precambrian accretionary tectonics in the bou azzer el graara region anti atlas morocco ophiolites of the anti atlas of morocco occur as highly dismembered slivers of mafic and ultramafic rocks sandwiched between a precambrian craton and a magmatic arc tectonic slices of accretionary melange ophiolites and forearc basins have been juxtaposed by oblique subduction and transpression in the forearc region of a late proterozoic subduction zone synkinematic to postkinematic calc alkalic magmatism in these terranes indicates migration of the magmatic front over the deformed forearc terranes

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

low latitude valley region irregularities studied using the gadanki radar 1 a study of valley region irregularities vri and structures utilizing a database of observations obtained by the gadanki radar is presented it is shown that vri occur both in the presence and absence of equatorial spread f esf activity vri are found to occur initially near 2000 lt at an altitude as high as 180 km and then descend with time notably the occurrence pattern of vri resembles closely to that of the low latitude intermediate layers the above occurrence pattern is observed irrespective of the type of esf event present during esf plume events however significant temporal structures in vri with close relation to the esf structures are observed which are otherwise missing vri velocities in relation to esf are in the range of 70 m s 1 to 100 m s 1 while in the absence of esf vri velocities are in the range of 50 m s 1 further snr and spectral width are considerably higher when they are associated with esf plumes it is suggested that vri are generated on the intermediate layers possibly through gradient drift instability it is also suggested that the valley region ambient fields are quite capable of generating these irregularities the role of the unstable f region electric fields on vri is found to be important for structuring in vri and may be critical for the generation of irregularities when the valley region is stable by itself

pan african displaced terranes in the tuareg shield central sahara concepts developed in the recently published model of the air region eastern tuareg shield niger africa as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the tuareg shield 500000 km 2 the 23 terranes recognized have their own lithological metamorphic magmatic and tectonic characteristics and are separated by subvertical strike slip megashear zones that can be traced for hundreds of kilometres or by major thrust fronts some of these boundaries have ophiolitic assemblages or molassic deposits the tuareg shield was shaped and partly accreted during the pan african orogeny 750 550 ma but not as a homogeneous body

proterozoic collisional basins in a pan african suture zone anti atlas mountains morocco abstract the anti atlas mountains constitute a late proterozoic suture zone produced by northward subduction of oceanic lithosphere culminating in the pan african orogeny southward migration of thrust slices associated with the destruction of the forearc terrane resulted in the uplift and erosion of previously deposited basin sediments these sediments were subsequently reincorporated into collisional basin deposits of the tiddiline formation the tiddiline formation consists of coarsening upwards sequences of maroon siltstones sandstones and intraformational conglomerates these rocks unconformably overlie metamorphosed volcaniclastic rocks of the relict forearc basin and accretionary terrane syn and post depostional deformation has resulted in folding about gently plunging fold axes folds were subsequently cut by strike slip faults that strike at a high angle to the basin axis deformation of the tiddiline formation is attributed to transpressional suturing of the relict forearc terrane to the west african craton to the south collisional basins of the anti atlas mountains serve as ancient analogs for the destruction of forearc basins in an obliqueconvergent margin setting such as those of the western pacific region

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

parallel plate capacitor analogy of equatorial plasma bubble and associated fringe fields with implications to equatorial valley region irregularities vhf radar echoes from the valley region plasma irregularities displaying ascending pattern are often observed during the active phase of equatorial plasma bubble in the close vicinity of the geomagnetic equator and have been attributed to bubble related fringe field effect these irregularities however are not observed at a few degrees away from the equator in this paper we attempt to understand this contrasting observational result by comparing fringe field at the geomagnetic equator and low latitudes we use parallel plate capacitor analogy of equatorial plasma bubble and choose a few capacitor configurations consistent with commonly observed dimension and magnetic field aligned property of plasma bubble for computing fringe field results show that fringe field decreases significantly with decreasing altitude as expected further fringe field decreases remarkably with latitude which clearly indicates the role of magnetic field aligned property of plasma bubble in reducing the magnitude of fringe field at low latitudes compared to that at the geomagnetic equator the results are presented and discussed in the light of current understanding of plasma bubble associated fringe field induced plasma irregularities in the valley region

