uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

las canadas caldera tenerife canary islands a review or the end of a long volcanological controversy abstract the las canadas caldera is one of the best exposed volcanic calderas in the world and one of the few known evolved alkaline volcanoes it truncates the pre teide pico viejo central volcanic edifice the las canadas edifice which started to take shape at the end of the formation of a large basaltic shield that forms the main part of island of tenerife historically the origin of the las canadas caldera has been controversial as it has been interpreted as the result of either multiple vertical collapses or due to a giant sector collapse the available stratigraphical structural volcanological geochronological and geophysical data as well as its comparison to other well known collapse calderas should not offer any doubt as to its direct relationship with a long history of phonolitic explosive volcanism however the existence of large landslide events on tenerife which have significantly modified the flanks of the las canadas volcano have also been used as a potential explanation for the origin of the las canadas depression this contribution reviews the available information on the las canadas caldera the causes of this controversy and rationalises the most plausible explanation for the origin of las canadas caldera based on the current evidence gathered from all previous studies

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

paleomagnetic test of the emperor fracture zone hypothesis late cretaceous paleomagnetic data from the pacific plate were used to test farrar and dixon s 1981 hypothesis that 1700 km of strike slip accumulated during the early tertiary along the emperor fracture zone system an 8000 km long nw se trending feature which consists mainly of the emperor trough the gardner seamounts and the line islands the data strongly disagree with this hypothesis instead the data are consistent with no motion having occurred between seafloor east of the emperor fracture zone and seafloor west of the emperor fracture zone

minimum continental velocities with respect to the pole since the archean abstract paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea floor magnetic anomalies minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole such latitudinal velocities have been determined for the center of mass of the north american baltic siberian and african shields since the archean the results indicate that plate velocities in the past have at times easily exceeded those for present day continents and that they often were equivalent to present day oceanic velocities although there are peaks and troughs through time north american velocities are significantly greater prior to one billion years ago than those of siberia or baltica

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

vertical and lateral collapses on tenerife canary islands and other volcanic ocean islands recent studies demonstrate that tenerife has undergone large lateral collapses this has led to the suggestion that the las canadas caldera one of the best exposed calderas in the world is the result of lateral collapse we have tested this idea using the available structural stratigraphic volcanological and geochronological data we conclude that the las canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of tenerife our results indicate however that vertical collapses may have played a major role in triggering lateral collapses we propose that the association of vertical and lateral collapse events such as inferred for tenerife can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

multiple related flank collapses on volcanic oceanic islands evidence from the debris avalanche deposits in the orotava valley water galleries tenerife canary islands abstract catastrophic flank collapses are common on oceanic islands of volcanic origin and often recognizable from the resulting morphology as large u shaped embayments however post collapse volcanic activity can infill such features thereby obscuring them this study takes advantage of a dense network of long sub horizontal galleries for groundwater extraction in the flank collapse structure of the orotava valley tenerife canary islands this impressive landform is located in the overlapping zone of the ne rift and the canadas edifice of this volcanic island three debris avalanche deposits dads have been identified inside the waterworks bored into the valley s western sector the deeper layer the lower dad l dad was previously described under the local name of mortalon this deposit lies unconformably over older volcanic rocks where a prominent shear zone is developed here interpreted as the detachment plane of a massive rockslide the l dad was therefore produced during the large failure event the orotava landslide ol which carved the depression two younger dads much smaller in volume and not previously described were also identified in the underground strata the intermediate i dad and upper dad u dad both correlate well with the thick breccia deposits cropping out at the base of the marine cliff along the western coast of the valley the i dad and u dad are conformably intercalated between the lava flows and other volcanics infilling the depression and their bases are erosive depositional features without structural deformation zones below them their stratigraphic setting and geometrical reconstruction indicate that both deposits were emplaced after the valley was formed by the ol event in two successive failures here called western orotava landslides wols the younger called wol 2 was dated at 494 22 ka using the 40ar 39ar method these two subaerial failures affected the upper eastern flank of the canadas iii edifice previously destabilized when the ol failure event carved a steep wall at its foot the western lateral escarpment of the orotava valley

