Crustal temperatures near the Lithoprobe Southern Canadian Cordillera Transect

Abstract

Heat flow and radioactive heat generation have been measured and the data compiled across southern British Columbia in the region of the Lithoprobe Southern Canadian Cordillera Transect. Heat flow in the trench-arc zone between the continental margin and the Garibaldi volcanic belt is very low, but in the volcanic belt it is high and very irregular. Farther inland, to the east, the heat flow is moderately high, with the highest values in southeastern British Columbia, associated with high surface radioactive heat production. The thermal data from the central and eastern interior of southern British Columbia define a single heat-flow province with a reduced heat flow of 63 mW/m2 flowing into the upper crust. This indicates a warm, thin lithosphere similar to that of the Basin and Range of the United States to the south. Occurrences of seismic reflective bands in the lower crust of the Cordillera were compared with temperatures calculated from surface heat flow and heat generation using a simple one-dimensional conductive model. The 450 °C isotherm corresponds approximately to the brittle– ductile transition, and deeper crust may be rheologically detached from the upper crust. Where the thermal data approximately coincide with the transect seismic reflection lines, the 450 °C isotherm often corresponds to the top of characteristic sub-horizontal reflector bands, as found in Phanerozoic areas elsewhere around the world. The lower limit of the reflective band in a number of Cordilleran reflection sections is near the 730 °C isotherm, which corresponds to the transition from present "wet" amphibolite- to "dry" granulite-facies conditions. This control of the depth to the deep crustal reflective bands by present temperature provides support for the model of the reflectors being produced by fluids trapped at lithostatic pressure (layered porosity), a model that can also explain the high electrical conductivity in the deep crust of the area. The probable rheological detachment of the lower crust and a possible nonstructural origin of the deep reflectors require that interpreted lower crustal structural boundaries such as the top of the basement of the North American craton under the Lithoprobe Southern Canadian Cordillera Transect be treated with caution. However, there is no doubt that many seismic reflectors are related to crustal structures, and the model is presented as an explanation for bands of seismic reflectors in the lower Phanerozoic crust, not as a model for all seismic reflectors.

Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone

“…In addition to the large reflective bands beneath Vancouver Island, smaller but very "bright" reflectors have been observed at shallower depths beneath the mainland (Cook et al, 1988). Lewis et al (1992) tried to correlate these bright spots with fluids in the crust. The shallow low V p region in the tomographic image may indicate the presence of hot fluids such as melts or water in the crust, perhaps driven by a lower crustal magma intrusion.…”

Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

“…Surface heat flow for the warm slab models shown in Figure 6 compared with observations at northern Cascadia. Small black dots, white circles, squares, and the grey triangle are heat flow values obtained from bottomsimulating reflectors, marine probes, shelf wells, and an ODP borehole, respectively [Davis et al, 1990;Lewis et al, 1992;Hyndman et al, 1993;Wang et al, 1995]. Grey circles are heat flow observations complied by Currie et al depths than shown in Figure 6.…”

Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

“…The heat flow decreases inland of the trench as a result of the heat sink of the cold upper part of the oceanic plate that is underthrusting the margin (e.g., Hyndman and Wang, 1993;Wang et al, 1995). Heat flow data are available from land boreholes and petroleum exploration wells on the continental shelf (Lewis et al, 1988(Lewis et al, , 1991, marine probe measurements on the continental slope and Cascadia Basin Spence et al, 2000b;Riedel, 2001;Riedel et al, 2006), and from the depth of the BSR on the continental slope Ganguly et al, 2000;Riedel, 2001;He et al, 2003).…”