The structural, thermal, and fluid evolution of the Livingstone Range anticlinorium, and its regional significance to the southern Alberta foreland thrust and fold belt

Abstract

The Livingstone Range anticlinorium (LRA) is a long (>65 km) narrow (<5 km) structural culmination that coincides with a major hanging-wall ramp across which the Livingstone thrust cuts ~1000 m up-section eastward from a regional décollement in the upper part of Devonian Palliser Formation to another regional décollement within Jurassic Fernie Formation. The presence of Precambrian basement fluids prior to thrusting and folding is recorded in the LRA by deformed jasper+/-fluorite+/-sphalerite veins, and adjacent haloes of altered dolomitic host rock with high 87Sr/86Sr ratios (0.7094 to 0.7100) relative to most host rocks. Basement fluids are a possible source for anomalously radiogenic strontium that occurs in the diagenetically altered Paleozoic carbonate rocks in the LRA and throughout the Western Canada Sedimentary Basin, but underlying thick shale strata such as the Exshaw Formation are also a possible source. The earliest stages of thrusting deformation involved the development of distinctive chevron-style, flexural-slip thrust-propagation folds that have conspicuous blind thrust faults along their hinge zones. The hinge-zone thrust system of the Centre Peak anticline consists of a series of stacked detachment thrusts, each of which emerges from a different zone of interbed slip in the backlimb of the anticline and deflects the hinge zone eastward. Each successively higher detachment thrust dies out in the hinge zone at approximately the same stratigraphic level at which an overlying detachment thrust fault emerges from a bedding detachment zone in the backlimb. Fluid flow during thrust-propagation folding is recorded by dolomite+/-calcite veins with isotopic compositions that are similar to those of host rocks. Fluid flow occurred along faults related to thrust-propagation folding, and also along many tear faults and larger thrust faults. Veins in these fault zones have slightly higher 87Sr/86Sr ratios relative to adjacent host rocks and are interpreted to have formed from a mixture of formation fluids and hotter basement fluids in a rock-dominated system. Oxygen isotope thermometry of four syn-folding veins indicates they precipitated at anomalously high temperatures (>250°C). The youngest episode of fluid flow along thrust faults and tear faults is recorded by calcite veins with very low δ18O values (-18 to -9‰ PDB), which are interpreted to have precipitated along faults that were active while the LRA was being transported eastward and elevated by underlying thrust faults, and cooled by infiltrating meteoric water. The conduits along which significant meteoric fluid circulation occurred are marked by visibly altered host rocks that have anomalously low δ18O values and slightly lower δ13C values relative to most host rocks. Rapid cooling due to deep infiltration of meteoric water into the shallow brittle surface of the deforming earth is almost certainly not restricted to thrust and fold belts, nor is its thermal effect necessarily restricted to the upper few kilometers. This model for fluid flow has significant implications for predicting thermal conditions in deep metamorphic rocks that lie beneath the brittle crust, the most obvious effect being to push down the brittle/ductile transition zone, which would enhance even deeper meteoric fluid circulation and cause the deflection of underlying isotherms.