conjugate ductile shear zones abstract the geometric character of conjugate ductile shear zones has been defined from the structures developed in a undeformed granitoid corresponding to an homogeneous and isotropic material in the plane perpendicular to the intersection line of conjugate zones plane 1 3 one notes that 1 the principal direction of finite shortening determinated from the orientations of internal structures bisects the acute angle of the conjugate sets 2 the zones cross along a sub rectangular intersection zone lengthened perpendicularly to the direction of finite shortening and thickened in this direction and 3 in the intersection zone the foliation has a crenulated shape conjugate ductile shear zones seem to have the same evolution as conjugate brittle shear zones and are characterized by the morphology of the intersection zone showing a local more complex strain since geometrical patterns of conjugate shear depend on the observation planes the angular relation between the conjugate sets appears insufficient to determine the bulk strain directions

depositional history and sequence stratigraphy of the precambrian cambrian boundary stratotype section chapel island formation southeast newfoundland abstract facies relationshps in the vendian to lower cambrian chapel island formation of southeast newfoundland are used to establish a sequence stratigraphic framework for this important boundary stratotype section the formation consists of five members of sandstone mudstone and minor limestone deposited in a variety of shoreline to shelf environments peritidal sandstone and shale units in member 1 are overlain by sandstone and silstone deltaic and shelf deposits of member 2 these make up a transgressive system tract in which small scale changes in relative sea level are superimposed on autocyclic deltaic processes the thickness 400 m of nearshore and inner shelf sediments resulted from high sediment accumulation rates which for member 2 nearly kept pace with subsidence upper member 3 and member 4 consist of inner shelf mudstone and peritidal limestone that were deposited in a low energy oxygen stratified basin these deposits are arranged in three large scale shoaling cycles representing three foresteping parasequences of one highstand systems tract an extensive limestone bed at the tope of member 4 contains a sequence boundary associated with or shortly postdating development of this surface is a significant epeirogenic event that resulted in basin reorganization increased subsidence and progradation of a thick clastic sequence represented by member 5 the deposits of this upper member consist of an upward coarsening storm dominated shelf to shoreface succession of sandstone and silstone deposits of nearly identical character are also found in new brunswick and suggest that these sediments and the underlying sequence boundary could be significant for interregional correlation global correlation of the paleobathymetric curve developed for the chapel island formation is considered unfeasible given the long history of epeirogenic movements during the vendian and early cambrian in this region

prolonged magmatism on 4 vesta inferred from hf w analyses of eucrite zircon the asteroid 4 vesta is the second most massive planetesimal in the solar system and a rare example of a planetary object that possibly can be linked to a specific group of differentiated meteorites the howardite eucrite diogenite suite the 182hf 182w chronometry of individual zircon grains from six basaltic eucrites revealed distinct growth episodes ranging from 4532 11 6 ma to 4565 0 0 9 ma and constrains the early thermal history of 4 vesta indicating that its mantle generated basaltic melts for at least 35 million years myr initially the energy needed for melting was provided by decay of short lived isotopes mostly 26al the long duration of magmatism despite the short lifetime of 26al implies that the asteroid must have accreted within the first 4 myr of solar system formation similar to the formation of iron meteorite parent bodies and that its interior must have been thermally well insulated by an early formed crust that prevented heat loss

origin of zoned metal grains in the que94411 chondrite laser ablation inductively coupled plasma mass spectrometry la icp ms was used to measure distributions of the siderophile elements p v cr fe co ni mo ru rh pd w re os ir and pt in metal grains in the metal rich chondrite que94411 with a spatial resolution of similar to 30 mum the platinum group elements pges except pd exhibit radial zoning in these grains that mimics that previously observed in ni and co the concentrations of these elements decreases from the cores to the rims of the grains the pge distributions support a condensation origin for the enhanced refractory element abundances in the zoned grains the lack of zoning in pd refutes an origin by a redox controlled process and none of the pge ni relationships support an origin by fractional crystallization from a metallic melt several models of grain formation were explored including equilibrium fractional condensation which failed to yield the correct radial zoning the zoning may be the product of a nonequilibrium fractional condensation process in which the refractory siderophiles remained supersaturated in the cooling solar nebula or of diffusion between refractory enhanced fe ni cores and other fe ni metal that may have been deposited later from the solar nebula copyright c 2001 elsevier science ltd

displacement accumulation from earthquakes on isolated normal faults abstract displacement profiles for isolated normal faults are most often triangular rather than semi elliptical as predicted for linear elastic materials simple modelling indicates that triangular displacement profiles can arise from gutenberg richter earthquakes randomly distributed on fault surfaces of constant dimensions near triangular profiles are generated after a small number of earthquake cycles and for earthquake magnitude ranges as small as 0 5 the results are only weakly dependent on the shape of earthquake slip profiles and reproduce the geometry of displacement profiles on the non propagating active cape egmont fault in new zealand by contrast growth models in which fault displacement and length increase in proportion result in unrealistic displacement profiles for realistic slip profiles irrespective of whether earthquake populations are characteristic or gutenberg richter

martin brasier s contribution to the palaeobiology of the ediacaran cambrian transition martin brasier s work spanned almost the entire geological column but the origin of animals and the nature of the cambrian explosion were areas of particular interest to him martin adopted a holistic approach to the study of these topics that considered the interplay between multiple geological and biological phenomena and he sought to interpret the fossil record within the broad context of geological biogeochemical and ecological changes in the earth system here we summarize martin s main contributions to this area of research and assess the impact of his findings on the development of this field karl popper would have said that palaeontology is not real science because you can t go out and sample it i think absolutely the opposite i think this is actually where science is it s trying to guess what lies over the hill and map terra incognita when people come in and colonize that s just technology martin brasier 2013 excerpt from a phone interview in moor 2016

