microstructural deformation record of an orogen parallel extension in the vepor unit west carpathians abstract deformation microstructures of quartzo feldspathic rocks have been used to analyse the orogen parallel stretching of the vepor unit in the west carpathians during cretaceous convergence this process occurs contemporaneously with the burial of the vepor basement related to the overthrusting palaeozoic volcano sedimentary sequences of the gemer unit the microstructures develop along a metamorphic gradient indicating an increase of pressure and temperature towards the structural footwall the recrystallization of quartz occurs in the field of subgrain rotation recrystallization sgr while the plagioclase and k feldspar disintegrate into a neocrystallized muscovite albite matrix palaeopiezometry of the recrystallized quartz in conjunction with p t estimates yield strain rates of 9 10 12 to 6 10 14 s 1 depending on temperature 380 480 c and choice of piezometer and flow law calibration strain analysis of the recrystallized quartz aggregates and quartz texture analysis indicate an overprint of the variscan magmatic and high grade fabrics by a cretaceous plane strain deformation resulting in vertical shortening and horizontal orogen parallel east west stretching spatial distribution and conflicting sense of shear of the asymmetrical quartz lattice preferred orientation patterns indicate that the large scale pure shear deformation of the vepor unit is locally accommodated by simple shear zones observed metamorphic gradients across the vepor unit are interpreted to result from later large scale folding and heterogeneous exhumation of the orogen parallel deformation fabric

paleomagnetic and 40ar 39ar geochronologic data bearing on the structural evolution of the silver peak extensional complex west central nevada the silver peak extensional complex located in the silver peak range of west central nevada is a displacement transfer system linking the furnace creek fish lake valley fault system and transcurrent faults of the central walker lane late neogene northwest directed motion of an upper plate composed of lower paleozoic sedimentary rocks and late tertiary volcanic and volcaniclastic strata exhumed a lower plate assemblage of metamorphic tectonites with proterozoic and mesozoic protoliths paleomagnetic investigation of miocene pliocene pyroclastic and sedimentary rocks of the upper plate and miocene mafic dikes in the lower plate reveals modest horizontal axis tilting northwest side up and vertical axis rotation clockwise within the extensional complex eight to ten samples from each of 123 sites were demagnetized 95 sites yielded interpretable results dual polarity results from one population of mafic dikes in the lower plate assemblage indicate moderate northwest side up tilting declination d 329 inclination i 37 95 4 3 number n 30 sites in situ 95 the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction some dikes yield exclusively normal polarity results that are interpreted to indicate modest clockwise vertical axis rotation d 021 i 57 95 4 3 n 19 sites in situ concurrent with uplift of the lower plate rocks and nine sites yield magnetization directions that are north directed with positive inclinations of moderate steepness similar to an expected miocene field late miocene pyroclastic rocks in the upper plate yield normal polarity magnetizations suggestive of moderate clockwise vertical axis rotation d 032 i 53 95 8 8 n 10 sites the apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field because the overall dispersion of the vgp virtual geomagnetic pole positions is high for the latitude of the site location middle miocene sedimentary rocks probably were remagnetized shortly after deposition of eight 40ar 39ar determinations from mafic dikes in the lower plate five groundmass concentrates yield saddle shaped age spectra and one separate provided a plateau date of low confidence isochron analysis reveals that all six groundmass concentrates contain excess ar if rapid cooling and ar retention below 250 c are assumed the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10 5 ma the 40ar 39ar age spectrum data are consistent with existing fission track cooling and k ar isotopic age information from lower plate granitic rocks and indicate rapid cooling of the lower plate assemblage from well above 300 c to 100 c between 13 and 5 ma rapid cooling may explain the overall distribution of paleomagnetic results from lower plate intrusions such that the earliest acquired magnetizations reflect both northwest side up tilt and clockwise rotation and the younger magnetizations reflect northwest side up tilt overall the paleomagnetic data from the silver peak extensional complex are interpreted to suggest that vertical axis rotation of crustal scale blocks associated with displacement transfer in the central walker lane may play an integral part in accommodating strain within a continental displacement transfer system

strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

tertiary basin development and tectonic implications whipple detachment system colorado river extensional corridor california and arizona a stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly miocene deposits of the whipple detachment system colorado river extensional corridor in the mohave mountains and aubrey hills of arizona and the eastern whipple mountains of california near parker dam these deposits comprise four unconformity bounded sequences composed of locally derived epiclastic and volcanic rocks and the peach springs tuff the three older sequences represent syntectonic units that were deposited coeval with detachment faulting and the fourth is interpreted to be postextensional the sequences are correlated between four fault bounded regions which are the remnants of four different depositional basins similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units the basins developed in about the same positions relative to each other and to volcanic sources as they occupy at present this is shown by gradational changes of pre tertiary rock types between regions systematic variations in the abundance of magmatic units and correlative volcanic units that occur in two adjacent regions the basins formed in the early miocene from segmentation of the upper crust into blocks bounded by high angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low angle detachment fault extreme rotation of one large crustal block which constitutes the central mohave mountains is recorded by a major unconformity in the lower miocene section of one basin because coeval sections that formed in adjoining basins do not record this rotation the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently these proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the whipple detachment system

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

paleomagnetic data bearing on style of miocene deformation in the lake mead area southern nevada abstract paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late miocene hamblin cleopatra volcano which was structurally dismembered during the development of the lake mead fault system lmfs provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the basin and range province sinistral separation along the fault system dissected the volcano into three major blocks the eastern cleopatra lobe of the volcano is structurally the most intact section of the volcano normal and reverse polarity data from paleomagnetic sites collected along traverses in the cleopatra lobe yield an in situ grand mean of declination d 339 inclination i 54 95 3 1 k 27 2 n 81 sites the rocks of the central core of the volcano yield an in situ grand mean of d 3 i 59 95 6 8 k 42 5 n 11 sites six normal five reverse polarity sites collected within the western hamblin lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of d 168 i 58 95 6 5 k 28 9 n 18 sites interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area considers the possibility of two different types of structural corrections a stratigraphic tilt correction involves restoring flows to the horizontal using the present strike this correction assumes no initial possibly radial dip of flows of the volcano and is considered invalid a structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of d 9 i 53 95 3 1 k 27 2 and d 58 i 78 95 6 8 k 42 5 for the cleopatra lobe and the central intrusive core respectively the data from the cleopatra lobe are slightly discordant in a clockwise sense from expected middle to late miocene field directions the data from the volcano are not consistent with a proposed structural model of uniform moderate magnitude statistically significant counter clockwise vertical axis rotation of fault bounded blocks during overall sinsitral displacement along the lmfs we also analyzed dikes of the northernmost part of the miocene wilson ridge hypabyssal igneous complex strata of the triassic chinle formation and basalt flows of the miocene west end wash callville mesa volcanic centers dikes in the wilson ridge pluton and the triassic strata yield magnetizations with directions suggestive of statistically significant clockwise vertical axis rotations consistent with local large magnitude shear of crustal fragments near some of the faults of the lmfs late cenozoic deformation of the hamblin cleopatra volcano area appears to have been non uniform in scale and magnitude and no single structural model involving strictly strike slip faulting can account for the observed paleomagnetic data

