strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

large scale crustal structure of the northwestern yilgarn craton western australia evidence from nd isotopic data and zircon geochronology u pb zircon geochronology and nd isotope geochemistry have been used in the northwestern yilgarn craton region of western australia to map gneiss units of different ages and to provide a model for late archean crustal evolution despite the inherent difficulties of minimal exposure and variations in the appearance of units brought about by heterogeneous strain the 3300 3730 ma gneisses and intercalated metasedimentary units crop out in a 100 km wide tract bounded on both sides by areas containing 2920 3000 ma gneisses between 2750 and 2620 ma several generations of granitoids were emplaced throughout the region regardless of whether the granitoids intruded the 3300 3730 ma or 2920 3000 ma gneisses modeling of their initial nd isotopic compositions shows that they were most likely formed by partial melting of predominantly 2920 3000 ma gneisses only a minor input from early archean sources is indicated for granitoids cutting the 3300 3730 ma gneisses and intercalated metasediments some of the granitoids that intrude the 3300 3730 ma gneisses contain 2920 ma inherited zircons supporting the nd evidence that their source is dominated by 2920 3000 ma gneisses the 3300 3730 ma gneisses are interpreted as belonging to an allochthonous terrain emplaced over a younger terrain comprising the 2920 3000 ma gneisses subsequently partial melting concentrated in the underlying 2920 3000 ma terrain gave rise to the late archean granitoids

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 liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

early archaean gneisses from the yilgarn block western australia the yilgarn block is an ancient crustal block which extends over 650 000km2 in the south west of western australia and consists of high grade gneiss and granite greenstone terrains1 being one of the largest segments of archaean crust in the world it is important to studies of early crustal evolution we report here a geochronological study of banded gneisses near mount narryer in the north west of the yilgarn block which has given a rb sr whole rock isochron age of 3 348 43 myr with an initial 87sr 86sr ratio of 0 7037 0 0005 the data indicate that the rocks had a prior crustal residence time of 200 myr the validity of this early archaean age is supported by model sm nd ages of 3 510 myr and 3 630 myr for two of the samples taken together these ages represent the oldest evolutionary sequence so far identified in the yilgarn block and are comparable with old ages from other archaean cratons elsewhere in the world

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

physical analog modeling of pull apart basin evolution abstract pull apart basins are structural depressions formed by localized extension along strike slip fault systems typically at releasing bends or steps in the fundamental strike slip system analog modeling is used to evaluate the sequence of structural evolution of pull apart basins and factors that control their degree of structural asymmetry and geometry basin evolution internal structure and overall symmetry are investigated for oblique releasing step angles and with varying rates of displacement between brittle and ductile crust on opposing sides of the strike slip system while maintaining relative rates between brittle crust on opposing sides of the basin pull apart basin evolution is separated into three stages incipient early and mature incipient pull apart basins are characterized by formation of a normal fault bounded graben or half graben parallel to the oblique step between main strike slip zones in the early stage of formation additional normal faults form toward the basin interior from the original bounding faults and cross basin strike slip faults cut diagonally across the basin interior basin bounding normal fault systems are characterized by lateral variations of fault throw and localized relay ramps in the mature stage of evolution strike slip and normal faults join to completely bound the pull apart basin analog model results indicate that displacement associated with cross basin faults causes development of a through going strike slip fault that links the two main strike slip displacement zones ultimately resulting in a decline in normal fault activity asymmetric symmetric and hybrid pull apart basins all follow the same overall deformation sequence just described the asymmetry of a pull apart basin is controlled by the degree of decoupling between brittle and ductile crust beneath the two crustal blocks in relative motion this is modeled by maintaining a constant relative rate of motion between opposing fault blocks in all models but varying the rate between the blocks and the fixed model basement absolute rate models in which one side of the detachment is fixed with respect to the basement form asymmetric pull apart basins defined by a half graben with the master fault on the mobile side in models where opposing sides of the fault system are equally decoupled from the basement symmetrical pull apart basins form defined by horst and graben structures and master fault dominance switching sides along the length of the basin fault segmentation associated with relatively immature pull apart basins may be capable of arresting earthquake rupture late linkage of main strike slip zones by a cross basin fault could extend potential rupture area dramatically increasing the possibility for large magnitude earthquakes

analog modeling of pull apart basins scaled sandbox models have successfully simulated the geometries and progressive evolution of pull apart basins developed in a weak sedimentary cover above right stepping releasing dextral strike slip fault systems in rigid basement synkinematic basin infill was added progressively as the models were deformed vertical and horizontal sectioning of the completed models has allowed the full three dimensional architecture of the pull apart system to be analyzed we present three representative end member experiments 30 underlapping releasing sidestep 90 sharp nonoverlapping releasing sidestep and 150 overlapping releasing sidestep the pull apart basin geometries are typically sigmoidal to rhombic grabens the geometries of which are dependent upon the architecture of the underlying basement fault systems underlapping releasing sidestepping faults offset angles of 30 75 typically form elongate rhomboidal grabens 90 releasing offset sidesteps produce shorter squat rhomboid pull apart basins and overlapping releasing sidesteps 115 150 produce box like grabens with highly kinked basin sidewalls the pull apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones pdz of the main dextral strike slip faults the sidewall faults show changes in kinematics from dominantly dip slip extension in copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received june 24 1996 revised manuscript received november 20 1996 final acceptance june 10 1997 2fault dynamics project geology department royal holloway university of london egham surrey tw20 oex united kingdom supported by the fault dynamics project sponsored by arco british limited petrobras u k ltd bp exploration conoco u k limited mobil north sea limited and sun oil britain we also acknowledge additional funding from sun oil britain landsat thematic mapper data courtesy of rtz mining and exploration limited south american division jurrian reijs is thanked for field data relating to the salina del fraile pull apart basin chris elders is thanked for reviewing earlier versions of this manuscript and j crowell c morley and k biddle are thanked for constructive reviews of the manuscript the contents of this paper have been

strain accommodation about strike slip fault discontinuities in granitic rock under brittle to ductile conditions abstract brittle and ductile structures adjacent to fault terminations and echelon fault steps of left lateral strike slip faults in the lake edison granodiorite of the central sierra nevada california are related to stress perturbations caused by fault slip the structures associated with fault terminations change from dilatant splay fractures only to splay fractures and ductile fabrics on opposing sides of the fault to ductile fabrics ahead of the fault with decreasing distance from a younger neighboring pluton and presumably with increased temperatures the distributions of deformation mechanisms correlate with the inferred local magnitudes and spatial distributions of the maximum tensile stress the mean stress and the maximum shear stress displacements along the faults are transferred across echelon stepovers by mineralized dilatant fractures in extensional steps to accommodate slip transfer across contractional steps rock apparently was squeezed vertically out of the step by ductile flow concentrations of mobile and immobile elements indicate that diffusive mass transfer was not significant increased mean stress in contractional steps apparently enhanced crystal plastic flow of quartz a flow law that includes a pressure effect through correlation with pressure dependent solidus temperatures is more successful than other experimentally derived flow laws in predicting the observed distribution of ductile fabrics

paleoenvironments of the upper pennsylvanian ames limestone and associated rocks near pittsburgh pennsylvania paleoenvironmental analysis of the upper pennsylvanian ames limestone and associated rocks east of pittsburgh pennsylvania reveals a range of depositional environments from shallow marine to deltaic the depositional history began with paludal sedimentation followed by eastward transgression of a clastic lagoon dominated by abundant molluscs and mud lloater brachiopods especially thin shelled neochonetes this period was followed by the eastward transgression of a tidal belt and a shoreward low energy carbonate lagoon tidal belt sediments were derived from 1 shoreward transportation of offshore shelf sediments carbonate mud and fine sand fine crinoid debris and trilobites by tidal currents and 2 erosion and transportation of underlying clastic lagoon sediments molluscs neochonetes and clay minerals in tidal channels the carbonate lagoon contained abundant neochonetes in carbonate mud abandonment of the tidal belt allowed the eastward transgression of open circulation offshore shelf carbonate sediments the far offshore sediments were dominated by crinoids whereas near offshore shelf sediments had a sparse fauna of crurithyris and crinoids in the final stage of deposition deltaic progradation from the east established the following offshore to onshore sequence of environments offshore carbonate shelf near shore clay miner shelf prodelta and delta front nearshore clay mineral shelf deposits were dominated by crurithyris a robust thick shelled brachiopod indicative of higher energy conditions than were existent for the thin shelled neochonetes

guidelines for reporting zircon hf isotopic data by la mc icpms and potential pitfalls in the interpretation of these data invited technical paper high quality analytical data are essential for the development of sound scientific interpretations to ensure the quality of the data published in chemical geology a new type of contribution has been introduced the invited technical paper when a particular technical issue needing discussion or clarification is identified specialists in the technique will be invited to share their expertise as is true for all articles published in the journal these contributions will be subject to peer review before publication the first topic to be treated concerns in situ hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry this is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world however if insufficient care is taken during analysis and data processing inaccurate results can be obtained notably because of the existence of very large isobaric interferences on the isotope of interest to address this issue we have asked christopher fisher jeffery vervoort and john hanchar to provide a set of guidelines that can be adopted to assure that reliable hf isotopic data are obtained by this technique abstract over the past decade the hf isotope composition of zircon as determined in situ by laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool there has been no uniformity however in the reporting of in situ hf isotopic data and quite often reviewers and readers of papers are not provided with sufficient information to assess data quality the goal of this invited contribution is to provide the non specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ hf isotopic data from zircon including details of how the challenging large isobaric interference corrections have been made in addition we discuss a number of potential pitfalls vis a vis the assignment of the incorrect age to the measured hf isotope composition non specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others

