japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

regional strike slip in the eastern continental margin of korea and its tectonic implications for the evolution of ulleung basin east sea sea of japan air gun profiles delineate a large scale strike slip fault system along the eastern continental margin of korea this system comprises two right stepping curvilinear master faults hupo and yangsan faults that are closely associated with en echelon nominal faults and reverse faults and folds these structural features indicate two phases of regional deformation in the earlier phase late oligocene to early miocene the margin experienced extensional shear strain with dextral strike slip movements along the hupo and yangsan faults that guided a pull apart opening of the pohangyoungduk basin the later phase deformation late miocene to early pliocene was governed by shortening strain that induced a convergent strike slip reactivation of the hupo fault as well as widespread contractile faulting and folding the regional deformation in the eastern korean continental margin reflects three stages of tectonic evolution of the ulleung basin 1 pull apart opening stage from late oligocene to early miocene 2 rotational opening stage with a differential rotation of the japanese arc in middle miocene and 3 back arc closing stage from the end of middle miocene to present

magnetic property zonation in a thick lava flow in this study grain size and composition dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick 30 60 m basaltic lava flow similar zonation occurs in this flow at three localities separated by tens of kilometers the magnetic properties subdivide the flow to three zones the upper layer representing the top 1 3 of the lava 20 m has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains approaching pure magnetite the central layer in the underlying 2 3 of the flow 35 m has larger magnetically less stable and less oxidized grains with relatively uniform magnetic properties the basal layer the bottom 1 10 of the flow 5 m has near primary least oxidized titanomagnetites ulv68mag32 the upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature above to colonnade below distinguishing between regions of faster and slower cooling microprobe data indicate that the intraflow oxidation state fe3 fe2 of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center the position of lowest equilibrium temperature in contrast curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water as well as unknown contributions of circulating air and percolating water from above the titanomagnetites of the basal layer of the flow remain essentially unaltered

synvolcanic and syntectonic sedimentation of the mixed volcaniclastic epiclastic succession in the miocene janggi basin se korea understanding the variability and complex interplay of volcanism tectonism and sedimentation is crucial for unraveling the evolution of sedimentary basins and volcanotectonic depressions in many settings the miocene janggi basin in se korea formed in association with backarc opening of the east sea sea of japan is filled by abundant volcanic deposits intercalated with fluvio lacustrine sedimentary deposits four lithostratigraphic units units i to iv in ascending order are identified in the southern part of the basin including extensive and voluminous dacitic lapilli tuffs units ii and iv and mixed dacitic volcaniclastic and epiclastic sedimentary deposits units i and iii these units suggest repetitive ignimbrite emplacement onto a braided stream to lacustrine environment paleoflow measurements show that the epiclastic conglomerates were deposited by a north flowing fluvial system that debouched from a drainage area to the south of the basin whereas the ignimbrites were introduced from the east because of the separate source areas the epiclastic and volcaniclastic depositional processes did not interfere with each other resulting in discrete alternations of epiclastic and mostly primary volcaniclastic deposits the studied succession also lacks an aggradation degradation cycle which results from the alternation of periods of rapid volcaniclastic sedimentation and relatively longer periods of fluvial incision the volcaniclastic and epiclastic deposits are therefore interpreted to have accumulated in rapid succession in association with rapid basin subsidence and contemporaneous ignimbrite forming eruptions the studied succession can thus be regarded as a syntectonic sequence in which the stratal geometry was significantly modulated by tectonostratigraphic controls with overprints of syntectonic contemporaneous volcanism

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

tectonically controlled vent migration during maar diatreme formation an example from a miocene half graben basin in se korea abstract understanding the roles of external controls such as hydrology and tectonics is crucial for interpreting the evolution of a phreatomagmatic volcano and its subsurface structure called the diatreme the yangpori diatreme in a miocene terrestrial half graben basin of korea provides an opportunity to assess the roles of synvolcanic fault movement in forming a maar diatreme volcano the exposed part of the diatreme fill consists of two offset stacked sequences with contrasting lithofacies characteristics a lower sequence early diatreme fill of locally stratified or bedded basaltic lapilli tuff and tuff in the northeastern part and an upper sequence later diatreme fill of dacite clast rich and disorganized tuff breccia in the southwestern part the boundary between these sequences is abrupt suggesting eruption of the volcano in two distinct phases the first phase of eruption was driven by phreatomagmatic explosions mainly within the basin fill some of the diatreme fill was deposited in a local pond by debris flows and suspension settling of fines whereas the rest by fallout from eruption plumes and jets the second phase of eruption was driven by phreatomagmatic explosions at a fracture aquifer within the basement accompanying sudden lowering of the explosion locus the later diatreme fill therefore contains abundant basement derived dacite clasts its deposition was accomplished by en masse fallout of volcanic debris from dense collapsing columns a number of faults and shear fractures were generated within and in the vicinity of the diatreme together with penecontemporaneous basaltic intrusions structural and geochemical analyses show that many of these structures are genetically related with the formation of the diatreme the early diatreme fill was also tilted toward the southwest suggesting southwestward shift of the locus of explosion and diatreme excavation the explosion locus of the yangpori volcano is therefore interpreted to have migrated both vertically and laterally the abrupt lithofacies boundary between the early and later diatreme fills and the proximity of the diatreme to the basin margin suggests that the migration was caused by an abrupt tectonic activity near the basin margin most probably the feeder dike was dislocated by movement along the basin margin fault which created abundant fractures deep within the basement that redirected ascending magma and opened a new hydraulically active zone in the basement synvolcanic faulting is therefore interpreted to have played a crucial role in forming the offset stacked architecture of the yangpori diatreme sequences the faulting is also inferred to have influenced significantly the morphology and internal facies architecture of unpreserved ejecta rimbeds

