age and origin of the boil mountain ophiolite and chain lakes massif maine implications for the penobscottian orogeny the boil mountain ophiolite complex of west central maine is widely interpreted to mark the lower ordovician penobscottian suture between the dunnage chain lakes and gander terranes the ophiolite consists of two distinct volcanic groups including a lower island arc tholeiite sequence and an upper mid ocean ridge basalt sequence a new middle ordovician 477 1 ma u pb age on a tonalite sill that intrudes the lower volcanic gabbroic sequence is younger than other ca 500 ma age constraints for the ophiolite and represents a maximum age for the ophiolite prior to final emplacement over gneissic rocks of the chain lakes massif a comparison of ages and paleogeography of the boil mountain ophiolite with ophiolitic sequences in quebec and newfoundland indicates that the taconian and penobscottian orogenies and ophiolite obduction occurred simultaneously although on different margins of the iapetus ocean the taconian ophiolite sequences were obducted onto the appalachian margin of laurentia during its col

using remote sensing to quantify aeolian transport and estimate the age of the terminal dune field dunas pampa blanca in southern peru abstract aeolian dunes are widely used to reconstruct paleoenvironmental conditions however terminal dune fields ergs in the coastal desert of southern peru where information regarding quaternary paleoenvironmental conditions is very limited have until now not been used for paleoenvironmental reconstructions and the time depth of their accumulation is unknown here different estimates are derived to constrain the time depth recorded in the dunas pampa blanca a terminal dune field in coastal southern peru dune field age is calculated using the volume of the dunas pampa blanca and i recent aeolian transport rate in migrating transverse dunes feeding the dunas pampa blanca derived from digital processing of sequential landsat and quickbird images and ii limitations posed by recent fluvial sediment supply to the source of aeolian transport the resulting maximum age estimate of 70 8 ka from aeolian transport compares with a minimum age estimate of 4 75 ka from sediment supply however a minimum age estimate of 110 450 ka is deduced from the tectonic and topographic evolution of the region this discrepancy contradicts the hypothesis of late quaternary stability in the peruvian coastal desert and indicates that recent conditions of aeolian sediment supply and transport are not representative for the late quaternary

massive dissociation of gas hydrate during a jurassic oceanic anoxic event in the jurassic period the early toarcian oceanic anoxic event about 183 million years ago is associated with exceptionally high rates of organic carbon burial high palaeotemperatures and significant mass extinction heavy carbon isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon which is isotopically light in contrast examples of light carbon isotope values from marine organic matter of early toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12c versus 13c refs 1 2 5 6 here however we report carbon isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic biospheric and atmospheric carbon reservoirs and that this occurred despite the enhanced burial of organic carbon we propose that as has been suggested for the late palaeocene thermal maximum some 55 million years ago the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental margin sediments

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

formation and emplacement of the josephine ophiolite and the nevadan orogeny in the klamath mountains california oregon u pb zircon and 40ar 39ar geochronology cordilleran ophiolites typically occur as basement for accreted terranes in the klamath mountains ophiolitic terranes were progressively accreted by underthrusting beneath north america the josephine ophiolite is the youngest of the klamath ophiolites and forms the basement for a thick late jurassic flysch sequence galice formation this ophiolite flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the klamath mountains above and a coeval plutonic volcanic arc complex below the outcrop pattern of the roof orleans thrust indicates a minimum displacement of 40 km and geophysical studies suggest 110 km of displacement the basal madstone cabin thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km a rapid sequence of events from ophiolite generation to thrust emplacement has been determined using 40 ar 39 ar and pb u geochronology ophiolite generation occurred at 162 164 ma a thin hemipelagic sequence was deposited from 162 to 157 ma and flysch deposition took place between 157 and 150 ma tight age constraints on thrusting and low grade metamorphism associated with ophiolite emplacement nevadan orogeny are provided by abundant calc alkaline dikes and plutons ranging in age from 151 to 139 ma deformation and metamorphism related to the nevadan orogeny appears to have extended from 155 to 135 ma most of the crustal shortening took place by thrusting constrained to have occurred from 155 to 150 ma on both the roof and basal thrusts minimum rates of displacement are 2 4 and 3 6 mm year for the basal and roof thrusts respectively but correlations with coeval thrusts yield rates of 8 4 and 22 mm year within the range of plate velocities the high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western north america and an active arc from 155 to 150 ma a steep thermal gradient was present in the josephine galice thrust sheet from 155 to 150 ma with amphibolite facies conditions developed along the basal thrust after accretion of the ophiolite by underthrusting the ophiolite and overlying flysch underwent low grade dynamothermal regional metamorphism from 150 to 135 ma the upper age limit is tightly constrained by a 135 ma k feldspar cooling age syntectonic plutons as young as 139 ma and a lower cretaceous angular unconformity very rapid exhumation is indicated by the late valanginian to hauterivian age 130 ma of the unconformably overlying strata suggesting unroofing by extensional tectonics

