GLS 342: Plate Tectonics
Assessing the angle of subduction using GeoMapApp
Introduction
The dip angle of the subducting oceanic lithosphere within a subduction zone has implications for the tectonic and petrologic history of a convergent plate boundary. There are numerous factors that may influence the angle of subduction. This lab will focus on two major factors: 1) age of the oceanic lithosphere, which is related to the temperature and density of the oceanic lithosphere; 2) and the rate of convergence.
Learning Goals
1) Use real data sets to investigate geologic questions and develop skills with organizing and deciphering large data sets.
2) Understand the relationship between subduction angle, age oceanic lithosphere, and convergence rate.
Overview
Students will complete three tasks using GeoMap App:
1) Plot earthquake data at each subduction zone and based on earthquake data, plot the subduction interface to determine the angle of subduction.
2) Determine the age of oceanic lithosphere adjacent to the oceanic trenches.
3) Determine the rate of convergence at oceanic trench.
The four subduction zones that students will analyze are:
1) Tonga–Kermadec trench: 174° W, 25° S
2) Sumatra–Java trench: 115° E, 9° 30’ S
3) Central American trench: 87° W, 11° 30’ N
4) Chile–Peru trench: 72° W, 21° S
GeoMapApp directions
Open geomapapp (download at GeoMapApp.org_
-choose Mercator (default)
Part 1: Collect earthquake data
Go to Datasets
 earthquakes and plate boundaries
 earthquakes >5.0 M (1973–2009)
This will plot earthquake data for the Earth. However, this is a lot of data and it not all of the earthquake data will be plotted. As one zooms in to specific areas, the program will update the earthquake data and plot all of the data for a smaller region.
Go to first subduction zone
Tonga-Kermadec trench
-use navigation symbols on GeoMapApp toolbar
1

-In order to add more detail to the dataset, see toolbar on right side of the program.
Choose color
 set color to depth from dropdown menu
-in order to be consistent with upcoming datasets, adjust color range by moving the deeper (right) arrow to 600 km (this may already be set to this range for the Tonga-Kermadec system)
Choose scale
 set scale to magnitude from dropdown menu
These data should give one a sense of how seismically active this subduction system is (i.e., how frequently there are earthquakes and their magnitude).
Complete the same steps for the other three subduction zones and compare the four subduction zones.
Save images of earthquake data from all four subduction zones.
Questions (include answers on separate homework report)
1) Based on these data, which subduction system is the most seismically active?
2) Which subduction zones have the largest earthquakes?
3) Which subduction zones have the deepest earthquakes?
4) Based on the depth and number of earthquakes, discuss the most prominent causes for the earthquakes at each of these subduction zones.
5) At this stage, estimate which subduction zones have downgoing slabs that dip more steeply than others.
2

Part 2: Determine the angle of subduction at each subduction zone
Choose Basemaps
 Regional grids
 subduction interface depth
 choose Syracuse and Abers (2006) for Chile–Peru (Andes)
 choose USGS Slab 1.0 for Central America, Tonga-Kermadec, Java
-adjust color gradient in “contributed grids” window by moving line of the subducted slab
-set contour interval and range (button that looks like small contoured hill)
 set interval to 50 m
-use distance/profile tool (grid button, next to contour button) to produce a profile
 click on button
-make sure the arrow tool is highlighted (on main map); then, click and drag across the subduction system
-this will produce a profile; from this profile, calculate the subduction angle.
If the subduction profile is complex, select arbitrary domains and determine the angle for each domain
NOTE: on the layer manager, one can click on the information button and will be linked to the actual data hosted on the USGS website
Questions (include answers on separate homework report)
1) What are the angles of subduction for each of these subduction zones?
2) Are these subduction angles what were expected? Explain.
3) What could be causes for the differences in angle of subduction for each of the regions?
Save images from each subduction zone with contours and subduction interface
3

Part 3: Assessing the factors that influence subduction angle
Determine the convergence rate
Basemaps
 seafloor ages, depths, sediments, and spreading
 seafloor spreading rate (Earthbyte)
-in layer manager, click on the legend for a color scale to estimate rates
-or click on the information button and find the data on the Earthbyte website
Determining seafloor age
Basemaps
 seafloor ages, depths, sediments, and spreading
 seafloor crustal ages
Questions
1) What are the (estimated) convergence rates at each of the subduction zones?
2) Based on the data, does the rate of convergence influence subduction angle?
3) What are the oceanic crustal ages at each of these subduction zones?
4) Based on the data, does the age of oceanic lithosphere influence subduction angle?
Summary questions
1) What other factors may have a significant influence of subduction angle? List these.
2) Why do some subduction zones have very deep earthquakes, whereas other subduction zones do not?
3) How do the factors that influence subduction angle also influence the type of accretionary orogen? Describe the possible types of accretionary orogens and determine what types of orogens these four subduction systems represent.
4