vp vs distribution in the northern taiwan area implications for the tectonic structures and rock property variations abstract the ratio of compressional wave velocity vp to shear wave velocity vs is often indicative of lithology and rock conditions in this study we determine the vp vs characteristics for the different tectonic features in northern taiwan and quantify the crack density and saturation by using the ob74 method the result shows that the highest vp vs values are located beneath the western foothills at depths between 4 and 21 km these vp vs highs are simultaneously characterized by low vp and low vs suggesting the presence of fluids the average crack density and saturation for each depth can reach as high as 25 and 91 at depths of 7 and 11 km respectively this distribution could be caused by the presence of less compacted sediments within the foreland basin and the progressive convergence of crustal faults with depth as the shallow portion of the foreland basin is highly fractured fluids could infiltrate into deeper parts to create a highly saturated environment however when the depth is deeper than 11 km the high crack density and saturation area migrate from the west to the east of the shanchiao fault this observation may indicate the presence of a preexisting syn rift basin buried at depth during the collision in the ilan plain the highest crack density and saturation are observed in its northeastern portion at a depth of 4 km with values of 17 1 and 60 3 respectively however slightly high saturation values spread throughout the plain at a depth of 15 km which may indicate a possible fluid conduit from a deep hot rock body to the surface and could be applied toward studies on geothermal fluid activity

brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

partition between collision and subduction accretionary prisms along an inherited transcurrent fault zone new insights on the taiwan fold and thrust belt a new geotectonic framework of the taiwan orogen is presented in accordance with the hypothesis of an oblique arc arc collision the colliding luzon arc is physically connected to the eastern coastal range in which a subduction complex remnant is preserved and backthrust with intra arc sediments in a small retroforeland basin a southern and extinct extension of the ryukyu arc is characterized in western taiwan it displays a duplex structure tananao and backbone horses and lishan triangle zone between a buried floor thrust located in the arc crust and a roof thrust developed in the arc cover hsfiehshan range and south backbone range westward the basal thrust climbs in the sedimentary series of the western proforeland foothills and hengchun peninsula and dies out in a buried tip line the northern part of the orogen including all the tananao arc core is shown as an intra oceanic continental arc arc collision belt characterized by an unroofed duplex culmination above a leading floor thrust and both proforeland and retroforeland basins the southern part which displays a roof thrust sequence above a buried duplex is shown as an accretionary prism built in a transition zone between continent and oceanic subduction transition from the asian continental crust including the former ryukyu arc to the oceanic old philippine sea crust the partition is believed to be induced by a deep intracontinental transcurrent fault zone able to influence the difference in shortening duplex pattern and leading thrust depth the evolution was controlled by the ryukyu subduction backarc extension arc magmatism extinction and cooling and intra arc collapse until the early middle miocene around 15 ma and then it was controlled by the luzon arc progression continental subduction collision indentation and hinterland uplift and frontal thrust propagation

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

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

the present and past bottom current flow regime around the sediment drifts on the continental rise west of the antarctic peninsula throughout the last decade large sediment drifts located on the upper continental rise west of the antarctic peninsula were the target of oceanographic measurements bathymetric mapping seismic investigations shallow sediment coring and deep sea drilling by ocean drilling program odp leg 178 these studies concluded that for most of the late neogene and quaternary a generally sw ward directed bottom current affected the deposition on the drifts in particular during glacial periods the deposition was additionally influenced by nw ward directed transport of terrigenous detritus supplied by turbidity currents from the antarctic peninsula continental slope in a recent study however the palaeomagnetic signal of the drift sediments recovered at two odp leg 178 sites 1095 and 1101 was interpreted to provide spatial and directional information on the physical record of the ne ward flowing antarctic circumpolar current acc here we investigate the link between the clockwise flowing acc and the generally sw wards flowing near bottom contour current we show that at the odp leg 178 sites on the western antarctic peninsula continental margin the acc only affects the ocean circulation above ca 1000 m water depth therefore the signal of acc flow might only be archived in the ice rafted debris ird content of the drift sediments however the ird from the leg 178 sites accounts for only a small proportion of the drift sediments and is dominantly of local origin past variability of bottom water flow around the sediment drifts was reconstructed on the basis of seismic studies and clay mineralogical and grain size analyses these reconstructions provide useful information about both downstream direction and velocity changes of the bottom current and point to its sw ward flow along the upper rise during at least the last 9 4 ma

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

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

basin inversion in central taiwan and its importance for seismic hazard proyectos intramurales 2006 301010 ministerio de ciencia e innovacion cgl2009 11843 bte and the csic predoctoral program junta para la ampliacion de estudios jae predoc

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

inversion of the paleogene chinese continental margin and thick skinned deformation in the western foreland of taiwan abstract new structural data available seismicity data together with mechanical constraints on the eurasian continental lithosphere and reconstruction of paleostress trajectories are combined in order to re assess and discuss the dominant deformation mode thin skinned vs thick skinned in the western fold and thrust belt of the active taiwan collision zone serial balanced cross sections and computed paleostress tensors suggest that structural styles and stress trajectories at the front of the taiwan mountain belt may vary rapidly along strike depending on the presence of pre orogenic paleogene troughs in the chinese continental margin taihsi and tainan basins that are favourably oriented to be reactivated and inverted in localities of the western foreland where basin inversion and thick skinned basement involved shortening predominate the crustal seismic activity is important and characterized by strike slip faulting in these domains of the fold thrust belt the stress deviations with respect to the regional transport direction are also important by contrast in domains of significant syn orogenic subsidence a thin skinned style of deformation may be prominent due to the lack of available pre existing features the seismic activity is limited to few major faulted boundaries such as the active chelungpu sani thrust and the stress deviations are limited the timing of deformation at the belt front seems to be independent of the structural styles so that frontal folds are active since probably 0 5 ma however in inner parts of the western foothills of taiwan where superimposed thick skinned and thin skinned deformation are required out of sequence thrusting occurs to maintain the topographic profile finally we suggest that kinematics and structural styles of the western fold thrust belt are controlled by the mechanics of the eurasian continental margin in agreement with a recent study at the scale of the orogen the limited shortening amounts across the wf and in the central range as well as the absence of continental hp rocks better support a thick skinned collision model for taiwan

dinoflagellate cysts from sediment traps deployed in the bellingshausen weddell and scotia seas antarctica dinoflagellate cysts have been recovered from six long term 1 2 yr sediment trap moorings deployed in the bellingshausen weddell and scotia seas antarctica these traps mostly moored near the sea bed to sample the nepheloid layer were located both within and to the north of the maximum sea ice limit the numbers of cysts together with the composition of the assemblages reinforce the importance of the maximum sea ice limit as a modern biogeographic boundary for the distribution of dinoflagellate cysts cysts derived from heterotrophic dinoflagellates make up the highest proportions within the assemblages recovered from the traps one trap sampled the export production revealing little difference in cyst flux over those sampling the nepheloid layer cyst flux appears to be highest in areas closest to the antarctic convergence north of the maximum sea ice limit and to high nutrient availability there are however differences between the sediment trap assemblages and those recovered from core top samples at the same or nearby sites these differences in the greater number of cysts and in the higher numbers of round brown protoperidinium cysts in the traps may reflect annual differences in the primary productivity and or cyst production in the area in some areas the sediment record may preserve little information about local surface water productivity because of the activity of bottom water currents for example those arising from the antarctic circumpolar current

deep crustal structure of an arc continent collision constraints from seismic traveltimes in central taiwan and the philippine sea the collision of continental crust of the eurasian plate with the overriding luzon arc in central taiwan has led to compression uplift and exhumation of rocks that were originally part of the chinese rifted margin though the kinematics of the fold thrust belt on the west side of the orogen has been described in detail the style of deformation in the lower crust beneath taiwan is still not well understood in addition the fate of the luzon arc and forearc in the collision is also not clear compressional wave arrival times from active source and earthquake seismic data from the taiwan integrated geodynamic research program constrain the seismic velocity structure of the lithosphere along transect t5 an east west corridor in central taiwan the results of our analysis indicate that the continental crust of the eurasian margin forms a broad crustal root beneath central taiwan possibly with a thickness of 55 km compressional seismic velocities beneath the central range of taiwan are as low as 5 5 km s at 25 km depth whereas p wave seismic velocities in the middle crust on the eastern flank of the taiwan mountain belt average 6 5 7 0 km s this suggests that the incoming sediments and upper crust of the eurasian plate are buried to midcrustal depth in the western flank of the orogen before they are exhumed in the central range to the east the luzon arc and forearc are deformed beneath the coastal range of central taiwan fragments of the rifted margin of the south china sea that were accreted in the early stages of the collision form a new backstop that controls the exhumation of eurasian strata to the west in this evolving mountain belt

magnetotelluric evidence for thick skinned tectonics in central taiwan taiwan is the type example of an arc continent collision numerous tectonic models have been proposed for this orogen and include both thin skinned and thick skinned lithospheric deformation these models predict very different structures at middle and lower crustal depths but insuffi cient geophysical data exist to unequivocally distinguish between them long period magnetotelluric mt data were collected in central taiwan in 2006 2007 to constrain the crustal resistivity structure a two dimensional inversion of these mt data revealed a prominent electrical conductor that extends across the decollement predicted by the thin skinned model this feature is interpreted to be due to 1 2 saline fl uids and is inconsistent with the thin skinned model in contrast the thick skinned model predicts this feature since fl uids are generated in the crustal root through metamorphism quantitative correlation of the resistivity and seismic velocity models supports small volume high salinity fl uids in a thickened crust as the cause of this conductor

biostratigraphic characterization and quaternary microfossil palaeoecology in sediment drifts west of the antarctic peninsula implications for cyclic glacial interglacial deposition abstract the sedano project recovered 19 gravity cores on sediment drifts from the pacific continental margin of the antarctic peninsula camerlenghi et al 1997a high resolution terrigenous sedimentary record of the sediment drifts on the antarctic peninsula pacific margin in ricci c a ed the antarctic region museo nazionale dell antartide siena pp 705 710 fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils diatoms planktonic and benthic foraminifera framed in a depositional process reconstruction barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles respectively analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles the unit boundaries were drawn more accurately by means of the microfossil distribution on the basis of micropalaeontological and sedimentological evidence interglacial unit c is correlated to oxygen isotope stage 5 thus dating the unit boundaries at 127 and 70 ka peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment a calcareous nannofossil cold taxa association is present in most cores examined and its consistent distribution within interglacial unit c indicates key environmental relationships the occurrence of calcareous nannofossils has been related to temperature tolerance sea ice cover reduction nutrient availability and factors limiting primary productivity our results confirm that coccolithophorids occurred at southern high latitudes in the western marginal basins of the antarctic peninsula during short periods of the late quaternary a foraminiferal assemblage made up of sinistral neogloboquadrina pachyderma and few benthics occurs only in interglacial units correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes

diagenetic magnetic enhancement of sapropels from the eastern mediterranean sea diagenetic dissolution of magnetic minerals has been widely observed in organic rich sediments from many environments organic rich sediments from the eastern mediterranean sea sapropels recovered during leg 160 of the ocean drilling program reveal a surprising catalogue of magnetic properties sapropels from all sites studied across the eastern mediterranean sea are strongly magnetic and the magnetization is directly proportional to the organic carbon content the magnetization of the sapropels is dominated by a low coercivity probably single domain magnetic mineral with an inverse magnetic fabric that exhibits a clear decay in magnetic properties when exposed to air during heating the magnetic particles irreversibly break down between 360 and 400 c the contrast between the magnetic properties of sapropels and surrounding sediments is marked with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments the contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non steady state diagenesis sulphate reducing conditions dominated during sapropel deposition while overlying sediments were deposited under oxic conditions the mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate reducing conditions that existed during times of sapropel formation attempts to identify this mineral have been unsuccessful but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase the influence of diagenesis on the magnetic properties of cyclically deposited eastern mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments

spatial and temporal variation of sediment yield in the landscape example of huanghe yellow river regional scale relations between specific sediment yield and drainage basin area in the huanghe basin reveal varying landscape sedimentary dynamics declining specific sediment yield with area is typical of disturbed agricultural areas while increasing yields indicate the remobilization of stored sediment by bank and floodplain erosion along major tributaries using the developed relations areally adjusted sediment yield maps are created over the entire huanghe basin maps of the loess plateau for periods before and after 1970 reveal major changes in sediment yield indicating that land management may have a major effect on sediment dynamics within a short period of time copyright 2008 by the american geophysical union

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

large volume loss during cleavage formation hamburg sequence pennsylvania green reduction spots in red slate of the hamburg sequence exposed near shartlesville pennsylvania have axial ratios of 1 42 1 0 0 28 on the limbs of near isoclinal folds and 1 0 0 79 0 41 in fold hinge zones conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44 preceded the tectonic strain associated with cleavage formation this strain having identical maximum extensions but greater shortening in fold hinges as compared to limbs was characterized by 41 extension in x no change in y 50 to 59 shortening in z and 29 to 42 tectonic volume loss the general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates where deformation was an almost constant volume process and extension in x compensated for shortening in z

kinematics of fault related folding in a duplex lost river range idaho u s a abstract the doublespring duplex located in the lost river range of idaho is a sevier age fault related fold complex in massive limestones of the upper mississippian scott peak formation folds within the duplex closely resemble fault bend fold geometrics with open interlimb angles and low angle bed cut offs narrow widely spaced bedding parallel shear zones with well developed pressure solution cleavage alternate with massive relatively undeformed layers on fold limbs shear zones are developed only on the limbs of anticlines and have similar but unique morphologies in each of three different folds incremental strain histories reconstructed from antitaxial fibrous overgrowths and veins within the shear zones constrain the kinematics of folding shear zones experienced distributed bedding parallel simple shear flexural flow towards pins near axial surfaces while adjacent massive layers experienced rotation through an externally fixed extension direction the absence of footwall synclines and morphological differences in shear zones from adjacent folds suggest that faulting preceded folding kinematic histories of folds that have experienced different translational histories are identical and are not compatible with strain histories predicted from previous kinematic models of fault bend folding shear zone development and fiber growth is instead interpreted to have occurred during low amplitude fixed hinge buckling in response to initial resistance to translation of the thrust sheet fault bend folding with mobile axial surfaces occurred with translation of the thrust sheets once the initial resistance to translation was overcome and resulted in no penetrative strain

how the structural architecture of the eurasian continental margin affects the structure seismicity and topography of the south central taiwan fold and thrust belt studies of mountain belts worldwide show that along strike changes are common in their foreland fold and thrust belts these are typically caused by processes related to fault reactivation and or fault focusing along changes in sedimentary sequences the study of active orogens like taiwan can also provide insights into how these processes influence transient features such as seismicity and topography in this paper we trace regional scale features from the eurasian continental margin in the taiwan strait into the south central taiwan fold and thrust belt we then present newly mapped surface geology p wave velocity maps and sections seismicity and topography data to test the hypothesis of whether or not these regional scale features of the margin are contributing to along strike changes in structural style and the distribution of seismicity and topography in this part of the taiwan fold and thrust belt these data show that the most important along strike change takes place at the eastward prolongation of the upper part of the margins necking zone where there is a causal link between fault reactivation involvement of basement in the thrusting concentration of seismicity and the formation of high topography on the area correlated with the necking zone the strike slip reactivation of east northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones here basement is not involved in the thrusting there is weak focusing of seismicity and localized development of topography we also show that there are important differences in structure seismicity and topography between the margins shelf and its necking zone

growth of mica porphyroblasts under low grade metamorphism a taiwanese case using in situ 40ar 39ar laser microprobe dating abstract mica porphyroblasts a common metamorphic microstructure are analyzed in the slate belt of northern taiwan where large fish like growths are found within a meta pyroclastics with constraints on the time temperature history from deposition through peak metamorphic state to exhumation in situ 40ar 39ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm scale mica porphyroblasts grown on a pressure solution seam because the peak metamorphic temperature and the porphyroblast formation condition 250 c remained far below the closure temperature of the k ar radioisotope system in muscovite and the absence of muscovite in the mafic protolith the dating results likely document the growth of the mica porphyroblast fabrics the syn kinematic nature of the analyzed porphyroblasts is confirmed by the 6 to 2 5 ma growth ages suggesting that the host rock was continuously deformed during the earlier two thirds of the taiwan orogeny the pattern of fiber growth in contrast to outward decreasing ages normally observed in peripheral recrystallization appears random and resembles void fills in boudin openings we postulate that syntaxial crack seal following tensile micro boudinage along with slips on sub grain boundaries as a viable mechanism for the development of mica porphyroblasts and fish especially in lower grade metamorphic rocks

fault geometries illuminated from seismicity in central taiwan implications for crustal scale structural boundaries in the northern central range abstract data sets of collapsed earthquake locations earthquake focal mechanisms gps velocities and geologic data are integrated to constrain the geometry and kinematics of a crustal block within the accreted continental margin rocks of taiwan s northeastern central range this block is laterally extruding and exhuming towards the north northeast the block is bound on the west southwest by the previously recognized sanyi puli seismic zone and on the east by a vertical seismic structure that projects to the eastern mountain front of the central range focal mechanisms from the broadband array of taiwan seismicity bats catalog consistently show west side up reverse displacements for this fault zone a second vertical structure is recognized beneath the slate belt metamorphic belt boundary as a post chi chi relaxation oblique normal fault bats focal mechanisms show east side up normal displacements with a minor left lateral component the vertical and lateral extrusion of this crustal block may be driven by the current collision between the philippine sea plate and the puli basement high indenter on the eurasian plate and or trench rollback along the ryukyu subduction zone in addition the vertical extent of the two shear zones suggests that a basal decollement below the eastern central range is deeper than previously proposed and may extend below the brittle ductile transition

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

kinematic analysis of the taiwan slate belt the taiwan slate belt can be subdivided into two structural domains based on fold and cleavage geometries and finite and incremental strain histories the two belts correspond generally to two topographic ranges in taiwan the hsuehshan range and the backbone range rocks in the hsuehshan range are characterized by upright symmetric folds and a steeply se dipping axial planar pressure solution cleavage synkinematic fibers in pressure shadows are typically straight parallel to cleavage and plunge downdip indicating that the rocks experienced a coaxial strain history in contrast rocks in the backbone range are characterized by inclined asymmetric folds that verge toward the foreland and display a moderately se dipping axial planar pressure solution cleavage synkinematic fibers are straight and parallel to the downdip stretching lineation in xy sections in xz sections however the fibers typically display a clockwise curvature viewed to the ne indicating a noncoaxial strain history we propose that the different strain histories are due at least in part to differences in the original tectonostratigraphic settings of the strata in the two ranges strata in the hsuehshan range are interpreted to have been deposited in a half graben associated with an east dipping normal fault the strata of the backbone range in contrast are considered to have been deposited on the unfaulted chinese continental margin we suggest that the basement horst on the east side of the half graben functioned as a steep faced rigid backstop that concentrated pure shear deformation throughout most of the hsuehshan range shear strains were apparently limited to zones near the base of the sedimentary sequence and adjacent to the backstop the noncoaxial strains characteristic of the backbone range are interpreted to be the result of a regional scale shear zone that formed as chinese basement and its sedimentary cover were underthrust southeastward beneath the luzon arc

apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite summary a mixed polarity zone representing alternations between remagnetized and non remagnetized strata has been documented within the lower few metres of the crp 1 core ross sea antarctica detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single domain particles of a ferrimagnetic iron sulphide mineral while the reversed polarity magnetization of non remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite fe3s4 based on microtextural relationships it is not possible to determine the relative timing of formation for much of the greigite however a significant proportion of the greigite has grown on the surface of authigenic siderite feco3 grains that occur as microconcretions and as cement surrounding detrital matrix grains in such cases microtextural relationships indicate that siderite post dates early diagenetic pyrite and that greigite post dates the siderite siderite usually forms in environments with abundant dissolved iron and carbonate but without dissolved pore water h2s this set of geochemical conditions occurs in methanic settings below the sulphate reduction zone in which early diagenetic pyrite forms we interpret the observed remagnetization of the lower part of the crp 1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide thereby causing precipitation of greigite the distribution of siderite and associated greigite in the lower part of the crp 1 core is patchy which accounts for the apparent alternation of polarities this study is part of a growing catalogue of remagnetizations involving greigite which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

magnetite dissolution diachronous greigite formation and secondary magnetizations from pyrite oxidation unravelling complex magnetizations in neogene marine sediments from new zealand detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of neogene marine sediments from the east coast of new zealand is dominated by the authigenic iron sulphide greigite the greigite is present as a mixed population of stable single domain and superparamagnetic grains which is consistent with authigenic growth from solution this growth can result from pyritization reactions soon after deposition which also leads to dissolution of most detrital magnetite however where constrained by field tests our data suggest that remanence acquisition can occur 1 myr after deposition and can vary in timing at the outcrop scale strong viscous overprints result from oxidation of the iron sulphides probably during percolation of oxic ground water this process can sometimes destroy any ancient remanent magnetization this complex magnetic behaviour particularly the presence of late forming magnetizations carried by greigite means that the remanence in new zealand cenozoic sediments and in similar sediments elsewhere cannot be assumed to be primary without confirmation by field tests the reversals test should be employed with caution in such sediments as patchy remagnetizations can lead to false polarity stratigraphies

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

magnetostratigraphy and environmental magnetism of two quaternary deep sea gravity cores from the west pacific southern ocean 1 we present new paleomagnetic data for quaternary u channel samples from two deep sea gravity cores from the west pacific southern ocean these cores were recovered in the antarctic summers 1995 1996 during the oceanographic cruise of the r v italica one was recovered beneath the present day antarctic polar front anta 95 157 and the other is from within the antarctic zone anta 96 16 the matuyama brunhes boundary has been identified in both cores with a longer history of sedimentation available in anta 96 16 a detailed rock magnetic study shows discrete occurrences of ice rafted debris which were influenced by 100 kyr orbital eccentricity variations during the brunhes chron this is consistent with the main periodicity of the climate system since the mid pleistocene climate transition

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

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

the structure and kinematics of the central taiwan mountain belt derived from geological and seismicity data 1 the structure of the taiwan mountain belt is thought to be that of an imbricate thrust and fold belt developed in a forward breaking sequence above a shallowly dipping basal detachment in recent years however a growing amount of seismicity data from the internal part of the mountain belt indicates the existence of widespread fault activity in the middle and lower crust suggesting that deeper levels of the crust must be involved in the deformation than predicted by the shallow detachment imbricate thrust belt model to address this issue we present new geological mapping together with earthquake focal mechanism and seismic energy release data from the central part of taiwan we concur with the interpretation that the foreland basin part of the western foothills comprises an imbricate thrust system that is developing as a forward breaking sequence that is structurally and kinematically linked to a basal detachment at between 7 and 10 km depth to the east of the foreland basin however in the hsuehshan and central ranges our data show the presence of two fault systems an earlier inactive thrust system with a well developed cleavage is cut by a system of steeply dipping active faults that penetrate to a depth of 25 to 30 km or more in the hsuehshan range the second fault system is best represented by a structural and kinematic model in which this part of the mountain belt forms a zone of transpression with a structural architecture similar to that of a crustal scale positive flower structure eastward in the central range mesozoic basement rocks are over thrusting strongly folded and cleaved deep water sediments of the first now inactive thrust system the involvement of deep crustal levels and mesozoic basement in the second fault system is suggestive of the reactivation of preexisting basin bounding faults that were located on the eurasian continental margin

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

characterization of topographic steady state in taiwan abstract the arc continent collision in taiwan has resulted in surface uplift high relief and high rates of erosion because the collision is older in the north and has propagated southward taiwan offers the opportunity to study the evolution of topography as it evolves towards a large scale steady state in this paper we analyze a 40 m digital elevation model of taiwan to determine how topographic steady state is expressed in the landscape particular attention is paid to the eastern central range where major drainage basins are approximately aligned in the direction of the collision propagation analyses of orogen size regional topographic slope drainage basin hypsometry and local topographic slope reveal a geomorphological transition 100 125 km north of the southern tip of the island to the south of this transition topographic characteristics change with distance along the island to the north these characteristics are relatively constant which we interpret as representing large scale topographic steady state assuming that the collision has propagated southward at a rate of 55 mm yr the time to topographic steady state is 1 8 2 3 myr after emergence above sea level within the steady state region we evaluate regional variability in the topography and find that the most prominent deviations from a steady form are associated with large scale structures though our analyses of local topographic slope and river profile form suggest that spatial variations in rock strength and rock uplift rate also contribute to regional topographic variability

investigating the kinematics of mountain building in taiwan from the spatiotemporal evolution of the foreland basin and western foothills the taiwanese range has resulted from the collision between the luzon volcanic arc and the chinese continental margin which started about 6 5 myr ago in the north and has since propagated southward the building of the range has been recorded in the spatiotemporal evolution of the foreland basin we analyze this sedimentary record to place some constraints on the kinematics of crustal deformation the flexure of the foreland under the load of the growing wedge started with a 1 5 myr long phase of rapid subsidence and sedimentation which has migrated southward over the last 3 5 myr at a rate of 31 10 5 mm yr reflecting the structural evolution of the range and the growth of the topography during the oblique collision isopachs from the toukoshan 0 to 1 1 ma and cholan 1 1 to 3 3 ma formations as well as the sedimentation rates retrieved from a well on the pakuashan anticline indicate that the foreland basement has been moving toward the center of mass of the orogen by 45 50 mm yr during the development of the basin from there we estimate the long term shortening rate across the range to 39 5 44 5 mm yr by considering available data on the thrust faults of the foothills of central taiwan we show that most if not all the shortening across the range is accommodated by the most frontal structures with little if any internal shortening within the wedge the range growth appears therefore to have been essentially sustained by underplating rather than by frontal accretion in addition only the upper 7 to 9 km of the underthrusted crust participates to the growth of the orogen this requires that a significant amount of the chinese passive margin crust is subducted beneath the philippine sea plate

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

kinematic analyses of the hsuehshan range taiwan a large scale pop up structure the hsuehshan range exposed in the northern and central taiwan slate belt is a fault bounded structural high cored by biotite grade slates and metasandstones syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern hsuehshan range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland near the eastern boundary of the hsuehshan range however pressure shadows record noncoaxial strain histories consistent with a top to the east sense of shear along a steep nw dipping shear zone this noncoaxiality is attributed to se directed backthrusting on the lishan fault which separates the higher grade eo oligocene rocks of the hsuehshan range from the lower grade miocene rocks of the backbone range because it is bounded to the east by the se vergent lishan fault and to the west by a series of nw vergent thrusts e g the chuchih fault the hsuehshan range is envisaged as a pop up structure strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces e g the lishan fault the hsuehshan range tapers in width to the south thus the pop up may be buried or die out to the south where the collision is younger

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

assessing the timing of greigite formation and the reliability of the upper olduvai polarity transition record from the crostolo river italy 1 the crostolo river section in italy yielded a detailed record of the upper olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field the paleomagnetic record is carried by the authigenic iron sulphide greigite which was interpreted to have formed shortly after deposition our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite which usually forms with greigite as a precursor this suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour

brittle frictional mountain building 3 low grade metamorphism an active fold and thrust belt in unchanging tectonic and climatic conditions exhibits a dynamic steady state with the flux of rocks accreted at the toe balanced by the flux of rocks eroded off the top rocks entering the toe are buried and heated before they are uplifted and eroded this results in a characteristic map pattern of low grade metamorphism on the surface metamorphic isograds are generally parallel to the regional strike of a fold and thrust belt with the grade increasing progressively from unmetamorphosed and zeolite facies near the deformation front up to greenschist facies in the highest mountains such a pattern is observed in the active fold and thrust belt of taiwan this paper examines the origin of this low grade syntectonic metamorphism using a previously developed mechanical and thermal model of a steady state fold and thrust belt as a basis to model the metamorphism we develop a petrogenetic grid for rocks of basaltic composition considering phases within the chemical system cao mgo al2o3 sio2 h2o equilibria diagnostic of stable low grade mineral assemblages are mapped onto a cross section of the taiwan fold and thrust belt using the calculated fluid pressure and temperature distributions within the wedge the variation of metamorphic grade along the surface is predicted by assuming that there is no retrograde metamorphism the most important mechanical factor controlling the degree of metamorphism in an active fold and thrust belt is the amount of frictional heating both on the basal decollement fault and within the deforming brittle wedge frictional heating raises temperatures in the deepest portions of the taiwan fold and thrust belt by 200 c to 250 c the resulting high temperatures in excess of 400 c are responsible for the extensive greenschist facies metamorphism the amount of heating is well constrained by the observed heat flow anomaly geochronological data and the critical wedge taper the best fitting coefficient of basal friction in taiwan is b 0 5 0 2 underplating of footwall rocks by the assimilation of duplexes along the decollement fault is not considered to be a significant process in taiwan because of the observed balance between the accretionary influx at the toe and the erosive efflux off the top in other fold and thrust belts however underplating can significantly increase the outcrop width of higher metamorphic grades on the surface by increasing the flux of rocks through the higher fluid pressure and temperature regions within the wedge

kinematics of decollement folding in the lost river range idaho decollement folding of massive 2 m thick carbonate beds of the scott peak formation within the willow creek anticline northern lost river range idaho was associated with the development of thin 0 25 m widely spaced deformation zones incremental strain histories determined from fibrous overgrowths in strain fringes from the deformation zones provide a basis for kinematic analysis of the anticline plane strain is indicated by coaxial up dip extension on cleavage xy planes and a lack of extension in the y direction in xz sections coaxial incremental strain histories characterize fold hinges and document pinned hinges during folding the sense of rotation on opposing limbs indicates that the fiber curvature records primarily the spin of layers through fixed moderately steep 66 12 dg l diverging extension directions the magnitude of the external rotation for each sample is typically less than the bedding dip suggesting that the external rotation also includes a component of internal rotation related to layer parallel shear toward the upper flat panel or the hinge pin line the magnitude of this internal rotation varies within and between layers

taiwan mountain building insights from 2 d thermomechanical modelling of a rheologically stratified lithosphere summary the taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes in this paper we present a fully coupled thermomechanical modelling of the taiwan collision based on a realistic viscous elastic plastic rheology it satisfactorily reproduces available thermochronometric data long short term deformation patterns heat flux and erosion sedimentation distribution across the taiwan orogeny we found that a deep seated flux of asian crustal material into the orogenic wedge should be invoked to counter balance observed exhumation and erosion in the central range however in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered asian crust we finally infer that such a model is in better agreement with the basic expectation that the hot young and buoyant chinese continental margin should hardly be subducted beneath the cold old and dense oceanic plate of the philippines sea

polyphase history and kinematics of a complex major fault zone in the northern taiwan mountain belt the lishan fault abstract the lishan fault is a major fault zone in the taiwan collision belt it separates two major units the hsuehshan range and the backbone range which differ in lithology ages of sediments metamorphic grades and deformation styles despite the importance of the lishan fault its evolution and tectonic behaviour remained poorly known and controversial we therefore carried out a tectonic study which includes both the identification of structures in and along the fault zone and the palaeostress analysis aiming at reconstructing the succession of fault mechanisms the lishan fault zone underwent polyphase evolution with reactivations under different tectonic regimes consistent with the cenozoic history of taiwan 1 1 on the hsuehshan range the earliest events reflect the palaeogene miocene extension of the chinese continental margin 2 2 serial cross sections and observation of ductile brittle structures show east vergent folding indicating that for the most recent compressional evolution related to late cenozoic taiwan collision the lishan fault was a steeply dipping east vergent back thrust the compression occurred along nw se trends inducing thrusting but also along n s ones inducing transpression with reverse sinistral slip the lishan fault thus underwent contraction as well as strike slip during the mountain building of the taiwan orogeny reverse and strike slip fault systems alternated because of permutations 2 3 under the same compressional stress regime of nw se 1 minor compressional events also occurred 3 3 a late extension accommodated by normal faulting reveals the influence of both the n s extension in the okinawa trough northeast off taiwan

a thermomechanical wedge model of taiwan constrained by fission track thermochronometry abstract we present 31 new apatite fission track aft ages for the island of taiwan that when combined with existing aft and zircon fission track zft data provide regional spatial coverage of the island with respect to low temperature thermochronometry the overall pattern of zft and aft ages in taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the central range and eastern taiwan this pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone in this model new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west the southward propagating collision is consistent with reset ages in the north where erosional exhumation has been ongoing for longer and unreset ages in the south where the younger collision implies less time for erosional exhumation despite the variation in the age of the collision along the strike of the island the widths of the aft and zft reset age zones remain nearly constant between 23 00 n to 24 00 n and 23 20 n to 24 00 n respectively suggesting that the orogen is in an exhumational steady state over these regions with respect to the aft and zft thermochronometers we use the fission track data in conjunction with observations of crustal structure crustal fabric and heat flow measurements to constrain a time dependent two dimensional thermomechanical model of orogen evolution by accounting for the heat transfer tectonic and erosion processes needed to predict aft and zft ages we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen with our model we conclude that 1 roughly half of the material accretion in taiwan occurs through underplating over an approximately 40 km wide region 2 current average erosion rates are 3 3 mm yr in the eastern central range and 2 3 mm yr over the whole island 3 the collision has been propagating southward at a rate between 20 and 51 km ma over the past 2 3 ma and 4 central taiwan is in a topographic thermal and exhumational steady state

from accretion to collision motion and evolution of the chaochou fault southern taiwan 1 the chaochou fault ccf is an important tectonic boundary of the taiwan orogenic belt between the metamorphosed slate belt to the east and the western foothills foreland fold and thrust belt to the west although the fault is known to be a high angle oblique sinistral thrust fault in places both its kinematic history and its current role in the development of the orogen are poorly understood field fabric data suggest that hanging wall structural orientations vary along strike particularly in the on land extension of the intersection of the eurasian continent ocean boundary and the luzon arc slip lineations also reveal a change in fault motion from dip parallel in the north to a more variable pattern in the south this correlates somewhat with recent gps results that indicate that the direction of current horizontal surface motion changes along strike from nearly perpendicular to the fault in the northern to oblique and nearly parallel to the fault in the south mountain front sinuosity and the valley floor width valley height ratio indicate higher activity and uplift in the north geodetic and geomorphic data together indicate that along the northern segment of the ccf the slate belt hanging wall is currently undergoing rapid uplift related to oblique arc continent collision between the eurasian continent and the luzon arc the southern segment of the fault is significantly less active perhaps because 1 the fault is blind or 2 this portion of the fault is south of the point where the eurasian continental platform intersects the orogenic front

a sedimentary paleomagnetic record of the matuyama chron from the western antarctic margin odp site 1101 a high resolution paleomagnetic record for part of the matuyama chron 0 7 2 1 ma is reported for ocean drilling program site 1101 leg 178 off the antarctic peninsula pacific margin a rock magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization nrm is carried by pseudo single domain magnetite progressive alternating field demagnetization of 83 m of u channels provided a polarity stratigraphy down to the olduvai subchron two geomagnetic events preceding the jaramillo subchron were identified including the cobb mountain polarity interval the bulk magnetic parameters vary by more than a factor of 20 over the entire time interval but by less than a factor of 6 over the 0 7 1 1 ma interval this latter interval was selected for paleointensity determinations which were carried out by normalizing the nrm by the anhysteretic remanent magnetization arm direct comparison of the site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record a more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0 95 1 1 ma interval this test yielded no outlier for the period considered confirming the geomagnetic character of the records we have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the jaramillo subchron fl 2001 elsevier science b v all rights reserved

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

diffusion creep modelling of fibrous pressure shadows abstract grain scale deformation of a bimineralic composite by normal stress driven grain boundary diffusive mass transfer has been modelled using diffusion creep software that for the first time allows different diffusion coefficients for interphase boundaries and grain boundaries as well as for different components by selecting suitable coefficients we have obtained a fully coupled mechanical and mass transfer numerical solution for the formation of fibrous pressure shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions the model is two dimensional and run in pure shear velocity mode to a stretch ratio of 1 4 the first results are reported which show dissolution and face controlled antitaxial growth of a soluble polycrystalline calcite matrix at the interface with a circular and relatively insoluble pyrite object the ratio of caco3 diffusion along calcite pyrite versus calcite calcite boundaries strongly influences the extent of pressure shadow growth and the pattern of strains in the matrix a low ratio produces little growth significant perturbation and deflection of total strain axes around the pyrite an intermediate ratio produces appreciable growth little perturbation and near parallel total strain axes a high ratio produces larger pressure shadows and total strain axes that bend in towards the pyrite as if it were a weak object sliding and rotation of matrix grains are also affected these results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour in particular assumptions such as perturbation of the flow by the object deflection of external foliation around pre deformational porphyroblasts and relationships of pressure shadow strain to bulk strain may need to be re examined in the light of this finding

late quaternary sediments from deep sea sediment drifts on the antarctic peninsula pacific margin climatic control on provenance of minerals 1 we present results of detailed paleomagnetic investigations on deep sea cores from sediment drifts located along the pacific continental margin of the antarctic peninsula high resolution magnetic measurements on u channel samples provide detailed age models for three cores collected from drift 7 which document an age of 122 ka for the oldest sediments recovered near the drift crest at site sed 07 and a high sedimentation rate 11 cm kyr at site sed 12 located close to the alexander channel system low and high temperature magnetic measurements in conjunction with microscopic and mineralogic observations from drifts 4 5 and 7 indicate that pseudosingle domain detrital titanomagnetite partially oxidized and with limited ti substitution is the dominant magnetic mineral in the drift sediments the titanomagnetite occurs in two magnetic forms 1 a low coercivity form similar to laboratory synthesized titanomagnetite and 2 a high coercivity form bcr 60 mt these two forms vary in amount and stratigraphic distribution across the drifts we did not find evidence for diagenetic magnetic iron sulfides as has been previously suggested for these drift deposits the observed change of magnetic mineralogy in sediments deposited during heinrich events on drift 7 appears to be related to warming periods which temporarily modified the normal glacial transport pathways of glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments with a different provenance understanding this sediment provenance delivery signature at a wider spatial scale should provide information about ice sheet dynamics in west antarctica over the last 100 kyr

strain partitioning and crack seal growth of chlorite muscovite aggregates during progressive noncoaxial strain an example from the slate belt of taiwan abstract chlorite muscovite aggregates from the slate belt of southeastern taiwan have an ellipsoidal geometry k 1 on a flinn diagram with an aspect ratio a c ranging from about 7 1 to 12 1 and a crystallographic preferred orientation with the basal planes of the chlorite and muscovite at a high angle to the slaty fabric in the rock the principal axes of the aggregates are approximately coaxial with the principal finite strain directions determined from fibrous pressure shadows in cleavage parallel sections straight fibers parallel to the downdip mineral lineation record plane strain in cleavage perpendicular sections however pressure shadows have curved fibers that are consistent with w directed thrusting we propose a model in which the chlorite mica aggregates grow syntectonically by cracking of detrital chlorite grains parallel to the basal planes and precipitation of secondary chlorite and mica in the open cracks this model involves 1 rotation of preexiting detrital chlorite grains during progressive noncoaxial deformation 2 cracking of these grains parallel to the basal plane when the 001 planes rotate into the incremental shortening field with nucleation and growth of secondary chlorite and muscovite in the open microcracks as the aspect ratio increases the grains stabilize in orientation with the long axes normal to the basal planes at a small angle to the shear direction extension normal to 001 is also accompanied by some shortening parallel to 001 but with an overall increase in size of the aggregates the final grain size is controlled by the cumulative displacement associated with crack seal events microprobe analyses of both detrital and secondary chlorite grains show distinct compositional differences within individual grains perpendicular to 001 particularly in the detrital grains which we interpret as primary and secondary chlorite consistent with a primary and secondary origin for the aggregates

detecting rock uplift across southern taiwan mountain belt by integrated gps and leveling data abstract rock uplift on the earth surface is a key observation for studies of tectonics and geodynamic processes geodetic measurements from global positioning system gps and leveling are able to track vertical deformation over a wide range of spatial and temporal scale taiwan mountain belt is subject to a rapid uplift rate of 20 mm yr as revealed by gps and leveling measurements in previous studies the extremely high rates motivate us to analyze a set of newly processed gps and leveling data along a nw se transect across southern taiwan by considering the effects of the earthquake related deformation as well as hydrological and surface processes our estimates of rock uplift rates from gps and leveling are from 12 to 14 mm yr across the southern central range of taiwan vertical velocity changes on the small spatial scale are primary related to fault locking in the seismic cycle the large spatial feature showing the mean uplift rate of 6 mm yr between the eastern central range and the frontal thrust faults can be explained by a large scale pop up structure assuming the depth of 15 20 km in the orogenic wedge and the horizontal shortening of 30 mm yr our results draw attention to carefully study geodetic vertical deformation in the mountain belts and seek relevant causes

crustal scale seismic profiles across taiwan and the western philippine sea abstract we have used combined onshore and offshore wide angle seismic data sets to model the velocity structure of the taiwan arc continent collision along three cross island transects although taiwan is well known as a collisional orogen relatively few data have been collected that reveal the deeper structure resulting from this lithospheric scale process our southern transect crosses the hengchun peninsula of southernmost taiwan and demonstrates characteristics of incipient collision here 11 km thick transitional crust of the eurasian plate eup subducts beneath a large rapidly growing accretionary prism this prism also overrides the n luzon forearc to the east as it grows just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure because this break is accompanied by intense seismicity we interpret that the forearc block is being detached from the n luzon arc and philippine sea plate psp at this point our middle transect illustrates the structure of the developing collision steep and overturned velocity contours indicate probable large scale thrust boundaries across the orogen the leading edge of the coherent psp appears to extend to beneath the east coast of taiwan deformation of the psp is largely limited to the remnant n luzon arc with no evidence of crustal thickening to the east in the huatung basin our northern transect illustrates slab continent collision the continuing collision of the psp and eup as the psp subducts the collisional contact is below 20 km depths along this transect ne of hualien this transect shows elements of the transition from arc continent collision to ryukyu arc subduction both of our models across the central range suggest that the paleozoic to mesozoic basement rocks there may have been emplaced as thick coherent thrust sheets this suggests a process of partial continental subduction followed by intra crustal detachment and buoyancy aided exhumation although our models provide previously unknown structural information about the taiwan orogen our data do not define the deepest orogenic structure nor the structure of western taiwan additional seismic active and passive geologic and geodynamic modeling work must be done to fully define the structure the active deformation zones and the key geodynamic process of the taiwan arc continent collision

monitoring interseismic activity on the ilan plain ne taiwan using small baseline ps insar gps and leveling measurements partitioning from arc continent collision and backarc extension

nappe structure revealed by thermal constraints in the taiwan metamorphic belt terra nova 23 85 91 2011 abstract vitrinite reflectance and raman spectroscopy of carbonaceous material data are used to better resolve the thermal history of the hsuehshan range which is accreted between the foreland fold thrust belt and bulldozer hinterland units in the taiwan mountain belt the observed thermal data indicate that the strata in the northern hsuehshan range underwent dynamic metamorphism during the neogene orogeny while the strata in the southern hsuehshan range may have predominantly experienced burial metamorphism during palaeogene sedimentation based on the thermal constraints the hsuehshan range is interpreted to consist of nappe stacks originating from the rifted eurasian continental margin this interpretation is consistent with well documented cases in the european alps and the himalayas and is also shown in physical modelling and thermo kinematic studies invoking underplating and erosion processes

late cenozoic metamorphism and mountain building in taiwan a review abstract the orogenic wedge of taiwan results from the late miocene collision of the chinese passive margin and the luzon volcanic arc because of the southward propagation of the collision over time and because of its extreme rates of deformation and erosion it has appeared as an ideal place to investigate mountain building processes here we review existing metamorphic and thermochronological data on late cenozoic burial and exhumation most of the island has been extensively investigated in particular in terms of zircon or apatite fission track and u th he thermochronology peak metamorphic temperatures have also been quantified from the raman spectroscopy of carbonaceous material along three transects in central taiwan the different datasets appear consistent and indicate that cooling rates and exhumation are more intense within the hsueshan range and within the tananao complex lateral variations from south to north are also observed and they can be interpreted in terms of different stages of exhumation and mountain building existing models provide a framework to analyze the data and discuss the limits of some previous interpretations in particular in terms of exhumational steady state or of recent acceleration of uplift and exhumation these models are also discussed in light of available data it appears that all of them fit thermochronological data within the tananao complex whatever the kinematics the depth of the wedge or the mechanism responsible for exhumation and that exhumation of the hsueshan range has been ignored in most models future models need therefore to consider the taiwan wedge as a whole additional constraints on peak metamorphic conditions on kinematics of shortening and exhumation or on the geometry of the wedge at depth need to be gathered to further unravel mountain building processes at the scale of the whole orogen

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

cleavage fronts and fans as reflections of orogen stress and kinematics in taiwan recent observations of cleavage patterns strain histories and kinematics across the taiwan mountain belt depict systematic orogen scale variations with respect to the synorogenic divide and suggest that the pattern of cleavage development is a predictable consequence of orogen stresses and kinematics in taiwan continental crust within the collision is accreted in the prowedge facing asia but is advected eastward into the east verging retrowedge where the most deeply exhumed rocks are exposed wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges the observation of a single cleavage in western taiwan suggests that the cleavage in the prowedge remains stable with respect to the stress orientation in contrast the existence of a second crenulation cleavage in the retrowedge is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics advection of a fabric across a topographic divide in a doubly vergent wedge provides an explanation for the occurrence of cleavage fronts and fans in natural systems such as taiwan

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

orientation history and rheology in slates kodiak and afognak islands alaska abstract theoretical and experimental studies have shown that slates have a roughly parabolic yield curve in and space where is the orientation of cleavage relative to the maximum compressive stress is the orientation of cleavage relative to the incremental shortening direction and is the deviatoric stress deformation mechanisms that operate in low grade rocks such as diffusive mass transfer in a grain boundary fluid dislocation creep and frictional grain boundary sliding may have stress strain rate relationships that depend on during a progressive strain history and may vary as a function of the strain path thus the sequence of dominant deformation mechanisms as well as the stress strain history may depend on the orientation history or variations in as strain accumulates in the kodiak formation a slate belt in southwest alaska the orientation history and deformation mechanisms were evaluated using incremental strain indicators and microstructures there are four successive textural events 1 development of a slaty cleavage s2 with antitaxial growth of fibrous quartz and mica in pressure shadows fi2 2 static growth of siderite and chlorite porphyroblasts 3 10 70 rotation of porphyroblasts relative to s2 with growth of curved syntaxial siderite fibers and 4 antitaxial growth of fibrous quartz and micas with local development of a crenulation cleavage s3 cumulative incremental strain and progressive finite strain histories were determined in 23 samples from syntectonic fibers and orientation histories were reconstructed from information provided by cumulative incremental strain histories textural and strain analyses indicate that 1 the strain history of nearly all samples can be approximated by two coaxial strain accumulations separated by a rigid rotation 2 development of crenulation cleavages was dependent on the orientation history and the magnitude of the early strain accumulation i e the degree of mica alignment at the completion of d2 3 unstable behavior or buckling occurred when suggesting that the angle of internal friction was close to 0 i e basal slip in micas may have been more important than frictional grain boundary sliding and 4 diffusive mass transfer was accompanied by anisotropic plastic deformation so the orientation history could be used to reconstruct the history

the determination of progressive deformation histories from antitaxial syntectonic crystal fibres abstract antitaxial syntectonic crystal fibres are modelled here as mechanically passive features the fibres representing displacement paths forming within a homogeneously deforming material evidence that many fibres develop in this way is widespread asymptotic curvature thinning of fibres and strained old fibres a measuring technique is proposed by which the complete deformation history excluding translation may be determined the method involves the iterative unstraining of two complete fibres whose development began and ended simultaneously a source of error is introduced by assuming that each of the two fibres developed at constant proportional rates although synthetic examples suggest this error to be minimal it is suggested that the kinematics of non passive fibres are unobtainable since the rheologies and boundary conditions of the material involved are relatively unknown and cannot be determined from geometry alone analysis of natural fibres found within the ordovician slates in new jersey u s a indicates that volume losses of up to 50 may occur during the formation of some slates general constraints concerning the geometrical analysis of syntectonic crystal fibres are briefly discussed and it is concluded that deformation histories must be interpreted with some modicum of caution

the shuilikeng fault in the central taiwan mountain belt for over 200 km along strike the shuilikeng fault of taiwan separates miocene rocks of the western foothills from the largely eocene and oligocene rocks of the hsuehshan range to the east despite its importance in the taiwan mountain belt the structure and kinematics of the shuilikeng fault are not well known here we present results from new geological mapping along 100 km of its strike length at the surface the shuilikeng fault is a steeply east dipping brittle fault with a series of splays and bifurcations along its southern part it cuts an earlier fold and fault system outcrop kinematic data vary widely from thrusting to strike slip the surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth these data indicate that the shuilikeng fault can be traced to greater than 20 km depth some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive from a regional perspective we interpret the shuilikeng fault to reactivate a pre existing rift related basin bounding fault to the east of which rocks in the hsuehshan range are being exhumed

sedimentation on the continental rise west of the antarctic peninsula over the last three glacial cycles abstract the continental rise west of the antarctic peninsula includes a number of large sediment mounds interpreted as contourite drifts cores from six sediment drifts spanning some 650 km of the margin and 4 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy interglacial sedimentation rates range from 1 1 to 4 3 cm ka glacial sedimentation rates range from 1 8 to 13 5 cm ka and decrease from proximal to distal sites on each drift late quaternary sedimentation was cyclic with brown biogenic burrowed mud containing ice rafted debris ird in interglacials and grey barren laminated mud in glacials foraminiferal intervals occur in interglacial stages 5 and 7 but not in the holocene processes of terrigenous sediment supply during glacial stages differed meltwater plumes were more important in stages 2 4 turbidity currents and ice rafting in stage 6 the terrigenous component shows compositional changes along the margin more marked in glacials the major oxides al2o3 and k2o are higher in the southwest and cao and tio2 higher in the northeast there is more smectite among the clay minerals in the northeast magnetic susceptibility varies along and between drifts these changes reflect source variations along the margin interglacial sediments show less clear trends and their ird was derived from a wider area downslope processes were dominant in glacials but alongslope processes may have attained equal importance in interglacials the area contrasts with the east antarctic continental slope in the se weddell sea where ice rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial the differences in glacial setting and margin physiography can account for these contrasts

kinematics of arc continent collision in the eastern central range of taiwan inferred from geomorphic analysis active arc continent collision in taiwan is an example of a typical collisional orogen with a deformed continental margin foothills belt hsueshan range and central range separated from an accreted island arc coastal range by a collapsed forearc basin longitudinal valley the geomorphology of the eastern central range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late quaternary time digitized drainage basin perimeters stream channels and the mountain piedmont junction of a 200 km long segment of the eastern central range provide data for calculating quantitative geomorphic parameters statistical analyses of these parameters define four domains of distinct landscape character the northern domain extends northward along the coast from the northern end of the longitudinal valley lv and has landscape characteristics suggesting uplift near the mountain front the central two domains are adjacent to the lv and show evidence of uplift concentrated in the interior of the range away from the front the boundary between the two central domains occurs near the middle of the lv the southern domain extends southward along the coast from the southern end of the lv and has characteristics suggesting a transient response to increased uplift rate analysis of fluvial terrace profiles in the central range adjacent to the southern half of the lv indicates a change in uplift pattern through time with migration of maximum uplift rate from the mountain front to the interior of the range this change probably occurred just prior to late pleistocene time