Subduction Zone Response to Spreading Ridge Collisions
Encounters between oceanic spreading ridges and subduction zones represent a transient, but common, tectonic setting. The resulting triple-junctions often leave a record of ridge collision in the subduction zone's upper plate and this record can be used to constrain the past position of oceanic plates. However, recognizing the signature of ridge-trench interaction in the geologic record requires a multi-disciplinary approach that documents changes in faulting, magmatism, and sedimentation in both space and time. This type of research formed the backbone of my PhD work at MIT, in which I documented the effects of Eocene triple-junction migration along the Washington margin. The results have helped constrain past ridge-positions and may help explain northward terrane transport during the Eocene.
Clast of flow-banded rhyolite within conglomerate in the uppermost Chumstick Formation, Washington. We used zircons separated from this clast to constrain the maximum depositional age of this unit. This age constraint, among many others, was used to make regional correlations between Eocene non-marine sedimentary units throughout Washington. These correlations helped create a robust age model for sedimentation, faulting, and magmatism during Eocene ridge-trench interaction.
Eocene Ridge-Trench Interaction in Washington
Plate reconstructions require that an oceanic spreading center intersected North America during the Paleogene. However, subduction has erased the magnetic spreading record needed to constrain the exact location of this ridge-trench interaction. One potential location for this intersection is modern Washington and southern British Columbia. To help test this hypothesis, our research team (Robert Miller [San Jose State University], Paul Umhoefer [Northern Arizona University], Jeff Tepper [University of Puget Sound], and Erin Donaghy [ConcoPhillips]) has worked to generate a temporally controlled history of faulting, sedimentation, and magmatism in central and western Washington ca. 50 Ma (Eddy et al., 2016; 2017). This record shows a period of basin disruption and magmatism followed by a dramatic change to regional right-lateral strike-slip faulting. Such a change is consistent with accretion of overthickened oceanic crust (Siletzia terrane) and the passage of a triple-junction. This record has helped constrain the geometry of oceanic plates in the northern Pacific basin during the Eocene. The recognition that the plate boundary along the triple-junction's northern arm had a strong component of right-lateral slip may provide an explanation for paleomagnetic data that suggests that the Chugach and Yakutat Terranes have slid several thousand km to the north since the Eocene.
Mylonitic orthogneiss within the Ross Lake fault zone. Part of this project has involved constraining the timing of motion on major strike-slip faults throughout Washington. Near this location, the Ross Lake fault zone was sealed at 48.5 Ma by the Skymo layered mafic intrusion. These relationships were constrained by an undergraduate thesis by Yuem Park (Berkeley) that I co-advised.
A large xenolith of ca. 51 Ma sandstone within a ca. 49 Ma basaltic dike. This dike is part of the Teanaway dike swarm that intruded a large forearc sedimentary basin immediately after the basin was folded ca. 50 Ma. We have linked basin disruption and emplacement of this dike swarm to events along the continental margin, including passage of a triple-junction.
Our proposed plate reconstruction for ca. 50 Ma modified from Bradley et al. (1993), showing southward migration of the Kula-Farallon oceanic spreading center along the British Columbian coast and accretion of the Siletzia terrane to Washington, Oregon, and southern Vancouver Island.