crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

seismic image of undercrusted serpentinite beneath a rifted margin during the continental rifting the upper mantle was unroofed and the mantle rocks were transformed into serpentinite at the ocean continent transition of the west galicia margin spain the serpentinite layer several km thick extends probably eastwards beneath the highly thinned continental crust of the margin the serpentinite layer was recently imaged by seismic reflection it is discontinuously and deeply layered as serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

the tectono stratigraphic evolution of distal hyper extended magma poor conjugate rifted margins examples from the alpine tethys and newfoundland iberia abstract in order to capture the general tectono stratigraphic framework of a magma poor conjugate rifted margin two examples have been analysed and compared the seismically imaged and drilled iberia newfoundland rifted margins e g seismic sections screech1 ise1 and the fossil distal tethys rifted margins exposed in the alps the lower austroalpine and the ligurian brianconnais units exposed in se switzerland and nw italy respectively are here considered and described for the first time as conjugate margins they form like the iberia newfoundland rifted margins conjugate upper and lower plate margin both pairs of margins show a similar first order morpho tectonic arrangement using the top of pre rift succession tprs and the base of post rift successions bprs as stratigraphic marker levels and two syn tectonic sequences sts 1 2 three major rift domains can be identified which experienced different evolutions and preserved different stratigraphic architectures i the proximal margin soled by normal 30 5 km thick continental crust it is characterized by widespread fault bounded graben half graben basins filled by the sts 1 showing growth structures followed by a succession time equivalent unit to sts 2 that does not show any evidence for syn depositional tectonic activity ii the upper plate distal margin floored by thinned to ultra thinned continental crust it can be subdivided into a distal high and a distal low separated by a major fault system the high is characterized by the occurrence of a marked syn rift unconformity leading to the erosion and or non deposition of the whole syn tectonic sequence sts 1 sts 2 the low displays complete stratigraphic successions in which the sts 2 becomes very characteristic and exclusive of this part of the margin iii the lower plate distal margin floored by hyper extended crust exhumed crust and subcontinental mantle the pre tectonic sequence and the sts 1 are only present inside extensional allochthons that are excised from the upper plate and passively transported on top of major exhumation faults the sts 2 deposited during the formation of the most distal domain is mostly formed by clasts derived from the dismantling of the extensional allochthons and of the surrounding exhumed basement it is overlain by deep marine turbidites and hemi pelagic sediments the nature of basement serpentinized mantle and hydrated upper or lower continental crust will selectively condition the composition of the sts 2 in the distal lower plate margin the recognition of a complex rift stratigraphy in the two conjugate pairs of margins shows that simple stratigraphic models are unable to describe the complete evolution of rifted margins from their initial stage preserved in the proximal margins to the final evolution leading to upper and lower plate margins in the conjugate distal margins this paper describes for the first time the stratigraphic relationships at distal conjugate rifted margins which is a key to understand the rift evolution and architecture of these yet little investigated systems

a palaeomagnetic study of charnockites from madras block southern granulite terrain india abstract the south indian craton is composed of low grade and high grade metamorphic rocks across different tectonic blocks between the moyar bhavani and palghat cauvery shear zones and an elongated belt of eastern margin of the peninsular shield the madras block north of the moyar bhavani shear zone which evolved throughout the precambrian period mainly consists of high grade metamorphic rocks in order to constrain the evolution of the charnockitic region of the pallavaram area in the madras block we have undertaken palaeomagnetic investigation at 12 sites chrm directions in 61 oriented block samples were investigated by alternating field af and thermal demagnetization titanomagnetite in cation deficient cd and multi domain md states is the remanence carrier the samples exhibit a chrm with reverse magnetization of d m 148 1 i m 48 6 k 22 2 95 9 0 and a palaeomagnetic pole at 37 5 n 295 6 e dp dm 7 8 11 8 this pole plots at a late archaean location on the indian apparent polar wander path apwp suggesting an age of magnetization in the pallavaram charnockites as 2600 ma the nearby st thomas mount charnockites indicate a period of emplacement at 1650 ma mesoproterozoic thus the results of madras block granulites also reveal crustal evolution similar to those in the eastern ghats belt with identical palaeopoles from both the areas

alpine serpentinites ultramafic magmas and ocean basin evolution the ideas of h h hess hess9 ideas related to the tectonic significance of ultramafic rocks ultramafic magmas evolution of oceanic crust and h 2 o outgassing all emanating from his abiding interest in serpentinite hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional alpine type mountain belts they are not magmatic intrusions as he proposed but tectonic remnants of fossil plate boundaries hess was the formulator of the concept of sea floor spreading his ideas on the nature of oceanic crust changed through the years but he repeatedly argued that oceanic crust was partly or predominantly serpentinite despite the lack of subsequent popularity of this view he was at least partly right serpentinite is a part of oceanic crust notably in tectonically thinned regions along rifted ridges near transform faults and along obliquely rifted continental margins such as the mesozoic western alpine continental margin

