533 545 ma also imply a relatively low cooling rate these data show that the massive injection of magmas in the continental crust of the bp during the brasiliano pan african orogeny produced a long lived thermal anomaly of regional extent the 40ar 39ar ages in the caruaru area are older than in other segments of the bp with similar geological characteristics this indicates a tectono thermal history for the bp that is more complex than previously recognized with either diachronous deformation magmatism and associated low p high t metamorphism or contrasting cooling of the different domains possibly due to differential vertical movements

a complete wilson cycle recorded within the riacho do pontal orogen ne brazil implications for the neoproterozoic evolution of the borborema province at the heart of west gondwana abstract the riacho do pontal orogen rpo is part of the extensive brasiliano pan african orogenic system in northeastern south america borborema province this region is crucial for understanding the neoproterozoic evolution of west gondwana because it occupies a strategic position between three of its most important constituents namely the sao francisco congo amazon and west africa cratons based on lithostratigraphic lithochemical geochronological and isotopic data a five stage model for the geodynamic evolution of the rpo and adjacent areas in the borborema province is proposed 1 rift phase development of a triple junction rift system at ca 900 820 ma ago probably related to the emplacement of a mantle plume and leading to intense mafic ultramafic magmatism of the brejo seco layered intrusion ca 900 ma this was followed by the development of a continental rift system testified by the paulistana complex metavolcanosedimentary sequence ca 882 ma 2 drift phase with continued lithospheric thinning the rift system further evolved to a broad passive margin in the northern sao francisco craton edge represented by the barra bonita formation platformal sediments the drift phase culminated in the development of true oceanic crust represented by the monte orebe ophiolite around 820 630 ma ago 3 convergence phase at circa 630 620 ma onset of subduction in the borborema province caused the inversion of the basins obduction of oceanic crust slices and sedimentation of the mandacaru formation syn orogenic greywackes 4 collisional phase continental collision between the sao francisco craton lower plate and the pernambuco alagoas block upper plate occurred around 620 590 ma with the stacking of the casa nova nappes upon the lower plate crustal thickening deformation metamorphism melt generation and intrusion of the syn collisional rajada suite two mica granites 5 lateral escape phase the collisional stage was followed by a lateral escape tectonics stage around 590 530 ma that generated the western branch of the e w trending pernambuco shear zone which truncates the northern part of the orogen and displaces its counterparts in the transversal zone in a dextral sense shear this phase was accompanied by extensive alkaline magmatism in the form of the serra da aldeia caboclo suites thus the rpo represents a complete plate tectonic cycle that spans the whole neoproterozoic and preserve excellent time markers for both the break up and dispersion of the rodinia supercontinent fragments and for the re amalgamation of those fragments in west gondwana the interpretation of a complete wilson cycle in this area challenges current views of the borborema province as a coherent block since paleoproterozoic times part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted to form the heart of west gondwana

ductile duplexing at a bend of a continental scale strike slip shear zone example from ne brazil abstract during the pan african orogeny the borborema province in ne brazil developed a continental scale shear zone system that comprises ne and ew trending ductile strike slip shear zones remote sensing and structural mapping has revealed a pattern of arcuate anastomosing strike slip shear zones separating sigmoidal lenses of less deformed material located at the western end of the ew trending patos shear zone which is one of the largest shear zones of the province this structure of imbricate shear zones was initiated under high temperature deformation conditions it is interpreted as a ductile strike slip duplex and may represent a kinematic pattern for strain accommodation in response to a bend of a ductile mega shear zone

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

incorporation and redistribution of locally derived lithic fragments within a pyroclastic flow the lower miocene peach springs tuff exposed in the newberry mountains california was deposited within a paleovalley trending s65 w exposures within the paleovalley contain lithic breccia intercalated with ash rich ignimbrite the clast assemblage of the lithic breccias matches the rock types of the paleovalley walls and therefore the clasts were not derived from a distant eruptive vent flow direction breccia bed thickness grain size data and sedimentary textures indicate that the lithic breccias were deposited from density currents within the pyroclastic flow that moved down tributaries and into the main paleovalley to be intermingled with the ash rich pyroclastic flow a model is proposed whereby a turbulent boundary layer at the base of the pyroclastic flow is induced by surface roughness of the substrate and incorporates loose material from the substrate to produce a high density ground layer that decouples from the lower density ash rich pyroclastic flow after decoupling has occurred the high density lithic rich ground layer moves independently from the ash rich pyroclastic flow the lithic breccia horizons have many characteristics of proximal lag breccias and caution must be used when inferring distance from vent in ancient ignimbrites based on the occurrence of coarse breccias

