DEEP REFRIGERATION OF AN EVOLVING THRUST AND FOLD BELT BY ENHANCED 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 evolution 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 provides enhanced pervasive permeability. These considerations may resolve an apparent paradox in the thermal evolution of the Lewis thrust sheet and the rocks beneath it. The intuitively appealing notion that tectonic burial beneath a thick thrust sheet must lead to substantial heating of the rocks that are overridden is contradicted in the case of the Lewis thrust sheet in a region where it was > 8 km thick. Coalification levels in Upper Cretaceous rocks beneath the Lewis thrust sheet are low (0.74 % VR); and the thermal history of rocks both within and below the thrust sheet, as deduced from apatite and zircon fission track analysis, requires rapid cooling at the time of thrusting. The paradox may be ascribed to unwarranted assumptions about the relative roles of conductive and advective heat transport during and after thrusting. Meso-scale tectonic fabrics record brittle dilation and associated solution transport and precipitation during thrusting and thrust-related folding. "Penetrative" deformation was accomplished mainly by fracturing and by slip on joints, bedding, and small faults. Reconnaissance regional stable-isotope geochemistry of vein-filling minerals indicates that deep penetration of meteoric water was widespread in the southeastern Canadian Cordillera (Nesbitt and Muehlenbachs, GSAB, v. 107, 1033-1050, 1995; CJES, v. 32, 1699-1719, 1995). Diachronous, foreland-migrating, Upper 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, probably just prior, to their incorporation into the evolving foreland thrust and fold belt (Enkin et al., GSAB, v. 112, 929-942, 2000). This "hot flush" evidently was driven ahead of the deeply penetrating cold meteoric water that refrigerated the developing thrust sheets and made them even more brittle.