Continental flood basalt magmatism contemporaneous with Deccan traps in the Mannar basin,offshore Sri Lanka

The Gulf of Mannar and adjoining Cauvery basin to the north between India and Sri Lanka are associated with a failed rift, which initiated during the late Jurassic to early Cretaceous as a precursor to the breakup of East Gondwana. Despite the occurrence of igneous rocks that can be noted in seismic profiles, offshore, and deep seated occurrence of those have lead only to the limited understanding of igneous activity in the Mannar basin. Rock cuttings recovered in the Barracuda exploratory well in the Mannar basin shows approximately 700 m thick basalt rock sequence interlayered with sediments at a depth of 3500–4200 m below mean sea level. Here, we analyzed samples recovered from the Barracuda well for major and trace element composition. Major and trace element data suggest that the basalts were crystallized from two different degrees of partial melts from a similar source. Chondrite normalized rare earth element (REE) patterns indicate that the basalts are similar to continental flood basalt, though they show a distinct Ba positive anomaly. Importantly, supported with previously available K–Ar data, we decipher that these basalts are contemporaneous with the Deccan traps. Rifting between Seychelles and India which had occurred at ~62 Ma approximately 3.5 Ma after the main Deccan eruption is synchronous with the Barracuda volcanism suggesting coeval rifting between Seychelles–India and India–Sri Lanka. Thus, our data suggest simultaneous rifting between Seychelles–India and India–Sri Lanka. Large plate reorganizations that took place during this time period in the Indian Ocean have likely caused consequent passive rifting in the Mannar basin.     

Anomalous subsidence on the rifted volcanic margin of Pakistan: No influence from Deccan plume

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced > 4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5–11.5 km. Curiously < 400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (> 600 °C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent–ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.     

Geophysical imaging of buried volcanic structures within a continental back-arc basin: The Central Volcanic Region, North Island, New Zealand

Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island.     

Ar–Ar age and thermal history of martian dunite NWA 2737

We report an ³⁹Ar–⁴⁰Ar age determination of a whole rock sample of the olivine-rich, martian meteorite Northwest Africa (NWA) 2737. Those extractions releasing 0–48% of the ³⁹Ar define an ³⁹Ar–⁴⁰Ar isochron age of 160–190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm–Nd age of 1.42 Ga. At least part of this excess ⁴⁰Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar–Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that ⁴⁰Ar was only partially degassed by an impact event ~ 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures > 600 °C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~ 11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~ 300–500 °C and buried in ejecta to a depth of ~ 1–20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~ 11 Ma ago probably heated NWA 2737 to no more than ~ 400 °C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.     

Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes

We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian.     

Basement control on dyke distribution in Large Igneous Provinces: Case study of the Karoo triple junction

Continental flood basalts consist of vast quantities of lava, sills and giant dyke swarms that are associated with continental break-up. The commonly radiating geometry of dyke swarms in these provinces is generally interpreted as the result of the stress regime that affected the lithosphere during the initial stage of continental break-up or as the result of plume impact. On the other hand, structures in the basement may also control dyke orientations, though such control has not previously been documented. In order to test the role of pre-dyke structures, we investigated four major putative Karoo-aged dyke swarms that taken together represent a giant radiating dyke swarm (the so-called “triple-junction”) ascribed to the Jurassic Karoo continental flood basalt (> 3 × 10⁶ km²; southern Africa). One of the best tests to discriminate between neoformed and inherited dyke orientation is to detect Precambrian dykes in the Jurassic swarms. Accordingly, we efficiently distinguished between Jurassic and Precambrian dykes using abbreviated low resolution, ⁴⁰Ar/³⁹Ar incremental heating schedules.Save-Limpopo dyke swarm samples (n = 19) yield either apparent Proterozoic (728–1683 Ma) or Mesozoic (131–179 Ma) integrated ages; the Olifants River swarm (n = 20) includes only Proterozoic (851–1731 Ma) and Archaean (2470–2872 Ma) dykes. The single age obtained on one N–S striking dyke (1464 Ma) suggests that the Lebombo dyke swarm includes Proterozoic dykes in the basement as well. These dates demonstrate the existence of pre-Karoo dykes in these swarms as previously hypothesized without supporting age data. In addition, aeromagnetic and air-photo interpretations indicate that: (1) dyke emplacement was largely controlled by major discontinuities such as the Zimbabwe and Kaapvaal craton boundaries, the orientation of the Limpopo mobile belt, and other pre-dyke structures including shear zones and (2) considering its polygenetic, pre-Mesozoic origin, the Olif ants River dyke swarm cannot be considered part of the Karoo magmatic event.This study, along with previous results obtained on the Okavango dyke swarm, shows that the apparent “triple junction” formed by radiating dyke swarms is not a Jurassic structure; rather, it reflects weakened lithospheric pathways that have controlled dyke orientations over hundreds of millions of years. One consequence is that the “triple-junction” geometry can no longer be unambiguously used as a mantle plume marker as previously proposed, although it does not preclude the possible existence of a mantle plume. More generally, we suggest that most Phanerozoic dyke swarms (including triple junctions) related to continental flood basalts were probably controlled in part by pre-existing lithospheric discontinuities.     

Fault architecture in the Main Ethiopian Rift and comparison with experimental models: Implications for rift evolution and Nubia–Somalia kinematics

The Main Ethiopian Rift (MER) offers a complete record of the time–space evolution of a continental rift. We have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models, and implemented with field controls. This analysis has been compared with the results of lithospheric-scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting. Integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along-axis variation of the rift development and southward decrease in rift evolution. The northernmost MER sector is in a mature stage of incipient continental rupture, with deformation localised within the rift floor along discrete tectono-magmatic segments and almost inactive boundary faults. The central MER sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase. The southernmost MER sector is instead in an early continental stage, with the largest part of deformation being accommodated by boundary faults and almost absent internal faults. The MER thus records along its axis the typical evolution of continental rifting, from fault-dominated rift morphology in the early stages of extension toward magma-dominated extension during break-up. The extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along-axis variations in rift architecture and evolutionary stage, being oblique rifting conditions controlling the MER evolution since its birth in the Late Miocene in relation to a constant post ca. 11 Ma ~ N100°E Nubia–Somalia motion.     

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.     

Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

A model for the early evolution of the Irish continental margin

It has been suggested that Porcupine Ridge, west of Ireland, represents a continental fragment displaced westwards relative to Europe at an early stage in the opening of the North Atlantic. This hypothesis presents difficulties, particularly in relation to the magnetic evidence for the onset of seafloor spreading at these latitudes. However, the structure of the Irish continental margin, so far as it is known, appears consistent with a westward rotation of Porcupine Ridge by some 23°; and there are still grounds for supposing that the adjacent Rockall Trough may represent a locus of Mesozoic seafloor spreading with which the rotation could have been associated.     

Origin of Deccan Trap lavas: evidence from combined trace element and Sr-, Nd- and Pb-isotope studies

Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar Formations can be shown to reflect three distinct end-members which provide new constraints for petrogenetic models. Pb-isotope data for selected basalts exhibit a wide range with²⁰⁶Pb/²⁰⁴Pb= 16.87–22.45, and a linear correlation on a Pb—Pb diagram. The least contaminated Ambenali basalts plot within the Pb-array, and interaction with mantle lithosphere involves a shift to less radiogenic Pb whereas contamination with crust is characterised by more radiogenic Pb. Unlike the Karoo and Parana continental flood basalt provinces only four flow units within the southern Deccan appear to contain a significant contribution from mantle lithosphere. The Mahabaleshwar and Ambenali Formation basalts exhibit a striking negative Pb—Sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle. It further suggests that basalts from the Walvis Ridge, Kerguelen and Ninetyeast ridge all remobilised such shallow level material, and that the Deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to T-type MORB, but related to the Reunion hotspot, and continental mantle lithosphere of the Indian plate.     

St. John's Island (Red Sea): a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins

New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm³) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm³) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable.     

Sedimentological constraints on hydrometeor-enhanced particle deposition: 1992 Eruptions of Crater Peak, Alaska

Water is a dominant component of volcanic clouds and has fundamental control on very fine particle deposition. Particle size characteristics of distal tephra-fall (100s km from source volcano) have a higher proportion of very fine particles compared to predictions based on single particle settling rates. In this study, sedimentological analyses of fallout from for the 18 August and 16–17 September 1992 eruptions of Crater Peak, Alaska, are combined with satellite observations, and cloud trajectory and microphysics modeling to investigate meteorological influences on particle sedimentation. Total grain size distributions of tephra fallout were reconstructed for both Crater Peak eruptions and indicate a predominance of fine particles < 125 μm. Polymodal analysis of the deposits has identified a particle subpopulation with mode ~ 15–18 μm involved in particle aggregation. Accounting for the magmatic water source only, calculated ice water content of the 3.7 hour old September 1992 Spurr cloud was ~ 4.5 × 10^− 2 g m^− 3 (based on an estimated cloud thickness of ~ 1000 m from trajectory modeling). Hydrometeor formation on particles in the volcanic cloud and subsequent sublimation may induce a cloud base instability that leads to rapid bulk (en masse) sedimentation of very fine particles through a mammatus-like mechanism.     

Distribution and geochronology of Oregon Plateau (U.S.A.) flood basalt volcanism: The Steens Basalt revisited

The timing and petrogenesis of mid-Miocene flood basalt volcanism in the northwest United States has been extensively addressed, yet the chemical characteristics and temporal details of the Steens Basalt, exposed on the Oregon Plateau, are poorly defined. Steens Basalt volcanism has generally been accepted to have occurred at ∼ 16.6 Ma, coeval and/or just prior to the onset of Columbia River Basalt Group volcanism to the north. New major and trace element analyses and nine ⁴⁰Ar/³⁹Ar ages ranging from 15.51 ± 0.28 to 16.58 ± 0.18 Ma were obtained on Oregon Plateau flood basalt lava flows from stratigraphic sections in close proximity to Steens Mountain. Additionally, new ⁴⁰Ar/³⁹Ar ages were obtained on the uppermost and thirty-first lava flow down from the top of the ∼ 1 km section of Steens Basalt exposed at Steens Mountain and yield eruption ages of 16.59 ± 0.10 and 16.55 ±0.10 Ma, respectively. Field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of Steens Mountain.     

Volcanological and tectonic control of stratigraphy and structure in the western Deccan traps

Many of the world's flood basalt provinces form elevated plateaux at the margins of continents, although in most cases their present large elevation is not the result of mountain building processes. Several explanations have recently been put forward to explain such occurrences of epeirogeny. The Deccan Trap basalt province forms one such elevated plateau, and results are presented here showing how the epeirogenic uplift in this region, combined with crustal subsidence probably associated with the rifting of the Indian continental margin, has affected the structure of the basalt sequence. Trace element analytical data are used for samples from numerous vertical sections through the Deccan Traps lava series along and around the Western Ghats ridge in India. The results reinforce the previously defined stratigraphy of the Mahabaleshwar area, and extend it over a region covering some 36 000 km², reaching as far south as Belgaum and the Trap/basement contact. These results show that the lava pile is not flat lying, but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0.3 ° over most of the area, although in the south there is evidence of a reversal of this plunge. The fold is interpreted as being the result of two tilting processes: (1) westward tilting near the coast, due to the foundering of the passive continental margin, and (2) epeirogenic uplift along the whole west coast of India producing the observed topography and the peninsula-wide drainage patterns, and also the easterly component of dip. Variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold, although the possibility of some component of depositional dip may also be important. This latter possibility can be modelled using a simple computer program. The results of this modelling show that a migrating linear volcanic edifice fits the observations best.     

Milankovitch forcing of bioturbation intensity in deep-marine thin-bedded siliciclastic turbidites

Trace fossils have been used to infer a Milankovitch-type control on pelagic and hemipelagic sediments, but there have been no comparable studies in deep-marine siliciclastic turbidite successions. Here, we present the results of a quantitative analysis of trace-fossil abundance and intensity in an essentially continuous, Middle Eocene, 230-m-long (Well A6) core, comprising very thin- and thin-bedded siliciclastic turbidites in the deep-marine Ainsa basin, Spanish Pyrenees. After removing the sediment slides and debris flow deposits, as these represent geologically instantaneous events approximately two orders of magnitude thicker than the typical laminated turbiditic sediments in the core, spectral analysis was conducted on the bioturbation intensity data. Spectral analysis of bioturbation intensity in the A6 core suggests, for the first time from a siliciclastic turbidite succession at a tectonically active plate margin (thrust-top basin in a foreland basin), a cyclicity that is interpreted to reflect the 41-kyr and an ~ 112-kyr (possibly an average of the 95- and 125-kyr) Milankovitch frequencies. We propose that this drove environmental changes in bottom-water conditions in the Ainsa basin, most likely leading to critically varying levels of oxygenation (stratification) of bottom waters. Our age model for the Ainsa basin (~ 4 km of deep-marine sediments in ~ 10 Myr) yields an average sediment accumulation rate of ~ 40 cm kyr^− 1, that is consistent with that inferred from the spectral analysis (~ 30 cm kyr^− 1) for fine-grained sedimentation. An implication of these results is that global environmental change, probably glacio-eustasy, acted as a driver on deep-marine siliciclastic sedimentation patterns during the Middle Eocene.     

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.     

Mantle flow and melting beneath oceanic ridge–ridge–ridge triple junctions

Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This study investigates how triple junction (TJ) geometry may influence mantle dynamics. An earlier study [Georgen, J.E., Lin, J., 2002. Three-dimensional passive flow and temperature structure beneath oceanic ridge-ridge-ridge triple junctions. Earth Planet. Sci. Lett. 204, 115–132.] suggested that the effects of a ridge–ridge–ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity model that focuses on the slowest-diverging ridge of a triple junction with geometry similar to the Rodrigues TJ. This spreading axis may be considered to be analogous to the Southwest Indian Ridge. Within 100 km of the TJ, temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by ~ 40 °C and 1 km, respectively. We also investigate the effects of differential motion of the TJ with respect to the underlying mantle, by imposing bottom model boundary conditions replicating (a) absolute plate motion and (b) a three-dimensional solution for plate-driven and density-driven asthenospheric flow in the African region. Neither of these basal boundary conditions significantly affects the model solutions, suggesting that the system is dominated by the divergence of the surface places. Finally, we explore how varying spreading rate magnitudes affects TJ geodynamics. When ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to the Azores TJ), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~ 150 °C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the TJ. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of TJ geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.     

Dalrymple Trough: An active oblique-slip ocean–continent boundary in the northwest Indian Ocean

The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere.     

Simulating the dispersal of tephra from the 1991 Pinatubo eruption: Implications for the formation of widespread ash layers

PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climatic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO₂ gas to higher altitudes (~ 25 km), but these pulses represent only a small fraction (~ 1%) of the total erupted material released during the simulation. Comparison with TOMS images of the SO₂ cloud after 71 and 93 h indicate that the SO₂ gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16–18 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and at a level below the calculated height of neutral buoyancy (~ 25 km). Simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO₂ cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.