counterclockwise rotation of northeast japan paleomagnetic evidence for regional extent and timing of rotation early to late miocene welded tuffs have been sampled from northeast honshu island and the oshima peninsula of hokkaido island japan for paleomagnetic study characteristic directions with a high unblocking temperature component above 560 c are isolated from 24 sites westerly declinations d 1 91 are identified in welded tuffs older than 17 ma from these areas when combined with previous paleomagnetic data from welded tuffs the early miocene paleomagnetic poles derived from northeast honshu island and the oshima peninsula are statistically identical and indicate that northeast honshu island and the oshima peninsula were subjected to counterclockwise rotation as a single tectonic block these results establish that the rotating block of northeast japan extends north to the oshima peninsula to the latitude 43 n the dimension of this rotating block is of the order of 500 km the paleomagnetic data provide a best curve for the rotation process with the counterclockwise rotation reaching a climax at 15 0 ma and a net amount of rotation of 46 4 the 500 km long northeast japan block was rotated counterclockwise through more than 45 and accompanied a clockwise rotation of the 600 km long southwest japan block at about 15 ma

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

lithostratigraphy of the volcanic sequences at hole 917a leg 152 se greenland margin abstract an overall high recovery rate up to 52 and 450 m of continuous logging data of good quality at odp hole 917a leg 152 se greenland margin allow us to refine the petrological descriptions made onboard and to establish a precise lithostratigraphy of the palaeocene volcanic succession changes in the dips of the flow banding between the upper series and the middle and lower series coincide with the transition from continental magmatism to post breakup volcanism established by the geochemistry

structural and petrophysical characterization of the upper basement crustal section at odp iodp site 1256 east pacific ocean 1 new ocean crust is constantly being formed from mid ocean ridge axis voluminous flows of lava are emplaced also away from the ridge axis adding off axis crustal layers to the crustal pile research on deep sea is of great importance to better understand the mechanisms and the nature of the crust forming and evolution in this work we decipher the first stages of the postmagmatic evolution of an intact volcanic section from the upper oceanic crust at ocean drilling program integrated ocean drilling program odp iodp site 1256 eastern pacific ocean using for the first time an innovative core log integration technique to match direct core related and indirect borehole related data by depth shifting and reorienting individual core pieces recovered by drilling we are able to identify the clusterization of structures and physical properties within distinct downdeep strong and weak lava zones reflecting the cooling and tectonic evolution of lavas rather than lithological variations we define the evolution of the structural zones that typically affect lava flows colonnades and entablature zones studying an off axis lava flow encountered in present day upper ocean crust for the same off axis flow we are also able to suggest the lava flow direction nw se and its relationships with the paleoridge axis despite the environmental difficulties in the study of the subseafloor under deep water and using only one dimensional data deriving from ocean drilling this work shows how an array of diverse data can be integrated into a coherent interpretation of lava flow history obtaining detailed information on the mechanism of submarine lava emplacement and flow

