forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

tsunamis in the aeolian islands southern italy a review the aeolian islands located in the south tyrrhenian sea to the west of calabria and to the north of sicily constitute the aeolian volcanic arc characterized by a relevant volcanic activity with an established tsunamigenic potential according to the documentary sources that are available for the aeolian tsunamis and that cover unfortunately only about the last 100 years stromboli is responsible for the majority of the events some cases are also known to have occurred at the islands of salina and vulcano in terms of intensities the most destructive events are those originated by the stromboli volcano reported to have produced severe destruction to boats and houses and to have even caused injuries and fatalities in this work we focus on the historical events and discard the paleotsunami occurrences that were probably associated with the ascertained flank collapses of the volcanic cones we also neglect the recent destructive tsunamis of 30 december 2002 which were caused by a series of mass failures at the sciara del fuoco since observations and analyses are of such an amount that deserve to be illustrated in devoted papers the bulk of the present research consisted in collecting all the available data on the historical cases the first in order of time is the july 1916 stromboli tsunami with the purpose of casting more light on their generation mechanism and on their effects and consequently of contributing to the studies on assessment of tsunami hazard and risk in the aeolian islands and in the southern tyrrhenian sea

discordant paleomagnetic data for middle cretaceous intrusive rocks from northern baja california latitude displacement tilt or vertical axis rotation 1 paleomagnetic results and u pb zircon dating from the san marcos dike swarm and the el testerazo pluton in the cretaceous peninsular ranges batholith of northern baja california are used to evaluate alternative pre neogene paleogeographic reconstructions of the baja california peninsula the san marcos dike swarm is a dense northwest striking regional dike swarm that is exposed over an 100 km long segment of the batholith and has yielded a u pb zircon crystallization age of 120 1 ma dike attitudes from the swarm suggest a regionally consistent average 320 e strike and 79 ne dip the el testerazo pluton is a younger tonalite intrusion that truncates the northern end of the dike swarm all but one of 36 sites sampled in this study show remanence of normal polarity paleopoles for the san marcos dike swarm and el testerazo pluton are indistinguishable and were combined into a paleopole at 248 1 e 86 6 n a95 4 8 which is displaced with respect to the 122 ma reference pole for stable north america at 198 2 e 72 3 n a95 3 3 the displacement may be described by an apparent clockwise rotation of 18 6 and an apparent northward shift of 8 5 restoring a northward shift of about 3 related to the separation of baja california from north america since 10 ma only a marginal northward displacement of 5 5 is left the clockwise rotation may be the result of crustal block rotations within the right lateral shear systems in northern baja california although there is no geological evidence that supports this possibility alternatively the difference between paleopole and reference pole may be due to tilting of the study area restoring a northeastward tilt of 11 based on the mean dip measured for the san marcos dike swarm in the study area yields a paleopole at 187 6 e 70 8 n a95 5 6 which is indistinguishable from the 122 ma north american reference pole the tilting hypothesis suggested previously as a potential explanation for low inclination of paleomagnetic data from baja california therefore seems to be viable for the discordant paleopoles from this segment of the batholith data obtained previously about 150 km further south and along the western edge of the batholith defined a concordant paleopole without any tilt correction indicating that regional tilting may not be valid for all of baja california

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

volcanic record of the subduction of an active spreading ridge taitao peninsula southern chile during the pliocene the taitao peninsula was the locus of the subduction of the chile active spreading ridge beneath the south american margin new field data allow us to distinguish within the taitao ophiolite a genuine ophiolitic body of undetermined age and two volcanic and volcaniclastic units one unit shows evidence for deposition in a shallow water environment and it unconformably overlies the chilean continental basement its established pliocene age demonstrates that it was deposited during the subduction of the chile ridge lavas from both units include enriched mid oceanic ridge basalts morbs together with nb depleted basalts andesites and dacites la nb ratios vary randomly from 1 to 4 in both units and are positively correlated with sio 2 content these chemical characteristics are consistent with upper crustal contamination of morb type basalts by the chilean crust as already suggested from isotopic data we consider these volcanic suites to be the result of eruptions of morb magmas that originated from the downgoing shallowly buried active spreading center and were contaminated during their rise through the chilean continental basement

pb sr and nd isotopic and chemical evidence for a primitive island arc emplacement of the el arco porphyry copper deposit baja california mexico the el arco porphyry copper deposit is located in central baja california and is a resource containing 600 mt of ore with 0 6 copper it was emplaced within a relatively primitive jurassic island arc and it was subsequently metamorphosed and intruded by the cretaceous peninsular ranges batholith the porphyritic stock intrusion and ore formation at el arco has recently been dated at 165 ma valencia et al geos 24 189 2004 this age is much older than aptian albian k ar ages previously reported from el arco barthelmy geology of el arco calmalli area baja california mexico msc thesis san diego state university ca 1975 baja california geology san diego state university ca pp 127 138 1979 the copper mineralization at el arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas mafic dikes that intruded the deposit are not mineralized but they are affected by post ore low grade metamorphism the dikes are compositionally the most primitive rocks while host rock andesites and the porphyry stock display typical volcanic arc characteristics the pb isotope data from sulfides feldspars quartz and whole rock samples indicate that 1 the copper bearing porphyry stock and the surrounding andesites evolved from a similar source with an average value of 9 43 2 no external pb was added during mineralization 3 some pb isotope compositions were slightly disturbed by a later metamorphic event strontium and nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust our data favor a model for the formation of the el arco deposit linked to a triassic to jurassic intra oceanic arc system cropping out at the western margin of central baja california in the cedros vizcaino region the intra oceanic arc together with the el arco deposit was accreted to the active continental margin of north america and metamorphosed during the early cretaceous this model is in disagreement to earlier models that favor the el arco deposit formation being linked to the cretaceous continental margin

seismic ridge subduction and topography foreland deformation in the patagonian andes abstract the patagonian andes recorded several episodes of active ridge subduction in the last 80 million years an analysis of the spatial and temporal relation between the present segment of collision of the chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the patagonia fold and thrust belt several magmatic episodes related to the collision such as near trench magmatism adakite emplacement oib plateau basalts in the retroarc and the arc volcanic gap are associated with the uplift and deformation of the patagonian cordillera based on these correlations a collision of the aluk or phoenix farallon ridge during paleogene times south of 43 30 is identified changes in magmatic patterns molasses deposits deformation and uplift of the patagonian cordillera constrain the region affected by the collision similar evidence implies a third period of collision in the late cretaceous based on the occurrence of adakitic rocks arc magmatic gap and deformation along the southern patagonian andes this earliest hypothesized collision would require the existence of a new oceanic microplate between the pacific and the aluk plates during late cretaceous times present rapid isostatic rebound related to the continental ice cap retreat in the patagonian andes is restricted to the region south of chile triple junction 46 30 s the uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the northern hemisphere and requires an abnormally hot mantle with low viscosity this abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat slab subduction proposed by some authors to explain the arc volcanic gaps

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

latest cretaceous early paleogene boom of porphyry cu mineralization associated with the laramide magmatic arc of mexico abstract porphyry copper and associated deposits are widespread through most northwestern mexico particularly in sonora and sinaloa in the northwestern part of the country these deposits belong to a nw se trending copper belt that extends from southwestern usa to southern mexico the copper mineralization is clearly linked to subduction related calc alkaline plutons formed during the eastward migration of the cordilleran magmatic arc largely coincident in time and space with the laramide orogeny 80 40 ma the economically most important porphyry copper deposits were emplaced in the southwest margin of the north american craton the porphyry copper mineralization was formed during the latest cretaceous to early paleogene between 75 and 52 4 ma but it was particularly fertile between 69 and 54 ma when the larger copper concentrations were deposited to form the famous copper cluster of arizona new mexico and sonora during this time more than half of the total porphyry copper related metals of the entire cordillera was deposited this large copper accumulation developed south of the classical flat slab laramide province characterized by uplift of large crustal blocks and mostly amagmatic conditions an inferred slab tear zone separated this zone from a slab segment above which asthenospheric magma flux favored copper concentration the mexican porphyry copper belt accounts for 30 deposits out of which the buenavista del cobre 30 mt cu and the la caridad 8 mt cu are the most relevant more to the south close to the boundary between sonora and sinaloa the porphyry copper deposits are smaller

the 2007 stromboli eruption event chronology and effusion rates using thermal infrared data accepted for publication in journal of geophysical research copyright 2009 american geophysical union further reproduction or electronic distribution is not permitted

