Cascadia

East Pacific subduction system

48

±

5

Ma

Episodic subduction

A250P

Schematic tectonic reconstruction of the Cascadia SZI event (modified from Stern and Dumitru et al., 2019 and Wells et al., 2014). A trench jump occurred due to the accretion of the Siletzia and Yakutat large igneous province (LIP) formed by the Yellowstone plume, initiating the new Cascadia subduction zone. Shown are the new subduction zone (pink line), other active (solid purple lines) and inactive (dashed purple lines) subduction zones, and spreading ridges (solid red lines).

The Cascadia SZI event formed the currently active Cascadia subduction zone at around 53 - 43 Ma (Hyndman et al., 1990; Priest, 1990; Schmandt and Humphreys, 2011; Stern and Dumitru, 2019) and induced subduction of the Farallon and Kula plates below the North America plate.

The Cascadia SZI event is believed to have occurred as an episodic subduction via a trench jump after the large igneous province (LIP) Siletzia accreted at the previous trench of the Farallon/Cordilleran subduction zone (Wells et al., 2014; Stern and Dumitru, 2019). There is also an indication of the presence of the prominent Yellowstone mantle plume during the time of the onset, which might have been facilitated by the breaking of the intact subducting plate (Stern and Dumitru, 2019). It is worth noting that at the time of the trench jump there was ongoing subduction to the north and south of the Cascadia subduction zone. Although the accretion of the Siletzia block is likely to be the main cause of the trench jump, the nearby subduction zones in the north and south might therefore also have had a role in this SZI event.


Siletzia volcanic rocks on northwestern Washington (Olympic Peninsula) were emplaced between 53.18 ± 0.17 Ma and 48.364 ± 0.036 Ma, based on zircon U-Pb ages (Eddy et al., 2017). Deformation associated with Siletzia emplacement has been suggested to be between 51.309 ± 0.024 and 49.933 ± 0.059 Ma, as inferred from shortening in the forearc basin (Eddy et al., 2017). The oldest ages of the Cascadia arc are found mostly in south-west Washington in basalts and basaltic andesites, and have age ranges of 45-36 Ma (du Bray and John, 2011). Wells et al. (2014) document a sill complex phase related to subduction initiation to 48-45 Ma.


The model of Müller et al. (2016) does not implement this SZI event, and east-dipping subduction along the western margin of North America remains uninterrupted in this model during this time interval.


P-wave seismic tomography images a slab attached to the surface at the Cascadia subduction zone that reaches 350 km depth (van der Meer et al., 2018). The anomaly is broken into two parts, separated by the Yellowstone plume. S- and P-wave vote map tomography images agree on a small fast anomaly between 200-250 km depth.

Seismic tomography VoteMap (Shephard et al., 2017) analysis of the Cascadia SZI event.

This SZI event is not implemented in the model of Müller et al. (2016).

du Bray, E. A., & John, D. A. (2011). Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California. Geosphere, 7(5), 1102-1133.


Hyndman, R. D., Yorath, C. J., Clowes, R. M., & Davis, E. E. (1990). The northern Cascadia subduction zone at Vancouver Island: Seismic structure and tectonic history. Canadian Journal of Earth Sciences, 27(3), 313-329.


Eddy, M.P., Clark, K.P., and Polenz, M. (2017). Age and volcanic stratigraphy of the Eocene Siletzia oceanic plateau in Washington and on Vancouver Island: Lithosphere, 9, 652–664.


Müller, R. D., Seton, M., Zahirovic, S., Williams, S. E., Matthews, K. J., Wright, N. M., Shephard, G. E., Maloney, K. T., Barnett-Morre, N., Hosseinpour, M., Bower, D. J., Cannon, J. (2016). Ocean Basin Evolution and Global-Scale Plate Reorganization Events Since Pangea Breakup. Annual Review of Earth and Planetary Sciences, 44, 107-138.


Priest, G. R. (1990). Volcanic and tectonic evolution of the Cascade volcanic arc, central Oregon. Journal of Geophysical Research: Solid Earth, 95(B12), 19583-19599.


Schmandt, B., & Humphreys, E. (2011). Seismically imaged relict slab from the 55 Ma Siletzia accretion to the northwest United States. Geology, 39(2), 175-178.


Shephard, G.E., Matthews, K.J., Hosseini, K., Domeier, M. (2017). On the consistency of seismically imaged lower mantle slabs. Scientific Reports 7.


Stern, R. J., & Dumitru, T. A. (2019). Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation?. Geosphere.


van der Meer, D. G., van Hinsbergen, D. J., & Spakman, W. (2018). Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics, 723, 309-448.


Wells, R., Bukry, D., Friedman, R., Pyle, D., Duncan, R., Haeussler, P., & Wooden, J. (2014). Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot. Geosphere, 10(4), 692-719.

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