Southwest Flank Research Articles

Phreatomagmatic deposits and stratigraphic reconstruction at Debunscha Maar (Mt Cameroon volcano)

The Debunscha Maar (DM) is located on the southwest flank of Mount Cameroon, an active stratovolcano on the Cameroon volcanic line (CVL). Here, we present the physical characteristics of the pyroclastic deposits at DM with the aim of deciphering tephra emplacement mechanisms, evolution of water–magma interaction and reconstructing the stratigraphy beneath the maar. From GPS measurements, the crater has long and short axes of 500 m and 320 m, respectively. Generally, the pyroclastic deposits are well stratified and present a variety of depositional bed forms including structureless/massive beds, massive beds with faint internal stratifications, inversely graded beds, lens- shaped units, impact sags, cross lamination, planar beds as well as dune-like beds. Clast sizes include ash, lapilli-tuff, bombs and blocks (pyroclastic breccia), with clast lithologies consisting of entrained lithics of porous ankaramite pillow lavas, lithified sediments (sandstone and shale) and juvenile material. The porous ankaramite pillow lavas have glassy margins and vesicle zonations typical of pillow lavas formed by subaqueous eruption. The pillow fragments are more common in early-formed eruption products at the base of the deposit. The lithified sandstones show planar laminations and together with the shales occur predominantly in stratigraphic positions above the ankaramite pillow lavas. The juvenile materials include basaltic bombs with low vesicularity (< 15%) and moderate vesicularity (15–50%). The bombs have chilled surfaces and their abundance increases towards the top of the deposit. The presence of accretionary lapilli, fragments of country-rock and juvenile clasts with ragged surfaces as well as curved and chilled margins, is unambiguous evidence in support of phreatomagmatic activity. Of the observed lithic clasts, only the pillow lavas would appear to have the porosity necessary to furnish the required amount of water to feed the phreatomagmatic maar eruption. The clast stratigraphy suggests that the maar is underlain by ankaramite pillow lava that erupted on a consolidated sedimentary substratum. Studying deposits resulting from maar eruptions has a direct implication for hazards assessment at areas of active maar volcanism because many surface processes occur around such volcanoes well after the eruptive activities have stopped.

Evidence for carbonate platform failure during rapid sea-level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufriere Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south-west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse-grained and either ungraded or poorly graded, and were deposited by non-cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub-units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub-units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi-stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea-level change.

Eruptive and earthquake activities related to the 2000 eruption of Mount Cameroon volcano (West Africa)

Mount Cameroon is an active volcano located in the Gulf of Guinea, west of Central Africa. After the March–April 1999 eruption on the SW flank, another eruption of the volcano occurred in 2000. It took place from three sites on the southwest flank and near the summit. The first eruptive site was located 500 m to the southwest of the summit, at 3900 m altitude. Activity on this site was mainly explosive with no lava flow. The second site was located between 3220 and 3470 m altitude. Lava was emitted along NNE–SSE fissures from this site and flew towards Buea, the main city of the area, stopping ~ 4 km from the first houses. The last site was located in the south western flank at 2750 m altitude. The lava ejected from an old cone near the first 1999 eruptive site was divided into two branches, for a total length of around 1 km. The location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics. The pre-eruptive period was characterized by a seismic swarm which may be a precursor recorded in March 2000 by an analogue seismic station. The main shock was a magnitude 3.2 event, and was felt by the population in Ekona town located on the eastern flank. It had a Modified Mercalli intensity of III–IV. When the eruption started, a temporary network of short period 3-component seismic stations was set up around the volcano to improve the monitoring of seismic activity. The co-eruptive period from late May to September was characterized by sequences of earthquake swarms, volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in August and early September. Some of the earthquakes were felt by the population in Buea and its environments. The largest seismic event recorded had a magnitude of 4. During the post-eruptive period from mid-September to December, seismicity returned to its background level of 1–3 earthquakes per 3 days. Hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano. The frequency/magnitude relationship revealed a b-value of 1.43 higher than those previously determined, but more representative of volcanic media. Seismic energy release was gradual after the 2000 eruption started.

