Late Cretaceous and Early Cenozoic Exhumation in the Foothills of the Brooks Range-Chukotkan Syntaxis Revealed by Apatite Fission Track Analysis

Abstract

The Brooks Range and Chukotka orogens converge into a syntaxial region located in the western North Slope and southern Chukchi shelf that is considered to be petroleum prospective in its frontal part. We present apatite fission track (AFT) analysis along a transect that extends ∼100 km N-S, orthogonal to the Brookian structural fabric, near the Chukchi sea coast, to better document the deformational history and regional petroleum system elements. The northern end of the transect crosses Aptian-Albian foreland basin strata, deformed into a series of detachment folds developed on pre-Jurassic autochthonous rocks. Four samples extend across one of these folds. Three of these samples from low structural levels yield structurally ordered, partially reset AFT ages of 90-65 Ma. The degree of AFT annealing is consistent with vitrinite reflectance of coal from nearby sites. In contrast, the deepest sample yields an AFT age of 67 Ma, and fission tracks appear to have been largely reset after deposition. Along this transect, the age-depth relationship shows that fold growth and exhumation initiated at ≥67 Ma. The southern part of the transect extends from the zone of detachment folds to south of the Brookian mountain front. It was not possible to sample across a large amount of relief on any single structure in this region. We collected seven samples from Valanginian-Aptian strata in both structural highs and lows. Structurally low samples exhibit cooling ages of 73-71 Ma and display age and track length distributions that record post-depositional resetting. Samples from the proximal hanging walls of N-vergent late Brookian thrust faults yield 60-53 Ma AFT ages. In the syntaxial region, evidence outlined above indicates regional exhumational cooling at 73 to 67 Ma. This exhumation overlaps in time with thrusting along the Herald arch (front of Chukotka thrust belt), recently documented with AFT data from the Chukchi Shelf, hinting at a possible linkage. Alternatively, this exhumation may indicate that Late Cretaceous quiescence of the Brooks Range ended earlier in the syntaxial foothills than in the central and eastern Brooks Range. The ca. 70 Ma exhumation was overprinted by N-vergent late Brookian thrust faults by 60 Ma, with similar timing to those in the central and eastern Brooks Range. These results can be used to establish the relative timing of hydrocarbon generation and the development of structural traps.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016