depth of penetration of lubricant fluids and water in queenston shale of southern ontario queenston shale is a rock formation located in southern ontario in canada that exhibits time dependent deformation behaviour induced by water penetration microtunnelling is an efficient technique that can be used to construct pipelines and tunnels in queenston shale in this technique lubricant fluids such as bentonite solution and polymer solution are utilized which can have a significant impact on the time dependant deformation i e swelling of queenston shale within their depth of influence a test procedure was developed in this research to investigate the depth of penetration of lubricant fluids and water in queenston shale these fluids were applied under pressure similar to that used in the microtunnelling process on queeenston shale specimens the depth of penetration of these fluids into the test specimens was monitored with time it was concluded that the penetration depth varied for the various fluids considered water had the deepest penetration into queenston shale based on the obs

late palaeogene quaternary geology of halmahera eastern indonesia initiation of a volcanic island arc the late palaeogene quaternary stratigraphy of halmahera is described and new formation names are proposed based on recent field investigations of the ne and central part of the island this stratigraphic information provides new insights into the neogene history of halmahera and the development of the present island arc the late palaeogene and younger rocks rest unconformably on an ophiolitic basement complex which formed part of a late cretaceous early tertiary fore arc after volcanic arc activity ceased in the eocene the former fore arc terrane was uplifted and deeply eroded in the late palaeogene some of the late palaeogene early miocene river valleys are currently being re excavated by the present rivers slow subsidence began in the mid late oligocene and by the end of the miocene all eastern halmahera was the site of shallow water carbonate deposition there is no evidence for arc volcanism in central halmahera at this time and the reported oligo miocene volcanism in nearby regions is interpreted as volcanism related to the sorong fault system the miocene shallow water region subsided rapidly in the early pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites with increasing amounts of calc alkaline volcanic debris from a pliocene volcanic arc built on the western arms of halmahera probably on the eroded early tertiary arc this phase of rapid subsidence in the pliocene back arc region resulted from the initiation of subduction of the molucca sea lithosphere eastwards beneath halmahera differential subsidence on nw se and ne sw sets of faults in the region immediately behind the active arc led to the formation of deep sediment filled basins adjacent to the eastern arms a major deformation event in the pleistocene resulted in folding and local thrusting at the junction between eastern and western halmahera and volcanism ceased in the pliocene arc the third halmahera arc the quaternary arc currently active in the northern part of the islands began activity within the last 1 ma and is built upon the deformed and partly eroded pliocene arc the pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward dipping molucca sea plate with adjacent plates either with a fragment of the australian continent in the sorong fault zone and or with the philippine sea plate beneath northern halmahera

collapse of the luzon volcanic arc during onset of arc continent collision evidence from a miocene pliocene unconformity eastern taiwan a regional miocene pliocene unconformity in the coastal range of eastern taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc continent collision the unconformity separates precollisional volcanic and volcaniclastic rocks of the miocene luzon arc and forearc basin from an overlying thick sequence of syncollisional pliocene pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of taiwan the duration of time represented by the unconformity averages 1 2 m y and locally ranges up to 4 m y the basal unit of the younger syncollisional sequence consists of bathyal mega slump folded mudstone pebbly mudstone and thin bedded turbidites sharp stratigraphic juxtaposition of these deep marine deposits directly above shallow marine limestone and epiclastic rocks upper part of the older arc sequence requires extremely rapid subsidence during initiation of the collisional basin the coastal range unconformity is interpreted to represent a period of latest miocene nondeposition and or erosion in the luzon arc that was related to initial subduction of the eurasian continental margin along the manila trench this period was followed in pliocene time by very rapid subsidence or collapse of the arc platform to 1 2 km water depth there are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin 1 extension rifting and crustal thinning in the luzon arc massif to form a series of small rift or pull apart subbasins or 2 flexural subsidence of oceanic lithosphere in a foreland style basin due to crustal loading in the expanding accretionary wedge of proto taiwan although data are insufficient to provide a unique solution the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east which may record eastward migration of an outer flexural bulge notable differences between the unconformity and paleogeography of the early collisional basin and analogous present day basins in the offshore region to the south suggest that arc continent collision and basin development in eastern taiwan have experienced secular change since late pliocene time

