the low δ18o late stage ferrodiorite magmas in the skaergaard intrusion result of liquid immiscibility thermal metamorphism or meteoric water incorporation into magma abstractwe report new laser fluorination oxygen isotope analyses of selected samples throughout the skaergaard intrusion in east greenland particularly relying on 1 mg separates of the refractory alteration resistant minerals zircon sphene olivine and ferroamphibole we also reexamine published oxygen isotope data on bulk mineral separates of plagioclase and clinopyroxene our results show that the latest stage strongly differentiated magmas represented by 3 to 6 km3 of ferrodiorites around the sandwich horizon sh where the upper and lower solidification fronts met became depleted in 18o by about 1 5 2 relative to the original skaergaard magma and the normal mantle derived mid ocean ridge basalt earlier studies did not recognize these low 18o ferrodiorite magmas 18o 3 4 because after the intrusion solidified much of the intrusion and its overlying roof rocks were heavily overprinted by low 18o meteoric hydrothermal fluids we consider three possible ways of producing these low

disequilibrium partial melting and rheomorphic layer formation in the contact aureole of the basistoppen sill east greenland the tholeiitic basistoppen sill was intruded into the upper part of the skaergaard complex shortly after the skaergaard magma had solidified heat from the cooling basistoppen magma caused disequilibrium partial melting in the adjacent skaergaard ferrogabbros olivine ferrobustamite and magnetite were selectively melted and removed from the rock as an iron rich melagabbro magma plagioclase acted as a refractory phase during partial melting and was left behind as an anorthositic gabbro restite modal and grain size layering formed rheomorphically in the previously solidified host rocks as a result of partial melting and recrystallization the rheomorphic layers are distinct from those found elsewhere in the intrusion

seismotectonics of northeastern united states and adjacent canada data for local earthquakes recorded by a network of stations in northeastern united states and adjacent canada were analyzed to study the seismicity the relationship between earthquakes and known faults the state of stress and crustal and upper mantle velocity structure in addition portable seismographs were deployed in the field to study aftershocks as a result accurate locations for about 364 local earthquakes 2 mb 5 and 22 focal mechanism solutions were determined a comparison of the spatial distribution of these events 1970 1979 with historical earthquakes 1534 1959 reveals that seismic activity in the northeast is relatively stationary in space those areas that have had little or no seismicity historically are relatively aseismic today whereas the historically active areas are also active today the instrumental locations historical seismicity and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces 1 the adirondack western quebec province is a northwesterly trending zone of seismic activity about 200 km wide and at least 500 km long extending from the se adirondacks into western quebec canada thrust faulting on planes striking nnw to nw appears to predominate and the inferred axis of maximum horizontal compression is largely uniform and trends wsw nearly parallel to the calculated absolute plate motion of north america little or no seismicity is found where anorthosite outcrops at the surface correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep ne or sw gradient in bouguer anomalies this zone does not appear to extend southeastward to boston as proposed by some workers 2 the appalachian province is a northeasterly trending zone of seismic activity extending from northern virginia to new brunswick canada highangle reverse or thrust faulting on n to ne trending planes appears to predominate the western margin of this province however appears to be relatively aseismic we attribute this relative lack of activity to one or more of the following the presence of igneous activity postdating rifting of africa from north america the occurrence of intense metamorphism during the acadian orogeny which may have annealed preexisting faults and the predominance of ductile as opposed to brittle deformation in the geologic past the inferred axis of maximum horizontal compression along the eastern margin of the appalachians is rather uniform and trends w wnw almost perpendicular to the magnetic lineations offshore we suggest that this w wnw compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers

anatectic migmatites from the roof of an ocean ridge magma chamber a well preserved contact aureole at the base of the sheeted dyke is likely to be centred at or very close to the sheeted complex in the troodos ophiolite cyprus records the partial melting dyke gabbro boundary this region of the crust is a of the roof of a mid ocean ridge magma chamber hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10 30 m of the sheeted dyke in rheology and thermal structure it is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust e g deand closest to the underlying gabbros 30 at water undersaturated conditions and temperatures if not all of the lower crust e g phipps morgan chen 875 c geological considerations indicate that the duration of 1993 quick denlinger 1993 boudier et al 1996 melting events was probably on the time scale of eruptive cycles i e and magmatic heat is rapidly transferred across a thin years to centuries this resulted in disequilibrium partial melting 100 m conductive boundary layer e g lowell and highly imperfect melt segregation within the contact aureole burnell 1991 and advectively transported out of the we hypothesize that partial melting is a common process within crust by hydrothermal fluids e g cann et al 1985 the roof zones of axial magma chambers at fast spreading ridges geophysical surveys along the east pacific rise epr and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties e g melt volumes proportion of melt and magma field relations suggest that where the highest degrees of crystals of amcs are not static and change on the time melting occurred a mixture of anhydrous hornfels and hydrous scale of eruptive cycles years to centuries wilcock leucocratic melt disaggregrated and was assimilated into the magma delaney 1996 singh et al 1998 chamber it is well known from ophiolite studies that the roofs of amcs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks particularly within the uppermost few hundred

geochemistry of dendritic anorthosites and associated pegmatites in the skaergaard intrusion east greenland evidence for metasomatism by a chlorine rich fluid abstract in the upper zone uza of the skaergaard intrusion dendritic anorthosites and pegmatites are found in a zone of deformed gabbro along the flank of a trough banded structure apatite biotite some having over 2 wt c1 and amphibole have higher chlorine contents than those found in the surrounding rocks and indicate the presence of a high temperature aqueous fluid having a high hcl hf fugacity ratio the fluid must have been derived locally by exsolution of magmatic volatiles because hydrous minerals in nearby hydrothermal veins have low chlorine contents and the structures are far from the margin of the intrusion apatite and quartz in the pegmatites have abundant fluid inclusions containing cubic halide crystals giving evidence for a separate aqueous fluid coexisting with an evolved residual melt the pegmatites are also characterized by mafic and felsic clots with compositions similar to immiscible silicate liquids dendritic anorthosites have equal or higher abundances of zr hf ta and rare earth elements than the surrounding gabbros yet they contain little pyroxene or fe ti oxides the unusual trace element enrichment patterns and the associated rare assemblage of zirconolite zircon baddeleyite and allanite suggest that the chlorine rich fluid was enriched in trace elements derived from the residual melt and the reacted mafic minerals deformation of the magma at a crystallinity below the critical melt fraction resulted in localized exsolution and channeling of a magmatic aqueous fluid and an evolved residual melt dissolution of pyroxene olivine and fe ti oxides allowed for further deformation and expulsion of the mafic melt leaving a plagioclase rich residue these structures are the product of enhanced reaction locally of an aqueous fluid which may have moved upward as a nearly planar reactive front throughout the lower crystallization zone in addition to the transport of trace elements migration of a high temperature fluid through the zone of crystallization could aid in the compaction of igneous cumulates and the development of layering this study presents the first evidence for a high temperature chlorine rich aqueous fluid in the layered series of the skaergaard intrusion and illustrates the important effect of such fluids on the physical and chemical evolution of layered intrusions

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

the evolution of chabahar beach ridge system in se iran in response to holocene relative sea level changes abstract the chabahar strandplain chs stretches along the chabahar bay at a width of more than 5 km along the northern coast of the gulf of oman and se iran an area that has been subjected to tectonic uplift as a part of the makran accretionary prism the chs comprises of beach ridges inter ridge swales sand dunes tidal channels and fluvial deposits we present the first documentation of the spatial distribution and the internal architecture of the chs based on topographic surveys sediment percussive coring trenching and ground penetrating radar gpr transects radiocarbon dating on marine shells from foreshore deposits of 8 representative beach ridges yielded ages between 4800 and 270 cal years bp at respective distances of 4800 to 670 m from the present shoreline we interpret the boundary between the foreshore beach and the foredune deposits as indicator of past sea levels by analogy to present shore processes this boundary is readily recognizable in sediment profiles from cores and trenches as well as gpr reflectors based on the age model and depositional features we estimate relative sea level fall of up to 15 m over the past 4800 years considering that the eustatic sea level changes for this period are negligible for the makran coast this relative sea level fall is related to tectonic uplift across the coastal makran the elevation of palaeo shorelines with similar ages decrease from east to west of the chs which suggest that alongshore variations in uplift rates are likely related to different movements of coastal fault blocks