Structural evaluation of column-height controls at a toe-thrust discovery, deep-water Niger Delta

Industry discoveries to date in the deep-water toe-thrust play have limited column heights and lack any unequivocal thrust-fault-dependent hydrocarbon columns. The causes of the limited success are controversial, the main issue being whether the encountered columns are caused by leaking thrust faults or other causes. The controversy is in large part caused by the presence of no-seismic-image zones that, even on three-dimensional (3-D) prestack-migrated data, obscure the possible thrust cutoffs. We use dipmeter, seismic, stratigraphic, and fluid pressure data to construct an integrated geometrically and kinematically balanced cross section through a recent toe-thrust discovery in which a prominent no-seismic-image zone on the southwest flank of the Alpha structure appeared to be a thrust fault zone holding about 100 m (328 ft) of thrust-fault-dependent oil column. To constrain the structure within the unimaged forelimb, dip panels and fold axial surfaces were constructed from dipmeter data recorded in both a vertical hole and a sidetrack well through the no-seismic-image zone. Stratigraphic tops from both wells were projected through the no-seismic-image zone using the dip panels and axial surfaces, maintaining the observed stratigraphic thicknesses. These data and analyses tightly constrain potential fault locations and offsets within the no-data zone. The resulting structural model shows that the no-seismic-image zone in the forelimb is not a thrust but instead an overturned limb without any fault offset of the pay section. The results show that the trapped hydrocarbons are confined completely within the four-way dip closure, and that the trap's potential is not limited by the thrust faults' seal capacities. In addition, top-seal analyses show that top-seal integrity is not a limiting factor for hydrocarbon column heights at the present time and has not been a limiting factor throughout the geological history of the structure. Taken together, these observations indicate that the column heights are most likely controlled by access to charge instead of trap integrity, consistent with an observed lack of thermogenic hydrocarbons in nearby four-way dip closures. These conclusions imply that where sufficient access to thermogenic charge is present, there may be additional thrust-fault-dependent hydrocarbon columns.

A photographic and acoustic transect across two deep-water seafloor mounds, Mississippi Canyon, northern Gulf of Mexico

In the northern Gulf of Mexico, a series of seafloor mounds lie along the floor of the Mississippi Canyon in Atwater Valley lease blocks 13 and 14. The mounds, one of which was drilled by the Chevron Joint Industry Project on Methane Hydrates in 2005, are interpreted to be vent-related features that may contain significant accumulations of gas hydrate adjacent to gas and fluid migration pathways. The mounds are located ∼150 km south of Louisiana at ∼1300 m water depth. New side-scan sonar data, multibeam bathymetry, and near-bottom photography along a 4 km northwest–southeast transect crossing two of the mounds (labeled D and F) reveal the mounds' detailed morphology and surficial characteristics. Mound D, ∼250 m in diameter and 7–10 m in height, has exposures of authigenic carbonates and appears to result from a seafloor vent of slow-to-moderate flux. Mound F, which is ∼400 m in diameter and 10–15 m high, is covered on its southwest flank by extruded mud flows, a characteristic associated with moderate-to-rapid flux. Chemosynthetic communities visible on the bottom photographs are restricted to bacterial mats on both mounds and mussels at Mound D. No indications of surficial gas hydrates are evident on the bottom photographs.

Geological setting of the 8 October 2005 Kashmir earthquake

The source of the 8 October 2005 earthquake of M 7.6 was the northwest-striking Balakot–Bagh (B–B) fault, which had been mapped by the Geological Survey of Pakistan prior to the earthquake but had not been recognized as active except for a 16-km section near Muzaffarabad. The fault follows the Indus–Kohistan Seismic Zone (IKSZ); both cut across and locally offset the Hazara–Kashmir Syntaxis defined by the Main Boundary and Panjal thrusts. The fault has no expression in facies of the Miocene–Pleistocene Siwalik Group but does offset late Pleistocene terrace surfaces in Pakistan-administered Jammu-Kashmir. Two en-echelon anticlines near Muzaffarabad and Balakot expose Precambrian Muzaffarabad Limestone and are cut by the B–B fault on their southwest sides, suggesting that folding and exposure of Precambrian rocks by erosion accompanied Quaternary displacement along the fault. The B–B fault has reverse separation, northeast side up; uplift of the northeast side accompanied displacement, producing higher topography and steeper stream gradients northeast of the fault. No surface expression of the B–B fault has been found northwest of the syntaxis, although the IKSZ and steeper stream gradients continue at least as far as the Indus River, the site of the Pattan earthquake of M 6.2 in 1974. To the southeast, northwest-striking faults were mapped by the Geological Survey of Pakistan. One of these faults, the Riasi thrust, cuts across the southwest flank of an anticline exposing Precambrian limestone. Farther southeast, in Indian-administered territory, Holocene activity on the Riasi thrust has been described. In the Kangra reentrant still farther southeast, active faulting may follow the Soan thrust, along which Holocene and Pleistocene offsets have been described. The Soan thrust, rather than the south flank of the Janauri anticline, may represent the surface projection of the 1905 Kangra earthquake of M 7.8.

Three-dimensional magnetotelluric characterization of the Coso geothermal field

A dense grid of 125 magnetotelluric (MT) stations plus a single line of contiguous bipole array profiling has been acquired over the east flank of the Coso geothermal system, CA, USA. Due to production related electromagnetic (EM) noise the permanent observatory at Parkfield, CA was used as a remote reference to suppress this cultural EM noise interference. These data have been inverted to a fully three-dimensional (3D) resistivity model. This model shows the controlling geological structures possibly influencing well production at Coso and correlations with mapped surface features such as faults and the regional geoelectric strike. The 3D model also illustrates the refinement in positioning of resistivity contacts when compared to isolated 2D inversion transects. The resistivity model has also been correlated with micro-earthquake locations, reservoir fluid production intervals and most importantly with an acoustic and shear velocity model derived by Wu and Lees [Wu, H., Lees, J.M., 1999. Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data. J. Geophys. Res. 104 (B6), 13217–13233]. This later correlation shows that the near-vertical low-resistivity structure on the eastern flank of the producing field is also a zone of increased acoustic velocity and increased V p/ V s ratio bounded by mapped fault traces. Over of the Devils’ Kitchen is an area of large geothermal well density, where highly conductive near surface material is interpreted as a smectite clay cap alteration zone manifested from the subsurface geothermal fluids and related geochemistry. Enhanced resistivity beneath this cap and within the reservoir is diagnostic of propylitic alteration causing the formation of illite clays, which is typically observed in high-temperature reservoirs (>230 °C). In the southwest flank of the field the V p/ V s ratio is enhanced over the production intervals, but the resistivity is non-descript. It is recommended that more MT data sites be acquired to the south and southwest of Devil's Kitchen to better refine the resistivity model in this area.

Maximizing Production Capacity With Intelligent-Well Systems in a Deepwater West Africa Field

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 103308, "Maximizing Production Capacity Using Intelligent- Well Systems in a Deepwater, West Africa Field," by D.J. Goggin, M.A. Ovuede, N. Liu, SPE, U. Ozdogan, SPE, P.B. Coleman, SPE, and D.P. Meinert, SPE, Chevron; I. Nygard, Statoil; and J. Gontijo, Petroleo Brasileiro Nigeria, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. Large, deepwater fields with a limited number of wells may require intelligent-well systems to maximize production capacity under facility constraints. Agbami field, a highly dipping reservoir with many producing zones and few wells, will use intelligent-well systems consisting of interval-control valves (ICVs) and many sensors to monitor, analyze, and control (MAC) injection and production at the zonal level. Introduction A key objective for optimizing asset performance is maximizing and accelerating recovery by producing wells at optimal rates, which will delay the arrival of water and gas while honoring the mechanical limitations of the production network (e.g., flowlines, manifolds, risers, and separators). The challenge for operators is to know where the excess production capacity exists in the network at any given time and what control settings can be adopted if well failures occur. Therefore, a MAC process is used to respond to unplanned operational interruptions and to be prepared for longer-term reservoir behavior caused by geologic uncertainties. Reservoir Description The overall field structure is a northwest/southeast-trending four-way-closure anticline on the Niger delta front, approximately 65 miles offshore Nigeria in the Gulf of Guinea. The anticlinal structure has dip angles as steep as 30° on the northeast and southwest flanks. From 3D-seismic data, several faults have been mapped across the anticlinal structure. A major thrust fault divides the Agbami field into inboard (northeast) and outboard (southwest) halves. Normal faults further subdivide the field into eight potentially noncommunicating areas. The high structural dip and the fault compartments are important factors in the well-placement strategy, injection patterns, and oil-recovery expectation. The field has four stacked reservoirs with distinct pressure profiles. The current geologic model includes three of these reservoirs designated as 14MY, 16MY, and 17MY. The 17MY reservoir contains 80% of the total-field oil-in-place of approximately 2 billion bbl. Field-Development Plan and Production Network The field-development plan recommends midflank oil production with crestal gas injection and peripheral water injection as the primary oil-recovery, pressure-maintenance, and gas-disposition scheme in the 17MY units. Oil properties of 47°API gravity and 0.23-cp viscosity are favorable to a combined crestal miscible-gas drive and peripheral water-injection scheme in a highly dipping reservoir. Thirty-eight wells are planned in three drilling stages. Plans include 17 Stage-1 wells drilled and completed before first oil production, comprising 10 producers, four water injectors, and three gas injectors. Stages-2/-3 drilling occurs after first oil production.

静岡県掛川‐相良油田地域における古〜新第三系前弧堆積盆地の根源岩と石油システム

Oil fields occur in the Sagara district of Shizuoka Prefecture, despite the disadvantageous geological conditions of a fore-arc setting. We investigated the petroleum system of this fore-arc basin in southern Shizuoka Prefecture based on hydrocarbon compositions of oils, source rock potential, organic matter type, and burial history of the Paleogene to Neogene sediments. Argillaceous rock samples from the MITI-Omaezakioki well and from outcrops in the Kakegawa and Sagara districts were analyzed for geochemical properties by CHNS elemental analyzer, Rock-Eval pyrolysis, and GC/MS.Various maturity parameters in the Sagara oils indicate differing maturity levels, ranging up to the condensate zone. The range in maturity parameters is wider in the northwest flank of the Megami anticlinorium than in the southwest flank. The upper layers of the Lower Miocene Towata Formation and the lower layers of the Lower Miocene Matsuba Formation have relatively high potential for hydrocarbon generation (up to TOC=0.9 wt%, S2=2.8mg/g and HI=350mg/gC). The highest-potential layer of the Matsuba Formation was probably deposited in a highly productive marine environment during deposition of siliceous clastic sediments. No equivalent high-potential layers were identified in the MITI-Omaezakioki well, suggesting that the organic-rich layer formed only in the northern part of the basin. In the Eocene-Oligocene Mikura Group, layers relatively rich in TOC (up to 0.7 wt% in over-matured black shale) were observed in the Kakegawa area and also in the MITI-Omaezakioki well. According to epimerization of sterane and hopane isomers, thermal gradient was higher in the northern part of the basin than in the south. In both areas the Kurami Group only reached medium-light oil zone, whereas the Mikura Group reached the condensate zone. The thermal structure of the basin thus changed significantly between the Paleogene and Neogene.On the basis of basin modeling simulation of the MITI-Sagara well, medium-light oils (Ro=0.7-1.0%) were generated in the Mikura Group at ca. 29 Ma and 15 Ma, and condensates (Ro=1.2%) at ca. 2 Ma. The Kurami Group generated medium-light oils (Ro=0.7-1.0%) at ca. 15 Ma and 2 Ma. Therefore, the ca. 2 Ma condensates from the Mikura Group migrated and were contaminated by oils from the Kurami Group and bitumens from the upper reservoir layers (e.g. Sagara Group).

Mississippian (Osagean) Shallow-Water, Mid-Latitude Siliceous Sponge Spicule and Heterozoan Carbonate Facies

Mixtures of biosiliceous and heterozoan-dominated carbonate deposits are commonly interpreted as recording cold-water polar or deep basinal conditions. However, a growing body of literature is documenting examples from the rock record that show these deposits accumulated in shallow-water middle- to low-latitude environments. The continued recognition of ancient neritic heterozoan carbonate and biosiliceous accumulations is broadening our understanding of the various paleoenvironmental controls on their development.&#x0D; Early-Middle Mississippian time was characterized by the development of biosiliceous and carbonate accumulations in North America. This study focuses on Osagean cherty dolomitic strata in cores from the Schaben field in Kansas, which is located in Ness County on the southwest flank of the Central Kansas uplift (CKU). During the Osagean, Kansas was located at approximately 20° S latitude, within the tropical to subtropical latitudinal belt. Study area strata are characterized by shallow-water inner-shelf carbonates that were deposited on a gently southward-sloping shelf (ramp). Two depositional sequences (DS1 and DS2) are identified in cores and are separated by a sequence boundary (SB1) that evidences subaerial exposure. The primary facies in the two depositional sequences include 1) Mudstone-Wackestone (MW); 2) Sponge Spicule-Rich Wackestone-Packstone (SWP); 3) Echinoderm-Rich Wackestone-Packstone-Grainstone (EWPG); and 4) Dolomitic Siltstones and Shale facies. Other features identified in cores include 1) Silica Cementation and Replacement; 2) Silica Replaced Evaporites; 3) Brecciation and Fracturing; and 4) Calcite Cementation and Replacement.&#x0D; The abundance of echinoderm facies with other diverse fauna, evidence of extensive reworking by burrowing organisms, and only rare occurrence of evaporites suggest subtidal deposition in a normal to slightly restricted marine inner-shelf setting for DS1. After the SB1 subaerial exposure event, marine conditions returned but the depositional environment over the study area changed compared to that for much of DS1 deposition. The volumetric increase of sponge-spicule wackestone and packstone (SWP) with less diverse fauna, abundance of early evaporites (replaced by silica), and evidence for shallowest water to subaerially exposed conditions throughout DS2 suggest deposition in more restricted environments that likely ranged from restricted inner shelf/protected embayment to evaporative lagoon and possibly supratidal flat.&#x0D; One of the more significant characteristics in DS2 is the dominance of siliceous sponge spicule facies and heterozoan carbonates that were deposited in shallow-water and restricted environments. This study and others from numerous periods in the geologic record are indicating that shallow-marine, mid-latitude biosiliceous and heterozoan carbonates may be more common than previously thought. Especially interesting are the examples from Mississippian (Osagean-Meramecian) strata in North America that show similar facies associations with DS2 strata of this study.&#x0D; The predominance of Early-Middle Mississippian heterozoan carbonate and biosiliceous (spiculitic) deposits, and lack of photozoan deposits, in the mid-latitude shallow-shelf setting in Kansas and surrounding areas was likely due to abundant nutrients and dissolved silica derived from basinal and/or terrestrial sources. Based on available evidence, upwelling of basinal waters rich in nutrients and dissolved silica appears to have been a primary control on shelf margin and shelf facies. Upwelling even may have had a primary imprint on shallow-water, inner-shelf areas, especially during transgression(s). Nutrients and dissolved silica from terrestrial sources may have contributed to the facies associations in shallowest water, inner-shelf areas. However, the available evidence suggests that terrestrial sourced nutrients and dissolved silica were not the dominant control.&#x0D; The results of this study have implications from a petroleum reservoir standpoint. The DS2 sponge spicule, heterozoan carbonate, and silica-replaced evaporite facies in this study form reservoirs in Schaben field and another nearby field composed of similar facies. Because regional upwelling is likely to have had at least some control, facies similar to DS2 strata may form important reservoirs in Lower-Middle Mississippian strata that were deposited in shallow-water inner shelf/ramp settings elsewhere in Kansas and North America.&#x0D; Continuing studies of the controls on biosiliceous and heterozoan carbonate deposition and diagenesis in mid-latitude neritic settings will improve our understanding and predictive capabilities.

Late Quaternary glacier advances in the upper catchment area of the Indus River (Ladakh and Western Tibet)

Geomorphological mapping in the Markha valley and the northern Nimaling Mountains (Northern Zanskar Range) has revealed eight prominent glacial stages. The two oldest glaciations are marked by glacial abrasion forms as ice-scour limits and glacial benches above the lower and middle Markha valley. These glaciations probably occurred prior to the last glacial cycle. Furthermore, terminal and lateral moraines, glacial tongue basins, basal moraines and tills provide evidence for three glacial stages in the Markha and Nimaling valleys, when the equilibrium-line altitude (ELA) depression was 670–350 m. The glaciations probably occurred during the LGM, the Lateglacial and the Lower Holocene as indicated by the topographic sequence, the ELA depressions and the tentative glacial stratigraphic divisions for the North Indian areas. Large moraine complexes in the vicinity of the present glaciers probably represent three glacial stages of Neoglacial and Historical glacial advances. The ELA depressions are 10–70 m. The present glaciation of the Nimaling Mountains, largely restricted to north flanks and the glacier basins below, covers an area of 45–55 km 2. The modern ELA is calculated at 5650–5750 m a.s.l. on north faces, rising to about 5900–6000 m a.s.l. on south faces. In the central and southern parts of the Ayila Ridge (Western Tibet) present ELAs range from 5700 to 5800 m a.s.l. on the north-facing slopes to 6000–6100 m a.s.l. on southern slopes. Eight former glacier stages are preserved on the southwest flank of these mountains by lateral and terminal moraine ramparts. The ELA depressions amount to 640–950 m.

Depleted swell root beneath the Cape Verde Islands

The hotspot swell—an area of uplifted bathymetry or topography surrounding regional volcanism—is a defining hotspot characteristic, yet its origin is poorly understood. To test current ideas about swell formation, we studied the crust and shallow mantle structure of the Cape Verdes in a passive seismic experiment. The Cape Verde Islands are ∼450 km west of Senegal in the tropical Atlantic Ocean and are on the southwest flank of the Cape Verde Rise, the largest bathymetric anomaly in the oceans, rising ∼2 km above the surrounding seafloor ([Crough, 1982][1]). The archipelago occupies a unique position, an approximately stationary one in the hotspot frame of reference ([Gripp and Gordon, 2002][2]) and therefore with respect to the melting source believed to have produced it. Here we present an analysis of compressional to shear (P to S) converted seismic phases, recorded on a temporary network of seismograph stations on the Cape Verde Islands, that indicate a crust thickened to 22 km is underlain by a high-velocity, low-density layer, which overlies a zone of low shear-wave velocity starting at ∼80 km depth. We also measured shear-wave splitting delay times for teleseismic SKS phases, which are ∼0.81 s, compatible with an origin in this same layer. We interpret these observations as effects of hotspot melting, which produces a thickened crust and a depleted swell root that buoys the ocean floor and spreads laterally as it grows over time. [1]: #ref-5 [2]: #ref-11

A New Perspective on Southern Hemisphere Storm Tracks

Abstract A detailed view of Southern Hemisphere storm tracks is obtained based on the application of filtered variance and modern feature-tracking techniques to a wide range of 45-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. It has been checked that the conclusions drawn in this study are valid even if data from only the satellite era are used. The emphasis of the paper is on the winter season, but results for the four seasons are also discussed. Both upper- and lower-tropospheric fields are used. The tracking analysis focuses on systems that last longer than 2 days and are mobile (move more than 1000 km). Many of the results support previous ideas about the storm tracks, but some new insights are also obtained. In the summer there is a rather circular, strong, deep high-latitude storm track. In winter the high-latitude storm track is more asymmetric with a spiral from the Atlantic and Indian Oceans in toward Antarctica and a subtropical jet–related lower-latitude storm track over the Pacific, again tending to spiral poleward. At all times of the year, maximum storm activity in the higher-latitude storm track is in the Atlantic and Indian Ocean regions. In the winter upper troposphere, the relative importance of, and interplay between, the subtropical and subpolar storm tracks is discussed. The genesis, lysis, and growth rate of lower-tropospheric winter cyclones together lead to a vivid picture of their behavior that is summarized as a set of overlapping plates, each composed of cyclone life cycles. Systems in each plate appear to feed the genesis in the next plate through downstream development in the upper-troposphere spiral storm track. In the lee of the Andes in South America, there is cyclogenesis associated with the subtropical jet and also, poleward of this, cyclogenesis largely associated with system decay on the upslope and regeneration on the downslope. The genesis and lysis of cyclones and anticyclones have a definite spatial relationship with each other and with the Andes. At 500 hPa, their relative longitudinal positions are consistent with vortex-stretching ideas for simple flow over a large-scale mountain. Cyclonic systems near Antarctica have generally spiraled in from lower latitudes. However, cyclogenesis associated with mobile cyclones occurs around the Antarctic coast with an interesting genesis maximum over the sea ice near 150°E. The South Pacific storm track emerges clearly from the tracking as a coherent deep feature spiraling from Australia to southern South America. A feature of the summer season is the genesis of eastward-moving cyclonic systems near the tropic of Capricorn off Brazil, in the central Pacific and, to a lesser extent, off Madagascar, followed by movement along the southwest flanks of the subtropical anticyclones and contribution to the “convergence zone” cloud bands seen in these regions.

TES spectroscopic identification of a region of persistent water ice clouds on the flanks of Arsia Mons Volcano, Mars

As part of a search for small‐scale spectrally anomalous regions within the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES) data set, we have identified a region ∼100 km in diameter on the southwest flanks of the volcano Arsia Mons that exhibits remarkable and persistent water ice cloud cover. High‐opacity clouds (by Martian standards) occur in this region in all of the 400 TES spectra covering this area, regardless of season, as well as in all of the MGS Wide‐Angle camera images of this same region. We have also found evidence for similar persistent cloudiness at other local solar times of day in this region based on Hubble Space Telescope UV imaging. Modeling of the TES spectra identifies the clouds as water ice and reveals average IR cloud optical depths near 0.5 and typical cloud particle sizes of ∼3 μm. The presence of persistent cloudiness in this area, even during the relatively warm and cloud‐free season near perihelion, may indicate unique local circulation processes, perhaps related to orographic effects or the interaction between the global circulation and the Tharsis region in general. Our analysis also demonstrates that through careful modeling it is possible to extend the search for unique spectral features in the TES data far into the wings of the strong 15‐μm CO2 band.

Earthquake swarms preceding the 2000 eruption of Miyakejima volcano, Japan

The first sign of magma accumulating beneath Miyakejima, an island volcano in the northern Izu islands, Japan, came at around 18:00 on 26 June 2000, when a swarm of earthquakes was detected by a volcano seismic network on the island. Earthquakes occurred initially beneath the southwest flank near the summit and gradually migrated west of the island, where a submarine eruption occurred the next morning. Earthquakes then migrated further to the northwest between Miyakejima and Kozushima, another volcanic island and developed to the most intense earthquake swarm ever observed in and around Japanese archipelago. To better image how the initial magma intrusion occurred, we relocated hypocenters by using a station-correction method and a double-difference method. The relocated epicenters are generally concentrated near the upper bound of dyke intrusions inferred from geodetic studies throughout the initial stages of the 2000 eruption at Miyakejima from 26 to 27 June 2000. As for seismic activity westward off Miyakejima in the morning on 27 June, hypocenters from both a nationwide seismic network that were relocated by the double-difference method, and those from the volcano seismic network relocated by the station-correction method, formed a very shallow cluster that ascended slowly with time as it propagated northwestward from Miyakejima. This suggests that the dykes have both a radial and upward component of movement.

Surface deformation associated with the March 1996 earthquake swarm at Akutan Island, Alaska, revealed by C-band ERS and L-band JERS radar interferometry

In March 1996, an intense earthquake swarm beneath Akutan Island, Alaska, was accompanied by extensive ground cracking but no eruption of Akutan volcano. Radar interferograms produced from L-band JERS-1 and C-band ERS-1/2 images show uplift associated with the swarm by as much as 60 cm on the western part of the island. The JERS-1 interferogram has greater coherence, especially in areas with loose surface material or thick vegetation. It also shows subsidence of similar magnitude on the eastern part of the island and displacements along faults reactivated during the swarm. The axis of uplift and subsidence strikes about N70°W, which is roughly parallel to a zone of fresh cracks on the northwest flank of the volcano, to normal faults that cut the island and to the inferred maximum compressive stress direction. A common feature of models that fit the deformation is the emplacement of a shallow dike along this trend beneath the northwest flank of the volcano. Both before and after the swarm, the northwest flank was uplifted 5–20 mm/year relative to the southwest flank, probably by magma intrusion. The zone of fresh cracks subsided about 20 mm during 1996–1997 and at lesser rates thereafter, possibly because of cooling and degassing of the intrusion.

Growth and collapse of Waianae Volcano, Hawaii, as revealed by exploration of its submarine flanks

Wai‘anae Volcano comprises the western half of O‘ahu Island, but until recently little was known about the submarine portion of this volcano. Seven new submersible dives, conducted in 2001 and 2002, and multibeam bathymetry offshore of Wai‘anae provide evidence pertaining to the overall growth of the volcano's edifice as well as the timing of collapses that formed the Wai‘anae slump complex. A prominent slope break at ∼1400 mbsl marks the paleoshoreline of Wai‘anae at the end of its shield‐building stage and wraps around Ka‘ena Ridge, suggesting that this may have been an extension of Wai‘anae's northwest rift zone. Subaerially erupted tholeiitic lavas were collected from a small shield along the crest of Ka‘ena Ridge. The length of Wai‘anae's south rift zone is poorly constrained but reaches at least 65 km on the basis of recovered tholeiite pillows at this distance from the volcano's center. Wai‘anae's growth was marked by multiple collapse and deformation events during and after its shield stage, resulting in the compound mass wasting features on the volcano's southwest flank (Wai‘anae slump complex). The slump complex, one of the largest in Hawai‘i, covering an area of ∼5500 km2, is composed of several distinct sections on the basis of morphology and the lithologies of recovered samples. Two dives ascended the outer bench of the slump complex and collected predominantly low‐S tholeiites that correlate with subaerial lavas erupted early during the volcano's shield stage, from 3.9 to 3.5 Ma. Pillow lavas from the outer bench have Pb, Sr, and Nd isotopic values that overlap with previously published subaerial Wai‘anae lavas. On the basis of the compositions of the recovered samples, the main body of the slump complex, as represented by the outer bench, probably formed during and shortly after the early shield stage. To the southwest of the outer bench lies a broad debris field on the seafloor, interpreted to have formed by a catastrophic collapse event that breached the outer bench. A dive within the debris field recovered subaerially derived volcaniclastic rocks; analyzed glasses are tholeiitic and resemble early shield stage compositions. The breach may have then been filled by slumping material from the main volcanic edifice. Finally, atop the northern main body of the slump is a rotated landslide block that detached from the proximal part of Ka‘ena Ridge after the volcano's late shield stage (3.2 to 3.0 Ma). From the inner scarp of this block we recovered subaerially erupted tholeiitic pillow breccias and hyaloclastites that are systematically higher in alkalis and more fractionated than those collected from the outer bench. These rocks correlate compositionally with late shield‐stage subaerial Kamaile‘unu lavas. None of the collected slump complex samples correlate with alkalic subaerial postshield lavas. Volcaniclastic rocks and glass disseminated in pelagic sediment, collected from north of Ka‘ena Ridge, originated from Wai‘anae's postshield stage and Ko‘olau's shield stage, respectively.

MAR volcanism in the Sierra Leone Fracture Zone region, Central Atlantic

We have studied major and trace element (including REE) geochemistries of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5 -7 10 0 N). The links of basalts of various compositions with particular ocean- floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwest flank of the MAR crestal zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of a deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are dierent for depleted tholeiites (44-55 km, 1320-1370 C) and enriched tholeiites (45-78 km, 1330-1450 C). Mantle plume impact is shown to aect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in the melts parental to the rift-related basalts occurring near the plume. T- and P-MORBs are also developed near the areas where mantle plumes are localized. The high-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov FZ) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. The likely causes of the temporal and spatial instability of mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In the sulfide-bearing rift morphostructures (the so-called Ore area and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. The functioning of these chambers is a combined eect of a singular geodynamic regime and plume activity. In these chambers, melts undergo deep dierentiation leading to progressively increasing concentration of the sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

Magmatic inflation at a dormant stratovolcano: 1996–1998 activity at Mount Peulik volcano, Alaska, revealed by satellite radar interferometry

A series of ERS radar interferograms that collectively span the time interval from July 1992 to August 2000 reveal that a presumed magma body located 6.6 ± 0.5 km beneath the southwest flank of the Mount Peulik volcano inflated 0.051 ± 0.005 km3 between October 1996 and September 1998. Peulik has been active only twice during historical time, in 1814 and 1852, and the volcano was otherwise quiescent during the 1990s. The inflation episode spanned at least several months because separate interferograms show that the associated ground deformation was progressive. The average inflation rate of the magma body was ∼0.003 km3/month from October 1996 to September 1997, peaked at 0.005 km3/month from 26 June to 9 October 1997, and dropped to ∼0.001 km3/month from October 1997 to September 1998. An intense earthquake swarm, including three ML 4.8–5.2 events, began on 8 May 1998 near Becharof Lake, ∼30 km northwest of Peulik. More than 400 earthquakes with a cumulative moment of 7.15 × 1017 N m were recorded in the area through 19 October 1998. Although the inflation and earthquake swarm occurred at about the same time, the static stress changes that we calculated in the epicentral area due to inflation beneath Peulik appear too small to provide a causal link. The 1996–1998 inflation episode at Peulik confirms that satellite radar interferometry can be used to detect magma accumulation beneath dormant volcanoes at least several months before other signs of unrest are apparent. This application represents a first step toward understanding the eruption cycle at Peulik and other stratovolcanoes with characteristically long repose periods.

Reservoir characterization of the Roar Gas Field, Danish North Sea

The Roar Field consists of a gentle inversion anticline in which porosity, permeability and hydrocarbon saturation distributions in the Danian–Upper Cretaceous Chalk reservoir have been mapped using seismic data. The seismic data are inverted to support porosity prediction in a stochastic modelling approach. The uniform chalk matrix causes seismic impedance to correlate mainly with porosity and fluid content. Replacement of oil by water has negligible effect on seismic impedance but correction for fluid content in the gas cap is needed to increase correlation between seismic impedance and porosity. Porosity in excess of 40% is characteristic, probably reflecting early invasion of hydrocarbons. A slight tilting since Miocene times is suggested by higher porosity on the southwest flank. Permeability is generally below 10 mD in the reservoir and around 1 mD below. A slight south-dipping free water level is consistent with a regional water zone pressure gradient, if local high permeability is considered. A gas cloud that produces a velocity anomaly not apparent in well velocity data hampers depth conversion. The velocity anomaly is revealed from horizontal wells and has led to the identification of a new culmination at the north end 15 m shallower than the culmination at the south end; this has increased the estimated initial hydrocarbons in place from around 510 to 720×10 9 SCF gas. Analysis of uncertainty on hydrocarbon in-place calculations resulting from the characterization is limited to studying the effects of the gas cloud anomaly and the value added by the two horizontal wells on porosity field determination.