on terrane analysis many recent papers on displaced crustal masses are organized around terranes given only geographic names and are bewildering for other than local experts self explanatory descriptive or genetic terms should be incorporated in most designations plate tectonic interpretations of aggregated terranes that incorporate awareness of how modern arc systems vary and evolve contain implicit and testable predictions but too many interpretations are based instead on invalid assumptions as actualistic models are applied to analyses of orogenic belts much more mobilistic interpretations than those generally now visualized will probably emerge an example is made of the carpathian region

metamorphic evolution of the luning fencemaker fold thrust belt nevada illite crystallinity metamorphic petrology and 40ar 39ar geochronology abstract the luning fencemaker fold thrust belt lftb of nevada accommodated substantial mesozoic shortening in the back arc region of the u s cordillera metamorphic grade in this belt has not been studied and timing of deformation is not well defined in this study we present new data on metamorphic grade and age of deformation in the northern lftb we focus on the d1 phase of deformation which accommodated most shortening in the belt metamorphic grade was established by petrographic study and microtextural analysis of deformed metasedimentary rocks bulk x ray diffraction and illite crystallinity analyses of slates and phyllites and microprobe analyses of metamorphic mica these techniques indicate that metamorphism during d1 deformation occurred under anchizone to epizone grade conditions at temperatures probably near 300 c timing of d1 deformation was investigated by 40ar 39ar whole rock dating of slates and phyllites age spectra indicate that d1 deformation occurred before 142 144 ma late

origin and tectonic significance of the aldrich mountains serpentinite matrix melange northeastern oregon the permian triassic aldrich mountains serpentinite melange consists of a matrix of massive and foliated serpentinite lizardite chrysotile that encases a variety of metamorphic igneous and sedimentary block types serpentinite matrix foliation trends ne sw dips steeply 75 and is parallel to long dimensions of blocks at block matrix contacts the foliation traces block margins the volumetrically most abundant block types are greenstone chert and metagabbro subordinate block types are amphibolite tectonites epiclastic sediments breccia conglomerate sandstone and argillite silicic volcanic rocks silicic plutonic rocks and ultramafic rocks three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix 1 serpentinization and related hydrous alteration confined to matrix ultramafic blocks and margins of mafic blocks 2 lower to middle greenschist and prehnite actinolite facies metamorphism characterized by static non penetrative fabrics of some blocks and 3 dynamothermal epidote amphibolite to upper amphibolite lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures 550 780 c and low to moderate pressures 3 5 kbar most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix the melange is interpreted to have formed within a permian to late triassic sediment starved tectonically active forearc fragments of abyssal oceanic crust and early permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc highly mobile serpentinite generated beneath the forearc was protrusively emplaced into the tectonized forearc and mixed with the blocks forming the melange melange generation probably began in the late early permian and ceased by the late triassic

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

collision driven slab breakoff magmatism in northwestern anatolia turkey abstract postcollisional eocene magmatism in northwestern anatolia produced two e w trending linear belts of plutons along and north of the izmir ankara erzincan suture zone iaesz whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution the suture zone granitoids szgs in the southern belt have ages around 54 48 ma are intrusive into blueschist rocks of the iaesz and are composed of diorite quartz diorite granodiorite and syenite the marmara granitoids mgs in the northern belt are slightly younger 48 35 ma intrusive into the paleozoic mesozoic crystalline basement rocks of the sakarya continent and composed of monzogranite granite and granodiorite both szgs and mgs have moderately to highly evolved medium to high k calc alkaline compositions and are predominantly metaluminous i type granitoids nd sr isotope systematics documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepacka

an expanded view of jurassic orogenesis in the western united states cordillera middle jurassic pre nevadan regional metamorphism and thrust faulting within an active arc environment klamath mountains california basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic ultramafic intrusive complexes delineate a middle jurassic arc terrane within the klamath mountain province of northern california exposures of the supracrustal volcaniclastic rocks are restricted to a single fault bounded terrane but the deeper level intrusive complexes were emplaced into most if not all the pre late jurassic terranes of the klamath mountain region the pre late jurassic terranes thus constitute the basement of the middle jurassic arc u pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus k ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of 177 159 ma the active arc and its basement were imbricated by a compressive deformational event the signature of which included thrust faulting isoclinal folding and regional metamorphism several diverse lines of evidence including k ar dating of metamorphic rocks crosscutting relations of dated intrusive complexes to thrust faults and u pb dating of synmetamorphic intrusive complexes establish a distinctly pre nevadan middle jurassic age ongoing at 169 ma and over by at least 161 ma for this compressive deformational episode outboard and structurally beneath the middle jurassic arc and its basement are several terranes that collectively comprise the western jurassic belt these terranes were deformed and regionally metamorphosed during the late jurassic nevadan orogeny which occurred within the time interval of 157 150 ma as upper jurassic plutons with 150 to 142 m y old zircon ages have contact aureoles that overprint the nevadan fabric and the 157 m y old rogue formation was deformed in the nevadan event the middle and late jurassic compressive deformational events were thus distinct and separated by as much as 15 20 m y the relation between middle and late jurassic magmatism and deformation suggests that the klamath mountain province records the evolution of a considerably long lived arc system that evolved above an east dipping subduction zone in addition we suggest that this are system may represent an oceanic continuation of the long recognized early mesozoic arc terrane of the western u s cordillera

