geochronology of early to middle paleozoic tectonic development in the southwest newfoundland gander zone southwest newfoundland s gander zone was once considered part of the collided eastern margin of an early paleozoic iapetus ocean distinguished from the rest of the gander zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered devonian in age overlying presumed precambrian basement it was later found that truly devonian strata were confined to the cape ray fault system the local northwestern boundary of the zone u pb zircon isotopic ages of 452 13 51 ma and 458 22 29 ma on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that ordovician and possibly older rocks underlie the gander zone itself a temporal framework for the three main stages of regional deformation corresponding to pre metamorphic recumbent folding d 1 and transpression d 2 and d 3 is presented using u pb and 40 ar 39 ar dates of granitoid and metamorphic rocks d 1 is bracketed as late o

strain trajectories above the main central thrust himalaya in southern tibet as a result of the collision of india with asia1 3 a thick pile of precambrian crystalline rocks the main central sheet was transported southwards over the indian continent a marked linear fabric a mineral stretching lineation within the main central sheet varies only slightly from a north south trend which is interpreted as paralleling the direction of transport of the nappe3 6 our study sought to extend the previous observations to the hitherto poorly known part of the main central sheet in southern tibet china the stretching lineation is dominantly north south in the lower part of the sheet but exhibits continuous and discontinuous variations from north south to east west in the upper part7 these variations of lineation trajectory indicate two shear components one subparallel to and one at a high angle with the himalayan belt the recognition of westward wrench shearing in the main central sheet implies a component of relative displacement parallel to the himalayan belt and not only a radial one as generally assumed in previous models this suggests that the kinematic story of the intracontinental deformation following the continental collision is probably not one of simple north south convergence

deformation resulting from regional extension during pluton ascent and emplacement central sierra nevada california solid state foliation lineation small scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the mt givens pluton mgp mylonite is common throughout much of the 1 2 km wide 10 km long shear zone with ultramylonite best developed near the contact with the mgp which itself lacks significant solid state deformation migmatization accompanies ultramylonite formation in the northern half of the zone but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear pure shear towards the mgp microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of 3 5 kb and 680 c respectively during shear zone formation at least at its late stages of development zircon pb u and 40 ar 39 ar ages constrain timing of the high temperature movement on the shear zone to 90 ma essentially the age of the mgp although movement immediately prior to that time appears likely we speculate that a regional extensional shear zone was developing prior to the emplacement of the mgp which as it ascended heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone the mgp was near its critical melt fraction during the last several kilometers of its ascent and could have possessed sufficient viscosity strength to impose a weak shear strain on the shear zone rocks although most of the foliation and extensional features in the zone are probably related to the regional strain field late stage folding of the foliation is attributed to shouldering aside of the wallrock by the mgp during the last increment of its ascent and final emplacement

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

some practical guidance for petroleum migration modeling abstract petroleum migration modeling is widely used to assess petroleum charge and fluid phase risk in exploration and appraisal there are three principal migration methods darcy flow invasion percolation and flowpath or ray tracing each method relies on different assumptions and yields different outcomes this paper analyzes the strengths and weaknesses of these methods as they are implemented in petromod providing a practical guide as to how and when to apply each migration method flowpath is the most simplistic and computationally fastest method and should only be applied when a quick screening is required invasion percolation delivers satisfactory results for most geological settings from structurally and stratigraphically simple to complex but does not consider expulsion from the source rock properly darcy flow is the preferred method for unconventional resource assessments and supports maximum downward migration a traditional modeling approach is to use two methods applying darcy flow to tight rocks and flowpath to highly permeable rocks another combined method can be used in such a way that darcy flow is applied to the source rock and invasion percolation to the carrier and reservoir beds the latter seems to deliver best results in most geological settings while also providing more realistic results of secondary migration losses

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

structural evolution of a major fault zone the cape ray fault zone southwestern newfoundland canada the cape ray fault zone is a major paleozoic structure in southwestern newfoundland and occurs at or close to the boundary between two major continental blocks laurentia and avalonia a detailed structural analysis demonstrates that the fault records early reverse sinistral thrusting of the grand bay complex at amphibolite grade d2 followed by a protracted event d3 characterized by reverse dextral thrusting of the grand bay complex rocks on top of the supracrustal rocks of the windsor point group and retrogression to greenschist facies as well as a pre 384 ma orogen parallel dextral transcurrent mylonite d4 during the later stages of the collision regional scale strain partitioning induced heterogeneity of strain both along and across the strike of the cape ray fault zone the east west oriented segment of the cape ray fault zone is a tear fault that accommodated differential displacement along the length of the fault later stages of the deformation include post 384 ma sinistral transcurrent r