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

apatite dissolution into peraluminous haplogranitic melts an experimental study of solubilities and mechanisms apatite ap dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750 c and 200 mpa ph2o as functions of ap grain size abundance melt ap ratio and melt asi alumina saturation index mol al2o3 na2o k2o cao between an asi of 1 1 1 metaluminous to mildly peraluminous the solubility of ap as measured by p2o5 in the melt is low 0 1 wt p2o5 which agrees with previous work in equilibrium runs with more strongly peraluminous compositions however ap solubility increases linearly with asi to p2o5 0 63 wt in melt at asi 1 3 and can be denned by a simple equation p2o5 3 4 3 1 asi r 0 833 which agrees within error with previous calculations reported in the literature in experiments where ap grains are sparce and widely separated i e simulating ap distribution in an ap bearing peraluminous protolith undergoing anatexis the dissolution of ap involves transient disequilibrium that promotes higher solubility of p2o5 up to nearly six times the equilibrium values adjacent to ap at a given bulk asi of melt and this increases the al si of melt in the ap dissolution aureole the approach to equilibrium between ap and melt is hastened by increasing the ap melt ratio within the local melt pool e g the capsule or by decreasing ap grain size for a similar melt ap ratio diffusivity calculations using inverse error function methods for the normal diffusion of p and ca yield dp 10 11 cm2 s and dca 10 10 cm2 s confirming prior results that diffusion of p away from ap surfaces is the rate limiting factor in ap dissolution these results combined with previous work at higher temperatures but on similar hydrous melts yield an activation energy e for p diffusion of 16 5 kcal mol and a frequency factor d0 of 1 4 10 3 cm2 s phosphorus diffusion through the melt is accompanied by uphill diffusion of al resulting in an increase of up to 2 wt al2o3 in melt near the ap within the ap dissolution aureole melt contains one additional al cation for every p cation i e al p 1 1 which is suggestive of an associated alpo4 complex the exchange reaction in melt is represented by alpsi 1 in which 2 to4 clusters are created in the place of 1 this exchange should be accompanied by discernable changes in melt properties e g density molar volume viscosity the results have applications to equilibrium and local disequilibrium effects during anatexis of apbearing metapelites and to the saturation limits of ap during crystallization of peraluminous magmas the dissolution of flux bearing minerals such as ap e g those containing p b or f may influence the stability and solubility of other trace element bearing minerals during anatexis of metapelites texture e g grain size and distribution and spacial relationship to other minerals plays an important role in reaction kinetics and at least temporarily in mineral solubilities the disequilibrium behavior of accessory minerals has critical consequences for ree modelling of granites

focal mechanisms of small earthquakes and the stress field in the western quebec adirondack region focal mechanisms were determined for 42 small earthquakes 1 4 m 3 4 that occurred in the adirondacks western quebec seismic region the analysis demonstrates the variability of the focal mechanisms and suggests that the region can be divided into two subprovinces with distinctive seismotectonic regimes 1 1 in the adirondacks the focal mechanisms are mostly strike slip with a comparatively smaller thrust component and indicate nne compression 2 2 in the western quebec seismic region the stress field is more heterogeneous and is dominated by two directions ne and nw of horizontal compression this variable pattern suggests that the seismicity and stress field are controlled by regional plate stresses but are affected by local structures and or by the predominant orientation of preexisting faults

40ar39ar dating of the skaergaard intrusion we report the first 40ar 39ar data for the skaergaard intrusion based on analyses of hornblende and biotite from a transgressive granophyre from upper zone a uza of the layered series hornblende separates heated in a resistance furnace and laser heated single crystals of biotite from the granophyre give plateau ages that are consistent with isochron ages of 55 48 0 30 and 55 40 0 14 ma respectively 2 internal uncertainty with initial 40ar39ar ratios of 292 4 19 0 and 291 1 7 2 if uncertainties in the ages of 40ar 39ar standard fluence monitors are considered external uncertainties 2 errors are 0 76 and 0 72 ma respectively resistance heating analysis of biotite separated from a gabbro from lower zone a lza of the skaergaard layered series indicates that the sample has been extensively contaminated by ar inherited from old crust and does not constrain the intrusion age apart from indicating that it is less than approximately 57 ma old the ar cooling ages are consistent within uncertainty with both pluton cooling models presented by norton and taylor 1 but the data agree best with their model s2 which assumes a smaller permeability 10 13 cm2 combination of the new ages and the norton and taylor cooling models constrains the emplacement and final crystallization of the skaergaard magma chamber to be at 55 75 0 30 ma and 55 65 0 30 ma respectively evidence for late crystallizing quartz and tridymite near the sandwich horizon sh combined with phase equilibria constraints on the near solidus crystallization temperature of the sh 975 25 c require the pressure of sh solidification to be 60 15 mpa this means that the tertiary basalts and sediments overlying the kangerdlugssuaq region were 3 2 0 8 km thick at 55 65 0 30 ma assuming that the 6 7 km thick tertiary section exposed further east along the east greenland coast is representative of that which accumulated above the skaergaard we infer that the skaergaard emplacement occurred during the main stage of the east greenland flood basalt event after approximately half of the tertiary section had erupted the onset of coastal flexure in the kangerdlugssuaq region was after 55 48 0 76 ma

characteristics of the western quebec seismic zone a comparison of geological data shows that most of the earthquakes of western quebec lie within or near the eastern and western boundaries of a grenville ca 1000 ma central metasedimentary belt and in the area of the younger fault zone along the ottawa river although most of the earthquakes have occurred within terrain of grenville age or older the younger rift zone is characterized by the larger earthquakes temiskaming m6 2 1935 cornwall massena m5 7 1944 the earthquakes also correlate with gradient zones around two prominent anomalies in the smoothed bouguer gravity field the pattern of aeromagnetic anomalies and drainage channels reflects preeambrian structural trends and indicates the central metasedimentary belt continues beneath paleozoic sediments to lake ontario the strikes of magnetic gravity and drainage features north of lake ontario are parallel to features mapped to the south of the lake geological data and geophysical trends suggest that a cataclastic zone is continuous alon

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

liquid segregation parameters from amphibolite dehydration melting experiments we have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 gpa and 750 1000 c the solidus for this calcic amphibolite 68 mg hb 32 pl an 90 is 750 c amphibolites begin to melt at relatively high temperatures in the garnet absent field but the solidus appears to backbend at 1 gpa coincident with the garnet in boundary hornblende breakdown due to garnet formation releases h 2o and causes melting thus in the garnet present field 1 gpa the amphibolite dehydration melting solidus may be coincident with the h 2o saturated solidus liquid interconnectivity may be achieved at 900 c and 5 vol liquid based on both physical and chemical data from solid rock runs mass balance calculations from powdered rock runs suggest that small amounts 5 15 vol of hydrous 4 wt h 2o low viscosity 10 3 10 4 pa s heavy rare earth element depleted felsic liquid may be segregated during amphibolite dehydration melting at 875 c the rapid breakdown of coarse grained hb cores may lead to the formation of transient h 2o saturated liquids with even lower viscosities 10 2 pa s although comprising only a small portion of the melting cycle these h 2o rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite e g increased deformation and liquid fracturing during anatexis of coarse grained natural rocks transient conditions may control the initial stages of liquid segregation