precise ages and isotopic constraints for the lewis hills bay of islands ophiolite preservation of an arc spreading ridge intersection the lewis hills the southernmost massif of the bay of islands ophiolite exposes a prominent shear zone the origin of this shear zone is pivotal with respect to the interpretation of the tectonic setting of the lewis hills and the genesis of the bay of islands ophiolite different models suggest that the shear zone represents a fracture zone that separates either oceanic lithospheres of similar origin or the lithospheres of an island arc and a marginal basin a contrasting model disputes the fracture zone setting new u pb zircon ages of the lewis hills combined with sm nd systematics are used to resolve this controversy in the western part of the lewis hills western lewis hills magmatic zircons from two trondhjemite bodies yield u pb ages of 500 6 2 0 ma and 503 7 3 2 ma initial nd 502 ma values for trondhjemites and gabbros range from 1 5 to 2 0 both the mineral ages and nd signature are similar to those of the island arc related little port complex which is located to the north of the western lewis hills in contrast apatite from a gabbro of the eastern part of the lewis hills eastern lewis hills yields a concordant u pb age of 485 0 1 0 ma this gabbro has an nd 485 ma of 7 4 the age and isotope signature link the eastern lewis hills with the northern marginal basin related massifs of the bay of islands ophiolite the significant differences of ages and isotope characteristics of the western and eastern lewis hills support the interpretation of the shear zone as the major tectonic boundary between an island arc and a marginal basin this complex tectonic setting may have resulted in a wide spectrum of magma compositions that involved both island arc and marginal basin sources

pulsation tectonics as the control of continental breakup abstract new data from the recent ipod drilling of dsdp site 534 in the blake bahama basin give a definitive age for the spreading center shift involved in the breakup of the north american atlantic margin a basal callovian age 155 m y is determined for the blake spur anomaly marking this spreading center shift that signals the birth of the modern north atlantic ocean this is some 20 m y younger than previously thought one implication of this result is that this spreading center shift starting north atlantic breakup is now of an age which could be assigned to the spreading center shift needed to end the rifting in the gulf of mexico it is suggested that this might be one and the same event another implication of this younger age for the blake spur event is that relatively high spreading rates are now required for the jurassic outer magnetic quiet zone along the north american margin this association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the mid cretaceous and implies a link between the processes controlling plate spreading which are in the upper mantle and the processes controlling the magnetic field which are in the outer core the cycles of fast and slow spreading and quiet and reversing magnetic field have a period of 60 100 m y a theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost d layer of the mantle could explain the correlation plumes carry heat away from the core mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading the pulse of fast spreading in the jurassic apparently caused the breakup of the north atlantic other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe implying that this might be a prevalent process i suggest rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics the term pulsation tectonics is appropriate for this new unifying theory explaining variations in the magnetic field plate motions and paleoenvironment the word pulsation was used by grabau 1934 for a long term rhythm nearly the length of a geological time period in which there were widespread transgressions and regressions of seas over whole continents thus pulsations are just the long term cycles of vail et al 1977 grabau s pulsation theory prophetically invoked large scale tectonic changes in the ocean basins as a cause for eustatic changes in contrast to local continental tectonics although he offered no explanation of the ocean changes the cyclic character of crustal movements called for in the theory of pulsation tectonics revives many ideas of these past advocates of non steady state tectonics and provides modern mechanisms that explain the observations on plate motions and the magnetic fields data which were totally unknown to these past authors

