cambrian volcanism in nova scotia canada cambrian volcanic rocks in nova scotia occur in small grabens or half grabens in the avalon zone composite terrane as part of a thin sequence of continental to shallow marine cambro ordovician ro

late devonian early carboniferous volcanism in western cape breton island nova scotia the fisset brook formation of western cape breton island and its equivalents at macmillan mountain and the north baddeck river are examples of late devonian and early carboniferous volcanic sequences associated with the formation of post acadian successor basins in the northeastern appalachians they consist of bimodal basalt rhyolite suites interbedded with alluvial fan lacustrine and rare fluvial sediments the earliest volcanic products are rhyolites and somewhat evolved basalts associated with coarse sediments followed by tholeiitic to transitional basalt flows interlayered with lacustrine type deposits geochemical studies on the fisset brook formation indicate extensive remobilization of alkalies ca rb and sr making these elements inappropriate for determining tectonic setting or magmatic affinity use of less mobile elements ti nb y and zr suggests that the basalts are tholeiitic and that the apparent alkalinity of the type section lavas is a result of alteration we conclude that volca

earthquake nests as natural laboratories for the study of intermediate depth earthquake mechanics article the physical mechanism of intermediate depth earthquakes is still under debate in contrast to conditions in the crust and shallow lithosphere at temperatures and pressures corresponding to depths 50 km rocks ought to yield by creep or flow rather than brittle failure some physical process has to enable brittle or brittle like failure for intermediate depth earthquakes the two leading candidates for that are dehydration embrittlement and ther mal shear runaway given their great depth intermediate depth earthquake processes can t be observed direct ly insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity in this paper we focus on three intermediate depth earthquake nests vrancea hindu kush and bucaramanga and what they reveal about the mechanics of intermediate depth earthquakes we review published studies of tectonic setting focal mechanisms precise earthquake locations and earthquake source physics at these locations with an emphasis on the bucaramanga nest all three nests are associated with subducting litho sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity in contrast the bucaramanga nest has a larger b value larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two we also report for the first time finding a signifi cant number of repeating earthquakes some with reverse polarity

geochronology of early to middle paleozoic tectonic development in the southwest newfoundland gander zone southwest newfoundland s gander zone was once considered part of the collided eastern margin of an early paleozoic iapetus ocean distinguished from the rest of the gander zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered devonian in age overlying presumed precambrian basement it was later found that truly devonian strata were confined to the cape ray fault system the local northwestern boundary of the zone u pb zircon isotopic ages of 452 13 51 ma and 458 22 29 ma on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that ordovician and possibly older rocks underlie the gander zone itself a temporal framework for the three main stages of regional deformation corresponding to pre metamorphic recumbent folding d 1 and transpression d 2 and d 3 is presented using u pb and 40 ar 39 ar dates of granitoid and metamorphic rocks d 1 is bracketed as late o

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

dynamic inversion of the 2015 jujuy earthquake and similarity with other intraslab events fondo nacional de desarrollo cientifico y tecnologico fondecyt of comision nacional de investigacion cientifica y tecnologica conicyt chile 11130230 programa riesgo sismico universidad de chile conicyt pcha magisternacional 2014 22140434

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

slab break off abrupt cut or gradual detachment new insights from the vrancea region se carpathians romania slab break off is a plate tectonic process which does not only return lithospheric material into the deeper mantle but also has severe effects on surface movements and seismic hazard slab pull induced seismicity is reduced when the subducted slab decouples from the overlying crust in the vrancea region se carpathians strong earthquakes frequently occur at intermediate depths 70 180 km in a laterally small volume while the crust shows low but distributed seismicity the stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust both features indicate a decoupling between slab and overlying crust either by slab break off or delamination however the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically thus a soft coupling is assumed that is strong enough to enable elongation of the slab but weak enough to inhibit quasi static stress transfer to the overlying crust

stress release in exhumed intermediate and deep earthquakes determined from ultramafic pseudotachylyte stresses released by coseismic faults during subduction toward lawsonite eclogite facies conditions in the alpine subduction complex of corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured congruent peridotite melting at ambient conditions of 1 5 gpa and 470 c requires a temperature increase of 1280 c to 1750 c we assume that more than 95 of the work is converted to heat during faulting hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain minimum estimates of released stress are typically greater than 220 mpa and as high as 580 mpa the abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth

microstructure metamorphism thermochronology and p t t deformation history of the port aux basques gneisses south west newfoundland canada the port aux basques gneisses comprise three lithostratigraphic units separated by major fault zones the grand bay complex the port aux basques complex and the harbor le cou group a similar regionally developed polyphase history of penetrative deformation characterizes each of these units thickening during d1 produced rare recumbent folds f1 and an axial planar schistosity s1 overprinted by d2 recumbent folds f2 and transposed during development of a locally penetrative differentiated crenulation cleavage s2 in western sectors of the area d2 was associated with nw directed reverse shearing the ne trending structural grain reflects d3 transpression partitioned into dextral transcurrent movement along major shear zones and development of upright to steeply inclined periclinal folds f3 and a variably penetrative schistosity s3 amphibolite facies metamorphism increases in grade from west to east across the area microstructures including porphyroblast matrix foliation relations and internal textural unconformities in garnet indicate episodic porphyroblast nucleation and growth which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism garnet kyanite and staurolite porphyroblasts are wrapped by the s2 foliation but each may contain trails of inclusions that define s1 commonly these trails preserve early stages of s2 crenulation cleavage development progressive and sequential reaction out of kyanite staurolite and muscovite in favour of sillimanite garnet biotite and k feldspar and the development of an increasing volume of anatectic migmatite in south eastern sectors of the area record syn to late d2 peak metamorphic conditions microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in p t space for units of the port aux basques gneisses peak metamorphic conditions are estimated to have been 620 650 c at 8kbar in the west and 700 750 c at 8 kbar in the east titanite from an upper amphibolite facies calc silicate gneiss yields u pb ages of c 420 ma interpreted to date cooling shortly after the thermal peak in these gneisses variable d3 strain was associated with some recrystallization of hornblende and micas 40ar 39ar hornblende plateau isotope correlation ages range from c 419 to c 393 ma from east to west across the area and are interpreted to record cooling through c 500 c coeval with or soon after d3 deformation the range in ages may record the effects of heterogeneous d3 deformation and differential uplift from south east to north west associated with displacement on major shear zones 40ar 39ar muscovite plateau ages cluster at c 390 ma and date cooling through c 375 c during regional exhumation cooling rates are moderate to fast and may indicate a component of tectonic exhumation the port aux basques gneisses are a product of silurian collisional tectonics the higher grade of metamorphism in comparison with adjacent areas of the canadian appalachians is interpreted to reflect greater thickening due to juxtaposition of the st lawrence promontory laurentian margin with the cabot promontory avalonian margin during closure of the iapetus ocean