bubble coalescence in basalt flows comparison of a numerical model with natural examples gas accumulation in magma may be aided by coalescence of bubbles because large coalesced bubbles rise faster than small bubbles the observed size distribution of gas bubbles vesicles in lava flows supports the concept of post eruptive coalescence a numerical model predicts the effects of rise and coalescence consistent with observed features the model uses given values for flow thickness viscosity volume percentage of gas bubbles and an initial size distribution of bubbles together with a gravitational collection kernel to numerically integrate the stochastic collection equation and thereby compute a new size spectrum of bubbles after each time increment of conductive cooling of the flow bubbles rise and coalesce within a fluid interior sandwiched between fronts of solidification that advance inward with time from top and bottom bubbles that are overtaken by the solidification fronts cease to migrate the model predicts the formation of upper and lower vesicle rich zones separated by a vesicle poor interior the upper zone is broader more vesicular and has larger bubbles than the lower zone basaltic lava flows in northern california exhibit the predicted zonation of vesicularity and size distribution of vesicles as determined by an impregnation technique in particular the size distribution at the tops and bottoms of flows is essentially the same as the initial distribution reflecting the rapid initial solidification at the bases and tops of the flows many large vesicles are present in the upper vesicular zones consistent with expected formation as a result of bubble coalescence during solidification of the lava flows both the rocks and model show a bimodal or trimodal size distribution for the upper vesicular zone this polymodality is explained by preferential coalescence of larger bubbles with subequal sizes vesicularity and vesicle size distribution are sensitive to atmospheric pressure because bubbles expand as they decompress during rise through the flow the ratio of vesicularity in the upper to that in the lower part of a flow therefore depends not only on bubble rise and coalescence but also on flow thickness and atmospheric pressure application of simple theory to the natural basalts suggests solidification of the basalts at 1 0 0 2 atm consistent with the present atmospheric pressure paleobathymetry and paleoaltimetry are possible in view of the sensitivity of vesicle size distributions to atmospheric pressure thus vesicular lava flows can be used to crudely estimate ancient elevations and or sea level air pressure

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

effect of compositional and structural variations on log responses of igneous and metamorphic rocks i mafic rocks well logging data provide continuous information about the physical in situ properties of the borehole wall it has become a standard method in oil industry for the investigation of subsurface geology interpretation techniques thus have been mainly developed for the use in sedimentary rocks and log responses of sediments are well known and calibrated to core data however this is not the case for igneous and metamorphic rocks we present a compilation of log responses of mafic rocks from different drill holes into oceanic and continental basement including holes drilled by deep sea drilling project dsdp and ocean drilling program odp and boreholes on land german continental deep drilling project ktb germany sancerre couy france hsdp usa these holes cover a variety of mafic rocks mid ocean ridge basalt morb flood basalt from large igneous provinces lip basalt and andesitic basalt from back arc basins gabbro metagabbro meta ultramafitites and amphibolites metamorphic rocks from continental holes are interpreted to represent ancient oceanic basement rocks

constraints on the evolution of the japan sea based on 40ar 39ar ages and sr isotopic ratios for volcanic rocks of the yamato seamount chain in the japan sea 40 ar 39 ar and sr isotope analyses were performed on basalts and andesites dredged from the yamato seamount chain in the japan sea the 40 ar 39 ar plateau ages range from about 11 to 17 ma though most samples show ages between 10 and 14 ma the seamounts seem to have formed within a period of a few million years although some of them might have formed earlier based on the present results together with previously reported radiometric age data it is thought that the yamato basin formed during some period prior to 17 ma and probably later than around 25 ma taking into account the radiometric age data on rocks from the japan basin it is conjectured that the opening of the japan sea might have started almost at this time or a little earlier the observed 87 sr 86 sr ratios range from 0 70357 to 0 70388 suggesting incorporation of some time integrated components enriched in incompatible elements such as continental crustal materials this may indicate that in the japan sea area at least the yamato basin had not developed enough to show the characteristics of typical n type morb source materials without being affected by pre existing continental crustal materials

evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties odp leg 183 the different effects of cooling and alteration on magnetic properties in single thick flows from subaerial and submarine eruptions cored and logged during ocean drilling program odp leg 183 at sites 1137 and 1140 kerguelen plateau are examined downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at site 1137 and two flows units at site 1140 covering transects from fresh to highly altered basalts changes in magnetic properties have previously been observed in several odp drill holes which penetrate basaltic basement in subaerial basalts a typical trend is that of high magnetic susceptibility and natural remanent magnetization nrm values in the altered flow top and lower values in the less altered massive flow interior in contrast submarine lava flows display the opposite behavior in their magnetic properties altered pillow rims have lower susceptibility and nrm values than the fresh pillow interiors it is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and hence the distribution of iron oxides the effects of low temperature alteration are most noticeable in the distribution of more mobile elements such as k consequently the spectral gamma ray sgr log which in basaltic basement is largely controlled by k concentration is an excellent proxy to the downhole identification of alteration the strong positive correlation observed for the subaerial basalts between the downhole total magnetic field ftot and sgr suggest a potential link with alteration in the drilled sections the alteration of the submarine basalts is not as pronounced and therefore no correlation is evident

using formation micro imaging wireline logs and onshore analogues to distinguish volcanic lithofacies in boreholes examples from palaeogene successions in the faroe shetland basin ne atlantic