circum pacific mid cretaceous deformation and uplift a superplume related event evidence of short lived episodes of mid cretaceous deformation metamorphism uplift and hiatus in sedimentation is widespread in the lower cretaceous rocks that bordered the cretaceous pacific basin i present a model in which these coeval but widely spaced events are linked to increased ridge push force at subduction zones triggered by large scale upwelling of mantle associated with the mid cretaceous superplume this model can account for mid cretaceous terrane accretion and ophiolite obduction events and can explain brief compressional phases in predominantly extensional orogenies as recognized in alaska british columbia california western south america and west antarctica

long term interactions between intermediate depth and shallow seismicity in north chile subduction zone we document interactions between intermediate depth and interplate seismicity in the north chile subduction zone over a 25 year period 1990 2015 we show that the 2005 mw 7 8 tarapaca slab pull earthquake was followed by 9 years of enhanced deep and shallow seismicity together with the decrease of eastward average gps velocities and associated interplate coupling eventually leading to the 2014 mw 8 1 iquique megathrust earthquake in contrast megathrust ruptures e g mw 8 0 antofagasta in 1995 or mw 8 1 iquique in 2014 initiate several years of silent background seismicity in the studied area both at shallow and intermediate depths the plunge of a rigid slab into a viscous asthenospheric mantle could explain the observed synchronization between deep and shallow seismicity and their long term interactions

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

arc continent collision in papua guinea constraints from fission track thermochronology the papua new guinea png mobile belt adjacent to the finisterre arc was formerly the leading ne corner of the australian plate that converged obliquely with the pacific plate forty new apatite and zircon fission track analyses of mobile belt rocks previously dated by k ar and rb sr analyses constrain neogene time temperature paths and tectonic models the paleogene arc along the southern margin of the caroline plate was juxtaposed against png in the early miocene coeval with locking up of the west dipping solomon subduction zone by the ontong java plateau these events initiated wrenching along the northern png margin and increased westward subduction of the solomon sea plate beneath the eastern margin the mobile belt underwent extension above the downgoing slab with rapid cooling of metamorphic rocks at 17 ma immediately prior to emplacement of the maramuni arc from 17 to 12 ma a change in plate motion at 12 10 ma terminated the arc and caused png caroline plate convergence creating the orogenic belt in new guinea from 12 to 4 ma this resulted in 4 5 km of uplift and 3 km of denudation and cooling of the entire mobile belt in the late miocene propagating westward along the mobile belt at 8 5 ma and southward into the fold belt at 5 4 ma the compression caused thrusting of miocene strata within the mesozoic type section a further change in plate motion at 4 3 ma returned the margin to transpression with local compression along strike slip faults and ongoing collision of the finisterre arc terrane

ophiolite obduction pulses as a proxy indicator of superplume events abstract a major new synthesis of ophiolite geochronology and map of global phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume related tectonism a marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction related minor intrusions this episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field formation of massive carbon rich deposits sea level highstands and formation of large flood basalt provinces all generally considered to be superplume proxy indicators a key to interpreting ophiolite obduction as a further proxy is the mid cretaceous superplume associated ocean margin compressional deformation during this event thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc terrane collision marginal basin shoaling back arc basin closure and ophiolite obduction convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

onset of mid cretaceous volcanism by elevation of the 670 km thermal boundary layer a pulse of ocean crustal production during the mid cretaceous begins at the same time that magnetic reversal frequency drops to zero we suggest that slab penetration into the lower mantle caused a thermal boundary layer normally at 670 km to be rapidly advected upward as the slabs descended through the lower mantle the onset of near surface melting due to the rise of this boundary layer coincides with the slabs9 arrival at d which drove up the heat flux across the core mantle boundary and stabilized the magnetic reversal process the slab penetration event was coincident with the breakup of the gondwana supercontinent and triggered the subsequent formation of oceanic plateaus in the central pacific and indian ocean basins

