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

charriage crustal anatexie et decrochements ductiles dans les maures orientales var france au cours de l orogenese varisque la cinematique et la signification tectonique des deformations enregistrees par les gneiss catazonaux des maures orientales socle varisque de provence france sont precisees les evenements successifs peuvent etre correles avec les evolutions metamorphiques et magmatiques l histoire orogenique des maures orientales apparait composee des evenements suivants 1 metamorphisme a haute pression uniquement conserve dans des reliques d eclogite et de quartzite a silicates calciques 2 metamorphisme catazonal accompagne d anatexie contemporain d un transport subhorizontal vers le nne d echelle crustale cet episode est responsable de la dilaceration des unites lithologiques de leur laminage et de leur mylonitisation 3 intrusion d une premiere generation de granitoides anatectiques 4 deuxieme episode tectonique dans des conditions epi a mesozonales responsable du deplacement vers le nord des maures orientales par rapport aux maures occidentales le long d un decrochement senestre de 4 km d epaisseur faille de ramatuelle plan de la tour dans lequel les granitoides d anatexie precoces sont mylonitises en dehors de la zone de faille cet episode est marque par des plis ouverts droits ou deverses vers l ouest orientes n s parallelement a la direction de transport 5 reactivation cataclastique moderee de la faille de ramatuelle plan de la tour avec un mouvement dextre localement accompagne dans la partie est de la region par des chevauchements mineurs vers le sud 6 intrusion d une deuxieme generation de granites anatectiques vers 320 ma

the role of strain heating in the evolution of megathrusts on the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws a model is developed for the interplay of thermal and mechanical processes along megathrust systems the model is based on the observation that thrust systems involving large portions of the lithosphere have a two part history during the early history of movement dry lower crustal upper mantle rocks are deformed in a shear zone where because of the nonlinear effect of temperature dependent diffusion strain heating rates are directly controlled by the initial thermal state that is heating is insignificant if initial temperatures are high but where temperatures are low large temperature increases are possible as a result strain heating acts as a thermal buffer such that upon arrival of upper crustal rocks at depths of 20 km or more the temperatures along the thrust would be controlled largely by the rheology of hanging wall rocks rather than the initial thermal state moreover the relatively high melting temperatures for dry lower crustal upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high as thrusting progresses this early history is successively eliminated as wet upper crustal rocks are emplaced beneath the preheated hanging wall the granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall either by hydrous fluxing of the hot hanging wall or by melting in the highest grade footwall rocks rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by tectonic pileup beneath the thrust thorough testing of the model will require thorough assessment of the time dependent problem however one dimensional steady state models together with energy considerations indicate that the general model is allowable the concept of thermal buffering by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology hence they represent real world systems where a thermal and mechanical system are intimately linked this rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

development of inverted metamorphic isograds in the western metamorphic belt juneau alaska an inverted metamorphic gradient is preserved in the western metamorphic belt near juneau alaska the western metamorphic belt is part of the coast plutonic metamorphic complex of western canada and southeastern alaska that developed as a result of tectonic overlap and or compressional thickening of crustal rocks during collision of the alexander and stikine terranes detailed mapping of pelitic single mineral isograds systematic changes in mineral assemblages and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km peak temperatures of metamorphism increase progressively from about 530 c for the garnet zone to about 705 c for the upper kyanite biotite zone silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9 11 kbar the metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt although truncation of the isograds north of juneau indicates that the sill intrusion continued after the isograds were established our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma garnet rim compositions and widespread growth of chlorite suggest partial re equilibration of the schists under pressures of 5 6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill the combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high t high p metamorphism associated with the collision of the alexander and stikine terranes was a long lived event extending from about 98 ma to about 67 ma

