During Leg 78B, we measured borehole temperatures in Hole 395A down to depths of 600 meters below the seafloor, more than five years after the hole had been drilled during Leg 45. Our attempts to measure an equilibrium temperature profile immediately after the initial re-entry were only partly successful, but we were able to obtain an apparent equilibrium profile by extrapolation of a subsequent pair of continuous temperature logs. This profile features an isothermal zone extending from the seafloor to 250 meters sub-bottom, or over 150 meters into basement, indicating a vigorous, pressure-driven downhole flow of bottom water into the upper levels of the basement. This flow probably began when the hole was first drilled through the relatively impermeable, 93-meter-thick sediment cover during Leg 45. We were unable to quantify the rate of flow during Leg 78B, partly because we could not reliably determine the geothermal gradient in the sediments in the adjacent Hole 395B. In the deepest 100 meters of Hole 395A, the temperature gradient approaches the predicted gradient for conductively cooling, 7.3-Ma-old crust. In combination with the extremely low permeability measured in this zone by Hickman et al. (this volume), this indicates predominantly conductive heat transfer deeper than about 500 meters below the seafloor. Low temperatures above 500 meters suggest shallow, lateral hydrothermal circulation in the permeable upper 300-400 meters of the basement, as modeled by Langseth et al. (this volume).
Finite-element geothermal models were used to compare the sensitivity of arctic subpermafrost gas hydrate at the Mallik borehole to temperate marine gas hydrate located offshore southwestern Canada. Considering the thermal signal alone accompanying the end of the last ice age, a 30 m gas-hydrate-bearing layer (porosity 51%, hydrate saturation 20%) at the base of gas hydrate stability 13.5 ka ago in the temperate marine environment would have disappeared by the present. In contrast, the same gas-hydrate-bearing layer underlying permafrost would persist until at least 4 ka after present, even with contemporary climate warming. These longer times for subpermafrost gas hydrate arise from thawing pore ice at the base of permafrost, at the expense of dissociation of the deeper gas hydrate. Overlying permafrost thus buffers the dissociation of underlying gas hydrate from climate surface warming.
On Leg 78B, the Glomar Challenger returned to Hole 395A on the Mid-Atlantic Ridge and logged the upper 0.5 km of the crust, obtaining excellent resistivity, natural-gamma, and caliper logs throughout the section and reasonable density and velocity logs near the base. The logs show that the crust may be divided into two distinct geophysical units. The upper 400 m of the crust displays high porosities and low velocities, densities and resistivities. Below this, between 400 and 500 m sub-basement, the crust displays a low crack porosity (1-2%), high resistivities (up to 1000 öm), and high velocities (up to 6.0 km/s). For the lower unit, a value of 2.2 was obtained for the exponent m in Archie's Law, which is consistent with low permeabilities measured in the same interval with a packer (2 to 9 µdarcies). Since the boundary between these two units is marked by a sharp increase in alteration products and the lower unit behaves as if it were sealed, we conclude that Hole 395A penetrated a major geophysical discontinuity.
A temporary array of land and ocean bottom seismograph stations was used to accurately locate microearthquakes on the Queen Charlotte fault zone, which occurs along the continental margin of western Canada. The continental slope has two steep linear sections separated by a 25 km wide irregular terrace at a depth of 2 km. Eleven events were located with magnitudes from 0.5 to 2.0, 10 of them beneath the landward one of the two steep slopes, some 5 km off the coast of the southern Queen Charlotte Islands. No events were located beneath the seaward and deeper steep slope. The depths of seven of these events were constrained by the data to between 9 and 21 km with most near 20 km. The earthquake and other geophysical data are consistent with a near vertical fault zone having mainly strike-slip motion. A model including a small component of underthrusting in addition to strike-slip faulting is suggested to account for the some 15° difference between the relative motion of the North America and Pacific plates from plate tectonic models and the strike of the margin. One event was located about 50 km inland of the main active zone and probably occurred on the Sandspit fault. The rate of seismicity on the Queen Charlotte fault zone during the period of the survey was similar to that predicted by the recurrence relation for the region from the long-term earthquake record.
A tectonic model for the formation, subsidence, and thermal history of Queen Charlotte Basin is developed. Based upon regional geological and geophysical data, subsidence data from offshore wells in Hecate Strait and Queen Charlotte Sound, and thermal criteria derived from present heat flow and vitrinite reflectance information, Queen Charlotte Basin is seen to have resulted from two distinct mechanisms. (1) During a period of broad regional uplift, rifting and crustal extension occurred in Queen Charlotte Sound up to about 17 Ma ago and the Queen Charlotte Islands were displaced northwards toward their present position by transcurrent motion along the Louscoone Inlet – Sandspit fault system. The rifting generated a significant thermal anomaly and a restricted deep basin as a consequence of crustal thinning and subsequent thermal cooling. (2) Beginning about 6 Ma ago, oblique underthrusting commenced along the margin, resulting in flexural uplift of the western part of the Queen Charlotte Islands and companion subsidence in Hecate Strait and Queen Charlotte Sound. The underthrusting caused rapid cooling of the old rift basins. This phase of subsidence has continued at a decreasing rate until the present.The tectonically generated subsidence in the basin has been estimated by correcting the well data for sediment compaction, paleo water depth, and sediment loading effects. At the site of the Harlequin well in the Queen Charlotte Sound rift, with the termination of extension and associated volcanism, the basin was 1500–2000 m deep and contained little sediment. Model calculations show that this depth is consistent with the estimated extension of about 70 km and a resulting crustal thinning to 8–10 km.Models for the lithosphere flexure generated by underthrusting are constrained by the geological evidence for uplift and erosion of over 5 km of material from the western portion of the Queen Charlotte Islands and the exponentially slowing subsidence to a present regional basement depth of 2 km in Hecate Strait. An excellent fit to the pre-erosion surface profile onshore and pre-Skonun basement surface offshore is obtained with a model having underthrusting on a 30° thrust at a 10 mm year−1 orthogonal component of convergence. The flexure generated by underthrusting, which is particularly well documented in the Queen Charlotte region, appears to be a feature of most subduction zones.Vitrinite reflectance data, present heat flow estimates from the wells, and thermal modelling indicate that the heat flux in Queen Charlotte Basin was much higher in the past than at present, particularly in Queen Charlotte Sound. A model is proposed with high heat flow generated by rifting prior to 17 Ma ago, followed by cooling from the underthrust oceanic lithosphere.
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