crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

crustal structure and surface zonation of the canadian appalachians implications of deep seismic reflection data in 1986 1181 km of marine seismic reflection data was collected to 18 20 s of two way traveltime in the gulf of st lawrence area the seismic profiles sample all major surface tectono stratigraphic zones of the canadian appalachians they complement the 1984 deep reflection survey northeast of newfoundland together the seismic profiles reveal the regional three dimensional geometry of the orogen three lower crustal blocks are distinguished on the seismic data they are referred to as the grenville central and avalon blocks from west to east the grenville block is wedge shaped in section and its subsurface edge follows the form of the appalachian structural front the grenville block abuts the central block at mid crustal to mantle depths the avalon block meets the central block at a steep junction that penetrates the entire crust consistent differences in the seismic character of the moho help identify boundaries of the deep crustal blocks the moho signature varies from uniform over extended di

biostratigraphy lithofacies and paleoenvironments of the gulf 718 1 well u s mid atlantic outer continental shelf abstract the gulf 718 1 well located on the southwestern flank of the schlee dome was drilled to a total depth of 3905 m the oldest sedimentary rocks sampled were not fossiliferous but are probably of late jurassic age sandstones and siltstones are the dominant lithologies except in the intervals between 1213 to 1450 m and 1993 to 2259 m where carbonates make up a major part of the section calcite is the primary cement throughout the section chert siderite diagenetic clay and dolomite are locally important the well contains coaly gas prone potential source rocks of neocomian age but these strata are only marginally mature for oil the paleoenvironments recorded in the strata represent a general upward progression from nonmarine to marginal marine to neritic depositional conditions these paleoenvironments are more marine than the stratigraphically equivalent sedimentary regimes of the new jersey coastal plain to the west but are generally more terrestrial than the paleoenvironments recorded in wells drilled seaward to the east

thermochronologic record of pluton emplacement deformation and exhumation at courtright shear zone central sierra nevada california thermochronology and thermobarometry of two plutons and a ductile shear zone at their contact in the central sierra nevada california as defined over the range 700 to 300 c by 40 ar 39 ar dating of hornblende and biotite and u pb dating of zircon indicate that ductile shear coincided closely with dramatically increased cooling and possible decompression of both plutons deformation occurred as the younger 90 ma mount givens pluton was emplaced against the older 102 ma dinkey creek pluton which was softened by conductive heating during extension these events coincided with significant regional exhumation possibly a result of crustal inflation recorded by a major pulse of rapid sediment accumulation in contemporaneous fore arc strata of the great valley sequence

synmagmatic deformation patterns in the old woman mountains se california abstract synmagmatic structures within granitic plutons may provide insight into the interplay between plutonism and regional deformation at mid crustal levels within the late cretaceous old woman pluton mojave desert se california synmagmatic structures include magmatic fabrics parallel to wall rock foliations melt filled shear zones folded gneissic xenoliths and schlieren with axial planar magmatic fabric deformation continued below granite solidus temperatures in shear zones on the pluton margins these structures permit reconstruction of a sheet like pluton that was emplaced under regional extension taken together with published thermochronometric data the synmagmatic structures constrain the role played by magmatic events in the collapse of a thickened crustal section the post plutonic cooling history suggests rapid uplift and denudation ending by 65 ma we argue that extensional deformation overlapped and post dated pluton emplacement and may have contributed to tectonic unroofing we suggest that magma emplacement can trigger subsequent extensional collapse in overthickened crustal blocks

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

the structure of the lower crust at the argentine continental margin south atlantic at 44 s abstract it is well established that the argentine passive margin is of the rifted volcanic margin type this classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope manifested by seismic data as a seaward dipping reflector sequence sdrs here we investigate the deep structure of the argentine volcanic margin at 44 s over 200 km from the shelf to the deep oceanic argentine basin we use wide angle reflection refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times the resulting p wave velocity depth model confirms the typical volcanic margin structure an underplated body is resolved as distinctive high seismic velocity vp up to 7 5 km s feature in the lower crust in the prolongation of a seaward dipping reflector sequence a remarkable result is that a second isolated body of high seismic velocity vp up to 7 3 km s exists landward of the first high velocity feature the centres of both bodies are 60 km apart the high velocity lower crustal bodies likely were emplaced during transient magmatic volcanic events accompanying the late rifting and initial drifting stages the lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the south atlantic rift evolved by instantaneous breakup of longer continental margin segments these segments are confined by transfer zones that acted as rift propagation barriers a lower crustal reflector was detected at 3 to 5 km above the modern moho and probably represents the lower boundary of stretched continental crust with this finding we suggest that the continent ocean boundary is situated 70 km more seaward than in previous interpretations

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

transpressional shearing and strike slip partitioning in the late cretaceous sierra nevada magmatic arc california the rosy finch shear zone rfsz represents an example of synmagmatic strike slip tectonics in the east central sierra nevada and provides information on transpressional tectonics within magmatic arcs the rfsz 1 4 km wide and 80 km long is contained in late cretaceous granitoids 92 83 ma of the mono pass intrusive suite dextral strike slip deformation is indicated by a continuous band of orthogneiss mylonitic and cataclastic deformation characterized by subvertical foliations with subhorizontal lineations the style of dextral shearing in the rosy finch shear varies along strike from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons deformation of the youngest granitoids is synmagmatic as constrained by both field observations and isotopic studies dextral shearing occurs concurrently in contemporaneous intrusive suites to the north and south tuolumne and mount whitney suggesting strike slip movement along the axis of active plutonism field studies also suggest that contraction across the arc acted concurrently with strike slip movement consistent with a transpressional setting for the late cretaceous sierra nevada magmatic arc comparison of the field data with strain models of oblique plate convergence suggests that the rfsz is a preserved ductile signature of transcurrent motion of strike slip partitioning within a transpressional orogenic system recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at 95 ma which is consistent with the timing constraints on the dextral movement on the rfsz