tectonic history of the illinois basin the illinois basin began as a failed rift that developed during breakup of a supercontinent approximately 550 ma a rift basin in the southernmost part of the present illinois basin subsided rapidly and filled with about 3 000 m of probable early and middle cambrian sediments by the late cambrian the rift bounding faults became inactive and a broad relatively slowly subsiding embayment extending well beyond the rift and open to the iapetus ocean persisted through most of the paleozoic era widespread deformation swept through the proto illinois basin beginning in the latest mississippian continuing to the end of the paleozoic era uplift of basement fault blocks resulted in the formation of many major folds and faults the timing of deformation and location of these structures in the forelands of the ouachita and alleghanian orogenic belts suggest that much of the deformation resulted from continental collision between north america and gondwana the associated compressional stress reactivated the ancient rift bounding faults upthrusting the northern edge of a crustal block approximately 1 000 m within the rift concurrently dikes radiometrically dated as early permian sills and explosion breccias formed in or adjacent to the reactivated rift subsequent extensional stress probably associated with breakup of pangea causedmore the crustal block within the rift to sink back to near its original position high angle northeast to east west trending normal faults with as much as 1 000 m of displacement formed in the southern part of the basin these faults displace some of the northwest trending early permian dikes structural closure of the southern end of the illinois basin was caused by uplift of the pascola arch sometime between the late pennsylvanian and late cretaceous less

an ancient rift complex and its relation to contemporary seismicity in the new madrid seismic zone gravity magnetic geologic and seismicity data have been combined in a seismotectonic analysis of the new madrid seismic zone previous studies have presented evidence for several rift zones in this area upper mississippi embayment including the reelfoot rift a late precambrian early paleozoic failed arm which extends n ne from the ancient continental margin in this study we propose that the northern terminus of the reelfoot rift forms a rift complex which appears to correlate well with the seismicity in the area this correlation suggests that faults associated with this rift complex are being reactivated in the contemporary stress field ene compression if this interpretation is valid it represents a seismotectonic model which can be used to predict the extent of future seismicity in the new madrid seismic zone the proposed rift complex also provides a coherent model for the tectonic development of this region of the north american midcontinent

model for the tectonic evolution of the mississippi embayment and its contemporary seismicity aeromagnetic and gravity surveys of the northern mississippi embayment have delineated a northeast trending basement depression about 70 km wide and more than 300 km long the feature the mississippi valley graben is remarkably linear over its known length has nearly parallel sides and has estimated depths relative to surrounding basement of more than 2 km both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben estimated magnetization directions for the plutons indicate a mesozoic age the graben of probable earliest paleozoic age underlies the northwest side of the embayment and contains the area of major seismicity in particular much of the instrumentally located contemporary seismicity and the estimated locations of the large magnitude 1811 1812 earthquakes are in the central part of the graben we interpret the early paleozoic graben the mesozoic plutonism and the sedimentation cycles as evidence of stretching events seismicity is attributed to strain in and below the central graben area which would appear to be a focal point of deformation in the stretching model we suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in pre cambrian time the postulated rotating stress field is presumably controlled by the system that drives the global plates

episodic reactivation of a late precambrian mylonite zone on the gondwanan margin of the appalachians southern newfoundland the grand bruit fault zone of southern newfoundland is a fundamental structure within late precambrian basement on the gondwanan margin of the appalachian orogen within the fault zone a sequence of structures documents changes in the sense of ductile displacement from 1 reverse dip slip to 2 dextral strike slip to 3 sinistral oblique slip and finally to 4 dextral lateral offsets fault movements along this structure were punctuated by emplacement of a variety of plutons and minor intrusions which when precisely dated allow these movements to be bracketed at between 571 ma and 564 ma 497 ma and 427 ma 424 ma and 420 ma and 421 ma and 387 ma respectively the tectonic evolution of the gondwanan inlier of southern newfoundland is mirrored in large part by the record of mylonite development within the grand bruit fault zone these tectonic events are attributable to well constrained regional orogenic events of both the pan african and appalachian cycles newly formed shear zones in the fault zone reactivate parts of much older faults of similar regional orientation and are in some cases kinematically indistinguishable from the ancestral structures integration of precise geochronological data with the sequence of overprinted fault structures demonstrates that although the role of progressive deformation in shear zone development was important the observed disposition of structures and rock units is primarily a function of polyorogenic accretion as a multiple reactivated structural lineament in a gondwanan basement inlier the fault zone exerted fundamental control over the tectonic development of the leading edge of the convergent southeast margin of the orogen

internal structure of the cadillac tectonic zone southeast of val d or abitibi greenstone belt quebec the kirkland lake larder lake cadillac break is a major fault zone of the abitibi greenstone belt well known for its spatially associated gold camps a detailed structural analysis of this fault zone in the val d or area has shown the presence of a 200 750 m wide zone of high strain defined here as the cadillac tectonic zone ctz which includes the narrower schist zone generally regarded as the cadillac break the boundaries of the ctz which coincide with major lithologic contacts strike approximately east west and dip 80 to the north the internal structure of the ctz is best analysed in terms of two increments of the same progressive deformation the d1 strain increment is characterized by an east west striking subvertical s1 foliation at low angle in strike to the bound aries of the ctz and containing subvertical elongation lineation primary lithological contacts have been transposed into s1 and folded by f1a folds f1b intrafolial folds of s1 ranging from noncylindrical subhorizontal f

