zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

age and depth evidence for pre exhumation joints in granite plutons fracturing during the early cooling stage of felsic rock this paper documents the fracture process associated with the early cooling stage of felsic magma characteristics of pre exhumation joints include their spatial distribution in granite bodies their fracture surface morphology and geological and petrological evidence for the depth of fracture initiation these characteristics allow inferences about the depth and the time of joint origin in the south bohemian pluton the intrusion levels of presently exposed granites of the pluton were 7 4 km in the northern part and 14 3 km in the southern part within the northern mrakotin granite borsov early nne joints propagated while the granite was at a temperature near the solidus and in part magma was still being injected post dated by thin granite dykes along nne joints evidence for the pre exhumation initiation of these joints comes from the geochronological dating of this late granite dykes 1 2 cm thick at 324 9 ma in age which were creating their own rupture in the rock the time of the pluton emplacement at 330 to 324 ma and the cooling ages of 328 to 320 ma were given by scharbert 1998 and gerdes et al 1998 from fluid inclusions within the late granite dyke that occupy joint surfaces the trapping depth of the analysed inclusions was calculated to 7 4 km near the solidus h2o separates by crystallization of anhydrous phases the associated increasing h2o pressure can initiate the first cracks and can generate a small portion of new granitic melt which forces the cyclic fracture propagation together with mobile low viscosity residual melt input into the fracture the determination of the level and time of the cooling dykes give evidence for the joint initiation at a depth of 7 4 km which was connected with the level and process of final emplacement and early cooling of the mrakotin granite long before the main exhumation at the earliest the erosion of the upper rock pile of 7 4 km thickness started significantly after generation of the early joint sets the nne trending joints are persistent in orientation throughout the south bohemian pluton but the joint surface morphology is varying in all sub plutons and occupy all sections of the stress intensity versus crack propagation velocity curve wiederhorn bahat curve statement of problems granite bodies can fracture in various characteristic manners relative to the density of fractures the number of joint sets and their distribution and the type of fractographic features on joint surfaces some granite plutons are dominated by only one orthogonal subvertical joint system e g in the hercynian south bohemian pluton czech republic fig 1 a whereas other plutons contain two or more subvertical joint systems of different generation time e g the cadomian lausitz pluton germany d the density of fractures in the earth s crust decreases with depth e g price 1966 ramsay huber 1987 this observation does not exclude fracture initiation within plutons at depths below 7 km we consider two problems 1 age and depth for propagation of pre

provenance of synorogenic foreland basin strata in southwestern montana requires revision of existing models for laramide tectonism north american cordillera

palaeozoic amalgamation of central europe new results from recent geological and geophysical investigations abstract multidisciplinary studies of geotransects across the north european plain and southern north sea and geological reexamination of the variscides of the north bohemian massif permit a new 3 d reassessment of the relationships between the principal crustal blocks abutting baltica along the trans european suture zone tesz accretion was in three stages cambrian accretion of the bruno silesian lysogory and malopolska terranes end ordovician early silurian accretion of avalonia and early carboniferous accretion of the armorican terrane assemblage ata palaeozoic plume influenced metabasite geochemistry in the bohemian massif explains the progressive rifting away of peri gondwanan crustal blocks before their accretion to baltica geophysical data faunal and provenance information from boreholes and dated small inliers and cores confirm that avalonian crust extends beyond the anglo brabant deformation belt eastwards to northwest poland the location and dip of reflectors along the tesz and beneath the north european plain suggest that avalonian crust overrode the baltica passive margin marked by a high velocity lower crustal layer on shallowly southwest dipping thrust planes forming the heligoland pomerania deformation belt the variscan orocline of southwest poland masks two junctions between the armorican terrane assemblage ata and previously accreted crustal blocks to the east is a dextrally transpressive contact with the bruno silesian and malopolska blocks accreted in the cambrian while to the north is a thrust contact with easternmost avalonia deeply buried beneath younger sedimentary cover in the northeast bohemian and rhenohercynian massifs devonian early variscide deformation dominated by wnw and nw directed thrusting records closure of ordovician devonian seaways between detached islands of the ata and avalonia

