diagenetic history of cambrian quartzarenites ras dib zeit bay area gulf of suez eastern desert egypt abstract in the northern corner of the el zeit range 370 m of sandstone overlie precambrian granite and underlie cretaceous marine strata the sandstones include the marine lower cambrian araba formation and the overlying dominantly fluvial upper cambrian naqus formation the framework compositions of both sandstones are almost entirely quartz with trace amounts of muscovite k feldspar and heavy minerals up to 21 oversize pores some filled with younger cements attest to extensive dissolution loss of detrital grains because the final mineralogical maturation of these quartzarenites reflects diagenesis they are diagenetic quartzarenites during burial diagenesis the introduction of up to 8 quartz cement inhomogeneously distributed mean 3 was followed by local pore occluding calcite cement which halted compaction sandstones without calcite cement underwent additional mechanical and chemical compaction sufficient to develop sutured quartz grain contacts and reduce porosity 27 these events were followed by 1 extensive dissolution loss of carbonate cement detrital feldspars micas and heavy minerals 2 formation of local patches of kaolinite mean 3 and 3 formation of extensive iron oxide cements including specular hematite these features suggest extensive invasion by oxidizing meteoric water the timing of this event can only be dated as post cambrian and pre cenomanian some outcrop samples contain pore occluding gypsum cement or mixtures of gypsum and halite sr87 sr86 ratios of four samples of gypsum cement have values 0 7079 and 0 7085 that indicate miocene and slightly younger seawater evaporites were evidently leached by modern meteoric water from nearby outcrops of miocene and younger marine evaporite bearing strata transported in surface and ground water to the topographically low terrain where the cambrian sandstones crop out and reprecipitated by evaporation initial primary porosity 45 was reduced to 27 by compaction in sands with minor quartz cement compaction ceased where calcite was precipitated subsequent dissolution of some calcite cement and unstable detrital grains generated considerable secondary porosity

lithostratigraphy and production characteristics of the wilcox group paleocene eocene in central louisiana 1 multiple discontinuous sandstones of the wilcox group in central louisiana were deposited in fluvial deltaic and marine sedimentary environments in the tertiary gulf of mexico basin prolific hydrocarbon reservoirs developed in association with positive structural features or where sedimentary characteristics were favorable regional stratigraphic correlations are difficult because the wilcox group contains numerous complex depocenters to delineate the occurrence of hydrocarbon bearing sediments and to promote further exploration a five fold lithostratigraphic framework units i v in descending order is proposed for the wilcox group on the basis of its sedimentary character as interpreted from well logs lithostratigraphic unit thickness and sandstone content were mapped within 22 parishes all units produce hydrocarbons although production is geographically variable production in the updip wilcox is the greatest in unit iii whereas units i and ii are most productive in the downdip deep wilcox shelf margin trend units v and iv represent the initial deposition of sand rich sediment into the gulf of mexico following the midway transgression transgression marked by unconformities and the cane river marl above unit i terminated wilcox deposition wilcox sediment sources were located to the northeast and northwest all units display a strong north south isopach grain in the northern two thirds of the study area however east west oriented sandstone packages are present along the paleoshelf margin alluvial valley progradational and aggradational and valley interfluve retrogradational coastal onlap deposition can be discerned from well logs

equatorial convergence of india and early cenozoic climate trends india s northward flight and collision with asia was a major driver of global tectonics in the cenozoic and we argue of atmospheric co2 concentration pco2 and thus global climate subduction of tethyan oceanic crust with a carpet of carbonate rich pelagic sediments deposited during transit beneath the high productivity equatorial belt resulted in a component flux of co2 delivery to the atmosphere capable to maintain high pco2 levels and warm climate conditions until the decarbonation factory shut down with the collision of greater india with asia at the early eocene climatic optimum at 50 ma at about this time the india continent and the highly weatherable deccan traps drifted into the equatorial humid belt where uptake of co2 by efficient silicate weathering further perturbed the delicate equilibrium between co2 input to and removal from the atmosphere toward progressively lower pco2 levels thus marking the onset of a cooling trend over the middle and late eocene that some suggest triggered the rapid expansion of antarctic ice sheets at around the eocene oligocene boundary