uranium lead ages from the dun mountain ophiolite belt and brook street terrane south island new zealand the dun mountain ophiolite and overlying maitai group is discontinuously exposed for 480 km in south island new zealand zircon u pb dates from plagiogranite are presented for relatively intact ophiolite ophiolitic melanges and for more silicic volcanic plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking step wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from 1 to more than 5 of their radiogenic pb relatively recently high precision 207 pb 206 pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 ma the zircon u pb data confirm correlation of petrologically distinct segments of the dun mountain ophiolite and show that mafic ultramafic ophiolite assemblages and moderately potassic high level granitoids developed within a short time interval probably during the extension of a volcanic arc marginal basin thick lenses of polymictic breccia and bio clastic limestone of the maitai group locally rest in depositional contact on relatively intact ophiolite within the dun mountain ophiolite comparison of inferred biostratigraphic ages from the limestone with the 280 ma ages from the plagiogranites indicate a gap of 20 m y following ophiolite formation a granite clast from conglomerate higher in the maitai group yielded a near concordant u pb date of 265 ma and provides a maximum age for this part of the sequence attenuation of the dun mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and maitai group sedimentation the dun mountain ophiolite and overlying maitai group are bounded to the west by triassic and jurassic volcanogenic sedimentary rocks of the murihiku terrane and in turn by the brook street terrane which is interpreted as remnants of an early permian oceanic arc a hornblende gabbronorite associated with a layered mafic ultramafic intrusion in the brook street terrane yielded a date of 265 ma significantly younger than dun mountain ophiolite such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the brook street terrane but which are not represented by detritus in the maitai group biotite granite occurs locally in the brook street terrane and is dated at 260 ma the absence of any clear stratigraphic correlation or provenance linkage between the brook street terrane and dun mountain maitai terrane suggests strike slip displacements on intervening terrane boundary faults

late devonian early permian a type granites in the southern altay range northwest china petrogenesis and implications for tectonic setting of a 2 type granites abstract the altay range is an important part of the central asian orogenic belt caob that contains a type granites that have been attributed to a post collisional setting during the late carboniferous permian however our new la icp ms zircon u pb age data demonstrates that there were two episodes of a type granite magmatism in the southern altay range a late carboniferous to early permian 308 291 ma suite does occur in the vicinity of qiakuerte but a distinct late devonian 382 367 ma suite is present in the kouan area the early kouan a type granites are mainly composed of alkali feldspar granites which are approximately synchronous with adakites boninites high ti basalts picrites ophiolitic rocks and high temperature low pressure metamorphic rocks whereas the late qiakuerte a type granites mainly consist of arfvedsonite and aegirine bearing granites which are approximately coeval with some mafic ultramafic rocks in the southern altay range a type granites are commonly classified as a1 and a2 sub types which are considered to be generated in anorogenic rift or plume related or post collisional settings respectively both suites of the southern altay range are geochemically similar to typical a2 type granites e g high k2o na2o feo mgo and ga al and y nb values and low cao ba sr and eu contents the kouan a type granites have relatively low la yb n 2 7 5 9 high end t 6 7 to 7 7 and nb la 0 59 1 67 and variable ehf t 5 0 to 14 1 values whereas the qiakuerte a type granites have comparatively high la yb n 4 2 9 2 low end t 5 3 to 6 0 and nb la 0 33 0 74 and variable ehf t 10 0 to 18 7 values the early kouan a type granites may have been generated by partial melting of a mafic source containing more depleted mantle derived components than the late qiakuerte a type granites we suggest that the late carboniferous to early permian qiakuerte granites were formed in a post collisional extensional setting as is typical of a2 type granites however the late devonian kouan a type granites were more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction in this case the upwelling of asthenosphere through the slab window provided the source of parental magmas or the heat for the generation of devonian magmas therefore our results suggest that a2 type granites can also form in a ridge subduction related extensional setting

axial morphology along the southern chile rise abstract morphology of four spreading segments on the southern chile rise is described based on multi beam bathymetric data collected along the axial zones the distribution of axial volcanoes the character of rift valley scarps and the average depths vary between segment 1 in the south terminating at the chile triple junction and segment 4 in the north which are separated by three intervening transform faults despite this general variability there is a consistent pattern of clockwise rotation of the southern most axial volcanic ridge within each of segments 2 3 and 4 relative to the overall trend of the rift valley a combination of local ridge transform intersection stresses and regional tectonics may influence spreading axis evolution in this sense

ritter island volcano lateral collapse and the tsunami of 1888 summary in the early morning of 1888 march 13 roughly 5 km 3 of ritter island volcano fell violently into the sea northeast of new guinea this event the largest lateral collapse of an island volcano to be recorded in historical time flung devastating tsunami tens of metres high on to adjacent shores several hundred kilometres away observers on new guinea chronicled 3 min period waves up to 8 m high that lasted for as long as 3 h these accounts represent the best available first hand information on tsunami generated by a major volcano lateral collapse in this article we simulate the ritter island landslide as constrained by a 1985 sonar survey of its debris field and compare predicted tsunami with historical observations the best agreement occurs for landslides travelling at 40 m s 1 but velocities up to 80 m s 1 cannot be excluded the ritter island debris dropped little more than 800 m vertically and moved slowly compared with landslides that descend into deeper water basal friction block models predict that slides with shorter falls should attain lower peak velocities and that 40 ms 1 is perfectly compatible with the geometry and runout extent of the ritter island landslide the consensus between theory and observation for the ritter island waves increases our confidence in the existence of mega tsunami produced by oceanic volcano collapses two to three orders of magnitude larger in scale

paleocurrent patterns of the sedimentary sequence of the taitao ophiolite constrained by anisotropy of magnetic susceptibility and paleomagnetic analyses abstract determination of paleocurrent directions is a robust approach to establish the mechanism of sediment transport and of deposition however in tectonically controlled or affected basins where rotations are suspected it is of critical importance to correct previously calculated paleocurrent directions to establish the original pre tectonic transport and deposition patterns in order to illustrate this we determined the orientations of the anisotropy of magnetic susceptibility ams fabrics from 14 sites distributed along the sedimentary deposits interbedded within the late miocene main volcanic unit mvu of the taitao ophiolite southern chile data were used to estimate characteristics directions and relative velocities of the sediment transporting paleocurrents and orientations of the depositional slope sedimentary deposits consist of a rhythmic sequence of conglomerate base sandstone and mudstone top beds that present a systematic decrease in grain size up sequence as a whole and within individual beds sedimentary deposits of the mvu as well as overlaying effusive volcanic products are disrupted by small and discrete dextral normal brittle faults the distribution of fault planes and their kinematic characteristics suggest the development of a dextral negative flower like fault zone in the area previously reported nw ward paleocurrent directions obtained from the orientations of sedimentary structures roughly agree with our calculations based on the orientations of ams fabrics calculated paleocurrent directions are sub parallel to the nw se trend of estero cono channel the main lineament in the area the spatial distribution of ams axes between and within sites suggest that coarse grain deposits were transported by relatively high velocity flows whereas fine grained deposits were by low to nearly null velocity flows previously reported paleomagnetic analyses indicate that counterclockwise rotations affected the whole taitao ophiolite including the mvu by applying the necessary corrections we obtained paleocurrent directions yielding mostly nnw ward directions restoration of the orientations of beddings indicates that the depositional surface dipped steeply at its easternmost edge and shallowly at the westernmost end we propose here that sediments were transported by turbidite flows generated by slope instabilities in the innermost part of the estero cono these turbidites followed the estero cono lineament while they filled the channel and transported sedimentary material eroded from the upper portions of the ophiolite and nearby units towards the pacific ocean sediments were deposited on a nw to w facing slope and sorted in decreasing grain size along the channel the instability of the slope was most probably triggered by the activity of a transtensional fault zone developed along the mid axis of the estero cono channel

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

ridge subduction and slab windows in the central asian orogenic belt tectonic implications for the evolution of an accretionary orogen abstract cenozoic ridge subduction and the resultant slab windows have been well documented worldwide especially along the western margins of north and south america the principal characteristics of ridge subduction which can be used to recognize the process in ancient orogens include intrusion of ridge generated magmas into a forearc in a near trench position this can be regarded as the hallmark of ridge subduction the key magmatic products are adakites that include plutonic and volcanic rocks with intermediate to high sio2 high sr y and la yb ratios elevated mgo na2o k2o ni and cr contents and high lile and lree elements they range from hornblende biotite bearing high mg diorites and granodiorites to hornblende pyroxene high mg andesites they are typically associated with a wide variety of coeval and compositionally diverse volcanic and plutonic rocks such as high ca boninites tholeiitic high t basalts alkaline basalts charnockites peraluminous ttgs i type granites and mid ocean ridge mor not supra subduction ophiolites the trace element chemistry of these rocks demonstrates that they were generated by partial melting of dehydrated subducted oceanic crust and of lower continental crust contaminated and metasomatized by juvenile melts that originated by upwelling of asthenospheric mantle it is this combination of mixed sources that gives rise to the diagnostic and distinctive features of coeval and associated ridge subduction rocks in a near trench environment other important features of ridge subduction are high temperature metamorphism closely associated with near trench plutons created by heat released through a slab window and porphyry gold copper zinc mineralization associated with mor ophiolites and generated in hydrothermal vents at a mor when a ridge subducts under a continental margin the diverging plates associated with the ridge continue to separate creating a slab window that forms between the separating plates adakitic melts tend to be generated close to the hot plate margins whereas a type granites and porphyry cu au deposits form in the centre of the opening slab window most of the above features have been described and analyzed in many parts of the central asian orogenic belt caob particularly in west junggar chinese altai russian altai east junggar beishan chinese tianshan inner mongolia kyrgyzstan and at alxa prominent in one of two slab windows defined in west junggar are two swarms of late carboniferous high mg hornblende biotite diorite dykes with sanukitoid chemistry that intruded through the slab window one generation was orthogonal to an ophiolite strewn paleotrench suture and the second was parallel to a transform fault that was parallel to the paleo trench the dikes provide the best evidence so far for the orientation and opening of a slab window in the caob and for the relative movement directions of the downgoing and overriding plates during the subduction and closure of the paleo asian ocean however these are early days because most parts of the caob have yet to be investigated for evidence of ridge subduction and slab windows so the prospects are high

