Temperature and Thermotectonic Histories from the Intermontane Belt in British Columbia
Jacek A. Majorowicz1, Kirk G. Osadetz2, C. A.
Evenchick3, Fil Ferri4, and Mark Hayes5
1 Northern Geothermal Consultants, Edmonton, AB
2 Geological Survey of Canada, Calgary, AB
3 Geological Survey of Canada, Pacific, Vancouver, BC
4 BC Energy and Mines, Victoria, BC
5 Geoscience Section, Victoria, BC
Present moderate to high heat flows in the Intermontane region preserve transients characteristics indicating a heat flow decay at undetermined times, probably since the Cretaceous or Tertiary. Currently significant lateral surface heat flow variations occur in Nechako and the Bowser basins. The surface heat flow varies by up to 30 +/- 10 mW/m2. In the same region Mantle heat flow variations, approximately 10 mW/m2, are also significant. Heat generation variations are between 0.6-6 mW/m3, suggesting that the large heat flow variations are related to heat generation changes. Variations in thermal lithosphere thickness, which is less than 70 km thick throughout the region, are also significant. Geotherms show large variations of mid-crustal temperature resulting in variations of the thickness of brittle upper crust. Depth to the conductive upper-mid crust from magnetotelluric (MT) data in Bowser and Nechako correlate with the top of 450°C isotherm (ductile-brittle transition) and this varies by 10-20km. Bottom-hole temperature data Nechako basin are consistent with a present geothermal gradient of 25 mK/m and a present heat flow of 80mW/m2. Temperatures that caused some of the highest observed thermal maturity levels, >2.5 %Ro, in the sedimentary succession were >300°C during a period when the heat flow is inferred to have been at least 100 mW/m2. Such heat flows are no longer characteristic of these sedimentary domains, where temperatures are about 120°C, and 245°C at depths of about 4 km and 9 km, respectively. This suggests changes in the thermotectonic regime since Cretaceous or younger time.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005