evolution of the northeast german basin inferences from a 3d structural model and subsidence analysis abstract a 3d structural model of the northeast german basin was evaluated with special emphasis on its evolution as an intracontinental depression the study includes investigations on subsidence history and structural setting of the basin thickness evolution and calculated tectonic subsidence volumes of permian to quaternary sediments in the northeast german basin indicate that the subsidence history was related to five stages of basin evolution which differ in their subsidence mechanisms for the initial rift phase in the late carboniferous to early permian a dominant thermal event and subordinate horizontal stresses were indicated by thickness variation evolution and by structural evidence the main part of basin subsidence occurred in a nw se oriented basin in the subsequent phase of thermal relaxation with maximum subsidence from early permian rotliegend to middle triassic muschelkalk from middle triassic the thermal subsidence pattern was superposed by further tectonic events in the middle triassic regional extension led to a reconfiguration of the southern part of the basin where new nne ssw trending troughs rheinsberg and gifhorn troughs developed in the jurassic the northwestern part of the basin was uplifted while in the south the keuper subsiding areas continued to sink and nw se trending depressions related to salt margins became important differentiation continued into cretaceous times when regional compression caused uplift of the southeastern part of the basin and basin margins a final subsidence phase occurred in the cenozoic this was accompanied by intensive salt movement recent basin configuration reflects the superposition of structural elements resulting from different evolution stages the main structural characteristics of the basin are 1 a vertical tectonic zonation in a pre zechstein succession which lacks significant internal structures and a strongly deformed post zechstein succession which was decoupled due to the thick zechstein salt and 2 a marked asymmetry of the basin with a shallow northern slope and a steep bounding fault at the southern margin elbe fault system the northwestern part of the basin shows the structural properties of an intracratonic sag basin with persisting subsidence and with minor salt mobilisation in contrast initial structures in the southeastern part are strongly overprinted by younger tectonic events including middle triassic to jurassic extension late cretaceous inversion and late to post cretaceous salt movements tectonic elements that deform the whole sedimentary succession are restricted to the basin s southern eastern and northeastern margins where salt thickness decreases combined volumetric and backstripping investigations show that 2 3 of the total subsidence was induced by the sediment load and 1 3 was caused by tectonics the tectonic subsidence history varies laterally across the basin while tectonic subsidence curves in the northwestern part of the basin show fast subsidence during the permian and early triassic decreasing exponentially with time subsidence curves in the southeastern part indicate repeated tectonic activity however the tectonic subsidence volume created during permian to late triassic is significantly higher than the tectonic subsidence during younger phases of basin history

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

maturation of organic matter in paleocene eocene wilcox group south texas relationship to clay diagenesis and sandstone cementation abstract thirty three mudstone core and cuttings samples ranging in depth of 1 6 4 7 km 5 200 15 400 ft and in temperature from 80 to 210 c 175 410 f were obtained from wells in south texas the results of closed system pyrolyses and bitumen and kerogen analyses are related to available data on clay mineralogy and sandstone cement this study examines the diagenesis of wilcox organic matter the migration of hydrocarbons and the importance of organically derived co2 in sandstone cementation the samples average 1 toc contain type iii kerogen and generally show bitumen contents less than or equal to about 150 mg gc modeling indicates that primary migration of methane and light hydrocarbons in aqueous solution is capable of producing giant 1 tcf gas and condensate fields because hydrocarbon generation does not occur until after the main stage of illitization smectite accounts for less than 25 of the total water involved pyrolysis experiments indicate that as much as 150 mg gc co2 may be liberated by wilcox organic matter during diagenesis to present day temperatures of 100 c 212 f the main zone of oil generation occurs at subsurface temperatures of 95 125 c 203 257 f the timing of these processes suggests that co2 could play an important role in creating secondary sandstone porosity for hydrocarbon migration the dgr13c values indicate that 25 of the carbonate cement present in wilcox sandstones may originate from decomposition and diagenesis of organic matter end of article last page 492