rapid rupture directivity determination of moderate dip slip earthquakes with teleseismic body waves assuming reduced finite source approximation for an earthquake with along dip rupture there is opposite directivity effects on down going direct p wave and up going depth phases pp and sp and this can be exploited for resolving fault plane and rupture direction we propose a method to calculate the reduced finite source synthetics for teleseismic p wave and determine the rupture directivity via waveform fitting we verified the effectiveness of this method with forward tests and investigated its robustness against station selection uncertainties in point source parameters and uncertainties in source finiteness parameters we applied this method to the 2011 mw5 8 virginia earthquake the 2008 mw6 0 nevada earthquake and six other dip slip earthquakes for most cases we obtained rupture directivity results consistent with previous studies but the method could fail for along strike ruptures and very shallow earthquakes our method may help mitigate earthquake hazard by improving shake map with rapid rupture directivity analysis

structural evolution of a major fault zone the cape ray fault zone southwestern newfoundland canada the cape ray fault zone is a major paleozoic structure in southwestern newfoundland and occurs at or close to the boundary between two major continental blocks laurentia and avalonia a detailed structural analysis demonstrates that the fault records early reverse sinistral thrusting of the grand bay complex at amphibolite grade d2 followed by a protracted event d3 characterized by reverse dextral thrusting of the grand bay complex rocks on top of the supracrustal rocks of the windsor point group and retrogression to greenschist facies as well as a pre 384 ma orogen parallel dextral transcurrent mylonite d4 during the later stages of the collision regional scale strain partitioning induced heterogeneity of strain both along and across the strike of the cape ray fault zone the east west oriented segment of the cape ray fault zone is a tear fault that accommodated differential displacement along the length of the fault later stages of the deformation include post 384 ma sinistral transcurrent r

subsidence in late paleozoic basins in the northern appalachians during the interval between continental collision in the devonian and continental breakup in the triassic the northern appalachians became the site of a wide plate boundary zone of dominantly right lateral strike slip as is typical of intracontinental transforms tectonism was both diachronous and rapidly variable along strike through regimes of pure strike slip transpressional deformation and rapid subsidence of extensional basins up to 9 km of mainly nonmarine clastic sediments accumulated in these local depocenters which subsided episodically in two stages 1 an initial phase of stretching and thinning of the lithosphere when subsidence was rapid fault controlled and often accompanied by volcanism and 2 a subsequent phase of gradual thermal subsidence during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement the largest depocenter the magdalen basin opened as a pull apart between strike slip faults in newfoundland and new brunswick from late devonian to early carboniferous subsequent thermal subsidence affected a large area during medial and late carboniferous a phenomenon that is well recorded to the north and west where no later tectonism occurred in areas to the south and east of the basin strike slip on other faults continued into the time of thermal subsidence introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull aparts

initiating intermediate depth earthquakes insights from a hp lt ophiolite from corsica abstract the hypocentres of intermediate depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals this relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field based research we evaluated the relationship of the coincidence of intermediate depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite the eocene schistes lustres complex in corsica these units comprise incompletely metamorphosed metagabbro and peridotite the wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals tremolite mg hornblende glaucophane in the metagabbro and serpentine tremolite and chlorite in the peridotite back scatter electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration vesicular and h 2 o rich melt veins are observed cross cutting partially molten pseudotachylyte fault veins and show evidence of h 2 o exsolution during melt solidification the crystallisation products of these melts indicate formation under high temperature high pressure conditions 1400 1700 c 1 5 gpa the peridotite hosted pseudotachylytes crystallised olivine orthopyroxene and diopside which are surrounded by interstitial al and h 2 o rich glass the metagabbro pseudotachylyte is dominated by al rich omphacite ilmenite and high fe anorthite xrf bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix entrained survivor clasts and the crystallisation products show that near total disequilibrium melting took place the peridotite hosted pseudotachylyte composition is skewed strongly towards chlorite however the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered indicating that preferential fusion of chlorite took place the metagabbro hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of mg hornblende and tremolite not all the pseudotachylytes are hydrous as the h 2 o content of the melts is highly variable the metagabbro pseudotachylyte ranges from 0 to 4 wt and the peridotite pseudotachylyte ranges from 0 to 14 wt the range in h 2 o content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate depth seismicity however we have observed that the pseudotachylytes with the most chaotic vein networks thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals possibly owing to repeated fluid ingress in between pseudotachylyte generating events this implies that free fluids enhance pseudotachylyte generation and possibly seismicity but are not a first order requirement microtextural and geochemical results from this study suggest that the presence of abundant h 2 o rich minerals in the slab exerts a strong rheological control during high strain rate deformation facilitating thermally triggered localising shear instabilities these field based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation and offer fresh insight into the debate of how intermediate depth earthquakes take place

the cheticamp pluton a cambrian granodioritic intrusion in the western cape breton highlands nova scotia the cheticamp pluton consists of biotite granodiorite locally megacrystic in the north and museovite biotite granodiorite in the south in probable faulted contact these two rock types especially the biotite granodiorite show a broad range in modal and chemical compositions they are interpreted to be cogenetic with the museovite biotite grandiorite derived from the biotite granodiorite by crystal fractionation involving mafic minerals plagioclase and sphene the overall peraluminous composition of the suite resulted from the fractionation process probably enhanced by alteration rather than from derivation from peraluminous source rocks a seven point whole rock rb sr isochron indicates an age of 525 40 ma the pluton intruded dioritic rocks and quartzo feldspathic gneisses thus indicating precambrian ages for these units it probably also postdates the western highlands volcanic sedimentary complex a major undated stratigraphic unit in the cape breton highlands although the age overlaps t

