extension subsidence and thermal evolution of the los angeles basin a two dimensional model abstract the los angeles basin is a narrow deep basin containing large oil fields its formation began in the middle miocene and its subsidence has been extraordinarily rapid we suggest that although the basin is in a predominantly strike slip tectonic environment the subsidence is largely a result of crustal extension simple one dimensional extension models ignoring the lateral flow of heat are inadequate to explain the rapidity of the subsidence we use a two dimensional numerical method to model the thermal evolution of the basin based on our model studies the central southwest northeast and palos verdes blocks are inferred to have been extended by 178 69 20 and 27 respectively the average crustal extension for the whole basin is 66 the lateral flow of heat out of the central block into adjacent cooler blocks accounts for its rapid subsidence during the early stage the deepest sediments are predicted to have been heated to about 210 c hydrocarbon maturity calculations indicate that the upper miocene sediments reached oil maturity a little over 4 m y b p no sediment is predicted to have achieved natural gas maturity

geomagnetism and age study of tyrrhenian seamounts summary we discuss the results of measurements of the magnetic anomaly field of several seamounts located on the ocean floor and on the bordering continental slopes of the tyrrhenian sea this geomagnetic study of seamounts is based on joint 3 d analyses of the magnetic anomalies and the bathymetry on measurements of the magnetic properties of rock specimens and on geochronological data the 3 d analysis of the magnetic anomalies of large volcanoes emplaced in the small oceanic basins of vavilov and marsili serves to shed light on the spatial distribution of the magnetized bodies by eliminating the effect of topography a new magnetic anomaly map of the southern tyrrhenian sea is obtained by the merging of shipborne local data measured on topographic highs with data of regional type the study of the magnetic patterns and age of the oceanic seamounts indicates that the volcanic activity mainly developed prior to chron c2 2 02 ma bp on the vavilov and magnaghi edifices vavilov basin whereas the volcanism of a big oceanic seamount emplaced in the small basin of marsili took place mainly during the period of chron c1 0 0 78 ma bp

hydrothermal activity in the se tyrrhenian sea an overview of 30 years of research subduction related volcanic activity in the southeast tyrrhenian back arc basin has resulted in a large variety of calcalkaline rocks ranging from arc tholeiitic to shoshonitic and rhyolitic composition which were emplaced in both oceanic and continental crust hydrothermal systems in the se tyrrhenian sea have leached these magmatic rocks generating a wide range of oxyhydroxide and sulfide deposits all the hydrothermal occurrences formed at much shallower depths than those from the mid ocean ridges oxyhydroxide deposits metalliferous sediments crusts and nodules are almost pure hydrothermal formations with insignificant hydrogenetic and diagenetic contributions they exhibit strong separation of mn rich and fe rich end members with mn rich occurrences dominant their geochemistry suggests a relative enrichment of mn in the hydrothermal systems of the se tyrrhenian the sulfides are zn and pb rich but poor in cu and relatively poor in fe this suggests either deep seated magma chambers or boiling of the hydrothermal fluids beneath the seafloor as a consequence 1 cu and a large proportion of the fe are deposited as sulfides in the basement rocks and sediment cover 2 the hydrothermal fluids become relatively enriched in mn zn and pb a number of accessory sulfosalts and selenides not typical of mid ocean ridge massive sulfides are present

