The rising mantle
'leachate' - arc complexes.
Paired metamorphic belts.
- last week we left off looking at the thermal
structure of a subduction-arc complex.
- the following pattern first worked out for
Japan by Miyashiro.
- the accretionary wedge is associated with
a lower geothermal gradient, and distinctive blueschist and eclogite
- what is eclogite??
- paper in Geology - when the earth
became cool enough to form eclogites, plate tectonics started.
- 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.
- the two belts of contrasting metamorphism
give rise to the term paired metamorphic belt.
- Japan has two pairs.
Production of arc magmas:
- range in composition: (3 common series)
- low-K tholeiitic series (mostly basalt).
- calc-alkaline series (mostly andesite).
- alkali series (alkaline basalts and shoshonites).
- partial melt mechanism(s):
- shear heating (minimal due to low friction).
- model of conditions at and above slab under
volcanic arc: 500-700 degrees C (Peacock); and significance of
low heat flow in arc-trench gap areas.
- fluid release from prograde metamorphic reactions:
- depths 80- 100 km: amphibolite -> eclogite
+ water. Water + pyrolite at these PT -> melt of asthenosphere.
- depths >100 km serpentine dehydrates
and partial melting of quartz eclogite crust produces acidic
magmas, which interact with the mantle pyrolite and fractionate
to produce calc-alkaline series. (Serpentine associated more
with fracture zones (?) could set up some predictions for this
- evidence for slab contributions:
- 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. May also be episodic.
- evidence for mantle contributions.
- the presences of basalts demands a mantle
- corner flow model
and replenishment of the source material.
Growth of arc root: Beneath
an arc can have an 18-20 km root (see diagram above). What are
mechanisms of production?
- tectonic processes.
- magmatic underplating (mafic lower crust).
- batholiths as columns versus blisters.
Spatial patterns of volcanic and plutonic
composition within arcs.
- distance from trench -> increase in slab
- 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.
map of volcanos associated with Marianas trench.
U.S. west coast volcanics.
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 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. Due next time.
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. Theidea 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:
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 backarc (extreme S Atlantic)
Characteristic traits of these basins:
- strong seismic wave attenuation in underlying
mantle above the slab.
- 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-rfiting event. MORB like flow.
- often thick sediment fill.
- 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.
Rear-arc fold-thrust belts.
- Good example the Andes (see exercise map)
and the Laramides.
- Trench axes not migrating in hotspot reference
- More common on continental margins.
- Crustal thickening definitely is a major tectonic
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.
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.
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.