EOCENE BREAKOFF AND ROLLBACK OF THE FARALLON SLAB – AN EXPLANATION FOR THE “CHALLIS EVENT”?

The early Eocene (~54-44 Ma) was a time of unusually widespread magmatism and extension in the Pacific Northwest, an episode commonly referred to as the “Challis Event”. Hypotheses proposed to account for this activity include low angle subduction, foundering of the slab, and passage of a slab window, but the data needed to evaluate these models has been lacking. Our ongoing petrologic studies and U-Pb dating of granites in NE Washington provide new constraints on the cause(s) of Challis magmatism and suggest this event may be a response to the accretion of Siletzia and subsequent detachment and rollback of the Farallon slab.

Challis activity began slightly before (or during) accretion of the Siletzia Terrane at ~50 Ma (Wells et al., 2014). Accretion was accompanied by breakoff/breakup of the Farallon slab that led to: (1) seismic evidence of relict Farallon slab fragments under the PNW (e.g., Schmandt and Humphreys, 2011), (2) a short-lived (~48-44 Ma) forearc magmatic belt in NW WA that contains oceanic basalts as well as adakites, S-type granites, and other rocks indicative of crustal melting, and (3) crustal uplift in NE WA, where >5 km of erosion occurred between the Late Cretaceous (epidote-bearing granites) and Eocene (epizonal granites). As Siletzia approached the margin subduction likely slowed, causing slab rollback that is recorded by a SW-ward migration of magmatism across NE WA between 52 – 46 Ma (Caulfield et al., 2015). Granites emplaced during this time have low B/Be (suggesting low water contents) and Sr-Nd isotopic compositions indicative of extensive deep crustal melting (Loewen et al., 2007). Nearby Cretaceous granites are similar isotopically and were apparently derived from the same lower crustal source(s). However they commonly display zircon inheritance and yield lower zircon saturation temperatures, suggesting that the Eocene melting event was hotter.

Taken together, these data support a model in which Siletzia accretion caused breakup and rollback of the Farallon slab. Asthenosphere upwelling through/around the foundering slab provided heat (but minimal fluid) that drove crustal melting as well as crustal uplift and extension. Ongoing U-Pb dating and geochemical studies will refine the temporal relations that have proven particularly helpful in deciphering the Challis event.