tectono stratigraphic modelling of the north sicily continental margin southern tyrrhenian sea abstract a two dimensional numerical modelling that simulate the kinematic and thermal response of the lithosphere to thinning was used for the quantitative reconstruction of the late neogene to recent times tectonic and stratigraphic evolution of the north sicily continental margin southern tyrrhenian sea the numerical study of the evolution of the north sicily margin builds on the crustal image and kinematic interpretation of the margin obtained by pepe et al tectonics 19 2000 241 on the basis of seismic data and gravity modelling tectonic modeling indicate that different segments of the margin were undergoing different vertical movements which are mainly expression of the rifting and thinning of the lithosphere occurred during tectonic evolution of the southern tyrrhenian sea a prediction of the pre rift basement topography and the moho along the margin converges to a value of 6 5 km for the depth of necking and a temperature dependent eet 500 isotherm the model fails to reproduce the morphology of the solunto high confirming its non extensional origin a polyphase evolution is required to reproduce the observed syn and post rift stratigraphy during the first rifting stage between 9 and 5 ma crustal thinning factors reach maximum values of 1 27 in the cefalu basin a similar value is predicted for the subcrustal thinning around 60 km nne of the profile margin crustal thinning factors increase during the second rifting stage from 4 to 2 ma and reach values of 2 and up to 3 5 in the cefalu basin and in the continent oceanic transition zone respectively similarly subcrustal lithospheric thinning factors reach values up to 2 5 in the distal sector of the margin an uplift of more than 100 m is predicted for the north sicily shelf and surrounding onshore areas during the post rift stage the evolution of thermal structure with time is very sensitive to the partial thinning factors describing the evolution of the thinning itself during time the lithosphere preserved part of its strength during extension the effective elastic thickness eet along the margin through time is 24 km at the onset of rifting and reaches values less to 8 km during the second rifting stage in the northeastern end of the margin

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

verifying discontinuous deformation analysis simulations of the jointed rock mass behavior of shallow twin mountain tunnels abstract this study extends discontinuous deformation analysis dda to simulate the interactions of twin tunnels in a jointed rock mass the ground subsidence and the rock stress distribution around the tunnels from the dda simulation are compared to those from the trap door model both the experimental and computational results show that the changing tunnel interactions in the jointed rock mass between the two tunnels are coincidently associated with the tunnel distance in addition the dip angle of the rock strata significantly affects the surface subsidence patterns and the stress distribution around the excavation zone therefore the dda method is a useful tool for describing the mechanical behavior of a jointed rock mass when a twin tunnel is constructed in mountain terrain

seismic study of the crust of the northern red sea and gulf of suez we report the results of fifteen expanding spread profiles esp s and a seismic wide angle reflection refraction une performed during march april 1986 in the gulf of suez and the egyptian part of the northern red sea area north of 25 n four 16 4 air guns were used as a sound source on board r v le suroit and a 96 channel 2 4 km long streamer was towed by a supply vessel the whity tide most of the profiles show good crustal reflection and refraction arrivals and often good moho arrivals obtained for a distance of 80 to 100 km we present the results of x t and p analysis obtained by a velocity inversion performed in the p plane and by ray tracing modeling of both the p and the x t sections the velocity models are computed for planar and linear gradient velocity layers the northernmost part of the red sea appears to be characterized by a continental type crust extremely thinned 4 lying at a mean depth of 7 8 km whereas the moho is at a mean 14 15 km depth the southern part shows a seismic velocity structure of an oceanic type except in the 40 km closest to the coastline in both parts seismic waves progressively get more attenuated with distance from the shore to the axial zone which is presently tectonically active the difference between the northern continental and southern oceanic zone is related to the termination of the levant fault the northern continental area appears to represent the termination of the levant fault as a zone of distributed deformation

effect of rock mass and discontinuity mechanical properties and delayed rock supporting on tunnel stability in an underground mine abstract in this paper the rock mass stability around the tunnels in an underground mine is investigated using the distinct element method a three dimensional model was developed based on the available geological geotechnical and mine construction information it incorporates the inclined lithologies including a weak interlayer the persistent and non persistent faults and a complex tunnel system the strain softening constitutive model was prescribed for the rock masses in the numerical model the continuously yielding joint model was used to describe the non linear behavior of the faults to account for the deformations occurred between the excavations and rock supporting the delayed installation of supports was implemented by using the stress relaxation method numerical analyses were performed to study the effects of the post failure constitutive parameters of the rock masses the mechanical properties of faults and the delayed supporting on tunnel stability the following results are presented a the rock mass deformations and failure zones around the excavations b the joint shear displacements of the major fault and c the failure conditions of the applied supports satisfactory comparisons are obtained between the predictions based on numerical modeling and the field deformation measurements the analysis procedures presented in this study are applicable for stability assessment of various underground rock excavation projects located in different parts of the world having different geological settings and complex engineering situations