reconstructing the avalonia palaeocontinent in the cambrian a 519 ma caliche in south wales and transcontinental middle terreneuvian sandstones an early cambrian caliche on the st non9s formation emended is the base of the caerfai bay formation unit term changed at caerfai bay south wales subaerial exposure and the caliche mean the two formations were not genetically related units the st non9s is an older sand sheet likely tidalitic not delta related referred to avalonian depositional sequence ads 2 and the caerfai bay is a shallow mud basin unit refered to ads 4a the similar random formation upper ads 2 in north american avalonia has a basal age of c 528 ma and is unconformably overlain by red mudstones or sandstones in fault bounded basins on the avalonian inner platform coeval british sandstones lower hartshill wrekin st non9s brand hills are unconformably overlain by latest terreneuvian ads 3 or epoch 2 ads 4a units dates of 519 ma on caerfai bay ashes give an upper bracket on the late appearance of avalonian trilobites and suggest an ads 2 4a hiatus of several million years post st non9s and post random basin reorganization led to abundant caerfai bay formation volcanic ashes and sparse brigus formation ashes in newfoundland the broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that east and west avalonia formed one palaeocontinent the inner platform in southern britain was larger than the midlands craton a tectonically defined later palaeozoic area unrelated to terminal ediacaran early palaeozoic depositional belts the cool water successions of early palaeozoic avalonia were distant from coeval west gondwanan carbonate platforms

development of shear zone related lozenges in foliated rocks abstract tectonic lozenges are elongate bodies bounded by relatively more deformed rocks the focus of this study is on the 2 d structure of tectonic lozenges developed during ductile shear in rocks with a pre existing mechanical anisotropy on the basis of a detailed analysis of shear zones in foliated rocks from the cap de creus area variscan of the eastern pyrenees five mechanisms to explain the development of different types of lozenges in foliated rocks are suggested these mechanisms are explained on the basis of the orientation of the previous foliation relative to the bulk shearing direction it is shown that the prevailing mechanism does not majorly depend on the bulk kinematics but on the angular relationship between the pre existing foliation and the bulk kinematic axes and on shear zone interaction this has implications on the use of lozenge shapes in tectonic interpretations the fact that there is a wide range of initial orientations propagation modes and coalescence types implies that the final lozenge geometry is not univocally related neither to the type of strain nor to the kinematic regime

the sou hills a barrier to faulting in the central nevada seismic belt bedrock zones transverse to regional structures may indicate barriers at depth that inhibit the propagation of fault ruptures in the basin and range province the tertiary bedrock of the sou hills separates dixie and pleasant valleys and is transverse to the trends of physiography and historic surface faulting in central nevada four lines of evidence indicate that the sou hills are a barrier to faulting in the seismic belt first total late cenozoic vertical displacement on range bounding faults decreases toward the sou hills second analyses of landforms that reflect rates of relative uplift show that quaternary uplift decreases where range bounding faults meet the sou hills third the most recent prehistoric faulting south of the transverse zone is several thousand years younger than faulting to the north fourth patterns of late quaternary fault scarps in the sou hills are similar to rupture patterns observed at the termination of faults elsewhere

mesoscale strike slip faults and damage zones at marsalforn gozo island malta well exposed strike slip faults in limestones in the north western part of gozo show damage zones that can be grouped into three categories based on their location along faults tip damage linking damage and distributed damage the predominant fracture types within damage zones include extension fractures and secondary faults tip damage zones usually show wedge shaped patterns formed by antithetic faults and extension fractures commonly accompanied by block rotation several fractures are combined at linking damage zones typically with the concentration of a high intensity of fractures structures in distributed damage zones are typically similar to the classical riedel shear pattern evolutionary and 3d models are proposed in terms of the geometries of damage zones for small displacement strike slip fault zones different evolutionary routes depend on fault tip modes and locations

the oldest zircons in the solar system abstract here we report the occurrence chemistry and u th pb isotopic systematics of three meteoritic zircon assemblages two from the vaca muerta mesosiderite and one from the simmern h5 chondrite one of the vaca muerta zircons occurs in the mesosiderite proper the other in a eucritic clast associated with chromite ilmenite and tridymite whereas the simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar like terrestrial zircons the meteoritic zircons are enriched in the heavy rare earth elements but unlike terrestrial zircons they do not show a positive ce anomaly this feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment under oxidising conditions ce4 which can substitute for zr4 in the zircon structure is stabilized whereas under relatively reducing conditions ce3 is stable the zircon from simmern has depletions in the relative abundances of tm and yb a characteristic of volatility fractionation of the ree the u concentrations of the two vaca muerta zircons are quite different at 1 and 50 ppm respectively both zircons have extremely low initial lead and give radiogenic ages that are the same albeit with relatively large errors on vm 1 zircon because of its much lower u concentration and hence radiogenic pb concentration the mean of the two analyses made on vm 2 zircon is concordant with a207pb 206pb age of 4563 15 ma 2 the simmern zircon has an exceptionally low u concentration of around 180 ppb and only a poorly constrained 207pb 206pb age of 4100 700 ma could be obtained

provenance of the pre devonian sediments of england and wales sm nd isotopic evidence nd values and clast petrography of the longmyndian sediments of shropshire indicate a provenance dominated by juvenile high nd t uriconian volcanic sources but with an increasing detrital contribution from older low nd t metamorphic sources up section by contrast most lower palaeozoic strata of england and wales have low nd t values typical of sediments derived from long established continental crust the sm nd isotopic evidence suggests that avalonia received sediment from an adjacent large continent in the cambrian to lower ordovician and in the silurian ordovician and silurian volcanicity supplied juvenile detritus only during and shortly after active volcanism

