emergent long lived gondwanaland vs submergent short lived laurasia supercontinental and pan african heat imparts long term buoyancy by mafic underplating gondwanaland was buoyant as indicated by nonmarine facies and laurasia was depressed as indicated by marine facies as a supercontinent gondwanaland lasted much longer than laurasia and was hotter from internal heat moreover the pan african orogenic cycle confined to gondwanaland augmented the heat supply which possibly generated a permanently buoyant lower crust by underplating a crustal layer in the australian proterozoic shield with subhorizontal reflectors and velocity vp 7 5 km s is interpreted as mafic underplating beneath latest neoproterozoic flood basalt the pan african terrane in east africa also contains evidence of mafic underplating and most of gondwanaland but not laurasia was affected by terminal pan african 0 5 ga uplift and cooling

tectonic geomorphology of the southeastern mississippi embayment in northern mississippi usa we analyzed drainage basin geometry in part of the southeastern mississippi embayment near the new madrid seismic zone in order to detect areas of lateral stream migration that could indicate recent tectonism toward this goal a field of drainage basin asymmetry vectors was generated from a digital terrain model and compared to geologic structure lithofacies seismicity remotely sensed lineaments and major drainage divides transverse topographical drainage basin asymmetry t index results define geomorphic domains in northern mississippi 33 to 35 n 88 5 to 90 25 w that may reflect deep crustal blocks bounded by active faults or flexures t index measurements a proxy for lateral stream migration of more than 4500 second order drainage basins produced a two dimensional field of 282 spatially averaged vectors from which we interpreted 12 domains some domain boundaries correspond to mapped faults of two paleozoic fault systems in our study area the northeast striking mississippi valley graben system and the northwest striking ouachita thrust belt and foreland faults several other prominent domain boundaries may be previously unmapped faults flexures parallel to these systems this interpretation is supported by satellite image analysis of the northeastern study area which reveals lineaments consistent with our domain boundaries that have strikes that reflect the regional subsurface structural grain

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

temporal and spatial correlations between changes in plate motions and the evolution of rifted basins in africa we present the results of a backstripping study incorporating over 200 wells in 14 different basins or groups of basins located on the african interior or continental margins these results presented as mean tectonic subsidence rates averaged over appropriate periods are compared with the opening histories and changes in plate motions of the atlantic and indian oceans and the tethys mediterranean area for seven time steps from the late jurassic until present the results indicate a temporal and spatial correlation between phases of rapid subsidence in african basins and changes in plate motions and therefore are promising in terms of applicability of large scale plate motions in the studies of paleo stresses in sedimentary basins especially at times of initial rifting and formation of subduction zones reactivation of the basins appears to be primarily controlled by the orientation of the basin and underlying basement structures with respect to the stress direction the basins are preferably formed within weaker zones mostly pan african shear zones and the distance from the active plate boundary to the weak zone plays a minor role in whether or not it will be reactivated the presence of three important hot spots which were active during the early stages of break up of gondwana may have been important in preweakening the lithosphere at their sites this may have played an additional role in the localization of the basin deformation

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

geochemistry and regional significance of the early cretaceous bimodal basalt felsic associations on grand banks eastern canada cretaceous mafic rocks are present in several wells on the grand banks of newfoundland near the southwest grand banks jurassic transform margin and along the trend of the collector magnetic anomaly in the brant and mallard wells felsic volcanic subvolcanic and pyroclastic rocks are several hundreds of meters thick seismic reflection profiles show that the volcanic rocks are locally bounded by rejuvenated faulted margins of early rift basins beneath the mid cretaceous postrift unconformity the early cretaceous age of the volcanic rocks is confirmed by k ar geochronology trace element and isotopic composition of both mafic and felsic rocks are quite different from those of plume related cretaceous rocks elsewhere on the eastern north american margin mafic rocks have a pb isotopic composition unusually enriched in 207 pb 204 pb which is similar to late triassic early jurassic mafic rocks along the eastern appalachians nd isotopic compositions are closer to typical ocean island basalt oib sources observed isotopic compositions suggest mafic magma was partially melted from continental margin lithospheric mantle enriched with small amounts of hydrous or silica rich phases of oib affinity whole rock geochemistry isotopic composition and mineral chemistry show that the felsic rocks resemble alkaline a type granite those from mallard have some crustal contamination those from brant which is located closer to the continental margin edge have almost pristine mantle isotopic characteristics indicating an oib source magma the spatial distribution of cretaceous volcanic products suggests that magma was generated along the reactivated transform margin and that major faults in continental crust acted as magma conduits partial melting of continental margin lithospheric mantle resulted from extensional decompression probably aided by a weak plume related thermal anomaly as on other transform margins magmatic products reflect a complex mantle history