on the meaning of peak temperature profiles in inverted metamorphic sequences inverted metamorphic sequences ims are common features of main thrust systems on earth they exhibit an upwards continuous increase in peak temperature conditions and thereby constitute evidence of the close relationship between the thermal field evolution and tectonic processes heat advection and shear heating are known to allow the formation of such metamorphic signatures heat diffusion also plays an important role in temperature distribution on both sides of the thrust other advection processes such as erosion or accretion may also cause a local peak temperature inversion each one of these processes therefore affects the thermal field around the thrust however despite the crucial importance of all these processes for the interpretation of the inverted peak temperature signatures their respective influences have never been quantified and compared all together to address this issue we propose an innovative coupled approach i we use two dimensional numerical models that simulate various thrust systems allowing for a wide diversity of setups to illustrate this study we focus on intracontinental thrust systems for which all processes listed are likely to play a key role in the thermal evolution we perform a parametric study including kinematic settings i e convergence erosion and accretion thermal properties mechanical strength and heat sources ii dimensionless numbers based on parameters are used to quantify the relative contributions of each process to the thermal budget evolution hence the three thermal processes i e heat diffusion heat advection and shear heating are compared with each other via three dimensionless combinations of the peclet and brinkman numbers rdif radv and rpro respectively erosion and accretion are compared separately based on a fourth dimensionless number rea iii we analytically examine the inverted peak temperature recorded along profiles that are perpendicular to the thrust zone defined in our numerical experiments each peak temperature profile presenting an inversion can then be characterized by a function of approximation involving six meaningful parameters the location ff and width ff of the maximum peak temperature inversion the characteristic peak temperature tcte and gradient glb beneath the inversion zone and the inversion related contrasts in the peak temperature t and gradient g this coupled approach linking numerical modelling and analytical treatment allows to quantitatively interpret ims in terms of the processes involved the application of our method to intracontinental thrust systems demonstrates that shear heating and erosion support significant inversions but that the relative contributions of each process have meaningful consequences our results reveal that competition between shear heating and heat diffusion on the one hand and between erosion and accretion on the other hand have a high impact in particular the variability in the rock s mechanical strength strongly influences the features of peak temperature inversions consequently none of these processes can be ignored our results highlight the major importance of the rheology of rocks in the thermal evolution of shear zones finally our methodology is not only restricted to the analysis of numerical data but also constitutes a way of broad interest to analyse peak temperature signatures around any shear zone

dynamic recrystallization and associated exsolution in perthites evidence of deep crustal thrusting mylonitic gneisses from parry sound ontario are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks 800 c 900 1100 mpa northward onto an amphibolite facies terrane we have examined by optical and electron microscopy microstructures in perthites from these mylonites that preserve the deformation recrystallization cycle which operated during high ductility deformation of the crystals the exsolution and recrystallization microstructures are intimately associated exsolution in the perthites occurred in two stages a calcic plagioclase oligoclase exsolved first and subsequently coarsened and deformed during the single crystal ductility phase of shear zone deformation a progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains the principal mode of recrystallization was subgrain rotation with a component of grain growth that followed the formation of mobile high angle boundaries misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters the second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions the perthites thus record deformation from crustal depths of the order of 34 42 km continental megathrusts such as the parry sound shear zone provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

metamorphic evolution of the central metamorphic belt klamath province california an inverted metamorphic gradient beneath the trinity peridotite metabasalts and metasedimentary rocks of the devonian central metamorphic belt comprise the lower plate of the east dipping trinity thrust system in the klamath province an inverted metamorphic gradient is preserved in the central metamorphic belt metamorphic conditions decrease from amphibolite facies adjacent to the trinity thrust through albite epidote amphibolite facies to upper greenschist facies at the base of the central metamorphic belt mineral chemistry mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 50 c at the trinity thrust to 500 50 c at the base of the central metamorphic belt under pressures of 5 3 kbar synmetamorphic ab qtz veins up to 1 m thick increase in abundance towards the trinity thrust infiltration of h2o co2 fluids derived from prograde devolatilization reactions in the central metamorphic belt caused extensive hydration and metasomatism of the trinity peridotite the hanging wall block of the trinity thrust zone geological relationships and the preserved inverted metamorphic gradient suggest that the central metamorphic belt formed in an east dipping devonian subduction zone in an oceanic environment the central metamorphic belt appears to represent a discrete slice of accreted oceanic crust several km thick rather than progressively accreted material metamorphic pressures recorded by the central metamorphic belt are intermediate between the 2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7 10 kbar preserved in subduction related inverted metamorphic gradients the lack of blueschist facies mineral assemblages in the central metamorphic belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth

metamorphism and near trench plutonism during initial accretion of the cretaceous alaskan forearc cenozoic uplift of the eastern aleutian forearc has exposed the tectonic join between the jurassic crystalline hanging wall of the arc and the oceanic assemblages of the footwall accreted during cretaceous and tertiary subduction the border ranges fault system brfs the early stages of accretion along the brfs in the knik river area were characterized by high temperature ductile thrusting and metamorphism at low to moderate p t p from 300 700 mpa tmax about 600 c peak metamorphic conditions at the currently exposed levels can be explained by heat carried to the area by tonalite and trondhjemite plutons generation of tonalite and trondhjemite melts within the cretaceous forearc however requires a heat source within or near the brfs ridge subduction cannot be ruled out as the heat source however simple thermal models presented herein indicate that the temperatures required for anatexis within the forearc can be explained by transient heating on the fault by shear heating of dry lithologies during the earliest stages of thrust motion water released from subducted oceanic crust would result in generation of partial melts in the hanging wall if shear heating is significant or if the initial thermal gradient within the hanging wall is high further accretion of oceanic sediment would quench the system and temperatures rapidly evolve to the high p t conditions of normal subduction zones thermochronologic data from this study show that the duration of high t low p t conditions was short of the order of about 15 m y or less exhumed forearcs here and elsewhere commonly record a period of low to moderate p t conditions during subduction initiation in which shear heating likely played an important role