distal sedimentation in a peripheral foreland basin ordovician black shales and associated flysch of the western taconic foreland new york state and ontario black shale is characteristic of early siliciclastic fill along the distal western flank of the middle to late ordovician taconic peripheral foreland basin this facies referred to as the utica black shale magnafacies includes at least five intervals bounded by unconformities and or condensed beds each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin progressive episodic shifts from carbonate to organic rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment in general silt poor organic rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source this mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east and possibly the south the resultant black shale does not necessarily represent the deepest part of the basin the deepest part of the basin the basin axis contained coarser grained axial submarine fan deposits throughout the later part of the middle ordovician and the first half of the late ordovician the basin axis migrated westward over 100 km yet the orientation of the axis and the basin margins were persistent this basin orientation generally parallel to the orogenic belt along with the large scale basin shape resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland smaller scale structural elements normal fault bounded basement blocks were superposed on large scale taconic foreland basin geometry abrupt shifts from carbonate ramp sedimentation to deeper water organic rich mud deposition may reflect movement along these faults during the orogeny

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

40ar 39ar dating of the emplacement of the muslim bagh ophiolite pakistan abstract the obduction related basal part of the muslim bagh ophiolite baluchistan pakistan and the underlying metamorphic sequence were studied structurally which demonstrated a wsw ene trending thrusting sequence for the initial obduction 40ar 39ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70 7 5 0 and 65 1 4 1 ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction the results indicate that the muslim bagh ophiolite represents a segment of ocean floor from the small and slow spreading ocean branch of the neo tethys located between the indo pakistani and the afro arabian plates the wsw ene oriented obduction of the muslim bagh ophiolite onto the indo pakistani continental margin occurred with the convergence of the neo tethys branch during the late cretaceous and before the tertiary collision of the indo pakistani plate with the eurasian plate

the early toarcian early jurassic and the cenomanian turonian late cretaceous mass extinctions similarities and contrasts abstract the early toarcian eto and cenomanian turonian c t mass extinctions are comparable from a wide range of scales and perspectives from a broad standpoint their similarities include virtually identical extinction intensity at the familial and generic levels widespread basinal facies deposited during sea level highstands an overall greenhouse climate and anoxia as an important causal mechanism the high resolution macroinvertebrate data analyzed here consisting of stratigraphic ranges diversity and abundance point to smaller scale similarities the two events resulted in significant ecological disruption and in both cases the biotic responses were very similar taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and in the eto belemnites in addition epifaunal taxa adapted to low oxygen conditions such as the buchiids posidoniids and inoceramids flourished in the post extinction environment during the survival interval as conditions ameliorated the biota became more diverse and gradually began to resemble pre extinction biotas furthermore the 13c curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions this points to the potential application of 13c as a tool for determining the repopulation modes and timing for other mass extinctions

precise 40ar 39ar ages from the metamorphic sole of the mersin ophiolite southern turkey abstract the mersin ophiolite is located on the southern flank of the e w trending central tauride belt in turkey it is one of the late cretaceous neotethyan oceanic lithospheric remnants the mersin ophiolite formed in a suprasubduction zone tectonic setting in southern turkey at the beginning of the late cretaceous the mersin ophiolite is one of the best examples in turkey in order to study reconstruction of ophiolite emplacement along the alpine himalayan orogenic belt 40ar 39ar incremental heating measurements were performed on seven obduction related subophiolitic metamorphic rocks hornblende separates yielded isochron ages ranging from 96 0 0 7 ma to 91 6 0 3 ma all errors 1 five of the seven hornblende age determinations are indistinguishable at the 95 confidence level and have a weighted mean age of 92 6 0 2 2 ma we interpret these ages as the date of cooling below 500 c intraoceanic thrusting occurred 4 ma soon after formation of oceanic crust the sole was crosscut by microgabbro diabase dikes less than 3 m y later the final obduction onto the tauride platform occurred during the late cretaceous early paleocene our new high precision ages constrain intraoceanic thrusting for a single ophiolite mersin in the tauride belt