E. E. Davis

An episode of seafloor spreading and associated plate deformation inferred from crustal fluid pressure transients

Three-year records of crustal fluid pressures and temperatures at four Ocean Drilling Program (ODP) sites on the northern Juan de Fuca Ridge and eastern ridge flank reveal a broad range of variations that include hydrologic transients that are contemporaneous with earthquakes along the ridge axis, the Nootka transform fault, and within the Juan de Fuca plate. One example of such a transient is the response to what is inferred to be a seafloor spreading event that triggered a swarm of earthquakes near the Endeavour ridge segment, beginning with a MW = 4.6 event on June 8, 1999. The largest transients were observed at ODP Sites 1024 and 1025 located 25.6 and 33.5 km east of the Endeavour axis. Pressures rose coseismically with the first earthquake of the swarm by roughly 1.6 and 1 kPa, continued to rise to maxima of 3.2 and 2.0 kPa within hours, then decayed to half the peak levels in 1 and 2 days at the two respective sites. A small (∼0.2 kPa) response of the same sign followed by a decay over 100 days was observed at Site 1027 situated 101 km east of the axis, and a similarly small response but of opposite sign was observed at Site 857 in Middle Valley, located along strike of the Endeavour segment roughly 50 km to the north of the earthquake swarm. The pressure transients are inferred to reflect a combination of the instantaneous internal plate deformation associated with extension at the ridge axis and lateral water flow in the crust following strain-induced fluid pressure gradients. The rate at which the transients dissipate constrains the regional-scale permeability of the upper igneous crust to be of the order of 10−10–10−9 m2. Instantaneous strain calculated from the initial amplitude of the transients ranges from 8×10−9 at the most distal site to 1.7×10−7 at the most proximal. The magnitude of regional strain is much larger than that which would result from the simultaneous earthquake, and we conclude that the first and all subsequent earthquakes of the swarm, and the crustal strain responsible for the hydrologic transients, are the consequence of a dominantly aseismic spreading “event” involving ∼12 cm of dilatation at the ridge axis. There were no clear indications of associated magmatic activity; hence episodic spreading may take place outside the influence of either dike injection or seismic rupture. Given the excellent sensitivity of pore pressure to strain, we anticipate that this simple observational technique can be applied to the investigation of seismic and aseismic deformation in a variety of tectonically active settings.

The measurement of marine geothermal heat flow by a multipenetration probe with digital acoustic telemetry and insitu thermal conductivity

The design and use of a marine heat probe with capability for measuring thermal conductivity insitu with high accuracy, and providing digital acoustic transmission of data to the ship, is described. The instrument employs the ‘violin bow’ strength member and parallel sensor string configuration suggested by C. R. B. Lister. Several hundred measurements have been made in the deep ocean on multipenetration or ‘pogostick’ profiles using a 3 m probe and in deep inlets of western Canada using a 7 m probe. The insitu thermal conductivity technique using a calibrated heat pulse has been studied in detail through laboratory calibration of the probe in materials of known conductivity, through numerical models, and through comparison of insitu measurements with needle probe measurements on sediment cores taken from the same sites. The insitu technique permits a conductivity accuracy of better than ±5% with a recording time of 7 minutes following 7 minutes in the bottom to establish the geothermal gradient. The pulse heating is also more energy efficient than the conventional continuous heating technique.

The permeability of young oceanic crust east of Juan de Fuca Ridge determined using borehole thermal measurements

Temperature measurements made in Ocean Drilling Program (ODP) Hole 1026B, drilled into a sediment-buried basement ridge in 3.5 m.y. old crust on the eastern flank of Juan de Fuca Ridge, revealed fluid flow from the formation and into the overlying ocean. This flow indicates that the upper basaltic crust at this site is naturally overpressured relative to hydrostatic. This finding is consistent with output from previously published numerical models of hydrothermal circulation in the upper crust that include appropriate crustal geometry, sediment thickness, and basal heat flow. The fluid flowing into Hole 1026B enters the hole through the upper 10 m of basalt below the sediment-basement contact. The flow rate estimated from the thermal data (80–120 m/hr), in combination with the inferred basement overpressure relative to hydrostatic (about 20–30 kPa), was used to estimate the bulk permeability of the shallowest oceanic crust: 5 to 9 × 10−12 m². Such high bulk permeability would allow regionally significant hydrothermal circulation, consistent with thermal homogenization of upper basement in this area.

Formation‐scale hydraulic and mechanical properties of oceanic crust inferred from pore pressure response to periodic seafloor loading

Observations of fluid pressure variations in young igneous oceanic crust have been made in five sealed and instrumented Ocean Drilling Program boreholes on the flanks of the Mid-Atlantic and Juan de Fuca Ridges. The holes penetrate locally well sedimented, and hence hydrologically well-sealed crust, and are situated 1 to 85 km from areas where water can flow freely through the seafloor at extensive basement exposures. Amplitudes and phases of formation pressure variations have been determined relative to tidal and nontidal pressure variations measured simultaneously at the seafloor for periods ranging from 4.8 hours to 14 days. Formation pressure variations are reduced to amplitudes between 98% and 28% relative to those at the seafloor and shifted in phase by up to 20°. Simple theory for porous media response to periodic loading predicts that the scale of diffusive signal propagation from locations of basement outcrop through buried parts of the igneous crust should he proportional to basement permeability and the inverse square root of the period of the variation. This behavior is consistent with the observations, and the characteristic half wavelength of the diffusive signal defined by the data from the sites near basement exposures is 14 km at diurnal periods. If signals propagate in a simple one-dimensional manner, this requires a formation-scale permeability of 1.7 X 10 10 m 2 . No constraints are provided on the thickness of material characterized by this permeability, hut the high-permeability pathway must be laterally continuous. At two sites near basement exposures the hulk modulus of the rock matrix estimated from the elastic component of the pore pressure response is 3 GPa. Where the igneous crust is regionally well sealed by sediment, the elastic response yields a hulk modulus of 16 GPa. The increase in hulk modulus with age and distance from basement outcrop is consistent with an observed increase in crustal alteration, an increase in seismic velocity, and a decrease in permeability, Observed lateral gradients of pressure, coupled with the estimated permeability, suggest that the amplitude of semidiurnal tidal volumetric flux (Darcy velocity) exceeds 10 -6 m s -1 ; semidiurnal fluid particle excursions would reach 0.25 m if the full volume of water contained in 10% porosity of the rock matrix were involved. If flow is channelized along discrete pathways, tidally modulated fluid flow velocities and particle excursions would he locally greater. The amplitude of tidal velocity is similar to that estimated for buoyancy-driven hydrothermal convection, hut the direction is generally different. Thus tidal flow may enhance water-rock interactions significantly. Energy dissipated in this manner would approach 0.3 μW m 3 .

Thermal effects of sediment thickening and fluid expulsion in accretionary prisms: Model and parameter analysis

We investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically. Variations landward across the wedge in the porosity-depth function are included in the model. The most important contribution to the thermal regime arises from the thickening of the wedge, which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent. This effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments. The total thermal perturbation is sensitive to the prism taper angle, the incoming sediment thickness, and the convergence velocity. Anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth. The thermal consequences of retarded fluid expulsion are significant, and result in locally depressed seafloor heat flow in this region. For the full observed range of accretion parameters, the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km.

Pore pressures and permeabilities measured in marine sediments with a tethered probe

A new probe for measuring pore fluid pressures in marine sediments has been constructed and tested in two hydrologic environments on the Juan de Fuca Ridge. The instrument utilizes a highly incompliant high-sensitivity differential pressure transducer, and a small (4mm) diameter sensing probe that telescopes within a larger diameter 1.5-m-long (currently 2.5 m) strength member. The small probe diameter and the low compliancy serve to minimize the time required for pressures to approach equilibrium after penetration. Observed decay times are short enough to permit the instrument to be used in a tethered mode. The mechanical configuration enables the small diameter probe to penetrate the sediments without buckling and to be pulled out without bending. This allows multiple penetrations to be completed during a single instrument lowering. Ten measurements were made during the development and testing of the instrument in fine-grained turbidite sediments on the Juan de Fuca Ridge. Four penetrations were uninterrupted by mechanical disturbances or instrumentational problems, and although equilibrium conditions were approached during only one penetration (2.7 hours maximum undisturbed decay period), all provided useful constraints on equilibrium pore pressures. Values at two sites where sediment cover is thick and heat flow relatively low were unresolvably different from hydrostatic. Values estimated from two penetrations located within a few hundred meters of a hydrothermal vent field within the sedimented axial valley of the northern Juan de Fuca Ridge, Middle Valley, exceeded hydrostatic and yielded pressure gradients of roughly 0.5 kPa m−1. Permeabilities were estimated from the rates of decay of the penetration transients. Values ranged from 5×10−16 to 1×10−14 m2 and agreed well with values measured on core samples collected near the probe penetrations which ranged from 8×10−16 to 1×10−14 m2. Using the values determined from the probe measurements from Middle Valley, the implied rate of fluid flow upward through the sediments in the vicinity of the vent field is roughly 5×10−10 m s−1, or 15 mm yr−1. With the current practical limit of resolution of roughly 10−10 m s−1 (3 mm yr−1) this direct measurement technique provides a means of determining pore fluid flow rates that is roughly 1 order of magnitude more sensitive than the method of estimating the rate of flow from the perturbation of the conductive thermal regime measured by typical marine heat flow instruments.

Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring

One primary objective of Integrated Ocean Drilling Program (IODP) Expedition 365, conducted as part of the NanTroSEIZE project, was to recover a temporary observatory, termed the “GeniusPlug“ emplaced to monitor formation, pore fluid pressure and temperature within a major splay fault that branches from the main plate interface, at a depth of ~400 m below sea floor (mbsf). The instruments were installed in Dec. 2010 and recovered in April 2016, yielding 5.3 years record of formation pressure and temperature within fault zone. Here, we use the pressure timeseries, and in particular the response to ocean tidal loading, to evaluate changes in physical properties of fault zone induced by several regional earthquakes. To accomplish this, we quantify: (1) the amplitude of the formation’s response to tidal loading, defined in terms of a tidal loading efficiency, governed primarily by the formation and fluid elastic properties; (2) the phase lag between the ocean tidal signal and the measured response in the observatory, which is governed by a combination of formation hydraulic diffusivity and the relative compressibilities of the formation and sensing volume; and (3) pressure steps associated with earthquakes, identified in formation pressure after removal of the tidal signal. We observe essentially no phase lag, but in for many events we detect both pressure steps and transient decreases in loading efficiency. To reveal the cause of these changes, we investigate the effects of static and dynamic crustal strains. Changes in theoretical static volumetric strain and the associated expected pressure step for each event are calculated based on Okada (1992), and using a conversion from volumetric strain to pore pressure based on formation properties defined by laboratory experiments. We find that, there is no clear correlation between observed pressure steps and predicted static volumetric strain; furthermore, the predicted pressure steps are ten to hundreds of times smaller than observed. As a proxy for dynamic strains, we calculate the integrated “pressure energy density” over a 30 minute window for each event, and show a positive correlation with both step changes in pressure and changes in loading efficiency. Most of the detected changes represent pressure increases and loading efficiency decreases. We speculate that disruption of grain contacts and subsequent pore collapse induced by dynamic strain produces changes of hydraulic properties in the fault zone. Alternatively, these changes could reflect exsolution of gas from pore fluids that drives pore pressures up while simultaneously reducing loading efficiency by increasing the compressibility of pore-filling fluids. Finally, the observed amplitude response and negligible phase lag of the formation pressure response to ocean tidal loading, taken together, allow an estimate of the minimum hydraulic diffusivity of splay fault of 8.9×10 m/s. U03-P02 JpGU-AGU Joint Meeting 2017

The structure and tectonic history of the western Canada subduction zone

Abstract As part of the Lithoprobe multidisciplinary earth sciences research program, multichannel Seismic profiles from the deep sea across the continental shelf and on Vancouver Island, coupled with a wide range of other geophysical and geological studies, have permitted detailed delineation of the structure and tectonic history of the Juan de Fuca subduction zone in western Canada. The modern continental margin was built against a pre-Tertiary continent consisting of amalgamation of older terranes, the oldest and westernmost being Wrangellia. No pre-Eocene continental margin record exists and transform fault displacement of margin rocks north to Alaska is inferred. Eocene subduction was interpreted by a seaward jump in the trench axis in latest Eocene, trapping a section of marine volcanics (Crescent Terrane), together with sections of Mesozoic marine sedimentary rocks (Pacific Rim Terrane). These were placed against and under the margin on steeply dipping thrust faults that are well imaged in the Seismic reflection data. The terrane positions are also well delineated by magnetic and gravity data. Seaward and underlying the Crescent Terrane is the modern accretionary prism which is up to 100 km wide. Development of the deformation front at the toe of the continental slope is clearly shown by landward and seaward verging faults in the sediments overlying the basaltic oceanic crust. SeaMARC II acoustic images illustrate the surficial effects of the deformation front. Beneath the deep ocean, the continental shelf and the western part of Vancouver Island, the top of the downgoing oceanic plate is imaged well by a one or two cycle continuous reflection at depths which are consistent with those determined for the top of the plate from Seismic refraction data and Wadati-Benioff zone earthquakes. Above the subducting slab, two broad bands of high reflectivity dip beneath Vancouver Island at angles less than that of the slab. The deeper band coincides with a dipping layer of high conductivity interpreted from a magnetotelluric survey. On the basis of profile heat flow measurements which were converted to isotherms, this layer is isothermal at about 450°. One tectonic model for the origin of the reflective bands suggests that they are thick sections of underplated volcanic and sedimentary material, the lower one of which would be saturated with hot saline fluids. An alternative interpretation, supported particularly by the dipping isotherms, suggests that the lower band is caused by fluids that are driven off the downgoing oceanii: plate and are trapped below an impermeable horizon formed at a metamorphic front.