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

geophysical evidence of a late pleistocene glaciation and paleo ice stream on the atlantic continental shelf offshore massachusetts usa abstract interpretations of seismic reflection data collected offshore massachusetts usa reveal the first conclusive geophysical evidence of a pre wisconsinan glaciation that extended beyond the limits of the last glacial maximum lgm in the region the data image numerous glacial geomorphic features that define the extent of a paleo ice stream including 1 a regionally distributed erosion surface that forms a 50 km wide trough with steeply eroded sidewalls 4 18 and nearly 100 m in relief at the margins 2 a network of sub ice sheet meltwater channels and 3 a transparent glacigenic seismic unit the orientation of the paleo ice stream trough indicates that the ice stream flowed to the south southwest toward the shelf break this suggests that the ice stream formed further to the north where it appears that georges bank southeast of the gulf of maine usa redirected ice flow limited well data constrain the glacial erosion event up to 300 m below sea level to occur within the pleistocene the glacial event represents a time of larger ice volume on the northern atlantic continental shelf as compared to the lgm thus we suggest that the event corresponds to marine oxygen isotope stage 12 late pleistocene when the first major pleistocene shelf crossing glaciation began offshore southeastern canada these geophysical constraints on a late pleistocene glaciation offshore massachusetts have important implications for 1 models of the laurentide ice sheet as the geomorphic evidence of pre lgm ice streams are difficult to characterize yet account for most of the ice sheet s mass flux and 2 the pore water salinity pattern offshore new england as sedimentary basins near an ice sheet margin often contain large volumes of glacially emplaced freshwater

rapidity of orogenesis in the paleoproterozoic halls creek orogen northern australia evidence from shrimp zircon data cl zircon images and mixture modeling studies combining u pb shrimp zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely spaced geological events important for understanding tectonothermal processes in the paleoproterozoic halls creek orogen of northern australia the youngest detrital zircon grains from a low grade quartz muscovite psammite of the tickalara metamorphics have a 207 pb 206 pb shrimp age of 1864 or 4 ma defining a maximum depositional age for the unit zircon crystals from a high grade garnet biotite metapelite approximately 5 10 volume percent leucosome from the same sequence are considerably more complex and shrimp analyses form a single large concordant group in the range approximately 1885 to 1830 ma the zircon crystals contain three distinct cl zoning patterns and individual shrimp spots show a corresponding variation in 207 pb 206 pb ages concentric oscillatory zoned zircon dominates pre 1850 ma ages and is interpreted as detritus from igneous source rocks narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning these rims comprise most of the post 1850 ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism reflecting the interaction of a robust pre existing detrital zircon suite with a very limited melt volume in addition some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 ma dividing the data into detrital and metamorphic suites solely on the basis of cl imaging yields an older group of 18 analyses 207 pb 206 pb age 1867 or 4 ma and a younger group of 10 analyses 207 pb 206 pb age 1843 or 4 ma not including five shrimp spots within areas of unzoned zircon mixture modeling of all 33 analyses in the post 1900 ma data set resulted in a best fit solution composed of two distinct components 1 an older group of 19 analyses with an age of 1867 or 4 ma and 2 a younger group of 14 analyses with an age of 1845 or 4 ma these results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores importantly the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of cl zoning patterns these data imply that high temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source such rapid rates of erosion deposition and burial have rarely been proposed for proterozoic rocks despite evidence for analogous orogenic processes in the mesozoic and cainozoic on a comparable timescale careful evaluation of geological and geochronological data in proterozoic provinces elsewhere may reveal similar patterns with potential implications for the possible rates of proterozoic orogenesis and crustal evolution

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

tectonothermal chronology of early cadomian arc development in guernsey and sark channel islands the cadomian orogen exposed in nw france and the channel islands comprises a collage of variably displaced terranes which record the late proterozoic evolution of a continental margin within a complex subduction zone on the islands of guernsey and sark calc alkaline quartz diorite stocks were emplaced during early stages of the cadomian orogeny zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions upper intercept ages for these discordant concentrates are c 1300 ma sm nd whole rock model ages t dm suggest older ages of c 2000 ma conversely the smallest and least magnetic zircons exhibit a distinct lead loss pattern suggesting that the 207 pb 206 pb dates of c 700 ma correspond to the time of crystallization of the stocks and provide a lower age constraint for initiation of subduction related magmatism in the cadomian orogen hornblende concentrates from deformed quartz diorite stocks exposed on guernsey and sark display discordant 40 ar 39 ar age spectra five samples record well defined isotope correlation ages ranging between 596 ma and 606 ma a hornblende concentrate from host orthogneiss sark yields a plateau age of 608 ma the hornblende ages are interpreted as dating post metamorphic cooling following late proterozoic deformation and metamorphism within the arc system both magmatic and cooling ages from this early cadomian arc contrast with those of younger initial tectonothermal activity c 570 ma and c 540 ma in other cadomian terrane elements and suggest palinspastic separation until very late stages of the orogeny the polyphase nature of this late proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of gondwana e g sw iberia west africa

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

mesozoic stratigraphy and paleoenvironments of the georges bank basin a correlation of exploratory and cost wells abstract the exxon 975 1 conoco 145 1 and mobil 312 1 hydrocarbon exploratory wells and the continental offshore stratigraphic test cost g 1 and g 2 wells were drilled in the southeastern part of the georges bank basin we used drill cuttings and logs from these wells to describe and correlate the dominant lithostratigraphic units and to document lateral changes in the depositional environments the strata penetrated by the exxon 975 1 and cost g 1 wells are much more terrestrial than at the seaward downdip cost g 2 conoco 145 1 and mobil 312 1 wellsites oldest rocks penetrated by the exploratory wells represent a middle jurassic carbonate evaporite sequence that correlates to the iroquois formation the iroquois records nonmarine to marginal marine sabkha tidal flat and restricted lagoonal paleoenvironments in the exxon 975 1 and cost g 1 wells but progressively changes to inner neritic littoral and lagoonal paleoenvironments at the mobil 312 1 wellsite the nonmarine deltaic siliciclastics of the overlying mohican formation misaine shale and mic mac mohawk formations are thicker and the marine carbonates of the scatarie and bacarro limestones are usually thinner in the jurassic strata of the exxon 975 1 and cost g 1 wells than at the other wellsites similarly lower delta plain and delta front facies of the early cretaceous missisuaga and logan canyon formations at the exxon 975 1 and cost g 1 wellsites reflect a greater terrestrial influence than the laterally equivalent shallow marine to delta front facies present at the downdip wellsites