geology and paleomagnetism of el potrero pluton baja california understanding criteria for timing of deformation and evidence of pluton tilt during batholith growth the 102 ma el potrero pluton in the western foothills of sierra san pedro martir in north central baja california was emplaced during a long period of contractional deformation bracketed between 132 and 85 ma that affected this segment of the peninsular ranges batholith the pluton records regional and emplacement related deformation manifested by 1 a solid state fabric developed on its eastern contact which is produced by eastward lateral pluton expansion 2 cleavage triple point zones in the host rock nw and se of the pluton 3 subhorizontal ductile shear zones indicative of top to the east transport 4 magmatic and tectonic foliations parallel to regional structural trends and regional shear zones 5 variable axial ratios of microgranitoid enclaves close to pluton wall rock contacts 6 evidence of brittle emplacement mechanisms in the western border of the pluton which contrast with features indicating mainly ductile mechanisms toward the east and 7 markedly discordant paleomagnetic directions that suggest emplacement in an active tectonic setting the overall mean for 9 accepted paleomagnetic sites is dec 34 6 i 25 7 k 88 3 95 5 5 and is deviated 35 with respect to the reference cratonic direction this magnetization is interpreted to indicate a combination of tilt due to initial drag during vertical diapiric ascent or westward lateral oblique expansion of the adjacent san pedro martir pluton and later rotation 15 by rosarito fault activity in the southwest this rotation may have occurred as eastward contraction acted to fill the space emptied by the ascending san pedro martir pluton the rosarito fault may have tilted several plutons in the area sierra san pedro martir el potrero san jose and encinosa magnetic susceptibility fabrics for 13 sites reflect mostly emplacement related stress and regional stress paleomagnetic data and structural observations lead us to interpret the el potrero pluton as a syntectonic pluton emplaced within a regional shear zone delimited by the main martir thrust and the younger rosarito fault

low temperature thermochronology of northern baja california mexico decoupled slip exhumation gradients and delayed onset of oblique rifting across the gulf of california 1 the northern gulf extensional province displays key structural relationships that characterize the magnitude direction and timing of neogene rift related transtension during the opening of the gulf of california apatite fission track and u th he thermochronology from the sierra san felipe document moderate cooling 4 c myr 7 c myr during the early paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral peninsular ranges beginning at 45 35 ma a period of tectonic quiescence with low cooling rates 1 c myr marks the development of a regional oligocene miocene peneplain rift related exhumation began at 9 7 ma and attains 2 5 km in the hinges of two antiformal megamullions decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension perpendicular folding of the fault surfaces the main faults of the detachment system were active contemporaneously thus forming a mechanically linked array of large displacement normal faults in the hanging wall of the main gulf escarpment the late miocene onset of transtension in the sierra san felipe suggests that widespread deformation may only have localized in the gulf extensional province between 9 and 7 ma some 3 5 ma after a major plate reorganization associated with cessation of subduction in the trench to the west between 12 ma and 9 7 ma plate boundary shearing was likely distributed between the continental borderland west of baja california and the southern basin and range province in mexico

the 15 march 2007 explosive crisis at stromboli volcano italy assessing physical parameters through a multidisciplinary approach basaltic volcanoes are dominated by lava emission and mild explosive activity nevertheless many basaltic systems exhibit from time to time poorly documented and little understood violent explosions a short lived multiblast explosive crisis paroxysmal explosion occurred on 15 march 2007 during an effusive eruptive crisis at stromboli italy the explosive crisis which started at 20 38 14 ut had a total duration of 5 min the combined use of multiparametric data collected by the permanent instrumental networks seismic acoustic and thermal records and a field survey carried out immediately after the event enabled us to constrain the eruptive dynamics and quantify physical parameters the eruption consisted of three major pulses in the first lithic blocks and ash were ejected at speeds of 100 155 m s and 130 210 m s respectively the high solid load of the eruptive jet resulted in the partial collapse of the column with the formation of a small volume pyroclastic density current the second 12 s long pulse emitted 2 2 2 7 107 kg of tephra mass discharge rate 1 9 2 3 106 kg s forming a 3 km high convective plume dispersing tephra up to the west coast and a dilute density current with limited dispersal downslope of the craters a final 30 s long phase formed a scoria flow with a volume of 1 5 1 7 104 m3 mass discharge rate 5 9 6 7 105 kg s a total runout of 200 m and a velocity of 45 m s the total gas volume involved in the explosion was 1 3 1 9 104 m3 with an initial overpressure of 7 9 0 4 mpa we compared the 15 march 2007 event with historical paroxysms in particular with that of 5 april 2003 which was remarkably similar

tectonics in the eastern central patagonian cordillera 45 30 47 30 s a subhorizontal angular erosional unconformity that separates subhorizontal upper palaeocene lower eocene fluvial deposits of the ligorio marquez formation from folded and faulted lower cretaceous tuffs and sedimentary rocks in the area south of chile chico in the northern patagonian fold and thrust belt of southern chile 46 45 s indicates tectonism prior to the previously recognized post early miocene contractional event similar observations between latitudes 45 30 and 47 30 s in argentina support the existence of a pre palaeocene eocene contractional tectonic phase and locally of a late early cretaceous event this indicates that the northern patagonian fold and thrust belt includes structures earlier than previously thought probably masked by the later neogene ones the timing of this earlier tectonism is not precisely known beyond being pre upper palaeocene lower eocene and post barremian one possibility is that it may be the earlier stages of a progressive deformation that in its later stages is represented by the post lower miocene tectonism previously documented in the region in this case it may represent the latest cretaceous palaeocene major contractional tectonism and uplift well documented along the southernmost patagonian cordillera south of latitude 51 s the upper palaeocene lower eocene ligorio marquez formation may represent the first accumulation in a foreland wedge top basin developed east of the orogenic front subsequently and probably during the middle miocene these beds were folded and faulted during the tectonism that generated the foreland basin where the fluvial deposits of the galera formation accumulated different speculative scenarios for the origin of this compression can be invoked depending on its age

comparison between the chile and mexico triple junction areas substantiates slab window development beneath northwestern mexico during the past 12 10 myr abstract when combined with the miocene recent volcanic record of baja california a parallel drawn between the chile and mexico triple junction areas substantiates slab window development beneath northwestern mexico during the past 12 10 myr the slab free zone manifestations challenge the notion that ridge subduction has not occurred beneath the southern baja california peninsula the geochemically distinctive rocks from the santa clara volcanic field of west central baja california including coeval adakites and niobium enriched basalt are commonly inferred to signal partial melting of the subducting plate at shallow depths and relatively high temperatures before slab dehydration occurs such pt conditions for slab melting have only been observed in association with spreading ridge subduction we propose that slab window development beneath southern baja california and mainland mexico 30 to 18 n resulted from subduction of the east pacific rise

conditions of magma storage degassing and ascent at stromboli new insights into the volcano plumbing system with inferences on the eruptive dynamics stromboli is known for its persistent degassing and rhythmic strombolian activity occasionally punctuated by paroxysmal eruptions the basaltic pumice and scoria emitted during paroxysms and strombolian activity respectively differ in their textures crystal contents and glass matrix compositions which testify to distinct conditions of crystallization degassing and magma ascent we present here an extensive dataset on major elements and volatiles co2 h2o s and cl in olivine hosted melt inclusions and embayments from pyroclasts emplaced during explosive eruptions of variable magnitude magma saturation pressures were assessed from the dissolved amounts of h2 oa nd co2 taking into account the melt composition evolution both pressures and melt inclusion compositions indicate that 1 ca basaltic melts entrapped in high mg olivines fo89 90 generate stromboli basalts through crystal fractionation and 2 the stromboli plumbing system can be imaged as a succession of magma ponding zones connected by dikes the 7 10 km interval where magmas are stored and differentiate is periodically recharged by new magma batches possibly ranging from ca basalts to basalts with a co2 rich gas phase these deep recharges promote the formation of bubbly basalt blobs which are able to intrude the shallow plumbing system 2 4 km where co2 gas fluxing enhances h2o loss crystallization and generation of crystal rich dense degassed magma chlorine partitioning into the h2o co2 bearing gas phase accounts for its efficient degassing 69 under the open system conditions of strombolian activity paroxysms however are generated through predominantly closed system ascent of basaltic magma batches from the deep storage zone in this situation crystallization is negligible and sulfur exsolution starts at 170 mpa chlorine remains dissolved in the melt until lower pressures only 16 being lost upon eruption finally we propose a continuum in explosive eruption energy from strombolian activity to large paroxysmal events ultimately controlled by variable pressurization of the deep feeding system associated with magma and gas recharges

fingerprints of late neoproterozoic ridge subduction in the pan african damara belt namibia subduction of mid ocean ridges is a common feature in recent convergent margins but is rarely documented in proterozoic to paleozoic orogenic belts here we describe evidence for ridge trench interaction in the deeply eroded late neoproterozoic damara orogenic belt central namibia the earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid ocean ridge metabasalts and trench metasediments u pb zircon ages of 550 540 ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction the thermal peak in the damara belt associated widespread granitic and alkalic plutonism and hydrothermal activity coincide with the waning stages of tectonism at 530 520 ma and are interpreted to indicate slab window widening and slab delamination we suggest that the proposed two stage thermal evolution of the damara belt comprising latest neoproterozoic ridge subduction and early cambrian slab delamination represents a fingerprint of ridge subduction in ancient orogens

mountain building across a lithospheric boundary during arc construction the cretaceous peninsular ranges batholith in the sierra san pedro martir of baja california mexico the jura cretaceous peninsular ranges batholith prb of southern and baja california contains a remarkable example of variation in crustal composition and structure across a batholith parallel lithospheric scale discontinuity this lithospheric boundary between western oceanic floored and eastern continental floored crust influenced contractional deformation arc magmatism and differential exhumation of western and eastern zones in the batholith during its evolution in the sierra san pedro martir of baja california mexico a ca 20 km wide doubly vergent fan structure occurs across the prb basement transition that consists of inward dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply dipping tectonized zone in the center a dramatic inverted metamorphic gradient occurs on the western side of this structure where mid crustal amphibolite metamorphic grade rocks with peak pressures of 5 6 kbar in the center of the fan were thrust over upper crustal sub greenschist grade rocks peak pressures 2 kbar in the western zone footwall an inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks peak pressures 4 5 kbar gradients in cooling ages determined by 40ar 39ar biotite and k feldspar and apatite fission track methods coupled with u pb zircon ages and al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the prb accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen the western zone of the batholith was a major forearc depo center for thick clastic sequences derived from the uplifting eastern prb and remained at low average elevation during the late cretaceous and paleogene in contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid cretaceous time this region had the isostatic potential for 4 5 km surface elevations and likely formed a topographically high orogenic plateau exhumation of the fan structure initiated after 100 ma and was largely complete by 85 ma eastward migrating unroofing of the rest of the eastern prb continued into the paleogene a variety of factors were responsible for exhumation in this region structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure much of which followed island arc accretion of the western zone along this segment of the batholith an episode of voluminous magmatism involving the intrusion of the 99 92 ma la posta type magmatic suite across the eastern zone of the prb triggered exhumation in the fan structure denudation in this region appears to have been solely by erosion no evidence has been found for extensional tectonics during this time this arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism it also displays the complex interrelationships among structural thickening exhumation and the role of magmatism in triggering exhumation episodes within orogens

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

the 1741 oshima oshima eruption extent and volume of submarine debris avalanche oshima oshima is a small volcanic island southwest off hokkaido in japan sea the violent eruption on august 29 1741 caused a sector collapse of the north flank very destructive tsunami with 2000 casualties was also documented but the generation mechanism has been controversial volume of subaerial sector collapse about 0 4 km was considered too small to generate such a devastating tsunami we compile and analyze recently obtained swath bathymetry data around the island the volcano is more than 2 3 km high from ocean bottom with its base diameter of 18 km the total slide volume both subaerial and submarine is estimated as 2 4 km nearly an order of magnitude larger than the subaerial part to the north of island hummocky terrain of debris deposits extends in an area of 69 km with the total volume of 2 5 km a value similar to the total slide volume it is one of the largest debris avalanches in historic time and probably responsible to the tsunami generation

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

multiple sector collapses at stromboli volcano italy how they work this paper demonstrates that four large sector collapses have affected the nw flank of the stromboli volcano in the past 13 ka alternating with growth phases in order to contribute to the evaluation of the critical conditions which trigger lateral collapses a reconstruction of the geometry of each collapse of the volcano edifice in the four stages that preceded the relative collapse events is also presented and a computation of the landslide volume this reconstruction is based on new field data plotted in three dimensions prior to the initial 13 ka collapse the volcano was 1125 100 m high above sea level the collapse had a volume of 2 23 0 87 km3 whereas the pre collapse volcano volume was 218 8 7 7 km3 the next edifice that failed was 900 70 m high a s l the collapse volume was 1 0 54 km3 with a precollapse volcano volume of 201 4 5 4 km3 the edifice then grew to 1000 60 m a s l the third collapse had a volume of 1 08 0 39 km3 and occurred within a volcano with a volume of 209 1 4 6 km3 this was followed by a new growth phase followed by the last collapse with a volume of 0 73 0 22 km3 the volcano volume was about the same as the present one the present active crater zone is at 780 m a s l in the first three collapses sliding surfaces cut the main magma conduit in the last collapse the upper scarp coincided with the conduit location dyking along a main ne trending weakness zone across the volcano summit exerted a lateral force for collapse inception the decrease of the landslide volumes with the age and the concentric scarps of the four collapses suggest that the younger sliding planes tended to become more superficial and to decrease the areal extent this is interpreted as due to a successively weaker eruptive products from dominantly lavas to dominantly pyroclastics and b the feedback effects between collapses and dykes that injected along the lateral segments of the first collapse slide plane

a case history of paroxysmal explosion at stromboli timing and dynamics of the april 5 2003 event abstract on april 5 2003 stromboli volcano italy produced the most violent explosion of the past 50 years the event was exceptionally well documented thanks to the presence on the island of several scientists and a large number of instruments deployed over the preceding months to monitor the effusive eruption that began in december 2002 integration of visual documentation deposit features and geophysical data allowed an accurate reconstruction of the explosive event and its dynamics the eruption consisted of a 8 min long explosive event which evolved through four phases whose timing was precisely recorded by an infrared thermometer located about 450 m from the summit crater phases 2 and 3 lasted 39 and 42 s respectively both had an impulsive character were responsible for ejecting almost the entire mass of the pyroclastic products phases 1 and 4 represented respectively a short lived precursory event and a waning tale during phase 2 meter sized ballistic blocks were launched with velocities of 170 m s to altitudes of up to 1400 m above the craters these fell on the volcano flanks and on the village of ginostra about 2 km distant from the vent a vertical jet rose above the craters which developed to feed a convective plume that reached a height of up to 4 km the calculated mass of the phase 2 fallout deposit and mass discharge rate were 1 1 1 4 108 kg and 2 8 3 6 106 kg s respectively during phase 3 a scoria flow deposit with an estimated volume of 0 9 1 1 104 m3 was erupted from the same vent that fed the ongoing sustained lava flow the average mass discharge rate for this phase was 2 5 3 1 105 kg s products emitted during phases 2 and 3 consisted of lithic and fresh magmatic material in similar proportions the juvenile fraction consisted of a deep originated almost aphyric highly vesicular pumice mingled with a shallow derived crystal rich moderately vesicular scoria similarities with the eruption dynamics of other historical paroxysms at stromboli makes the april 5 2003 explosion representative of these highly energetic events that constitute the most hazardous volcanic phenomena at stromboli volcano

a review on forearc ophiolite obduction adakite like generation and slab window development at the chile triple junction area uniformitarian framework for spreading ridge subduction this paper aggregates the main basic data acquired along the chile triple junction ctj area 45 48 s where an active spreading center is presently subducting beneath the andean continental margin updated sea floor kinematics associated with a comprehensive review of geologic geochemical and geophysical data provide new constraints on the geodynamics of this puzzling area we discuss 1 the emplacement mode for the pleistocene taitao ridge and the pliocene taitao peninsula ophiolite bodies 2 the occurrence of these ophiolitic complexes in association with five adakite like plutonic and volcanic centers of similar ages at the same restricted locations 3 the inferences from the cooccurrence of these sub coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature pressure p t path low grade greenschist facies overprint and amphibolite eclogite transition respectively 4 the evidences that document ridge jump events and associated microplate individualization during subduction of the scr1 and scr 1 segments the chonos and cabo elena microplates respectively the ridge jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation ophiolite emplacement and adakite like production and location in the ctj area kinematic inconsistencies in the development of the patagonia slab window document an 11 km westward jump for the scr 1 spreading segment at 6 5 to 6 8 ma the scr 1 spreading center is relocated beneath the north patagonia icefield npi we argue that the deep seated difference in the dynamically sustained origin of the high reliefs of the north and south patagonia icefield npi and spi is asthenospheric convection and slab melting respectively the chile triple junction area provides the basic constraints to define the basic signatures for spreading ridge subduction beneath an andean type margin

new paleomagnetic rock magnetic and petrographic results from the valle group baja california mexico exploring the causes of anomalously shallow paleomagnetic inclinations 1 a detailed paleomagnetic rock magnetic and sedimentologic study was conducted on cenomanian age strata of the valle group at el campito baja california to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of dec 342 6 inc 19 1 and 95 3 4 the inclination is 38 7 shallower than that predicted by the reference paleopole for north america laboratory experiments indicate that compaction could account for only 7 5 of the inclination shallowing fold tests at different scales yield both positive and negative results making a tight constraint on the remanence s age impossible a primary postdepositional remanence is supported by bedding parallel foliations in the anisotropy of anhysteretic remanence fabric although primary anomalously shallow inclinations could indicate significant southerly then northerly paleolatitudinal offset a more likely scenario is a late cenozoic low temperature remagnetization which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data the synthesis of the data indicates that the vizcaino terrane was contiguous with the cordillera margin during remagnetization and underwent a subsequent 13 5 5 4 clockwise rotation we tentatively interpret that the remagnetization resulted from burial and or miocene volcanism which caused an acquisition of thermochemical remanent magnetization tcrm this study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