the anisotropy of magnetic susceptibility in biotite muscovite and chlorite single crystals the anisotropy of magnetic susceptibility ams of single crystals of biotite muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock forming minerals the low field ams and the high field paramagnetic susceptibility are defined for the high field values it is necessary to combine the paramagnetic deviatoric tensor obtained from the high field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals this leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice the ellipsoid of paramagnetic susceptibility which was obtained for the three phyllosilicates is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c axes the anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements the degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite is smaller than previously published values the shape of the chlorite ams ellipsoid for all the samples is near perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks d 2003 elsevier science b v all rights reserved

late cretaceous stratigraphy deformation and intrusion in the madison range of southwestern montana dating of orogenic rock units in the central part of the madison range shows that laramide deformation was virtually completed by the end of the cretaceous early campanian k ar dates of about 79 m y were obtained from welded tuffs in the basal part of the livingston formation a volcanic and volcaniclastic assemblage that is conformable with underlying cretaceous clastic rocks and with the overlying sphinx conglomerate no datable materials were obtained from the sphinx but both it and the livingston were deformed by the hilgard fault system a series of thrust faults and associated folds which extend along the western side of the southern two thirds of the range this north trending fault system represents the culmination of laramide shortening within the range k ar and 40 ar 39 ar dating of hornblende from dacitic laccolithic rocks that intruded the hilgard fault system in the central part of the range indicates an approximate date of 68 69 m y b p for emplacement of the igneous suite the dacite postdates movement along faults of the hilgard fault system and postdates the synorogenic sphinx conglomerate

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

the magnetic susceptibility of granitic rocks as a proxy for geochemical composition example from the saruhan granitoids ne turkey the magnetic susceptibility of rocks is determined by their bulk chemistry and magnetic mineralogy in this case study of the saruhan pluton in turkey a detailed survey based on 925 field measurements and 37 whole rock analyses reveals a strong correlation between magnetic susceptibility and feot r 2 0 8 petrographic observations and calculations based on whole rock major element analyses indicate the presence of magnetite grains further calculations show that magnetite accounts for 88 of the magnetic susceptibility the spatial correlation between feototal and the magnetic susceptibility is particularly spectacular which suggests that km is a proxy for geochemical differentiation the zoning pattern of magnetic susceptibility across the pluton is concentric and reverse with the highest values of magnetic susceptibility in the center this type of zoning is consistent with a previously proposed petrogenetic model the comparison of the saruhan granites with a classic example of paramagnetic granites reveals two contrasted feot km trends this suggests that these variations can be a useful tool to distinguish between leucocratic paramagnetic and leucocratic ferrimagnetic granites 2007 elsevier b v all rights reserved

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

thrust faults and back thrusts in madison range of southwestern montana foreland in the rocky mountain foreland of southwestern montana a zone of late cretaceous thrust faults named the hilgard fault system extends along the west side of the madison range from hebgen lake northward for about 50 mi 80 km the thrust faults are steep at their leading edges but flatten westward beneath the associated plates where they commonly dip 25 30 structural lows and highs are apparent beneath the beaver creek plate the major thrust sheet of the system and correlate with salients and reentrants of the plate the beaver creek plate consists primarily of archean metamorphic rocks but phanerozoic strata are preserved along the northern part of the plate s leading edge only the forward part of the plate is preserved in the madison range because c nozoic normal faults of the madison range fault system dropped much of the plate beneath the madison valley on the west the kirkwood plate lies east of and beneath the beaver creek plate and contains structures of the eastern part of the hilgard system the central segment of the leading edge of the kirkwood plate is not completely detached from underlying strata the leading edge of the northern part of the plate defined by the cache creek fault is flanked on the west by an associated anticline the southern end of the cache creek fault is an eastward dipping back thrust which abruptly steepens on the west adjacent to the anticline both the fault and the anticline are believed to have formed above a concealed detachment fault the southern part of the kirkwood plate displays structures interpreted to represent displacement transfer from the beaver creek plate back thrusts are associated with both thrust plates and played a key role in accommodating strain associated with folding the strata at the leading edges of the plates the back thrusts project upward from the main thrust fault beneath each plate they dip eastward at moderate angles in their lower segments but steepen at structurally higher levels they become overturned locally and dip westward in their uppermost segments the northern part of the study area contains the northwest trending buck creek reverse fault which dips steeply to the northeast movement on this structure ceased before the north trending thrust faults of the hilgard system were emplaced