tectonic evolution of a continental magmatic arc from transpression in the upper crust to exhumation of mid crustal orogenic root recorded by episodically emplaced plutons the central bohemian plutonic complex bohemian massif the central bohemian plutonic complex cbpc consists of episodically emplaced plutons the internal fabrics of which recorded tectonic evolution of a continental magmatic arc the 354 350 ma calc alkaline plutons were emplaced by multiple processes into the upper crustal tepla barrandian unit and their magmatic fabrics recorded increments of regional transpression multiple fabrics of the younger 346 ma blatna pluton recorded both regional transpression and the onset of exhumation of mid crustal orogenic root moldanubian unit continuous exhumation related deformation during pluton cooling resulted in the development of a wide zone of sub solidus deformation along the se margin of the cbpc finally syn exhumation tabular durbachitic pluton of ultrapotassic composition was emplaced atop the intrusive sequence at 343 340 ma and the ultrapotassic tabor pluton intruded after exhumation of the orogenic root 337 ma we suggest that the emplacement of plutons during regional transpression in the upper crust produced thermally softened domain which then accommodated the exhumation of the mid crustal orogenic root and that the complex nature of the tepla barrandian moldanubian boundary is a result of regional transpression in the upper crust the enhancement of regional deformation in overlapping structural aureoles the subsequent exhumation of the orogenic root domain and post emplacement brittle faulting

u pb geochronological constraints on the tectonothermal evolution of the paleoproterozoic basement of cadomia la hague nw france abstract the la hague region of nw france provides some of the best exposures of paleoproterozoic basement in the northernmost segment of the cadomian orogen one of several neoproterozoic arc fragments in the circum atlantic region the basement rocks which include orthogneiss metasediments and metabsites contain a comparable series of structures with a common metamorphic history thus the basement units are regarded to have undergone the same broad tectonothermal evolution the metasedimentary succession includes migmatites that preserve relict kyanite overprinted by sillimanite associated with the development of garnet bearing leucosome biotite dehydration at a metamorphic peak of 700 800 c and 8 9 kbar is considered responsible for the anatexis evident in both the metasediments and orthogneiss units a u pb zircon crystallization age of 2061 2 7 ma for the culeron orthogneiss a member of the metamorphic suite confirms the paleoproterozoic protolith age idioblastic rims to 2 06 ga zircons from the culeron orthogneiss yield shrimp u pb dates of c 618 ma providing the first geochronological constraints on the likely age of high grade metamorphism and anatexis of the paleoproterozoic basement in cadomia foliated c 615 610 ma intrusions that intrude the metamorphic units record magmatic to solid state fabrics that parallel metamorphic fabrics within the basement consistent with deformation and metamorphism being late neoproterozoic in age

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part b the tectonic heritage abstract the dakhla 2002 experiment on the northwest african margin obtained deep penetration multichannel reflection mcs and wide angle seismic obs data these results were combined with total magnetic intensity data kinematic reconstructions and geological field studies to decipher the early tectonic and sedimentary history of the south moroccan margin adjacent to the reguibat shield the structural study of this margin shows that the crust thins from about 28 km beneath the un thinned continental domain to less than 10 km over a lateral distance of about 120 km the dakhla margin differs from the structural scheme of the adjacent african margin segments there the moho rise generally observed just above the deepening of the top of the basement is achieved with an offset of around 50 km with respect to the latter in early jurassic time the break up of continental crust in what is now the central atlantic led to the formation of two basins one on either side of the reguibat which correspond to the older parts of segments of the late palaeozoic hercynian orogen the dakhla segment appears as a precambrian cratonic zone squeezed between two orogenic segments which have remained unaffected by break up processes and the lower crust of this particular domain has behaved differently from all other neighbouring appalachian north america and mauritanide domains this observation points to the important role of tectonic inheritance in the structural development of passive continental margins

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

block island fault a paleozoic crustal boundary on the long island platform a major fault cutting through most of the crust can be identified and mapped on the long island platform using multichannel seismic reflection profiles and magnetic data the fault here called the block island fault bif strikes north northeast dips westward at low angle and does not resemble the thin skinned thrust faulting observed in the foreland of the appalachians the bif is located within the hinterland of the appalachian mountain belt in the collision zone between africa and north america we present several interpretations but favor one in which the fault originated as an east verging mid late paleozoic thrust fault possibly related to the collision of avalon or meguma with north america it was probably reactivated during early mesozoic continental breakup and again in the late cretaceous and tertiary causing the steeply dipping postrift new shoreham fault to form either as an antithetic normal or splay reverse fault

dynamic modeling of divergent margin formation application to the u s atlantic margin abstract a finite element model of extension in the continental lithosphere shows that on the u s atlantic margin the presence of pre existing weaknesses within the lithosphere can account for variations in subsidence and extension hinge zone development and some aspects of magmatic activity during rifting the lithosphere is treated as a layered viscoplastic material consisting of a quartz diorite crust and dunite mantle the crustal weakness may results from thrust faulting during the paleozoic applachian orogeny and is treated as a region within the crust with a weaker rheology the mantle weakness results from thickening of the crust below the pre rift appalachian orogen the model indicates that subsidence in the early stages of rifting is focused in the region encompassing in crustal weakness at the location where the triassic rift basins formed at about 225 195 ma extension within the mantle during this stage of rifting is diffuse and a shear zone develops within the lower crust connecting the crustal weakness to the mantle weakness in a manner akin to simple shear as rifting continues extension within the mantle becomes focused in a narrow region under the mantle weakness the crust above the mantle weakness is thermally weakened and the locus of subsidence and crustal extension shifts to the region overlying the mantle weakness the style of extension from this time onward is similar to pure shear seafloor spreading ultimately develops over the position of the original mantle weakness at about 175 ma the model accounts for a number of hitherto unexplained features of the u s atlantic margin including the shifting of the locus of extension from the triassic basins to the deep offshore basins hinge zone development timing and position of magmatic activity the location of the initiation of seafloor spreading and the asymmetry of the conjugate margins of the rift the predicted shift from simple shear to pure shear rifting style may be a characteristics of other continental rifts and passive margins