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

stratigraphy structure and potassic alteration of miocene volcanic rocks in the sleeping beauty area central mojave desert california the sleeping beauty area of the southeastern cady mountains central mojave desert contains a sequence of volcanic rocks 3 km thick which was erupted approximately 20 m y ago plate reconstructions indicate that the change from subduction to transform fault tectonics off the coast of california occurred about 20 m y ago as well and so these rocks contain a record of volcanism extensional faulting and potassic metasomatism during this time of tectonic transition the volcanic sequence comprises basalt to rhyolite flows and tuffs which are overlain by the widespread peach springs tuff and by terrestrial sediments bedding below the peach springs tuff generally dips 10 50 to the southwest and is cut by numerous steep northwest to north trending faults tilting predated deposition of the peach springs tuff bedding fault relationships indicate that deformation was not by tilted book geometry in its simplest form no significant low angle faults are exposed but such faults occur to the west and may underlie the area a major open southeast trending anticline in the area may be a drag fold related to the post late miocene right lateral north trending ludlow fault all units below the peach springs tuff were locally affected by severe potassic metasomatism which raised measured k2o contents to as high as 13 3 wt metasomatized rocks occur in irregular zones that follow northwest trending faults and are best developed around northwest trending breccia zones which have jasper matrices ba and mn prospects are invariably found in metasomatized rocks geologic and geochemical constraints indicate that metsomatism occurred at shallow depth 1 or 2 km and low temperature 150 c metasomatism occurred in at least two distinct pulses and apparently predated deposition of the peach springs tuff the k may have been derived from percolating closed basin brines or through hydrogen metasomatism of rocks deeper in the complex

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

passive and active margin history of the northern tatricum western carpathians slovakia the tatricum an upper crustal thrust sheet of the central western carpathians comprises pre alpine crystalline basement and a late paleozoic mesozoic sedimentary cover the sedimentary record indicates gradual subsidence during the triassic early jurassic initial rifting a jurassic early cretaceous extensional tectonic regime with episodic rifting events and thermal subsidence periods and middle cretaceous overall flexural subsidence in front of the orogenic wedge prograding from the hinterland passive rifting led to the separation of the central carpathian realm from the north european platform a passive margin rimmed by peripheral half graben was formed along the northern tatric edge facing the vahic south penninic oceanic domain the passive versus active margin inversion occurred during the senonian when the vahic ocean began to be consumed southwards below the tatricum it is argued that passive to active margin conversion is an integral part of the general shortening polarity of the western carpathians during the mesozoic that lacks features of an independent wilson cycle an attempt is presented to explain all the crustal deformation by one principal driving force the south eastward slab pull generated by the subduction of the meliatic triassic jurassic tethys oceanic lithosphere followed by the subcrustal subduction of the continental mantle lithosphere

mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development abstract to determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load bearing framework to an interconnected weak phase shear experiments were conducted on a fine grained gneiss 58 quartz interconnected 28 plagioclase dispersed 13 biotite aligned but dispersed at 745 and 800 c 1500 mpa and two strain rates 2 10 5 and 2 10 6 s 1 in experiments with a high phase strength contrast hpsc 25 and 45 between dispersed biotite and framework quartz grains biotite grains become interconnected due to stress concentration at their tips that allow local semi brittle flow of intervening quartz and cataclasis of intervening plagioclase in the higher temperature hpsc samples the biotite partially dehydrated in high strain zones a single narrow shear zone formed because the psc between the quartz and biotite reaction product layers remained high in the lower temperature hpsc samples there was no reaction and a penetrative s c fabric formed the fabric is defined by the many multiply interconnected biotite strands that formed because the psc between the quartz and biotite decreased as the biotites kinked in slower strain rate experiments with a much lower psc 10 biotite interconnection occurs by shearing into quartz quartz boundaries where new weak strain free recrystallized grains form at low strain a weak s c fabric forms but it evolves to an s c fabric as the psc decreases with strain thus the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection the degree of strain localization and the fabric

lateral extrusion in the eastern alps part 1 boundary conditions and experiments scaled for gravity lateral extrusion encompasses extensional collapse gravitational spreading away from a topographic high in an orogenic belt and tectonic escape plane strain horizontal motion of wedges driven by forces applied to their boundaries in the eastern alps it resulted from 1 an overall northerly compression apulia against eurasia 2 a strong foreland bohemian massif 3 lack of constraint along a lateral boundary carpathian region and 4 a previously thickened gravitationally unstable thermally weakened crust eastern alpine orogenic belt six indentation experiments reproduce lateral extrusion at lithospheric scale the models have two to four lithospheric layers with a mohr coulomb rheology for the upper and a viscous rheology for the lower crust the lithosphere rests upon a low viscosity asthenosphere a broad indenter a narrow deformable area and a weakly constrained eastern margin fullfill as closely as possible conditions in the eastern alps indentation produces both thickening in front of the indenter and escape of triangular wedges lateral variations in crustal thickness become attenuated by gravitational spreading the overall fault pattern includes domains of reverse strike slip oblique normal and pure normal faults strike slip faults in conjugate sets develop serially the narrow width of the deformable area and the strength of the foreland determine the angles between the sets gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin opposite the unconstrained margin the indenter front shows thrusts and folds intersecting with the conjugate strike slip sets a triangular indenter favors spreading high velocity of indentation favors escape high confinement limits lateral motion inhibits spreading and favors thickening lateral extrusion in the eastern alps is best modeled by 1 a weak lateral confinement 2 a broad and straight indenter 3 a narrow width of the deformable area and 4 a rigid foreland crustal thickening lateral escape and gravitational spreading all contribute to the overall deformation