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

non scaled analogue modelling of ams development during viscous flow a simulation on diapir like structures development of magnetic fabric within a diapirically ascending columnar body was investigated using non scaled analogue model made of plaster of paris containing small amount of fine grained homogeneously mixed magnetite the apparatus for the modelling consists of a manual squeezer with calibrated spring and a perspex container set of weak coloured layers at the bottom of the container was forced to intrude overlying fine grained sand through a hole in a board attached to the squeezer the development of ams fabric is correlated with complex flow pattern indicated by coloured and originally horizontal plaster layers strongly constrictional and vertical fabric in the base and in the lower domain of the diapir resulting from convergent and upwards flows is overprinted by subhorizontal oblate fabrics due to vertical flattening and initial divergent flow in the apical parts the measured ams fabrics are compared with natural examples of magmatic stocks and dykes

re os isotopic constraints on the source of platinum group minerals pgms from the vestřev pyrope rich garnet placer deposit bohemian massif abstract mineralogical studies of the heavy fraction from a holocene pyrope rich garnet placer deposit at vest ev krkonose piedmont basin bohemian massif have identified the presence of very rare grains of platinum group minerals pgm pt fe alloy grains are accompanied by os ir ru minerals native osmium iridium and ruthenium with inclusions of pt fe alloy and hongshiite ptcu this mineral assemblage is typical for several mantle settings including ophiolites the chemistry of the os ir ru minerals shows an enrichment of the pgm in ru which is typical of ophiolites the grain morphology of pgm and pyrope rich garnet mostly rounded with numerous euhedral subhedral grains does not exclude a common source in situ laser ablation mc icp ms was used to measure the re os isotopic compositions of single os rich grains which show heterogeneous subchondritic os isotopic compositions 187 os 188 os 0 12082 0 12505 0 00003 this precludes their low temperature origin and indicates derivation of platinum group elements pges essentially from mantle derived rocks without a significant contribution of crustal os the mantle model age t ma and re depletion model age t rd model ages range from 0 4 to 1 0 ga and most likely reflect a long history of melt depletion that affected the mantle sources of pgm

the magnetic fabric relationship between sedimentary and basement nappes in the high tatra mountains n slovakia abstract in the high tatra mountains n slovakia the magnetic fabrics are deformational in origin in sedimentary rocks of the kri na nappe in sedimentary rocks covering the underlying crystalline complex and in granitoids and metamorphic rocks of this complex the patterns of the principal susceptibilities are similar in all these units their most important feature is a girdle in magnetic foliation poles oriented nnw sse to n s in sedimentary formations the magnetic fabric pattern is compatible with that predicted by mathematical modelling to develop during a complex deformation comprising lateral shortening and simple shear which is characteristic of nappe deformation consequently the magnetic fabric in the high tatra mountains probably resulted from deformation associated with the formation and movement of the nappes during which not only the sedimentary strata but also the crystalline complex were thrust over the north european platform