anomalous opening of the equatorial atlantic due to an equatorial mantle thermal minimum abstract the geology of the equatorial atlantic is dominated by a broad east west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the mid atlantic ridge mar the origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum the notion of a mantle thermal minimum in the equatorial atlantic is supported by an equatorial minimum of zero age topography a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting indicated by the chemistry of mar basalts and peridotites this thermal minimum has probably been a stable feature since before the cretaceous separation of africa from south america it caused a pre opening equatorial continental lithosphere thicker and colder than normal the cretaceous benue trough in western africa and the amazon depression in south america are interpreted as morphostructural depressions created or rejuvenated by strike slip transpressional and transtensional tectonics ducing extension of the cold thick equatorial lithosphere the oceanic rift propagating northward from the south atlantic impinged against the equatorial thicker colder and therefore stronger than normal continental lithosphere that consequently acted as a locked zone this and a low magmatic budget due to the cold upper mantle caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt with diffuse deformation during mostly amagmatic extension the thick cold lithosphere prevented major cretaceous igneous activity from the st helena plume eventually initial weak isolated nuclei oceanic lithosphere were emplaced separated by e w continent continent transforms opening occurred largely by strike slip motion along these initial transforms the consequences were that the equatorial atlantic opened prevalently along an e w direction in contrast to the n s opening of the north and south atlantic and that sheared continental margins are particularly well developed in the equatorial atlantic after further continental separation the cold equatorial mantle caused a low degree of melting with na rich morb and alkali basalt rather than normal morb and with undepleted mantle peridotities thin crust depressed ridge segments and a prevalence of amagmatic extension similar conditions still exist today long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics with transform migration transpression and transtension causing strong vertical motion emersion and subsidence of lithospheric blocks development of deep pull apart basins and preservation of relict slivers of old lithosphere occasionally even of continental lithosphere within younger crust the equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa however they then create second order rebound thermal effects that help maintain the thermal minimum in the upper mantle it can be speculated that mantle thermal minima at the earth s equator might be related to true polar wander triggered by subduction of dense masses into the mantle

geochronology of the monteregian hills alkaline igneous province quebec three distinct groups of alkaline rocks can be recognized on the basis of silica content in the monteregian hills quebec strongly undersaturated slightly undersaturated to critically saturated and oversaturated the first two groups were derived from basanite and alkali picrite magmas respectively and yield distinct ages of intrusion clustered about 118 ma and 136 ma the age data are not readily explained by the translation of the north american plate across a fixed mantle hotspot but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas the data do suggest an upward migration of the zone of melting this change in the depth of melting may be a response to the upward transport of a metasomatic fluid perhaps with a coupled rise in mantle isotherms

age migrations of anorogenic ring complexes in northern nigeria forty rb sr isotopic measurements on minerals and whole rock samples from 11 anorogenic complexes in northern nigeria complement the evidence for systematic age variations along a series of en echelon ene wsw and n s lineaments following two major fracture directions of extensional deformation in the precambrian basement the new ages presented in this paper confirm our earlier discoveries of a general southerly age trend but substantiate for the first time that major local migrations of magmatic activity were concentrated along at least two ene wsw zones from the overlap in analytical error the age patterns also suggest that contemporaneous anorogenic magmatism may have been locally derived from several simultaneous high level magma chambers connected to a common deeper source different models that have been proposed for the origin of linear mid plate magmatic activity are critically reviewed in our opinion magma generation was most likely initiated in the asthenosphere lithosphere decoupling zone or