contemporaneous plutonism and strike slip faulting a case study from the tonale fault zone north of the adamello pluton italian alps 1 a field microstructural and geochronological study of contemporaneous plutonism and strike slip faulting along the eastern tonale fault zone provides new insights into the interrelationships between magmatic emplacement contact metamorphism and shearing and it places new time constraints for the periadriatic fault system although pluton emplacement and shearing were not caused by each other they mutually interacted during contact metamorphism the character of the composite tonale fault zone varies considerably with its proximity to the northern margin of the adamello pluton from a paired greenschist facies mylonite belt northern mylonite zone and cataclastic fault zone in the west to a fault zone consisting of northern mylonite zone cataclastic fault zone and a second mylonite belt southern mylonite zone formed by contemporaneous shearing and contact metamorphism in the east ongoing strike slip motion led to telescoping and advective cooling of the deforming contact aureole new u pb zircon ages for the northernmost adamello intrusions the avio at 34 6 1 0 ma and the presanella at 32 0 2 3 ma date the onset of contact metamorphism rb sr and k ar cooling ages and new zircon fission track ages provide evidence that postintrusive cooling to below 300 c was achieved rapidly i e approximately 30 myr ago being consistent with a shallow crustal emplacement at ambient temperatures of approximately 250 c cataclastic dextral strike slip faulting continued until about 20 ma when the tonale fault zone became offset by a number of minor sinistral shears and by the giudicarie fault hence dextral strike slip motion along the periadriatic fault system east of the bergell pluton lasted from approximately 35 20 myr ago

automatic grain boundary detection and grain size analysis using polarization micrographs or orientation images abstract a simple procedure for creating grain boundary maps from petrographic thin sections termed the lazy grain boundary lgb method has been devised it is based on a set of macro commands programmed for nih image a public domain image processing software the grain boundary detection is based on gradient filtering the procedure makes use of multiple input images sets of regular polarized micrographs or where possible orientation misorientation images on each image of a given input set only the most significant grain boundaries are detected and by combining those a single grain boundary map is obtained image models and criteria for grain boundary identification are discussed briefly the principal aim of the lgb method is to facilitate the production of large grain boundary maps containing several thousand cross sectional areas to provide statistically valid samples for grain size analysis a section of black hills quartzite was used to test the lgb method five different input sets were prepared and used for automatic lgb analysis one set was additionally used for an interactive analysis and for a manual tracing from all seven grain boundary maps the 2 and 3 d grain size distributions were calculated the results are compared and the different image sets and procedures are rated for precision over and undersegmentation and speed of preparation

tectonometamorphic evolution of the koralm complex eastern alps constraints from microstructures and textures of the plattengneis shear zone abstract lattice preferred orientations lpo of quartz have been investigated along a south north oriented section across the plattengneis of the koralm complex eastern alps the plattengneis forms an important shear zone within the austroalpine nappe complex of the eastern alps which has developed during the cretaceous evolution of the alpine orogen the quartz c axes form small circle distributions in the southernmost parts of the koralm complex which represent the uppermost structural level of the plattengneis further to the north two maxima between the y and z directions of the finite strain can be interpreted in terms of preferred slip on the rhomb planes these fabrics continuously grade into type i and type ii crossed girdle distributions in a northward direction a strong maximum near the y axis with the tendency to be distributed along a single girdle with three corresponding maxima of a axes near the margin of the pole figure can be observed in the central and northern parts such lpo are characteristic for both high grade metamorphic conditions and high finite strain the microstructures display that the deformation within the plattengneis shear zone was synmetamorphic a continuous increase of peak temperatures and pressure from approximately 550 c to approximately 750 c from the south to the central parts can be inferred from geothermometric calculations the temperatures then decrease to approximately 650 c from the central parts to the north the related pressures increase from 8 to 16 kbar and then decrease to 10 kbar the lpo changes that have been observed in the study area are best interpreted in terms of temperature dependence of the activation of glide systems within quartz aggregates the temperature and pressure evolution may indicate that the central parts of the koralm complex have been exhumed by larger amounts than the northern and southern parts this is also documented by the lpo evolution therefore we assume that the plattengneis shear zone formed during the exhumation of the koralm complex and is related to the exhumation of high pressure units in the footwall of this shear zone accordingly the kinematics of the plattengneis shear zone is rather extensional than thrust related the implications for the structural evolution of the eastern alps are shortly discussed

rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada 1 we use paleomagnetic data from tertiary volcanic rocks to address the rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada samples from the upper miocene 9 ma eureka valley tuff suggest clockwise vertical axis block rotations between ne striking left lateral faults in the bridgeport and mono basins results in the bodie hills suggest clockwise rotations r r 95 confidence limits of 74 8 since early to middle miocene 12 20 ma 42 11 since late miocene 8 9 ma and 14 10 since pliocene 3 ma time with no detectable northward translation the data are compatible with a relatively steady rotation rate of 5 2 ma 1 2 since the middle miocene over the three examined timescales the average rotation rates have probably not varied by more than a factor of two over time spans equal to half of the total time interval our paleomagnetic data suggest that block rotations in the region of the mina deflection began prior to late miocene time 9 ma and perhaps since the middle miocene if rotation rates were relatively constant block rotation in the bodie hills is similar in age and long term average rate to rotations in the transverse ranges of southern california associated with early transtensional dextral shear deformation we speculate that the age of rotations in the bodie hills indicates dextral shear and strain accommodation within the central walker lane belt resulting from coupling of the pacific and north america plates

origin of bimodal volcanism southern basin and range province west central arizona miocene volcanic rocks in the castaneda hills area west central arizona are interbedded with continental clastic sedimentary rocks minor limestone gravity glide blocks of precambrian and paleozoic rocks and monolithologic megabreccia the sedimentary and volcanic units dip to the southwest and are offset by northwest trending listric and high angle normal faults the listric faults coalesce at the rawhide detachment fault which overlies mylonitic gneiss the volcanic suite is strongly bimodal rocks with 55 to 71 wt sio 2 are rare on the basis of age geomorphic position and petrography five volcanic units can be distinguished older basalts 18 7 and 16 5 m y old quartz bearing basalts 13 7 and 12 4 m y old rhyolite lavas and tuffs 15 1 to 10 3 m y old mesa forming basalts 13 1 to 9 2 m y old and megacryst bearing basalts 8 6 to 6 8 m y old most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali olivine basalts many rhyolites contain more than 75 wt sio 2 the initial whole rock sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle the chemical composition of some of the megacrysts in megacryst bearing basalts with 87 sr 86 sr i equal to 7035 and 7038 supports a high pressure mantle origin the low 7034 sr ratio and lack of evidence for mixing with young rocks indicate that the quartz bearing basalts were also derived from the mantle other basalts with 87 sr 86 sr i 0 705 probably were derived from old lithospheric mantle with a high rb sr ratio and do not appear to be contaminated with old upper crustal material the rhyolites have initial sr isotopic ratios of 0 7093 and 0 7141 these ratios indicate that the rhyolites were not differentiated from the basalts partial melting of 1 3 b y old lower crustal material with rb sr 0 10 to 0 19 satisfactorily explains the isotopic ratios of the rhyolites granulite which may constitute the lower crust in this part of arizona has rb sr ratios similar to those required to produce the rhyolites k substituted for na during cooling and devitrification in some of the rhyolites partial melting of upper mantle peridotite and old lower crustal granulite from 19 to 7 m y ago in the castaneda hills area produced the bimodal volcanic suite the nearly contemporaneous production of basaltic and rhyolitic magma from the earth9s crust and mantle requires extremely heterogenous source regions asthenospheric upwelling associated with basin range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma the basaltic and rhyolitic magmas formed in separate source regions rose independently and erupted at the same time and place