timing of crust formation deposition of supracrustal sequences and transamazonian and brasiliano metamorphism in the east pernambuco belt borborema province ne brazil implications for western gondwana assembly abstract the main structural feature of the central domain of borborema province ne brazil is a network of dextral and sinistral shear zones these shear zones rework an older regionally developed flat lying foliation in orthogneisses and supracrustal belts which in the east pernambuco belt was formed under amphibolite facies conditions this study reports la icp ms u pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre transcurrent tectonothermal evolution in the eastern pernambuco domain ages of 2125 7 and 2044 5 ma in a mafic layer of banded orthogneiss are interpreted as the age of the protolith of the orthogneiss and of high grade transamazonian metamorphism respectively the latter age is consistent with the occurrence of low th u metamorphic zircon xenocrysts dated at 2041 15 ma in the leucosome of a migmatitic paragneiss a granitic orthogneiss dated at 1991 5 ma reflects late to post transamazonian magmatic event a similar age 1972 8 ma was found in rounded zircon grains from a leucocratic layer of banded orthogneiss ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the archean to neoproterozoic with peak ages at ca 2220 2060 1940 1200 1150 and 870 760 ma detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 ma magmatic zircons with the age of 626 15 ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the brasiliano high temperature metamorphism a lower intercept age of 619 36 ma from a deformed granodiorite dated at 2097 5 ma and the crystallization age of 625 24 ma of the felsic layer of banded orthogneiss also confirm the late neoproterozoic metamorphism these results show that the present fabric in basement and supracrustal rocks was produced during the brasiliano orogeny paleoproterozoic ages reported in this study are similar to those found in other sectors of the borborema province the cameroon and nigeria provinces and the sao francisco congo craton they show the importance of the transamazonian eburnean event and suggest that these tectonic units may have been part of a larger single continental landmass likewise similarities in post transamazonian metamorphic and magmatic events in the borborema nigeria and cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the atlantic ocean

an alternative kinematic interpretation of thetis boundary shear zone venus evidence for strike slip ductile duplexes 1 on venus a 1000 km long 50 to 200 km wide anastomosing shear zone system thetis boundary shear zone tbsz separates the eastern ovda and northwestern thetis regiones previous workers mapped a part of this shear zone and interpreted it as a sinistral strike slip shear zone containing an extensional jog in the present study it is reexamined for a detailed structural and kinematic analysis and an alternative kinematic interpretation is provided the tbsz is divided into western central and eastern segments the western segment is narrow and is oriented in a ene wsw direction along its northern boundary the preexisting folds of tessera are cut by the shear zone and the folds are dragged into parallelism with the shear zone in response to dextral strike slip motion both the western and central segments contain a pair of conjugate deformation bands sigmoidal folds and linear ridges which appear similar to s c like structures of large scale ductile shear zones the acute angle between the s and c bands suggests dextral sense of displacement the eastern segment forms the northeastern restraining bend of the tbsz where the shear zone is characterized by a wide branching fan like anastomosing shear zone system with well developed contractional strike slip ductile duplexes the internal fabric defined by the sigmoidal folds consistently shows dextral sense of displacement therefore the tbsz is a dextral strike slip shear zone showing s c like structures and contractional strike slip duplexes similar to the continental scale ductile shear zones on earth

the influence of geometry on the ascent of magma in open fissures during steady eruption the flow conditions emitted mass flux exit velocity and exit pressure depend on the geometry of the conduit in which the eruption occurs this dependence is examined for the onedimensional isothermal ascent of a homogeneous basaltic magma with an aqueous volatile phase and newtonian rheology by fixing the geometry of the feeding fissure the mass flux flowing in steady conditions can be determined at any depth as well the magma pressure and vertical velocity flow behaviour is analysed for three fissure shapes constant width slowly upward narrowing and lenticular in all the cases examined the magma arrives at the earth s surface with a pressure greater than atmospheric the results are compared with those obtained when a lithostatic pressure gradient is assumed for the magma column some speculations are made moreover about the change in eruption style if conduit geometry varies during a non steady phase

lithospheric shear zones and mantle crust connections abstract a crustal scale ductile shear zone network in the precambrian granulite facies crust of madagascar is examined to determine the nature of the connections between the mantle and lower crust based on three independent data sets field and satellite mapping c and o isotope geochemistry and gravimetry this crust is divided into three zones 1 outside of shear zones 2 minor shear zones that are 350 km long up to 1000 km and 20 35 km wide the mantle is uplifted by about 10 km beneath the major shear zones the major shear zones are rooted in and are inferred to be controlled by the mantle they directly tapped mantle derived co 2 the small scale minor shear zones were controlled by crustal processes and focused crustally derived h 2 o rich co 2 fluids the regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context the propagation of these mechanical instabilities promoted and channelled fluid flow these major pan african shear zones thinned the crust and were reactivated during the subsequent drifting of madagascar and opening of the indian ocean during jurassic to cretaceous times they also controlled many of the brittle fault zones in the overlying sedimentary basins mantle rooted large scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