the timing of late cretaceous alkalic igneous activity in the northern gulf of mexico basin southeastern usa upper cretaceous igneous rocks in the northern gulf of mexico basin are poorly understood due both to the paucity of material recovered from drillholes in the sedimentary basins and lack of geochronological data rocks from the alkalic igneous complexes at magnet cove and granite mountain arkansas the monroe uplift and the jackson dome louisiana and mississippi yielded 40ar 39ar ages ranging from 94 ma to 65 ma high precision plateau ages reveal that the magnet cove complex 94 ma predates granite mountain by 4 myr igneous activity in the monroe uplift may have been contemporaneous with that at magnet cove and continued sporadically for 30 myr a single specimen from the jackson dome indicates volcanism at 65 ma coincident with waning igneous activity at the monroe uplift based on geochemical and geochronological data the source for volcanoclastic material in the woodbine formation arkansas are rocks belonging to the magnet cove phase of magmatism cretaceous alkalic rocks of central ark

structural segmentation and the influence of basement structure on the namibian passive margin passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles the namibian margin is bounded by a hinge line that is offset by segment boundaries the most important is the cape cross segment boundary here pre existing basement lineaments and karoo rift elements were reactivated during late mesozoic rifting it marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break up and formation of the south atlantic a similar change occurs across other important segment boundaries but smaller examples lack this major segment boundaries with a temporal change in rifting and break up exhibit a strong connection to the basement province

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

heterogeneous decadal glacier downwasting at the mt everest qomolangma from 2000 to 2012 based on multi baseline bistatic sar interferometry abstract remote sensing based geodetic observations can be used as alternatives data to map glacier height changes because the harsh environment complicates in situ observations in this study we analysed five pairs of x band bistatic terrasar x tandem x images from 2011 and 2014 for mt everest qomolangma glacier height changes were derived using the d insar method respect to srtm dem of 2000 an iterative d insar method using multi baseline bistatic sar interferograms was employed which increased reliability and accuracy of glacier height changing observations from 2000 to 2012 the geodetic glacier mass balance for the mt everest and the surrounding region was 0 38 0 04 m w e a 1 the spatial pattern of the glacier mass loss was heterogeneous the regional heterogeneity may possibly reflect debris covering rates terminating type temperature rising rates and glacier flow rates comparison to long period geodetic glacier mass balance data provided by previous studies since 1970 revealed that our results showed more rapid increases in the glacier mass loss rates after 2000 in the area around khumbu glacier in the southern slope of the mt everest whereas glacier mass loss rates kept stable in the rongbuk catchment at its northern slope

transient crustal deformation from karst aquifers hydrology in the apennines italy abstract the increasing accuracy and spatiotemporal resolution of space geodetic techniques have positively impacted the study of shallow crustal deformation in response to the redistribution of water masses measurable deformations have been documented in areas where snow and water variability is large and persists over sufficiently long periods here we analyze gps time series and hydrological data from the central southern apennines a tectonically active region hosting large karst aquifers we document the occurrence of regional scale horizontal and vertical transient deformation that is clearly correlated to seasonal and multiyear hydrological variability these transient signals which are most strongly observed at gps sites surrounding the main karst aquifers modulate long term tectonic deformation our results suggest that the karst aquifers in this region experience alternating periods of expansion and contraction in response to increasing decreasing precipitation and consequently higher lower hydraulic head in the aquifers thanks to the availability of a dense continuous gps network and complementary hydrological datasets we are able to verify the processes causing the observed deformation we model the shallow crust in the region as a continuous anelastic solid and use green s functions for finite strain cuboid sources to estimate the strain rate distribution associated with the gps observations we use the mw 6 1 l aquila earthquake which struck the central apennines in 2009 and whose effects are evident in geodetic data to document the potential effects of moderate earthquakes on karst aquifers and to demonstrate the importance of correctly discerning tectonic from nontectonic signals in geodetic time series enhanced understanding of the karst aquifers behavior is of primary interest for improved management of this vital water resource and for a better understanding of the possible interactions between groundwater content and pore pressure variations in the crust and seismicity

