dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

geoid pangea and convection abstract we show that the present geoid has a simple low order configuration with an axis of symmetry in the equatorial plane we show further that it is a tennis ball pattern with an equatorial high belt and a polar low one which is clearly controlled by the rotation of the earth finally we show that the outline of pangea between at least 200 ma and 125 ma ago lay along a great circle passing through the paleo poles of rotation thus it also had an axis of symmetry in the equatorial plane this hemispheric super continent configuration ended in middle cretaceous time during a major geologic catastrophe which was accompanied by high rates of spreading hotspot outbreaks and high sea level stands we interpret this evidence in terms of separate steady state lower mantle convection responsible for the present geoid weakly coupled to the upper mantle one this weak coupling leads to the hemispheric continent configuration which ends when excessive heating of the upper mantle due to the insulating continental cap leads to continent dispersal the complete cycle from one supercontinent to the next might be of the order of 400 ma

phenomena of pulsation tectonics related to the breakup of the eastern north american continental margin abstract new data from the recent ipod drilling of dsdp site 534 in the blake bahama basin give a definite age for the spreading center shift involved in the breakup of the north american atlantic margin a basal callovian age 155 m y is determined for the blake spur magnetic anomaly marking this spreadingcenter shift that signals the birth of the modern north atlantic ocean this is some 20 m y younger than previously thought an implication of this younger age for the blake spur event is that very high spreading rates are now required for the jurassic outer magnetic quiet zone along the north american margin this association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the mid cretaceous and implies a link between the processes controlling plate spreading which are in the upper mantle and the processes controlling the magnetic field which are in the outer core a theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost mantle could explain the correlation plumes carry heat away from the core mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading the pulse of fast spreading in the jurassic apparently followed the breakup of continents bordering the north atlantic ocean other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe implying that this might be a prevalent process rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics

volcanism and vertical tectonics in the pacific basin related to global cretaceous transgressions abstract the dominance of volcanic processes and the importance of vertical tectonics in the geological evolution of the pacific basin has been recognised since the time of charles darwin data gathered on several legs of the deep sea drilling project dsdp and numerous marine expeditions in the past decade have confirmed menard s postulate that the pacific basin was the scene of volcanism on an enormous scale in mesozoic time widespread mid plate volcanism between 110 and 70 m y b p characterised the area bounded by the line islands the mid pacific mountains and the nauru basin marshall islands heating of the pacific lithospheric plate during this period of volcanism resulted in regional uplift and the bathymetric evolution of the area diverged significantly from a normal parsons sclater subsidence curve the farallon plate now almost entirely subducted was also the scene of mid plate volcanism that produced such features as the nicoya plateau now found as an allochthonous ophiolitic terrain landward of the middle america trench large benthonic reef associated foraminifera comprising a pseudorbitoid fauna hitherto considered to be largely restricted to central america have now been additionally recorded from dsdp sites 165 315 and 316 in the line islands site 462 in the nauru basin and in new guinea the distribution of this fauna of campanian maastrichtian age is interpreted as indicating stepping stone connections aseismic ridges plateaus and seamounts between the caribbean farallon and pacific plates 70 80 m y b p similarities between the geology of the nauru basin and the caribbean ocean crust reinforce the interpretation of the latter as a former part of the farallon plate estimates of the sea level and continental freeboard change caused by the thermally induced uplift of the pacific and farallon plates as well as substantial areas in the atlantic and indian ocean basins indicate that such shallowing of the sea floor could have been the major factor in causing global cretaceous transgressions

thermal infrared and visual observations of a water ice lag in the mars southern summer 1 we present thermal infrared and visual evidence for the existence of water ice lags in the early southern summer the observed h2o ice lags lay in and near a chasma and appears to survive between 6 8 sols past the sublimation of the co2 possible sources of the h2o that compose the lag are 1 atmospheric h2o that is incorporated into the seasonal cap during condensation 2 cold trapping of atmospheric water vapor onto the surface of the cap in the spring or 3 a combination of the 2 processes where water is released from the sublimating cap only to be transported back over the cap edge and cold trapped we refer to this later process as the houben effect which may enrich the amount of water contained in the seasonal cap at 85 s by as much as a factor of 15 this phenomenon which has already been identified for the northern retreating cap may present an important water transport mechanism in the southern hemisphere