40ar 39ar isotope constraints on the age of deformation in charnwood forest uk 40 ar 39 ar dating by a combination of spot analysis and step heating has been carried out on mica fabrics from slaty cleavage and a shear zone in the neoproterozoic to cambrian rocks of charnwood forest leicestershire the results indicate that crust adjacent to the eastern margin of the midlands microcraton was affected by localized epizonal metamorphism and penetrative deformation between 425 and 416 ma silurian to earliest devonian somewhat earlier than the acadian phase of the caledonian orogeny recorded elsewhere in southern britain the charnwood cleavage geometry suggests that the deformation arose within a dextral transpressional regime along the eastern margin of the midlands microcraton this tectonism may have overlapped with pre acadian sinistral transtensional movements documented for the welsh basin and lake district areas to the west and north of the midlands microcraton

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

u pb and hf w dating of young zircon in mesosiderite asuka 882023 mesosiderites are unique stony iron meteorites composed of eucrite like silicates and fe ni metals their formation including silicate metal mixing and metamorphisms provide important insights into early planetary processes in the inner solar system this report describes the first in situ u pb and hf w dating of zircon in a mesosiderite asuka 882023 the u pb 4502 75 ma and hf w 4532 8 5 7 10 5 ma ages may represent timing of the zircon formation which is considerably younger than crustal differentiation of the parent body this evidence combined with earlier studies of chronology implies that mesosiderites were reheated at 4530 4520 ma clearly after the silicate metal mixing

initial 182hf 180hf in meteoritic zircons zircons from the simmern h5 chondrite and pomozdino eucrite have been analyzed for their hf w isotope systematics zircons have high intrinsic hf contents and coupled with low w make them ideal hf w chronometers however measurements of 182hf 180hf 0 are far from straightforward with low signals of radiogenic 182w and difficulties in calibrating hf w ratios zircons were analyzed from the simmern chondrite and the pomozdino eucrite the simmern zircon has an ultrarefractory enriched trace element pattern a feature commonly associated with refractory inclusions analyses of simmern zircon show variable hf w that is likely due to surface contamination simmern chondrite zircon yields a 182hf 180hf 0 of 7 2 4 5 10 5 pomozdino eucrite zircon analyses show very high hf w values indicating 182hf 180hf 0 of 1 7 1 1 10 5 the simmern value is in good agreement with the initial value of the solar system indicated from hf w systematics of chondrites the pomozdino value is lower than expected if eucrites formed within several million years of refractory inclusions as suggested from al mg systematics of eucrites

analysis of the north anatolian shear zone in central pontides northern turkey insight for geometries and kinematics of deformation structures in a transpressional zone abstract the western part of the north anatolian shear zone at the southern boundary of the central pontides in turkey was investigated in the kursunlu arac area by means of a geological structural field study in this area the north anatolian shear zone results in a transpressional deformation zone that extends between two master faults striking parallel to the main shear direction the main systems of structures identified in the deformation zone appear to be oriented parallel to the directions predicted by riedel theoretical model nevertheless the strain partitioning is more complicated than predicted by theory the structural analysis suggests a polyphase deformation characterized by a steady component of transcurrence associated with alternance of compression and extension along each of theoretical directions the combination of double verging structures can be observed with folds and thrust surfaces root into high angle shear zones according to flower type geometries the discrepancies of directions kinematics and geometries from theoretical models are due to transpressive and or transtensive nature of the deformation according to the observed outcropping structures we propose a conceptual model for the north anatolian shear zone interpreting it as a crustal scale positive flower structure

early cambrian teichichnus dominated ichnofabrics and palaeoenvironmental analysis of the caerfai group southwest wales uk abstract the early cambrian caerfai group of pembrokeshire in southwest wales yields an intensely bioturbated teichichnus dominated ichnofabric and provides evidence for the early evolution of infaunal organisms three units of the caerfai group comprise the focus of this study in sections exposed at caerfai bay in the st non s sandstone few primary sedimentary structures are observed bedding being predominantly massive with subordinate parallel and current ripple cross lamination the topmost st non s sandstone contains common soft sediment deformation phenomena and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep fronted delta system teichichnus is the dominant trace with subordinate planolites palaeophycus and possible rhizocorallium the overlying caerfai bay shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front sandstone debrites are common as are tuffaceous beds with evidence of reworking by bottom currents sandstone and tuff event beds contain common skolithos and arenicolites with less frequent planolites interbedded mudstones contain infrequent teichichnus the caerfai bay shales coarsen and thicken upwards into the caerbwdy sandstone debrite sandstone beds dominate and bed thickening upsection indicates progradation of the delta system the dominant trace fossil within the unit is teichichnus which is constrained to the top few centimetres of individual sandstone beds less common are planolites and palaeophycus the size and abundance of teichichnus in the caerfai group are similar to or exceed those in contemporaneous sections in avalonia and baltica local environmental factors were probably the primary control on trace fossil distribution substrate and salinity are proposed as the main influences on ichnodiversity and size in the caerfai group teichichnus is most abundant in the st non s sandstone where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels burrow diameter progressively decreases upsection and bioturbation becomes less pervasive in subsequent units of the caerfai group this indicates that by the cambrian stage 2 significant advances into reduced salinity environments had occurred along with the dispersal of the teichichnus producer throughout the avalonia baltica seaway correlation within avalonia indicates that a transgression in the cambrian stage 2 initiated the deposition of the caerfai group marking the onset of the welsh basin