imaging a lithospheric detachment at the continent ocean crustal transition off morocco abstract the sismar seismic survey on the moroccan atlantic margin recorded deep penetration images of the continent ocean boundary this paper focuses on the 3d observation of a landward dipping reflector overlaid by a layered unit the deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named s1 marks the continent ocean boundary location as the landward dipping reflector crosses the moho it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle observations are used i to build a 2d model to explain the mechanisms of the lithospheric breaking off between moroccan margin and its conjugate canadian margin the landward dipping reflector allowing mantle exhumation ii to propose an ene wnw trend for the initial rifting extension in this part of atlantic during late triassic direction compatible with that of the southern grand banks transform margin

deep velocity structure of rifted continental crust u s mid atlantic margin from wide angle reflection refraction data we present new ocean bottom wide angle seismic data collected during the 1990 edge mid atlantic multichannel seismic experiment which provide a measurement of the deep velocity structure of rifted appalachian continental crust beneath the u s east coast continental margin reflections from the entire crust and moho are visible from offsets of zero to 100 km one dimensional inverse traveltime modeling reveals a 34 km thick crust consisting of four layers beneath the post rift sediments with velocities of 5 9 6 3 6 4 and 6 85 km s this velocity structure is indistinguishable from that found beneath several appalachian terranes in the northern appalachians of new england the average velocity of 6 85 km s in the lower crust limits the landward extent of the high velocity 7 2 7 4 km s lower crust found farther seaward and supports the interpretation of that layer as the result of rift related magmatic underplating rather than as thinned pre existing lower continental crust

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

variations in deformation fields during development of a large volume magmatic arc central sierra nevada california mid to late cretaceous plutons in the central sierra nevada magmatic arc show widely preserved magmatic foliation whereas regionally developed solid state foliation is absent relatively slow cooling of these plutons and expected strain rates 10 14 suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime domains of solid state ductile shear of only slightly younger age than the plutons on the other hand indicate a contractional regime timing of pluton emplacement and movement on the shear zones have been constrained using pb u zircon and 40 ar 39 ar hornblende and biotite geochronology both plutons and ductile shear zones become younger toward the east the four more westerly shear zones which were active between ca 100 and 90 ma show steeply plunging stretching lineations whereas the most easterly and or youngest zones active between ca 88 and 78 ma show mostly oblique and or subhorizontal stretching lineations indicating a change in kinematics at ca 90 ma the above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional we propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid and upper levels probably as dikes via fractures slab flattening caused the neutral or weak extensional regime to move eastward away from the trench increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons and domains of ductile shear formed in progressively younger plutons to the east the above events were accompanied by an oblique convergence vector between north america and farallon plates engebretson et al 1985 which imposed a relatively small component of right lateral shear onto the arc that increased with time we estimate that at ca 100 ma the convergence vector made an angle obl 20 to the arc normal and we suggest that around ca 90 ma obl passed through a critical value conceivably 20 oblcrit 30 at this juncture the component of right lateral shear became sufficiently large to induce significant arc parallel strike slip movement on the most easterly shear zones these kinematics continued as the dominant scheme possibly as late as ca 78 ma

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

time span of plutonism fabric development and cooling in a neoproterozoic magmatic arc segment u pb age constraints from syn tectonic plutons sark channel islands uk abstract new u pb zircon and titanite dates from syn tectonic plutons on the british channel island of sark constrain the time span of plutonism fabric development and cooling in this part of the neoproterozoic cadomian magmatic arc the tintageu leucogneiss is a mylonitic unit that was dated previously at 615 6 4 2 2 3 ma the port du moulin quartz diorite which intruded the tintageu unit contains a high strain solid state deformation fabric that is less intense than but parallel to fabrics in the leucogneiss and yields a u pb zircon date of 613 5 2 3 1 5 ma the little sark quartz diorite also displays solid state deformation fabrics in addition to relict magmatic textures and yields a u pb zircon date of 611 4 2 1 1 3 ma the north sark granodiorite is largely penetratively undeformed exhibits mainly magmatic fabrics and textures and has a u pb zircon date of 608 7 1 1 1 0 ma two fractions of titanite from each intrusion are essentially concordant and are identical within error with mean dates of 606 5 0 4 ma port du moulin quartz diorite 606 2 0 6 ma little sark quartz diorite 606 4 0 6 ma north sark granodiorite the new u pb data in combination with previous u pb and 40 ar 39 ar data and previous field studies confirm the syn tectonic nature of the sark plutons and quantify the time span ca 7 m y required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation titanite u pb and hornblende 40 ar 39 ar dates mark final cooling about 2 m y after intrusion of the last pluton

significance of a hiatus in down temperature fabric development within syn tectonic quartz diorite complexes channel islands uk detailed fabric development histories have been deduced for neoproterozoic foliated quartz diorite complexes of the channel islands uk the plutons were emplaced during regional d2 deformation magmatic fabrics formed at low crystal contents pre rheologically critical melt percent are only locally preserved due to overprinting by solid state deformation which occurred at moderate temperatures 400 550 c this is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization marginal recrystallization and myrmekite development on alkali feldspar ductile deformation of quartz to form types 2 3 ribbons quartz recrystallization by sub grain rotation associated with a slip but not c slip biotite recrystallization within micro shears and variable amphibole recrystallization the deformation histories display a distinct division between magmatic state and moderate temperature solid state deformation features with little evidence for intervening high temperature solid state fabrics this contrasts with criteria widely considered diagnostic of syn tectonic plutons which envisage development of a continuum of magmatic through high temperature solid state deformation fabrics however we consider the plutons syn tectonic and attribute the hiatus to contrasting strain and pluton cooling rates length and nature of deformation events and host rock ambient temperatures intermediate magmas emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than sio 2 richer granitoids

quantification and restoration of extensional deformation along the western iberia and newfoundland rifted margins 1 many recent papers describe the structure of the iberia and newfoundland rifted margins however none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading in our study we use two pairs of cross sections considered as conjugate lines one across the galicia bank flemish cap and the other across the southern iberia abyssal plain flemish pass both transects have been imaged by reflection and refraction seismic methods and have been drilled during ocean drilling program legs 103 149 173 and 210 drilling penetrated parts of the rift stratigraphy and the underlying basement the cross sections can therefore be considered as the best documented conjugate transects across present day hyperextended magma poor rifted margins the aim of this paper is threefold 1 provide a detailed description of the crustal architecture of the two conjugate sections 2 define the extensional structures and their ages and 3 quantify the amount of strain and strain rate accommodated along these lines this paper proposes a quantitative description of extension along the iberia newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

