NWEA Speaker Program for 2017-2018

April 19, 2018 Luncheon

Cenozoic deformation of the Cascadia convergent margin – implications for the history of the Coos Bay Basin

Ray E. Wells  U.S. Geological Survey, Emeritus

Laird Thompson  UF3 Managing Partner

Geoscience investigations of the Cascadia convergent margin, first for energy resource and later for earthquake hazard assessments, have revealed a 50 million-year history of terrane accretion, marginal rifting, clockwise rotation, northward migration, and breakup of the margin. New geologic mapping, paleomagnetism, calcareous nannoplanktonic biostratigraphy, and U/Pb and Ar/Ar dating document six main events along the margin:

56-49 Ma        Creation of the ocean island terrane of Siletzia in the NE Pacific

50 Ma             Accretion of Siletzia to the convergent margin, forming Coast Range basement

42 Ma             Post accretion, margin-parallel stretching, Tillamook magmatism, strike slip and basin formation

50-0 Ma          Clockwise rotation and northward migration of the forearc

30-0 Ma          Margin-parallel shortening of the forearc

~ 10 Ma          Breakup of the Juan de Fuca plate and uplift of the Coast Range

These events controlled the formation and modification of forearc basin subsidence and uplift. The Coos Bay Basin has a structural history that reflects the major tectonic episodes in the forearc.  We will look at the deformation pathway through time from specific outcrops that provide a detailed picture of episodic tectonic activity that has generated the spectacular coastal outcrops at and near Cape Arago.

Dr. Wells has been a research geologist with the USGS for 40 years, where he used field geology, paleomagnetism, and GPS to understand the tectonic evolution and seismic hazards of active continental margins. He has studied subduction zones around the world to better understand the controls on great megathrust earthquakes and has applied that understanding to the Cascadia convergent margin. Ray is particularly interested in how the oblique component of convergence is partitioned into permanent deformation of the forearc, producing faults, earthquakes, and tectonic rotation of the upper plate. Dr. Wells is a recipient of the Distinguished Service Award of the Department of the Interior and the 2017 recipient of the Geological Society of America’s Geologic Mapping Award in honor of Florence Bascom. Recently retired, Ray is a Research Geologist Emeritus stationed at the USGS Oregon Water Science Center and is a Research Associate with the Geology Department at Portland State University.

Dr. Thompson is an adjunct professor at Utah State University and the Managing Partner of UF3, fractured reservoir consulting LLC.  He is an industry recognized expert in borehole imaging interpretation and in the characterization of fractured reservoirs. He managed the R&D program for Mobil Oil Corp. in fractured reservoirs.  He is a respected teacher in Mobil’s training system and for AAPG. Since 2000 he has been a managing for UF3, Utah Faults, Fractures and Fluids, and has been teaching and consulting on fractured reservoirs, borehole imaging and microseismic analysis of permeability fields.  Laird is currently involved in microseismic R&D and application for fracing effectiveness and reservoir optimization.


Northwest Energy Association