formation and suppression of strike slip fault systems strike slip faults are a defining feature of plate tectonics yet many aspects of their development and evolution remain unresolved for intact materials and or regions a standard sequence of shear development is predicted from physical models and field studies commencing with the formation of riedel shears and culminating with the development of a throughgoing fault however for materials and or regions that contain crustal heterogeneities normal and or thrust faults joints etc that predate shear deformation kinematic evolution of strike slip faulting is poorly constrained we present a new plane stress finite strain physical analog model developed to investigate primary deformation zone evolution in simple shear pure strike slip fault systems in which faults or joints are present before shear initiation experimental results suggest that preexisting mechanical discontinuities faults and or joints have a marked effect on the geometry of such systems causing deflection lateral distribution and suppression of shears a lower limit is placed on shear offset necessary to produce a throughgoing fault in systems containing preexisting structures fault zone development observed in these experiments provides new insight for kinematic interpretation of structural data from strike slip fault zones on earth venus and other terrestrial bodies

the geologic history of vesta inferred from combined 207pb 206pb and 40ar 39ar chronology of basaltic eucrites abstract eucrites represent samples of mafic crust of the parent body likely vesta and record a complex geologic history involving magmatism metamorphism impact processing and metasomatism for better understanding of the geologic history we present combined 207pb 206pb and 40ar 39ar chronology of four basaltic eucrites showing distinct petrologic features agoult an equilibrated eucrite with granulitic textures camel donga a brecciated equilibrated eucrite enriched in metallic iron dag 380 a partially equilibrated monomict eucrite and nwa 049 a metasomatized polymict eucrite comprising mostly unequilibrated clasts the 207pb 206pb dating in combination with the acid washing technique was applied to pyroxene rich and plagioclase rich fractions while the 40ar 39ar dating was performed mainly on plagioclase grains using the laser incremental heating method the pb isotopic data for acid leaching residues are more radiogenic than those for acid washes reflecting efficient removal of non radiogenic pb components by the acid washing among the residues however only those of the agoult and camel donga plagioclase rich fractions yielded isochrons rather than errorchrons with 207pb 206pb ages of 4532 39 0 87 ma and 4515 43 0 42 ma respectively the trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase on the other hand the ar isotopic data for agoult and camel donga yielded plateau ages with weighted means of 4494 9 ma and 3749 25 ma in contrast to those for dag 380 and nwa 049 that did not return any plateau the 207pb 206pb and 40ar 39ar ages of agoult are interpreted to reflect the time of the u pb and k ar system closure during a prolonged thermal metamorphism by combining the ages reported here and in the literature with the calculated closure temperatures the cooling rate of agoult is determined to be 11 2 c ma this cooling rate corresponds to a burial depth of 10 20 km indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed the 20 ma younger 207pb 206pb ages of camel donga than agoult are considered to reflect delayed burial metamorphism in the source region whereas the plateau 40ar 39ar age would record the time of an impact causing brecciation and efficient ar degassing of the buried crust furthermore the model 207pb 206pb dates calculated for acid washed pyroxenes imply that the burial metamorphism of agoult and dag 380 were broadly contemporaneous and also that nwa 049 was metasomatized by fluids within the uppermost crust later than 4 4 ga

three dimensional geometry and growth of conjugate normal faults abstract good quality 2 d and 3 d seismic reflection data from the timor sea are used to determine the threedimensional geometry displacement patterns and development of intersecting conjugate normal faults these data are supplemented by data from previous physical modelling studies conjugate structures which comprise two intersecting opposed dipping normal faults or fault sets form synchronously on a geological time scale and develop due to the incidental intersection of the faults factors which affect both the formation and imaging of these structures include the fault density the spatial distribution of opposed dipping faults the seismic resolution and the vertical extent of the imaged fault data large conjugate structures grow from smaller ones larger conjugates are associated with more numerous and larger faults than small structures on the scale of the seismic data fault throws range from ca 10 400 m synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone and a corresponding increase in ductile strain of this region high strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults and are believed to be accommodated by numerous small sub seismic faults intersection of two opposed dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system

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

micrometer scale u pb age domains in eucrite zircons impact re setting and the thermal history of the hed parent body meteoritic zircons are rare but some are documented to occur in asteroidal meteorites including those of the howardite eucrite diogenite hed achondrite clan rubin a 1997 meteorit planet sci 32 231 247 the heds are widely considered to originate from the asteroid 4 vesta vesta and the other large main belt asteroids record an early bombardment history to explore this record we describe sub micrometer distributions of trace elements u th and 235 238 u 207 206 pb ages from four zircons 7 40lm separated from bulk samples of the brecciated eucrite millbillillie ultra high resolution 100 nm ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios our new u pb zircon geochronology shows that vesta s crust solidified within a few million years of solar system formation 4561 13 ma in good agreement with previous work e g carlson r w lugmair g w 2000 timescales of planetesimal formation and differ entiation based on extinct and extant radioisotopes in canup r righter k eds origin of the earth and moon university of arizona press tucson pp 25 44 younger zircon age domains ca 4530 ma also record crustal processes but these are interpreted to be exogenous because they are well after the effective extinction of 26