evidence of intermediate layer characteristics in the gadanki radar observations of the upper e region field aligned irregularities 1 the gadanki vhf radar observations of the upper e region field aligned irregularities are presented these are the first observations of low latitude upper e region irregularities that resemble the characteristics of intermediate layers observed over arecibo the most interesting aspect of these observations is their occurrence at altitudes as high as 160 km which requires an interpretation in terms of their source mechanism these irregularities were found to trigger about 21 lt first at higher altitude 140 160 km and propagate downward with time the signal intensities are found to be lower by about 12 db and the doppler spectra narrower by more than a factor of two compared to that of the normal e region echoes the doppler velocities are found to be both upward and downward with values less than 40 m s 1

finescale shear at 1660 and 2850 dbar over the mid atlantic ridge in the eastern brazil basin abstract two drifting shearmeter instruments have provided records of the magnitude of shear the magnitude of the vertical derivative of horizontal velocity measured over a 10 m aperture in the vicinity of the brazil basin tracer release experiment the floats drifted at roughly 1660 and 2850 dbar where the buoyancy frequencies were n 1 1 and 0 5 cph respectively two additional floats provided deep trajectories but no shear data the records of hourly mean values of shear magnitude with mean values denoted sh exhibit strong temporal variability with time scales comparable to those of fortnightly tidal modulation the deep record exhibits stronger mean sh near the seafloor than above with the exception of the bin closest to the seafloor sh from neither float correlates strictly with tidal forcing shallow sh varies in apparent response to atmospheric forcing the mean squared shear of the shallow record is 4 times the n dependent value associated with the garrett munk spectral model and that

thermal and geodynamic setting of the buem volcanic rocks near tiele northwest benin west africa abstract the mafic volcanic rocks of tiele occur as intercalations within very low grade metasediments mainly metapelites and meta arenites these proterozoic supracrustal rocks are considered as part of the buem structural unit bsu which is the most external tectonic unit of the pan african dahomeyide fold belt the geodynamic significance of the bsu is still debated it could represent either a portion of the west african passive margin during the rifting and the drifting of the pan african ocean affaton 1990 or a continental rift zone jones 1990 or the pan african oceanic crust itself burke and dewey 1972 and burke and dewey 1973 shackleton 1976 the mafic lavas derived from tholeiitic magmas which originated by partial melting of the lithospheric mantle with a possible asthenospheric contribution they are quite similar to morbs and free of any crustal contamination they are related to a strongly attenuated continental lithosphere and a passive margin would be the most likely tectonic environment for the coeval volcanic and sedimentary activities on the other hand these rocks demonstrate effects of early static recrystallizations related to a very low grade metamorphism of the prehnite pumpellyite facies developed under temperatures of 200 to 300 c such a thermal imprint clearly marks an extensional tectonic stage later the early parageneses were re equilibrated under slightly higher pressures during the pan african collision both magmatic features and early metamorphic characters of these metavolcanic rocks are in good agreement with a context of lithospheric thinning characterized by mantle derived magmatism and high thermal gradient these results fit with previous interpretations mainly based on sedimentary and structural features the bsu represents a portion of a proterozoic passive margin which suffered strong lithospheric stretching with deep seated normal faults and mantle derived magmatism and a subsequent severe subsidence evidenced by the prominent detrital sedimentation such a tectonic setting developed during the pan african oceanic drifting