margin segmentation and volcano tectonic architecture along the volcanic margin off argentina uruguay south atlantic abstract some 25 000 kilometers of regional multichannel seismic data acquired by bgr along the continental margins off argentina and uruguay document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusives manifested in the seismic data by huge wedges of seaward dipping reflectors sdrs the emplacement of the deeply buried 60 120 km wide sdrs was probably episodic as documented by at least three superimposed sdrs units distinct along margin variations in architecture volume and width of the sdrs wedges are probably related to margin segmentation it is suggested that the margin can be divided at least in four compartments bounded by the falkland fracture zone falkland transfer the colorado transfer the ventana transfer and the salado transfer the individual margin segments reflect in the distribution and thickness of the post rift sediments the individual transfer zones may have acted as rift propagation barriers selectively directing rift segments in left stepping patterns along the western south atlantic margin although we found extensive variations in the architecture style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the sdrs wedges decreases northward within the individual margin segments we suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along margin variations in the volcano tectonic architecture and volumes of extruded magmas

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

interplay of plutonism and regional deformation in an obliquely convergent arc southern coast belt british columbia the coast plutonic complex is an extensive zone of continental growth that formed along the mesozoic convergent margin of northwestern north america the orogeny creating this belt involved terrane accretion and assembly massive upward transfer and emplacement of sial in the form of batholiths constituting a magmatic arc and transformation of broad tracts of sedimentary and volcanic rocks into crystalline continental crust all operating in more or less the same space and time the mechanisms and interplay of these orogenic processes are well displayed in the harrison lake area of the southern coast belt british columbia great structural relief across the area exhibits a present day architecture defined by thin thrust stacked terrane sheets and early concordant pluton sheets folded on a macroscopic scale all truncated by oblique dextral reverse faults and overlain by later floored plutons construction of this complex began with terrane assembly on orogen normal thrusts during a lull in plutonism in the early cretaceous low grade metamorphism during this event indicates only modest crustal thickening subsequent plutons intruded into the assembled terranes appear to be composites of sheets early pluton sheets are now steeply dipping due to folding but were likely intruded as horizontal bodies large ovoid post folding plutons are mostly subhorizontal floored bodies at least in part sheeted these plutons are underlain by barrovian mineralogic aureoles that indicate downward vertical displacement of 10 km or more during plutonism suggesting pluton emplacement by vertical inflation magmatic fabrics in these bodies and the discordant relation of plutons to regional structures preclude emplacement in active fault zones penetrative strain aureoles flanking plutons are mostly limited to zones a few hundred meters wide and regional tectonic structures are widely preserved tectonic deformation of the arc is characterized by contraction and strike slip not orogen normal extension plutons played a greater role than terrane accretion in causing crustal thickening and high grade metamorphism

deformation partitioning in a slow spreading ridge undergoing oblique extension mohns ridge norwegian sea although oceanic spreading is often perpendicular to the ridge trends in some cases the angle between these two directions can be significantly less than 90 40 50 this occurs because of either a bend of the ridge trend or a change of the spreading direction we here describe oblique spreading in the mohns ridge resulting in deformation partitioning between the valley walls which are dominantly affected by strike slip displacements and the axial valley which is subject to nearly pure extension the axial valley walls are characterized by en echelon normal faults affecting the walls while the axial valley is affected by parallel faults grouped into oblique sets these fault sets define different structures horst or tilted blocks that are regularly spaced inside the axial valley moreover some ridge segments mainly undergo pure extension whereas others are affected by oblique extension we explain this faulting pattern including the along strike and transverse variations as a consequence of depth variations of the brittle ductile transition

structural and stratigraphic development of the newark extensional basin eastern north america evidence for the growth of the basin and its bounding structures the newark basin is a half graben bounded on its northwestern margin by a system of mostly normal slip faults many of which are reactivated paleozoic thrust faults and were active at least sporadically during the deposition of the entire basin fill as a result of along strike variations in displacement on the border fault system subsidence increased from the lateral edges toward the center of the basin and from the hinged margin toward the border fault system some intrabasinal faults also formed during the deposition of units currently preserved in their hanging walls along strike variations in the density of intrabasinal faults probably reflect changes in the dip angle of the border fault system variations in the amount of extension accommodated on these intrabasinal faults resulted in the formation of a transfer fault along strike variations in fault displacement probably produced the transverse folds in the hanging walls immediately adjacent to border faults and major intrabasinal normal faults some folds formed syndepositionally progressively younger strata onlap basement rocks along the hinged margin of the basin and younger splays of the border fault system propagated into the footwall the basin therefore grew deeper wider and longer through time

role of storm in ramgarh and west bokaro coalfields and its implication in adjacent peninsular gondwana coalfields india abstract the permian gondwana sediments in the damodar koel valley rajmahal and deoghar basins were deposited on a northwest palaeoslope in broad valleys within uplands e g aravalli vindhyan chattisgarh chotanagpur and santhal parganas upland the main winds during permian period were mostly the westerly winds coming from the northwest low pressure belt in the tethys which found no barrier between the vindhyan and santhal parganas uplands and then changing its direction moving towards east frequent storm activity also took place from the northwest tethyan low pressure belt and followed the same path of wind possibly there was a barrier in the form of a dense rain forest or elevated land at the place of east bokaro coalfield and its adjoining southern part which acted as a deterrent and minimized the effect of storm towards further east of the damodar valley intense tropical hurricanes for the generation of storm generated bedforms cannot be ruled out which probably in some cases causes marine inundation flooding deep into the landmass and also mass uprooting of trees

