DEEP REFRIGERATION OF A THRUST AND FOLD BELT BECAUSE OF ENHANCED SYNTECTONIC PENETRATION OF METEORIC WATER: THE LEWIS THRUST SHEET, SOUTHERN CANADIAN ROCKY MOUNTAINS

Refrigeration because of advective heat transport by deeply penetrating meteoric water is an integral part of the development of a foreland thrust and fold belt. Topographic relief generated during thrusting and folding drives large-scale deep penetration of cold meteoric water; and dilation due to the brittle deformation that accompanies thrusting and folding enhances pervasive permeability. These concepts resolve an apparent paradox in the thermal evolution of the Lewis thrust sheet and of the rocks that occur beneath it. The idea that relatively cool sediments in the footwall of a large thrust sheet must be thermally modified by burial beneath the relatively hot rocks in the hanging wall is intuitively appealing. However, it is contradicted in the case of the Lewis thrust sheet (which is up to 8 km thick) by coalification levels in rocks beneath the thrust, and by the thermal history of rocks both within and below the Lewis thrust sheet, as deduced from apatite and zircon fission track analysis. The paradox originates with the unwarranted assumption that heat transport during and after thrusting is dominantly conductive rather than advective. Meso-scale tectonic fabrics within and below the thrust sheet record syn-thrusting dilation and solution transport and precipitation. They show that "penetrative" deformation during thrusting and related folding was accomplished mainly by fracturing and by slip on bedding, joints, and small faults. They also show that solution of carbonate minerals and quartz and precipitation along fractures (as veins) and along shear surfaces (as slicken-fibres) was widespread. Reconnaissance stable-isotope geochemistry of vein-filling minerals indicates that deep penetration of meteoric water was widespread in the southeastern Canadian Cordillera. Diachronous, foreland-migrating, Cretaceous chemical remagnetization of Devonian and Mississippian carbonate rocks in the Front Ranges of the southern Canadian Rocky Mountains provides evidence of hot pore fluids flowing through these rocks prior, possibly just prior, to their incorporation into the evolving foreland thrust and fold belt. This "hot flush" evidently was driven ahead of the deeply penetrating cold meteoric water that refrigerated the developing thrust sheets.