paleoproterozoic subduction within the yangtze craton constraints from nb enriched mafic dikes in the kongling complex abstract niobium enriched mafic rocks constitute a rare suite of rocks formed in the subduction related settings and provide important insights into mantle sources and geodynamic processes associated with plate subduction in this contribution we report newly defined nb enriched mafic rocks from the archean paleoproterozoic kongling complex within the northern yangtze craton which shed new light on the paleoproterozoic subduction process of the craton la icp ms zircon u pb dating reveals two episodes of paleoproterozoic intermediate mafic magmatism 1 the ca 2 15 ga dioritic dikes and the younger nb enriched mafic dikes formed in 2 05 2 03 ga the ca 2 15 ga diorites have high nb contents of 14 7 18 0 ppm but low nb lapm 0 27 0 31 nb u 9 0 to 13 5 and nb thpm values 0 24 to 0 31 the enriched zircon hf isotope hf t 4 8 1 1 and whole rock nd isotope nd t 4 2 3 7 of the older diorites are similar to those of the coeval high mg andesites 2 12 ga from the northern huangling dome indicate an enriched mantle source the geochemical and isotopic data collectively suggest that they were derived from partial melting of a metasomatized sub arc lithospheric mantle at the initial stage of subduction the younger mafic rocks ca 2 05 2 03 ga possess high nb contents 10 9 24 2 ppm and lower magnitude of negative nb anomalies with nb u 17 2 51 6 nb thpm 0 40 0 88 and nb lapm 0 49 0 97 which is different from those of normal island arc rock suites the mafic dikes have negative zircon hf t values 12 6 10 5 dominantly negative nd t values 4 9 to 1 0 except one with 2 5 indicate an enriched mantle source the nb enriched mafic rocks were likely produced by partial melting of mantle wedge that has been modified by the recycled continental sediments along the subduction zone we infer that a slab window regime likely prevailed in the kongling complex at ca 2 05 ga during the subduction termination the opening of a slab window and asthenosphere upwelling possibly triggered partial melting of the enriched mantle wedge and account for the generation of the younger nb enriched mafic dikes

submarine landslide deposits of the historical lateral collapse of ritter island papua new guinea abstract the march 13th 1888 collapse of ritter island in papua new guinea is the largest known sector collapse of an island volcano in historical times one single event removed most of the island and its western submarine flank and produced a landslide deposit that extends at least 70 km from the headwall of the collapse scar we have mapped and described the deposits of the debris avalanche left by the collapse using full coverage multibeam bathymetry side scan sonar backscatter intensity mapping chirp seismic reflection profiles towcam photographs of the seafloor and samples from a single dredge applying concepts originally developed on the 1980 mount st helens collapse landslide deposits we find that the ritter landslide deposits show three distinct morphological facies large block debris avalanche matrix rich debris avalanche and distal debris flow facies restoring the island s land and submarine topography we obtained a volume of 4 2 km3 for the initial collapse about 75 of which is now forming the large block facies at distances less than 12 km from the collapse scar the matrix rich facies volume is unknown but large scale erosion of the marine sediment substrate yielded a minimum total volume of 6 4 km3 in the distal debris flow and or turbidite deposits highlighting the efficiency of substrate erosion during the later history of the landslide movement although studying submarine landslide deposits we can never have the same confidence that subaerial observations provide our analysis shows that well exposed submarine landslide deposits can be interpreted in a similar way to subaerial volcano collapse deposits and that they can in turn be used to interpret older incompletely exposed submarine landslide deposits studying the deposits from a facies perspective provides the basis for reconstructing the kinematics of a collapse event landslide understanding the mechanisms involved in its movement and deposition and so providing key inputs to tsunami models

biochronology paleoenvironments and stratigraphic sequences of the late albian middle eocene fore arc vizcaino basin western baja california mexico the vizcaino fore arc basin accumulated approximately 4 km 13 000 ft of upper albian middle eocene siliciclastic marine sedimentary rocks derived from the peninsular ranges in baja california data from eight exploratory wells document the micropaleontological content and lithological characteristics of these rocks the strata studied represent mostly neritic upper bathyal marine environments and overlie a basement composed of cretaceous granitic rocks or aptian albian volcaniclastic sedimentary rocks correlative with the alisitos formation we recognize four major depositional sequences within the basin that are related to the regional geology the basal albian turonian sequence 1 represents the initiation of fore arc basin sedimentation contains continental conglomerates that change to bathyal shales and correlates with the lower part of the valle group of the vizcaino peninsula sequence 2 is coniacian paleocene includes basal conglomeratic sandstones grading into maastrichtian bathyal shales and usually overlies a coniacian santonian unconformity sequence 2 is represented at the surface by the rosario group in northwestern baja california and the upper part of the valle group in the vizcaino peninsula sequence 3 is paleocene middle eocene represents continuity of fore arc sedimentation in neritic upper bathyal conditions is capped by a major unconformity and correlates with the sepultura and bateque formations to the north and south of the basin respectively the uppermost miocene pliocene sequence 4 is composed of marine sandstone siltstone unconformably overlying sequence 3 and is correlative with the tortugas formation that represents sedimentation after the end of subduction of the farallon plate beneath the north america plate

an evolving tectonic environment of late carboniferous to early permian granitic plutons in the chinese altai and eastern junggar terranes central asian orogenic belt nw china abstract the central asian orogenic belt caob represents one of the most important sites of juvenile crustal growth during the phanerozoic located in the central part of the caob the chinese altai and eastern junggar terranes record the collisional processes between the peri siberian and kazakhstan orogenic systems however the precise timing of collision between the two terranes remains controversial the wukuli and kadelat plutons in the chinese altai belt are dated at 305 and 280 ma respectively whereas the aketas pluton in the eastern junggar terrane is dated at 308 ma granites from the wukuli and kadelat plutons are strongly peraluminous a cnk 1 1 and are characterized by low al 2 o 3 na 2 o mno mgo cao and heavy rare earth element hree contents but with high sio 2 k 2 o and rb contents as well as high rb sr ratios granites from the wukuli pluton have low e nd t and e hf t values of 3 7 to 3 4 and 9 7 to 4 9 whereas those from the kadelat pluton have values of 3 6 to 3 4 and 8 0 to 2 6 these features suggest s type affinity for the wukuli and kadelat plutons with magma derivation through partial melting of mesoproterozoic metasediments the aketas pluton is composed of weakly peraluminous quartz monzonites that have a cnk values ranging from 0 92 to 1 08 with high na 2 o sr and sr y and low y yb nb and ta these rocks display positive e nd t 4 8 to 6 4 and e hf t 9 7 to 14 6 values and low initial 87 sr 86 sr ratios 0 703357 0 703868 similar to modern adakites suggesting that the quartz monzonites were derived from the partial melting of lower crustal material the geochemical characteristics suggest that the aketas pluton was formed in a subduction related setting the wukuli pluton in a syn collisional setting and the kadelat pluton in the subsequent post orogenic strike slip related setting in combination with data from other granitoids in these two terranes the aketas pluton represents the youngest record of subduction related environments suggesting that the final collision between the chinese altai and eastern junggar terranes might have occurred between 308 and 304 ma

a doubly vergent fan structure in the peninsular ranges batholith transpression or local complex flow around a continental margin buttress 1 the jura cretaceous peninsular ranges batholith prb of southern and baja california contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs jura cretaceous deformation in the prb is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith the western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures from 2 to 6 kbar and temperatures from 650 c elsewhere in the north american cordillera similar lithospheric boundaries are thought to accommodate various amounts of orogen parallel displacements of outboard tectonic fragments to test this proposal in the prb we examined an unusually well preserved part of the transition zone in the southern sierra san pedro martir sspm here a spectacular 20 km wide doubly vergent fan structure which developed over a period 40 m y during both jurassic and cretaceous magmatism occurs along the transition zone this structure consists of moderately inward dipping outward vergent fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic vertically sheeted plutons mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center a number of constraints on possible displacements rotation of units and duration of fabric formation limit the amount of orogen parallel displacement to a few kilometers across the fan structure in the sspm instead we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid pre mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex focused deformation along the lithospheric discontinuity and subsequent strain softening we suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression emphasizing the need for caution in interpreting such features as zones of significant orogen parallel displacement