supracrustal faults of the st lawrence rift system quebec kinematics and geometry as revealed by field mapping and marine seismic reflection data abstract the st lawrence rift system from the laurentian craton core to the offshore st lawrence river system is a seismically active zone in which fault reactivation is believed to occur along late proterozoic to early paleozoic normal faults related to the opening of the iapetus ocean the rift related faults fringe the contact between the grenvillian basement to the nw and cambrian ordovician rocks of the st lawrence lowlands to the se and occur also within the grenvillian basement the st lawrence rift system trends ne sw and represents a se dipping half graben that links the nw se trending ottawa bonnechere and saguenay river grabens both interpreted as iapetan failed arms coastal sections of the st lawrence river that expose fault rocks related to the st lawrence rift system have been studied between quebec city and the saguenay river brittle faults marking the st lawrence rift system consist of ne and nw trending structures that show mutual crosscutting relationships fault rocks consist of fault breccias cataclasites and pseudotachylytes field relationships suggest that the various types of fault rocks are associated with the same tectonic event high resolution marine seismic reflection data acquired in the st lawrence river estuary between rimouski the saguenay river and forestville identify submarine topographic relief attributed to the st lawrence rift system northeast trending seismic reflection profiles show a basement geometry that agrees with onshore structural features northwest trending seismic profiles suggest that normal faults fringing the st lawrence river are associated with a major topographic depression in the estuary the laurentian channel trough with up to 700 m of basement relief a two way travel time to bedrock map based on seismic data from the st lawrence estuary and comparison with the onshore rift segment suggest that the laurentian channel trough varies from a half graben to a graben structure from sw to ne it is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated quaternary deposits are possibly related to degassing processes of basement rocks and that hydrocarbons were drained upward by the rift faults

a tectonostratigraphic synthesis of the sub andean basins inferences on the position of south american intraplate accommodation zones and their control on south atlantic opening it has been shown in the accompanying paper that the sub andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains to test the hypothesis that changes in palaeo depositional setting rely on the presence of a series of transverse zones of structural accommodation data have been digitally compiled from across the south american continent spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries structural accommodation zones and patterns of subsidence and intraplate deformation the results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which on a regional scale interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization their strong spatial relationship with mesozoic intraplate alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during gondwana breakup these localized zones of high heat flow have important implications for source rock maturity in the interior phanerozoic intracratonic basins of south america on the south atlantic margin the majority of these crustal lineaments correlate with failed arms of triple junction rifts and define the boundaries to tectonostratigraphic domains recognized along the south atlantic rift system

the cretaceous peri atlantic alkaline pulse paap deep mantle plume origin or shallow lithospheric break up abstract although the early jurassic initial phases of atlantic rifting were marked by the tholeiitic basalt flows and dikes of the central atlantic magmatic province camp the cretaceous era was characterised by widespread alkaline igneous activity on widely separated peri atlantic continental margins nearly half the peri atlantic alkaline rocks dated between 250 to 50 ma fall within the 125 80 ma range this period of intense alkaline activity during the mid cretaceous formed the peri atlantic alkaline pulse paap the mechanisms for the opening of the atlantic ocean as well as for the camp and later paap magmatic events have been attributed either to deep mantle plumes or to a combination of tensional forces lithospheric rifting and structural controls we describe the paap event for the first time and test the plume and structural models the paap displays evidence of strong structural control and appears to be related to reactivation of pre existing lithospheric structures throughout the peri atlantic zone early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre existing structures mid cretaceous pulses in alkaline magmatic activity were closely associated with major atlantic tectonic events the opening of the south atlantic the change in intensity and direction of both seafloor spreading and plate motion and the consequent fluctuation in intra plate stresses were responsible for two spikes in peri atlantic alkaline magmatism at 125 ma and 85 80 ma the general lack of evidence for a deep hotspot associated with the peri atlantic zone seems incompatible with a mantle plume origin on the other hand the spatial distribution of the paap along passive continental margins suggests an ideal setting for the development of edge driven convection we propose that the periodic reactivation of deep seated pre existing zones of weakness during the major stages of atlantic tectonic evolution combined with coeval asthenospheric upwelling due to edge driven convection and continental insulation flow enhanced the ascent of alkaline magmas this mechanism of shallow small scale upwelling during periodic structural reactivation provides a more likely explanation for the paap than the commonly proposed deep mantle plume scenario

an early cambrian deltaic fluvial model for an iapetan rift arm drainage system southeastern quebec based on the model that during the rifting stage beginning at ca 590 ma the ottawa graben an iapetan failed arm localized a large river that flowed into the nascent iapetus a search was made for the related fluvial and deltaic deposits the search led to the identification of fluvial deposits as predicted by the model deltaic deposits were also identified despite complications brought about by deformation metamorphism and thrusting although they probably belong to a late phase of delta buildup in the early cambrian older deltaic deposits of the river probably lie buried beneath a volcanic shield that built up at the proximal end of the graben ca 554 ma the successful application of the model supports the paradigm of rift arm failed arm rivers and their deltas as proposed by k burke and j f dewey nearly two decades ago investigations based on this paradigm in appropriate geological situations may provide insights into problems related to continental rifting and breakup and provide inform

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

petrology of the alkaline volcanism of gorringe bank southwest portugal abstract alkaline volcanic rocks of the gorringe bank overlie typical oceanic crust made up of spilitized dolerites differentiated gabbroic and ultramafic rocks two main series are distinguishable in alkaline volcanics one from nephelinite to phonolite the other from lamprophyre to trachyte each sequence is differentiated by fractional crystallization with separation of clinopyroxene in the first one and kaersutite and mica in the second an intermediate suite is rich in lamprophyric and trachytic ocelli and in cognate ijolitic inclusions hypotheses on the origins of the more mafic lavas and on the influence of fluids like h 2 o and co 2 are discussed these series have a similar petrology to that of the sines and monchique complexes which intrude the continental crust at the same time on the same structural trend the alkaline rocks of gorringe are among the most alkaline series of the atlantic islands close to those of fernando de noronha and trindade where similar rocks occur

a plume head melting under a rifting margin abstract a large igneous province lip in the form of a long narrow band of thickened oceanic crust runs along the atlantic margin of north america abutting the rifted continental shelf we propose that this like many other lips has a mantle plume origin there is evidence that when the central atlantic ocean opened the rift was underlain by the flattened head of a mantle plume and that the rift site had drifted away from the plume tail by the time of the rifting so that the tail took little part in the formation of the lip we carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced we found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere to fit observations of the lip a thin flat plume head is required such a plume head results when a mantle plume with temperature dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth however an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep by decoupling the mantle flow from the lithosphere to avoid decoupling we propose that there must be significant topography on the lithosphere and the rift site is a region of thinned lithosphere very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

late cretaceous reactivation of major crustal shear zones in northern namibia constraints from apatite fission track analysis abstract namibia s passive continental margin records a long history of tectonic activity since the proterozoic the orogenic belt produced during the collision of the congo and kalahari cratons in the early proterozoic led to a zone of crustal weakness which became the preferred location for tectonism during the phanerozoic the pan african damara mobile belt forms this intraplate boundary in namibia and its tectonostratigraphic zones are defined by ductile shear zones where the most prominent is described as the omaruru lineament waterberg thrust oml wt the prominance of the continental margin escarpment is diminished in the area of the central and northern zone of the damara belt where the shear zones are located this area has been targeted with a set of 66 outcrop samples over a 550 km long 60 km broad coast parallel transect from the top of the escarpment in the south across the damara sector to the kamanjab inlier in the north apatite fission track age and length data from all samples reveal a regionally consistent cooling event thermal histories derived by forward modelling bracket this phase of accelerated cooling in the late cretaceous maximum palaeotemperatures immediately prior to the onset of cooling range from ca 120 to ca 60 c with the maximum occurring directly south of the omaruru lineament because different palaeotemperatures indicate different burial depth at a given time the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust we interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the south atlantic and south west indian oceans

the structure of the guinean continental margin implications for the connection between the central and the south atlantic oceans the results of a recent geological geophysical survey conducted off guinea are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the west african continental margin three sectors are distinguished a northwestern portion of the margin comprises a wide and deeply submerged plateau the guinean marginal plateau underlain by a thick sedimentary sequence and facing westward toward the gambia abyssal plain scismic stratigraphy and structures show clear analogies to the jurassic margins of the central atlantic including the presence of a cretaceous paleoslope covered by cenozoic deposits a southern area of the margin comprises a series of aligned w e trending acoustic basement features extending along the slope and bounding the guinean plateau to the south these features basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data the bordering deep sierra leone abyssal plain also dissected by e w trending oceanic fracture zones contains a sedimentary cover apparently not older than middle cretaceous between both sectors and between two nw se trending scarps lies an intermediate area the seismic profiles show that here the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the s e and covered by untectonized but partly eroded upper cretaceous to cenozoic sediments

the permo jurassic alkaline province of tadhak mali geology geochronology and tectonic significance the tadhak alkaline complexes located along the eastern edge of the west african craton in northeastern mali intrude the 2 ga old basement or the overlying pan african nappes and are covered by cretaceous sediments this magmatic activity was accompanied by doming and rifting northnortheast trending normal faults define two crustal segments exhibiting variously sized 1 km to 17 km intrusive complexes these comprise eroded undersaturated ring complexes with occasional carbonatites in the west and high level phonolitic plugs with shallow intrusive nepheline syenites in the east new rb sr isotopic data indicate 100 ma of within plate magmatic activity in the area without any obvious age progression in addition to the adrar tadhak complex 262 7 ma the following intrusions have been dated tirkine 215 11 ma anezrouf 184 14 ma and tidjerazraze in imanal 161 5 ma a clear break thus exists with the cambrian alkaline granite province of the iforas located on the opposite side of the pan african suture 87sr 86sr initial ratios are similar throughout between 0 7044 and 0 7049 which indicates a bulk earth isotopic composition the conclusion of exclusively mantle origin dupal composition previously demonstrated for the adrar tadhak complex can now be extended to the other complexes this implies repeated tapping of a single mantle source throughout the life of the province the described magmatic and tectonic permo jurassic activity has been triggered by intraplate stresses focussed along the pre existing pan african suture zone together with similar features in europe and africa the permo jurassic tadhak alkaline province heralds the progressive break up of pangaea and the opening of the atlantic ocean

a note on the geochronology of the iberian alkaline province abstract a critical assessment of previously published ages and new k ar results for two complexes in the iberian alkaline province monchique and sintra lead to the conclusion that two separate magmatic episodes occurred around 72 and 82 ma since emplacement of these alkaline rocks is undoubtedly related to rifting and subsequent changes in the atlantic ocean floor spreading geometry accurate timing of magmatic events within the iberian alkaline province is of fundamental significance