understanding basement tectonics of an interior cratonic basin southern illinois basin usa abstract although the illinois basin is one of the world s most studied interior cratonic basins little is known of its deep structure or seismotectonic framework in this study seismic reflection profiles over the major structures of the basin reveal reverse faults that penetrate the upper precambrian crust disrupt the surface of precambrian basement and accommodate folding of paleozoic sediments observations of deformation indicate concordant folding of the basement surface and overlying strata and or possible laramide style basement controlled folding most profiles show evidence of dual fold vergence and some profiles suggest transpression involving lower paleozoic strata and the basement surface the moderate seismicity in the southern illinois basin just north of the new madrid seismic zone nmsz defines an active deformation regime of interpreted nne striking dextral strike slip and reverse faulting upper crustal fault structure such as shown in this study could provide a fabric capable of reactivation by stress manifested as contemporary seismicity a tectonic framework for the possible kinematic linkage of deformation in the nmsz and the southern illinois basin is proposed on the basis of a hypothetical distribution of branched fault and or fold patterns in southern illinois that broaden northward beyond the nmsz such a dispersive deformation pattern is thought to be associated with the gradual cessation in displacement north of the nmsz i e displacement is dissipated over many faults the abrupt change in seismotectonic regime from a relatively discrete highly seismic nmsz with infrequent great earthquakes to a multi stranded fault pattern with more frequent moderate earthquakes in southern illinois is typical of active strike slip systems involving a bend or shift in direction

the decompression of volcanic jets in a crater during explosive volcanic eruptions during explosive volcanic eruptions fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10 100 atm and at the speed of sound of the mixture we show that for magmatic volatile contents no in the range 0 03 no 0 06 this has the approximate value 0 95 0 05 nort 12 where r is the gas constant and t is the eruption temperature this speed is nearly independent of the vent pressure and vent radius by assuming there is negligible mixing with the air we have modelled the decompression of such jets following eruption from the vent for free decompression into the atmosphere the velocity of the decompressed jet has the approximate value 1 85 0 05 nort 12 this is nearly independent of the eruption rate for decompression into a crater the process is more complex at low eruption rates with low vent pressure the material becomes underpressured as it rises in the crater a shock then forms in the crater this recompresses the magma volatile mixture which then issues from the crater as a relatively slow subsonic jet at atmospheric pressure as the eruption rate increases such shocks move towards the top of the crater and ultimately cannot form in the crater instead the material issues from the crater as a high speed supersonic jet the upward thrust provided by the crater walls on this high pressure jet then increases the upward velocity above that of a freely decompressing jet the presence of a crater may therfore cause collapse in relatively small eruptions whereas it may promote formation of buoyant eruption columns at higher eruption rates if a crater grows through erosion during an eruption column collapse will typically ensue even if the mass flux remains steady

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

continental scale rheological heterogeneities and complex intraplate tectono metamorphic patterns insights from a case study and numerical models abstract continental plates are built over long periods of time through successive extensional and compressional cycles they are therefore rheologically heterogeneous this heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes the study of the neoproterozoic borborema shear zone system of northeast brazil highlights a systematic link between marked changes in its tectono metamorphic pattern and the pre existing structure of the plate that is characterized by juxtaposition of continental domains either comprising an old basement palaeo to eoproterozoic or accreted during an extensional event between 1 0 and 0 7 ga in neoproterozoic time when the shear zone system was developed these domains displayed different geotherms and lithospheric thicknesses and therefore contrasted rheological behaviours we use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate from the very beginning of the deformation weak and stiff heterogeneities induce strain localization due to a lower initial effective viscosity or to stress concentrations at their tips respectively shear zones propagate from the heterogeneities and finally coalesce forming a network of high strain zones bounding almost underformed blocks within this network shear zones transfer strain between the different heterogeneities and model boundaries the evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere the resulting finite strain field is heterogeneous and displays rapid lateral variations in vertical and or rotational deformation such a heterogeneous strain distribution may induce contrasted magmatic metamorphic and uplift evolutions within an orogenic belt as observed in the borborema shear zone system and other collisional belts

