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

slip rates on the chelungpu and chushiang thrust faults inferred from a deformed strath terrace along the dungpuna river west central taiwan the chelungpu fault produced the september 1999 m w 7 6 chi chi earthquake central taiwan the shortening rate accommodated by this structure integrated over several seismic cycles and its contribution to crustal shortening across the taiwanese range have remained unresolved to address the issues we focus our study on the chelungpu and chushiang thrust faults within the southernmost portion of the chi chi rupture area structural measurements and available seismic profiles are used to infer the subsurface geometry of structures the chushiang and chelungpu faults appear as two splay faults branching onto a common ramp that further north connects only to the chelungpu surface trace we survey a deformed strath terrace along the dungpuna river buried under a 11 540 309 years old fill deposit given this age the dip angles of the faults and the vertical throw determined from the offset of the strath terrace across the surface fault traces we estimate slip rates of 12 9 4 8 and 2 9 1 6 mm yr on the chelungpu and chushiang faults respectively these yield a total shortening rate of 15 8 5 1 mm yr to be absorbed on their common decollement at depth this total value is an upper bound for the slip rate on the chelungpu fault further north where the chushiang fault disappears and transfers shortening to adjacent faults combining these results with the recently constrained shortening rate on the changhua blind thrust reveals that all these frontal faults presently absorb most of the long term horizontal shortening across the taiwanese range they thus stand as the major sources of seismic hazards in this heavily populated area the return period of earthquakes similar to the chi chi event over a 80 km long stretch of the western foothills is estimated to be 64 years this value is an underestimate because it assumes that all the faults locked during the interseismic period slip only during such large events comparison with historical seismicity suggests that episodic aseismic deformation might also play a major role in accommodating shortening

structures associated with the northern end of the 1999 chi chi earthquake rupture central taiwan implications for seismic hazard assessment abstract the surface rupture of the 1999 chi chi earthquake m w 7 6 trendsmore than 100 km in a north south direction surface deformation at the northernend stops abruptly at an area between the tachia river and the taan river where abroad pop up structure with east to northeast strike can be found that has a trenddifferent from the north south striking main thrust we combine the absolute ele vation data before and after the chi chi earthquake to obtain the regional verticaldisplacement and the magnitude of the pop up structure the greatest uplift couldreach as high as 15 16 m using deformation magnitude and the area balancingmethod we measure the depth of the detachment to show the subsurface geometryof the chelungpu fault at its northern end this shows that the geometry of thechelungpu fault controls termination of the surface rupture and the depth of thedetachment controls the amount of deformation introduction perhaps the most unexpected geological feature asso ciated with the chi chi earthquake

crustal deformation in taiwan results from finite source inversions of six mw 5 8 chi chi aftershocks 1 we inverted strong motion data for the finite source parameters of six large aftershocks of the 1999 chi chi taiwan earthquake to investigate seismogenic structure in taiwan for each event we derived a preferred model by testing different focal mechanisms hypocenters rupture velocities and dislocation risetimes as well as different combinations of stations in more than 1000 inversions we documented how the fits between the waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model if the deviation in focal mechanisms and hypocenters is less than 20 and 5 km respectively we generally recovered 80 of the preferred model s synthetic waveform fit unlike the dislocation risetime the rupture velocity used in the inversion had a strong influence on the waveform fits in this study we also used the slip models to study fault geometry two of the aftershocks ruptured on the southern extension of the main shock slip area one strike slip aftershock nucleated within the basement but ruptured mainly within the overlying sedimentary strata suggesting that seismogenic deformation in the basement can influence shallow structures p axes of the derived models have azimuths consistent with current plate motion finally gps displacement derived from the six slip models can explain 80 of the postseismic deformation observed in the aftershock regions indicating that studies of postseismic deformation must take into account the cumulative effects of large shallow aftershocks

preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the bushveld layered intrusion abstract the preferred orientation of plagioclase and pyroxene in gabbronorites from the bushveld layered igneous intrusion south africa was investigated using electron backscatter diffraction in two localities 200 km apart belfast and rustenburg strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane defined by 010 planes of plagioclase and 100 planes of pyroxene the pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure primary magmatic oxides are also influenced by the preferred orientation of plagioclase generating an oblate magnetic susceptibility fabric parallel to the foliation plane plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p waves traveling 4 8 faster in the vertical than in the horizontal direction

horizontal coseismic deformation of the 1999 chi chi earthquake measured from spot satellite images implications for the seismic cycle along the western foothills of central taiwan 1 the 1999 chi chi earthquake mw 7 6 broke a major thrust fault along the western foothills of the central range of taiwan we have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake these data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall at the surface coseismic slip increases from 5 6 m near the epicenter to 10 11 m to the north in the epicentral area we observe a left lateral strike slip zone trending n125 e and farther north a fault zone trending n s with a right lateral component the data were modeled using elastic dislocations the fault geometry consists of a shallow