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

contrast of metamorphic and structural histories across the work channel lineament coast plutonic complex british columbia a prominent topographic lineament along the western margin of the coast plutonic complex of southeast alaska and adjacent parts of british columbia separates terranes of contrasting metamorphic histories where it enters british columbia near prince rupert as the work channel lineament the western terrane preserves a regional medium high pressure metamorphic facies series increasing from chlorite grade in the west to kyanite muscovite migmatite near the lineament the metamorphic structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the jurassic this terrane was uplifted and cooled by cretaceous time 85 ma east of work channel lineament high temperature relatively low pressure assemblages are overprinted onto high pressure assemblages the low pressure metamorphic assemblages developed in the eocene during rapid uplift 2 mm yr of rocks which initially may have been metamorphosed at the same time and along the same pt gradient as those of the western terrane structures along the work channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane because the metamorphic discontinuity across the lineament is 3 5 kbar at about 700 the lineament in the prince rupert area marks a major crustal break

petrologic structural and age relations of serpentinite amphibolite and blueschist in the shuksan suite of the iron mountain gee point area north cascades washington part of the shuksan blueschist terrane near iron mountain and gee point north cascades washington has associated serpentinite amphibolite barroisite schist blueschist rare eclogite and blackwall type metasomatic rock field petrographic and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source the serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism isotopic dating gives metamorphic ages of 148 5 to 164 6 m y for the amphibolite and barroisite schist and 129 5 m y for nearby regional shuksan blueschists the origin of the serpentinite amphibolite blueschist assemblage is interpreted to be the result of sequential events in a subduction zone as subduction began oceanic crustal materials underwent high temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist this hypothesis may be applicable to the origin of similar rock associations in the franciscan terrane and other orogenic belts

major role of shear heating in intracontinental inverted metamorphism inference from a thermo kinematic parametric study abstract inverted metamorphism corresponds to the stacking of high temperature metamorphic units structurally on top of lower temperature units and is commonly observed along main thrusts in major orogens yet in spite of many existing models the origin and preservation of the metamorphic inversion in intracontinental collision belts are still debated in this study we use a crustal scale 2d thermo kinematic model in order to investigate the key parameters controlling the inversion of the geothermal gradient at crustal scale our results confirm that the kinematic framework strongly impacts the thermal evolution around the thrust erosion velocity and thermal conductivity of rocks are two parameters that control the spatial location of the thermal perturbation and the intensity of inversion respectively however even in extreme kinematic configurations i e convergence velocities 3 cm yr 1 and relatively high thrust dip angles 30 the thermal inversion is fleeting and thrust temperatures cannot reach the high temperature peak values characteristic of natural occurrences 600 c if shear heating is not taken into account conversion of mechanical energy into heat represents a main contribution to the thermal budget along main crustal shear zones it leads to high temperature conditions in the thrust zone and our results attest that it is the only process that allows the preservation through time of an intense thermal inversion our quantification shows that shear heating is much more efficient than other processes such as accretion and surface denudation and is compatible with the observations of inverted metamorphism in the himalayan or variscan belts for example this comparison with natural occurrences suggests that the formation and preservation of intracontinental inverted metamorphism require shear zone viscosity values of the order of 10 20 10 21 pa s for convergence velocities between 1 and 3 cm yr 1

thermal and structural evolution of the intracratonic arltunga nappe complex central australia the thermal and structural evolution of a paleozoic thrust system in central australia the arltunga nappe complex has been examined through field mapping petrographic microstructural and chemical analyses k ar and 40ar 39ar thermochronology and multi diffusion domain time temperature modeling of k feldspar 40ar 39ar data the arltunga nappe complex contains three distinct tectonic units bounded by regional faults and shear zones that have undergone contrasting deformational and metamorphic histories the cooling history of the three tectonic units has been constrained by k ar and 40ar 39ar isotopic age data from k feldspars micas and amphiboles results are consistent with a piggyback history of crustal imbrication spanning over 100 m y with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland implications for the intracratonic paleozoic tectonics of central australia are 1 uplift within the structurally highest tectonic unit was initiated before 400 ma 2 continued piggyback imbrication footwall failure and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325 300 ma 3 the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust resulting in 85 km of crustal shortening

deep crustal deformation textures along megathrusts from newfoundland and ontario implications for microstructural preservation strain rates and strength of the lithosphere lithospheric scale thrusts from the west newfoundland ophiolite belt white hills peridotite shear zone and the south western grenville province parry sound shear zone involve rocks of lower crustal and or upper mantle origin that exhibit intense crystal plastic deformation of plagioclase k feldspar orthopyroxene and clinopyroxene minerals that are commonly viewed as representative of low ductility phases the occurrence of this extreme deformation in shear zones that exhibit similar lower crustal syntectonic p t conditions suggests a phenomenological link between the megathrust environment and both the generation and subsequent preservation of the observed deformation microstructures an empirical homologous parameter is constructed in an attempt to characterize conditions for similar behaviour among different minerals and to explore the feasibility of refining a threshold recovery preservation condition within the megathrusts studied this parameter predicts at the estimated syntectonic temper