termination of a continental scale strike slip fault in partially melted crust the west pernambuco shear zone northeast brazil continental scale strike slip faults rooted in the upper mantle have been recently interpreted in deep seismic profiling these shear zones transect the entire crust and it may be assumed that the deformation pattern associated with their propagation at different depths was strongly influenced by the rheological properties of the rocks the continental scale west pernambuco shear zone provides information on the deformational behavior of a hot and partially melted crust that has undergone large scale shearing this late precambrian shear zone terminates eastward in a fanlike structure passing to a wide transpression zone at a high angle to the shear direction the main fault branches into numerous splay faults over a distance of about 50 km severe perturbation of the mylonitic flow pattern commonly occurs at branch points where subsidiary shear zones diverge from the main zone in these areas problems of strain compatibility are raised by the sharp bending and decrease in dip of the foliation surface we propose that this unusual deformation pattern is a result of the low strength of the continental crust at the considered level of observation

the proximal facies of the tosu pyroclastic flow deposit erupted from aso caldera japan the tosu pyroclastic flow deposit a low aspect ratio ignimbrite lari has widely distributed breccia facies around aso caldera japan the proximal facies 9 34 km away from the source consists of 3 different lithofacies from bottom to top a lithic enriched and fines depleted fd facies a lithic enriched li facies with an ash matrix and a fines and pumice enriched ni facies modes of emplacement of fd li and ni are interpreted as ground layer 2b lithic concentration zone and normal ignimbrite respectively these stratigraphic components in the tosu originated from the flow head fd and the flow body li and ni and were generated by a single column collapse event remarkably thick fd and li in contrast to thin ni suggest that due to high mobility most ash and punice fragments in the tosu were carried and deposited as ni in the distal area heavier components were selectively deposited as fd and li in the proximal area the rate of falloff of lithic clast size in the tosu shows an inflection at 20 km from the source in a survey of well documented pyroclastic flows the inflection distance of a lari is generally greater than that of a high aspect ratio ignimbrite so that the eruption of the former is probably more intense than the latter

shear zone junctions of zippers and freeways abstract ductile shear zones are commonly treated as straight high strain domains with uniform shear sense and characteristic curved foliation trails bounded by non deforming wall rock many shear zones however are branched and if movement on such branches is contemporaneous the resulting shape can be complicated and lead to unusual shear sense arrangement and foliation geometries in the wall rock for y shaped shear zone triple junctions with three joining branches and transport direction at a high angle to the branchline only eight basic types of junction are thought to be stable and to produce significant displacement the simplest type called freeway junctions have similar shear sense in all three branches the other types show joining or separating behaviour of shear zone branches similar to the action of a zipper such junctions may have shear zone branches that join to form a single branch closing zipper junction or a single shear zone that splits to form two branches opening zipper junction all categories of shear zone junctions show characteristic foliation patterns and deflection of markers in the wall rock closing zipper junctions are unusual since they form a non active zone with opposite deflection of foliations in the wall rock known as an extraction fault or wake shear zipper junctions can form domains of overprinting shear sense along their flanks a small and large field example are given from ne spain and eastern anatolia the geometry of more complex 3d shear zone junctions with slip parallel and oblique to the branchline is briefly discussed

the bishop tuff new insights from eruptive stratigraphy the 0 76 ma bishop tuff from long valley caldera in eastern california consists of a widespread fall deposit and voluminous partly welded ignimbrite the fall deposit f exposed over an easterly sector below and adjacent to the ignimbrite is divided into nine units f1 f9 with no significant time breaks except possibly between f8 and f9 maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for f1 f8 as ca 90 hrs the ignimbrite ig is divided into chronologically and or geographically distinct packages of material earlier packages ig1 were emplaced mostly eastward are wholly intraplinian coeval with fall units f2 f8 lack phenocrystic pyroxenes and contain few or no glass mountain derived rhyolite lithic fragments earlier packages ig2 were erupted mostly to the north and east are at least partly intraplinian interbedded with fall unit f9 to the east contain pyroxenes and have lithic fractions rich in g