progressive deformation structures associated with ductile thrusts in the moine nappe sutherland n scotland in the deeper parts of mountain belts polyphase structural sequences observed at outcrop scale can arise due either to overprinting of regionally distinct deformation phases e g reworking changes in orogenic boundary conditions or to localized controls that bring about transient changes in the patterns of ductile flow these are unlikely to be mutually exclusive processes and examples from scotland demonstrate that once regionally separate events are delimited using radiometric evidence it is possible to isolate complex deformation sequences arising due to local controls in the western moine nappe of sutherland the dominant structures were formed during caledonian ductile thrusting towards the wnw whilst earlier precambrian phases are relatively minor in importance two groups of caledonian folds and fabrics are recognized in many exposures main phase d2 structures which are broadly contemporaneous with ductile thrust fabrics and later secondary phase f3 folds the latter can be divided into two geometric groups sheath fold types which formed initially as wnw overturned buckles subsequently modified by ductile shearing and asymmetric types which are commonly open folds apparently formed with axes close or sub parallel to the thrust transport direction secondary structures show a close spatial association with high strain zones along ductile thrusts and can be shown to have formed during the later stages of thrusting in certain critical exposures i propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites where differential shearing occurs due to flow normal perturbations wrench related asymmetric fold types may form in contrast secondary sheath fold structures may result from localized compression phases caused by flow parallel perturbations local flow perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and or there is a strong geometric and kinematic similarity between sets of folds and fabrics if such features are not observed a regional polyphase sequence is more likely and it is therefore necessary to obtain radiometric dates or some other independent source of data to demonstrate a time separation

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

acadian dextral transpression and synorogenic sedimentary successions in the appalachians the successive seboomook littleton northern appalachians and catskill pocono central appalachians clastic wedges suggest oblique convergence and southwestward migration of acadian orogeny beginning in early devonian and continuing into early mississippian wrench fault movement in maritime canada coincided with deposition of all but the earliest components of the catskill pocono clastic wedge and continued into the pennsylvanian contrasts between a thin lower to middle devonian shallow shelf facies in the alabama appalachian fold thrust belt and a time equivalent thick shallowing upward sedimentary to volcanic succession in the adjacent talladega slate belt are interpreted to reflect a wrench fault basin a wrench fault setting for devonian rocks in alabama integrated with manifestations of oblique convergence during the acadian orogeny in the central and northern appalachians can be accommodated in dextral transpression along the entire length of the acadian appalachian orogen

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

origin of kinkbands and shear band cleavage in shear zones an experimental study specimens of artificial kcl mica schist have been deformed in simple shear with different initial orientations 10 intervals between 0 and 170 of the mica schistosity s1 relative to the imposed simple shear plane for most experiments the angular shear strain was 30 the strain rate 4 8 10 4s 1 and the stress normal to the imposed shear plane 20 mpa all experiments were conducted at room temperature deformation of the specimens is achieved largely by slip parallel to the s1 mica foliation however two new types of slip surfaces also develop as either a single orientation set or as a pair of conjugate sets these new slip surfaces are kinkbands and a shear band cleavage the type and number of sets of new slip surfaces which develop depends on the orientation of s1 relative to the shortening and extension fields of the instantaneous strain ellipsoid our experiments indicate that kinkbands or shear band cleavage form in shear zones in which s1 is inclined to the margin of the zone which structure forms and whether it occurs as a single set of structures or as a conjugate pair depends on the initial orientation of s1 in situations where s1 is parallel to the margins of the zone neither structure develops if the deformation is a simple shear the presence of kinkbands or shear band cleavage in such zones where s1 is parallel to the margins is an indication that the zones were transtensional or transpressional respectively we conclude that shear band cleavage is not only an excellent indicator of shear sense but also a qualitative indicator of the ratio of s1 normal shortening to s1 parallel shear conjugate shear bands indicate that s1 normal shortening was relatively strong and single shear band sets indicate that s1 parallel shear was relatively strong

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

geological interpretation of a high resolution reflection seismic survey at the buchans mine newfoundland sixteen kilometres of high resolution vibroseis reflection seismic data have been acquired in the vicinity of the former buchans mine direct identification of the cause of several reflectors is possible because the geology is tightly constrained by underground workings and drill holes both of which locally exceed 1 km depth many of the mine scale thrust faults are imaged as reflectors but conformable and intrusive contacts generally responded poorly a significant shallow dipping thrust the powerline fault is recognized below the orebodies and traced throughout the buchans area primarily as a result of the seismic survey it truncates ore stratigraphy and forms the floor thrust of a large duplex stack which hosts all the orebodies its presence has negative implications for exploration in the immediate mine area several lines of evidence suggest that this fault has a significant component of out of sequence movement a strong reflector 4 5 km below buchans is correlated with the surface expression