late devonian carboniferous detachment faulting and extensional tectonics in western cape breton island nova scotia canada abstract shallow dipping detachment faults have played a key role in the evolution of the late devonian to carboniferous maritimes basin of western cape breton island the faults are responsible for large scale post orogenic crustal thinning and denudation of mid crustal levels tournaisian and westphalian clastic sedimentation can be related to detachment faulting along the margaree shear zone msz and the ainslie detachment ad respectively the msz consists of thick shallow dipping retrogressive mylonite overprinted by cataclastic horizons brittle detachment faults and chloritic breccia the shear zone crosscuts and transports low grade late devonian bimodal volcanic rocks and redbeds of the fisset brook formation in its hanging wall towards the west and juxtaposes them against exhumed medium to high grade metamorphic rocks and basement in its footwall locally the mylonite is 200 700 m thick and outcrops along strike from north to south for approximately 60 km tournaisian coarse clastic units and debris flow deposits of the horton group unconformably overlie ductile mylonite of the msz due to incision of brittle faults into the msz regional transgression and marine flooding of the extensional complex and exhumed units are marked by the thick visean carbonate and evaporite deposits of the windsor group the carbonates reflect a relatively quiescent tectonic regime and widespread subsidence was likely due to the cessation of extension related thermally induced buoyancy in a tectonically thinned crust the ainslie detachment nucleated as a regional flat lying fault along the carbonate evaporite contact at the top of the macumber formation forming calc mylonite near the base of the windsor group significant stratigraphic omissions in the windsor group above the detachment provide clear evidence of an extensional regime during faulting clastic deposits of the westphalian cumberland group are bounded by the detachment and sedimentation is related to listric normal faults which cut down section towards the west southwest the documentation of detachment faults has widespread implications for the interpretation of the post orogenic evolution of the northern appalachians an extensional regime can now be recognized deep reflection seismic data are interpreted in light of the new findings and can be related to late devonian carboniferous basin formation

carboniferous volcanic rocks of the magdalen islands gulf of st lawrence volcanic rocks of mississippian age occur on the magdalen islands as cap rocks and within collapse breccias above salt diapirs that have formed the islands they consist of coarse volcaniclastic deposits and basaltic flows intruded by minor mafic dykes and plugs petrologic studies of the basaltic rocks show that they are extensively altered original plagioclase clinopyroxene olivine and interstitial glass are partially to entirely replaced by mixtures of chlorite sericite smectite sphene carbonate epidote albite potassium feldspar and iron oxides and the samples display a relatively wide range in chemical compositions especially mobile were k na and ca and most samples are classified as potash spilites poenites using standard discriminant diagrams for mafic igneous rocks it can be seen that the basalts appear to range from continental tholeiitic to continental alkalic however relict clinopyroxene compositions and the presence of kaersutitic amphibole and titaniferous biotite in so

devonian carboniferous tectonics and basin deformation in the cabot strait area eastern canada the magdalen basin in the cabot strait bay st george area was a major depocenter for devonian carboniferous sediments in eastern canada structure within the basin is complex and is characterized by northeast and east trending mainly dextral strike slip faults associated with the bend in the appalachian orogenic belt known as the st lawrence promontory under the cabot strait two linear grabens parallel the major fault trends and preserve up to 6 km of devonian carboniferous sedimentary rocks comprising mainly coarse grained terrigenous clastics with a lower carboniferous transgressive episode recorded by marine carbonates evaporites and fine grained clastics several unconformities within the sedimentary succession record deformation associated with movement alo g the regional strike slip faults minor local unconformities are interpreted within the upper horton early visean upper barachois late westphalian and post pennsylvanian successions a major unconformity cuts out most of the namurian and westphalian section in the study area and can be correlated on a regional scale kinematic strain partitioning along the master cabot fault led to the development of doubly vergent compressional structures within a wrench borderland this was complicated by transpressive deformation at a restraining bend in the master fault and by deformation associated with overstepped ends of splays of the regional fault system the magdalen basin probably formed in an early phase of post acadian extension and was later reactivated by regional strike slip faults the presence of thick source and reservoir rocks as well as diverse trapping possibilities and a favorable burial and maturation history indicate that the cabot strait area has good potential for accumulating and preserving petroleum

hydrodynamic effects on mission canyon mississippian oil accumulations billings nose area north dakota mission canyon oil production on the south flank of the williston basin provides an example of an area in the mature stage of exploration that shows significant hydrodynamic effects on oil accumulations related to stratigraphic traps the effects are illustrated by the billings nose fields and the elkhorn ranch field the reservoirs have low hydraulic gradients of about 2 m km 10 ft mi tilted oil water contacts with gradients of 5 m km 25 ft mi and variable formation water salinities that range from brackish to highly saline oil accumulations in some zones are displaced off structure and downdip to the northeast parallel to porosity pinch outs other zones are pure hydrodynamic traps lacking both structural and stratigraphic closure future success in exploration and development in the play will depend on recognizing the hydrodynamic effects and predicting oil displacement

a cascading failure during the 24 may 2013 great okhotsk deep earthquake on 24 may 2013 the largest ever recorded deep earthquake occurred beneath sea of okhotsk a multiple point source inversion procedure is applied to constrain source process of this earthquake based on waveform modeling of both direct p and sh waves and near surface reflected pp and ssh waves our results indicate that the earthquake consists of six major subevents separated in space and time encompassing a horizontal dimension of 64 4 km along n160 e and a downward depth extension of 35 4 km the geographic distribution and focal mechanisms of the inferred subevents and foreshock aftershock locations do not fit into plane rupture we suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region with the perturbation of stress generated by a shear instability triggering another

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

chronology of devonian to early carboniferous rifting and igneous activity in southern magdalen basin based on u pb zircon dating fifteen u pb zircon radiometric age determinations have been made on igneous rocks of middle devonian to early carboniferous age from the southern margin of the magdalen basin in cape breton island and northern mainland nova scotia volcanic rocks interbed with early rift basin sedimentary rocks with some palynological biostratigraphy dated intrusive rocks cut these sedimentary units our biostratigraphically constrained ages are in close agreement with the current devonian time scale combined with previously published data the age determinations show that igneous activity occurred in four pulses middle devonian 390 385 ma early late devonian 375 370 ma latest devonian to early tournaisian 365 354 ma and late tournaisian to early visean ca 339 ma middle devonian 385 389 ma volcanic rocks are confined to the guysborough group the fisset brook formation basalt and minor rhyolite in the type area and elsewhere in cape breton island and northern mainland nova scotia is late devonian c