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

shrimp u pb and 40ar 39ar age constraints for relating plutonism and mineralization in the boulder batholith region montana the composite boulder batholith montana hosts a variety of mineral deposit types including important silver rich polymetallic quartz vein districts in the northern part of the batholith and the giant butte porphyry copper molybdenum pre main stage system and crosscutting copper rich main stage vein system in the southern part of the batholith previous dating studies have identified ambiguous relationships among igneous and mineralizing events mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at butte have all been considered to be late stage differentiates of the boulder batholith however previous dating studies indicated that the boulder batholith plutons cooled from about 78 to 72 ma whereas copper rich main stage veins at butte were dated at about 61 ma recent efforts to date the porphyry copper molybdenum pre main stage deposits at butte resulted in conflicting estimates of both 64 and 76 ma for the mineralizing events silver rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously to resolve this controversy we used the u s geological survey stanford shrimp rg ion microprobe to date single age domains within zircons from plutonic rock samples and 40 ar 39 ar geochronology to date white mica biotite and k feldspar from mineral deposits u pb zircon ages are rader creek granodiorite 80 4 1 2 ma unionville granodiorite 78 2 0 8 ma pulpit rock granite 76 5 0 8 ma butte granite 74 5 0 9 ma altered steward type quartz porphyry dike i 15 roadcut 66 5 1 0 ma altered steward type quartz porphyry dike continental pit 65 7 0 9 ma and quartz monzodiorite of boulder baldy big belt mountains 66 2 0 9 ma zircons from rader creek granodiorite and quartz porphyry dike samples contain archean inheritance the 40 ar 39 ar ages are muscovite silver rich polymetallic quartz vein basin district 74 4 0 3 ma muscovite silver rich polymetallic quartz vein boulder district 74 4 1 2 ma muscovite early dark micaceous vein continental pit 63 6 0 2 ma biotite early dark micaceous vein continental pit 63 6 0 2 ma potassium feldspar early dark micaceous vein continental pit 63 to 59 ma and biotite biotite breccia dike continental pit 63 6 0 2 ma outlying silver rich polymetallic quartz veins of the basin and boulder mining districts probably are directly related to the 74 5 ma butte granite whereas steward type east west quartz porphyry dikes and butte pre main stage deposits are parts of a 66 to 64 ma magmatic mineralization system unrelated to emplacement of the boulder batholith the age of the crosscutting main stage veins may be about 61 ma as originally reported but can only be bracketed as younger than the 64 ma pre main stage mineralization and older than the about 50 ma eocene lowland creek intrusions the 66 ma age for the quartz monzodiorite of boulder baldy and consideration of previous dating studies in the region indicate that small ca 66 ma plutonic systems may be common in the boulder batholith region and especially to the east the approximately 64 ma porphyry copper systems at butte and gold mineralization at miller mountain are indicative of regionally important mineralizing systems of this age in the boulder batholith region resolution of the age and probable magmatic source of the butte pre main stage porphyry copper molybdenum system and of the silver rich polymetallic quartz vein systems in the northern part of the boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events

new timing constraints for the onset of laramide deformation in southwest montana challenge our understanding of the development of a thick skinned structural style during flat slab subduction

diapiric ascent of magmas through power law crust and mantle there has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust we argue here that this is mainly because the country rocks have previously been assumed to be newtonian and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten the ascent velocity of diapirs is calculated through an equation with the form of the hadamard rybczynski equation for the rise of spheres through newtonian ambient fluids this well known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid these correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of newtonian drops through power law fluids the equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks the velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth the depth of solidification of the diapir is systematically studied as a function of the geothermal gradient buoyancy of the body solidus temperature of the magma and rheological parameters of the wall rock the results show that when the wall rock behaves as a power law fluid the diapir s ascent rate increases without a similar increase in the rate of heat loss in this way diapirs rising at 10 to 102 m yr can ascend into the middle or upper crust before solidification strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates the thermal energy of the diapir is used to soften the country rock only at late stages of ascent the transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures

degree of magnetic anisotropy as a strain intensity gauge in ferromagnetic granites anisotropy of magnetic susceptibility data for a ferromagnetic granite godhra granite nw india are presented and it is shown that the degree of magnetic anisotropy p is not controlled by the mean susceptibility k m analyses carried out across a high strain zone lying between granite and adjacent gneiss show that p values are highest in samples that lie close to the contact and decrease away from it based on these results it is concluded that if p is not controlled by k m then the former can be used to gauge strain intensity variations in ferromagnetic granites

are the oroclines of the variscan belt related to late variscan strike slip tectonics terra nova 23 241 247 2011 abstract early variscan structures and magnetic anomalies delineate four oroclines along the variscan belt the arcs bend the lithostratigraphic zones of the autochthon and the allochthonous terranes which include ophiolites of the rheic suture and units of gondwanan derivation subducted on both sides of the rheic ocean a transcurrent model dominated by right lateral motion explains the genesis and evolution of the oroclines and a simple plate tectonics scenario links late variscan dextral strike slip movements in the variscides to a broad intracontinental shear zone kinematically equivalent to a transform fault terra nova 00 1 7 2011

stoped blocks in plutons paleo plumb bobs viscometers or chronometers abstract stoped blocks potentially provide information about paleohorizontal during emplacement of plutons and about the timing of and viscosities and kinematics during magmatic fabric formation in the mount stuart batholith washington we have examined magmatic fabrics patterns around stoped blocks in tonalite located near the pluton roof and completed 1 1 scale three dimensional mapping of fabric patterns around stoped blocks in diorite located 1000 m below the roof in both settings no structural record of magma ascent early emplacement or settling of the blocks is preserved instead fabric formation largely post dates the settling and trapping of stoped blocks two dimensional finite difference thermal modeling and calculations of sinking rates support short