the opening of the atlantic the mesozoic new england igneous province and mechanisms of continental breakup abstract plume and tensionally induced rifting models have been proposed as causes of continental rifting and breakup mantle plumes should produce early lithospheric doming events associated with alkaline magmatism however the triassic alkaline rocks on the east north american e n a margin are structurally localized the uplifts are mostly jurassic in age and voluminous plateau magmatism post dates the initiation of taphrogenesis by 10 to 20 ma in addition there are no chains of anorogenic igneous rocks leading to any of the postulated triassic plumes these facts argue against the plume generated rifting model for the e n a margin but are consistent with the predicted effects of the tensionally induced rifting model the genesis of linear chains of alkaline rocks has been attributed to passage of a lithospheric plate over a mantle plume this explanation has been proposed for the cretaceous monteregian and white mountain intrusive series and the kelvin seamount chain however these rocks are distributed along structurally controlled e w trending lineaments the monteregians are localized by the presence of a paleorift ottawa bonnechere graben in the underlying grenvillian basement and the kelvin seamounts are emplaced along the early jurassic kelvin fracture zone the structures localizing the southern white mountain lineament are less well defined the spatial temporal and geochemical relationships along and between these lineaments are consistent with an origin as a southeastward propagating tensional event the intra plate stresses are probably the result of shifts in the pole of rotation of the north american plate due to irregular northward extensions of seafloor spreading during the cretaceous

a plate tectonic setting for mesozoic rifts of west and central africa abstract africa by virtue of its central position within gondwana has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the early mesozoic continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of africa and the formation of rifted sedimentary basins deep within the african continent the multi stage opening of the atlantic ocean and associated rifting in west and central africa provide one of the most impressive examples of ocean continent tectonic interactions during the early cretaceous the onset of rifting along the future margins of the south atlantic is contemporaneous with intra continental rifting generating both strike slip and extensional basins within west and central africa the syn rift phase of intra continental basin development continued until the santonian by which time africa and south america had been physically separated for approximately 10 ma except for minor rifting during the senonian and tertiary the short lived phase of deformation at about 80 ma marks the transition into the post rift or sag phase of basin development this deformational event can be correlated with a period of plate motion changes recognised from fracture zone geometries seen in both the central and south atlantic oceans using present day stress analogies cretaceous rifting in africa can provide a means of indicating the regional palaeostress directions within africa at this time

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

chronology and geodynamic setting of cretaceous cenozoic rifting in west and central africa abstract the development of the early cretaceous to palaeogene west and central african rift system which extends from nigeria benue trough to kenya anza trough can be related to the build up of intraplate tensional stresses during the break up of gondwana which caused reactivation of pre existing zones of lithospheric weakness repeated changes in the intraplate stress regime of africa are reflected by phases of crustal extension alternating with episodes of compression many of these events can be correlated with changes in rates of seafloor spreading in the central and south atlantic oceans as reflected in flowline patterns the west and central african rifts can be considered as typical passive rifts which evolved in response to the build up of intraplate stresses however the st helena hot spot appears to have been located beneath the equatorial plate boundary at approximately 120 ma and may have played an important role in weakening the lithosphere during extension

thermal evolution of the central atlantic passive margins continental break up above a mesozoic super plume a near synchronous early jurassic 200 ma magmatic event along the north american and west african passive continental margins can be explained by the upwelling of a large scale mantle plume super plume beneath the west african craton lateral deflection of the outflow from the plume to the ne by the ambient sub lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from brazil to southern spain continental break up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle the dynamic model is a scaled version of that recently proposed for the hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of central atlantic oceanic crust

the late cretaceous alkaline igneous province in the iberian peninsula and its tectonic significance abstract the iberian province consists of the following the three subvolcanic syenitic major intrusive complexes of monchique sines and sintra in w and sw portugal together with their basanitic lamprophyric minor intrusive suites basanitic volcanic complexes around lisbon at least some of a widespread suite of basanitic to theralitic minor intrusives in west central portugal about 80 small basanitic lamprophyric to nepheline syenitic intrusions scattered through the pyrenees ne spain the french corbieres and off the coast of nw spain and the ormonde seamount of the gorringe bank off the sw coast of portugal most of these occurrences have been dated isotopically or from field evidence as late cretaceous geological and petrological details of the various occurrences are compiled and reviewed primary basanitic magmas were probably parental to the entire province and generated syenitic magmas by differentiation processes oversaturated rocks were produced by alkali loss and perhaps also by crustal involvement the iberian province is related to the opening of the n atlantic specifically that of the bay of biscay

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt