structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

a search for seamounts in the southern cook and austral region the existence of uncharted seamounts in the cook austral region of the south pacific has been investigated using geos 3 and seasat radar altimeter observations three previously uncharted submarine seamounts provisionally named geos a to geos c have been located between mauke and rimatara and a fourth geos d has been located east of rurutu this confirms that the aitutaki mauke islands of the southern cooks are a continuation of the austral chain a second group of uncharted seamounts has been provisionally located some 200 km south of rimatara and maria and this is suggestive of a second seamount chain south of the first that includes raratonga and mangaia fabert bank to the south of mangaia appears to be mislocated by about 2 sup 0 in longitude

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

metamorphic zonation migmatization and leucogranites along the everest transect of eastern nepal and tibet record of an exhumation history

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

simulation of the eastward 4 day wave in the antarctic winter mesosphere using a gravity wave resolving general circulation model 1 the eastward moving 4 day wave in the antarctic winter mesosphere is investigated using a high resolution middle atmosphere general circulation model that directly simulates the spontaneous generation propagation and dissipation of gravity waves the results are also compared with the simulated seasonal march of the meridional structures of the westerly jet streams in the southern hemisphere upper stratosphere and mesosphere in order to investigate baroclinic barotropic instability as a possible excitation mechanism for the 4 day wave the model successfully reproduces the dynamically unstable double jet structure of the mesospheric westerly winds the simulated 4 day wave develops in association with the baroclinic and barotropic instability of the mesospheric mean flows and has similar characteristics to those observed at 50 60 s near the stratopause the 4 day wave has strong equatorward heat flux associated with strong baroclinicity in the antarctic winter mesosphere mainly attributable to poleward overturning circulation driven by gravity wave forcing eastward forcing due to the 4 day wave occurs within the double jet structure and offsets part of the westward forcing due to gravity waves such an effect partially acts to stabilize the unstable mean flow structure in the antarctic winter mesosphere

timing constraints of orogeny to cratonization thermochronology of the paleoproterozoic trans hudson orogen manitoba and saskatchewan canada abstract a comprehensive thermochronometric investigation across a 600 km transect of the paleoproterozoic trans hudson orogen tho in canada has resolved two first order thermal events 1 widespread peak metamorphism at 1 82 1 79 ga associated with terrane amalgamation and 2 a late collisional metamorphic episode at ca 1 77 ga that is characterized by significant strike slip deformation and rapid cooling geophysical observations of the lithosphere beneath the tho reveal crust that is markedly different on either side of the crustal scale tabbernor sturgeon weir tsw strike slip fault system which offsets lower crustal reflectors the tho east of the fault system records u pb titanite and total pb monazite domain ages that are relatively indistinguishable from 40ar 39ar hornblende and mica cooling ages indicating rapid cooling and perhaps exhumation following 1 77 ga metamorphism seismic imaging indicates the eastern orogen is an irregular package of stacked crustal blocks and our thermochronometric data demonstrate it behaved as a single coherent element shortly 20 30 m y following cessation of convergence in contrast rocks located west of the tsw fault system cooled distinctly later preserving 40ar 39ar muscovite and biotite ages between 1 74 and 1 72 ga this crustal architecture delineated from seismic surveys illustrates a more homogeneous slightly 5 km thinner crust in the case of this orogen apparent stabilization and cratonization occurred ca 100 m y after peak terrane amalgamation and notably the orogen was likely cratonized as two blocks east and west of the tsw fault system