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

39ar 40ar dating of mesosiderites evidence for major parent body disruption 4 ga ago 39ar 40ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of ar by one or more thermal events less than 4 0 ga ago three samples gave release patterns of constant age whereas the other samples typically showed 200 400 ma increases in 39ar 40ar age with increasing gas extraction temperature these stepwise releases indicate ages of 3 4 3 8 ga 1 ga 109 a averaged ages for individual meteorites are 3 45 3 82 ga no substantial evidence exists in the temperature releases for 39ar 40ar ages older than 4 0 ga and the ar ages appear to be in contradiction to 244pu fission track ages previously reported for a few mesosiderites the ar ages probably date the event that caused disturbance in previously reported rb sr sm nd and pb pb ages of estherville and other mesosiderites data on release of ar as a function of extraction temperature were used in arrhenius plots to estimate activation energies for ar diffusion of 18 52 kcal mol these values were then used to calculate closure temperatures against ar loss of 2 340 c and 180 600 c for assumed cooling rates of 1 c ma and 1 c a respectively the ar data suggest that for certain assumed models of mesosiderite formation the 39ar 40ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of 1 c ma models which call for metal silicate mixing 4 5 ga ago followed by cooling at 1 c ma from temperatures 1150 c seem unlikely single event models which produce the silicate metal mixture at high temperatures rapidly cool the objects to temperatures of 500 c followed by cooling of 1 c ma and are not consistent with the ar data unless the thermal event occurred 500 c and left them deeply buried to undergo slow cooling

correlation for the lower palaeozoic the global correlation standards now agreed for the ordovician and silurian systems and partly so for the cambrian system are summarized correlation of the international series and stages with the traditional and revised british series and stages is reviewed as well as with those in north america laurentia

curving cross joints and the lithospheric stress field in eastern north america cross joints are late formed nonsystematic fractures that extend across intervals between systematic joints traces of such cross joints are seen on bedding plane surfaces of devonian catskill clastic sedimentary rocks of the appalachian plateau of western new york state where the maximum horizontal principal stress s h is oriented n65 e as indicated by in situ stress measurements between pairs of closely spaced systematic joints traces of cross joints are commonly planar and orthogonal to the preexisting joints however in the mid region between some widely spaced systematic joints in western new york cross joints strike parallel to the s h of the present lithospheric stress field but then curve to abut the preexisting joints at right angles a curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints and the perpendicular termination indicates that the preexisting joints were open depending on their age the strike of the mid region of curving cross joints denotes the orientation of either the neotectonic stress field or its tertiary predecessor in the north american lithosphere

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

confluence and intersection of interacting conjugate faults a new concept based on analogue experiments abstract examination of the interaction area of conjugate faults yields evidence of a yet unknown structural pattern analogue experiments revealed the evolution of this pattern during the interaction process a fault propagating towards the highly active section of another conjugate fault becomes increasingly deflected towards the obtuse angle side of the conjugate system while approaching until joining it during this process the tip of the curved propagating fault retains its shear sense until immediately before joining with the highly active fault a process called confluence in this report under constant remote stress the deflected fault now becomes dominant a fault branch splits off at the point where deflection commences propagating straight ahead of the original direction and may dissect the former master fault alternating activity at both of the faults finally produces the known intersection patterns further studies of the proposed concept on natural examples are recommended due to the close geometric similarity of the evolutionary structural stages and natural fault patterns

u pb and 207pb 206pb ages of zircons from basaltic eucrites implications for early basaltic volcanism on the eucrite parent body we have undertaken petrologic and shrimp u th pb isotopic studies on zircons from basaltic eucrites yamato y 75011 y 792510 asuka a 881388 a 881467 and padvarninkai with different thermal and shock histories eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene and typically have rounded to subrounded shapes due to recrystallization superchondritic zr hf ratios of eucritic zircons indicate that they crystallized from incompatible element rich melts after crystallization of ilmenite concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite y 75011 are higher than those in metamorphosed eucrites the u pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance radiogenic lead loss up to 48 from zircons is observed in the shock melted eucrite padvarninkai the 207pb 206pb ages of zircon in y 75011 4550 9 ma n 5 are nearly identical within analytical uncertainty to the ages of zircons from the metamorphosed eucrite y 792510 4545 15 ma n 13 the highly metamorphosed eucrites a 881388 4555 54 ma n 5 and a 881467 4558 13 ma n 8 and the shock melted eucrite padvarninkai 4555 13 ma n 18 the averaged 207pb 206pb age of zircon from five eucrites analyzed in this study is 4554 7 ma 95 confidence limits n 49 indistinguishable from the averaged u pb age 4552 9 ma of the same samples because of the high closure temperature of lead in zircon tclosure 1050 c with a cooling rate of 0 2 c yr the 207pb 206pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas this fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the solar system history 7 20 ma after cai formation 4567 2 0 6 ma thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body the u pb ages of eucritic zircons are older than the u pb sm nd and rb sr ages of some basaltic eucrites which is consistent with differences in closure temperatures of each isotopic system and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene plagioclase and phosphates

hf w internal isochrons for ordinary chondrites and the initial 182hf 180hf of the solar system abstract three ordinary chondrites forest vale h4 ste marguerite h4 and richardton h5 have been studied with 182 hf 182 w system t1 2 9 million years m y in order to provide constraints on the initial 182 hf 180 hf for the chondritic parent body and the solar system all three samples display positive correlations between 182 w 184 w and 180 hf 184 w providing confirmation that live 182 hf was present in these samples the best fit lines define comparable slopes passing through or close to the data for the bulk carbonaceous chondrites allende and murchison at the current level of resolution this indicates a common and well mixed chondritic reservoir in terms of the hf w isotope system forest vale yields the most precise results with an apparent initial 182 hf 180 hf of 1 87 0 16 10 4 and an initial 182 w 184 w of 0 864710 15 if the pb pb phosphate age of forest vale 5 m y younger than the pb pb age of allende cai is a formation age the bulk solar system initial 182 hf 180 hf is estimated to be 2 75 0 24 10 4 comparable to previous estimates based on the differences in w isotopic compositions between iron meteorites and the mean of carbonaceous chondrites the solar system 182 w 184 w initial of 0 86457 4 ew 4 99 0 46 is similar to the least radiogenic w reported for iron meteorites suggesting an extremely early origin of their parent bodies and the segregation of metallic cores from planetesimals the solar system 182 hf 180 hf initial obtained in this study is high relative to the initial abundances of other short lived nuclides including those with longer half lives such as 129 i t1 2 16 m y however this result is explicable with the model of wasserburg et al wasserburg g j busso m gallino r 1996 abundances of actinides and short lived nonactinides in the interstellar medium diverse supernova sources for the r processes astrophys j 466 l109 113 who have proposed that 182 hf is synthesized in the same r process supernovae events that produced the actinides over most of galactic history