the initiation of pull apart basins and transform continental margins results from numerical experiments of kinematic partitioning in divergent settings we present a set of numerical models to investigate how transform faults initiate in pull apart basins or transform continental margins the model represents an elastic plate with three aligned weak spots applying strain on the edge of the model firstly propagates extensional cracks from the pre existing damaged spots the subsequent evolution depends on the angle obliquity between the applied strain and the alignment of the damaged spots for obliquity 60 transtensional transfer zones connect the divergent cracks and no transform fault forms these experiments suggest that pull apart basins do not form as previously assumed by connection of overlapping transform faults but that transform faults form as connections of extensional areas

reworking of hadean crust in the acasta gneisses northwestern canada evidence from in situ lu hf isotope analysis of zircon abstract the acasta gneiss complex of northwestern canada comprises early archean orthogneisses and includes the oldest known terrestrial rocks 3 94 4 03 ga previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4 2 ga in the oldest rocks indicating that the source of early archean granitoids contained hadean 4 03 ga crust in this study we have determined the lu hf isotopic compositions of zircon grains extracted from the early archean acasta gneisses to evaluate the extent of the hadean crust contribution to the formation of these granitoids analyses of the lu hf isotopes were carried out using in situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains two ca 3 59 ga granitoids have e hf t of 2 4 2 2 and 3 2 2 5 suggesting that the source of the granitoids was extracted from the mantle as far back as 3 8 ga this is consistent with the presence of abundant zircon xenocrysts with ages up to 3 9 ga the hf isotopic composition of a 3 72 ga granitoid is significantly less radiogenic with an e hf t of 6 1 2 5 this indicates that the granitoid formed by remelting of very old crust the e hf t values for the two oldest rocks the ca 3 97 ga tonalites are 1 2 3 3 and 3 3 1 7 respectively indicating that even the oldest known granitoids contain reworked older crustal materials these results suggest that hadean crust had significantly contributed to the genesis of some of the early archean acasta granitoids

transtensional faulting patterns ranging from pull apart basins to transform continental margins an experimental investigation abstract seventeen small scale experiments were performed to study the deformation induced in brittle ductile models by a releasing stepover between two transform faults and by intersection between transform and divergent plate boundaries in both cases faulting depends on the rheological layering presence and strength of a ductile layer at depth and the width of the basal stepover successive types of pull apart basins are observed in the first set of experiments which are compared with natural examples firstly a lazy z shaped basin appears which is bounded by y faults above transform boundaries and r faults propagating from each corner of the stepover then r faults replace r faults leading to a rhomb shaped graben at every stage the basin length to width ratio remains between 2 2 and 3 8 suggesting that scale independence of pull apart basins is related to the geometrical shape of bounding faults in a second set of experiments deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin within the horsetail splay block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin a feature that can be compared with marginal ridges at transform margins

shear fracture pattern and microstructural evolution in transpressional fault zones from field and laboratory studies abstract zones of transpressional shear deformation accommodate strike slip and oblique slip displacements field work in a transpressive shear zone and transpressional analogue clay box modelling show that a p oriented foliation and associated p shears are preferentially developed over the more common r 1 riedel shears the carboneras fault system cfs in se spain is a left lateral transpressional shear zone with an internal geometry characterized by first order y oriented faults and widespread p oriented second order faults the mesoscopic to microscopic gouge fabric reflects the regional architecture of the shear zone being dominated by a pervasive poriented foliation and discrete y and p shears friction experiments carried out to investigate the textural evolution of gouge fabrics showed four textural stages of fabric development from foliation formation to extreme shear localization resulting in cross gouge failure transpression clay box models favoured the formation of secondary p oriented shear fractures and p oriented shear lenses further deformation caused differential shear lens rotation and shear lens orientations closer to the mean displacement direction our field studies and laboratory analogue experiments indicate that shear zones dominated by p shears are diagnostic of a transpressional deformation regime

global climate change and active tectonics effective tools for teaching and research abstract multidisciplinary student teams foster innovative research projects that can discern the locations times and styles of late quaternary faulting field studies in the area of the 1992 landers magnitude 7 3 earthquake in southeastern california provide excellent opportunities for teaching and research in climatic and tectonic geomorphology graduate student work begins with descriptions of type sections for mappable units of quaternary alluvium climate change induced aggradation events these are stair stepped flights of alluvial fan and terrace surfaces topographic profiles along faulted stream channels and fans provide estimates of vertical tectonic deformation and insight into the responses of fluvial systems to wrench fault tectonics time lines in this tectonically deforming landscape are the result of global climate changes that caused synchronous pulses of alluviation in the mojave desert at about 125 55 and 10 12 and 8 ka these dated alluvial surfaces record tectonic deformation resulting from cumulative late quaternary surface ruptures tectonic geomorphologic analyses use topographic pedogenic stratigraphic and structural data but do not require expensive and time consuming trenching projects ages of faulted and unfaulted alluvium constrain the dates of two pre 1992 earthquakes the 1992 landers event is not characteristic of earlier faulting because it ruptured through adjacent segments of the emerson fault that previously ruptured in separate events one at 3 1 2 ka and the other before 8 ka styles of tectonic deformation and locations of active faults however may not have changed during the past 125 ka

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

a three dimensional 3d structural model for an oil producing basin of the brazilian equatorial margin abstract hydrocarbon discoveries in regions such as the brazilian equatorial margin equatorial africa and french guiana have recently confirmed their importance as new exploration frontiers the mundau sub basin located on the brazilian equatorial margin is an oil and gas producing region with four producing fields in shallow water xareu atum espada e curima in order to understand the structural and seismic stratigraphic frameworks of an oil producing area of the brazilian equatorial margin this work addresses the 3d geometry and spatial distribution of main faults in the curima and espada fields the occurrence of hydrocarbons in the mundau sub basin is compared with fields in other parts of the brazilian equatorial basin and in equatorial africa data from 12 wells and a 3d post stack time migrated multichannel seismic volume are used to define nine 9 main seismic stratigraphic units the syn rift mundau formation units 1 2 3 and 4 the transitional paracuru formation unit 5 and the drift ubarana uruburetama and itapage members units 6 and 7 tibau and guamare formations units 8 and 9 the study area is dominated by nw se planar normal faults basinward dipping that formed multiple half grabens and tilted blocks with small anticlines and synclines genetically related to a transtensional system three types of plays are recognised in the mundau sub basins structural combined structural stratigraphic and stratigraphic turbiditic in the eastern part of the region where the basement is shallow no oil was found conversely oil was discovered in an anticlinal trap formed on a hanging wall block analogous to fields in cote d ivoire ghana transform margin this work shows that combined traps on footwall blocks are successful plays near the shelf break of the mundau sub basin in similarity with the espoir and baobab fields in ivory coast basin furthermore turbiditic reservoirs in drift units are analogous to the stabroek block in guyana and prospects in the gulf of guinea the structural and petroleum play analyses in this work are therefore crucial to understand the multiple geological processes leading to the trapping of hydrocarbons in the larger equatorial atlantic ocean

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

origin of cenozoic reverse faults in northeastern china an example from the rongxintun fault in the liaohe basin china abstract in northeastern china cenozoic reverse faults can be found in most basins it was proposed that these faults were the products of inversion tectonics during a cenozoic regional compressional event this explanation is inconsistent with regional structures that are dominated by normal faults on seismic profile as a typical example the rongxintun fault in northeastern liaohe basin is systemically analyzed on the basis of deformation style sedimentary character and systemic mapping using seismic data we conclude that the rongxintun fault was related to right lateral strike slipping of the yannan main fault during the late paleogene when the dongying deposition occurred and it was a secondary right lateral reverse fault p plane fault of the yannan strike slip fault system the fold was a fault related fold formed as a result of movement along the rongxintun fault according to this analysis of the rongxintun fault we conclude that the cenozoic reverse fault in northeastern china was a secondary right lateral reverse fault of a strike slip fault system and did not represent a compressional event this kind of mechanism of reverse faulting is consistent with the fact that there was large scale strike slip faulting in northeastern china during the cenozoic

fault patterns associated with domes an experimental and analytical study experimental clay and analytical models suggest that regional strain either extension or compression significantly affects the fault patterns produced by doming our models specifically simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied regional horizontal strain the models of circular domes show that without regional strain normal faults develop on the crests and flanks on the flanks the normal faults have radial trends with regional extension normal faults on the crests of circular domes trend perpendicularly to the regional extension direction whereas many normal faults on the flanks trend obliquely to it strike slip faults trending 60 from the regional extension direction fo m near the peripheries with regional compression many normal faults on the crests and flanks of circular domes strike parallel with the regional compression direction strike slip faults trending 30 from the regional compression direction also form on the flanks and reverse faults striking perpendicularly to the regional compression direction develop on the peripheries our models show that regional strain affects the fault patterns produced by elliptical doming similarly the fault patterns of our models resemble fault patterns of domed strata above salt diapirs and above uplifted blocks of basement

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

structural study of a semiductile strike slip system in the central iberian zone variscan fold belt spain structural controls on gold deposits the villalcampo shear system is a regional dextral strike slip fault zone that affects late variscan granites and their metamorphic country rocks over an area of about 150 km2 the detailed geometry of this subvertical north west south east shear zone is outlined the system forms an extensional fan to the northwest and extends to the south east as a broad extensional duplex particular attention is focused on the distribution of fault rocks and associated veins in its north west splay a structural study of the shear bands encompassing both geometric and kinematic criteria and a microscopic study of the fault rocks has led to the interpretation of the system as a brittle ductile shear zone calculations give a shear strain value of 1 5 and a minimum displacement of s 3700 m the localization of gold mineralization in mylonite filled subvertical extensional veins is a product of the formation of the villalcampo shear system the subvertical faults and veins underwent a process of cyclical sealing and reopening as such they acted as valves controlled by fluid pressure regulating fluid rock interactions and gold deposition conditions favouring these processes occur near the base of the seismogenic zone in the vicinity of the frictional quasi plastic transition at mid greenschist metamorphic conditions t 350 c and 10 15 km depth

structural evolution of the neogene gar basin western tibet implications for releasing bend development and drainage patterns we investigated the interaction among basin bounding faults basin fi ll and geomorphic features of the southern gar basin one of only two known releasing double bend basins along the karakoram fault to better understand their structural evolution and role in basin development the southern gar basin is bounded by an 44 km long n20 wstriking central fault segment fl anked by two n40 w striking segments that parallel the regional strike of the karakoram fault system the central fault segment is composed of a system of strike slip and normal faults that young basinward and incorporate basin fi ll in their uplifted footwalls the oldest faults along the extensional portion of the bend are dominantly strike slip and they strike 15 w from the main strike of the karakoram fault basin fi ll is broadly folded about a nnwtrending axis and can be explained by e se directed slip along a listric normal fault cross sections across the basin and associated faults suggest the geometry is best described as an extensional fl ower structure forward structural modeling of the intrabasinal faults shows that the system has accommodated 8 km of east west extension we interpret the bend to have formed from linking of r and p shears into a through going principal displacement zone at shallow levels in the crust low confi ning pressures r shears are exploited at deeper levels these faults merge with the principal displacement zone forming the extensional fl ower structure geometry we estimate that the shear zone is 50 35 km wide based on the aerial distribution of p and r shears restoration of r shears on the west and east sides of the gar valley indicates 55 km of right lateral separation along the karakoram fault which is a minimum slip estimate for the karakoram fault system

a paleopedologic and ichnologic perspective of the terrestrial pennsylvanian landscape in the distal appalachian basin u s a abstract the conemaugh group of southeastern ohio contains numerous ichnofossil bearing paleosols in both the lower glenshaw and upper casselman formations which were deposited during the kasimovian and gzhelian 305 301 ma along the distal edge of the appalachian basin this study examined the small scale twenty four stratigraphic sections were measured from ten sites in athens county ohio from which thirty paleosols were identified and described detailed field descriptions allowed differentiation of four distinct pedotypes one in the glenshaw formation and three in the casselman formation the glenshaw pedotype gpt is a vertisol that formed on a proximal to distal floodplain the three casselman pedotypes represent entisols inceptisols cpti alfisols cptii and vertisols cptiii all four types contain evidence of alteration by post pedogenic processes such as drowning by migrating river channels or rising sea level or a gradual transition from one alluvial subenvironment to another as a result of channel migration eight morphologically distinct ichnofossils were observed in the conemaugh group paleosols including three types of rhizoliths lined and unlined vertical to subhorizontal shafts chambers sinuous burrows with and without bioglyphs and general burrow mottling these ichnofossils occur in distinct assemblages that form unique ichnocoenoses within each of the four pedotypes they were produced by various plants arthropod detritivores carnivores and herbivores as well as amphibians and early reptiles as temporary to permanent members of complex soil ecosystems pedogenic properties combined with ichnofossil diversity abundance and distribution increase the understanding of the complexity of late pennsylvanian soil ecosystems and provide details on the small scale variations in paleosols due to short term autogenic changes in alluvial environments although broad regional studies across 10 2 10 3 of kilometers are important small scale variations over less than 1 km are needed to understand the effects of localized changes in hydrology topography organisms and sedimentary processes on soil formation and soil biodiversity consideration of these types of localized studies is also vital when attempting to interpret regional to global scale aspects of environment such as climate from paleosols

transform margins development controls and petroleum systems an introduction this paper provides an overview of the existing knowledge of transform margins including their dynamic development kinematic development structural architecture and thermal regime together with the factors controlling these this systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock seal rock and reservoir rock distribution expulsion timing trapping style and timing and migration patterns the paper then introduces individual contributions to this volume and their focus supplementary material location table and map of specific transform examples structural elements of the romanche transform margin and glossary of terms used in this article is available at https doi org 10 6084 m9 figshare c 3276407 gold open access this article is published under the terms of the cc by 3 0 license this volume shows dynamic linkage between new transform margin research and increasing transform margin exploration it offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins and their controlling factors our introduction to this volume reviews the current knowledge of the transform margins focusing on their dynamic development kinematic development structural architecture thermal regimes and petroleum systems the last part of this paper introduces the contributions to this volume

lithospheric scale centrifuge models of pull apart basins abstract we present here the results of the first lithospheric scale centrifuge models of pull apart basins the experiments simulate relative displacement of two lithospheric blocks along two offset master faults with the presence of a weak zone in the offset area localising deformation during strike slip displacement reproducing the entire lithosphere asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1 g experiments and thus provide a better approximation of the dynamic evolution of natural pull apart basins model results show that local extension in the pull apart basins is accommodated through development of oblique slip faulting at the basin margins and cross basin faults obliquely cutting the rift depression as observed in previous modelling studies our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull apart basins the basins are lozenge shaped in the case of underlapping master faults lazy z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults model cross sections show significant along strike variations in basin morphology with transition from narrow v and u shaped grabens to a more symmetric boxlike geometry passing from the basin terminations to the basin centre a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models these geometries are also typical of 1 g models and characterise several pull apart basins worldwide our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone a rise of the base of the lithosphere occurs beneath the basin and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence i e the basin depocentre

paleomagnetism and rotation constraints for the middle miocene southwestern nevada volcanic field middle miocene rocks of the southwestern nevada volcanic field swnvf lie across the projection of the walker lane belt within the basin and range province and thus provide an interesting opportunity to test for late cenozoic vertical axis rotation paleomagnetic data from individual ash flow sheets document no significant relative vertical axis rotation among localities within central swnvf an area of relatively low stratal tilts and widely spaced faults a time averaged mean paleomagnetic direction d 351 4 i 52 7 95 4 5 calculated from data from numerous separate rock units suggests that the central swnvf underwent minimal counterclockwise vertical axis rotation r 7 1 6 6 with respect to the north american craton no clockwise vertical axis rotation is found to support projection of dextral faults of the walker lane beneath the central swnvf clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field these clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central swnvf block and more strongly extended areas to the south and southwest of the field negligible rotation of 11 45 ma to 13 25 ma tuffs relative to the central swnvf was found at the southeast margin of the field where 90 clockwise rotation at the northwest termination of the las vegas valley shear zone had been postulated any clockwise rotation in this area must predate 13 25 ma and thus dextral shear within this part of the walker lane belt was not synchronous or connected across the southern margin of the field small counterclockwise vertical axis rotation relative to the craton as found for the central swnvf block might be a regional feature in the western great basin

pull apart stepover structures in an asphalted road surface a geological curiosity abstract an unusual and perhaps unique example of strike slip displacement and pull apart stepover structures is described from a relatively newly asphalted road surface in central norway the structures which include pull apart basins show a remarkable similarity to those developed both in natural strike slip fault systems and in physical analogue models a surface of decollement is believed to have developed at the interface between the new asphalt layer and the old bitumen emulsified planed metalled surface perhaps aided by the presence of excess rainwater gravitationally induced slippage of the asphalt on this 1 in 15 gradient hill road actually occurred several months after the asphalt had been laid

the transition from passive to active margin tectonics a case study from the zone of samedan eastern switzerland the zone of samedan is part of a fossil early mesozoic rift system originally situated in the distal lower austro alpine domain of the adriatic passive continental margin an early mesozoic configuration of asymmetrical rift basins bounded by relative structural highs compartmentalized late cretaceous active margin tectonics jurassic half grabens were folded into arcuate synclines whereas relative structural highs engendered thin imbricated thrust sheets west directed thrusting and folding initiated at the surface and continued to depths favoring mylonitization under lower greenschist facies conditions at this time liguria piemontese ophiolites were accreted to lower austro alpine units directly underlying the zone of samedan late cretaceous orogenic collapse of the adriatic active margin involved the reactivation of west directed thrusts as low angle top to the east normal faults these faults accommodated extensional uplift of liguria piemontese ophiolites and lower austro alpine units beneath and within the zone of samedan during paleogene collision some late cretaceous faults in the zone of samedan were reactivated under lower anchizonal conditions as north directed thrusts the latter stages of this early tertiary thickening were transitional to brittle high angle normal faulting associated with top to the east extension and spreading above the warm uplifting lepontine dome

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

investigating the crustal structure of a strike slip step over zone along the great glen fault strike slip step overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the earth s crust experimental reprocessing of a marine deep seismic reflection profile cutting across an 35 km wide right stepping step over developed on the great glen fault system north of the inner moray firth basin east of northwest highlands scotland reveals a restricted zone of prominent reflections and diffractions beginning at 7 8 km depth continuing as deep as the interpreted moho discontinuity at 26 27 km geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a 12 km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step over region the known kinematic history of the great glen fault system together with the observed direction of the step over implies that a zone of contraction would have formed within the step over during early caledonian sinistral strike slip the seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near vertical zone of strike slip which passes deeper into a layer of low angle dipping thrusts or shear zones the results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike slip faults