origin and age of the border ranges fault of southern alaska and its bearing on the late mesozoic tectonic evolution of alaska local geologic relationships in the western chugach mountains together with regional considerations suggest that in the early cretaceous a subduction zone was formed along the southern edge of the wrangellia peninsular alexander composite terrane talkeetna superterrane of csejtey et al 1982 of southern alaska the effects of this event include 1 a shattering of older pre cretaceous crystalline rocks along a complex fault system 2 emplacement of a tectonic melange beneath the shattered crystalline rocks and 3 a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks this event created a regionally mappable structural contact between broken crystalline rocks and the melange the border ranges fault of mackevett and plafker 1974 regional stratigraphy as well as radiometric ages of rocks predating and postdating the border ranges fault appear to bracket the age of the border ranges fault and its associated deformational effects to the interval between about 135 and 120 ma further regional tectonostratigraphic associations suggest that the deformation along the border ranges fault represents the nucleation of a north and or east dipping subduction zone beneath the talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the gravina nutzotin belt of southeast alaska mismatches in the distribution of different elements of this early cretaceous arc trench system are probably a result of late cretaceous or early tertiary strike slip motion but early cretaceous ridge trench interaction suggested by the occurrence of early cretaceous near trench plutons may have played a role as well the talkeetna superterrane is generally thought to be an exotic block that collided with the cordillera in the middle cretaceous coney et al 1980 the age data commonly cited as evidence for a middle cretaceous age for the collision are however misleading a model is proposed here in which the initiation of convergence along the trailing edge of the talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle cretaceous deformation recognized along the leading edge of the talkeetna superterrane csejtey et al 1982 records destruction of a syncollisional flysch basin during the final phase of the collision

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

transient discontinuities revisited pseudotachylyte plastic instability and the influence of low pore fluid pressure on deformation processes in the mid crust abstract occurrences of pseudotachylyte cyclically introduced as melts into the outer hebrides thrust are demonstrably synkinematic with crystal plastic mylonites classical interpretations of these rock types as respectively products of pressure dependent frictional melting and thermally activated intracrystalline deformation create a paradox in that these processes are to a large extent mutually exclusive detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy the ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 c and stresses of 140 210 mpa on which were imposed much higher transient stresses in response to heterogeneous non uniform flow the latter conditions are sufficient for the development of plastic instabilities in feldspar rich crust if pore fluid pressures are sufficiently low the latter is consistent with the absence of hydration during exhumation observed in the rocks under study low pore fluid pressure during thrust exhumation of deep crust enables activation of high strength ductile processes dominated by dislocation glide which may be a prerequisite to instability whereas other studies have demonstrated the possible occurrence of such melt generating instabilities it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure temperature conditions grain size reduction to produce ultramylonites dominated by grainsize sensitive flow is achieved by both deformation induced dynamic recrystallization and crystallization of instability generated melts

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

a model of granitic melt formation by frictional heating on shear planes abstract based on geological data derived from structural investigations in the field a hypothesis of anatexite formation along shear planes by frictional heating is proposed the hypothesis involves a physico mathematical model which was tested for different mechanisms and by numerical calculations with different parameters the formation of frictionites melt rocks of small volumes m3 requires a depth of deformation in the crust of more than 5 km and a particular tectonic regime and should be possible on the assumption of a stick slip process in association with a trigger mechanism of source activation the thermodynamical model employed needs further verification from the mechanical and material points of view

deformation and emplacement of the lizard ophiolite complex sw england based on evidence from the basal unit the basal unit of the lizard ophiolite complex preserves evidence of a protracted deformation and accretion history the earliest recognizable events record the construction of ocean crust and the generation of the lizard ophiolite complex in a slow spreading ridge axis environment the initial stages of deformation involved the development of a sub horizontal crustal scale shear zone and its progressive deformation by ductile thrusting and folding subsequent deformation progressed with the out of sequence stacking of ocean lithosphere in which detachment initiated at mid crustal levels culminated in mantle decoupling as thrust slices were derived from progressively deeper crustal levels the progressive emplacement of a coherent slice of ocean lithosphere over previously accreted units and the generation of a dynamothermal aureole are consistent with models suggesting rapid overthrusting of young ocean lithosphere evidence from the dynamothermal aureole indicates that syn emplacement extensional collapse of the complex occurred in response to thrust loading foreland directed offsliding and lateral thrusting mark the final stages in the collapse of the complex

