ALBERT

Beschreibung: The 2012 Mw 7.8 Haida Gwaii earthquake confirmed very oblique subduction and slip partitioning at the southern Queen Charlotte margin. In this study, we re‐examine the thermal regime near the earthquake using new model constraints and with the recognition that hydrothermal circulation in the subducting oceanic crust can significantly affect the margin thermal regime. The observed heat flow values are extremely high just seaward of the trench but decrease rapidly landward. We explain this pattern as the consequence of very vigorous hydrothermal circulation in the subducting oceanic crust. Using a finite‐element model, we simulate the thermal effect of the circulation using a high‐conductivity proxy that represents a very high Nusselt number in an aquifer along the top of the oceanic plate. Our thermal model indicates that the temperature at the intersection of the megathrust and the strike‐slip Queen Charlotte fault (QCF) just seaward of the coast is about 350° C, approximately the limit of seismogenic behavior, and cooler than previous models that did not include hydrothermal circulation. The change of plate motion kinematics across the QCF approximately coincides with a down‐dip transition of the thermally controlled seismogenic behavior of the megathrust. Seaward of the QCF, the shallow megathrust accommodates mainly the margin‐normal component of relative plate motion, with the strike‐slip component accommodated by the QCF. This portion of the megathrust exhibits stick slip and produced the 2012 Haida Gwaii earthquake. Landward of the QCF, the megathrust fully accommodates the very oblique motion of the oceanic plate beneath the continental crust and exhibits creep.

Beschreibung: This article provides a summary of the structure and tectonic history of the Queen Charlotte transform fault zone off western Canada, as background to understanding the 2012 Mw 7.8 thrust earthquake off Haida Gwaii. There was margin subduction prior to the Eocene. The fault zone then became the mainly transcurrent Pacific–North America boundary. There was mid‐Tertiary oblique extension, then 15°–20° oblique convergence from ∼6  Ma to the present that resulted in underthrusting and subduction initiation. The total underthrusting has been too small for Benioff–Wadati seismicity or arc volcanics but is indicated by (1) a trench, the Queen Charlotte Trough, into which the oceanic plate bows downward and an offshore flexural bulge, the Oshawa rise; (2) the Queen Charlotte terrace, an accretionary sedimentary prism; (3) seismic receiver function delineation of the underthrusting Pacific plate; (4) heat flow decreasing landward as predicted for underthrusting; (5) low gravity offshore and high onshore, consistent with underthrusting; and (6) late Tertiary uplift and erosion of the west coast of the islands. Oblique convergence is partitioned into nearly margin‐normal underthrusting (i.e., Mw 7.8 event) relative to the terrace, which is moving along the margin, and margin parallel on the Queen Charlotte strike‐slip fault just off the coast that produced the 1949 Ms 8.1 earthquake. Landward on the mainland, Global Positioning System data suggest slow coast‐parallel shear with no historical seismicity. The convergence rate decreases to the north of Haida Gwaii off Dixon Entrance, but large thrust earthquakes are possible. To the south, underthrusting of the Winona basin margin also could generate large earthquakes.

Beschreibung: Downhole electrical resistivity measurements can be exploited for gas hydrate concentration estimates. However, to do so requires that several assumptions be made, in particular about in situ pore water salinity and porosity. During Integrated Ocean Drilling Program Expedition 311, electrical resistivity was measured in four boreholes along a transect across the northern Cascadia margin, offshore Vancouver Island, Canada. Logging-while-rilling and conventional wireline logging data of resistivity, density, and neutron-porosity are used together with measurements of porosity and pore water salinity of the recovered core to systematically estimate gas hydrate concentrations at these four sites. Using Ar- chie’s law, empirical parameters a and m are determined from gas hydrate–free zones by means of a Pickett plot. The in situ salinity baseline trend for each site was estimated directly from the measured pore water salinity values, as well as indirectly by calculating the trend using Archie’s law and simultaneously solving for gas hydrate saturation and in situ salinity. Results showed that the in situ salinities at Sites U1325, U1326, and U1329 were well determined from a smooth trend through the highest measured salinity values of the recovered core. Only Site U1327 exhibitsstrongly decreasing pore water salinity, reaching 22‰ at the base of the gas hydrate stability field. This regional low salinity requires special analyses to estimate gas hydrate concentration from resistivity and introduces a large uncertainty. It is suggested that the decreased salinity results largely (90%) from a deeper fresh water source with the remaining freshening being the result of dissociation of pervasive gas hydrate (~3% of the pore space). Considering estimates from density porosity to be the most accurate, gas hydrate saturations average ~9% ± 7% at Site U1326, ~10% ± 7% at Site U1325, and 11% ± 7% at Site U1327 over the entire range of gas hydrate occurrence. No significant gas hydrate is inferred at Site U1329, although small am ounts may be present just above the bottom-simulating reflector. In two localized zones at Site U1326 (60–100 mbsf [meters below seafloor]) and Site U1327 (120–150 mbsf), significantly higher gas hydrate concentrations of 〉30% of the pore space were encountered.

Beschreibung: The 2012 Mw 7.8 Haida Gwaii earthquake confirmed very oblique subduction and slip partitioning at the southern Queen Charlotte margin. In this study, we re‐examine the thermal regime near the earthquake using new model constraints and with the recognition that hydrothermal circulation in the subducting oceanic crust can significantly affect the margin thermal regime. The observed heat flow values are extremely high just seaward of the trench but decrease rapidly landward. We explain this pattern as the consequence of very vigorous hydrothermal circulation in the subducting oceanic crust. Using a finite‐element model, we simulate the thermal effect of the circulation using a high‐conductivity proxy that represents a very high Nusselt number in an aquifer along the top of the oceanic plate. Our thermal model indicates that the temperature at the intersection of the megathrust and the strike‐slip Queen Charlotte fault (QCF) just seaward of the coast is about 350° C, approximately the limit of seismogenic behavior, and cooler than previous models that did not include hydrothermal circulation. The change of plate motion kinematics across the QCF approximately coincides with a down‐dip transition of the thermally controlled seismogenic behavior of the megathrust. Seaward of the QCF, the shallow megathrust accommodates mainly the margin‐normal component of relative plate motion, with the strike‐slip component accommodated by the QCF. This portion of the megathrust exhibits stick slip and produced the 2012 Haida Gwaii earthquake. Landward of the QCF, the megathrust fully accommodates the very oblique motion of the oceanic plate beneath the continental crust and exhibits creep.

Beschreibung: Integrated Ocean Drilling Program Expedition 311 is based on extensive site survey data and historic research at the northern Cascadia margin since 1985. This research includes various regional geophysical surveys using a broad spectrum of seismic techniques, coring and logging by the Ocean Drilling Program Leg 146, heat flow measurements, shallow piston coring, and bottom video observations across a cold-vent field, as well as novel controlled-source electromagnetic and seafloor compliance surveying techniques. The wealth of data available allowed construction of structural cross-sections of the margin, development of models for the formation of gas hydrate in an accretionary prism, and estimation of gas hydrate and free gas concentrations. Expedition 311 established for the first time a transect of drill sites across the northern Cascadia margin to study the evolution of gas hydrate formation over the entire gas hydrate stability field of the accretionary complex. This paper reviews the tectonic framework at the northern Cascadia margin and summarizes the scientific studies that led to the drilling objectives of Expedition 311 Cascadia gas hydrate.