development and recent activity of the san andres landslide on el hierro canary islands spain abstract extremely voluminous landslides with a long run out also known as megalandslides on oceanic volcanic islands are infrequent denudational processes on such islands at the same time they represent a major geological hazard that must be looked into to avoid negative consequences for the inhabitants of these islands their occurrence can be related to periods of intense seismo volcanic activity similar to that which occurred on el hierro island over 2011 2012 landslides on volcanic islands are studied using onshore and offshore geological geophysical and geomorphological records considering their unique triggering conditions e g lava intrusions eruptive vents magma chamber collapses previous work has pointed out similarities between specific cases of landslides on volcanic islands and deep seated gravitational slope deformations dsgsds which are typical in high mountain settings nevertheless the methodological approaches and concepts used to investigate dsgsds are not commonly applied on volcanic islands studies even though their use may provide new information about the development stage recent movements and future hazards therefore this approach for studying the san andres landslide sal on el hierro canary islands has been developed applying a detailed morphological field mapping an interpretation of digital elevation models structural measurements kinematic testing and a precise movement monitoring system the acquired information revealed a strong structural influence on the landslide morphology and the presence of sets of weakened planes acting as the sliding surfaces of the sal or secondary landslides within its body the presence of secondary landslides deep erosive gullies coastal cliffs and high on shore relative relief also suggests a high susceptibility to future landslide movement direct monitoring on the landslide scarps and the slip plane performed between february 2013 and july 2014 using an automated optical mechanical crack gauge with a precision of up to 10 2 mm detected creep movement in the order of 1 mm yr 1 with a persistent sinistral component as well as episodic horizontal and a subtle vertical movement this monitoring will continue in the future in order to verify the initial observations which instead of long term movement patterns could represent a landslide response to the period of an intense seismo volcanic activity during 2011 12

long term 17 ma turbidite record of the timing and frequency of large flank collapses of the canary islands volcaniclastic turbidites on the madeira abyssal plain provide a record of large volume volcanic island flank collapses from the canary islands this long term record spans 17 ma and comprises 125 volcaniclastic beds determining the timing provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the canary islands especially the western islands of tenerife la palma and el hierro these turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers furthermore the last large volume failures from tenerife coincide with explosive volcanism at the end of eruptive cycles many large volume canary island landslides also occurred during periods of warmer and wetter climates associated with sea level rise and subsequent highstand however these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant

proximal record of the 273 ka poris caldera forming eruption las canadas tenerife lithofacies architecture analysis is applied to the most proximal exposures of the 273 ka poris formation in the diego hernandez caldera wall of las canadas here the succession records an opening plinian phase followed by generation of a pyroclastic density current pdc that was partly blocked by near vent topography the upper part of the current surmounted the barrier and deposited an ignimbrite veneer during a hiatus in energetic flow ash and various ash aggregates were deposited from a hybrid cloud comprising parts of both the plinian and the co ignimbrite plumes phreatomagmatic explosions then generated currents rich in fine grained lithics current waxing with local erosion was followed by pulsing unsteady waning recorded in repeated sets of graded tuff reversion to magmatic activity was marked by hybrid deposition as coarse pumice fallout and juvenile ballistics entered pyroclastic currents to form stratified pumice block tuff increasingly energetic flow is recorded in 9 m of mainly massive lapilli tuff that includes evidence of widespread scour and bypass lithic rich layers record incremental onset of edifice disruption due to magma withdrawal laterally variable stratification and load structures show that the pdc was unsteady and highly non uniform at times with locally rapid aggradation of gas rich ignimbrite climactic edifice disruption is marked by a lithic block layer in erosional contact with the underlying ignimbrite recording intense pyroclastic current activity and widespread bypass of material to distal volcano slopes pumice block breccia 10 m thick and with blocks up to 1 2 m in diameter forms the top of the succession and registers modification of the magmatic plumbing the timing of collapse to form a major summit depression is uncertain however the abrupt termination of the proximal aggradation of coarse breccia with no record there of any waning stage pyroclastic currents suggests subsidence at that time conceivably with formation of a caldera that contained late erupted material

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

multi event oceanic island landslides new onshore offshore insights from el hierro island canary archipelago abstract this paper presents a combined onshore offshore morpho structural characterization of the el golfo giant landslide island of el hierro canary archipelago spain offshore data from multibeam echosounders chirp sub bottom profiles and multichannel seismic reflection data and onshore data coming from water wells and galleries have been analyzed the geomorphology and the internal architecture of this giant flank collapse are defined in order to determine its multi event nature the subaerial headscarp shows a non continuous arcuate profile formed by two nested semi circular amphitheaters that extend offshore along a smooth chute suggesting the occurrence of at least two large retrogressive events channels gullies and escarpments developed along the submarine sector of the scar also indicate smaller scale events and predominance of sediment bypass in the lower slope two subunits of submarine mass transport deposits mtds debris avalanche are identified on multichannel seismic reflection profiles with a total estimated volume of 318 km3 84 km3 and 234 km3 for the younger and older mtds respectively data from wells and galleries show abrasion platforms with beach deposits at sea level 0 masl formed after the landslide scar and buried by the post collapse el golfo lavas infill suggesting an age at least older than 23 5 82 5 ka for the landslide