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

evolution of the miocene waup basin se korea in response to dextral shear along the southwestern margin of the east sea sea of japan abstract the waup basin se korea is a nonmarine basin created coevally with the back arc opening of the east sea sea of japan during the miocene the basin is bounded by normal faults in the northwestern and southeastern margins and by dextral strike slip faults in the northeastern and southwestern margins a number of intrabasinal faults with dextral strike slip and normal senses of slip are also present delineating three structural domains of the basin i e the ne central and sw blocks the basin fills composed of alluvial to lacustrine deposits and dacitic to basaltic volcanogenic rocks have the geometry of a half graben in the ne and central blocks and a graben in the sw block they are younging from the ne to the sw blocks suggestive of propagating rifting of the basin and associated depocenter migration structural and stratigraphic data suggest that the ne and central blocks were produced by nw se extension during the early miocene 22 17 ma involving northwestward block tilting and subsequent clockwise horizontal block rotation on the other hand the sw block formed at about 17 ma by nw se extension and experienced neither block tilting nor block rotation the reconstructed configuration of the waup basin is that of a pull apart basin produced by nnw directed dextral strike slip stress and associated nw se extensional stress the ages of the basin fills ca 22 to

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

dips and extension plots of dip measurements taken from syntectonic deposits in extended terranes provide a simple and useful technique that can help to unravel complex deformation histories in the castaneda hills signal area of west central arizona dip analysis has enabled us to identify an unconformity that is at best equivocally displayed in the field this unconformity marks the inception of extension that formed the highly extended terranes het the technique also highlights a later unconformity that marks the onset of basin range br extension and one that formed because extension ceased and through flowing drainage developed northeastward decrease of dips related to het type extension is evidence for a decrease in deformation toward the colorado plateau conversely a mild increase of dips of younger basin fill deposits in that direction suggests that the locus of strongest deformation shifted toward the colorado plateau in br time the variability in amount and timing of tilting displayed by the dip data supports listric and rolling hinge models of extensional tectonism more than the domino model of upper plate het type normal faults dip data and stratigraphic data enable us to construct a sequence of events for middle tertiary extension in the castaneda hills signal area the first phase started ca 27 ma and consisted of upwarp of the area now occupied by the buckskin rawhide metamorphic core complex no rotation of blocks or volcanism accompanied this phase the second phase 19 to 13 ma is characteristic of het type extension and was marked by movement on a basal detachment fault and upper plate normal faults rotation of blocks along near horizontal axes and widespread basalt rhyolite volcanism the third phase about 13 to 7 5 ma is characteristic of br extension and was marked by movement on high angle normal faults minor rotation of blocks and basaltic volcanism the fourth and final phase ca 7 5 to 0 ma saw the waning of tectonism the inception of through flowing drainage related to the colorado river and the effusion of thin megacryst bearing basalt flows the early uplift probably dome like and thermally driven the areal variability of rotation and the migration of deformation toward the northeast do not support the concept that the extended terranes result from simple shear along a low angle fault of crustal dimensions or from a rolling hinge mechanism a more attractive mechanism is a thermal welt in the upper mantle and lower crust that moves northeast with time in a manner analogous to that proposed for the tyrrhenian sea apennine system

ultra high temperature metamorphism and its significance in the central indian tectonic zone abstract in the present study from the southern margin of the central indian tectonic zone it is demonstrated how the metamorphic p t path of ultrahigh temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later fmas solid solid reaction the extreme metamorphism in these rocks caused two stage biotite melting resulting in initial porphyroblastic garnet 1 and later sapphirine spinel 1 incongruent solid mineral assemblages during this process the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica oversaturated to a later silica undersaturated domain in the melanocratic layer this allowed localized concentration of sapphirine spinel 1 and residual sillimanite 1 producing an extremely restitic assemblage at the culmination of peak metamorphism bm 1 bm 1 is constrained at 1000 c at relatively deep crustal levels p 9 kbar from the stability of ferroaugite in a co metamorphosed iron formation granulite during subsequent metamorphism bm 2 the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine spinel 1 and sillimanite to produce coronal garnet 2 corundum assemblage via a fmas univariant reaction in the final stages of reaction history biotite 2 sillimanite 2 spinel 2 assemblage was produced after garnet 2 corundum due to localized melt crystal interaction the metamorphic sequence when interpreted with the help of a newly constructed qualitative kfmash petrogenetic grid reveals successive stages of heating increasing pressure and cooling around the kfmash invariant point opx crd which is consistent with a counterclockwise metamorphic p t path the near isobaric nature of post peak cooling t 250 300 c is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the iron formation granulites this study provides the first tight constraint for ultrahigh t metamorphism along a counter clockwise p t trajectory in the central indian tectonic zone and has important bearing for terrane correlations in this part of east gondwanaland in addition the new kfmash grid allows evaluation of metamorphic phase relations in ultrahigh t corundum bearing and corundum absent aluminous granulites

magma sources and plumbing systems during break up of the se greenland margin preliminary results from odp leg 152 abstract odp leg 152 drilled a transect across the se greenland seaward dipping reflector sequence the earliest and most landward lava flows hole 917a lower and middle series represent a pre break up continental sequence showing an overall variation from olivine basalt to dacite evolution in deep crustal magma reservoirs with a dwindling supply of primitive magma and an increasing role of crustal contamination can account for the variation eruption of picrite and olivine basalt magmas 917a upper series marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final phase of break up the younger parts of the seaward dipping reflector sequence 915a and 918d comprise compositionally uniform basalt 7 6 0 8 mgo suggesting that an effective magmatic filtering system was established soon after break up in magma chambers associated with a spreading axis the continental to oceanic magmatic transition is marked by clear changes in composition the continental magmas had a garnet lherzolite source indicated by low y zr and were contaminated with continental lithospheric mantle and crust high ba zr the majority of the break up and oceanic volcanic rocks show no evidence for lithospheric contamination low ba zr and appear to have had a source in progressively shallower depleted morb source asthenosphere increasing y zr low nb zr

structural evolution of the southwestern margin of the ulleung basin east sea japan sea and tectonic implications abstract structural mapping and restoration of depth converted seismic profiles from the southwestern ulleung basin in the east sea japan sea back arc basin reveal the structural evolution of the area since the middle miocene the back arc closure began in the earliest middle miocene ca 15 ma and caused compressional deformation that has propagated southwestward from the dolgorae to the gorae structures and further to the unnamed inversion structure immediately adjacent to se korea the accelerated convergence of the young

paleomagnetism of tertiary rocks on the korean peninsula tectonic implications for the opening of the east sea sea of japan abstract two different distributions of declinations with dual polarities are found in the tertiary rocks of the yeonil basin and yangnam basin along the yangsan fault in the southeastern part of the korean peninsula these are northward declinations in the middle miocene yeonil group and strong eastward declinations in the early miocene yangbuk group and in the eocene volcanic rocks this is interpreted by invoking 1 the yangsan fault as a dextral wrench fault system by spreading in the uleung basin of the east sea sea of japan and 2 a clockwise vertical rotation caused by the movement of the fault at around 17 3 ma just prior to the sedimentation of the yeonil group the timing of the deflection found in korea is distinguished from that in southeastern and northwestern japan at around 15 ma two stages of spreading are thus constrained in the east sea area parallel at 23 15 ma and pivotal at around 15 ma crustal thinning also occurred a reliable pole position 84 9 n 292 6 e d p d m 3 3 4 8 for the miocene about 15 ma in korea is obtained from the yeonil group

early neogene biochemostratigraphy of pohang basin a paleoceanographic response to the early opening of the sea of japan east sea abstract the location of the pohang basin near the korea strait in the southwest of the sea of japan east sea makes this area appropriate for providing a record of paleoenvironmental biotic changes associated with the early neogene 16 5 ma opening of this gateway to the pacific ocean stable isotopic and planktonic foraminiferal records are presented that assist with the understanding of paleoenvironmental changes in the sea of japan resulting from the opening of the korea strait the oldest sediments of early neogene age of the pohang basin overlie cretaceous basement and are early miocene 16 5 ma shallow estuarine facies containing a benthic foraminiferal assemblage dominated by ammonia beccarii l the oldest early neogene planktonic foraminiferal assemblages in the basin are 16 5 ma in age latest early miocene foraminiferal zone n8 the migration of these planktonic assemblages to the sea of japan at that time appears to have resulted from the initial opening of the korea strait the overlying early neogene marine sequence of the pohang basin extends from 16 5 ma zone n8 through 14 ma zone n10 change in the oxygen isotopic record of the pohang basin sequence suggests strong local paleoenvironmental control related to the early opening of the korea strait early middle miocene isotopic temperatures of planktonic foraminifera are relatively cool at 15 ma at a time when global temperatures were high in the middle low latitude regions it was not until 14 8 ma that isotopic temperatures of planktonic foraminifera increased markedly this distinct warming is inferred to reflect the major intrusion of the kuroshio current into the sea of japan probably as a result of further critical opening and deepening of the korea strait at this time planktonic foraminifera increased in abundance reflecting expanding oceanic influence a cooling that followed at 14 5 ma when the strait was well open is unlikely to reflect local tectonic control on the paleoceanography but global cooling during the early middle miocene associated with the expansion of the east antarctic ice sheet

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

synrift stratigraphic geometry in a transfer zone coarse grained delta complex miocene pohang basin se korea the gojusan fan delta is a coarse grained delta complex on the western margin of the miocene pohang basin se korea the deposits consist of five facies associations fa alluvial fan conglomerates fa i shallow marine mouthbar sandstones fa ii fluvial and mouth bar conglomerates fa iii gilberttype foreset conglomerates fa iv and hemipelagic mudstones fa v different facies associations characterize the northern central and southern parts of the delta complex to the north fa iv is laterally juxtaposed with fa i or the basement with scarp derived breccias along the contact centrally and to the south fa i is laterally juxtaposed with fa ii with an abrupt facies change and local inliers of basement rocks along the contact early alluvial fan and mouth bar deposits are overlain by the topset fa iii and truncated by the foreset fa iv of a gilbert type delta in the central part of the fan delta complex whereas fa ii passes transitionally upward into fa iii in the south with the latter extending basinward into a gently inclined shoal water delta front gilbert type and shoal water geometries are therefore developed in the same delta complex the composite delta geometry is interpreted as reflecting 1 its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike and 2 the operation of intrabasinal faults this interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull apart basin margin

neogene stress field in sw japan and mechanism of deformation during the sea of japan opening we present new structural data from sw japan and discuss the mechanism of deformation during the opening of the sea of japan we studied the miocene basins at the southern margin of the sea of japan northern coast of sw japan and of the median tectonic line mtl by means of fault slip analysis to determine stress field directions during the opening of the sea of japan in early and middle miocene time on the southern margin the stress field was extensional or transtensional with a nw se trending extension direction the mtl was a normal fault in shikoku during early middle miocene and probably a normal fault with a left lateral component in central japan before having been strained by the collisions of the tanzawa and izu blocks with central japan we combine our results with published data for the sea of japan area and show that early and middle miocene stress field directions are remarkably consistent on the eastern southern and western margins of the sea of japan with hmax trending ne sw and hmin trending nw se the stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin this stress field distribution is in agreement with the model of the opening of the sea of japan in an extensional transfer zone between two n s right lateral strike slip zones proposed by jolivet et al 1991 analogue modelings and kinematic reconstructions showed that this model accounts for 20 to 30 of clockwise rotation in sw japan while paleomagnetic rotations reached 40 to 50 during the opening we present structural data showing that sw japan was strained during the rotation and did not behave as a rigid block our observations suggest that rotations of small blocks may have occurred we propose that sw japan has been sheared between the right lateral strike slip zones which bound the sea of japan and that the missing 10 to 30 of rotation are to be found in its internal deformation in our model the mtl is a normal fault with a strike slip component rotating in a right lateral shear zone

miocene tectonic evolution of the basins and fault systems se korea dextral simple shear during the east sea sea of japan opening crustal deformation of se korea caused by back arc opening of the east sea sea of japan commenced locally in the late oligocene intense deformation occurred during the early miocene accompanying extension of parallelogram shaped pull apart basins between nnw striking principal displacement zones clockwise rotation and northwestward tilting of blocks and southwestward propagating rifting at about 17 ma the crustal deformation suddenly expanded westward and northward owing to activation of the yeonil tectonic line resulting in a dramatic change of depositional environment and the subsidence of wedge shaped pull apart basins the yeonil tectonic line and western border faults of the pohang basin are the westernmost limit of the miocene crustal deformation all the features of crustal deformation in se korea support the pull apart model for the east sea opening however they indicate that the nnw striking faults such as the yeonil tectonic line acted as the principal displacement zones rather than the nne striking yangsan fault under a consistent dextral simple shear at about 16 ma the collision of the philippine sea plate with the japanese islands caused a tectonic inversion this inversion resulted in a compositional change of basaltic magma at about 15 ma and crustal uplift in se korea which caused the cessation of sedimentation at about 10 ma

dip domain method applied to the mesozoic connecticut valley rift basins this paper presents a new approach to structural analysis of u s east coast mesozoic basins a contoured compilation of numerical values of dip data for the connecticut valley rift basins reveals distinct domains related to their broader tectonic development additional more standard geologic constraints are then used to interpret the dip domain patterns most of the tilt deformation involved and hence postdated early jurassic basaltic flow units and overlying jurassic thick alluvial deposits normal drag characterizes the attitude of mesozoic strata along the western border fault of the main hartford basin whereas reverse drag is common along its larger listric eastern border fault strata within the basin mostly along its eastern margin include several subgrabens and or strongly tilted zones associated with early subbasins marked by local changes in strike of the early border fault the meriden cross graben may be a similar structure that extended much farther southwestward across the basin within the hartford basin linear domains of steeper dips are interpreted as surface expressions of the edges of a 5 to 15 km wide late stage central graben that trends n15 e diagonally across the basin possibly a reflection of very late strike slip stress systems

alternate senses of displacement along the tsushima fault system during the neogene based on fracture analyses near the western margin of the japan sea abstract the western margin of the japan sea is characterized by the existence of a n20 e trending transcurrent fault system in the korea strait between korea and kyushu this fault system called here the tsushima fault system played a major role during the opening of the japan sea during the tertiary in order to understand its displacement history we analysed the deformation recorded by neogene rocks exposed on the korea strait islands tsushima goto and hirado islands a synthesis of published stratigraphical and structural data together with our field observations reveals that two tectonic events occurred in succession in the study area an early miocene nw se oriented extension event i and a middle miocene nw se oriented shortening event ii two distinct families of fractures with contrasted stratigraphic occurrences have been recognized in the neogene rocks exposed on the korea strait islands and allow us to define and date the stress fields characterizing each tectonic event stress field i is of intermediate type the vertical axis is either 1 or 2 with a n45 e hmax and n135 e hmin and was active between 22 and 16 ma stress field ii is compressional the vertical axis is 3 with a n135 e hmax and a n45 e hmin and was active between 15 and 10 ma the trends of stress field i principal axes imply that the displacement along the tsushima fault system was dextral during the early miocene conversely the trends of stress field ii principal axes imply that the displacement along the same fault system was sinistral during the second half of the middle miocene the inferred displacement history agrees with 1 a pull apart model for the opening of the japan sea during the early miocene and 2 transition from tensional to compressional stress fields in the japanese arc and back arc region at the end of the middle miocene

the iceland plume and its influence on the evolution of the ne atlantic the papers printed below were contributed by scientists who attended the second arthur holmes european research meeting held in reykjavik iceland in early july 1994 the igneous rocks produced by mantle plumes and at seafloor spreading centres provide a powerful probe for investigating the state of the mantle beneath the lithosphere iceland is a particularly good window into the convecting mantle because the north atlantic spreading centre crosses directly above the mantle plume the plate divergence allows decompression melting to occur to depths as shallow as 15 km below the surface in other mantle plumes such as hawaii the thick lithospheric lid above the plume prevents the convecting mantle rising shallower than the base of the lithosphere which lies at a depth of many tens of kilometres a further feature of the ne atlantic is that decompression melting beneath the spreading centres adjacent to iceland samples the asthenospheric mantle consistently at all distances from the mantle plume this allows the spatial and temporal extent and variability of the plume to be investigated the arthur holmes meeting was held early july 1994 in reykjavik iceland fittingly enough at a location almost on top of the european north american plate boundary and directly above the iceland plume the presentations covered six main themes melt generation the relative importance of plume depleted mid ocean ridge basalt and other magma sources breakup processes and effects on marginal basins the timing and expression of volcanic and tectonic events and geochemical and geophysical manifestations of the plume through space

miocene pleistocene volcanism and tectonics in southern korea and their relationship to the opening of the japan sea abstract miocene pleistocene volcanism and tectonics in southern korea are discussed the miocene volcanism and related tectonics are represented by the eoil orogeny which occurred in the yangnam and pohang basins in the southeastern corner of the korean peninsula the yangnam and pohang basins represent the uplifted central part of the pohang ulsan graben which was rifted between the proto korean peninsula and paleo ulleung land probably in late oligocene early miocene times the eoil orogeny is correlated with the green tuff orogeny in japan evidence of pliocene volcanism is observed on ulleung island located in the western part of the central japan sea volcanic and tectonic events represented on ulleung island are in chronological order 1 volcanic eruption 2 subsidence and marine transgression 3 dome like uplift and 4 volcanic eruption evidence of pleistocene volcanism is present on cheju island located at the southwestern entrance to the korea tsushima strait volcanic and tectonic events represented on cheju island are in chronological order 1 volcanic eruption 2 subsidence and marine transgression 3 uplift and volcanic eruption and 4 doming and volcanic eruption three grabens namely the pohang ulsan graben the ulleung basin and the yamato basin were formed in the southern half of the japan sea which was above sea level in early miocene times paleo ulleung land was present between the pohang ulsan graben and the ulleung basin in miocene pliocene times these three grabens are active rifts and the tectonic events that occurred in this region originated from the action of rising magma the extension and deepening of the southern half of the japan sea resulted from the tectonic movements of these active rifts and their surrounding regions thus it is suggested that the opening of the japan sea progressed by diapiric tectogenesis

controls and evolution of fan delta systems in the miocene pohang basin se korea abstract the miocene pohang basin southeastern korea occurs in the eastern part of the ne sw trending yangsan fault which has experienced right lateral strike slip deformation since the eocene during the miocene however the strike slip movement was negligible and instead was under extensional regime large scale drainage systems were developed at the junction between the strike slip zone and the oblique transfer faults acting as major sources for coarse grained sediments high rate of sediment supply and rapid subsidence of the hanging wall resulted in the progradation of large scale gilbert type fan deltas doumsan duksung and gohyun systems which all show a radial distributional pattern along the steeply inclined footwall scarps created by ne sw trending faults the drainage systems were relatively small here low rate of sediment supply formed scree apron type malgol and yugye systems and steep faced slope type maesan system fan deltas based on variations in sedimentary facies and distribution architecture and overlapping patterns the fan delta systems in the pohang basin can be divided into four stages of evolution during the early stage early miocene alluvial fans prograded into shallow marine setting in the doumsan and duksung systems high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal water type to gilbert type foresets in the malgol and maesan systems rapid sea level rise accompanied with low rate of sediment supply caused transformation of alluvial fans to scree apron type and steep faced slope type fan deltas respectively in the second stage subsidence of the hanging wall formed large scale truncation surfaces in the doumsan and duksung fan deltas coarse grained sediments were continuously supplied in the doumsan system whereas only fine grained sediments were deposited in the duksung system in the malgol system sediment supply was almost terminated in this stage in the third stage middle miocene sediment supply rate of the doumsan fan delta decreased abruptly and the system was overlain by fine grained gilbert type deposits the duksung fan delta was overlain by the maesan and gohyun systems the maesan system prograded further basinward during the last stage late miocene sediment supply of the doumsan and maesan systems was also terminated and the entire basin was draped by fine grained biogenic materials mainly diatoms

