The Cracked and Broken Plate on Which Our Islands Rest

THE ONGOING TECTONIC PROCESSES of the Cascadia Subduction Zone offshore of and beneath Western Washington and British Columbia have been dishing up earthquakes, tsunamis, and volcanism that regularly alter the geological surface features of the Pacific Northwest. Indeed, these deformational processes along with past glaciation are what make our region so inviting — its diversity of scenery, environments, and ecosystems. It is a region of tectonic plate collision where the Juan de Fuca and Explorer microplates are steadily diving beneath the North American Plate. These microplates move under the continental edge at 42 to 45 millimeters per year, or nearly as fast as our fingernails grow. This motion compresses the western edge of the continent, forcing up the upper plate, which is consequently deformed through folding and faulting. In addition, as the down-plunging plates move under the upper plate, materials on them are off-scraped and accreted to it, forming an “accretionary wedge.” This wedge, along with exotic tectonic terranes transported by plate motion northward from California and Oregon — huge blocks torn from the Sierra Nevada and Oregon Coast Ranges — is what built the San Juan Islands.

The rocks that make up the San Juan Islands are as diverse as their physiography. The underlying and exposed basement rocks consist of volcanic, granitic, and metamorphic types of the Paleozoic to Mesozoic eras (formed from about 420 million to 65 million years ago), which are overlain locally by sedimentary types of bedrock that were deposited in the Cenozoic era (from about 55 million to 2 million years ago). These surface rocks were severely sculptured during the last major glacial ice advance (about 25 to 30 thousand years ago) when vegetation and soil were eroded followed by deposition of glacial till during the Pleistocene Epoch. The weight of the overlying ice depressed the basement rock surface, which rebounded after the ice retreated or melted in place.

Although the process of plate motion and collision continues today, recent earthquakes of moderate to large magnitudes (greater than magnitude 4) generated from local tectonic stress release have not been recorded within the San Juan Archipelago. Regionally, however both crustal (located in the upper 37 km or 23 miles) and subduction (located at deeper depths along the subducting slab) earthquakes have occurred. Crustal earthquakes of low to moderate magnitudes in the greater Puget Sound region occur primarily due to north-south compressive stresses along western Washington. Based on paleoseismology studies, at least five and as many as seven ground-rupturing earthquakes have occurred in Puget Sound lowlands during the Holocene Epoch (the last 11,500 years). The most recent large-magnitude earthquake to occur in the greater Puget Sound region was the 2001 magnitude 6.8 Nisqually event that originated on the interface between the down-going slab of the Juan de Fuca Plate and upper plate of the Cascadia Subduction Zone at a depth of 52 km (or about 32 miles).  No recently reported large earthquakes have occurred either on the Seattle or the Devils Mountain fault zones, which are located in the upper, crustal plate.

Recent Marine Geologic Investigation of the Archipelago

During the past two decades, a suite of marine bathymetric and geophysical data was collected for the first time and interpreted to map geologic structures such as faults and folds on and beneath the floor of central Salish Sea; they have been used to document the tectonic history of the region. The interpretation of these data indicates that tectonic deformation with considerable faulting and folding is taking place in this part of the upper plate. Deformed sediments suggest that many of these faults are active, since they displaced sediment from the Quaternary Period (from 2.6 million years ago to the present day). This activity occurs along faults that lie between the two major east-west oriented lateral fault zones, the Skipjack Island and Devils Mountain fault zones that bound the San Juan Archipelago on the north and south; it may result from stress associated with movement along these fault zones.

The recently mapped Skipjack Island fault zone separates the Canadian Gulf Islands from the San Juan Islands; it extends from Vancouver Island (where it appears to connect with the Fulford fault) to just offshore of northern Lummi Island. Lateral movement along the fault zone has rotated the islands of Matia, Sucia, and Patos, and is producing zones of compression along a ridge between southern Alden Bank and northern Lummi Island.

Along the southern boundary of the San Juan Archipelago lies the Devils Mountain fault zone, which extends from southern Vancouver Island immediately offshore of Victoria to the mainland of Washington State. Several faults splay northwestward from the primary trace of the Devils Mountain fault zone. These faults form the bays and peninsulas of southern Lopez Island as well as shape the southwestern shoreline of San Juan Island and extend across Haro Strait to the Saanich Peninsula. These faults appear to be active, as they displaced Quaternary sediments deposited in the bays of southern Lopez Island and pockets of sediment within Vancouver Island’s offshore shelf.

If those of you living in the San Juan Islands at times feel a bit dizzy, it may be (tongue-in-cheek) from the gradual rotation of this Archipelago. Horizontal strike-slip movements along the faults bounding the Archipelago appear to be rotating it clockwise, with local areas such as the seabed north of Lummi Island and southern Lopez Island being compressed. In addition, on 5 April 2019, a shallow (about 13 km or 8 miles deep), magnitude 2.9 to 3.2 earthquake and aftershocks struck along a reactivated fault, possibly the Haro fault, near the center of the Archipelago. This event suggests that internal deformation is occurring within the San Juan Islands that may be a precursor of more to come.

Our recent investigations indicate that the San Juan Islands are presently undergoing tectonic deformation with active folding and faulting. This conclusion is not unexpected, given the proximity of the islands to an active subduction zone, the Cascadia Subduction Zone, located offshore of the North American coast and running beneath the islands. But the history of previous episodes of seismic and aseismic activity in the area is poorly understood along with the relationship of lower-slab movement to upper-plate deformation. While the plate collision continues, stresses are building within the rocks of the Archipelago and further deformation and minor to moderate earthquakes can be expected hereabouts with potential larger magnitude earthquakes on the Devils Mountain and Skipjack Island fault zones.

Tectonic map of the San Juan Archipelago showing previously mapped faults. White lines with yellow labels are faults constructed from mapping on the San Juan Islands and other islands; red lines represent seafloor bounding faults of the Archipelago including the recently mapped Skipjack Island fault zone (SJIfZ) and the previously mapped Devils Mountain fault zone (DMfZ). Other faults include LfZ = Lopez fault zone or Ltf = Lopez thrust fault; Ff = Fulford fault on Vancouver Island, BBf = Buck Bay fault, Rtf = Rosario thrust fault or RfZ = Rosario fault zone, Ot = Orcas thrust fault, and Hf = Haro fault. Islands include DI = Decatur Island, LOI = Lopez Island, SJI = San Juan Island, SI = Shaw Island, BlI = Blakely Island, LI = Lummi Island, CI = Clark Island, OI = Orcas Island, WI = Waldron Island, StI = Stewart Island, PI = Patos Island, and MI = Matia Island.

This article is a synopsis of a recently published paper by H. Gary Greene and J. Vaughn Barrie in a special issue of the Journal of the Geological Society of London:

Greene, H.G., Barrie, J.V., 2020. Faulting within the San Juan-Gulf Islands Archipelagos, upper plate deformation of the Cascadia Subduction Complex. In Asch, K., Kitazato, H., and Vallius, H., eds., From Continental Shelf to Slope: Mapping the Ocean Realm. Geological Society of London Special Publication 505.

Author Profile

Gary Greene is a marine geologist who has been mapping the sea floor of the Salish Sea for more than two decades. He served as Director — and is currently Professor Emeritus — of the Moss Landing Marine Labs on Monterey Bay in California and was a research faculty member at Friday Harbor Labs. Now heading the Tombolo Mapping Lab at the SeaDoc Society on Orcas Island, he has pioneered sophisticated sonar techniques to map the sea floor in many regions of the Pacific Ocean, discovering previously unknown benthic habitats and geological features such as submarine faults and folding.

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