calamity peak layered granite pegmatite complex black hills south dakota part i structure and emplacement the calamity peak pluton is a satellitic granite pegmatite intrusion located 2 km southwest of the main harney peak granite dome in the black hills south dakota many aspects of the structural development of the calamity peak pluton 1 km 2 mimic those of the larger harney peak dome 100 km 2 thus the model described here for the calamity peak pluton provides a general basis for evaluating the evolution of proterozoic granite pegmatite intrusives of the harney peak granite system the overall structure of the intrusive rocks at calamity peak is that of a dome as shown by the general geometry of the pluton contacts and by a domical pattern of internal textural layering textural layering is defined by alternating bands of pegmatite and fine grained granite 0 1 2 0 m thick fine grained layers are in turn banded on a scale of 2 20 mm owing to modal variations of tourmaline quartz and feldspars textural differentiation of at least some of these layers resulted from segregation of initially homogeneous magmas into pegmatitic upper and aplitic lower components during crystallization relationships between the pluton and regionally dipping schistose country rocks however do not support a simple domal structure schistosity conforms to the contact only along the southern margin of the pluton the side facing with the regional dip country rock schistosity is at a high angle to the contact at the apparent floor of the pluton which is exposed in the core of the dome and along the northern margin the side that opposes the regional dip in order to explain the structural observations it is inferred that the initial intrusive body at calamity peak consisted of a flat lying discordant sheet intruded into steeply dipping quartz mica schist country rock however an arcuate sheet concave downward cannot be ruled out new magma pulses possibly several thousands were successively emplaced along the hanging wall contact of earlier pulses as indicated by the upward truncation of early layers creating a composite layered pluton intrusions of individual magma pulses were localized in three major composite sheets which form structurally and chemically discrete portions of the pluton growth of the pluton proceeded by forcible distension and plastic deformation of overlying country rocks schistosity was molded into a conformable attitude in portions of the hanging wall and pre intrusive country rock lineations were deflected around the pluton the layered granite pegmatite series is crosscut by four successive stages of fracture filling bodies the orientations and compositions of these bodies record the late stages in the structural and fluid evolution of the plutonic system

age of crustal melting and leucogranite formation from u pb zircon and monazite dating in the western himalaya zanskar india new u pb data from leucogranites in the high himalayan slab in the western himalaya of zanskar reveal that crustal melting occurred at 21 19 5 ma and that anatexis occurred along the himalaya at least from kashmir zanskar to eastern nepal at approx 24 19 5 ma high u monazites from leucogranitic melt pods in migmatites within the 35 km wide sillimanite k feldspar zone at the deepest structural levels exposed umasi la have u pb ages of 20 6 19 5 ma higher level migmatitic melt pods from shafat in the suru valley also contain magmatic monazites with u pb ages of 20 8 0 3 ma zircons from the leucogranites contain an approx 460 ma inherited component as do some umasi la monazites showing that the protoliths were at least ordovician in age

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

patterns of pervasive shear strain near the boundaries of the la ronge domain inner trans hudson orogen western canadian shield abstract the saskatchewan segment of juvenile paleoproterozoic crust inner trans hudson orogen reindeer zone is widely regarded as a nw dipping stack of litho structural domains the metavolcanic metasedimentary la ronge domain lrd whose least deformed portion has the geological map pattern of an archean greenstone belt is underlain by metasedimentary rocks of the kisseynew domain and overlain by migmatitic rocks of the rottenstone domain in easily accessible parts of lrd boundary walls the horizontal patterns of tangential boundary parallel strain accumulated under peak metamorphic conditions are investigated utilizing the strain patterns and recently published reflection seismic profiles we estimated the direction and derived the sense of tangential shear strain components on various horizontal scales in all parts of boundary walls investigated the dip shear has the sense of ductile thrusts but the strike shear sense varies with position on scales ranging from 100 m to 10 km the regional persistence of thrust shear in both lrd walls accords with tectonic scenarios of protracted horizontal convergence of juvenile crust after initial collision between the archean superior and hearne continents the patterns of strike shear on the other hand are difficult to reconcile with indentation hypotheses or counter clockwise oroclinal rotation

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

the wathaman batholith largest known precambrian pluton the early proterozoic wathaman batholith in northern saskatchewan and manitoba is a 900 km long megacrystic granite granodiorite intrusion that straddles the junction between ensialic miogeoclinal and probably ensimatic eugeoclinal island arc terranes of the trans hudson orogen of the western churchill province although the largest precambrian batholith known it is apart from marginal complexities remarkably homogeneous throughout and unlike comparably sized and situated phanerozoic batholiths shows no evidence of multiple intrusion nor does it have comagmatic early mafic phases however it may be considered as just one phase of a larger batholithic belt that also includes numerous smaller plutons taken as a whole the composite batholithic belt is similar in many aspects to mesozoic pacific rim batholithic belts and like them probably was emplaced during plate collision the batholith is affected by pervasive internal deformation is bounded on the northwest by major blastomylonite zones 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

volatile assisted intrusion and autometasomatism of leucogranites in the khumbu himalaya nepal abstract leucogranite crops out in the upper imja khola khumbu himalaya nepal where it makes up approximately 50 of the exposed bedrock leucogranites and their associated aplite pegmatites typically intruded as concordant sills millimeters to kilometers wide the exceptional nuptse granite is an ellipsoidal pluton of nearly 1 km radius which probably resulted from the ballooning of a sill and is wrapped in concordant country rock foliation small scale structures and mineral paragenesis indicate syntectonic intrusion into high temperature low pressure sillimanite muscovite cordierite bearing country rocks penetrative deformation of the country rock and sills is characterized by foliation striking n 60 e 40 nw and by north northwest plunging mineral lineation subparallel to fold axes of decimeter to meter scale isoclinal folds and centimeter to decimeter scale kink folds and crenulation a volatile rich phase played a fundamental role in the development of the imja khola leucogranites it

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

accretion history of the trans hudson orogen in manitoba and saskatchewan from paleomagnetism lithoprobe s sponsorship has led to the acquisition of paleomagnetic data from 20 units throughout the paleoproterozoic trans hudson orogen tho of saskatchewan and manitoba essentially the first such data for the region discussed summarily in this paper they provide an initial framework for the tho s tectonic evolution they show that the archean hearne and superior cratons were at subtropical and subpolar paleolatitudes respectively at 1875 ma with the lynn lake laronge arc midway between them in the manikewan ocean by 1855 ma this ocean was still 5500 700 km wide and its seafloor was subducting northwestward under the hearne craton and pericratonic peter lake domain margin with the coeval development of an andean type continental magmatic arc the wathaman chipewyan batholith between 1855 and 1810 ma coalescing apparent polar wander path apwp segments record closing of the ocean at a rate of 12 cm a trapping and accreting the several separate intervening terranes flin flon hanson lake lynn lake laronge rottenstone and presumably also other tho terranes from 1815 to 1775 ma the assembled terranes drifted as a coherent craton yielding a stillstand and hairpin in the apwp

a cordilleran type batholitic belt in the churchill province in northern saskatchewan abstract the rottenstone and western la ronge domains of the south central churchill province in saskatchewan consist mostly of hudsonian batholithic complexes spatially and structurally related to an aphebian hudsonian volcanic arc the batholithic belt is of comparable dimensions and compositional variation to cordilleran type phanerozoic batholiths and appears to be emplaced in close proximity to a prior late archean early aphebian continental crustal margin a quartz diorite line separating dominant quartz diorite tonalite granodiorite plutonic rocks from dominant granodiorite quartz monzonite granite intrusives can be defined these general characteristics as well as more detailed features of aphebian deposition and hudsonian thermotectonism within and on either side of the rottenstone domain suggest a continental margin rather than an intracontinental location for evolution of the hudsonian orogenic belt of the south central churchill province and provide some insights into the possible development of this part of the canadian shield as part of a hudsonian wilson cycle

archean rocks in the southern rottenstone domain significance for the evolution of the trans hudson orogen an augened strongly flasered and multiply folded monzonitic gneiss occurs in a structural dome in the black bear island lake region of northern saskatchewan within the ca 1850 ma paleoproterozoic southern rottenstone domain of the trans hudson orogen tho a sample of this rock has yielded thermal ionization mass spectrometry tims and sensitive high resolution ion microprobe shrimp zircon data consistent with a formation age of at least 2500 ma zircons also show somewhat younger ca 2380 ma overgrowths indicating a complex history the nd model age tdm of 2726 ma also confirms the late archean age of the rock as does the isotopic composition of common pb from a k feldspar sample u pb analyses of titanites yield ages of ca 1800 ma indicating recrystallization during terminal closure of the tho it is unlikely that the archean rocks are a part of the archean sask craton in the glennie domain for lithoprobe seismic sections indicate that the sask craton dips westward beneath the la ronge a

regional setting and kinematic features of the needle falls shear zone trans hudson orogen needle falls shear zone is the southern part of a major northeast trending ductile shear system within the paleoproterozoic trans hudson orogen in saskatchewan throughout its exposed length of 400 km the shear zone separates reworked archean continental crust and infolded paleoproterozoic supracrustals of the cree lake zone to the northwest from mainly juvenile paleoproterozoic arc terrains and granitoid plutons of the reindeer zone to the southeast it also defines the northwest margin of the ca 1855 ma wathaman batholith which forms the main protolith to shear zone mylonites although not precisely dated available age constraints suggest that the shear zone formed between ca 1855 and 1800 ma toward the end of peak thermotectonism in this part of the orogen in the needle falls study area shear zone mylonites exhibit varied sequentially developed ductile to brittle fabric features including c s fabrics winged porphyroclasts especially delta type small scale compressional and extensional

silicic magma chambers as traps for basaltic magmas the cadillac mountain intrusive complex mount desert island maine the cadillac mountain intrusive complex located on mount desert island maine provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma the complex consists of three units the cadillac mountain granite cmg the somesville granite sg and gabbro diorite g d basaltic magmas were emplaced before during and after the emplacement and crystallization of the granitic plutons and the chambers of silicic magma acted as traps for basaltic magma basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma the g d unit about 1 5 km thick consists of interlayered gabbroic dioritic and granitic rocks although thin 1 m thick gabbroic layers in the g d unit are typically chilled on all margins thicker layers of gabbro commonly form the lower parts of macrorhythmic units

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

igneous emplacement in a transpressive shear zone ox mountains igneous complex the central and southwestern ox mountains inlier in western ireland comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone granitoid emplacement occurred in two distinct styles in the ox mountains shear zone the older pluton the ox mountains granodiorite was emplaced syn kinematically with respect to the main sinistral transpressional events in its wall rocks it was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient dilational sites occurred in response to the transpressive deformation the younger lough talt and easky lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the ox mountains inlier the different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation

the development of flanking folds during simple shear and their use as kinematic indicators abstract we used a numerical finite element simulation to model the formation of flanking folds around rotating planar structures e g veins faults or dykes in a non linear viscous medium during ideal simple shear if the planar structure is much more viscous than its host it does not deform and flanking folds with no displacement along the structure develop their vergence is consistent with the overall sense of shear however if the planar structure is much less viscous than its host strain is concentrated within the structure and a secondary shear zone is developed in which slip is opposite to the overall sense of shear then flanking folds develop that have a vergence which is incompatible with the drag on structure if the deflection of markers is not clearly preserved then such flanking folds can be easily misinterpreted as shear bands indicating a wrong shear sense the deflection of the foliation in flanking folds is very similar to deformed asymmetric pull aparts and can therefore help to interpret these otherwise ambiguous shear sense criteria because our model ideal simple shear boundary conditions fails to describe back rotation of planar structures in rocks we speculate that shear band geometries are indicative for general shear

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

emplacement of himalayan leucogranites by magma injection along giant sill complexes examples from the cho oyu gyachung kang and everest leucogranites nepal himalaya abstract the upper part of the high himalayan slab in north central nepal is comprised of a thick layer parallel sheet of biotite muscovite tourmaline garnet sillimanite cordierite leucogranite up to 3 4 km thick and dipping north at 5 20 these strongly peraluminous magmas were emplaced into high temperature low pressure sillimanite and cordierite bearing gneisses calc silicates and rare amphibolites which were metamorphosed at temperatures of 600 650 c some time during the oligocene early miocene parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks in the mountains of cho oyu gyachung kang pumori lingtren and the base of the everest massif these leucogranites form part of a single structural horizon bounded at the top by the lhotse detachment the lower of two n dipping normal faults of the south tibetan detachment std system and below by the khumbu thrust kt an out of sequence fault which was partly responsible for the uplift erosion and exhumation of the leucogranites a model for the emplacement of these leucogranites is proposed where they represent viscous minimum melts produced by melting of a pelitic protolith similar to the underlying sillimanite grade gneisses through muscovite breakdown either during fluid absent melting at

buckle folding and deep crustal shearing of high grade gneisses at the junction of two major high strain zones central gneiss belt grenville province ontario low plunging transport parallel f3 folds are common at all scales in the central gneiss belt of the grenville province but few of these folds are sheath folds where the d1 d2 parry sound shear zone intersects the d3 shawanaga shear zone ssz at a high angle f3 folds formed at several scales centimetre to greater than outcrop scale in layered d1 d2 straight gneisses at the start of their evolution the f3 folds formed just beyond the ssz with hinges near orthogonal to the d3 shear direction and with typical buckle features e g wavelengths vary with layer thickness and hinges are discontinuous and bifurcate the buckle folds evolved within the ssz by rotation of hinges towards the shear direction even though hinges initiated at a high angle to the shear direction sheath folds were not produced in addition to tightening the buckles the ductile reorientation produced thin thick extended shortened limb pairs and very straight ridge like fold hinges and removed small folds from the extended

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18