tectonic setting and regional correlation of ordovician silurian rocks of the aspy terrane cape breton island nova scotia interlayered mafic and felsic metavolcanic rocks and metasedimentary rocks of ordovician to silurian age are characteristic of the aspy terrane of northwestern cape breton island these rocks were affected by medium to high grade metamorphism and were intruded by synkinematic granitoid orthogneisses during late silurian to early devonian times they were intruded by posttectonic devonian granitic plutons and experienced rapid devonian decompression and cooling the chemical characteristics of the mafic metavolcanic rocks indicate that they are tholeiites formed in a volcanic arc setting the volcanic rocks of the aspy terrane differ from many other silurian and silurian devonian successions in atlantic canada which have chemical and stratigraphic characteristics of volcanic rocks formed in extensional within plate settings and are somewhat younger than the aspy terrane sequences aspy terrane units are most similar to ordovician silurian volcanic and metamorphic units in southwestern newfoundland incl

petrochemistry and tectonic significance of carboniferous volcanic rocks in new brunswick carboniferous volcanic rocks from the new brunswick platform in the maritimes basin are divided into three age groups late tournaisian to early visean volcanic rocks are tholeiitic basalts and andesites that in southern new brunswick are inter bedded with abundant calc alkalic rhyolite late visean to namurian volcanic rocks consist of an interbedded sequence of alkalic basalts and trachyandesites late westphalian volcanic rocks change in composition up section from trachyte to peralkalic rhyolite all three age groups display petrochemical features indicative of an intraplate tectonic setting the volcanic geochemistry is consistent with the development of the maritimes basin either as a failed rift formed along the margin of a late paleozoic ocean or as a rhomb graben formed within a transcurrent zone the former model is preferred the change in basaltic composition from tholeiitic to alkalic apparently coincided with a decrease in rate of extension between the tournaisian and namurian local peral

pluton emplacement by wall rock thrusting hanging wall translation and extensional collapse latest devonian plutons of the cobequid fault zone nova scotia canada

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

long lived fault zones and their influence on the tectonic development of the southwestern barents sea the southwestern barents sea shows a complicated network of fault complexes and systems with various geometrical and genetic characteristics trend relation to basement reactivation fault plane geometry and regional significance using these parameters a classification of the fault complexes has been made and a correlation to onshore faults has been attempted the area is subdivided into several fault blocks separated by deep seated zones of weakness first and second class fault systems these are reactivated in connection with later mesozoic movements in the area and have acted as foci of strain so leaving the central area of the blocks relatively strain free a third class of fault systems is of local significance and reflects deformation limited to the interior of the fault blocks

tectonic burial thrust emplacement and extensional exhumation of the cabot nappe in the appalachian hinterland of cape breton island canada silurian imbricate thrusting early devonian high grade metamorphic nappe emplacement and devonian carboniferous extensional denudation characterize deformation in the appalachian hinterland of cape breton island compressional deformation following early silurian arc volcanism features imbrication of cambrian precambrian basement rocks of gondwana derivation with ordovician silurian cover sequences across thick zones of mylonite during south directed transport high grade metamorphism and gneissic rocks of late silurian age in the region indicate that significant tectonic burial and crustal thickening occurred as a result of the thrusting partial denudation of the high grade assemblages occurred during early devonian thrust emplacement of the cabot nappe toward the northwest along the highlands shear zone the nappe is characterized by an amphibolitic gneiss and high grade schist complex defining a large folded klippe above silurian units kyanite is widespread within the nappe and a distinctive feature of the thrust sheet is the dynamothermal metamorphism of cooler greenschist grade footwall rocks producing inverted isograds staurolite is regionally distributed and occurs in pelitic units in the immediate footwall of the highlands shear zone forming a discontinuous halo around the klippe greenschist grade footwall rocks are exposed in structural windows as a result of folding and faulting shear sense indicators along the margins of the cabot nappe have been rotated into their present positions due to superposed folding providing apparent movement directions for the nappe complete exhumation to surface occurred during late devonian extension along the low angle margaree shear zone

dual polarity early carboniferous remagnetization of the fisset brook formation cape breton island nova scotia summary red siltstones and volcanic flows of the fisset brook formation of cape breton island nova scotia were folded or tilted in two phases one of late tournaisian and the other of mid namurian age upon thermal demagnetization both rock types yield three components of magnetization herein denoted as l i and h the l component has low unblocking temperatures and a direction conforming to that of the present day geomagnetic field l is inferred to be of recent origin the intermediate i component carried by haematite is of synfolding origin directions could be obtained through vector subtraction for the demagnetization interval of 300 550 c and yield a mean of d i 160 38 k 90 8 ag5 s lo after 39 per cent of the tilt correction is applied the h component has even higher unblocking temperatures which overlap to a large degree with those of the i component but analysis of intersecting great circles appears to yield a best fit direction for h which is nearly antipodal to that of the i component h is also synfolding and yields a mean direction of d i 342 38 k 120 9 ag5 6 9 after 60 per cent of tilt correction both components are interpreted as secondary chemical remanent magnetizations of late tournaisian to early namurian age a comparison of all available carboniferous results from the craton and the northern appalachians indicates that palaeolatitudes for nova scotia changed from about 20 s to about 10 s in the interval between the early and late namurian and that they changed again from about 10 s to 0 between the late namurian and the stephanian declinations show good agreement between newfoundland nova scotia new brunswick and the craton with the exception of the meguma terrain which underwent a counterclockwise rotation with respect to the mainland in the late carboniferous as noted previously by spariosu kent keppie 1984

the deep peru 2015 doublet earthquakes abstract on 24 november 2015 two events of magnitude mw 7 5 and mw 7 6 occurred at 600 km depth under the peru brazil boundary these two events were separated in time by 300 s deep event doublets occur often under south america the characteristics that control these events and the dynamic interaction between them are an unresolved problem we used teleseismic and regional data situated above the doublet to perform source inversion in order to characterize their ruptures the overall resemblance between these two events suggests that they share similar rupture process they are not identical but occur on the same fault surface dipping westward using a p wave stripping and stretching method we determine rupture speed of 2 25 km s from regional body wave inversion we find that stress drop is similar for both events they differ by a factor of two the similarity in geometry rupture velocity stress drop and radiated energy suggests that these two events looked like simple elliptical ruptures that propagated like classical sub shear brittle cracks

recurring large deep earthquakes in hindu kush driven by a sinking slab hindu kush subduction zone produces large intermediate depth earthquakes within a small volume every 10 15 years here we study the last three m 7 events within the cluster and find complex and diverse rupture processes however their main subevents appear to recur on the same fault patch dipping 70 to the south this recurrence requires an average of 9 6 cm yr slip rate on the patch much higher than the 1 cm yr surface convergence rate measured geodetically the high slip rate is likely caused by significant slab internal deformation such as localized slab stretching necking we infer that the hindu kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm yr