the relocation of microearthquakes in the northern mississippi embayment three component seismograms recorded by a small array of digital instruments in the northern mississippi embayment consistently show a high amplitude phase on the vertical component that arrives approximately 0 8 s before the shear wave on the basis of its timing and apparent velocity this phase is identified as an s p conversion from the boundary between the unconsolidated cenozoic sediments and the underlying paleozoic rocks synthetic models of ground motion in the mississippi embayment indicate that vertical displacement amplitudes are higher for s p conversions than for transmitted shear waves at all angles of incidence the models and their agreement with observations of the three component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area the travel times of converted phases are used together with crustal velocity models derived from a recent seismic refraction survey to relocate approximately 500 microearthquakes recorded by the central mississippi valley regional seismic network since the network data are recorded by vertical component seismographs we assume that the s phases observed at the stations sited on unconsolidated sediments are s p conversions this assumption significantly reduces the average rms residual and provides well constrained hypocentral locations for 350 earthquakes the relocated microearthquakes cluster tightly along previously identified epicentral trends focal depths range from 0 5 to 22 km but events deeper than 14 km are rare and events shallower than 3 km are confined almost exclusively to the area between ridgely tennessee and new madrid missouri focal mechanisms are generally consistent with the results of previous studies the axial seismicity trend which extends from caruthersville missouri to marked tree arkansas is characterized by right lateral strike slip motion on a northeast trending nearly vertical fault plane the events between ridgely tennessee and new madrid missouri commonly show strike slip focal mechanisms with minor components of both normal and reverse dip slip

structure of the new madrid seismic source zone in southeastern missouri and northeastern arkansas new seismic reflection data in the southwestern part of the new madrid seismic zone nmsz show that a major zone of disrupted reflectors and a large antiform coincide with a prominent trend of earthquake epicenters between caruthersville missouri and marked tree arkansas we speculate that the antiform may be caused by igneous intrusions the strong correlation between the seismicity in this trend and the extent of the disrupted zone provides a geologic basis for defining the source zone for large earthquakes in this part of the nmsz

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

a new lower ordovician arenig faunule from the coy pond complex central newfoundland and a refined understanding of the closure of the iapetus ocean a newly discovered fossiliferous horizon within sediments belonging to the coy pond complex of the exploits subzone in central newfoundland yields the graptolite undulograptus austrodentatus s l and cyclopygid trilobite cyclopyge grandis brevirhachis this late arenig faunule constrains the upper age limit of the ophiolite complex and is the first record of an early ordovician cyclopygid trilobite in north america this is consistent with a paleogeographic affinity for south central newfoundland with the northern oceanic margin of avalonia in a peri gondwanan position during the early ordovician and contrasts with coeval shelly and graptolitic faunas from the notre dame subzone of central newfoundland which show marked laurentian affinities the exploits subzone is generally considered equivalent to the region of scotland lying south of the southern upland fault the newfoundland discovery which is supported by faunal data from elsewhere in newfoundland and in ireland suggests that the region around t

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

40 ar 39 ar amphibole ages from the kingston complex new brunswick evidence for silurian devonian tectonothermal activity and implications for the accretion of the avalon composite terrane the kingston complex of southern new brunswick canada is a regional ne trending belt of mafic and felsic dikes bordered by dextral ductile shear zones the complex separates two contrasting late precambrian early paleozoic tectonostratigraphic assemblages near the nw margin of the avalon composite terrane rhyolitic dikes within the complex record an early silurian crystallization age dike orientations suggest that emplacement of the complex accompanied sinistral movement interpreted to reflect accretion of the avalon composite terrane to cratonic north america mafic dikes within the complex display greenschist to epidote amphibolite facies metamorphic assemblages that developed during dextral transpression amphibole concentrates from seven metamorphosed mafic dikes record late silurian to early devonian ca 416 390 ma 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages interpreted to date closely the dextral transpressive deformation together with other recent age data the 40 a

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

aulacogens of ancient platforms problems of their origin and tectonic development abstract riphean and palaeozoic linear graben like platform depressions aulacogens jointly with linear swell uplifts in a platform cover intraplatform folded zones and meso cenozoic epi platform rift zones are different morphological manifestations of a single group of linear platform structures aulacogens are characterized by the long duration of their development and ability for regeneration their existence is certain mainly since the late proterozoic riphean usually they inherit more ancient linear attenuated tectonic zones of platform basement or adjust to them aulacogens pass through common stages of development forming a single cycle or in many cases even several similar though not identical cycles the cycle starts with the incipience or regeneration of a linear graben like depression limited by normal faults and after a more or less deep and long subsidence accompanied often by basic or alkaline basic volcanism it is completed in many cases by an inversion stage at this stage horst like uplifts appear limited by thrust and overthrusts in the aulacogen basement while linear swell like uplifts or their systems and even linear folded structures occur in its cover the early stages of an aulacogen evolution occur under horizontal extension conditions whereas the inversion stage takes place under a compressional regime cycles of aulacogen development resemble in attenuated miniature form the development cycle of adjacent geosynclinal areas coincide with them in time and many through and penetrating aulacogens are their direct offsets the late proterozoic cycle of aulacogen evolution corresponds to a pre plate stage of the platform history aulacogen stage by bogdanov and is characterized by the widest development of a network of differently oriented aulacogens over the whole area of ancient platforms which have been subjected to some general horizontal extension crawling away and only partially compensated by compression during the inversion stage the subsequent aulacogen development in palaeozoic and on laurasian platforms partly in meso cenozoic was spatially and genetically connected with the history of geosynclinal later orogenic belts bordering these platforms with regard to gondwana platforms and later the north american and siberian the mesozoic saw the development of many aulacogens or rift zones ranging parallel to transversally with respect to the recent margins of platforms this process was connected with the break up of the gondwana supercontinent and the formation of secondary oceanic depressions as a rule it has not been completed by inversion but took place under extension conditions and probably some general expansion of the earth activated during the meso cenozoic