submarine landslides around the canary islands the morphology and structure of the submarine flanks of the canary islands were mapped using the gloria long range side scan sonar system bathymetric multibeam systems and sediment echosounders twelve young 2 ma giant landslides have been identified on the submarine flanks of the canary islands up to now older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the canary islands most slides were found on the flanks of the youngest and most active islands of la palma el hierro and tenerife but young giant landslides were also identified on the flanks of the older 15 20 ma but still active eastern islands large scale mass wasting is an important process during all periods of major magmatic activity the long lived volcanic constructive history of the islands of the canary archipelago is balanced by a correspondingly long history of destruction resulting in a higher landslide frequency for the canary islands compared to the hawaiian islands where giant landslides only occur late in the period of active shield growth the lower stability of the flanks of the canaries is probably due to the much steeper slopes of the islands a result of the abundance of highly evolved intrusive and extrusive rocks another reason for the enhanced slope instability is the abundance of pyroclastic deposits on canary islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas dike induced rifting is most likely the main trigger mechanism for destabilization of the flanks flank collapses are a major geological hazard for the canary islands due to the sector collapses themselves as well as triggering of tsunamis in at least one case a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse

slope failures on the flanks of the western canary islands abstract landslides have been a key process in the evolution of the western canary islands the younger and more volcanically active canary islands el hierro la palma and tenerife show the clearest evidence of recent landslide activity the evidence includes landslide scars on the island flanks debris deposits on the lower island slopes and volcaniclastic turbidites on the floor of the adjacent ocean basins at least 14 large landslides have occurred on the flanks of the el hierro la palma and tenerife the majority of these in the last 1 million years with the youngest on the northwest flank of el hierro as recent as 15 thousand years in age older landslides undoubtedly occurred but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments landslides on the canary island flanks can be categorised as debris avalanches slumps or debris flows debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence up to a maximum thickness of 1 to 2 km they are the commonest type of landslide mapped in contrast slumps move short distances and are deep rooted landslides which may affect the entire thickness of the volcanic edifice debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks some landslides are complex events involving more than one of the above end member processes individual debris avalanches have volumes in the range of 50 500 km3 cover several thousand km2 of seafloor and have runout distances of up to 130 km from source overall debris avalanche deposits account for about 10 of the total volcanic edifices of the small relatively young islands of el hierro and la palma some parameters such as deposit volumes and landslide ages are difficult to quantify the key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks broadening into a depositional lobe below 3000 m debris avalanche deposits have a typically blocky morphology with individual blocks up to a kilometre or more in diameter however considerable variation exists between different avalanche deposits at one extreme the el golfo debris avalanche on el hierro has few large blocks scattered randomly across the avalanche surface at the other icod on the north flank of tenerife has much more numerous but smaller blocks over most of its surface with a few very large blocks confined to the margins of the deposit icod also exhibits flow structures longitudinal shears and pressure ridges that are absent in el golfo the primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes observations in experimental debris flows show that the differences between the el golfo and icod landslide deposits are probably controlled by the greater proportion of fine grained material in the icod landslide this in turn relates to the nature of the failed volcanic rocks which are almost entirely basalt on el hierro but include a much greater proportion of pyroclastic deposits on tenerife landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands with landslides propagating perpendicular to the rift orientation however this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a weakness that can be carried forward during island development this may occur because certain island flanks are steeper extend to greater water depths or are less buttressed by the surrounding topography and because volcanic production following a landslide my be concentrated in the landslide scar thus focussing subsequent landslide potential in this area landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight although the actual triggering factors are poorly understood they may include or be influenced by dyke intrusion pore pressure changes related to intrusion seismicity or sealevel climate changes a possible relationship between caldera collapse and landsliding on tenerife is not in our interpretation supported by the available evidence

the evolution of volcanic systems following sector collapse abstract sector collapses affect volcanic edifices across all tectonic settings and involve a rapid redistribution of mass comparable in scale to the largest magmatic eruptions the eruptive behaviour of a volcano following sector collapse provides a test of theoretical relationships between surface loading and magma storage which imply that collapse driven unloading may lead to changes in eruption rate and erupted magma compositions large sector collapses are infrequent events globally with all historical examples being relatively small in comparison to many of the events documented in the geological record as a result exploration of the impacts of sector collapse on eruptive behaviour requires detailed investigation of prehistoric collapses but this is often hindered by poorly resolved stratigraphic relationships and dating uncertainties nevertheless observations from a number of volcanoes indicate sharp changes in activity following sector collapse here a global synthesis of studies from individual volcanoes in both arc and intraplate settings is used to demonstrate a number of common processes in post collapse volcanism multiple examples from large 5 km3 sector collapses in arc settings show that collapse may be followed by compositionally anomalous large volume and often effusive eruptions interpreted to originate via disruption of a previously stable upper crustal reservoir these anomalous eruptions highlight that magma compositions erupted during periods of typical i e unperturbed by sector collapse volcanism may not be representative of the range of compositions stored within a vertically extensive crustal reservoir if eruptible magma is not present upper crustal reservoirs may rapidly solidify following collapse without further eruption allowing more mafic compositions to ascend to the surface with only limited upper crustal modification resulting in edifice regrowth at temporarily elevated eruption rates subsequent re establishment of an upper crustal reservoir further supports a relationship between surface loading and crustal storage but long term chemical and mineralogical differences between pre and post collapse evolved magmas imply that a newly developed reservoir can overprint the influence of a preceding reservoir forming a spatially and compositionally distinct plumbing system these broad patterns are replicated in intraplate settings despite differences in scale and melting processes current evidence suggests that post collapse evolution of intraplate volcanoes can be explained by unloading induced destabilisation of the magma plumbing system rather than increased melt production what emerges from an apparently diverse set of observations is a systematic behaviour that strongly supports a coupling between edifice growth and magma ascent storage and pressurisation eruption rates erupted compositions and the style of volcanism at any particular system may thus be modulated from the surface and long term shifts in surface behaviour may occur without any changes in the deep parts of magmatic systems observations of sharp post collapse changes in erupted compositions including the ascent of primitive mafic magmas also require a crystal dominated mid to upper crustal reservoir consistent with recent models of crustal magmatic systems

internal flow variations and diachronous sedimentation within extensive sustained density stratified pyroclastic density currents flowing down gentle slopes as revealed by the internal architectures of ignimbrites on tenerife during a protracted explosive eruption at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of las canadas volcano tenerife each pyroclastic current developed marked local and regional spatial variations in response to the incised gently concave substrate topography the locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography this complex shifting response of the density currents to minor topographic variations has been reconstructed in high resolution over a wide area 500 km2 using the internal architecture of cryptic time surfaces entrachrons marked by compositional zoning in the deposit including variations in clast types valley side field relations reveal that the currents were density stratified yet at a single instant in time the lower levels of each current comprised a granular fluid at some locations but were fully dilute and turbulent at others moreover the locations of these variations shifted geographically as the topography changed during the eruption the variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies both laterally and in the downcurrent direction individual currents were regionally widespread and travelled 15 km but deposited only in longitudinally restricted localised zones that spanned several small valleys and interfluves the currents bypassed slopes up and downcurrent of the restricted depositional zones without depositing the locations of deposition then gradually shifted with time such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow units are markedly diachronous deposition of a flow unit commenced and ceased at different times in different places this study suggests that in hazard assessments a models of density currents that incorporate only pre existing topography e g from dems may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement altering flow paths b frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under estimated because currents commonly bypass without leaving a deposit record and c even where preservation appears to be complete an ignimbrite at a single exposure commonly will not record the current s entire flow history at that site

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

mega tsunami conglomerates and flank collapses of ocean island volcanoes abstract marine conglomerates at high elevation on the flanks of ocean islands are usually interpreted as evidence of mega tsunamis generated by volcano flank collapses although their origin is sometimes debated elevated littorals vs tsunami in this review we introduce case studies of well documented examples of tsunami conglomerates in hawaii pacific ocean the canary and cape verde islands atlantic ocean and mauritius island indian ocean other less documented marine conglomerates are also presented as tsunami candidates then we build a comprehensive picture of the general characteristics of these conglomerates and the different methods that can be applied to date them different perspectives of research are proposed especially on the use of tsunami conglomerates as proxies for better constraining numerical models of ocean island flank collapses and associated tsunamis we also discuss the possible links between volcano growth flank instability and climate

seismic evidence of continental margin influence on the ninetyeast ridge in the bay of bengal the ninetyeast ridge ner one of the most enigmatic features in the indian ocean is covered by thick bengal fan sediment north of 9 n we present seismic reflection data on the eastern flank of the ner at 10 n that show the presence of 4 5 km thick sediments beneath the bengal fan sediments these sediments can be imaged up to 60 km beneath the andaman fore arc accretionary wedge suggesting that the decollement lies above these sediments the presence of thick sediments above the northernmost segment of the ner suggests that this segment was close to a continental margin during its emplacement we propose that these sediments were deposited soon after the breakup of india and antarctica between 130 and 100 ma and might act as source rocks for oil and gas generation beneath recent bengal fan sediments furthermore subducting thick sediments can significantly change the seismogenic behavior of the andaman subduction zone

new sonar evidence for recent catastrophic collapses of the north flank of tenerife canary islands previous sonar surveys show that the north flank of tenerife has been subject to at least four major landslides during the past 1 ma the youngest icod affected the region to the north of the teide pico viejo complex the world s third highest oceanic volcano recently we obtained the first detailed acoustic images of icod using a deep tow side scan sonar the images suggest that tenerife s north flank has experienced at least two types of flow deposit in the recent past the older flow deposit icod i is characterised by a 15 to 20 km wide 65 km long chaotic debris avalanche deposit which includes several very large blocks we believe the deposit to be 170 ka and that it represents the mass wasting products of the canadas edifice remnants of which are now found in the las canadas caldera wall the younger flow deposit icod ii associated with a shute in its proximal part appears to have produced a less chaotic deposit in its distal part which clearly preserves flow structures such as latitudinal boulder ridges and longitudinal shear structures the sonar images cannot determine how much younger icod ii is than icod i although it is likely that they are a consequence of the same lateral collapse event there is evidence from the shute area for erosional scour and sediment deposition since the icod landslide if this is correct then it suggests that mass wasting is an ongoing process that has already started to modify the teide pico viejo complex itself

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

stratigraphy of the bandas del sur formation an extracaldera record of quaternary phonolitic explosive eruptions from the las canadas edifice tenerife canary islands explosive volcanism has dominated the large phonolitic shield volcano of tenerife the las canadas edifice for the last 1 5 m y pyroclastic deposits of the bandas del sur formation are exposed along the southern flanks and record the last two of at least three long term cycles of caldera forming explosive eruptions each cycle began with flank fissure eruptions of alkali basalt lava followed by minor eruptions of basanite to phonotephrite lavas minor phonotephritic to phonolitic lava effusions also occurred on the flanks of the edifice during the latter stages of the second explosive cycle non welded plinian fall deposits and ignimbrites are the dominant explosive products preserved on the southern flanks of these a significant volume has been dispersed offshore many pyroclastic units of the second explosive cycle exhibit compositional zonation banded pumice occurs in most units of the third youngest explosive cycle and ignimbrites typically contain mixed phenocryst assemblages indicating the role of magma mixing mingling prior to eruption at least four major eruptions of the third cycle began with phreatomagmatic activity producing lithic poor accretionary lapilli bearing fallout and or surge deposits the repeated brief phase of phreatomagmatism at the onset of these eruptions is interpreted as reflecting an exhaustive water supply probably a small caldera lake that was periodically established during the third cycle accidental syenite becomes an increasingly important lithic clast type in ignimbrites up sequence and is interpreted as recording the progressive development of a plutonic complex beneath the summit caldera successive eruptions during each explosive cycle increased in volume with the largest eruption occurring at the end of the cycle more than ten major explosive eruptions vented moderately large volumes 1 ges 10 km 3 of phonolitic magma during the last two cycles culminating each explosive cycle was the emplacement of relatively large volume 5 10 km 3 ignimbrites with coarse vent derived lithic breccias interpreted to record a major phase of caldera collapse in the extracaldera record explosive cycles are separated by 0 2 m y periods of non explosive activity repose periods were characterized by erosion remobilization of pyroclastic deposits by discharge events and pedogenesis the current period of non explosive activity is characterized by the construction of the teide pico viejo stratovolcanic complex within the summit caldera this suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield building phases within the summit caldera alternating effusive and explosive cycles have thus been important in the volcanic evolution of the las canadas edifice

the quaternary pyroclastic succession of southeast tenerife canary islands explosive eruptions related caldera subsidence and sector collapse a much revised quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the bandas del sur in southern tenerife new ar 41 ar 39 data obtained for the arico granadilla fasnia poris la caleta and abrigo formations are presented allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments we also provide a minimum age of 287 7 ka for a major sector collapse event at the gaimar valley the bandas del sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics including widespread basal plinian layers predominantly phonolite composition ignimbrites with similar extensive geographic distributions thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns lateral gradations into localized massive facies within palaeo wadis and widespread lithic breccia layers that probably record caldera forming eruptions each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean the succession indicates that las canadas volcano underwent a series of major explosive eruptions each starting with a plinian phase followed by emplacement of ignimbrites and thin ash layers some of coignimbrite origin several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption because passage of each pyroclastic density current was characterized by phases of non deposition and erosion the entire course of each eruption is incompletely recorded at any one location accounting for some previously perceived differences between the units because each current passed into the ocean estimating eruption volumes is virtually impossible nevertheless the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the bandas del sur succession and probably formed a complex nested collapse structure detailed field relationships show that extensive ignimbrite sheets e g the arico poris and la caleta formations relate to previously unrecognized caldera collapse events we envisage that the evolution of the nested las cahadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite related caldera collapse events and large sector collapse events interspersed with edifice building phases