seismic noise tomography in the chile ridge subduction region summary we used cross correlation of ambient seismic noise recorded in the chile triple junction ctj region to estimate interstation surface wave time domain green s functions and then inverted traveltimes to obtain crustal surface wave velocity models interstation distances within the chile ridge subduction project crsp temporary seismic network ranged from 40 to 100 km we selected 365 d and cross correlated and stacked 24 hr of vertical component data at 38 stations pairs resulting in nominally 703 traveltimes along assumed straight interstation paths velocities in 2 d cells of 30 km 30 km were calculated using a linear least squares inversion of the rayleigh wave group velocity traveltimes furthermore we performed a rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3 d shear velocity model of the patagonian crust the process was applied to cross correlation pairs determined in two period bands 5 10 s corresponding to shallow crustal velocities down to approximately 10 km depth and 10 20 s for velocities down to around 20 km our results show that cell velocities correlate well with known geological features we find high crustal velocities where the patagonian batholith outcrops or is likely present at depth and low velocities correlate with the active volcanic arc of the southern volcanic zone and the subducted chile ridge in taitao peninsula where thermal activity of hot springs is present high velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units low velocity in the east correlate with sequences of volcaniclastic deposits

lower plate deformation at the chile triple junction from the paleomagnetic record 45 30 46 s during the ctj cruise geophysical surveys were conducted between 45 s and 48 s in the region of the chile triple junction ctj where the nazca and antarctica plates are subducting beneath the south america plate near the ctj the south chile rise scr which separates the nazca and antarctica lower plates consists of three spreading segments trending n160 separated by a series of parallel fracture zones the active spreading centers of the three segments consist of grabens with various widths and depths bounded by steep fault scarps we provide robust data showing that the scr recorded remote and long term effects of ridge subduction far from the subduction front magnetic profiles multibeam bathymetric and seismic data were acquired at intervals of 13 km along a n80 e direction across the scr during the ctj cruise of r v l atalante deformation of the oceanic lithosphere includes 1 a segmentation of the spreading axes along strike 2 some ridge jumps and 3 local constriction and changes in trend of the fracture zone valleys off axis volcanism is observed in places that may suggest a link with an abnormal stress field induced by ridge subduction the tectonic and volcanic anomalies which occurred in response to the subduction of the scr1 axis may be correlated with geochemical anomalies and slab fragmentation recognized by previous works

sedimentological and geochemical evidence for multistage failure of volcanic island landslides a case study from icod landslide on north tenerife canary islands 1 volcanic island landslides can pose a significant geohazard through landslide generated tsunamis however a lack of direct observations means that factors influencing tsunamigenic potential of landslides remain poorly constrained the study of distal turbidites generated from past landslides can provide useful insights into key aspects of the landslide dynamics and emplacement process such as total event volume and whether landslides occurred as single or multiple events the northern flank of tenerife has undergone multiple landslide events the most recent being the icod landslide dated at 165 ka the icod landslide generated a turbidite with a deposit volume of 210 km3 covering 355 000 km2of seafloor off northwest africa the icod turbidite architecture displays a stacked sequence of seven normally graded sand and mud intervals named subunits sbu1 7 evidence from subunit bulk geochemistry volume basal grain size volcanic glass composition and sand mineralogy combined with petrophysical and geophysical data suggests that the subunit facies represents multistage retrogressive failure of the icod landslide the basal subunits sbu1 3 indicate that the first three stages of the landslide had a submarine component whereas the upper subunits sbu4 7 originated above sea level the presence of thin non bioturbated mud intervals between subunit sands suggests a likely time interval of at least several days between each stage of failure these results have important implications for tsunamigenesis from such landslides as multistage retrogressive failures separated by several days and with both a submarine and subaerial component will have markedly lower tsunamigenic potential than a single block failure

a review of batholiths and other plutonic intrusions of mexico abstract granitic plutons constitute a major portion of the phanerozoic continental crust of mexico with the great majority ca 90 associated to the laramide late cretaceous eocene orogeny and the eastward subduction of the pacific ocean plates as well as to magmatic arcs essentially built since the early mesozoic at the western margin of north america exposed mainly as a wide up to 300 km and over 3000 km long batholithic belt at the mexican pacific margin from baja california to chiapas granitoids conform large intrusive complexes and hundreds of smaller plutons the age of which vary from ca 1400 ma mesoproterozoic to ca 10 ma late miocene in many cases uplift and erosion have revealed the deep roots of the batholiths whereas in other places many intrusions were emplaced in upper crustal environments as suggested by the extremely variable cooling rates of 200 c ma very shallow to 1 10 c ma very deep lithologies and isotopic data indicate unambiguously the central participation of the local lower crust in the genesis of the batholiths and plutons imprinting on them marked petrologic geochemical and structural zoning across the paleozoic paleomargins and through the present nw trending mexican continental edge according to the lithospheric component involved laurentia in the northern and northwestern regions of mexico accreted mesozoic terranes in western mexico and oaxaquia gondwana in eastern and southern mexico major problems related to the evolution of the mexican main granitoids are outlined in time slices for the proterozoic paleozoic permo triassic jurassic early cretaceous late cretaceous paleogene and neogene which represent in mexico major epochs of crustal growth and continental recycling mainly associated not only with pacific north america convergence but also with extensional and transpressional events that altogether marked not only the lithological and structural evolution of most of the country but also its extraordinary mineral wealth finally some preliminary comparisons differences and similarities are made between the mexican batholiths and other plutonic complexes in central asia lhasa terrane and japan

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

coupling eruption and tsunami records the krakatau 1883 case study indonesia the well documented 1883 eruption of krakatau volcano indonesia offers an opportunity to couple the eruption s history with the tsunami record the aim of this paper is not to re analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions indeed though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits they may include juvenile material e g fresh pumice or be interbedded with distal pyroclastic deposits ash fall surges due to their simultaneity with the eruption five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of java and sumatra 1 bioclastic tsunami sands and 2 pumiceous tsunami sands deposited respectively before and during the plinian phase 26 27 august 3 rounded pumice lapilli reworked by tsunami 4 pumiceous ash fall deposits and 5 pyroclastic surge deposits only in sumatra the stratigraphic record on the coasts of java and sumatra which agrees particularly well with observations of the 1883 events is tentatively linked to the proximal stratigraphy of the eruption

emplacement of the resurrection peninsula ophiolite in the southern alaska forearc during a ridge trench encounter the 57 1 ma resurrection peninsula ophiolite of southern alaska s chugach terrane formed in a near trench environment as the kula farallon ridge was being subducted beneath north america the magmatic sequence includes pillow lavas sheeted dikes gabbros trondhjemites and a poorly exposed ultramafic section the lavas show mid ocean ridge basalt and arc like geochemical signatures interpreted to reflect compositionally diverse melts derived from near fractional melting of a variably depleted mantle source mixed with variable amounts of assimilated continentally derived flysch a sedimentary sequence overlying the ophiolite here named the humpy cove formation of the orca group preserves a continuous 3 6 plusmn 1 4 ma record of turbidite sediments that were deposited on the ophiolite as it was transported to north america and emplaced in the chugach accretionary prism a transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent hydrothermal metals are not resolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite suggesting that they were diluted by turbidite sedimentation the top of the section is truncated by the fox island shear zone a 1 km thick greenschist facies west over east thrust related to the emplacement of the ophiolite into the accretionary wedge the fox island shear zone is intruded by a 53 4 0 9 ma granite showing that the ophiolite formed was transported to the north american continent overthrust by a major accretionary prism related thrust and intruded by granite all within 3 6 1 4 ma using these estimates for the timing of formation and emplacement of the ophiolite with palinspastic reconstructions and plate motion vector triangles we suggest that the resurrection peninsula ophiolite was transported up to 500 km 461 79 km relative to north america from the place of its formation at the kula farallon ridge prior to its emplacement into the north american margin since the best estimates of the amount of translation of the chugach terrane come from paleomagnetism of the resurrection peninsula ophiolite current estimates of margin parallel strike slip need to be reduced considerably

new geochemical constraints for the origin of ridge subduction related plutonic and volcanic suites from the chile triple junction taitao peninsula and site 862 leg odp141 on the taitao ridge abstract several features of neogene and quaternary magmatism in the region south of the present day chile triple junction ctj at 46 12 s are directly related to the migration of the triple junction due to the obliquity of the ridge orientation with respect to the subduction front the triple junction migrated from south to north during the last 14 ma the taitao peninsula the westernmost promontory of the chile coast and the taitao ridge a submarine promontory north of the taitao peninsula provide the most complete collection of ridge subduction related magmatic products in the region the emplacement of near trench volcanics the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the chile spreading centre and the continental crust we present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the taitao magmas the new geochemical data were obtained on samples collected from the taitao peninsula during a field expedition in 1995 and from samples of the taitao ridge during leg odp141 site 862 which have been re sampled in 1996 by one of us selected major and trace element compositions of 20 volcanic rocks from the taitao ridge are discussed together with 53 analyses from different rock types from the taitao peninsula including 24 unpublished analyses nd and sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the taitao peninsula together with the oxygen isotope composition of four separated clinopyroxenes six main magmatic types are identified 1 n type morb 2 e type morb 3 lree depleted morb showing some trace element features typical of arc basalts 4 moderately nb depleted e morb 5 calc alkaline andesites dacites and rhyolites and 6 andesites and dacites with adakitic signature chemical similarities exist between some forearc magmas of the taitao ridge and the taitao peninsula and magmas emplaced at the chile active spreading ridge one important result based on isotope data is that the lavas emplaced over the continental crust taitao peninsula did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle derived magmas the likely source of these basalts could be the hot convective oceanic mantle of the southern chile spreading ridge buried at moderate depth 10 30 km

paleozoic to triassic ocean opening and closure preserved in central iran constraints from the geochemistry of meta igneous rocks of the anarak area the anarak area belongs to an ophiolitic belt along the northern border of the central east iranian microcontinent and is thought to contain fragments of the former paleotethys and neotethys oceans a wide range of meta igneous rocks from the late paleozoic to triassic anarak metamorphic complex amc and nearby meraji area have been studied to constrain the origins and modes of emplacement of oceanic remnants in central iran our samples occur as layers and lenses embedded in extensive sequences of deformed meta sediments and smaller bodies of serpentinized ultramafic rocks petrographical and geochemical data combined with field and satellite observations allow recognition of seven types of meta igneous rocks preserved from low grade to blueschist facies conditions their origins based on relative abundances of immobile trace elements include subduction zone mid ocean ridge ocean intraplate and continental rift settings these data and existing geochronological constraints show the amc formed an accretionary complex formed exhumed incrementally during the carboniferous permo triassic and triassic igneous rocks from meraji formed in the early devonian due to opening of the paleotethys and belong to a rift sequence extending over 300 km along the edge of the central east iranian microcontinent the amc and nearby rock associations record the evolution of the paleotethys during a complete wilson cycle between ca 450 and 225 ma with implications for 1 continental rifting 2 ocean opening 3 subduction initiation 4 ocean intraplate and continued mid ocean volcanism 5 ridge subduction and 6 final closure of the ocean during continent continent collision alternate interpretations of the anarak metabasites are possible but require radical departures from the widely accepted model for tectonic evolution of the paleotethys with the existence of paleotethyan backarc basin s and permian or earlier collision of continental blocks in central iran in any case our results show accretionary complexes preserved along suture zones contain an important record of the evolution of oceanic crust from ancient ocean basins

evidence for a ridge subduction event in the ordovician rocks of north central maine modern and past ridge subduction events are characterized by the intrusion of midocean ridge basalt morb magmas into an overlying accretionary prism the fi eld relationships and trace element geochemistry of ordovician mafi c igneous rocks of the weeksboro lunksoos and munsungun anticlinoria of north central maine indicate that they resulted from such an event the bean brook gabbro intrudes the hurricane mountain melange and other related sedimentary strata of continental derivation the gabbro and associated dry way volcanics have morb trace element chemistries while the bluffer pond and stacyville volcanics are more enriched e morb all of which indicate derivation from a mid ocean ridge on petrogenetic diagrams mafi c samples plot in morb fi elds or when th is used as a discriminator in arc fi elds along trends that originate from morb fi elds and extend toward the composition of upper continental crust these trends are consistent with the presence of silicic and metasedimentary xenoliths in the dry way volcanics and bean brook gabbro and indicate the magmas were not subduction products but were contaminated by th rich upper continental crust the nearby chain lakes massif likely represents the basement to the chain lakes microcontinent and the geographic relationship and ages of the massif hurricane mountain melange and dry way bean brook magmatic rocks indicate northwestdirected subduction modern coordinates on the southeastern margin of the chain lakes microcontinent within the early to middle ordovician taconic ocean subduction at this boundary probably terminated because of the ridge subduction analogous to neogene california

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

magmatic tectonic effects of high thermal regime at the site of active ridge subduction the chile triple junction model abstract high thermal gradients are expected to be found at sites of subduction of very young oceanic lithosphere and more particularly at ridge trench trench rtt triple junctions where active oceanic spreading ridges enter a subduction zone active tectonics associated with the emplacement of two main types of volcanic products 1 morb type magmas and 2 calc alkaline acidic magmas in the forearc also characterize these plate junction domains in this context morb type magmas are generally thought to derive from the buried active spreading center subducted at shallow depths whereas the origin of calc alkaline acidic magmas is more problematic one of the best constrained examples of ridge trench interaction is the chile triple junction ctj located southwest of the south american plate at 46 12 s where the active chile spreading center enters the subduction zone in this area there is a clear correlation between the emplacement of magmatic products and the migration of the triple junction along the active margin the ctj lava population is bimodal with mafic to intermediate lavas 48 56 sio 2 and acidic lavas ranging from dacites to rhyolites 66 73 sio 2 previous models have shown that partial melting of oceanic crust plus 10 20 of sediments leaving an amphibole and plagioclase rich residue is the only process that may account for the genesis of acidic magmas due to special plate geometry in the ctj area a given section of the margin may be successively affected by the passage of several ridge segments we emphasize that repeated passages will lead to the development of very high thermal gradients allowing melting of rocks of oceanic origin at temperatures of 800 900 c and low pressures corresponding to depths of 10 20 km depth only in addition the structure of the ctj forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike slip faults the occurrence of such a deformed domain implies that an important tectonic coupling may exist between the upper and the lower plates leading to the partitioning of the continental lithosphere and to the tectonic underplating of very young oceanic lithosphere below the continental wedge we assume that in the case of the ctj the uncommon situation of three successive ridge segments entering the trench at 2 3 ma intervals only resulted in a strong and finally long lived thermal anomaly this anomaly caused remelting of underplated portions of very young still hot oceanic lithosphere only particular geometrical rtt configurations are able to produce such features these include linear continental margin short ridge segments slightly oblique to the trench and short transform faults finally the ctj example shows that a possible scenario for the origin of calc alkaline acidic rocks in the near trench region involves coeval tectonic coupling and repeated passage of thermal anomalies due to successive subduction of short ridge segments therefore the local abundance of calc alkaline acidic rocks associated with morb type lavas in ancient series could be the tracer of plate tectonic configurations involving the subduction of short ridge segments in a relatively short duration

new geologic mapping and shrimp u pb zircon data in the peninsular ranges batholith baja california mexico evidence for a suture geologic and shrimp u pb zircon studies in the sierra san pedro martir area of northern baja california mexico suggest that the western and eastern parts of the peninsular ranges batholith originated as separate arcs they are now juxtaposed along a well exposed ductile thrust main martir thrust marked by an age discontinuity of at least 10 to 15 m y and sharp changes in rock type metamorphic grade and deformation history the east dipping thrust and adjacent highly deformed rocks were stitched by 108 97 ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs lack of chemical isotopic and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin or development of the arc on a prism of continentally derived sediments adjacent to the continental margin instead the western arc may have originated as an island arc above a subduction zone outboard of north america if so a second concurrent subduction zone along the north american margin must have driven convergence and suturing of the two arcs and the main martir thrust may mark a nonterminal suture within a wider convergence related deformation zone that formed ca 115 108 ma

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

devonian volcanic rocks of the southern chinese altai nw china petrogenesis and implication for a propagating slab window magmatism induced by ridge subduction during accretionary orogenesis abstract ridge trench interaction is a common tectonic process of the present day pacific rim accretionary orogenic belts and this process may facilitate slab window magmatism that can produce significant thermal anomalies and geochemically unusual magmatic events however ridge trench interaction has rarely been well documented in the ancient geologic record leading to grossly underestimation of this process in tectonic syntheses of plate margins the chinese altai was inferred to have undergone ridge subduction in the devonian and a slab window model is proposed to interpret its high temperature metamorphism and geochemically unique magmatic rocks which can serve as an excellent and unique place to refine the tectonic evolution associated with ridge subduction in an ancient accretionary orogeny for this purpose we carried out geochemical and geochronological studies on devonian basaltic rocks in this region secondary ion mass spectrometry sims zircon u pb dating results yield an age of 376 2 2 4 ma suggesting an eruption at the time of late devonian geochemically the samples in this study have variable sio2 43 3 58 3 wt low k2o 0 02 0 07 wt and total alkaline contents 2 16 5 41 wt as well as fe2o3t mgo ratios showing typical tholeiitic affinity on the other hand the basaltic rocks display morb like ree patterns la yb n 0 90 2 57 and ga yb n 0 97 1 28 and have moderate positive end t values 4 4 to 5 4 which collectively suggest a derivation from a mixing source comprising morb like mantle of a mature back arc basin and subordinate arc mantle wedge these basaltic rocks are characterized by low la yb 1 26 3 69 dy yb 1 51 1 77 and sm yb 0 83 1 32 ratios consistent with magmas derived from low degree 10 partial melting of the spinel lherzolite source at a quite shallow mantle depth considering the distinctive petrogenesis of the basaltic rocks in this region the late devonian basalts in the southern chinese altai is suggested to have witnessed the propagating process of slab window magmatism that was induced by ridge subduction in a nascent rifting stage of a back arc basin