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

large wave forms from the fish canyon tuff colorado wave like bedforms of large amplitude and wavelength 6 m 60 m occur in an exposure at the base of the fish canyon tuff colorado an ignimbrite ash flow tuff of great volume 3 000 km 3 the bedforms are located some 30 40 km from the center of the source caldera they are interpreted to be deposits from a series of large pyroclastic surges precursors to the main pyroclastic flows the pyroclastic surge beds have bigger wave forms than those previously recognized such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber the depositional sequence of this great ignimbrite forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence

strike slip fault terminations at seismogenic depths the structure and kinematics of the glacier lakes fault sierra nevada united states 1 structural complexity is common at the terminations of earthquake surface ruptures similar deformation may therefore be expected at the end zones of earthquake ruptures at depth the 8 2 km long glacier lakes fault glf in the sierra nevada is a left lateral strike slip fault with a maximum observed displacement of 125 m within the fault pseudotachylytes crosscut cataclasites showing that displacement on the glf was accommodated at least partly by seismic slip the western termination of the glf is defined by a gradual decrease in the displacement on the main fault accompanied by a 1 4 km wide zone of secondary faulting in the dilational quadrant of the glf the secondary faults splay counterclockwise from the main fault trace forming average angles of 39 with the main fault slip vectors defined by slickenlines plunge more steeply west for these splay faults than for the glf static stress transfer modeling shows that the orientations of the splays and the plunge of displacement on those splays are consistent with displacement on the main fault the glf termination structure shows that structural complexity is present at the terminations of faults at seismogenic depths and therefore ruptures that propagate beyond fault terminations or through step overs between two faults will likely interact with complex secondary fault structures models of dynamic rupture propagation must account for the effect of preexisting structures on the elastic properties of the host rock additionally aftershock distributions and focal mechanisms may be controlled by the geometry and kinematics of structures present at fault terminations

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

vent migration and changing eruptive style during the 1800a taupo eruption new evidence from the hatepe and rotongaio phreatoplinian ashes the 1800a taupo eruption was one of the most complex silicic eruptions worldwide within the past 5000 years new work on phases 3 and 4 the hatepe and rotongaio ashes has identified a mappable internal stratigraphy for each enabling detailed isopach and isopleth measurements for subunits within the deposits the new data indicate that the vent configuration for the taupo eruption was more complex than previously thought and involved at least three sources on a ne sw fissure centred on horomatangi reefs phases 1 3 of the eruption were from a southwestern vent s phase 4 from a northeastern source and phases 5 and 6 probably from the horomatangi reefs area a separate source for the rotongaio ash phase 4 helps explain the contrast between the pumice rich phases of the eruption and the dense juvenile clasts of the rotongaio ash the rotongaio magma resided in a separate initially blind conduit and was degassing prior to and during earlier phases of the taupo eruption this new work on the hatepe and rotongaio ashes underscores the importance of a detailed stratigraphic framework in deciphering extremely fine grained fall units of largescale sustained eruptions

erosion processes in volcanic conduits and application to the ad 79 eruption of vesuvius abstract the flow of gas magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall erosion may be produced by the impact of pyroclasts on the conduit wall fluid shear stress at the wall conduit wall collapse and volcanic tremor using a two phase flow non equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process this abrasion process was found to be the largest near the conduit exit where the gas magma velocities are the largest the erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit depending on the wall roughness and yield strength of wall rocks this shear stress is the largest near the magma fragmentation level where the gas magma viscosity and velocity gradients are very large the collapse of conduit wall due to the difference between the gas magma and lithostatic pressures can occur below and above the magma fragmentation level causing the production of lithics directly when the wall collapses inward and indirectly when the wall collapses outward the effectiveness of different erosion mechanisms was tested with the magma characteristics conduit geometry and wall rock properties pertaining to the ad 79 eruption of vesuvius it was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit the conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

ignimbrite morphology and the effects of erosion a new zealand case study ignimbrite morphology previously generalised using aspect ratios is here quantified as the relationships between the various thicknesses of material forming an ignimbrite and the areas and volumes represented by those thicknesses the morphology can be measured for the deposit in its present day eroded condition or reconstructed for the original deposit the reconstructed morphology of the 22 500 year bp ca 11 500 km2 ca 300 km3 oruanui ignimbrite in new zealand is documented to illustrate the latter approach the oruanui ignimbrite is an intermediate aspect ratio deposit and shows broadly linear relationships between 1 in thickness and the cumulative area occupied by that thickness or less of material and 2 thickness and the volume represented by that thickness or less of material two theoretical morphologies one where thicknesses exponentially decay with distance from a maximum and the other of uniform thickness slab are compared with the oruanui data limited comparative data suggest that low aspect ratio violently emplaced ignimbrites will show upward concave curves at one extreme following the exponential decay model and high aspect ratio gently emplaced ignimbrites downward concave curves with the slab model as an extreme when plotted on diagrams where the oruanui data show linear trends the effects of erosion on oruanui and model ignimbrite morphologies are modelled using two theoretical erosion scenarios 1 material is evenly removed from the land surface and 2 thinner non welded material is preferentially removed for the oruanui ignimbrite data area is lost much more rapidly in the first instance than volume for example 5 m of erosion is sufficient to remove 50 area whereas 40 m scenario 1 or 120 m scenario 2 of erosion is required to remove 50 volume in old ignimbrites volume estimates may be reasonably accurate even after strong erosion provided the original thicknesses of ponded landscape forming material can be inferred but estimates of original area and aspect ratio will be inaccurate an envelope enclosing all known outcrops of an ignimbrite will give a better estimate of original area and aspect ratio than simply summing the areas of known outcrops

