Geochemical and Thermal Evidence from Pythia's Oasis Seep Indicate Strike-Slip Faults May Regulate Megathrust Pore Fluid Pressure Offshore Central Oregon
Abstract
In subduction zones, seep sites commonly coincide with fault zones that extend to the plate boundary. As such, fault-hosted seep sites provide a unique opportunity to directly sample fluids that have traveled from plate boundary depths, informing on the state of pore fluid overpressure, the permeability structure, and the water budget of subduction zone forearcs. Pore fluid pressure, regulated by the relative rates of fluid production and fluid escape, impacts the strength and slip behavior of the megathrust fault through its influence on effective stress. We present geochemical and thermal evidence from a seafloor seep called Pythia's Oasis indicating strike-slip faults may regulate drainage of the megathrust and underthrust section offshore of central Oregon. The main Pythia's Oasis vent emits warm (~12˚C), highly altered fluids at a rate of ~10-30 cm s-1, approximately 3-8 orders of magnitude greater than other known Cascadia subduction zone seeps. Heat flow values at the seep are 260 mW m-2, relative to a background of 80 mW m-2, and are consistent with focused discharge and high water flux. Geochemical analyses of the emitted fluids reveal enrichment in B (6660 µM), Li (202 µM), and Sr (1190 µM), depletion in Cl (340 mM), non-radiogenic 87Sr/86Sr (0.7066), and the presence of ethane and propane. These chemically and isotopically distinct values reflect high-temperature reactions at and below plate boundary depths. Pythia's Oasis is coincident with the Alvin Canyon strike-slip fault on the central Cascadia margin (~45˚ N), one of three strike-slip faults which together mark a tectonic transition to landward thrust-vergence, sediment over-consolidation, and an increased locking fraction. The unprecedented flow rates, deeply-sourced geochemical signatures, and co-location of Pythia's Oasis with the Alvin Canyon strike-slip fault, indicate that central Cascadia strike-slip faults act as high permeability hydraulic conduits which dewater the megathrust by channeling overpressured fluids to the seafloor. If similar seeps occur along the other strike-slip faults, they may collectively play a role in the transitions in sediment consolidation and locking fraction observed offshore of central Oregon by regulating pore fluid pressure and effective stress.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMOS53A..06A