the 3d quantitative lattice and shape preferred orientation of a mylonitised metagranite from monte rosa western alps combining neutron diffraction texture analysis and synchrotron x ray microtomography abstract two complementary 3d techniques neutron diffraction and synchrotron x ray microtomography sxr ct were used to compare the shape and lattice preferred orientations of a mylonitised metagranite from the monte rosa unit western alps italy the goal of using these techniques was to obtain two different orientation distribution functions although the two functions describe relatively independent characteristics of the rock fabric nonetheless they also exhibit close relationships to macroscopic fabrics and may be complementarily used to quantify rock fabrics and microstructures thereby highlighting 3d features that cannot be obtained with either technique if used independently we describe an approach that can be potentially useful in various disciplines e g structural geology rock mechanics tectonics and geophysics when a complete data set of preferred orientations and size distribution is needed micas display a strong orthorhombic symmetry between mesoscopic lineation and microscopic spo and lpo whereas quartz and feldspars are characterised by a monoclinic symmetry between mesoscopic lineation and lpo these observations suggest a rheological decoupling between the weak phase mica layers and the stronger quartz feldspar layers this mechanical decoupling occurred during the alpine subduction collision when the monte rosa unit was part of the insubric line system and accommodated large vertical strain

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

gps measurement of postseismic deformation following the 1999 chi chi taiwan earthquake 1 in the 15 month period september 1999 to december 2000 after the 1999 chi chi taiwan earthquake mw 7 6 about 80 campaign surveyed gps stations in central taiwan were occupied up to seven times furthermore seven continuous gps stations were put into operation in the epicentral region mostly within three weeks after the main shock gps data from these near field stations and some pre existing far field stations were utilized to study the postseismic deformation following the chi chi earthquake the postseismic displacements of the gps stations were approximated by a combination of an exponential transient decay function with a relaxation time of 86 days and a postseismic linear rate change stations on the hanging wall displayed west northwesterly horizontal displacements of up to 252 mm and uplift as large as 229 mm postseismic displacements are much larger to the south near the epicenter of chi chi main shock than in the north where the maximum coseismic displacement occurred stations on the footwall show only small displacements the postseismic gps data were inverted to infer the deeper fault geometry and afterslip distribution based on a four segment fault model with its shallow part constrained by coseismic fault geometry deduced from gps and seismological studies the optimal model requires the lower fault segment to be a horizontal plane at a depth of 10 4 km consistent with a model based on gps data taken during the first three months after the main shock and other geological or geophysical studies the maximum afterslip 459 mm occurs in the hypocentral region and in the northern part of the decollement slip of about 430 mm also occurs in the southern part of the decollement the afterslip moment inferred from 15 months of gps data is 4 7 1019 n m approximately 2 3 times the seismic moment released by aftershocks this implies that a major part of the postseismic deformation is aseismic

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

an experimental steady state foliation the history of the intensity and orientation of a grain shape foliation was investigated in octachloropropane deformed in simple shear at 80 of its absolute melting temperature and a shear strain rate of 4 10 5s 1 foliation orientation developed from the beginning of the deformation remains steady throughout the deformation foliation intensity becomes steady after a bulk shear strain of about 0 9 has been accumulated the steadiness of foliation orientation and intensity is achieved by some balance between foliation strengthening and foliation weakening processes the main foliation strengthening process is intragranular plastic deformation foliation weakening processes include dynamic recrystallization by migration of straight or or slightly wavy grain boundaries grain dissection rotational recrystallization grain amalgamation relative rigidity of hard grains and grain boundary sliding a small but definite angle although not constant is observed between the foliation and the long axis of the bulk total strain ellipse the average aspect ratio of grains is lower than that of the bulk total strain ellipse by a factor of about 0 4 at total shear strain of 1 3 the average grain size also stabilizes becoming steady from a shear strain of 0 5 onwards a steady grain shape foliation may be a possible paleostress or a paleostrain rate indicator but cannot be a paleostrain indicator

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

deformation experiments on anhydrite rocks of different grain sizes rheology and microfabric miiller w h schmid s m and briegel u 1981 deformation experiments on anhydrite rocks of different grain sizes rheology and microfabric in g s lister h j behr k weber and h zwart editors the effect of deformation on rocks tectonophysics 78 527 543 three anhydrite rocks of different grain size were experimentally deformed in the 20 to 45o c temperature range the strength of the finegrained varieties becomes strongly temperature and strain rate dependent above 3oo c and a flow law of the form 6 a exp h rt sinh o a with an activation energy h of 27 36 kcal mole and a very low value for the exponent n of 1 5 2 was found the course grained rock remains relatively insensitive to strain rate and temperature even at 45o c microstructural and x ray texture analyses suggest that intracrystalline glide and twinning are the major deformation mechanisms the transition from strong work hardening observed at high stresses into steady state flow at low stresses was found to correlate with the onset of dynamic recrystallization by grain boundary migration

preferred orientation and elastic anisotropy of illite rich shale shales display significant seismic anisotropy that is attributed in part to preferred orientation of constituent minerals this orientation pattern has been difficult to quantify because of the poor crystallinity and small grain size of clay minerals a new method is introduced that uses high energy synchrotron x rays to obtain diffraction images in transmission geometry and applies it to an illite rich shale the images are analyzed with the crystallographic rietveld method to obtain quantitative information about phase proportions crystal structure grain size and preferred orientationtexture that is the focus of the study textures for illite are extremely strong with a maximum of 10 multiples of a random distribution for 001 pole figures from the three dimensional orientation distribution of crystallites and single crystal elastic properties the intrinsic anisotropic elastic constants of the illite aggregate excluding contribution from aligned micropores can be calculated by appropriate medium averaging the illitic shale displays roughly transverse isotropy with c11 close to c22 and more than twice as strong as c33 this method will lend itself to investigate complex polymineralic shales and quantify the contribution of preferred orientation to macroscopic anisotropy

plagioclase microfabrics in a ductile shear zone from the jotun nappe norway abstract gabbroic and anorthositic rocks from the jotun nappe are transected by small ductile shear zones in which a high grade paragenesis and a new foliation are formed most plagioclase grains show evidence of plastic deformation and have recrystallized by subgrain rotation and bulge processes to fine grained mylonite during these processes a plagioclase grain shape fabric was destroyed and with increasing strain a pre existing plagioclase crystal fabric was successively modified enhanced and finally obliterated this could be evidence of superplastic flow inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that 021 a could be an operative slip system and planes close to 132 could be possible slip planes

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

high temperature plastic deformation of quartz plagioclase multilayers by layer normal compression it is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts respectively such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase to test this assumption we performed deformation experiments on synthetic layered and particulate quartz anorthite 50 50 composites and aggregates of the end members in a paterson gas medium apparatus at a confining pressure of 300 mpa a constant strain rate of 10 5 s 1 and temperatures from 1273 to 1473 k under these conditions anorthite deforms by recrystallization accommodated dislocation creep while quartz is semibrittle electron backscatter diffraction measurements show a strong lattice preferred orientation lpo of anorthite developed in deformed specimens and the lpo pattern is interpreted as a result of dislocation slip on the 010 100 system our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz anorthite mixture i e particulate composites with the same modal composition but weaker than pure quartz aggregates our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that 1 compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and 2 thin layered rocks compressed normal to the layering are rheologically stronger than homogeneous isotropic mixtures under the same deformation conditions thus weak phase based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

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

strain softening and microstructural evolution of anorthite aggregates and quartz anorthite layered composites deformed in torsion torsion experiments of anorthite an aggregates and layered composites with equal volume fractions of quartz qtz and an were performed in a gas medium apparatus at a confining pressure of 400 mpa temperatures from 1373 to 1473 k and twist rates from 1 0 x 10 4 to 3 0 x 10 4 rad s dense specimens were fabricated from an glass and qtz crystalline powder using hot isostatic pressing hip techniques both an aggregates and qtz an layered composites show a continuous strain weakening from a peak stress at gamma 0 2 0 3 to gamma 3 2 and steady state flow has not reached under the experimental conditions the weakening is even more pronounced in the layered composites than the monolithic aggregates suggesting channeling or localization of flow into the weak material between strong layers the sheared an specimens developed pervasively c s c structures which are similar to those observed in natural ductile shear zones tem and electron backscattering diffraction ebsd fabric analyses suggest that grain boundary migration recrystallization accommodated dislocation creep with 010 100 as the dominant slip system was operating in the an the strain softening may be due to the development of crystallographic preferred orientation cpo the operation of dynamic recrystallization and the formation of extremely fine grained recrystallized material in the narrow c shear bands c 2004 elsevier b v all rights reserved