growth and demise of cenozoic isolated carbonate platforms new insights from the mozambique channel seamounts sw indian ocean although long term evolutions of isolated shallow water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades their driving processes are still the subject of active debates the mozambique channel sw indian ocean is characterized by several flat topped seamounts ranging from 11 s to 21 s in latitudes based on a comprehensive geomorphologic study and on dredged samples analysis we show that these features correspond to tropical isolated shallow water carbonate platforms coupling strontium isotopy and foraminifera biostratigraphy well constrained chronostratigraphy results indicate that shallow water carbonate production started in the mozambique channel during distinct cenozoic periods ranging from paleocene to early miocene our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism while some of them kept developed until present days forming modern carbonate systems some others were drowned during late neogene and subsided to form guyots although different factors can be discussed tectonic and volcanism appear as good potential triggers for demise episodes during late miocene early pliocene times chronology and location of this geodynamical activity tend to emphasize influence of east african rift system until southern mozambique channel

depositional and diagenetic relationships between gulf of elat aqaba and mesozoic of united states east coast offshore the tectonic setting and depositional environments along the ancient margin during the mesozoic of the east coast of the united states may have been similar to those in the present gulf of elat aqaba the gulf of elat aqaba is the northern continuation of the red sea rift zone where carbonates are accumulating contemporaneously with clastics under arid conditions clastics are deposited primarily in alluvial fan complexes which spill out onto a narrow shelf carbonate deposits including reef complexes occur along the shelf break in comparison along the united states east coast margin triassic to early jurassic rifting coincided with carbonate and clastic deposition the middle jurassic and cretaceous strata reflect the transition from synrift to postrift stages f tectonic evolution seismic and well data of the jurassic and cretaceous sections show an increase in the carbonate content approaching the united states east coast shelf break a discontinuous belt of carbonate buildups is inferred along the united states east coast shelf and slope the gulf of elat aqaba represents an analogous model for the early jurassic east coast margin of the united states beginning in the middle jurassic the east coast shelf edge carbonate sequences were being deposited on a passive margin carbonate growth may have outpaced subsidence and may be similar stratigraphically to the gulf of elat aqaba where carbonate deposits are prograding seaward in the gulf of elat aqaba calcium carbonate cementation has significantly reduced the porosity 28 and permeability 0 01 md of still submerged carbonate reefs however pleistocene carbonates on uplifted blocks in the adjacent onshore have undergone leaching and development of secondary porosity as a result of dissolution by meteoric fresh water the uplifted carbonates contain high secondary porosity 60 and permeability 10 000 md emergent carbonate strata exposed to percolating meteoric waters generally develop such high porosities in the subsurface former emergent surfaces are recognized as unconformities among examples of mesozoic carbonate reservoirs formed during subaerial exposure and marked by unconformities are the casablanca upper jurassic spain and golden ane cretaceous mexico oil fields unconformities in the mesozoic carbonate rocks of the united states east coast suggest that potential carbonate reservoirs one jurassic and one cretaceous formed during periods of subaerial exposure hydrocarbons generated during the early or late cretaceous may have moved into the reservoirs prior to deep burial possibly inhibiting further subsurface diagenesis including porosity and permeability changes and preserving the reservoirs

paleomagnetic evidence for a stationary marion hotspot additional paleomagnetic data from madagascar abstract the island of madagascar experienced widespread magmatism at ca 90 ma due to its interaction with the marion hotspot previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the marion hotspot has remained fixed for the past 90 ma we report paleomagnetic data from northeastern madagascar d analava complex samples were collected from basalts rhyolites gabbros and a dolerite dyke sixty samples from 5 sites yield a paleomagnetic pole at 66 7 s 43 5 e a95 10 7 and a grand mean pole gmp calculated from 10 different studies covering the entire island of madagascar falls at 68 9 s 49 0 e a95 4 4 this pole translates to a paleolatitude for the volcan de l androy focal point of the hotspot at 45 2 6 5 s compared to the current location of the marion hotspot at 46 s our results confirm and expand upon previous studies that argue for the fixity of the marion hotspot for the past 90 ma

late cretaceous intraplate thrusting in central europe effect of africa iberia europe convergence not alpine collision a well established event of intraplate basin inversion and basement thrusting affected central europe in late cretaceous time it is widely accepted to have resulted from the collision of the alpine orogen with europe s margin at that time an early alpine orogen located on the leading edge of the adria microplate still lay far southeast of its present day position and had entered a phase of extension after a first orogenic event characterized by w to nw directed thrusting this configuration is not likely to have induced ssw nne directed thrusting and folding in the future european foreland that was still separated from the alpine wedge by a strip of oceanic lithosphere by contrast the onset of intraplate contraction coincides with an important change in relative motion between the european and african plates at ca 90 ma africa s sse directed sinistral transform motion relative to europe changed to ne directed convergence this agrees well with the timing and kinematics of intraplate thrusting in central europe structures of similar age and kinematics occurring in southern france spain and north africa suggest that the late cretaceous pulse of contraction was caused by pinching west central europe s thin lithosphere between baltica and africa only since the onset of n directed thrusting in the alps in paleocene or eocene time are the kinematics of the alps and their european foreland compatible indicating that mechanical coupling between africa europe and the adria microplate had been achieved

paleomagnetism and rock magnetism of early cretaceous rajmahal basalts ne india implications for paleogeography of the indian subcontinent and migration of the kerguelen hotspot abstract we present here new paleomagnetic and rock magnetic results from rajmahal traps rt india the results are comprising of 350 standard specimens collected from 18 sites 90 samples the detailed rock magnetic studies indicate that a single domain sd magnetite fe3o4 is the dominant magnetic carrier with ilmenite pyrrhotite and maghemite in the accessory form the calculated characteristic remanent magnetization chrm directions yield a mean declination of 317 51 mean inclination of 61 92 95 3 6 k 93 39 the obtained virtual geomagnetic pole vgp position of the rt at 117 ma is 11 37 n 297 58 e 62 42 w with a95 3 5 k 99 99 dp 4 33 dm 5 58 and the calculated paleolatitude is at 43 1 s indistinguishable from that of earlier works this paleomagnetic pole of rt basalts has minimal effect of secular variation and comply with the geocentric axial dipole gad model the vgp is plotted on synthetic apparent polar wander path apwp for india constrained better and agrees well with the previously reported poles the postulation of migration of mantle plume and the global mantle circulations are tested and the present study results are in accord with this suggesting the southward migration of the kerguelen hotspot mantle plume by at least 6

feldspar alteration and diagenetic characteristics of the parsora sandstones son basin india abstract moderate to poorly sorted immature parsora sandstones rich in k feldspar show much of the feldspar during early diagenesis transformed to kaolinite after prolonged interaction with acidic pore solutions the kaolinitic epimatrix formed and was later partially or wholly converted as an orthomatrix producing chert phyllosilicate assemblage ferric oxide bleached biotite kaolinite and quartz cement denote an oxy acidic early diagenetic environment late diagenesis involved neoformation of primary or secondary matrix illitisation of montmorillonite and muscovite authigenesis the high pressure temperature regime required for these transitions resulted from tectonic activity during triassic jurassic times carbonate chlorite appeared late in the sediments denoting an alkaline reducing condition at the late part of the diagenesis finally the secondary porosity developed through carbonate dissolution was later filled up with allochemical ferric iron cement receiving ions from the percolating meteoric water

depositional and diagenetic controls on reservoir quality in first wilcox sandstone livingston field louisiana livingston field is a large oil field approximately 25 million bbl in southeastern louisiana the field produces from the lower eocene first wilcox sandstone a 40 to 50 ft 12 to 16 m thick barrier island deposit from subsea depths of 9 917 10 146 ft 3 023 3 092 m the presence and quality of the reservoir are the direct result of diagenetic events that were strongly influenced by depositional facies early primary porosity was significantly reduced by clay and quartz overgrowths and by carbonate cements dissolution of the carbonate cement and leaching of feldspars and other unstable grains restored porosity to 65 75 of original values the highest degree of secondary porosity was created in the facies of highest primary porosity the eolian beach and upper sho eface while the initially less porous middle and lower shoreface developed little or no secondary porosity the formation of late stage diagenetic clays kaolinite illite chlorite and smectite have decreased permeability by reducing or blocking pore throat openings