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

waning magmatic activity along the southern explorer ridge revealed through fault restoration of rift topography 1 we combine high resolution bathymetry acquired using the autonomous underwater vehicle abe with digital seafloor imagery collected using the remotely operated vehicle ropos across the axial valley of the southern explorer ridge ser to infer the recent volcanic and tectonic processes the ser is an intermediate spreading ridge located in the northeast pacific it hosts the magic mountain hydrothermal vent we reconstruct the unfaulted seafloor terrain at ser based on calculations of the vertical displacement field and fault parameters the vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief forming processes over the last 11 000 14 000 years the restored topography indicates that the axial morphology evolved from a smooth constructional dome 500 m in diameter to a fault bounded graben 500 m wide and 30 70 m deep this evolution has been accompanied by changes in eruptive rate with deposition of voluminous lobate and sheet flows when the ser had a domed morphology and limited extent low effusion rate pillow eruptions during graben development most of the faults shaping the present axial valley postdate the construction of the dome our study supports a model of cyclic volcanism at the ser with periods of effusive eruptions flooding the axial rift centered on the broad plateau at the summit of the ridge followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes with minor low effusion rate eruptions confined to the axial graben the asymmetric shape of the axial graben supports an increasing role of extensional processes with a component of simple shear in the spreading processes

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

magnetotelluric evidence for layered mafic intrusions beneath the voring and exmouth rifted margins abstract marine magnetotelluric mt surveys at two volcanic passive margins have revealed an enigmatic layer of extremely high conductivity 0 1 m at 10 km depth at the voring plateau off the northwest shelf of norway 2d inversion of data from nine sites along a 54 km line resolves a layer with a conductance of 10 4 s at the exmouth plateau off the northwest shelf of australia 2d inversion of 122 sites in 17 lines finds a similar layer at similar depth but an order of magnitude higher conductance at both plateaus the depth of the high conductivity layer coincides roughly with what seismic studies have identified as an assemblage of sills we propose that the extremely high conductance is due to well connected conductive cumulates e g magnetite precipitated in layered mafic intrusions in contrast to sill emplacement the nature of layered intrusion formation requires connection to a magma source over time such a connection would not be likely during rifting when the rift provides a preferential pathway for pressure release this implies emplacement prior to or during a pause in the early stage of continental breakup

styles of rifting and fault spacing in numerical models of crustal extension extension of the earth s crust can result in differing styles of rifting such as horst and graben half graben metamorphic core complexes and areas of distributed crustal thinning faulting patterns can range from either being distributed to highly localized observations indicate that the factors controlling the extensional deformation symmetry and fault spacing include rheological aspects such as the yielding mechanism and strain softening and physical aspects such as initial heterogeneities and the strength of the lower crust compared to the upper crust time dependent numerical models of extension are presented which investigate the influence of the yielding mechanism lower crust strength strain weakening and initial heterogeneity in the crust have on a the style of rifting b fault spacing and c integrated strength in the upper crust models with an anisotropic yielding mechanism result in more realistic lithospheric strength profiles slip plane angle distributions and fault interaction than models with an isotropic yielding mechanism heterogeneity type and yielding mechanisms have the largest effect on the resulting symmetry of deformation whereas the amount of strain weakening has the greatest influence on asymmetry the likelihood of the metamorphic core complex mode occurring is primarily controlled by lower crust strength crustal thinning is encouraged by both low amounts of strain weakening and a strong lower crust lower crust viscosity exerts the primary control on whether the resulting deformation is distributed or localized the degree of strain weakening has the largest influence on the average strength of the upper crust and the slope of the strength profile in the upper crust

palaeomagnetic study of archaean banded hematite jasper rocks from the singhbhum orissa craton india abstract the banded hematite jasper formation within the iron ore supergroup of the singhbhum craton in eastern india comprises fine alternating layers of jasper and specularite it was deposited at 3000 ma and deformed by a mobile episode at 2700 ma hematite pigment 10 m is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts 1 2 of magnetite are present palaeomagnetic study identifies a dual polarity remanence resident in hematite d i 283 60 95 12 which predates deformation studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite the palaeopole 32 e 24 n dp dm 14 18 records the earliest post metamorphic magnetisation event in the orissa craton a minimum apparent polar wander motion of the orissa singhbhum craton of through 80 is identified during late archaean times 2900 2600 ma