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

ultra high temperature and subsolidus shear zones examples from the poe mountain anorthosite wyoming abstract ultra high temperature shear zones formed with interstitial melts present during the emplacement of the poe mountain anorthosite wyoming the shear zones are characterized by coarse grained plagioclase with flat extinction and rare deformation microstructures dissected grain microstructures suggest that plagioclase underwent recrystallization by fast grain boundary migration orthopyroxene crystallized from interstitial melts as strain free foliation parallel tablet grains plagioclase has a magmatic petrofabric characterized by poles to 010 normal to the shear foliation and 100 parallel to the foliation and movement direction poles to 001 have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation 100 defines a point maximum perpendicular to the shear foliation and 010 and 001 have great circle distributions approximately parallel to the shear foliation microstructures of the ultra high temperature shear zones contrast with those of granulite grade shear zones cutting across the intrusion the granulite grade shear zones initiated as fractures which evolved into shear zones by dynamic recrystallization deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains

the geochemistry and petrology of the late precambrian georgeville group a volcanic arc rift succession in the avalon terrane of nova scotia

revealing surface deformation of the 1999 chi chi earthquake using high density cadastral control points in the taichung area central taiwan the 1999 chi chi earthquake m w 7 6 ruptured a major thrust fault along the western foothills of the central mountain range of taiwan here we use cadastral control points to detect horizontal displacement in taichung central taiwan the cadastral control points were used to identify the coordinates of buildings and acreage the coordinates of each point are measured by total station and connected to global positioning system gps control points the density of these control points is nearly 36 points km2 and the accuracy of each point is within less than 2 3 cm the government of taiwan measured cadastral control points before and after the chi chi earthquake in the taichung area northern end of the chelungpu fault 1269 control points were measured over a 35 km2 area with most control points being located on the hanging wall of the chelungpu fault on the footwall the displacement is about 1 1 1 2 m toward 123 to 128 which is consistent with gps data at the hanging wall the displacement direction concentrates at 326 to 330 in the eastern section the displacement is from 7 5 to 8 m which is consistent with gps and strong motion data but in the western part displacement changes from 8 m to 4 6 m and then increases again to 8 m along the 326 trending we conclude that the changes in displacements were controlled by the geometry of the fault plane online material horizontal displacement of cadastral control points

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

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

plagioclase preferred orientation by tof neutron diffraction and sem ebsd abstract the lattice preferred orientation lpo of an anorthosite composed of andesine sampled from a highly deformed anorthositic mylonite grenville province quebec was measured by tof neutron diffraction and sem ebsd the quantitative texture analysis of neutron data was accomplished by using the rietveld texture analysis with the wimv algorithm implemented in the program package materials analysis using diffraction maud the texture calculations of the ebsd data were performed by using the program beartex analyses from neutron and electron diffraction data gave similar results if ebsd data are smoothed to account for grain statistics the principal pole figures show 010 roughly parallel to the rock foliation 001 poles exhibiting a low angle 25 to the pole to foliation and 100 poles close to the y direction perpendicular to the lineation and foliation pole the 100 crystallographic direction shows a maximum in the lineation direction 010 directions concentrate near the foliation pole the geological deformation conditions and the constructed pole figure patterns indicate that the preferred orientation could be attributed to intracrystalline slip dominantly on 010 with 100 as slip direction elastic properties calculated by averaging document weak anisotropy that has implications for the seismic structure of the lower crust

seismic reflectivity of a finely layered granulite facies ductile shear zone in the southern grenville province quebec abstract lithoprobe reflection profile 54 across the granulite facies morin shear zone msz in the grenville tectonic province quebec canada reveals a west dipping zone of strong reflections the origin of these reflections has been investigated by detailed field mapping laboratory measurements of p wave velocities and densities of representative rock samples from the main lithological units and forward synthetic modeling petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin layer clusters likely resulted from intensive tectonic transposition and metamorphic differentiation lattice preferred orientation lpo induced anisotropy of granulite facies mylonites except for those containing abundant sillimanite biotite or amphibole is very low and hence unlikely to be the primary cause for reflections confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 mpa since composite reflections of thin layer clusters can cause high reflectivity the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin layer mafic clusters within dominantly felsic rocks

experimental and texture derived p wave anisotropy of principal rocks from the transalp traverse an aid for the interpretation of seismic field data a representative suite of deformed metamorphic rocks from the transalp reflection seismic traverse in the eastern alps was studied in the laboratory with respect to elastic properties and whole rock texture compressional wave p wave velocities and their anisotropies were measured at various experimental conditions dry wet confining pressure and compared to the texture related component of anisotropy here texture refers to crystallographic preferred orientations cpos which were determined by neutron texture goniometry in gneisses and schists p wave anisotropies are mainly controlled by the microcrack fabric in marbles and amphibolites cpo contributes very significantly to anisotropy at 200 mpa confining pressure the degree of anisotropy is between 5 and 15 depending on rock composition and or cpo intensity special emphasis was also put on discussing possible effects of fluids on seismic velocity and anisotropy distributions of water filled microcracks and pores are distinctly anisotropic with maximum contribution to bulk rock velocity mostly parallel to the foliation pole decreasing p wave velocity and increasing anisotropy of immersed samples may be explained by crack induced changes of the elastic moduli of bulk rock the main conclusion regarding interpretation of transalp data is that strong reflections in the deep alpine crust are probably due to marble gneiss and metabasite gneiss contacts although p wave anisotropy and boundaries between zones of dry or wet series may contribute to reflectivity to some extent

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

great earthquakes in low strain rate continental interiors an example from se kazakhstan the lepsy fault of the northern tien shan se kazakhstan extends e w 120 km from the high mountains of the dzhungarian ala tau a subrange of the northern tien shan into the low lying kazakh platform it is an example of an active structure that connects a more rapidly deforming mountain region with an apparently stable continental region and follows a known palaeozoic structure field based and satellite observations reveal an 10 m vertical offset exceptionally preserved along the entire length of the fault geomorphic analysis and age control from radiocarbon and optically stimulated luminescence dating methods indicate that the scarp formed in the holocene and was generated by at least two substantial earthquakes the most recent event dated to sometime after 400 years b p is likely to have ruptured the entire 120 km fault length in a m w 7 5 8 2 earthquake the lepsy fault kinematics were characterized using digital elevation models and high resolution satellite imagery which indicate that the predominant sense of motion is reverse right lateral with a fault strike dip and slip vector azimuth of 110 50 s and 317 343 respectively which is consistent with predominant n s shortening related to the india eurasia collision in light of these observations and because the activity of the lepsy fault would have been hard to ascertain if it had not ruptured in the recent past we note that the absence of known active faults within low relief and low strain rate continental interiors does not always imply an absence of seismic hazard

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

evidence for azimuthal and temporal variations of the rupture propagation of the 1999 chi chi taiwan earthquake from dense seismic array observations data recorded by the smart 2 array and its nearby accelerometers in eastern taiwan were used to make direct measurements of the rupture propagation of the 1999 mw 7 6 chi chi taiwan earthquake the n th root stack method was employed to window and stack seismic waveforms and it revealed that coherent seismic energy showed clear temporal changes with regard to azimuth the results of this study elicited unambiguous observations as to rupture propagation direction rupture duration time and spatial distribution of the major asperities on the fault plane the rupture propagation of the chi chi earthquake was reconstructed and demonstrated that the source of the early arrived coherent energy was close to the epicenter and that the subsequent major energy moved southward twelve seconds after the initial rupture clear arrival energy was resolved and then propagated northward the horizontal apparent rupture velocity is estimated at 2 1 km sec for its northward propagation the largest source of radiated energy was identified and its location was near the northern end of the earthquake fault the strong shaking was continuous for about 24 seconds the analysed results of this study provide direct observations of the rupture properties of this earthquake these properties may serve as valuable constraints for other chi chi earthquake rupture propagation studies based on other indirect measurement methods

coseismic displacement bilateral rupture and structural characteristics at the southern end of the 1999 chi chi earthquake rupture central taiwan 1 the 1999 chi chi earthquake mw 7 6 was due to slip along the chelungpu fault a major n striking thrust fault in the fold thrust belt of western taiwan the surface rupture extends over 100 km in a n s trend with nw trending horizontal displacement increasing from 2 to 10 m from south to north the central segment of the chelungpu fault is characterized by bedding slip along the pliocene chinshui shale which has resulted in the development of a monoclinal structure on the hanging wall at the southern end of the chi chi earthquake rupture however the chelungpu fault connects with the ne striking tachienshan fault and cuts into miocene strata with complex structures on the hanging wall in this study we use digital cadastral data to calculate coseismic horizontal displacement around the chushan area results show the amount and direction of horizontal displacement to be ca 1 2 to 2 6 m and ca 220 to 260 respectively on the hanging wall and ca 1 4 m and 105 respectively on the footwall coseismic displacement and seismological data pertaining to the hanging wall at the southern end of the chi chi earthquake rupture indicate the existence of three distinct blocks horizontal displacement indicates the chushan block s movement trended sw this result reflects the chushiang and luliao faults being reactivated with different faulting mechanisms and slip azimuths this study considers a complex coseismic displacement field resulting from a bilateral rupture process whereby a southward rupture with sw trending movement results in right lateral strike slip faulting of the tachienshan fault and thrusting with a right lateral component on the ns strike of the chelungpu fault the northward rupture is associated with nw trending movement that results in activation of the luliao and chushiang faults

preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the insubric line western alps abstract preferred lattice orientations of plagioclase measured by the universal stage from deformed amphibolite and greenschist facies rocks near the insubric line western alps reflect the type and orientation of the strain system either 001 or 010 are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction by the preferred orientation patterns dominant 010 001 slip is indicated and additionally slip on 001 parallel to the a direction possibly at somewhat higher temperatures and or at a higher an content of the plagioclase it is suggested that obliquities of the symmetry planes of the orientation patterns against the principle axes of strain are related to the sense of shearing

determination of stress directions from plagioclase fabrics in high grade deformed rocks alem paraiba shear zone ribeira fold belt southeastern brazil abstract a new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high temperature mylonitic rocks from the alem paraiba shear zone ribeira fold belt southeastern brazil we have measured the lattice preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations the maximum compressive stress direction 1 determined for all studied samples is a function of the mechanical twin orientations of a number of distinct plagioclase populations the 1 direction is generally subperpendicular to the 010 plane the statistical treatment for most of the plagioclase grains examined for each sample shows that 1 is almost perpendicular to the foliation plane suggesting a significant coaxial component in the deformation process of these rocks

plagioclase fabric development in a high grade shear zone jotunheimen norway abstract shear zones of precambrian age transform meta anorthositic rocks to mylonites of high metamorphic grade in jotunheimen norway a plagioclase microfabric study is presented for one of these shear zones the shear zone is heterogeneously and asymmetrically developed regarding the degree of recrystallization and this is inferred as reflecting strain variations pre shear zone plagioclase is preserved in the deformed rocks as porphyroclasts which display large variations in grain elongation in the plane of mylonitic foliation with aspect ratios long dimension average short dimension as large as 25 1 the preferred orientation of lattice and optical directions in the plagioclase porphyroclasts indicates that they are preferentially oriented with the pole to 010 at a moderate angle to the foliation pole in such a way that they are unfavorably oriented for mechanical twinning on the albite and pericline laws in a flattening strain across the plane of the mylonitic foliation this is taken to indicate that the porphyroclasts represent those pre shear zone plagioclase grains which were oriented in order to accommodate such a flattening strain by mechanical twinning while grains otherwise oriented accommodated the strain by kinking with an axis of external rotation at 100 and glide which accumulated lattice strain and eventually led to recrystallization the plagioclase porphyroclasts with large aspect ratios are preferentially oriented with 100 at a large angle to the inferred translation direction of simple shear strain they are the products of preferential recrystallization of zones of large lattice strain in grains which were strongly kinked due to their special orientation this implies that grain elongation is not applicable as a measure of strain the microstructures indicate that the recrystallization was dynamic the produced neoblasts display a preferred orientation which is weak although with a preference for 010 to have a low angle to the foliation plane which mimics the preferred orientation of the porphyroclasts there is a slight indication that the orientation of neoblasts is controlled by the plagioclase host such that 100 of the neoblasts prefer intermediate rather than large angles against 100 of the porphyroclast these features and microstructural observations indicate that rotation recrystallization is a basic recrystallization mechanism under high grade metamorphic conditions

complete texture analysis of a deformed amphibolite comparison between neutron diffraction and u stage data abstract this paper describes a new technique of analysing complex neutron diffraction patterns allowing the quantitative texture analysis of each rock forming mineral up to trielinic lattice symmetry in polymineralic rocks the method is demonstrated using hornblende and plagioclase fabrics in alzenau amphibolite from the spessart mountains mid german crystalline rise hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements for control purposes crystallographic preferred orientations were also determined independently by individual grain measurements using a u stage from the neutron texture analyses we obtain only superposed pole figures hornblende and hornblende hornblende and plagioclase and plagioclase with plagioclase of low indexed lattice planes therefore the critical crystallographic directions were recalculated by the orientation distribution function odf which is given by texture components the optical and neutron derived pole figures are in a good agreement all pole figures display strong preferred orientation although the plagioclase texture is less pronounced the hornblende texture consists of a 001 maximum parallel to the stretching lineation with 010 showing a tendency to develop a girdle around the lineation 100 axes are distributed with a clearly split maximum representing the monoclinic angle between 100 and 001 the plagioclase texture is a rare one with 001 subparallel within the foliation and 100 oriented subparallel to the lineation 010 of plagioclase is concentrated normal to lineation within the foliation the technique outlined here is potentially the most useful one for texture analysis of polyphase rocks independent of crystal symmetry