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

emplacement kinematics of a granite diapir the chindamora batholith zimbabwe the geometric features of a large 50 40 km granitic batholith and its surrounding greenstone belt envelope are described the batholith is made up of several intrusive components and an intrusion sequence based on contact relations and xenoliths has been established the igneous rocks show a local foliation s fabric and preferred orientation of minerals the intensity of this fabric accords with the finite strain determined from xenolith shape analysis when the strain variations are analysed in detail it appears that the batholith was built up by a succession of magmatic pulses each of which led to a stretching of the previously consolidated igneous material so called balloon tectonics evidence is presented that xenolith deformation commences only when the surrounding igneous material has crystallized and a method is suggested whereby the position of the surface separating crystal supported phase plastic behaviour from melt supported phase liquid behaviour at different stages in the plutonic history can be determined ductile shear zones are present and their geometry accords well with the shape changes necessary to allow the outer parts of the pluton to stretch

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

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

overprinting of magnetic fabrics in granites by small strains numerical modelling abstract the anisotropy of magnetic susceptibility ams is used to determine weakly defined mineral preferred orientation fabrics in granites however the effects of small strains superimposed on the low intensity ams fabrics typical of these rocks is not well understood numerical simulations are presented which investigate the effects of small superimposed strains on the shapes magnitudes and orientations of pre existing ams fabrics which are similar to those reported in the literature for undeformed granites a computer generated biotite petrofabric is subjected to progressive strain using march s strain response model for pure shear plane strain axially symmetric shortening and simple shear regimes results indicate that a pre existing ams fabric might be significantly modified or overprinted by small strains 10 15 strain in the pure shear and axially symmetric shortening regimes 0 5 1 0 for simple shear a new ams fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric at low strains the evolution of the shape of the ams ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework the ams ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain this modelling suggests that small strains affecting a pluton following its emplacement could mask ams fabrics developed earlier the results also suggest that in simple shear the orientation of the ams ellipsoid may serve as a shear sense indicator for small strains even if the earlier fabric is not isotropic

four magmatic fabrics in the tuolumne batholith central sierra nevada california usa implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust for the first time four distinct magmatic fabrics are documented in a composite plutonic body the tuolumne batholith central sierra nevada california usa one type of fabric was formed by strain caused by highly localized magma flow type 1 whereas the other three chamber wide fabrics recorded strain increments during boundary processes along batholith margins type 2 superimposed by increments of heterogeneous regional tectonic strain types 3 and 4 our present work indicates that in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static actively deforming and rheologically complex crystal mushes at lower melt percentages we demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem placement regional tectonic strain however it may be commonly very problematic to infer the exact nature of flow or even fabric forming process from the preserved rock record our study also exemplifies how examination of magmatic fabric patterns in plutons complemented with geochronology may provide crucial constraints on the interplay among successive magma emplacement fabric preservation temporal evolution of strain fields in a crystallizing magma chamber and the development of crystal mush zones

the magnetic zone axis a new element of magnetic fabric for the interpretation of the magnetic lineation abstract the magnetic zone axis is the common direction of the intersection of the magnetic foliations of different samples it is easily determined using tensors from the minimum susceptibility axes it provides useful information for the interpretation of the magnetic fabric particularly of the maximum axis a bootstrap method reveals small local disturbance of the magnetic foliation it yields also the confidence zone of the magnetic zone axis

