Deep Structure of Siletzia in the Puget Lowland: Imaging an Obducted Plateau and Accretionary Thrust Belt With Potential Fields

Abstract

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.

Key Points

• Includes map interpretation and models of the upper crust utilizing constraints from gravity, aeromagnetics, seismology and geology

• Modeled structures show an accretionary fold and thrust belt, wrapping around the northern edge of an obducted northern margin of Siletzia

• Interpreted structures suggest the Seattle fault could have an earliest Eocene history as a tear fault within the fold and thrust belt

Plain Language Summary

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. We 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 layer of rock, called Siletzia, to the continent. We show 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. We argue 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. Our images also provide information that can improve models of ground shaking from future earthquakes affecting the greater Seattle urban area.