late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

elevated lateral stress in unlithified sediment midcontinent united states geotechnical and geophysical indicators for a tectonic origin abstract indirect and direct geotechnical measurements revealed the presence of high lateral earth pressure ko in shallow unlithified sediment at a site in the northernmost mississippi embayment region of the central united states results from pile load and pressuremeter tests showed maximum ko values greater than 10 however the complex geologic environment of the midcontinent made defining an origin for the anomalous ko based solely on these measurements equivocal although in situ sediment characteristics indicated that indirect tectonic or nontectonic geologic mechanisms that include transient overburden loads e g fluvial deposition erosion glacial advance retreat and dynamic shear loads e g earthquakes were not the dominant cause they were unable to provide indicators for a direct tectonic generation localized stresses induced anthropogenically by the geotechnical field tests were also considered but ruled out as the primary origin a high resolution shear wave sh reflection image of geologic structure in the immediate vicinity of the test site revealed compression style neotectonism and suggested that the elevated stress was a tectonic manifestation post paleozoic reflectors exhibit a tertiary structural inversion as evidenced by post cretaceous fault displacement and pronounced positive folds in the hanging wall of the interpreted faults the latest stratigraphic extent of the stress effects i e all measurements were in the late cretaceous to tertiary mcnairy formation as well as the relationship of stress orientation with the orientation of local structure and regional stress remain unknown these are the subjects of ongoing studies

near surface deformation in the new madrid seismic zone as imaged by high resolution sh wave seismic methods over 1 5 kilometers of high resolution sh wave refraction and reflection profiles have been performed in the madrid bend area of northwestern tennessee approximately 8 km north of tiptonville tennessee to establish the existence of neo tectonic near surface deformation the specific area was chosen because of the access it provided to the tiptonville dome a holocene flexure and the trend of contemporary seismicity within the new madrid seismic zone nmsz we believed the highest likelihood of near surface deformation to be over these features partially reversed refraction reflection walk away tests and common depth point cdp seismic profiling using the non traditional sh wave method have traced antiformal flexure and numerous faults into the late eocene and holocene sediments of the tiptonville dome province of the lake county uplift lcu vertical displacements exhibited across the faults range from approximately 3 to 30 m these structures have been interpreted to be neo tectonic features associated with tensional stresses across the tiptonville dome

earthquake faulting related deformation in soil evidenced in s wave shallow reflection data field results from portugal abstract expressions of ductile soft sediment deformations induced by ground movements due to past earthquakes are difficult to recognize in near surface soils we have carried out shallow s wave reflection studies in a seismically active area located northeast of metropolitan lisbon portugal identifying shallow disturbed zones and hidden fault segments in this area is important but quite difficult because of small vertical slips due to earthquakes the holocene alluvial cover hiding the fault segments and a high rate of surficial sedimentation we have performed s wave reflection profiling at two sites vila franca xira and castanheira de ribatejo we detected different but interrelated evidence of soft sediment deformation in the seismic data this evidence includes sharp lateral changes in the s wave velocity field changes in the reflection horizons in stacked sections aligned diffractions in unmigrated sections discontinuities in common offset gathers and discontinuities backscattered and d

strain histories from the eastern central range of taiwan a record of advection through a collisional orogen abstract in the eastern central range of taiwan there is a regional variation in the orientation of maximum finite stretch across the slate belt with down dip maximum stretch found in the western central range and along strike maximum stretch in the eastern central range incremental strain histories from syntectonic fibers in pyrite pressure shadows indicate a progressive change in extension direction from down dip to along strike during deformation there is a corresponding temporal variation in stretching direction shown in samples from the eastern edge of the central range a pattern that mimics the regional west to east spatial variation these observed temporal and spatial strain distributions are used to evaluate the kinematics associated with slaty cleavage development during advection through the taiwan orogenic system the subduction zone beneath the island of taiwan is influenced by two types of obliquity that have the potential to generate the observed along strike stretching first the plate motion vector of the philippine sea plate relative to the eurasian plate is slightly oblique to the regional strike of the mountain range which could result in partitioning of strike slip shearing into the interior of the collision second the north south luzon volcanic arc on the philippine sea plate is obliquely oriented relative to the northeast southwest edge of the eurasian continental margin which could result in lateral extrusion of the ductile core of the range incremental strain histories in cleavage parallel samples represent a time for space equivalence where the stretching direction is fixed relative to the position within the mountain belt architecture e g the topographic divide and temporal variations in the eastern central range reflect lateral advection through the strain field in response to accretionary and erosional fluxes incremental strain histories in cleavage perpendicular samples show both clockwise and counter clockwise rotation of strain indicating the possibility of bulk coaxial strain and lateral extrusion

