Paper No. 67-6
Presentation Time: 9:45 AM
LONG DISTANCE PLUTONIC RELATIONSHIPS DEMONSTRATE 33 MY OF STRAIN PARTITIONING ALONG THE DENALI FAULT
Strike-slip faults commonly display variations in Pleistocene-Holocene slip rates with the missing slip often taken up by distributed deformation. The presence of mountain ranges along strike-slip faults also indicates that slip partitioning is a 106-107 yrs process. We identify two piercing point pairs to access long-term strain partitioning along a ~500 km transect of the arcuate Denali fault which currently accommodates right-lateral slip at rates decreasing east to west from 13 to 5 mm/yr. A suite of ~39-37 Ma composite plutons are located on both sides of the Denali fault, and we applied geochronology, geochemistry, and petrolophysical datasets to establish these are a dismembered plutonic complex. Our data indicate that the Foraker pluton (S of Denali fault) shares the same age range, geochemical variability, magnetic susceptibility, Hf isotopic composition, and inferred magmatic flux as the Nenana-Schist Creek-Panorama plutons (N of the Denali Fault). In addition, they both intrude Alaska Range suture zone rocks and McKinley-sequence aged (~58 Ma) granitoids along their eastern contacts. This correlation establishes ca. 155 km of right lateral separation since 37 Ma at a rate of ~4.2 mm/yr. Detailed U-Pb detrital zircon geochronology paired with metamorphic zircon analysis was performed on the Maclaren Schist (S of the Denali fault) and compared to the narrow Cottonwood Terrane (N of the Denali fault) near the Alaska – Yukon border. Multi-dimensional analysis of the detrital spectra show regionally unique similarities, both rock units contain ~33 Ma metamorphic zircon growth and the Maclaren Schist is intruded by ~33 Ma plutons. The eastern Denali fault Maclaren-Cottonwood Terrane geochronology correlation establishes ~305 km of displacement on the eastern Denali fault since 33 Ma at a rate of ~9.2 mm/year. The ratio of Pleistocene-Holocene slip rates between the western (5.3 mm/yr) and eastern (12.9 mm/yr) Denali fault is 0.41 and our new constraints yield a Late Eocene-Holocene ratio of 0.46. Hence, we interpret that the overall arcuate geometry of the Denali fault master strand was established by 33 Ma. We infer that the persistent long-term geometric stability of the Denali fault and other highly slip partitioned fault systems is related to long-term -highly oblique transpressive environs.