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

opening mode of the japan sea inferred from the palaeomagnetism of the japan arc the western edge of the pacific plate is marked by a chain of back arc basins stretching from the aleutians through japan and the philippines to new zealand most of the basins lie behind volcanic island arcs far from continents and are the result of extension of the oceanic lithosphere1 but the japan sea is bounded on one side by continental crust the sikhote alin and korean peninsula of the asian continent and on the other by an active arc with continental fragments it can thus be said to be associated more with continental rifting1 4 than with oceanic processes we report here palaeomagnetic data from the japan arc which indicate that since the early miocene north east japan has rotated counter clockwise through 47 around a vertical axis while south west japan has rotated clockwise through 56 refs 5 8 we attribute this differential rotation which took place concurrently between 21 and 11 myr ago to the back arc opening of the japan sea8 with a double door mode that is there are two fan shaped spreading basins facing each other such a fan shaped opening is probably peculiar to the geological phenomenon of rifting of a continental fragment from thick continental lithosphere

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

a miocene pull apart deformation zone at the western margin of the japan sea back arc basin implications for the back arc opening mode abstract opening mode of the japan sea is studied from the viewpoint of geologic structure around the western marginal fault zone of the back arc basin latest reflection seismic survey in the fukue basin located at the western end of the rifted sliver of southwest japan delineates an nne ssw transcurrent fault that is an extension of the yangsan fault running along the eastern coast of the korean peninsula as suggested by paleomagnetic studies in korea the yangsan fault was activated with dextral motions during the early miocene rifting event of the japan sea our offshore seismic study indicates that the nne ssw fault zone was simultaneously active with dextral sense because it constitutes a pull apart depression on a right stepping of the fault the fukue basin was under continuous transtensional stress regime during the japan sea rifting accompanied with a clockwise rotation of southwest japan it is discordant from the previous tectonic model of two stage parallel and fan shaped back arc opening which requires compressive regime with sinistral fault motions to the west of rotation pivot of southwest japan in the second stage the southward transportation of the rifted sliver with formation of pull apart basins was the dominant mode along the western margin of the japan sea throughout the opening event the southern part of the marginal fault zone has been immune from post opening tectonic events such as the late miocene strong inversion on the back arc margin of southwest japan or the quaternary e w tectonic stress in the japan sea reactivating the northern sector of the fault zone yangsan fault

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

new constraints on the age and style of continental breakup in the south atlantic from magnetic anomaly data abstract we present new constraints on the opening of the south atlantic ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles we use 45 000 km of recently collected commercial ship track data combined with 561 000 km of publically available data the new data cover the critical ocean continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations within the final grids the mean cross over error is 14 nt computed from 8 227 ship track intersections the forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep seismic reflection data we find the oldest magnetic anomalies to date from m10r 134 2 ma late valanginian north of the falkland agulhas fracture zone and m3 129 3 ma barremian south of the rio grande fracture zone hence assuming the gpts used is correct continental breakup was contemporaneous with the parana and etendeka continental flood basalts many of the landward linear anomalies overlap seismically mapped seaward dipping reflectors sdrs we interpret this to mean that a significant portion of the sdrs overlay crust formed by subaerial seafloor spreading here crustal accretion is envisaged to be similar to that at mid ocean ridges but sheet lava flows that later form the sdrs rather than pillow basalts form the extrusive component segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional submarine oceanic crustal generation

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

paleomagnetic evidence for the clockwise rotation of southwest japan abstract over 500 oriented samples of felsic rocks of cretaceous to middle miocene age were collected along the go river in the central part of southwest japan in an attempt to detect the process of tectonic rotation of southwest japan from the paleomagnetic view point thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities the paleomagnetic results establish that southwest japan began to rotate clockwise through58 14 later than 28 ma and ceased its motion by about 12 ma southwest japan has undergone no detectable north south translation since 28 ma these results imply that southwest japan was rotated about the pivot around 34 n 129 e between 28 ma and 12 ma in association with the opening of the japan sea