structural framework of the gulf of elat aqaba northern red sea the gulf of elat aqaba occupies the southern part of the dead sea rift the rift is considered to be a plate boundary of the transform type partially leaky which connects seafloor spreading in the red sea with the sagros taurus zone of continental collision the deep water in the gulf of elat up to 1850 m provides a rare opportunity to examine the process of continental rifting by marine geophysical techniques the bathymetry alone provides much information about fault patterns in this area the fragmentation of the once continuous arabian african platform is a complicated process it shapes the structure of the gulf which has developed through continuing tectonism primarily consisting of faulting recent geophysical and geological studies of the gulf of elat including bathymetry bottom photographs continuous seismic profiles seismic refraction gravity magnetics heat flow and coring provide new information about the shallow and deep crustal structure of this important segment of the world rift system the shallow structure of the gulf is dominated by three elongated en echelon basins which strike n20 25 e undulations in the floors of the basins produce several distinct deeps these basins are interpreted as pullaparts the new data from the gulf suggest that classical models for the formation of these structures should be modified only one of the longitudinal faults of each depression is a strike slip fault while the other is predominantly a normal fault the situation with the other two sides of the basin which are supposed to be composed of normal faults may also be more complex than previously thought at least in one case one such boundary is composed of a strike slip fault while the other has no significant fault crustal models of the gulf of elat based on gravity data indicate that the basins are rather shallow and do not extend into the lower crust the fill of the basins extends to about 5 km below the seafloor in the northern and southern basins and less in the central basin most significant magnetic anomalies in the gulf of elat extend from land into the sea none of them however extends from one coast to another across the gulf this supports the geological evidence for a shear between the arabia and sinai plates the magnetic field over the southern third of the gulf is rather smooth and markedly different from that of the other parts the nature of the magnetics the crustal structure of the western margin and the heat flow values indicate a thinner crust and different tectonic processes in the southern part of the gulf overall it seems that within the southern gulf of elat a transition occurs between crustal spreading that takes place in the red sea to a rifting without spreading that takes place along the dead sea transform spreading activity propagates from south to north the most active place is the central basin which is being propagated northward into the shallow northern basin

new aspects of rhomb structures abstract the acute angles of 48 rhombic trapezoidal and triangular grabens horsts and upthrusts throughout the world were determined the results indicate an angular spread of 7 73 in most structures the two acute angles differ considerably most rhomb structures are grabens which show affinities to seismic faults and only a few are horsts and upthrusts in this connection modes of static crustal fracture propagation are compared to dynamic propagation rhomb structures occur along strike slip faults and in rifts these results question the universality of the pull apart model as a mechanism for rhomb structure development the two mechanisms that can lead to the formation of rhomb structures are 1 interaction of en echelon or non coplanar fractures by shear and extensional modes and 2 fracture bifurcation in an extensional mode the two mechanisms may result in faults of different shapes characterized by curved and straight boundaries respectively the grabens and horsts in extensional regions are the consequence of early fracture and later vertical displacements

modelling the influence of pre existing brittle fabrics on the development and architecture pull apart basins abstract we use new analogue modelling experiments to analyse the development of pull apart basins in an upper crust characterised by the presence of pre existing discrete fabrics as in previous models lateral movement of rigid basal plates induced strike slip deformation of a sand pack local extension allowing the formation of a pull apart basin was produced by the step over geometry of the master faults in this area a basal silicone layer was introduced to distribute deformation and reproduced a weaker crust in the basin conditions of neutral overlapping and underlapping interacting master faults were reproduced the model upper crust modelled by a sand mixture was characterised by the presence of pre existing structures the orientation of these inherited heterogeneities was systematically varied in different experiments model results indicate that depending on their orientation with respect to the strike slip displacement pre existing structures can be reactivated both within and at the margins of the pull apart basins inside the basin reactivation occurs when the pre existing structures are orthogonal or sub orthogonal to the strike slip displacement in this case the pre existing fabrics delay the development and linkage of cross basin faults and increase the complexity of the deformation pattern giving rise to a new set of faults characterised by an atypical trend pre existing fabrics oblique to the local extension direction may be partly reactivated in the central part of the basin as segments of cross basin faults at the margins of the pull apart reactivation occurs if the fabrics spatially coincide with the lateral boundaries of the silicone layer in these conditions reactivation allows a faster development of the border faults which are less segmented than in the homogenous models this also results in a more regular final geometry of the pull apart

zirconology of the meeberrie gneiss yilgarn craton western australia an early archaean migmatite abstract the meeberrie gneiss a major component of the early archaean narryer gneiss complex of western australia is a polyphase migmatite comprising monzogranites together with minor tonalitic and trondhjemitic components ranging in age from 3730 to 3300 ma with important resolvable age components at 3670 3620 and 3600 ma ion probe u pb studies of zircons from numerous localities show that almost all hand specimens of the gneiss consist of multiple generations of magmatically crystallized zircons even down to the scale of individual centimetre sized bands the earliest age components generally are not found in inherited cores but rather in separate zircon grains usually indistinguishable from the younger grains in terms of morphology internal structure and trace element composition only in rare low strain zones are original cross cutting relationships between individual magmatic components and pegmatite segregations preserved for the remainder late archaean deformation has blended units and erased pre existing structures however deformation state appears to have little bearing on the zircon isotopic systematics geochemical data for such rocks are thus composite as are nd isotopic compositions which nonetheless show that the younger generations of material in the gneisses are produced predominantly by reworking of older crust

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

evolution of the pannonian basin system 1 tectonics the carpathian arc formed during cretaceous to miocene time by continental collision between europe and smaller continental fragments following southward and westward subduction of ocean floor during the last stages of thrusting in the outer carpathians a set of discrete basins formed behind the carpathian loop these basins are regions of middle to late miocene extension and are connected to each other and to areas of coeval shortening in the outer carpathian thrust belt by a system of strike slip faults sinistral northeast trending and dextral northwest trending sets of conjugate shears reflect overall east west extension of the intra carpathian region during middle to late miocene time palinspastic reconstruction of the basins indicates about 100 50 km of east west extension across the intra carpathian region during this time we interpret the vienna basin which is superimposed partly on the flysch nappes of the outer west carpathians as the result of thin skinned extensional tectonics above a shallow detachment surface within the crust extension of upper crustal rocks above this detachment was probably accomodated by thrusting of the outermost nappes of the west carpathians over the european platform extension of basins farther inside the carpathian loop probably involved rocks at deeper crustal levels and within the upper mantle the most internally situated basins formed by extensional processes that involved the entire lithosphere thus the depth to which extension occurred seems to be closely related to the proximity of each basin to the active thrust front in the carpathians this can be related to the geometry of the subduction zone extension within the intra carpathian basins began first in the northernmost basins and migrated southward and eastward over a ten million year period palinspastic reconstruction of the basins at three different times during the miocene 17 5 16 5 and 13 0 ma predicts eastward migration of the zone of thrusting and shortening within the carpathian mountains to accomodate the synchronous eastward and southward migration of areas of extension from these reconstructions about 120 60 km of east west shortening is predicted across the outer east carpathians from about 17 5 ma until present both predictions are in good agreement with the timing and magnitude of shortening in the carpathian thrust belt as determined directly from rocks within the belt from this we infer a close relationship for middle to late miocene extension of the intra carpathian region and middle to late miocene crustal shortening within the outer carpathians during middle and late miocene time deformation of the pannonian fragment was dominated by extensional and wrench tectonics except along a narrow zone in the outer carpathian thrust belt middle and late miocene thrusting in the carpathians is interpreted as the result of simple shear caused by relative motion between europe and the overriding pannonian fragment at the subduction boundary rather than of regional compression of lithospheric rocks at depth the initiation of extensional tectonics within the intra carpathian region in middle miocene time can be related to reorganization of major fragment boundaries of the eastern european alpine system

straightening of the northern san jacinto fault california as seen in the fault structure evolution of the san jacinto valley stepover abstract we investigate a releasing stepover between the casa loma and claremont strands of the northern san jacinto fault zone to evaluate the late quaternary structural evolution of the fault zone and to assess the likelihood of a rupture jumping across the stepover our new cone penetration test cpt and trench observations along the claremont fault at mystic lake indicate that the main strand of the claremont fault has jumped nearly a half kilometer westward into the san jacinto releasing stepover during the late quaternary multiple faults are inferred from the cpt data within a small sag at the northeast side of the stepover that cuts through younger stratigraphy to the west of the basin bounding fault near mystic lake previous seismic reflection data also suggest the presence of a young fault that cuts basin strata beneath the middle of mystic lake farther west of our study area numerous tectono geomorphic features observed in satellite and light detection and ranging digital elevation model lidar dem imagery are interpreted to delineate the location of the currently active faults including a zone of faults that cut across the basin from the northern end of the casa loma fault to the claremont fault seismicity observations suggest the presence of many faults within the stepover zone finally new paleoseismic data from the mystic lake site suggest that some late holocene earthquakes may have jumped the stepover all of these observations suggest that the san jacinto stepover which has been used as the primary basis for segmenting the northern san jacinto fault zone is being bypassed and that the fault zone may now be capable of larger earthquakes than previously expected

brachiopod geochemical records from across the carboniferous seas of north america evidence for salinity gradients stratification and circulation patterns abstract stable isotopic analyses of 100 well preserved carboniferous brachiopod shells representing two time slices from across north america reveal systematic regional changes in environmental conditions for both of the time slices studied the chesterian latest mississippian and the virgilian latest pennsylvanian 18o and 13c show a stepwise decrease moving across the continent from modern day west to east higher 18o in the west reflects greater water depths less freshwater input and more direct upwelling influence from cool open marine waters than in the east modelling based on these regional isotopic data indicates salinity reductions on the order of 1 4 psu in the appalachian basin relative to the midcontinent basin for the virgilian time interval over a range of reasonable thermal gradient and 18ofw assumptions the magnitude of this gradient is similar to those of modern day hudson bay and the panama bight which exhibit permanent haloclines driven by freshwater discharge considering existing paleoecological evidence for freshening the lack of more dramatic benthic freshening suggests that the depth of a regional halocline was shallower than the depths inhabited by thick shelled stenotopic brachiopods although other lines of evidence are necessary to verify the strength of the halocline estimated 18o temperatures for appalachian basin brachiopods are 3 5 c cooler than those of coeval conodonts due to either slightly warmer fresher surface waters or uncertainty in the phosphate 18o paleothermometer despite discrepancies in calculated temperatures the magnitude of the regional isotopic gradient derived from brachiopod and conodont proxies is similar after making appropriate glacioisotopic and water depth corrections a decreasing 13c trend toward the east southern paleolatitude likely reflects greater restriction and terrestrial influence near the sea s tropical shoreline trace element trends are less consistent and are affected by species and metabolism erratic variations in mg ca concentrations even among pristine samples of the same genera with similar 18o values and unrealistic mg ca derived temperatures justify continued caution in applying existing mg ca thermometer equations to paleozoic brachiopods