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

deposition of the bear gulch limestone a carboniferous plattenkalk from central montana the carboniferous bear gulch limestone of the central montana big snowy trough is a lithographic limestone analogous to the plattenkalk carbonates of the mesozoic european middle eastern tethyan belt it contains an excellently preserved chitinous and phosphatic marine fauna petrographic and field studies synthesized with palaeontological data provide insights into depositional history tectonic activity initiated development of small en echelon basins rims of these basins were sites of high primary productivity photosynthesis by subtidal cyanobacterial mats and algae caused in situ precipitation of micrite cyanobacteria and algae also provided a food source for benthic and nektonic organisms which provided food for predators respiration in turn provided carbon dioxide for continued photosynthesis thus a balanced ecosystem evolved productivity in the water column used up available oxygen resulting in dysaerobic to anaerobic conditions in the bottom waters and sediments this plus high sedimentation rates of carbonate ooze transported downslope aided in preservation of whole body fossils there are no likely modern analogues known for the bear gulch limestone however comparisons with mesozoic plattenkalke demonstrate that it is most closely analogous to those deposited in shallow water back reef areas of the tethyan carbonate platform rather than to those deposited in the deeper water shelf edge environment this is in agreement with the water depth of roughly 40 m estimated for the bear gulch limestone

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

early tertiary subduction zones and hot spots hot spots and subduction zones may control the position of the earth s rotation axis paleomagnetic reconstructions in the hot spot framework require a relative motion of about 10 between the hot spot reference frame and the earth s spin axis since the early tertiary if the viscosity of the mantle permits a changing distribution of mass anomalies can control the position of the earth s rotation axis as it tracks the maximum principal axis of the perturbations the present plate geometry should give the best indication of what features are associated with the controlling density anomalies for the present geoid anomalies indicate that subducting slabs are major mass contributions removing these effects a simpler residual geoid remains with highs strongly correlated with hot spot locations although neither the hot spots alone nor subducting slabs alone have maximum principal axes near the present pole adding the contributions of the two gives a combined axis within a degree of the pole suggesting that these two effects control the location of the spin axis to establish whether changes in plate geometry could account for the observed polar motion the locations of subduction zones are reconstructed in the hot spot framework for the early tertiary a time of significantly different plate motions for this reconstruction the combined maximum principal axis of the subductionhot spot systems is not at the present pole but about 10 away in a location similar to the pole position required by paleomagnetic studies this suggests that changes in subduction geometry may cause a shift in the position of the rotation pole and furthermore that inertia tensor effects may be the link between plate velocities and the spin axis

causal link between quaternary paleoclimatic changes and volcanic islands evolution 1 giant landslides and resulting tsunamis represent the main geologic hazards linked to volcanic island evolution from offshore and onland studies flank failures have been identified around numerous islands in most geodynamic contexts however the triggering conditions are still poorly understood and several causes may act simultaneously to reach a critical threshold here we show that most large volume 10 km3 landslides occur at glacial stages termination and we propose that a causal relationship between flank collapse of volcanic islands and global climatic changes has existed at least since 900 kyr moreover ages reported here favour the hypothesis that major collapses occurred during the onset of glacial to interglacial transitions when sudden influx of melt water from polar ice caps causes rapid sea level rise we propose that rapid sea level rise induces enhanced coastal erosion and sudden changes of pore pressure conditions within basal layers which favour edifice failure

the montesbelos mass flow southern amazonian craton brazil a paleoproterozoic volcanic debris avalanche deposit the present contribution documents the extremely well preserved paleoproterozoic architecture of the montesbelos breccia named here for the first time which is interpreted as a rare example of a subaerial paleoproterozoic 1 85 ga granular dominated mass flow deposit few of which are recorded in the literature montesbelos deposit is part of the andesitic sobreiro formation located in the sao felix do xingu region southern amazonian craton northern brazil the large volume high variability of textural features presence of broken clasts angular low sphericity fragments mono to heterolithic character and the size of the outcrops point to a volcanic debris avalanche flow fluviatile sandy material and debris flows are associated with the deposit as a result of post depositional reworking processes

conditions favouring catastrophic landslides on tenerife canary islands catastrophic failures of volcano flanks represent one of the most hazardous geological phenomena these immense mass movements originate either by increasing the destabilizing forces driving forces or by reducing the strength of the materials involved or both the study of large volcanic landslides on tenerife suggests that the presence of weak residual soils palaeosols in combination with the pre existence of deep narrow canyons created by fluvial erosion have played a fundamental role in the initiation of large scale sector collapses of the volcanic edifice regardless of the triggering mechanism considered residual soils strongly reduce the material strength during undrained loading while pre existing canyons control the lateral limits of the landslide the existence of a wet climate in some sectors of the island favours these circumstances