modern u pb chronometry of meteorites advancing to higher time resolution reveals new problems in this paper we evaluate the factors that influence the accuracy of lead pb isotopic ages of meteorites and may possibly be responsible for inconsistencies between pb isotopic and extinct nuclide timescales of the early solar system instrumental mass fractionation and other possible analytical sources of error presence of more than one component of non radiogenic pb migration of ancient radiogenic pb by diffusion and other mechanisms possible heterogeneity of the isotopic composition of uranium u uncertainties in the decay constants of uranium isotopes possible presence of freshly synthesized actinides with short half life e g 234u in the early solar system possible initial disequilibrium in the uranium decay chains and potential fractionation of radiogenic pb isotopes and u isotopes caused by alpha recoil and subsequent laboratory treatment we review the use of 232th 238u values to assist in making accurate interpretations of the u pb ages of meteorite components we discuss recently published u pb dates of calcium aluminum rich inclusions cais and their apparent disagreement with the extinct nuclide dates in the context of capability and common pitfalls in modern meteorite chronology finally we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early solar system based on the combined use of the u pb system and extinct nuclide chronometers

the proterozoic cambrian transition within the charnian supergroup of central england and the antiquity of the ediacara fauna a revised interpretation for the charnian supergroup is suggested from ichnological and lithological comparisons with the cambrian of nuneaton and other parts of the avalonian superterrane late neoproterozoic volcaniclastic sediments contain an ediacara fauna in the maplewell group these are succeeded by one or more inferred erosional breaks and overlain by passive margin sediments quartzarenites glauconitic greywackes pelites of the brand group which yield cambrian trace fossils including abundant large teichichnus this transformation is compared with that between the neoproterozoic caldecote volcanic formation and the cambrian hartshill formation at nuneaton the hanging rocks formation of the brand group lies above a major erosional break but shares a volcanogenic island arc petrology with the underlying maplewell group conglomeratic sandstones at the base of the brand hills formation contain purple pelite clasts comparable with underlying lithologies plus clasts of weathered diorite showing granophyric textures identical to those seen in the enigmatic south charnwood diorites nd sm isotope studies confirm the geochemical similarity between these diorites and those that intruded the caldecote volcanics at nuneaton previously u pb zircon dated at 603 2 ma these strong similarities with the nuneaton succession suggest that deposition of the brand hills formation in the cambrian was preceded by the local unroofing of diorite and erosion of underlying mudrocks much as seen in other parts of avalonia a date of 603 2 ma for the south charnwood diorites provides a minimum age for the local ediacara fauna but this is in need of substantiation

deformation of an island arc rates of moment release and crustal shortening in intraplate japan determined from seismicity and quaternary fault data the historical record of large m 6 9 earthquakes and geologically determined rates of slip on quaternary faults in intraplate japan honshu and shikoku are used to estimate the average rate of seismic moment release m for the last 400 years and during the late quaternary respectively values of m estimated from the two data sets are similar in regions where seismic activity is concentrated on land we interpret this observation to suggest that m in intraplate japan has been constant during the late quaternary and is relatively free from secular variation when averaged over periods of several hundreds of years m in shikoku may be attributed almost solely to right lateral slip of the median tectonic line mtl the easterly strike of the mtl is consistent with a compressive stress field that trends northwest crustal shortening of the izu peninsula taken up on a set of strike slip faults that show north by northwest compression is 1 mm yr northeast honshu is characterized by a set of reverse type faults that trend northerly conversion of m in northeast japan to strain rates suggests that about 5 of the relative plate motion between japan and the pacific plate 9 7 cm yr is accommodated as a permanent east west shortening 5 mm yr of northeast honshu the predominant deformation in central and western honshu takes place as slip on a conjugate system of strike slip faults that strike northeast and southeast and show right lateral and left lateral motion respectively crustal shortening resulting from slip on faults in central and western honshu is 5 and 0 5 mm yr respectively in an easterly direction central and western honshu are in closest proximity to the nankai trough and hence the stress field in these regions cannot simply be attributed to the accommodation of the relative northwesterly convergence of the philippine sea plate the northward impingement of the izu peninsula into honshu may influence stresses in central and western japan but a conclusive explanation of the stress field in central and western honshu remains an enigma

zipper junctions a new approach to the intersections of conjugate strike slip faults intersecting pairs of simultaneously active faults with opposing slip sense present geometrical and kinematic problems such faults rarely offset each other but usually merge into a single fault even when they have displacements of many kilometers the space problems involved are solved by lengthening the merged fault zippering up the conjugate faults or splitting it unzippering this process can operate in thrust normal and strike slip fault settings examples of conjugate pairs of large scale strike slip faults that may have zippered up include the garlock and san andreas faults in california usa the north and east anatolian faults turkey the karakoram and altyn tagh faults tibet and the tonale and giudicarie faults southern alps intersecting conjugate ductile shear zones behave in the same way on outcrop and micro scales zippering may produce complex and significant patterns of strain and rotation in the surrounding rocks depending on the angle between the faults and the relative strength of the blocks they bound a zippered fault will have a slip rate equal to the vector sum of the slip rates on the merging faults unless that displacement is transferred into or out of the system by distributed strain in the surrounding rocks

