| Abstrakt: |
Active strike‐slip fault systems commonly display along‐strike Quaternary slip rate gradients associated with fault bends and splay faults, which generate surface uplift by dip‐slip faulting or distributed "off fault" deformation. By analogy, the documentation of long‐term (107 yr) slip gradients on some continental strike‐slip fault systems implies long‐term coevolution of strike‐slip and dip‐slip fault systems. Here we leverage the observed ≥33 Myr right‐lateral slip gradient on the Denali fault, Alaska, USA to investigate the role of splay thrust systems in accommodating the slip gradient. We focus on the Broxson Gulch thrust system, which splays southwestward from the Denali fault in the eastern Alaska Range. Apatite and zircon (U‐Th)/He and fission‐track cooling ages from metasedimentary and metaplutonic rocks intersected by the thrust system record an along‐strike decrease in cooling ages commensurate with an increase in late Oligocene‐Neogene bedrock exhumation and shortening with proximity to the Denali fault. The dominant structure in the Broxson Gulch thrust system is the Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone–the main Late Cretaceous suture between western North America and outboard accreted arc terranes. After reactivation of the Valdez Creek shear zone at ca. 30 Ma, the thrust system grew by south‐vergent imbrication of the upper crust along thrust and reverse faults until at least 6 Ma. Incorporating results from the Broxson Gulch thrust system into the regional structural evolution of the Denali fault system reveals significant spatiotemporal heterogeneity in shortening adjacent to the Denali fault. Moreover, nearly all of the late Oligocene‐Neogene shortening south of the Denali fault was focused along reactivated terrane boundaries inherited from Mesozoic assembly of the North American Cordillera, and the spatial distribution of the inherited structures appears to control slip partitioning behavior of the Denali fault system across time scales ranging from 101 (historic seismicity) to 107 yr. The slip partitioning behavior of the Denali fault system highlights the mechanical importance of inherited structures leading to protracted shortening on splay thrust systems, which siphon slip from the master strike‐slip fault. We contend that the weakness of nearby reactivated terrane boundaries should be considered among other mechanisms commonly evoked to explain the partitioning behavior of continental strike‐slip fault systems (e.g., stress field rotation, obliquity angle, and strength of master strike‐slip fault). Plain Language Summary: Lateral motion across vertical faults, such as the San Andreas strike‐slip fault in California or the Denali fault in Alaska, becomes complicated when that fault bends. The complex motion in fault bends is often taken up by several faults that each play a different role: some faults move the crust horizontally and some faults uplift the crust and create mountains. For decades, geologists have been interested in fault bends in vertical faults because the earthquakes and resulting mountains associated with bent faults are different from those that form along straight faults. To help understand fault bends, we studied the Denali fault in southern Alaska because it has been curved for at least 45 million years and has had several large earthquakes that built the large mountains of the Alaska Range during that time. In our study area, we found that the mountains started forming at about 30 million years ago because an older fault related to formation of the North American continent formed a weakness that reactivated. When we compared our findings with previously published data from other faults through the curved part of the Denali fault, we found that most of the faults that uplifted the crust and made mountains in the last 30 million years are also old faults related to assembly of North America. The results from our study suggest that the location of the older faults strongly influences which faults accommodate vertical motion, seen as uplift of the mountain ranges, and which faults accommodate lateral motion in fault bends. Key Points: Oligocene‐Neogene increase in convergence across the dextral Denali fault system reactivated a suture, which developed a splay thrust systemProtracted shortening on splay thrust systems stabilized the ca. 107 yr slip gradient observed on the Denali faultThe spatial distribution of reactivated terrane boundary faults near the master Denali fault controls the slip partitioning behavior [ABSTRACT FROM AUTHOR] |
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