an integrated geophysical study east of the southern chagos laccadive ridge complex central indian ocean basin implications for the rodriguez triple junction dynamics in the late cretaceous abstract the central indian ocean basin east of the southern chagos laccadive ridge complex has evolved due to seafloor spreading in the n s direction since the late cretaceous the trace of the rodriguez triple junction rtj has been inferred in this basin but its exact location and dynamics prior to c29o are not known about 20 000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the rtj in the late cretaceous synthetic seafloor spreading models endorse the presence of late cretaceous to early palaeogene magnetic anomalies c34y to c24o in the study area half spreading rates are initially low 32 mm yr between c34y to c33y and increase up to 78 mm yr by c28y thereafter a decreasing trend is seen 57 mm yr towards the west in the study area higher spreading rates 120 mm yr between c33y and c32ay and distorted anomalies between chrons c32ay and c30y are observed detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation plate reconstruction models suggest that the rtj at c34y time lay southwest of comorin ridge which coincided with the northern edge of the conrad rise until c33y the rtj left a linear trace parallel to the fracture zone trend on both the indian and antarctic plates while it gradually moved eastwards on the african plate the rtj migrated westwards until c31y causing the transfer of a considerable amount of crust from the african plate to both the indian and antarctic plates thereafter the rtj rapidly reversed its migration direction towards the east until c29o causing the formation and rotation of a microplate on the indian plate and lengthening of the southwest indian ridge the present study suggests that this episodic migration of the rtj may be due to ridge propagation under the influence of the marion hotspot which was in the vicinity during that period this rtj migration resulted in microplate formation lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the pacific ocean thus the origin of anomalous structures such as the crozet bank may be related to triple junction dynamics under the influence of marion hotspot further the counter clockwise rotation of the southwest indian ridge between c33y and c29y may have a causal link with the formation and rotation of the microplate which occurred during the same period the present study thus facilitated the inference of the dynamics of the rtj trace between c34y and c29o on the three plates

weathering mantles and their significance for geomorphological evolution of central and northern europe since the mesozoic abstract weathering mantles saprolites are very widespread in central western and northern europe where they form thick near surface residual deposits often in excess of 50 m they vary in the type of weathering the bedrock lithology involved the topographic setting and the age and stratigraphic context this review paper provides information about the distribution and major characteristics of weathering mantles from a number of key areas including the british isles the central european belt of mid mountains and uplands the scandinavian mountains and the fennoscandian shield it also examines the evidence for weathering etched origin of various landforms including boulders tors inselbergs basins and some regional landforms assemblages major types of regionally important weathering mantles include ferrallitic lateritic saprolites kaolinite rich saprolites sandy to grus mantles and various residuals left after partial dissolution of carbonate rocks although dating of weathering mantles is often circumstantial it appears sufficiently precise to argue for various ages of deep weathering mesozoic pre late cretaceous mantles are particularly widespread but there are also early tertiary and miocene saprolites widely present whilst most grus weathering is likely to be of miocene to pleistocene age individual upland areas have been subjected to multiple episodes of weathering whose legacy is the present day co existence of different weathering products continental and shallow marine deposits around the uplands particularly in the north sea and intra continental graben structures are often dominated by quartz and kaolinite pointing to deeply weathered land surfaces as the source areas and extending the record of weathering on the geological time scale they demonstrate that weathering mantles repeatedly formed and were subjected to erosion and new cycles of weathering were initiated we suggest that deep unroofing of basement structures in many parts of europe has been mainly accomplished by deep weathering and stripping of pre weathered rock the phenomenon of deep weathering has been very important for geomorphology etched landforms vary in size from small residual boulders up to regional characteristic landscapes etchsurfaces such as joint valley landscapes of southern sweden or undulating multi convex and multi concave landscapes of many uplands in central europe deep weathering has played a major part in the geomorphological evolution of europe since at least the late triassic and the concept involving long term etching and stripping provides an appropriate framework to describe and explain both erosional landscapes and sediment features