supersaturation and crystal growth in the roof zone of the skaergaard magma chamber the coarse grained upper border series rocks of the skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite skeletal and hopper crystals of apatite and less abundant sector zoned augite crystals and hopper zircon crystals in addition the melanogranophyres which occur as pods and lenses in the lower part of the upper border series and the upper part of the layered series are characterized by very coarse grained dendritic ferrohedenbergite crystals skeletal hopper and sectorzoned crystals are not present in the layered series gabbros the development of these unusual crystal morphologies in the upper border series requires that the roof zone magma was intermittently supersaturated and indicates that the skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient it is suggested that supersaturation developed in the roof zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization and as a result of sudden volatile loss during the later stages of crystallization when the upper border series rocks became rigid enough to fracture

fluid present disequilibrium melting in neoarchean arc related migmatites of daeijak island western gyeonggi massif korea the melting process of meta igneous rocks was investigated via field petrographic and geochemical analyses of the neoarchean 2 51 ga migmatite complex in daeijak island western gyeonggi massif this complex consists primarily of garnet free amphibolites and tonalitic migmatites both of which contain hornblende plagioclase and quartz as major constituents neosomes and leucosomes in the migmatite have dioritic tonalitic and tonalitic trondhjemitic compositions respectively compositions of hornblende xfe 0 39 0 42 and plagioclase an24 27 vary little between the neosomes and leucosomes the amphibolites show distinct depletions in nb ta zr and ti relative to large ion lithophile elements suggesting an arc related origin for their basaltic protolith leucosomes have lower contents of k2o mgo feo tio2 zr rb and rare earth elements ree than amphibolites and neosomes but are higher in sio2 na2o and sr contents leucosomes and neosomes have positive eu eu n 1 32 7 26 and negative 0 71 0 97 eu anomalies respectively which are attributed to the variable degree of plagioclase fractionation during the partial melting the p t condition for the migmatite formation was estimated to be 700 730 c and 4 7 5 5 kbar primarily based on the hornblende plagioclase thermobarometry and phase equilibria various lines of textural evidence such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn deformation crystallization of melt chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the ree modeling between melt product and source rock disequilibrium process is most likely attributed to the rapidity of melt extraction or migration compared to chemical diffusion rate in summary the fluid present disequilibrium melting of dioritic tonalitic protoliths has produced tonalitic trondhjemitic leucosomes in a dynamic arc environment prevalent in the neoarchean crust of the korean peninsula as well as the shandong peninsula north china craton

neotectonic movement earthquakes and stress state in the eastern united states abstract a regional pattern of crustal movement across the eastern united states is suggested by relations found by investigations into the cause of earthquakes the pattern consists of zones of fractures and vertical movement most of the seismic source areas appear to be related to northwest trending fracture zones that commonly have apparent righ lateral strike slip offset where they cross broad northeast trending belts of vertical movement the fracture zones also appear to control the positions of local northwest trending basins embayments on the atlantic and gulf coastal plains these embayments seem to be relatively subsiding at present and have earthquakes associated with them intervening arches have very few earthquakes some earthquakes are also spatially related to north trending extensional fault zones that commonly lie north of northwest trending fracture zones a slight northward tilt of the entire region and possible lingering effects of glacial rebound in the northeast show no relation with seismicity both the broad belts of vertical movement and embayments are apparently those initiated by the late cretaceous as the north atlantic basin widened and the northwest trending fracture zones can be directly or indirectly related to the offshore transform fracture zones the interplate movements in the eastern united states are thus directly related to those in the north atlantic basin the fracture pattern is also similar to that of the western united states but the movements less these tectonic movements in the eastern united states can be accounted for by a maximum horizontal compressive stress oriented approximately north northwest south southeast to north south and an extension oriented approximately east west