Today we are looking at plate tectonics, giving a brief introduction to the Aleutian Arc, which marks the boundary between the North American Plate and the Pacific Plate. The Arc is both seismically and volcanically active. Volcanic activity is of interest because the Arc lies under very busy North American and Asian aviation routes .
The Pacific Plate is moving north westward at a rate of 59mm per year in the north east and 92mm per year in the north west. It subducts orthogonally under the North American Plate on its north-eastern margin, obliquely further westward, and parallel to the North American plate at the transform boundary on its north-western margin. The subduction zone comprises the island arc, the Aleutian Trench, and distinct Wadati-Beniof zones, which extend down to around to between 100 km and 250 km.
The Aleutian Arc is 3,000 km long from the junction with the Fairweather Fault (an extension of the Queen Charlotte Fault) in the east to the triple junction of the Ulakahan Fault, Aleutian Trench and Kuril-Kamchatka Trench in the west. The arc includes a 2,500 km long chain of basaltic andesitic stratovolcanoes and calderas. Behind the arc are basaltic lava fields in the Bering Sea. Most volcanoes are in uninhabited or sparsely populated areas so they pose more of a risk to aviation.
Origins of the Aleutian Arc
In the early Cenozoic, 60 million years ago, the Farallon Plate and Kula Plates covered what is now the northern Pacific Ocean; the Kula Plate moved northwards, while the Farallon Plate moved eastwards. There were active continental margins: in the north west there was in island arc; and, in the north east there was subduction of oceanic crust beneath the Bering Shelf volcanic belt. The Kula-Pacific Transform Fracture Zone may have separated the north west Pacific from the north east.
In the middle of the Eocene (50 million years ago to 47 million years ago) the movement of the Pacific Ocean plates changed from the NNW subduction of the Kula Plate, followed by ridge subduction, to the NW subduction of the Pacific Plate. This change in motion resulted in the formation of the Aleutian Arc and its back-arc basin in the Bering Sea. The Shirshov Ridge and Bowers Ridge originated from the ongoing displacement of the North American Plate in relation to the Eurasian Plate in the mid Eocene. Subduction beneath the Kamchatka margin is associated with the late Cenozoic. The Komandorsky Basin formed in the Miocene.
The oblique subduction of the Pacific Plate towards the centre of the Aleutian Arc has caused clock-wise rotation of island arc blocks and breaches in the island chain, notably at the Near, Buldir, Amchitka and Amukta Straits. This motion moved the Komandorsky block from its location at the subduction zone near the junction with the Bowers Ridge to its current location at the transform boundary; Eocene volcanicsnin the block ceased after its movement away from the subduction zone. Recent tomographic studies have shown a possible slab under the Bering Sea that may be a remnant of the Kula Plate.
The current Aleutian Arc is seismically very active; it has produced several large earthquakes with magnitudes in excess of 7.0M.
We looked at the earthquakes between 1 January 1975 to 30 November 2020 with a magnitude greater than 4.5M from 47.04°N, 142.559°W to 66.548°N, 198.984°W downloaded from USGS Earthquake search (see Sources below). This includes the triple junction at the west of the Arc and the junction with the Fairweather Fault in the east, and the Bering Sea. It also picks up some of the subduction zone to the west at the northern end of the Kuril-Kamchatka Trench. We also identified seismic swarms in the period; for this purpose, swarms are defined here as more than 30 earthquakes per day (normally there are less than 10 per day).
Figs 2a and 2b show that most of the action is at the southern section of the Arc where the subduction of the Pacific Plate changes from orthogonal to oblique. The geoscatter plot shows that most of the swarms are occurring here, confirmed by the geodensity plot. There is very little seismic activity behind the arc, except for a swarm in Kamchatka north of the arc.
Earthquakes with magnitudes greater than 7.0M occur round the arc, with the exception of the region near the Fox Islands; the lack of earthquakes here greater than 7.0M may be due to the nature of the crust, or, the comparatively short time period selected (45 years is a short time in geological terms).
If we look closer at sections of the arc (see figs 3a to 3e below), we can see that that earthquakes tend to follow a block pattern with gaps in between; the gaps may be gaps between crustal blocks or they may be areas likely to have swarms in the future. We can also see the well-defined Wadati-Benioff Zones which extend to a depth of 250 km in the Eastern and Central Blocks (blocks here are the sections of the arc that we have selected to plot and are not intended to represent crustal blocks).
The lack of an active Wadati-Beniof Zone under the western segment of the Aleutian Arc explains why there is little current volcanism there. Active volcanism in Kamchatka is south of the Aleutian arc, driven by the subduction of the Pacific Plate under the Okhotsk Plate. There is some seismicity in Kamchatka, north of the Aleutian Arc; this does not appear to be connected to an active subduction zone, although looking at Google Maps there may be an old trench in the area.
Clearly, the western and eastern segments of the Aleutian Arc are complex junctions and deserve a closer look. We will examine this in more detail when we discuss volcanic activity.
The Armchair Volcanologist.
19 December 2020
© Copyright remains with the author, all rights reserved, 2020.
Sources & Further Reading
Map base for Fig 1: Google Maps
Raw earthquake data from USGS Earthquake Catalogue Search: https://earthquake.usgs.gov/earthquakes/search/
The Aleutian Arc, Wikipedia: https://en.wikipedia.org/wiki/Aleutian_Arc
“Volcanoes of the World”, Third Edition, Lee Siebert, Tom Simkin, and Paul Kimberly, Smithsonian Institution, 2010, University of California Press
“Cenozoic Geodynamics of the Bering Sea Region”, V. D. Chekhovich, A. N. Sukhov, O. G. Sheremet, and, M. V. Kononov, Geotectonics, 2012, Vol. 46, No. 3, pp 212-231.
“Bowers Ridge (Bering Sea): An Oligocene – Early Miocene Island Arc”, Maren Wanke, Maxim Portnyagin, Kaj Hoernle, Reinhard Werner, Folkmar Hanff, Paul van den Bogaard, and Dieter Garbe-Schönber, Geology (2012) 40 (8): 687–690.