comparison of terrane accretion in modern southeast asia and the mesozoic north american cordillera the western part of the north american cordillera has been divided into numerous suspect terranes some of these terranes have traveled hundreds to thousands of kilometres between the time of their formation and their final incorporation into the accretionary belt of the cordillera most of the terranes appear to be relics of island arcs oceanic plateaus and islands continental margin fragments and complex accretionary terranes the latter including melange belts ophiolite fragments and thrust faulted forearc provinces because the terranes have complex histories and diverse stratigraphies it has been difficult to envisage an actualistic setting for the mesozoic cordillera here we propose an analogy with the tectonic setting of the indo pacific region from the tonga trench on the east to eastern indonesia on the west this region comprises several distinct island arcs several large oceanic plateaus numerous accretionary terranes melange zones ophiolite fragments and a variety of continental fragments all moving toward the central collision zone like the cordillera this part of the indo pacific region is undergoing oblique convergence driven by relatively high rates of movement between the oceanic plates and the collision zone also analogous to the cordillera is the formation of a foreland fold and thrust belt developing between the collision zone and the australian continent and a zone of basement rooted foreland folds suggestive of an early laramide style of deformation

relationships between kinematics of arc continent collision and kinematics of thrust faults folds shear zones and foliations in the nevadan orogen northern sierra nevada california abstract the mesozoic nevadan orogeny in the northern sierra nevada metamorphic belt california may be attributed to arc continent collision stratigraphic data and macroscopic cross cutting relations suggest successive accretion of two arcs along an active continental margin the younger accretion event involved the early jurassic slate creek terrane which is a 3 5 km thick pseudostratigraphic arc fragment the slate creek thrust an isoclinally folded fault with a subhorizontal median surface carries the slate creek terrane at least 40 km eastward continentward over the pre existing continental margin terrane amalgam no rock units can be correlated across the slate creek thrust which is thus interpreted as an arc continent suture in addition to the slate creek thrust the nevadan orogen includes a major east vergent imbricate thrust set east of and beneath the slate creek thrust and steep west vergent reverse shear zones macroscopic upright folds and steep foliations that overprint and cut the east vergent structures these data suggest a model for the relationships between the kinematics of arc collision and nevadan orogenic structures the slate creek arc terrane accreted by westward partial subduction of the continental margin along the east vergent slate creek thrust the continental margin was imbricated along an east vergent thrust set the structurally higher inactive slate creek thrust suture was deformed by steep west vergent shear zones folds and foliations which may have accommodated shortening of the east vergent thrust sheet this deformation occurred within an active continental margin arc that probably initiated by subduction flip after collision of the slate creek arc this kinematic model is consistent with the structural geometry and chronology of the nevadan orogen while qualitatively maintaining lithosphere scale strain compatibility this model has implications for problems related to emplacement of large crystalline thrust sheets displacements beneath and at the margins of shortened crustal segments and interaction of oppositely verging structures the nevadan orogen is a slate belt and the structural plate tectonic model presented for the nevadan orogeny may be testable in slate belts of other orogens