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

welded tuff infilling a volcanic vent at weolseong republic of korea a plug of parataxitically welded tuff with an elliptical 700 400 m cross section crops out in weolseong district republic of korea it intrudes late cretaceous granodiorite and extrusive tuffs of the chisulryoung volcanic formation and displays a steeply inclined welding foliation concordant with the intrusive contacts and a subvertical stretching lineation these fabrics are interpreted as resulting from laminar shearing in a boundary layer during deposition of the tuff by agglutination along the walls of a vent which served as a feeder for extrusive ignimbrites

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

eastern termination of the altyn tagh fault western china constraints from a magnetotelluric survey the left lateral altyn tagh fault forms the northern boundary of the tibetan plateau the strike slip rate of the active altyn tagh fault decreases northeastward and reduces close to zero as it passes north of the qilian shan this geometry raises controversies on whether and how the fault terminates or extends further east to address these controversies wide band magnetotelluric data were collected along four profiles across the altyn tagh fault ranging from 135 to 261 km in length all four profiles are located in the foreland of the qilian shan ranges and are oriented perpendicular to the inferred fault zone that could be the continuation of altyn tagh fault both the two dimensional and three dimensional electrical resistivity models derived from our magnetotelluric data show that the hexi corridor crust is generally of low resistivity whereas the crust of the huahai jinta basin is in general of high resistivity with a local and isolated low resistivity anomaly within the mid lower crust the generally high resistivity crust of the huahai jinta basin may be rheologically unfavorable for the altyn tagh fault passing through the basin toward the northeast the entirely different electrical structure between the hexi corridor and its northern neighbors indicates the existence of a tectonic boundary that coincides with the altyn tagh fault in the west and reverse faults in the east the two dimensional electrical conductivity models suggest that the altyn tagh fault transfers from a single fault in the west to a branching set of mainly dip slip faults in the east

miocene rhyolitic welded tuff infilling a funnel shaped eruption conduit shiotani southeast of matsue sw japan at shiotani sw japan rhyolitic welded tuff forms a steep sided funnel shaped body confined by paleogene granitic rocks to an elliptical area 1 1 5 km across the shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70 90 in contrast nearby contemporary volcaniclastic deposits are non welded and gently inclined near the contact with the granite the tuff is plastically deformed and shows lineations that plunge inward at angles of 40 65 lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff the geometry and internal structures suggest that the shiotani welded tuff was emplaced and welded in a funnel shaped eruption conduit upon collapse of a plinian or phreatoplinian eruption column the majority of the conduit filling pyroclasts probably fell back en masse into the conduit heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit especially at deep levels the hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma these processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall emplacement of nearby correlative volcaniclastic mass flow deposits in a shelf to upper bathyal environment suggests a possibility that when active the shiotani conduit was under the sea welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access

two stage terrane assembly in western gondwana insights from structural geology and geophysical data of central borborema province ne brazil abstract combined geophysical and structural data from the transversal subprovince of the borborema province ne brazil highlight the internal structure of and interrelationships between the constituent terranes radiometric and magnetic maps show distinctive signatures for the archean paleoproterozoic alto moxoto and meso to neoproterozoic alto pajeu and pernambuco alagoas terranes mapped radiometric and first and second order magnetic lineaments associated with euler deconvolution enable correlation between geophysical data and major structures in addition to early related deformation of the alto moxoto terrane combined analysis of late transposition foliations lineations and kinematic criteria reveal a complex structural evolution marked by two distinct assembly stages the first phase is characterized by thrust tectonics with top to the south vergence resulting in the juxtaposition of the allochtonous alto pajeu terrane with the structurally underlying alto moxoto terrane the terrane boundary is delineated by the serra de jabitaca shear zone which is associated with low dipping collisional granitic sheets and ca 1 0 ga mafic ultramafic rocks interpreted as obtudcted ophiolite remnants later strike slip movements strongly folded and obliterated thrust related markers and the continental scale e w pernambuco lineament is interpreted as the result of lateral assembly between the composite alto pajeu alto moxoto terranes and the pernambuco alagoas terrane during the metamorphic peak of the brasiliano orogeny evolution of the borborema province reflects accretionary processes during the assembly of western gondwana