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

on the forces driving plate tectonics inferences from absolute plate velocities and intraplate stress summary absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress the absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar while the global pattern of absolute velocity is probably close to those predicted by such models the absolute motions do not therefore provide a strong test of the driving mechanism comparison of predicted intraplate stress with the long wavelength features of the global stress field however as determined by in situ measurements earthquake mechanisms and stressinduced geological structures does prove to be a powerful test of possible driving forces all absolute velocity models have several interesting properties lithosphere in the equatorial half of the earth is moving significantly faster than lithosphere in the polar half some connection to the earth s rotation is implied since this statement is demonstrably untrue for co ordinate poles much different from the geographic pole subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate confounding attempts to explain the dip angles of benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere such rapid migration may be a necessary but is not a sufficient condition for ridge jumps force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere from the rms absolute velocities of individual plates there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere the predicted intraplate deviatoric stress cannot match the well determined stress fields in north america and europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system the mid plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities

variations in the velocities of the major plates since the late cretaceous abstract we present models for the relative and absolute velocities of the major plates from 80 m y ago to the present as tests of several alternative plate reconstructions and as a guide in the evaluation of the hypothesis that pronounced changes in plate speeds or directions arise directly from changes in the configuration of plate boundary forces digitized plate boundaries were rotated backwards in time using published rotations to determine positions at 5 m y intervals back to the late cretaceous the oceanic plate boundaries were then corrected so as to properly match magnetic anomalies relative plate velocities were obtained by differencing finite rotations absolute velocities were determined by assuming that no net torque is exerted on the lithosphere rms absolute speeds of each plate were calculated as functions of time as a conceptually simple measure of plate velocities and accelerations we have tested several possible reconstructions for the antarctic and pacific plates proposed to explain the failure of simple rigid antarctic and or pacific plate models to match paleomagnetic data hotspot tracks and equatorial crossings defined by pelagic sediment deposition we find that 1 1 plate reconstruction models with either two antarctic or two pacific plates in the early tertiary are distinguishable from models with single antarctic and pacific plates principally on the basis of faster speeds for the kula plate and the northern pacific greater areas for the kula and farallon plates and faster subduction at the aleutians and along the west coast of south america for that time 2 2 oceanic plates as a group have moved faster than those with continental lithosphere only since 50 m y ago active subduction appears to be a sufficient condition for rapid plate motion regardless of the fractional plate area comprised of continental lithosphere 3 3 the absolute motion of india in the late cretaceous was anomalously fast 15 20 cm yr compared to the speeds of the present plates 4 4 the rms velocities of a number of plates and of the lithosphere as a whole are much higher in the late cretaceous than through most of the cenozoic 5 5 the predicted apparent polar wander for north america is in fairly good agreement with paleomagnetic data with the slight discrepancy attributable to a net contribution to the torque balance from plate boundary forces

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

geochronology of mesozoic cenozoic magmatism in central chile lat 31 36 s abstract radioisotope dates indicate magmatism in central chile lat 32 36 s has been episodic and migratory throughout mesozoic and cenozoic time dated plutonic and metaplutonic rocks appear to cluster into the following age groups 1 415 380 m y 2 310 270 m y 3 230 170 m y 4 170 140 m y 5 120 80 m y 6 63 56 m y and 7 24 7 m y rocks of meso and cenozoic age are distributed in remarkably well defined parallel linear belts which trend n s and are older away from the central andean cordillera this pattern suggests that the continental margin has increased in width by as much as 200 km in the last 200 m y by the emplacement of successive plutonic belts volcanic strata of lower middle jurassic age 7000 m and early cretaceous age 7000 13000 m occur on the west side of the present coastal cordillera and on both flanks of the andean cordillera these rocks mainly andesites rhyolites and volcano clastic sediments are intercalated with fossil bearing marine sediments which provide age control up until the final emergence of the andean cordillera in late cretaceous time k ar dates indicate this continental volcanism continued through the late cretaceous then ceased during the period between 60 and 30 m y this hiatus in volcanism coincides with the absence of plutonic rocks in this age range in the central andean cordillera volcanism began again about 25 m y ago and has persisted with variable intensity to the present post oligocene volcanic strata are generally nearly conformable above late cretaceous volcanic strata and miocene regional folding has deformed the entire stratigraphic sequence into broad n s trending folds volcanic strata younger than 4 m y are undeformed the presently active chain of strato volcanoes appears to be less than 500 000 years old and west of older extinct volcanoes 1 2 m y old suggesting a westward shift of the volcanic are during the pleistocene

