Citation by Richard J. Blakely
It is rare to have a geologic mapper extraordinaire, an accomplished geophysicist, and an insightful tectonic modeler all in one person. That person is Ray Wells, the 2022 recipient of the George P. Woollard Award.
Ray has an uncanny ability to meld discoveries from diverse fields of earth-science into regional tectonic models. His most cited work used Sierra Nevada block motion, paleomagnetism, potential-field analysis, and geologic insights to define Pacific Northwest microplates and their secular velocities. His forearc-block model provides a foundation for understanding crustal strain throughout Cascadia and continues to be the starting kinematic framework for many of us working in the Pacific Northwest.
Ray’s work on Siletzia, the Paleogene LIP that cores Cascadia's forearc, used his geologic mapping, along with isotopic ages, biostratigraphy, and paleomagnetic data to place Siletzia in a global plate-motion model. By accounting for 40 million years of clockwise forearc rotation, he showed that the timing of Siletzia’s magmatism and its accretion to North America is consistent with the location of the Yellowstone hotspot.
Ray recognized that variations in Cascadia tremor at 30-40-km depth are spatially associated with mapped forearc faults. He postulated that these faults extend through the upper plate to near the over-pressured megathrust, thereby providing pathways for fluid escape and modulating tremor on the megathrust. Fluid transfer, along with stress transfer, may be another critical link between the megathrust and upper plate faults.
Ray’s impact extends well beyond the Cascadia subduction zone. He was among the first to use satellite gravity anomalies to investigate the source zones of great earthquakes, showing that epicenters were clustered along the gradient bounding the forearc high, while large slip occurred beneath adjacent basin-centered lows. This work linked the seismic cycle to development of forearc structure in a new way and helped foster more global studies of subduction zones.
Ray’s innovative and impactful contributions to our understanding of subduction zone tectonics epitomize the spirit of the George P. Woollard Award. Congratulations, Ray!
Response by Ray E. Wells
Thank you, Rick for your efforts on my behalf. Thank you to the award committee and to Gene Humphreys, Walter Mooney, Kelin Wang, Richard Gordon, and Bob Butler for your support. I owe a lot to my wife Sally, fellow students, and colleagues who helped me find my way. I am deeply honored to be the 2022 recipient of the George P. Woollard Award.
I began my career as a USGS field geologist, mapping in the accreted basalts of the Oregon and Washington Coast Range with Parke Snavely. Subsequent encouragement by Rob Coe and Eli Silver sent me on a path to paleomagnetism and tectonics at UC Santa Cruz. While running the night shift on the Stanford magnetometer, I absorbed plate reconstructions from the other night owls, Dave Engebretson, Richard Gordon, Doug Wilson, and Jim Magill. Upon re-joining the USGS, I further explored Cordilleran reconstructions with Myrl Beck, Sherm Gromme', and Jack Hillhouse. By the end of the 1990s, paleomagnetists had made it clear that Cenozoic clockwise rotation was characteristic of much of the Cascadia convergent margin.
The eruption of Mount St Helens and the recognition of past earthquakes on the Cascadia megathrust changed everything. The idyllic backdrop on the side of apple crates was revealed to be a dynamic environment with M 9 earthquakes and erupting volcanoes. I was lucky to work with Rick Blakely, Craig Weaver, Bob Simpson, Tom Brocher, Alan Nelson, Jon Hagstrum, and others at the USGS. We used potential fields, seismology, lidar, and geology, along with the paleomagnetism to document the active crustal architecture of the Cascadia convergent margin. We proposed a microplate model to explain the large clockwise rotation of the Oregon forearc and the resulting changes in arc productivity, crustal strain, and seismicity along the margin. Our secular velocity field for the Oregon forearc, derived in part from its paleomagnetic rotation rate proved useful for evaluating permanent deformation of the upper plate.
I then joined Rob McCaffrey and his colleagues who were inverting Cascadia's GPS velocity field for block rotations and locking on the bounding faults. This work revealed variable locking on the megathrust and the velocities of upper plate blocks. We noticed that the secular block velocity field from GPS could explain the clockwise offset of the Neogene Cascade arc from the present arc, although the Oregon magmatic arc lags behind the faster rotating forearc. This indicates that the rotating block is driven in part by northward motion of the Sierra Nevada and not just slab rollback.
Cascadia's block rotations are largely the result of a Pacific-North America shear overprint on the convergent margin and may affect the behavior of the subduction zone. For example, tremor variability on the megathrust is correlated with upper plate lithology, forearc block boundaries, and topography of the forearc. This structural segmentation of the forearc could produce rupture mode diversity on the megathrust. The relation of the megathrust seismic cycle to the permanent deformation and seismicity of the upper plate is a fundamental problem that continues to inspire scientists working in Cascadia and other hazardous subduction zones. We have made some progress, but there is much more to do, and I'm happy to see young scientists fully engaged with these problems. Thank you again for this honor. I'm very happy to be able to share this occasion with colleagues, friends, and family.