chronology of nappe assembly in the pan african dahomeyide orogen west africa evidence from 40ar39ar mineral ages abstract the pan african dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the west african craton was incorporated into gondwana orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in ghana and togo external nappes include the atacora composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c 2 0 ga foreland basement the suture zone is represented by high pressure granulite facies garnet homblende gneiss we have analyzed muscovite and homblende concentrates from external and suture zone nappes in incremental heating 40 ar 39 ar experiments muscovites which define the penetrative foliation in the atacora nappes and a shear foliation in the basement parautochthon record 40 ar 39 ar plateau ages of 579 4 0 8 ma and 578 1 0 5 ma respectively in contrast c 600 km to the north muscovite in quartz schist near the base of the atacora nappes displays a spectrum which gives a minimum age of 608 1 1 2 ma whereas muscovite in the overthrust kara nappes yields a plateau age of 633 8 0 5 ma the c 30 m y difference between the atacora nappes in the northern dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication however forward imbricating thrusting is also suggested by the older age of kara nappes relative to atacora nappes in the northern segment the youngest ages are inferred to date orogen parallel nappe transport at c 575 ma hornblende from the structurally lower sections of the suture zone nappes record a 36 ar 40 ar versus 39 ar 40 ar isotope correlation age of 587 4 3 ma hornblende from a tonalitic vein in the host garnet mafic gneiss yields an isotope correlation age of 581 9 2 4 ma these ages are interpreted to date the exhumation of the suture zone nappes and thus provide a minimum age of crustal thickening in the dahomeyide orogen the age data are consistent with coeval forward and hindward propagating thrusting and nappe imbrication and constrain the amalgamation in northwest gondwana to have occurred in the latest proterozoic rather than cambrian

u pb dating of the end of the pan african orogeny in the tuareg shield the post collisional syn shear tioueine pluton western hoggar algeria abstract the tioueine pluton intrudes the neoproterozoic series of the iskel terrane located in the tuareg shield western hoggar the consistency of the internal structures as well as the nature and organization of the associated microstructures demonstrate that the tioueine pluton was emplaced syn kinematically while n s strike slip shear zones were active the syn tectonic emplacement of the tioueine massif implies that this pluton although belatedly crystallized entirely belongs to the concept of post collisional magmatism in order to date precisely the late pan african tectono metamorphic event in the studied area an u pb age of 523 1 ma was obtained from abraded zircons of a late quartz syenite from the tioueine pluton this early cambrian age is younger than the other plutons of the tuareg shield which were mainly emplaced between 630 ma and 580 ma this dating also shows that the tuareg shield was not a single coherent block at 525 ma but rather an amalgam of active terranes moving each other along major shear zones finally the tioueine massif represents probably the final welding of the tuareg shield assembly of terranes and consequently the end of the post collisional orogenic episode in the whole pan african belt

mid latitude e region field aligned irregularities observed with the mu radar fine structures e region field aligned irregularities were observed on june 24 25 1989 with the mu radar at shigaraki japan 34 9 n 136 1 e geomagnetic latitude 25 0 n the 3 2 m scale irregularities were observed with the mu radar in five main beam directions each of which was nearly perpendicular to the geomagnetic field at 100 km altitude doppler spectra were obtained every 20 s with a range resolution of 600 m field perpendicular echoes appeared from 2130 to 2330 lt and from 0400 to 1100 lt times that correspond to postsunset and postsunrise period in the e region a preliminary examination of the doppler spectra indicates spectral widths of 50 120 m s 1 and the mean doppler velocities are well below the ion acoustic speed these spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets and have been attributed to the gradient drift instability the echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time height distribution for this reason they are classified into two types continuous and quasi periodic the appearance of the continuous echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period the appearance of the quasi periodic echoes was intermittent with periods of 5 10 min and situated above 100 km altitude during the postsunset period the quasi periodic echoes showed phase propagation toward the radar while the averaged mean doppler velocity was away from the radar by measuring the time delays in echo regions from five directions an apparent westward motion approximately 120 m s 1 of the irregularity regions was estimated

radar observations of daytime 150 km echoes from outside the equatorial electrojet belt over gadanki 1 we present systematic observations of daytime 150 km echoes from gadanki 13 5 n 79 2 e 6 3 dip latitude the echoing region is confined to 142 162 km altitude and show forenoon descent and afternoon ascent displaying a necklace shape quite similar to that of the equatorial 150 km echoes signal to noise ratio velocity and spectral width are as high as 13 db above noise 5 to 35 m s 1 and 2 to15 m s 1 respectively the observed necklace shape is first of its kind from gadanki and the present observations have almost all characteristics of equatorial daytime 150 km echoes the gadanki radar observations hence provide first experimental evidence on the occurrence of daytime 150 km echoes outside the equatorial electrojet belt