large eruption triggered ocean island landslide at tenerife onshore record and long term effects on hazardous pyroclastic dispersal an extensive debris avalanche deposit has been discovered on canadas volcano tenerife canary islands the onshore component of the 733 3 ka abona landslide deposit exposes classic block facies and mixed facies across 90 km 2 three lines of evidence together show that the avalanche was triggered by an ignimbrite forming explosive eruption 1 the deposit is enclosed by phonolitic ignimbrites and is draped by a plinian fallout layer all within a single eruption unit 2 it contains prismatic jointed pumice blocks that were hot during landslide emplacement indicated by chilled rims and breadcrust surfaces 3 these blocks yield the same 40 ar 39 ar date as the associated ignimbrite and fall deposit landslide hummocks dammed surface water forming ephemeral lakes perched on the volcano flank phonolite dome growth destabilized the southeast sector of a mid pleistocene canadas caldera wall and created a major breach that affected the passage of destructive pyroclastic density currents on tenerife for 0 5 m y showing that landslides can have enduring consequences for pyroclastic dispersal and hazards

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

plate driving forces at anomaly 23 time abstract a world wide set of plate boundaries and relative velocities was calculated for anomaly 23 time about 55 m y ago at the paleocene eocene boundary just before the breakup of australia antarctica the set was used as input data for harper s recently published model of plate driving and resisting forces for the present day the model enabled estimates to be made for the age of the oceanic lithospheric sinking at present day and at 55 ma old subduction zones plausible ages were found for most zones the worst relative error in any torque component for the 55 ma reconstruction was less than 8 per cent of the largest driving torque component this is about one and a half times the corresponding error obtained for the present day plate driving force model it is speculated that the transference of a subduction zone from the indian to the australian antarctic plate at about 55 ma could have caused the breakup of australia antarctica the model was also useful when preparing the plate reconstruction by giving physically implausible results for some regions on closer examination our initial data in these regions were found to be in error the substitution of more recent data generally improved the results

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

stratigraphy structure and geochronology of the las canadas caldera tenerife canary islands after a long period of subaerial fissure fed extrusions of basaltic magmas 12 to 3 ma volcanic activity was then concentrated in the central part of tenerife phonolitic magma chambers formed and a central volcanic complex was constructed the las canadas edifice the formation of a large depression the las canadas caldera truncated the top of the edifice the active twin strato cones teide pico viejo are sited in this depression the history of the las canadas caldera and edifice are established from stratigraphy geochronology k ar dates and volcanological studies two different groups are recognized separated by a major unconformity the lower group is dated at 2 to 3 ma and includes the products of several volcanic centres which together represent several cycles the upper group ranges from 1 56 to 0 17 ma and includes three different formations representing three long term 100 to 300 ka volcanic cycles the periods of dormancy between each formation were of 120 to 250 ka duration the las canadas caldera is a multicyclic caldera which formed over the period 1 18 0 17 ma each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the las canadas edifice geological observation and geochronology support an origin by collapse into a magma chamber the minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume 45 km3 but approaches the inferred volume of the original caldera depression 140 km3 after the formation of the caldera sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall

correlation between rate of sea level change and frequency of explosive volcanism in the mediterranean volcanic activity has frequently been linked to quaternary environmental change either by driving climate modification1 2 or in response to environmental changes3 although a link has been established between large explosive eruptions and small 0 5 c brief 1 2 years falls in global temperatures4 both the evidence and mechanisms responsible for longer episodes of eruption induced planetary cooling remain questionable1 2 5 6 in contrast recent research based on ice core data suggests that rapid climate changes during the past 110 000 years increased explosive volcanic activity7 here we present a statistical analysis relating the frequency of explosive activity of mediterranean volcanoes based on dated8 9 10 11 tephra layers in deep sea sediment cores to the rate of late quaternary sea level change the nonlinear correlation between the two is tentatively explained in terms of dynamic responses of the volcanoes to stress related influences on various spatial scales the correlation supports a mechanism or mechanisms by which the climate driven growth and decay of large ice sheets can influence the eruptive chronologies of distant volcanic edifices via changes in global sea level