More on subduction zones
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.
- What is eclogite??
- high pressure and low geothermal gradient metamorphic product of basalt, high density - two diagnostic minerals oomphacite (green pyroxene) and garnet.
- fairly rare.
- Image of an eclogite from Norway. In addition to the red garnet, and the greenish pyroxene omphacite, there is blue kyanite and a bit of white mica known as phengite. Could the phengite carry water to even deeper mantle depths? Image source: https://commons.wikimedia.org/wiki/File:Eclogite_Norway.jpg
- Possible that when earth
became cool enough to form eclogites, subduction and arc magmatism as we know it started - Bjørnerud, M & Austrheim, H, 2004, Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust; Geology, v. 32 no. 9 p. 765-768.
- Japan has two sets of paired metamorphic belts.
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):
- low-K tholeiitic series (mostly basalt).
- calc-alkaline series (mostly andesite, most common?).
- alkali series (alkaline basalts and shoshonites).
- Diagram to right is an AFM diagram showing two of the compositional trends. A is Na2O + K2O, F is FeO + Fe2O3, and M is MgO. Image source: https://en.wikipedia.org/wiki/Calc-alkaline_magma_series .
Partial melt mechanism(s) to generate the igneous activity:
- shear heating? Thought to be minimal due to low friction, and low geothermal gradient. The arc is also not in the right place.
- model of conditions at and above slab under
volcanic arc: 500-700 degrees C (Peacock)
- fluid release from prograde metamorphic reactions:
- depths 80-100 km: amphibolite -> eclogite
+ water. water + pyrolite (mantle rock) at these PT -> melt of asthenosphere.
- depths >100 km serpentine dehydrates
and which could aid partial melting of quartz-eclogite crust, producing acidic
magmas, which interact with the mantle pyrolite and fractionate
to produce calc-alkaline series. Serpentine is possibly more associated with fracture zones (?), and this could set up some predictions for this
- evidence for slab contributions to arc magmas:
- isotopic signature of continental derived
sediments in volcanics.
- Peacock suggests will have slab contributions
only for anomalously warm subducted slab, i.e. when subducting
a spreading ridge. Refers to models with a significant slab contribution
as the minority view.
- introduces idea of temporal evolution of
magmas. Magma generation is also episodic.
- evidence for mantle contributions.
- the presence of basalts in the suite demands a mantle
- what does the volume of arc magmatism indicate?
- corner flow model
for convection above the slab and replenishment of the source material.
Diagram attempting to identify the various factors that produce arc volcanism.
Growth of arc root: Beneath
an arc can have an 18-20 km crustal root (see diagram above). What are
mechanisms of production?
- tectonic processes (crustal thickening).
- magmatic underplating (mafic lower crust).
- batholiths as columns versus blisters.
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?
- shallower dip yield wider arc and better
- H2O soluble elements (B, Cs, As, and Sb)
decrease away from trench - thought to reflect dewatering of
slab (Ryan & others).
- K2O increases away from trench.
- very complex story that involves, free fluids,
fluid expelled in dehydration reactions, potentially two source
materials (slab and wedge), and contamination and fractionation.
Along strike spacing of volcanoes?
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
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?
basins and arc spreading
Some major examples:
- Japan Sea.
- Bering Sea.
- Gulf of Mexico and Amerasian basins ( Lundin & Doré, 2017).
Bathymetry of the Bering Sea. Image source: NOAA site http://www.pmel.noaa.gov/np/pages/seas/bseamap2.html .
- Lau basin in back of Tonga arc.
- Mariana trough assoc. with Marianas trench.
- East Scotia Sea back-arc (extreme S Atlantic)
Characteristic traits of these basins:
- strong seismic wave attenuation in underlying
mantle above the slab (suggesting more distributed mantle melt).
- trench axes migrating seaward in hotspot
reference frame for arcs that have growing back arc basins.
- underlain by oceanic crust, sometimes with
coherent magnetic anomalies, often not.
- a distinctive magmatic-volcanic suite is
associated with the arc-rifting event. MORB like.
- often thick sediment fill/cover.
- sites of abundant hydrothermal activity.
Mechanical models for generation:
- Sinking subducted plate pulls overriding plate with
it (trench suck) producing tensile stresses in overriding plate
which fails at weakest point, eventually causing lithospheric
thinning and generation of basaltic crust. If it continues long
enough a seafloor spreading center organizes.
- Alternate model for back-arc basin - capture
by outboard trench jump. They may be polygenetic.
- Lundin and Dore´ (2017) arue for a distinctive high-angle back arc, where spreading is oblique to the subduction zone and often has a very significant rotational component.
Rear-arc fold-thrust belts
These provide a distinct contrast with rear-arc basins where extension dominates.
- Good example the Andes (see exercise map, Haschke et al. 2002)
and the Sevier fold-thrust belt (in Idaho-Wyoming).
- Trench axes not migrating in hotspot reference
- More common on continental margins.
- Crustal thickening definitely is a major tectonic
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.
- Haschke, M., SIebel, W., Guntar A. & Scheuber, 2002, Repeated crustal thickening and recycling during the Andean Orogeny in northern Chile (21°-26° S); JGR, 107, B1 # 2019, 10,1029/2001JB000328.
- Hamilton, W., 1979, Tectonics of the Indonesian
region; Geological Survey Professional Paper 1078, 345 p. This
has a wealth of informaton and one incredibly beautiful map.
- Lundin, E. R. & Dore' , 2017, The Gulf of Mexico and Canada Basin: Genetic Siblings on Either Side of North America; GSA Today, 27, 4-11 - http://www.geosociety.org/gsatoday/archive/27/1/abstract/i1052-5173-27-1-4.htm .
- Peacock, S., 1996, Thermal and Petrologic Structure
of Subduction Zones: in Bebout et al. (eds.) Subduction Top to
Bottom, American Geophysical Union Geophysical Monograph 96, p.
119-133. This is a nice recent compilation looking at the basic
question of how arc volcanics are generated.
- Ryan, J., Morris, J., Bebout, G. & leeman,
B., 1996, Describing Chemical Fluxes in Subduction Zones: Insighs
from "Depth-Profiling" Studies of Arc and forearc Rocks;
in Bebout et al. (eds.) Subduction Top to Bottom, American Geophysical
Union Geophysical Monograph 96, p. 119-133.
- Taylor, B., 1995, Backarc Basins; Plenum Press,
N. Y. 524 p. This has a succinct evolutionary history of backarc
basins in the preface, and then articles with loads of details.
- Uri ten Brink, 1990, Volcano spacing and plate rigidity; Geology, 19, 397-400.
- Worrall, D. M., 1991, Tectonic History of the
Bering Sea and the Evolution of tertiary Strike-Slip Basins of
the Bering Shelf; GSA ASpecial Paper 257, 120 p and many plates.
A very detailed and richly supported history for the Bering Sea.
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.