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