mid mesozoic mid jurassic to early cretaceous evolution of the georges bank basin u s north atlantic outer continental shelf sedimentology of the conoco 145 1 well abstract the conoco 145 1 exploratory well located in the southeastern portion of the georges bank basin was drilled to a total depth of 4303 m below the sea floor the oldest sedimentary rocks sampled are of middle jurassic age late bathonian callovian a dolomite limestone evaporite sequence dominates the section below 3917 m limestone is the predominant lithology in the intervals of 3271 3774 m 2274 3158 m and 1548 1981 m siliciclastics dominate the remainder of the drilled section calcite tightly cements most of the rocks below 1548 m dolomite silica siderite and diagenetic clay cements are locally important restricted inner marine environments representing lagoonal and tidal flat conditions prevailed at the wellsite during much of the deposition recorded by the callovian bathonian age iroquois formation these environments gave way to a carbonate platform which formed part of the 5 000 km long bahama grand banks gigaplatform that lasted through the end of the late jurassic encompassing the uppermost portion of the iroquois formation and the scatarie limestone and bacarro limestone members of the abenaki formation the absence of a skeletal reef association and the dominance of muddy limestone fabrics are evidence that the 145 1 wellsite was located on the platform interior major periods of siticiclastic deposition interrupted carbonate deposition and they are recorded by stratigraphic equivalents of the mohican formation misaine shale member of the abenaki formation and the mohawk and mic mac formations a series of sustained prograding delta systems the earliest of which is preserved as the missisauga formation buried the carbonate platform following its drowning in the early cretaceous berriasian valanginian the sparser primarily allochthonous lignite content and better sorted glauconite bearing sands of the missisauga strata at the 145 1 wellsite suggest that shallow marine or barrier bar environments were more prevalent than the low delta plain facies recorded farther shoreward at the cost g 1 wellsite

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the sierra nevada batholith tehachapi mountains california we present thermodynamic estimates of pressures temperatures and volatile activities in variably deformed gabbroic to granitic cretaceous 115 100 ma batholithic and framework rocks of the tehachapi mountains southernmost sierra nevada california al contents of hornblende in granitoids imply igneous emplacement at 8 kbar in the southernmost tehachapi mountains with lower pressures 3 7 kbar to the north metamorphic pressures and temperatures for garnet bearing paragneisses and metaigneous rocks were estimated on the basis of garnet hornblende plagioclase quartz and garnet biotite plagioclase quartz thermobarometers disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite based thermobarometers to rocks of contrasting composition and mineralogy preferred pressures cluster at 7 1 9 4 and 3 6 4 3 kbar incomplete knowledge of reaction histories however limits our interpretation of the lower pressures because they are minimum estimates the 4 kbar samples are all from a small area and if our interpretation is correct they imply a local more shallow event superimposed on crust once residing at deeper structural levels garnet hornblende and garnet biotite temperatures are less coherent likely owing to retrograde fe mg exchange and range from 570 to 790 c the majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling they are not granulites country rock paragneisses are typically migmatized at peak metamorphic conditions near that of the wet granite solidus 690 c veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase thermodynamic estimates of volatile activities in these garnet bearing assemblages suggest variable mostly co 2 rich fluid compositions in the absence of any pervasive fluid flux the igneous rocks of the tehachapi mountains were thus intruded at depths of 30 km making them the deepest known exposed components of the cretaceous sierra nevada batholith metamorphism occurred at these great depths and perhaps locally after 15 km of uplift before 87 ma implying an uplift rate of 1 2 mm yr a minimum uplift rate is 0 6 mm yr this original uplift and possible subsequent uplift events may have been related to underthrusting of a block of rand schist from what is now the southeast with concomitant widespread ductile deformation the deduced pressure temperature and uplift history is similar to those of high pressure high temperature cretaceous batholithic rocks in salinia and the san gabriel mountains but direct correlation is not wan anted when compared with higher level intrusive rocks from analogous portions of the sierra nevada batholith to the north the tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average but displaying a similar level of isotopic hybridization involving mantle and crustal sources the batholith is quartz rich at these levels suggestive of a weak ductile middle crust susceptible to prolonged deformation and possible delamination

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

high mantle temperatures following rifting caused by continental insulation the continents are thought to insulate and heat the underlying mantle geochemical analyses of lava samples formed at a mid ocean ridge in the atlantic ocean immediately after continental breakup show that the mantle was up to 150 c hotter than today and took about 70 million years to cool the distribution of continents is thought to influence the temperature of the underlying mantle over geological timescales insulation effects generate a build up of heat that may cause increased magmatism1 2 3 4 such as flood basalt volcanism5 6 and ultimately rift the continents causing them to break apart and new ocean basins to form here we use analyses of the major element geochemistry of lava samples collected from oceanic crust in the atlantic and pacific oceans to quantify the effect of continental insulation the lavas formed at mid ocean ridges following continental rifting and break up and preserve a record of upper mantle temperatures over the past 170 myr we find that samples from the pacific ocean formed more than 2 000 km from the nearest continental margin do not record raised mantle temperatures in contrast samples from the atlantic ocean that formed close to the margin of the rifted continent reveal an upper mantle temperature immediately after continental rifting that was up to 150 c higher than the present day average mantle temperatures remained high for 60 70 myr we conclude that the atlantic thermal anomaly was created by continental insulation and persisted in the mantle beneath the atlantic ocean long after the continental fragments had dispersed

the kennack gneiss of the lizard peninsula cornwall sw england commingling and mixing of mafic and felsic magmas accompanying givetian continental incorporation of the lizard ophiolite the kennack gneiss comprises a suite of interlayered mafic and felsic igneous rocks that intruded the 397 ma lizard ophiolite complex cornwall at 376 4 1 7 ma u pb single zircon date and were shortly thereafter c 370 ma metamorphosed to the amphibolite facies weakly deformed examples at the type locality of kennack sands reveal net veining of the mafic by the felsic component dispersed enclaves of the former in the latter and flame like interfingering of the two features indicative of magmatic commingling and mixing the silicic components of the kennack gneiss range from granodiorite to syenogranite and from metaluminous to peraluminous the least silicic rocks being the most peraluminous they were generated through anatexis of a predominantly metasedimentary crustal source having trace element and isotopic compositions comparable to those of the devonian gramscatho group of south cornwall the mafic component of the gneiss includes members group 1 which are weakly enriched in light ree have minor negative ta nb p and ti anomalies in extended trace element plots and display primitive time corrected 143 nd 144 nd and 87 sr 86 sr values and others group 2 which are more strongly enriched in all incompatible trace elements exhibit more prominent negative ta nb p and ti anomalies and have isotopic ratios intermediate between group 1 mafic components and the associated felsic gneiss group 2 represents mixtures of group 1 and granitic melts whereas group 1 rocks are comparable to emorb and were probably generated through partial melting of a weakly enriched mantle source emplacement of two commingled magmas into the base of the lizard ophiolite one representing the products of crustal anatexis implies that the ophiolite was removed from its oceanic setting and was in the process of obduction in the givetian

crustal structure fossil subduction and the tectonic evolution of the newfoundland appalachians evidence from a reprocessed seismic reflection survey reprocessed lithoprobe seismic reflection data across the appalachian orogen in newfoundland provide images of an ordovician devonian collision zone that separates laurentia from ganderia an accreted peri gondwanan microcontinent prominent reflectivity within ganderian basement tapers westward and merges with reflections that project beneath the moho outlining a probable ordovician to devonian subduction zone reflectivity within ganderian basement likely originates from transposed compositional layering within cambrian neoproterozoic arc basement migmatites and other high grade rocks of the meelpaeg allochthon were likely extruded in the devonian toward the northeast the reflection moho may have been established in the devonian in parts of newfoundland by partial melting of the lower crust reflection truncations outline a near vertical carboniferous strike slip fault zone that cuts the entire crust

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

tectonic subsidence flexure and global changes of sea level tectonic models for the evolution of passive continental margins predict that following rifting sediments should progressively onlap basement at the edge of a margin as the lithosphere cools and increases its flexural rigidity with age the pattern of modelled onlap is strikingly similar to that used by vail and colleagues to estimate sea level rise through geological time this similarity suggests that major portions of stratigraphical sequences at margins may have a tectonic rather than eustatic control the patterns of onlap used by vail and colleagues may be widespread however because several widely separated passive margins rifted at similar times but they are unlikely to be worldwide

the protracted development of the continent ocean transition in afar continental breakup and the transition to seafloor spreading is characterized by extensional faulting thinning of the lithosphere and at magmatic margins voluminous intrusive and extrusive magmatism1 2 3 4 it is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent ocean transition is buried beneath thick layers of volcanic and sedimentary rocks5 6 and the tectonic activity that characterized breakup has ceased instead the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed ancient rifted margins1 5 7 8 ongoing rifting in ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in ethiopia we show that although intrusion of magma maintains crustal thickness during the early stages of the continent ocean transition subsidence of the margin below sea level and eruption of voluminous basalt flows is initiated by late stage thinning of the heavily intruded weakened plate just before the onset of seafloor spreading we thus conclude that faulting stretching and magma intrusion are each important but at different times during breakup

a dynamic model of extension in the baltimore canyon trough region a dynamic model of extension in the baltimore canyon trough region of the central north atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the appalachian orogen the crustal weakness is hypothesized to result from thin skinned thrust faulting in the appalachian foreland and the upper mantle weakness results from crustal thickening beneath the appalachian hinterland during the early stages of rifting 225 185 ma the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear extension and thinning of the upper crust at this time results in the formation of late triassic early jurassic rift basins in the appalachian piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust the pressure gradient may also have influenced magma transport moving melt generated beneath the eastern baltimore canyon trough into the more westerly triassic basin province during the late stages of rifting 185 175 ma extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region the loci of extension in the crust and mantle are then coincident and the style of rifting becomes similar to whole lithosphere pure shear the shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the triassic basins the onset of rapid subsidence in the baltimore canyon trough and the formation of a basement hinge zone separating the highly thinned crust beneath the baltimore canyon trough from moderately thinned crust beneath the triassic basin province extension becomes increasingly focused beneath the eastern edge of the baltimore canyon trough until the onset of seafloor spreading approximately 50 m y after rifting began the resulting conjugate margins are asymmetric with most of the region encompassing the triassic basins stranded on the north american plate

integration of cocorp deep reflection and magnetic anomaly analysis in the southeastern united states implications for origin of the brunswick and east coast magnetic anomalies integration of magnetic anomaly analysis with cocorp deep reflection data from the southeastern united states provides three new constraints on the interpretation of the brunswick and east coast magnetic anomalies as well as on the reflection data these are as follows 1 the source of the brunswick anomaly lies within the deep crust this anomaly is not caused by a mesozoic rift basin as proposed by some workers 2 a simple seaward dipping high susceptibility slab model can explain both the brunswick and east coast magnetic anomalies the along strike change in character of the two anomalies results largely from a change in azimuth of the source body 3 beneath the southeastern united states this source body dips south lies immediately on the south flank of the prominent southward dipping reflective zone revealed on cocorp surveys and was previously associated with the alleghanian suture between north america and africa these results imply that a dipping highly magnetized zone in the upper plate of the alleghanian suture is responsible for both the brunswick and east coast magnetic anomalies the high susceptibility material responsible for these anomalies might be mafic lower continental or oceanic crust thrust upward during alleghanian continental collision or mafic igneous material intruded into the upper plate of the suture zone during subsequent mesozoic rifting or both the latter hypothesis implies that the alleghanian suture acted as a zone of weakness a repository which was reactivated to control the site of ultimate atlantic rifting and possibly initial sea floor spreading

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

geochronological constraints on late precambrian intrusion metamorphism and tectonism in the anti atlas mountains the bou azzer inlier anti atlas morocco is a critical element for understanding gondwanan geology because it exposes rocks formed during the paleosuturing of the gondwanan margin with peri gondwanan arc terranes numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the gondwanan margin to be bracketed detailed examination of plutons within the inlier reveal down temperature magmatic to solid state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes u pb geochronological data for the ao t abdulla diorite ma and 653 8 1 6 bou offroh granodiorite ma provide a limit to the onset of collision and regional metamorphism a 653 0 1 3 precise age of ma for the ousdrat quartz diorite provides an indication of the lower limit of the collision 640 8 1 4 in the region tectonothermal activity in the anti atlas at ca 650 ma correlates with similar periods of tectonism in the peri gondwanan terranes of avalonia and cadomia and points to the emergence of continent wide tectonism in western gondwana as early as 650 ma

controls on the stratigraphic development of the triassic fundy basin nova scotia implications for the tectonostratigraphic evolution of triassic atlantic rift basins abstract the late triassic synrift succession of the minas sub basin bay of fundy nova scotia includes a fluvio alluvial fan dominated unit lower and middle wolfville formation a transitional fluvio playa unit upper wolfville formation and an upper playa dominated package blomidon formation we describe the facies development in the 240 m thick upper wolfville formation examine vertical facies stacking patterns and develop a depositional model for this succession the unit comprises channelized and unconfined fluvial deposits interbedded with aeolian and playa sediments that grade upward into playa deposits the succession comprises progradational and retrogradational fluvial packages and thin aggradational playa claystones comparison with 11 other atlantic margin continental triassic rift basins allows an assessment of the controls that govern rift basin development the transition from fluvial to playa or lacustrine conditions occurs in all basins at different times the lack of synchronicity indicates that global climate change was not a controlling factor we suggest that the fluvial to lacustrine or playa transition was due to a decrease in source area relief related to a decline in regional tectonic activity and that the basin was hydrologically closed throughout sedimentation this tectonostratigraphic model may be appropriate for other triassic basins developed along the atlantic margin