magnetic fabrics in the cabanas granite ne brazil interplay between emplacement and regional fabrics in a dextral transpressive regime the late neoproterozoic cabanas granite northeastern brazil is a tabular body that was emplaced south of the dextral east pernambuco shear zone system anisotropy of magnetic susceptibility ams measurements reveal that the cabanas granite has fabric patterns more complex than usually reported in plutons intruded in similar settings most structures have ew to ene trending magnetic lineations consistent with the overall regional structural direction in the north these structures parallel the mylonitic fabric encountered in shear zones that bound or cross cut the pluton in the central and southern portions of the pluton magnetic foliation trajectories the overall banded disposition of the iso susceptibility lines and shape parameters of the ams ellipsoids suggest that the cabanas granite was subjected to ene trending folding along with its underlying and enclosing country rocks it is concluded that emplacement took place during regional dextral transpression a secondary but well developed family of structures consists of n to nw plunging lineations which is interpreted to represent the trace of the vertical conduits that fed the pluton this study emphasizes the utility of the ams to unravel both the internal structure of low anisotropy plutons and the interplay between emplacement and regional straining during magma crystallization

laminar flow in shear zones the pernambuco shear zone ne brazil abstract the pernambuco shear zone psz is a 700 km long branch of one of the largest and best exposed precambrian strike slip shear zone systems in the world the shear zones are part of the eroded record of a large scale collision zone foliation and lineation orientations along the shear zone indicate that local deformation perturbations have produced variable fabric orientations with up to 40 change in dip and strike however the dominant flow pattern is consistently horizontal strike slip across a vertical shear plane well developed dextral shear sense criteria are observed at most localities although oblate strain fabrics are common and lineations are frequently absent very few small folds were found to refold the shear fabric developed in late precambrian granites which intrude along most of the shear zone but sheath folds are common in mylonitized pre shear gneisses this geometrical evidence indicates that steady laminar shear flow took place in the initially isotropic granites and more irregular but not turbulent flow preferentially developed in rocks with a pre existing fabric the deformation pattern is similar to many other deep crustal shear zones laminar shear flow is thought to be common in granites and coarse crystalline gneisses deformed in upper amphibolite to granulite facies conditions the dramatic weakening of the middle crust produced by syn shear injection of granites and mafic intrusions into vertical shear zones may cause the predominant movement pattern in collision zones to be strike slip lateral extrusion rather than compressional thrusting

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

oblique lineations in orthogneisses and supracrustal rocks vertical partitioning of strain in a hot crust eastern borborema province ne brazil detailed structural work conducted at the eastern area of the neoproterozoic brasiliano zpan african borborema province northeastern brazil has shown two orientations of stretching lineations with ese trend in supracrustal rocks and ne trend in underlying orthogneisses in the metasedimentary sequence numerous kinematic indicators showing a top to the northwest sense of shear denote a welldeveloped non coaxial deformation in the orthogneisses lineations formed dominantly during coaxial deformation although a component of ne directed shear is locally observed the two lineations were produced under similar high temperature metamorphic conditions and are interpreted as the result of a protracted nw se contractional strain regime where i subhorizontal non coaxial shear with superimposed flattening led to an initial phase of nw directed thrusting ii flattening strains mainly accumulated in the orthogneisses with progressive deformation leading to a lineation oblique to the transport direction iii the subhorizontal fabric in basement and cover rocks was refolded by overturned folds and then iv cross cut by conjugate ene striking dextral and nne striking sinistral shear zones contemporaneous with ne trending upright folds it is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse q 2005 elsevier ltd all rights reserved

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

assessing the tectonic significance of a large scale transcurrent shear zone system the pernambuco lineament northeastern brazil abstract the pernambuco lineament borborema province northeastern brazil is traditionally considered a continuous 700 km long dextral e w strike slip shear zone it is interpreted as a fundamental tectonic element of the brasiliano pan african orogeny and proposed to continue into west africa as the sanaga or amadoua faults recent structural work reveals that the pernambuco lineament is segmented into two branches separated by more than 100 km this study shows that the eastern branch the east pernambuco shear zone system consists of two high temperature amphibolite facies and several low temperature greenschist facies mylonitic belts which are laterally and transversely discontinuous the main protoliths of the mylonites are magmatic rocks country rocks proximal to the shear zones are little affected by strike slip shearing either at high or low t conditions the high t mylonitic belts are located at the southern border of a major granitoid batholith and the low t belts form a series of overlapping right stepping shear zones to the south and east of them these observations strongly indicate that the pernambuco lineament is not a transcontinental structure that it was not responsible for significant displacements or for emplacement of large magmatic bodies and consequently that its tectonic role during the brasiliano event was secondary

