GROWTH AND EXHUMATION OF THE CORDILLERAN THRUST WEDGE (SOUTHWESTERN ALBERTA) INFERRED FROM DETRITAL MINERAL U-PB GEOCHRONOLOGY AND ND ISOTOPES

The provenances of sedimentary strata that accumulate in foredeep basins contain an important record of growth and denudation of the adjacent orogenic belt. We use U-Pb zircon and monazite geochronology, in tandem with Sm-Nd whole rock data, from synorogenic clastic sediments in the Cordilleran foredeep of southwestern Alberta to provide new perspectives on the growth and decay of the orogenic wedge. Foredeep clastics are up to 3 km thick and comprise three major pulses of sediment delivery into the basin: the Fernie-Kootenay (1000 m; 140-125 Ma), the Blairmore (800 m; 120-95 Ma), and Belly River-Porcupine (2100 m; 83-55 Ma). Detrital zircon and monazite from the uppermost Kootenay are recycled entirely from Triassic-Ordovician sandstones within thrust imbricates of the miogeocline. This is consistent with the Nd data which imply a mix between Triassic-Devonian sources and preDevonian sources within the miogeocline. Juvenile Nd signatures appear in the lower part of the volcanogenic Blairmore. U-Pb ages on zircon confirm derivation from juvenile arc rocks of Quesnel and also detect the presence of syndepositional magmatic material in the orogenic hinterland. The upper part of the Blairmore shows a transition to less juvenile Nd signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Belly River-Porcupine interval with juvenile material occurring early in the sequence, accompanied by syndepositional grains of magmatic affinity, and less radiogenic (for Nd) material in the upper part of the sequence. We suggest that in general these patterns can be interpreted in terms of relative rates and patterns of tectonic growth of the orogenic wedge versus fluvial incision. During periods when the foreland fill is overwhelmed by juvenile material one can envision that the thrust wedge must have contributed limited detritus to the foreland fill and have grown by predominantly subcutaneous processes. Periods of wedge incision are marked by the reintroduction of a miogeoclinal provenance into the foreland fill. The common presence of syndepositional magmatic grains may indicate a component of wedge inflation and growth due to magmatic intrusion.