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

structure and evolution of the galicia interior basin atlantic western iberian continental margin abstract an industrial multichannel seismic reflection data set is presented on the galicia interior basin g i b a sedimentary basin that has been poorly studied until now seismic stratigraphic units defined in other areas of the western iberian continental margin as well as some deeper reflections can also be identified within this basin although they show some particular characteristics the g i b is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs western banks the structural framework of the area consists of nnw sse trending normal faults cross cut by ne sw transfer faults the inception of the g i b probably ocurred during triassic times within the late jurassic early cretaceous rifting phase it probably experienced several extensional episodes the most intense one being during valanginian times evidence for compressional tectonic activity in this basin during cenozoic times has also been found when comparing the g i b with the adjacent deep galicia margin a gap in the timing of the main extensional events as well as some differences in the structural styles have been found in a wider regional context an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western iberian margin is consistent with a northward propagating rift for the late jurassic early cretaceous phase

continental breakup by detachment faulting field evidence and geochronological constraints tasna nappe switzerland the lower tasna detachment ltd is a low angle fault contact between serpentinized peridotite below and continental basement above it was formed during jurassic to early cretaceous rifting of a tethyan continental margin and later captured in a thrust nappe during tertiary plate convergence foliated gabbro gabbro mylonite and granitoid mylonite occurring along the ltd record shearing under decreasing temperatures u pb dating of zircon from the gabbro mylonite yielded a permian age interpreted as the age of gabbro intrusion whereas the breakup of the passive margin occurred as late as early cretaceous this suggests that the gabbro belongs to a prerift lower to middle crustal intrusion smeared out along the detachment by extensional faulting the juxtaposition of mantle and upper crust along the lower tasna detachment may serve as a model for several seismic reflectors observed in distal passive continental margins e g s reflector of the galicia margin

the crustal structure and tectonic development of the continental margin of the amundsen sea embayment west antarctica implications from geophysical data the amundsen sea embayment of west antarctica represents a key component in the tectonic history of antarctic new zealand continental breakup the region played a major role in the plate kinematic development of the southern pacific from the inferred collision of the hikurangi plateau with the gondwana subduction margin at approximately 110 100 ma to the evolution of the west antarctic rift system however little is known about the crustal architecture and the tectonic processes creating the embayment during two rv polarstern expeditions in 2006 and 2010 a large geophysical dataset was collected consisting of seismic refraction and reflection data ship borne gravity and helicopter borne magnetic measurements two p wave velocity depth models based on forward travel time modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the amundsen sea embayment seismic reflection data image the sedimentary architecture and the top of basement the seismic data provide constraints for 2 d gravity modelling which supports and complements p wave modelling our final model shows 10 14 km thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf the homogenous crustal architecture of the continental rise including horst and graben structures are interpreted as indicating that wide mode rifting affected the entire region we observe a high velocity layer of variable thickness beneath the margin and related it contrary to other normal volcanic type margins to a proposed magma flow along the base of the crust from beneath eastern marie byrd land west antarctica to the marie byrd seamount province furthermore we discuss the possibility of upper mantle serpentinization by seawater penetration at the marie byrd seamount province hints of seaward dipping reflectors indicate some degree of volcanism in the area after break up a set of gravity anomaly data indicate several phases of fully developed and failed rift systems including a possible branch of the west antarctic rift system in the amundsen sea embayment

a seismic refraction and reflexion study of the continent ocean transition beneath the north biscay margin the structure of the northern margin of the bay of biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent ocean boundary these blocks formed by rifting in late jurassic early cretaceous time and are now thinly covered by post rift sediments of aptian to recent age seismic refraction profiles were occupied on the shelf on either side of and across the continent ocean transition to the shelf using p ubs and o bs with explosives and a 4 x 1000 in 3 4 x 16400 cm 3 airgun array two ship expanding spread multichannel 48 trace seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition a two ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf conventional 48 trace single ship multichannel profiles were located along all the refraction and two ship reflexion lines interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms conventional seismic processing techniques have been used to prepare the two ship multichannel seismic data for interpretation the survey is believed to be the first attempt to apply two ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust the results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide the seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone however extension values calculated in the upper crust from the rotation of fault blocks are much less 1 1 1 4 and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust

structure of the gabon margin from integrated seismic reflection and gravity data in the south gabon basin deep multi channel seismic reflection and gravity modeling analysis have shed light on key features of the structure of the margin the thinned continental crust beneath the gabon margin appears to be composed of two distinct layers separated by a clear strong and more or less continuous reflector running in the 7 10 s twt window the lower crust is characterized by a higher density intermediate between the lower values of the upper crust and the denser values of the mantle the lower crust is irregularly shaped and presents lateral thickness variations along the direction of thinning and along the coast in the offshore thinned continental domain the lower and upper crust form a 20 25 km thick body crustal thicknesses point to a relatively sharp and narrow transition along a few tens of kilometers between the unthinned and the thinned continental crust the high density layer identified offshore gabon presents similar characteristics in density geometry and spatial distribution as the underplated magmatic bodies observed along volcanic margins e g along the south atlantic namibia margin or the north atlantic voring margin although this lower crustal body could possibly represent ultra mafic serpentinized rocks or high grade metamorphic crustal rocks we suggest that it could be composed of mafic rocks magmas resulting from partial melting during rifting may underplate the crust and or be intruded in the lower crust through a system of dykes and sills in this view the present day crustal thicknesses along rifted margins characterized by magmatic underplating and or intrusion are not representative of the thinning that the crust experienced during rifting results of this study point to relatively shallow sedimentary basins along the south gabon margin the deepest offshore depocenters located under the westernmost side of the continental platform appear to be associated with the deepest syn rift basins these basins seem to extend along 20 to 40 km in the ne sw direction with a present day average thickness of 7 3 km offshore gabon whereas the crustal thinning appears significant the syn rift deposit are not thick we suggest that the area was anomalously uplifted during the rifting phase due to an elevated thermal lithospheric gradient the conclusions derived from our seismic and gravity analysis are consistent with the implications such a thermal anomaly would have on the tectonic evolution of a rifted margin with 1 an underplated high density lower crustal layer 2 shallow depth syn rift basins associated with a relatively thin crust and subsequently 3 elevated recorded subsidence rates in the initial post rift stages research highlights the gabon margin appears characterized by an underplated high density lower crustal layer shallow depth syn rift basins are associated with a relatively thin crust seismic and gravity analysis points to an anomalous uplift of the gabon margin during the rifting phase crustal structure syn rift infill and post rift subsidence are consistent an uplift of the gabon margin during the rifting phase the uplift of the gabon margin is possibly due to an elevated thermal lithospheric gradient

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

seismic image of the deep crust at the eastern margin of the alps austria indications for crustal extension in a convergent orogen abstract a 39 km long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the eastern alps austria the ese wnw trending low fold seismic line crosses austroalpine basement units and extends approximately from 20 km west of the penninic window group of rechnitz to 60 km sse of the alpine thrust front the explosive source seismic data reveals a transparent shallow crust down to 5 km depth a complexly reflective upper crust and a highly reflective lowermost crust the upper crust is dominated by three prominent west dipping packages of high amplitude subparallel reflections the upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low angle normal shear zones related to the miocene exhumation of the rechnitz metamorphic core complex in the western portion of the upper crust east dipping and less significant reflections prevail the lowermost package of these reflections is suggested to represent the overall top of the european crystalline basement along the western portion of the line the lower crust is characterised by a 6 8 km thick band of high amplitude reflection lamellae typically observed in extensional provinces the moho can be clearly defined at the base of this band at approximately 32 5 km depth due to insufficient signal penetration outstanding reflections are missing in the central and eastern portion of the lower crust we speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower moho in the east preferable at about 30 5 km depth the high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust the latter interpreted to represent broad extensional mylonite zones emphasises the latest extensional processes in accordance with eastward extrusion