fault development and interaction in distributed strike slip shear zones an experimental approach analogue model experiments using both brittle and viscous materials were performed to investigate the development and interaction of strike slip faults in zones of distributed shear deformation at low strain bulk dextral shear deformation of an initial rectangular model is dominantly accommodated by left stepping en echelon strike slip faults riedel shears r that form in response to the regional bulk stress field push up zones form in the area of interaction between adjacent left stepping riedel shears in cross sections faults bounding push up zones have an arcuate shape or merge at depth adjacent left stepping r shears merge by sideways propagation or link by short synthetic shears that strike subparallel to the bulk shear direction coalescence of en echelon r shears results in major through going faults zones master faults several parallel master faults develop due to the distributed nature of deformation spacing between master faults is related to the thickness of the brittle layers overlying the basal viscous layer master faults control to a large extent the subsequent fault pattern with increasing strain relatively short antithetic and synthetic faults develop mostly between old but still active master faults the orientation and evolution of the new faults indicate local modifications of the stress field in experiments lacking lateral borders closely spaced parallel antithetic faults cross faults define blocks that undergo clockwise rotation about a vertical axis with continuing deformation fault development and fault interaction at different stages of shear strain in our models show similarities with natural examples that have undergone distributed shear

experimental fracture strain and subsidence patterns over en echelon strike slip faults implications for the structural evolution of pull apart basins abstract claybox experiments show that with progressive left lateral displacement on basal left stepping en echelon strike slip faults an extremely complicated zone of evolving fracture strain and subsidence patterns develops in the overlying clay the fracture pattern is more complex than predicted by theoretical models and consists of a progressively widening array of riedel shears conjugate riedel shears and tension gashes above basal master faults conjugate riedel shears rotate anticlockwise and lengthen forming s shaped fractures riedel shears rotate both anticlockwise and clockwise those that rotate anticlockwise eventually open as tension gashes other tension gashes open where the ends of riedel and conjugate riedel shears link strain contours in the deformed clay show an asymmetrical z shaped pattern over the stepover area between basal en echelon faults in the early stages of displacement strain is distributed penetratively over a progressively broadening zone in later stages of displacement strain is partitioned into a narrow middle zone maximum strain is sustained always in the middle of the sheared zone and increases exponentially with progressive displacement subsidence is accommodated in the stepover area by oblique slip on many riedel and conjugate riedel shears distributed throughout the stepover area subsidence y exhibits a linear relationship with master fault displacement x described by y 0 36 x 1 4 the experimental fracture strain and subsidence patterns in clay over en echelon faults can be used as interpretive and predictive tools with which to constrain interpretations of static field examples especially in pull apart basins on the crustal scale where many similarities with the models exist

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

hf isotopes in detrital and inherited zircons of the pilbara craton provide no evidence for hadean continents predictions of large volumes of stabilized continental crust by the early archaean stand in stark contrast to the actual amount of pre 3 5 ga rocks presently exposed on earth s surface the pilbara craton of western australia one of the best preserved paleoarchean crustal blocks on earth is believed to have developed on a cryptic possibly 3 8 ga continental basement if substantiated this could support the notion of a widespread and enduring hadean ca 4 5 4 0 ga felsic intermediate crust to test this and to elucidate the earliest evolution of the pilbara craton we report hf isotope data from previously dated detrital zircon grains and inherited zircon crystals hosted by granitic gneisses the crystallization ages 3 80 3 55 ga of which substantially exceed those of the oldest exposed igneous rocks of the craton 3 52 ga the hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the pilbara craton being extracted from near chondritic mantle between 3 7 and 3 6 ga with little or no input from significantly older crust these new data suggest either that the pilbara craton developed remote from the isotopic influence of the putative eoarchean to hadean continental masses or that the stabilized volumes of the earliest continents have been overestimated the latter scenario would be consistent with the extreme scarcity of 3 9 ga rocks and minerals and the dominantly chondritic hf isotope composition of the oldest continental rocks in earth s most ancient archaean cratons

shear zone geometries in experimentally deformed clays the influence of water content strain rate and primary fabric in experimentally deformed clays three parameters which influence the geometry of shear zones are water content strain rate and the orientation of the shear zones with respect to any primary fabric the shear zones are the main microstructures induced in the clays within the experimental conditions triaxial compression at water contents between 20 and 35 w w and strain rates between 10 4 and 10 8 s 1 small changes in water content produce a significant change in the appearance of shear zones structures in wetter sediments are more complex and more numerous than those in drier sediments which tend to produce a small number of discrete planar shear zones strain rate is a far less important influence on shear zone geometry the orientation of the zones with respect to a primary fabric is also significant shear zones which are parallel to the fabric have a simpler geometry than zones which intersect the fabric at a high angle knowledge of these factors may help interpret the conditions within which shear zones formed in naturally deformed soft sediments

excess hafnium 176 in meteorites and the early earth zircon record 1 the long lived176lu to 176hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation detrital hadean zircons 3 8 gyr from the jack hills metasedimentary belt of western australia record extremely enriched hf isotope signals suggesting early extraction of a continental crust 4 5 gyr but fail to identify a prevalent complementary depleted mantle reservoir suggesting that crust formation processes in the early earth were fundamentally distinct from today however this conclusion assumes that the hf isotope composition of bulk chondrite meteorites can be used to estimate the composition of earth prior to its differentiation into major silicate reservoirs namely the bulk silicate earth bse we report a176lu 176hf internal mineral isochron age of 4869 34 myr for the pristine sah99555 angrite meteorite this age is 300 myr older than the age of the solar system confirming the existence of an energetic process yielding excess 176hf in affected early formed solar system objects through the production of the 176lu isomer t1 2 3 9 hours thus chondrite meteorites contain excess 176hf and their present day composition cannot be used to infer the lu hf parameters of bse using a revised bse estimate based on the sah99555 isochron we show that earth s oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as 4 3 gyr in earth s history with little evidence for the existence of continental crust prior to 4 4 gyr this new view suggests continuous juvenile crustal growth and recycling throughout the hadean and archean eras perhaps analogous to modern plate tectonics

metallic lead nanospheres discovered in ancient zircons zircon zrsio4 is the most commonly used geochronometer preserving age and geochemical information through a wide range of geological processes however zircon u pb geochronology can be affected by redistribution of radiogenic pb which is incompatible in the crystal structure this phenomenon is particularly common in zircon that has experienced ultra high temperature metamorphism where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages in some cases yielding apparent hadean 4 ga ages from younger zircons documenting the composition and mineralogy of these pb enriched domains is essential for understanding the processes of pb redistribution in zircon and its effects on geochronology using high resolution scanning transmission electron microscopy we show that pb rich domains previously identified in zircons from east antarctic granulites are 5 30 nm nanospheres of metallic pb they are randomly distributed with respect to zircon crystallinity and their association with a ti and al rich silica melt suggests that they represent melt inclusions generated during ultra high temperature metamorphism metallic pb is exceedingly rare in nature and previously has not been reported in association with high grade metamorphism formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic pb from zircon both the redistribution and phase separation of radiogenic pb in this manner can compromise the precision and accuracy of u pb ages obtained by high spatial resolution methods

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

the interpretation of complex zircon u pb systems in archaean granitoids and gneisses from the jack hills narryer gneiss terrane western australia abstract shrimp u pb ages of zircons from archaean ttg granitoids and gneisses from the jack hills region of western australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon u pb systems under metamorphic and magmatic conditions strong light rare earth element lree enrichment and similarities in the major and trace element chemistry suggest that ttg gneisses formed by fractional melting of a garnet bearing amphibolitic source maximum ages of 3 75 to 3 6 ga obtained from low u zircon cores are interpreted as the period of formation of the ttg parent rocks however most zircons in these populations are complex consisting of zoned and unzoned parts and have disturbed u pb systems which form a concordant to discordant array between 3 7 and 3 3 ga on a concordia plot suggesting these grains experienced one or more disturbance events within this time span the range of complexity of u pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of pb and possibly u has occurred within and from zircons causing variable isotopic discordance and yielding a range of apparent ages within a single grain disturbance of the zircon isotopic systems appears to be due to a combination of early archaean events together with recent pb loss the presence of 3 3 ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite together with the exclusive presence of 3 3 ga zircons in one sample of porphyritic granite is interpreted as evidence of a major magmatic and possibly metamorphic event at this time the presence of ca 3 5 ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization evidence for 3 3 ga magmatism appears to be restricted to zircons from ttg rocks south of the jack hills metasedimentary belt raising the possibility that the present position of the belt defines a major suture within the narryer gneiss terrain the post tectonic monzogranites have lree enriched patterns with strong negative eu anomalies similar to the older gneisses but in contrast to these rocks zircons from the monzogranites have relatively simple u pb systems which define an age of 2654 7 ma and only rarely do zircons have inherited cores with nearly concordant ages of ca 3 6 ga the very low content of inherited zircon raises doubts about the formation of the granites by reworking of older ttg gneisses and an alternative explanation that the monzogranites formed from new 2 65 ga crust with minor 3 6 ga crustal contamination is seen as more likely

sedimentation in pull apart basins active examples in eastern turkey the erzincan susehri niksar and lake hazar basins are actively evolving pull apart basins in eastern turkey their structural drainage and depositional systems generally support a pull apart basin sedimentation model based on ancient pull apart basin deposits this model involves marginal coarse grained sedimentation adjacent to master faults axial fine grained sedimentation away from master faults and high sedimentation rates however studies of the turkish pull apart basins suggest the following refinements in the model 1 detailed sedimentary facies patterns and the dominant facies of a pull apart basin vary according to basin shape climate drainage character and volcanic activity depositional environments near fault bounded margins include alluvial fan fan delta and alluvial apron depositional environments in central areas away from fault bounded margins include active braid plain inactive braid plain meander plain salt playa marsh lacustrine mouth bar delta and volcanic 2 i

quaternary faulting in queen valley california nevada implications for kinematics of fault slip transfer in the eastern california shear zone walker lane belt new geologic map tectonic geomorphologic and terrestrial cosmogenic nuclide tcn geochronologic data document the geometry style kinematics and slip rates on late quaternary faults within the queen valley california nevada area these data provide important insight into the kinematics of fault slip transfer from the dextral white mountains fault zone northward into the mina defl ection queen valley is an 16 kmlong ne trending basin bounded to the south by the white mountains and underlain by four major pleistocene to holocene alluvialfan surfaces four different fault types and orientations cut and offset all but the youngest surfaces 1 the normal slip queen valley fault which consists of a set of ne striking nw and se dipping normal fault scarps that cut across the se side of the valley and offset all but the youngest surfaces 2 discontinuous ne striking sinistral faults exposed on the north side of the valley 3 the nw striking dextral coyote springs fault which merges into 4 a set of e w striking thrust faults measured offsets across normal fault scarps developed within 10 be tcn dated surfaces yield minimum late pleistocene horizontal extension rates of 0 1 0 3 mm yr documented fault geometries and slip orientations across queen valley suggest that fault slip transfer models such as the extensional displacement transfer block rotation and simple shear models within the dextral fault system proposed for the eastern california shear zone walker lane belt are not applicable to this part of the mina defl ection rather dextral fault slip is transferred by both a restraining westward step and a releasing eastward step restraining and releasing bends have been extensively documented at a range of scales in strike slip fault tectonic settings globally and they have been simulated in analog models thus it is not surprising to document both within the 630 km long dextral shear zone that makes up the northern eastern california shear zone walker lane belt our results combined with published slip rates for the dextral white mountain fault zone and the eastern sinistral coaldale fault suggest that transfer of dextral slip into the mina defl ection is partitioned into three different components horizontal extension along the queen valley fault thrust faulting that merges into the dominantly dextral slip along the coyote springs fault and dominantly sinistral slip along the coaldale fault a velocity vector diagram of fault slip partitioning across queen valley predicts a small component of contraction across the coyote springs and western coaldale faults contraction across the mina defl ection is consistent with global positioning system data an observed reduction in late pleistocene fault slip rates at the northern end of the eastern california shear zone and across the southwestern part of the mina defl ection may be explained by distribution of slip across a much broader zone than generally thought

mechanics and mechanisms of magmatic underplating inferences from mafic veins in deep crustal mylonite dioritic to gabbro dioritic veins with extreme length to width ratios 1000 1 are localized along an amphibolite facies shear zone the cmb line between exposed segments of originally middle and lower continental crust strona ceneri and ivrea verbano zones northern italy the geometry of these veins and their mutual cross cutting relationships with the mylonitic foliation indicate that veining was coeval with noncoaxial flattening of the lower crust in early permian time the veins formed as closely spaced extensional shear fractures and propagated parallel to the originally gently to moderately dipping 30o mylonitic foliation vein opening at high angles 60 90o to the inferred1 direction and subparallel to the pre existing planar fabric requires that melt pressure slightly exceeded the lithostatic pressure and that differential stress was low 10 20 mpa in the vicinity of the veins the interaction of regions of tensile stress concentration at vein tips caused the concordant veins to curve and link up across the mylonitic foliation once interconnected the veins served as conduits for the rapid movement of mafic melt along the shear zone thermal modelling constrains the mafic melt in the narrowest 1 mm wide veins to have crystallized almost instantaneously such veins extend no more than a meter from host veins into the country rock indicating that the minimum rate of vein tip propagation and melt flow was at least several m s maximum crystallization times of only hundreds to thousands of years for even the thickest mafic veins 10 100 m in the ivz are short compared to the 15 20 ma duration of early permian crustal attenuation and magmatism in the southern alps this suggests that veining in the lower crust occurred episodically during extended periods of mylonitic creep concordant vein networks within deep crustal shear zones that are inclined as the cmb line may have been can also channel overpressurized mafic melt from deeper sources e g lower crustal magma chambers into cooler intermediate crustal rock this locally widens the depth interval of combined viscous and brittle deformation within the crust and can trigger partial melting of the country rock 1999 elsevier science b v all rights reserved

the oldest zircons of africa their u pb hf o isotope and trace element systematics and implications for hadean to archean crust mantle evolution abstract more than 450 detrital zircon grains from two limpopo belt quartzite samples were investigated by a combination of scanning electron imaging u pb dating 18 o lu hf isotope and trace element analyses in order to get robust information about the early earth s crust mantle evolution the detrital zircon grains have crystallization ages between 3 95 ga and 3 18 ga show hf t between 1 to 15 1 unit and 18 o vsmow mostly between 5 5 and 8 1 0 2 pristine zircon domains reveal ti in zircon temperatures between 700 and 865 c and th u of 0 3 2 3 trace elements point to zircon formation in predominately granitoid rocks metamorphic zircon rims have ages 2 65 ga hf 2 65ga 15 18 o 7 0 8 1 and th u mostly hf t age array i which starts from a chondritic uniform reservoir chur at about 4 5 ga and requires 176 lu 177 hf 0 020 indicative for mafic crust most zircon grains however plot on or above an hf t age array ii which runs parallel to array i 176 lu 177 hf 0 021 and starts from chur at 4 01 ga oxygen isotope compositions of 18 o 5 5 indicate that the magmatic host rocks of the zircons have been formed either by melting of altered mafic crust which interacted with cold water prior to granitoid formation and or that ancient sedimentary and or magmatic rocks were involved in the melting process the new u pb hf 18 o datasets together with compiled data from worldwide sources indicate a significant gap of about 5 epsilon units between arrays i and ii furthermore they illustrate that many hadean zircon analyses plot well below array i and some above chur these findings support an interpretation that the hadean earth was covered by a long lived mafic protocrust perhaps forming a partially open stagnant lid this protocrust was affected by internal reworking but also injected and overlain by ultra mafic rocks derived from chondritic and highly depleted mantle sources at

the transfer of slip between two en echelon strike slip faults a case study from the 1992 landers earthquake southern california detailed mapping of the fresh surficial ruptures in the right step between the homestead valley fault and the emerson fault two right lateral en echelon faults that broke in the 1992 landers earthquake indicates that the transfer of dextral slip between the faults is accommodated primarily by a series of obliquely striking right lateral strike slip cross faults right lateral slip on the cross faults and counterclockwise rotation of the interior blocks are sufficient to transfer virtually 100 of the dextral slip and to accommodate the extension across the jog asymmetry of the dextral slip curves along the cross faults indicates that some of them may have been induced to fail by slip on the homestead valley fault while others were induced to fail by slip on the emerson fault comparison of the magnitude of slip in 1992 to bedrock and geomorphic offsets suggests that the homestead valley fault and several secondary faults have experienced about 100 nominal landers events and that the stepover structure originated at the same time as the homestead valley fault about 1 m y ago because the emerson fault has an order of magnitude greater total offset but only slightly greater surficial slip in 1992 we conclude that it is a significantly older fault repetition of 1992 like events should eventually lead to a merging of the homestead valley fault with the emerson

origin of cariaco basin southern caribbean sea abstract the cariaco basin an anoxic basin in the central eastern venezuelan continental borderland is located in the area of overlap of two en echelon right lateral strike slip fault zones the moron and el pilar fault zones on the basis of 28 acoustic reflection profiles across it the basin is interpreted as a pull apart basin due to east west tension in the area of the right step between the two fault zones 125 km of overlap and 35 km separation the offset along the fault zones is less than 100 km and at least 25 km its age is most probably quaternary as suggested by the thickness of the sedimentary fill and the estimated rate of deposition and because strike slip displacement along the master faults probably began no earlier than the late tertiary the cariaco basin is the largest pull apart basin along the bocono moron el pilar fault system which is a part of the caribbean south america plate boundary

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

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

gradient analysis of faunal distributions associated with rapid transgression and low accommodation space in a late pennsylvanian marine embayment biofacies of the ames member glenshaw formation conemaugh group in the northern appalachian basin usa abstract the ames sea situated in a shallow marine embayment in the appalachian basin was the result of the most extensive marine transgression of the pennsylvanian in this region gradient analysis of the proportional abundance of taxa reveals an environmental continuum across the ames embayment along which four biofacies were distributed biofacies distribution is interpreted to be controlled by salinity turbidity and oxygen gradients associated with the geometry of the ames depositional basin and relative sea level change the eastern portion of the ames sea resembled a foredeep trough located on the detrital slope of the recently uplifted appalachian highlands biofacies 1 and 2 occur exclusively in the eastern part of the ames depositional basin and are dominated by eurytopic molluscs tolerant of terrigenous influx biofacies 3 occurs in both the eastern and western portions of the ames depositional basin this biofacies is dominated by the opportunistic brachiopod neochonetes which would quickly colonize the seafloor in the wake of transgression d biofacies 4 occurs exclusively in the western part of the ames depositional basin and is composed primarily of stenotopic crinoids bryozoans brachiopods and epifaunal bivalves that inhabited a clear water cratonic ramp on the eastern flank of the cincinnati arch vertical variations of faunal assemblages in the ames member were the result of variations in relative sea level associated with glacio eustatic cycles biofacies 3 dominated by neochonetes was deposited at the base of the ames member during the initial rapid transgression in a mostly dysoxic environment with low turbidity and marine salinity during the first regression low density surface water from eastern sources restricted vertical circulation established a stratified water column and promoted estuarine conditions in the ames embayment well oxygenated surface waters were unable to circulate to the benthic habitat which led to periodically dysoxic conditions and the establishment of biofacies 1 and 2 composed of diminutive eurytopic molluscs a second stratigraphically higher occurrence of biofacies 3 is interpreted to represent a second transgression however unlike the first transgression lithologic and faunal data suggest that the benthic habitat was well oxygenated above the mid ames marine zone occurrence of biofacies 3 biofacies 1 and 2 were deposited in the final regressive phase as local terrigenous source areas brought increasing turbidity and fluctuating salinity to the ames embayment previous paleoecological research on the ames member cited freshwater and or clastic influx as the major controls on biotic distribution but the results of the present study suggest that oxygen availability was also a major control stratification of the water column during the ames sea level cycle promoted estuarine circulation that prevented well oxygenated surface waters from reaching the benthic habitat this led to the deposition of dark gray to black fissile shales that contain fossil assemblages dominated by diminutive molluscs and brachiopods that were tolerant of low oxygen conditions

controls on clastic sequence geometries in a shallow marine transtensional basin the bohemian cretaceous basin czech republic the bohemian cretaceous basin combines features of a shallow water mostly 100 m epicontinental seaway formed during a global transgression with those of a tectonically active transtensional setting the basin formed under a greenhouse climate and was affected by strong axial currents dense well log coverage combined with locally high quality exposures and biostratigraphic control make it possible to examine in three dimensions the geometries of genetic sequences and interpret their controlling variables sand dominated deltas formed sequences at several spatial scales that reflect nested transgressive regressive cycles with durations ranging from tens of thousands of years to millions of years progradation directions and distances thicknesses and internal geometry of the individual sequences were controlled primarily by intrabasinal faulting basin scale changes in subsidence rate eustatic fluctuations and localized bathymetric changes due to successive filling of the basin along strike change in sediment input from different parts of the source area and a short lived uplift of a secondary clastic source provided additional controls on the sequence geometry efficient hypopycnal transport combined with redeposition of fine clastics in shallow water promoted development of steep slopes of sand dominated deltas while preventing downlap of muddy clinoforms most of the suspended load became deposited downcurrent in subhorizontal or gently dipping bottomsets long term accommodation rates were low during the early to middle turonian with minor intrabasinal faulting but became accelerated in the late turonian and early coniacian this acceleration was caused at least partly by increased subsidence rate accompanied by structural partitioning of the depocentre and partly compensated by increased sediment input indicating increased uplift rates in the western sudetic island source area this event probably reflected an increase in the regional strain rate in central europe the succession of two major flooding events in the early turonian and late early coniacian separated by a low accommodation interval in the middle turonian shows a close similarity to published estimates of long term eustatic curves however the eustatic component of accommodation rate in the bohemian late turonian and coniacian is difficult to separate from accelerated subsidence in several cases evidence for short term 100 kyr scale forced regressions independent of basinal structural activity suggests small scale eustatic falls at rates which as presently understood cannot be explained other than by a glacio eustatic mechanism

structures in natural serpentinite gouges abstract serpentinite gouges from cataclastic fault zones in the voltri massif nw italy are dominated by few discrete boundary parallel y shears and oblique synthetic riedel r 1 shears separating domains of massive gouge unlike natural clay rich fault gouges the cohesive microbreccia domains only rarely show foliations these structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge the dominant gouge forming mechanisms were fracturing of olivine and pyroxene grains along serpentinized grain boundaries and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation syn tectonic serpentinization added to the weakening of fragments textural similarities with experimentally formed gouges suggest that the voltri fault zones were dominated by stick slip sliding hence that they may represent fossil seismogenic faults a notable aspect of the gouge texture in the voltri fault zones is a sigmoidal shape of the riedel shears which bend into parallelism with the y shears it is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on y or p shears the average spacing between the r 1 shears increases linearly with 0 47 times the distance between bounding y shears this proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes

the central european graben system rifting imitated by clay modelling abstract central europe is traversed by an active graben belt between the north sea and the mediterranean along with a lateral rift rift offset between some segments along these offsets connecting the individual segments specific patterns have been observed which may be physiographically compared with transform faults a series of clay experiments were carried out to understand rhinegraben type graben formation especially the transform like features were modelled to learn more about the kinematics of transform faulting in the continental domain it has been attempted to approach natural structural and lithological boundary conditions of graben formation by varying the experimental conditions in such a manner that experimental and in situ conditions resemble in an optimal way the experiments are related to the paleogene extensional stage of the evolution of the rift system and do not consider the pliocene to recent kinematics of shear rifting

the antrim galway line a resolution of the highland border fault enigma of the caledonides of britain and ireland the westward continuation of the highland border fault of scotland hbfz into ireland is problematic it is widely thought to follow a pronounced magnetic and gravity lineament the fair head clew bay line fcl the advantage of this interpretation is that it places all the ordovician ophiolitic complexes and associated sedimentary basins to the south of the fcl which would represent the contact between laurentia and the outboard terranes its main shortcomings are that both the deep structure and timing of strike slip are different on the hbfz and fcl in ireland the fcl is a north dipping feature that can be traced to the moho on birps profiles while the hbfz has no such signature terrane amalgamation in western ireland was completed by the late ordovician while in scotland the midland valley terrane did not finally dock until the early devonian these considerations suggest that in western ireland a branch of the hbfz exists which was active in post ordovician time and must lie south of connemara an examination of irish geological geophysical and image processed magnetic data shows that a profound lineament can be traced from antrim to galway bay the antrim galway line stitching plutons date movement on it as pre 405 ma we propose that the antrim galway line represents the continuation of the scottish hbfz while the fcl is a preserved ordovician splay of the hbfz system whose northdipping geometry is a product of ordovician collapse of the orogen in western ireland

cenozoic evolution of the tan lu fault zone east china constraints from seismic data abstract the tan lu fault zone tlfz is a continental scale strike slip fault zone in the east china that contains rich and significant information about the mesozoic cenozoic evolution of eastern asia it experienced a long term and complex superimposed deformation characterized by mesozoic sinistral and cenozoic dextral motions the fault has a well documented mesozoic sinistral displacement history but its cenozoic history is poorly known in this study based on the observations of the interrelationships of fault kinematics fault geometry and concomitant sedimentation the cenozoic evolution of the tlfz is constrained by abundant seismic data from oil fields numerous bends along the cenozoic master fault sensitively record changes in the stress field during strike slip motion taking into account the cenozoic evolution of the bohai bay basin east china the evolutionary history of these strike slip bends is investigated and the results indicate that dextral motion along the tlfz began at about 40 ma furthermore the cenozoic evolutionary history of the tlfz is suggested to include an extensional deformation with a possible weak sinistral shear from 65 to 40 ma from 40 to 0 ma dextral motion occurred due to a change in the subduction direction of the pacific plate relative to the eurasian plate causing the fault to experience dextral shear with a component of extensional deformation dextral motion was most intense from 40 to 25 ma and then the dextral direction may have suddenly changed from a nearly ne sw trend to a nearly n s trend at around 25 ma dextral motion was relatively enhanced again after 12 ma and was particularly strong after 5 ma episodic and ephemeral thrusting deformation might have occurred during the quaternary the cumulative cenozoic dextral displacement of the tlfz is far smaller than its mesozoic sinistral strike slip displacement may be no more than 21 km

initiation and development of pull apart basins with riedel shear mechanism insights from scaled clay experiments typical pull apart structures were created in scaled clay experiments with a pure strike slip geometry riedel type experiments a clay slab represents the sedimentary cover above a strike slip fault in the rigid basement at an early stage of the development of the deformation zone synthetic shear fractures riedel shears within the clay slab display dilatational behaviour with increasing basal displacement the riedel shears rotate and open further developing into long narrow and deep troughs the shear displacement and the low angle with the prescribed principal basal fault set them apart from tension gashes at a more evolved stage synthetic segments y shears parallel to the basal principal fault develop and accommodate progressive strike slip deformation the y shears connect the tips of adjacent troughs developed from the earlier riedel shears resulting in the typical rhomb shaped structures characteristic for pull apart basins the strait of sicily rift zone with major strike slip systems being active from the miocene to the present comprises pull apart basins at different length scales for which the structural record suggests development by a mechanism similar to that observed in our experiments

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

three dimensional structural architecture and evolution of the eocene pull apart basin central maracaibo basin venezuela abstract three dimensional seismic mapping of interpretative sub surface time slice sections incorporating age data and detailed structural observations places constraints on the structural architecture and stratigraphic evolution of the icotea pull apart basin in the central maracaibo basin venezuela all four fault bounded sides of the icotea basin its 3 km thick eocene basin fill of clastic sediments and faults pre dating the opening of the basin can be mapped in three dimensions the development of the icotea basin provides an excellent observational basis for understanding the structural history of three sets of regionally extensive faults and testing two models for the formation of pull apart basins in general three main fault trends produced at different periods in the formation of the cretaceous recent maracaibo basin are mapped using 3d seismic data in the central part of the basin 1 n ne striking normal faults including the icotea fault these faults formed originally during late jurassic early cretaceous rifting and reactivated as left lateral strike slip faults during late paleocene eocene convergent deformation between the caribbean and south american plates 2 nw striking faults formed by late paleocene eocene overthrusting of the caribbean plate and downward flexure of the south american plate to create a major foreland basin depocenter and 3 ne striking normal faults of pre cretaceous age were reactivated during late paleocene eocene plate convergence three dimensional architecture of the icotea basin interpreted from five time slices through the basin and its flank areas supports the simple pull apart model for the eocene opening of the icotea basin the amount of extension across the suite of normal faults ranges between 0 8 and 2 25 km this range of offset is consistent with some previous estimates of minor left lateral displacement along the icotea fault but is inconsistent with either low angle or high angle thrusting during eocene time as inferred from previous interpretations of widely spaced two dimensional seismic lines the normal faults that formed the pull apart basin reactivated pre existing faults due to plate flexure

chondritic lu hf in the early crust mantle system as recorded by zircon populations from the oldest eoarchean rocks of yilgarn craton west australia and enderby land antarctica abstract a persistent question in early earth investigations is the depletion history of the mantle as recorded in the isotopic compositions of the oldest rocks and minerals here we present new in situ sensitive high resolution ion microprobe shrimp u th pb geochronology and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms lu hf tracing data for individual zircons extracted from the oldest rocks of the antarctic and australian continents these are 3 88 and 3 85 ga orthogneisses from the gage ridge and mt sones in enderby land and a 3 73 ga meeberrie gneiss from the yilgarn craton the zircons from each sample show complex u th pb systematics due to the mobile behaviour of pb resulting in several age populations with discordant to reversely discordant arrays on concordia plots in contrast measured 176hf 177hf compositions from the same analysis sites show narrowly defined compositional ranges with 3 or fewer groups for each sample reflecting the resistance of 176hf 177hf in zircons to disturbance even under high temperature granulite conditions the initial 176hf 177hf calculated for the most primitive hf population for each sample using the best age estimate yields near chondritic weighted mean compositions of 1 6 0 5 1 6 0 4 and 0 6 0 7 epsilon units deviations in parts per 10 000 from a chondritic uniform reservoir chur reference at 3877 62 3850 50 and 3731 4 ma respectively these results provide no evidence for significant lu hf fractionation on the early earth expected to result from processes including accretion intra mantle differentiation or massive hadean crustal growth

fault zone structure from topography signatures of en echelon fault slip at mustang ridge on the san andreas fault monterey county california 1 we used high resolution topography to quantify the spatial distribution of scarps linear valleys topographic sinks and oversteepened stream channels formed along an extensional step over on the san andreas fault saf at mustang ridge california this location provides detail of both creeping fault landform development and complex fault zone kinematics here the saf creeps 10 14 mm yr slower than at locations 20 km along the fault in either direction this spatial change in creep rate is coincident with a series of en echelon oblique normal faults that strike obliquely to the saf and may accommodate the missing deformation this study presents a suite of analyses that are helpful for proper mapping of faults in locations where high resolution topographic data are available furthermore our analyses indicate that two large subsidiary faults near the center of the step over zone appear to carry significant distributed deformation based on their large apparent vertical offsets the presence of associated sag ponds and fluvial knickpoints and the observation that they are rotating a segment of the main saf several subsidiary faults in the southeastern portion of mustang ridge are likely less active they have few associated sag ponds and have older scarp morphologic ages and subdued channel knickpoints several faults in the northwestern part of mustang ridge though relatively small are likely also actively accommodating active fault slip based on their young morphologic ages and the presence of associated sag ponds

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

extensional duplexing in the york cliffs strike slip fault system southern coastal maine abstract a 150 m long sinistral strike slip fault system consisting of left stepping en echelon fault segments cuts deformed paleozoic metasediments and early mesozoic dike intrusions in coastal terrace outcrops in southern maine the longer fault segments follow the vertical bedding in the metasediments and accommodate approximately 1 m of displacement detailed mapping at scales down to 1 6 indicates that overlap zones between offset layer parallel fault segments consist of high dilation angular breccias rounded attrition breccias and extensional duplexes where the offset segments straddle upright quartzite layers in one 10 m long duplex r and r shears cut the layering in the quartzite at 28 and 69 respectively relative to the dominant layer parallel fault segments creating a doubly tapered imbricate r shear lens segmentation structure individual imbricate lenses accommodate additional layer parallel extension through conjugate x x shear fractures with average orientations of 109 and 60 respectively relative to the dominant fault segments extensional fractures oblique to the dominant fault segments are rare within this duplex but commonly form distinctive horsetail arrays at fault terminations that average 44 to the layer parallel faults strain analysis shows that approximately 60 80 cm of measured strike slip displacement is accommodated internally within this duplex as layer parallel extensional strain using the r shear and x x shear fracture mechanisms the strain distribution and observed cross cutting relations suggest an outward symmetric growth of the duplex structure from left stepping sinistral fault segments that developed sequences of tip to plane r shear overlap linkages during continued displacements cyclic r r to x x shear fracturing may relate to stress reorientation following r shear ruptures during episodic growth of the duplex and incremental slip of the fault system

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

identifying hierarchical spatial patterns within paleocommunities an example from the upper pennsylvanian ames limestone of the appalachian basin abstract identifying ecological mechanisms that produce hierarchically arrayed spatial variation in community structure can be difficult in the fossil record due to conflation of spatial and temporal patterns however this difficulty can be mediated by minimizing the temporal duration of deposition within the unit examined in this study the fauna of the upper pennsylvanian ames limestone conemaugh group was analyzed to explore whether ames paleocommunities exhibited hierarchical structure in a spatial dimension this widespread carbonate unit was deposited during the maximum flooding interval of a glacio eustatically influenced fifth order sea level cycle and preserved taxa are contemporaneous within only a few thousand years paleocommunity structure and variability was assessed at multiple spatial scales using samples collected from seven outcrops of the ames limestone throughout southeastern ohio which form a northeast to southwest trending transect parallel to the paleoshoreline abundance data were collected using quadrat sampling for brachiopods bivalves gastropods bryozoa corals crinoids echinoids trilobites and foraminifera paleocommunity structure was analyzed via cluster ordination guild and abundance analyses at multiple spatial scales within a single locality among localities and within the total study area to provide insight on geographic partitioning of paleocommunity variation multiple levels of paleocommunity organization were recovered within the ames fauna all levels exhibited spatial partitioning but the inferred proximate controls shifted from abiotic environmental controls at higher hierarchical levels to biotic controls at the lowest level at the highest level differentiation into a northern and southern regional paleocommunity was controlled primarily by substrate consistency and habitat heterogeneity related to variation in fluvial input within the basin local paleocommunity differentiation reflects biotic responses to topographic and environmental conditions that were geographically distributed within the region whereas within outcrop variation was due largely to biotic feedback mechanisms

lu hf systematics of the ultra high temperature napier metamorphic complex in antarctica evidence for the early archean differentiation of earth s mantle abstract the napier complex of the east antarctic craton comprises some of the oldest rocks on earth 3 8 billion years old overprinted by an ultra high temperature uht metamorphic event near the archean proterozoic boundary garnet orthopyroxene sapphirine osumilite rutile and a whole rock representing a fully equilibrated assemblage from this uht granulite belt have yielded a lu hf isochron age of 2403 43 ma the first ever determined on a uht mineral assemblage preservation of the uht mineral assemblage in the rock analyzed without any significant retrogression suggests rapid cooling with closure likely to have occurred for the lu hf system at post peak uht conditions near a temperature of 800 c this mineral whole rock isochron yields an initial 176 hf 177 hf ratio corresponding to an e hf value of 14 1 acquired during uht metamorphism such a low value demonstrates that overall uht granulites evolved in a low lu hf environment probably formed when the rocks were first extracted from a highly depleted mantle zircon e hf values we have measured see through the uht metamorphism and show that the source materials for the magmas that formed the napier complex were extremely depleted 5 6 e hf at 3 85 ga relative to the chondritic uniform reservoir chur these results also suggest significant depletion of the early archean mantle in agreement with the early differentiation of the earth that the latest core formation models require

low seasonality in central equatorial pangea during a late carboniferous highstand based on high resolution isotopic records of brachiopod shells the relationship between permian carboniferous glacial cycles and low latitude climate remains a subject of vigorous debate this study investigated seasonality and regional environmental variability in a portion of central equatorial pangea during a late pennsylvanian highstand using stable isotope and trace element analyses of brachiopod shells from the virgilian ames member of the conemaugh group in the appalachian basin u s a well preserved thick shelled neospirifer dunbari specimens were serially sampled across growth bands to elucidate a record of seasonal variability during the life of the organisms because neospirifer only colonized this marginal basin during near maximum highstands when stable near marine salinities were established these data are a direct proxy for intra annual climate fluctuations during interglacial times neospirifer specimens show remarkably little internal chemical variability with 18 o generally fluctuating by 0 4 or less and 13 c fluctuating by less than 1 5 within a single specimen moreover total 18 o variability between all specimens is only 1 5 this lack of variation reflects a homogeneous nonseasonal to weakly seasonal climate during the ames highstand both 18 o and 13 c are 1 5 lower than those of other virgilian specimens from regions with a more proximal connection to the open ocean suggesting at least moderate freshwater influence in the appalachian basin during this time although salinities remained close enough to marine levels to maintain stenohaline fauna thus brachiopod seasonal records indicate normal moist tropical conditions during this penultimate carboniferous interglacial with no evidence for strong monsoonal variations in temperature or rainfall

structural variety and tectonic evolution of strike slip basins related to the philippine fault system northern luzon philippines the northern part of the philippine fault zone in luzon corresponds to a complex braided system of left lateral strike slip faults the nw oriented main active branch which emerges from the philippine sea splits into an array of north striking splays responsible for the tectonic evolution of the central cordillera this complex fault pattern has favored local stress field variations strike slip basins have evolved in this framework along or in the vicinity of the main splays of the philippine fault apart from the classical pull apart tectonics on a releasing fault termination overlap or bend we describe other mechanisms such as the strike slip tilting or the warping of a strip limited by two strike slip faults the strike slip basins are good recorders of the evolution of the philippine fault system those located along the north striking cordilleran faults individualized in late early pliocene to pleistocene time when the present day philippine fault initiated but their main control is the fault shape acquired in upper middle miocene time the recent tectonic evolution of the fault system is best recorded in central luzon where the active basins trace an asymmetrical v shape with the longest branch trending ne parallel to the east luzon trough and the shortest one trending nw related to the philippine fault both the fault pattern and the basin distribution demonstrate the influence of the benham rise in the tectonic evolution of luzon the structural setting is interpreted as the result of an early miocene collision event between the benham rise and the eastern margin of luzon and subsequent inception of the nw striking strand of the philippine fault the present locations of the basins result from the interaction between the structural heritage and the present day regional plate motion