differentiation and magmatic activity in vesta evidenced by 26al 26mg dating in eucrites and diogenites abstract the 26 al 26 mg short lived chronometer has been widely used for dating ancient objects in studying the early solar system here we use this chronometer to investigate and refine the geological history of the asteroid 4 vesta ten meteorites widely believed to come from vesta 4 basaltic eucrites 3 cumulate eucrites and 3 diogenites and the unique achondrite asuka 881394 were selected for this study all samples were analyzed for their 26 mg and 27 al 24 mg ratios in order to construct both whole rock and model whole rock isochrons mineral separation was performed on 8 of the hed s in order to obtain internal isochrons while whole rock al mg analyses of hed s plot on a regression that could be interpreted as a vestan planetary isochron internal mineral isochrons indicate a more complex history crystallization ages obtained from internal 26 al 26 mg systematic in basaltic eucrites show that vesta s upper crust was formed during a short period of magmatic activity at 2 66 0 58 1 39 million years ma after calcium aluminum inclusions after cai we also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of vesta while 26 al was still extant cumulate eucrites crystallized progressively from 5 48 0 60 1 56 to 7 25 ma after cai model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2 88 0 12 0 14 ma after cai i e contemporaneously from the onset of the basaltic eucritic crust based on their cumulate texture we suggest cumulate eucrites were likely formed deeper in the crust of vesta diogenites have a more complicated history and their 26 al 26 mg systematics show that they likely formed after the complete decay of 26 al and thus are younger than eucrites this refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4 vesta from which the basaltic eucrites rapidly crystallized in order to explain the younger age and the complex history of diogenites we postulate that a second episode of magmatism was possibly triggered by a mantle overturn we bring a refined chronology of the geological history of vesta that shows that the asteroid has known a more complex differentiation than previously thought

strains at the intersections of synchronous conjugate normal faults abstract processes accommodating the mutual offsetting of intersecting conjugate faults are investigated by outcrop examination of three pairs of conjugate normal faults with maximum throws from 5 mm to 10 cm in two cases resolved fault throws decrease sharply towards the intersection zones which on the scale of observation are zones of ductile thinning of inter fault layers that accommodate the throw reductions strain within the intersection zone of conjugate faults in fluvio glacial sands is accommodated by inter grain slip and layer thinning is complemented by layer parallel extension at constant volume in a second example strain within the intersection zone of conjugate faults in inter layered chalk and marl is accommodated by volume reduction without layer extension in a third example also in fluvio glacial sands strain in the intersection zone is accommodated by small faults throws 1 mm when throws on all resolvable faults are aggregated there is no decrease in throw towards the conjugate intersection the presence or absence of a decrease in resolved throw towards conjugate intersections and of a complementary ductile zone accommodating the change in throw is a function not only of the resolution limit of the observations but also of the scale of the deformation process within the conjugate zone

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

the distinction between caledonian burial and regional metamorphism in metapelites from north wales an analysis of isocryst patterns in snowdonia and ll n three prograde stages in metapelites gwna group to wenlock are recognized stage i with white mica crystallinity values 0 43 2 stage ii 0 43 0 26 2 stage iii isocrysts at 0 03 2 intervals are drawn separately for pelites above and below the sub arenig unconformity approximately concordant isocryst patterns characterize the unconformity in the northwestern and southeastern limbs of the snowdonia syncline whereas discordant patterns are developed northwest of the aber dinlle fault and at the closure of the ynyscynhaiarn anticline the contrasting patterns are the result of a pre arenig burial metamorphism in post tremadoc pelites belts of steep metamorphic gradient coincide with belts of high strain and a broad area of higher grade caradoc pelites coincides with the core of the snowdonia syncline the later metamorphism is related to caledonian deformation rather than to burial pelite alteration adjacent to small intermediate to acid plutons suggests emplacement before the caledonian regional metamorphism a close correlation exists between metabasite zones and metapelite stages

evolution of fault patterns in clay experiments abstract the evolution of systems of strike slip faults from a single fault to intricate patterns is examined here in a series of clay experiments in the experiments plates of clay are loaded laterally under constant strain rate in plane strain with continuous monitoring of the stresses the experiments indicate that an individual fault appears as a linear trace about 1 mm in length that grows by in plane propagation of one or both ends next the fault evolves by out of plane propagation and interacts and coalesces with other subparallel faults this process generates a crooked nonlinear fault which at this point usually ceases to grow younger faults developing close to the older fault may cross and displace it the fault pattern in a sample changes in nature during a single run the early pattern is either domainal or conjugate depending on the initial fault distribution the second stage is usually domainal when subparallel faults develop close to each other during the final stage young faults cross old ones and generate conjugate patterns the relation between rheology and growth mechanisms of the faults is discussed faults in brittle elastic solids grow by out of plane propagation into zones of reduced normal stress whereas many of the faults in the present experiments grow by in plane propagation this propagation direction and the fault pattern evolution is interpreted here as indicating relaxation of nonuniform stresses at the proximity of the faults it is further argued that stress relaxation may be applicable to faulting of rocks in the upper crust