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

tectonic evolution of southwestern newfoundland as indicated by granitoid petrogenesis four granitoid suites are recognized in the region of the cape ray fault zone of southwestern newfoundland the two oldest ordovician silurian suites represent partial melts of their enclosing host rocks the port aux basques granite is modelled as a partial melt of the gneissic component of its host port aux basques complex the cape ray granite forms a dominantly tonalitic terrane derived by partial melting of ophiolitic material the red rocks granite and a megacrystic phase of the cape ray granite form coherent lines of geochemical descent from the parental tonalite but show evidence of some continental crust contamination the late devonian windowglass hill granite is a subvolcanic equivalent of felsic volcanic rocks in the windsor point group both units were derived as partial melts of continental crust the post tectonic late devonian to early carboniferous strawberry and isle aux morts brook granites constitute the youngest granitoid suite in the region these a type granitoids were derived as partial melts of an underlying depleted granulitic felsic crust the depleted nature of the source may have resulted from previous generation of the windowglass hill granite and windsor point group the only possible protolith for the granulitic source is precambrian grenvillian gneiss the presence of this gneiss beneath the cape ray fault zone of southwestern newfoundland implies that the complete series of lithologies is allochthonous

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

abundant aftershock sequence of the 2015 mw7 5 hindu kush intermediate depth earthquake the 2015 m w7 5 hindu kush earthquake occurred at a depth of 213 km beneath the hindu kush region of afghanistan while many early aftershocks were missing from the global earthquake catalogues this sequence was recorded continuously by eight broad band stations within 500 km here we use a waveform matching technique to systematically detect earthquakes around the main shock more than 3000 events are detected within 35 d after the main shock as compared with 42 listed in the advanced national seismic system catalogue or 196 in the international seismological centre catalogue the aftershock sequence generally follows the omori s law with a decay constant p 0 92 we also apply the recently developed double pair double difference technique to relocate all detected aftershocks most of them are located to the west of the hypocentre of the main shock consistent with the westward propagation of the main shock rupture the aftershocks outline a nearly vertical southward dipping plane which matches well with one of the nodal planes of the main shock we conclude that the aftershock sequence of this intermediate depth earthquake shares many similarities with those for shallow earthquakes and infer that there are some common mechanisms responsible for shallow and intermediate depth earthquakes

zooming into the hindu kush slab break off a rare glimpse on the terminal stage of subduction abstract the terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere this process although common in geological history is short lived and therefore rarely observed the deep seismicity under the hindu kush central asia including the 2015 m w 7 5 event is a rare case that testifies to this process here we use new seismological data to create a high resolution picture of slab break off and infer its dynamics high precision earthquake locations and tomographic images show subduction of continental crust down to 180 km a large dataset of source mechanisms indicates sub vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab below the subducted crust extends at higher rates 40 km ma most m w 7 earthquakes between 1983 2015 relocated relative to our new well constrained earthquake catalog cluster in a small volume below 180 km and indicate shearing on an overturned interface a slip model for the latest 2015 m w 7 5 event suggests that it ruptured into a seismic gap on this interface from this configuration we conclude that a horizontal slab tear develops along strike of the hindu kush seismic zone at the base of the subducted continental crust below the subducted crust the deepest and also largest earthquakes 180 265 km are likely associated with deformation in the mantle lithosphere from the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along strike from simple to pure shear the fastest detachment rates and largest earthquakes occur during the simple shear dominated stage earthquakes in the upper part 60 180 km above the rapidly extending slab might be triggered by processes related to the subduction of crustal rocks

syndeformational antigorite dehydration produces stable fault slip we conducted deformation experiments in which temperature was increased above the thermal stability of antigorite to directly test the dehydration embrittlement hypothesis and its potential link to intermediate depth earthquakes the experiments were designed to maximize the possibility for instability pore pressure was undrained pre dehydration stress was high and deformation was localized prior to initiation of reaction however stick slip instabilities or unstable fault behavior did not develop rather we find that weakening occurs stably at a rate that depends on the ratio of the temperature ramp rate to the strain rate resulting in laboratory slow slip or stress relaxation events

continental lithospheric subduction and intermediate depth seismicity constraints from s wave velocity structures in the pamir and hindu kush abstract the pamir has experienced more intense deformation and shortening than tibet although it has a similar history of terrane accretion subduction as a primary way to accommodate lithospheric shortening beneath the pamir has induced the intermediate depth seismicity which is rare in tibet here we construct a 3d s wave velocity model of the lithosphere beneath the pamir by surface wave tomography using data of the tipage tien shan pamir geodynamic program and other seismic networks in the area we imaged a large scale low velocity anomaly in the crust at 20 50 km depth in the pamir overlain by a high velocity anomaly at a depth shallower than 15 km the high velocity anomalies colocate with exposed gneiss domes which may imply a similar history of crustal deformation partial melting and exhumation in the hinterland as has occurred in the himalaya tibet system at mantle depths where the intermediate depth earthquakes are located a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the hindu kush and eastern pamir respectively moreover the geometry of the low velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting asian and indian lithospheric mantle beneath the pamir and hindu kush respectively metamorphic processes in the subducting lower crust may cause the intermediate depth seismicity down to 150 180 km depth beneath the pamir and hindu kush we inverted focal mechanisms in the seismic zone for the stress field differences in the stress field between the upper and lower parts of the indian slab imply that subduction and detachment of the indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep hindu kush

paleomagnetic evidence for post devonian displacement of the avalon platform newfoundland the possibility that the avalon platform where the avalonian lithotectonic belt is best developed was involved in late paleozoic displacement was tested by paleomagnetic study of red sandstones of the upper devonian terrenceville formation of eastern newfoundland two magnetization directions were identified by thermal demagnetization analysis of 60 oriented samples from 10 sites a high blocking temperature thermally discrete a component of normal and reversed polarity and an intermediate blocking temperature thermally distributed b component of reversed polarity the b component d 185 9 i 3 3 a95 7 2 for n 8 sites is interpreted as a postfolding secondary magnetization and gives a paleomagnetic pole position latitude 43 6 n longitude 117 1 e near early to late permian paleopoles for north america the a component d 181 6 i 28 0 a95 10 1 for n 9 sites is interpreted as the characteristic magnetization possibly dating from near the time of deposition of the terrence ville formation the corresponding paleomagnetic pole position latitude 27 4 n longitude 123 5 e falls within a group of late devonian early carboniferous paleopoles obtained from the acadia displaced terrain encompassing the coastal areas of new england and the canadian maritimes which form another part of the avalonian belt these paleopoles are systematically offset by 15 to 20 in latitude from coeval pole positions obtained from cratonic north america thus the avalonian belt of the northern appalachians which is thought to represent a remnant of a precambrian and early paleozoic microcontinent on tectonostratigraphic considerations appears to correspond to a late paleozoic displaced terrain on the basis of paleomagnetic evidence

the pucallpa nest and its constraints on the geometry of the peruvian flat slab abstract seismic nests or clusters are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface e g zarifi and havskov 2003 zarifi et al 2007 prieto et al 2012 in this study we focus on a seismic nest located in central peru beneath the amazon basin city of pucallpa the location of this nest is just beyond the eastern most extent of the peruvian flat slab similar to the settings of two other south american clusters the bucaramanga and pipanaco nests while the pucallpa nest is visible in the figures of many earlier papers on intermediate depth south american seismicity it has not to our knowledge been described as a seismic nest before we present the first detailed description of the pucallpa nest compare it to other established nests and discuss its implications for our understanding of the peruvian flat slab our results indicate that the pucallpa nest demarcates the northern margin of a sag in the horizontally subducting nazca plate in central peru the position of the nest along the projected location of the downgoing mendana fracture zone is consistent with local variations in b values and could help to explain both the nest s genesis and the co located change in slab geometry the nest is also spatially well correlated with the northern margin of the thick skinned shira mountains and with the high heat flow associated with the agua caliente dome hermoza et al 2006 navarro comet 2018 further study is needed to understand the effects of the complex peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding south american continent

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

new insights into the appalachian orogen from geology and geochronology along the cape ray fault zone southwest newfoundland reverse oblique overthrusting of high grade port aux basques gneiss over the lower grade supracrustal windsor point group along the cape ray fault zone occurred during late silurian to early devonian time and is attributed to major oblique collision between gondwanan and laurentian continental blocks new u pb zircon 40 ar 39 ar and k ar data limit the timing of this orogenic event the megacrystic granitic phase of the cape ray igneous complex and the cape ray tonalite have been dated at 488 3 ma and 469 2 ma respectively a black rhyolite from the windsor point group yielded an ordovician age 453 5 4 ma a pretectonic gabbro sill within the windsor point group dated at 424 4 3 ma is cut by the windowglass hill granite dated at 424 2 ma synkinematic hornblende and biotite from the grand bay complex mylonites of the port aux basques gneiss have 40 ar 39 ar ages of 407 4 ma and 403 4 ma respectively which is indicative of a late silurian early devonian age for the amphibolite grade deformation these ages are compatible with a previously determined 415 ma monazite age interpreted as the time of peak metamorphism in the port aux basques gneiss and suggests rapid exhumation the east west flexure of the cape ray fault zone represents a tear fault accommodating differential shortening between the cape ray fault zone and the correlative fault system in central newfoundland mid devonian granites 386 3 384 2 ma intervene between the thrusting events and a younger sinistral transcurrent movement this deformation and magmatic history can be correlated throughout the northern appalachians along other major fault boundaries

timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history van der plmjm b a 1987 timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tectonophysics 135 15 24 the newfoundland appalachians have been interpreted as an area where lower paleozoic plate convergence culminated in collision between an ordovician volcanic chain and the north american craton in middle ordovician times closure of the intervening proto atlantic iapetus ocean was considered incomplete subsequent deformation gave rise to regional folding and faulting recent studies in the newfoundland dunnage zone have revealed that the deformation history is far more complex than previously recognized large scale thrusting folding and faulting occurred in silurian devonian times furthermore it has been suggested that the dunnage zone is an allochthonous terrane underlain by dominantly continental crust rather than representing remnants of a rooted ocean basin in view of these results a revision of tectonic scenarios and zonal subdivision is warranted and a multi stage collision history will be discussed with emphasis on the spatial distribution and significance of silurian devonian deformation in central newfoundland subduction in lower paleozoic times gave rise to the formation of a volcanic terrane concurrently to the southeast a marginal sea was formed mariana type subduction in middle ordovician times the volcanic terrane collided with the north american craton first stage collision and back arc spreading terminated continued crustal shortening resulted in the formation of a silurian accretionary terrane telescoped marginal sea and its subsequent deformation second stage collision devonian carboniferous strike slip faulting represents the third stage in the collision history the model is applicable to large tracts of the caledonian appalachian chain its main characteristics are a the revised zonal subdivision of the area is based on characteristics of silurian and older rocks rather than middle ordovician and older rocks only b the central part of the orogen represents a telescoped marginal sea that formed to the southeast of the ordovician volcanic chain rather than a remnant of the incompletely closed iapetus ocean c the earliest deformation is progressively younger toward the southeast d the appalachian collision history is a result of the activity of a single deformation regime over a long period of at least 75 ma

the mechanism of the deep bolivia earthquake of june 9 1994 we analyzed the iris broad band seismograms for the june 9 1994 bolivia earthquake the largest deep focus earthquake ever recorded we inverted the record sections with a duration of 330 sec which include p pp and sp phases and obtained a dip slip mechanism dip rake strike 13 4 1 or 89 103 95 with a scalar moment of mo 2 9 10 21 nm mw 8 2 this mechanism is similar to that obtained from long period 175 to 250 sec rayleigh and love waves 83 100 90 mo 3 0 10 21 nm mw 8 3 the main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1 2 10 20 nm mw 7 3 the main rupture starts at about 20 km east or northeast of the initial break and propagates horizontally in the east to northeast direction the source duration is 40 sec and the rupture dimension is about 40km if a fault area of 40 40km is assumed the stress drop is 110 mpa an inversion of body waves p pp and sp with 6 basis moment tensors yielded a best fit solution with a 10 implosive component this result is of marginal significance and its confirmation would require more detailed information on the structures near the source region pp bounce points and stations the observed amplitude of the radial mode 0s 0 1226 sec is consistent with a double couple model with less than 2 isotropic component

panafrican to hercynian deformations in the mauritanides and tectonic significance of gravity anomalies abstract the mauritanides rokelides chain which extends along the western margin of the west african craton is characterized by roughly northward structural trends and at least for the northern two thirds eastward thrusting of large amplitude the style and timing of deformation change from south to north along the chain in sierra leone guinea and southeastern senegal the rokelides the main event is attributed to the panafrican orogeny apparently with only limited thrusting the belt of gravity highs associated with that part of the orogen is interpreted as a west dipping suture zone in northeastern senegal and central mauritania this panafrican event is masked by caledonian and hercynian events which resulted in strong folding and far travelled thrusting the positive gravity anomaly closely parallel to the belt records the trace of the collision of two continental blocks a taconian event in northern mauritania and south morocco the age of emplacement of large nappes is hercynian and their geodynamic interpretation requires that a trench or ocean has closed farther west presumably beneath the eastern u s atlantic coastal plain

paleomagnetism and geochemistry of carboniferous sandwich bay dykes from coastal labrador paleomagnetic petrographic and geochemical data are used to characterize a suite of dykes in the sandwich bay area south of cartwright labrador these dykes are carboniferous in age and their separate identity is indicated by the following i distinct paleomagnetic signatures ii homogeneous geochemistry iii cross cutting relations with other dated rock units in the area and iv a whole rock k ar date of 327 13 ma the dikes have a soft present earth9s field component superposed on a stable magnetization at d 325 5 i 26 9 n 14 k 91 6 95 4 6 with the corresponding paleopole at 16 2 n 157 6 e dp dm 2 8 5 2 the stable magnetization is shown to be primary from baked contact tests no kiaman overprint is present the paleopole determined for the dykes suggests emplacement of the dykes at equatorial latitudes during the carboniferous times

global rupture parameters for deep and intermediate depth earthquakes we investigate the global rupture parameters for deep and intermediate depth earthquakes from measurements of rupture duration and radiated seismic energy we estimate stress drop apparent stress and radiation efficiency and obtain a detailed earthquake energy budget from scaling of the source parameters we highlight differences between crustal and deep seismicity with the latter showing larger fracture energies the observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate depth events differs in agreement with previous studies we observe along strike variability of rupture properties for deep and intermediate depth earthquakes correlating with slab morphology plate age or presence of volcanic structures

les bassins d avant pays de la chaine hercynienne au carbonifere inferieur the evolution of marine sedimentary basins deformed by the hercynian orogeny was studied in germany france iberia northwest africa and north america in the lower carboniferous these basins opened along strike slip faults and were filled with turbidites and shelf deposits until the main compressive phase at the end of visean and the namurian the infilling was interrupted with frequent volcanic episodes and gravity induced movements according to sedimentary vergence these basins were set into two festoons on both sides of an elongated and emergent swell structured during the devonian and corresponding to the internal zones of the orogen these basins were filled during a general tectonic compression phase they correspond to the foreland basins of the hercynian belt

implications of spore evidence for late devonian age of the piskahegan group southwestern new brunswick the predominantly volcanic piskahegan group has commonly been considered early carboniferous based on its stratigraphic position however spores recently discovered in the carrow formation an alluvial fan deposit in the exocaldera facies indicate that most if not all of the group is of late devonian late famennian age the spore assemblage includes several species reported previously from ireland belgium and eastern europe some of them apparently restricted to the southern parts of the old red sandstone continent in late devonian time comparison of records of earliest occurrences suggests that the incoming of some species was diachronous volcanic rocks of the piskahegan group are coeval with post acadian tin tungsten bearing granites elsewhere in new brunswick and are considered the surface expression of plutonism that resulted from acadian continental collison

uranium and thorium in paleozoic rhyolites of nova scotia the concentrations of u and th in paleozoic rhyolites of nova scotia are closely related to their contents of k the rhyolites were affected by secondary processes including low grade metamorphism that could lead either to enrichment or depletion of these elements the average abundances of u ppm th ppm and k in rhyolites are cambrian 2 69 9 77 and 2 58 respectively silurian 4 50 17 3 and 4 09 respectively and devono carboniferous 4 64 18 3 and 5 67 respectively the depletion of u th and k in cambrian rocks compared with younger paleozoic rhyolites is probably related to secondary processes in silurian and devono carboniferous rhyolites these elements were mainly redistributed during the secondary processes but not removed

paleomagnetism of permian sedimentary rocks from tanzania and the permian paleogeography of pangea abstract we report new paleomagnetic results from karoo permian rocks in tanzania k1 and k3 redbeds and k5 siltstones and limestones from the songwe kiwira basin ndeke redbeds from the mikumi basin and group viii redbeds from the nyakatitu basin were sampled and thermally demagnetized palynologic data and lithologic correlations suggest that the k1 redbeds are lower permian and that the k3 ndeke and group viii redbeds as well as the k5 siltstones and limestones are upper permian stable magnetic directions were obtained from the k3 ndeke and group viii redbeds yielding an upper permain pole at 26 n 267 e characteristic remanence directions were also obtained from the k5 siltstones and limestones however because these directions define a pole 66 n 269 e that is very close to the upper cretaceous pole for africa and significantly different from the upper permian redbed poles these rocks may have been remagnetized the k1 redbeds did not yield stable results using our new results together with published permian poles for africa and the other gondwanan continents we revise the mean lower and upper permian poles for africa west gondwana and gondwana and investigate whether or not these revised poles can discriminate between the various pangea reconstructions proposed for the permian with any greater certainty than previous unrevised mean poles when compared to the mean lower and upper permian poles for laurussia we find that our revised west gondwanan and gondwanan poles do not necessarily provide stronger support for any particular pangea reconstruction than the previous poles therefore even though this study improves the paleomagnetic data base for gondwana for the permian it is not clear that the combination of our results with other paleomagnetic data from the gondwanan continents yields significant new insights about the permian configuration of pangea as suggested previously strong non dipole fields during the permian may provide an explanation for why it is not easy to discriminate with certainty between the various permain pangea reconstructions even with new paleomagnetic results