intracontinental subduction and hinged unroofing along the salmon river suture zone west central idaho the salmon river suture zone in west central idaho juxtaposes volcanic arc rocks of the wallowa terrane directly against cratonic north america detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure temperature p t histories from lowest to highest structural level the wallowa terrane shows only subgreenschist metamorphism the rapid river plate rrp records unroofing and cooling from 8 kbar and 550 c to 6 kbar and 475 500 c and the pollock mountain plate pmp shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9 11 kbar and 600 625 c ar ar hornblende ages combined with the p t data suggest that currently exposed levels of the rrp and pmp were juxtaposed against one another at 15 20 km depth at or prior to 118 ma indicating that 10 20 km of uplift and hence also the onset of collision related metamorphism occurred before 118 ma correlation of the metamorphic and age data with geometric constraints from the initial sr 0 706 line and the dimensions of the rrp and pmp permit construction of large scale retrodeformable sections of the west side of the suture from late jurassic through late cretaceous time the abrupt nature of the sr 0 706 line implies that the arc continent boundary extends vertically through most of the crust which requires sharp downwarping of the arc lithosphere in order to account for the pmp metamorphic data narrow zoned overgrowths on pmp garnets record this burial event and require initially rapid 3 km m y uplift rates in order to be preserved we suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc allowing buoyant rise of fragments of thickened crust detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which hanging walls rise this mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high pressure metamorphic terrains subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone the consumption of lithosphere during 40 km of shortening between two crustal blocks implies that the salmon river suture is the trace of an intracontinental subduction zone burial and collision apparently began before about 130 ma and thus any precollision strike slip faulting or tectonic escape of intervening terranes was likely accomplished in jurassic and earliest cretaceous time

stratigraphic record of triassic jurassic collisional tectonics in the blue mountains province northeastern oregon sedimentary and volcanic rocks in the blue mountains province bmp of northeastern oregon preserve a well studied record of triassic jurassic magmatism basin evolution and terrane accretion terranes of the bmp represent two magmatic arcs wallowa and olds ferry terranes an intervening oceanic subduction and accretionary complex baker terrane and a complex thick succession of sedimentary rocks commonly known as the izee terrane we divide volcanic and sedimentary rocks into two regionally correlative unconformity bounded megasequences 1 ms 1 late triassic to early jurassic deposits that change up section from 1a older volcanic and volcaniclastic deposits of the wallowa and olds ferry arcs to 1b marine turbidites shale and argillite with chert clast conglomerate and olistostromes derived from the emergent baker terrane and 2 ms 2 early to late jurassic marine deposits that overlap older rocks and structures and record 20 to 40 m y of deep crustal subsidence in a large marine basin many of the known stratigraphic relationships in the blue mountains cannot be explained using the existing model for a middle triassic to late jurassic west facing non collisional volcanic arc and forearc basin we propose a new tectonic model for the bmp based on prior studies and comparison to modern analogues which includes 1 middle triassic magmatism in the wallowa and olds ferry arcs during subduction and progressive closure of an ocean basin 2 late triassic collision between facing accretionary wedges of the wallowa and olds ferry arcs and growth of marine basins on both sides of the emergent baker terrane thrust belt 3 early to middle jurassic terrane continent collision which resulted in closure of a wide back arc basin crustal thickening and loading in nevada and growth of a large marine collisional basin in the bmp and 4 late jurassic thrusting regional shortening and final accretion of the basin and underlying terranes to western north america this analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove triassic jurassic crustal thickening mountain building and basin development in the western north american cordillera

evolution of a late mesozoic back arc fold and thrust belt northwestern great basin u s a abstract the luning fencemaker fold and thrust belt was active from the middle or late jurassic through the early cretaceous and involves rocks of the mesozoic marine province of the northwestern great basin rocks of the marine province were deposited in a back arc basin bound on the west by the sierran arc they are underlain by simatic crust formed either in a paleozoic marginal basin formed on the west coast of north america or in an exotic oceanic arc accreted to north america in the permo triassic deposition in the marine province was localized by the underlying simatic crust which later controlled the locus of back arc thrusting to the south in areas of the back arc region overlying continental crust mesozoic rocks are terrestrial volcanic and volcanogenic deposits rocks of this area are not disrupted by thrusts of significant magnitude until the southwestern end of the sevier thrust belt is encountered during contraction the luning fencemaker thrust belt underwent several hundred kilometers of nw se shortening which was not developed in the sierra nevada to the west the arc and back arc regions were separated by a regionally extensive left lateral fault system that was active during back arc thrusting the marine province was partially coupled with the sierra nevada during transpressional motion on the left lateral fault resulting in imbrication and emplacement of the luning fencemaker thrust belt in the continental region to the south decoupling was more complete and only minor thrusting was associated with the southerly displacement of the sierran arc farther south where the paleozoic miogeoclinal hingeline was encountered conditions apparently changed and the sevier thrust belt developed timing relations suggests that the andean type margin of the western united states had paired back arc thrust belts luning fencemaker and sevier for much of the late mesozoic the belts were separated by a region that underwent relatively minor internal shortening the intervening region probably was decoupled at depth and the thrust belts may have shared a decollement development of the luning fencemaker thrust belt and the associated left lateral fault system extended over a period of 40 50 m y and is best explained as a product of plate convergence the nw se shortening axis for the thrust belt and the inferred left slip on the strike slip fault are not compatible with the generally accepted right oblique subduction proposed for the sierran arc since the early triassic they suggest an interval of left oblique convergence for the middle to late jurassic and early cretaceous regionally developed structures indicate a return to right oblique convergence in the late early cretaceous

emplacement and deformation history of the western margin of the idaho batholith near mccall idaho influence of a major terrane boundary cretaceous plutons of the western margin of the idaho batholith were emplaced along and to the west of the major terrane boundary separating middle proterozoic and paleozoic continental rocks from mostly mesozoic accreted oceanic arc terranes of the blue mountain province this boundary is marked by a change in the lithology of pendants and inclusions within the batholith plutons form two newly named complexes of igneous and metamorphosed igneous rocks the hazard creek complex emplaced west of the boundary between the oceanic arc and the continental margin consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons the little goose creek complex which intruded the boundary between the oceanic arc and the continental margin is primarily porphyritic granodiorite to granite orthogneiss a preliminary u pb age of 111 ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic arc continent boundary the plutonic rocks were deformed both during and after emplacement in response to east west compressive stresses cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary the major deformation of the hazard creek complex occurred during its emplacement the dominant fabric in the little goose creek complex is due to subsolidus ductile deformation the localization of two deformation events along the pre existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long lasting crustal flaw

punctuated thrust deformation in the context of doubly vergent thrust wedges implications for the localization of uplift and exhumation understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing punctuated thrust activity is widely documented from field studies in a range of mountain belts the potential for climatic forcing of thrust activity has been a subject of recent debate here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges the model predicts that rates of surface uplift frontal accretion and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates this time scale ranges from 0 1 to 5 m y for various settings and should be calculated before external forcings such as climate are invoked

late triassic arc related potassic igneous rocks in the north american cordillera igneous rocks of late triassic age are widespread in the cordillera of western north america and except in wrangellia consist of subduction related plutonic and volcanic suites many of these including those in the stikinia quesnellia rattlesnake creek and jackson terrenes and in southern california are clinopyroxene rich and belong to high potassium and shoshonitic rock series features that are generally absent from older and younger igneous rocks in the same terranes the late triassic subduction related rocks are exposed in two discontinuous belts that lie east and west of the cache creek terrane in canada and correlative melange terranes farther south stratigraphic and structural data suggest that these belts were spatially separate magmatic arcs in late triassic time tectonic implications of this analysis include an explanation of middle jurassic cordilleran deformation as the result of collision of the western with the eastern belt absence of late triassic links between stikinia and quesnellia disassociation of stikinia with terranes in northwestern nevada and tentative correlation of the wallowa seven devils terrane with stikinia rather than wrangellia

structure and evolution of the magnitogorsk forearc basin identifying upper crustal processes during arc continent collision in the southern urals the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the intraoceanic magnitogorsk volcanic arc and its forearc basin sediments accreted to the east european craton during the devonian the magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc and by comparing its surface geological features with those in active arc continent collision settings it is possible to identify upper crustal processes that were active in the southern urals the arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized the late lower to middle devonian aktau formation represents a remnant of the intraoceanic to collisional forearc basin to the irendyk volcanic front whereas the middle devonian to lower carboniferous ulutau koltubanian and zilair formations were deposited in a suture forearc basin to the east magnitogorsk volcanic front it was not until the late devonian that these two basins were joined structural mapping combined with reflection seismic profiling shows these basins to be affected by open nonlinear volcanic basement cored synsedimentary folds the karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin the forearc region is affected by minor thrusting that involves the volcanic basement although it is not clear if these thrusts reactivate preexisting trench parallel faults synsedimentary deformation slumping and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the late devonian when the full thickness of the continental crust is interpreted as having arrived at the subduction zone

geodynamic processes of taiwan arc continent collision and comparison with analogs in timor papua new guinea urals and corsica abstract the taiwan arc continent collision involves four geodynamic processes intra oceanic subduction initial arc continent collision advanced arc continent collision and arc collapse subduction these processes now occur simultaneously in 19 24 30 n but have operated sequentially southward since the late miocene although the geological and geophysical features appear to change progressively from north to south across the island they are distinct within individual tectonic regimes using the present scenario as a basis for comparison it is suggested that timor is at the initial arc continent collision stage while papua new guinea and urals proceeded to the advanced arc continent collision stage however corsica represents an even more advanced stage than in taiwan where a previously accreted arc has collapsed and been totally removed

paleomagnetism and mesozoic tectonics of the seven devils volcanic arc in northeastern oregon upper triassic volcanogenic rocks of the seven devils group and huntington formation of brooks 1979b in northeastern oregon record two episodes of magnetization the older magnetization which predates late jurassic deformation of the volcanic arc terrane comprising the seven devils group was probably acquired in late triassic time the inclinations of both normal and reversed polarity magnetizations indicate that the triassic rocks originated approximately 18 from the paleoequator the results are consistent with paleomagnetic data from similar triassic rock assemblages on vancouver island and in south central alaska we interpret these assemblages as displaced fragments of wrangellia a once coherent late paleozoic volcanic arc and early mesozoic volcanic plateau that was torn apart and carried north by plate motion along the continental margin we interpret the younger magnetization which fails the fold test as a widespread thermochemical overprint caused by late jurassic and early cretaceous plutonism this magnetization yields a paleomagnetic pole at 60 n 295 e 95 7 and is similar to the pole obtained from plutons that intrude the seven devils group the angular difference between the pole from the plutons and the late jurassic early cretaceous reference pole for stable north america suggests that the seven devils volcanic arc has undergone a large clockwise rotation 66 21 the rotation may reflect the last stages of the arc s accretion to the continental margin or later large scale block movements in the pacific northwest

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

papua new guinea highlands palaeomagnetic constraints on terrane tectonics abstract oblique convergence since the early cenozoic between the northward moving australian plate westward moving pacific plate and almost stationary eurasian plate has created a world ranking tectonic zone in the eastern indonesia new guinea southwest pacific region tonga sulawesi megashear that is notorious for its complex mix of tectonic styles and terrane juxtapositions unlike an ancient analog the mesozoic cenozoic cordillera of north america palaeomagnetic constraints on terrane motions in the zone are few to improve the framework of quantitative control on such motions and therefore our understanding of the development of the zone results of a palaeomagnetic study in the highlands region of papua new guinea png in the southern part of the new guinea orogen are reported the study yields new insights into terrane tectonics along the australian craton s active northern margin and confirms the complexity of block rotations to be expected at the local scale in tectonically intricate zones the study is based on more than 500 samples 21 localities collected from an interior and an exterior zone of new guinea s central cordillera the two zones are separated by the tahin and stolle lagaip kaugel fault zones and collectively represent the para autochthonous northern margin of the australian craton samples from the interior zone which in the study area comprises a cratonic spur of uncertain probably displaced origin come from triassic to miocene sediments and subordinate volcanics of the kubor anticline jimi terrane and yaveufa syncline 16 localities in the central and eastern highlands samples from the exterior zone which represent a basement involved pliocene foreland fold and thrust belt come from middle eocene to middle miocene carbonates and clastics five localities in the southern highlands of the papuan fold belt results permit us to constrain the tectonic evolution of the two zones palaeomagnetically using mainly thermal demagnetization techniques three main magnetic components have been identified in the collection 1 a recent field overprint of both normal and reverse polarity 2 a pervasive overprint of mainly normal polarity that originated during extensive middle to late miocene intrusive activity in the central cordillera and 3 a primary component which has been identified in only 7 of the 21 localities 5 of 11 stratigraphic units represented in the collection all components show patterns of rotation that are consistent within the zones but differ between them in the interior zone central and eastern highlands large scale counterclockwise rotations of between 30 and 100 have been established throughout the kubor anticline and jimi terrane with some clockwise rotation present in the southern part of the yaveufa syncline in contrast in the mendi area of the exterior zone southern highlands clockwise rotations of between 30 and 50 can be recognized these contrasting rotation patterns across the tahin and stolle lagaip kaugel fault zones indicate decoupling of the two tectonic zones probably along basement involved faults the clockwise rotations in the southern highlands of the papuan fold belt are to be expected from its structural grain and are probably governed by regional basement faults and transverse lineaments in contrast the pattern of counterclockwise rotations in the kubor anticline jimi terrane cratonic spur of the central and eastern highlands was unexpected the pattern is interpreted to result from non rigid rotation of continental terranes as they were transported westward across the northeastern margin of the australian craton this margin became reorganised after the middle miocene when the steadily northward advancing australian craton impinged into the westward moving pacific plate buffer plate system transpressional reorganisation under the influence of the sinistral tonga sulawesi megashear became enhanced with mio pliocene docking and subsequent southward overthrusting of the finisterre terrane onto the northeastern margin of the australian craton

upper plate deformation induced by subduction of a volcanic arc the snellius plateau molucca sea indonesia and mindanao philippines abstract the northern molucca sea shows the incipient subduction of a composite oceanic and arc volcanic block the snellius halmahera block shb multi beam reflectivity seismic and gravity data obtained during the modec marine survey showed that the shb disappears beneath the accretionary wedge and the outer ridge of the sangihe arc to the north in mindanao island ongoing convergence generated shortening of the forearc basin and the backthrusting of the shb meanwhile a classical system of paired subduction philippine trench and strike slip fault philippine fault was installed the transition from lithospheric subduction to crustal overthrusting is located where the philippine trench sensu stricto begins and also coincides with the off shore extension of the philippine fault we observe a reversal of the thrusts from an eastward vergence in the molucca sea to a westward vergence in mindanao island this reversal takes place at the latitude where the forearc area emerges by uplift and the downgoing crust shb deepens resulting in a strong gravity low centered above the accretionary wedge the philippine fault initiated in a place where the crust was sliced off by a transfer zone which marked the northern termination of the molucca sea and drags northward a sliver of the previously accreted shb the northward drifting of this sliver created an extension which however cannot account for the gravity low we propose that the shortening and the uplift of the upper plate were induced by the buoyancy of the subducted unit shb and triggered the thrust reversal

87 sr 86 sr and 18 o 16 o isotopic systematics and geochemistry of granitoid plutons across a steeply dipping boundary between contrasting lithospheric blocks in western idaho a spatially abrupt geochemical boundary is preserved within four plutonic complexes along the western margin of the cretaceous idaho batholith near mccall idaho these intrusives ranging in composition from tonalite to granite were emplaced across a regional boundary between accreted oceanic arc terranes and the continental margin and their isotopic major element and trace element geochemistry provide detailed information about this change in crustal characteristics at depth indicating that the boundary is nearly vertical and extends deep into the lithosphere the hazard creek complex emplaced west of the transition in wall rock lithology has initial 87sr 86sr ri less than 0 7045 and 18o greater than 7 5 indicating little or no continental crust in its source region however elevated 18o requires some incorporation of rocks formed or altered at the earth s surface a large shift in ri and 18o is observed across the 5 8 km wide little goose creek complex which was emplaced across the wall rock boundary this is interpreted as mixing between 1 a basaltic or andesitic magma with low k2o and high na2o al2o3 and sr similar to that forming the hazard creek complex and 2 materials similar to precambrian sedimentary sedimentary rocks with low sr high 18o 15 and high ri 0 83 at 100 ma the payette river complex emplaced east of the wall rock boundary exhibits at least one additional component with low 18o 6 moderate ri 0 708 and mafic composition this component is inferred to be old basaltic material in the lower crust or upper mantle similar to that inferred to be a minor part of the peninsular ranges batholith in se california silver et al 1979 hill et al 1986 the easternmost complex in the idaho transect is made up of granites that may contain a component of granitic cratonal basement the entire west to east geochemical transition from oceanic arc magmas to cratonal magmas takes place over a lateral distance of less than 20 km although the zone of transitional protolith dominated by metasedimentary rocks is unusually narrow and may have been in part tectonically removed the striking geochemical similarities between this traverse and several other transects across much broader areas of nevada and california suggest that the craton itself was not rifted apart but that juxtaposition of the accreted oceanic arc terranes occurred along the preexisting craton margin the data confirm that the isotopic geochemistry of granitoid plutons can be used as a probe of deep lithospheric character and that major lateral variations in the lithosphere on the order of one to two kilometers in width can be recognized in favorable circumstances

similarity structure in the convective boundary layer of a lake natural convection influences a variety of geophysical phenomena including daily mixing cycles in the atmosphere and in surface waters the seasonal overturn of lakes and global climate variability conceptual models of convection in water are primitive and based on very limited data particularly at small dissipative scales here i present an analysis of microscale data from the convective boundary layer of a lake indicating a complex depth dependence in the rate at which temperature fluctuations are dissipated suitably scaled this vertical structure is remarkably similar to that found in the atmospheric convective boundary layer1 2 stirred by rising thermals of buoyant air heated at the earth s surface this comparison consistent with recent documentation of similarity in the more uniform vertical structure of energy dissipation3 5 suggests another unifying link in the structure and dynamics of convection in boundary layers of lakes oceans and the atmosphere6

timing and structural expression of the nevadan orogeny sierra nevada california the nevadan orogeny was a very short lived event in the late jurassic that involved the deformation of a great variety of rock types and paleozoic and mesozoic terranes throughout the extent of the sierra nevada the nevadan structures show great variation in style but relatively constant orientations these relations can be explained by considering the prior histories of the various terranes slaty cleavages and tight folds are the characteristic main phase structures in the western belt of jurassic island arc volcanic rocks and flysch type sedimentary rocks a strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar jurassic rocks that were deformed and metamorphosed at greater depths probably during underthrusting of the western belt beneath the central belt the central belt of paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main phase structures from weak spaced to crenulation cleavages in the south where polyphase deformed rocks formed a structural basement to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north where most of the paleozoic basement rocks lack penetrative pre nevadan fabrics eastward directed thrust faulting apparently was important only in the northern part of the range where main phase deformation was most intense the eastern belt of jurassic and triassic magmatic arc volcanic and sedimentary rocks defines the core of a major synclinorium and the rocks contain penetrative slaty cleavages and asymmetric tight to isoclinal folds a late phase of nevadan structures consisting of northeast trending cleavages and minor folds also shows a marked variation in style from relatively intensely developed in the north to very weakly developed in the south the regional extent and geometry of the nevadan structures indicate that the nevadan orogeny involved underthrusting of island arc rocks on the west and significant crustal shortening in the central and eastern belts these features suggest that the nevadan orogeny resulted from the collision of the island arc western belt with an andean type arc eastern belt situated at the western edge of north america

cretaceous sedimentation and tectonics tyaughton methow basin southwestern british columbia the mesozoic tyaughton methow basin straddles the fraser yalakom pasayten straight creek fypsc strike slip fault zone between six tectono stratigraphic terranes in southwestern british columbia data from hauterivian cenomanian basin fill provide constraints for reconstruction of fault displacement and paleogeography the early cretaceous eastern margin of the basin was a region of uplifted jurassic plutons and active intermediate volcanism detritus shed southwestward from that margin was deposited as the marine jackass mountain group albian inner to mid fan facies of the jackass mountain group can be correlated across the yalakom fault suggesting 150 25 km of post albian dextral offset deposits of the jackass mountain group overlap the major strike slip zone fypsc if that zone represents the eastern boundary of the tectono stratigraphic terrane wrangellia then accretion of wrangellia to terranes to the east occurred before late early cretaceous time the western margin of the basin first b

anomalous early jurassic deformation in the western u s cordillera new data from the black rock desert of northwest nevada indicate that paleozoic and early mesozoic magmatic arc rocks in this area were affected by regional shortening deformation and metamorphism in the early jurassic ongoing at 201 ma this is 30 50 m y prior to the time at which significant jurassic shortening deformation is thought to have begun in the western u s cordillera and is further anomalous because the early jurassic time frame in most other arc assemblages of the cordillera is characterized by accumulation of strata and neutral to extensional tectonism these conflicting relations support the following conclusions 1 the black rock desert is probably separated from other arc assemblages to the west and south in the klamath mountains and sierra nevada by a younger strike slip fault boundary that juxtaposes arc elements with different early jurassic structural histories 2 tectonic conditions varied along the early jurassic plate margin such that shortening occurred to the north but not to the south an aspect of the mesozoic evolution of the cordillera that has not been previously appreciated at present regional early jurassic deformation is documented only in the black rock desert but it may have impacted a much larger area in this part of the cordillera based on relations in the blue mountain province of oregon and the luning fencemaker fold and thrust belt of central nevada