laccolithic as opposed to cauldron subsidence emplacement of the eastern mourne pluton n ireland evidence from anisotropy of magnetic susceptibility the structural evolution and emplacement of the eastern mourne pluton was investigated using anisotropy of magnetic susceptibility ams measurements carried out on 112 oriented block samples and structural data from the host rocks from these new data cauldron subsidence as the emplacement mechanism is disputed and evidence for an alternative laccolithic style model involving inflation is presented this includes deflection and uplift of host rock bedding close to contacts and the magnetic fabric pattern which has a gentle dome geometry even close to contacts the magnetic lineations usually plunge down dip near the external margins but otherwise have a general ssw nne trend that diverges northward this suggests a northward directed inflow direction the model for the emplacement of the eastern mourne pluton is a laterally fed laccolith emplaced south to north the eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical breached laccolith

ascent of a granitoid diapir in a temperature varying medium a relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth dependent temperature to predict emplacement histories ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it ascent velocity is described by the expression u 2 gd3z 0r where is the density contrast between diapir and country rock g is gravitational acceleration d is the deformation zone width z is a parameter which describes the increase in viscosity across the zone 0 is the country rock viscosity at the contact and r is the diapir radius the width of the deformation zone as dictated by the temperature distribution and rheological function is the point at which the viscosity increases by a factor f from the contact value although ascent rates vary with choice of this parameter ascent distances are not strongly dependent on f results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock but only weakly dependent on the initial diapir temperature ascent velocity varies with the diapir size with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons as a result of the proportionately larger amount of thermal energy available for ascent larger bodies will rise greater distances than smaller diapirs however the additional ascent appears nominal relative to the contrast in available energy modeling diapirs using a no slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress free boundary condition the time scale of granitoid diapirism over crustal distances cannot be less than 104 years due to the rate limiting effect of thermal conduction above the body and not greater than 105 years reflecting solidification times of kilometer sized plutons a consequence of ascent by a deformation zone mechanism is the generation following emplacement of a mechanically weakened crust which extends from beneath the diapir to near the source perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust

distinguishing diapirs from inflated plutons an integrated rock magnetic fabric and structural study on the roundstone pluton western ireland granitoid plutons account for much of the continental crust and are critical in the generation of several economic resources despite over a century of research the fundamental process by which large felsic bodies intrude remains controversial two contrasting models persist 1 mass ascent and subsequent cooling of magma diapirism 2 sheet ascent and subsequent emplacement laccoliths the latter hypothesis is different from the former as distinct ascent and emplacement processes must occur and be identifiable traditional field methods rarely allow this important distinction to be made we present field petrographic and anisotropy of magnetic susceptibility data from the roundstone pluton connemara western ireland which was previously considered a diapiric intrusion the new data reveal anomalous subvertical magmatic lineations in the core of this circular intrusion as well as a suite of magmatic and submagmatic fabrics that parallel nnw sse country rock faults and coeval g2 sheets within the pluton we deduce that these structures reflect a distinct centralized ascent process along a nnw sse conduit lateral emplacement and pluton inflation followed ultimately forming a punched laccolith we conclude that the combined application of the above techniques provides a unique insight into pluton architecture that removes ambiguity between contrasting diapiric and inflated pluton models supplementary material complete rock magnetic data tables and sample site coordinates irish grid are available at http www geolsoc org uk sup18837

granitoid diapirism during protracted tectonism in an archaean granitoid greenstone belt yilgarn block western australia abstract the bullfinch parker dome area is an archaean granitoid greenstone terrain in the yilgarn block of western australia affected by a complex protracted tectonic event the majority of the deformation history was progressive rather than multiple discrete events finite strain patterns and structural and metamorphic zonation suggest that deformation in the greenstones was caused by regional ene wsw shortening and broadly simultaneous upward movement of granitoids by solid state diapirism and or ballooning plutonism rather than by core complex formation or multiple folding evidence for pluton related tectonism includes the foliation trends steeply plunging radial mineral lineations indications of upward movement of plutons increase of strain towards the batholith margins concordant structures in the pluton and the surrounding rocks such as lineation and foliation patterns constrictional tectonics near east west trending granitoid greenstone contacts syn to post kinematic peak metamorphism contact metamorphic zoning and pressure differences metamorphism largely of amphibolite facies was probably related to uprise of the granitoids at moderate crustal depths 10 12 km p 3 kbar causing both regional and contact metamorphism as a single thermal event relative and absolute timing defines an overlapping sequence of metamorphism progressive deformation and plutonism in this part of the yilgarn craton during the late archaean

crustal regime of deformation and ascent of granitic magma abstract structural studies and gravity data demonstrate the magma flow pattern of granitic plutons and their morphology at depth the deepest zones calculated from gravity data provided they show vertical lineations in outcrop are interpreted as the feeder channels which fed the pluton their morphology at depth as well as the position and number of root zones vary according to the type of deformation suggesting that regional deformation determines the emplacement of granitic plutons therefore deformation must also influence the segregation and the ascent of acid magmas a thermal model is constructed which demonstates the structure and heat generation of the hercynian crust according to that model the thermal gradient changes the conditions for the deformation of the crust obviously a large amount of heat must be supplied to induce melting of the lower crust as a result magma segregation and ascent occur entirely within the ductile field when upwelling magma reaches the brittle ductile transition zone the hydrodynamic force buoyancy is insufficient to fracture the upper brittle crust parameters which control diapirism wavelength viscosity thickness do not match those related to the volume of erupted magma as a consequence diapiric upwelling appears to be limited under favourable conditions to the lower ductile crust and does not apply to intrusions in a brittle crust shearing helps to segregate magma and to feed conduits toward the surface since it creates the required zones of local extension for the magma to infiltrate

deformation around a rising diapir modeled by creeping flow past a sphere a new experimental method for determining the pattern of flow and three dimensional progressive finite deformation of fluid elements in low reynolds number re viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder this is intended as a model analogue of natural rock deformation associated with magmatic and solid state diapirs rising through ductile crust the kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures the experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent newtonian test fluid used sgm36 are discussed in detail the progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis the progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere the results compare favorably with the external strain pattern associated with post tectonic mesozonal diapiric intrusions

geological controls on gas content distribution of coal seams in the kinik coalfield soma basin turkey abstract the miocene soma coal basin may be one of the most promising areas for low rank coalbed methane in turkey so far the coalbed methane potential of the coal seams in the kinik coalfield soma basin has not been adequately studied a systematic investigation was performed on the gas content distribution of the kp1 and km2 coal seams and its geological controls which has been based on the geological data and the experimental results of 84 coal samples taken from 37 coal exploration wells in the kinik coalfield soma basin samples were analyzed in terms of coal properties and gas content the gas content was measured by the direct method based on the canister desorption test the gas content of the kp1 and km2 coal seams range from 0 90 to 2 61 m3 t and 0 51 to 3 86 m3 t respectively on as received basis with increasing trend from west to east areas in both seams diffusivity values of the kp1 and km2 coal seams vary from 2 84 10 5 min 1 to 56 4 10 5 min 1 and from 0 64 10 5 min 1 to 54 3 10 5 min 1 respectively the gas content of the kp1 seam has not been correlated with the coal properties whereas the gas content of the km2 seam has positive correlation with fixed carbon content and gross calorific value and negative correlation with moisture and ash content normal faults are well developed in the study area play an important role at gas migration causing gas dissipation and reduced gas content of each coal seam in the northern part the low gas content may result from the combined effects of the geological structure thickness of overlying unit burial depth and coal thickness in the southern part the gas content of coal seams is controlled by the combined effects of relatively favorable factors including fewer normal faults thicker overlying unit high burial depth and thick coal seam the variation of the gas content for the coal seams is a comprehensive reflection of geological factors with each coal seam has its own pattern of gas distribution and this should be considered for the future coalbed methane exploration and development studies in soma basin

diapirism and gravity tectonics report of a tectonic studies group conference held at leeds university 25 26 march 1980 ir is now almost two decades since hans ramberg began his well known research of simulating diapiric structures in the laboratory and his application of this model work to the origin of major structures in orogenic belts ramberg 1963 1967 since then there has been a revolution of ideas in structural geology ideas ofhorizontal tectonics based on the movements of the earth s plates have largely taken preference to ideas based on vertical tectonics related to diapirism it was therefore considered timely to review the relative role of diapirism in regional deformation and hence the tectonic studies group affiliated to the geological society of london held a meeting on this theme on 25 26 march 1980 at the university of leeds this report includes the abstracts of all the contributions and it also summarises some of the conclusions most of which emerged during discussion following the contributions

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

syn orogenic extensional and contractional deformation related to granite emplacement in the northern tasman orogenic zone australia abstract the hodgkinson province of the northern tasman orogenic zone comprises uniform low grade metasediments of mid palaeozoic age that host numerous permian granite intrusions three widely developed sequential deformations d2 d4 are recognized on the basis of penetrative fabrics and folds which show clear regionally consistent overprinting relationships d3 and d4 were contemporary with phases of granite emplacement as shown by relationships of s3 and s4 microstructures to porphyroblasts which grew in thermal aureoles adjacent to intrusive contacts granitic dyke relationships with s3 and the deflection of s3 and s4 foliation trajectories around plutons support this view granites of d3 and d4 affiliation have a close spatial relationship and strong geochemical similarities suggesting that they were closely associated in time isotopic dating of granites of d4 association show that this deformational phase represents the hunter bowen orogeny which has been recognized as the terminal permian triassic episode of tectonism for the eastern sector of the tasman orogenic zone d3 fabrics are best developed in zones of enhanced metamorphic grade and are typically subhorizontal or of gentle inclination and associated with recumbent folds when not affected by subsequent deformation d3 is attributed to a phase of tectonism linked to the hunter bowen orogeny but preceding the main episode of compression uplift above tracts of rising magma combined with the thermal weakening of crustal material and the reduction of compressive stress due to plate convergence sponsored widespread ductile extension at a high crustal level the zone of failure was a discontinuous carapace above the magmatic front d4 is attributed to the main episode of the hunter bowen orogenic compression and involved crustal shortening with the imposition of upright structures and the widespread emplacement of granite s3 foliation surfaces have been reoriented in the vicinity of d4 granite bodies which behaved as rigid bodies during d4 compression the preparation of crustal pathways by d3 granite was utilized by granites of the d4 generation with the hodgkinson province displaying an intimate interplay of magmatism stress regime and structural evolution over a time period of perhaps 15 20 ma

internal kinematics of salt diapirs the internal structure of intrusive and extrusive salt bodies elucidates their external shape and the mechanism and history of diapiric emplacement information relevant to petroleum exploration trace amounts of brine can act like heat to weaken salt and promote geologic creep by solution transfer mechanisms we compare natural strain markers in salt with the artificial markers used by fluid dynamicists stream lines are the smooth flow paths followed by fluid particles stream surfaces separate flow laminae and contain the stream lines grain shape fabrics in salt anneal readily and have strain memories so short that lineations map the stream lines and foliations map the stream surfaces of the last stages of deformation by contrast larger structures like folded beddi g have much longer strain memories as a result multiple generations of tonguelike recumbent flow folds formed by unsteady flow in the mother source layer rotate to upright sheath folds or spines in the stems of diapirs here they are constricted and refolded into steep curtain folds to form crescentic and other interference patterns visible in the roofs of salt mines mature diapirs can form crestal bulbs which may be balloon or mushroom shaped and which potentially form larger petroleum traps than previously supposed screens of country rock may be entrained between the stem and peripheral overhanging lobes caused by episodic sedimentation

granite emplacement in orogenic compressional conditions the la alberca bejar granitic area spanish central system variscan iberian belt abstract the la alberca bejar granitic area comprises a set of intrusions of upper carboniferous to lower permian age located in the hinterland of the variscan belt of western spain gravity data anisotropy of magnetic susceptibility and microstructural studies carried out in the granitoid and its country rocks indicate that the granitoids of this region have tabular and inverted drop shapes and show sub horizontal magmatic fabrics and sw ne lineations the granitoid bodies were intruded episodically with single intrusive pulses recorded by deformation in the country rocks all intrusive events took place during d 3 compressive stage in the core of large d 1 anticlines and are not related to structures in the country rock a syn orogenic emplacement model for granite intrusion in such conditions is proposed the proposed model implies the activity of coeval emplacement mechanisms and room generation

the role of extensional tectonics at different crustal levels on granite ascent and emplacement an example from tuscany italy abstract the role of regional extension on the rise and emplacement of granites in the crust is still debated pluton ascent and emplacement widely occurred in tuscany italy since late miocene during the post orogenic collapse of the inner apennines and are presently occurring in the geothermal areas of amiata and larderello tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels ductile extension enhanced the segregation and ascent of granitic melts in the lower crust controlling pluton emplacement in correspondence with the brittle ductile transition in the brittle crust magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction pluton emplacement mainly occurred by means of roof lifting the case of tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle extending crust the estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates this suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes roof lifting accommodated the space required for pluton emplacement

the chinamora batholith zimbabwe structure and emplacement related magnetic rock fabric abstract the origin of dome and keel structural geometries in archean granite greenstone terrains appears to lack any modern analogues and is still poorly understood the formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility ams data for the chinamora batholith in zimbabwe the roughly circular shaped batholith is surrounded by ca 2 72 2 64 ga greenstones the batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units i banded basement gneisses ii granodioritic gneisses iii equigranular granites and iv central porphyritic granites in the gneissic granites a partial girdle n s of poles to the magnetic foliation is developed that has been folded around a consistent flat lying magnetic lineation plunging at shallow angles to the e or w in the equigranular granites the magnetic lineation generally plunges to the nw the magnetic foliation has a variable strike no clear trends can be distinguished the ams measurements of the porphyritic granite revealed a nw se striking foliation and showed subhorizontal magnetic lineations the magnetic foliation is subparallel to the macroscopic foliation wall rocks are moderately inclined and show radial or concentric lineations triaxial strain ellipsoids and kinematics that demonstrate off the dome sliding and coeval pluton expansion the results of the observations do not point to a single emplacement process neither the observed structural data nor the magnetic fabric support a model envisaging spherically ballooning it is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses whereas the fabrics in the porphyritic granites reflect emplacement as laccolith like sheets

emplacement related fault patterns around the northern granite arran scotland conspicuous faults in the new red sandstone nrs mainly permian triassic of arran are shown to be kinematically related to the intrusion of the northern granite in paleocene time early inflation of the granite pluton in the upper crust induced radial stretching in the overlying sedimentary cover recorded in predominantly normal offsets on the faults in the nrs continued ballooning of the pluton superimposed strong concentric dip slip displacement over its flanks both stretching and uplift were heterogeneous strongly controlled by a nonuniform stratigraphy and by reactivation of older faults striking predominantly north northwest and northeast and dating back at least to permian time the paleocene displacements on these discrete weak sub nrs faults diffused upward through nrs sandstones on numerous minor faults each with only small offset this behavior resulted from slip hardening on each fault in the poorly cemented nrs rocks late in the intrusion history the major faults on the southeast side of the granite formed a conjugate strike slip system that accommodated radial expansion of the pluton mainly during intrusion of a discrete magma batch that now forms the inner fine grained granite other structures around and within the northern granite are shown to be more compatible with this two stage ballooning model than with a vertical plug emplacement model previous assessments of the ballooning process in general have been made on deep level lateral margins of plutons where radial shortening dominates in the envelope north arran is an important example of the complexities of ballooning at shallow levels in a heterogeneous crust

on the role of diapirism in the segregation ascent and final emplacement of granitoid magmas only bodies of magma with a high crystal content and partially molten crustal country rocks can ascend as diapirs once such an envelope is pierced the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content which may then continue ascent via fractures and dykes the details of the mechanisms of granitoid magma segregation are still unknown but it appears that many magmas hegin their ascent through the crust as mushes with at least 50 melt and that such magmas are rheologically able to ascend through a thickness of crust this ascent mechanism explains the dearth of structures attributable to the ascent of granitoids in contrast to the abundance of structures that developed during their final emplacement when a magma becomes too crystalline melt 25 to continue its ascent via dykes it is immobilised at approximately this stage a hydrous magma may become saturated with water and release fluids into the aureole making it particularly susceptible to deformation magma that continues to arrive at this level is also immobilised and the pluton grows as a ballooning diapir these characteristically deform themselves and their aureoles by bulk shortening magmas that are able to ascend to shallow depths largely by virtue of lower water contents and higher initial temperatures tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence high level intrusions lend to be tabular are also fed by dykes or conduits and assemble in tabular batholiths

