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

We hypothesize that the dominant structural grains of the Ancestral Rockies (NW-trending) and Laramide Rockies (N-S trending) represent reactivation of Proterozoic normal fault systems that formed ca.1.1 Ga and 0.8 Ga respectively. 40Ar/39Ar K-feldspar thermochronology can help test this hypothesis by providing data on the thermal history of different crustal blocks. K-feldspar age spectra can be understood via a model that incorporates multiple diffusion domains where smaller diffusion domains retain argon between ~150 and 200°C, and larger domains retain argon between ~275 and 325°C. This empowers a single K-feldspar with the ability to record segments of a T-t history as it cools from 325° to 150° C.

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.