precambrian tectonics of the south indian shield inferred from geophysical data geophysical parameters such as magsat landsat gravity deep seismic sounding seismicity and heat flow data have identified cratonic and mobile units in the precambrian continental crust of the south indian shield landsat imagery and gravity field data suggest that the shield is divided into four geophysical domains from north to south namely 1 the deccan trap covered gneissic basement and the gneiss terrain around hyderabad 2 the dharwar craton subdivided into its eastern and western blocks 3 the granulite terrains of kerala karnataka and tamil nadu and 4 the eastern ghats granulite belt the smooth transition in the gravity field from the dharwar greenstone terrain to the deccan trap covered area to the north indicates the northward continuity of the granite gneiss basement however the trap covered region lacks any significant short wavelength anomalies of the type associated with the dharwar greenstone belts indicating the probable absence of any such large scale high density bel

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

paleoconstraints during rifting of the northeast atlantic passive margins the northeast atlantic passive margins formed during the early cretaceous phase of rifting from goban spur to galicia bank these margins are characterized by a tilted fault block geometry well defined by numerous single channel and multichannel seismic data stress directions cannot be deduced from these structural data but recently withjack and jamison 1986 have described the strain states and fault patterns produced by oblique rifting using experimental and analytical techniques we have applied these techniques to the northeast atlantic continental margins all types of extensional margins are represented there with pure extensional rifting goban spur and west galicia margins oblique rifting north spanish and celtic margins and close to pure wrenching armorican margin the rifting directions are parallel to the adjacent oceanic fracture zones or flow lines defined in the northeast atlantic and in the bay of biscay except for the west galicia margin where a 20 difference exists this difference could be within the error range of the method or could be due to a slight relative motion between galicia bank and iberia during rifting globally the coherence between rifting and spreading directions is an argument in favor of the validity of the method on the basis of kinematic arguments the motion of iberia with respect to eurasia has an eastward component related to the west to east motion of africa with respect to eurasia while iberia was a small plate whose motion was roughly coupled to africa this is confirmed by our analysis we conclude that during both the rifting and spreading episodes in the bay of biscay the motion of iberia with respect to eurasia was in the southeastward direction recently boillot 1984 suggested that iberia moved about 150 km southwestward with respect to eurasia during the rifting phase and then moved in the southeastward direction during the spreading phase we suggest that the boillot hypothesis could be wrong

kinematics of peridotite emplacement during north atlantic continental rifting galicia northwestern spain abstract the western part of the galicia passive continental margin western edge of spain is bordered by serpentinized peridotites over a n s distance of about 125 km these peridotites were emplaced at the end of continental rifting and or at the very beginning of oceanic accretion they have been observed in situ and sampled in five sites with the nautile submersible during the galinaute cruise 1986 some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge most of the escarpments covered by the nautile on the peridotite ridge have yielded only ultramafic rocks except at dive 10 where the peridotites are overlain by a sheared chlonte bearing schist and at dive 14 where the ultramafic rocks are covered by basalts all the peridotites are plagioclase bearing harzburgites and iherzolites locally cross cut by rare plagioclase rich veins and dioritic intrusives some pyroxenites and gabbros are associated with the peridotites at dive 14 the plagioclase bearing peridotites are similar in composition to those drilled during odp leg 103 primary structures have not been found except in the northern part of the studied zone where a few samples exhibit magmatic textures elsewhere the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high temperature fabric at these localities boudinage structures and ultramylonitic bands referred to as shear bands occur and the rocks have a disrupted mylonitic texture a few dioritic dykes have also suffered this mylonitization shearing occurred in a rotational regime at high and decreasing temperature 1000 850 c and under a high deviatoric stress 102 mpa i e under lithospheric conditions the structural pattern of the peridotite ridge deduced from measurements on oriented samples is complex the trend of the mylonitic foliation appears nearly constant from south to north n125 n105 but its dip increases 20 45 northward from south to north the stretching lineation pitch varies from subvertical to subhorizontal stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area to the north it occurred in a northwesterly direction with a strike slip component stretching in the chlorite bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation this study confirms the uplift of the galicia peridotites during the continental rifting of the north atlantic ocean in cretaceous times during their ascent the peridotites underwent some melting and high temperature low stress ductile deformation followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures the geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress strain field prevailing in the continental lithosphere during the rifting episode it is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point the kinematics of emplacement agree well with an e w opening of the north atlantic ocean