mechanism of emplacement of the cannibal creek granite with special reference to timing and deformation history of the aureole abstract the ovoid cannibal creek granite consists of a minor phase emplaced during the regional d 3 deformation and a major phase emplaced during the regional d 4 deformation it possesses an intense contact parallel foliation on its margin and in the adjacent porphyroblastic schist that formed through partitioning of the shearing component of deformation against the crystallized pluton margin during synchronous e w d 4 shortening during this process a pre existing shallow s 3 foliation was updomed around the pluton and re used reactivated by d 4 shear with associated dissolution of quartz resulting in the formation of an ovoid closed foliation around the pluton thus the granite is located within a domal shaped approximately n s trending pluton scale f 3 4 antiform that formed synchronous with emplacement three successive stages of porphyroblast growth record the history of processes operating during progressive emplacement and the structures that developed space problems were solved through upward movement of the rocks above the granite due to movement on the s 4 cleavage as well as on pre existing subvertical bedding and cleavages and by reactivation of steep d 2 faults

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

evidence for significant granite space creation by the ballooning mechanism the example of the ardara pluton ireland the emplacement mechanism of the ardara granite a caledonian pluton located on the northwestern seaboard of ireland has been a widely cited and a key point of reference in the debate about the granite space problem for nearly half a century early models related its subcircular tailed shape concentric foliation pattern and concentrically deformed envelope to diapirism it was subsequently reinterpreted using strain data as a ballooning pluton that was created by 70 in situ expansion at the emplacement site and was later cut by a regional shear zone recently it was the leading example in a wide ranging review of forcefully emplaced plutons of this type and was reinterpreted as a series of nested diapirs associated with only modest amounts 30 of forceful space creation but a large component of inferred early block stoping we report the results of the complete remapping and extensive collection of new structural data from this 8 km diameter normally zoned three phase calc alkaline pluton we confirm the existence of the steeply inclined internal foliation pattern and that it was formed in the magmatic state although this and the outline of the pluton are modified by late stage high temperature solid state regional transcurrent shear zones along two of the pluton contacts structural mapping has failed to identify a macroscopic lineation in the magmatic foliation planes and this is confirmed by strain determinations using mafic enclave populations and magmatic crystal separations for the magmatic state deformation that show nearly pure flattening type strains k 0 strains measured by these techniques show an apparently smooth gradient increasing outward toward the pluton contacts from an inner low strain zone we argue that these features together with the simple normal petrographic zonation a lack of small scale structures that would indicate the pluton had moved up relative to the wall rocks and an exposure level that is not in the roof zone are inconsistent with diapirism either singly or multiply but are most easily explained by a ballooning emplacement mechanism synmagmatic shear sense data from both the pluton and the adjacent country rocks indicate however that within the overall context of radial ballooning there was a limited amount of late stage nonradial northward injection of magma an inversion of the deformed enclave and crystal spacing strain determinations and strain determinations within the country rocks suggest that more than 80 of the volume of the pluton was accommodated by the ballooning mechanism

the identification of granitic diapirs diapirism is a themomechanically viable ascent mechanism for granitic magmas however there is very little direct evidence of actual granite diapirs many of the characteristics we may expect to use to determine ascent mechanisms are destroyed by changes in the shapes of intrusions resulting from the interaction of buoyancy and tectonic forces during emplacement many features of ballooning plutons are identical to those we would expect for diapirs notably patterns of strain deduced from deformed xenoliths and concentric foliations cannot be used as unambiguous evidence for diapirism diapirism can only be proven beyond doubt where it is possible to demonstrate vertical uplift of country rocks in excess of the vertical thickness of the intrusion