Fluid Budget of Metasedimentary Rocks from a Tertiary Accretionary Prism and Connections to Seismicity, Olympic Peninsula, Northwest Washington State

Author

Holly Makena MacFadden Rotman, Central Washington University

Document Type

Thesis

Date of Degree Completion

Summer 2010

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Chris Mattinson

Second Committee Member

Careyt Gazis

Third Committee Member

Tim Melbourne

Fourth Committee Member

Audrey Huerta

Abstract

Metamorphic dehydration reactions and fluid movement in accretionary prisms have been linked to the recently discovered episodic tremor and slip (ETS) earthquake events along subduction zones, but prior studies lack the detail to effectively test the hypothesis that fluid flow triggers ETS events. I conducted field work along a 52.5 km transect on the Olympic Peninsula metasedimentary accretionary prism of the Cascadia subduction zone, and collected approximately 40 representative samples of sandstone and mudrock that were buried to 6–15 km. This depth range intersects the 10–50 km depth range of ETS events. My objectives are to quantify the water flow recorded in rocks of the Olympic Peninsula via petrographic, whole rock, and isotopic analyses to test the prediction that water release increases at ~10 km depth, creating fluid overpressure needed to trigger seismicity.

I calculated that on the Olympic Peninsula 1 km³ of 50% sandstone and 50% mudrock loses ~10⁵ kg H₂O/yr during burial from 6–14 km depth, comparable to the values expected from large-scale fluid budget models. Quartz veins that compose 0.5–1% of the Obstruction Peak site (~14 km burial depth) are important records of fluid flow quantity and origin. δ¹⁸O values of +11.8‰ to +15.2‰ indicate that vein H₂O originated from metamorphic reactions. Flow recorded by 1 km³ of rock containing 0.5–1% quartz veins is >10⁶ kg H₂O/yr, comparable to the values 2 × 10⁷ to 2 × 10⁸ kg H₂O/yr modeled at compositionally similar subduction zones to produce fluid overpressure conditions. I observed fibrous quartz veins, which also indicate fluid overpressure conditions were reached and support my H₂O flow estimates. Therefore, Olympic Peninsula rocks at depths of ~10–14 km record dehydration and fluid overpressure large enough to trigger subduction zone seismicity.

Recommended Citation

Rotman, Holly Makena MacFadden, "Fluid Budget of Metasedimentary Rocks from a Tertiary Accretionary Prism and Connections to Seismicity, Olympic Peninsula, Northwest Washington State" (2010). All Master's Theses. 1460.
https://digitalcommons.cwu.edu/etd/1460