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

thermal structure of active thrust belts in order to study the thermal structure of active thrust belts we have developed a numerical model of conductive heat transfer between thrust sheets during deformation our finite difference approach alternates small instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms in each step conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging wall rocks decrease while temperatures in foot wall rocks increase over distances of up to 10 km from the thrust surface when the effects of internal heat production are also calculated heating of foot wall rocks exceeds cooling of hanging wall rocks rocks located between two thrusts may experience a complicated temperature time path of early heating followed by cooling these models help to explain the rapid metamorphism of rocks in the taconian thrust belt in the northern appalachians of new england soon after deposition of the youngest sediments

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

microseismicity tectonics and seismic potential in the western himalayan segment nw himalaya india abstract the tectonics and seismic potential of the western himalayan segment 30 33 n 76 80 e of the nw himalayan india region have been determined in this study 423 earthquakes were located in the nw himalaya between 2004 and 2013 using more than 4495 p and 4453 s differential travel times to determine the moment tensors for 8 mw 4 0 of these earthquakes using their broadband regional waveforms the geometry of the main himalayan thrust mht plane which varies along the strike of the himalaya in flat and ramp segments with a dip ranging between 2 5 to 4 to 19 below the himalayan frontal thrust hft in the south to the south tibetan detachment std in the north has also been deduced in this study two crustal ramps were reported in this study with a depth variance below the main central thrust mct and to the south tibetan detachment std between 12 to 22 km and 28 to 40 km depth respectively the estimated earthquake potential prevailing in the western himalayan seismic gap lying between the epicentral zone of the 1905 kangra earthquake and the 1975 kinnaur earthquake reveals that the total amount of energy released since the last great event is only a fraction 3 5 of the accommodated energy i e 1 1e 28 dyne cm yr this suggests that if an earthquake hits this nw himalayan segment in the future its magnitude might be around mw 8 0

on the inter relationship between grain size sensitive creep and dynamic recrystallization of olivine abstract the recent theoretical results of twiss 1976 and goetze 1978 suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain boundary sliding or nonlinear coble creep mechanisms this would result following the production of a fine grain size by dynamic recrystallization for the high temperature experimental work performed by karato et al 1982 dry single crystals of olivine were almost totally recrystallized during creep and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates however the activation energy for creep determined by karato et al 1982 was far higher than that predicted by the models of twiss 1976 or goetze 1978 although the conditions required for operation of at least the model of twiss 1976 apparently were satisfied the data for the highly recrystallized specimens from the higher stress lower temperature experiments of zeuch and green 1979 and zeuch 1980 are in good agreement with the results of karato et al 1982 these latter experiments were conducted under conditions where either the model of twiss 1976 or goetze 1978 should have been applicable i tentatively conclude that although fine grain sizes were produced in the experiments of karato et al 1982 zeuch and green 1979 and zeuch 1980 by dynamic recrystallization there is no evidence for a transition to grain boundary diffusional creep mechanisms at high or low stresses despite the predictions of twiss 1976 or goetze 1978 instead deformation is dominated by dislocation movement with recovery by dynamic recrystallization

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

heat sources of cretaceous metamorphism in the eastern alps a discussion abstract the alpine orogenic cycle is characterised by a series of early metamorphic events that occurred in the cretaceous the eoalpine events a collection of data on peak metamorphic conditions pt paths and on relationships between structure and metamorphism for these events in the eastern alps shows that 1 eoalpine metamorphism occurred largely simultaneously with pervasive deformation 2 the eoalpine thermal peak was reached near the pressure peak 3 lower grade structurally higher rocks reached their metamorphic peak earlier than higher grade structurally lower rocks and there is a wide spread of metamorphic ages 4 low grade metamorphic conditions are probably more perturbed than high grade conditions and 5 there is a conspicuous regional distribution of metamorphic grade with amphibolite and eclogite facies metamorphism occurring predominantly in two regions located symmetrically west and east of the tauern window possible heating mechanisms are discussed that can satisfy these observations and a review of previous thermal models is provided it is shown that models which rely on conductive heat transfer cannot account for all of these observations in order to assess other heat sources the effects of shear heating are evaluated here with simple model calculations and a straightforward comparison of length and time scales of dissipative heat production the results suggest that shear heat episodically released during the stacking of the austroalpine nappe pile may be a significant contributor to the observed thermal perturbation

evidence for an inverted metamorphic gradient associated with a precambrian suture southern wyoming an inverted metamorphic gradient associated with the northern mylonite zone of the cheyenne belt a deeply eroded precambrian suture in southern wyoming has been documented within metasedimentary rocks of the early proterozoic snowy pass supergroup metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite garnet and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone a minimum temperature increase of approximately 100 c over a km wide zone is required for this transition parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the snowy pass supergroup field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite grade rocks over rocks of the snowy pass supergroup along the northern mylonite zone mineral equilibria and garnet biotite geothermometry on synkinematic mineral assemblages within the snowy pass metasedimentary rocks indicate deformation at minimum temperatures of 480 c and pressures of 350 400 mpa 3 5 4 0 kbar this implies tectonic burial or upper plate thickness of 13 15 km the narrow character of metamorphic zonation and microtextures within the snowy pass supergroup which indicate late synkine matic growth of garnet and staurolite preclude rotation of pre existing isograds by folding as a mechanism for development of the inverted gradient conductive transport of heat from the upper into the lower plate across the originally low angle thrust is insufficient to produce the necessary temperatures in the lower plate shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated up dip advection of metamorphic fluids is a feasible but unproven mechanism for heat transport the possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present

fabric development in syn tectonic intrusive sheets as a consequence of melt dominated flow and thermal softening of the crust the temperature dependence of ductile deformational processes suggests that the thermal anomaly associated with a cooling syn tectonic intrusion will produce a crustal low viscosity horizon that will concentrate deformation in the vicinity of the pluton during cooling this effect should be most prominent when magma is present but should also occur after solidification of the magma when flow is by solid state processes this paper evaluates this hypothesis for sheet like intrusions using one dimensional time dependent thermal models and accompanying predictions of viscosity vs time history based on experimental flow laws these models predict that for systems with an initial normal geothermal gradient e g extensional systems strike slip systems or thrust systems with low displacement rates the base of a large plutonic sheet cools more slowly than the top and fabric development should be most pronounced on the floor of the pluton in contrast in megathrust systems where displacements are sufficiently rapid to produce temperature inversions cooling is also upside down and fabric development should preferentially occur along the top of plutonic sheets moreover when a pluton is emplaced within a zone of inverted isotherms the heat may be trapped within the inversion a natural system characterized by this history should show a sharp thermal front coincident with the top of a paleo temperature inversion if a pluton is weaker than its country rock it will form a weak horizon in the crust throughout its cooling history and the plutonic sheet should take up the bulk of the deformation throughout its cooling history if a pluton is stronger than its country rock under conditions of solid state flow however models predict a two phase deformational history prior to solidification the deformation should be concentrated in the pluton with the magma representing a weak horizon in the crust but upon crystallization the pluton should become relatively rigid with respect to its country rock and deformation should be concentrated in the wall rocks these two cases predict markedly different structural histories and the history could easily be misinterpreted in the first case static mineral growth might be prominent in the wall rocks and these textures could be misinterpreted as evidence for post tectonic emplacement in the second case the extensive high t deformation of the wall rocks could be readily ascribed to non tectonic emplacement related deformation rather than a consequence of syn tectonic emplacement the model predictions are tested with three examples of well exposed plutonic sheets a neogene extensional system in death valley california and two megathrust systems in southern alaska border ranges fault system and mclaren metamorphic belt structural observations from these systems are broadly consistent with the model predictions and lend support to first order characteristics of the models nonetheless more sophisticated 2 d models of the coupled thermal mechanical system are needed to thoroughly test the hypothesis

thermomechanical evolution of a ductile duplex the thermomechanical evolution of a midcrustal ductile duplex in central australia has been reconstructed through space and time using 40ar 39ar thermochronology flow stress estimates cross sectional restoration of dislocation creep microstructures and microstructural and structural analysis a critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased in dating these micas the mylonitic microstructures have effectively been dated the time temperature history of the duplex has been constrained through multidomain thermal modeling of k feldspar 40ar 39ar data the modeling demonstrates that a temperature gradient existed across the duplex during its formation the concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing this analysis shows that the duplex formed as a forward propagating thrust system accommodating 60 km of convergence between the upper and lower plates of the megathrust with a significant fraction of the displacement occurring after microstructural freezing finally using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex although uncertainties are clearly large the timing of highest estimated strain rates during duplex evolution does indeed correlate with the highest rates of convergence between the upper and lower plates of the megathrust system according to regional cooling history studies and with coeval sedimentation in adjoining molasse basins

sea level change during the last 2500 years in new jersey usa relative sea level changes during the last w2500 years in new jersey usa were reconstructed to test if late holocene sea level was stable or included persistent and distinctive phases of variability foraminifera and bulk sediment d 13 c values were combined to reconstruct paleomarsh elevation with decimeter precision from sequences of salt marsh sediment at two sites using a multi proxy approach the additional paleoenvironmental information provided by bulk sediment d 13 c values reduced vertical uncertainty in the sea level reconstruction by about one third of that estimated from foraminifera alone using a transfer function the history of sediment deposition was constrained by a composite chronology an ageedepth model developed for each core enabled reconstruction of sea level with multi decadal resolution following correction for land level change 1 4 mm yr four successive and sustained multi centennial sea level trends were objectively identified and quantified 95 confidence interval using error in variables change point analysis to account for age and sea level uncertainties from at least 500 bc to 250 ad sea level fell at 0 11 mm yr the second period saw sea level rise at 0 62 mm yr from 250 ad to 733 ad between 733 ad and 1850 ad sea level fell at 0 12 mm yr the reconstructed rate of sea level rise since w1850 ad was 3 1 mm yr and represents the most rapid period of change for at least 2500 years this trend began between 1830 ad and 1873 ad since this change point reconstructed sea level rise is in agreement with regional tide gauge records and exceeds the global average estimate for the 20th century these positive and negative departures from background rates demonstrate that the late holocene sea level was not stable in new jersey