revealing coseismic displacements and the deformation zones of the 1999 chi chi earthquake in the tsaotung area central taiwan using digital cadastral data 1 the 1999 chi chi taiwan earthquake mw 7 6 was the largest earthquake to strike taiwan in the twentieth century this earthquake is associated with a 100 km long surface rupture in order to reveal the details of displacement near the surface rupture we use a digital cadastral system to calculate coseismic displacement around the tsaotung area central taiwan the digital cadastral system was originally conceived to survey land and building boundaries in the tsaotung area taiwan authorities have taken digital cadastral measurements before and after the chi chi earthquake the cadastral system affords high density control points that reach 1421 points km2 a system denser than that of the gps accuracy is to within 11 cm a level that is higher than spot imaging and one that allows us to study surface deformation in detail coseismic displacement is 4 3 4 6 m at distance from the surface rupture and decreases to 3 4 m near the surface rupture the azimuth of horizontal displacements is 310 315 and rotates to 280 305 near the surface rupture this produced a compression left lateral deformation zone with 10 3 compression strain near the surface rupture coseismic displacement of the footwall is 1 1 2 m in 110 which is similar to that from using gps data in the tsaotung thrust slice we observed that the azimuth of horizontal displacement rotates from a nw trend to a south trend as a result of slip partitioning and gravity slide effect

the initiation of strain localisation in plagioclase rich rocks insights from detailed microstructural analyses abstract in order to shed light on the cause for onset of strain localisation in plagioclase rich rocks we have performed detailed microstructural analyses on a sheared anorthosite leucogabbro using optical microscopy electron backscatter diffraction ebsd and chemical analyses the analysed sample is from an archaean unit sw greenland deformed at lower to mid crustal conditions t 675 700 c and moderate pressure the initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation recrystallised plagioclase grains average size 80 m are dominantly found in i clusters ii lenses and iii continuous bands subparallel to shear zone boundaries recrystallised grains in clusters and lenses display inherited crystallographic orientations their bulk crystallographic preferred orientation cpo is random however crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems recrystallised grains in continuous bands show a cpo with a single dominant active slip system 001 aligned with the structural xyz framework for these parent and daughter grains misorientation axes are random and the dominant slip system is different grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast we infer that the cause for strain localisation is recrystallisation and development of a cpo in continuous recrystallised bands microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation accommodated grain boundary sliding disgbs in continuous bands this inferred shift in dominant deformation mechanism would lower the strength of the shear zone

slip vectors of the surface rupture of the 1999 chi chi earthquake western taiwan about 100 slip vectors were surveyed along the 1999 chi chi earthquake rupture 100 km long western taiwan to understand the coseismic reverse faulting processes the surveyed slip azimuths vary locally over 80 where the rupture is irregular or associated with flexure even where the rupture is straight and has consistent slip azimuths the fault scarp heights horizontal slips and fault dip angles oscillate in a distance of hundreds of meters along the rupture despite these local variations the net slip up to 11 5 m vertical and horizontal slips are significant greater in the north on which maximum shortening 6 m by pure thrust and strike slip 8 m by left lateral fault occur in association with the bend of the rupture the slip azimuths also progressively rotate from north northwest in the north to west southwest in the south these patterns generally agree with those revealed by gps surveys except that the slips by gps in the south move to the west northwest and have values unexpectedly smaller than our measurements our obtained sw directed slips resulted in dominantly right lateral faulting up to 2 4 m where the rupture ends to the southeast this however does not correspond to focal mechanisms of any main shocks or aftershocks

on the measurement of plagioclase lattice preferred orientations abstract there are two major difficulties in measuring plagioclase lattice preferred orientations lpos by u stage the first which is caused by dependence of the lpo determination on anorthite content cannot be overcome using the u stage method at present the second difficulty which is caused by the dependence on orientation of thin sections can be overcome by measuring all twinned grains in any two orthogonal sections compared with previous methods published in the literature the present method using two rather than three orthogonal sections is more statistically meaningful and time saving in practice this method was used for measuring lpos of plagioclase an 50 from a typical anorthosite mylonite deformed under the granulite facies conditions in the morin ductile shear zone within the canadian grenville tectonic province

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

preferred orientation of anorthite deformed experimentally in newtonian creep abstract synthetic anorthite aggregates were deformed in a paterson gas deformation apparatus at confining pressures up to 400 mpa in torsion and axial compression at temperatures between 950 c and 1200 c samples deformed in torsion under newtonian creep display development of texture or crystallographic preferred orientation as documented with synchrotron x ray diffraction measurements complex diffraction patterns were deconvoluted with the rietveld method to obtain quantitative texture information torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0 3 develop clear textures texture and shape preferred orientation spo of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide these results show that a material deforming in linear viscous creep can develop a strong texture in striking contrast to the paradigm that the presence of a texture precludes low stress newtonian behavior our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological mechanical regime as sometimes applied for interpretation of seismic anisotropy in the earth

fluid pressure stress field and propagation style of coalescing thrusts from the analysis of the 20 may 2012 ml 5 9 emilia earthquake northern apennines italy terra nova 25 72 78 2013 abstract two major earthquakes of ml 5 9 and 5 8 and hundreds of aftershocks affected the emilia region po plain italy between 20 may and june 2012 the events concentrate in the uppermost 10 km of the crust with few events up to 30 km two buried sub parallel n100 e striking thrusts of the northern apennines belt are reactivated these thrusts coalesce at the interface between the metamorphic basement of the belt and the overlying about 10 km thick sedimentary succession focal mechanisms indicate a compressive stress field with a sub horizontal roughly n s striking 1 this stress field is consistent with active shortening in northern apennines suprahydrostatic pore pressure and 1 2 3 are required to reactivate the thrusts the involved fluids could be gas and brines hosted in the folded sedimentary successions the time and spatial evolution of the seismicity indicates a foreland to hinterland propagation of the ruptures and a thin skinned deformation of northern apennines

fault rupture process of the 20 september 1999 chi chi taiwan earthquake we modeled the source rupture process of the 20 september 1999 chi chi taiwan earthquake mw 7 6 using an integrated dataset of near field strong ground motion and global positioning system observations this large inland thrust earthquake produced a surface break more than 80 km long along the chelungpu fault the event triggered over 700 strong motion instruments on the island of tai wan this large ground motion dataset provides the best opportunity to date to un derstand earthquake source processes in a major earthquake first we develop a 3d curved fault model consistent with the surface rupture data and aftershock distribu tion the subsurface fault plane has an average dip of 30 to the east and measures 96 km along strike and 40 km down dip next we applied a genetic algorithm to reconstruct the source rupture process the inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down dip most of the displacement occurred over a 15 km depth range in the northern segment of the fault with the maximum slip exceeding 20 m the average slip is 3 8 m and the average rupture velocity is 2 6 km sec the mainshock slip distribution clearly complements the aftershock location distribution slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north this relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations the rake angle rotates across the rupture plane from predominantly dip slip in the southern segment to mostly left lateral oblique slip in the north we find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior at fault bends rupture tends to decelerate the rake angle rotates to a near dip slip orientation and the shear stress increases as rupture tries to overcome the bending barriers the total seismic moment from our inversion is 2 9 10 27 dyne cm in good agreement with the harvard and the u s geological survey moment estimation

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

geomorphological identification of pre existing active chelungpu fault in central taiwan especially its relation to the location of the surface rupture by the 1999 chichi earthquake abstract pre existing active faults were identified on the chelungpu fault based on airphoto interpretation and field survey and their precise position was mapped active faults are divided into four types from i to iv depending on certainty of the presence of active faults and accuracy of their location most of the chelungpu fault shows evidence of late quaternary repeated activities the location of the 1999 chichi earthquake surface ruptures were plotted from the sites where they were accurately mapped by gps measurements by the central geological survey supplemented by our own observations on the active fault map there is a very close coincidence between the active fault trace and the 1999 surface rupture indicating that the mapping of active faults is fundamental for the estimation of location of rupture by future earthquakes

the influence of three dimensional grain size distributions on the rheology of polyphase rocks abstract in this study the relation between the deformational behaviour of two phase rocks and the grain size distributions of the constituent phases is investigated samples of hot isostatically pressed 50 50 volume percent mixtures of calcite and anhydrite were experimentally deformed in a triaxial apparatus at a temperature of 500 c a confining pressure of 300 mpa and a strain rate of 2 10 5 s 1 for calcite and anhydrite these are the deformational conditions for dislocation and diffusion creep respectively and for a grain size of 8 m these are the conditions of the equiviscous point it was found that samples from different batches of the synthetic material while having an identical average two dimensional grain size displayed significantly different rheological behaviour the ensuing full analysis revealed that the volumetric three dimensional grain size distributions of the different phases and the different batches varied significantly since the anhydrite is in the transition between the dislocation and the grain size sensitive diffusion creep regime the deformational behaviour of the bulk depends critically on the grain size distribution of the anhydrite phase the usefulness of the concept of an average grain size is discussed

crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep abstract the crystallographic preferred orientations of a series of experimentally deformed fine grained albite aggregates were measured by synchrotron source x ray diffraction most samples were deformed and extensively recrystallized by low temperature recrystallization accommodated dislocation creep in axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on 001 the sheared samples have a marked asymmetry of the maxima with respect to the shear zone boundaries one sample was axially compressed by solution precipitation creep it developed a somewhat different but equally strong preferred orientation perhaps reflecting crystallographic anisotropy in rates of dissolution and growth

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling