superimposed tectono metamorphic episodes of jurassic and eocene age in the jadeite uplift myanmar as revealed by 40ar 39ar dating abstract the myanmar jadeite uplift forms an important link between the indo burma range and the tagaung myitkyina belt two contrasting ages of jurassic 152 4 1 5 ma of glaucophane from blueschist and eocene 44 8 1 1 and 45 0 1 3 ma of phengitic muscovites from quartz schists in the uplift were yielded using 40 ar 39 ar dating the jurassic age of the glaucophane even older than the forming age of the jadeitite from recent relevant literatures and for the reason of possible excess argon involved is interpreted as the lower limit of the subduction age previous studies correlated this subduction with the woyla intra oceanic arc or the incertus arc to the west the eocene ages of phengitic muscovites are interpreted as the time of an intra continental shearing deformation event against the timing of hp lt metamorphism as previously suggested combined with other studies it is suggested that the tagaung myitkyina belt and the indo burma range belonged to a single belt which has been separated by the sagaing fault leaving the jadeite uplift straddling along the fault between the belt and the range we propose a rapid exhumation model for the myanmar jadeitite at 45 ma coeval with onset of the sagaing fault

sediment deformation and hydrogeology of the nankai trough accretionary prism synthesis of shipboard results of odp leg 131 the main objective of leg 131 was to provide data on the deformational processes and associated hydrogeology of the nankai prism toe drilling succeeded for the first time in the history of ocean drilling in penetrating the complete sedimentary sequence to basaltic basement reaching 1327 mbsf metres below seafloor with good core recovery 55 excellent correlation of the lithology and structure including the frontal thrust and the decollement with seismic reflection images was also determined bedding dips faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to nw se shortening as expected from the plate tectonic convergence vector below the decollement no significant deformation features were observed indicating that the decollement is a sharp discontinuity in stress transmission physical properties data show major discontinuities at the decollement notably an increase in porosity below the later this may indicate excess pore pressure in the subducted section and decollement zone a less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature attempts to make downhole measurements were severely hampered by unstable hole conditions but useful constraints have been placed on the thermal regime and some calibration of laboratory physical properties toin situ conditions has been provided andin situ stress and pore pressure were measured in the uppermost sediments evidence of channelized fluid flows is inconclusive no sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found thermal measurements are not significantly different from those predicted by a purely conductive heat flow model a signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events we conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site

raw material quality and oldowan hominin toolstone preferences evidence from kanjera south kenya the role of raw material quality in oldowan technology has not been fully explored there are numerous studies suggesting oldowan hominins preferred certain types of stone for artifact manufacture previous studies of the artifact assemblage from the early pliocene oldowan locality of kanjera south south rachuonyo district kenya show that raw material selection and transport was an important aspect of late pliocene hominin adaptations yet the exact properties of stones that hominins were selecting remain enigmatic two potentially important features of artifact raw material are durability and fracture predictability we investigate fracture predictability through mechanical tests of stone and investigations of the affect of stone properties on fracture patterns in archaeological collections we investigate stone durability with actualistic studies of edge attrition combined with further mechanical tests of various lithologies oldowan hominins at kanjera appear to have selected raw materials based on their durability the ability for a stone to fracture consistently does not appear to be as important in hominin toolstone preference as previously assumed hominins that produced the assemblages at kanjera south appear to have incorporated an extensive understanding of various attributes of raw material in the transport and production of stone artifacts when combined with previous research on the transport patterns at kanjera the results of this study provide evidence for a more complex raw material acquisition strategy than has previously been suggested for late pliocene oldowan hominins

lithology and biostratigraphy of franciscan like chert and associated rocks in west central baja california mexico we present the result of biostratigraphic and lithologic studies of ribbon chert and associated rocks in a mesozoic subduction complex in west central baja california mexico the subduction complex terrane is divided into three subterranes each of which consists of an originally coherent sequence of interbedded oceanic rocks including ocean floor basalt terrigenous siliciclastic turbidites radiolarian ribbon chert and limestone parts of the original stratigraphy are preserved in the least deformed least metamorphosed subterrane ocean floor pillow basalt on cedros island is overlain by about 40 m of radiolarian ribbon chert ranging in age from early jurassic or older to early cretaceous the chert is overlain by thin bedded turbidites on san benito west island radiolarian ribbon chert is interbedded with volcanogenic rocks including a megabreccia that contains blocks of metabasites folded late jurassic early cretaceous chert and marble in a mid cretaceous radiolarite matrix chert samples from tectonically thinned sequences elsewhere on cedros and san benito islands contain late triassic to early cretaceous radiolarian assemblages the 40 m thick chert section on cedros island probably accumulated atop oceanic crust in a large ocean basin for at least 65 m y and was later capped by terrigenous turbidites as it approached the subduction zone along the western margin of north america faulted chert sections on cedros and san benito central and west islands are subsets of the main section on cedros island the chert megabreccia section on san benito west island is interpreted to have accumulated at the foot of a volcanic edifice on the ocean floor for example seamount fracture zone or during extension and normal faulting of the downgoing oceanic plate in a subduction zone sedimentary structures and stratigraphic relations indicate that all ribbon chert in this subduction complex was deposited by sediment gravity flows that is they are turbidites

kinematic analysis of melange fabrics examples and applications from the mchugh complex kenai peninsula alaska permian to cretaceous melange of the mchugh complex on the kenai peninsula south central alaska includes blocks and belts of graywacke argillite limestone chert basalt gabbro and ultramafic rocks intruded by a variety of igneous rocks an oceanic plate stratigraphy is repeated hundreds of times across the map area but most structures at the outcrop scale extend lithological layering strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation forming broken formation and melange deformation was noncoaxial and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing generally thinly sedimented oceanic plate soft sediment deformation processes do not appear to have played a major role in the formation of the melange a model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to s1 are contracted in both the brittle and ductile regimes layers at 30 458 to s1 are extended in the brittle regime and contracted in the ductile regime and layers at angles greater than 458 to s1 are extended in both the brittle and ductile regimes imbrication in thrust duplexes occurs at deeper levels within the wedge many structures within melange of the mchugh complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge a displacement field for the mchugh complex on the lower kenai peninsula includes three belts an inboard belt of late triassic rocks records west to east directed slip of hanging walls a central belt of predominantly early jurassic rocks records north south directed displacements and early cretaceous rocks in an outboard belt preserve southwest northeast directed slip vectors although precise ages of accretion are unknown slip directions are compatible with inferred plate motions during the general time frame of accretion of the mchugh complex the slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates which became more oblique with time they are not considered indicative of strain partitioning into belts of orogen parallel and orogen perpendicular displacements because the kinematic data are derived from the earliest preserved structures whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion related fabrics 1999 elsevier science ltd all rights reserved

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

internal dynamics of a paleoaccretionary wedge insights from combined isotope tectonochronology and sandbox modelling of the south central chilean forearc forearc accretionary wedges are cyclic systems in which material is frontally and or basally accreted material cycling involves underthrusting subduction underplating exhumation erosion transfer to the trench and underthrusting again in this study we present a novel tectonochronologic approach to constrain long term exhumation rates of basally accreted wedge complexes based on isotopic dating of structural features on petrological data and sandbox analogue simulations congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics for the present day surface material of the paleoaccretionary wedge of south central chile valdivia area 40 s published u pb ages of detrital zircon place a maximum age of 278 ma for subduction prograde metamorphism at transitional greenschist to blueschist facies conditions 420 c 8 9 kbar was immediately followed by progressive penetrative deformation associated with basal accretion dated at 250 245 ma using rb sr internal mineral isochrons the accretion process involved duplex tectonics and antiformal stacking with formation of near horizontal mylonitic shear zones at around 241 ma continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels both semi ductile small scale extensional shear zones and post kinematic vein mineralizations yield rb sr ages of 235 ma tension gashes representing the latest isotopically dateable stage of structural evolution were formed at 210 ma at conditions of 230 c at 1 5 3 kbar as constrained by fluid inclusion data zircon fission track data indicate final cooling to below 200 c at 186 24 ma the results suggest continuous basal accretion for at least 50 ma with long term average exhumation rates of 0 6 0 2 mm a most probably outbalanced by similar long term average erosion rates changing plate boundary conditions at about 210 200 ma terminated the accretion process as evident from a dramatic decrease of exhumation rates at that time since then the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the andean active margin the tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high temperature increments of p t trajectories

oldowan behavior and raw material transport perspectives from the kanjera formation the archaeological record of oldowan hominins represents a diverse behavioral system it has been suggested that exploitation of lithic resources by oldowan hominins was simplistic and represented mostly use of local sources of stone here we investigate the raw material selection and transport behaviors of oldowan hominins reflected in the stone artifact assemblages from the kanjera south formation south rachuonyo district kenya using geochemical methods ed xrf artifacts are linked to primary and secondary source outcrops throughout southwestern kenya these data show that hominins selected raw materials for transport at frequencies that are significantly different from their availability on ancient landscapes furthermore a substantial proportion of the assemblage represents transport over relatively long distances 10 km our study further suggests that in the early stages of stone tool use hominins used a wide variety of raw materials and selected these materials at some distance from their eventual discard locations early hominin behavior may have incorporated an understanding of raw material source distributions across a more extensive landscape than has been previously documented this supports the growing perspective that oldowan technology represents a more complex behavioral pattern than is usually associated with the beginnings of hominin tool use

horizontal shortening versus vertical loading in accretionary prisms 1 the analysis of two phased deformation of subducted turbidites within the shimanto belt on kyushu japan shows strongly contrasted kinematics between an earlier noncoxial fold and thrust type stage and a later metamorphic coaxial vertical shortening stage the deep deformation is characterized by the development of a subhorizontal muscovite chlorite metamorphic foliation associated with pure shear deformation which overprinted the structures inherited from the earlier stage we associate the metamorphic deformation stage with the lower levels of accretionary prism landward segment inner wedge which was therefore deformed by vertical loading and horizontal extension much in contrast with prism trenchward segment outer wedge which often shows fold and thrust structures indicative of horizontal shortening a possible explanation based on wang and hu s 2006 model for these longitudinal variations in deformation kinematics of the prism may lie in the contrasted rheology of its basal surface strain weakening in the seismic portion below the inner wedge while strain strengthening in the aseismic portion below the outer wedge the vertical shortening active within the inner wedge revealed by this study possibly represents a large contribution to the exhumation of the deeper levels of accretionary prisms

effect of heat treatment on siliceous rocks used in prehistoric lithic technology stone tool manufacture by many prehistoric and recent societies was characterized by deliberate heating of fine grained siliceous rocks to improve their flaking properties extensive mechanical testing of heated and unheated cryptocrystalline and macrocrystalline quartz lithologies has shown that thermal treatment causes a consistent marked reduction in fracture toughness this mechanical property can be used as an objective measurement of the flaking qualities of stone materials and a reliable criterion for the recognition of intentionally heated artefacts in the archaeological record x ray diffraction studies and scanning electron microscopy have demonstrated that the change in fracture toughness with heating is the result of recrystallization the poorly ordered strongly interlocking cryptocrystalline fabric of the unheated samples becomes more equigranular and better crystallized with thermal treatment as a consequence fractures propagate more readily in heated samples accounting for their better flaking properties

recognition of ocean plate stratigraphy in accretionary orogens through earth history a record of 3 8billion years of sea floor spreading subduction and accretion abstract ocean plate stratigraphy ops is a term used to describe the sequence of sedimentary and volcanic rocks deposited on oceanic crust substratum from the time it forms at a spreading center to the time it is incorporated into an accretionary prism at a convergent margin in this study we review the major geological characteristics of relict cenozoic to early archean oceanic crust and ops persevered in alaska japan california franciscan complex central asia british isles canada slave province australia pilbara craton and greenland isua and ivisaartoq belts an assessment of ops in accretionary orogens spanning the duration of earth s rock record shows remarkable similarities between ops of all ages in terms of structural style major rock components sequence of accretion and trace element geochemical signatures volcanic rocks preserved in the orogenic belts are characterized predominantly by oceanic island arc basalts island arc picrites mid ocean ridge basalts back arc basalts oceanic plateau basalts ocean island basalts and boninites with extremely rare komatiites this demonstrates that sea floor spreading lateral movement of oceanic plates with accompanying sedimentation over the oceanic substratum and accretion at convergent margins have been major earth processes since at least 3 8 ga ago there have been some secular changes in the rock types in ops such as changes in carbonates and radiolarian cherts whose sources were in the biota in existence in phanerozoic times but absent in the precambrian but overall there have been few changes in the style of ops accretion with time komatiites and banded iron formations occur predominantly in archean orogenic belts reflecting higher mantle temperatures and less oxic seawater composition respectively before 2 5 ga this is clear documentation that plate tectonics including the lateral movement of oceanic lithosphere has been a major heat loss mechanism on earth since the early precambrian

ophiolite accretion in the lachlan orogen southeastern australia slivers of dismembered cambrian ophiolites are preserved in major fault zones of the turbidite dominated western and central lachlan orogen of southeastern australia geometrical chronological and metamorphic constraints indicate that these slivers did not undergo classic tethyan style obduction but were incorporated into evolving turbidite wedges as offscraped slices imbricated fault slivers or duplexes and as blocks in melange the processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho formation and metamorphism of serpentinite talc matrix melange up to blueschist conditions and accretion via underplating or offscraping during underthrusting these processes have produced major large scale fault zones within the chevron folded turbidites that show marked along strike variation in structural complexity that relate to magnitude of displacement the fault zones may have formed preferentially where topographic highs occurred in the oceanic crust promoting fold and fault nucleation within irregularities in the layering these fault zones have predominantly brittle character due to their formation under a low geothermal gradient duplex like structures in the fault zones and the inferred development through buckling of then faulting of the oceanic lithosphere have analogues in modern oceanic environments e g indian ocean and young subduction systems e g sulu sea southeast asia they are interpreted to have formed due to convergence in a non collisional intra oceanic setting during closure of a cambrian backarc basin

twenty five million years of subduction accretion exhumation during the late cretaceous tertiary in the northwestern caribbean the trinidad dome escambray complex central cuba the trinidad dome escambray complex central cuba forms part of an accretionary wedge built up during intra oceanic subduction in the caribbean from the late cretaceous 75 ma to tertiary fieldwork microscopic studies p t estimates and new 40ar 39ar isotopic data indicate a tectono metamorphic evolution involving five deformation phases three main stages d1 d2 and d3 were responsible for the subduction exhumation of the units during metamorphic development 40ar 39ar isotopic data allow for the first time an evaluation of the complete time interval from subduction collision to final exhumation d1 deformation took place at 60 5 0 6 ma during subduction and was characterized by a prograde path with hp peak metamorphic conditions in eclogite facies in the monforte nappe the highest structural nappe and greenschist metamorphism in the la sierrita yaguanabo and rio chiquito nappes at lower structural levels d2 deformation 56 32 0 40 ma included a short lived heating event in the lower nappes that caused incipient recrystallization at lower amphibolite facies but was generally linked to decompression and retrograde metamorphism to greenschist conditions during folding and nne thrusting of all nappes parallelism of the main s2 foliation and serpentinite lenses indicates d2 was the main phase of exhumation during d3 exhumation and nne thrusting continued as the units were progressively incorporated into the growing accretionary wedge until about 50 ma structures related to d4 are post metamorphic and reflect the domal structure d5 strike slip and normal faults cut the tectonic pile and low angle normal faults are more abundant towards the dome rim indicating exhumation of the escambray field observations structures and the inverted distribution of the p t conditions in the wedge suggest exhumation mechanisms by underthrusting underplating and erosion combined with extension at the surface subduction accretion exhumation in the trinidad dome thus occurred from 75 to 50 ma indicating a period of 25 million years of deformation history in the northern caribbean realm

formation of taconic melanges and broken formations in the hamburg klippe central appalachian orogenic belt eastern pennsylvania abstract the hamburg klippe of the central appalachian orogenic belt exposed in eastern pennsylvania displays a complex record of poly phase melange and broken formation development in a convergent margin setting it includes an imbricate stack of tectonic slices which consist of upper cambrian to upper ordovician deep water and continental slope sedimentary rocks emplaced by gravity sliding onto the laurentian passive margin during deposition of the upper ordovician martinsburg formation based on their internal structure and stratigraphy the block matrix ratios and relations and the inferred tectonic settings of origin we have differentiated the following melanges and broken formations whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision i sedimentary broken formations without exotic blocks formed by in situ and local down slope remobilization during the early stages of closure of the octoraro sea basin in the early ordovician ii two types of sedimentary melanges with exotic blocks formed at the front of the advancing accretionary wedge during the middle ordovician subduction of the laurentian continental margin beneath a microcontinent magmatic arc tectonic assembly iii layer parallel extensional broken formation and a diapiric melange formed in the outer trench and at the toe of the accretionary wedge respectively during the early late ordovician iv precursory olistostromes formed during the late ordovician collisional episodes as the hamburg klippe was emplaced in the martinsburg formation on the downgoing laurentian continental margin and v contractional deformation related broken formations formed at the base of main thrust faults overprinting the previously formed melanges and broken formations this sequential development of different melange types in the central appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences the structural level of melange formation within the accretionary wedge and the kinematics of deformational processes

matrix deformation mechanisms in hp lt tectonic melanges microstructural record of jadeite blueschist from the franciscan complex california abstract exhumed high pressure low temperature metamorphic melanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction rigid blocks of widely varying size are embedded in a weak matrix which takes up the deformation and controls the rheology of the composite the microfabrics of a highly deformed jadeite blueschist from the franciscan complex california are investigated to help understand deformation mechanisms at depth the specimen shows a transposed foliation with dismembered fold hinges and boudinage structures several generations of open fractures have been sealed to become veins at high pressure metamorphic conditions the shape of these veins frequently restricted to specific layers indicates distributed host rock deformation during and after sealing small cracks in jadeite and lawsonite are healed with tiny quartz inclusions aligned along the former fracture surface large jadeite porphyroblasts show strain caps and strain shadows open fractures are sealed by quartz and new jadeite epitactically grown on the broken host monophase glaucophane aggregates consist of undeformed needles with a diameter between 0 1 and 2 m grown after formation of isoclinal folds only quartz microfabrics indicate some stage of crystal plastic deformation followed by annealing and grain growth aragonite in the latest vein generation shows retrogression to calcite along its rims the entire deformation happened under hp lt metamorphic conditions in the stability field of jadeite and quartz at temperatures between 300 and 450 c and pressures exceeding 1 1 4 gpa the microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism accompanied by brittle failure and vein formation at quasi lithostatic pore fluid pressure this indicates low flow strength and combined with high strain rates expected for localized deformation between the plates a very low viscosity of material in the interplate shear zone at a depth 30 45 km