hit and run collision model for the laramide orogeny western united states the formation of the rocky mountains an event referred to as the laramide orogeny has presented a dilemma since the introduction of plate tectonics in scale and tectonic style the late cretaceous early tertiary orogeny resembles collisional orogenies such as the himalayas however no collider remains adjacent to the continental margin at the latitude of laramide features i e california to washington terranes of western british columbia and southeast alaska known together as baja bc may constitute the colliding plate geologic studies and paleomagnetic data suggest that baja bc collided with north america at the latitude of modern baja california the hit and was subsequently translated northward during oblique convergence the run to its present latitude between 94 ma and ca 40 ma northward translation of baja bc is spatially and temporally coincident with the ca 80 to 45 ma laramide orogeny suggesting a causal link between the events

the alkaline peralkaline granitic post collisional tin zebane dyke swarm pan african tuareg shield algeria prevalent mantle signature and late agpaitic differentiation abstract the tin zebane dyke swarm was emplaced at the end of the pan african orogeny along a mega shear zone separating two contrasting terranes of the tuareg shield it is located along the western boundary of the archaean in ouzzal rigid terrane but inside the adjacent tassendjanet terrane strongly remobilized at the end of the precambrian the tin zebane swarm was emplaced during post collisional sinistral movements along the shear zone at 592 2 5 8 ma 19wr rb sr isochron it is a dyke on dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites all rock types have sr and nd isotopic initial ratios sr i 0 7028 and e nd 6 2 typical of a depleted mantle source similar to the prevalent mantle prema at that period no crustal contamination occurred in the genesis of the tin zebane swarm even the samples showing evidence of fluid interaction essentially alkali mobility have the same isotopic signature the peralkaline granites have peculiar geochemical characteristics that mimic subduction related granites this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine zircon titanite allanite and possibly fergusonite separated from the liquid in the swarm itself due to magmatic flow turbulence the tin zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation in a post collisional setting

synsedimentary faulting and palaeocurrent patterns in the triassic sandstones of the high atlas morocco the influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow marine sandstones of upper triassic basins in the high atlas the synsedimentary nature of faults is deduced from hydroplastic slickensides thickness variations due to block tilting and dislocation of layers next to the fault scarp on a regional scale it is shown that the major normal fault trend of n050 070 controlled the overall palaeocurrent pattern which was directed towards the west southwest i e in the direction of the future atlantic ocean some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the triassic

sedimentary and volcanic evolution of a neoproterozoic continental margin bleida anti atlas morocco abstract a tentative geodynamic evolution of the neoproterozoic continental margin of the northern border of the west african craton is reconstructed using both sedimentological observations and geochemical arguments from volcanic rocks the rifting stage was marked first by intermittent subsidence of a very shallow shelf environment leading to deeper water sedimentation conditions with local surges of deltaic channels this rifting stage was also characterized by the eruption of a 500 m thick pile of submarine basic lava flows with geochemical characteristics similar to those generally observed in basaltic lavas erupted during continental rifting this first rifting stage 790 ma clauer 1976 was succeeded by a drastic change both in sedimentation and volcanism the younger sediments are deep water distal turbidites with associated pyritic black shales they include a few highly differentiated tholeiitic flows and sedimentary exhalative copper deposits linked to felsic volcanism these changes are attributed to the development of deep water oceanic basins along the northern border of the west african craton the closure of this pan african oceanic domain occurred by transpression tectonics along this major lithospheric transform zone which is roughly e w trending