bursting the bubble of ballooning plutons a return to nested diapirs emplaced by multiple processes a popular model for the emplacement of roughly spherical plutons is that of ballooning or in situ inflation of a magma chamber in a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent while the hotter tail of magma continues to rise and expand the already crystallized outer margin this expansion forms a concentric gneissic to mylonitic foliation and flattening type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures our reexamination of three supposedly ballooned plutons ardara ireland cannibal creek australia papoose flat california and evaluation of published descriptions of many others indicate that this model is largely incorrect deflections of country rock structures strains and porphyroblast matrix relationships indicate that only minor to moderate expansion usually 30 or less occurred during emplacement that other emplacement mechanisms must have occurred and that regional deformation continued during and after emplacement internal structures indicate that when magma chamber expansion did occur it did so by flow of magma that magmatic foliations and lineations formed late in the magma chamber evolution that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion and that only minor internal subsolidus deformation results from emplacement this study indicates that many plutons previously interpreted as post tectonic ballooning plutons are better viewed as syntectonic nested diapirs emplaced by a variety of country rock material transfer processes this nested diapir model implies that magma ascent may occur by rise of large magma batches instead of transport in dikes followed by ballooning that magma chamber dynamics differ from that in the ballooning model and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt

model ages of fracture fillings and mineralogical and geochemical evidence for water rock interaction in fractures in granite the melechov massif czech republic abstract the mineral chemical and pb isotope composition were investigated in the host granite and low temperature open fracture fillings from the 100 m deep borehole pdm 1 located in the melechov massif of the moldanubian batholith the fillings are dominated by limonite and clay mineral illite kaolinite montmorillonite nontronite chlorite mixtures accompanied by minor amounts of residual minerals from the parent rock quartz feldspars zircon monazite chemically contrasting fracture assemblages occur mainly at lesser depths

elastic anisotropy and pore space geometry of schlieren granite direct 3 d measurements at high confining pressure combined with microfabric analysis pore space geometry of granitic rocks and its evolution with depth are key factors in large scale seismics or in projects of enhanced geothermal systems or of deep hazardous waste repositories in this study we studied macroscopically anisotropic schlieren bearing granite by experimental p wave velocity v p measurements on spherical sample in 132 directions at seven different confining pressures in the range 0 1 400 mpa in order to discriminate the phenomena affecting the rock elastic properties we analysed the orientation of microcracks and of grain boundaries and we measured the anisotropy of magnetic susceptibility of the rock three sets of microcracks were defined with two of them linked to the massif exfoliation process and one to cooling contraction of the massif during pressurization the measured mean v p and v p anisotropy degree at ambient pressure and at highest confinement 400 mpa yielded 3 3 km s 1 and 24 per cent and 6 2 km s 1 and 3 per cent respectively the associated v p anisotropy pattern was transversely isotropic and governed by the schlieren with a minimum v p direction perpendicular to them and a girdle of high v p directions parallel to them the highest change in v p was observed between 0 1 and 10 mpa suggesting a significant closure of porosity below depths of 500 m change of the v p anisotropy pattern to orthorhombic together with increase of mean v p and v p anisotropy degree during depressurization was attributed to inelastic response of one of the sets of microcracks to the loading unloading cycle

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

in defense of magmatic diapirs abstract diapirism as a crustal magma ascent mechanism has been recently criticized we contend that this reflects an overly simplistic view that diapirs must resemble modeled hot stokes diapirs and the perception that magma ascent in dikes is a more problem free mechanism for the construction of plutons we describe four cordilleran plutons that have characteristics much more compatible with diapirs than dike fed chambers these plutons were emplaced at depths ranging from 10 to 30 km and record different parts of diapiric ascent paths most ascended during complex visco elastic flow of host rock during regional deformation have narrow structural aureoles indicating power law behavior of host rock and were constructed of multiple batches of magma attributes enabling them to ascend greater distances than single hot stokes diapirs some features of these plutons are not typically attributed to diapirs and thus we introduce the term visco elastic diapir for bodies consisting of one or more batches of magma rising together with length to width ratios

