The Aleutian Trench is part of a subducting zone in the Pacific Ocean connecting Alaska to Kamchatka. The trench is formed where the Pacific plate is being subducted under the North American plate. The boundary is visibly defined on map, see the embedded link below.
Modelling the large-scale motion of the lithosphere is a fundamental topic in structural geology, plate tectonics is the current thinking to this end. The theory states that the lithosphere is broken up into tectonic plates, the boundary between plates is well defined in an appropriate geological framework . We will investigate the Aleutian Trench, a convergent boundary between the Pacific plate and the North American plate. A map of the earth's major (and some minor) plates is presented in Figure 1.
In a recent journal, Zahirovic, Sabin, et al analyse a plate motion model and extract topologies in 1 Myr intervals. The model uses paleomagnetic data sampled over the last 200 Myr, present day plate velocities from eight plates, and post-Pangea plate tectonic reconstructions . The embedded link below models the break-up of Pangea using data from Zahirovic, Sabin, et al; the video is hosted by AAAS via Earthbyte.org. The model projects the Pacific plate forming around 190 Myr ago and the Aleutian Trench forming around 40 Myr ago.
When a tectonic plate is subducted, stress builds in the boundary between the two plates. At some critical value of stress, the plates slip against each other and energy is relieved. This large scale displacement (and the mechanical waves which follow) is an earthquake. Figure 2 presents earthquake foci recorded over a ten year period around the Aleutian Islands, the foci are recorded on both plates along the boundary.
During 1980 and 1981, the U.S. Geological Survey completed a reconnaissance along the Aleutian Arc and the Aleutian Trench . The locations investigated are presented in Figure 3, for our investigation we examine the (analysed) section L9-12.
McCARTHY et al. analyse the time and depth section from the seismic survey and build a structural model, these are presented in Figure 4. Relative to the two plates, the contact is a thrust fault over the oceanic crust. An accretion layer of off-scraped sediment forms as the oceanic crust subducts . The faults formed on the oceanic crust suggest that a principle stress is in the direction of general vergence (Anderson fault theory). This is to say that the oceanic crust is extending and active. A master thrust fault runs along the plate boundary, a duplex fault system is forming on the hanging wall. The Pacific and North American plate form a convergent boundary.
The seismic data was processed at the U.S. Geological Survey marine seismic processing facility in Menlo Parktime . To generate such data sets, elastic waves are introduced to the crust. Bedded surfaces act well as reflectors, sensors are run along the cross section. Fault planes act as pseudo reflectors, in the depth section they can be seen cutting across the general vergence of the bedding. The time section assumes mild lateral velocity variations and so cannot resolve the faults.
The Aleutian Trench is an active convergent boundary between the Pacific and North American plates. The subduction is continuous, but the internal forces are high in the lithosphere so the displacement is expressed in discrete intervals. The North American plate is thrusting towards the Pacific plate, and the Pacific plate is extending towards the North American plate. This is apparent from the normal faults along the extending oceanic crust and the duplex system on the hanging wall. Relative to each other, the North American and Pacific plates form a contractional margin.
- Wikipedia contributors. "Plate tectonics." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 22 Mar. 2015. Web. 22 Mar. 2015 http://en.wikipedia.org/wiki/Plate_tectonics.
- Zahirovic, Sabin, et al. "Tectonic speed limits from plate kinematic reconstructions." Earth and Planetary Science Letters 418 (2015): 40-52.
- Stauder, William. "Tensional character of earthquake foci beneath the Aleutian Trench with relation to sea‐floor spreading." Journal of Geophysical Research73.24 (1968): 7693-7701
- McCARTHY, J. I. L. L., and David W. Scholl. "Mechanisms of subduction accretion along the central Aleutian Trench." Geological Society of America Bulletin 96.6 (1985): 691-701