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

present day and future tectonic underplating in the western swiss alps reconciliation of basement wrench faulting and decollement folding of the jura and molasse basin in the alpine foreland abstract the western alps form a geodynamically active mountain belt showing the typical features of an evolving orogenic wedge with its pro wedge geometry to the nnw and its retro wedge structures to the sse renewed tectonic underplating of european continental crust occurred after the orogenic wedge underwent major dynamic disequilibrium following the break off of the southward subducting slab of the european passive margin the most important of these basement imbricates are the mont blanc aiguilles rouges and gastern aar crystalline massifs also forming the alps highest mountains the upper plate present day orogenic wedge of the western alps includes the molasse basin and the jura fold and thrust belt both decoupled from the basement over a basal decollement surface the overall geometry of this wedge appears to be strongly unstable according to simple wedge models in its attempt to regain stability out of sequence thrusts form in the existing basement nappes but also new basement nappes should develop beneath the southern portion of the molasse basin new out of sequence thrusts in the cover trigger higher than average uplift rates concentrated around the newly forming structures and are accompanied by a concentration of earthquakes tectonic underplating is further corroborated by neotectonics and the tectonic structures observed in the prealpes molasse basin and jura similarly uplift rates and earthquakes along the southern edge of the jura mountains seem to witness the development of a new incipient basement nappe at depth partial inversion of former permo carboniferous grabens in the basement a possible spatial coincidence of areas with strong earthquake activity and zones with uplift rates above surrounding values suggest a common mechanism for their origin in the western swiss alpine foreland combined with information from basement geometry and wedge dynamics it is proposed that the common mechanism is the development of basement imbricates by tectonic underplating the proposed model for ongoing and possible future tectonic underplating beneath an active alpine orogenic wedge also allows to reconcile the models of basement wrench faulting in the molasse basin and jura with the distant push theory where the molasse basin and jura develop over a basal decollement horizon

equatorial quasiperiodic echoes from field aligned irregularities observed over jicamarca this is a report of so far undescribed equatorial quasiperiodic eqp echoes from field aligned irregularities from the e and lower f region these echoes were observed on october 20 1993 at the jicamarca radio observatory in conjunction with a spectacular spread f event we have identified two regions where striated eqp echoes are clearly observed the time periods are between 1 and 5 min in the first region 120 140 km the slope of the striations is negative apparent downward propagation while in the second 160 200 km the slope is positive moreover in the first region the time periods striation spacings are smaller and the altitude rates of change are higher than those observed in the second region although these equatorial echoes appear somewhat similar to those seen at midlatitudes we believe that the two phenomena are different we have no explanation as yet for these eqp observations our purpose in presenting these data is primarily to describe their properties in more detail than has been done in the past

role of synsedimentary strike slip faults in the formation of moroccan triassic basins upper triassic sandstone and siltstone infilled basins in the massif ancien of the high atlas morocco are distributed along a complex east northeast trough at the boundary between the hercynian orogen and the west african craton the formation of these basins involved a dense pattern of pre existing faults hydroplastic slickensides which affect nonlithified sediments help identify not only dip slip but also strike slip faults together with other tectonic and sedimentary structures the directional distribution of these faults leads to a model in which extension varies from north northwest during lower unit sandstone infilling to northwest during upper unit siltstone infilling the latter direction is compatible with the formation of the lower liassic intrusional doleritic dikes of the anti atlas

ionospheric irregularities in the low latitude valley region observed with the equatorial atmosphere radar 1 the geomagnetically low latitude valley region between the upper e region and the lower f1 region is studied with the equatorial atmosphere radar ear in indonesia three meter scale field aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread f esf in the period from sunset to midnight the valley region echoes usually appear at above 150 km and propagate downward with time rapid beam scanning of the ear revealed that spatial structure temporal variation and drift velocity of the valley region echoes resemble those of esf which indicates that the dynamoelectric field in the equatorial f region controls the low latitude valley region irregularities perturbed electric fields associated with esf map down to the low latitude valley region and can produce the perturbed plasma density structures as images of esf structures image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the ear it is suggested that intermediate layers should supply a plasma density gradient for excitation of 3 m scale irregularities in the valley region through gradient drift instability