integration of p and sh wave high resolution seismic reflection and micro gravity techniques to improve interpretation of shallow subsurface structure new madrid seismic zone abstract shallow high resolution seismic reflection surveys have traditionally been restricted to either compressional p or horizontally polarized shear sh waves in order to produce 2 d images of subsurface structure the northernmost mississippi embayment and coincident new madrid seismic zone nmsz provide an ideal laboratory to study the experimental use of integrating p and sh wave seismic profiles integrated where practicable with micro gravity data in this area the relation between deeper deformation of paleozoic bedrock associated with the formation of the reelfoot rift and nmsz seismicity and shallower deformation of overlying sediments has remained elusive but could be revealed using integrated p and sh wave reflection surface expressions of deformation are almost non existent in this region which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment since p and sh waves respond differently to the rock and fluid properties and travel at dissimilar speeds the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability p wave profiles acquired in southwestern illinois and western kentucky usa detect faulting of deep paleozoic bedrock and cretaceous reflectors while coincident sh wave surveys show that this deformation propagates higher into overlying tertiary and quaternary strata forward modeling of micro gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and cretaceous strata the integration of the two seismic and the micro gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

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

the olmsted fault zone southernmost illinois a key to understanding seismic hazard in the northern new madrid seismic zone geological deformation in the northern new madrid seismic zone near olmsted illinois usa is analyzed using integrated compressional wave p and horizontally polarized wave sh seismic reflection and regional and dedicated borehole information seismic hazards are of special concern because of strategic facilities e g lock and dam sites and chemical plants on the ohio river near its confluence with the mississippi river and because of alluvial soils subject to high amplification of earthquake shock we use an integrated approach starting with lower resolution but deeper penetration p wave reflection profiles to identify displacement of paleozoic bedrock higher resolution but shallower penetration sh wave images show deformation that has propagated upward from bedrock faults into pleistocene loess we have mapped an intricate zone more than 8 km wide of high angle faults in mississippi embayment sediments localized over paleozoic bedrock faults that trend north to northeast parallel to the ohio river these faults align with the pattern of epicenters in the new madrid seismic zone normal and reverse offsets along with positive flower structures imply a component of strike slip the current stress regime favors right lateral slip on northeast trending faults the largest fault the olmsted fault underwent principal displacement near the end of the cretaceous period 65 to 70 million years ago strata of this age dated via fossil pollen thicken greatly on the downthrown side of the olmsted fault into a locally subsiding basin small offsets of tertiary and quaternary strata are evident on high resolution sh wave seismic profiles our results imply recent reactivation and possible future seismic activity in a critical area of the new madrid seismic zone this integrated approach provides a strategy for evaluating shallow seismic hazard related targets for engineering concerns

late miocene to early pliocene blueschist from taiwan and its exhumation via forearc extraction blueschist facies rocks in the yuli belt of taiwan s central range record ongoing subduction of the eurasian plate we present a prograde lu hf garnet whole rock age of 5 1 1 7 ma from a retrogressed blueschist in the yuli belt this age is considerably younger than the previously assumed age of 14 8 ma for high pressure metamorphism in the yuli belt and represents the youngest lu hf garnet age ever recorded for blueschist facies metamorphism the age sheds new light on the palaeogeographic origin and exhumation scenario of the yuli belt we propose that the yuli belt originated from the ocean continent boundary of the chinese passive margin it was subducted eastward during collision with the luzon island arc and rapidly exhumed when the forearc lithosphere was removed from above the continental slab by discrete subduction extraction this process reduces the pressure above the continental slab and may prompt the ascent of subducted crust into the opening gap thus it can control the exhumation of high pressure rocks

quaternary faulting in the southern mississippi embayment and implications for tectonics and seismicity in an intraplate setting the recently recognized saline river fault zone in the southwestern mississippi embayment strike 135 is characterized by moderate historic seismicity we document paleoseismicity possibly strong on the saline river fault zone that is outside the known region of neotectonism in the northern embayment six surface faults were excavated within the saline river fault zone and all faults displace marine eocene units and fluvial pliocene pleistocene units five show post wisconsin loess movement thermoluminescence tl age 23 6 3 ka and three deform middle to late holocene eolian silt tl age 5 1 0 6 ka and 3 6 0 5 ka as fault tip flexures along the principal excavated fault fault plane braiding and plunging drag folding suggest a strong component of left slip and subsidiary faults show both normal and reverse displacement 1m of pliocene pleistocene wisconsin and holocene units we interpret these faults as components of a left slip flower structure a shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m supporting a strike slip interpretation in addition the only focal plane mechanism available in the area indicates left slip with a small reverse component we conclude that the saline river fault zone is a paleozoic mesozoic basement fault zone reactivated as a left slip system in a quaternary east west compressive stress field field evidence suggests that earthquakes with several meters of strike slip may be characteristic and the width of a liquefaction field on the saline river fault zone suggests an event of magnitude 5 5 6 considering a similarity of strike and sense of holocene movements of the meers fault of southern oklahoma and of the saline river fault zone there may be additional active fault zones that pose seismic hazards concealed by sediments in southern north america

shear wave seismic reflection studies of unconsolidated sediments in the near surface we have successfully applied of sh wave seismic reflection methods to two different near surface problems targeting unconsolidated sediments at the former fort ord where the water table is approximately 30 m deep we imaged aeolian and marine aquifer and aquitard stratigraphy to a depth of approximately 80 m we identified reflections from sand clay and sand silt interfaces and we mapped these interfaces along our transects at an aggregate study site in indiana where the water table is at a depth of 1 to 2 m we imaged stratigraphy in alluvial sand and gravel and observe a strong reflection from the 32 m deep bedrock surface in both cases we exploited the high resolution potential of sh waves their insensitivity to water content and the possibility of reducing love wave contamination by working along a roadway we accomplished our results using only sledgehammer sources and simple data processing flows

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

evidence for recurrent faulting in the new madrid seismic zone from mini sosie high resolution reflection data a mini sosie high resolution seismic reflection survey was conducted on reelfoot scarp in the northwestern tennessee portion of the new madrid seismic zone interpretation of the mini sosie data revealed the need to reinterpret previously collected reflection data obtained from explosive source and vibroseis surveys interpretation and integration of the three data sets have resulted in a new model for the subsurface of reelfoot scarp and provide evidence for recurrent movement along reelfoot fault the major reverse fault associated with reelfoot scarp estimated displacements on reelfoot fault vary from 60 m 60 ms for late paleozoic rocks to 15 m 20 ms for late eocene sedimentary units no clear offsets are observed on this particular fault for units younger than late eocene age however uplift folding and related structures are observed in younger sediments an observed variation of offset with depth age and the presence of the younger structures are evidence of reactivation of reelfoot faul

early history of earth s crust mantle system inferred from hafnium isotopes in chondrites the 176lu to 176hf decay series has been widely used to understand the nature of earth s early crust mantle system1 2 3 4 5 6 the interpretation however of lu hf isotope data requires accurate knowledge of the radioactive decay constant of 176lu as well as bulk earth reference parameters a recent calibration of the value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3 9 gyr implying widespread continental crust extraction from an isotopically enriched mantle source more than 4 3 gyr ago7 but does not provide evidence for a complementary depleted mantle reservoir here we report lu hf isotope measurements of different solar system objects including chondrites and basaltic eucrites the chondrites define a lu hf isochron with an initial 176hf 177hf ratio of 0 279628 0 000047 corresponding to 1 983 0 033 10 11 yr 1 using an age of 4 56 gyr for the chondrite forming event this value indicates that earth s oldest minerals were derived from melts of a mantle source with a time integrated history of depletion rather than enrichment7 the depletion event must have occurred no later than 320 myr after planetary accretion consistent with timing inferred from extinct radionuclides8 9 10

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

tectonic erosion and the removal of forearc lithosphere during arc continent collision evidence from recent earthquake sequences and tomography results in eastern taiwan a very common but important observation of collided and accreted volcanic arcs is that the forearc regions of these arcs are largely or even entirely missing the processes and mechanisms responsible for the removal and transport of the forearc materials from the collisional belts are thus important issues in understanding tectonics and crustal growth the young and ongoing collision between the luzon volcanic arc and the eurasian continental margin that forms the island of taiwan provides a rare opportunity to examine these processes and mechanisms as they occur from observations of a new detailed 3 d tomography combined with relocated hypocenters of two earthquake sequences occurred in 2003 and 2006 we found that the luzon forearc lithosphere initially underthrusts westward after the collision began as the collision proceeds the forearc basement then subducts eastward beneath the colliding and accreting luzon arc along a major fault system in eastern taiwan thus the luzon forearc lithosphere appears to be removed by tectonic erosion and is being transported eastward into the mantle our results from the active taiwan orogen will provide important insights for interpreting rock records from many old arc continent collisional belts in the world

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

paradigms new and old for ultrahigh pressure tectonism regional ultrahigh pressure uhp metamorphic terranes exhibit a spectrum of lithological structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them at least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths i continental margin subduction ii microcontinent subduction iii sediment subduction iv intracontinental subduction v subduction erosion and vi foundering of a crustal root most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of uhp rocks at least six geodynamic processes can be envisioned to have exhumed uhp continental rocks i eduction ii microplate rotation iii crustal stacking iv slab rollback v channel flow and vi trans mantle diapirs most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one uhp terrane more detailed and systematic field investigations are warranted to assess the predictions of numerical models and more sophisticated and realistic numerical models are required to replicate and explain the petrological structural and chronological data obtained from uhp terranes

subduction collisional processes between the eurasian and philippine sea plates constraints from thermal age paths of the taiwan orogen abstract records of the thermal age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc tectonic events and thermochronological data of the taiwan orogen have been extensively investigated we use the zircon u pb youngest ages as the initial subduction age the raman spectroscopy of carbonaceous material 40ar 39ar age of mica porphyroblasts whole rock lu hf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion and the apatite fission track data for cooling age of the taiwan orogen to illuminate the subduction and collisional processes between the eurasian plate and the luzon arc the hsuehshan range nappe in the west of the taiwan orogen whose subduction was initiated at 12 ma reached peak metamorphism with a temperature of 250 c to 490 c at a depth of 8 km during the interval 6 2 5 ma mainly by basal superimposed accretion the isothermal dwelling during peak metamorphism condition of the hsuehshan range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the taiwan orogen and led to a sudden increase of subduction and decollement depths and slow downward thickening of the orogenic wedge and mountain root growth located in the east of the taiwan orogen the yuli beltsubduction was initiated at 15 4 16 ma and attained peak metamorphism at a depth of 40 km with a temperature of 550 560 c and a pressure of 10 5 13 kbar during the interval 12 2 4 4 ma due to basal superimposed accretion the exhumation of the hsuehshan range commenced at 2 5 ma which is later than that of the yuli belt at 3 4 ma such distinct and different thermal age paths of the hsuehshan range and the tananao complex indicate that different tectonic wedges in the taiwan orogen are formed through different subduction and accretional processes

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

shallow subsurface structure of the wasatch fault provo segment utah from integrated compressional and shear wave seismic reflection profiles with implications for fault structure and development integrated vibroseis compressional and experimental hammer source shear wave seismic reflection profiles across the provo segment of the wasatch fault zone in utah reveal near surface and shallow bedrock structures caused by geologically recent deformation combining information from the seismic surveys geologic mapping terrain analysis and previous seismic first arrival modeling provides a well constrained cross section of the upper 500 m of the subsurface faults are mapped from the surface through shallow poorly consolidated deltaic sediments and cutting through a rigid bedrock surface the new seismic data are used to test hypotheses on changing fault orientation with depth the number of subsidiary faults within the fault zone and the width of the fault zone and the utility of integrating separate elastic methods to provide information on a complex structural zone although previous surface mapping has indicated only a few faults the seismic section shows a wider and more complex deformation zone with both synthetic and antithetic normal faults our study demonstrates the usefulness of a combined shallow and deeper penetrating geophysical survey integrated with detailed geologic mapping to constrain subsurface fault structure due to the complexity of the fault zone accurate seismic velocity information is essential and was obtained from a first break tomography model the new constraints on fault geometry can be used to refine estimates of vertical versus lateral tectonic movements and to improve seismic hazard assessment along the wasatch fault through an urban area we suggest that earthquake hazard assessments made without seismic reflection imaging may be biased by the previous mapping of too few faults

erosion rates and orogenic wedge kinematics in taiwan inferred from fission track thermochronometry new apatite and zircon fission track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active taiwan arc continent collision we present 20 new apatite fission track ages and 6 new zircon fission track ages apatite and zircon ages are all reset in the northern and eastern parts of taiwan although the region of reset apatite ages is larger we interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone a onedimensional thermal model including erosion provides prediction of the fission track ages the distribution of reset ages is best explained with an erosion rate of 4 6 mm yr given a propagation velocity of 60 mm yr this erosion rate implies that nearly 25 km of material has been eroded from northern taiwan the lack of reset 40 ar 39 ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component a result consistent with an eroding orogenic wedge model

using raman spectroscopy of carbonaceous materials to track exhumation of an active orogenic belt an example from eastern taiwan abstract the burial and exhumation history of an orogen can provide insights into the thermal history of rocks in the mountain belt asynchronous collision will exhume materials to the surface at different times and expose different metamorphic grades along the orogenic axis by tracking the thermal signals of eroded materials it is possible to compare spatial patterns of exhumation along the mountain belt previous studies such as bulk mineralogy of sandstones crystallinity of clay minerals and fission track thermochronology all suggest that the pliocene to pleistocene meta sedimentary deposits within the preserved basins of the coastal range reflect the unroofing history of the central range in eastern taiwan in this study modern sediments from six drainages of the east central range liwu mugua laku laku luye jinlun and hsuhai rivers and three basin sections of meta sedimentary deposits from the coastal range shuilien chimei and madagida sections were sampled to reconstruct the thermal signals of each drainage along the modern orogenic axis and reconstruct temporal variations of thermal signals in preserved plio pleistocene sedimentary rocks the temperature index t i of modern sediments determined by raman spectroscopy of carbonaceous materials are medium to high 0 4 0 8 in conjunction with bio and magneto stratigraphy our data suggest that the northern shuilien area has been receiving detritus of medium metamorphic grade from the exhumed orogeny in the north from 3 4 ma and higher grade since 2 ma meanwhile the southern madagida section has been mostly taking in juvenile detritus from the south part of the orogen until 1 ma the central chimei section has started collecting mature detritus since 1 24 ma the data suggest that the plio pleistocene basins contain a good record of orogen evolution

exhumation of high and ultrahigh pressure metamorphic rocks by slab extraction exhumation of high and ultrahigh pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks erosion of their overburden or extensional thinning of the overburden some high pressure terranes such as the adula nappe in the central alps fit none of these scenarios we propose an additional way in which part of the overburden may be removed it may sink off into the deeper mantle slab extraction structural and metamorphic relationships in and around the adula nappe indicate that the emplacement of this alpine high to ultrahigh pressure nappe to 3 2 gpa in a pile of lower pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins ultimately leading to the extraction of the intervening slab in terms of mechanics the cause of the exhumation is in this case not the buoyancy of the high pressure rocks but the negative buoyancy of the extracted slab

evolution of early crust in chondritic or non chondritic earth inferred from u pb and lu hf data for chemically abraded zircon from the itsaq gneiss complex west greenland zircon grains in rocks collected from the itsaq gneiss complex southwest greenland were analyzed for u pb and lu hf in the same grain using isotope dilution thermal ionization mass spectrometry tims and multicollector inductively coupled plasma mass spectrometry mc icp ms grains were pretreated using chemical abrasion or air abrasion to assure that only zircon material unaffected by the migration of parent and daughter elements was analyzed the data are consistent with derivation of all studied rocks from a single enriched mantle source or mafic crustal protolith with 176lu 177hf of 0 022 0 003 that was repeatedly melted and produced tonalitic magmas the assessment of the primary mantle source from which this mafic protolith was derived at or before 3 85 ga greatly depends on the assumed composition of the bulk silicate earth using the currently accepted lu hf bulk earth parameters based on the analysis of chondrites yields ehf t of 0 to 1 for the 3 80 3 86 ga rocks suggesting that

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

matrix mosaics brittle deformation and elongate porphyroclasts granulite facies microstructures in the striding athabasca mylonite zone western canada abstract the late archean striding athabasca mylonite zone western canadian shield is a 400 km long linked system of granulite facies mylonite belts developed in the continental crust the granulite facies mylonites 800 1000 c at 0 8 1 1 gpa developed in subsets of the anhydrous mineral assemblage clinopyroxene orthopyroxene garnet plagioclase quartz hornblende comparison of the microstructures of the striding athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the striding athabasca mylonites in addition to dislocation creep and dynamic recrystallisation extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism the microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain scale and aggregate scale diffusion grain boundary mobility and mass transfer at relatively slow strain rates despite their annealed appearance the granoblastic matrix mosaics developed synkinematically deformation induced dilation may enhance metamorphic reactions where products are more voluminous than reactants in the porphyroclast population highly elongate 20 1 monocrystalline orthopyroxenes appear to be fragments of dismembered kinked parent grains rather than stretched porphyroclasts granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites in particular it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained high temperature shear zone rocks

basal accretion a major mechanism for mountain building in taiwan revealed in rock thermal history abstract deep tectonic processes are key integral components in the evolution of mountain belts while observations of their temporal development are generally obscured by thermal resetting retrograde alteration and structural overprinting here we recorded an integrated rock time temperature history for the first time in the pro wedge part of the active taiwan arc continent collision starting from sedimentation through cleavage forming state to its final exhumation the integrated thermal and age results from the raman spectroscopy of carbonaceous material rscm method zircon u pb laser ablation dating and in situ 40ar 39ar laser microprobe dating suggest that the basal accretion process was crucial to the development of the taiwanese orogenic wedge the basal accretion process commenced early in the mountain building history 6 ma and gradually migrated to greater depths as constrained by persistent plate convergence and cleavage formation under nearly isothermal state at similar depths until 2 5 ma recorded in the early accreted units such development essentially contributed to mountain root growth by the increased depth of the wedge detachment and the downward wedge thickening during the incipient to full collision stages in the taiwan mountain belt

seismic investigation of near surface geological structure in the paducah kentucky area application to earthquake hazard evaluation abstract seismics method were used to evaluate shallow geological conditions at 33 sites in the vicinity of paducah kentucky a combined set of p and s wave seismic refraction and reflection soundings were used in addition to local borehole information to produce structure maps of 1 a shallow 160 m deep shear wave velocity contrasts across the shallow unconformity were generally 2 to 1 while the contrast at the top of the paleozoic bedrock exceeds 5 to 1 these seismic boundaries have been determined to be very important in modelling and interpreting earthquake ground motion amplification in the paducah area the quality and accuracy of the data and the cost effective nature of the methods suggest that other communities in areas at risk to damage from seismic activity with foundation conditions comparable to paducah might benefit from similar characterization in order 1 to identify seismically hazardous near surface geological conditions and 2 to develop geological models that could be used in computer simulations of site response

multiple late holocene earthquakes along the reelfoot fault central new madrid seismic zone the reelfoot fault is an east vergent reverse fault underlying the lake county uplift a low amplitude late holocene anticline bordered on the east by the 32 km long reelfoot scarp fluvial deposits across the scarp define an 8 m high east facing monocline most near surface deformation along the scarp is accommodated via folding rather than faulting we interpret the scarp as a fault propagation fold developed over a wcst dipping reverse fault interpreted from shallow seismic reflection data trench exposurcs provide evidence for three episodes of deformation along the reelfoot fault within thc past approximately 2400 years between a d 780 and 1000 between a d 1260 and 1650 and during a d 1812 our best estimate of the average recurrence interval for deformation along the scarp is 400 500 years each episode of deformation had a slightly different style the third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault the second most recent earthquake produced about 1 3 m of throw in the graben as well as folding along the updip projection of the reverse fault and development of the scarp thesc relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude the 1811 1812 episode of deformation produced abundant liquefaction prominent folding of fluvial strata along the scarp and minor faulting in the graben

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

signature of fault zone deformation in near surface soil visible in shear wave seismic reflections small throw seismogenic fault segments hidden in the holocene sediments are crucial but difficult targets in seismic exploration we report here the detection of the deformation pattern and a concealed fault segment in the unconsolidated sediments at vila franca xira portugal through identification in shear wave reflection data of multiple signatures of ductile deformation associated with faulting we find step like changes in the stacking velocity along a shallow subsoil layer boundary indicating synsedimentary faulting we also recognize a consistent distortion in the moveout of the reflection events in the raw shear wave data synthetic modeling of seismic data helps in interpreting these observations and identifying backscattered energy from a steeply dipping shallow fault zone prior to this finding there was no evidence for holocene activity of this fault although the fault is considered to be the most probable source for the disastrous 1531 earthquake

microdiamond discovered in the seve nappe scandinavian caledonides and its exhumation by the vacuum cleaner mechanism when a continent collides with an island arc or other continent continental crust of the subducted continent may be buried to depths exceeding 100 km and exposed to pressures that can cause formation of coesite and diamond this process leads to substantial density increase in sio2 rich rocks and in turn to a reduction of the buoyancy of the subducted material which should inhibit exhumation nevertheless coesite and diamond bearing continental crustal rocks are known from several occurrences worldwide we report on the discovery of microdiamond in kyanite garnet gneiss from allochthonous metasediments of the seve nappe complex in the scandinavian caledonides our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust we propose that the diamond bearing rocks were subducted in an arc continent collision setting and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block