current challenges and future developments in magnetic fabric research abstract magnetic fabrics are used extensively as proxies for mineral fabrics and to correct paleomagnetic data and have been proposed to quantify pore fabrics today s understanding and interpretation of magnetic anisotropy benefits from decades of carefully observing anisotropic mineral and rock properties establishing and testing empirical relationships between magnetic and mineral fabrics and developing models to quantify aspects of anisotropy each advance was preceded by some puzzling observations i e data that could not be explained based on the models available at the time e g oblique or inverse fabrics or lithology dependent anisotropy strain relationships these observations led to numerous experimental and numerical techniques designed to characterize magnetic fabrics and determine their origin despite the successful application of magnetic fabrics in many structural and tectonic problems there are still phenomena and measurements that cannot be explained based on today s magnetic fabric theory with the purpose of fostering future development i will touch on three main areas where i see challenges in magnetic fabric research 1 experimental characterization of magnetic anisotropy including the non uniqueness of reported tensors and non linearity 2 anisotropy modelling and defining the carrier minerals and origin of the magnetic fabric 3 selection of adequate sets of anisotropy tensors to correct paleomagnetic data the description of the challenges presented here will hopefully help define directions for future research inform those that are new to anisotropy and advance our field

upper mantle peridotites in the bay of islands ophiolite newfoundland formation during the final stages of a spreading centre abstract the structure of upper mantle rocks in ophiolites reflects the high temperature flow beneath an oceanic spreading centre locally overprinted by detachment related strain the structure of the mantle section in the ophiolitic table mountain massif bay of islands complex boic newfoundland was investigated in detail it is correlated with other massifs in the boic and the spreading related mantle flow history is reconstructed six structural units can be recognized on the basis of changes in the attitude of the high temperature mineral stretching lineation the relative age relationships between the structural units can partly be established they support the plate thickening model for oceanic lithosphere formation the structural history of the mantle section suggests that early plate driven flow was followed by forced flow underplating of an off axis diapir to the base of the lithosphere and detachment this complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite during this final history the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up in the blow me down bmd massif of the boic all spreading related flow structures are parallel to the ridge suggesting that the structures and the massif were frozen in near the ridge itself the presence of several kilometres of massive dunites in the bmd massif might indicate that in the final stages of spreading melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust lherzolites occurring at the base of the boic are considered as product of non adiabatic mantle upwelling prevalent in the final stages of spreading

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

the structures of two ophiolite massifs bay of islands newfoundland a model for the oceanic crust and upper mantle abstract for a detailed analysis of their main structures accumulation planes foliation planes and lineations the peridotite gabbro units of table mountain and blow me down mountain bay of islands newfoundland have been divided into several structural units each one being characterized by specific microstructures of deformation or accumulation a fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it is postulated for the structure of the lithosphere formed at a spreading center the tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle and the direction of the ridge is inferred from the lineation trend in the peridotites the structures in the cumulates are related to subsidence of the magma chamber floor during spreading at the base of the massifs the tectonic structures in the mafic metamorphic rocks are parallel to those observed the latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites from these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere the direction of thrusting is given by the lineation trend and its sense by the analysis of shear all these data are combined into a paleogeographic scheme for the newfoundland caledonides

the boundary between the ossa morena and southportuguese zones southern iberian massif major suture in the european hercynian chain the ossa morena zone omz and the southportuguese zone spz represent the southernmost domains of the iberian massif european hercynian belt lithostratigraphic petrographic and structural differences exist between both zones the boundary between the omz and the spz is marked by a narrow belt of amphibolites of tholeiitic oceanic affinity bard mmunha et al 1986 and associated serpentinites flaser gabbros pillowed metabasalts castro et al 1987 which are highly sheared along their southern border these beja acebuches ophiolites op cit are exposed for over 200 km in the southernmost part of the omz and the northernmost area of the spz several zones have been distinguished on the basis of structural and lithological criteria the southernmost area of the omz which bounds the acebuches amphibolites is composed of high grade metamorphic rocks and an another important shear zone mark the omz northern boundary the boundary between the omz and the spz is considered to be a suture zone between two continental areas represented by the northern omz and the southern spz sinistral transpression associated with large scale recumbent folds are characteristic of the suture which can be tentatively correlated through the ibero armorican arc with the lizard complex sw england

the role of melt in the uplift and exhumation of orogenic belts abstract in some orogenic belts rapid pulses of uplift occur when melt is present detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks and especially anatectic migmatite can be physically transported from the lower to middle crust on decollements the decollements form in the lower crust because even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms the crust is substantially weakened where melt forms or is intruded melt in the lower crust is present long enough to accomodate strain whereas in the upper crust it crystallizes too fast to be a factor in strain localization following crustal loading due to thrusting exhumation results in the formation of additional melt in the migmatites due to decompression melting support of this sequence of events includes 1 evidence for near isothermal decompression of high temperature metamorphic rocks with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation 2 the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks in either thrust or normal configurations 3 nearly concordant k ar cooling dates for biotite and hornblende suggesting rapid cooling rates and 4 post tectonic intrusions of leucogranite or leucotonalite that may represent the products of decompression melting the crystalline slabs of the high himalaya the compressional surges of the coast orogen of british columbia and alaska the cordillera of southern british columbia and the southern belt of the hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred during the terminal phases of uplift and following uplift these orogenic belts were exhumed during extension while melt was still present

state of stress in the lithosphere recent estimates of the magnitude and orientation of lithospheric stresses are reviewed data come from a variety of sources including earthquake focal mechanisms fault slip data young volcanic dikes and feeders in situ stress measurements at depth lithospheric flexure models microstructure paleopiezometry and consideration of constraints on maximum stress differences from laboratory determined friction and flow laws

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

lithospheric stress and deformation the study of the mechanisms magnitudes and modes of lithospheric stress and deformation occupies a central position in the discipline of tectonophysics it is therefore difficult in assembling a comprehensive review to place limits on what should be included for example one cannot discuss stress and deformation apart from considerations of the rheological models of the lithosphere which link the two and ultimately control the behavior of plates at their margins and in their interiors once admitting to the importance of rheological considerations in discussing stress and deformation one is then compelled to include information relating to the thermal state of the lithosphere since temperature is one of the foremost environmental variables controlling rheology furthermore lithospheric processes cannot be considered in isolation from those affecting the asthenosphere since surely there exists some coupling between the motion of the lithosphere and convection in the earth s interior in fact an understanding of lithospheric stress and deformation can be considered a prerequisite to solving problems in geodynamics because it is through the lithospheric filter that one detects the consequences of thermal and compositional anomalies at greater depth the topic of this report cannot even be cleanly separated from reviews in other disciplines such as seismology and geodesy the magnitude and orientation of lithospheric stress is often indicated by earthquakes while lithospheric strain can be observed geodetically the philosophy i have adopted here is that some overlap with other review papers in this series is unavoidable and probably desirable from the standpoint of emphasizing how recent work n other areas of geophysics has contributed to the advancement in our understanding of the lithosphere s state of stress and mechanisms of deformation i have however resisted the temptation to include extraterrestrial studies presumably covered in the planetary section despite the similar causes for deviatoric stress and deformational mechanisms operating on other terrestrial planets certainly ideas of how silicate materials behave and how lithosphere responds to internal forces benefit from the perspective of omparative planetology finally in defining the scope of this portion of the u s national report to the iugg i did not consciously exclude papers by non u s citizens work done at non u s institutions projects not funded by the u s government or however else foreign might be defined topics in geophysical research cannot be divided along political boundaries however i did not attempt to seek out articles written in languages other than english or published in journals not readily available in the united states and i apologise for this obvious shortcoming

cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows 1 cooling rate effects on the intensity of thermoremanent magnetization trm have been well documented in ceramics in that case laboratory cooling is generally more rapid than the initial cooling leading to an overestimate of the paleofield by 5 10 in thellier type paleointensity experiments the reverse scenario however has never been tested we examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass sbg samples from 13 sites at 17 30 18 30 s on the east pacific rise absolute cooling rates determined by relaxation geospeedometry at five of these sites range from 10 to 330 c min 1 at the glass transition 650 c over the dominant range of remanence blocking temperatures 200 400 c the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the thellier experiment these results suggest that while the cooling rate effect might introduce some within site scatter it should not result in a systematic bias in paleointensity from sbg paleointensity estimates from the 15 sites range from 29 to 59 t with an average standard error of 1 t comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent indistinguishable from present day and the oldest is at least 500 and probably several thousand years old these age estimates are consistent with available radiometric ages and geologic observations

petrochemistry and origin of sub ophiolitic metamorphic and related rocks in the oman mountains the semail ophiolite complex of the northern oman mountains was obducted during cenomanian to campanian upper cretaceous times onto the arabian continental margin during this time span schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice whole rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid ocean ridges a comparison of trace element geochemistries however suggests that the protoliths were not equivalent to the haybi alkaline and transitional tholeiites found in the underlying structural slice or to the semail ophiolite volcanic rocks which lie structurally higher in the sequence it is therefore suggested that the amphibolites may have been derived from jurassic or lower cretaceous oceanic basalts not recognized elsewhere in the oman mountains structurally beneath the amphibolites within the metamorphic sheet greenschist facies metasedimentary rocks including quartzites and marbles are interbanded with metabasite horizons containing actinolite and less commonly blue crossitic amphiboles it is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement and were accreted to the base of the ophiolite during progressive underthrusting