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

geotectonic framework of the blueschist unit on anglesey lleyn uk and its role in the development of a neoproterozoic accretionary orogen abstract a 560 550 ma 5 km 25 km blueschist unit extends ne sw on the island of anglesey wales uk and continues to the southwest for more than 70 km along the northern coast of the lleyn peninsula it has the shape of a shallow dipping slab or sheet up to a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest today the top boundary of the slab is commonly a thrust that was originally a low angle normal fault at its top in the subduction zone above it is an accretionary complex and a unit of high grade gneisses the bottom boundary is today a thrust that was originally a thrust below which is a unit of low grade or unmetamorphosed rocks belonging to an olistostrome type accretionary complex this thrust was originally at the bottom of the slab when it was exhumed in the subduction zone the sandwiched assemblage was created by the tectonic insertion of the blueschist between the other units folding particularly during late doming of the tectonic sandwich rotated the top normal fault boundary into a thrust later high angle ne sw trending normal faults displaced the tectonic stratigraphy in horst and graben a 200 km long and 200 km wide 680 450 ma calc alkaline volcano plutonic belt extends southeastwards from the blueschist belt via northern wales to central england suggesting that the subduction of oceanic lithosphere was to the southeast and this is consistent with the extrusion of the thin high pressure slab into the accretionary complex to the northwest this took place on the western accretionary margin of avalonia finally we outline the key steps involved in unravelling the structure of a pacific type accretionary complex

blueschist metamorphism during accretion in the lachlan orogen south eastern australia low t intermediate to high p assemblages indicative of the prehnite pumpellyite greenschist and blueschist facies are preserved in melange zones and slivers of oceanic crust within two major fault zones of the turbidite dominated lachlan orogen in one of these fault zones governor fault zone blueschists occur as franciscan like blocks in a serpentinite talc matrix that is interleaved with phyllites and slates and structurally overlain by a fault slice or duplex of predominantly pillow basalt chert and turbidite the blueschist metavolcanics are interpreted to have formed at 450 c and at a depth of approximately 21 27 km the presence of blue amphibole in the blocks rinds and matrix indicate that the metavolcanics were emplaced in the matrix prior to blueschist metamorphism blocks and matrix were partially exhumed interleaved with tectonic slices of phyllite and slate and subsequently folded at about 10 12 km depth inferred from bo values of the dominant mica fabric in the phyllites and slates metamorphic p t is highest in the structurally lowest slice melange zone and lowest in the overlying ophiolitic fault slice suggestive of an accretionary burial metamorphic pattern formed by underplating of the melange in the other fault zone heathcote fault zone blueschists transitional to greenschist facies are interpreted to have formed at 450 c and at a depth of approximately 15 21 km they occur as blocks in serpentinite talc matrix melange and are also associated with fault slices of oceanic crust textural and mineralogical evidence suggests that the protoliths for the blueschists in both fault zones were boninitic pillow lavas the metamorphic facies and patterns and the structural and lithological associations can be interpreted in terms of disruption of oceanic crust and overlying sediments during subduction and formation of serpentinite matrix melange overprinted by blueschist metamorphism either prior to or during underplating of the melange and duplex formation the presence of blueschist metavolcanics indicate that these processes occurred at considerable depth these interpretations have implications for the evolution of large scale fault zones in noncollisional convergent oceanic settings

structure and tectonic evolution of the southwestern trinidad dome escambray complex central cuba insights into deformation in an accretionary wedge abstract the trinidad dome escambray complex cuba forms part of an accretionary wedge built during intra oceanic subduction in the caribbean from the late cretaceous to cenozoic the structure reflects syn subduction exhumation during thickening of the wedge followed by extension field mapping metamorphic and structural analysis constrain the tectonic evolution into five stages three ductile deformation events d1 d2 and d3 are related to metamorphism in a compressional setting and formation of several nappes d1 subduction fabrics are only preserved as relict s1 foliation and rootless isoclinal folds strongly overprinted by the main s2 foliation the s2 foliation is parallel to sheared serpentinised lenses that define tectonic contacts suggesting thrust stacks and underthrusting at mantle depths thrusting caused an inverted metamorphic structure with higher grade on top of lower grade nappes exhumation started during d2 when the units were incorporated into the growing accretionary wedge along nne directed thrust faults and was accompanied by substantial decompression and cooling folding and thrusting continued during d3 and marks the transition from ductile to brittle ductile conditions at shallower crustal levels the d4 5 events are related to extension and contributed to the final exhumation likely as a core complex d4 is associated with a regional spaced s4 cleavage late open folds and numerous extension veins whereas d5 is recorded by normal and strike slip faults affecting all nappes the p t path shows rapid exhumation during d2 and slower rates during d3 when the units were progressively incorporated into the accretionary prism the domal shape formed in response to tectonic denudation assisted by normal faulting and erosion at the surface during the final stages of structural development these results support tectonic models of sw subduction of the proto caribbean crust under the caribbean plate during the latest cretaceous and provide insights into the tectonic evolution of accretionary wedges in an intra arc setting

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

a 100 m y long window onto mass flow processes in the patagonian mesozoic subduction zone diego de almagro island chile diego de almagro island was formed by the subduction and accretion of several seafloor derived tectonic slices with very heterogeneous ages and pressure temperature time p t t paths the highest element of the pile the lazaro unit evidences subduction in the high p granulite field 1 3 gpa 750 c at ca 163 ma below it a thin tectonic sliver the garnet amphibolite unit preserves eclogite facies remnants 570 c and 1 7 gpa formed at ca 131 ma in situ u pb zircon rim ages peak assemblages were nearly fully amphibolitized during decompression down to 1 2 gpa and 600 c at 125 120 ma rb sr multimineral dating the underlying blueschist unit has 50 m y younger metamorphic ages and exhibits slightly cooler peak burial conditions 520 c 1 7 gpa ca 80 ma in situ white mica ar ar ages and multimineral rb sr dating and is devoid of amphibolitization the mylonites from the sinistral strike slip seno arcabuz shear zone bounding diego de almagro island to the east also exhibit amphibolite facies 620 c and 0 9 gpa deformation at ca 117 ma multimineral rb sr ages in situ white mica ar ar dating and multimineral rb sr dating of low t mylonites 450 c along the base of the lazaro unit reveal partial resetting of high t assemblages during tectonic displacement between 115 and 72 ma and exhumation of the slice stack detrital zircon u th pb ages indicate that the material accreted on diego de almagro island has been mostly recycled from a permian triassic accretionary wedge madre de dios accretionary complex exposed along the subduction buttress geological and geochronological constraints suggest that the rocks of the seno arcabuz shear zone and the lazaro unit were tectonically eroded from the buttress while the underlying garnet amphibolite and blueschist units instead derive from the subducted oceanic basin with increasingly younger maximum depositional ages the very long residence time of the rocks 90 m y for the lazaro unit along the hanging wall of the subduction interface recorded long term cooling along the patagonian subduction zone during the mesozoic diego de almagro island therefore represents a unique window onto long term tectonic processes such as subduction interface down stepping tectonic erosion and episodic underplating near the base of an accretionary wedge 40 50 km

toolstone constraints on knapping skill levallois reduction with two different raw materials lithic raw material constraints are widely assumed to be a determining factor of flaked stone tool morphology but this assumption remains largely untested we conducted a controlled experiment to determine whether a knapper s growing replication skills would be hindered if the toolstone used was switched from large flakes of an easily worked chert to nodules of less tractable one two batches of preferential levallois cores were knapped an earlier series made from standardised large flakes of sediments dominated by chalcedonic quartz followed by a more challenging one using variably shaped cortical nodules of microcrystaline quartz that varies in the completeness of quartz replacement of calcite and dolomite skill level markers were designed to measure the knapper s ability to achieve a series of set goals these were quantified and subjected to statistical testing in all but one test significant increases in skill could be detected from the earlier to the later batch of reductions despite the drop in toolstone quality significant improvements in the consistency of the knapper s output could also be detected however the switch to a more challenging nodular chert did require extra shaping which resulted in more waste this masked visible progress towards producing a less costly core overall our results do not support the assumed primacy of toolstone constraints over other factors in influencing the morphology of flaked stone tools

deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions abstract we conducted triaxial deformation experiments using a griggs type solid pressure medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the sambagawa metamorphic belt a greenschist and an epidote amphibole schist at a confining pressure of 1 gpa 400 c and strain rate of 10 5 1 s differential stresses for all mafic schists were higher than 1 gpa the greenschist samples were weaker than epidote amphibolite samples under all experimental conditions microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi brittle deformation samples of the epidote amphibole schist deformed at 500 c show shear localization along a single fault oriented parallel to the foliation while mechanical data shows no rapid stress drop on the surface of the fault we observe the formation of slickensides with little gouge all sample types exhibit a high stress exponent n 15 and strain rate strengthening i e positive a b values frictional behavior that inhibits earthquake nucleation differential stress increased with increasing confining pressure while the friction coefficient decreased with increasing confining pressure and temperature greenschist and epidote amphibolite facies metamorphism typically occurs at hot subduction zones such as cascadia and southwest japan the lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi brittle deformation or frictional sliding with a positive a b during deformation of these types of metamorphic rocks weak and aseismic fault zones in subducting slabs might promote slab mantle decoupling at the corner of the mantle wedge and detachment and underplating of oceanic crust from the subducting slab to forearc crust

quartz and silcrete raw material use and selection in late holocene assemblages from semi arid australia both quartz and silcrete cobbles are abundant in the stony desert regions of western new south wales australia and were used by aboriginal people who occupied these regions from the mid to late holocene archaeologists often characterise quartz as an inferior material for flaking when compared to silcrete but aboriginal people made intensive use of both materials here we investigate the degree to which archaeologists can draw inferences about the choices people made in the past regarding the selection and use of different raw materials different types of raw material i e microcrystalline silcretes and macrocrystalline quartzes were flaked more or less intensively but it is the utilization of the products of this flaking not simply their manufacture that allows inferences to be made about past intentions

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

ophiolitic melanges in crustal scale fault zones implications for the late palaeozoic tectonic evolution in west junggar china the baijiantan and darbut ophiolites in west junggar are exposed in steep fault zones 70 containing serpentinite melange in contact on either side with regionally distributed upper devonian lower carboniferous ocean floor peperitic basalts and overlying sedimentary successions the ophiolitic melanges show classic structural features created by strike slip faulting and consistent shear sense indicators of left slip kinematics sandstone blocks within the melanges resemble the surrounding sediments in lithology and age indicating that the ophiolitic melanges consist of locally derived rocks the ophiolitic melanges therefore originated from left slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones sandstone is the youngest lithology involved in the melange and provides a maximum age for the melange of 322 ma whereas stitching plutons are younger than 302 ma multiple clusters in zircon ages from single gabbro blocks in the melange at 375 360 354 and 340 ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers late paleozoic tectonic evolution of west junggar involved late devonian to carboniferous relative motion between the junggar block and west junggar ocean basin which triggered the left slip fault zones within a remnant ocean basin along which the oceanic crust was disrupted to form linear ophiolitic melanges final filling of this remnant ocean basin and its dismemberment by strike slip faulting occurred in the late carboniferous followed by crustal thickening by juvenile granites at the carboniferous permian boundary

well preserved underplating structure of the jadeitized franciscan complex pacheco pass california duplex structures several hundred metres thick bounded by marker beds exhibiting basalt chert association have been found in the franciscan complex near pacheco pass central diablo range california the terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 c and 7 8 kbar detailed mapping of the basalt chert sequence has clarified the change of environment from mid oceanic ridge to subduction zone eastward convergence of duplexing indicates eastward subduction of the oceanic plate during early cretaceous time the presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the franciscan coherent unit which has long been believed to have an in situ origin interpreted as either near trench volcanism or accreted terranes in which geosynclinal type volcanism occurred

thickening of fault zones a mechanism of melange formation in accreting sediments sediments accreted at subduction zones undergo stratal disruption and form a type of melange the thickness of the disrupted zones grows with progressive deformation this suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment factors causing the abandonment of fault surfaces during continuing deformation include 1 strengthening owing to porosity loss during consolidation 2 localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits and 3 reorientation of fault surfaces the disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass

implication for the two stage layer parallel faults in the context of the izu forearc collision zone examples from the miura accretionary prism central japan abstract the southern tip of the miura peninsula central japan exposes nice outcrops of forearc sediment accreted to another forearc which occurred during a series of izu arc collision with the honshu arc neogene deep marine volcaniclastic sedimentary sequences the misaki formation show many types of faulting and folding associated with layer parallel faults besides many imbricate thrusts of the main deformation stage the layer parallel faults are divided into two stages one is related to northwestward gravity driven slump deposits the other is southeastward thrust faulting and the both are associated with many layer duplications with liquefaction of sediments the slumping is commonly developed in the upper part of the sequence in the surface of the down going slab before accretion whereas the thrusting is dominant in the lower part within the toe of the accretionary prism probably along the decollement zones in the prism thus the misaki formation represents a good example of deformation just before and after shallow forearc accretion in the arc collision zone

kinematics and age constraints of deformation in a late carboniferous accretionary complex in western junggar nw china abstract the karamay area situated in the eastern part of western junggar southern altaids contains an ophiolitic melange with ultramafic rocks gabbro basalt chert and limestone which show typical block in matrix structures and coherent turbidites and tuffs these lithological associations are interpreted as incoherent and coherent series formed in an accretionary complex on the basis of detailed field mapping and analyses of the asymmetry of imbricate thrusts duplexes tilted structures shear band cleavages and the nw verging inclined to overturned folds we conclude that the overall movement in the accretionary complex was top to the nw the youngest tuff involved in the deformation contains detrital zircons that have a u pb age la mc icp ms of 308 7 ma 39ar 40ar resistance furnace step heating of amphibole separates from a diorite dike which cuts the folded and imbricated rocks in the accretionary prism yielded a plateau age of 307 2 ma consequently the age of the deformation in the prism is tightly constrained at 307 308 ma implying that the deformation occurred in an extremely short time span during se ward subduction combined contemporaneous occurrence of baogutu adakite high mg sr enriched and y poor dioritic dikes miaoergou charnockite and maliya mafic rocks we further suggest the accretionary complex was cut by near trench volcanic rocks and plutons possibly due to interaction with a spreading ridge

melanges through time life cycle of the world s largest archean melange compared with mesozoic and paleozoic subduction accretion collision melanges abstract melanges represent a significant component of collisional and accretionary orogenic belts we present a comparison of subduction accretion melanges of different ages to test how melange forming processes at convergent margins changed through time we describe the mesozoic mchugh melange in alaska and the paleozoic proto tethyan laohushan melange of the qilian orogenic belt of the alpine himalayan chain together with documentation of a neoarchean melange that marks the suture between the eastern block and central orogenic belt of the north china craton ncc the mchugh is a classic accretionary melange that has not yet experienced a collision and the laohushan is an accretionary collisional melange with structures related to both stages of its evolution we compare the characteristics of these melanges with the world s largest archean melange the newly documented circa 2 5 ga 1600 km long jianping zunhua zanhuang dengfeng jzzd melange of the north china craton the jzzd is a classical melange in terms of map patterns lithological relations structural characteristics including mixed rock types of different ages origins and metamorphic histories and preserves both paired metamorphic features and local blocks preserving ultra high pressure uhp assemblages it shows high stratal disruption with both block in matrix and highly mixed thrust bounded tectonic slices types of map patterns the melange is part of an arc accretionary complex that was emplaced over a 2 7 2 5 ga passive margin sequence on the western edge of the eastern block of the ncc at 2 5 ga the frontal zone of the accretionary complex marks the suture between the eastern block and an oceanic arc terrane in the central orogenic belt cob and exhibits significant along strike n s variations in the north the jianping pyroxenite melange contains blocks of 2 52 ga metapyroxenite metabasalt tonalite trondhjemite granodiorite diorite ttgd gneisses in a metasedimentary and serpentinite matrix intruded by 2 49 2 47 ga plutons metamorphosed to amphibolite granulite conditions at 2 47 ga the zunhua shangying ophiolitic melange contains relatively intact 2 5 ga ophiolitic sheets in a 2 5 ga metasedimentary melange intruded by 2 48 ga granitic dikes the zunhua ultra high pressure uhp podiform chromite melange has blocks of pillow basalt bif dikes gabbros pyroxenite harzburgite some with 2 55 ga podiform chromites in dunite envelopes enclosed in a 2 52 2 46 ga metasedimentary matrix metamorphosed and intruded by granitic dikes at 2 46 ga the chromites and dunite pods formed through reaction of a boninitic melt and harzburgite during subduction initiation bewtween 2 6 2 55 ga uhp conditions are documented by tio2 ii inclusions in chromites indicating subduction to at least 270 km depth the zanhuang melange includes blocks of metapsammites metagabbros metabasalts and ultramafic rocks in a metapelitic matrix intruded by 2 49 2 54 ga granites then imbricated with 2 65 ga marble metapelite quartzite of the passive margin of the eastern block the dengfeng melange includes upper plate blocks of metagabbro metabasalt bif and felsic rocks in a sandy to pelitic matrix thrust over a turbiditic broken formation melange unit in the east circa 2 5 ga metamorphic conditions record a relatively high t p anatexis 950 c gpa in the west whereas the central part of the belt records intermediate t p metamorphism

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

metamorphism of franciscan tectonostratigraphic assemblage pacheco pass area east central diablo range california coast ranges the pacheco pass area well known for the relatively high pressure low temperature mineral assemblages developed in a well ordered section of chiefly metaclastic strata was restudied in order to elucidate the geologic paragenetic and textural relationships among the various lithologic units the mapped 7 minute quadrangle extends from 4 km west of the range crest eastward to the ortigalita fault tectonic contact of the franciscan assemblage with coeval great valley strata semicontinuous metachert beds underlain by scraps of na cpx and or na amphibole bearing metabasaltic rock mark the base of terrigenous tectonostratigraphic units evidently decoupled from the downgoing oceanic crust capped plate underplating along newly recognized bedding plane faults has juxtaposed members of an apparently conformable franciscan sedimentary sequence four depositional units each consisting of monotonous metagraywacke and interlayered metashale are stacked within the accretionary section from top down these are tectonostratigraphic members a b c and d unit d differs from overlying members in possessing a greater abundance of bluestone greenstone pods minor serpentinite bodies and rare metaconglomerate lenses varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation the ortigalita fault is nearly vertical strikes north northwest and exhibits apparent dextral offset it truncates the east west striking gonzaga fault and other franciscan structures the topographic low occupied by san luis reservoir is a cenozoic tectonic depression sited at a releasing bend on the ortigalita fault newly acquired textural chemical mineralogic and areal relations of albitic and jadeitic pyroxene bearing franciscan metagraywacke near pacheco pass demonstrate that 1 na cpx and na amphibole are metamorphic minerals not clastic materials 2 pumpellyite is a minor neoblastic phase in albite bearing metagraywacke west of pacheco pass and in the north western corner of the quadrangle but it does not occur in jadeitic pyroxene bearing metaclastic rocks 3 higher textural grade metagraywacke specimens generally contain higher modal proportions of jd 4 abundance textural grade and chemistry of na cpx are not related to tectonostratigraphic units and 5 because of sluggish reaction rates albite persisted metastably into the higher pressure jadeitic pyroxene quartz p t field production of na cpx resulted from a reaction of the sort ab chl jd iws qtz gln h 2 o irregular oscillatory zoning within single jadeitic pyroxene prisms and variable jd ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a p t transition zone high pressure overstepping of the phase boundary explains variable modal proportions of ab and jd as well as chemical heterogeneity of the jd elevated p attended metamorphism of na cpx bearing rocks exposed in the pacheco pass area as indicated by the wide spread coexistence of quartz and naalsi 2 o 6 rich pyroxene inferred physical conditions of prograde metamorphism were 150 50 c at 7 8 kbar or more franciscan rocks in the east central diablo range underwent subduction zone metamorphism accompanying paleo pacific lithospheric plate descent during mid cretaceous convergence underplating contraction and sequestering of the tectonostratigraphic assembly at depths of 25 30 km during underflow followed by gradual rise took place under conditions of refrigeration due to protracted descent of the oceanic slab paleogene diapiric uplift and extension followed indicating more complete decoupling of the buoyant trench complex from the downgoing plate finally passage of the east pacific rise triple junction in neogene time initiated the present dextral slip regime in the california coast ranges

raw material quality and prepared core technologies in northeast asia the design and assembly of lithic toolkits is mediated by a number of factors including the abundance and quality of raw materials available in general low raw material abundance and high raw material quality are thought to lead to formal tool designs whereas high raw material abundance and low raw material quality lead to informal designs low raw material quality is seen as the overriding factor producing informal tool designs even where low raw material abundance would favour formal designs in north china the predominance of simple flake and core technologies based on relatively poor quality raw materials and the near absence of sophisticated prepared core technologies seems to corroborate the importance of raw material quality in toolkit design recent studies of a late pleistocene lithic assemblage from the lower grotto at tsagaan agui cave mongolia suggest however that raw material quality is not an absolute constraint on the development of sophisticated core reduction strategies levallois like and other prepared core forms based on large flake blanks are conspicuous in the lower grotto assemblage despite the poor quality of raw material available at the site contrary to expectations these core forms appear to have developed directly in response to poor raw material quality the implication is that raw material quality alone cannot explain the apparent technological simplicity of the north chinese middle and upper paleolithic

structural development of a high pressure collisional accretionary wedge the samana complex northern hispaniola abstract the samana metamorphic complex exposes a segment of a high pressure collisional accretionary wedge built during caribbean island arc north america continental margin convergence combined detailed mapping structural and metamorphic analysis and 40ar 39ar geochronology show that the deformation can be divided into five main events early subduction related d1 deformation and high p low t m1 metamorphism under lawsonite blueschist 325 425 c 12 18 kbar rincon marbles and santa barbara schists lower structural nappes and eclogite facies conditions 425 450 c 18 20 kbar punta balandra upper structural nappe was followed by m2 decompression and cooling in the blueschist facies conditions during d2 folding thrusting and nappe stacking 40ar 39ar plateau ages and t t p t estimations revealed late eocene to earliest miocene retrograde m2 metamorphism in the different nappes for a consistent d2 top to the ene tectonic transport which suggests a general northeastward progradation of deformation the d3 event substantially modified the nappe stack and produced open to tight folds with amplitudes up to kilometer scale and the d4 ductile to brittle normal shear zones and faults and related subhorizontal folding record a late extensional deformation which also affects the whole nappe pile non penetrative d3 and d4 fabrics indicate m3 cooling in and under the m3 greenschist facies conditions from the miocene to the present the nappe pile was cut and laterally displaced by a d5 sinistral strike slip and reverse fault system associated with the septentrional fault zone

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

effects of distance from stone source on landscape scale variation in oldowan artifact assemblages in the paleo olduvai basin tanzania abstract ongoing excavations at olduvai gorge tanzania reveal tremendous variability among stone artifact assemblages across the plio pleistocene olduvai lake basin during bed i and lower bed ii times theoretically stone artifact traces of oldowan hominin land use are determined by the distribution of larger mammal carcasses and arboreal refuge from predation as well as proximity to stone material sources we provide an initial evaluation of these theoretical expectations focusing on the effects of distance from stone source on four parameters of oldowan artifact assemblages from the lowermost bed ii eastern olduvai basin quartzite artifact assemblages show expected distance from material source trends relative to their straight line proximity to three of four points along naibor soit a local quartzitic inselberg the weight density and proportionate weight of quartzite assemblages decrease with increasing distance from naibor soit as do the size of flaked pieces and the proportion of these that are minimally reduced the results demonstrate predicted behavioral patterning in broad scale traces of hominin land use but proximity to naibor soit explains the majority of variability in only the weight proportion of stone artifacts made on quartzite ecological factors appear to have also influenced the landscape distribution of oldowan hominin activity traces

mechanical characterization of raw material quality and its implication for early upper palaeolithic moravia abstract raw material mechanical tests were conducted to answer the question whether differences in raw material procurement among early upper palaeolithic populations in moravia czech republic may have been driven by different mechanical properties of those materials characterization of mechanical properties of erratic flints and krumlovsk les i type chert show that the relatively finer grained erratic flints preferred by local aurignacian populations are more easily and probably also predictably knapped at higher speeds such as reached with soft antler wood percussors whereas cherts of krumlovsk les i type exploited by both szeletian and aurignacian populations are more resistant to fracture propagation this implies the suitability of the former material for fine blade and bladelet production and of the latter to projectile e g szeletian leaf points manufacture and possibly explains the export of leaf points from szeletian areas the krumlov forest to bohunician and aurignacian sites within moravia exploitation of erratic flints was easier as regards aurignacian and probably entire upper palaeolithic knapping technology certain tasks however were better met with other raw materials thus reflecting the relativity of chipped stone raw material quality perception in the palaeolithic

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

shelfbreak current over the canadian beaufort sea continental slope wind driven events in january 2005 the shelfbreak current over the beaufort sea continental slope is known to be one of the most energetic features of the beaufort sea hydrography in january 2005 three oceanographic moorings deployed over the canadian eastern beaufort sea continental slope simultaneously recorded two consecutive shelfbreak current events with along slope eastward bottom intensified flow up to 120 cm s 1 both events were generated by the local wind forcing associated with two pacific born cyclones passing north of the beaufort sea continental slope toward the canadian archipelago over the mooring array the associated westerly wind exceeded 15 m s 1 these two cyclones generated storm surges along the beaufort sea coast with sea surface height ssh rising up to 1 4 m following the two westerly wind maxima we suggest that the westerly along slope wind generated a surface ekman onshore transport the associated ssh increase over the shelf produced a cross slope pressure gradient that drove an along slope eastward geostrophic current in the same direction as the wind this wind driven barotropic flow was superimposed on the background baroclinic bottom intensified shelfbreak current that consequently amplified summer fall satellite altimetry data for 1992 2013 show that the ssh gradient in the southeastern beaufort sea is enhanced over the upper continental slope in response to frequent storm surge events because the local wind forcing and or sea ice drift could not explain the reduction of sea ice concentration over the beaufort sea continental slope in january 2005 we speculate that wind driven sea level fluctuations may impact the sea ice cover in winter

biostratigraphic constraints on formation and timing of accretion in a subduction complex an example from the franciscan complex of northern california the determination of the total age coverage of pelagic bedded chert is particularly important in studies of ancient accretionary complexes because the time span represents the minimum travel time of an oceanic plate before accretion at an island arc or continental margin the yolla bolly terrane of the franciscan complex consists of rare metabasalt overlain by bedded radiolarian chert which in turn is overlain by metagraywacke and subordinate slaty mudstone and conglomerate the bedded chert ranges in age from aaleian early middle jurassic to tithonian latest jurassic a lithologic gradation from pelagic bedded chert through hemipelagic siliceous mudstone occurred during the tithonian and thus marks the arrival of the oceanic sediments at the continental margin the tithonian age also agrees with previously reported tithonian to valanginian early cretaceous fossils in the overlying terrigenous metagraywacke both chert and metagraywacke are intruded by gabbroic sills suggesting off ridge volcanism

accreted kula plate fragment at 94ma in the yokonami melange shimanto belt shikoku japan abstract the cretaceous shimanto belt that is distributed along the pacific side of southwest japan contains one of the best records of accretionary processes and history of oceanic plate subduction in the circum pacific region yokonami melange shimanto belt shikoku sw japan is composed of pillowed basalt that erupted in the equatorial region about 200 m thick bedded radiolarian chert sequence that contains radiolarians with age determinations spanning ca 135 ma upper valanginian to 93 9 ma cenomanian younging southward with repetition and alternated sandstone and mudstone although the chronology of the bedded chert sequence is well studied using the radiolarian the chronology of the clastic units was not reported sufficiently we performed a detailed geologic mapping investigation of rock outcrops along the goshiki no hama coast the yokonami melange we have determined u pb ages of zircon grains extracted from tuffs and sandstones to determine the depositional age of the turbidite and hemipelagic units the youngest depositional age of these clastic sediments is interpreted to be the arrival time at the trench we reconstructed the ocean plate stratigraphy with our new chronological data which includes mid oceanic basalt bedded chert hemipelagic sediments and trench turbidite deposits we recognize that the turbidite sequence has been repeated by layer parallel thrust in the study area we have delineated the travel trajectory of the oceanic lithosphere based on 1 the relative plate motion model of the paleo pacific ocean 2 the arrival time of the subducted lithosphere at the trench 94 32 ma and 3 paleomagnetic measurement of the basalt and turbidite the trajectory suggests that the morb in the goshiki no hama coast yokonami melange erupted at the kula pacific ridge in the paleo pacific ocean traveled northward and eventually arrived at the continental margin of east asia at 94 32 ma

lithic raw material quality of middle pleistocene artefacts from the han river danjiangkou reservoir region central china abstract the east asian acheulean is often considered distinct from african and european acheulean technologies as large cutting tools lcts are thicker and heavier on average this distinction is generally true but exceptions occur particularly in thickness as more chinese sites are reported it has been suggested that the raw materials available in east asia have played a part in determining this dissimilarity extensive raw material studies on east asian technologies have not yet been done to determine if this is indeed the case it is thus important to investigate the impact of raw materials quality on the east asian acheulean this paper focuses on raw material attributes for lcts from the danjiangkou reservoir region central china drr with specific focus on middle pleistocene artefacts from terrace 3 of the han river extensive experimental studies were done to determine raw material attributes and how those attributes affect the knapping process in the creation of lcts a set of variables was recorded for the experimental tools and compared with a sample of archaeological lcts the variables recorded included the impurity encounter rates of various raw materials their weathering states grain sizes and shapes the results indicated that hominids occupying the drr exhibited selective behaviours for the raw materials they used to make lcts despite not having a wide variety of materials from which to select it is also evident that they showed adaptability and made strategic decisions with regards to the selected raw materials for manufacture of lcts to meet their needs

a middle permian ophiolitic melange belt in the solonker suture zone western inner mongolia china implications for the evolution of the paleo asian ocean

a useful measure of the desirability of different raw materials for retouch within and between assemblages the raw material retouch index rmri abstract due to their predictable fracture and the quality of the edge that can be produced on them fine grained raw materials such as chert have long been favoured by prehistoric toolmakers for the production of relatively small retouched artefacts it is common to find far higher frequencies of retouch in such materials than is the case in all other rock types in order to quantify the desirability of different materials for retouch i propose a simple index relating the frequency of retouched tools in a raw material to the frequency of that material in the entire assemblage using the raw material retouch index rmri one can compare relative raw material desirability both within and between assemblages

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

lower paleolithic bipolar reduction and hominin selection of quartz at olduvai gorge tanzania what s the connection abstract numerous researchers have noted that at lower paleolithic sites in east africa hominins largely exploited quartz toolstone with the bipolar reduction technique the choice to pursue bipolar knapping on quartz is often attributed to raw material constraints thus at some east african lower paleolithic sites the abundance of bipolar knapping may have simply constituted a response to the local absence of lithic resources other than small quartz pebbles however at olduvai gorge tanzania where a variety of other stone raw materials were available the hominin use of bipolar reduction is still predominately tied to quartz while quartz raw material constraints may explain the use of bipolar reduction on quartz at olduvai what they do not explain is the virtual absence of bipolar reduction on non quartz toolstones thus we ask here two separate but related questions 1 why did hominins use bipolar on quartz and 2 why did hominins avoid bipolar on non quartz to begin to understand this tight technology toolstone connection we formulated two simple hypotheses which we tested via experimental stone tool replication 1 quartz bipolar reduction produces flakes that possess superior functional characteristics to those produced via non quartz bipolar reduction 2 bipolar reduction is more expedient on quartz than on non quartz toolstones our experimental tests indicated that while quartz and basalt bipolar reduction yield flakes with similar attributes bipolar reduction on quartz is significantly more expedient than it is on basalt as such the close technology toolstone association between quartz and bipolar can be explained by constraints and advantages of both quartz and basalt alike bipolar reduction is already widely acknowledged to be an expedient technology requiring little to no skill by applying bipolar reduction exclusively to quartz hominins at olduvai appear to have only enhanced this reduction strategy s features overall our experimental results are consistent with the widely held notion that lower paleolithic hominins recognized the differences in the physical properties of different types of raw material and that it is the interplay between raw material and reduction strategy that governed hominin association between bipolar reduction and quartz at olduvai

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated