https://www.abovethenormnews.com/2025/10/06/explorer-microplate-slab-tear-vancouver-island/


The Birth of a Microplate, The Death of a Trench
By
David Freeman - October 6, 2025



At 07:42 on July 18, 2021, aboard the research vessel Marcus G. Langseth, seismic channel four lit up with a signal no one expected. The ship was forty nautical miles off the Vancouver Island shelf, holding position over the Nootka Fault Zone. Air guns below the waterline pulsed rhythmically into the deep, bouncing acoustic waves off layers of subsurface rock. What returned was a discontinuity. A sharp drop in the oceanic crust. A vertical break in the slab. The image built slowly. Layer by layer, a tear in the Earth opened on the screens.

It was not a theoretical model or a retrospective reconstruction. This was the moment a subduction zone began to die.

The Nootka Fault Zone is a narrow corridor of left-lateral shear where the Explorer and Juan de Fuca plates grind past each other before plunging beneath North America. But the grinding is no longer balanced. And the plunging is no longer continuous. A fracture in the descending Explorer slab, thirty-five kilometers long, nearly vertical, and descending to forty kilometers depth, has formed, slicing through crust and mantle alike. Across it, the slab has dropped by more than three kilometers. Subduction, the very force that drives the Cascadia arc, is shutting down in real time.

This is not happening all at once. The process began nearly four million years ago when a segment of spreading ridge approached the trench at an oblique angle. That ridge carried young, buoyant oceanic crust that resisted sinking. The trench met resistance. Slab pull weakened. The ridge jumped west, leaving behind an orphaned block of crust. That block is now the Explorer microplate, wedged between the Pacific Plate to the west, the Juan de Fuca Plate to the south, and the North American Plate to the east. A triple junction fractured. A fault zone formed.

Early deformation was broad and chaotic. Ridge-parallel faults inherited from seafloor spreading reactivated under shear. Seawater penetrated deep into the mantle. Serpentinization altered peridotite, weakening it further. The crust cracked. Hydrothermal circulation surged. Over time, the stress narrowed into a defined shear zone. The Nootka Fault Zone became a twenty kilometer wide belt of left-lateral motion marked by twin fault strands, the Northern and Southern Nootka faults. Today they bound the decoupling edge of a tectonic microplate.

The CASIE21 expedition brought a suite of high-resolution seismic imaging tools to the region. With an array of multichannel sensors towed behind the vessel and a grid of ocean-bottom seismometers deployed along the fault corridor, researchers created a complete acoustic cross-section of the incoming plate, the overriding crust, and the slab interface. Each ping from the air guns sent a wave of energy through the crust and mantle, captured and processed into precise subsurface imagery.

Seismic images from the survey show clear slab offsets in both the Explorer and Juan de Fuca plates. On profile PD02, the Explorer slab shows a vertical break. Earthquakes from the past decade align precisely along this fault plane, including a magnitude 6.6 rupture in 2014. The pattern of seismicity is near-vertical. The focal mechanisms are normal faulting. The interpretation is unavoidable: this is a mature slab tear, active and deepening.

On the Juan de Fuca side, the tear is still in its early stage. It appears as a broad buckle rather than a sharp offset. Seismicity is more scattered. But the geometry matches the Explorer tear. Both terminate where they intersect the subducted trace of the Nootka Fault Zone, evidence that the transform fault cuts and offsets the slab tears themselves.

The implications go beyond structure. Methane vents at Maquinna Mound mark where fluids escape along the southern strand. In seismic sections they appear as packets of low-frequency reflections capped by bright, inverted signals. This is the fingerprint of gas beneath compacted sediment and confirms that faults within the zone act as deep conduits from mantle to seafloor.

Submersible dives at these sites have observed columns of gas bubbles and warm fluid discharge. These anomalies suggest ongoing hydrothermal alteration and deep fluid migration. They are not residual features from ancient faulting. They are modern signatures of an actively fragmenting plate boundary.

Along the transform corridor, seismicity is not evenly distributed. Earthquake density is highest along the southern strand where the most pronounced structural offset occurs. The Northern Nootka Fault exhibits less displacement but remains active. Between them, a complex network of splay faults branches outward, accommodating localized extension and transtension. In the upper mantle, seismic reflectivity reveals fault-plane echoes that dip at sixty degrees, often in en echelon patterns typical of strike-slip deformation.

Further landward, the Explorer slab deepens, but its seismic signature fades. Tomographic models show a region of reduced velocity consistent with slab detachment. In that same area, tremor and low-frequency earthquakes disappear. On the Juan de Fuca side, those signals continue, indicating ongoing coupling between the slab and the overriding plate. This asymmetry is critical. It confirms that the Explorer segment is no longer mechanically linked to the surface plate. It is tearing free.

Heat flow measurements taken along the margin support this interpretation. Offshore of the Explorer domain, thermal gradients are steeper. Onshore, temperatures are anomalously low. The contrast indicates that the cold slab, once descending into the mantle and drawing heat away, is no longer present. Instead, warm mantle is likely rising to fill the void.

That void is known as a slab window. As the detached slab pulls away, it leaves an open conduit between the asthenosphere and the base of the crust. In past events, such windows have triggered unusual volcanic activity. Along the Garibaldi Volcanic Belt in British Columbia, geochemical anomalies in alkali basaltic rocks suggest input from deep, dry mantle sources. The timing and location match the projected slab tear beneath the Explorer segment.

The broader tectonic consequences are still unfolding. The Queen Charlotte Fault to the north, once a secondary transform, may re-establish itself as the primary boundary accommodating Pacific and North America motion. If the Nootka Fault continues to propagate downward and inland, the triple junction may jump south, as it did thirty million years ago during the Farallon breakup. In that ancient scenario, abandoned microplates and fossil ridges were left embedded in the Pacific Plate. Similar structures may already be forming offshore Cascadia.

Slab detachment is not rare in Earth’s history, but catching one in progress is unprecedented. Previous reconstructions rely on seismic tomography and geologic remnants. Here, for the first time, researchers can correlate live earthquake data, active deformation, and deep imaging in one place. The discovery redefines how microplates form and how subduction zones end.

The final act may take hundreds of thousands of years. But the record is already clear. Subduction beneath the Explorer plate is no longer sustainable. The forces that once pulled the slab beneath North America have weakened beyond recovery. In their place, a new tectonic regime is forming. The slab is tearing. The trench is retreating. And the edge of a continent is rewriting its boundary.

This is not conjecture but a record built from seismic images, earthquake catalogs, and bathymetric maps. It is the first time the death of subduction has been imaged as it happens. Beneath Vancouver Island a microplate is being born. Offshore, the trench that once consumed it is dying. And for the first time, scientists can watch the change unfold in real time.

Source:

https://www.science.org/doi/10.1126/sciadv.ady8347