What is Causing (Permanent) Uplift in Southern Cascadia?

Abstract:

Coastlines above and near locked patches of subduction zones typically subside inter-seismically as coupling causes the upper plate to be dragged down. Over multiple seismic cycles, this inter-seismic subsidence is balanced by co-seismic or post-seismic uplift. This elastic deformation cycle should result in no net vertical deformation accumulating along subduction margins. In southern Cascadia we observe both permanent uplift in the geologic record (uplifted marine terraces and high regional river incision rates) and geodetic inter-seismic uplift. This raises several questions: (1) what drives geodetically observed uplift in southern Cascadia?, (2) does it also drive permanent uplift in southern Cascadia?, and (3) is it a subduction zone process or associated with other tectonic interactions?

By decomposing the present-day GPS velocity field into a subduction cycle and Mendocino Crustal Conveyor (MCC) component to produce subduction coupling models for the Cascadia margin, we investigate the drivers of vertical motions in southern Cascadia. Preliminary models show that coupling terminates within the Franciscan terrane in southern Cascadia. The relatively weak rheology of the Franciscan terrane (compared to the relatively strong Siletzia terrane in central Cascadia) may localize subduction-related shortening generated uplift. However, these subduction coupling effects alone cannot produce the observed present-day observed uplift rates in southern Cascadia. Pacific-North America shear motion causes minor uplift but this process is also insufficient to produce the residual uplift needed. The deforming Gorda plate (versus the older Juan de Fuca plate to the north) may affect vertical motions although its effect is unclear. Uplift associated with North American shortening ahead of the northward migrating MTJ can provide the remaining components of the observed present-day high uplift rates. We suggest that the superposition of the crustal deformation effects associated with the northward migration of the MTJ combined with non-elastic subduction deformation may produce the high uplift rates observed in this region of Cascadia.