PROTEROZOIC AND PALEOZOIC ANCESTRY OF REGIONAL LARAMIDE FAULT NETWORKS: USING 40AR/39AR K-FELDSPAR THERMOCHRONOLOGY TO EVALUATE EARLY COOLING/EXHUMATION HISTORIES ACROSS DISCRETE FAULT ZONES
Several sample traverses were made across faults in southern Colorado. The results show different cooling histories at regional and local scales. At the regional scale, samples document important events that may represent periods of exhumation during 1) Grenville orogenesis (1200-1100 Ma), 2) Late Precambrian rifting at 800-700 Ma (western Cordillera) and perhaps 600-550Ma (Oklahoma Aulocogen trend), and 3) Ancestral Rockies deformation (ca. 350 Ma). At more local scales, feldspars may resolve disparate cooling paths across discrete faults.
40Ar/39Ar age spectrum analyses of K-feldspars across a segment of the Ilse fault reveal significantly different overall ages. On the west side of the fault, age spectra show a steep initial age gradient from ~600 to 1000 Ma for the first 5% of the spectrum followed by a more gradual gradient from 1000-1100 Ma for the remainder of the spectrum. On the east side, age spectra climb from 320 Ma to ~800 Ma for the first 10% of the spectrum and gradually climb from 800 to 900 Ma for rest of the spectrum. The contrasting age spectra observed is interpreted to record distinct thermal histories across the Ilse fault and a complex history for the Ilse fault that evidently has experienced multiple episodes of reactivation beginning in the Latest Mesoproterozoic.