chronological constraints on the thermal and tilting history of the sierra san pedro martir pluton baja california mexico from u pb 40ar 39ar and fission track geochronology the tectonothermal history of the four major phases of the sierra san pedro martir pluton and surrounding metamorphic rocks of the mesozoic peninsular ranges batholith of baja california is presented on the basis of u pb 40 ar 39 ar step heating and fission track dating in combination with al in hornblende geobarometry a previous model proposed up to 90 of east side up tilting of the pluton exposing 20 km of crustal section to account for its crescent shape asymmetrical zoning internal structure the eastward younging of k ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks from greenschist to amphibolite facies the u pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97 0 4 1 ma and 93 8 1 1 ma all except one of the 105 40 ar 39 ar age spectra have well defined plateaus and are interpreted as cooling ages samples from the pluton give hornblende and biotite 40 ar 39 ar plateau dates and apatite fission track dates that young from west to east thus hornblende dates decrease from 95 to 91 ma biotite dates decrease from 94 to 88 ma and apatite dates decrease from 72 to 57 ma muscovite biotite and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40 ar 39 ar dates of 88 ma the exposed part of the pluton underwent rapid cooling approximately equal to 40 c ma down to 250 c in the first 10 m y after intrusion modeling of track length distribution in apatite is consistent with monotonic slow cooling from ca 80 ma to the present the data do not support a history that includes major tilting of the pluton eastward younging of 40 ar 39 ar and fission track dates may be explained by approximately equal to 15 of east side up tilting of the pluton at or after 88 ma about a north south horizontal axis furthermore the fission track data suggest that part or all of this tilting may have taken place at or after 57 ma and therefore may be a consequence of regional scale crustal extension associated with the opening of the gulf of california in neogene time such tilting is in agreement with the al in hornblende geobarometry for the hornblende biotite intrusive phase that yields pressures of 5 2 0 6 kbar an approximately equal to 15 northeast side up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic north america and militates against large scale northerly transport of baja california

geology and eruptive history of unzen volcano shimabara peninsula kyushu sw japan abstract during the past 500 thousand years unzen volcano an active composite volcano in the southwest japan arc has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben the volcano can be divided into the older and the younger unzen volcanoes the exposed rocks of the older unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200 300 ka drill cores recovered from the basal part of the older unzen volcano are dated at 400 500 ka the volcanic rocks of the older unzen exceed 120 km3 in volume the younger unzen volcano is composed of lava domes and pyroclastic deposits mostly younger than 100 ka this younger volcanic edifice comprises nodake myokendake fugendake and mayuyama volcanoes nodake myokendake and fugendake volcanoes are 100 70 ka 30 20 ka and

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

the storegga slides evidence from eastern scotland for a possible tsunami abstract the second storegga slide on the continental slope off western norway has been dated at between 8000 and 5000 yrs b p a prominent sand layer in flandrian holocene deposits along the coast of eastern scotland and dated at approximately 7000 yrs b p may have been deposited by a tsunami generated by the slide the altitude and stratigraphy of the layer allow estimates to be made of the magnitude of the earthquake which initiated the slide

the landslides and tsunamis of the 30th of december 2002 in stromboli analysed through numerical simulations on the 30th of december 2002 two tsunamis were generated only 7 min apart in stromboli southern tyrrhenian sea italy they represented the peak of a volcanic crisis that started 2 days before with a large emission of lava flows from a lateral vent that opened some hundreds of meters below the summit craters both tsunamis were produced by landslides that detached from the sciara del fuoco this is a morphological scar and is the result of the last collapse of the northwestern flank of the volcanic edifice that occurred less than 5 ka b p the first tsunami was due to a submarine mass movement that started very close to the coastline and that involved about 20 106 m3 of material the second tsunami was engendered by a subaerial landslide that detached at about 500 m above sea level and that involved a volume estimated at 4 9 106 m3 the latter landslide can be seen as the retrogressive continuation of the first failure the tsunamis were not perceived as distinct events by most people they attacked all the coasts of stromboli within a few minutes and arrived at the neighbouring island of panarea 20 km ssw of stromboli in less than 5 min the tsunamis caused severe damage at stromboli

geochemical diversity of late miocene volcanism in southern baja california mexico implication of mantle and crustal sources during the opening of an asthenospheric window abstract five main petrologic and geochemical groups can be identified among the middle to late miocene lavas from the western part of southern baja california 1 calc alkaline and k rich andesites emplaced between 15 5 and 11 7 ma 2 adakites and 3 associated niobium rich basalts erupted between 11 7 and 8 5 ma in the santa clara volcanic field vizcaino peninsula 4 10 6 9 2 ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near san ignacio and 5 magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11 7 ma south of san ignacio and between 9 7 and 8 8 ma near la purisima these lavas although spatially and temporally related display very different geochemical signatures their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface these magmas were eventually trap

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

glacial interglacial trench supply variation spreading ridge subduction and feedback controls on the andean margin development at the chile triple junction area 45 48 s during the chile triple junction ctj cruise march april 1997 em12 bathymetry and seismic reflection data were collected in the vicinity of the chile triple junction 45 480s where an active spreading ridge is being subducted beneath the andean continental margin results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction erosion to subduction accretion the andean continental margin and the chile trench exhibit a strong segmentation which reflects the slab segmentation and the chile triple junction migration three segments were identified along the andean continental margin the presubduction the synsubduction and the postsubduction segments from north to south both climate induced variations of the sediment supply to the trench and the tectonic reorganization at the nazca antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin along the survey area we infer the succession of two different periods during the last glacial interglacial cycle a glacial period with ice rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the andean ice cap retreat allowed the andes to be drained off during these warm periods rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction accretion 1 along the presubduction segment after the last deglaciation and 2 along the postsubduction segment after the interglacial episode at 130 117 ka conversely a nonaccretion or subduction erosion mode characterized the presubduction and postsubduction segments during glacial maximums the major effects of subduction of the buoyant chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the nazca antarctica plate boundary we suggest that a postsubduction westward jump of the chile ridge occurred during the past 780 kyr it produced slab fragmentation and individualization of an ephemeral microplate north of the taitao fracture zone the chonos microplate in 780 kyr two episodes of subduction accretion separated by an episode of subduction erosion occurred in relation with the chonos microplate individualization and subduction the current northward migration of the triple junction along the chonos microplate south america plate boundary introduces a sharp change in the tectonic mode from subduction erosion to the north to subduction accretion to the south the data collected along the taitao ridge have revealed the complex three dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite the taitao ridge ophiolite no connection exists between the taitao ridge ophiolite and the bahia barrientos ophiolite cropping out onland in the taitao peninsula

recent eruptive history of stromboli aeolian islands italy determined from high accuracy archeomagnetic dating 1 the large sample method of archeomagnetic dating was applied to stromboli 127 samples 10 sites yielded paleofield directions with 95 confidence intervals less than 2 coupled with a reference curve for secular variation in western europe these allow accurate dating of volcanic events a lava flow underlying san bartolo village gave a minimum age of ad 100 100 a lava overflow predating the last sector collapse was dated from 1350 60 suggesting the recent occurrence of highly hazardous events lava spatters and a hot avalanche high on the northern flank were emplaced during the xxth century lava spatters on the lower flanks could date the onset of the still ongoing phase from ad 550 50 these results are different from those obtained in a recent study probably because traditional paleomagnetic sampling cannot yield sufficient precision considering the characteristics of the archeomagnetic secular variation curve

miocene to late quaternary patagonian basalts 46 47 s geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction miocene to quaternary large basaltic plateaus occur in the back arc domain of the andean chain in patagonia they are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the south chile ridge scr we have investigated three volcanic centres from the lago general carrera buenos aires area 46 47 s located above the inferred position of the slab window corresponding to a segment subducted 6 ma ago 1 the quaternary rio murta transitional basalts display major trace elements and sr and nd isotopic features similar to those of oceanic basalts from the scr and from the chile triple junction near taitao peninsula e g 87sr 86sr o 0 70396 0 70346 and end 5 5 3 0 we consider them as derived from the melting of a chile ridge asthenospheric mantle source containing a weak subduction component 2 the plio quaternary 1 and tio2 2 wt these intermediate basalts differ from their strictly alkalic equivalents by having lower high field strength element hfse and higher end up to 5 4 these features are consistent with their derivation from an enriched mantle source contaminated by ca 10 rutile bearing restite of altered oceanic crust the petrogenesis of the studied mio pliocene basalts from mlba and mcc is consistent with contributions of the subslab asthenosphere the south american subcontinental lithospheric mantle and the subducted pacific oceanic crust to their sources however their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment scr 1 which entered the trench at 6 ma indeed magmatic activity was already important between 12 and 8 ma in mlba and mcc as well as in southernmost plateaus i e 6 ma before the subduction of the scr 1 segment we propose a geodynamic model in which oib and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear in the slab which formed when the southernmost segments of the scr collided with the chile trench around 15 ma during their ascent they interacted with the patagonian supraslab mantle and locally with slivers of subducted pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region

tracking dynamics of magma migration in open conduit systems open conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi permanent volcanic activity this persistent activity limits our ability to read changes in the monitored parameters making the assessment of possible eruptive crises more difficult we show how an integrated approach to monitoring can solve this problem opening a new way to data interpretation the increasing rate of explosive transients tremor amplitude thermal emissions of ejected tephra and rise of the very long period vlp seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate during the 2014 flank eruption of stromboli this magma input preceded the effusive eruption by several months when the new lateral effusive vent opened on the sciara del fuoco slope the effusion was accompanied by a large ground deflation a deepening of the vlp seismic source and the cessation of summit explosive activity such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent we show how this model successfully reproduces the measured rate of effusion the observed rate of ground deflation and the deepening of the vlp seismic source this study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open conduit system and to track magma drainage during the effusive crisis with a great impact on hazard assessment