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

the geology and structural history of the cape ray fault zone in southwestern newfoundland structural studies within the cape ray fault zone southwestern newfoundland indicate that three tectonic events d1 to d3 have deformed all regional lithologic units these lithologies are distributed as a tonalitic terrain intrusive into ophiolitic remnants to the northwest a central volcanic sedimentary terrain and a staurolite kyanite gneiss terrain to the southeast d1 produced the dominant northeasterly striking planar fabrics in the host rocks that are axial planar to isoclinal folds the fold axes parallel a synkinematic mineral elongation lineation d2 formed open asymmetrical folds with curvilinear fold axes and local crenulation cleavage the d3 event produced local conjugate kink folds radiometric dates on deformed plutons indicate these deformational events were post 369 12 ma post middle devonian and pre 352 6 ma pre late carboniferous the fault was previously interpreted as a cryptic suture along which opposed margins of the iapetus ocean were juxtaposed supposedly this sut

a kinematic model for the collision and complete suturing between gondwanaland and laurussia in the carboniferous maps of middle to late carboniferous strike slip faults in western europe northern africa and eastern north america have been compiled from geological gravity and magnetic onshore and offshore data a large majority of these faults can be interpreted in terms of a logical syn to post collisional pattern superimposed on the initial suture between the african i e gondwana and north american european laurussian continental margins the pre collision positions of the major continents are derived from paleomagnetic data and when combined with the general distribution of the strike slip faults lead to a kinematic model of the complex middle late carboniferous collision in this model the west african precambrian craton acted as a rigid promontory or die with respect to the partially rigid plastic zones between it and the large north american craton these partially rigid plastic zones comprise the external appalachian and canadian maritime margins the mauritanide and hercynian margins of north

geochemical evolution of earliest carboniferous continental tholeiitic basalts along a crustal scale shear zone southwestern maritimes basin eastern canada abstract continental tholeiites were extruded locally within the extensional wrench maritimes basin from about 375 to 330 ma late devonian early carboniferous basalts of the early carboniferous diamond brook formation are up to 1 5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness the volcanic rocks and co genetic plutons occur along a crustal scale strike slip fault that facilitated the transport of relatively primitive magma resulting in basalts with high mg number ni and cr end 4 and relatively low tio2 v y and zr fractional crystallization of plagioclase and lesser clinopyroxene in the absence of iron oxide fractionation resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in tio2 and other high field strength elements hfses rare andesites are interpreted as products of partial melting of underplated gabbro the relatively primitive basalts resemble low ti basalts found in other continental flood basalt provinces and have trace element and isotopic characteristics that have been ascribed in the literature either to partial melting of or contamination of asthenospheric melts by subcontinental lithospheric mantle in the maritimes basin similar continental tholeiites were extruded intermittently over a 45 ma period high feot 12 normalised to 8 mgo suggests high potential temperature in the source mantle presumably within a mantle plume the youngest eruption products geochemically and isotopically resemble himu oib plume activity may have gradually migrated eastward from late devonian to permian from the maritimes basin through scotland to norway and north germany

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

depositional facies textural characteristics and reservoir properties of dolomites in frobisher alida interval in southwest north dakota the mississippian frobisher alida interval is an upward shoaling cycle that began with open marine sedimentation and culminated with the deposition of a widespread sabkha salina evaporite this cycle is the most prolific oil producing interval in the north dakota portion of the williston basin most frobisher alida production in the southern williston basin is from dolomite reservoirs the six major facies defined in this paper are lithologic suites that represent sediments and precipitates deposited in similar environments dolomite formed as a replacement of muddy sediments deposited in three facies ii iv and v facies ii sediments consisted primarily of muds deposited in offshore lagoons facies iv sediments were composed of burrowed muds deposited in a restricted marine environment in front of a barrier bar and island complex fossiliferous muds which formed in an open marine environment were the dominant sediments deposited in facies v the original sediment textures strongly influenced the textures and distribution of replacement dolomite some dolomite zones display relatively uniform textural and petrophysical properties over large portions of southwest north dakota porous dolomite zones with larger average crystal sizes display better producing characteristics than zones with smaller average crystal sizes the average crystal size in frobisher alida dolomites ranges from about 8 to 44 m most large frobisher alida dolomite reservoirs in southwest north dakota occur in combination structural stratigraphic hydrodynamic traps stratigraphic traps in dolomites are found where facies changes occur or where an updip decrease in the dolomitization intensity formed lower permeability rocks a regional hydrodynamic gradient aids stratigraphic trapping in many dolomite zones that would not trap oil under hydrostatic conditions

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

geometry of the pamir hindu kush intermediate depth earthquake zone from local seismic data 1 we present new seismicity images based on a two year seismic deployment in the pamir and sw tien shan a total of 9532 earthquakes were detected located and rigorously assessed in a multistage automatic procedure utilizing state of the art picking algorithms waveform cross correlation and multi event relocation the obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the pamir hindu kush intermediate depth seismic zone with improved detail and resolution the relocated seismicity clearly defines at least two distinct planes one beneath the pamir and the other beneath the hindu kush separated by a gap across which strike and dip directions change abruptly the pamir seismic zone forms a thin approximately 10 km width curviplanar arc that strikes east west and dips south at its eastern end and then progressively turns by 90 to reach a north south strike and a due eastward dip at its southwestern termination pamir deep seismicity outlines several streaks at depths between 70 and 240 km with the deepest events occurring at its southwestern end intermediate depth earthquakes are clearly separated from shallow crustal seismicity which is confined to the uppermost 20 25 km the hindu kush seismic zone extends from 40 to 250 km depth and generally strikes east west yet bends northeast toward the pamir at its eastern end it may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth in the upper part events form a plane that is 15 25 km thick in cross section and dips sub vertically north to northwest seismic activity is more virile in the lower part where several distinct clusters form a complex pattern of sub parallel planes the observed geometry could be reconciled either with a model of two sided subduction of eurasian and previously underthrusted indian continental lithosphere or by a purely eurasian origin of both pamir and hindu kush seismic zones which necessitates a contortion and oversteepening of the latter

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths

microstructure metamorphism and tectonics of the western cape breton highlands nova scotia microstructural and petrological data from the jumping brook metamorphic suite western cape breton highlands suggest that a single episode of syntectonic prograde metamorphism followed by uplift cooling and associated retrogression affected these rocks during mid palaeozoic times microstructures indicative of progressive crenulation foliation development can be traced from low grade chlorite zone through high grade kyanite zone rocks allowing a clear sequence of porphyroblast growth to be established metamorphic reactions and p t calculations suggest metamorphic conditions of 700 750 c at 8 10 kbar were achieved in kyanite zone rocks although a complete p t t path was not defined combined petrological and geochronological data can be used to constrain computed p t t models these models suggest that a component of post metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift the microstructural and petrological data to not support the interpretation that the high grade rocks represent pre existing crystalline basement indeed the metamorphic history geochronology and computed tectonic models all point to a single short lived episode of silurian devonian volcanism intrusion convergence regional metamorphism and uplift probably resulting from collision tectonics at an irregular continental margin