The Queen Charlotte triple junction is a geologic

triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can be ...

where three

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

s meet: the Pacific plate, the North American plate, and the

Explorer plate The Explorer plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Vancouver Island, Canada, which is partially subducted under the North American plate. Along with the Juan de Fuca plate and Gorda plate, the Expl ...

. The three plate boundaries which intersect here are the

Queen Charlotte Fault The Queen Charlotte Fault is an active transform fault on the boundary between the North American plate and Pacific plates. It is Canada's right-lateral strike-slip equivalent to the San Andreas Fault to the south in California. The Queen Charlott ...

, the northern

Cascadia subduction zone The Cascadia subduction zone is a convergent plate boundary, about off the Pacific coast of North America, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ m ...

, and the Explorer Ridge. The Queen Charlotte triple junction is currently positioned adjacent to the Queen Charlotte Sound near the Dellwood Knolls off the coast of

Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian province of British Columbia. The island is in length, in width at its widest point, and in total area, while are of land. The island is the largest ...

.Spence, G. D., & Long, D. T. (1995) Transition from oceanic to continental crustal structure: Seismic and gravity models at the queen charlotte transform margin. Canadian Journal of Earth Sciences, 32(6), 699–717. 10 Ma to 1.5 Ma prior to the triple junction's current location, it was located southwest of Vancouver Island The movements of the triple junction have been characterized by two major shifts in the Pacific-North American Tertiary plate tectonic record. First, at approximately 40 Ma the relative plate motions switched from orthogonal convergence to right-lateral strike slip. The variance in location of the triple junction may have also been related to the formation of an independent basin block. This formation could have been produced by fore-arc bending of the Pacific plate, due to oblique underthrusting prior to 1 Ma which produced stresses sufficient to break the Pacific plate and isolate the block. Transpression of 15–30 mm/yr since 5 Ma has been taking place, as well as varying amounts of both transpression and transtension occurring before then.Rohr, K. M. M., & Furlong, K. P. (1995). Ephemeral plate tectonics at the Queen Charlotte triple junction. Geology, 23(11), 1035–1038. To the northwest of the triple junction the Pacific plate currently has 15 degrees of oblique convergence, passing under the North American plate along the Queen Charlotte transform fault zone. The Explorer plate is a small chunk of the Juan de Fuca plate that broke away from the Juan de Fuca plate about 3.5 Ma and has moved much slower with respect to North America.

Plate kinematics and overview

The relative plate motions of this region have been difficult to determine due to the complicated nature of the Pacific, Juan de Fuca and Explorer plate triple junction

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

triangle not being understood. The Juan de Fuca plate, created at the spreading ridge southwest of the triple junction, is moving at a rate of 45.7 mm/yr at an azimuth of 244˚ in relation to the North American plate, and the Pacific plate is moving at 58.6 mm/yr in relation to the Juan de Fuca plate. It has been proposed as well that the Explorer plate which makes up one of the three points of the junction is an

ephemeral Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...

(short lived) plate that behaved independently for a brief period. Around 4 Ma it rapidly evolved and culminated as a new transform plate boundary. On the east it is becoming coupled with North America, while the western side becomes part of the Pacific plate. Earthquakes also occur due to the separating of the Pacific and North American plates along the Queen Charlotte Basin. Seismic recording studies have been made in the region with the two most active regions being the immediate area surrounding the Dellwood knolls and the Dellwood-Revere fracture zone (refer to fig.1).Riddihough, R. P., Currie, R. G., & Hyndman, R. D. (1980). Dellwood Knolls and Their Role in Triple Junction Tectonics off Northern Vancouver Island. Canadian Journal of Earth Sciences, 17(5), 577–593. The magnitudes ranged from 0.2 to 3.2 in 76 events over the course of the 15-day study; however, magnitudes in the region have gone up to a magnitude 6.4 within the last 5 years. Using the locations of these epicentres it is possible to map the Pacific plate boundary along the Dellwood valley where the concentrations of events occur.

Explorer plate

The Explorer ridge has been migrating since 5 Ma to the west at a rapid pace(~22 mm/yr), while the Juan de Fuca plate has remained stable. The migration was due to a combination of jumps, asymmetric spreading, and segment propagation. This model implies that the Queen Charlotte fault is lengthening to the south, while fragmenting the Explorer plate. Furthermore, this means that the Explorer plate is most likely being captured by the Pacific plate. However, the previously subducted parts will remain in place, coupled with the North American plate. A similar process to this is taking place in the Rivera triple junction where small ephemeral plates were also formed. The Explorer ridge is in the process of becoming extinct however, and high seismicity in the Explorer plate indicates that it is being severed by the establishment of this new simpler plate boundary configuration.

Queen Charlotte Basin

The Queen Charlotte Basin was formed during the last 43 Ma by episodes of extension paired with a belt of subsidence and uplift. Periods of

igneous activity Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

in the

Queen Charlotte Islands Haida Gwaii (; / , literally "Islands of the Haida people"), previously known as the Queen Charlotte Islands, is an archipelago located between off the northern Pacific coast in the Canadian province of British Columbia. The islands are separ ...

have corresponded with periods of

extension Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (proof theory) * Extension (predicate logic), the set of tuples of values that ...

. The

heat flow Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

in the Queen Charlotte Basin has been calculated to be 69±5 mW/m². To the southeast the heat flow is reduced through cooling by the subducting plate, and increased to the northwest through crustal extension. There is much evidence for

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

in the Queen Charlotte Basin such as steeply dipping to vertical basins, deep and narrow asymmetric depocentres, upward branching complex fault patterns and contemporaneous normal and reverse faults within the same structure or local area.Rohr, K. M. M., & Dietrich, J. R. (1992). Strike-slip tectonics and development of the tertiary queen charlotte basin, offshore western Canada: Evidence from seismic reflection data. Basin Research, 4(1), 1–19. Longitudinal asymmetry is a classic indicator of strike-slip tectonics and can be observed at many scales in the Basin.

References

{{Triple Junctions Geology of British Columbia Triple junctions Plate tectonics Oceanography of Canada