More on subduction zones

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Paired metamorphic belts

Last week we left off looking at the thermal structure of a subduction-arc complex. The accretionary wedge is relatively cold because the cooler subducting slab is below, and the area beneath the arc is hot because of the heat transfer by ascending magma. The former is associated with a low geothermal gradient and the later with a high geothermal gradient. This influences the metamorphism that occurs in these areas. This paired belt pattern of metamorphic rocks was first worked out for Japan by Miyashiro. The accretionary wedge is associated with a lower geothermal gradient, and distinctive blueschist and eclogite metamorphic facies. In contrast the arc-plutonic complex is associated with high heat flow a high geothermal gradient. The associated metamorphic facies are greenschist and amphibolite facies, and associated intrusive migmatites are common.


Production of arc magmas

Considerable compositional variation, especially in comparison to basalts in ocean basins.

Elongate granitic batholiths such as found in the Sierra Nevada Mountains of California are often the roots of subduction related volcanic arcs. Are continents growing incrementally in size due to the formation of such batholiths through time?

In this geologic map to the left of California from the USGS the large red body in eastern California is the Cretaceous age Sierra Nevada batholith (image source: https://commons.wikimedia.org/wiki/File:Geologic_map_California.jpg ). Note how it is truncated by a large fault at its southern end (the Garlock fault), but other batholiths continue southward. Indeed the La Paz batholith at the tip of the Baja peninsula is also of Cretaceous age, and indicates that subduction used to be occurring along the length of western North America in the past. More on this tectonic evolution when the San Andreas plate boundary is discussed. To the right is a small part of this immense Sierra Nevada batholith that can be enjoyed in all its splendor within Yosemite National Park. Granite plus glaciation has formed a very distinctive landscape here.

Range in magmatic and volcanic composition (3 common geochemical series):

Partial melt mechanism(s) to generate the igneous activity:

Diagram attempting to identify the various factors that produce arc volcanism.


from http://pubs.usgs.gov/of/2000/ofr-00-0365/report.htm

Growth of arc root: Beneath an arc can have an 18-20 km crustal root (see diagram above). What are mechanisms of production?


Spatial patterns of volcanic and plutonic composition within arcs

With distance from trench there is an increase in slab depth, which should influence magma generation conditions and hence influence ultimate composition. What across strike patterns are seen?

Along strike spacing of volcanoes?


Exercise: Along strike arc segmentation: case history of the Andes. Take the xerox copies of plate tectonic map handed out and focus on the arc geology along its length. Identify distinct segments on the basis of volcanic, earthquake, fault and other behavior. Then try and relate the segment boundaries to along strike changes in the incoming subducting material.

Fitch faults and oblique subduction: Sunda style tectonics

The devastating earthquake and tsunami of 2004 have led to more detailed information being available for this tectonic system

Image from USGS site http://walrus.wr.usgs.gov/tsunami/sumatraEQ/tectonics.html, that shows how the oblique component of subduction increases northward along the trench. Yet studies of first motions of related earthquakes indicate that the movement was roughly trench orthogonal. The idea behind Great Sumatran Fault takes up the strike-slip component, and the area between this fault and the trench, the arc-trench gap, is acting as a microplate. In other words, the oblique subduction has been decoupled, separated into convergent and strike-slip components that are localized in different areas.

This block diagram nicely displays the difference between coupled versus decoupled oblique subduction. More specifics can be found at http://walrus.wr.usgs.gov/tsunami/sumatraEQ/tectonics.html, where this diagram was taken from. A question that naturally develops is - what determines whether oblique subduction is coupled or decoupled?


Back-arc basins and arc spreading

Some major examples:

Bathymetry of the Bering Sea. Image source: NOAA site http://www.pmel.noaa.gov/np/pages/seas/bseamap2.html .

Characteristic traits of these basins:

Mechanical models for generation:


Rear-arc fold-thrust belts

These provide a distinct contrast with rear-arc basins where extension dominates.

This is a strip of a satellite image taken from NASA's Visible Earth site (source: http://visibleearth.nasa.gov/view.php?id=69385 ) that goes from the Pacific coast across the central part of the Andes. The dark country border is between Bolivia (upper right), Chile (left) and Argentina (lower right). The glaciers and ice fields are located in the area of the volcanic arc, and a close look will find the volcanoes. Of interest here are the arc-parallel ridges in the right (eastern) half of the image. These are the surface expression of the rear-arc fold-thrust belt, and a close look will find fold structures. The repeated ridges indicate that the same layers are being tectonically repeated, a pattern consistent with an underlying detachment and a thin-skinned tectonic style.


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Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.