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

major marine cycles in the mesozoic the cycles postulated by fischer 1979 are considered in the light of the mesozoic stratigraphical and palaeontological record it is suggested that the most important events were i the late permian regression ii the late mid triassic transgression iii the late late transgression iv the late early jurassic transgression v the late mid jurassic transgression vi the end jurassicre gression vii the late early cretaceous transgression viii the end cretaceous regression these are related to episodicity in ocean floor spreading climatic changes and major happenings in the history of the shallow marine benthos

source of marine magnetic anomalies some results from dsdp leg 83 leg 83 of the deep sea drilling project represents the deepest penetration of undisturbed marine crust yet achieved providing samples from the lower part of the extrusive layer and the upper 300 m of the sheeted dike complex we present here measurements of the magnetic properties of rocks from leg 83 and address the question of their suitability as sources for marine magnetic anomalies we find that due to extensive hydrothermal alteration the extrusive part of the leg 83 section is insufficiently magnetized to be part of the magnetic source layer the sheeted dike complex on the other hand does have sufficient magnetization and contrary to recent models of marine magnetization based on ophiolite studies may be a contributor to magnetic anomalies the strong dependence of magnetic properties on the alteration history of the rocks however makes extension of these results to other regions problematical

evidence of displaced terranes from permian to jurassic faunas around the pacific margins for the permian to jurassic time interval tethyan marine invertebrate faunas which are generally accepted as having lived at low latitude can be distinguished from less diverse higher latitude faunas displacement of these low latitude faunas to positions of high latitude around the pacific margins therefore provides key evidence for the movement of so called suspect or displaced terranes the fullest story so far has been worked out for the western margin of n america as far north as southern alaska but there is also convincing evidence for the substantial northward movement of continental segments along the ne asian margin as regards the southern hemisphere the torlesse terrane of new zealand appears to have moved a considerable distance southwards but there is no fauna1 evidence of comparable far travelled suspect terranes along the andean margin of south america not only tethyan and boreal but e and w pacific elements are detectable in the faunas and the distribution of these argues for a panthalassia studded with islands of various sizes fringing the adjacent continents rather than for the disintegration of a single continent pacifica

evidence for late pleistocene climate changes from buried soils on the southern slopes of mt kilimanjaro tanzania abstract the distribution of the altitudinal vegetation zones on the slopes of mt kilimanjaro is well known but no reliable data are available about the vegetation shifts during the quaternary here i present palaeopedological results elemental biomarker stable isotope and radiocarbon analyses indicating that the frequently occurring buried black a horizons in the montane forest belt are developed under ericaceous vegetation since erica species are not abundant at this altitude nowadays but are characteristic for the sub alpine zone the palaeosols reflect palaeoclimatically driven fluctuations and changes of the vegetation zones on mt kilimanjaro the downhill descent of the ericaceous belt coincided with cold and dry periods like the late glacial and the last glacial maximum older palaeosols document that such events also took place during the marine isotope stages 3 and 4

marine triassic faunas of north america their significance for assessing plate and terrane movements the distribution of marine triassic invertebrate faunas in north america is reviewed to recognize forms that may have paleolatitudinal significance

sea floor lava fields on the east pacific rise a seabeam and seamarc i survey of a seamount group located several kilometres west of the axis of the east pacific rise epr at lat 9 53 n defines the existence of a large submarine lava field at the base of one of the volcanoes mib the field must be relatively young with respect to the surrounding terrain because lavas lap up against the base of mib seamount obscuring older erosional terrain and extend out over the faulted sedimented terrain of the flank of the epr volcanic cones within the lava field are aligned with the strike of fault scarps that are buried by the lavas mib seamount is characterized by a large well developed caldera and we suggest that the lava field was created during a recent volcanic episode when magmas migrated laterally away from the seamount9s reservoir along preexisting faults parallel to the ridge axis the withdrawal of magmas from the mib reservoir into the flanking parasitic eruptive plumbing system created the lava field and deflated the seamount summit this event contributed to the development of the mib caldera draining of shallow level magma chambers under seamounts by pressure controlled basal flank eruptions that form lava fields is an important mechanism for the development of seamount morphology

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

crustal movements and geodynamics in fennoscandia abstract morner n a 1981 crustal movements and geodynamics in fennoscandia in p vyskocil r green and h malzer editors recent crustal movements 1979 tectonophysics 71 241 251 the fennoscandian shield the classical area of glacial isostasy has been subjected to a complex sequence of motions throughout the late cenozoic still the geometry of the areas deformed has been roughly the same an asthenosphere of low viscosity 1020 1021 poises and a lithosphere of high flexural rigidity are established the present uplift represents an other mechanism than the rapid glacial isostatic readjustment of the low viscosity asthenosphere and seems to originate from mesospheric motions at around 1900 1910 there was a distinct geodynamic change which seems to be controlled by changes in the earth s rate of rotation the postglacial uplift of fennoscandia is found to be associated with intensive neotectonism faulting fracturing and seismicity

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

cordilleria a newly defined canadian microcontinent palaeomagnetic data and geological field evidence indicate that most of the western canadian cordillera lay 1 500 km south of its present position in the early cretaceous we suggest that this region comprised a microcontinent termed cordilleria which moved on the kula plate and collided with the craton 100 myr ago to form first the mackenzie mountains and then the rockies

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

age and structure of the southern rockall trough new evidence abstract the rockall trough separates the rockall plateau microcontinent from the shelf and slope west of the british isles the structure and age of the trough has been the source of considerable discussion although widely considered to be of oceanic origin postulated ages for the spreading range from permian to cretaceous new seismic profiles linked to the ipod sites in the bay of biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern rockall trough it is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the albian maastrichtian interval