heat flow in the balearic and tyrrhenian basins western mediterranean we present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the balearic and tyrrhenian basins in the western mediterranean analysis of 12 measurements within a 10 km radius from 40 01 4 55 e in the balearic abyssal plain shows a mean heat flow of 92 10 mw m 2 after correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late oligocene age proposed for the basin a similar survey with 18 measurements around 40 16 n 11 19 e in the western tyrrhenian gives a flux of 134 8 mw m 2 while the third survey of 26 measurements in the southern tyrrhenian abyssal plain at 37 17 n 12 58 e yields a slightly higher heat flow of 151 10 mw m 2 these values are within the range predicted by simple plate cooling models for the late miocene ages of the deep tyrrhenian basin thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins the thermal signature is similar also the trend of increasing heat flow from west to east through the balearic and tyrrhenian basins is in agreement with models of the formation of the western mediterranean behind an eastwardly migrating trench system in all three areas the measured flux shows significant local variability in the two westernmost surveys this can be attributed to the presence of buried high conductivity salt structures but in the third area se tyrrhenian the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone instead evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area

modeling the effect of water excavation sequence and rock reinforcement with discontinuous deformation analysis abstract a powerful numerical method that can be used for modeling rock structure interaction is the discontinuous deformation analysis dda method developed by shi in 1988 in this method rock masses are treated as systems of finite and deformable blocks large rock mass deformations and block movements are allowed although various extensions of the dda method have been proposed in the literature the method is not capable of modeling water block interaction sequential loading or unloading and rock reinforcement three features that are needed when modeling surface or underground excavation in fractured rock this paper presents three new extensions to the dda method the extensions consist of hydro mechanical coupling between rock blocks and steady water flow in fractures sequential loading or unloading and rock reinforcement by rockbolts shotcrete or concrete lining examples of application of the dda method with the new extensions are presented simulations of the underground excavation of the unju tunnel in korea were carried out to evaluate the influence of fracture flow excavation sequence and reinforcement on the tunnel stability the results of the present study indicate that fracture flow and improper selection of excavation sequence could have a destabilizing effect on the tunnel stability on the other hand reinforcement by rockbolts and shotcrete can stabilize the tunnel it is found that in general the dda program with the three new extensions can now be used as a practical tool in the design of underground structures in particular phases of construction excavation reinforcement can now be simulated more realistically however the method is limited to solving two dimensional problems

rapid subsidence of small oceanic basins a study of the marsili basin central mediterranean many small backarc basins show subsidence rates significantly greater than those in major ocean basins of similar ages in this study we test by numerical modeling the hypothesis that small backarc basins are formed by the same fundamental processes as major oceanic basins i e by accretion spreading and cooling and that the rate of subsidence is modified and accelerated by lateral heat loss from the hot subbasinal asthenosphere to the surrounding cooler continental lithosphere bathymetric seismic and drilling data in the marsili basin a small and young backarc basin in the central mediterranean are used to constrain the models the results of this analysis confirm the above hypothesis they further suggest that for actively spreading backarc basins the most rapid subsidence occurs in a zone near the margin of the basin during the earliest stage of basin spreading whereas after spreading terminates the location for rapid subsidence migrates towards the basin center

potential field modeling of collapse prone submarine volcanoes in the southern tyrrhenian sea italy 1 hydrothermal alteration may weaken volcanic rocks causing the gravitational instability of portions of active volcanoes with potentially hazardous collapses here we show high resolution multibeam magnetic and gravity surveys of the marsili seamount the largest active volcano of europe located in the southern tyrrhenian back arc basin these surveys reveal zones with exceptionally low densities and with vanishing magnetizations due probably to the comminution of basalts during hyaloclastic submarine eruptions and to their post eruptive hydrothermal alteration the location of these regions correlates with morphological data showing the occurrence of past collapses similar evidence has been obtained from pre existing data at vavilov seamount another older volcanic system in the tyrrhenian back arc basin here a large volume of at least 50 km3 may have collapsed in a single event from its 40 km long western flank given the similarities between these volcanoes a large collapse event may also be expected at marsili