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

oxygen and carbon isotope stratigraphy of early cambrian carbonates in southeastern newfoundland and england carbonate rocks have been sampled through predominantly siliciclastic sediments above the precambrian cambrian global stratotype level in southeastern newfoundland to assess their potential for oxygen and carbon isotope stratigraphy comparable successions were sampled at nuneaton and comley in england greatly depleted 18 o signals are attributed to widespread thermal alteration during deep burial and granitic intrusion including within the stratotype region carbon isotope ratios appear to have been less affected and these are described from nine sections a provisional composite 13 c curve is based on non ferroan pink nodular and bedded micrites several 13 c excursions occur in the fossiliferous bonavista group and allow the position of the tommotian atdabanian boundary to be identified chemostratigraphic correlation of the new precambrian cambrian boundary stratotype may however prove difficult because of the lack of suitable well preserved carbonates the search must begin for a comparable reference section allowing global correlation of the boundary level using chemostratigraphy as well as biostratigraphy

new w isotope evidence for rapid terrestrial accretion and very early core formation the short lived hf 182 w 182 isotope system is an ideal clock to trace core formation and accretion processes of planets planetary accretion and metal silicate fractionation chronologies are calculated relative to the chondritic hf 182 w 182 isotope evolution here we report new high precision w isotope data for the carbonaceous chondrite allende that are much less radiogenic than previously reported and are in good agreement with published internal hf w chronometry of enstatite chondrites if the w isotope composition of terrestrial rocks representing the bulk silicate earth is homogeneous and 2 24 epsilon 182w units more radiogenic than that of the bulk earth metal silicate differentiation of the earth occurred very early the new w isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion early terrestrial core formation implies rapid terrestrial accretion thus permitting formation of the moon by giant impact while hf 182 was still alive this could explain why lunar w isotopes are more radiogenic than the terrestrial value copyright c 2002 elsevier science ltd

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

th pb ion microprobe dating of allanite allanite which is a common accessory mineral in a wide variety of rock types typically contains high concentrations of th and u thus an in situ method of u th pb dating of this phase would have broad application we describe a method to permit th pb ages of allanite to be determined with approximately 10 accuracy using a high resolution ion microprobe knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ion ization efficiencies of th and pb the chemical compositions of three allanite samples used as age standards cima d asta pluton 275 5 1 5 ma atesina volcanic complex 276 3 2 2 ma la posta pluton 94 2 ma were determined using an electron microprobe permitting an assessment of matrix effects on ionization an ion microprobe calibration curve involving elemental and oxide species of th and pb i e 208 pb th vs tho 2 th yields highly scattered apparent ages when allanite age standards with different fe contents are used however a three dimensional plot of 208 pb th vs tho 2 th vs feo sio improves the accuracy of the calibration to about 10 even though this level of uncertainty is substantially greater than that expected for u th pb ion microprobe analyses of zircon or monazite th pb ages of allanite can still be used to address important geologic questions we used this method to date two metamorphic allanite grains from the footwall of the main central thrust nepal himalaya and an allanite grain from the pacoima canyon pegmatite california allanite inclusions in garnet from nepal yield significantly older ages than the coexisting monazite indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault the pacoima canyon allanite grain yields a younger age than that reported for zircon implying pb loss during cooling of the pegmatite

the formation of conjugate normal fault systems in folded sandstone by sequential jointing and shearing waterpocket monocline utah 1 we report conjugate normal fault development by the formation and subsequent shearing of joints during flexure and associated extension of the jurassic wingate and navajo sandstones at the core of the waterpocket monocline utah geometry and structural components across fault zones were quantified from an incipient stage to a relatively well developed stage where the zone is 120 m wide and accommodates more than 3 5 m of slip shearing of joints leads to the formation of splay fractures near the joints tip subsequently these splay fractures are sheared and a new generation of splay fractures forms multiple generations of splay fractures cause the width of the fault damage zone to expand into the surrounding host rock producing unusually wide fault damage zones with respect to fault offset these extensive fault damage zones are stratigraphically confined and develop with low strains at small bed dips during folding the faults formed by this sequential process develop a characteristic fault architecture distinguished by the type of component structures their geometric relationships and their distribution we conclude that cyclic joint formation shearing accompanied by splay fracture formation and repeated shearing produce a geometric pattern similar to a classical conjugate fault system however the mechanism identified here has different implications for fault intersection angles stress inversion and fault architecture

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory

reappraising the neoproterozoic east avalonian terranes of southern great britain abstract inliers of late neoproterozoic basement rocks exposed in southern great britain described in summary herein have been previously attributed to a series of terranes bounded by major crustal lineaments that form a dissected remnant of a single subduction complex together these terranes have been termed east avalonia and interpreted as correlative with west avalonia of the northern appalachian orogen a review of age and nd and o isotopic data shows an essentially ensialic complex with two broad overlapping divisions 1 the monian composite terrane of nw wales and fenland terrane of eastern england that are characterised by felsic intrusions at around 615 ma and have more evolved end t 7 5 to 2 5 older t dm 1868 ma to 1269 ma and generally more elevated 18 o up to 13 7 and 2 a more diverse assemblage represented by the cymru wrekin and charnwood terranes including more substantive volcano sedimentary successions and intrusions ranging in age between around 710 ma and 566 ma in which 615 ma magmatic rocks are not represented less evolved end t 2 5 to 3 5 younger t dm 1501 994 ma and lower 18 o 6 5 to 10 1 comparison with other neoproterozoic basement units throughout the northern appalachian orogen shows that both divisions from southern great britain have closer isotopic affinity with igneous components of ganderia which generally have evolved compositions interpreted to reflect an ensialic setting and the input of gondwanan cratonic source components they are mostly dissimilar from magmatic rocks of west avalonia which typically have less evolved compositions interpreted to reflect the contribution of juvenile mesoproterozoic source components