magnetic fabric and modeled strain distribution in the head of a nested granite diapir the melechov pluton bohemian massif abstract the melechov pluton bohemian massif is interpreted as a mid crustal nested granitic diapir with an apical part exposed at the present day erosion level the diapir head exhibits a concentric structure defined by lithologic zoning and by the anisotropy of magnetic susceptibility ams in concert with theoretical models outward dipping margin parallel magnetic foliations are associated with oblate shapes of the susceptibility ellipsoids and higher degree of anisotropy passing inward into weaker triaxial to prolate fabric by contrast magnetic fabric in an inner granite unit is in places oriented at a high angle to internal contacts and is interpreted as recording an internal diapir circulation we use inverse modeling to calculate strain variations across the diapir from the ams data the magnetic fabric parameters and calculated strains are in agreement with strain distribution in heads of model newtonian diapirs traveling a distance of two body radii and suggest granitic magma ascent as a crystal poor suspension followed by crystallization of fabric markers and their response to strain near the final emplacement level the intrusive fabric thus formed late but though generally weak was still capable of recording incremental strain gradient in the granite diapir

numerical modeling of the motion of rigid ellipsoidal objects in slow viscous flows a new approach abstract a simple algorithm for modeling the rotation of rigid ellipsoidal objects in viscous flows based on jeffery s 1922 proceedings of the royal society of london a102 161 179 theory is presented and is implemented in a fully graphic mathematics application mathcad http www mathsoft com the orientation of ellipsoidal objects is specified in terms of polar coordinate angles that can be easily converted to the trend and plunge angles of the three principal axes rather than the euler angles with the mathcad worksheets presented in the supplementary data associated with this paper modeling the rotation paths of individual rigid objects the development of inclusion trail geometry within syn kinematic porphyroblasts and the development of preferred orientation and shape fabrics for a population of rigid objects becomes as easy a task as using a spreadsheet the shape and preferred orientation fabrics for a population of rigid objects can be presented in both a three dimensional form and a two dimensional form allowing easy comparison between field data and model predictions the modeler can customize the type and format of the output to best fit the purpose of the investigation and to facilitate the comparison of model predictions with geological observations application examples are presented for various types of modeling involving rigid objects

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

paleomagnetic results from the upper cretaceous maudlow and livingston formations southwest montana a paleomagnetic investigation of welded tuff units from the late cretaceous livingston and maudlow formations was undertaken to refine the late cretaceous to paleocene apwp for cratonic north america after progressive af and thermal demagnetization the maudlow formation 46 1 n 248 9 e yields a paleomagnetic pole at 70 n 208 e 11 sites a95 9 k 22 tectonic correction improves the directional grouping and the change is statistically significant two normal polarity intervals bounding a single reversed polarity interval are present we interpret the reversed interval to be chron 33r on the basis of 1 stratigraphic correlation of the maudlow formation with the campanian sedan formation located 40 km to the east and 2 k ar dates of 83 2 ma at the middle of the sampled section and 74 9 1 ma near the top of the sampled section the livingston formation 45 5 n 250 1 e yields a high hc magnetization with a negative fold test suggesting a post folding secondary magnetization the livingston mean direction yields a paleomagnetic pole at 72 n 218 e 10 sites a95 4 k 107 and is clearly distinguishable from a co existing lower hc component reflecting a recent viscous overprint we interpret the high hc magnetization as late cretaceous to paleocene in age neither the maudlow formation pole nor the pole derived from the high hc overprint in the livingston formation agree with previously published poles for either the late cretaceous or tertiary but they do lie on a simple continuation of the apwp between late cretaceous and paleocene mean poles secondary magnetizations found in the franciscan complex of california also yield poles which lie on a proposed extension of the apwp if the franciscan secondary poles represent an overprint event associated with early tertiary accretion and if the livingston secondary pole represents a deformational event related to laramide uplift of the nearby beartooth range then together these three poles suggest that the north american apwp may continue farther to the east and south to define a cretaceous tertiary cusp