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

timing of cadomian deformation and magmatism within la hague nw france the la hague peninsula normandy nw france contains the most northerly components of the late neoproterozoic cadomian orogeny u pb dates for single grain and small multiple grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region the syntectonic jardeheu quartz diorite and omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist facies fabric pervasive across the peninsula u pb zircon dates of 610 1 ma for the jardeheu quartz diorite and 608 2 ma for the omonville granodiorite are similar to a previously reported sensitive high resolution ion microprobe u pb date for metamorphism of the basement and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 ma an emplacement age of 573 9 1 3 0 9 ma for the syntectonic thiebot granodiorite places limits on the timing of deformation along the thiebot shear zone zircons from the undeformed st martin monzonite give a precise u pb age of 581 3 1 1 ma the new dates point to reactivation of magmatism and localized greenschist facies deformation between 582 and 573 ma

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

reprocessing and enhanced interpretation of the initial cocorp southern appalachians traverse abstract reprocessing of the 1978 1980 cocorp southern appalachian seismic reflection data has produced improved images of structures related to the emplacement of the blue ridge inner piedmont allochthon the results enhance and extend the interpretation presented previously that the blue ridge and inner piedmont are allochthonous above a shallow and shallow dipping detachment that can be followed from outcrop at the blue ridge valley and ridge transition to at least beneath the carolina terrane the continuity of reflections in the new images supports the interpretation that the southern appalachian detachment is not rooted on the east side of the inner piedmont but rather projects as a low angle detachment or zone of decoupling to beneath the coastal plain an implication of this geometry is that terranes such as the carolina terrane between autochthonous north america and the alleghanian suture beneath the coastal plain are detached thin flakes

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

pre mesozoic basement rocks offshore nova scotia canada new constraints on the accretion history of the meguma terrane basement samples from offshore wells and geophysical data show that the meguma terrane forms the basement of mesozoic cenozoic sedimentary basins as far as 200 km offshore nova scotia the data provide constraints on avalon meguma terrane interaction during middle to late paleozoic events in the appalachian orogen the basement consists of metasedimentary rocks intruded by late paleozoic granitoid plutons principally magnetite bearing granodiorites which contrast with more peraluminous ilmenite bearing devonian granodiorite and granite onshore offshore tonalites have a subduction related trace element signature that is similar to the signature in minor devonian gabbro and tonalite intrusions on land nd sr and pb isotopes suggest three sources for meguma terrane plutons 1 2 ga basement possibly saharan shield and sediments derived therefrom this basement may be present beneath the southwestern scotian shelf 2 late proterozoic underthrust avalonian basement and basement autochthonous to the meguma terrane and 3 melts from mantle last remobilized in the late proterozoic pan african orogeny seismic reflection profiles show meguma terrane thrust northward over a wedge of avalon basement extending 30 km south of the surface suture there is no evidence that the devonian tonalites are directly related to subduction the lack of either ophiolites or paleozoic pelagic sediment suggests that convergence of the avalon and meguma terranes was achieved principally by strike slip motion resulting from low angle continent continent convergence with only a minor thrust component the widespread devonian carboniferous igneous activity of the amalgamated meguma and avalon terranes was a consequence of regional extension which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material

batholith emplacement at mid crustal levels and its exhumation within an obliquely convergent margin abstract emplacement of the central part of the coast mountains batholith of northern coastal british columbia occurred within a regime characterized by oblique convergence between the farallon kula and north american plates we use new structural kinematic and u pb isotopic data to show that the locations geometry and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones these data also show that the most active period of pluton emplacement from 67 to 51 ma coincided with a change in style of deformation within the batholith prior to 67 ma plutons were emplaced within an arc dominated by regional scale contractional shear zones in contrast emplacement of 67 51 ma plutons occurred in an arc increasingly dominated by normal faults with arc parallel to oblique displacement and by sinistral transtensional shear zones we have identified and mapped the structure of three plutonic complexes composed of 67 to 51 ma plutons the khyex sill complex arden lake plutonic complex and quottoon plutonic complex shear zone controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes during and after this latest cretaceous paleogene period of intense plutonic activity and accompanying deformation the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 ma

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

u s geological survey deep seismic reflection profile across the gulf of maine deep seismic reflection and magnetic data suggest that the gulf of maine is underlain by four crustal blocks of differing reflection and magnetic character two of these blocks the gulf of maine fault zone and adjacent central plutonic zone can be correlated with avalonian rocks in southern new england and new brunswick the boundary between them the fundy fault projects onshore near the ponkapoag fault in southeastern massachusetts in a region where no major crustal boundary has yet been identified a third block called the southern plutonic zone is interpreted as avalonian although the reflection and magnetic data are ambiguous the fourth block along the rifted continental margin is correlated with meguma rocks of nova scotia the late paleozoic variscan front crosses the gulf of maine within the gulf of maine fault zone and indicates significant compressional movement rather than strike slip the moho surface throughout the region is essentially flat and may have been produced by mesozoic crustal extension associated modification of the lower crust was minimal in the northern gulf and may have been moderate in the central and southern gulf the franklin rift basin formed by reactivation of the inferred avalon meguma boundary beneath georges bank as a low angle detachment

generating melt during lithospheric extension pure shear vs simple shear the uniform stretching model has been used to calculate the volume and composition of melt generated by adiabatic decompression during extension of the continental lithosphere the consequences of this approach for melt generation by litbospheric simple shear were investigated the results show that given an initially planar detachment fault it is extremely difficult to generate melt from the asthenosphere under either normal or elevated asthenospheric potential temperatures our conclusion is independent of the initial dip of the detachment fault and also holds when the initial dip in the lithospheric mantle is double that in the crust in the north sea lithospheric simple shear fails to account for the existence and location of magmatism these conclusions may also apply more generally to passive margin development

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california