Fluid Escape Research Articles

Sector collapse of mud volcanoes, Azerbaijan

Abstract: Field data collected from mud volcanoes in Azerbaijan are used to describe a process in mud volcano development that involves portions of the constructional edifices collapsing outwards in ‘thin-skinned' slides. These events create kilometre-scale scarps that are tens of metres in height, arcuate in plan view, elongate and facing downdip. Similar morphological features occur on igneous volcanoes and have been described as ‘sector collapse' structures. The largest sector collapses in igneous volcanoes involve some 10 12 tons of mobilized material; equivalent structures in mud volcanoes are several orders of magnitude smaller. We employ a shape parameter that can be utilized in field and satellite-based mapping, to distinguish between slope failure and eruptive deposits. Three mud volcanoes with kilometre-scale sector collapses are described and controlling mechanisms are reviewed. The updip domains of these collapses are characterized by fluid escape, showing that there is also linkage to deeper mud volcano structure. The observations are reconciled in a model consisting of a deflating mud chamber that triggers thin-skinned sector collapse. The updip domain of the sector collapse is localized above a deep-seated zone of volume loss and the downdip domain of the collapse runs down the edifice flank onto the surrounding plain.

Palaeoproterozoic metavolcanic and metasedimentary succession hosting the Dannemora iron ore deposits, Bergslagen region, Sweden

The Dannemora supracrustal inlier is located in the north-eastern part of the Bergslagen region in south-central Sweden and hosts the second largest iron ore deposit in the region. The metasupracrustal succession of the inlier consists of c. 1.9 Ga Palaeoproterozoic rocks that are mainly sub-alkaline, rhyolitic to dacitic, pyroclastic deposits, reworked pyroclastic deposits and metalimestone. It is c. 700–800-m thick and termed the Dannemora Formation. The formation is divided into lower and upper members and the former is in turn subdivided into subunits 1 and 2. The great thickness of individual pyroclastic deposits indicates deposition within a caldera. The rocks show characteristics of a pyroclastic origin by containing abundant pumice, cuspate and Y-shaped former glass shards, and fragmented crystals of quartz and subordinate feldspars. Scattered spherulites and lack of welding-compacted fiamme suggest that the lower member was slightly welded, where as the upper member contains sericite-replaced glass shards with preserved primary shapes indicating no welding. Undisturbed layers of ash-siltstone with normal grading and fluid–escape structures are attributed to subaqueous deposition below storm wave base in the eastern part of the inlier, where as erosion channels and cross-bedding in some of the volcaniclastic deposits imply deposition and reworking above wave base in the central part of the inlier. Epidote spots, previously interpreted as altered limestone fragments and an indicator for subaquatic deposition, are here reinterpreted as the result of selective alteration related to the intrusion of mafic dykes and to Ca release during dolomitisation of limestone.

Evaluation of the Stability of Boston Type I Keratoprosthesis–Donor Cornea Interface Using Anterior Segment Optical Coherence Tomography

To evaluate the anatomic stability of an implanted Boston type I keratoprosthesis (KPro)-donor cornea interface and assess the presence or absence of a potential space (gap) between the KPro front plate and donor cornea using anterior segment optical coherence tomography (AS-OCT). The presence of a gap would raise concerns of a possible pathway for the exchange of extraocular fluid with the anterior chamber. Fifteen eyes implanted with a Boston type I KPro were studied by the noncontact technique of AS-OCT (AC Cornea OCT prototype; OTI, Canada). All the KPro devices had been implanted at least 4 weeks before the study (mean: 7 months, range: 1-22 months). Eight eyes had aphakic Kpros, and the other 7 had pseudophakic implants. Anesthetized eyes were imaged before and during pressure application using sterile cotton-tip applicators. Pressure was applied for 10 seconds on the nasal or temporal side of the eye. Images were analyzed for any possible changes in the KPro-donor cornea interface during the application of pressure. Of 15 eyes, 10 had the threaded front plate model with a T-shaped silhouette and corrugated sides, whereas 5 had the threadless type with a T-shaped silhouette and smooth sides on cross-sectional optical coherence tomography. Of the 15 eyes, 2 revealed a gap between the front plate and the surface of the donor cornea. The rest revealed no gaps. With pressure, none of the eyes, including the 2 with gaps, demonstrated any change in the KPro-donor cornea interface during dynamic imaging (eg, gaping or evidence of fluid escape along the KPro-donor cornea borders). In all eyes, the position of the titanium locking ring was visible and verified to be in an adequate position. The implanted KPro-donor cornea interface seems to be stable dynamically using AS-OCT. A gap that has been documented with this imaging tool showed neither gaping nor escape of anterior chamber fluid during dynamic cross-sectional imaging. Further studies will be needed to assess the cause of these gaps and to determine if they are possibly related to the hydration status of the cornea, suture tension on the donor cornea, or other factors that remain to be determined.

Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme. The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes. Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.

Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran accretionary prism and its implications for hydrocarbon fluid seepage

Using high‐resolution seismic profiles and other geophysical data, collected during R/V Meteor Cruise M74/2, we investigate the distribution patterns of shallow sediments, their structure and deformation processes, and their role in the migration, accumulation and seepage of hydrocarbon‐rich fluids. Here, we show that rapid syn‐kinematic sedimentation at the frontal Makran accretionary prism provides a mechanism by which emerging imbricated thrust packets override the footwall at the seafloor without significant mass‐wasting and destruction of fault‐related anticline in the hanging wall. These anticlines may rise high above the seafloor to form plate‐boundary‐parallel ridges, and distinguish from simple thrust blocks seen at convergent margins elsewhere. With the fast burial of many thrust faults by the syn‐kinematic sediments, anticlinal growth structures form in these syn‐kinematic sediments by continuous thrust activity. The anticlinal structures preserved within the cores of the ridges or formed from these syn‐kinematic sediments act as structural traps for migrating hydrocarbon‐rich fluids, above which fluid escape structures are generated leading to seafloor seeps. Most of the discovered hydrocarbon seeps around Sixth Ridge are sourced from these traps. Despite the compressional environment and the rapid syn‐kinematic sedimentation rates, shallow subsurface of the frontal Makran is a normally pressured regime, in which the buoyancy of hydrocarbons may account for the fluid migration. In this important respect, the Makran accretionary prism differs from many other convergent margins and accretionary prisms, where fluid flow is largely driven by tectonically induced overpressure.

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event.

3D seismic expression of km‐scale fluid escape pipes from offshore Namibia

ABSTRACTThis paper documents a large number of large km‐scale fluid escape pipes with complex seismic expression and a diatreme‐like geometry from the mapping of a 3D seismic survey, offshore Namibia. These pipes are crudely cylindrical, but occasionally have steep conical geometry either narrowing upwards or downwards. They are generally ovoid in planform and their ellipticity varies with pipe height. Vertical dimensions range from ca. 100 to >1000 m and diameters range between 50 and 600 m. The lower part of the typical pipe is characterised by a sag‐like or collapse type of structure, but this is only imaged well in the wider pipes. The upper part of the typical pipe is characterised by gently concave upwards reflections, with a negative relief of tens of metres. There is some evidence (pipe cross‐section geometrical variations and amplitude anomalies) that these concave upwards reflections are vertically stacked palaeo‐pockmarks. A conceptual model for pipe formation is proposed that involves hydraulic fracturing and localisation of focused vertical fluid escape with volume loss at the base of the pipe inducing collapse within the pipe. Continuing episodic fluid migration through the pipe produces further collapsing of the core of the pipe and pockmark structures at the top of the pipe. Longer term seepage through pipes is manifested in zones of amplification of reflections above the top of the pipe.

Cenozoic mud volcano activity along the Indus Fan: offshore Pakistan

ABSTRACTThis study documents the tectono‐stratigraphic setting and expulsion history of a major, previously undescribed mud volcano (MV) province in the Indus Submarine Fan, offshore Pakistan. A buried MV field of nine composite MVs has been recognized using two‐dimensional (2D) and 3D seismic reflection data in a confined area of 50 × 65 km2. Conduits are recognized on each of these MVs connecting the pre‐Eocene parent beds to the stacked mud cones. The buried MVs are up to 8.4 km wide (4.5 km average) with a central conduit of 1.23 km average diameter and an average mud cone thickness of 0.33 km. Three major phases of fluid and mud remobilization occurred in the Early to Middle Miocene, intra‐Middle Miocene and in the Late Miocene to Plio‐Pleistocene transition. Most of the mud source (parent beds) seems to be of pre‐Eocene origin. Geometrical information from 21 mud cones allows an estimate of the volume required to build these fluid escape features. The calculated volume of remobilized sediments is 71.5±9 km3. The location of the MV field is limited to the pre‐Eocene main depocentre, with major tectonic deformation occurring along the wrench system of the Indo–Arabian plate boundary, i.e. the southern edge of the Murray Ridge. The Indus MV field is, to our knowledge, the longest lived (∼22 Myr) remobilized, Cenozoic sedimentary system observed worldwide. No evidence of present‐day mud flow activity is seen on the seabed seismic reflection in the study area.

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part I. Miscibility and foaming of poly( l-lactic acid) in carbon dioxide + acetone binary fluid mixtures

Miscibility and foaming of poly( l-lactic acid) (PLLA) in carbon dioxide + acetone mixtures have been explored over the temperature and pressure ranges from 60 to 180 °C and 14 to 61 MPa. Liquid–liquid phase boundaries were determined in a variable-volume view-cell for polymer concentrations up to 25 wt% PLLA and fluid mixtures containing 67–93 wt% CO 2 over a temperature range from 60 to 180 °C. Even though not soluble in carbon dioxide at pressures tested, the polymer could be completely solubilized in mixtures of carbon dioxide and acetone at modest pressures. Foaming experiments were carried out in different modes. Free-expansions were carried out by exposure and swelling in pure carbon dioxide in a view-cell followed by depressurization. Foaming experiments were also carried out within the confinement of specially designed molds with porous metal surfaces as boundaries to direct the fluid escape path and to generate foams with controlled overall shape and dimensions. These experiments were conducted in pure carbon dioxide and also in carbon dioxide + acetone fluid mixtures over a wide range of temperatures and pressures. Foaming in carbon dioxide + acetone mixtures was limited to 1 and 4 wt% acetone cases. Microstructures were examined using an environmental scanning electron microscope (ESEM). Depending upon the conditions employed, pore diameters ranging from 5 to 400 μm were generated. At a given temperature, smaller pores were promoted when foaming was carried out by depressurization from higher pressures. At a given pressure, smaller pores were generated from expansions at lower temperatures. Foams with larger pores were produced in mixtures of carbon dioxide with acetone.

Widespread active seepage activity on the Nile Deep Sea Fan (offshore Egypt) revealed by high-definition geophysical imagery

Fluid escape structures on the Nile Deep Sea Fan were investigated during the MEDIFLUX MIMES expedition in 2004. Mud volcanoes, pockmarks and authigenic carbonate structures were surveyed for the first time with a high-resolution deep-towed 75 kHz sidescan sonar and a 2–8 kHz Chirp sediment echosounder. In combination with existing multibeam bathymetry and detailed seafloor in situ geological observations, these new data allowed detailed seep analyses. About 60 gas flares were detected acoustically in the water column from the sidescan sonar raw data at water depths from 770 to 1700 m. These gas flares coincide at the seabed with 1) the centres of the mud volcanoes where mud is also extruded, 2) the borders of the mud volcanoes where the emitted gases contribute to the precipitation of authigenic carbonates, and 3) to the edges of broad sheets of authigenic carbonates. Subsurface sediments are commonly disturbed by ascending fluids throughout the delta, with an abundance of seep-related carbonate structures on the seafloor. The feeder channels below mud volcanoes, similar to the gas conduits below the widespread carbonate crust structures and pockmarks, are relatively narrow and, for the vast majority of them, do not exceed a few metres in diameter. The seeps on the Nile Deep Sea Fan clearly follow lineations on the seafloor that we can relate to faults.

Controls on the 129I/I ratio of deep-seated marine interstitial fluids: ‘Old’ organic versus fissiogenic 129-iodine

Iodine and its radioisotope 129I have been successfully used to trace the origin of pore waters in submarine fluid escape structures because of their close association with organic material in deeply buried sediments. We report here halogen concentrations and 129I/I ratios for fluids of five mud volcanoes sampled along an E–W transect across the Gulf of Cadiz in the NE Atlantic Ocean. Concentrations of iodide and bromide increase consistently seaward accompanied by a decrease in 129I/I ratios from 6490 × 10 − 15 to 663 × 10 − 15 . The exceptionally high 129I/I ratios at the near-shore locations reflect the presence of fissiogenic 129I, produced in situ by spontaneous fission of 238U within terrigenous sediments and released into pore water during clay mineral diagenetic processes. The observed 129I/I and halogen trends, together with similar changes in 87Sr/ 86Sr ratios, indicate a progressive seaward transition from inorganic-terrigenous to organic-marine fluid sources. Comparison of our results with literature data for varying geological settings reveals a general relationship between fissiogenic 129I, radiogenic 87Sr and the lithology or provenance of rocks and sediments, respectively. While 129I/I ratios in continental rock-hosted aquifers and terrigenous sedimentary systems are dominated by in situ production of fissiogenic 129I, iodine isotopes in oceanic settings or volcanogenic marine sediments reflect the release of ‘old’ iodine from deeply buried organic matter. The Gulf of Cadiz represents the full transition between these continental and oceanic 129I/I and 87Sr/ 86Sr end members. This is the first systematic investigation of fissiogenic 129I production in marine sedimentary environments.

Lithium isotope geochemistry of marine pore waters – Insights from cold seep fluids

Lithium concentration and isotope data (δ 7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ 7Li (δ 7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ 7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite–illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a −1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.

Old marine seismic and new satellite radar data: Petroleum exploration of north west Labrador Sea, Canada

This paper presents some new concepts in the petroleum systems of the northern Labrador and southern Baffin Island offshore region of eastern Canada. The focus of this work is the region of the Hekja O-71 gas discovery of 1979 by Aquitaine, one of only five wells drilled between 1976 and 1980 within an area covering some 166,000 square kilometers within the Saglek Basin. This study emerged from a broad scale re-examination of the petroleum potential of this area “from the crust up” using regional reflection, refraction and potential field data. An opportunity to use SAR (synthetic aperture radar) data from the RADARSAT-1 Earth observation satellite was taken to incorporate alternative data sources to support this exploratory review and resource assessment. Examination of the final map of interpreted slick-like features on the sea surface, revealed a close correlation of some of these occurrences to several bathymetric features which have underlying seismic signatures similar to previously identified gas hydrate “pipes” or chimney anomalies as seen in data from offshore Nova Scotia and in the Irish Sea. Globally, many active marine hydrocarbon seeps appear on the seafloor as “pockmarks”; in this case no seafloor depressions were seen on conventional multi channel 2-D seismic data associated with active seepage. Instead, very distinct mound-like structures are seen associated with seeps at two locations east of Hudson Strait. The presence of significant cold water coral in close proximity to the active seepage sites may show a link between seafloor petroleum seepage and coral reef development similar to that observed offshore Norway. Close examination of the near surface seismic character led to the interpretation of what was originally seen as seismic noise in vintage data, as likely being fluid escape chimneys or gas pipes, along the eastern edge of the Hekja structural complex, where favourable hydrocarbon trapping conditions are present. No active seeps were seen there; this may be due to other factors, such as the nature of the fluids that are escaping or the possibility of self sealing activity by biological agents such as bacterial matting with associated carbonate hard-ground formation. One seep anomaly was noted relatively close to the major basaltic eruptive complex near the Gjoa G-37 well. This may be due to the presence of volcaniclastic- hosted hydrocarbons similar in style to those recently discovered at the Rosebank field in Paleocene volcanics in the Faroe-Shetland Basin. The occurrence of an active seep in a volcanically-dominated terrain raises the potential for a radical shift in accepted types of petroleum leads and where they may occur. The presence of apparent oil seepage versus the past history of finding only gas-prone accumulations is a strong indicator that a second, oil-prone petroleum system exists in the Saglek Basin. The signs of active petroleum systems as shown by oil seep anomalies with confirming seismic evidence of discrete sea floor structures in close proximity makes a compelling argument for re-examining these areas with an eye towards stratigraphic instead of structural plays.

Effect of acute postural variation on diabetic macular oedema

This study aimed to study the pathophysiology of diabetic macular oedema (DMO) by analysis of concomitant changes in macular volume (MV), mean arterial blood pressure (MABP), intraocular pressure (IOP), and retinal artery and vein diameters in response to acute postural changes in patients with DMO and healthy subjects. Thirteen patients with DMO (13 eyes) and five healthy subjects (five eyes) were examined after resting in a chair for 15 mins using optical coherence tomography to measure MV and fundus photography to assess retinal vessel diameters. The patients then lay down for 60 mins, during which they were examined repeatedly before they were reseated and examined again. Intraocular pressure was measured using pulse-air tonometry, arterial blood pressure by sphygomanometry and fluid columns using rulers and a spirit level. In healthy subjects, retinal artery (p = 0.02) and vein (p = 0.001) diameters decreased when subjects lay down, whereas MV remained stable. In patients with DMO, no orthostatic variation in retinal vessel diameters could be demonstrated, whereas MV had increased by 2.4 +/- 0.6% (mean +/- standard error of the mean; p = 0.006) 50 mins after assuming a recumbent position. In both healthy subjects and DMO patients, MABP decreased and IOP increased in a recumbent position, with no significant difference between the groups. The increase in MV that occurs in DMO when changing from a seated to a recumbent position is associated with a failure of retinal artery contraction, a response seen in healthy subjects that appears to counter-regulate the increase in ocular perfusion pressure caused by assuming a recumbent position.

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out‐of‐sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well‐drained, sand‐rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated décollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Effect of the Presence of a Heat Conducting Horizontal Square Block on Mixed Convection inside a Vented Square Cavity

Finite element method is used to solve two-dimensional governing mass, momentum and energy equations for steady state, mixed convection problem inside a vented square cavity. The cavity consists of adiabatic left, top and bottom walls and heated right vertical wall; but it also contains a heat conducting horizontal square block located somewhere inside the cavity. Forced flow conditions are imposed by providing an inlet at the bottom of the left wall and an exit at the top of the right wall, through which the working fluid escape out of the cavity. The aim of the study is to describe the effect of such block on the flow and thermal fields. The investigations are conducted for various values of geometric size, location and thermal conductivity of the block under constant Reynolds and Prandtl numbers. Various results such as the streamlines, isotherms, heat transfer rates in terms of the average Nusselt number, average fluid temperature in the cavity and the temperature at the center of solid block are presented for different parameters. It is observed that the block size and location have significant effect on both the flow and thermal fields but the solid-fluid thermal conductivity ratio has insignificant effect on the flow field. The results also indicate that the average Nusselt number at the heated surface, the average temperature of the fluid inside the cavity and the temperature at the center of solid block are strongly dependent on the configurations of the system studied under different geometrical and physical conditions.

Pockmarks, collapses and blind valleys in the Gulf of Cádiz

Herein we describe a suite of fluid escape depression features, including pockmarks and collapse structures, discovered in the Gulf of Cadiz (Spain) during several recent cruises. We also establish an evolutionary model for these depressions and discuss the generation of bottom undercurrent furrows from fluid-flow structures, considering the oceanographic and tectonic framework and gas expulsion mechanisms. We describe for the first time blind valleys, which we define as giant, elongated (3 to 10 km long), collapsed and complex fault-strike features comprising mega-collapses and mega-pockmarks, generated in gas-venting areas and not associated to the collapse of mud-volcano complexes. We detected the blind valleys above diapiric structures. The collapse processes associated to blind valleys result from fluid escape through migration pathways which, in turn, are created by distension due to diapiric activity or to later tectonic reactivation of these diapirs. The evolution of these blind valleys, and their present-day morphology as furrows, derives from progressive fluid migration as well as from interaction of Mediterranean Outflow Water with the seafloor.

An early ecosystem of Archean tidal microbial mats (Moodies Group, South Africa, ca. 3.2 Ga)

Abundant microbial mats from the Mesoarchean Moodies Group (Barberton Greenstone Belt, South Africa, ca. 3.2 Ga) are densely interbedded with coarse-grained and gravelly sandstones in a nearly mud-free setting. They apparently grew in marginal marine and possibly subaerial coastal and tidal environments. Characteristic sedimentary structures include anastomosing bedding, gas or fluid escape structures, sand volcanoes, biomat doming, patchy silicified microstromatolites, and microbial sand-chip conglomerates. They indicate rapid growth of mechanically tough microbial mats, possibly aided by early seafloor silicification, in a high-energy, high-sedimentation-rate environment. The observations expand the knowledge of the habitat of Archean microbial mats and life on early Earth. If tidal environments were more widespread in the Archean than today, biomats in extensive intertidal settings may have contributed to immobilize large volumes of sand.

Aseismic controls on in situ soft‐sediment deformation processes and products in submarine slope deposits of the Karoo Basin, South Africa

AbstractIntervals of soft‐sediment deformation features, including vertical fluid escape and load structures, are common and well‐exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine‐grained material transported through the flame onto the palaeo‐seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment‐rich fluids. Flames are oriented parallel to the deep‐water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north‐west‐facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep‐water succession. The driving mechanisms that resulted in the final configuration of the soft‐sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground‐penetrating radar profiles confirm the overall north‐east orientation of the flame structures and provide a basis for recognition of potential larger‐scale examples of flames in seismic reflection data sets.

Effects of rapid sedimentation on developing the Nyegga pockmark field: Constraints from hydrological modeling and 3‐D seismic data, offshore mid‐Norway

Three‐dimensional (3‐D) seismic data in the Nyegga region expose hundreds of seafloor pockmarks and fluid escape chimneys in close proximity to the northern sidewall of the Storegga Slide, on the formerly glaciated Norwegian margin. As the up to 350 m wide individual craters of the Nyegga pockmark field postdate the Late Glacial Maximum (∼25 calendar (cal) ka B.P.) and because modern fluid flux is characterized as microseepage at most, it appears that the pockmarks developed nearly simultaneously with some specific event. External types of forcing such as earthquake loading, the Storegga Slide (8.1 cal ka B.P.), or rapid sediment loading represent potential trigger mechanisms for developing hundreds of pockmarks within a millennial time window. We integrate the 3‐D seismic observations with a two‐dimensional (2‐D) sedimentation–fluid flow model to quantify the effects of sediment loading and erosion on overpressure generation and fluid expulsion. The models simulate the temporal evolution of compaction‐driven overpressure and fluid expulsion based on sedimentation rates estimated from seismic data and hydrologic and sediment properties obtained from laboratory experiments and petrophysical data. The 2‐D model predicts rapid overpressure generation in response to high sedimentation rates during the last (Weichselian) deglaciation period (25–18 cal ka B.P.). The high pressures significantly increase fluid expulsion, which peaks between 19 and 16 cal ka B.P. The modeled high‐flux period and the following declining flux period exhibit an excellent correlation with existing age dates of methane‐derived authigenic carbonate retrieved from Nyegga pockmarks. Accordingly, rapid sediment loading provides a mechanism to drive extensive cold seep formation. Our model suggests that the Nyegga pockmark field developed between 19 and 16 cal ka B.P. and therefore predates the giant Storegga Slide event.