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

a late cretaceous ca 90 ma kimberlite event in southern india implication for sub continental lithospheric mantle evolution and diamond exploration abstract we report groundmass perovskite u pb sims ages perovskite nd isotopic la icpms composition and bulk rock geochemical data of the timmasamudram diamondiferous kimberlite cluster wajrakarur kimberlite field in the eastern dharwar craton of southern india the kimberlite pipes gave similar mesoproterozoic ages of 1086 19 ma tk 1 microcrystic variant and 1119 12 ma tk 3 however a perovskite population sampled from the macrocrystic variant of tk 1 gave a much younger late cretaceous age of ca 90 ma this macrocrystic kimberlite phase intrudes the mesoproterozoic microcrystic phase and has a distinct bulk rock geochemistry the nd isotope composition of the 1100 ma perovskites in the cluster show depleted end t values of 2 1 0 6 to 6 7 0 3 whereas the 90 ma perovskites have enriched end t values of 6 3 1 3 the depleted mantle dm model age of the cretaceous perovskites is 1 2 ga whereas the dm model age of the proterozoic perovskites is 1 2 to 1 5 ga bulk rock incompatible trace element ratios la sm gd lu la nb and th nb of all timmasamudram kimberlites show strong affinity with those from the cretaceous group ii kimberlites from the bastar craton india and kaapvaal craton southern africa as the late cretaceous age of the younger perovskites from the tk 1 kimberlite is indistinguishable from that of the marion hotspot linked extrusive and intrusive igneous rocks from madagascar and india we infer that all may be part of a single madagascar large igneous province our finding constitutes the first report of cretaceous kimberlite activity from southern india and has significant implications for its sub continental lithospheric mantle evolution and diamond exploration programs

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

active concomitant counterclockwise rotation and northwards translation of africa during the albian campanian time a paleomagnetic study on the wadi natash alkaline volcanic province 104 78ma south eastern desert egypt abstract africa was a key tectonic unit in the geologic evolution of the tethyan realm in the middle late cretaceous during the rifting of the south equatorial atlantic ocean this research sheds light on the tectonic evolution of africa during the albian campanian time the two end members of the wadi natash alkaline volcanic province wnavp 104 78 ma in the south eastern desert of egypt were studied paleomagnetically the wnavp 24 5 n 34 25 e is made up of a thick succession of differentiated lava flows grading from alkali olivine basalt aob followed by hawaiite mugearite and benmoreite flows with abundant sandstone interbeds all intruded by a large number of trachyte phonolite tr ph circular lava domes and ring dykes forming conspicuous landmarks in the province the oldest flows of the aob 104 7 ma and the youngest tr ph plugs and ring dykes 86 78 ma as well as the interbedded turonian taref sandstone formation previously known as nubian sandstone were sampled all over the province which covers more than 500 km 2 the isothermal remanent magnetization irm study pointed to magnetite family as the major remanence in wadi natash volcanics with little contribution from goethite hematite minerals on the other hand goethite and or hematite are the dominant remanence carriers in the interbedded taref sandstone after the progressive stepwise thermal demagnetization of all samples the characteristic remanence chrm direction for each rock type was visually isolated and its best fit line was calculated followed by the site means rock unit means and the paleomagnetic poles this paleomagnetic study revealed that 1 in the tilt corrected coordinates the mean chrm of the oldest aob flows n 12 sites was dec inc 340 10 k 54 95 6o yielding a north pole at 55 n 250 e the chrm of the youngest tr ph ring dykes and plugs n 10 sites was dec inc 354 3 5 k 60 95 6 3o yielding a north pole at 66 5 n 229 e 2 on the other hand the site mean chrms of the taref sandstone interbeds n 8 sites in the present day in situ coordinates are clustered around the present day geomagnetic field with a mean dec inc 3 33 k 37 95 9 3 yielding a north pole at 83 n 191 e the two paleomagnetic poles of the aob and the tr ph are in accordance with albian campanian poles from africa including madagascar and those rotated from the main cratons such as the north american craton europe including greenland and iberia south america and india using the published plate kinematics and paleomagnetic euler pole pep rotation parameters paleogeographically the two obtained poles of wadi natash volcanics place the african plate at two different azimuths and paleolatitudes during albian cenomaniam and santonian middle campanian tectonically these two paleomagnetic poles point to an active synchronous northwards translation associated with a counterclockwise rotational convergence of africa towards eurasia during the albian campanian time between the time of the aob eruption 104 7 ma and the intrusion of the tr ph 86 78 ma africa concomitantly rotated counterclockwise about 14 and translated northwards towards eurasia about 6 of latitude