mesozoic and early tertiary rift tectonics in east africa abstract a complex history of crustal extension occurred in east and central africa during the mesozoic and early tertiary beginning in the late jurassic this resulted in a large system of rifts the central african rift system that spanned from central sudan to southern kenya late jurassic rifting is best documented in the white and blue nile rifts of the sudan and records east west extension in half graben that were connected by large scale shear zones and pull apart basins early cretaceous rifting re activated jurassic basins and spread to the large south sudan rifts and anza rift in kenya by the late cretaceous the extension direction shifted to the ne sw and the presently observed large scale rift geometry was established in the early tertiary some mesozoic basins were again reactivated while other regions underwent wrench faulting and basin inversion the large number of basins preserved in the central african rift system can be used to construct an evolutionary model of continental rift tectonics early phases of extension at low strains produced alternating half graben accommodation zone geometries similar to those observed in most young and active continental rifts at higher strains some border faults were abandoned so that through going simpler active fault systems could evolve this is interpreted as representing a switch from complex oppositely dipping detachment structures with strike dimensions of 50 150 km to regional detachment structures that continue for hundreds of kilometers parallel to the rift this change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate non rotational high angle normal faulting dominates in the development of these late basin geometries deciphering similar rift basin histories from passive continental margins may in many cases exceed the limits of available reflection seismic data east west striking shear zones including the umm hani and south sudan shears served as basin terminating structures in the late jurassic rift system these shear zones were later reactivated during the cretaceous and early tertiary controlling the positions of some accommodation zones and jogs apparent offsets in rift trends the south sudan shear may have continued to play a role in neogene extensional tectonics as it connects jogs in both the abu gabra anza rift jurassic to early tertiary and turkana chew bahir rift neogene to recent

how to deal with missing overburden investigating exhumation of the fragment of the mid polish anticlinorium by a multi proxy approach abstract tectonic inversion and erosion conceal basin geological history as simple stratigraphic reconstructions can be inaccurate thermal modelling techniques are often applied in this paper we show the advantages of a multi proxy approach for exhumation studies which explores full range of all possible solutions the part of the mesozoic central european basin that was inverted into the mid polish anticlinorium in the late cretaceous is chosen as a case study the analysis consists of three steps first a wide range of possible geological scenarios is constructed accounting for two alternatives of the overburden sequence southern and northern that result from the impact of the holy cross fault on mesozoic deposition the second stage comprises apatite fission track and vitrinite reflectance analyses carried out on a 1500 m long vertical profile inverse modelling of these data to obtain thermal histories and 1d thermal modelling of burial erosion using the petromod software in the third step results of modelling exercises are combined with probabilities of the overburden variants to define an ensemble of the most likely geological scenarios by applying this workflow we conclude that exhumation of the mid polish anticlinorium began in the latest turonian early campanian and that 1 7 2 3 km of uppermost triassic cretaceous rocks were removed the heat flow was similar or slightly higher than the present day value and during the jurassic the study area was located on the northern side of the holy cross fault where deposition was faster we also investigate the impact of elevated late palaeozoic heat flow on our samples and find that it was overprinted by high late cretaceous temperatures all this is achieved by presenting the results as an ensemble of the most likely solutions within the wide modelling space

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

extensional and contractional deformation in the blue ridge province virginia basement rocks of the blue ridge anticlinorium in central and northern virginia were involved in two temporally distinct deformation episodes the older event is characterized by a locally pervasive low strain fabric and high strain shear zones a preferred grain shape orientation in recovered quartz feldspar and biotite combined with a poorly developed symmetric quartz c axis pattern defines the low strain fabric kinematic indicators are typically symmetric but where asymmetric they consistently indicate top down to the southeast extensional motion discrete shear zones are within and close to late proterozoic plutons kinematic evidence fron these zones records large noncoaxial strains also with a down to the southeast extensional shear sense feldspar microstructures range from cataclastic fractures to complete recovery suggesting that deformation occurred at greenschist to amphibolite grade both low strain foliations and high strain shear zones are cut by paleozoic thrust related tectonites on the basis of kinematic evidence and spatial association with late proterozoic plutons cross cutting relations with thrust related structures and regional geologic relations we suggest that extensional deformation is related to late proterozoic rifting of the laurentian margin the kinematic histories of the rockfish valley fault zone and related shear zones appear to be relatively simple and consistent with paleozoic west to northwest directed thrusting the rockfish valley fault zone does not separate distinctly different basement terranes and thus its displacement is thought to be small mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars under lower to middle greenschist facies conditions solution transfer processes were an important deformation mechanism in these tectonites thrust related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust the rockfish valley fault zone appears to predate the formation of the south mountain cleavage in the valley and ridge province but it may be part of a lengthy progressive thrust related deformation

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

promontory promontory collision in the canadian appalachians in cape breton island and southwestern newfoundland the appalachian orogen is extremely narrow the rocks were strongly deformed and underwent high grade barrovian type metamorphism in silurian time and the orogen parallel silurian thrusts are dominantly west vergent antithetic to the westward subduction this area is bounded by two transverse dextral wrench faults the canso fault to the southwest and the gunflap hills fault to the northeast these tectono stratigraphic structural and metamorphic features are explained by silurian collision between the st lawrence promontory on the laurentian margin and the cabot promontory on the avalon margin during the final closure of the iapetus ocean the west vergent antithetic thrusting is interpreted to be related to tectonic wedging while the two transverse wrench faults accommodated differential movement from promontories to reentrants during the collision promontory promontory collision should be a common process and probably played a significant role in shaping the tectonic framework of many orogenic belts

diapiric origin of the blytheville and pascola arches in the reelfoot rift east central united states relation to new madrid seismicity most of the earthquakes in the new madrid seismic zone correlate spatially with the blytheville arch and part of the pascola arch which are interpreted to be the same structure both arches may have formed by diapirism along the axis of the reeffoot rift seismic geophys ical and drill hole data indicate that the rocks in the arches are highly deformed and fractured and have gross lithologic properties that make them weaker than rocks adjacent to the arches the weaker rocks are inferred to fail seismically more readily than the stronger rocks adjacent to the arches

comparative petrology of gneissic rocks in southwestern newfoundland in southwestern newfoundland pelitic migmatites of the meelpaeg subzone of the gander zone are separated by faults and plutons from metasedimentary rocks of the port aux basques gneiss complex pb

tectonic setting and regional significance of the port aux basques gneiss sw newfoundland the port aux basques gneiss of southwest newfoundland comprises three paragneiss and schist assemblages the grand bay and port aux basques complexes and the harbour le cou group they are intruded by large volumes of basic sheets and coeval middle ordovician granitoids the margaree and kelby cove orthogneiss which also cut a tectonic slice of earlier ultrabasic to basic rocks within the grand bay complex the big barachois assemblage the grand bay and port aux basques complexes are juxtaposed by the late silurian isle aux morts fault zone against the harbour le cou group to the east which dominantly comprises sillimanite and garnet bearing pyritiferous metasediments with amphibolitic intrusions and meta pillow basalts in its lower stratigraphic units geochemical studies have shown that metabasites of the grand bay complex port aux basques complex margaree and kelby cove orthogneiss comprise fractionated and crustally contaminated morb like and within plate tholeiites suggestive of an ensialic okinawa type back arc basin these have comparable characteristics to metabasites in the harbour le cou group and suggest that these units comprise remnants of an extensive mid ordovician marginal basin developed within a promontory on the peri gondwanan continental margin in contrast the big barachois assemblage in part comprises a distinct suite of pre mid ordovician island arc tholeiites

new insights into the appalachian orogen from geology and geochronology along the cape ray fault zone southwest newfoundland reverse oblique overthrusting of high grade port aux basques gneiss over the lower grade supracrustal windsor point group along the cape ray fault zone occurred during late silurian to early devonian time and is attributed to major oblique collision between gondwanan and laurentian continental blocks new u pb zircon 40 ar 39 ar and k ar data limit the timing of this orogenic event the megacrystic granitic phase of the cape ray igneous complex and the cape ray tonalite have been dated at 488 3 ma and 469 2 ma respectively a black rhyolite from the windsor point group yielded an ordovician age 453 5 4 ma a pretectonic gabbro sill within the windsor point group dated at 424 4 3 ma is cut by the windowglass hill granite dated at 424 2 ma synkinematic hornblende and biotite from the grand bay complex mylonites of the port aux basques gneiss have 40 ar 39 ar ages of 407 4 ma and 403 4 ma respectively which is indicative of a late silurian early devonian age for the amphibolite grade deformation these ages are compatible with a previously determined 415 ma monazite age interpreted as the time of peak metamorphism in the port aux basques gneiss and suggests rapid exhumation the east west flexure of the cape ray fault zone represents a tear fault accommodating differential shortening between the cape ray fault zone and the correlative fault system in central newfoundland mid devonian granites 386 3 384 2 ma intervene between the thrusting events and a younger sinistral transcurrent movement this deformation and magmatic history can be correlated throughout the northern appalachians along other major fault boundaries

when do faults in sedimentary basins leak stress and deformation in sedimentary basins examples from the north sea and haltenbanken offshore norway faults may be barriers or conduits for fluid flow in sedimentary basins the properties of faults however depend on stress conditions and rock properties at the time of deformation and subsequent diagenesis of the fault zone several recent publications have suggested that petroleum reservoirs in the north sea and at haltenbanken offshore mid norway have experienced leakage along faults caused by imposed stresses related to glacial loading during the quaternary the jurassic reservoirs in these areas are however bounded by faults produced during the upper jurassic rifting when the sediments were still soft and for the most part uncemented these faults do not represent zones of weakness because of strain hardening and later diagenesis in sandstones and cementation in mudstones the fault zones are commonly stronger than the adjacent rocks they are therefore not likely to be reactivated tectonically furthermore there is little evidence of glacial deformation in the quaternary sediments overlying these oil fields it has been proposed that very large horizontal stresses inferred to be related to periods of glacial loading caused shear failure at pore pressures below fracture pressure and subsequent leakage along these shear zones we argue that this is not a likely mechanism during progressive burial in sedimentary basins very high horizontal effective stresses up to 60 mpa at about 3 km 1 8 mi depth at haltenbanken would have caused more mechanical compaction and grain crushing than that observed in situ external stress i e plate tectonic stress from spreading ridges ridge push will be transmitted primarily through the basement and not through the much more compressible overlying sedimentary rocks during progressive basin subsidence chemical compaction i e caused by quartz cementation causes rock shrinkage which will relax differential stresses this makes brittle deformation shear failure resulting in open fractures less likely to occur at stresses below the fracture pressure in subsiding sedimentary basins with progressive compaction horizontal stress will normally not exceed the vertical stress except when there is significant shortening of the underlying basement

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

interference between pluton expansion and non coaxial tectonic deformation three dimensional computer model and field implications abstract structures in orogenic belts are greatly controlled by interactions between pluton related and tectonic related strain fields however many aspects of the geometry and evolution of these structures are poorly known for example 1 what is the geometry of these structures if regional deformation is coaxial 2 how does this geometry change with progressive deformation this paper describes a three dimensional simulation of strain shape and magnitude as well as foliation and lineation patterns formed by interactions between pluton expansion and homogeneous coaxial deformation this simulation suggests that interactions between pluton expansion and regional uniaxial shortening generate patterns that are distinct from the patterns produced by regional uniaxial extension or regional non coaxial deformation for example shortening produces doughnut shaped constrictional rings about the pluton whereas extension produces apple shaped flattening regions around the pluton in addition shortening produces one continuous doughnut shaped low strain region around the pluton on the other hand extension creates two separate ellipsoidal low strain regions at the ends of the pluton furthermore shortening forms foliation rings that have triangular two dimensional sections and lineation is concentric around the axis of maximum shortening in contrast extension produces two separate foliation and lineation cones at the ends of the pluton all these structural and strain elements migrate away from the pluton with increasing expansion rates and towards the pluton with increasing tectonic deformation rates although structural studies around plutons should always integrate microstructural radiometric and field measurements simulation generated patterns can be used to evaluate 1 origin geometry and evolution of structures around plutons 2 mechanisms that explain inclusion trails within porphyroblasts 3 contribution of pluton related strains to strains measured in the country rock and 4 type and three dimensional orientation of regional kinematic schemes

the geology and structural history of the cape ray fault zone in southwestern newfoundland structural studies within the cape ray fault zone southwestern newfoundland indicate that three tectonic events d1 to d3 have deformed all regional lithologic units these lithologies are distributed as a tonalitic terrain intrusive into ophiolitic remnants to the northwest a central volcanic sedimentary terrain and a staurolite kyanite gneiss terrain to the southeast d1 produced the dominant northeasterly striking planar fabrics in the host rocks that are axial planar to isoclinal folds the fold axes parallel a synkinematic mineral elongation lineation d2 formed open asymmetrical folds with curvilinear fold axes and local crenulation cleavage the d3 event produced local conjugate kink folds radiometric dates on deformed plutons indicate these deformational events were post 369 12 ma post middle devonian and pre 352 6 ma pre late carboniferous the fault was previously interpreted as a cryptic suture along which opposed margins of the iapetus ocean were juxtaposed supposedly this sut

