MIGRATING EOCENE MAGMATISM AND TECTONISM ACROSS THE PACIFIC NORTHWEST: SLAB ROLLBACK, SLAB WINDOW, OR BOTH?

Immediately prior to establishment of the modern Cascade arc the Pacific Northwest experienced widespread magmatism and extension extending as far inland as Montana. Termed the Challis Event, this brief (52-45 Ma) episode has previously been attributed to a number of processes including slab window passage and slab rupture/rollback. Based on a synthesis of precise U-Pb dating, petrology of igneous units, basin analysis, and evolving deformation styles we suggest the Challis Event resulted from slab breakoff and rollback. However, slab windows also played an important role in bounding the area affected by rollback.

Geochemical (B/Be) and Sr-Nd isotopic data indicate Eocene granitoids in NE WA formed by melting of arc-like lower crust under anomalously hot, dry conditions. U-Pb ages of these plutons (52–45 Ma) define a younger-to-the-SW age progression. At the same time (49.4-45 Ma) extensive swarms of basaltic to rhyolitic dikes were emplaced in the Cascades from the latitude of Seattle to southern British Columbia, granodiorite to leucotonalite magmas were intruded at depth in the Cascade core, and there was a regional transition from contraction to extension. These observations support a slab rollback model in which lower crustal melting was driven by hot mantle upwelling around a foundering slab. To the west of the rollback region is a ~160 km long belt of bimodal volcanics and S-type plutons with 51.6-48.1 Ma U-Pb ages, which we interpret as demarking a zone of slab breakoff. Restoration of offset on the Straight Creek Fault places these rocks atop the inferred edge of Siletzia in the subsurface.

In our model the accretion of Siletzia triggered at least two ruptures of the shallowly dipping Farallon (or Kula) slab. An initial breakoff event farther inland >52 Ma (Duke et al., 2014) marked the beginning of margin-normal rollback beneath WA, and was ultimately followed a second rupture under western WA at ~51 Ma. The area undergoing rollback was likely bounded to the N and S by slab windows, one near the Canadian border (Kula-Resurrection) and one near the WA-OR border (Kula-Farallon). A high velocity ‘curtain’ seismically imaged under WA and ID (Schmandt and Hemphreys, 2011) may be this detached fragment of slab.