The Quimper Geological Society (QGS) hosts lectures on Saturdays. Many of us enjoy (and rely on) the social aspect of the get-togethers at Port Townsend’s First Baptist Church and conversations before and after the lectures. The day of the lecture is filled with a flurry of activities and it is difficult to get to our members’ more “personal geologic” questions. So, QGS is extending our geologic events to provide more opportunities for our Quimper community to socialize and ask questions.
QGS is happy to help you identify a particular rock. We ask that you bring questions and rock samples to the pub the day after the lecture, where we can spend more time examining your specimens and answering your queries.
At the first meeting of “Meet Your Local Geologists”, on September 16, 2024, the day after Darrel Cowan’s lecture, several local geologists met at the Port Townsend Pour House to discuss a number of geological processes. Some interesting rocks got examined. A good time was had by all! Some folks hoped that QGS would continue to meet and discuss more geology…
So now, QGS will hold an event we will call “Meet Your Local Geologists”. We plan to meet the Sunday after our Saturday hosted lecture. These gatherings will probably take place after every other lecture. Here is information about our first such event.
SPECIFICS:
“Meet Your Local Geologist”
When: November 17, 2024
Time: 3-5 PM
Location: Pour House (2231 Washington St, Port Townsend)
Please check the QGS website frequently for the next occurrence… quimpergeology.org.
The Lecture: What Happens When You Crash Iceland into North America? A view of Washington 50 million years ago…
The Puget Lowland of Washington State contains several potentially dangerous seismic faults, including the Seattle fault, which runs south of downtown Seattle. To accurately assess the earthquake hazard in this region, we need to understand the architecture and geologic history of the rocks that host these faults, deep below the Puget Lowland. Geologists do this by using small changes in Earth’s gravity and magnetic fields to create images of the Earth’s subsurface. These rocks formed in a subduction zone 50 million years ago when a set of volcanic islands, similar to modern-day Iceland, collided with the edge of North America. This added a mass of rock, called Siletzia, to the continent. Megan showed us that as the islands piled up, they broke and folded into mountain ranges. South of Seattle, Siletzia was pushed up and over ancient North America, whereas to the north, Siletzia was pulled down and under the continent. She argued that a tear in Siletzia between these two zones eventually became the proto-Seattle fault, which provides a story for the Seattle fault’s origin and earliest history. Her images provided information that can improve models of ground shaking from future earthquakes affecting the greater Seattle urban area. *AGU abstract below.
This was an in-person only lecture, 4 PM on Saturday, November 16, 2024.
* AGU article abstract: Deep Structure of Siletzia in the Puget Lowland: Imaging an Obducted Plateau and Accretionary Thrust Belt with Potential Fields
Detailed understanding of crustal components and tectonic history of forearcs is important due to their geological complexity and high seismic hazard. The principal component of the Cascadia forearc is Siletzia, a composite basaltic terrane of oceanic origin. Much is known about the lithology and age of the province. However, glacial sediments blanketing the Puget Lowland obscure its lateral extent and internal structure, hindering our ability to fully understand its tectonic history and its influence on modern deformation. In this study, we apply map-view interpretation and two-dimensional modeling of aeromagnetic and gravity data to the magnetically stratified Siletzia terrane revealing its internal structure and characterizing its eastern boundary. These analyses suggest the contact between Siletzia (Crescent Formation) and the Eocene accretionary prism trends northward under Lake Washington. North of Seattle, this boundary dips east where it crosses the Kingston arch, whereas south of Seattle the contact dips west where it crosses the Seattle uplift (SU). This westward dip is opposite the dip of the Eocene subduction interface, implying obduction of Siletzia upper crust at this southern location. Elongate pairs of high and low magnetic anomalies over the SU suggest imbrication of steeply-dipping, deeply rooted slices of Crescent Formation within Siletzia. We hypothesize these features result from duplication of Crescent Formation in an accretionary fold-thrust belt during the Eocene. The active Seattle fault divides this Eocene fold-thrust belt into two zones with different structural trends and opposite frontal ramp dips, suggesting the Seattle fault may have originated as a tear fault during accretion.
About the Speaker:
Megan Anderson is an earthquake geophysicist at the Washington Geological Survey. Megan spent her early years in Kent, WA, during which the eruption of Mt. St. Helens spurred her fascination with geology, which was her major at Carleton College in Minnesota. She studied subduction processes and earthquakes in South America for her Ph.D. at the University of Arizona. She has studied numerous tectonic regions of the world but has always made her way back to the Pacific Northwest because there is so much left to discover. She taught for ten years at Colorado College, dragging her students and equipment across the country to do research in Washington. She is now firmly planted in Olympia as home.
Uncovering ancient subduction mega-faults in the Olympic Mountains
On October 5, 2024 the Quimper Geological Society hosted a lecture by Harold Tobin, professor of Seismology and Geohazards at the University of Washington and Director of the Pacific Northwest Seismic Network. Harold’s research employed field mapping and geophysical techniques to investigate processes that operate in fault zones, particularly those associated with subduction zones. One of his study areas is the Olympic Mountains, where recently (2023 and 2024) he and his graduate students have been studying a remote area west of Mount Olympus, last visited by geologists when Rowland Tabor and Bill Cady where they mapped more than 50 years ago. Tabor and Cady identified the Olympic Structural Complex, the highly deformed sedimentary rocks of the accretionary wedge associated with the Cascadia subduction zone. In a region ranging from Snow Dome to Mt. Tom and the White Glacier, Tobin’s team mapped a zone of concentrated brittle and ductile deformation they interpret as a fossil megathrust fault, the plate boundary fault responsible for great Cascadia earthquakes. Harold discussed the evidence for this conclusion and how analysis of an ancient fault helps us understand how and why these megathrust faults slip and generate earthquakes, as well as how the Olympics were built.
About the Speaker
Professor Harold Tobin holds the Paros Endowed Chair in Seismology and Geohazards at the Department of Earth and Space Sciences at the University of Washington. He also serves as the Director of the Pacific Northwest Seismic Network and Washington’s State Seismologist. Despite that title, his scientific roots are in subduction zone geology and the structure of plate boundary fault zones. With a B.S. in geology and geophysics from Yale University and a PhD from University of California, Santa Cruz, Harold has held faculty positions at New Mexico Tech and the University of Wisconsin-Madison prior to moving to Seattle in 2018. His first taste of subduction geology was as an undergraduate field assistant in the Olympic Mountains in 1987, and after 30 years of offshore and onshore research in Japan, Alaska, New Zealand, Costa Rica, and California, he has come full circle to explore the core rocks of the Olympic mountains.
Please check the QGS website frequently for the next occurrence… 
The Puget Lowland of Washington State contains several potentially dangerous seismic faults, including the Seattle fault, which runs south of downtown Seattle. To accurately assess the earthquake hazard in this region, we need to understand the architecture and geologic history of the rocks that host these faults, deep below the Puget Lowland. Geologists do this by using small changes in Earth’s gravity and magnetic fields to create images of the Earth’s subsurface. These rocks formed in a subduction zone 50 million years ago when a set of volcanic islands, similar to modern-day Iceland, collided with the edge of North America. This added a mass of rock, called Siletzia, to the continent. Megan showed us that as the islands piled up, they broke and folded into mountain ranges. South of Seattle, Siletzia was pushed up and over ancient North America, whereas to the north, Siletzia was pulled down and under the continent. She argued that a tear in Siletzia between these two zones eventually became the proto-Seattle fault, which provides a story for the Seattle fault’s origin and earliest history. Her images provided information that can improve models of ground shaking from future earthquakes affecting the greater Seattle urban area. *AGU abstract below.
Megan Anderson is an earthquake geophysicist at the Washington Geological Survey. Megan spent her early years in Kent, WA, during which the eruption of Mt. St. Helens spurred her fascination with geology, which was her major at Carleton College in Minnesota. She studied subduction processes and earthquakes in South America for her Ph.D. at the University of Arizona. She has studied numerous tectonic regions of the world but has always made her way back to the Pacific Northwest because there is so much left to discover. She taught for ten years at Colorado College, dragging her students and equipment across the country to do research in Washington. She is now firmly planted in Olympia as home.
Professor Harold Tobin holds the Paros Endowed Chair in Seismology and Geohazards at the Department of Earth and Space Sciences at the University of Washington. He also serves as the Director of the Pacific Northwest Seismic Network and Washington’s State Seismologist. Despite that title, his scientific roots are in subduction zone geology and the structure of plate boundary fault zones. With a B.S. in geology and geophysics from Yale University and a PhD from University of California, Santa Cruz, Harold has held faculty positions at New Mexico Tech and the University of Wisconsin-Madison prior to moving to Seattle in 2018. His first taste of subduction geology was as an undergraduate field assistant in the Olympic Mountains in 1987, and after 30 years of offshore and onshore research in Japan, Alaska, New Zealand, Costa Rica, and California, he has come full circle to explore the core rocks of the Olympic mountains.