R. D. Hyndman

The heat flow in the northern Canadian Cordillera between 59 and 64 N is very high, 105 ± 22 mW/m². The average upper crust heat generation is 4.6 ± 2.4 microW/m³, giving a reduced deep heat flow of 59 mW/m², similar to the southern Canadian Cordillera, 49–53 N. Estimated Moho temperatures of 800–1000°C and lithosphere only 50–60 km thick are substantiated by slow upper mantle seismic velocities and by high elevations associated with crust only 32–35 km thick. The new data support the conclusion that all of the North and South America Cordilleras are hot. The Paleoproterozoic Wopmay orogen to the east has a remarkably high heat flow of 90 ± 15 mW/m² for such an old and stable region. However, basement samples indicate high heat generation and estimated deep temperatures are intermediate between the Cordillera and the Slave Craton further to the east. Heat flow from the Yellowknife area of the Slave Craton, 53 mW/m², implies low lithosphere temperatures in agreement with kimberlite xenolith P‐T data. The high temperatures in the Cordillera result in a very thin and weak lithosphere and may allow a lower crust detachment. The north and central Cordillera have been inland of the coastal Queen Charlotte transform fault since subduction stopped in the Eocene. Thus some process has maintained high temperatures in the former back arc for more than 40 Ma.

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

The heat flow pattern from the deep sea across the Queen Charlotte Terrace to the west coast of the Queen Charlotte Islands has been determined through 20 marine heat flow measurements and by 11 borehole measurements at one land site. There is a continuous transition from very high deep sea heat flow, through intermediate values on the terrace, to low continental heat flow. The deep sea values are scattered, probably reflecting hydrothermal circulation in the crust, but their mean is in agreement with the theoretical value for the 7 Ma deep sea oceanic crust. The mean heat flow at the land site of 47 mW m⁻² is similar to the average for the coast Insular Belt to the south and east. The measurements on the 30 km wide 2 km deep terrace are less scattered and much lower than those on the deep sea floor. The main thermal contrast is at the seaward edge of the terrace rather than near the coast where the earthquakes of the main plate boundary are located. Numerical models suggest that no reasonable steady state ocean–continent boundary can explain the observed heat flow transition. The data are consistent with a model of oblique underthrusting of the deep sea floor beneath the terrace. The continental margin heat flow transition will tend to mask any thermal anomaly generated by fault motion, but the results imply that there is no large frictional heat generated by motion on the Queen Charlotte fault.Nine marine heat flow measurements were made at the southwestern end of Queen Charlotte Sound south of the Queen Charlotte fault zone to test the hypothesis that the sound is a zone of recent crustal extension or rifting. The measurements were limited to isolated pockets of soft sediment and to sea floor depths greater than 800 m where bottom water temperature transients are negligible. The heat flow values are fairly uniform with an average of 86 mW m⁻², which is about double the mean of other Insular Belt values. The Queen Charlotte Sound values could be affected by a nearby ocean spreading centre or by sediment erosion, but they are consistent with the rift hypothesis.