magnetic evidence for the nature of the crust beneath the southern red sea magnetic data over the southern red sea are used to examine various models for the evolution of the region the linear nature of the magnetic anomalies over the coastal margins and adjacent portions of the main trough together with their decrease in amplitude with elevation is shown to be compatible with deep thin sources that are strongly magnetized the general lack of correspondence between gravity anomalies and deskewed magnetic anomalies indicates that the anomalies are due to intrabasement bodies rather than basement relief associated with density contrasts across axis correlations of individual anomalies indicate a symmetrical disposition of magnetic sources about a line close to the present deep water axis the regular symmetrical pattern of linear anomalies strongly suggests a seafloor spreading origin the combined geologic and geophysical evidence therefore supports a model in which the margins are underlain by material more typically oceanic in character than continental the magnetic evidence further implies that this oceanic crust formed during an earlier phase of spreading in the red sea although the age of this crust is unknown anomaly correlations suggest spreading may have taken place during the late oligocene early miocene

heat flow and lithospheric thinning in the ligurian basin n w mediterranean we present preliminary results of 78 new heat flow measurements collected on a transect across the central ligurian basin south france to corsica margins the basin wide flux has an average of 79 21 mwm 2 with significant local and regional variability correction for the deposition of up to 5 km of post rift sediment gives a mean value of 102 mwm 2 which is similar to values for normal oceanic crust of the same age 21 ma the total tectonic subsidence tts calculated for the region with probable oceanic crustal structure is comparable to or somewhat greater than 21 ma crust of the major ocean basins the heat flow data overall are compatible with the observed tts in showing substantial lithospheric thinning but do not tightly constrain thermal models of continental extension and rifting

subsidence of the shikoku back arc basin abstract sources of basaltic tephras found in miocene cores at dsdp sites 443 and 444 in the eastern side of shikoku basin are likely to be the kinan seamounts which were emergent during middle to early late miocene abnormally high vesicularity of dredged and dsdp pillow basalts together with existence of a thin layer of limestone or chalk overlying the basalts seem to indicate that the axial zone of shikoku basin was much shallower than the present depth immediately after solidification of the basement and that it subsided quickly shallow water depths of the back arc basins are characteristic of the present lau basin and okinawa trough whereas the mariana trough possesses water depths about 1000 m deeper than normal oceans of the same age most of inactive back arc basins have depth age relations consistent with that of mariana trough very shallow water depths of the axial zone of back arc basins similar to those of the shikoku basin axis during their opening period appear to be due to non isostatic uplift caused by excess mass and to isostatic elevation by buoyancy of hot partially molten materials accumulated above the subducting slab

numerical simulation on excavation induced damage of rock under quasi static unloading and dynamic disturbance during the excavation of underground opening the rock may experience a complex loading path that includes the highly confined compression before excavation unloading of confining stress and further disturbance of dynamic loading after excavation by using rock failure process analysis for dynamics rfpa dynamics the failure of rock sequentially subjected to this complex loading path is numerically simulated in order to examine the rock failure mechanism induced by excavation the rfpa dynamics is firstly used to reproduce the failure of rock under confined compression followed by unloading of confining pressure and it is validated against with the existing experimental observation then the failure characteristics of rock specimen sequentially subjected to the quasi static triaxial loading unloading of confining pressure and dynamic disturbance are numerically simulated where the effect of magnitude of axial loading and confining pressure and duration and amplitude of the dynamic disturbance on the final failure patterns of rock are examined the numerical results indicate that the arc shaped spalling damage zone is prone to develop with the increase in the axial pressure and lateral pressure coefficient as for the effect of dynamic disturbance the contribution of duration and amplitude of dynamic disturbance on the energy input are similar where the area of damage zone increases with the energy input into the rock specimen in this regard the area of the damage zone is influenced by both the magnitude of in situ stress and waveform of dynamic disturbance this study denotes that it is of great significance to trace the complex loading path induced by excavation in order to capture the rock failure mechanism induced by underground excavation

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

numerical methods in rock mechanics the purpose of this civilzone review paper is to present the techniques advances problems and likely future development directions in numerical modelling for rock mechanics and rock engineering

an implicitly coupled hydro geomechanical model for hydraulic fracture simulation with the discontinuous deformation analysis in this article we present and verify an algorithm for modeling hydraulic fracturing in complex fracture geometries the method is built upon an existing model which uses the discontinuous deformation analysis for simulation of rock mechanics and upon a finite volume fracture network model for simulation of compressible fluid flow in fractures improvements are made to the fluid contact and coupling components of the existing algorithm to increase its accuracy and stability the model is successfully benchmarked against the analytical solution for the opening profile of an internally pressurized griffith fracture and against the semi analytical solution for the pressure and opening profile of a bi wing hydraulic fracture propagating under conditions of no leakoff and no toughness additionally the model is verified through comparison with the results of a hydraulic fracturing experiment that examined propagation of a high viscosity fracturing fluid in an impermeable medium

a numerical procedure for unsaturated seepage analysis in rock mass containing fracture networks and drainage holes abstract it is computationally difficult to model unsaturated flow through a highly fractured rock system that contains porous rock matrix densely distributed fracture networks and narrowly spaced drainage holes we present an efficient numerical procedure that uses a three dimensional finite element model to solve richards equation with embedded zero thickness elements for fractures and line elements for drainage holes a mixed boundary condition is applied at potential seepage surfaces and a water retention relation with extremely narrow unsaturated zone is specified to precisely locate the phreatic surface the procedure compares favorably with results from other numerical solutions and experimental data a case study is developed to evaluate seepage through a densely fractured rock slope the simulated results demonstrate that fractures disorder the hydraulic potential distribution with fracture patterns acting as significant influential factors and drainage holes dramatically lower the phreatic surface due to the effects of drainage and depressurization the proposed numerical procedure shows to be capable of investigating the unsaturated flow in highly fractured rock mass

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

thermal and mechanical evolution of u s atlantic continental margin abstract the postrifting tectonic subsidence of the u s atlantic continental margin appears to be due to thermal contraction following continental breakup this subsidence is greatly increased by the effect of sediment loading models for the response of the lithosphere to loads show that the rigidity of the lithosphere is a strong function of temperature hence the flexural rigidity at the u s margin should vary in both space and time the thermal structure and subsidence of the margin has been modelled using a two dimensional extension model where the amount of extension has been allowed to vary across the margin and lateral conduction of heat has been included the equivalent elastic thickness of the lithosphere has been approximated as the depth to an isotherm thermoelastic effects of the cooling lithosphere as well as the flexural loading of the sediments have been included the lateral flow of heat and flexure have contrasting effects the lateral heat flow tends to cause uplift of the coastal plain region landward of the hinge zone and to increase the subsidence rate in the regions of the transitional crust in addition the effects of lateral heat flow modify the thermal history of the sediments and heat flow flexure on the other hand causes subsidence of the coastal plain and decreases the sediment accumulation beneath the transition zone the stratigraphy of the u s margin is seen to be the result of the complex interaction of these effects as their contributions vary through the history of the margin end of article last page 1671

calc alkaline magmatism and rifting of the deep water volcano of marsili aeolian back arc tyrrhenian sea abstract geochemistry and mineral chemistry data indicate that new lava samples recovered from a large depth interval of marsili volcano southern tyrrhenian sea are calc alkaline the deep to intermediate portions of the volcano are made up by calc alkaline basalts whereas the summit consists of high potassium calc alkaline andesites the contents of mgo cao total iron cr ni and v decrease with increasing silica while k 2 o rb ba th nb and lree exhibit the opposite behavior the andesite lavas of the top at waterdepth shallower than about 800 m are younger than 0 2 ma composition of the basalts varies with the waterdepth overall the deepest products of marsili are geochemically more mafic than those erupted in the higher physiographic positions geochemical features suggest the existence of a mantle source that was modified by metasomatic acquirement of subduction derived incompatible elements the seamount lies on oceanic crust which formed above subducting lithosphere not earlier than about 2 ma ago it is currently undergoing extension along linear faults the major distensional fractures trend n s to n15 e paralleling the physiographic elongation of the volcano at the summit later faults have an oblique ne sw orientation which is associated with the eruption of high potassium andesites the transition from basalts to high potassium andesites is discussed as a possible response to the extension and rapid subsidence that dominate the volcano and surrounding region

late oligocene to recent episodes of magmatism in and around the tyrrhenian sea implications for the processes of opening in a young inter arc basin of intra orogenic mediterranean type abstract five episodes of magmatic activity as deduced from geochronological data affected the tyrrhenian and peri tyr rhenian areas from the late oligocene 32 ma to recent times the episodes of w e migration of magmatism from the islands of sardinia and corsica to the eolian islands and the roman comagmatic region of central italy are related to the anti clockwise drift movements of the apenninic chain and clockwise drifting of the sicilian part of the maghrebian chain magmatic activity in the tyrrhenian sea initiated in late miocene times 8 5 6 5 ma in concomitance with a significant change of the geodynamic regime from compressional to tensional the tyrrhenian bathyal plain was affected by two episodes of spreading the first episode occurred around 7 ma and the second from about 1 9 to 1 3 ma these spreading events apparently developed during times of magmatic quiescence in the peri tyrrhenian areas two subsequent episodes of volcanism of the seamount type occurred in the bathyal plain one from 5 to 1 8 and the other from 1 3 to recent they are contemporaneously with extensive manifestations in the peri tyrrhenian areas the different types of magmatism of the late miocene back arc spreading in the central bathyal plain and acidic manifestations of intra crustal anatectic origin in the northern continental platform and slope i e islands of capraia elba and montecristo and seamount vercelli are possibly related to movements of upwelling of the astenosphere which in the southern tyrrhenian sea developed with a much stronger intensity timing of the volcanism in relationship with the overall structural evolution through time in the basin and neighbouring lands is considered

predictions of hydrothermal alteration within near ridge oceanic crust from coordinated geochemical and fluid flow models coordinated geochemical and hydrological calculations guide our understanding of the composition fluid flow patterns and thermal structure of near ridge oceanic crust the case study presented here illustrates geochemical and thermal changes taking place as oceanic crust ages from 0 2 to 1 0 myr using a finite element code we model fluid flow and heat transport through the upper few hundred meters of an abyssal hill created at an intermediate spreading rate we use a reaction path model with a customized database to calculate equilibrium fluid compositions and mineral assemblages of basalt and seawater at 500 bars and temperatures ranging from 150 to 400 c in one scenario reaction path calculations suggest that volume increases on the order of 10 may occur within portions of the basaltic basement if this change in volume occurred it would be sufficient to fill all primary porosity in some locations effectively sealing off portions of the oceanic crust thermal profiles resulting from fluid flow simulations indicate that volume changes along this possible reaction path occur primarily within the first 0 4 myr of crustal aging

tectonic focusing of voluminous basaltic eruptions in magma deficient backarc rifts highlights shield type volcanoes in young backarc rifts may be linked to arc segmentation arc transverse faults act as magma conduits for voluminous basaltic eruptions the nifonea volcano hosts the first large lava lake described in a submarine backarc effusive eruptions produced short live hydrothermal venting and an event plume the coriolis troughs of the new hebrides subduction zone are among the youngest backarc rifts in the world they reach depths of 3 km despite their small size 100 km in length and only 25 45 km wide and their proximity to the arc front 50 km the narrow deep graben morphology is characteristic of magma deficient arc rifts in the early stages of backarc extension where the rate of extension and subsidence exceeds the magmatic input unexpectedly the youngest graben the vate trough contains a centrally located 1000 m tall and 14 km wide shield volcano with a large 5 8 km breached summit caldera the nifonea axial volcano has a volume of 126 km3 reflecting unusually high extrusion rates given its young age 3 ma and the summit caldera hosts the remnants of a large lava lake the first described from a submarine backarc setting extensive diffuse hydrothermal venting and several clusters of black smoker chimneys with the highest recorded fluid temperatures 368 c in the sw pacific occur on the youngest lava flows comparison with similar axial volcanoes on the mid ocean ridges suggests that the 46 106 m3 of sheet flows in the caldera could have been erupted in 30 hours the focusing of voluminous basaltic eruptions into an otherwise magma deficient backarc has been linked to strong left lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d entrecasteaux ridge this study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc rifting and could explain the wide range of melt compositions volcanic styles and mineral deposit types found in nascent backarc rifts

thermal evolution of a rift basin the tyrrhenian sea existing geological and geophysical data support the view that the opening of the tyrrhenian sea was by rifting of a formerly continuous continental lithosphere which began in upper miocene on the upper sardinian margin and migrated southeastward to the calabria margin heat flow across the tyrrhenian sea shows a pronounced asymmetry from about 50 70 mw m 2 over the sardinian margin to more than 150 mw m 2 over its southeastern margin off southern italy and calabria in this study we use the newly acquired data from ocean drilling program leg 107 heat flow seismic velocity and bathymetric data to constrain a group of models that supposes the tyrrhenian sea opened by detachment of a formerly continuous lithosphere along a southeastern facing low angle normal fault specific models tested include 1 the wernicke model in which thinning and extension of the continental lithosphere are supposed to be accomplished by simple shear along a large scale gently dipping detachment fault zone which cut through the entire lithosphere and 2 the delamination model in which the low angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust below which concurrent pure shear may occur a versatile two dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models and the results are compared with observations to test the validity of the models delamination models with large pure shear components in the lower lithosphere and the wernicke model predict heat flow within 1 of the measurements the wernicke model however predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from rayleigh wave dispersion results the delamination model with a large component of pure shear in the subbasinal lower lithosphere on the other hand predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin

architecture and neogene to recent evolution of the western calabrian continental margin an upper plate perspective to the ionian subduction system central mediterranean the western calabria continental margin forms the transition between the pliocene to recent marsili spreading center and continental calabria all parts of the upper plate of the ionian subduction zone integrating high resolution and crustal seismic images constrained by gravity modeling we provide a detailed reconstruction of the architecture of the margin and develop a new scheme for its miocene to present evolution this time span encompasses the continent continent collision between africa and eurasia subsequent orogenic collapse and rifting apart between the two continental masses and the pliocene to recent emplacement of oceanic crust in the vavilov and marsili basins the crust of the margin thins from the calabria coast 25 km to the marsili continent ocean transition 12 km on the whole upper and lower crusts thin proportionally with pure shear geometry the continental margin is covered by an oligocene to present sedimentary succession reaching a maximum thickness of 6 0 km in the paola basin during the miocene the continental margin experienced regional shortening accommodated by a large number of mainly west vergent thrusts possibly associated with the late stages of the kabilo calabrian chain shortening continued through pliocene to recent but was accommodated by a limited number of west vergent thrust faults located in the western part of the margin and by a few tens of kilometers wide syncline located in the eastern part of the profile the accommodation space created in the syncline core hosted a 4 5 km thick plio quaternary sedimentary succession the paola basin no significant extensional fault is observed along the profile miocene to recent subsidence was controlled by 1 a short wavelength component related to shortening and responsible for the formation of the paola basin syncline and possibly contributing to the uplift of onshore calabria and 2 a long wavelength component responsible for the regional subsidence and oceanward tilting of the calabria margin short wavelength subsidence ended in the late pliocene but long wavelength downward movements persisted and even accelerated during late pliocene to quaternary times when the present day bathymetry was achieved both horizontal deformations and vertical movements are difficult to explain in the context of a normal back arc basin without taking into consideration patterns of secondary mantle flow generated by the retreat of the subducting slab