the chaparral shear zone deformation partitioning and heterogeneous bulk crustal shortening during proterozoic orogeny in central arizona the proterozoic chaparral shear zone of central arizona is one of a network of subvertical northeast striking shear zones that divide the proterozoic orogenic belt of arizona into tectonic blocks the zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation stretching lineations plunge shallowly northeast and displacement across the zone was dominantly right lateral several lines of evidence constrain displacement across the zone to be greater than 5 km but probably less than tens of kilometers 1 stratigraphic and plutonic rocks can be correlated across the zone 2 structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar and 3 integration of shear strain estimated by deflection of earlier fabric suggests greater than 5 km of strike slip more important than scale of movement across the zone structural studies have clarified several aspects of the assembly history of the orogen early north to northwest striking s 1 foliation on both sides of the chaparral shear zone records one or more deformational events that took place between 1 75 and 1 7 ga early fabrics record northeast southwest or east west shortening and west verging thrusting that we interpret to have formed during development of a primitive arc complex in contrast intense deformation in the chaparral shear zone took place during regional northwest southeast shortening d 2 that produced the dominant subvertical northeast striking foliation and the present block architecture of the orogen d 2 is interpreted to have been in part synchronous with emplacement of 1 70 ga granitoids and to record assembly of volcanic belts to north america of general interest is the character of d 2 deformation partitioning in and across the shear zone two types of high strain domains were generated during shortening one type represented by rocks southeast of the shear zone involved extreme shortening and transposition by folding the other type represented by the shear zone involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones overprinting relationships suggest that the second type may have nucleated on the first implying an important component of deformation partitioning in time as well as space mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing conjugate shear bands suggest a shallow southeast plunging 1 late during strike slip deformation this is consistent with regional d 2 shortening but not with the incremental shortening direction expected during right lateral simple shear this supports a regional kinematic model in which the chaparral shear zone right lateral and a temporally related left lateral shear zone were regional scale conjugate shear zones that accommodated heterogeneous d 2 northwest southeast orogenic shortening via escape block tectonics thus orogen parallel strike slip displacements in the chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

past present and future of basin and petroleum system modeling basin and petroleum system modeling bpsm has had increasing impact on industry decisions related to exploration and new venture opportunities over the last decade basin and petroleum system modeling technology usability and user group size have grown as a result of its capacity to reduce exploration risk based on current statistics improvements in bpsm have significant potential to reduce future well failures particularly when caused by lack of petroleum charge to bring bpsm practitioners from academia and industry together an aapg hedberg research conference was organized to discuss the latest developments and issues in this field a survey was conducted during the conference in santa barbara california april 2016 and this paper summarizes the results a key takeaway was an overarching consensus throughout the bpsm community that improved understanding of hydrocarbon migration and more flexible workflows are necessary to better assess charge and migration risk in exploration and new ventures in addition bpsm is increasingly used to predict pore pressure and porosity at field scales which opens new opportunities to integrate bpsm workflows with other technologies such as seismic rock property analysis and reservoir quality modeling in this paper we discuss these and other issues that arose from in depth discussion and an online survey

clay modeling of rift asymmetry and associated structures abstract a clay model is used to simulate the development of half graben of alternating asymmetry in a rift zone the sense of asymmetry is determined by the spatial arrangement and movement pattern of plates below the clay the plates can be thought of as analogous to low angle detachment horizons below rifts the change in half graben asymmetry occurs across a transfer zone marked by a highly faulted anticlinal saddle whose axis is oblique to the rift trend

graben inversion in nature and experiments the inversion of crustal scale basement grabens is studied here through laboratory experiments on small scale models and available oil industry seismic lines from the southern north sea two basic configurations are considered first both the basement and the sedimentary cover are brittle and inversion does not involve any potential decollement between them second the basement and sedimentary cover are separated by a weak ductile layer e g salt which can allow decollement of the cover during both extension and later compression and inversion the second configuration is more complicated and can lead to a large variety of geological structures laboratory experiments were carried out on brittle ductile models built with sand to represent brittle layers basement and sedimentary cover and silicone putty to simulate the decollement layer between basement and cover a mechanically based classification of inversion structures is proposed the effects of some crucial parameters are investigated including obliquity between the direction of shortening and normal faults as well as strength profiles and the presence or absence of salt diapirs the experimental investigation leads to the following conclusions 1 the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45 2 if there is a superficial decollement e g basement cover interface inversion initiates low dipping thrust faults in the cover localized at graben borders 3 salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults and 4 when the cover is decoupled from the basement by a decollement layer inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike slip faults within the graben trending oblique to the graben borders experimental results are compared with field examples in particular from the southern north sea

model of the southeastern margin of the mississippi valley graben near memphis tennessee from interpretation of truck magnetometer data the geometry and magnetic properties of the southeastern margin of the mississippi valley graben ate studied by analyzing detailed magnetic anomaly profiles that are based on measurements made along roads near memphis tennessee the closely spaced measurements were obtained with a truck mounted magnetometer system developed by the u s geological survey modeling results suggest that the graben margins represent both structural boundaries and conduits for ascending magma about 2 km of vertical offset associated with normal faulting occurs within an interpreted 5 5 km wide zone in which magnetic basement has an average dip of 20 nw into the graben this zone may contain several large normal faults formed during a stage of rifting in late precambrian or early paleozoic time subsequent erosion of the scarps probably aided in forming the gentle northwestward slope of the buried precambrian surface across the fault zone the high apparent susceptibility about 3 10 3 emu of magnetic basement associated with this fault zone and with the upblock suggests either that ascending magma intruded the upblock or that the two blocks differed lithologically prior to the formation of the graben