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

tilt and rotation of the footwall of a major normal fault system paleomagnetism of the black mountains death valley extended terrane california paleomagnetic data have been obtained from miocene intrusions including crosscutting mafic and felsic dikes and proterozoic crystalline rocks to evaluate deformation during miocene and younger extension and unroofing of the black mountains death valley california synrift intrusions contain a well defined and at the site level well grouped magnetization interpreted to be of dual polarity the in situ direction of which is discordant in declination and inclination with an expected late cenozoic reference direction the oldest of these intrusions 11 6 0 2 ma gives a magnetite dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures about 550 to 580 c that is interpreted to predate unroofing from midcrustal depths in situ site mean directions of this magnetization are directed toward the west and west northwest with moderate to shallow positive down and negative up inclinations the variation in direction of magnetization particularly inclination with site locality around the turtleback structures along the western flank of the black mountains is interpreted to result from folding of the intrusion after remanence acquisition younger intrusions 8 7 ma generally give magnetizations with inclinations similar to expected miocene values two populations of in situ site means are identified one with southwest declination and negative inclination the other with northward declination and positive inclination a preferred interpretation for footwall deformation involves from oldest to youngest 1 southwest side down tilting of the entire range block of some 20 to 40 to possibly 70 and at least locally folding of the crystalline rocks on a trend parallel to the death valley turtlebacks between 11 6 and 8 7 ma 2 progressive east to west footwall unroofing between 8 7 and ca 6 5 ma and 3 clockwise rotation 50 to 80 of much of the black mountains after the core detached from stable terrane to the west we interpret late rotation of the black mountains as oroflexure related to right lateral shear along the death valley fault zone

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

carbon cycle history through the jurassic cretaceous boundary a new global δ13c stack we present new carbon and oxygen isotope curves from sections in the bakony mts hungary constrained by biostratigraphy and magnetostratigraphy in order to evaluate whether carbon isotopes can provide a tool to help establish and correlate the last system boundary remaining undefined in the phanerozoic as well provide data to better understand the carbon cycle history and environmental drivers during the jurassic cretaceous interval we observe a gentle decrease in carbon isotope values through the late jurassic a pronounced shift to more positive carbon isotope values does not occur until the valanginian corresponding to the weissert event in order to place the newly obtained stable isotope data into a global context we compiled 31 published and stratigraphically constrained carbon isotope records from the pacific tethyan atlantic and boreal realms to produce a new global 13c stack for the late oxfordian through early hauterivian interval our new data from hungary is consistent with the global 13c stack the stack reveals a steady but slow decrease in carbon isotope values until the early valanginian in comparison the late jurassic early cretaceous 13c curve in gts 2012 shows no slope and little variation aside from the well defined valanginian positive excursion chemostratigraphic correlation durschning the jurassic cretaceous boundary interval is difficult due to relatively stable 13c values compounded by a slope which is too slight there is no clear isotopic marker event for the system boundary the long term gradual change towards more negative carbon isotope values through the jurassic cretaceous transition has previously been explained by increasingly oligotrophic condition and lessened primary production however this contradicts the reported increase in 87sr 86sr ratios suggesting intensification of weathering and a decreasing contribution of non radiogenic hydrothermal sr and presumably a concomitant rise in nutrient input into the oceans the concomitant rise of modern phytoplankton groups dinoflagellates and coccolithophores would have also led to increased primary productivity making the negative carbon isotope trend even more notable we suggest that gradual oceanographic changes more effective connections and mixing between the tethys atlantic and pacific oceans would have promoted a shift towards enhanced burial of isotopically heavy carbonate carbon and effective recycling of isotopically light organic matter these processes account for the observed long term trend interrupted only by the weissert event in the valanginian

large magnitude extension along metamorphic core complexes of western arizona and southeastern california evaluation with paleomagnetism cordilleran metamorphic core complexes mccs and their associated structures detachment faults and mylonites are the products of large magnitude cenozoic extension paleomagnetic data from the harquahala harcuvar buckskin and whipple mccs of western arizona and southeastern california 1900 samples 191 of 278 sites accepted for inclusion in group and grand means allow us to estimate the magnitude of footwall tilting associated with extension all mcc crystalline rocks sampled are in the footwalls of associated brittle detachment faults rocks sampled include synkinematic miocene intrusions dikes and plutons late cretaceous granites and proterozoic host rocks these rocks with different levels of success at the site level yield dual polarity high unblocking temperature and high to moderate coercivity magnetizations demagnetization behavior reveals that those sites with well grouped magnetizations and high laboratory unblocking temperatures and or high to moderate coercivities have characteristic remanent magnetizations carried by either magnetite with minor hematite or by hematite with minor magnetite results of baked contact tests show that these footwall rocks possess thermoremanent magnetizations or high temperature thermochemical remanent magnetizations acquired early in their cooling history either before or during extension our overall interpretation of the paleomagnetic data is that during cenozoic extension mcc footwalls tilted flattened only a modest amount 15 in the direction of tectonic transport we also interpret harcuvar and whipple mcc paleomagnetic data to indicate tectonic tilting steepening of the mylonitic zone to form a back dipping mylonitic front the whipple bullard eagle eye detachment fault system and associated mylonites are interpreted to have originated with a low angle dip 10 to 25 ne we relate the small amount of footwall flattening in the direction of tectonic transport and the steepening of the back dipping mylonitic front to the buoyant rise of exhumed footwalls according to rolling hinge isostatic uplift models our interpretation refutes the widespread applicability of models that predict mccs to represent strongly tilted crustal blocks originally bounded by high moderate angle normal faults additionally a fold test of harquahala mcc data yielded negative results this suggests that antiformal footwall arches represent the oldest structures of mccs that developed prior to or during mylonitization of footwall rocks before acquisition of the remanent magnetizations carried by these rocks

cretaceous collision and indentation in the west carpathians view based on structural analysis and numerical modeling 1 a model of indentation of a rigid promontory into weak metasedimentary rocks during cretaceous convergence is suggested for tectonic evolution of southern part of west carpathians an early arcuate cleavage fan has developed in front of northward moving southern rigid basement block the interaction of the moving indenter with western stationary basement promontory is responsible for development of the boundary parallel shear zone along which the main southern indenter is shifted to the east this results in development of a new steep transpressional cleavage overprinting the early fabric eastward displacement of the southern indenter causes the development of a thrust zone parallel to the margin of the eastern stationary promontory a proposed numerical model of the deformation of a thin viscous sheet in front of oval rigid indenter reliably simulates the development of the observed deformation pattern modeled discrete partitioning between the western promontory and the indenting block fully agrees with the observed secondary cleavage associated with the transpressional shear zone our numerical model interconnects this complex kinematic frame with finite strain pattern which was to date possible only for simple boundary conditions in addition the model explains the polyphase cleavage patterns in terms of complex shapes of promontories and changes in movements of indenting blocks

age and significance of core complex formation in a very curved orogen evidence from fission track studies in the south carpathians romania abstract twenty four new zircon and apatite fission track ages from the getic and danubian nappes in the south carpathians are discussed in the light of a compilation of published fission track data a total of 101 fission track ages indicates that the getic nappes are generally characterized by cretaceous zircon and apatite fission track ages indicating cooling to near surface temperatures of these units immediately following late cretaceous orogeny the age distribution of the danubian nappes presently outcropping in the danubian window below the getic nappes depends on the position with respect to the cerna jiu fault eocene and oligocene zircon and apatite central ages from the part of the danubian core complex situated southeast of this fault monitor mid tertiary tectonic exhumation in the footwall of the getic detachment while zircon fission track data from northwest of this fault indicate that slow cooling started during the latest cretaceous the change from extension getic detachment to strike slip dominated tectonics along the curved cerna jiu fault allowed for further exhumation on the concave side of this strike slip fault while exhumation ceased on the convex side the available fission track data consistently indicate that the change to fast cooling associated with tectonic denudation by core complex formation did not occur before late eocene times i e long after the cessation of late cretaceous thrusting core complex formation in the danubian window is related to a larger scale scenario that is characterized by the nnw directed translation followed by a 90 clockwise rotation of the tisza dacia block due to roll back of the carpathian embayment this led to a complex pattern of strain partitioning within the tisza dacia block adjacent to the western tip of the rigid moesian platform our results suggest that the invasion of these southernmost parts of tisza dacia started before the late eocene i e significantly before the onset of miocene age rollback and associated extension in the pannonian basin

preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks abstract grain size sensitive flow of fine grained rocks such as are often found in mylonitic shear zones is commonly inferred to be accomplished by grain boundary sliding so that no preferred crystallographic orientation is expected to develop pre existing fabrics are supposed to weaken and grain shapes to remain equant however many fine grained mylonites commonly exhibit marked preferred crystallographic orientation whilst displaying a stable but weak grain shape fabric we have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic ultrafine grained 700 c the strong crystal plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow this is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain boundary sliding from experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working it was shown that although the microstructure could be totally annealed the early formed fabric survived even subsequent high temperature crystal plastic or superplastic flow this may explain the frequent occurrence of strong low temperature fabrics in calcite ultramylonites

alpine metamorphic evolution and cooling history of the veporic basement in northern hungary new petrological and geochronological constraints petrological and geochronological investigations were carried out on metamorphic rocks of the veporic unit inner western carpathians in northern hungary k ar and ar ar data on micas and amphibole show only alpine ages mostly in the range of 87 95 ma in this basement unit thermobarometric calculations yield lower amphibolite facies peak conditions ca 550 30 c and 9 1 kbar for the eoalpine metamorphic event complex evolution of gneissic rocks is reflected by the presence of discontinuously zoned garnets the cores of which may represent relics of a pre alpine presumably variscan thermal event zircon fission track ft data in the narrow range of 75 77 5 ma indicate that this portion of the veporic unit was emplaced to shallow crustal levels already during the senonian time the relative minor difference between zircon ft and k ar or ar ar ages suggests very rapid cooling during the late cretaceous most probably related to the extensional unroofing of the veporic core complex the obtained cooling ages do not support previous models of tertiary uplift and exhumation of the veporic unit along the hurbanovo diosjen line

petrological and kar and40ar39ar age constraints for the tectonothermal evolution of the high pressure meliata unit western carpathians slovakia abstract the meliata high pressure unit exposed in the southern part of the western carpathians consists of four groups of rocks which are characterized by different lithologies and metamorphic histories 1 the most common are triassic marbles intercalated with metabasalts and glaucophane bearing phyllites phengite from metabasalts and phyllites record k ar and40ar 39ar plateau ages of 152 155 ma which are interpreted to date the high pressure metamorphism 2 blueschist facies mafic rocks and subordinate glaucophane bearing micaschists devoid of marbles which contain relic low si muscovite and newly formed high si phengite the40ar 39ar data obtained for the micas indicate palaeozoic ages of 375 380 ma for the early amphibolite facies muscovite and a 250 ma age for the secondary blueschist facies phengite 3 glaucophane free phyllites and phengite quartzite with low si and high si micas respectively textural features indicate a polyphase evolution characterized by a greenschist facies metamorphism followed by a blueschist facies overprint the high si micas provide evidence for an early middle jurassic age 172 ma of the high pressure metamorphism 4 amphibolite facies slices and blocks overprinted by blueschist facies metamorphism represent old basement rocks regarding the differences in lithologies metamorphic conditions and radiometric ages of high pressure micas older age for continental vs younger age for oceanic rocks subduction of a continental wedge prior to that of oceanic crust can be assumed this interpretation is based also on palaeogeographical reconstruction of the mesozoic western tethys between the apulian promontory and the tisza bihor continental block

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

the alpine overprints of the magnetic fabrics in the basement and cover rocks of the veporic unit western carpathians slovakia abstract in the veporic unit building up the central areas of the western carpathians the magnetic fabrics are more or less coplanar and coaxial in metamorphic early palaeozoic in age granitic early to late palaeozoic in age and sedimentary cover rocks permian triassic in age the magnetic fabrics in metamorphic and granitoid basement rocks as well as in sedimentary volcanic cover rocks appear to reflect a very wide p t conditions of cretaceous originally variscan basement reactivation connected with a prograde metamorphism of the sedimentary cover rocks in lower greenschist in the north veporic area or the upper greenschist to lower amphibolite facies in the south veporic area during collisional burial in a collisional wedge in addition there are deformation zones where the magnetic fabrics formed due to a ductile deformation associated with extensional exhumation along sinistral lateral strike slip and normal faults the late open folds did not principally change the former magnetic fabrics the magnetic fabrics in sedimentary cover rocks of the north veporic area are either related to still recognizable bedding planes in competent layers with parallel superimposed metamorphic schistosity or they are conformable to subparallel or oblique metamorphic cleavages in the south veporic area they are undoubtedly related to the metamorphic mylonitic fabrics of the basement granitoids and their metamorphic mantle rocks the magnetic fabrics in granitoid rocks of the south veporic area s of the pohorela sinistral strike slip fault are influenced with micromechanisms of dominated low temperature ductile deformation and partial dynamic recrystallization we ascribe the observed coaxial magnetic fabrics in metamorphic granitic and sedimentary rocks to an alpine collision 110 95 ma p max 8 9 kbar t max 530 c and extension tectonometamorphic events 95 80 ma up to p 4 5 kbar t 300 c of cretaceous age the magnetic fabrics seem not to be strongly dependent on temperatures until the medium part of the amphibolite facies as in this example but they are strongly following simultaneous deformational low temperature to medium temperature meso and microfabrics

miocene low angle normal faulting and dike emplacement homer mountain and surrounding areas southeastern california and southernmost nevada homer mountain and surrounding regions are within or adjacent to the western part of a broad region of low angle normal faults exposed within the lower colorado river trough during middle miocene time upper plate rocks in the homer sacramento dead and newberry mountains moved eastward or northeastward relative to the lower plate above single or multiple low angle normal faults deposition of coarse clastic sedimentary rocks occurred during extensional faulting and was accompanied by and closely followed by eruption of basaltic volcanics upper plate fault blocks of miocene volcanic and sedimentary rocks and older underlying crystalline rocks are tilted gently to steeply to the west or southwest low angle normal faults have complex sinuous traces due to the irregular form of fault surfaces antiformal warping and uplift of the lower plate about a north south trending axis due to tectonic denudation and isostatic rebound divided the regionally east thickening extensional allochthon into separate synformal and wedge shaped components the homer mountain area lies within or adjacent to the synformal component lower plate rocks are intruded by numerous middle miocene east west trending dikes which are in turn intruded by subhorizontal dikes both sets of dikes are cut and displaced by low angle normal faults except on the west flank of the newberry mountains where an east west trending dike cuts the basal low angle normal fault k ar geochronologic and field data establish the approximate synchroneity of dike emplacement and low angle faulting the state of stress in the lower plate as inferred from dike orientations differed radically from the state of stress in the upper plate as inferred from fault geometry low angle faulting and east northeast west southwest distension of upper plate rocks reflect regional reduction of compression in the east northeast west southwest direction and associated large scale east northeast west southwest crustal extension in contrast concave upward flexure of the lower plate in response to tectonic denudation and resultant isostatic uplift is inferred to have produced local subhorizontal compression at shallow crustal levels in the lower plate that overwhelmed the regional extensional stress and prevented emplacement of dikes oriented perpendicular to the direction of regional extension north south crustal extension during dike emplacement appears to have resulted from minor divergence of lower plate crustal blocks as they were displaced in a west southwest direction away from the colorado plateau

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

paleomagnetism and tectonic rotation of the lower miocene peach springs tuff colorado plateau arizona to barstow california we have determined remanent magnetization directions of the lower miocene peach springs tuff at 41 localities in western arizona and southeastern california an unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the colorado plateau to barstow california a distance of 350 km the peach springs tuff was apparently emplaced as a single cooling unit about 18 or 19 ma and is now exposed in 4 tectonic provinces west of the plateau including the transition zone basin and range colorado river extensional corridor and central mojave desert strike slip zone as such the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes from 4 sites on the stable colorado plateau we have determined a reference direction of remanent magnetization i 36 4 d 33 0 95 3 4 that we interpret as a representation of the ambient magnetic field at the time of eruption a steeper direction of magnetization i 54 8 d 22 5 95 2 3 was observed at kingman where the tuff is more than 100 m thick and similar directions were determined at 7 other thick exposures of the peach springs tuff the steeper component is presumably a later stage magnetization acquired after prolonged cooling of the ignimbrite when compared to the plateau reference direction tilt corrected directions from 3 of 6 sites in the central mojave strike slip zone show localized rotations up to 13 in the vicinity of strike slip faults the other three sites show no significant rotations with respect to the colorado plateau both clockwise and counterclockwise rotations were measured and no systematic regional pattern is evident our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post middle miocene strike slip faults in the central mojave desert most of our sites in the transition zone and basin and range province have had no significant rotation although small counterclockwise rotation in the mccullough and new york mountains may be related to sinistral shear along en echelon faults southwest of the lake mead shear zone the larger rotations occur in the colorado river extensional corridor where 8 of 14 sites show rotations ranging from 37 clockwise to 51 counterclockwise these rotations occur in allochthonous tilt blocks which have been transported northeastward above the chemehuevi whipple mountains detachment fault upper plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation from this relation we infer that rotations are accommodated along numerous low angle faults at higher structural levels above the detachment surface

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

evolution of quartz microstructures and textures during polyphase deformation within the tauern window eastern alps the interior of the tauern window exposes underplated penninic continental lithosphere and the overlying obducted penninic oceanic crust within a large antiformal dome in the internal zone of the eastern alps these units have been affected by a polyphase deformation history generally three deformation events are distinguished d1 is related to underplating of and top to the n nappe stacking within the penninic continental units of the tauern window deformation stage d2 is interpreted to reflect the subsequent continent collision between the penninic continental units and the european foreland d3 is related to the formation of the dome structure within the tauern window during thickening of continental lithosphere and nappe stacking d1 and subsequent intracontinental shortening d2 these tectonic units have been ductilely deformed close to a plane strain geometry conditions for the plastic deformation of the main rock forming mineral phases quartz feldspar dolomite calcite have prevailed during all three phases of crustal deformation generally two types of quartz microstructures that are related to d1 are distinguished within the tauern window a equilibrated and annealed fabrics without crystallographic preferred orientations cpo have only been observed in the central part of the southeastern tauern window corresponding with amphibolite grade metamorphic conditions b in the northeastern and central part of the tauern window microstructures are characterized by quartz grains that show equilibrated shape fabrics but well preserved cpo with type i cross girdle distributions indicating a deformation geometry close to plane strain during d2 two types of quartz microstructures are distinguished too a quartz grains that show equilibrated shape fabrics but well preserved cpo the c axes distributions generally are characterized by type i cross girdles locally by type ii cross girdles and in places oblique single girdle distributions b a second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain boundary migration and strong cpo this type is restricted to the southern sections of the western and eastern tauern window the c axis distributions show type i cross girdles in the western part of the tauern window and single girdles in the southeastern part in the western part of the tauern window a continuous transition from type b microstructures in the south to type a microstructures in the north is documented the microstructural evolution also documents that the dome formation in the southeastern and western tauern window has already started during d2 and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism d3 is restricted to distinct zones of localized deformation d3 related quartz fabrics are characterized by the formation of ribbon grains the c axes show small circle distributions around the z axis of the finite strain ellipsoid during exhumation and doming d3 deformation occurred under continuously decreasing temperatures

late cenozoic history and styles of deformation along the southern death valley fault zone california late cenozoic deposits in the southern death valley region have been offset 35 km by right lateral strike slip faulting on the southern death valley fault zone since miocene time virtually all slip took place prior to 1 m y ago along western traces of the fault zone during the past 1 m y the eastern traces of the fault zone have been active and characterized by oblique slip with a lateral component of only a few hundred metres movement along these eastern traces has formed normal faults and gentle to isoclinal folds that have uplifted fan gravel and lacustrine sediments as much as 200 m above the modern alluvial fan surface surveying of the longitudinal profile of the amargosa river which flows within the eastern traces of the fault zone suggests that vertical deformation continues today the 35 km of right lateral offset which is based on matching offset alluvial fan gravel with its source area refines earlier estimates of 8 to 80 km of movement for the southern death valley fault zone and it is consistent with the geometry of a pull apart basin model for central death valley causes for the observed differences in the style and timing of movement of the eastern and western subzones are not well understood the study area however is located a few kilometres north of the intersection of the southern death valley and garlock fault zones the garlock fault zone changes its sense of movement from left lateral strike slip to east vergent thrusting against the southern death valley fault zone the resulting compression may have caused the shift in activity and the change in style of deformation along branches of the southern death valley fault zone

cretaceous evolution of a metamorphic core complex the veporic unit western carpathians slovakia p t conditions and in situ40ar 39ar uv laser probe dating of metapelites alpine metamorphism related to the development of a metamorphic core complex during cretaceous orogenic events has been recognized in the veporic unit western carpathians slovakia three metamorphic zones have been distinguished in the metapelites 1 chloritoid chlorite garnet 2 garnet staurolite chlorite 3 staurolite biotite kyanite the isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system k2o feo mgo al2o3 sio2 h2o kfmash the isograds are roughly parallel to the north east dipping foliation related to extensional updoming along low angle normal faults thermobarometric data document increasing p t conditions from c 500 c and 7 8 kbar to c 620 c and 9 10 kbar reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies 40ar 39ar data obtained by high spatial resolution in situ ultraviolet uv laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the late cretaceous mean dates are between 77 and 72 ma however individual white mica grains record a range of apparent 40ar 39ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous the reconstructed metamorphic p t t path is clockwise reflecting post burial decompression and cooling during a single alpine orogenic cycle the presented data suggest that the veporic unit evolved as a metamorphic core complex during the cretaceous growth of the western carpathian orogenic wedge metamorphism was related to collisional crustal shortening and stacking following closure of the meliata ocean exhumation was accomplished by synorogenic orogen parallel extension and unroofing in an overall compressive regime

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif abstract investigations of variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the tepla barrandian unit bohemian massif the study focuses on determining differential stress deformation mechanisms and geometry of cadomian metagreywackes and cambrian granitoids which show different grade of variscan barrovian type metamorphism ranging from the greenschist subgreenschist facies boundary semibrittle deformation regime to the high grade amphibolite facies the semibrittle deformation regime t ca 300 c is characterized by large scale thrusting within a folding environment and by similar intensities of cataclasis diffusive mass transfer and crystal plasticity of quartz the latter including low temperature migration recrystallization within cadomian quartz veins of the greenschist facies part dislocation creep of quartz operated at differential stresses ranging from ca 70 mpa in the lowermost greenschist facies to ca 20 mpa in the uppermost greenschist facies these values have been derived by paleopiezometry using the grain size of recrystallized quartz they are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes a marking increase in recrystallized grain size of quartz occurs within the lower greenschist facies where low temperature grain boundary migration is replaced by subgrain rotation thus causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level pressure solution of quartz and development of discrete crenulation cleavage was common in fine grained metagreywackes of the lower greenschist facies whereas the upper greenschist facies t ca 500 570 c is characterized by increasing intensity of metamorphic reactions grain coarsening and development of a schistosity the entire greenschist facies level forms a suprastructure where viscosity contrasts between mica and quartz rich layers were sufficiently high for buckle folding strain protected intercalated metagranitoids on the other hand do not show pervasive deformation fabrics the amphibolite facies part forms an infrastructure characterized by pervasive mylonitic shearing of both cambrian granitoids and metagreywackes it is suggested that the combination of the following processes has contributed to weaken the amphibolite facies level making nonlocalized flow possible 1 increased recrystallization of feldspar 2 crystal plastic softening of quartz due to high temperature grain boundary migration and activity of prism slip and 3 enhanced diffusive mass transfer in the form of metamorphic reactions the structural breaks at the semibrittle crustal level and at the greenschist amphibolite facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

neogene tilting of crustal panels near wrangell alaska a late oligocene miocene igneous complex south and west of wrangell alaska contains mafic dikes that yield a discordant paleomagnetic direction inclination i 70 4 declination d 39 3 95 4 8 n 72 sites combined with local and regional geobarometric metamorphic and structural observations the discordant paleomagnetic direction indicates east side up tilt by 16 about a tilt axis with azimuth 8 neogene tilt of crustal blocks in the insular superterrane accounts for much of the paleomagnetic discordance in cretaceous plutons without the coastwise translation of 1000 km as suggested by the baja british columbia hypothesis

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

paleomagnetic vectors and tilted dikes abstract where tectonic deformation reorients rocks without penetrative strain their paleomagnetic vectors may be restored to their original attitudes by untilting for strata paleomagnetic inclination is readily restored but the tilt axis must be precisely known if paleodeclination is required for dikes without the knowledge of the rotation s neither declination nor inclination of the paleomagnetic vector can be uniquely defined furthermore back rotating dike orientations to an upright attitude assumes primary verticality whereas primary dike dips are bimodal across the spreading axes e g troodos ophiolite cyprus in the cyprus ophiolite the dikes of the limassol forest transform zone are tilted due to uplift of the mantle sequence rocks and deflected against the arakapas fault their paleomagnetic vectors may be restored rotating about the two axes defined by the strike and the vertical or about a net axis that is possibly the actual tectonic rotation axis this net axis is determined from the tectonic regional dispersion of the dike orientations in this test case the results of the restorations differ slightly but underline the difficulty in selecting the best restoration procedure and the greater difficulty of restoring the paleomagnetic data from dikes vis a vis strata for dikes it is recommended that the paleomagnetic vectors are restored using average dike orientations to minimize the inaccuracies due to the large primary variation in dike orientation

timing of relay ramp growth and normal fault linkage upper galilee northern israel we investigate a kilometer scale steep relay rampstructure in the galilee northern israel the data indi cate an asymmetric fold above the buried tip of thesouthern boundary fault as well as at the lower partof the ramp structural analysis suggests that the buriedtip of the bounding fault is only a few hundred metersbelow the surface a sequence of colluvial wedges exposed at the base of the southern flank of the relayramp over the blind tip of the boundary fault presentssuccessively decreasing angles from 67 at the bottomto 30 at the top the lower three wedges rest at anglesgreater than the angle of repose suggesting tilting dur ing and after deposition we suggest that the increasingangle of the wedges is the result of the development ofthe monocline over the buried tip ofthe boundaryfault which in turn reflects the development of the entirerelay ramp structure paleomagnetic measurements testthis hypothesis samples from three of the four lowerwedges and the carbonate fill in the fractured bedrockyield northerly declination and positive inclination inclination anomalies are 19 4 3 3 and 11 3 1 8 for the fractured bedrock and the wedges respec tively these anomalies reflect part of the tilt paleo magnetic measurements and optically stimulatedluminescence osl constrain the time of initial defor mation of the monocline above the southern boundaryfault between 780 ka and 461 75 ka osl age ofwedge 1 and the termination of deformation to 176 22 ka osl age of wedge 5 these bounding agessuggest tilting rates that range between 1 16 kyrand 1 8 kyr termination of tilting along the south ern flank of the relay ramp and the fact that the tipof the buried normal fault is only a few hundreds ofmeters below the surface suggest that the relay rampmay be close to being breached by a connecting fault

stratigraphic and structural synthesis of a miocene extensional terrane southeast california and west central arizona detailed stratigraphy and isotopic dating of stratigraphic sections in the colorado river extensional corridor support a regional correlation of highly faulted tertiary stratigraphic sequences and provide a chronologic framework for interpreting the evolution of low angle normal detachment faults on the basis of this correlation we define six tilting domains in the upper plate of the whipple chemehuevi and rawhide detachment faults and identify three discrete episodes of detachment faulting that began in the early miocene and ended in middle miocene time episodes of rapid detachment faulting are indicated by extreme tilting of upper plate fault blocks and overlying miocene sequences fanning dips of basinal deposits and angular unconformities that represent short time gaps in the accumulation of syntectonic sequences during the first episode of detachment faulting at about 20 ma the upper plate segmented to form the domains basin subsidence and extreme tilting of upper plate fault blocks and syntectonic deposits characterized the eastern topock crossman aubrey parker dam and buckskin rawhide domains whereas the western mopah domain was the site of abundant volcanic activity but no basins or tilting a second episode of extension at about 18 ma produced extreme tilts in the buckskin rawhide domain but upper plate blocks in the mopah domain tilted moderately a third regionwide faulting episode between 14 and 12 ma was due to localized uplift of middle and lower crust and eventual exposure of the detachment faults and their footwalls the upper plate fault blocks responded passively to localized slip on the detachment faults rapid extension began on the whipple chemehuevi detachment fault at 20 ma and had shifted southward to the buckskin rawhide detachment fault by 18 ma volcanic activity also shifted southward to the buckskin rawhide domain at this time the southward shift of rapid extension and volcanism probably represents buildup and release of strain at localized sites in the lower plate otherwise stratigraphic and structural relations indicate that the locations of upper plate basins faulting and tilting of upper plate blocks and position of the breakaway zone remained stable throughout the major phases of extension

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

paleomagnetic quantification of upper plate deformation during miocene detachment faulting in the mohave mountains arizona 1 paleomagnetic data from miocene 20 ma volcanic rocks and dikes of west central arizona reveal the tilt history of proterozoic crystalline rocks in the hanging wall of the chemehuevi whipple mountains detachment fault we obtained magnetization data from dikes and flows in two structural blocks encompassing crossman peak and standard wash in the mohave mountains in the crossman block the dike swarm records two components of primary magnetization 1 cnh a normal polarity high unblocking temperature or high coercivity component inclination i 48 5 declination d 6 4 and 2 crhm a reversed polarity high temperature or high coercivity component i 33 6 d 197 5 argon age spectra imply that the dikes have not been reheated above 300 c since their emplacement and a baked contact test suggests that the magnetization is likely to be miocene in age crhm deviates from the expected direction of the miocene axial dipole field and is best explained as a result of progressive tilting about the strike of the overlying andesite flows these data suggest that the crossman block was tilted 60 to the southwest prior to intrusion of the vertical dike swarm and the block continued to tilt during a magnetic field reversal to normal polarity cnh miocene dikes in the crossman block are roughly coplanar so the younger dikes with normal polarity magnetization intruded along planes of weakness parallel to the earlier reversed polarity swarm an alternative explanation involves cnh magnetization being acquired later during hydrothermal alteration associated with the final stages of dike emplacement in the standard wash block the primary component of magnetization is a dual polarity high temperature or high coercivity component swhl i 7 2 d 0 7 to produce agreement between the expected miocene magnetic direction and the swh component requires 1 correcting for a 56 tilt about the strike of flow bedding and 2 removing a counterclockwise vertical axis rotation of 20 the two rotations restore the standard wash dikes to vertical make parallel the dike layering in the crossman and standard wash blocks and align the strikes of bedding in both blocks geologic mapping geochemical evidence and paleomagnetic data are consistent with the upper plate of the mohave mountains having tilted in response to formation of the underlying detachment fault

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate bearing faults in nature abstract layers of fine grained muscovite were hot pressed then sheared between alumina sliders to shear strains up to 2 at temperatures between 300 and 700 c confining pressures of 206 mpa and various pore water pressures high pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used shear strain rates between 10 3 s 1 and 10 7 s 1 were accessed using constant shear strain rate and stress relaxation testing except for strain rates 5 s 1 at 700 c deformation was strain rate and temperature insensitive but effective normal stress sensitive with a friction coefficient at yield of 0 3 rising with strain to 0 5 steady state sliding was not attained from the mechanical data and microstructural study deformation was inferred to have occurred by a mixture of brittle frictional and crystal plastic processes at 700 c and low strain rates the shear strength falls rapidly with a linear viscous characteristic in a way not previously reported this is tentatively attributed to rate control by viscous glide of basal dislocations extrapolating these results to geological strain rates we expect mica rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0 25 and 0 5 giving way at mid crustal conditions to a rapid strength drop as viscous creep supervenes thus mid to lower crustal mica dominated faults of any orientation and terrains of schistose metapelitic rocks in the cores of orogeneic belts are expected to be very weak supporting shear stresses in the range 1 10 mpa the frictional behaviour of mica rich faults in upper crustal regions is however expected to be too strong to account alone for the proposed weakness of some major fault zones

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma