Earl E. Davis

A mechanism for the formation of methane hydrate and seafloor bottom‐simulating reflectors by vertical fluid expulsion

Bottom-simulating reflectors (BSR) are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation. They are believed to correspond to the deepest level at which methane hydrate (clathrate) is stable. We present a model in which BSR hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field. In this model, most of the methane is generated below the level of hydrate stability, but not at depths sufficient for significant thermogenic production; the methane is primarily biogenic in origin. The model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid. The model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field, for the source of the large amount of methane contained in the hydrate, and for BSRs being common only in special environments. Strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes, so BSRs are uncommon. Upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation. In these areas hydrate BSRs are common. The most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field. The critical boundary in the phase diagram between the fluid-plus-hydrate and fluid-only fields is not well constrained. However, the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep-sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production. In most fluid expulsion regimes, the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation. Thus, as observed in borehole data, there are no large chloride or isotope anomalies remaining in the local pore fluids. The differences in the concentration of methane and probably of CO2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions.

Accretion and recent deformation of sediments along the northern Cascadia subduction zone

New multichannel seismic reflection profiles, acoustic imagery, and swath bathymetric data from the Vancouver Island continental margin reveal the surface morphology, cross-sectional geometry, and deformation style of this part of the Cascadia accretionary prism. The incoming 3- to 5-km-thick sediment section fails most commonly along landward-dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust. The faults are listric with approximately circular cross sections, and hanging-wall sediments are back-tilted in a planar manner. High, near-lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement, and from the long distances over which the decollement has propagated seaward. The faults dip more steeply at 40°-45° near the surface where Pleistocene turbidites are displaced. Similar folds have developed where the turbidites are particularly thick (>4 km), and near-hydrostatic pore pressures are inferred to occur in the turbidite part of the section. An increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading, particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites. The top of the downgoing Juan de Fuca plate, which defines the base of the accreted sedimentary wedge beneath the margin, is well imaged in the new seismic reflection profiles and is further defined by land reflection data, seismic refraction, and earthquake hypocenters. The landward-dipping continental backstop to the prism is also well located. If these constraints are used for the geometry of the accretionary prism, a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42-m.y. accretion history. This result, along with the observation that deformation-front faults penetrate most or all of the sediment section, suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the Eocene. Application of the critically tapered wedge model of Davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism, particularly the seawardmost part beneath the lower slope. Beneath the continental shelf, the dip of the subducting slab increases to greater than 11°, a dip that according to the critically tapered wedge model, is too steep to allow upward growth of an accretionary prism. This provides a simple explanation for ongoing subsidence of the Tofino sedimentary basin over the older part of the accretionary complex. To the southeast, the Juan de Fuca plate dips much more gently beneath the Olympic Peninsula of northern Washington, allowing the accretionary prism to grow upward well above sea level to form the Olympic Mountains.

Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge

We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin . Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO 2 , sulfate, K, Li, U, O 2 , nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H 2 S, H 2 , CH 4 , 222 Rn, and 226 Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4–13 L/s and 2–3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.

An unequivocal case for high Nusselt number hydrothermal convection in sediment-buried igneous oceanic crust

New observations of seafloor heat flow, precisely located along seismic reflection profiles crossing a buried ridge on the eastern flank of the Juan de Fuca Ridge, show a nearly exact inverse correlation between heat flow and sediment thickness, such that the basement-sediment contact appears isothermal to within 10 K, despite a factor of three local variation in sediment thickness. We have used these observations with numerical models to infer hydrothermal heat-transport properties of the upper oceanic crust at this 3.5 Ma site. Model results show that, while fluid circulation is stimulated by the effects of basement topography even at sub-critical Rayleigh number conditions, the creation of a nearly isothermal basement surface requires very high heat-transport efficiency. Lower limits for the Nusselt number ( Nu ≥ 25), for the Rayleigh number ( Ra ≥ 4000), and for the permeability (κ ≥ 10−11 m2), are provided by assuming that high permeability is distributed throughout the uppermost 600 m of relatively low-velocity igneous crust at this site. Relatively high permeability can also be inferred by considering the calculated fluid pressure regime in light of what is known about the relationship between fluid seepage through the sediment section and the underlying basement topography from geochemical data: Local super-hydro-static pressure and fluid discharge above buried basement ridges can occur only if basement permeability is higher than 10−13 m2. Unfortunately, no constraint on the actual distribution of high permeability below the top of the igneous crust is provided by the thermal regime based on the heat-flow and seismic observations. Equally uniform upper basement temperatures can be produced by fluid flow in a thinner layer (of thickness h) having a correspondingly higher Nusselt number and permeability. Only the products of Nu × h, and κ × h2 are constrained. Bulk permeabilities (averaged over intervals a few hundred meters thick) measured in boreholes that have penetrated the upper oceanic crust are typically less than 10−13 m2. The much higher formation-scale permeability we infer may be a consequence of the relative youth of the crust at this site, although it is more likely that the interconnected fractures and extrusive volcanic unit contacts and voids that contribute most to the bulk formation permeability are relatively infrequent and not representatively sampled by drilling.

On the cause of the asymmetric distribution of seamounts about the Juan de Fuca ridge: ridge-crest migration over a heterogeneous asthenosphere

Abstract The distribution of non-hotspot seamounts in the northeast Pacific is highly asymmetric; small seamount chains and isolated edifices are numerous on the Pacific plate, but nearly absent on the Juan de Fuca plate. We propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference. Plate motion solutions indicate that the Juan de Fuca ridge is migrating to the west at a ridge-perpendicular rate of 20 mm a−1, which is large compared to the half-spreading rate of 30 mm a−1. Migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre. If slightly enriched, lower melting temperature heterogeneities are present in the upper mantle, they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite. Seamount volcanism will occur preferentially on the Pacific plate because of two factors: Firstly, more asthenosphere and hence more early-melt heterogeneities must ascend to supply the Pacific plate which is translating more rapidly than the relatively stationary Juan de Fuca plate. Secondly, the asthenosphere that is required to ascend to supply the thickening of the Juan de Fuca plate will have been flushed of its significant shallow-level heterogeneities by the previous advance and passing of the ridge; few should remain to cause volcanism on the Juan de Fuca plate. Testable physical and petrologic predictions of this model can be identified. For example, within a small seamount chain, the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis. Variation in lava chemistry along a small seamount chain, which is interpreted to represent magmatism from a single large heterogeneity, will provide the major discriminant of this hypothesis. Systematic variations should be observed, with earlier formed edifices being constructed of more enriched lavas, and later (younger) ones of more depleted lavas. Isotope ratios should be unaffected by varying degrees of partial melting, but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant.

Theory for the propagation of tidally induced pore pressure variations in layered subseafloor formations

Tidally induced pore pressure variations below the seafloor depend on the elastic moduli and transport properties of the pore fluid and formation. Hence observations of pore pressure variations, in conjunction with model predictions, can provide important constraints on these formation properties. In this paper, we study the propagation of tidally induced pore pressure variations in a layered poroelastic medium. We derive an analytic solution and use the solution to investigate the effects of various parameters, in particular, the bulk modulus of the formation, the bulk modulus of the pore fluid, and the formation permeability. Specific examples are considered that include the typical continuous depth variation of properties that occurs through normal sediment consolidation and imbedded layers of contrasting properties. Diffusive propagation of tidal pressure variations from the seafloor depends on the hydraulic diffusivity. The depth limit of diffusive propagation scales with the inverse square root of permeability and the period of the signal; for typical fine-grained marine sediments the depth scale at tidal frequencies is only a few meters. Any internal contrast in elastic properties, due to the presence of free gas for example, can give rise to large instantaneous pressure changes across a layer boundary, which in turn can induce diffusive propagation of signals above and below the interface. Long-term pressure records from sealed deep-ocean boreholes that included tidal signals are considered in light of the model results. In Ocean Drilling Project (ODP) Hole 857D on the Juan de Fuca Ridge, the observed attenuation of the seafloor tidal signal to 15% is consistent with the relatively low compressibility of the hydrothermally indurated section intersected by the open part of the borehole and with the high compressibility of the hot formation fluid. In ODP Hole 892B in the Cascadia accretionary prism, the attenuation to 55% and several degree phase lead of the formation tidal signal are probably the result of the open part of the hole being connected to an overlying interval bearing a few percent free gas via a high-permeability fault zone.

Observations of natural-state fluid pressures and temperatures in young oceanic crust and inferences regarding hydrothermal circulation

Abstract Four boreholes, drilled a few tens of meters into igneous basement on the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, were sealed and instrumented for long-term monitoring to observe the hydrologic state of young sediment-sealed oceanic crust. The thermal regime is dominated by the effects of rapid fluid circulation in uppermost igneous basement driven by very small non-hydrostatic pressure gradients. Upper basement temperatures are uniform laterally between pairs of holes over distances of hundreds of meters to kilometers. In the case of two holes drilled into a sediment-buried basement ridge and adjacent valley, basement temperatures differ by less than 2 K despite the 2.2 km lateral separation of the sites and the 2.5:1 contrast in sediment cover thickness. Under conductive conditions, upper basement temperatures would differ by roughly 50 K. By comparison with modeling results, the observed degree of isothermality suggests a fluid flux of at least 10−6 m s−1 (30 m yr−1), and an effective permeability in the range of 10−10–10−9 m2 in the uppermost igneous crust. The pressure difference available to drive this rapid flux between the ridge and valley, estimated by comparing the observed pressures via the isothermal upper basement hydrostat that is inferred to connect the two sites, is small (≈2 kPa) and also suggests high permeability. Relative to the hydrostats defined by the local conductive sediment geotherms, substantial super-hydrostatic pressure (+18 kPa) is present within the buried basement ridge, and sub-hydrostatic pressure is present in the adjacent valley (−26 kPa). Such pressure differentials are the direct consequence of the advection-dominated thermal regime and small pressure losses in high-permeability basement, and are available to drive fluid seepage through sediment sections vertically up above and horizontally away from buried ridges, and down above valleys. No constraints are provided by any of the observations on the depth in the crust to which thermally or chemically significant flow might extend, although just as in the overlying sediments, the pattern of deep flow may be affected by the near-isothermal and near-hydrostatic conditions present in the permeable uppermost crustal section.

Detailed geomorphology and neotectonics of the Endeavour Segment, Juan de Fuca Ridge: New results from Seabeam swath mapping

A 36-km-wide corridor of Seabeam bathymetry was collected on the Juan de Fuca Ridge, north of 48°00′N, in May 1983. These data, merged with previous Seabeam bathymetric compilations for the Cobb Offset, as well as with Sea MARC I and Sea MARC II side-scan sonar, seismic-reflection, magnetometer, and deep-tow photography data, have been used to identify morphological variations along the spreading center of the Endeavour Segment of the Juan de Fuca Ridge (between the Cobb Offset and Sovanco Fracture Zone). The Endeavour Segment consists of two ridge sections, separated by the 13-km-wide Endeavour Offset, which formed when spreading jumped from Middle Valley to West Valley < 200,000 yr ago. The locus of spreading occurs along the entire Endeavour Segment as a narrow (1–2 km wide), shallow (10–30 m deep) inner rift, which is superimposed upon larger structures that range from broad (5–10 km wide), deep (as much as 3,000 m) median valleys to narrow (5 km wide), shallow (2,100 m) volcanic ridges. An abrupt discontinuity in the tectonic fabric occurs at about 48°05′N. South of this latitude, spreading occurs within an axial high (Endeavour Ridge), which gradually deepens to the south (South Endeavour Valley) as it approaches the zone of overlap with the propagating rift at the Cobb Offset. North of 48°05′N, the axis is characterized by broad, deep, median valleys (North Endeavour Valley and West Valley). These large-scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism, waning magma supply at the distal ends of ridge axes, thermal contrasts across ridge offsets, propagating rifts, and an unstable triple junction. The pronounced axial deep of West Valley is interpreted as reflecting the youth of the structure (< 200,000 yr) and may be primarily due to collapse, with little extension and magmatism. The rift axis jump to West Valley fortuitously isolated Endeavour Seamount, the youngest edifice in the Heck Seamount chain, from the Pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed Endeavour Offset. Development of a remarkably simple overlapping conjugate rift pair at Endeavour Offset has resulted, in spite of the complex and highly variable crustal thickness, magma supply, and pre-existing structural grain. Rifting in South West Valley has caused pre-existing topography generated at the Endeavour Ridge to be destroyed by subsidence and burial, thereby creating the apparent discontinuity in morphology north of 48°05′N.

INFLUENCE OF BASEMENT TOPOGRAPHY ON HYDROTHERMAL CIRCULATION IN SEDIMENT-BURIED IGNEOUS OCEANIC CRUST

Abstract Hydrothermal convection in the upper oceanic crust has been inferred to be a globally common and important process. Under the simplest conditions of planar boundaries, lateral dimensions of convection cells provide a strong constraint on the vertical extent of significant permeability, and on the depth of penetration of convection. It has been suggested, however, that topography can exert a strong influence on the pattern of circulation, potentially making it impossible to be use patterns of heat-flow variations to constrain the depth of circulation. We have investigated convection as it is influenced by sediment-buried basement topography for a range of Rayleigh numbers from well below critical conditions ( Rac) up to the limit of steady-state convection (∼ 10 Rac), and for a variety of topographic wavelengths and amplitudes, using an advanced numerical modelling scheme. We find that convection is always stimulated by topography at sub-critical conditions, although flow velocities are in general too small to be thermally significant. Sub-critical flow is controlled by the buried topography, with upwelling beneath warm basement troughs, lateral flow rising along the basement-sediment interface, and descending flow beneath basement ridges. The opposite direction of flow occurs, only when the slope of isotherms in basement is reversed, as in the case where the sediment cover is conformal and thin. At Rayleigh numbers between critical and about 5 Rac, convection occurs as a result of Rayleigh instability, with a natural half-wavelength between 1 and 2 times the convective layer thickness, similar to that which is present in the absence of topography. Buried topography influences the pattern of flow only weakly. At Rayleigh numbers higher than about 5 Rac, whether the pattern of convection is controlled by topography depends on the slope and wavelength of the topography. Over most of the ranges of these parameters representative of the topography of typical oceanic crust, however, the convection pattern follows topography similar to the sub-critical topographically driven convection, but with much higher velocities. The results are not applicable to Rayleigh numbers above about 10 Rac, conditions that are probably common in buried young oceanic crust, which requires transient simulations. Fluid flow is also stimulated under the influence of topography both through the sediments that bury the permeable upper crust, and in deeper basement. Owing to low characteristic permeabilities, this flow is thermally insignificant, but it may be geochemically important because it must persist as long as the perturbations caused by the buried basement topography and the convection in the permeable part of the crust are present.

New evidence for age variation and scale effects of permeabilities of young oceanic crust from borehole thermal and pressure measurements

In 1996, long-term sealed-hole hydrological observatories with subseafloor temperature and pressure sensors were installed in four cased holes drilled by the Ocean Drilling Program into sedimented young oceanic crust east of the Juan de Fuca Ridge. Data recovered over a year later showed that all four holes displayed temperature profiles indicative of vertical fluid flow immediately prior to their being sealed. Warm water was being produced from basement in two cases, and cool ocean bottom water was being drawn into basement at the others. Linear flow rates of ∼60–200 m/h were estimated from the perturbation of the temperature profiles relative to undisturbed geothermal gradients at the sites. The pressure differentials driving the flow were also measured at the time of the observatory installations, allowing estimates of permeabilities of the upper crustal sections penetrated by the holes. Estimated permeabilities vary systematically with age, ranging from about 10−10 m2 in the youngest site (0.9 Ma) to 10−12 m2 in the oldest site (3.6 Ma), confirming an apparent reduction of permeability with age determined with packer experiments at three of the same sites. Combined with other estimates of permeabilities in the same holes using methods with different scales of investigation, the new permeability estimates also provide evidence for a significant scale dependence of permeability in the upper oceanic crust.