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

the significance of microgranitoid enclave shapes and orientations abstract enclaves are often incorrectly used to measure magmatic strains in plutons we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons 1 adjacent enclaves may form at different times and different places and initially have non spherical shapes with axial ratios up to 2 7 2 the final shapes and orientations of enclaves are a complex function of a initial shape and temperature of enclaves b subtle changes in composition melt percents volatiles grain sizes and thus temporally variable viscosity contrasts between the enclave and magma c a competition between strain and interfacial energies and d deformation path which may include internal strain and rigid rotations caused by magma flow during ascent convection expansion chamber boundary processes and tectonism 3 enclaves spend much of their time in magma as relatively rigid objects and thus rigidly rotate and potentially break apart rather than strain at matrix strain rates 4 in some instances enclaves do not record or track finite strain because of the above final enclave populations are heterogeneous and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses on the other hand a comparison of the preserved characteristics of igneous layering mineral fabrics and carefully evaluated enclave fabrics including internal mineral alignment in enclaves may provide qualitative data on the changing magnitude and kinematics of magmatic strains

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

magnetic fabric shape preferred orientation and regional strain in granitic rocks since granites do not preserve easily mappable foliations lineations and strain markers determining the degree of shape preferred orientation spo in them is challenging the aim of this paper is to present a methodology for determining variation in the degree of spo in granites and to test the feasibility of correlating the same with regional strain the case of the godhra granite located in the southern parts of aravalli mountain belt india is taken as an example degree of spo is determined using two different techniques a anisotropy of magnetic susceptibility ams and b strength of mineral lineation determined by calculating the concentration parameter k of von mises distribution by digital image analysis of biotite kbi and feldspar kf in thin sections prepared parallel to the magnetic foliation plane spo data obtained using the above two techniques from 20 samples distributed across the entire granite are analysed samples with a higher k also have a stronger magnetic fabric magnetic lineation l and degree of magnetic anisotropy p 0 also the degree of spo is greater for granites from the southern parts as compared to the northern parts it is argued that this variation in degree of spo cannot be attributed to rheological differences or strain resulting from difference in aspect ratios regional strain is inferred as the dominant factor since the emplacement of the godhra granite is synchronous with the tectonic rejuvenation along the central indian tectonic zone citz that lies to its south the higher degree of spo in the southern parts is attributed to its proximity to the citz whilst caution needs to be exercised to directly apply magnetic data as a measure of strain intensity on a regional scale it can be a useful guide to select samples for detailed spo analysis using some alternative technique especially for granites and other magmatic rocks that are devoid of mesoscopic strain markers 2006 elsevier ltd all rights reserved

basement and cover rock deformation during laramide contraction in the northern madison range montana and its influence on cenozoic basin formation two major laramide fault systems converge in the northwestern madison range the northwest striking southwest vergent spanish peaks reverse fault and the north striking east vergent hilgard thrust system analysis of foliation attitudes in basement gneiss north and south of the spanish peaks fault indicates that the basement in thrusted blocks of the hilgard thrust system has been rotated by an amount similar to that of the basement cover contact steeply dipping north striking breccia zones enclosing domains of relatively undeformed basement may have permitted domino style rotation of basement blocks during simple shear between pairs of thrusts in most places along the hilgard thrust system a large basement overhang produced by thrusting of archean blocks above rocks as young as late cretaceous overlies a tight footwall syncline this tight folding is largely concentric and was accommodated by flexural slip resulting in severe crowding in synclinal hinges that resulted in observed or inferred features such as bedding plane slip imbricate and out of syncline thrusting and hinge collapse the north striking madison normal fault system a zone of tertiary and quaternary valley forming normal faults is approximately parallel to the hilgard thrust system in some places normal faults are reactivated thrusts on which large basement overhangs of the hilgard thrust system were dropped back into the madison valley and covered by tertiary basin fill deposits leaving only the rocks of the footwall synclines exposed in other places both the thrusted archean blocks and the near isoclinal footwall synclines are well preserved this paired fault system the madison normal fault system and the hilgard thrust system of the northern madison range is strikingly similar to other paired systems in southwestern montana along and adjacent to the western margins of the ruby range snowcrest range greenhorn range tobacco root mountains and bridger range such systems may be the result of collapse of the crestal zones of large laramide basement uplifts arches during tertiary extension no hydrocarbon discoveries have been made in this unique structural province however petroleum exploration here has focused on basement cored anticlines both surface and subthrust related to the two major laramide fault systems and on the fault bounded blocks of tertiary rocks within the post laramide extensional basins the interplay of the two laramide fault systems during both laramide shortening and tertiary extension has produced a variety of possible structural traps in the madison range that have not yet been thoroughly investigated