Collaborative Research: How do accretionary sutures evolve into Cordilleran strike-slip faults?

The western margin of North America formed over hundreds of millions of years through the attachment of numerous crustal blocks. Geologists recognize that the modern-day configuration of western North America fault systems from southern California to Alaska partly reflects the distribution and motions of these blocks. Little is known about how these block-bounding fault systems evolve over the long term after they have formed.

The PIs and their team will address long-term fault evolution by focusing on the Denali Fault Zone in Alaska. The Denali Fault Zone is the boundary between North American crust and the Wrangellia Composite Terrane, a crustal block that collided with North America about 90 million years ago. This project will constrain the timescales, physical mechanisms, and pressure/temperature conditions of the continental crust during the transition from collisional to lateral plate motion along the Denali Fault Zone.

This project will contribute to an improved understanding of regional seismicity that poses a threat to nearby infrastructure (e.g., Trans-Alaska Pipeline, Richardson Highway, and Denali National Park and Preserve) and the genesis and distribution of mineral endowments (Valdez Creek mining district and the Juneau Gold Belt). The project investigators will also contribute to STEM curriculum development for UAF programs that provide supplemental instruction and cohort formation for marginalized undergraduate students.

In addition, the project will support and train three graduate students, at least three undergraduate students, and three early-career scientists.

The northern North American Cordillera margin is an archetypal accretionary orogen bisected by numerous margin parallel lithospheric-scale strike-slip fault systems. Collectively, the interplay of accretionary tectonics with major strike-slip margins is a natural consequence of long-lived oblique convergence. The Denali fault is perhaps the best studied owing to its impressive topographic expression and active seismicity.

It is 2000 km long, preserves >480 km of net right-lateral displacement over the last 52 million years, and corresponds to a ~10 km offset in the Moho. For a significant portion of its trace, the Denali fault delineates the boundary of North American affinity rocks on the north from Wrangellia Composite Terrane rocks on the south. The strike-slip reactivation of a suture formed by subduction and eventual collision of distinct lithosphere results in a geometric and mechanical conundrum wherein shallowly dipping convergent structures appear to be reactivated and transformed into sub-vertical strike-slip structures.

The Maclaren Glacier metamorphic belt (MGmb) is a package of strongly deformed amphibolite-facies para- and orthogneiss exhumed along the south-vergent Valdez Creek shear zone – the exposed suture between the Insular terranes and North America in the northern Cordillera where a continuous zone of inverted Barrovian metamorphism is preserved across a thrust sense ductile shear zone. The northern MGmb is exposed along the Denali fault system, and thus preserves fabrics that formed during terminal suturing of the Insular terranes and development of the modern Denali fault.

We will integrate quantitative structural and kinematic analysis with high-resolution P-T-t histories of mid-lower crustal metamorphic rocks in the MGmb and thermochronology of superjacent upper crustal rocks along two transects through the suture zone. The MGmb is ideal for this study as the traverses offer exceptional lateral and vertical exposure providing unprecedented three-dimensional analysis suitable for kinematic studies.

Moreover, the bulk lithologies are amenable to detailed petrographic and thermobarometric analysis as well as contain a rich petrochronometer record as demonstrated on MGmb rocks elsewhere. These attributes will facilitate a direct link between structural and kinematic analysis to the P-T-t evolution of the entire transect with minimal extrapolation between separate lithologies.

Data from this study will be used to constrain an integrated perspective on how moderately dipping sutures evolve to margin parallel strike-slip faults globally.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.