magnetic and magmatic structures of the emas granodioritic pluton cachoeirinha belt ne brazil relationships with pan african strike slip fault systems abstract the emas granodiorite was intruded during the brasiliano pan african orogeny into the metapelites of the cachoeirinha belt ne brazil this pluton was chosen for a petrofabric study of magmatic emplacement structures using magnetic anisotropy because of its conspicuous position in the junction area between the large e w trending patos shear zone and the ne sw trending cachoeira da mina strike slip fault the magnetic behavior of this calc alkaline granite is dominated by paramagnetic minerals which is optimal for the application of the anisotropy of magnetic susceptibility technique used for this study almost all microstructures correspond to the magmatic state so the measured magnetic lineations and foliations may be interpreted respectively as directions and planes of magmatic flow these magnetic magmatic lineations and foliations are dominantly n s and ne sw oriented and are gently to moderately dipping the anisotropy of magnetic susceptibility ratios range from 1 to 5 4 most of these values corresponding to magmatic alignment the lowest ratios are located chiefly in the centre of the pluton and the highest on the northern and western margins from these data we propose that the emplacement of the emas pluton was controlled by a ne sw trending left lateral strike slip zone parallel to the cachoeira da mina fault giving rise to openings in the metapelitic country rocks infilled by two succesive magmas the principal result of this study is the unexpected independence between the emplacement of the emas pluton and the functioning of the patos dextral shear zone

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

the influence of conduit geometry on the dynamics of caldera forming eruptions abstract during caldera collapse events new conduits may open and dramatically modify the style of the associated eruption in order to investigate the effect of conduit geometry on the dynamics of caldera forming eruptions we performed numerical simulations of magma ascent in single vent cylindrical conduits and ring fissure conduits the results show that for a given volume of conduit the discharge rate is an order of magnitude smaller in ring fissure conduits due to the higher friction furthermore the transition from a sustained plinian column to a collapsing fountain feeding pyroclastic flows occurs at a discharge rate an order of magnitude higher for ring fissure conduits due to the higher rate of air entrainment in a curtain jet than in a cylindrical jet the production of pyroclastic flows from ring fissure conduits therefore requires a much larger conduit volume than from a cylindrical conduit we argue that this should be correlated with a much larger volume of lithics in the deposit we confront these theoretical considerations with two geological examples the taupo ignimbrite has been emplaced by a high discharge rate pyroclastic flow and contains a small volume of lithics this is consistent with its emplacement during a single vent eruptive phase the bishop tuff has been emplaced by a lower discharge rate pyroclastic flow and yet contains much more lithics this is consistent with an eruption through successive vents migrating along a ring fissure the lithic content of an ignimbrite and the eruption discharge rate can therefore give some insights into the mechanism of caldera collapse and the role of ring fissures as magma conduits a conclusion of our analysis is that perfectly annular ring fissure conduits do not favour the formation of pyroclastic flows the occurrence of pyroclastic flows in virtually all caldera forming eruptions suggests that magma ascent is mainly localised in more restricted conduits a corollary conclusion is that widespread ignimbrites which record high discharge rate eruptions are not necessarily the result of ring fissure opening during caldera collapse

a major mid crustal decollement of the paleozoic convergent margin of western gondwana the guacha corral shear zone argentina abstract the guacha corral shear zone gcsz is one of the most significant and widest thrusts in the western gondwana margin of argentina it is 10 16 km wide and thrusts 525 515 ma forearc migmatites of the pampean orogeny eastwards onto greenschist facies rocks metamorphosed at 460 450 ma during the famatinian orogeny it is divided into two sections at the highest structural levels the shear zone is 7 5 km wide and records high t amphibolite facies deformation at lower structural levels this is overprinted by a 2 5 km wide low t greenschist facies shear zone the two are separated by a transitional zone where overprinting relationships are preserved motion in the two sections had the same kinematics syn anatectic structures in migmatites indicate that thrusting started during pampean forearc melting and continued into subsolidus during hanging wall exhumation and cooling greenschist facies reactivation at the end of the famatinian orogeny thrust the pampean migmatites over famatinian age metamorphic rocks at this stage the shear zone became part of a system of contractional shear zones defining the eastern boundary of a 300 km wide belt of famatinian foreland shortening thus the uniquely wide gcsz is a major tectonic boundary developed in two steps first as an amphibolite facies mid crustal detachment controlling the widening of the compressional pampean forearc then as the easternmost foreland thrust system of the famatinian orogeny